Displays

Models for the formation of the Rustenberg Layered Suite of the Bushveld Igneous Complex continue to be debated. The consensus timescale over which magmatism took place has reduced hand in hand with advancements in geochronological techniques and data precision. The most recent studies by double spiked (²⁰²Pb-²⁰⁵Pb) zircon CA-ID-TIMS U-Pb have indicated emplacement in less than 1 Myrs [1][2]. Increasing analytical precision has also seemingly permitted individual magmatic layers to be resolved, leading to the “out of sequence sill” emplacement model [2], albeit contested [3].

We present two new high-precision zircon dates obtained from two continuous core intervals collected  <4m apart in a single Ni-Cu-PGE rich pyroxenite unit in the Turfspruit section of the Platreef, Northern Limb of the Bushveld Complex [4]. Grobler et al. [5] correlate this pyroxenite with the Merensky Cyclic Unit of the Upper Critical Zone in eastern and western limbs. Assuming the recommended zircon ²³⁸U/²³⁵U of Hiess et al. [6] without uncertainties propagated as per previous studies e.g. [1][2], the age interpretations of these two samples define a minimum and maximum temporal interval between 1.01 ±0.16 Myrs and 1.28 ±0.22 Myrs that brackets, or overlaps with, the entirety of previous dates from all preceding studies. The pyroxenite is continuous, without intrusive contacts, and the stratigraphically lower sample produces an apparently younger zircon age than the overlying sample.  It seems highly unlikely the entire longevity of the Bushveld’s magmatic evolution was apparently captured within this 4 m section. Therefore, it now seems highly improbable that the Bushveld was emplaced and cooled in less than 1 Myrs, as the current paradigm states [1].

The older date from the Platreef now aligns the isotopic age relationships with the field observations of the overlying Main Zone, in contrast to the interpretation of Mungall et al. [2]. The new dates alone neither support nor contradicts the “out of sequence” sill emplacement model. Rather they merely indicate that melt related process that crystallised zircon was protracted within narrow vertical intervals, and that future work should acknowledge this potential complexity. It raises questions which age of event(s) introduced or modified sulfides within the ore bearing horizon. This requires greater integration of the geochronological record with ore textures at a high sampling density.

However, there also remains a substantial, yet previously overlooked caveat to all geochronological interpretations presented thus far; “out of sequence” sills in particular. This caveat is that the variations in the ²³⁸U/²³⁵U between samples over observed magnitudes of variations in zircon [4] could account for any offsets in ²⁰⁷Pb/²⁰⁶Pb dates interpreted as real temporal differences. This issue remains to be tested.

References:

[1] Zeh A et al. (2015) EPSL 418:103-114; [2] Mungall J et al. (2016) Nat. Coms. 13385; [3] Latypov R et al. (2017) South African Jour of Geol. 120.4, 565-574; [4] Nodder SM (2015) MESci dissertation, Cardiff University, 257pp; [5] Grobler D et al. (2019) Min Dep 54, 3-28; [6] Hiess J et al. (2012) Science 418,103-114

How to cite: Tapster, S., McDonald, I., Holwell, D., and Grobler, D.: Are protracted timescales of magmatism documented in the Platreef, Bushveld Complex?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19851, https://doi.org/10.5194/egusphere-egu2020-19851, 2020.

The association of platinum-group elements (PGE) and the chalcophile elements Te, As, Bi, Sb and Sn (TABS) has been documented in several magmatic sulfide deposits. These groups of elements are either hosted within sulfide minerals, or combine to form discrete platinum-group minerals (PGM) associated with sulfide minerals. However, the concentration of TABS in parental magmas from which magmatic sulfide deposits formed was still missing. This study presents the distribution of TABS and Se in B-1, B-2 and B-3 rocks of the Marginal Zone of the Bushveld Complex. These rocks have been proposed as representative of the parental liquids from which the Bushveld Complex crystallized, thus allowing us to assess the concentration of Se and TABS in the liquids from which some of the largest PGE deposits in the world have formed. Concentrations of As and Sb in the initial Bushveld liquid (B-1) are significantly higher than in primary magmas, whereas the Se and TABS of later magmas (B-2 and B-3) are similar to primary magmas. We attribute the difference due upper crustal contamination of the B-1 magma, whereas the B-2 and B-3 magmas were most likely contaminated with a plagioclase-rich residuum formed upon the partial melting of the upper crust. Moreover, we modeled the concentrations of the TABS in the Merensky Reef using a mixture of two of the magma types present in the Marginal Zone (the B-1 and B-2) as the initial silicate liquid. The modeled concentrations closely resemble the measured values obtained for a section across the Merensky Reef at the Impala mine. This supports the B-1 and B-2 mixture as an appropriate initial liquid for the crystallization of the Merensky Reef. The modeling also shows that the distributions of Se, Te and Bi across the Merensky Reef are controlled by the sulfide liquid component. In contrast, As and Sb distributions are influenced both by the amount of silicate melt component in the cumulates and the sulfide liquid component. This is because Se, Te and Bi are moderately to strongly chalcophile elements, but As and Sb are only slightly chalcophile elements. Consequently, the effect of crustal contamination for elements with high partition coefficients between sulfide and silicate liquid (Te, Bi and Se) is obscured by the interaction of sulfides with a large volume of silicate magma. Therefore, the concentrations of these elements are higher in samples with greater proportions of sulfide minerals. In contrast, for elements with lower partition coefficients (As and Sb), the whole-rock concentrations are not upgraded by the presence of sulfide minerals, and thus the effect of crustal contamination can be more readily assessed.

How to cite: Mansur, E. and Barnes, S.-J.: Concentrations of Te, As, Bi, Sb (TABS) and Se in the Marginal Zone of the Bushveld Complex: evidence for crustal contamination and the nature of the magma that formed the Merensky Reef, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12277, https://doi.org/10.5194/egusphere-egu2020-12277, 2020.

The Upper Group 2 (UG2) chromitite layer in the upper Critical Zone of the Bushveld Complex, South Africa, is the world’s largest PGE orebody. The UG2 is typically 0.5 to 1.5 m thick and it consists of 75–90 modal % chromite with interstitial silicate and sulfide minerals. Although a minor component, phlogopite is important because it is a hydrous phase. It has been noted that the UG2 chromitite contains more abundant phlogopite (locally > 1 modal %) than the surrounding pyroxenite layers (Mathez and Mey, 2005). More and more studies suggest that water plays an important role in the UG2 chromite formation and in PGE enrichment or remobilization (e.g., Li et al., 2004; Mathez and Mey, 2005; Schannor et al., 2018). The source of the water is controversial, and this motivated our ongoing study of hydrous minerals in the UG2.

We determined the chemical composition and hydrogen isotope ratio of phlogopite from the chromitite layer and from the surrounding pyroxenite in drill cores from two sites the eastern and western Bushveld (Nkwe and Khuseleka, respectively). The δD values of phlogopite in chromitite from the eastern site are -38.2 to -25.5‰ (mean = -29.7‰, n = 6). The corresponding values from the western site are similar, with -34.6 to -31.6‰ (mean = -33.2‰, n = 6). The δD values of phlogopite from pyroxenite are more variable, ranging from -43.1 to -26.1‰ for the eastern site (mean = -32.9‰, n = 4) and from -38.7 to -26.1‰ for the western site (mean = -31.7‰, n = 3).

Published whole-rock δD values for silicate cumulate rocks in the upper Critical Zone are -93 to -55‰ (Mathez et al., 1994), which are similar to mantle values (-70±10%; Boettcher and O'neil, 1980) and are interpreted as magmatic.  In comparison, our δD values of phlogopite from UG2 are much higher and suggest a significant contribution of crustal fluids. Harris and Chaumba (2001) estimated a δD value of -22‰ for paleo-meteoric water in the Bushveld Complex. Given the relative homogeneity of the phlogopite δD data in both sites of the complex, and the primary appearance of the grains in thin section, we argue that the crustal fluids were incorporated in the magma before the crystallization of the UG2 layer. Triple oxygen isotopes will test our hypothesis further.

References: Boettcher & O'neil (1980) Amer. Jour. Sci. 280A, 594–621. Harris & Chaumba (2001) J. Petrol. 42, 1321–1347. Li et al. (2004) Econ. Geol. 99, 173–184. Mathez et al. (1994) Econ. Geol. 89, 791–802. Mathez & Mey (2005) Econ. Geol. 100, 1616–1630. Schannor et al. (2018) Chem. Geol. 485, 100–112.

How to cite: Zhou, H., Trumbull, R., Veksler, I., Glodny, J., and Bindeman, I.: Hydrogen isotopes in phlogopite indicate crustal fluids in the UG2 chromitite layer, Bushveld Complex, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10852, https://doi.org/10.5194/egusphere-egu2020-10852, 2020.

The liquid line of descent of the Skaergaard magma intersects a binodal creating an immiscible conjugate pair comprising a dense Fe-rich liquid and a buoyant Si-rich liquid. These two liquids have different wetting properties: the Si-rich liquid wets plagioclase, whereas the Fe-rich liquid wets oxides, pyroxene and olivine. The two liquids may therefore undergo differential migration within a gabbroic crystal mush: the Fe-rich liquid sinks and accumulates in mafic layers, while the Si-rich liquid rises and accumulates in plagioclase-rich regions.

Field-scale evidence of metre-scale differential migration of unmixed immiscible interstitial liquids is provided by paired felsic and mafic lenses spatially associated with gabbroic pegmatite bodies in the Skaergaard floor cumulates. These represent small batches of late-stage liquids rising from the pegmatite bodies into the overlying mush, and their subsequent separation into immiscible conjugates. The paired lenses form irregular, approximately layer-parallel clusters in thick mush, but thin concordant dendritic structures within strongly foliated thin mush. Invariably the melanocratic component lies stratigraphically below the felsic component.

Differential migration within the floor cumulates is also recorded by mm-scale mafic and felsic rims developed on the top and bottom margins of anorthositic blocks derived from the roof. Highly tabular blocks have an upper mafic rim and a lower leucocratic rim. As the block aspect ratio decreases, the rims disappear, with the mafic rim retained at lower aspect ratios than the leucocratic rim. We interpret rim formation as a consequence of trapping migrating unmixed interstitial liquid against the relatively impermeable blocks: tabular blocks are most effective at trapping these liquids.

On a smaller scale, the different wetting properties of the two immiscible conjugates result in post-accumulation pattern formation in rapidly deposited modally graded layers, imposing cm-scale internal layering on the overall modal grading. The tops of the modally-graded layers may also develop felsic flame-like structures interpreted as a consequence of upwards-migration of the immiscible Si-rich conjugate from high-porosity rapidly deposited layers into the overlying cumulates.

