HFSE AND REE FRACTIONATION BETWEEN RUTILE AND SUPERCRITICAL FLUID-MELT AT 5.5-7.8 GPA AND 850-1090°C: IMPLICATION TO THE PROCESSES IN SUBDUCTION ZONES
I.S. Karputin1, A.G. Sokol1, O.A. Kozmenko1, A.L. Ragozin1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: supercritical fluid-melt, subduction, high-pressure high-temperature experiments, distribution coefficient, rutile, pelite, schist, mantle metasomatism
Abstract
Previously, in experiments at pressures of 5.5–7.8 GPa and temperatures of 850–1090°C, it was established that dehydration of naturally doped quartz-muscovite-chlorite schist results in the formation of a supercritical fluid-melt (SiO2>Al2O3≥K2O>Na2O, >30 wt.% H2O and ~10 wt.% CO2) and a restite (garnet, phengite, clinopyroxene, coesite, kyanite, and accessory rutile, zircon, monazite). In this work, the compositions of rutile and the supercritical fluid-melt were determined by LA-ICP-MS, and based on the obtained data, the fractionation patterns of HFSE and REE between them were reconstructed. It is shown that at pressures of 5.5–7.8 GPa, with increasing temperature, the distribution coefficient of Nb between rutile and the supercritical fluid-melt decreases from 400–800 (850–940°C) to 70–80 (1030–1090°C), and the distribution coefficient of Ta decreases from 100–200 to ~10. The ratio of the distribution coefficients we established, D(Nb/Ta), is 2–10, which is significantly higher than the values for rutile–silicate melt or rutile–carbonatite melt pairs. It is concluded that the separation of volatile-rich supercritical fluid-melts during the dehydration of rutile-bearing metasediments at depths >170 km may lead to the formation of eclogitic restite with increased Nb/Ta ratio. Analysis of the obtained data demonstrated the possibility of using the Zr-in-rutile thermometer for rutile- and zircon-bearing metasediments in the pressure range of 5.5–7.8 GPa. However, a systematic underestimation of 22–30% was found for the values calculated using the thermometer (calibrated up to 3.0 GPa) relative to the experimental temperatures.
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