Thermodynamic Modeling of High-grade Metabasites: A Case Study Using the Tso Morari UHP Eclogite

Thermodynamic modeling is an important technique to simulate the evolution of metamorphic rocks, particularly the poorly preserved prograde metamorphic reactions. The development of new thermodynamic modeling techniques and availability of updated thermodynamic databases and activity–composition (a–X) relations, call for an evaluation of best practices for modeling pressure–temperature (P–T) paths of metabasites. In this paper, eclogite from the Tso Morari UHP terrane, NW India, is used as a representative metabasite to directly compare the outputs (pseudosections and P–T paths) generated from recent versions of the widely used THERMOCALC and Theriak-Domino programs. We also evaluate the impact of using the most updated thermodynamic database (ds 62, Holland and Powell in J Metamorph Geol 29(3):333–383, 10.1111/j.1525-1314.2010.00923.x, 2011) relative to an older version (ds 55, Holland and Powell in J Metamorph Geol 16(3):309–343, 10.1111/j.1525-1314.1998.00140.x, 1998), and the effect of the user’s choice of mineral a–X relations while considering the effect of garnet fractionation on the rock’s effective bulk composition. The following modeling protocols were assessed: (1) TC33; THERMOCALC version 3.33 with database ds 55 and garnet a–X relations of White et al. (J Metamorph Geol 25(5):511–527, 10.1111/j.1525-1314.2007.00711.x, 2007); (2) TC47; THERMOCALC version 3.47 with database ds 62 and garnet a–X relations of White et al. (J Metamorph Geol 32(3):261–286, 10.1111/jmg.12071, 2014a); (3) TDG; Theriak-Domino with database ds 62 and garnet a–X relations of White et al. (2014a), and (4) TDW; Theriak-Domino with database ds 62 and garnet a–X relations of White et al. (2007). TC47 and TDG modeling yield a similar peak metamorphic P–T of 34 ± 1.5 kbar at 544 ± 15 °C and 551 ± 12 °C, respectively. The results are 5–8 kbar higher in pressure than that determined from TC33 modeling (26 ± 1 kbar at 565 ± 8 °C), and TDW modeling (28.5 ± 1.5 kbar at 563 ± 13 °C). Results indicate that all four modeling protocols generally provide consistent metamorphic phase relations and thermodynamic simulations regarding fractionation of the bulk composition and prograde metamorphism within uncertainty. In all model calculations, the initial bulk composition measured by XRF does not represent the effective bulk composition at the time of garnet nucleation. The choice of garnet a–X relations can affect predictions of peak pressure, regardless of program choice. This study illustrates the importance of careful consideration of which a–X relations one chooses, as well as the need for comparison between modeling predictions and evidence from the geochemistry and petrography of the rock(s) themselves.