These observations demonstrate the complexity of behaviour in a crystal mush containing a two-phase interstitial liquid. Understanding cumulate evolution necessitates a consideration of the scale of migration of interstitial liquid and the possibility of the differential loss of one of the two conjugates.

How to cite: Holness, M., Honour, V., and Nicoli, G.: Differential migration of interstitial immiscible liquids in the Skaergaard Layered Series, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2334, https://doi.org/10.5194/egusphere-egu2020-2334, 2020.

In order to test whether the crystal mushes that form layered mafic intrusions can behave as open systems, we investigated mineral-scale textural, chemical and Sr isotopic heterogeneity in the c. 60 Ma Rum intrusion, Scotland. Within Unit 10 of the Rum intrusion, intercumulus plagioclase and clinopyroxene crystals in peridotite 1-2 cm above and below millimetric Cr-spinel seams exhibit complex optical and chemical zoning (Hepworth et al. 2017). These Cr-spinel seams are closely associated with sulphide and platinum-group element (PGE) mineralization. High precision Sr isotopic analyses (undertaken by thermal ionisation mass spectrometry) of individual intracrystal zones (sampled by micromilling) in intercumulus plagioclase and clinopyroxene from within the PGE-enriched Cr-spinel seams have revealed significant intra-crystalline heterogeneity. ⁸⁷Sr/⁸⁶Sr heterogeneity is present between plagioclase crystals, between clinopyroxene and plagioclase, and within plagioclase crystals, throughout the studied section. The preservation of Sr isotope heterogeneities at 10-100 µm length-scales implies cooling of the melts that formed the precious metal-rich layers at rates >1 °C per year, and cooling to diffusive closure within 10s-100s of years. The combined textural observations and intra-crystal plagioclase ⁸⁷Sr/⁸⁶Sr data also highlight the importance of repeated cycles of dissolution and recrystallization within the crystal mush, and together with recent documentation of ‘out-of-sequence’ layers in other layered intrusions (Mungall et al. 2016; Wall et al. 2018), raise the prospect that basaltic magmatic systems may undergo repeated self-intrusion during solidification.

Hepworth, L.N., O’Driscoll, B., Gertisser, R., Daly, J.S. and Emeleus, H.C. 2017. Journal of Petrology 58, 137-166; Mungall, J. E., Kamo, S. L. & McQuade, S. 2016. Nature Communications 7, 13385; Wall, C. J., Scoates, J. S., Weis, D., Friedman, R. M., Amini, M. & Meurer, W. P. 2018. Journal of Petrology 59, 153–190.

How to cite: Daly, J. S., Hepworth, L., O'Driscoll, B., Johnson, C., Gertisser, R., and Emeleus, C. H.: Sr isotopes indicate millennial-scale formation of metal-rich layers by reactive melt percolation in an open-system layered intrusion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20272, https://doi.org/10.5194/egusphere-egu2020-20272, 2020.

The composition of the magmas from which the chromites that form the massive chromite layers of the Stillwater, Great Dyke and Bushveld Complexes are of interest both to understand the economic importance of the resources in the layers (Cr and PGE), but also in understanding how these layers form.  Magmas that have been suggested as parental to the intrusions are boninites or crustally contaminated komatiites.  Another magma that could be considered in recognition of the continental setting of the Bushveld and Great Dyke is picrite associated with continental flood basalts. In order to investigate whether any of these magmas are suitable parental magmas for the chromites we have determined major and trace elements in komatiites of low metamorphic grade, boninites and chromites from low-Ti and high-Ti picrites of the Emeishan Provence.

In order to test whether the chromites are in equilibrium with volcanic magmas we first modelled the major and minor element composition of the chromites that should have crystallized from the komatiite, boninites and picrite liquids using SpinMelt v2.  The compositions are approximately correct.  In terms of major and minor elements none of the chromites from the layered intrusions match boninite chromites.  The Great Dyke chromites are similar to chromites from komatiites.  The chromites the Bushveld are slightly more evolved with higher Ti contents and lower Cr# and resemble the chromites from the low-Ti picrites of Emeishan.  The Stillwater chromites have similar Ti contents to the Emeishan low-Ti picrites, but have lower Cr#.  Their compositions resemble chromite compositions reported from the North Atlantic Igneous Provence.

Hafnium, Ta, Cu, Sn, Sc, Ti, Mn, Ni, Co, Mn, Ga, V and Zn were determined by LA-ICP-MS.  To compare the composition of the chromites an estimate of their partition coefficients into chromite was made based on the concentrations of elements in komatiite chromite divided by element in komatiite.  The elements were then arranged in order of compatibility and the chromites normalized to the median komatiite chromite.  Podiform chromites from boninites are depleted in most elements and none of the layered intrusions chromites resemble them.  The chromites from the Great Dyke have essentially flat patterns close to 1 times komatiite, but with negative Cu anomaly and a slight positive Sn anomaly.  The Bushveld and Stillwater chromites are richer in Al, Ga, V and Ti than the komatiite chromite and are depleted in Cu.  The patterns resemble the chromites form the low Ti-picrites form Sn to Zn, but differ from picrites from Hf to Cu.  The picrites are enriched in Hf, Ta and Cu.

The chromite compositions suggest that boninite magmas are not involved in forming the chromites from layered intrusions.  The Great Dyke chromites appear to have a komatiitic affinity.  The Bushveld and Stillwater chromites appear to have a low-Ti picrite affinity.

How to cite: Barnes, S.-J., Mansur, E., Pagé, P., Meric, J., and Arguin, J.-P.: A Comparison of Major and Trace Element Compositions of Chromites from the Stillwater, Bushveld and Great Dyke Layered Intrusions with Chromites from Komatiites, Boninites and Large Igneous Provinces. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9768, https://doi.org/10.5194/egusphere-egu2020-9768, 2020.

Recent developments in high definition mineral chemistry at the grain scale are shedding new light on the processes and rates of magma storage, differentiation and eruption. However, the complementary physics and fluid dynamics of magma as a granular material are still based on viscous compaction theory, which may not be relevant in sub-volcanic settings where magma is being deformed by external shear. We present a quantitative model for shear deformation of a crystallised dense magma (>70% solid) with poro-elastic properties where the critical link between the mechanics and associated compositional changes in the melt are governed by dilation (volume increase) of the granular skeleton. Key material parameters governing the dilatancy effect include magma permeability, mush strength, the shear modulus and the contact mechanics and geometry of the granular assemblage. Calculations show that dilation reduces the interstitial fluid (melt) pressure to produce a ‘suction’ effect. At shear strain rates in excess of the tectonic background, deformation-induced melt flow can redistribute chemical components and heat between regions of crystallising magma with contrasting rheological properties, at velocities far in excess of diffusion or buoyancy forces, the latter of course the driving force behind fractional crystallisation and compaction. Unlike static magmas, there is no ‘lock-up’ state above which the interstitial melt cannot percolate. Co-mingling of hotter, indigenous melt has the potential to interrupt (or locally reverse) fractionation trends and produce reverse zoning or resorbtion of crystals, mimicking some of the textural effects attributed to magma mixing. Post-failure instabilities include hydraulic rupture of the mush along shear zones with potential for larger scale extraction and redistribution of evolved melt. A novel feature of congested, sub-volcanic granular magma is that the eruption itself helps drive rapid melt extraction, negating the requirement to first segregate large volumes of evolved melt as a precursor. 

How to cite: Petford, N., Clemens, J., and Koenders, C.: Differentiation in Sheared Granular Magma, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10584, https://doi.org/10.5194/egusphere-egu2020-10584, 2020.

Boron is a distinctly crustal element in that it is strongly enriched in the surface reservoirs, such as continental crust, seawater, sediments, serpentinites and altered oceanic crust, relative to the mantle. These B-enriched reservoirs are also isotopically very distinct from the regular depleted upper mantle (d¹¹B = -7.1 ±0.9 ‰ [10.1016/j.gca.2017.03.028]). This has encouraged the idea that boron could be an ideal tracer for subducted surface materials in the deep mantle in the form of isotopically anomalous recycled components in ocean island basalts (OIB) and enriched MORB. Yet, the potential of a geochemical tracer of this type is weakened by its extraction from the slab at the onset of subduction by dewatering and metamorphic dehydration, because this process depletes the recycled components in fluid-mobile elements. As such, this “subduction barrier” diminishes the deep recycling efficiency of incompatible, fluid-mobile tracers like B.

This study focuses on the B abundances and B isotopic compositions of glasses and melt inclusions that show low Cl/K ratios and are thought to represent the uncontaminated mantle signal from the HIMU (Tuvalu and Mangaia), EM1 (Pitcairn) and EM2 (Samoa) sources. Strikingly, all samples are depleted in boron by a factor of approximately 1.5 to 4 relative to non-fluid-mobile elements of similar incompatibility (e.g. LREE, P, Be). This negative boron anomaly is ubiquitous in OIB and is consistent with the results of previous studies [10.1016/0016-7037(95)00402-5; 10.1016/j.epsl.2018.12.005]. It also mirrors their characteristic negative Pb anomaly. These anomalies show that the mantle sources of OIB are depleted in B (and Pb) relative to non-fluid-mobile elements of similar incompatibility and relative to the MORB-source mantle. This is best explained by the presence in the OIB sources of recycled components that are enriched in all incompatible elements except for the fluid-mobile B (and Pb). The fluid mobile elements must have been preferentially extracted in the subduction barrier and returned to the surface on the short path via arc magmas. Arc magmas consistently show a general enrichment in isotopically heavy boron [10.1007/978-3-319-64666-4_9] with positive B anomalies.

Despite of the low recycling efficiency of boron into the convecting mantle, OIB still have B isotope signatures that are distinct from those of MORB. Previous studies have reported OIB signatures slightly lighter than MORB and the primitive mantle [10.1016/j.epsl.2018.12.005]. However, our study exclusively finds isotopically heavy B with a range in d¹¹B from MORB-like values (-8.6 ±2.0 ‰) up to -2.5 ±1.5‰ for EM1 and HIMU lavas. The total OIB range is small but significant, and is consistent with the deep recycling of material that is strongly depleted in boron, but isotopically distinct (with isotopically heavy B in the case of our EM1 and HIMU samples). The B depletion combined with the B isotopic anomaly in OIB shows that B is efficiently (but not quantitatively) removed from the slab during subduction, and that isotopically distinct mantle domains are thus produced. The subduction barrier for boron increases its strength as a tracer in arcs, but it diminishes its potential as a tracer of deep mantle recycling.

How to cite: Marschall, H. and Jackson, M.: Low recycling efficiency of boron into the deep mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10054, https://doi.org/10.5194/egusphere-egu2020-10054, 2020.

