Mossbauer spectroscopy of omphacite and garnet pairs from eclogites: Application to geothermobarometry
YI-LIANGLI,1,2 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, U.S.A.
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 Mossbauer s~oscopy was Iileasured
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+/EFe ratios of 0.026 to 0.082 in garnet but
high Fe3+/EFe 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.
