GMPV4.6 | Garnet and its inclusions: an archive of geological processes
EDI
Garnet and its inclusions: an archive of geological processes
Convener: Gabriele Cruciani | Co-conveners: Silvio FerreroECSECS, Lorraine Tual, Mattia GilioECSECS, Iwona KlonowskaECSECS
Orals
| Fri, 19 Apr, 10:45–12:30 (CEST)
 
Room -2.33
Posters on site
| Attendance Fri, 19 Apr, 16:15–18:00 (CEST) | Display Fri, 19 Apr, 14:00–18:00
 
Hall X1
Posters virtual
| Attendance Fri, 19 Apr, 14:00–15:45 (CEST) | Display Fri, 19 Apr, 08:30–18:00
 
vHall X1
Orals |
Fri, 10:45
Fri, 16:15
Fri, 14:00
Garnet preserves and records in an exceptionally way the geochemical conditions at which it equilibrates. Because it is stable across most of the lithosphere and found in very different protoliths, garnet provides a unique perspective to track deep geochemical processes, from magmatic ones to those responsible for the formation of ore deposits. Its stability over an extremely wide range of pressure and temperature (P–T) and its durability and resilience as detrital material make garnet an invaluable archive to unravel crustal evolution and planetary processes across geological timescale. Using this mineral as an archive where one can read the history of natural rocks, petrologists can reconstruct the P-T history and evolution of igneous and metamorphic rocks. Microstructures, compositional zoning, and elastic behaviour of its inclusions are just a few of the many aspects of great interest to better comprehend the history and fate of garnet-bearing rocks. Indeed, for instance the relatively slow diffusivity of trace elements in garnet allows the preservation of growth zones resulting from complex evolutions. When detrital, meticulous work on a high number of crystals allows to discover unexpected new evidences of UHP metamorphism and/or to collect new data on the tectono-metamorphic evolution of ancient basements. Finally, recent analytical improvements in dating of individual growth zones with high resolution or in-situ provide new opportunities to date the evolution and rate of a wide spectrum of geological and tectonic processes, while stable isotope zoning in garnet is the new frontier in exploring fluid/melt-rock interactions at depth.
We invite geoscientists from all kind of geological expertise to contribute to our session with their insights and approaches to unlock the secrets of this remarkable and useful mineral. Studies of fluid, melt and mineral inclusions within garnets and the application of innovative analytical techniques methodological approaches are also welcome.

Session assets

Orals: Fri, 19 Apr | Room -2.33

Chairpersons: Gabriele Cruciani, Silvio Ferrero, Lorraine Tual
10:45–10:50
10:50–11:00
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EGU24-6665
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On-site presentation
Sarah Penniston-Dorland, Alejandro Cisneros de León, William Hoover, Besim Dragovic, Philip Piccoli, and Christiana Hoff

Fluids released within subduction zones affect fundamental Earth processes, including seismicity and the generation of arc magmas, the formation of continental crust, and the geochemical evolution of the mantle. However, very little is understood about processes of fluid transport within subduction zones. Bulk-rock variations in Li isotopic compositions (δ7Li) are observed in fluid-related features in subduction-related metamorphic rocks at the centimeter-scale suggesting a short duration of fluid infiltration events – weeks to centuries. These measurements capture a time-integrated record, while in situ measurements in metamorphic minerals such as garnet can record individual events experienced by the rock. In our work measuring δ7Li in situ in garnet from several exhumed subduction zone metamorphic localities, we have found variations in δ7Li occurring within crystals over a scale of a few hundred microns, including troughs of negative values of δ7Li. Variations in δ7Li are associated with evidence for fluid release and fluid-rock reaction suggesting a role for fluids fluxing through the slab. The negative δ7Li excursions suggest that diffusion played a role in the history recorded by these garnets - in some cases garnet experienced intracrystalline diffusion of Li on the scale of a few hundred microns, and in others garnet growth zones incorporated variations of δ7Li within the metamorphic fluid surrounding the garnet, caused by diffusion of Li within the intergranular fluid on at least a centimeter scale. Multiple troughs in some of the garnets record the episodicity of fluid flow. Ongoing work is focusing on investigating garnets from a wide range of natural samples to look for patterns in fluid flow episodicity. Additionally, experiments determining the diffusivity of Li within garnet are being performed in order to quantitatively constrain timescales of intracrystalline diffusion.

How to cite: Penniston-Dorland, S., Cisneros de León, A., Hoover, W., Dragovic, B., Piccoli, P., and Hoff, C.: Garnet as an archive of fluid flow processes during subduction metamorphism: Evidence from in situ measurement of Li isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6665, https://doi.org/10.5194/egusphere-egu24-6665, 2024.

11:00–11:10
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EGU24-10096
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ECS
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On-site presentation
Jan Kulhánek and Shah Wali Faryad

Compositional zoning of trace elements in garnet serves as a valuable tool for reconstructing petrogenetic evolution, supplementing major element analyses. This is particularly applicable to trace elements exhibiting a strong affinity for garnet and characterized by slow diffusion rates, such as Y and heavy rare earth elements (HREE). The present study examines various zoning patterns of trace elements observed in large garnet porphyroblasts within micaschist samples from the Variscan high-pressure (HP) metamorphic terrain of the Krušné hory Mts. (Saxothuringian zone, Bohemian Massif).

Using electron probe micro-analyser and laser ablation-inductively coupled plasma mass spectrometry, three distinct types of compositional zoning in garnet were identified by compositional mapping. These zoning types were classified as a continuous core-to-rim change, concentric annular changes, and overprinting (or mimicking) of a pre-existing distribution. The study focuses on the formation mechanisms of each type of zoning, their dependence on pressure-temperature change, and fluid availability.

The significantly elevated concentrations of Sc, Y, and HREE in the garnet's central core suggest a rapid diffusion of these elements from the matrix into the garnet after nucleation, challenging a description solely through Rayleigh fractionation. The observed prograde growth of pressure-temperature (PT) conditions of the rock samples to HP–medium temperature (MT) aligns well with the compositional zoning patterns exhibited by the garnet, encompassing major and trace elements, as well as other minerals. Specific compositional patterns include: (1) gradual increase in Co and Zn contents towards the rim, mirroring Mg and inversely related to Mn, indicative of a continuous rise in temperature; (2) overprint zoning of Ti and partly Ca, Sm, Eu, Gd, and Tb in the central part, transitioning to purely concentric annular zoning in the rim, suggesting an increase in temperature; (3) well-developed overprint zoning of Cr throughout the garnet grain, indicating temperatures only up to MT; and (4) depletion of Y, and most of rare earth elements (HREE, Ho, Dy, Tb, Gd, Eu, and Sm – REE) in the rim, accompanied by enrichment of coupled VIII(Na, Li)+ + IVP5+ substitution elements, experimentally documented from HP to ultra-HP conditions.

The observed inverse annular oscillatory distribution of Sc and V is discussed to be attributed to fluctuating oxygen fugacity during garnet growth, influenced by changes in the availability of the fluid matrix medium carrying trace elements. Higher fluid availability corresponds to increased Sc, Y, and REE incorporation into garnet, evident in well-correlated annular elevations, while V exhibits the inverse trend. Elevated trace element contents in garnet are linked to the breakdown of main and accessory phases carrying these elements during garnet growth, incorporating them into the garnet. The presence of a fluid medium in the system appears to predominantly influence the extent and frequency of annular variations in trace element concentrations. Thus, annular zoning in garnet is associated with both the decomposition of trace element-bearing phases and fluid medium availability.

Acknowledgement: This work was supported by the Czech Science Foundation (Grant No. 24-12845S), Grant Agency of Charles University (Grant No. 1194019), and by Charles University through the Cooperatio Program (Research Area GEOL).

How to cite: Kulhánek, J. and Faryad, S. W.: Formation of various trace element zoning patterns in high-pressure metamorphic garnet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10096, https://doi.org/10.5194/egusphere-egu24-10096, 2024.

11:10–11:20
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EGU24-3555
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Highlight
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On-site presentation
Simon Wagner, Roland Köchl, Bianca Zerobin, Peter Tropper, Gert Goldenberg, Gunda Barth-Scalmani, Christoph Hauzenberger, Gerald Degenhart, and Walter Ungerank

Garnet has been utilized for various applications for centuries. It is an important phase for interpreting geological and mineralogical processes due to its wide range of stability and occurrence in various geochemical environments. In the late 18th century, the Zemmgrund in the innermost parts of the Zillertal became an important source of raw material for garnet jewelry. The garnets are found within ductile shear zones in the “Zentralgneisse” of the Venediger nappe system, which is part of the Penninic Tauern Window. There are several comparable shear zones in the innermost Zillertal, with the most relevant for industrial purposes being at the Rossrugg ridge (Zemmgrund). The shear zone rocks can be described as garnet-bearing chlorite mica schists from a petrographic perspective. To better comprehend the deposit's genesis, geochemical composition analyses were conducted on Rossrugg samples using µ-XRF, EPMA, and LA-ICP-MS. The garnet samples exhibit continuous zoning, with the almandine component having the highest proportions of 60 mol% (core) and 73 mol% (rim). The distribution of trace elements, such as Co, Zn, or Zr, correlates with this pattern. In contrast, Ti, HREEs and Y shows a decreasing concentration towards the core. The calculations for garnet formation conditions were performed using Thermocalc v3.45 and TC_Comb, resulting in T=612±34°C and P=7.5±1.4 kbar. The PerpleX and Theriak-Domino software was used to calculate garnet isopleths, which showed that garnet growth occurred at decreasing pressure and increasing temperatures.