The chemical diversity observed in the rock record of volcanic arcs is determined by a multitude of processes operating between the magma source region and the surface. A fundamental step in producing this variability is fractional crystallisation, assimilation and melting in the lower crust which drives magmas to more evolved and hydrous compositions. During extensive fractionation of hydrous magmas in the lower crust, amphibole (± garnet) is stabilized in the fractionating assemblage and plagioclase is suppressed resulting in melts with elevated Sr, an absence of strong negative Eu anomalies (both elements being compatible in plagioclase), and depleted Y (compatible in amphibole and garnet). The high Sr/Y values that result can be used to provide insights into arc magma evolution, evaluate whether a magmatic system has the potential to form a porphyry-related ore deposit and track crustal thickness. However, this deep fractionation history may be obscured due to differentiation and mixing upon ascent to the shallow crust. Since arc rocks are a product of this multi-stage, polybaric process, unravelling the complete history of magmatic evolution using bulk-rock chemistry alone can be challenging. However, accessory minerals such as apatite, are capable of capturing discrete periods of melt evolution during differentiation [1]. For example, apatite has been shown to record the Sr content of the melt at the time of its crystallization which has been used to reconstruct host rock compositions in provenance studies [2, 3].

Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of Central Chile. These rocks formed during magmatism that culminated in high Sr/Y magmas and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu* and Mg in apatite to demonstrate that apatite tracks the multi-stage differentiation of arc magmas. We apply fractional crystallization modelling to show that early crystallizing apatite inherits a high Sr/Y and Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modelling shows that crystallisation of the in-situ host rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk magma chemistry. Understanding this decoupling behaviour is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies. We suggest our approach is widely applicable for unravelling the composite evolution of arc magmas and studying magmatic processes conducive to porphyry ore deposit formation.

References

[1] Miles, A.J., Graham, C.M., Hawkesworth, C.J., Gillespie, M.R., and Hinton, R.W., 2013, Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon: Contributions to Mineralogy and Petrology.

[2] Jennings, E.S., Marschall, H.R., Hawkesworth, C.J., and Storey, C.D., 2011, Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so: Geology.

[3] Bruand, E., Storey, C., and Fowler, M., 2016, An apatite for progress: Inclusions in zircon and titanite constrain petrogenesis and provenance: Geology, v. 44, p. 91–94.

How to cite: Nathwani, C., Loader, M., Wilkinson, J., Buret, Y., Sievwright, R., and Hollings, P.: Multi-stage arc magma evolution recorded by apatite in volcanic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1405, https://doi.org/10.5194/egusphere-egu2020-1405, 2020.

The presently active La Fossa cone, Vulcano, widely considered the most hazardous volcano in the Aeolian Islands, is characterised by alternating periods of Vulcanian to subplinian explosive events and lava flow effusion. It has formed over 5.5 kyr, last erupting in 1888-90 [1], and presently behaves in a quiescent, fumarolic stage. The volcanic deposits from the cone comprise 7 major formations: Punte Nere, Grotta dei Palizzi 1, 2, and 3, Caruggi, Pietre Cotte and Gran Cratere. Many of these commence with dilute pyroclastic density current (PDC) deposits and tephra fallout capped by lava flows, with a compositional range from shoshonite to rhyolite (52-74 wt.% SiO₂) [1]. Crustal xenoliths in some of the lava flows and PDC deposits signify the importance of crustal contamination in the La Fossa magmatic system [1]. Here, we present new oxygen isotope data of mineral (clinopyroxene, plagioclase) and glass separates and combine these with petrological and textural analyses as well as clinopyroxene crystal chemistry and thermobarometry to constrain the extent of crustal contamination and to determine if and where crustal contamination took place in the magmatic system of La Fossa.

Oxygen isotope data are presented for pumice, scoriae, breadcrust bombs, lavas and mafic magmatic enclaves of all formations of La Fossa. δ¹⁸O values range from +6.0‰ to +6.7‰ (SMOW) for clinopyroxene (n=19), from +7.0‰ to +8.1‰ for feldspar (n=15) and from +8.3 ‰ to +8.7 ‰ for obsidian glass (n=2). Estimated δ¹⁸O_melt values are higher than that of mantle-derived magmas, indicating that crustal contamination is ubiquitous in the La Fossa magma plumbing system. δ¹⁸O_fsp increases with the degree of magmatic differentiation, indicating feldspar is more contaminated in the more evolved products of La Fossa. However, no systematic variation is observed between δ¹⁸O_px and whole-rock SiO₂, indicating disequilibrium between clinopyroxene and plagioclase. The disequilibrium observed at La Fossa suggests that clinopyroxene is mostly xenocrystic in the more evolved samples. This is supported by clinopyroxene equilibrium tests. Single-crystal X-ray diffraction to determine clinopyroxene crystal structures is presented to constrain crystallisation pressures. Crystallisation pressure of magmas feeding explosive eruptions to between approximately 2 and 6 kbar, while magmas feeding effusive eruptions appear to have crystallised at a narrower pressure range. Our results indicate that crustal contamination is an important process at La Fossa that accompanies fractional crystallisation and magma mixing/mingling processes throughout the entire (deep to shallow) crustal magma plumbing system.

References:

[1] De Astis et al. 2013. Geol. Soc. London Memoirs. 37. 281-349.

How to cite: Wiltshire, R., Gertisser, R., Halama, R., Boyce, A., Petrone, C., Nazzareni, S., Lucchi, F., Tranne, C., and Sulpizio, R.: Insights into the magma plumbing system of La Fossa di Vulcano (Aeolian Islands, Italy) using oxygen isotopes and clinopyroxene crystal structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13433, https://doi.org/10.5194/egusphere-egu2020-13433, 2020.

Silicic magmatism in the Central Andes forms rhyolitic to dacitic volcanic deposits that range from large-volume ignimbrites (>1000 km3) to small local dome eruptions. The mass proportion between mantle-derived magmatic contributions to crustal melting was previously estimated to range from 20 to 70 % based on Sr-O isotope data obtained on separated feldspar and quartz contained as crystal cargo. New O-Hf isotope data from in-situ ion-probe and laser ablation measurements of U-Pb-dated zircons further constrain type, proportion, and processes of crustal input into silicic magmas. Variations in time and space of these geochemical parameters are documented here using representative samples that cover the entire Central Andes over 20 Ma and 800 km distance. Systematic covariations in isotope tracers relate to increasing crustal thickening through time during Andean orogenesis. Collectively, Sr-Nd-Pb-Hf-O isotopic signatures vary in space and time and temporally reflect increasing crustal input during ignimbrite flare-ups as the crust becomes thermally matures. Spatial variations derive from different crustal domains in the Central Andes and reflect the different age and composition of crustal components.

Remarkably, inherited zircon representing basement involved in crustal assimilation is exceedingly rare over the entire province. This most probably reflects high temperatures that exceed zircon saturation temperatures of crustal melts in ignimbrite-forming magmas. This observation distinguishes silicic ignimbrite-forming magmatism from typical granitoid-forming magmatism in orogenic settings where abundant older zircons inherited from the crust are commonly found.

How to cite: Wörner, G., Belousova, E., Turner, S., Kemann, J., Schmitt, A. K., Gerdes, A., and de Silva, S.: Ignimbrite flare-ups in the Central Andes: Crustal sources and processes of magma generation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3519, https://doi.org/10.5194/egusphere-egu2020-3519, 2020.

We present the new approach to AFC modelling published as Editor’s Choice in the July 2019 issue of Journal of Petrology [1].

Our new, Equilibrated Major Element – Assimilation with Fractional Crystallisation (EME-AFC) approach simultaneously models the major element, trace element, and radiogenic and oxygen isotope compositions during such magmatic differentiation (including a new approach to oxygen modelling); addressing the lack of current AFC modelling approaches for felsic, amphibole- or biotite-bearing systems. We discuss the application of this model to granitic magmatism in SE Asia and Antarctica, with particular focus on the Mt Kinabalu granitic intrusion of Borneo. We discuss the background to the model, and explain how it can be freely accessed via GitHub [2], and applied to other scenarios of magmatic differentiation; not just granitic magmatism.

We present new geochemical data for the composite units of the Mount Kinabalu, and use this to explore the discrimination between crustal- and mantle-derived granitic magmas. The isotopic data (oxygen, Hf, Sr, Nd, and Pb) indicate that the magma cannot be the result only from fractional crystallisation of a mantle-derived magma. Alkali metal compositions show that crustal anatexis is also an unsuitable processes for genesis of the intrusion. Using the new EME-AFC modelling approach, we show that the high-K pluton was generated by fractional crystallisation of a primary, mafic magma followed by assimilation of the partially melted sedimentary overburden. We propose that Mt Kinabalu was generated through low degree melting of upwelling fertile metasomatised mantle driven by regional crustal extension in the Late Miocene.

[1] Burton-Johnson, A., Macpherson, C.G., Ottley, C.J., Nowell, G.M., Boyce, A.J., 2019. Generation of the Mt Kinabalu granite by crustal contamination of intraplate magma modelled by Equilibrated Major Element Assimilation with Fractional Crystallisation (EME-AFC). J. Petrol. 60, 1461–1487.

[2] https://github.com/Alex-Burton-Johnson/EME-AFC-Modelling

How to cite: Burton-Johnson, A., Macpherson, C., Ottley, C., Nowell, G., and Boyce, A.: A new approach to modelling differentiation (with particular focus on granitic magmatism): Equilibrated Major Element Assimilation with Fractional Crystallisation (EME-AFC), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22106, https://doi.org/10.5194/egusphere-egu2020-22106, 2020.

