Displays

Distorted olivines of enigmatic origin are ubiquitous in erupted products from a wide range of volcanic systems (e.g., Hawai'i, Iceland, Andean Southern Volcanic Zone). At Kīlauea volcano, distorted olivines are commonly attributed to ductile creep within dunitic bodies located around the central conduit, or within the deep rift zones (~5–9 km depth). However, a recent suggestion that lattice distortions are produced by an early phase of branching dendritic growth, followed by textural ripening and the merging of misoriented crystal buds, has gained considerable traction.

A quantitative examination of the microstructures in distorted olivines by electron backscatter diffraction (EBSD) reveals striking similarities to microstructures observed in deformed mantle peridotites, but significant differences to the crystallographic signatures of dendritic growth. This suggests that lattice distortions record the application of differential stresses at high temperatures within the magmatic plumbing system, rather than rapid crystal growth. Previous petrological work has suggested that differential stresses are produced by ductile creep within Kīlauea’s deep rift zones. Crucially, this has fuelled suggestions that significant quantities of magma must travel along these rift zones in order to acquire distorted olivines, despite the paucity of geophysical evidence for these magma transport paths. In contrast, we show that the spatial distribution of eruptions containing distorted olivines is consistent with their derivation from the main magma storage reservoir. This model not only aligns petrological and geophysical observations at Kīlauea, but also accounts for the occurrence of distorted olivines in a wide variety of basaltic systems worldwide (which lack deep rift zones).

Application of piezometers developed for mantle peridotites reveals that distorted olivines have experienced differential stresses of ~3–12 MPa. Assuming that mush piles behave as granular materials, and form force chains, these stresses can be generated within cumulate piles of ~180–720 m. Based on available constraints on the magma supply rate and the geometry of Kīlauea’s summit reservoir, these thicknesses accumulate in a few centuries (consistent with residence times inferred from melt inclusion records).

Overall, we demonstrate that microstructural investigations of erupted olivine crystals by EBSD generates rich datasets which provide quantitative insights into crystal storage within mush piles. Under the increasingly prevalent view that crustal magmatic systems are mush-dominated, constraining the geometry and dynamics of crystal storage regions is crucial to further our understanding of magmatic plumbing systems. The presence of distorted olivines in many different volcanic settings highlights the global applicability of the methods developed in this study. Furthermore, assessments of deformation conditions using EBSD need not be restricted to olivine-bearing lavas. Microstructural fabrics types in natural and experimental samples have been established for a wide variety of igneous phases (e.g. diopside, plagioclase, hornblende), so similar approaches may be utilized in more evolved volcanic systems.

How to cite: Wieser, P., Edmonds, M., Maclennan, J., and Wheeler, J.: Microstructural Constraints on Magmatic Mushes under Kīlauea Volcano, Hawai'i, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-352, https://doi.org/10.5194/egusphere-egu2020-352, 2020.

Melt inclusions originate from small juvenile melt droplets trapped at magmatic pressures and temperatures during crystallization of their host mineral. Thus, melt inclusions retained by their host crystals can uniquely preserve evidence for the thermochemical conditions in the magma during crystal growth. Zircon is a resistant mineral even under magmatic conditions, and it is common in many different rock types (igneous, metamorphic, and sedimentary). Moreover, zircon crystallization significantly affects trace element concentrations of the melt during processes such as fractionation, melt separation, and/or retention of accessory phases in the residual melt. Its potential as a host mineral for melt inclusions, however, has not been fully realized, mainly because of the small size of zircon and its inclusions.

Here, we developed a new technique for ion imaging of elemental distributions in melt inclusions in zircon, and applied it to melt-inclusion bearing zircon crystals from selected Miocene ignimbrites of the Central Anatolian Volcanic Province, Turkey. The high-sensitivity ion imaging of zircon provides information about the 2-D distribution of critical elements in the crystal and its inclusions, and element distributions can be directly compared to cathodoluminescence (CL) patterns of the host. High-sensitivity element maps were obtained using a CAMECA 1280-HR IMS at Heidelberg University for areas of 25×25 µm at 2-3 µm lateral resolution. Ion images for each element containing 128×128 pixels raw intensity values were initially processed using instrumental software (WINImage©) to accumulate data from measurement cycles into a single image data. Each element map was then recorded as a grayscale image with intensities encoded in each pixel. The raster images for each element was further processed using ImageJ© and ARCGIS© programs, where each element map was converted to a color scale expressing the appropriate value ranges and the data obtained on the same trace element for each zircon in different units were reduced to the same legend values. The color ion images obtained from the grayscale images of each map were overlaid onto CL images to correlate trace element abundances with growth regions visible in CL images.

Imaging has the important advantage compared to spot analyses of melt inclusions that contamination from the wall of the host mineral can be mitigated. For this, Zr ion images were used as controls for selecting ROIs (Regions of Interest) in order to eliminate pixels with mixed signals at the interface between zircon and the inclusion due to the finite width of the ion beam. High resolution imaging of melt inclusions and zircon allowed re-evaluating zircon-melt partitioning behavior of important trace elements for natural melt compositions. Partitioning values for elements with comparatively low abundances in the melt relative to zircon (Y, Th, U and Dy) are slightly lower than spot analyses and previously published results but they all follow a similar trend with predicted partitioning coefficients. 

This research was financed by The Scientific and Technological Research Council of Turkey within the research program number of 2214/A. 

How to cite: Akin, L., Aydar, E., and Schmitt, A. K.: High Sensitivity Mapping of Melt Inclusions in Miocene Zircons of Central Anatolian Volcanic Province (CAVP), Cappadocia, Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-36, https://doi.org/10.5194/egusphere-egu2020-36, 2020.

Kimberlites are the deepest melts that reach Earth’s surface and, therefore, can provide unique insights into the composition and evolution of the convective mantle through time. Application of isotope geochemistry to trace the composition of kimberlite sources has thus far been hindered by the ubiquitous alteration and incorporation of xenocrystic material in kimberlite rocks. Bulk-kimberlite analyses are typically considered reliable for Nd and Hf isotopes due to their overwhelmingly higher concentrations in kimberlite melts compared to common mantle and crustal contaminants. Conversely, Sr and Pb isotope compositions of bulk kimberlite samples are seldom considered representative of their parental melts thus requiring analysis of robust magmatic phases, primarily perovskite. Addressing the primary (i.e. magmatic) isotopic composition of volatile elements, such as N and noble gases, requires analyses of volatile-rich phases, and fluid inclusions in olivine represent a typical primary target in mantle-derived magmas. However, fluid inclusions in kimberlitic olivine are dominantly secondary in origin. Secondary inclusions can form at any time after crystallisation of their mineral host, which requires assessment of the origin of trapped fluids (i.e. pristine magmatic fluids, crustal fluids of external derivation, or combination thereof) before their isotopic composition can be used to make inferences about kimberlite mantle sources.

Here we present trace-element and Sr-Nd-Pb-He-N isotopic compositions of multiple olivine aliquots representing two different magmatic units of the ~88 Ma Wesselton kimberlite (Kimberley, South Africa). The Sr and Nd isotopic composition of olivine analysed by isotope-dilution (ID) TIMS are within the narrow range of perovskite ⁸⁷Sr/⁸⁶Sr (0.7043-0.7046) and whole-rock ¹⁴³Nd/¹⁴⁴Nd (eNd_i = 0.4–2.2) for the Kimberley kimberlites. These results indicate that the secondary fluid inclusions, which dominate the incompatible trace-element budget of olivine separates, have a pristine magmatic origin devoid of crustal contribution.

Helium isotope compositions were measured by laser heating of 1.6 to 9.8 mg of olivine using an ultrahigh-sensitivity compressor-source noble gas mass spectrometer. ³He/⁴He ratios are between 1.6 R_A and 3.7 R_A (where R_A indicates the atmospheric ³He/⁴He ratio), values more radiogenic than MORBs but comparable to HIMU OIBs. These results indicate a high time-integrated (U+Th)/He ratio in the source of the Kimberley kimberlites, which is consistent with the moderately high (i.e. HIMU-like) time-integrated U/Pb ratio implied by elevated initial ²⁰⁶Pb/²⁰⁴Pb in Wesselton olivine (19.1-19.5), Kimberley kimberlites (up to 19.9) and megacrysts in southern African Cretaceous kimberlites (up to 20.5). The combination of low ³He/⁴He, moderately radiogenic ⁸⁷Sr/⁸⁶Sr, and negative d³⁴S values (-2.6‰ to -5.7‰) require a contribution from subducted recycled material in the source of the Kimberley kimberlites. Conversely, a preliminary N isotope analysis of Wesselton olivine by in-vacuo crushing using a noble gas mass spectrometer returned a mantle-like d¹⁵N of -2.9‰, which might suggest limited recycling of surface N (d¹⁵N >0‰) in the source of these kimberlites. We conclude that the combination of Sr-Nd-Pb and He-N isotope tracing of fluid inclusions in olivine can provide a robust new approach to address the composition of kimberlite sources and, therefore, the evolution of the deep mantle through time.

How to cite: Giuliani, A., Koornneef, J. M., Barry, P., Will, P., Busemann, H., Maden, C., Maas, R., Greig, A., and Davies, G. R.: A preliminary assessment of the application of Sr, Nd, Pb, He and N isotope analysis to fluid inclusions in kimberlite olivine: A new approach to trace deep-mantle sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5267, https://doi.org/10.5194/egusphere-egu2020-5267, 2020.

The petrologic study of olivine-hosted melt inclusions (MIs) from alkaline primary Cenozoic basalts of Northern Victoria Land (Antarctica) provide new insights on the role of volatiles in the onset of rift-related magmatism. The concentration of volatile species (H₂O, CO₂, F, Cl) have been determined by Secondary Ion Mass Spectrometry (SIMS) on a selection of MIs which have been previously re-homogenized at high pressure and temperature conditions in order to avoid any heterogeneity and reducing the H diffusion. The least differentiated MIs vary in composition from basanitic to alkaline basalts, analogously to what is found in McMurdo volcanics, while their volatile concentrations reach up to 2.64 wt% H₂O, 3900 ppm CO₂, 1377 ppm F and 1336 Cl. Taking into account the most undegassed MIs a H₂O/(H₂O+CO₂) ratio equal to 0.88 was determined, which in turn brings the CO₂ content in the basanitic melt with the highest water content up to 8800 ppm.