This mineral has been utilized by mankind for thousands of years due to its abundance in a wide variety of rocks, especially as a gemstone. Almandine garnets from the Zemmgrund (Ziller Valley, Tyrol) and Radenthein (Carinthia) have played a significant role in jewelry production in the Alpine region since the end of the 18th century. Until the early 20th century, a small industry was established in the Zillertal, which supplied raw materials to Bohemia. Due to the export to Bohemia, the 'Tyrolean' garnets were mixed with the pyropes and lost their identity to some extent. However, there are various methods available to determine the origin of the raw materials used. Besides size and color, another criterion for differentiation is the inclusion pattern of the garnets. For example, Zillertal garnets typically contain chlorite, zircon, apatite, quartz, ilmenite, and epidote as inclusions. In contrast, the garnets from Radenthein exhibit oriented inclusion growth of ilmenite and rutile. However, to clearly differentiate between various garnets from the alpine deposits in a piece of jewellery, destruction-free chemical analyses using suitable methods (e.g. µ-XRF) are necessary. By using PCA and comparing specific oxides (e.g. CaO-MgO), individual deposits can be effectively distinguished. However, local differences, due to different small deposits of the Zemmgrund, presents a challenge as the chemical differences are subtle because of similar geological conditions and little differences in local bulk compositions. Nonetheless, by applying PCA to the collected data, it can be subdivided into five groups, including samples from individual deposits and a warehouse in Zell am Ziller.

How to cite: Wagner, S., Köchl, R., Zerobin, B., Tropper, P., Goldenberg, G., Barth-Scalmani, G., Hauzenberger, C., Degenhart, G., and Ungerank, W.: Orogenic treasures: Ziller Valley garnets and their transformation from petrogenetic indicator to gemstone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3555, https://doi.org/10.5194/egusphere-egu24-3555, 2024.

11:20–11:30
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EGU24-17450
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ECS
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On-site presentation
Ben Knight and Chris Clark

The variation in element concentrations within a mineral play a critical role in understanding the evolution of geological margins over time. Diffusion is a process that alters element concentrations during crystal growth and subsequent cooling, with modelling of this process primarily conducted in one-dimension (1D), significantly contributing to understanding the thermal and tectonic evolution of various igneous and metamorphic regions. However, the complexity of geological systems requires a more detailed approach encapsulating the multidimensional nature of mineral evolution. We present the initial developments of diffusion modelling, extending from 1D models to two-dimensional (2D) and three-dimensional (3D) modelling of garnet diffusion using the Underworld code. These models are designed to better constrain geological settings where garnet diffusion occurs, concentrating on the growth and extraction of the mineral from various bulk rock compositions. 2D and 3D models are compared to previously published work to validate the development of the models. Furthermore, the implications of these models for geochronological and petrological interpretations are explored, highlighting their potential to provide further insights into metamorphic processes when compared to 1D models. Future work will focus on the anisotropic properties of the garnet, including fast diffusion pathways within the mineral, as well as anisotropic properties of the rock itself, due to varying diffusion coefficients across minerals. This research paves the way for a more comprehensive understanding of diffusion in rocks, offering a robust tool for the community to unravel the complex history recorded in metamorphic rocks and enhance the accuracy of thermobarometric estimations. Through these new tools, we aim to provide better insights into the evolution of geological margins over time.

How to cite: Knight, B. and Clark, C.: Extending (Garnet) Diffusion Modelling into Multidimensional Domains, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17450, https://doi.org/10.5194/egusphere-egu24-17450, 2024.

11:30–11:40
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EGU24-5398
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On-site presentation
Evangelos Moulas and Kurt Stüwe

Extracting apparent timescales or cooling rates of rocks via inverse diffusion modelling allows the testing of geodynamic consequences on the petrological record.  Indeed, inverse modelling of diffusion in minerals such as garnet has been used extensively to constrain thermal history timescales [1] and/or cooling rates [2]. Although inherently such models do not have unique solutions, [3], they can be used to place constraints on the thermal history of rocks and regions [4].

This is interesting because different geodynamic processes will have thermal histories with different attributes [5], in particular qualtiative differences in the shape of cooling curves. In this contribution, we investigate aspects of different apparent cooling rates extracted via inverse diffusion modelling in garnet. Our results suggest a clear distinction in thermal history depending on wether the rocks experienced “active” (driven from tectonic forces) or “passive” (purely conductive) cooling [5]. We emphasize that active cooling implies slow cooling rates at high-grade conditions whereas passive cooling can have very large cooling rates at high-grade conditions (Fig. 1). For petrologic systems with relatively high closure temperatures (>400 °C), the spatially varying aparent cooling rates allow the identification of local heat sources such as intrusions or heat-producing shear zones (Fig. 1). Our results help to identify processes that have transient and local characters such as the thermal affects of heat-producing shear zones and magmatic intrusions.

 

Figure 1 – (a) Thermal histories from rocks in the vicinity of a shear zone, note the higher temperatures experienced by the shear-zone rocks. Such thermal histories will lead to spatial gradients of diffusion relaxation. (b) Absolute cooling rates as a function temperature for the curves shown in (a). The cooling rate versus temperature curve was calculated considering the thermal histories shown in (a) for the time period after 1Myr. (after [2]).

 

REFERENCES

[1] Chakraborty, S. Diffusion in Solid Silicates: A Tool to Track Timescales of Processes Comes of Age. Annu. Rev. Earth Planet. Sci. 36, 153–190 (2008).

[2] Burg, J.-P. & Moulas, E. Cooling-rate constraints from metapelites across two inverted metamorphic sequences of the Alpine-Himalayan belt; evidence for viscous heating. J. Struct. Geol. 156, 104536 (2022).

[3] Moulas, E. & Bachmayr, M. Petrology as an ill-posed inverse problem. in 73 (Mainz Institute of Multiscale Modelling, 2023).

[4] Braun, J., Beek, P. van der & Batt, G. Quantitative Thermochronology: Numerical Methods for the Interpretation of Thermochronological Data. (Cambridge University Press, 2006). doi:10.1017/CBO9780511616433.

[5] Stüwe, K. & Ehlers, K. Distinguishing Cooling Histories using Thermometry. Interpretations of Cooling Curves with some Examples from the Glein-Koralm Region and the Central Swiss Alps. Mitteilungen Österr. Geol. Ges. 89, 201–212 (1998).

How to cite: Moulas, E. and Stüwe, K.: Distinguishing cooling histories via Diffusion Geospeedometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5398, https://doi.org/10.5194/egusphere-egu24-5398, 2024.

11:40–11:50
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EGU24-14917
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ECS
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On-site presentation
Jan Schönig

Documenting metamorphic conditions through the geologic record is a key for understanding the evolution of plate tectonics on Earth. Minerals characteristic for deep subduction processes (i.e. modern-style plate tectonics) like glaucophane, coesite, and diamond are commonly replaced by their low-pressure polymorphs during exhumation. However, when entrapped as inclusions in resistant host minerals like garnet, these mineral phases are shielded from external metamorphic fluids and may be preserved. Finding evidence for deep subduction processes in host garnets of large volumes of (partially) re-equilibrated crystalline rocks is challenging, time consuming, and often hampered by poor outcrop conditions due to weathering and soil formation. In contrast, by analyzing detrital garnet, natural processes such as erosion and sedimentary transport can sample garnet grains sourced from fresh as well as altered crystalline rocks located in the drainage area, enabling large crustal volumes to be screened using a comparatively low number of samples. Case-studies from Norway (Schönig et al. 2018, Sci. Rep.), Germany (Schönig et al. 2019, Geology; Schönig et al. 2020 Gondwana Res.), Austria, Papua New Guinea (Baldwin et al. 2021, PNAS), and Greenland (Schönig et al. 2023, Eur. J. Mineral.) demonstrate mineral inclusion analysis of detrital garnet integrated with major-element chemistry (Schönig et al. 2021, Contrib. Mineral. Petrol.) to be an efficient tool for screening tectonometamorphic units on the presence or absence of rocks related to modern-style plate tectonic processes (Schönig et al. 2022, Earth-Sci. Rev.). This contribution gives a synopsis of the main findings from the five spatially, chronologically, and tectonically distinct localities.

How to cite: Schönig, J.: Detrital Garnet Petrology: Inclusions as a main source of information, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14917, https://doi.org/10.5194/egusphere-egu24-14917, 2024.

11:50–12:00
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EGU24-15529
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Highlight
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On-site presentation
Bernardo Cesare, Enrico Mugnaioli, Tommaso Tacchetto, Sofia Lorenzon, Cristian Biagioni, Fabrizio Nestola, Nicola Campomenosi, and Giancarlo Della Ventura

Since the first report on the tetragonal structure of common (OH- and andradite-free) garnet in metamorphic rocks (Cesare et al., 2019) , many new occurrences have been discovered.

Non-cubic anhydrous garnets are widespread in low-temperature metamorphic terranes worldwide, including the world-renowned blueschists from the Franciscan Complex (USA), Syros (Greece) and Aosta Valley (Italy), and the phyllites from the iconic Barrow garnet zone of Scotland.

