SREL Reprint #2844

Mössbauer spectroscopy of omphacite and garnet pairs from eclogites: Application to geothermobarometry

Yi-Liang Li1,2, Yong-Fei Zheng1, and Bin Fu1

1School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
2Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29802, USA

Abstract: Cation partition among coexisting minerals has been widely applied to eclogite thermometry, but an accurate estimation of Fe3+ content compared to total Fe is crucial in obtaining reasonable temperatures for petrologic studies. Room-temperature Mössbauer spectroscopy was measured for garnet-omphacite pairs in high-pressure (HP) and ultrahigh-pressure (UHP) eclogites from the Dabie terrane in east-central China. The results show very low Fe3+/ΣFe ratios of 0.026 to 0.082 in garnet but high Fe3+/ΣFe ratios of 0.240 to 0.689 in omphacite. The hyperfine parameters of minerals record the HP-UHPconditions that the eclogites experienced. Fe2+ in clinopyroxenes with low Na + Ca contents in their M2 sites shows pressure-induced occupation in M1 site. The quadrupole splitting of Fe2+ in HP-UHPgarnets (3.61 to 3.77 mm/s) andomphacites(2.77 to 3.06mm/s) are among the highest values ever reported, indicating effectively pressure-regulated polyhedral sites. After the Fe3+ was corrected, Fe2+-Mg partitioning not only significantly narrow the ranges relative to those without Fe3+ correction, but also yield temperatures about 8 to 370 °C lower than the uncorrected temperatures for the same garnet-pyroxene pairs. The recalculated temperatures are constrained to narrow ranges of 477 to 647 °C for quartz-bearing eclogites and 624 to 843 °C for coesite-bearing eclogites. These maximum values provide close proxies to peak metamorphic temperatures provided that the retrograde exchange of Fe-Mg cations by diffusion between minerals during exhumation is taken into account.

SREL Reprint #2844

Li, Y., Y. Zheng, and B. Fu. 2005. Mössbauer spectroscopy of omphacite and garnet pairs from eclogites: Application to geothermobarometry. American Mineralogist 90:90-100.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).