Major and trace element melting modelling indicate that basanite and alkali basalt composition can be reproduced by 3 and 7% of partial melting of an amphibole-bearing spinel lherzolite respectively. Assuming a perfect incompatible behavior for H₂O and CO₂ these melting proportions allow to constrain the water and CO₂ contents in the mantle source in the range 780-840 and 264-273 ppm respectively. The resulting CO₂/Nb, CO₂/Ba and H₂O/Ce ratio are lower than those estimated for Depleted MORB Mantle (DMM), suggesting that the NVL Cenozoic alkaline magmatism could be originated by an enriched mantle source composed by a range from 70% to 60% of Enriched Mantle (EM) and from 30% to 40% of Depleted Morb Mantle (DMM).

A global comparison of fluid-related, highly incompatible and immobile/low incompatible elements such as Li, K, Cl, Ba, Nb, Dy and Yb allow to put forward that the prolonged (~500 to 100 Ma) Ross subduction event played a fundamental role in  providing the volatile budget into the lithospheric mantle before the onset of the Cenozoic continental rifting.

How to cite: Giacomoni, P. P., Ferlito, C., Bonadiman, C., Casetta, F., Ottolini, L., Zanetti, A., and Coltorti, M.: Geochemistry of basic magmatism of Western Antarctic Rift: implications for volatiles storage and recycling in the mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16794, https://doi.org/10.5194/egusphere-egu2020-16794, 2020.

Magnetoplumbite (yimengite-hawthorneite, HAWYIM), crichtonite (lindsleyite-mathiasite, LIMA) and hollandite (priderite) minerals are exotic titanate phases, which formed during metasomatism at the conditions of high alkali activity, especially K, in the fluids in the upper mantle peridotites. The paper presents data on experiments on formation of K-end-members priderite, yimengite and mathiasite, as the result of the interaction of chromite, chromite+rutile and chromite+ilmenite assemblages in the presence of a small amount of silicate material with H₂O-CO₂-K₂CO₃ fluids at 3.5 and 5 GPa and 1200°C. The experiments demonstrated the principal possibility of the formation of the titanates in the reactions of chromite with alkaline aqueous-carbonic fluids and melts. However, the formation of these phases does not proceed directly on chromite, but requires additional titanium source. The relationship between titanates is found to be a function of the activity of the potassium component in the fluid/melt. Priderite is an indicator of the highest potassium activity in the mineral-forming medium. Titanates in the run products are constantly associated with phlogopite. Experiments prove that the formation of titanates manifests the most advanced or repeated stages of metasomatism in mantle peridotites. Association of titanates with phlogopite characterizes a higher activity of the potassium component in the fluid/melt than the formation of phlogopite alone. The examples from natural associations, reviewed in the paper, well illustrate these conclusions. Experiments revealed the following features of crystallization of these phases and allowed interpretation of the titanate associations in metasomatized mantle peridotites.

(1) The principal possibility of the formation of minerals of crichtonite and magnetoplumbite groups and priderite in the reactions of chromite with alkaline aqueous-carbonic fluids and melts is confirmed. Such substances are considered as main agents of potassium metasomatism, leading to the formation of titanates in the upper mantle (Konzett et al., 2013; Rezvukhin et al., 2018).

(2) The formation of these phases does not proceed directly on chromite (e.g. Haggerty et al. 1983; Haggerty, 1983; Nixon, Condliffe, 1989), and requires additional titanium source. They are rutile and ilmenite, which are themselves usually are products of modal metasomatism of peridotites. This experimental fact demonstrates that the formation of titanates marks probably the most advanced or repeated stages of metasomatism in mantle peridotites.

(3) This is also proved by the relationships of titanates with phlogopite. Association of titanates with phlogopite characterized by a higher activity of the potassium component in the fluid/melt than the formation of phlogopite alone. Such conditions can again be created at the most advanced or repeated stages of mantle metasomatism.

(4) The relationship between titanates is also a function of the activity of the potassium component in the fluid/melt. Priderite is an indicator of the highest potassium activity in the mineral-forming medium. The above examples from natural associations (Zhou, 1986; Konzett et al., 2013; Almeida et al., 2014) well illustrate this conclusion.

How to cite: Vorobey, S., Butvina, V., and Safonov, O.: Syntheses of rare K-titanates (yimengite, mathiasite and priderite) at high TP conditions: application to modal mantle metasomatism., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-203, https://doi.org/10.5194/egusphere-egu2020-203, 2020.

The Ventura Espiritu Santo Volcanic Field (VESVF) and the Sierra Chichinautzin (SCN) are two monogenetic volcanic fields originated in different tectonic environments in the central portion of Mexico (continental rift and subduction). The VESVF is located 35 km NE of the city of San Luis Potosí in the south of the Basin and Range extensional province. This volcanic field was formed by the eruption of alkaline magmas of mafic composition transporting mantle xenoliths described as spinel lherzolites and pyroxenites (Luhr et al., 1989; Aranda -Gómez and Luhr, 1996). The SCN is a Quaternary volcanic field located in the Trans-Mexican Volcanic Belt (TMVB) between two Quaternary arc-volcanoes (Popocatepetl and Nevado de Toluca[AR1] ). Some authors believe that its origin has been related to the subduction of the Cocos plate beneath the North American plate (Marquez et al., 1999; Meriggi et al., 2008); however, the basalts present in the SCN are geochemically similar to OIBs.

New isotopic data of noble gases and CO₂ in fluid inclusions from the VESVF and SCN are presented in this work, since these two areas offer a great opportunity to study the local lithospheric mantle features and related processes (e.g., metasomatism, partial melting) occurring beneath Mexico. Twelve fresh xenoliths from the VESVF and two aliquots of olivine phenocrysts of andesites from SCN were selected. Based on the petrographic analysis, it was determined that the set of xenoliths exhibit same paragenesis: Ol> Opx>> Cpx> Spinel; all samples are plagioclase-free and are classified as spinel-lherzolites and harzburgites. Both the boundaries and the fractures of the crystals develop veins composed of yellowish glass and tiny crystals of carbonates. Lavas from SCVF were previously described as olivine andesites mainly aphanitic and porphyritic with few (<10%) phenocrysts of olivine and orthopyroxene (Marquez et al., 1999; Straub et al., 2011).

The mantle xenoliths and the olivine phenocrysts have comparable Rc/Ra values (where Rc/Ra is the ³He/⁴He corrected for air contamination and normalized to air He). We find Rc/Ra compositions of 6.9-7.7 and 7.2-7.3, respectively, which are within the MORB-like upper-mantle range (Graham, 2002). The highest CO₂ concentrations are observed in olivine phenocrysts from SCN (9.2·10^-7 mol/g and 1.3·10^-6 mol/g), while the xenoliths cover a wide range of concentrations with values as high as 3.9·10^-7 mol/g in Cpx. The isotopic composition of CO₂ (d¹³C vs PDB) in the olivine phenocrysts is around -6.2‰ with CO₂/³He ratios of 3.3·10⁹, which are comparable to MORB-like range (-8‰<d¹³C<-4‰); the mantle xenoliths in contrast, although displaying similar CO₂/³He ratios (2.8·10⁹), exhibit more positive d¹³C signature between -1.0 and -2.7%. We propose that these differences testify for isotopic heterogeneity in the mantle beneath the two areas, with and reflect mantle metasomatism underneath VESVF driven by interaction with carbonate rich-melts (likely consequence of carbonate recycling during the subduction process), as also evidenced by the petrographic analysis.

How to cite: Sandoval-Velasquez, A. L., Aiuppa, A., Rizzo, A., Frezzotti, M. L., Straub, S., Gomez-Tuena, A., and Espinasa-Perena, R.: Geochemistry of noble gases and CO2 in mantle xenoliths and arc lavas from central Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17010, https://doi.org/10.5194/egusphere-egu2020-17010, 2020.

Fluids play key roles in many geological processes across wide ranges of spatial and temporal scales. A major challenge in establishing the impacts of fluids is that partial replacement of minerals by dissolution-precipitation produces gaps in the rock record. Finding the records of such processes can help in understanding and reconstructing the processes of fluid flow, mineral dissolution and related volume changes.

The ultra-high pressure metamorphic (UHPM) Lago di Cignana Unit (Zermatt-Saas Zone, Western Alps) has been intensively studied because it is a piece of exhumed coesite- and diamond-bearing oceanic lithosphere. In this unit, schistose quartzite hosts coesite-bearing garnet and contains lenses of garnetite, which previously have been attributed to local bulk-compositional differences. Almost the entire quartzite consists of a retrograde mineral assemblage, and therefore processes occurring during subduction are best recorded in garnet.

A combined microstructural and petrological study of the garnetite lenses and their host rock reveals evidence for compaction by dissolution during subduction, partially driven by intergranular pressure solution. As the host rock matrix is removed, garnet is preferentially preserved and concentrated into garnetite. Garnet-garnet contacts then result in pressure solution and grain boundary migration. Different garnet densities and microstructures in the garnetite, alongside dissolution-reprecipitation structures in host rock garnet, suggest a complex process driven by fluid pulses. Linking garnet composition and structures to P-T through barometry on inclusions reveals an evolving fluid pathway during prograde to peak metamorphism, resulting in significant mass removal by pressure solution in metasediments subducted to UHPM conditions.

The occurrence of pressure solution and mass removal at UHPM conditions in combination with the large amounts of fluids produced by slab dehydration suggests that dissolution may play a significant role in metasediments during subduction.

Acknowledgements: This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro)

How to cite: van Schrojenstein Lantman, H., Wallis, D., Scambelluri, M., and Alvaro, M.: High volumes of mineral dissolution by localized fluid pulses in UHPM metasediments of Lago di Cignana, Western Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15965, https://doi.org/10.5194/egusphere-egu2020-15965, 2020.