Peralkaline magmatism is mostly sustained by extensive feldspar fractionation from mafic parents at shallow depths in intraplate settings. In this case, silica saturation is critical as it controls the differentiation trend that a peralkaline magma follows. SiO₂-oversaturated parents fractionate towards rhyolites, and SiO₂-undersaturated towards phonolitic compositions. The Miocene post-shield stage of Gran Canaria records both differentiation trends, which has previously been ascribed to changes in the mantle source. Such stage has been divided in the Mogan and Fataga Group based on silica saturation. Here, we propose that contamination plays a key role in the differentiation of Gran Canaria volcanics. This assumption is supported with new ⁴⁰Ar/³⁹Ar geochronology, mineral, glass and juvenile clast chemistry (oxygen isotopes, major and trace elements) merged with a detailed stratigraphy. Two types of contaminants were identified, one being cogenetic feldspar-dominated cumulates and the second one being sediments within the island crust. We propose that barium-rich trachytic magmas with positive europium anomalies are linked to melting of the feldspar cumulates left after extensive fractional crystallisation. The chemistry of such trachytes does not follow a liquid line of descent and contains reverse-zoned alkali-feldspars. The shift in silica saturation took less than 1 Ma and is marked by an increase in peralkalinity from 0.9 to 1.5 and a decrease in oxygen isotopes ratio from 7.0 to 5.0 ‰. We interpret these observations as the consequence of maturation of the shallow magma reservoir towards less sediment contamination. Such assimilation of sediments is limited thermally, and compositionally because the system should remain alumina deficient. Crustal assimilation in Gran Canaria did not produce voluminous silicic melts by itself but allowed the deviation of the differentiation trend of a more primitive, initially SiO₂-undersaturated magma. The tightrope of differentiation is represented by the thermal divide between the granite and phonolite minima (i.e. feldspar join in petrogeny’s residua system). Contamination by sediments produces a transient SiO₂-oversaturated system (Mogan Group). Cogenetic assimilation of cumulates by thermal rejuvenation of the reservoir attracts the magma towards the thermal divide (ubiquitous during the peralkaline stage). Armouring against sediment assimilation through time relaxes the system back to the initial SiO₂-undersaturated conditions (Fataga Group).

How to cite: Cortes Calderon, E. A., Ellis, B., Neukampf, J., Harris, C., Mark, D., Wolff, J., and Bachmann, O.: The role of contamination in the tightrope of Gran Canaria felsic magma differentiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13633, https://doi.org/10.5194/egusphere-egu2020-13633, 2020.

The 8-9 km thick Dufek layered mafic intrusion of Antarctica was emplaced at approximately 182 Ma associated with the Ferrar dolerites and the breakup of the supercontinent Gondwana.  It is rivaled in thickness only by the Bushveld Complex of South Africa and shows a similar progression in mineral compositions all the way to the uppermost contact with an overlying granophyre layer.  This progression in mineral composition suggests that it crystallized from the bottom to the top and did not form an upper solidification front (a.k.a., Upper Border Series) typical of smaller intrusions such as the Skaergaard Intrusion.  Unlike the Bushveld Complex, however, the Dufek Intrusion is exposed in only two ~1.8 km thick sections: the lowermost Dufek Massif, and the uppermost Forrestal Range, which are separated from one another by a ~50km wide snowfield.  The remainder of the stratigraphy is inferred from geophysics, evolution of mineral compositions, and projection of the dip of the layering through the snowfield. 

            We obtained precise CA-ID-TIMS U-Pb zircon ages from samples from the Dufek Massif and Forrestal Range in order to determine the timescale of solidification of a large layered mafic intrusion.  What we found is surprising - zircons from the bottom of the intrusion record younger ages than those from the top of the intrusion.  Two samples from the Dufek Massif have zircon U-Pb ages of 182.441±0.048 Ma and 182.496±0.057 Ma; whereas three samples from the Forrestal Range have zircon U-Pb ages of 182.601±0.064 Ma, 182.660±0.10 Ma, 182.78±0.21 Ma.  Thus, the lower section of the Dufek Intrusion solidified approximately 160,000 years after the upper.  We explore two possibilities for this reverse-age stratigraphy, (1) that the ages reflect the solidification of interstitial melt in a single magma chamber cooling from the top down, or (2) that the Dufek Massif and Forrestal Range are two separate magma chambers that are not connected at depth.  Our results have implications for the stratigraphic thickness estimates of the Dufek Intrusion as well as the duration of magmatism associated with continental breakup.

How to cite: VanTongeren, J., Taylor, A., and Schoene, B.: Solidification timescale for the Dufek Intrusion, Antarctica determined by U-Pb zircon ages, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10794, https://doi.org/10.5194/egusphere-egu2020-10794, 2020.

Study of phenocryst and megacryst mineral associations of alkali rocks is the key to understanding an evolution and a source of the rocks.

In the Devonian Kola alkaline province (KAP) along with large mafic-ultramafic massifs there are several synchronous swarms of lamprophyre dykes. The dyke swarms occur mainly in the Kandalaksha graben. As suggest, their compositional diversity is caused by fractional crystallization and crustal contamination and a primary melt of lamprophyre was generated from a common source.  

We have studied two dykes of the Turiy Cape swarm and  the Kandalaksha swarm in the vicinity of the Kandalaksha town. The aim of  study was to determine the source of lamprophyre melts based on petrography, geochemistry and detailed investigation of clinopyroxene and olivine.

Two principal petrographical types: alkali lamprophyres (Cb-Anl monchiquite) and ultramafic lamprophyres (Cpx ailikites and mela-alikites) in the Kandalaksha swarm were observed. Alkali lamprophyres contain medium size (0.3 – 1 cm) phenocrysts of olivine, clinopyroxene, magnetite, phlogopite. A groundmass contains analcime, clinopyroxene, various amounts of carbonate (from 0 to 30-40 %), pyrite, apatite, ilmenite.

Ultramafic lamprophyres contain medium size (0.5 – 1 cm) phenocrysts of olivine, clinopyroxene, phlogopite and amphibole. A groundmass contains phlogopite, carbonate, apatite, clinopyroxene, garnet, titanite and opaque minerals.

The most important chemical features of the  alkali lamprophyres are undersaturation of SiO2 (31.04-40.54 wt%), high alkali contents (3.86 – 6.47 wt% K2O+Na2O) and their sodium specification (K2O/Na2O - 0.36-0.68), whereas ultramafic lamprophyres  have lower alkali contents (1.73-3.39 wt% K2O+Na2O), and potassium specification (K2O/Na2O -  1 -2.31). They also contain less SiO2 (27.73 – 34.11 wt%).

The Turiy Cape dykes are characterized by only a single petrographic type  - alkali lamprophyres (Cb-Anl and Ne-Anl monchiquites). They contain small and medium size (0.1 – 1 cm) phenocrysts of olivine, clinopyroxenes, amphiboles, magnetite, phlogopite. A groundmass contains analcime, nepheline, aegirine, phlogopite, garnet, perovskite, apatite and opaque minerals.

Rocks of the Turiy Cape dykes are SiO2 undersaturated (33.93 - 41.86 wt%), and contain extremely high alkalis (5.62-14.51 wt% K2O+Na2O) and all of them have sodium specification (0.11-0.68 K2O/Na2O).

The most primitive core of clinopyroxenes in the Kandalaksha dykes are high magnesian (#Mg – 0.76 – 0.87), low titanian (0,5 – 1,09 wt% TiO2) and contains chromium (0.1-1.1 wt% Cr2O3). The clinopyroxenes of the Tyriy Cape dykes have high magnesian core (#Mg 0.79-0.83, 1.48-2.05 wt% TiO2, 0.12-0.4 wt% Cr2O3).

Olivines in the Kandalaksha lamprophyres have more primitive composition in comparison with olivines from the Turiy Cape ones. The #Mg of Kandalaksha olivines varies from 0.84 to 0.87, nickel concentration varies from 1500 to 2500 ppm and the  #Mg of Turiy Cape olivines varies from 0.82 to 0.85, nickel concentration varies from 500 to 1000 ppm.  

Based on composition of primary minerals we suggest that compositional diversity of both dyke swarms were formed due to crystal fractionation processes. Though, the significant difference in chemistry of whole rocks and clinopyroxene and olivine composition do not support a common source for of the  Turiy Cape and Kandalaksha dykes.

This work was supported by the Russian Science Foundation under Grant No. 19-17-00024.

How to cite: Vozniak, A., Sazonova, L., and Nosova, A.: Lamprophyres from Turiy Cape and Kandalaksha Devonian dykes (Kola peninsula, Russia) : petrography,geochemistry and mineral composition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-784, https://doi.org/10.5194/egusphere-egu2020-784, 2020.

The redox sensitive element sulfur is used for reconstructing the oxygen fugacity during magmatic melt evolution applying the sulfur isotopic composition of sulfide and sulfate minerals. Especially fast ascending sulfur-rich alkaline magma from the upper mantle provides the possibility for determining the oxidation state of Earth`s mantle via a detailed investigation of the sulfur cycling. Here we present the first sulfur isotope data of sulfides, sulfates as well as carbonate associated sulfate (CAS) of carbonatite (sövite) from two well-studied locations (Orberg and Badberg) of the Kaiserstuhl volcanic complex, situated in the southern part of the Upper Rhine Graben (Germany). Based on our results, sövites are 25000 times more enriched in sulfate than in sulfide. Sulfides display a δ³⁴S value of 0.6 ‰ (V-CDT), whereas water-soluble sulfate (e.g. anhydrite) show a sulfur isotopic composition between 3.8 ‰ and 6.1 ‰. δ³⁴S_CAS data are at 6 ‰ at the Orberg and 9 ‰ at Badberg locality. Our sulfur isotope data are comparable to other carbonatite occurrences worldwide (e.g. Phalabora, South Africa), emplaced at similar temperatures (ca. 860 °C). However, the strongly elevated sulfate content recorded here for sövites formed at this high temperature is unique and indicates an enhanced oxidation state during sövite formation in the Kaiserstuhl volcanic complex.

How to cite: Kuhn, B., Peters, C., and Schurr, S.: Sulfur cycling in carbonatite of the Kaiserstuhl volcanic complex (Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2574, https://doi.org/10.5194/egusphere-egu2020-2574, 2020.

Despite the largest explosive eruptions posing significant potential hazards, the recurrence rate of these so called ‘super-eruptions’ remains poorly constrained. The younger portion of the Yellowstone-Snake River Plain province is well-known for large-scale explosive volcanism; however, the older history within the Snake River Plain remains poorly-known and partially obscured by later basaltic volcanism. To address this, we characterised the mineral cargo of four widely spaced rhyolitic ignimbrites found at the margins of the Snake River Plain that reveal a strong compositional similarity in bulk geochemistry, major crystal phases (e.g. pyroxene and ilmenite), and radiogenic isotopes. To test whether these four compositionally similar units may have had a common origin we used a tandem in-situ and isotope dilution method for U/Pb geochronology of zircon crystals. The youngest populations of zircons from all four samples are equivalent in age, and together define a pooled weighted mean ²³⁸U/²⁰⁶Pb age of 11.030 ± 0.006 (MSWD = 1.44, n=24). These results reveal an event with a conservatively estimated erupted volume ~1,470 km³, of similar magnitude to the largest Yellowstone eruptions. Numerous widely dispersed tephra deposits found across the western portions of North America with geochemical affinities to the Snake River Plain province hint at the existence of other such voluminous ignimbrites. The improved ability to correlate deposits of an individual eruption shown by this and other recent studies implies that ‘super’ eruptive events are more common than previously thought.

