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Abstract Synchrotron‐based high‐pressure/high‐temperature single‐crystal X‐ray diffraction experiments to ~24 GPa and 700 K were conducted on eclogitic garnets (low‐Fe: Prp28Alm38Grs33Sps1and high‐Fe: Prp14Alm62Grs19Adr3Sps2) and omphacites (low‐Fe: Quad57Jd42Ae1and high‐Fe: Quad53Jd27Ae20), using an externally heated diamond anvil cell. Fitting the pressure‐volume‐temperature data to a third‐order Birch‐Murnaghan equation of state yields the thermoelastic parameters including bulk modulus (KT0), its pressure derivative (K′T0), temperature derivative ((∂KT/∂T)P), and thermal expansion coefficient (αT). The densities of the high‐Fe and low‐Fe eclogites were then modeled along typical geotherms of the normal mantle and the subducted oceanic crust to the transition zone depth (550 km). The metastable low‐Fe eclogite could be a reason for the stagnant slabs within the upper range of the transition zone. Eclogite would be responsible for density anomalies within 100–200 km in the upper mantle of Asia.
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Eclogite thermobarometry: The consistency between conventional thermobarometry and forward phase‐equilibrium modelling
Abstract 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 assumedXFe3+of a rock (or mineral), therelativedifferences between methods are not particularly sensitive to the choice of bulk‐rockXFe3+, except at high temperatures (>650°C, amphibole absent) and for very large differences in assumedXFe3+(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 meaningfulPmaxat 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 meaningfulTmaxat a typical eclogite facies pressure of 2.5 GPa.
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- Award ID(s):
- 2127195
- PAR ID:
- 10470031
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Metamorphic Geology
- Volume:
- 42
- Issue:
- 1
- ISSN:
- 0263-4929
- Format(s):
- Medium: X Size: p. 89-108
- Size(s):
- p. 89-108
- Sponsoring Org:
- National Science Foundation
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