Subducted metapelites are more prone to re-equilibrate during exhumation than mafic or ultramafic rocks to the point that recognizing high-pressure (HP) relicts is often very challenging. Geologic evidence from the Cima Lunga Unit (Central Alps) show this apparent discrepancy between high to ultra-high pressure metamorphism (28 kbar and 780 °C) recorded in mafic/ultramafic lenses, and Barrovian metamorphism (<10 kbar, 650°C) in the adjacent metapelitic rocks. We collected a white mica – garnet – biotite – plagioclase – kyanite (+ quartz, + zircon, + rutile) bearing metapelite adjacent to the garnet metaperidotite lens that displays an apparently well equilibrated Barrovian mineral assemblage (garnet + plagioclase + biotite), with no macroscopic or microtextural indication of a HP and/or HT metamorphic event (e.g. omphacite crystals; migmatitic texture; polyphase inclusions). Nevertheless, microstructures like atoll-like garnet or large white mica flakes surrounded by biotite and ilmenite replacing rutile suggest incomplete re-equilibration. We investigated garnet and phengite crystals by electron probe and laser ablation-ICP-MS mapping. Major and trace element mapping reveals very complex mineral zoning in both minerals. In particular, high Ti content in phengite and increasing P and Zr contents in pyrope-rich garnet indicate that the studied rock underwent a HP-HT event. This is also supported by Zr in rutile thermometry that indicates temperatures well above the Barrovian metamorphism (T > 700 °C). We combined detailed textural analysis with petrological-geochemical data and thermodynamic modelling to reconstruct the metamorphic evolution of the studied rock. We show that, thank to incomplete re-equilibration, the rock documents an evolution from prograde to UHP-HT peak (27 kbar and 800 °C) to retrograde (Barrovian) conditions (10 kbar and 620 °C). Noteworthy, peak metamorphic conditions of metapelite coincide with peak metamorphic conditions of the garnet metaperidotite. Lastly, geochemical evidence for minor wet melting of the studied metapelite at HP-HT conditions was recognized and is likely linked to the dehydration of chlorite to form garnet peridotite in the adjacent ultramafic body. We propose that metapelites and ultramafic rocks were coupled before subduction or at least in its early stage. This finding opens new scenarios for the geodynamic interpretation of the Cima Lunga unit. We propose that the ultramafic lenses at Cima di Gagnone were parts of the exhumed and serpentinised mantle emplaced at the hyper-extended European continental margin of the Piemont-Ligurian ocean. Slices of the margin were detached and tectonically mixed in the subduction channel. These new constraints call for re-evaluation of the paleogeographic position of the Adula-Cima Lunga nappe.

How to cite: Piccoli, F., Lanari, P., Hermann, J., and Pettke, T.: How to disclose local equilibrium in subducted metapelites from the Cima Lunga Unit (Central Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2674, https://doi.org/10.5194/egusphere-egu2020-2674, 2020.

Small portions of pristine melt with diameters of 2 to 50µm are increasingly recognized as a rather common occurrence in high grade metamorphic terranes which experienced melting. Their study delivers crucial chemical information on partial melts at depth. But they are also unique "natural experimental charges" where the behaviour of the silicate melt can be investigated, directly in the natural rocks, under P-T-t conditions which cannot be completely reproduced in the laboratory.

Each nanogranitoid case study has consistently shown H₂O-bearing, silica and alkali-rich melt. However, rather than a classic granitoid assemblage consisting mainly of quartz and feldspar(s), on cooling these isolated melt droplets produce a plethora of mineral phases identified via microRaman spectroscopy that are rarely –or never- observed as rock-forming minerals. Cristobalite (tetragonal) and tridymite (orthorhombic) are often present as SiO₂ polymorphs, and hexagonal kokchetavite as a polymorph of KAlSi₃O₈. NaAlSi₃O₈ occurs as orthorhombic kumdykolite, whereas CaAl₂Si₂O₈ may occur either as monoclinic svyatoslavite or trigonal dmisteinbergite. Two presently unidentified phases have been also recognized via Raman and analysed via electron microprobe. One has the main peak at 426-430 cm^-1 and has the composition of a granitic glass, whereas the second has a main peak at 412 cm^-1 and a variable composition depending on the inclusion in which it occurs. As their main peaks occur in the same region of most tectosilicates, it is likely that they are two new polymorphs of feldspar, to the best of our knowledge never reported before. These polymorphs have been so far identified in inclusions mainly hosted in garnet, zircon and, in one case, sapphirine and trapped under an extremely variable range of metamorphic conditions (from low P migmatites to UHP eclogites) in very different rock types (metagranitoids, metasediments, mafic and ultramafic rocks).

Microstructures confirm that all of these phases crystallize directly from the trapped melt on cooling, independently of the internal P of the inclusions or the original conditions of melt entrapment. They appear to be the result of metastability in the inclusions, possibly during rapid crystallization of a melt, not caused by rapid cooling but by the peculiar undercooled and supersaturated conditions achieved on cooling by a melt confined in a small cavity (Ferrero & Angel, 2018). According to this possibility, these polymorphs can be regarded as kinetically stabilized, yet possibly thermodynamically metastable, phases as recently proposed by Zolotarev et al. (2019) for dmisteinbergite. A preliminary crystallization experiment on a haplogranitic melt at undercooled conditions however failed to reproduce such phases. Another possibility is that under natural cooling the confined inclusions experience underpressurization, and the system (i.e. the trapped melt) reacts crystallizing phases, i.e. the polymorphs, less dense than their common counterparts. This would result in the decreasing of the P gradient between inclusions and surrounding rock, equivalent to reducing the free energy of the system.

References

Ferrero, S. & Angel, R. 2018. JPet 59, 1671–1700.

Zolotarev, A.A. et al. 2019. Minerals  9, 570.

How to cite: Ferrero, S., Angel, R. J., Borghini, A., Wannhoff, I., Fuchs, R., Tedeschi, M., Gianola, O., O'Brien, P. J., Wunder, B., and Ziemann, M. A.: Strange polymorphs and where to find them: a melt inclusion story , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9085, https://doi.org/10.5194/egusphere-egu2020-9085, 2020.

Over the recent years, Raman elastic barometry has been developed as an additional method to calculate metamorphic conditions in natural systems. A major advantage of Raman elastic barometry is that it does not depend on thermodynamic databases and classic geobarometry methods but relies on mechanical calculations. As a consequence, Raman elastic barometry offers an independent method for estimating the pressure conditions that prevailed at the time of entrapment of minerals during growth of their hosts.

The difference between the pressure calculated using elastic geobarometry and that calculated by phase equilibria methods has recently been employed to estimate the extent of metamorphic reaction overstepping in natural systems. Quantification of the latter however implicitly assumes that the rheology of the inclusion-host system is perfectly elastic. This assumption may not hold at high temperatures, where viscous creep of minerals takes place.

The amount of viscous relaxation of a host-inclusion system is a path-dependent quantity which mostly depends on the temperature-time (T-t) path followed. Here, we present examples of visco-elastic relaxation of mineral inclusions and calculate the apparent reaction overstepping which results by assuming that the mechanical system is purely elastic. Our modelling shows that host-inclusion systems that experienced large peak temperatures for long periods of time will retain inclusion residual pressures that cannot be simply related to the growth of their hosts and should therefore not be used for reaction overstepping calculations.

How to cite: Moulas, E., Zhong, X., and Tajcmanova, L.: Viscous relaxation of mineral Inclusions and its implications for reaction overstepping calculations in metamorphic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7568, https://doi.org/10.5194/egusphere-egu2020-7568, 2020.

Trapped and sheltered inside other crystals, mineral inclusions preserve fundamental and otherwise lost information on the geological history of our planet. In the last decade, quartz inclusions in garnet have become a fundamental tool to estimate pressure and temperature of metamorphic rocks at the time of inclusion entrapment. In these approaches, as well as in all other applications, inclusions are regarded as immutable objects and the possibility of a change in their shape has never been considered.

With a detailed characterization of samples from greenschist and granulite facies, performed by optical and electron microscopy, EBSD, X-ray tomographic microscopy, laser Raman spectroscopy and FIB serial slicing, we show that after being trapped with irregular (“scalloped”) shape in low-temperature rocks, quartz inclusions in garnet from granulites formed at 750-900 °C and various pressures acquired a polyhedral “negative crystal” shape imposed by the host garnet, and almost exclusively defined by the facets of dodecahedron and icositetrahedron. A similar behaviour is also observed in biotite inclusions. The 3-fold and 4-fold morphological symmetry axes of the polyhedral negative crystals are parallel to corresponding crystallographic axes in the host garnet.

The systematic presence of a fluid film at the quartz-garnet boundary is not supported by Raman and FIB investigation.

Strengthened by microstructures indicating the process of “necking down” of polycrystalline quartz inclusions, our data support that - like in fluid inclusions changing shape to negative crystals - shape maturation of mineral inclusions occurs by temperature-assisted dissolution-precipitation via grain boundary diffusion. This process tends to minimize the surface free energy of the host-inclusion system by forming energetically favored facets and by decreasing the inclusion surface/volume and aspect ratios.

Optical investigation of numerous samples of worldwide provenance suggests that the negative crystal shape of quartz inclusions in garnet from granulites is a widespread microstructure that underpins a systematic phenomenon so far overlooked.

How to cite: Cesare, B., Parisatto, M., Mancini, L., Peruzzo, L., Franceschi, M., Tacchetto, T., Reddy, S. M., Spiess, R., and Marone Welford, F.: Quartz inclusions in garnet from high-temperature metamorphic rocks change their shape, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4143, https://doi.org/10.5194/egusphere-egu2020-4143, 2020.

With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that ⁴⁰Ar/³⁹Ar geochronology using this mineral, produces cooling ages alone.

Further, we show that high-definition ultra-high-vacuum (UHV) ³⁹Ar diffusion experiments using phengitic white mica routinely allow the extraction of muscovite sub-spectra in the first 10-30% of ³⁹Ar gas release during ⁴⁰Ar/³⁹Ar geochronology. The muscovite sub-spectrum is distinct and separate to the main spectrum which is dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the cooling history of the eclogite-blueschist belt. Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones.

