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1. Thermodynamic modelling on the UHP metamorphism and fluid infiltration of the Tso Morari coesite-bearing eclogite in NW India

   Pan, R. ; Macris, C. ; Menold, C. ( January 2021 , Annual VM Goldschmidt Conference)

   null (Ed.)

   The Tso Morari terrane within the Himalayan orogeny underwent ultrahigh-pressure (UHP) metamorphism due to northward subduction under the Eurasian continent during the early Eocene. The advancement of computational petrology and availability of relevant thermodynamic databases provide the mechanism to more precisely quantify metamorphic processes. In this study, we model the eclogite’s prograde pressure-temperature (P-T) path as well as multiple fluid infiltration events during exhumation using Theriak-Domino with dataset ds62 and garnet[1] and other metabasic mineral activity-composition relations. The effect of garnet fractionation on the rock’s effective bulk composition is considered in simulating prograde garnet growth. A “fishhook” shape clockwise P-T path is obtained with a peak pressure of ~28.5 kbar at ~563 °C, followed by a peak temperature of ~613 °C at ~24.5 kbar[2]. Thermodynamic modelling using P-M(H2O) pseudosections on Tso Morari eclogites indicates three distinct phases of fluid infiltration during exhumation. Fluid infiltration Ⅰ occurs at ~610 °C and ~23.5 kbar with ~3.1 mol % fluid expulsion due to the destabilization of lawsonite. The modelling results are consistent with petrographic observations in the eclogite: we found ~6.0 vol % epidote and ~21.0 vol % amphibole and the possible pre-existence of lawsonite evidenced by its pseudomorph (as epidote and paragonite aggregates) in a garnet core and rim[3], and CNASH modelling on the epidote and its inclusion paragonite. Fluid infiltration Ⅱ occurs at ~9.2 kbar and ~608 °C with >2.6 mol % fluid infiltration at amphibolite-facies. This phase of fluid infiltration is characterized by aggressive amphibolization from the boudin core to rim. Fluid infiltration Ⅲ occurs at ~610 °C and ~8.7 kbar, caused by breakdown of phengite as predicted through modelling the symplectitic association (plagioclase, biotite, and amphibole) surrounding omphacite. In summary, this study not only illustrates the application of thermodynamic modelling in quantifying metamorphic processes, but also the need of comparison between modeling predictions and petrographic observations. [1] White et al. (2007), J Metamorph Geol 25, 511–527. [2] Pan et al. (2020), Contrib Mineral Petrol 175, 1–28. [3] St-Onge et al. (2013), J Metamorph Geol 31, 469–504. 

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2. Thermodynamic Modeling of the Tso Morari UHP Eclogite, NW Himalaya

   Pan, R ; Macris, C.A. ; Menold, C.A. ( December 2019 , AGU Fall Meeting 2020)

   Thermodynamic modeling is an important technique to interpret metamorphic phase relations and calculate model pressure-temperature (P-T) paths for metamorphic rocks. This study uses representative, coesite-bearing eclogites from the Tso Morari UHP terrane of the NW Himalaya to simulate its prograde metamorphism using multiple modeling programs and thermobarometry. Our modeling yields a peak metamorphism P-T of ~32-33 kbar and ~560-570 °C by the THERMOCALC345 and Theriak-Domino programs (Green et al., 2016), which is ~5 kbar higher in pressure and ~15 °C lower in temperature than that determined by using THERMOCALC333 (White et al., 2007) (~27.8 kbar and ~580 °C). The significantly higher pressure obtained using the THERMOCALC345 and Theriak-Domino is likely a result of the upgrade of thermodynamic parameters of minerals (i.e. garnet Wpy-gr and agr) in the newer a-x relations. The modeled effective bulk compositions and mineral stabilities along the calculated P-T path show different patterns under the two modeling techniques. Modeling by the Theriak-Domino programs is preferred in this case because the results are more consistent with the measured mineral compositions of our rocks. Multiple thermobarometers by garnet-omphacite-phengite, garnet-omphacite, garnet-phengite on the garnet rim, high-Si phengite and matrix omphacite yield a peak metamorphism of ~ 28.5-29.0 kbar and ~ 650-728 °C, which is generally consistent with the modeled P-T path. Based on our model calculations, the initial bulk composition measured by XRF does not represent the reactant bulk composition at the time of garnet nucleation, and this compositional discrepancy possibly is caused by the crystallization of pre-garnet minerals (i.e. hematite), reaction overstepping, or partial reequilibration. In summary, by implementing and evaluating multiple modeling strategies and considering the petrography and metamorphic mineralogy of the rocks, this study finds that the eclogite modeling using Theriak-Domino programs in the Tso Morari terrane provide more consistent metamorphic phase relations and more reasonable thermodynamic simulations regarding fractionation of the bulk composition and prograde metamorphism. References: Green et al. J Metamorph Geol 34, 845-869 (2016) White et al. J Metamorph Geol 25, 511-527 (2007) 

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3. Chemical and mechanical controls on metasomatic assimilation of eclogite blocks in high-pressure host gneiss

   Menold, C.A. ; Grove, M. ; Macris, C.A. ( December 2019 , AGU Fall Meeting 2019)

