Evaluation of multiple thermodynamic modeling protocols on metabasites:
using the Tso Morari UHP eclogite as a case study
Abstract
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.