This study thus illustrates a new method for the quantitative determination of the timing of movement in mylonites and/or in strongly stretched metamorphic tectonites. Shear bands formed in such structures are rarely coarsely crystalline enough to allow mineral grains that can be individually dated using laser spot analysis. Where phengitic white mica is involved, interlaying is usually so fine as to preclude the application of laser methods. In any case, laser methods do not have the capability of extracting exact and detailed age-temperature spectra, and can never achieve the definition of the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments.

Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses. In consequence, we discover a robust correlation between microstructure and age, even down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections.

A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of ³⁹Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. To apply percentage-release formula for the estimation of diffusivity, there must be a sharp rise to the temperature in question, then that temperature must be maintained at a constant value, then dropped sharply to relatively low values.

How to cite: Forster, M., Nie, R., Yeung, S., and Lister, G.: Microstructural shear bands in mylonites dated using muscovite sub-spectra from high-definition ultra-high-vacuum (UHV) argon diffusion experiments with phengitic white mica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1813, https://doi.org/10.5194/egusphere-egu2020-1813, 2020.

Microstructures and the different thermoelastic properties of minerals ensure that no rock is ever under perfect hydrostatic stress at the grain level. If deviatoric stresses and strains significantly modify thermodynamic properties of minerals so that the equilibrium assemblage and compositions are different from that predicted from hydrostatic conditions, it is crucial to be able to measure the stress state of minerals in-situ in rocks. Forty years ago it was considered that ‘Analysis of residual stresses at the scale of mineral grains within a polycrystalline aggregate such as a rock is virtually intractable’ [1]. This is no longer true.

Confocal Raman spectroscopy allows spectra to be collected from small volumes of mineral grains within a section. The positions of Raman peaks depends on the elastic strains in the minerals through the phonon-mode Grüneisen tensors [2]. The development of precise DFT simulations of crystal structures and their Raman spectra now allows the components of the phonon-mode Grüneisen tensors to be calculated [3]. With these tensors it is possible to determine the strains from measured Raman peak positions, to thereby map the strain, and hence the stress state, of individual mineral grains. We have now extended the DFT simulations to show that the Raman shifts of crystals subject to symmetry-breaking stresses (e.g. around inclusions) are, as expected, not solely determined by the phonon-mode Grüneisen tensors of the ideal crystal. We have also recently developed the measurement of the change in peak intensities in cross-polarised Raman spectra to determine the stress [4] in these cases. For minerals such as garnets, this effect is stronger and therefore more sensitive to stress than the shifts in peak positions and offers at the moment the possibility to quickly visualise stress and strain fields in minerals in-situ in rocks. Quantitative stress values from this method await the determination of the piezo-phonon tensors for garnets, but comparison of peak positions and intensities show that the two methods return consistent results.

This work was supported by ERC-StG TRUE DEPTHS grant (number 714936) to M. Alvaro. N. Campomenosi was also supported by the University of Genova.

[1] Holzhausen & Johnson (1979) Tectonophysics 58, 237.

[2] Angel et al. (2019) Zeitschrift für Kristallographie, 234, 129.

[3] Murri et al. (2018) American Mineralogist, 103, 1869.

[4] Campomenosi et al. (2020) Contributions to Mineralogy and Petrology, accepted.

How to cite: Angel, R., Murri, M., Campomenosi, N., Mihailova, B., Prencipe, M., and Alvaro, M.: Measuring stress and strain in rocks by spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2869, https://doi.org/10.5194/egusphere-egu2020-2869, 2020.

Ion probe ²⁰⁸Pb/²³²Th fissure monazite ages from high pressure regions of the Western Alps and from the Argentera Massif provide new insights on the tectonic evolution of the Western Alps during Cenozoic times. Fissure monazite is a hydrothermal mineral crystallizing during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Fissure monazite ages directly record chemical disequilibrium occurring in a fissure environment, but growing evidences indicate that fissure monazite commonly register tectonic activity. Fissure monazite age domains from this study show that monazite crystallization occurred between ~32-30.5 Ma and ~31.5-30 Ma in the Piémontais and Briançonnais Zone of the High Pressure regions, and between ~17-15 Ma and in the north-eastern border of the Argentera Massif. So far, monazite ages were recorded between ~32-23 Ma and at ~20.5 Ma in the Briançonnais Zone and in the south-western border of the Argentera Massif respectively. Thus the presented dataset corroborate and complement already reported fissure monazite ²⁰⁸Pb/²³²Th ages from the Western Alps. This new fissure monazite ages compilation supports that Late Oligocene thrusting affected the High Pressure regions of the Western Alps, and that Early and Middle Miocene dextral strike-slips movements respectively affected the south-western and north-eastern margins of the Argentera Massif. Chemical observations provide new hints on fissure monazite growth conditions (e.g. leached host-rock minerals, oxidation conditions) encouraging to pursue chemical studies with a larger dataset on natural fissure monazite to better understand growth conditions under cleft environment.

How to cite: Ricchi, E., Gnos, E., Rubatto, D., and Pettke, T.: Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and Piemontais and Briançonnais Zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18344, https://doi.org/10.5194/egusphere-egu2020-18344, 2020.

Phlogopite is accepted as a major mineral indicator of the modal metasomatism in the upper mantle within a very wide P-T range and fluid/melt compositions. It extensively forms in mantle peridotites transforming initial harzburgites and lherzolites to phlogopite wehrlites both in garnet and spinel-facies. A reaction 5En + Grt + [K₂O + 2H₂O in fluid] = Phl + Di (Grt – pyrope-grossular garnet CaMg₂Al₂Si₃O₁₂) is considered as the major mechanism for phlogopite formation in garnet-facies peridotites. This reaction is commonly accompanied by regular compositional changes of primary garnet and pyroxenes. In order to illustrate the regularities, we report result of experimental study of the phlogopite-forming reactions in the model systems pyrope-enstatite, grossular-pyrope-enstatite and knorringite-pyrope-enstatite systems in presence of a H₂O-KCl fluid at pressure 3 and 5 GPa and temperatures of 900 and 1000°C. The experiments were aimed at the tracing of variations of grossular and knorringite contents in garnet, as well as Al content of pyroxenes, with variations of the KCl content in the fluid.

The increase of X_KCl in the fluid is accompanied by gradual decomposition of garnet and Al-bearing enstatite in all systems. The Al₂O₃ content in orthopyroxene decreases in the pyrope – enstatite system at 5 GPa and 900°C. In the system enstatite-pyrope-grossular at 5 GPa and 1000°C phlogopite forms at the KCl content 10 mol. % in the fluid. Further increase of the KCl content in the fluid results in gradual disappearance of garnet and orthopyroxene and stronger domination of phlogopite and clinopyroxene. Grossular content in garnet increases with the KCl concetration in the fluid up to 10 mol. %, but further increase of the KCl concentration to 20 mol. % results in decrease of the grossular content in garnet. In the system enstatite-pyrope-knorringite at the KCl content in the fluid 0 – 10 mol. %, garnet contains 8-9 mol. % of knorringite. Cr-bearing phlogopite (about 2 wt. % Cr₂O₃) appears in this system at 10 mol. % KCl in the fluid, and its formation results in a slight increase of the knorringite content in garnet. Because of relatively high SiO₂ bulk content in comparison to the typical peridotite, Cr-bearing kyanite (not spinel) forms at 20 mol. % KCl in the fluid resulting in a decrease of the knorringite content in garnet down to 3-5 mol. %. The Cr₂O₃ content in the coexisting phlogopite concomitantly decreases by about 1 wt. %.

The above experiments reproduced some characteristic regularities in variations of garnet and pyroxene compositions in the course of phlogopite formation in mantle peridotites. The applicability of the experimental results is illustrated by examples from peridotite xenoliths from kimberlites. These effects can be applied for the quantitative and qualitative estimates of variations in K activity during the modal mantle metasomatism.

How to cite: Limanov, E., Butvina, V., and Safonov, O.: Experimental study of phlogopite-forming reactions in the system orthopyroxene+garnet in presence of the H2O-KCl fluids., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-228, https://doi.org/10.5194/egusphere-egu2020-228, 2020.

Amphibole is one of the most abundant ’water’-bearing minerals in the Earth’s upper mantle. Amphiboles occur as interstitial grains, lamellae within pyroxenes or as daughter minerals within fluid inclusions.  Most commonly amphibole formation is related to mantle metasomatism, where the agent has a subducted slab (e.g. Manning 2004) or an asthenospheric origin (e.g. Berkesi et al. 2019).  After the formation of fluid inclusions, a subsolidus interaction can take place where the H₂O content of fluid inclusions may crystallize pargasite (e.g. Plank et al. 2016).

Here we present amphibole lamellae formation in mantle xenoliths from the Persani Mountains Volcanic Field that is interrelated to a reaction between fluid inclusions and host clinopyroxene.  Newly formed amphibole lamellae occur only in the surroundings of the fluid inclusions and grow within the host clinopyroxene in a preferred crystallographic direction.  Studied lamellae do not reach the rim of the host mineral implying that components needed for formation of amphibole lamellae in clinopyroxene could have only originated from the fluid inclusion itself.  We measured the major element composition of amphibole lamellae and host clinopyroxene (1) and used Raman spectroscopy and FIB-SEM on fluid inclusion study situated next to the lamellae (2).  Results support the hypothesis that chemical components (dominantly H⁺) migrated sub-solidus from the fluid inclusion into the host mineral after fluid entrapment via subsolidus interaction.  Beyond the clinopyroxene-hosted fluid inclusions, fluid inclusions in orthopyroxenes were also studied as a reference.  Our study shows that post-entrapment diffusion from a fluid inclusion into the host mineral changes the solid/fluid ratio of the mantle  which could modify the rheology of the lithospheric mantle.

Berkesi, M. et al. 2019. Chemical Geology, 508, 182-196.

Kovács et al. (2017) Acta Geodaetica et Geophysica, 52(2), 183-204.

Manning C. E. 2004. Earth and Planetary Science Letters, 223, 1-16.

Plank, T. A. et al. 2016. In AGU Fall Meeting Abstracts.