These garnets share compositional features such as grossular >20% and pyrope <7%, which are common in both metabasites and metapelites metamorphosed at T < ~500 °C.

The non-isotropic nature of these garnets determines a very weak birefringence, which is generally overlooked, yet it can be easily detected by polychromatic polarization microscopy on conventional 30-µm thin sections. Similarly, this deviation from an isotropic behaviour can be observed by Raman microspectroscopy using crossed-polarized geometry.

Optically, the birefringence pattern suggests the presence of twelve growth sectors (twins) arranged in a dodecahedron, displaying sector quartets with three crystallographic orientations having optical axes swapped by 90°.

Preliminary single-crystal XRD and HRTEM investigations have demonstrated that these anisotropic garnets have a pseudocubic tetragonal structure with a minimal (~ 1/1000) difference between the a,b and c axes.

Due to the minimal departure from the cubic cell, conventional EBSD orientation analysis could notdiscriminate the orthogonal orientation of axes in the optically different sectors. However, EBSD revealed consistent, small (<1°) structural disorientations coincident with the rhythmic Ca-Fe compositional variations, similar to what is observed in zircon growth zoning.

This presentation will report the new results obtained from FPA-FTIR, HRTEM, single-crystal XRD and synchrotron micro-XAS investigations, aiming to better constrain the crystallographic structure of tetragonal garnets as well as their OH- content, thereby providing a better understanding of the origin of their departure from a cubic structure.

 

References

Cesare, B., Nestola, F., Johnson, T. et al. Garnet, the archetypal cubic mineral, grows tetragonal. Sci Rep 9, 14672 (2019). https://doi.org/10.1038/s41598-019-51214-9

How to cite: Cesare, B., Mugnaioli, E., Tacchetto, T., Lorenzon, S., Biagioni, C., Nestola, F., Campomenosi, N., and Della Ventura, G.: Common, non-cubic garnet in metamorphic rocks: the state of the art, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15529, https://doi.org/10.5194/egusphere-egu24-15529, 2024.

12:00–12:10
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EGU24-9809
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ECS
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Highlight
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On-site presentation
Alfredo Idini, Celestino Angeli, Franco Frau, and Roberto Argazzi

Tsavorite is the trade name for the green vanadium-chromium variety of grossular. The occurrence of tsavorite plays a key role in the geological research inherent to the formation of the Gondwana continent because it is hosted exclusively in the metamorphic unit from the Neoproterozoic Metamorphic Mozambique Belt (NMMB). The studies on the tsavorite deposit of the NMMB contributed to determining the metamorphic evolution of these Precambrian terrains. The areas of Merelani Hills (Tanzania) and Tsavo Park (Kenya) are by far the most important source of high-quality specimens of tsavorite that are extensively used for petrological studies. Furthermore, the tsavorite crystals from Merelani Hills exhibit a peculiar feature: the fluorescence is easily recognizable lighting up centimetric crystals with a common portable LED lamp. Using the long UV we observe a bright pink-orange colour, while exciting with the short UV the effect is a pale yellow. To the best of our knowledge, this phenomenon is unusual among the members of the garnet group and only a few research papers have investigated this phenomenon in natural garnets. To characterize the fluorescence, up to 25 grams of tsavorite crystals were meticulously sampled from the rocky matrix under the UV lamp and then pulverized in an agate mortar to perform an accurate XRPD acquisition. The results show that no other phases than garnet are present and, thanks to the high quality of the XRPD acquisition, the Electron Density Map was calculated and plotted against a CIF grossular standard, showing that an excess of negative charge is clearly pinpointed in the Y(6) crystallographic site, occupied by Al+3 in the grossular standard structure. The bulk elemental analysis, performed on the same powder used for XRPD acquisition, shows that the contents of Fe, Mg and Mn are < 0.5 wt.%, while V2O3  and Cr2O3 are respectively 0.32  and 0.015 wt.%, showing a good consistency with bibliographic data. The fluorometry with an excitation beam at 408 nm indicates a complex emission pattern with the most intense emission at 701 and 716 nm and subordinately at 592 nm. The colour perception of the emitted light is dominated by the emission band at 592 nm which is close to the peak sensitivity of the human eye at 555 nm, while the contribution of the red band, though more intense, is perceived as much weaker due to the lower eye sensitivity and modulates the colour ranging from bright orange to pink-red. Because of the characteristic colour perceived under UV light, the use of a common led lamp can serve as a diagnostic tool to identify tsavorite whenever a rapid test is required, e.g. in the case of field survey. The emitted photoluminescence lines, besides the very uncommon low andradite molarity, allow precise identification of the emissions of Mn2+, Cr3+ and V3+.

 

 

How to cite: Idini, A., Angeli, C., Frau, F., and Argazzi, R.: Role of manganese, chromium and vanadium in the photoluminescence emission spectrum of grossular var. tsavorite from Tanzania., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9809, https://doi.org/10.5194/egusphere-egu24-9809, 2024.

12:10–12:20
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EGU24-18033
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ECS
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On-site presentation
Joëlle D'Andres, John Mavrogenes, Elena Belousova, and Fabrizio Magrini

This study addresses a significant gap in the geochemical literature by systematically investigating garnet compositions within and around base metal deposits -- a topic that remains relatively understudied, despite the mineral's known significance in various geological contexts. Garnet, traditionally associated with diamond exploration and recognized for its potential as an indicator mineral for deposits like skarns,emerges in our research as a key geochemical tracer in the context of base metal exploration.

Focusing on the Eastern Succession of the Mount Isa Inlier in Australia, a globally significant mineral province, we characterized the major and trace element compositions of garnets associated with diverse base metal systems. Our new dataset, containing more than 2500 datapoints, was complemented by literature data compiled from a range of mineral systems including skarns, IOCGs, subaqueous volcanic-related (VMS, Broken Hill, sedimentary-exhalative), porphyry-epithermal and granite-related pegmatite systems. Comparative analyses with background metamorphic/igneous garnets reveal distinct trends in Mn-Fe and Ca-Mg space in ore-associated garnets, along with unique trace element patterns (e.g., Eu anomalies) and chalcophile element variations (e.g., Zn, Cd, Ga) in diverse ore systems.

To clarify the importance of these anomalous compositions, we refined our dataset by considering specific garnet associations and their related geological context. Garnets in the database were categorized depending on their occurrence within the ore bodies, the alteration haloes, or the background lithologies. Additionally, we carried out a multivariate statistical analysis through a principal component analysis (PCA) and subsequent cluster analysis, to identify hidden spatial patterns. Our results shed light on the geochemical variations in garnet composition across various ore systems and provide new insights into the sources of these variations.

This research not only contributes valuable geochemical data for base metal exploration but also establishes the efficacy of multivariate statistical analyses in deciphering complex garnet data structures. The implications of our findings extend beyond the Eastern Succession of the Mount Isa Inlier, fostering a broader understanding of the geochemical dynamics associated with base metal deposits globally.

How to cite: D'Andres, J., Mavrogenes, J., Belousova, E., and Magrini, F.: Exploring Garnet’s Geochemical Variations: Insights into Base Metal Deposits , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18033, https://doi.org/10.5194/egusphere-egu24-18033, 2024.

12:20–12:30
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EGU24-9255
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ECS
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On-site presentation
Nicola Campomenosi, Ross John Angel, Matteo Alvaro, and Boriana Mihailova

Mineral inclusions represent a real treasure in earth sciences because of their large range of applicability to unravel metamorphic and geodynamic processes. For instance, the contrast in the thermal expansion and compressibility coefficients between a mineral inclusion and its surrounding host leads to a residual pressure in the inclusion (Pinc) that, once determined at ambient conditions, can be used to back-calculate the pressure and temperature of entrapment (e.g. Rosenfeld and Chase, 1961; Angel et al. 2015). The Pinc can be quantified via in situ Raman spectroscopy, using ab initio calibrations (e.g. Murri et al. 2018). In addition, Raman spectroscopy can be used to explore the evolution of residual pressure in mineral inclusions under non-ambient conditions. We have analysed quartz-in-garnet (QuiG) host-inclusion systems in situ under high external pressure applied with a diamond-anvil cell at room temperature. The evolution of quartz Pinc calculated from the Raman data collected on fully encapsulated inclusions at high pressure agrees with the predictions calculated from the equations of state and confirm that the garnet host acts as a pressure shield to the softer quartz inclusion. The pressure-dependent sharpening of the A1 mode near 207 cm-1 indicates that quartz inclusions become metastable against coesite at values of Pinc of ~ 2.4 GPa, which corresponds to the pressure of the quartz-coesite phase boundary at room temperature of free crystals (Bose and Ganguly 1995). However, the external applied pressure may exceed the Pinc of more than 2 GPa. Finally, we show that “partially” encapsulated inclusions undergo significant non-hydrostatic stress with evident symmetry-breaking due to heterogeneous host-shielding effects. At room temperature, such deviatoric stresses do not affect the metastability of quartz inclusions with respect to coesite.

Financial support by the Alexander von Humboldt Foundation to N. Campomenosi., and by the European Research Council (ERC) grant agreements 714936 to M. Alvaro

Angel, R. J., et al. (2015).  Journal of Metamorphic Geology33(8), 801-813.

Bose, K., & Ganguly, J. (1995). American Mineralogist80(3-4), 231-238.

Kaminsky, F. (2012). Earth-Science Reviews110(1-4), 127-147.