How to cite: Ellis, B. and Schmitz, M.: Reconstructing a Snake River Plain ‘super-eruption’ via compositional fingerprinting and high-precision U/Pb zircon geochronology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8223, https://doi.org/10.5194/egusphere-egu2020-8223, 2020.

Sweden is responsible for over 90% of the iron ore production in the European Union, the bulk of which originates from the Kiruna-Malmberget region in northern Sweden, the type locality for Kiruna-type apatite-iron oxide ores. Despite thorough investigations of these long known deposits, their origin is still debated. Currently, two main formation theories are discussed: formation by orthomagmatic processes (Nyström & Henriquez 1994; Troll et al. 2019), versus hydrothermal processes (Hitzman et al. 1992; Smith et al. 2013).

Secondary ion mass spectrometry (SIMS) analysis allows gathering of more detailed information regarding intra-crystal variations, such as core to rim growth zonations, than bulk analysis do. Measurements of δ⁵⁶Fe and δ¹⁸O in Kiruna-type magnetites by SIMS would therefore aid in the determination of their main formation process. However, there are conflicting studies regarding crystallographic orientation effects of δ⁵⁶Fe and δ¹⁸O in magnetite, and while some authors found that the isotope ratios varied depending on how the crystal was oriented (e.g. Huberty et al. 2010), others found no such effects (e.g. Marin-Carbonne et al. 2011). This research project thus aims to further examine any effects of crystal orientation on Fe and O isotope signatures and identify a suitable magnetite reference material for SIMS analysis. To enable comparison between isotope ratios and crystal orientations, the sample orientations will therefore be determined by electron backscatter diffraction (EBSD) prior to SIMS analysis. SIMS analysis require reference material mounted next to the sample for continuous corrections during analysis. Different magnetite samples will now be tested for usage as reference materials. If a homogeneous reference material is found, future studies can utilise it for further investigations of the formation of Kiruna-type magnetite, as well as any other research concerning δ⁵⁶Fe or δ¹⁸O in magnetite.

Hitzman, M.W., Oreskes, N., & Einaudi, M.T. (1992). Geological characteristics and tectonic setting of proterozoic iron oxide (Cu-U-Au-REE) deposits. Precambrian Research. Precambrian Metallogeny Related to Plate Tectonics, vol. 58 (1), pp. 241–287. DOI:10.1016/0301-9268(92)90121-4.

Huberty, J.M., Kita, N.T., Kozdon, R., Heck, P.R., Fournelle, J.H., Spicuzza, M.J., Xu, H., & Valley, J. W. (2010). Crystal orientation effects in 18O for magnetite and hematite by SIMS. Chemical Geology, vol. 276 (3), pp. 269–283. DOI:10.1016/j.chemgeo.2010.06.012.

Marin-Carbonne, J., Rollion-Bard, C., & Luais, B. (2011). In-situ measurements of iron isotopes by SIMS: MC-ICP-MS intercalibration and application to a magnetite crystal from the Gunflint chert. Chemical Geology, vol. 285 (1), pp. 50–61. DOI:10.1016/j.chemgeo.2011.02.019.

Nyström, J.O. & Henriquez, F. (1994). Magmatic features of iron ores of the Kiruna type in Chile and Sweden; ore textures and magnetite geochemistry. Economic Geology, vol. 89(4), pp. 820–839. DOI:10.2113/gsecongeo.89.4.820.

Smith, M.P., Gleeson, S.A., & Yardley, B.W.D. (2013). Hydrothermal fluid evolution and metal transport in the Kiruna District, Sweden: Contrasting metal behaviour in aqueous and aqueous–carbonic brines. Geochimica et Cosmochimica Acta, vol. 102, pp. 89–112. DOI:10.1016/j.gca.2012.10.015.

Troll, V.R., Weis, F.A., Jonsson, E., Andersson, U.B., Majidi, S.A., Högdahl, K., Harris, C., Millet, M.-A., Chinnasamy, S.S., Kooijman, E., &Nilsson, K.P. (2019). Global Fe–O isotope correlation reveals magmatic origin of Kiruna-type apatite-iron-oxide ores. Nature Communications, vol. 10(1), pp. 1712. DOI:10.1038/s41467-019-09244-4.

How to cite: Andersson, M., Troll, V., Whitehouse, M., Deegan, F., Högdahl, K., Jonsson, E., Kenny, G., and Andersson, U.: Characterizing magnetite reference material for secondary ion mass spectroscopy (SIMS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10738, https://doi.org/10.5194/egusphere-egu2020-10738, 2020.

The olivine-rich troctolites are Mg-rich rocks forming by open-system magmatic crystallization in response to primitive melt injections into the growing lower oceanic crust (e.g., Renna et al., 2016).

In the present study, whole-rock highly siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au and Re) and chalcogen (S, Se and Te) element compositions, and Re-Os isotopes of the olivine-rich troctolites from the Jurassic Alpine ophiolites were determined with the aim to investigate the control that the formation of lower oceanic crust may exert on the fractionation of HSE and other incompatible chalcophile elements in MORB.

The olivine-rich troctolites have initial γOs (160 Ma) ranging from +0.2 to +5.9, and Primitive Mantle (PM)-normalized HSE-Te-Se-S patterns showing a gradual increase from Os to Au, and nearly flat Au-Te-Se patterns. These patterns are similar to those of little-fractionated mantle melts and are parallel, at higher concentrations levels, to those typical of MORB. The olivine-rich troctolites have higher Te and Os/Ir, and lower Se/Te than MORB, which may be reconciled with a process of sulfide accumulation. Sulfide precipitation could be promoted by interaction between melts interstitial to olivine and melts relatively rich in silica, which could migrate from an underlying gabbroic framework (cf. Renna et al., 2016). Melts residual to the formation of olivine-rich troctolites are inferred to have a markedly HSE-fractionated signature comparable to that of MORB.

Renna M.R., Tribuzio R., Ottolini L. (2016). J Geol Soc Lond 173, 916–932

How to cite: Tribuzio, R., Renna, M. R., Armandola, S., Becker, H., Sanfilippo, A., and Wang, Z.: Fractionation of Highly Siderophile Elements during reactive melt infiltration in lower oceanic crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7141, https://doi.org/10.5194/egusphere-egu2020-7141, 2020.

Granitic rocks in Thailand and South East Asia have been divided into Eastern belt granite, Central belt granite, and Western belt granite.The Central belt granite widely expose in Thailand including Dan Chang area. According to the field investigation, there are two granitic bodies in Dan Chang area. The main granitic body is composed of mainly K-feldspar, plagioclase, quartz, and biotite with some accessory minerals of opaque mineral and zircon. The granitic rocks are obviously characterized by porphyritic texture of coarse-grain K-feldspar.Another granitic body is small body which is similar mineral assemblage to those granite in the main body with different proportion of K-feldspar. The amount of K-feldspar in the small body granite is less than the main body granite. However, both granitic bodies are generally characterized by porphyritic texture of K-feldspar. Moreover, aplite and pegmatite are associated with both of them. The results of field investigation and petrographic study of the granitic rocks in Dan Chang District, Suphan Buri Province, Thailand can be compared with the Central belt granite which may be resulted from the Sibumasu and Indochina collision in Late Triassic.

How to cite: Tukpho, T. and Fanka, A.: Petrology of granitic rocks in Dan Chang District, Suphan Buri Province, Thailand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12359, https://doi.org/10.5194/egusphere-egu2020-12359, 2020.

An experimental study of three examples of the sub-volcanic body of Ary-Bulak ongonite was carried out to determine the composition of the liquidus phases and the order of crystallization of minerals. Phase relations in the samples of porphyritic ongonites (1), porphyritic ongonites with a high content of Ca and F (2)  and aphyric rocks with a high content of Ca and F (3) were studied at temperatures of 700–800 °C, a pressure of 1 kbar in Ni-NiO and Mt– Hem buffer conditions.

Rock samples have specific petrochemical aspects. In this series of rocks from (1) to (3), the general tendency is directed towards a decrease in the content of alkalis and silicon and an increase in the content of F and Ca.

Liquidus is achieved for porphyritic and aphyric ongonites with a high content of Ca and F. The liquidus phases for them are fluorite, topaz and plagioclase. Crystallization of porphyry ongonites begins at a temperature below 700 ° C.

The phase relationship and composition of the liquidus phases are independent of oxygen fugacity.

How to cite: Novikova, A. and Alferyeva, Y.: Experimental Study Of Phase Relations In The Ca-Ongonite From Ary-Bulak Massive (Transbaykal Region, Russia) at 700–800 °C, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13270, https://doi.org/10.5194/egusphere-egu2020-13270, 2020.

Phenocrysts of olivine with high Ni, low Mn and Ca relative to global MORBs are usually attributed to a stronger role of the pyroxenite melting (Soblev et al., 2005). The Hawaiian shield stage lavas (high Si group) with high bulk-rock and olivine Ni have usually been attributed to the role recycled oceanic crust. However, the Hawaiian plume also produces lavas with Si-undersaturated alkali basalt (low Si group) and relatively low Ni, whose origin has not been well understood. In this study, we examine the role of deep carbon on the magma compositions and their influences on olivine geochemistry. Here by comparing the whole rock and olivine geochemistry data of Hawaiian high Si group basalts with Hawaiian low Si group basalts, we find that the primary magmas of the latter have relatively lower Ni but comparable concentrations of Mn and Ca. However, the high Si group basalt olivines have indistinctive partition coefficient of Ni but significantly lower Mn and Ca than those of the high Si group basalts.

The deep Earth is a large reservoir of carbon, which when participates in mantle melting would significantly influence the mantle residual minerals and melt compositions. For example, mantle melting with CO₂ is commonly shown to reduce SiO₂ in the melts. Thus, the genesis of the Si-undersaturated alkali basalts has usually been attributed to the role of CO2 (Zhang et al., 2017). The role of CO2 in the genesis of Hawaiian alkali lavas have also been predicted in previous studies. Based on the observations from Hawaiian lavas, we suggest that CO2 played a key role in lowering the partition coefficients of Mn and Ca. We have conducted high pressure-temperature melting experiments on mantle rocks with CO2, and find that CO2 has a potential influence on the partition of Ni, Mn and Ca between olivine and silicate melts, more experiments remain to be further conducted. This work was financially supported by the National Natural Science Foundation of China (91858206, 41876040).

How to cite: Zhan, M.-J., Zhang, G.-L., and Wang, S.: Partition of Ni, Ca and Mn between olivine and carbonated silicate melt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13763, https://doi.org/10.5194/egusphere-egu2020-13763, 2020.