   Mica- and garnet-rich selvages are often developed around eclogitized mafic blocks within felsic gneiss in HP to UHP metamorphic terranes. The development of these metasomatic features ranges from readily identified reaction zones between the eclogite and host gneiss to shear zones where the spatial relationships between eclogite blocks and host gneiss are completely obscured. Block-selvage relationships within the Luliang Shan HP/UHP belt (North Qaidam, China) and the Tso Morari UHP terrane (NW Himalaya, India) approximate end members of the selvage preservation process. Here we apply whole-rock and incompatible trace element compositions coupled with B and O isotopic data in white mica to constrain the relationship of metasomatism vs. deformation during selvage formation. Within the Luliang Shan, extensive fluid flow formed thick, compositionally hybridized phengite- and garnet-bearing selvages between eclogite (SiO2 ~ 50%) and quartzofeldspathic gneiss (SiO2 ~ 80%). The Luliang Shan HP selvages have intermediate SiO2 and range from 5-10 m in thickness as "halos" around spheroidal eclogite blocks. Volatile enrichment at near-UHP conditions in the selvage is indicated by enrichment of Li, Cs, Ba, Ar, and δ18O and very light δ11B values in phengite. The retrograde muscovite from the host gneiss is low in Li, Cs, Rb, and Sr but possess remarkably high B concentrations (up to 3000 ppm) and positive δ11B values that are best explained by interaction with fluids devolatilized from accreted sediments within cooler regions of the subduction zone. Alternatively, the Tso Morari UHP terrane features boudinaged discoids of eclogite encased within highly strained quartzofeldspathic gneiss. Whole rock major element sampling performed normal to the foliation reveal consistently high SiO2 (78-80%). Highly variable degrees of metasomatic recrystallization occur within the phengite-rich rocks spatially associated with eclogite. The selvage rocks exhibit heterogeneous degrees of enrichment in Li, Be, B, and Ba and yield δ11B values of -4 to -6‰ typical of undevolatilized oceanic and continental crust. We conclude that fluid-mediated metasomatic reaction between eclogite and gneiss at Tso Morari is sheared out into lenses that are incorporated into, and heterogeneously distributed throughout, the host gneiss. 

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4. Accessory phases as recorders of subduction redox: Sulfide–oxide– silica equilibria during high-pressure metamorphism

   Walters, JB ; Cruz-Uribe, AM ; Marschall, HR ( December 2018 , 2018 Fall Meeting, AGU)

   Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides: (1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites. As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer. In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs. 

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5. Eclogite thermobarometry: The consistency between conventional thermobarometry and forward phase‐equilibrium modelling

   https://doi.org/10.1111/jmg.12747  

   Hernández‐Uribe, David ; Holder, Robert M. ; Hernández‐Montenegro, Juan D. ( October 2023 , Journal of Metamorphic Geology)

   Eclogite thermobarometry is crucial for constraining the depths and temperatures to which oceanic and continental crust subduct. However, obtaining the pressure and temperature (P–T) conditions of eclogites is complex as they commonly display high‐variance mineral assemblages, and the mineral compositions only vary slightly withP–T. In this contribution, we present a comparison between two independent and commonly used thermobarometric approaches for eclogites: conventional thermobarometry and forward phase‐equilibrium modelling. We assess how consistent the thermobarometric calculations are using the garnet–clinopyroxene–phengite barometer and garnet–clinopyroxene thermometer with predictions from forward modelling (i.e. comparing the relative differences between approaches). Our results show that the overall mismatch in methods is typically ±0.2–0.3 GPa and ±29–42°C although differences as large as 80°C and 0.7 GPa are possible for a few narrow ranges ofP–Tconditions in the forward models. Such mismatch is interpreted as the relative differences among methods, and not as absolute uncertainties or accuracies for either method. For most of the investigatedP–Tconditions, the relatively minor differences between methods means that the choice in thermobarometric method itself is less important for geological interpretation than careful sample characterization and petrographic interpretation for derivingP–Tfrom eclogites. Although thermobarometry is known to be sensitive to the assumedX_Fe³⁺of a rock (or mineral), therelativedifferences between methods are not particularly sensitive to the choice of bulk‐rockX_Fe³⁺, except at high temperatures (>650°C, amphibole absent) and for very large differences in assumedX_Fe³⁺(0–0.5). We find that the most important difference between approaches is the activity–composition (a–x) relations, as opposed to the end‐member thermodynamic data or other aspects of experimental calibration. When equivalenta–xrelations are used in the conventional barometer,Pcalculations are nearly identical to phase‐equilibrium models (ΔP < 0.1). To further assess the implications of these results for real rocks, we also evaluate common mathematical optimizations of reaction constants used for obtaining the maximumP–Twith conventional thermobarometric approaches (e.g. using the highestaGrs² × aPrp in garnet and Si content in phengite, and the lowestaDi in clinopyroxene). These approaches should be used with caution, because they may not represent the compositions of equilibrium mineral assemblages at eclogite facies conditions and therefore systematically biasP–Tcalculations. Assuming method accuracy, geological meaningfulP_maxat a typical eclogite facies temperature of ~660°C will be obtained by using the greatestaDi,aCel, andaPrp and lowestaGrs andaMs; garnet and clinopyroxene with the lowest Fe²⁺/Mg ratios may yield geological meaningfulT_maxat a typical eclogite facies pressure of 2.5 GPa.

    

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