Murri, M., et al. (2018). American Mineralogist103(11), 1869-1872.

Rosenfeld, J. L., & Chase, A. B. (1961). American Journal of Science259(7), 519-541.

How to cite: Campomenosi, N., Angel, R. J., Alvaro, M., and Mihailova, B.: Mineral inclusions under non-ambient conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9255, https://doi.org/10.5194/egusphere-egu24-9255, 2024.

Posters on site: Fri, 19 Apr, 16:15–18:00 | Hall X1

Display time: Fri, 19 Apr 14:00–Fri, 19 Apr 18:00
Chairpersons: Silvio Ferrero, Iwona Klonowska, Mattia Gilio
X1.90
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EGU24-449
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Federica Boero, Stefano Ghignone, Marco Bruno, Mattia Gilio, Alessia Borghini, and Emanuele Scaramuzzo

The occurrence of Ultrahigh Pressure (UHP) index minerals (i.e., coesite, microdiamond, overall) in tectono-metamorphic belts is of paramount importance to attest the depths attained during subduction of the lithosphere. Recently, many studies focuses on the importance of the inclusions stored in host garnet that preserve information on the trapping conditions (i.e., elastic properties). Detailed inclusions study allows to recognize UHP mineral phases that escaped re-equilibration. Locally, these minerals are no more present in rock matrix, making inclusions the unique evidence of early rock evolution. In this work, we perform a detailed inclusion study through a multi-technique approach to obtain new constraints on the peak metamorphic conditions of a deeply subducted oceanic slab (> 100km).

We focused on two tectono-metamorphic units outcropping in the Western Alps, characterized by a metamorphic peak in UHP conditions. These units belong to the meta-ophiolites of the Internal Piedmont Zone (IPZ) and are located in (i) the Susa Valley (Ghignone et al., 2021), and (ii) Pellice Valley (Colle del Baracun; Ghignone et al., 2023). The units consist of oceanic lithosphere (serpentinite, metagabbro) and metasedimentary cover (micaschist, calcschist). For each unit we selected three samples of Grt-bearing metasediments, making a detailed characterization of the garnet inclusions with optical microscope and micro-Raman spectroscopy. Measured Raman spectra allowed to identify coesite in the Susa Valley (previously unrecognized). We applied Raman-based elastic geothermobarometry (Angel et al., 2015) on quartz and zircon inclusions in garnet as well as Zr-in-rutile thermometry to define the metamorphic path of the units. Furthermore, we combined the distribution of inclusions in zoned garnet with multispectral compositional maps (SEM-EDS), for obtaining a detailed mineral assemblage related to each garnet growth stage.

Our results highlight substantial differences of inclusion mineralogy and their microstructural position within the garnet shells between the two studied units. The IPZ in the Susa Valley recorded (i) the peak-PT under UHP conditions, (ii) a second event under HP eclogitic conditions, and (iii) a late re-equilibration event under LP conditions. Instead, the IPZ in the Pellice Valley recorded i) an early event corresponding to the prograde stage, ii) a peak pressure in UHP conditions, iii) a late re-equilibration event under LP conditions. Despite these differences, the two studied meta-ophiolitic units show similar metamorphic evolutions down to UHP conditions. This evolution follows a similar gradient to that of the coesite-bearing Lago di Cignana unit (e.g., Groppo et al., 2009). This may point out that in the Western Alps three major ophiolite slices underwent UHP metamorphism, thus suggesting that a large volume of oceanic lithosphere was subducted at ca. 100 km depth.

Angel, R., J., Nimis, P., Mazzucchelli, M., L., Alvaro, M., Nestola, F. (2015). J. Metamorph. Geol. 33 (8), 801-813. [10.1111/jmg.12138]

Ghignone, S., Borghi, A., Balestro, G., Castelli, D., Gattiglio, M., Groppo, C. (2021). J. Metamorph. Geol. 39 (4), 391–416. [10.1111/jmg.12574]

Ghignone, S., Scaramuzzo, E., Bruno, M., Franz, A. L. (2023). Am. Mineral. 108, 1368–1375. [10.2138/am-2022-8621]

Groppo, C., Beltrando, M., Compagnoni, R. (2009). J. Metamorph. Geol., 27(3), 207-231. [10.1111/j.1525-1314.2009.00814.x]

How to cite: Boero, F., Ghignone, S., Bruno, M., Gilio, M., Borghini, A., and Scaramuzzo, E.: Detailed study of garnet inclusions from the UHP meta-ophiolites of the Western Alps: new petrological constraints on the deep history of a subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-449, https://doi.org/10.5194/egusphere-egu24-449, 2024.

X1.91
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EGU24-3436
Dražen Balen and Hans-Joachim Massonne

The area of Mt. Papuk in Croatia, which is protected as a UNESCO Global Geopark due to its exceptional geological diversity, is known for polycyclic metamorphic rocks. The study of metamorphic and igneous rocks demonstrate the occurrence of pre-Variscan, Variscan and Alpine orogenic events. Within the metamorphic lithologies (but also igneous ones, e.g., S-type granite), garnet is a key mineral to decipher details of the geological evolution even if it represents a rock volume of up to 2-3% only. The Variscan orogeny has been recognized here as the most imprinting one producing garnet-bearing medium- to high-grade rocks accompanied by partial melting. This is the only case in the Mt. Papuk area where garnet in metamorphic complex coexisted with melt, i.e. reached high temperatures.

The rock samples (mica schist, gneiss, migmatite) show a schistose fabric and a well-preserved medium to coarse-grained granolepidoblastic texture. Some of the rocks reached high-grade conditions discernible by partial melting. Schistosity is defined by the preferential orientation of elongated feldspar grains, micaceous (biotite and minor muscovite) domains and quartz bands. Feldspar (~40-50 vol%; oligoclase up to 28 mol% An) is the predominant phase, followed by biotite (20-30 vol%), quartz (20 vol%), white mica, reddish garnet and sillimanite (fibrolite). Zircon, apatite, monazite, ilmenite, rutile and titanite are accessory minerals. Monazite grains with high Ce2O3 content (approx. 28 wt%) were dated with the electron microprobe and yielded an age peak at 384.5±9.0 Ma (1σ).

Garnet occurs in almost all quartz- and mica-rich lithologies of the upper amphibolite facies and exhibits two size populations: 1) small, up to 100 μm large and chemically homogeneous (Alm=64, Prp=9, Grs=2 and Sps=25 mol%) garnets that are generally devoid of inclusions except some containing quartz, and 2) 1-2 mm large garnets with a slightly different chemical composition (Alm=66, Prp=12, Grs=2 and Sps=20 mol%) in inner zones containing numerous apatite, rutile, biotite, plagioclase and quartz inclusions. The thin rim of the large garnets corresponds chemically to the composition of the small garnet population.

Using Ti-in-biotite (average 645°C) and garnet-biotite thermometry (590-650°C at 500 MPa) and pseudosection modelling, a clockwise P-T path was reconstructed with maximum pressure conditions within the rutile P-T stability field where the garnet core grew at 800-900 MPa and a temperature of 590-600°C. These conditions were followed by a significant pressure drop, accompanied by melting, partial resorption of garnet and the formation of a thin garnet rim, to 450 MPa (~15 km depth) at temperatures of 680°C.

The geothermobarometric data, additionally constrained by microtextural evidences and mineral stability fields, suggest a convergence-related model reflecting burial, heating and finally rapid (tectonic?) exhumation of the moderately thickened crust. This model suggests a complex tectonometamorphic evolution of the crystalline terrain filling a gap in the Variscan belt between Central and Eastern Europe.

How to cite: Balen, D. and Massonne, H.-J.: Geochemistry and microtextural characteristics of garnet from the Variscan medium- to high-grade metamorphic complex of Mt. Papuk (Croatia), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3436, https://doi.org/10.5194/egusphere-egu24-3436, 2024.

X1.92
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EGU24-4106
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ECS
M. Thereza A. G. Yogi, Fred Gaidies, Olivier K. A. Heldwein, and A. Hugh N. Rice

The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi-equilibration of their major components across the rock matrices during interface-controlled, size-independent garnet growth. There is microstructural evidence for foliation-parallel, small-scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of c. 630˚C, which is c. 40˚C higher than the temperature obtained from resorbed rims of the largest garnet crystal. The use of a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the entire garnet population, and the higher, more realistic peak temperature, results in an estimated duration of 1 to 4.9 Myr for garnet growth in the Kolvik Nappe. This is approximately one order of magnitude faster than duration estimates calculated when using the apparent, lower temperature obtained from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for c. 2.5 Myr at metamorphic peak temperatures of c. 560˚C. The garnet growth durations obtained in this work are comparable with the uncertainty on the Lu–Hf garnet–whole-rock isochron ages published previously for the studied rocks (Gaidies et al., 2022). The results provide new insight into the timescales of repeated Barrovian-type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the final stages of the Caledonian Orogeny in Arctic Norway.

Reference: Gaidies, F., Heldwein, O. K. A., Yogi, M. T. A. G., Cutts, J. A., Smit, M. A., Rice, A. H. N. (2022). Testing the equilibrium model: An example from the Caledonian Kalak Nappe Complex (Finnmark, Arctic Norway). Journal of Metamorphic Geology, 40(5):859–886.