In-situ trace element analysis of cumulus minerals may provide a clue to the parental magma from which the minerals crystallized. However, this is hampered by effects of the trapped liquid shift (TLS). In the Main Zone (MZ) of the Bushveld Complex, the Ti content in plagioclase grains shows a clear increase from core to rim, whereas most other elements (e.g., rare earth elements (REEs), Zr, Hf, Pb) do not. This is different from the prominent intra-grain variation of all trace elements in silicate minerals in mafic dikes and smaller intrusion, which have a faster cooling rate. We suggest that crystal fractionation of trapped liquid occurred in the MZ of Bushveld and the TLS may have modified the original composition of the cumulus minerals for most trace elements except Ti during slow cooling. Quantitative model calculations suggest that the influence of the TLS depends on the bulk partition coefficient of the element. The effect on highly incompatible elements is clearly more prominent ­­than on moderately incompatible and compatible elements because of different concentration gradients between cores and rims of cumulate minerals. This is supported by the following observations in the MZ of Bushveld: 1) positive correlation between Cr, Ni and Mg# of clinopyroxene and orthopyroxene, 2) negative correlation between moderately incompatible elements (e.g., Mn and Sc in clinopyroxene and orthopyroxene, Sr, Ba, Eu in plagioclase), but 3) poor correlation between highly incompatible elements and Mg# of clinopyroxene and orthopyroxene or An# of plagioclase. Modeling suggests that the extent of the TLS for a trace element is also dependent on the initial fraction of the primary trapped liquid, with strong TLS occurring if the primary trapped liquid fraction is high. This is supported by the positive correlation between highly incompatible trace element abundances in cumulus minerals and whole-rock Zr contents.

We have calculated the composition of the parental magma of the MZ of the Bushveld Complex. The compatible and moderately incompatible element contents of the calculated parental liquid are generally similar to those of the B3 marginal rocks, but different from the B1 and B2 marginal rocks. For the highly incompatible elements, we suggest that the use of the sample with the lowest whole-rock Zr content and the least degree of TLS is the best approach to obtain the parental magma composition. Based on calculation, we propose that a B3 type liquid is the most likely parental magma to the MZ of the Bushveld Complex.

How to cite: Yang, S., Maier, W. D., Godel, B., Barnes, S.-J., Hanski, E., and O'Brien, H.: Parental magma composition of the Main Zone of the Bushveld Complex: Evidence from in-situ LA-ICP-MS trace element analysis of silicate minerals in the cumulate rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17903, https://doi.org/10.5194/egusphere-egu2020-17903, 2020.

Al-in-olivine thermometry, based upon the temperature-dependent solubility of Al in the olivine crystal structure [1], has become a widely adopted method to investigate the crystallisation temperatures of primitive mantle melts on Earth [2]. The thermometer is calibrated using the Al contents of co-existing olivine and spinel: these phases are on or near the liquidus of primitive magmas, so the thermometer should access liquidus temperatures of mantle melts, thereby constraining the minimum mantle melting temperature. CFB-associated primitive melts have average olivine crystallisation temperatures well in excess of MORB, and back-calculation to the potential temperature of their mantle source regions suggests mantle thermal anomalies of several hundred degrees [3].

Whilst mantle thermal anomalies are moderately well-understood on Earth, relatively little is known about the melting conditions in the mantles of the Moon and Mars that led to the production of Maria basalts and Martian surface basalts and associated volcanic activity. Several samples returned from the Moon and basaltic meteorites from Mars (shergottites) are primitive and rich in both olivine and spinel, so would appear ideal samples for the application of Al-in-olivine thermometry to unravel their respective mantle melting conditions and, more generally, the thermal structures of those planetary interiors. In this study, we present preliminary investigations into a) five Apollo 12 primitive lunar basalts, and b) two olivine-phyric shergottites. We find that pervasive shock features make the trace Al concentrations of shergottitic olivines difficult to use, because high Al concentrations are associated with a fine micron to sub-micron network of K-rich melt veins, suggestive of fluid-mediated melt transport. On the other hand, olivine phenocrysts in all five lunar samples yield clear trends in Al contents and are excellent targets for Al-in-olivine studies. We present preliminary thermal results, as well as a newly-calibrated set of relevant thermodynamic parameters needed for back-calculating lunar melting temperatures. A fully quantitative assessment of lunar maria liquidus temperatures is, however, currently hampered by the limited calibration range of the Al-in-olivine thermometer and the unconstrained effect of high spinel TiO₂ contents on the results.

[1] Coogan, L. A., Saunders, A. D. & Wilson, R. N. Chem. Geol. 368, 1–10 (2014).

[2] Trela, J. et al. Nat. Geosci. 10, 451–456 (2017).

[3] Jennings, E. S., Gibson, S. A. & Maclennan, J. Chem. Geol. 529, 119287 (2019).

How to cite: Jennings, E., Buisman, I., and Coull, P.: Investigating mantle melting temperatures on Earth, Mars and the Moon using Al-in-olivine thermometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19579, https://doi.org/10.5194/egusphere-egu2020-19579, 2020.

The 2010 eruption of Merapi produced pyroclastic deposits and lava flows that are compositionally very similar, raising the question as to the underlying reason of the differences in eruptive styles between the various phases of the 2010 eruptive events. To test whether primary magmatic volatile content is the reason for the different eruption styles, we analyzed magmatic water contents in nominally anhydrous clinopyroxene crystals contained in lava and ash from the 2010 eruptive events. We utilized two analytical approaches: (i) Fourier-transform infrared spectroscopy (FTIR) analysis of fresh clinopyroxene from the ash and lava samples and (ii) FTIR analysis of clinopyroxene both prior to and after experimental re-hydration. By employing calculated partition coefficients, we determined the magmatic water content of the magma from which the various crystals grew. The magmatic water content determined from the unmodified clinopyroxenes from lava samples yield a range of 0.35 wt.% to 2.02 wt.% H₂O, whereas magmatic water contents determined from untreated clinopyroxene contained in the ash samples range between 0.04 and 3.25 wt.%, with two outliers at 4.62 and 5.19 and wt.%, respectively. In contrast, for the rehydrated crystals the range for lava derived clinopyroxene crystals is between 1.94 and 2.19 wt.% and for ash between 1.74 and 2.66 wt.%, with two crystals at extreme values of 0.85 and 3.20 wt.%. We interpret these results to indicate that crystals from different populations are present in the 2010 eruptive products, with the dominant group reflecting relatively low magmatic H₂O contents (around 2 wt.%) due to storage in shallow magma reservoirs and pockets at high levels within the Merapi plumbing systems (e.g. top 3 km). The overall higher H₂O range and the occasionally more extreme values recorded in clinopyroxenes from ash deposits may then represent the presence of a crystal population that last equilibrated at deeper levels and at higher water contents, i.e. these crystals derive from the replenishing magma that activated the shallow portion of the plumbing system during the 2010 events. While this is work in progress, our results so far seem to suggest that the pyroclastic deposits of the 2010 Merapi eruption may contain a higher fraction of clinopyroxene derived from ‘deeper magma’ with higher H₂O contents then what we have detected in associated lavas.

How to cite: Dimitriou, D., Troll, V., Weis, F., Seraphine, N., Deegan, F., Skogby, H., Humaida, H., and Gertisser, R.: Investigating H2O contents in clinopyroxene from explosive versus effusive eruption products from Merapi volcano, Indonesia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20063, https://doi.org/10.5194/egusphere-egu2020-20063, 2020.

The generation and source of ~230,000 km³ of total erupted volume of the predominately silicic (>90 %; Pankhurst et al., 2000) magmas which comprise the Jurassic Chon Aike Large Silicic Igneous Province (CASP) of Southern Patagonia is currently debated. In this study, we conducted a widespread sampling of multiple eruptive units, primarily ignimbrites and minor rhyolitic flows, along the Eastern Andean front (~47°S to 49°S), owning to the third and youngest eruptive episode of the CASP (El Quemado Complex; EQC). To determine the magmatic source and potential role of a significant crustal contribution proposed in the generation of these magmas, we analyzed the in-situ δ¹⁸O composition of both quartz and zircon by SIMS. We combined these data with LA-ICP-MS U/Pb analyses on single zircon crystals to characterize the potential for changing oxygen isotopic values through time and space within the EQC units along this ~230 km long transect.

The northern-most units sampled have the lightest average δ¹⁸O (relative for the EQC) analyzed in zircon and quartz (7.7 and 10.4 ‰, respectively). Oxygen isotope values increase towards the South, with the highest δ ¹⁸O values previously reported in El Chaltén, reaching up to 10.1 ‰ for zircon and 12.5 ‰ for quartz (Seitz et al., 2018). Eruptive units from the same locality appear to be homogeneous in their oxygen isotopic composition.

U/Pb zircon ages for the EQC range overall from ~148 to 155 Ma, though no obvious trend from North to South in zircon crystallization ages is noticeable. Multiple inherited zircon cores (at ~230, 460, 500, 1300 Ma) with Jurassic magmatic overgrowths were discovered. Isotopic compositions of these inherited magmatic cores are variable in their δ¹⁸O values throughout time. However, and more significantly, most of these inherited cores record high δ ¹⁸O values, with the highest value at 9.5 ‰ measured for a ~460 Ma core. These high values measured within inherited cores are found at all locations sampled for the EQC.

The δ¹⁸O values of the EQC rocks are significantly higher than what would be expected for silicic magmas formed by simple closed-system fractionation from any mantle-derived melt (6­-7‰; Valley, 2003). Thus, our oxygen isotope data support significant input of crustal material - of either a sedimentary origin or from hydrothermally altered crust - to generate these ignimbrites and rhyolites with elevated δ¹⁸O values all along this transect.

How to cite: Foley, M., Putlitz, B., Baumgartner, L., Guillermin, Z., and Bégué, F.: Magmas of the El Quemado Complex (Chon Aike Silicic Igneous Province, Patagonia): Elevated Oxygen Isotope Signatures Across Space and Time, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21265, https://doi.org/10.5194/egusphere-egu2020-21265, 2020.

Calcite (CaCO₃) and magnesite (MgCO₃) are among the most common carbonates on Earth. The presence of Ca‑Mg‑carbonates in the mantle affects the melting and phase relations of peridotites and eclogites and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset of melting and the incipient melt composition of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO₃‑MgCO₃‑system. Thus, a deeper insight into the phase relations and melting behavior of the CaCO₃‑MgCO₃‑system is crucial to better understand the carbon cycle in the Earth’s mantle.

We performed quenched multi-anvil experiments at 6 and 9 GPa to (a) examine the phase relations of the nominally anhydrous CaCO₃‑MgCO₃‑system and to (b) establish partition coefficients for Li, Na, K, Sr, Ba, Nb, Y and rare earth elements (REEs) between carbonates and dolomitic melt. We used a rotating multi-anvil press to overcome quenching problems as observed in previous studies. Rotation of the multi‑anvil press is, additionally, indispensable to establish equilibrium between solid carbonates and dolomitic melt.