How to cite: Lange, T. P., Pálos, Z., Patkó, L., Berkesi, M., Liptai, N., Aradi, L. E., Szabó, Á., Szabó, C., and Kovács, I. J.: Amphibole lamellae formation in the upper mantle due to interaction of fluid inclusions and host minerals: a case study from Persani Mountains Volcanic Field, Transylvania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1192, https://doi.org/10.5194/egusphere-egu2020-1192, 2020.

Naturally, olivine reacts with CO₂-rich fluids, producing carbonates and silica. If in completion, this reaction will cause a large increase in the solid volume (~85%), which can generate a significant stress/force when it occurs in a confined space. This may be used to fracture the surrounding rocks in the context of the injection of industrially captured CO₂ into peridotites for permanent sequestration. Contrarily, this volume-increasing reaction may also clog transport paths and thus inhibit CO₂ access, leading to little or no volumetric increase at industrial time scales. Although observations from natural systems suggest that reaction-induced fracturing during peridotite carbonation can occur, the fracturing mechanism has not been experimentally reproduced under in-situ stress-temperature-chemical conditions. Here, we report 9 flow-through experiments performed on pre-compacted Åheim dunite (containing ~85% olivine) powders (grain size 36-50 µm) during carbonation reaction under controlled σ-P-T conditions. This was done using a purpose-built apparatus, consisting of a flow-through system accommodated with a uniaxial servo-controlled loading system. Before experiments, the dunite powders were compacted stepwise up to 250MPa to form a disc-shape sample with starting porosity of ~25%. The sample was covered by a thin Teflon sleeve plus Vaseline to reduce the friction against the vessel wall. The experiments were performed at a constant temperature of 150℃ and constant (Terzaghi) effective stress of 1, 5, 15MPa, respectively. The sample was first exposed to deionized (DI) water at a pore fluid pressure of 10MPa, and then the DI water was replaced, maintaining constant pore pressure of 10MPa, by flow-through of a certain chemical fluid, such as CO₂ saturated brine (containing 1M NaCl plus 0.64M NaHCO₃, pH~3), CO₂ saturated water (pH~3), NaHCO₃ saturated solution (pH~9) and NaHSO₄ solution (pH~3). The permeability was measured for all experiments using the flow-through system by means of the steady-state method, and each experiment took 2-4 weeks. The experiments show that the samples exhibited 0-0.37% compaction strain when CO₂ saturated brine, CO₂ saturated water, and NaHCO₃ saturated solution flow through, independently of poroelastic effects, and the sample permeability drops in the order from 10^-17 to 10^-20 m². By contrast, for the NaHSO₄ flow-through experiment where no carbonation reaction occurred, the sample permeability increased from 2*10^-17 to 7*10^-17 m², associated with 0.05% compaction. The sample mass after the NaHSO₄ flow-through experiment reduced ~5%, suggesting that magnesium and silica may be partly leached out from the sample. Microstructure observations and XRD analysis on these samples demonstrate a drastic reduction in porosity of the reaction zone where CO₂ was integrated into the crystal structure of the product carbonates by means of carbonation reactions. The mechanism responsible for the observed behavior seems to be that the dissolution of olivine that occurred first at the grain contact surface leads to compaction, followed by precipitation of carbonates at porous that clogs the transport paths and thus reduces the permeability, though the detailed chemical analysis is still performing. As a result, our current findings suggest that the volume-increasing precipitation produced via the carbonation reaction under in-situ subsurface conditions will clog transport paths.

How to cite: Liu, J., Wolterbeek, T., and Spiers, C.: Volumetric response of crushed dunite during carbonation reaction under controlled σ-P-T conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3047, https://doi.org/10.5194/egusphere-egu2020-3047, 2020.

In the eclogite facies shear zone of Bottarello, Monte Rosa, Western Alps [1], fault recrystallization around 600 °C gives concordant Lu-Hf (garnet) and ³⁹Ar-⁴⁰Ar (white mica, WM) 47 Ma ages whereas <100 m from the fault the unsheared rock at the same T preserves Mesozoic inheritance. The Ar retentivity of WM is not accurately predicted by hydrothermal laboratory experiments, because the latter are plagued by massive dissolution artefacts [2]. Independent field observations confirm that WM only starts losing Ar in dry rocks above 600 °C [3-8], but when retrograde reactions occur, WM can recrystallize and be totally reset below 230 °C [9]. The Bottarello fault obliterated all relict WM from the protolith; the neoformed WM records its own formation age.

The island of Naxos (Cyclades, Greece) is the classic example of multiple, coexisting WM generations [10]: relict pre-eclogitic basement WM, and eclogitic phengite retrograded to muscovite. Electron microprobe element maps demonstrate intergrowths at a scale <5 µm, which makes laser microprobe dating useless. Bulk mica dissolution for Rb-Sr gives Eocene ages [11], which agree with bulk K-Ar ages. This is paradoxical, as Ar diffusivity is c. 4 orders of magnitude higher than that of Sr [12]; the only explanation is that both chronometric systems record formation ages around 500-600 °C. The WM generations can be unravelled by their Ca/Cl/K signatures; coarse and fine sieve fractions are never isomineralic. Ages of pure mica generations are obtained by extrapolating Ca/Cl/K-vs-age trends.

The in-sequence thrusts of the Garhwal Himalaya add one complication: thrusting was long-lived. Microstructures combined with chemical microanalysis distinguish three monazite generations (dated by U-Pb) and three WM generations: relicts in microlithons, foliation-defining mica, and static coronas. As in the previous examples, intergrowths are <<10 µm and only combining Ca/Cl/K systematics with the observed differences in structural breakdown temperatures can assign the different WM ages in the same sample to chemically distinct generations [13]. WM formation ages overlap with Mnz ages and date the onset of faulting, the kinematic peak, and the post-faulting corona formation.

There is no free lunch: dating deformation is extremely labor-intensive and requires, always, establishing the context between microtextural, microchemical, petrological and multichronometric analyses. Whenever one of these four is missing, the tectonic reconstruction is invariably faulty [14].

[1] Villa &al, J Petrol 55 (2014) 803-830

[2] Villa, Geol Soc London Spec Pub 332 (2010) 1-15

[3] Di Vincenzo &al, J Petrol 45 (2004) 1013-1043

[4] Itaya &al, Island Arc 18 (2009) 293-305

[5] Heri &al, Geol Soc London Spec Pub 378 (2014) 69-78

[6] Laurent &al, Lithos, 272-273 (2017) 315-335

[7] Airaghi &al, J Metam Geol 36 (2018) 933-958

[8] Imayama &al, Geol Soc London Spec Pub 481 (2019) 147-173

[9] Maineri &al, Mineralium Deposita 38 (2003) 67-86

[10 Wijbrans & McDougall, Contrib Min Petr 93 (1986) 187-194

[11] Peillod &al, J Metam Geol 35 (2017) 805-830

[12] Cherniak & Watson, EPSL 113 (1992) 411-425

[13] Montemagni &al, Geol Soc London Spec Pub 481 (2019) 127-146

[14] Bosse & Villa, Gondwana Res 71 (2019) 76-90

How to cite: Villa, I. M.: Dating deformation: multichronometric examples from the Western Alps, Naxos, and the Garhwal Himalaya, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4133, https://doi.org/10.5194/egusphere-egu2020-4133, 2020.

Melt inclusions have been for almost 150 years an exclusive feature of magmatic rocks. However, intensive research activity in the last decade has shown that melt inclusions, or nanogranitoids, are also a widespread feature of high grade metamorphic rocks. Such inclusions rapidly became fundamental tools to unravel partial melting and melt-related processes taking place during orogenesis.

One of the latest discoveries in this field has been the identification of nanogranitoids and glass inside the mega almandine-pyrope garnets of Barton Mine (Gore Mountain, NY State, US). These crystals are arguably the world’s largest garnets and occur within garnet hornblendite. Their size is ca. 35 cm in average, while garnet diameters up to 1 m were reported in historical record. Fluid is often invoked in the formation of large crystals, but so far no study has identified clear witnesses for the presence of fluid during garnet formation, e.g. primary fluid inclusions.

Polycrystalline inclusions of primary nature were instead reported by Darling et al. (1997) to occur inside the garnet: such inclusions are the main target of our study. Their shape ranges from tubular (2-100 µm in length) to negative crystal shape (2-50 µm). They mainly contain cristobalite/quartz, kumdykolite and amphibole. Minor phases such as biotite/phlogopite, enstatite, rutile, ilmenite and a second, Ca-richer plagioclase (or its rare polymorphs dmisteinbergite and svyatoslavite) may be also present. The inclusions were re-homogenized to a silicate-rich glass via piston cylinder experiments at 1.0-1.5 GPa and 925-940°C. Experimental results prove that such inclusions are former droplets of melt, in agreement with the finding of preserved residual glass in one single inclusion before the experimental runs. The melt composition measured in situ via electron microprobe is tonalitic-trondhjemitic with 5-6 wt% H₂O.

The identification of melt inclusions points toward a melt rather than a fluid as the medium which favored extreme garnet growth under low nucleation rate conditions. The elements necessary to grow garnets – mainly Fe, Al, Si, Mg- are indeed far more effectively transported by a silicate melt rather than simple aqueous fluid, at least at the limited depth envisioned for this process. In conclusion, the finding of melt inclusions in metamorphic rocks brought us forward along the path toward the solution of the enigma represented by the formation of these giant garnets.

References
Darling, R.S., Chou, I.M., Bodnar, R.J., 1997. An Occurrence of Metastable Cristobalite in High-Pressure Garnet Granulite. Science 276, 91.

How to cite: Ferrero, S., Wannhoff, I., Darling, R., Wunder, B., Oscar, L., O' Brien, P. J., Ziemann, M. A., and Günter, C.: Melt inclusions in metamorphic rocks: how localized melting promoted the formation of the Gore Mountains mega garnets (Adirondacks, US), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4543, https://doi.org/10.5194/egusphere-egu2020-4543, 2020.

Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression, Bohai Bay Basin, East China. Based on the study of vein petrography and fluid inclusions features, we demonstrate the vein was the product of hydrocarbon generation and expulsion from organic-rich shales. Consequently, the primary inclusions in the fibrous calcites recorded the fluid conditions during maturation of these source rocks. In most cases, the calcite-hosted primary inclusion assemblages are composed of the two-phase (oil + gas) hydrocarbon inclusions, with or without coexisting aqueous inclusions. Less commonly, the assemblages are made up of inclusions with only liquid hydrocarbon (i.e., monophase, high-density petroleum inclusions). In addition, many bitumen-bearing oil inclusions could also be observed in the fibrous calcite veins. By modelling the isochores of two-phase oil inclusions and coexisting aqueous inclusions, in light of the burial history for the basin, we conclude the fluid overpressure up to approximately twice (2x) the hydrostatic value (i.e., ~0.5–0.6x lithostatic) are the most common during the hydrocarbon generation and primary migration. The highest degrees of overpressure are recorded by the rare monophase petroleum inclusions. The resulting isochores of these highest density inclusions project to pressures that overlap with the lithostatic gradient. Thus, the monophase inclusions indicate pressures approaching and in some cases exceeding lithostatic. Our results indicate that fluids present during hydrocarbon generation and expulsion in organic-rich shales were indeed overpressured, but that lithostatic pressures were not the norm and evidently not a prerequisite for vein dilation, which means the fluid pressures during dilation of horizontal veins are not necessarily equal to the overburden throughout the history of the opening. This suggests that at least some of the vein dilation is accommodated and offset by concomitant narrowing of the adjacent wall rock laminae, likely by scavenging (dissolution/reprecipitation) of CaCO₃ from the adjacent wall rock, owing to the positive pressure dependence of calcite solubility, and presence of organic acids as byproducts of hydrocarbon generation.

How to cite: Wang, M., Chen, Y., and Steele-MacInnis, M.: Beef veins record fluid overpressure during oil primary migration in source rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5045, https://doi.org/10.5194/egusphere-egu2020-5045, 2020.

Elastic geobarometry allows one to recover the PT entrapment conditions of a host-inclusion pair from measurements of the residual pressure of the inclusion which develops upon exhumation due to differences of its thermo-elastic properties from the host (Angel et al., 2015). At the present, calculations assume that the inclusion is a single phase. For a soft inclusion in a stiffer host, the volume change of a free inclusion crystal would be greater than that of the host, which leads to the inclusions being compressed into a smaller volume than expected and thus positive inclusion pressures. Conversely, an inclusion stiffer than the host should develop a negative pressure.

Rutile-in-garnet would be a good candidate for elastic geobarometry because of its common occurrence in high-pressure high-temperature (HP-HT) metamorphic rocks, its simple structure and chemistry and its broad PT stability field. However, recent work by Zaffiro et al. (2019) demonstrated that rutile trapped in garnet should always exhibit negative pressure upon exhumation because rutile is stiffer than garnet, making this pair unsuitable for elastic geobarometry.

Nevertheless, rutile inclusions in garnets from the Pohorje eclogite seem to challenge this thermodynamic prediction. Rutile inclusions show no Raman peak shifts relative to free crystals within the measurement error, despite there being strain birefringence in the garnet host around the rutile which indicates the relaxation of stressed inclusions. High resolution 3D Raman mapping on one of these rutile inclusions revealed the presence of tiny (2-3 µm thick) amphibole crystals located between the garnet and rutile, with the amphibole occupying about 25-30% of the volume of the inclusion. The presence of this amount of amphibole lowers the bulk modulus of the composite inclusion (rutile + amphibole) to less than the bulk modulus of the garnet, hence leading to pressurization of the inclusions upon exhumation. This study shows that careful characterization of host inclusion systems, linked to thermodynamic modelling, is thus necessary to interpret residual pressures (P_inc) in terms of entrapment conditions.

This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro).

References:

Angel R.J. et al. 2015. J. Metamorph. Geol., 33(8), 801-813.

Zaffiro G. et al. 2019.  Mineralogical Magazine, 83(3), 339-347.

How to cite: Musiyachenko, K., Murri, M., Angel, R. J., Prencipe, M., Alvaro, M., and van Schrojenstein Lantman, H.: Elastic geobarometry of multiphase inclusions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5725, https://doi.org/10.5194/egusphere-egu2020-5725, 2020.

Thermal signatures as well as timing of fault motions can be constrained by thermochronological analyses of fault-zone rocks (e.g., Tagami, 2012, 2019).  Fault-zone materials suitable for such analyses are produced by tectocic and geochemical processes, such as (1) mechanical fragmentation of host rocks, grain-size reduction of fragments and recrystallization of grains to form mica and clay minerals, (2) secondary heating/melting of host rocks by frictional fault motions, and (3) mineral vein formation as a consequence of fluid advection associated with fault motions.  The geothermal structure of fault zones are primarily controlled by the following three factors: (a) regional geothermal structure around the fault zone that reflect background thermo-tectonic history of studied province, (b) frictional heating of wall rocks by fault motions and resultant heat transfer into surrounding rocks, and (c) thermal influences by hot fluid advection in and around the fault zone.  Geochronological/thermochronological methods widely applied in fault zones are K-Ar (⁴⁰Ar/³⁹Ar), fission-track (FT), and U-Th methods.  In addition, (U-Th)/He, OSL, TL and ESR methods are applied in some fault zones, in order to extract temporal information related to low temperature and/or recent fault activities.  Here I briefly review the thermal sensitivity of individual thermochronological systems, which basically controls the response of each method against faulting processes.  Then, the thermal sensitivity of FTs is highlighted, with a particular focus on the thermal processes characteristic to fault zones, i.e., flash and hydrothermal heating.  On these basis, representative examples as well as key issues, including sampling strategy, are presented to make thermochronological analysis of fault-zone materials, such as fault gouges, pseudotachylytes and mylonites, along with geological, geomorphological and seismological implications.  Finally, the thermochronological analyses of the Nojima fault are overviewed, as an example of multidisciplinary investigations of an active seismogenic fault system.

References:

1. Tagami, 2012. Thermochronological investigation of fault zones. Tectonophys., 538-540, 67-85, doi:10.1016/j.tecto.2012.01.032.
2. Tagami, 2019. Application of fission track thermochronology to analyze fault zone activity. Eds. M. G. Malusa, P. G. Fitzgerald, Fission track thermochronology and its application to geology, 393pp, 221-233, doi: 10.1007/978-3-319-89421-8_12.

How to cite: Tagami, T.: Low-temperature thermochronology of fault zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6060, https://doi.org/10.5194/egusphere-egu2020-6060, 2020.

Using partition coefficients is extremely useful to model melting processes and fluid-rock interactions. However, partition coefficients values remain scarce in regard of their sensitivity to mineral composition and to the variability of mineral composition. In addition, the inferred equilibrium between phases is not necessarily reached, even in high-grade metamorphic conditions associated to melting. Disequilibrium may dramatically hamper the effective mobility of species and lead to element distribution far from the predicted values.

This contribution aims at estimating partition coefficients for chromium (Cr) between garnet and clinopyroxene, and testing them in natural rocks of various metamorphic grades. As a poorly mobile trivalent element, Cr is chosen as a proxy to rare earth elements.

Theoretical partition coefficients for Cr between garnet and clinopyroxene are calculated ab initio from structures where Cr³⁺ is modelled as a defect in Al³⁺ sites using CRYSTAL17 (Dovesi et al., 2014) and the thermodynamic description of Dubacq and Plunder (2018). Results are compared to electron microprobe measurements in mineral assemblages containing tens to thousands of ppm of Cr, where element mapping brings much information.

Results of ab initio computations highlight the role of crystal-chemistry over the strain field around point defects, controlling the dynamics of the Cr³⁺ = Al³⁺ exchange between clinopyroxene and garnet. As expected, the partitioning of Cr between garnet and clinopyroxene depends strongly on the grossular and pyrope content: Cr incorporates grossular preferentially to jadeite, but jadeite incorporates Cr preferentially to pyrope.

Comparison between predicted and measured partition coefficients allowed to estimate the deviation from equilibrium. Disequilibrium is evidenced even for samples metamorphosed around 850°C, as shown by the distribution of Cr-rich and Cr-depleted domains. Disequilibrium is attributed to slow diffusivity of Cr in fluid and at grain boundaries during crystal growth, leading to interface-coupled dissolution-precipitation.

Dovesi, R., Orlando, R., Erba, A., Zicovich‐Wilson, C. M., Civalleri, B., Casassa, S., ... & Noël, Y. (2014). CRYSTAL14: A program for the ab initio investigation of crystalline solids. International Journal of Quantum Chemistry, 114(19), 1287-1317.

Dubacq, B., & Plunder, A. (2018). Controls on trace element distribution in oxides and silicates. Journal of Petrology, 59(2), 233-256.

How to cite: Dubacq, B., Figowy, S., Noël, Y., and D'Arco, P.: First-principle partitioning and disequilibrium of chromium in garnet – clinopyroxene assemblage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6962, https://doi.org/10.5194/egusphere-egu2020-6962, 2020.

Absolute dating of rock deformation is often hampered by the observation that the affected minerals are only partly re-equilibrated with respect to their isotopic composition.

Generally, three different processes enable mineral grains to adjust isotopically during the deformation event, i.e. volume diffusion, recrystallisation as well as dissolution and reprecipitation.

The degree to which the crystal structure is affected by these processes is different and thus the extent of isotopic equilibration during these processes generally differs in a way that diffusive element exchange is believed to be the most ineffective and slowest process, whereas re- and neo-crystallization seem to be fast and thorough.

Fluid-induced dissolution and reprecipitation is a very common mineral reaction mechanism in the solid Earth and as the crystal lattice is intensively reworked during this process, elemental and isotopic exchange between matrix and the newly formed crystal should be facilitated.   

Commonly, element and isotopic exchange during such mineral reactions is thought to occur via aqueous solutions, but new experimental as well as natural data show that the element transfer during mineral dissolution and reprecipitation can also occur in an amorphous material that forms directly by depolymerization of the crystal lattice.

Furthermore, precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface, hence the entire element and isotopic transfer between reactant and product mineral might not involve equilibration with the intercrystalline transport medium with important consequences for the interpretation of age data from re- and neo-crystallized grains.