How to cite: Yogi, M. T. A. G., Gaidies, F., Heldwein, O. K. A., and Rice, A. H. N.: Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Kalak Nappe Complex (Arctic Norway), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4106, https://doi.org/10.5194/egusphere-egu24-4106, 2024.

X1.93
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EGU24-6669
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ECS
Olga Turek, Karolina Kośmińska, Jarosław Majka, Mattia Gilio, Iwona Klonowska, Alessia Borghini, Adam Włodek, Daniel Buczko, and Simon Cuthbert

The Western Gneiss Region (WGR) is a well-known UHP metamorphic terrane in SW Norway, where eclogite bodies crop out among gneisses. These rocks experienced high- and ultrahigh-pressure (HP-UHP) metamorphism during the Caledonian Orogeny and display pressure-temperature (P-T) gradient increasing from the south to the north (e.g., Cuthbert et al., 2000). Eclogites from nine localities along the entire length of WGR, namely Drøsdal, Vårdalsneset, Verpeneset, Halnes, Saltaneset, Grytting, Ulsteinvik, Solholm, and Juvika, have been chosen for P-T condition estimates. Here we present a reevaluation based on the new geothermobarometric techniques in the WGR regional study.

Eclogite varies in the amount of amphibole, kyanite, phengite, zoisite, and inclusions of quartz and rutile in garnet. Phengite occurs mainly in the eclogites in the south, whereas those from the north contain orthopyroxene. Garnet in the south is almandine-rich with the average composition of Alm0.41–0.57Grs0.15–0.31Prp0.10–0.37Sps0.01–0.07, with increasing Mg in the rims and often showing complex compositional zoning. Garnet from the other localities is homogenized and dominated by pyrope with the composition of Prp0.41–0.56Alm0.33–0.46Grs0.09–0.13Sps0.01–0.04. Clinopyroxene in the northern part is poorer in jadeite component with XNa=0.20-0.29 and XFe=0.12-0.17 than clinopyroxene in the south with XNa=0.40-0.52 and XFe=0.12-0.19. Silicon in phengite is up to 3.30 atoms per formula unit (apfu) in most localities, with a maximum of 3.48 apfu in the Verpeneset locality.

Coesite and polycrystalline quartz inclusions in garnet, evidence of UHP metamorphism, are typical of Saltaneset and Verpeneset localities, which is also validated by thermobarometric calculations. We estimated the peak P-T equilibration condition by Grt-Cpx-Phe thermobarometry, Ti-in-Quartz and Zr-in-Rutile thermometry, along with Quartz-in-Garnet elastic geobarometry. Preliminary estimates give slightly higher P values than previously reported in the two southernmost localities Drøsdal and Vårdalsneset [720-830°C and 1.9-2.1 GPa (Foreman et al., 2005), 635°C and 2.3 GPa (Engvik et al., 2007), accordingly] yielding the peak conditions of 680-740°C and 2.7-2.85 GPa. The obtained results from other localities give P conditions close to the boundary of quartz and coesite stability fields and T in the range of 650-750°C, with the highest P of 3.28 GPa in the Verpeneset locality. The highest T of 875°C has been obtained in Ulsteinvik locality. Those results may help refine previous studies and understand the history of the WGR.

This work was funded by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

 

References:

Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., Wain, A. (2000). Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides. Lithos, 52 (1–4), 165-195.

Foreman, R., Andersen, T.B., Wheeler, J. (2005). Eclogite-facies polyphase deformation of the Drøsdal eclogite, Western Gneiss Complex, Norway, and implications for exhumation. Tectonophysics, 398, 1-32.

Engvik, A.K., Andersen, T.B., Wachmann, M. (2007). Inhomogeneous deformation in deeply buried continental crust, an example from the eclogite facies province of the Western Gneiss Region, Norway. Norwegian Journal of Geology, 87, 373-389.

How to cite: Turek, O., Kośmińska, K., Majka, J., Gilio, M., Klonowska, I., Borghini, A., Włodek, A., Buczko, D., and Cuthbert, S.: P-T conditions of high- and ultrahigh-pressure metamorphism along the Western Gneiss Region, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6669, https://doi.org/10.5194/egusphere-egu24-6669, 2024.

X1.94
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EGU24-8683
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ECS
Caroline Lotout, Aphrodite Indares, Jeffrey Vervoort, and Etienne Deloule

In the Grenville Province, high-P rocks that are discontinuously exposed along the margin of the allochthonous belt preserve the record of deep crustal processes and are an essential puzzle piece to understanding Mesoproterozoic geodynamics. The Manicouagan Imbricate zone (MIZ) is one of the three high-P domains from the Grenville Province. It is located in the Central Grenville between the Parautochthonous Belt to the North and the orogenic hinterland to the South, that were metamorphosed during the 1005–980 Ma Rigolet and 1080–1020 Ma Ottawan orogenic phases, respectively. In the western MIZ, the Lelukuau Terrane (LT) mostly consists of Labradorian-age (~1650 Ma) mafic suites with a fringe of aluminous rocks at its southern edge. Metamafic samples from the Western and Eastern parts of the MIZ display a peak assemblage of garnet, clinopyroxene, plagioclase, rare pargasite or edenite, and quartz ± kyanite. Pseudosection modelling suggests high-P granulite peak conditions at ca. 14 to 16 kbar and 800–900°C, with the scarcity of hydrous phases and quartz explaining the lack of evidence for partial melting. Zircon cores from the Western LT sample show a maximum magmatic age of ca 1.6 Ga. Lu–Hf and Sm–Nd dating on garnet from this sample yield ages of 1020 ± 7 Ma and 1005 ± 13 Ma, respectively, overlapping within error and inferred to represent peak metamorphic conditions followed by fast cooling. In the Eastern LT sample, garnet Lu–Hf dating yields two ages that are consistent with a petrographically preserved two stage growth, at 1033 ± 6 Ma and 1013 ± 6 Ma, while the Sm–Nd age indicates cooling at 1003 ± 8 Ma. The recorded high-P granulite facies conditions highlight a late Ottawan to Rigolet-age localized crustal thickening at the margin of the hinterland during the propagation of the orogen to the NW, with a possible younging of the high-P granulite-facies metamorphism from the Eastern to Western LT. These new results indicate that the high-P belt in the Central Grenville does not represent the exhumed base of an Ottawan age orogenic plateau, as previously proposed, and that no tectonic hiatus exists between the two orogenic phases, as generally thought. Finally, this publication highlights the diversity and diachronicity of the high-P domains in the Grenville Province.

How to cite: Lotout, C., Indares, A., Vervoort, J., and Deloule, E.: High-P metamorphism in the Mesoproterozoic: Petrochronological insights from the Grenville Province, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8683, https://doi.org/10.5194/egusphere-egu24-8683, 2024.

X1.95
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EGU24-9789
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ECS
Alexandre Peillod, Benjamin Hess, and Uwe Ring

Understanding the mechanisms of mountain building and its destruction poses significant challenges. Following subduction of a slab, an increase in the subduction rate and/or a decrease of the convergence rate, may lead to slab rollback. During slab rollback, segments of the middle-lower crust may undergo heating and partial melting due heating of thinned lithosphere by the asthenosphere and the migration of the magmatic arc. Alternatively, after accretion of radiogenic crust, the lower parts of the crust may relax through radiogenic decay. In both scenarios, the heating of the middle-lower crust reduces crustal strength, resulting in the development of metamorphic core complexes. The timing of lower crust heating is crucial for understanding the switchover from lithospheric shortening to extension.

The Hellenides in the eastern Mediterranean constitute an arcuate orogen located north of the present-day active Hellenic margin, marking the site of NNE-ward subduction of the African plate beneath Eurasia. The Aegean Sea region in the Hellenide is a world-class example of large-scale continental extension above a retreating subduction zone. In the Cyclades, the Hellenide orogeny began in the early Cenozoic, causing subduction and sustained high-pressure (HP) metamorphism between approximately 53 and 30 Ma. The timing of slab rollback is subject of intense debate. A decrease in the convergence rate was interpreted to suggest that slab rollback initiated at around 35–30 Ma, during or even after the waning stages of HP metamorphism. In contrast, the formation of extensional sedimentary basins and radiometric dating of extension-related mylonite place rollback at approximately 23 Ma, distinctly after the final stages of HP metamorphism.

Between about 30 Ma and 20 Ma, isobaric heating during the exhumation of some HP rocks has been proposed on some islands (Syros, Tinos, Andros, and Naxos). However, the precise timing and duration of this isobaric heating remains largely unconstrained. On Naxos, previous geothermobarometry estimates indicate isobaric heating occurred from 500 to 550°C from a middle segment of the nappe stack (Peillod et al., 2021). Garnet chemical zonation formed during exhumation allows for a heating timescale to be estimated via diffusion chronometry. We conducted Monte Carlo diffusion simulations to determine the best-fitting timescale based on Chi-square statistics, assuming three different heating paths. Diffusion model results for a heating path from 500 to 550°C indicate a 10 Myr timescale. A lower temperature path (from 400 to 450°C) requires heating over an unreasonable geological timescale (>100 Myr). Conversely, a higher temperature path (from 600 to 650°C) requires a timescale of <1 Myr. This higher temperature path corresponds to temperatures recorded near the migmatite core at the bottom of the Naxos nappe stack. At such temperatures and within a 10 Myr period, chemical heterogeneities in garnet would have relaxed, as observed in most garnets near the migmatite core. The results of this approach indicate that the heating of the lower crust occurs over a 10 Myr period, suggesting that the heating may result from radiogenic decay during the Oligocene before the slab roll during the Miocene.