The melting temperature and phase relations of Ca‑Mg‑carbonates depend on the Mg/Ca‑ratio. For instance, at 6 GPa Ca‑rich carbonates with a molar Mg/(Mg+Ca)‑ratio (X_Mg) of 0.2 will transform into a dolomitic melt (X_Mg=0.33‑0.31) and calcite crystals (X_Mg=0.19‑0.14) at 1350‑1440 ℃. Partial melting of Mg‑rich carbonates (X_Mg=0.85) will produce a dolomitic melt (X_Mg=0.5‑0.8) and Ca‑bearing magnesite (X_Mg=0.89‑0.96) at 1400‑1600 ℃. Trace element distribution into Ca-Mg-carbonates depends on X_Mg, temperature and seems to follow lattice constraints for divalent cations. Partition coefficients of REEs between magnesite (Ca_0.11Mg_0.89CO₃) and dolomitic melt (Ca_0.5Mg_0.5CO₃) at 6 GPa and 1400 ℃ are uniform scattering marginal between 0.1‑0.2. The partition coefficient of Lu (D=0.1) is unmodified to lower Ca-content in magnesite (Ca_0.04Mg_0.96CO₃) and higher temperature (1600 ℃), but the partition coefficients between such Ca-poor magnesite and dolomitic melt (Ca_0.2Mg_0.8CO₃) decrease continuously from heavy-REEs to light‑REEs from 0.1 to 0.001, respectively.

Our findings have important implications for the cycle of carbon and trace elements in the mantle because Ca‑Mg‑carbonates will (partially) melt at 6 GPa and temperatures above ~1300 ℃ producing a dolomitic melt. Consequently, CO₂ will be liberated by partial melting of an upwelling carbonated mantle at a depth of ~200 km considering the thermal structure of the upper mantle. The results also affirm that carbonates are stable in the subducting slab even for hot subduction zone geothermal gradients unless carbonate-bearing lithologies in the slab are infiltrated by aqueous fluids.

How to cite: Sieber, M., Wilke, F. D. H., Reichmann, H.-J., and Koch-Müller, M.: Phase relations of Ca-Mg-carbonates and trace element partition coefficients between carbonates and dolomitic melt at 6 and 9 GPa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2990, https://doi.org/10.5194/egusphere-egu2020-2990, 2020.

High-Mg^# (molar 100 × Mg/(Mg + Fe)) diorites can provide significant insights on the mantle metasomatism under the subduction zone. Here we investigate the genesis of Neoproterozoic high-Mg^# diorites in the western Yangtze Block to constrain mantle metasomatism during the subduction process. Zircon U-Pb dating results display new weighted mean ²⁰⁶Pb/²³⁸U ages of 850.1 ± 1.7 Ma, 840.9 ± 2.4 Ma, and 836.6 ± 1.9 Ma for these high-Mg^# diorites. They are metaluminous and calc-alkaline rocks, and characterized by moderate SiO₂ (57.08–61.12 wt.%), high MgO (3.36–4.30 wt.%) and Mg^# values (56–60). The relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703406 to 0.704157), highly positive whole-rock εNd(t) (+3.26 to +4.26) and zircon εHf(t) values (+8.43 to +13.6) imply that they were predominantly sourced from depleted lithospheric mantle. These high-Mg^# diorites also show the enrichment of large ion lithophile elements (LILEs, e.g., Rb, Ba, K, and Sr) and depletion of high field strength elements (HFSEs, e.g., Nb, Ta, Zr, and Hf), resembling typical arc magma affinity. The highly variable Rb/Y, Th/Ce, Th/Sm, and Th/Yb ratios indicate the significant incorporation of subduction-related fluids and sediment-derived melts into primary mantle source. We therefore propose that the ca. 850–835 Ma high-Mg^# diorites in this study were formed by the partial melting of metasomatized mantle source influenced by subduction fluids and sediment melts. Our new data, in conjunction with numerous studies of metasomatized mantle magmatism from the western Yangtze Block, suggest that Neoproterozoic mantle sources were progressively metasomatized by the subduction-related compositions from slab fluids, sediment melts, to oceanic slab melts during persistent subduction process.

How to cite: Zhu, Y. and Lai, S.: Genesis of ca. 850-835 Ma high-Mg# diorites in the western Yangtze Block, South China: Implications for mantle metasomatism under the subduction process, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3985, https://doi.org/10.5194/egusphere-egu2020-3985, 2020.

Our numerical study evaluates the settling rate of solid particles, suspended in a highly 
vigorous, finite Prandtl number convection of a bottom heated fluid. We explore a broad 
range of model parameters, covering particle types appearing in various natural systems, 
and focus in particular on crystals nucleating during the cooling of a magma ocean. The 
motion of inertial particles within thermal convection is non-trivial, and under idealized 
conditions of spherical shaped particles with small Reynolds number it follows the 
Maxey-Riley equation (Maxey and Riley, 1983). Two scaling laws exist for the settling 
velocities in such system: for particles with small but finite response time, the Stokes' 
law is typically applied. For particles with a vanishing response time, a theoretical model 
was developed by Martin and Nokes (1989), who also validated their prediction with analogue 
experiments. 

We develop a new theoretical model for the settling velocities. Our approach describes 
sedimentation of particles as a random process with two key constituents: i) transport 
from convection cells into slow regions of the flow, and ii) the probability of escaping 
slow regions if a particle enters them. By quantifying the rates of these two processes, 
we derive a new equation that bridges the gap between the above mentioned scaling laws. 
Moreover, we identify four distinct regimes of settling behaviour and analyze the lateral 
distribution of positions where particles reach the bottom boundary. Finally, we apply our 
results to the freezing of a magma ocean, making inferences about its equilibrium vs 
fractional crystallization. The numerical experiments are performed in 2D cartesian geometry 
using the freely available code CH4 (Calzavarini, 2019).

References:
Maxey, M. R. and Riley, J. J.(1983): Equation of motion for a small rigid sphere in a nonuniform flow. 
Physics of Fluids, 26(4), 883-889.

Martin, D and Nokes, R (1989): A fluid-dynamic study of crystal settling in convecting magmas. 
Journal of Petrology, 30(6), 1471-1500.

Calzavarini, E (2019): Eulerian–Lagrangian fluid dynamics platform: The ch4-project. Software Impacts, 1, 100002.

How to cite: Patocka, V., Calzavarini, E., and Tosi, N.: Settling behaviour of particles in Rayleigh-Benard convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5908, https://doi.org/10.5194/egusphere-egu2020-5908, 2020.

The Remal granite-gneiss is situated close to the tectonic boundary between the Singhbhum Craton and the Rengali Province in the state of Odisha, eastern India. This granite-gneiss contains two prominent fabric elements - a sub-horizontal to gently dipping felsic fabric S_ign, believed to be of igneous origin that predates a sub-vertical gneissosity S₁ which is of tectonic origin. S_ign layers have a non-uniform, arcuate geometry and grain-size within the layers show systematic variations. S₁ is defined by metre-scale segregations of biotite-poor and biotite-rich domains whose orientations are constant. S_ign layers are arranged rhythmically in cross-section and either curve into parallelism with or truncate against layers above and below; the entire assembly resembles cross beds developed in sediments. Some of the layers develop trough cross-bedding similar to those seen in mafic intrusions such as the Skaergaard Complex, indicative of slumping of a crystallizing mush along an inclined depositional plane at the time of crystallization. The S_ign layers are composed of quartz, K-feldspar and plagioclase with abundant graphic intergrowths and myrmekite, and lack any evidence of compaction. Plagioclase grains are often zoned, and dihedral angles between mineral grains is significantly different from the equilibrium value of 120°, testifying to the preservation of the igneous nature of this fabric without significant solid state modification. In contrast, S₁ is sub-parallel to localized mylonite zones within the granite-gneiss composed of chlorite and epidote, indicative of deformation under greenschist facies conditions. The mylonitized zones contain prominent dextral shear sense indicators and is believed to have originated due to the amalgamation of the Rengali Province with the Eastern Ghats Mobile Belt along the east-west trending, sub-vertical Brahmani Shear Zone further to the south. The S₁ gneissosity appears to have developed as a result of this deformation event. EBSD analyses of quartz grains within the granite-gneiss reveal distinct variations in the distribution of <c> axes in different domains. Close to the mylonite zones, deformation of quartz has been dominantly accommodated by basal <a> slip with a dextral shearing overprint while away from these zones and S_1, the <c> axes are distributed in clusters without any systematic pattern. The persistence of an earlier igneous layering, despite the subsequent development of a gneissosity concomitant with localised mylonitisation, indicates that the later deformation event has not obliterated the earlier formed igneous fabric. The study also demonstrates that development of a gneissosity does not necessarily require deformation operating at moderate to high temperature, and can stabilize even under greenschist facies conditions.

How to cite: Dobe, R. and Gupta, S.: Orthogonal Tectonic and Magmatic Fabrics in a Layered Granite-Gneiss at Remal Dam Site, India: Implications for Fabric Generation and Superposition , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6526, https://doi.org/10.5194/egusphere-egu2020-6526, 2020.

Transition metals are of special interest for understanding the conditions of differentiation processes such as core formation. Those that have more than one oxidation state can also provide powerful constraints on changing redox conditions in the mantle over time. The ability to determine isotopic fractionations associated with differentiation processes has provided a new dimension to exploration of the conditions in the early Earth in particular. It has been recently shown that Cr isotope variations in igneous systems are strongly affected by redox conditions and chromite crystallisation.

In this study, we have investigated the variations in chemical composition and Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed under controlled redox conditions. The Cr isotopic compositions measured in the silicate melts in our experiments are strongly influenced by oxygen fugacity and experiments performed at 1300 °C and -12 < logfO₂ < -6 are correlated with fO₂. This suggests that Cr isotopes are a powerful tool to study changes in redox conditions in high temperature processes. The Cr isotopic composition of silicate melt reacted under more oxidising conditions (logfO₂ > -6) are isotopically much lighter compared to melts reacted at lower oxygen fugacity. Three hypotheses are proposed to explain such variations: (i) a change in Cr bonding environment in the silicate melt (ii) a change in Cr bonding environment in the chromite (iii) volatile loss of Cr from the silicate melt. More work is needed to definitively determine the factors that control the isotopic behaviour of Cr in silicate melts.

How to cite: Bonnand, P., Bruand, E., Matzen, A., Jerram, M., Schiavi, F., Wood, B., Boyet, M., and Halliday, A.: Redox control on chromium isotope behaviour in silicate melts in contact with magnesiochromite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11044, https://doi.org/10.5194/egusphere-egu2020-11044, 2020.