How to cite: Konrad-Schmolke, M. and Halama, R.: How trustworthy are absolute age data from reprecipitated mineral grains?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7963, https://doi.org/10.5194/egusphere-egu2020-7963, 2020.

Dehydration reactions are metamorphic reactions that release water (aqueous fluids). They are crucial for the dynamics and chemical recycling in subduction zones. The dehydration of serpentinites is of particular importance as it represents the main source of volatiles in subduction zones and it occurs in a narrow temperature interval. It is generally assumed that dehydration reactions proceed at near-equilibrium. In such situation, the pressure of the fluid produced by the reaction approaches the lithostatic pressure. Deviations from the equilibrium model have been invoked for antigorite dehydration based on natural observations of anisotropic olivine grain growth at high-pressure conditions (1.5-2.0 GPa) [1,2]. Here we show another occurrence of texturally non-equilibrated olivine growth at low pressure (0.4 GPa) in olivine-talc veins crosscutting serpentinites in the Bergell intrusion contact aureole at Alpe Zocca (Malenco Unit, Northern Italy) [3].

The dehydration reactions, which resulted in the replacement of serpentinites by olivine-talc metaperidotites, occurred under quasi-static conditions. The main reaction front, which defines the equilibrium ol + tlc isograd, is a 150 m wide zone outcropping at ~750 m from the Bergell intrusion. In the metaperidotites, olivine has a crystallographic preferred orientation (CPO) correlated with the precursor antigorite CPO, with [010]_Ol axes parallel to [001]_Atg. These CPOs are accompanied by shape-preferred orientations (SPO) that mark the foliation in both rock types. Talc crystals are also oriented and often bent around olivine crystals, suggesting local compaction. We interpret the foliated metaperidotites as formed at near equilibrium conditions, with pervasive fluid extraction from the metaperidotite by viscous metamorphic compaction.

However, downstream (<100 m) of the equilibrium dehydration reaction front, ol + tlc bearing veins with variable width and shapes are common. These veins are often surrounded by centimeter- to decimeter-scale dehydration reaction zones that propagate into the serpentinite wall-rock. Olivine crystals in dehydration veins also have a strong SPO and CPO that define a jackstraw texture within the plane of the vein. They are elongated (reaching up to 50 cm) parallel to [001] within the vein plane and have their [010] axes normal to the plane of the vein. We interpret the olivine–talc assemblage in the veins and in the nearby reaction zones as resulting from premature dehydration reactions at lower temperature than the equilibrium conditions owing to effective fluid extraction from the wall-rock into the veins. The jackstraw texture indicates fast kinetics, with the crystal orientation controlled by anisotropic growth under a fluid pressure gradient. These textures record local displacement of the reactions towards lower temperature conditions owing to the formation of extensional veins, which acted as high permeability channels allowing for effective draining of the system. These brittle ‘sparkles’ during or before the massive pervasive dehydration seem to have little impact on the overall reaction. 

[1] Padrón-Navarta et al. (2011). Journal of Petrology 52, 2047-2078. [2] Dilissen et al. (2018). Lithos 320-321, 470-489. [3] Clément, M., Padrόn-Navarta, J. A. & Tommasi, A. (2019). Interplay between fluid extraction mechanisms and antigorite dehydration reactions (Val Malenco, Italian Alps). Journal of Petrology, in press.

How to cite: Padrón-Navarta, J. A., Clément, M., and Tommasi, A.: Olivine dis-equilibrium growth in the Val Malenco contact aureole (Northern Italy), sparks before the fireworks?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9268, https://doi.org/10.5194/egusphere-egu2020-9268, 2020.

Investigation of mantle xenoliths can provide information on the architecture and evolution of subcontinental lithospheric mantle through time. These reconstructions rely also on correct estimates of the pressures and temperatures (P-T) experienced by these rocks over time. Unlike chemical geothermobarometers, elastic geobarometry does not rely on chemical equilibrium between minerals, so it has the potential to provide information on over-stepping of reaction boundaries and to identify other examples of non-equilibrium behaviour in rocks. Here we introduce a method that exploits the elastic anisotropy of minerals to determine the unique P and T of equilibration from a measurements of single-crystal mineral inclusions trapped in a crystalline host from an eclogite xenolith [1]. We apply it to perfectly preserved quartz inclusions in garnet from eclogite xenoliths in kimberlites. We show that the elastic strains of inclusions calculated from in-house Raman spectroscopy measurements of the inclusions are in perfect agreement with those determined from in-situ X-ray diffraction measurements performed both in-house and at the synchrotron. Calculations based on these measured strains demonstrate that quartz trapped in garnet can be preserved even when the rock passes into the stability field of coesite (high pressure and temperature polymorph of quartz). This supports a metamorphic origin for these xenoliths that provides constraints on mechanisms of craton accretion from a subducted crustal protolith. Furthermore, we show that some key inclusion minerals do not always indicate the P and T attained during subduction and metamorphism.

This project has received funding from the European Research Council under the H2020 research and innovation programme (N. 714936 TRUE DEPTHS to M. Alvaro)

[1] M Alvaro, ML Mazzucchelli, RJ Angel, M Murri, N Campomenosi, M Scambelluri, F Nestola, A Korsakov, AA Tomilenko, F Marone, M Morana (2020) Fossil subduction recorded by quartz from the coesite stability field, Geology, 48, 24-28

How to cite: Alvaro, M., Mazzucchelli, M. L., Angel, R. J., Murri, M., Campomenosi, N., Scambelluri, M., Nestola, F., Korsakov, A., Tomilenko, A., Marone, F., Morana, M., and Alabarse, F.: Fossil Subduction Recorded By Quartz From The Coesite Stability Field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10711, https://doi.org/10.5194/egusphere-egu2020-10711, 2020.

The Kovdor massif is a part of the Paleozoic Kola alkaline province and located in the eastern part of the Baltic Shield. Kovdor carbonatites host a unique complex baddeleyite-apatite-magnetite deposit from which iron ores and zirconium have been mined. New data on melt inclusions in olivine crystals from phoscorites and olivinites of the ore complex are presented in this contribution. Daughter minerals in crystallized melt inclusions were identified by Raman spectroscopy and scanning electron microscopy. The trace element composition of inclusions was determined using LA-ICP-MS.

Melt inclusions in olivine from Kovdor phoscorites are negative crystal or round in shape, with sizes ranging from 5 to 50 microns. They form groups or line up. According to the mineral composition, two types of melt inclusions can be distinguished: carbonate and silicate-carbonate. In the first type, Ca-Na-Mg- (Sr?) - REE carbonates are dominant among daughter phases. In the second one, silicate phases (phlogopite, monticellite, diopside), Ca-Na-Mg carbonates and magnetite are found together. Melt inclusions in olivine from olivinites are isometric or elongated, 5–25 μm in size. They form groups or occur as isolated inclusions. Benstoneite, geylussit, ankerite, calcite and hydroxyl-bastnesite along with phyllosilicates (phlogopite, paragonite?) were identified among daughter minerals.

The rare earth elements composition of melt inclusions from both types of rocks is characterized by the predominance of light REE. The content of REE, especially light ones, in inclusions from phoscorites is higher. Strontium and barium contents in most melt inclusions have negative correlations with niobium and zirconium concentrations.

Melt inclusions from phoscorites and olivinites contain carbonate and silicate mineral phases in various proportions, which may imply heterogeneous trapping of crystalline phases and two immiscible melts, silicate and carbonatite. Inclusions from phoscorite represent a more evolved magma with higher concentrations of rare metals.

This work was supported by the Russian Science Foundation, grant No 19-17-00013.

How to cite: Redina, A., Wohlgemuth-Ueberwasser, C., Mikhailova, J., and Ivanyuk, G.: Melt inclusions in olivines from phoscorites and olivinites of the Kovdor massif., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12611, https://doi.org/10.5194/egusphere-egu2020-12611, 2020.

The Northern Apennines (NA) are a characteristic example of foreland fold-and-thrust belt (FTB) migrating towards its foreland. The progressive and quite regular eastward migration of the NA has been classically constrained in time by relying on the age of the syn-orogenic foreland basins, mainly determined by means of foraminifera and nannofossil biostratigraphy.

The well-known age of deformation makes the NA a perfect area where to test the reliability of the K-Ar illite dating applied to Cenozoic deformation involving siliciclatic deposits. In particular, we present the results of the first attempt to directly date, by K-Ar on illite separated from fault rocks, Neogene thrusts within the Trasimeno Tectonic Wedge (TTW), an imbricate thrust complex mainly made up of Tertiary siliciclastic rocks, located in the inner-central part of the NA, which represents the external front of the so-called Tuscan Nappe.

We sampled two WSW-dipping thrust faults, whose fault cores are composed of scaly gouge formed at the expense of the pelitic component of the host rocks. X-ray diffraction (XRD) and K-Ar isotopic analysis of multiple grain-sizes (from < 0.1 to 10 µm) allowed us to discriminate between syn-kinematic illite crystals formed during thrusting and detrital illite crystals inherited from the host rock. XRD data show a mineralogical association composed of quartz, calcite, albite, K-feldspar, chlorite, kaolinite and two populations of illite polytypes (1M_d and 2M₁). After the X-ray semiquantitative analysis, the results of the K-Ar dating of the two samples were regressed by the Illite Age Analysis (IAA) approach to assessing the effects of potential host rock contamination. Fault slip along the thrusts is then constrained to 15.2 ± 7.6 Ma and 15.4 ± 16.6 Ma.

Despite the large errors, the obtained dates are in excellent agreement with the timing of deformation along the base of the TTW, bracketed between the late Aquitanian and the latest Burdigalian – earliest Langhian and, more in general, with the proposed time evolution of the Northern Apennines.

Even if based on a limited dataset, our results suggest that the application of K-Ar dating of fault gouge can be extended to tectonic settings where independent constraints are not available and thus it becomes a valuable tool to study and constrain the time-space evolution of FTBs in recent and even active orogens.