How to cite: Peillod, A., Hess, B., and Ring, U.: Slab reequilibration and slab roll back timing in the Cyclades: Evidence from garnet diffusion and P-T estimates (Naxos, Greece), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9789, https://doi.org/10.5194/egusphere-egu24-9789, 2024.

X1.96
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EGU24-9913
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ECS
Iris Wannhoff, Jan Pleuger, Xin Zhong, Timm John, Leo J. Millonig, Axel Gerdes, and Richard Albert

The Koralpe-Saualpe-Pohorje Complex (KSPC) in the Eastern Alps stretches from SE Austria to NE Slovenia and hosts the type locality for eclogite. Although the KSPC has been studied for decades, some aspects of its tectonometamorphic evolution are still controversial. There is, for example, an ongoing discussion, if the Pohorje unit experienced ultra-high-pressure (UHP) conditions during the Eoalpine orogeny. The KSPC is part of the Austroalpine basement units and was interpreted to represent a coherent (U?)HP nappe consisting mainly of gneisses and metasedimentary rocks, with abundant eclogite lenses embedded. Some of the Austroalpine basement units, including the KSPC, experienced a long-lived Permian-Triassic tectonometamorphic event, where gabbros intruded into a thinned crust, which experienced eclogite facies conditions during the Late Cretaceous. A metamorphic field gradient with an increase in peak pressure-temperature (PT) conditions from NW to SE, and UHP conditions for Pohorje, has previously been proposed based on thermodynamic modelling, geothermobarometry and the discovery of diamond in fluid inclusions in garnet. To unravel the metamorphic evolution of the KSPC, we applied quartz-in-garnet elastic barometry, Zr-in-rutile thermometery and in-situ U-Pb dating of garnet and rutile from eclogite and metasedimentary rock samples along a NW-SE transect. This is the first application of quartz in garnet elastic barometry within the KSPC in order to determine the entrapment pressures of the quartz inclusions. The eclogite samples yielded maximum pressures of 1.9 GPa across the KSPC, indicating no pressure increase from the NW to SE. The metasedimentary rocks show overall lower pressures with a maximum of ca. 1.4 GPa. Zr-in-rutile thermometry yielded uniform temperatures of 640 (±30)°C, indicating no temperature gradient. The novel approach of in-situ garnet U-Pb dating was conducted to decipher potentially different metamorphic events. Garnet from the Koralpe, Saualpe and Pohorje metasedimentary rocks yielded Early Cretaceous dates ranging from ~95 to 105 Ma, similar to eclogitic garnet from the Koralpe with ~112 Ma. Additionally, garnet from a Saualpe micaschist yielded Late Triassic cores (~224 Ma) and Early Cretaceous rims (~115 Ma). Rutile throughout the KSPC yielded Late Cretaceous U-Pb dates of ~98–83 Ma (eclogites) and ~87–80 Ma (metasedimentary rocks).The results of this study suggest that the KSPC represents a coherent nappe. The recorded maximum pressures and temperatures are identical throughout the KSPC. The lower pressure for the metasedimentary rocks is interpreted to be the result of viscous relaxation in garnet due to the presence of fluids during metamorphism. The obtained garnet U-Pb dates from both eclogites and metasedimentary rocks are interpreted as Late Cretaceous bulk crystallization ages that reflect prograde to peak metamorphic garnet growth. The Late Triassic U-Pb dates from the Saualpe garnet cores are in line with the existing literature proposing a polymetamorphic cycle for the KSPC. The rutile dates are interpreted as cooling ages. The lack of a metamorphic field gradient may imply a different tectonical setting for the KSPC than previously proposed, and warrants further investigation.

How to cite: Wannhoff, I., Pleuger, J., Zhong, X., John, T., Millonig, L. J., Gerdes, A., and Albert, R.: New petrological and geochronological results from the Koralpe-Saualpe-Pohorje Complex (Eastern Alps), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9913, https://doi.org/10.5194/egusphere-egu24-9913, 2024.

X1.97
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EGU24-11318
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ECS
Renelle Dubosq, Alfredo Camacho, Anna Rogowitz, David Schneider, and Baptiste Gault

Garnet is a common high-pressure mineral in the Earth’s lithosphere. Considered a high-strength mineral stable across a wide range of pressure and temperature conditions, it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, the trace and major element compositions of garnet are commonly used for geothermobarometers and geochronometers to provide the conditions and timing of metamorphic events. We combine electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and atom probe tomography (APT) on garnet from an eclogite facies mylonite in the Musgrave Province (central Australia) to investigate the mechanisms of element mobility during high-strain deformation under dry (<0.002 wt% H2O), lower crustal conditions. Previous investigations suggest that mylonitization occurred at 1.2 GPa and 650°C. Herein, we focus on two garnet clasts that are located within one common high-strain zone. EBSD and ECCI data from garnet clast 1 reveal a micro-shear zone characterized by recrystallized strain-free garnet crystals. APT analysis of one of the recrystallized high-angle grain boundaries shows Fe enrichment in the form of  equally spaced (4.5–6.5 nm), planar arrays of Fe-rich nanoclusters (3.0–7.5 nm). The combined data suggests that these nanoclusters formed as a result of enhanced Fe diffusion along high-angle grain boundaries of recrystallized garnet during high-strain deformation. Garnet clast 2 evinces crystal-plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low-angle grain boundary development at the rim of the garnet porphyroclast. APT analysis of a low-angle grain boundary within the highly-strained clast shows Ca enrichment and Mg depletion along dislocations, suggesting crystal-plasticity enhances element mobility via 'pipe' diffusion, with dislocations acting as high-diffusivity pathways. Our data reveal the interaction of chemical and mechanical processes at the nanoscale through deformation-induced enhanced element diffusivity. Consequently, caution should be exercised when using deformed garnets as petrological tools because of this enhanced major element mobility during high-strain deformation. To evaluate this hypothesis, we modelled the Ca diffusion profiles across undeformed and deformed garnet rims within clast 2. Simulations based on the measured Ca concentration profiles of the undeformed rims yield time estimates of 0.8 to 1 Ma for the duration of high-strain deformation at eclogite facies along the shear zone, whereas simulations across the deformed rims yield longer time estimates of 5 to 22 Ma. Our study thus highlights the importance of conducting a thorough microstructural and geochemical analysis on garnet prior to utilizing the mineral for petrological applications.

How to cite: Dubosq, R., Camacho, A., Rogowitz, A., Schneider, D., and Gault, B.: Nanostructural and chemical evolution of garnet during high-strain deformation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11318, https://doi.org/10.5194/egusphere-egu24-11318, 2024.

X1.98
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EGU24-21602
Dirk Spengler

This study analyses the H2O content in nominally anhydrous minerals (NAMs) in 10 eclogites from W Norway. Each sample has oriented lamellar to acicular inclusions in clinopyroxene, which are either quartz with pargasite or quartz/albite without pargasite [1]. Low-Al orthopyroxene and polycrystalline quartz inclusions in several of these samples provide evidence for UHP metamorphism in the stability fields of diamond and coesite. The H2O content is quantified using Fourier transform infrared spectroscopy (FTIR), unpolarised infrared radiation, spectra deconvolution, and the calibration of [2].

Preliminary data was obtained from the analysis of the first 5 eclogites yield for garnet 22-379 and 16-31 µg g-1 structural H2O for samples with and without pargasite lamellae, respectively. The highest value occurs in a zoisite-bearing eclogite. If regarded separately, then the variation in the 4 zoisite-free eclogites shrinks to 16-32 µg g-1 H2O. Absorption bands characteristic for molecular H2O in garnet (centered at wavenumbers <3460 cm-1) were not observed. The ranges for structural H2O in clinopyroxene from these 5 samples are 125-380 and 183-564 µg g-1, respectively. The highest value occurs in a sample with intense recrystallization of clinopyroxene (but not garnet) after peak metamorphism. If regarded separately, then the total range is 125-380 µg g-1 H2O. The obtained clinopyroxene–garnet H2O partition coefficient has ranges of 1.0-11.0 and 11.6-18.2, respectively. The extreme values belong to the zoisite-bearing (1.0) and the strongly recrystallized (18.2) samples. If regarded separately, then the total range is reduced to 3.9-11.6.

 

Combining the preliminary data of the quantified structural water with petrological information tends to suggest the following relationships. (1) The current H2O content in NAMs is affected by the presence of hydrous minerals during peak metamorphism and the retrogression history. (2) The peak UHP garnet is water-deficient unless zoisite forms part of the mineral assemblage. (3) The current H2O content of clinopyroxene (containing oriented inclusions of quartz with and without pargasite) from "diamond-facies" UHP eclogite is lower compared to that of "graphite-facies" UHP eclogite from a similar tectonic setting that lacks such inclusion microstructures [3]. (4) Samples with oriented inclusions of pargasite in clinopyroxene tend to have lower clinopyroxene–garnet H2O partition coefficients than those without pargasite, which suggests that pargasite lamellae formed by clinopyroxene dehydration during early decompression. Additional data will be presented to test these preliminary indications.

This study received funding from Norway Grants 2014–2021 operated by the National Science Centre (Poland) under project contract no. 2020/37/K/ST10/02784.