The Yoko-Dovyren ultramafic-mafic intrusion (the northern Baikal region, Russia) has excellent outcropping as well as layering falls vertically. It`s age is 728 Ma. Length of the main body is 26 km. The modal layering of its central part (~3 km thick) includes a basal reversal (from chilled rocks to plagiolherzolites) followed with Pl-bearing to adcumulate dunite, troctolite and gabbroic sequence.

Over the past 20 years, several sections of the massif have been studied in detail. (Ariskin et al 2018) have determined two major types of parental magmas recorded in the FeO vs MgO trends for the Ol cumulates through the first 500 m of the cross-section. These two geochemically similar magmas are consistent with equilibrium olivine Fo88 and Fo86 in the range of temperatures from 1290°C to ~1200°C.

We present the results of quantification of CSD of olivine from the dunite succession, which argue for two types of olivine grain populations differing for the more primitive and relatively evolved magma.

The slope of the log-linear CSD function in the lower-temperature magmas has a less steep as compared to the higher temperature ones.  Both populations can be considered to represent intratelluric olivine crystallized at a pre-emplacement stage. At a stratigraphic level of 200 m from the lower contact, in some of the samples we observed changes in the CSD patterns, which evidence a coarsening of the populations within the Dovyren chamber. Starting from 350-400 m coarsening is noticeable everywhere, so that the CSD cease to be log-linear. In addition, in a narrow zone of 500-550 m dunite are found to display a pronounced bimodal (kinked) distribution of olivine. In a larger population, olivine has highest aspect ratio (up to 3-3.5) compared to other dunite samples. The origin of such dunite can be explained by the intrusion of hot portions of magma into the colder cumulus. In this case such elongated crystals may be due to the increased growth rate of the original olivine grains. The smaller population may be due to a new nucleation event after emplacement. CSD in cumulates above the «kinked dunites» demonstrate coarsening of olivine, with the most coarse-grained populations typical of highly contaminated dunite.

Thus, a rather narrow zone is distinguished in dunite, where we can observe primary intratelluric CSD, which is not substantially altered nither by peritectic reactions in the loose cumulus of the reversal sequence, where the temperature drops rapidly, nor by coarsening during long history of temperature oscillations close to the primary magmas condition above this zone.

This work support from the Russian Science Foundation (RSF, grant No. 16-17-10129)

Ariskin Alexey, Danyushevsky Leonid, Nikolaev Georgy, Kislov Evgeny, Fiorentini Marco, McNeill Andrew, Kostitsyn Yuri, Goemann Karsten, Feig Sandrin, and Malyshev Alexey. The dovyren intrusive complex (southern siberia, russia): Insights into dynamics of an open magma chamber with implications for parental magma origin, composition, and cu-ni-pge fertility. Lithos, 302:242–262, 2018.

How to cite: Sobolev, S., Ariskin, A., Tarquini, S., Pshenitsyn, I., Nikolaev, G., and Shkurskii, B.: Genetic interpretation of CSD for olivine through the dunite section of the Dovyren layered intrusion: linking with geochemistry and probable dynamics of the cumulate mush., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11377, https://doi.org/10.5194/egusphere-egu2020-11377, 2020.

In the last two decades, improved fine scale analysis in crystalline profiles has improved our understanding of igneous processes, while opening our sight to more complexities. As an example, plagioclase crystal profiles in Holyoke flood-basalt flow revealed that the crystals got exposured to different melt environments as the layer underwent fractional crystallization. Fractional crystallization is an essential process for determining the compositional evolution of magmatic systems. The process requires a reactive segregation process, where crystals precipitate from the melt and segregate from their residual melt. In this study, we are motivated by the subtleties in the crystalline record to model the segregation component of fractional crystallization, or crystal fractionation.

We build a numerical model with individually resolved, denser-than-melt crystals in a convective flow. We test the low to intermediate crystallinity regime, where the physical processes leading to efficient fractionation are less clear than at high crystallinity. We simulate the physical segregation of crystals from their residual melt at the scale of individual crystals using a direct numerical method. By resolving each of the crystals, we do not require a priori parameterization of crystal-melt interactions. We use tracers in the melt to track the different melts around the crystals.

We find that collective sinking of crystal-rich clusters dominate settling at low particle Reynolds numbers. The relatively rapid motion of this cluster strips away the residual melt around the cluster. Compared to individual settling, the resulting crystal fractionation is efficient but heterogeneous at the crystalline scale. Similar to the Holyoke flood-basalt plagioclase profiles, the crystals in our analysis show exposure to different melt environments as they drive crystal fractionation. Our results suggest that cluster driven fractional crystallization will vary in efficiency. At the system scale, this result would suggest a bell curve compositional abundance distribution in volcanic systems.

How to cite: Culha, C., Suckale, J., Keller, T., and Qin, Z.: Crystal fractionation by crystal-driven convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12618, https://doi.org/10.5194/egusphere-egu2020-12618, 2020.

On the Moon, mare basalts were the results of explosive volcanic eruptions which sampled mantel material during the ascent. Apollo 15 and Apollo 17 missions have landed on the edge of Mare Imbrium and Mare Tranquillitatis respectively and collected numerous volcanic material, including basaltic lavas, mantle and crustal xenoliths, and magnesium rich green glasses. Studies of the green glass indicate that the melt from which it formed originated about 400 kilometres below the Moon's surface.

Due to the absence of tectonic reworking, a protracted mantle convection history and the lack of weathering, and notwithstanding meteorite impacts, the pristine nature of the lunar samples can be used to both better constrain magma-storage depth during plume-like volcanic activity and provide better understanding on the crystallization of magma oceans. Unlike most erupted volcanic material on Earth, whole rock lava and xenolith samples present at the Moon’s surface likely preserve pressure and temperature at which they have formed or have reequilibrated. In this study, we used thermodynamic modelling to constrain the minimum depth of magma storage and the equilibrium depth of mantle and crustal xenoliths (i.e. picrite, dunite, troctolite).

Our results indicate that there were two levels of magma storage beneath the Mare Imbrium at the time of the eruption, at 140 ± 11 km depth and at ~ 82 km depth below the KREEP layer (~ 60 km). Picrite and dunite are equilibrated at 130-150 km depth, troctolite at 80 km depth and anorthosite between 0 and ~ 35 km depth. The maximum equilibrium depth for forsterite-rich olivine in picrite xenoliths and green glass beads is estimated at 490 ± 10 km. Estimated lunar mantel potential temperature (Tp) is 1490 °C, which is similar to the Icelandic Tp (~ 1490 °C) and close to the North Atlantic Province Tp (1350 °C).

There are strong petrological similarities in the internal architecture of the first 150 km of the Moon presents Shiant Isles Main Sill (135 m) (SIMS) in Scotland), suggesting similar formation processes. The SIMS formed with a significant crystal cargo (~ 15 vol%), which then differentiates through settling of crystals from a vigorously convective magma and the concomitant rising of buoyant melt giving rise to a sandwich horizon significantly above the mid-point (~ 75 %) of the sill total thickness. On the Moon, the predominant current theory of lunar formation suggests the formation of a flotation anorthosite crust on the top of a rapidly convecting magma ocean. However, in such environment (Ra ~ 10³⁰), anorthosite crystals are likely to be re-entrained, suggesting the crust might have only formed once the magma ocean had an aggregate crystal cargo of roughly 50%. 

Hence, the petrological information contained in picritic sills on Earth might give direct insights into the formation and evolution of the magma ocean on the Moon. Based on our observations, we argue that lunar differentiation would have then been driven by the formation of a stagnant lid, compaction through buoyant flow of anorthite-rich melt and then further refinement through magmatism on the moon.  

How to cite: Nicoli, G., Neufeld, J., and Holness, M.: The Moon in the Skye: insights into the formation and evolution of the lunar magma ocean , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15016, https://doi.org/10.5194/egusphere-egu2020-15016, 2020.

Pados-Tundra ultramafic complex belong to Serpentinite belt in the northern Fennoscandian Shield and composed of dunite-harzburgite-orthopyroxenite with 7 rhythms and 4 Cr layers. The associated massif named as Malyi Pados are considered as a satellite intrusion (Mamontov, Dokuchaeva, 2005) or dislocated block detached according by (Barkov et al., 2016). Nevertheless the complex includes of Dunite Zone with podiform and stratiform chromitite with Ir subgroup PGE (Ru, Os, Ir – IPGE) and associated with chromian spinel in ophiolite (Joban, 2006). Fiestly unusual microtextures and mineralogical features with clinochlore, laurite and native Ru was found (Barkov et al., 2017).

Isotope U-Pb data on baddeleyite in core of zircon from mafic gabbronorite rocks of the Malyi Pados gave 2083±7 Ma and are coeval to ages of Cu-Ni Pechenga (1980 Ma) and PGE Bushveld deposits. Notably are measured new U-Pb ages with 2087±3 Ma for baddeleyite and metamorphic rutile with 1804±10 Ma from hornblendite dyke which are cutted ultramafic rocks of the Pados-Tundra complex.

New Sm-Nd mapping data for the main rocks of the complex are reflected model T_DM ages of primary protolith from 2.78 Ga to 2.36 Ga and 3.13 Ga for host rock with positive εNd values from +2.7 to +2.1. New Sm-Nd investigations to podiform chromitites of the Pados-Tundra complex are similar to Sopcheozerskoe Cr-deposit (Dunite Block) of the Monchegorsk ore region with positive εNd and young protolith ages about 2.7 Ga for primary magma sources instead of Paleoproterozoic Co-Cu-Ni and PGE layered intrusions of the Fennoscandian Shield with 2.4 Ga to 2.5 Ga for origin and 3.2 - 3.5 Ga of the protolith EM-1 enriched mantle plume reservoir (Bayanova et al., 2009, 2014, 2018). All new U-Pb on baddeleyite and Sm-Nd studies to whole rocks of the Pados-Tundra complex infer about ophiolite (spreading or oceanization of the crust) and presence diamond in podiform chromitites according to new highlights of (Ballhause et al., 2017).

All investigations are supported by RFBR 18-05-70082 (Arctic resources), 18-35-00152, 18-35-00246, Scientific Research Contract N0.0226-2019-0053 and Program of Presidium RAS 8.48.

How to cite: Bayanova, T., Pavel, S., Evgeniy, K., Ekaterina, S., and Elena, B.: Podiform and stratiform chromitite with PGE in Paleoproterozoic (2.1 Ga) Pados-Tundra ultramafic (ophiolite) complex (N-E part of the Fennoscandian Shield, Arctic region), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1737, https://doi.org/10.5194/egusphere-egu2020-1737, 2020.