From Fractionation to Thermometry : Evaluating Equilibrium Isotope Fractionation Among Bridgmanite, Akimotoite and Majorite

Iyer, Sriparameshwaran (2025) From Fractionation to Thermometry : Evaluating Equilibrium Isotope Fractionation Among Bridgmanite, Akimotoite and Majorite. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Sriparameshwaran Iyer (20MS085)) 20MS085_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

Understanding the distribution of stable isotopes among coexisting high-pressure phases is essential for probing deep Earth and extraterrestrial processes. This study investigates magnesium and silicon isotope fractionation among three high-pressure polymorphs of MgSiO3 - Bridgmanite, Akimotoite and Majorite. Bridgmanite, the most abundant lower mantle phase, along with Akimotoite and Majorite which are stable in the transition zone and uppermost lower mantle, can coexist in certain regions of the Earth’s interior, with a possible triple-point at around the 660 km discontinuity. These minerals have been reported to coexist in the shock-melt veins of chondritic meteorites like Katol L6 and Tenham L6. In this study, we have calculated isotope ω-factors (reduced partition function ratio) for the Pure Mg end-members and Fe-doped systems (→10% in each phase) at 0 GPa and 24 GPa. We found that Bridgmanite is depleted in heavier Si, Akimotoite is enriched in heavier Mg and Majorite is enriched in heavier Si & depleted in heavier Mg. Pressure significantly increases both Mg and Si isotope ω-factors of all phases. Doping Fe at 24 GPa affects the isotope ω-factors differently for the three phases under consideration. While the changes observed for Majorite are significant, doping Fe only slightly alters the ω-factors for Bridgmanite and Akimotoite when compared with those observed for Pure Mg end members at 24 GPa. Using results from this and previous studies, we explored the Mg and Si isotope fractionation between Mantle Transition zone and Lower Mantle. We concluded that while Mg fractionation may not be detectable, significant Si fractionation is possible between these two layers. But, the expected signature could be destroyed by mantle convection. Fractionation studies can be used to construct Isotope thermometers. In the context of Earth, these thermometers can tell us the depth at which the samples were present in the Earth’s interior before ejecting out. In the context of meteorites, this can be used to get the peak conditions of the shock events when these high pressure phases crystallized. i

Actions (login required)

View Item