[1] Spengler et al., 2023, Eur. J. Mineral. 35:1125-1147

[2] Bell et al., 1995, Am. Mineral. 80:465-474

[3] Gose & Schmädicke, 2022, J. Metamorph. Geol. 40:665-686

How to cite: Spengler, D.: Water in eclogitic garnet and clinopyroxene with oriented quartz and pargasite inclusions, W Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21602, https://doi.org/10.5194/egusphere-egu24-21602, 2024.

X1.99
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EGU24-17060
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ECS
Rene Asenbaum, Rainer Abart, and Martin Racek

Mafic–ultramafic lenses embedded in felsic granulites of the Gföhl Unit, Moldanubian Zone, are considered as mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m-sized mafic lens mainly formed by garnet-pyroxenite. Several samples were collected from loose boulders. Petrographic features provide evidence for an early HP-HT eclogite-facies peak metamorphism overprinted to variable degrees by HT granulite-facies metamorphism at lower pressures.

The primary eclogite facies mineral assemblage comprises garnet, sodium-rich clinopyroxene, kyanite, rutile and quartz. The rocks are characterized by

compositional layering on the mm-scale, which is reflected by corresponding systematic variation of the compositions of garnet porphyroblasts. The garnets show homogeneous compositions in their internal domains defining plateaus, the compositional characteristics of which correlate with the compositional layering of the rocks and vary from Alm19 Prp55 Grs27 to Alm15-18 Prp42-50 Grs32-43. The systematic variation of garnet compositions with the bulk rock compositional layering testifies to lack of equilibration between the compositionally distinct layers on the mm-scale during HP-HT eclogite-facies metamorphism.

The HT-granulite-facies overprint is manifested by the breakdown of the eclogite facies mineral assemblage. This is evident, for example, from the formation of sapphirine–spinel–an-rich plagioclase symplectites in garnet supposedly replacing garnet-hosted kyanite and clinopyroxene inclusion. Another peculiar feature is represented by the partial resorption of garnet by plagioclase and clinopyroxene in the form of corrosion tubes penetrating the garnet in a worm-like fashion. Finally, garnet is partially or entirely replaced by plagioclase–spinel–orthopyroxene—clinopyroxene symplectite, where Grs-rich garnets are systematically more strongly affected by this replacement than Grs-poor garnets. Quartz is consumed during the decompression reactions and can only be found as rare relic grains. When a clinopyroxene matrix surrounds relic quartz, the clinopyroxene becomes successively more Si-rich due to inverse Tschermak substitution towards the relic quartz grain.

Throughout the samples and irrespective of the layer they pertain to, the garnets show similar pronounced secondary compositional zoning in the outermost 200 µm. The zoning is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the rim. The compositional changes in garnet are gradual suggesting diffusion-mediated re-equilibration at decreasing pressures, and the composition of the garnet at the interface to the rock matrix is the same throughout the specimen indicating that the rock equilibrated on the cm scale during the HT overprint.

Pressure and temperature were estimated based on equilibrium phase diagrams. They indicate peak pressures above 1.8 GPa and temperatures of around 950 °C for the primary mineral assemblages and the different garnet cores. In accordance with peak P-T, the garnet rims indicate pressures of around 1.3 GPa at the same temperature.

Considering the regional metamorphic setting of the Moldanubian Zone, the relatively localized secondary chemical zoning of garnet at its rim indicates that the granulite-facies metamorphism was remarkably short-lived and suggests rapid transport of the mafic–ultramafic lithologies from mantle depths to the mid-crustal level. Very likely incorporation of the relatively hot mafic lens into a supposedly cooler dominantly felsic environment led to immediate cooling of the mafic lens.

How to cite: Asenbaum, R., Abart, R., and Racek, M.: Decompression of high-grade metamorphic mafic rocks with small-scale compositional layering, Gföhl Unit, Moldanubian Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17060, https://doi.org/10.5194/egusphere-egu24-17060, 2024.

X1.100
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EGU24-11813
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ECS
Alessia Borghini, Silvio Ferrero, Patrick O'Brien, Bernd Wunder, Peter Tollan, Jarosław Majka, Rico Fuchs, and Kerstin Gresky

Garnets in the eclogites of Pfaffenberg, Granulitgebirge (Bohemian Massif, Germany) contain primary granitic melt inclusions with a continental crust signature. The inclusions are up to 30 µm in diameter and polycrystalline with a main mineral assemblage dominated by phlogopite/biotite, kumdykolite, quartz/cristobalite, two unknown phases with main Raman peaks at 412 and 430 cm-1 respectively, osumilite and plagioclase. In minor amounts, the inclusions contain also white mica, K-feldspar, amphibole and kokchetavite with the local presence of a fluid phase composed of CO2, CH4 and N2.

The inclusions were successfully re-homogenized at 975ºC and 2.7 - 3 GPa and the melt is from trondhjemitc to granitic, peraluminous and hydrous (average H2O = 4.82 wt%). The melt trace elements patterns revealed similarities with melts produced by partial melting of metasediments part of the continental crust. The melt is in fact enriched in Cs, Pb, Rb, Th, U, Li and B and most likely it originated from the continental crust itself. Interestingly, in situ analyses of Cl and calculation of F partitioning between apatite and melt show that the melt is exceptionally halogens-rich with an average Cl content of 0.41 wt% and a calculated F content of 0.23 wt%.

Pfaffenberg eclogites occur as lenses in garnet peridotite and they are surrounded by continental rocks. They can be regarded as the product of crust-mantle interaction taking place during subduction at mantle depth with the agent of the interaction, i.e., the melt, now preserved as inclusions in the eclogite garnets. The melt is responsible for crustal material mobilization and transfer in the mantle and can be used to constrain and quantify the elements, especially volatiles, transported from the crust to the mantle.

This research is part of the project No. 2021/43/P/ST10/03202 co-funded by the National Science Centre of Poland and the European Union Framework Programme for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339.

How to cite: Borghini, A., Ferrero, S., O'Brien, P., Wunder, B., Tollan, P., Majka, J., Fuchs, R., and Gresky, K.: High-pressure eclogites preserve a halogen-bearing metasomatizing agent in primary melt inclusions (Bohemian Massif, Germany)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11813, https://doi.org/10.5194/egusphere-egu24-11813, 2024.

X1.101
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EGU24-8787
Silvio Ferrero, Gabriele Cruciani, Marcello Franceschelli, Dario Fancello, Leila Rezaei, and Kerstin Gresky

Garnet in high grade rocks often contains primary melt (MI) and fluid inclusions (FI), fundamental to reconstruct the suprasolidus history of the rocks in which it occurs. MI and FI-bearing garnets have been recently found at Punta de li Tulchi, on the NE coast of Sardinia, Italy. They were identified in a single, decimetre-thick leucocratic body of overall granitic composition, hosted in migmatitic orthogneiss. Whereas in the host rock garnet is generally rare or absent, in the leucocratic body it reaches up to 5 vol% and occurs as subhedral grains up to several mm in size. Garnet composition is mainly almandine-spessartine (Alm60-70, Sps20-30 Pyr~7 Grs~3).

The inclusions occur in clusters in the inner part of the garnet, i.e., they are primary in nature and therefore trapped during garnet growth. A preliminary investigation via optical microscopy and MicroRaman Spectroscopy (MRS) shows the presence of (at least) two types of inclusions. Type 1 inclusions are polycrystalline, ≤30 microns in size, and contain cristobalite +white mica ±carbonate, an assemblage overall consistent with their interpretation as nanogranitoids, or crystallized melt inclusions of anatectic origin. Type 2 inclusions contain two phases, i.e., liquid and vapour (70-30 vol% approximately). MRS display the presence of liquid H2O and a CH4-rich bubble, with minor amounts of CO2 and N2 detected in some cases. Often type 2 inclusions are characterized by the presence of a carbonate – either ankerite (CaFe carbonate) or rhodochrosite (Mn carbonate), based on the features of its Raman spectrum.

Previous studies in the area suggest that these crustal rocks underwent high T metamorphism and re-melting in presence of fluid at 1.0 -1.1 GPa (Fancello et al., 2018) during the Variscan orogeny. The finding of melt-bearing garnets suggests that anatexis here also involved dehydration melting, at least locally, with formation of peritectic garnet. Moreover, this took place under conditions of primary fluid-melt immiscibility, as testified by the coexistence of MI and FI in the same cluster, a feature already found in many other nanogranitoid studies (see Ferrero et al., 2023). Finally, this is the first case study where nanogranitoids are found in almandine-spessartine garnet rather than in almandine-pyrope.

References

Fancello, D., et al. Trondhjemitic leucosomes in paragneisses from NE Sardinia: Geochemistry and P-T conditions of melting and crystallization. Lithos, https://doi.org/10.1016/j.lithos.2018.02.023

Ferrero, S., et al. H2O and Cl in deep crustal melts: the message of melt inclusions in metamorphic rocks. EJM, https://doi.org/10.5194/ejm-35-1031-2023

How to cite: Ferrero, S., Cruciani, G., Franceschelli, M., Fancello, D., Rezaei, L., and Gresky, K.: Fluid-melt immiscibility during lower crustal melting recorded in the orthogneiss of Punta de li Tulchi,  Sardinia (Italy)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8787, https://doi.org/10.5194/egusphere-egu24-8787, 2024.

