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Evaluation of multiple thermodynamic modeling protocols on metabasites: using the Tso Morari UHP eclogite as a case study
  • Catherine Macris,
  • Carrie Menold
Indiana University Purdue University at Indianapolis

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Catherine Macris
Indiana University – Purdue University Indianapolis
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Carrie Menold
Albion College
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The development of thermodynamic modeling techniques and availability of updated thermodynamic databases and activity-composition (a-X) relations, call for an evaluation of modeling pressure-temperature (P-T) paths of metabasites. In this study, eclogite from the Tso Morari UHP terrane, NW India, is used as a representative metabasite to compare P-T paths generated from the widely used THERMOCALC (TC) and Theriak-Domino (TD) programs. We also evaluate the effect of using the most updated thermodynamic database ds 62 (Holland and Powell 2011) relative to an older version ds 55 (Holland and Powell 1998), and the most updated garnet a-X relations of White et al. (2014) (W14) relative to an older version of White et al. (2007) (W07), while accounting for the effect of garnet fractionation. The following modeling protocols were assessed: (1) TC33: TC v3.33 with ds 55 and garnet a-X relations of W07; (2) TC47: TC v3.47 with ds 62 and garnet of W14; (3) TDG: TD with ds 62 and garnet of W14, and (4) TDW: TD with ds 62 and garnet of W07. TC47 and TDG modeling protocols yield a similar peak metamorphic P-T of 34 ± 1.5 kbar at 544 ± 15 °C and 551 ± 12 °C, respectively; while TC33 and TDW modeling yield similar peak P-T results: 26 ± 1 kbar at 565 ± 8 °C and 28.5 ± 1.5 kbar at 563 ± 13 °C, respectively. Results indicate that all four modeling protocols generally provide consistent thermodynamic simulations regarding metamorphic compositional and temperature evolution; however, the pressure generated by protocols using W14 (TC47 and TDG) is 5–8 kbar higher than that predicted by protocols using W07 (TC33 and TDW). The difference in peak pressure results for the modeling protocols (TC47 and TDG vs. TC33 and TDW) are beyond the suggested uncertainty using mineral isopleth thermobarometry in pseudosections: ± 50 °C and ± 1 kbar at 2σ (Powell and Holland 2008). This study illustrates that the choice of garnet a-X relations can affect predictions of peak pressure regardless of program choice, as well as the need of comparison between modeling predictions and petrological observations.