How to cite: Carboni, F., VIola, G., Aldega, L., van der Lelij, R., Brozzetti, F., and Barchi, M. R.: K-Ar dating of recent thrusts: an application to the Tertiary clay gouges in the Northern Apennines of Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13888, https://doi.org/10.5194/egusphere-egu2020-13888, 2020.

The study of fluid inclusions (FI) composition (He, Ne, Ar, CO₂) integrated with the petrography and mineral chemistry of mantle xenoliths representative of the Sub Continental Lithospheric Mantle (SCLM) is a unique opportunity for constraining its geochemical features and evaluating the processes and the evolution that modified its original composition. An additional benefit of this type of studies is the possibility of better constraining the composition of fluids rising through the crust and used for volcanic or seismic monitoring.  

In this respect, the volcanic areas of Eifel and Siebengebirge in Germany represent a great opportunity to test this scientific approach for three main reasons. First, these volcanic centers developed in the core of the Central European Volcanic Province where it is debated whether the continental rift was triggered by a plume (Ritter, 2007 and references therein). Second, Eifel and Siebengebirge formed in Quaternary (0.5-0.01 Ma) and Tertiary (30-6 Ma), respectively, thus spanning a wide range of age. Third, Eifel is characterized by the presence of CO₂-dominated gas emissions and weak earthquakes that testify that local magmatic activity is nowadays dormant, but not ended (e.g., Bräuer et al., 2013). It is thus important to better constrain the noble gas signature expected in surface gases in case of magmatic unrest.

This work focuses on the petrological and geochemical study of mantle xenoliths sampled in the West Eifel and Siebengebirge volcanic areas (Germany) and aims at enlarging the knowledge of the local SCLM. Gautheron et al. (2005) carried out the first characterization of noble gases in FI of crystals analyzed by crushing technique (as in our study) but limited to olivines and to West Eifel eruptive centers. Here, we integrate that study by analyzing olivines, orthopyroxenes and clinopyroxenes from a new suite of samples and by including two eruptive centers from Siebengebirge volcanic field (Siebengebirge and Eulenberg quarries).

Xenoliths from the Siebengebirge localities are characterized by the highest Mg# for olivine, clinopyroxene and Cr# for spinel, together with the lowest Al₂O₃ contents for both pyroxenes, suggesting  that the mantle beneath Siebengebirge experienced high degree of melt extraction (up to 30%) while metasomatic/refertilization events were more efficient in the mantle beneath West Eifel.

In terms of CO₂ and noble gas concentration, clinopyroxene and most of the orthopyroxene show the highest gas content, while olivine are gas-poor. The ³He/⁴He varies between 5.5 and 6.9 Ra. These values are comparable to previous measurements in West Eifel, mostly within the range proposed for European SCLM (6.3±0.4 Ra), and slightly below that of MORB (Mid-Ocean Ridge Basalts; 8±1Ra). The Ne and Ar isotope ratios fall along a binary mixing trend between air and MORB-like mantle. He/Ar* in FI and Mg# and Al₂O₃ content in minerals confirm that the mantle beneath Siebengebirge experienced the highest degree of melting, while the metasomatic/refertilization events largely affected the Eifel area.

References

Bräuer, K., et al. 2013. Chem. Geol. 356, 193–208.

Gautheron, C., et al. 2005. Chem. Geol. 217, 97–112.

Ritter, J.R.R., 2007. In: Ritter, J.R.R., Christensen, U.R. (Eds.), Mantle Plumes: A Multidisciplinary Approach. Springer-Verlag, Berlin Heidelberg, pp. 379–404.

How to cite: Rizzo, A. L., Coltorti, M., Faccini, B., Casetta, F., Ntaflos, T., and Italiano, F.: Geochemistry of noble gas and CO2 in fluid inclusions from lithospheric mantle beneath Eifel and Siebengebirge (Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14005, https://doi.org/10.5194/egusphere-egu2020-14005, 2020.

Olivine is an important mineral phase in naturally cooled basaltic rocks. The texture and composition of olivine are strictly related to the interplay between the degree of magma undercooling and crystal growth rate. Crystals formed at low undercoolings and growth rates generally show polyhedral-hopper textures and quite homogeneous compositions, while skeletal-dendritic textures and evident crystal zonations occur at high undercoolings and growth rates. In this context, we have performed equilibrium and disequilibrium (i.e., cooling rate) experiments to better understand, by a comparatively approach, the effects of crystallization kinetics on the incorporation of major and trace cations in olivine lattice. The experiments were carried out in a 1 atm vertical tube CO-CO2 gas-mixing furnace to perform experiment at atmospheric pressure and oxygen fugacity of QFM-2 using a basaltic glass (i.e., OIB) as starting materials. The equilibrium experiment was performed at 1175 °C. These target temperatures were kept constant for 240 h and then quenched. Conversely, the disequilibrium experiments were performed at the superliquidus temperature of 1250, and 1300 °C, which was kept constant for 2 h before cooling. The final target temperatures of 1150 (undercooling -ΔT = 50 °C), and 1175 °C (-ΔT = 25 °C) were attained by applying cooling rates of 2 °C/h, 20 °C/h, and 60 °C/h. Then the experimental charges were quenched. Results show that the olivine texture shifts from euhedral (i.e., equilibrium) to anhedral (i.e., disequilibrium) under the effect of cooling rate and rapid crystal growth. In equilibrium experiments, the composition of olivine is homogeneous and non chemical gradients are found in the melt next to the crystal surface. In contrast, a diffusive boundary layer develops in the melt surrounding the olivine crystals growing rapidly under the effect of cooling rate and degree of undercooling. The compositional gradient in the melt increases with increasing cooling rate and undercooling, causing the diffusive boundary layer to expand towards the far field melt. Because of the effects of crystallization kinetics, skeletal-dendritic olivines incorporates higher proportions of major and trace elements that are generally incompatible within their crystal lattice under equilibrium conditions.

How to cite: Lang, S., Mollo, S., France, L., Nazzari, M., Misiti, V., Gurenko, A. A., and Devidal, J.-L.: Kinetic aspects of major and trace elements in olivines from variably cooled basaltic melts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19696, https://doi.org/10.5194/egusphere-egu2020-19696, 2020.

We present various examples of age variations in Potassium-bearing minerals of tectonites obtained by ⁴⁰Ar/³⁹Ar in situ dating. Age variations in pre-kinematic clasts and syn-kinematic blasts both span large age ranges of significantly different ages values at the cores compared to the rims, calling for petrologic interpretation. Some of the synkinematic grains are overgrown by late- to post-kinematic blast that show consequently the youngest ages values within the samples. The concurrence of textural relation and age value in the case of late- to post-kinematic growth seem to be a robust tool to date the termination of deformation.  

Additional examples where break down reactions lead to dissolution of the prekinematic texture and crystallization of new minerals as coronas, within fractures of strain shadows also yield partly reset age values with larger scatter. The interpretation of those age values is more challenging and might be obscured by the 3D textural geometry of the analysed volume as well as disequilibrium between reactants and products.      

Commonly based on the percentage errors age values are interpreted as being geological significantly different, when errors do not overlap, or as natural geologic scatter, in the case errors overlap. This interpretation is biased by the absolute age value itself and might lead to over- and underestimations of geological events in geologic history. There is a strong need of error calculation that enables geological interpretation of tectonic event independent from their absolute age.

How to cite: Schneider, S.: Inter- and Intragranular age variations: Diffusion or mineral growth? – And what is wrong with the error? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21192, https://doi.org/10.5194/egusphere-egu2020-21192, 2020.

Ar/Ar-in-situ geochronology by laser ablation NGMS (noble gas mass spectrometry) provides a powerful tool to determine inter- and intra-granular age variations of potassium-bearing minerals while maintaining the structural integrity of a sample. This makes it an excellent method in targeting the understanding of the post-collisional evolution of an orogen by dating different mica generations. In order to investigate the timing of exhumation related to extensional deformation in the Internal Dinarides, we sampled paragneisses from the upper greenschist- to amphibolite-grade mylonitic detachment zones of two metamorphic core complexes (MCC’s). The MCC’s are located at the distal Adriatic passive margin (Cer MCC, central western Serbia) and within the Late Cretaceous suturing accretionary wedge complex (Motajica MCC, northern Bosnia and Herzegovina) that separates Adria-derived units from blocks of European affinity.

Mica grains were assigned to either pre-kinematic or syn-kinematic growth, according to their structural context, texture and grainsize. Pre-kinematic growth is characterized by large, deformed minerals of up to 3.5 mm in size, while rather fine-grained, recrystallized mineral aggregates that usually formed in the strain shadow of larger clasts represent syn-kinematic growth.

The ages of pre-kinematic white mica from paragneisses of the Motajica detachment range from approx. 80 to 25 Ma. They partly show a large intra-granular age spread characterized by significantly older core ages becoming progressively younger towards the rim. This pattern likely suggests diffusive loss of radiogenic Ar. Ages between 80-55 Ma in the central parts of the grains, associated with a top-W transport direction, are interpreted as the time interval of mineral growth and subsequent deformation in an accretionary wedge during E-ward subduction of the Adriatic passive margin underneath European units.

Syn-kinematic white mica from Motajica yielded ages between 22 and 16 Ma, which are interpreted as the time of peak activity of extension. This also corresponds with the time of crustal extension in the Pannonian Basin to the north. At Cer MCC, located roughly 150 km ENE of Motajica MCC and structurally below the accretionary wedge complex, ages of deformed white mica indicate exhumation between 19 and 15 Ma with a top-N directed transport.  

Our results suggest that the opening of the Pannonian Basin in response to slab-retreat underneath the Carpathian orogen resulted in the extensional reactivation of suturing thrusts that separated Adriatic from European units, leading to exhumation of parts of the accretionary wedge (Motajica MCC). This event was followed by the progressive exhumation of the passive Adriatic margin (Cer MCC) that occupied a structural position below the suturing accretionary wedge.

How to cite: Löwe, G., Schneider, S., Pfänder, J. A., and Ustaszewski, K.: Dating extensional deformation to unravel exhumation patterns in the Internal Dinarides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21425, https://doi.org/10.5194/egusphere-egu2020-21425, 2020.