X1.102
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EGU24-11567
Thomas Griffiths, Victoria Kohn, Taisia Alifirova, Nina Daneu, Eugen Libowitzky, Olga Ageeva, Rainer Abart, and Gerlinde Habler

Almandine-spessartine garnet in a Moldanubian peraluminous pegmatoid  (Bohemian Massif, Austria) shows asymmetric morphology, compositional zoning, and microstructural zoning, indicating directed crystal growth. Sector-specific variations in inclusion abundance and microstructures in {112} and {110} garnet sectors indicate facet-specific crystallization processes, associated with individual garnet surface configurations and a compositional boundary layer (CBL) present in the melt adjacent to growing garnet (Kohn et al., 2024). Rutile inclusions show distinct changes in abundance, aspect ratio, shape preferred orientations (SPOs) and crystallographic orientation relationships (CORs) between garnet growth zones. We quantified the SPOs of > 2400 rutile needles in two crystallographically equivalent {112}Grt rim sectors, and recorded the COR, location, habit and SPO of > 350 rutile inclusions in a transect across core and rim zones within one {112} Grt sector.

Rutile inclusions are elongated parallel to the four ⟨111⟩ Grt directions, the three ⟨100⟩ Grt directions and one ⟨112⟩ Grt direction. The most frequent SPO for a given {112} Grt sector is the one closest to the garnet growth direction (i.e. the facet normal), whereas the SPO lying in the facet is exceedingly rare. Sectioning effects cannot explain these frequency variations. Based on the facet-specific SPO and COR statistics, we infer rutile inclusions formed by nucleation at the advancing garnet surface and subsequent co-growth with the host.

Based on directly correlated SORs and CORs between elongate rutile inclusions and garnet host, specific CORs were pooled into three COR groups: 103R/111G (“one <103>Rutile direction one <111>Garnet direction”), 001R/111G and 001R/100G. Within one {112} Grt sector, core domains exhibit lower aspect ratios and higher abundance of rutile inclusions, with COR group 103R/111G being predominant. Contrastingly, the rim domain exhibits highly elongate rutile needles with lower abundance. The dominant COR group changes to 001R/111G and COR group 001R/100G appears. We suggest the decrease in inclusion abundance signals a decrease in the ratio of rutile nucleation rate to rutile growth rate, while the increase in aspect ratio signals an increase in the growth rate of rutile compared to the garnet growth rate (normal to the facet). The needle-bearing rim supposedly crystallized from a melt with higher Na, Si and OH content compared to the core (Kohn et al. 2024). A corresponding increase in diffusion rates of components in the melt is hypothesized to have decreased supersaturation with respect to rutile in a CBL, decreasing rutile nucleation rates and affecting relative growth rates. The preference for particular SPO-COR combinations should be influenced by the garnet surface configuration upon heterogeneous nucleation of rutile. Radial and lateral variations of rutile CORs and SPOs are thus attributed to changes in the nature of the garnet/melt interface, and/or the garnet growth mechanism.

Based on comparison with previous studies, changes in rutile COR group frequencies associated with increasing Si- (and likely OH) content of the melt are a systematic feature of magmatic fractional crystallization in peraluminous pegmatitic systems containing rutile-bearing garnet.

Funded by Austrian Science Fund (FWF): I4285-N37 and Slovenian Research Agency (ARRS): N1-0115

Kohn et al. (2024), Lithos, DOI: 10.1016/j.lithos.2023.107461

How to cite: Griffiths, T., Kohn, V., Alifirova, T., Daneu, N., Libowitzky, E., Ageeva, O., Abart, R., and Habler, G.: Systematic variations in shape preferred orientation and crystallographic orientation relationships of rutile inclusions in garnet upon fractional crystallization of pegmatoid melt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11567, https://doi.org/10.5194/egusphere-egu24-11567, 2024.

X1.103
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EGU24-5164
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ECS
Simon Schorn, Evangelos Moulas, and Kurt Stüwe

Hydration and retrogression are common processes in many reworked polymetamorphic terranes. The incorporation of water into a new, hydrated assemblage obliterates precursor high-grade parageneses and releases significant latent heat proportional to the degree of hydration. The hydration of dry gneisses (containing 1–2 wt.% water) at greenschist-facies conditions, may result in an approximate doubling of the bulk water content (4–5 wt.% water). This equates to a gain of 20–40 g of water per kg of retrogressed rock. Considering an average latent heat release of 4 kJ per g of water accommodated in hydrous minerals (Connolly & Thompson, 1989), hydration leads to the release of 80–160 kJ per kg of retrogressed rock. Consequently, this effect causes a significant, albeit short-lived, perturbation of the local thermal conditions. Cooling of the affected rock pile is delayed while the additional energy enhances thermally-activated processes such as diffusive loss of radiogenic Argon. This, in turn, may contribute to a significant rejuvenation of apparent 40Ar/39Ar ages in white mica of up to ~10 % (Schorn et al., 2023). In this study, we present results of multicomponent diffusion modelling of garnets hosted in pervasively hydrated micaschist from the polymetamorphic eclogite-type locality (Koralpe–Saualpe, Austria). Using simple thermokinematic models, we explore exhumation paths that encompass variable degrees and conditions of hydration, along with latent heat to, constrain a set of plausible, yet non-unique, temperature–time histories for the investigated rocks. These cooling paths serve as input for garnet diffusion modelling (Fig. 1), used to determine an appropriate set of thermal parameters for the investigated samples. Our findings contribute to discussions on the broader implications of pervasive fluid–rock interaction in a collisional setting, emphasizing the significance of the often-overlooked effects of retrogression in reworked metamorphic terranes.

Figure 1 - Multicomponent diffusion modelling of a polyphasic garnet. Yellow dots: EMPA data; red line: initial composition; blue line: calculated composition.

Figure 1 - Multicomponent diffusion modelling of a polyphasic garnet. Yellow dots: EMPA data; red line: initial composition; blue line: calculated composition.

REFERENCES

Connolly, J. A., & Thompson, A. B. (1989). Fluid and enthalpy production during regional metamorphism. Contributions to Mineralogy and Petrology, 102(3), 347-366.

Schorn, S., Moulas, E., & Stüwe, K. (2023). Hot when wet: the consequences of exothermic hydration on geochronology (No. EGU23-5769). Copernicus Meetings.

How to cite: Schorn, S., Moulas, E., and Stüwe, K.: Constraining latent heat of hydration using combined thermal- and garnet diffusion modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5164, https://doi.org/10.5194/egusphere-egu24-5164, 2024.

Posters virtual: Fri, 19 Apr, 14:00–15:45 | vHall X1

Display time: Fri, 19 Apr 08:30–Fri, 19 Apr 18:00
Chairpersons: Silvio Ferrero, Lorraine Tual, Iwona Klonowska
vX1.12
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EGU24-10532
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ECS
Tanya Srivastava, Mallickarjun Joshi, Alok Kumar, Amar Nath Tiwari, and Shubham Patel

In Kumaun Lesser Himalaya, North Ramgarh Thrust and South Ramgarh Thrust define the northern and southern boundaries of the Almora Nappe. The Almora Nappe consists of two tectono-stratigraphic units, viz. the Ramgarh Group and the Almora Group. The Ramgarh Group consists of mylonitised granite gneisses capped by low-grade metamorphic rocks and the Almora Group consists of an interbanded sequence of metapelites and metapsammites progressively metamorphosed from the greenschist to upper amphibolite facies conditions. The Almora Group rocks comprise quartzites, garnet-mica schists, and K-feldspar–sillimanite gneisses. The garnet-mica schists are coarse to medium-grained, with well-developed foliation defined by chlorite-biotite-muscovite-garnet-plagioclase-quartz mineral assemblage, and the accessory minerals are apatite and zircon. The pelitic gneisses consist of garnet, kyanite, cordierite, sillimanite, and K-feldspar. Garnet is a common and important mineral in these metamorphic rocks to constrain P-T conditions.  In this study, we specifically focus on the textural details of garnets from the schists and gneisses of the Almora Group. The most common garnet is type II garnet which is synkinematic with S-shaped inclusions of quartz and biotite, while the older type-I garnets occur as stretched out grains in the matrix with the stretch direction generally parallel to the foliation. The garnet porphyroblasts are euhedral to subhedral and consist of quartz, plagioclase, muscovite, and apatite inclusions, which are wrapped around by muscovite and biotite lepidoblasts. The hematite leachings are observed around the periphery of the garnet porphyroblasts that suggest high fO2 in the last stages of garnet growth. The micas microfolded in a few samples, with the presence of quartz in the folded hinge region. The garnets can be classified into at least two generations based on textural and petrographic attributes. Moreover, the presence of an idioblastic rim in the garnets suggests that the later phase of metamorphism outlasted the deformation. The occurrence of two generations of garnet i.e. garnet within garnet documented by (Joshi & Tiwari, 2004; Joshi & Tiwari, 2009) suggests a hiatus in crystallization and the associated metamorphic processes are likely attributed to two generations of pre-Himalayan metamorphisms.

 

How to cite: Srivastava, T., Joshi, M., Kumar, A., Tiwari, A. N., and Patel, S.: Petrological and textural characteristics of Garnet mica schists and Gneisses from Almora Group: Insights into pre-Himalayan metamorphism from Kumaun Himalaya, India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10532, https://doi.org/10.5194/egusphere-egu24-10532, 2024.