Mineralogical Controls on the Physical Properties of the Earth’s Upper Mantle: A First-principles Study

Gholamimahmoodabadi, Zeinab (2026) Mineralogical Controls on the Physical Properties of the Earth’s Upper Mantle: A First-principles Study. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Zeinab Gholamimahmoodabadi (19RS077)) 19RS077.pdf - Submitted Version Restricted to Repository staff only Download (17MB)

Abstract

The structure and dynamics of the Earth’s upper mantle is largely governed by how minerals respond to variations in pressure, temperature, and chemical composition. These responses control major phase transitions, seismic velocity contrasts, and interactions between the lithosphere and asthenosphere. Using first-principles density functional theory, we provide three complementary investigations to elucidate the mineralogical origins of key upper-mantle seismic discontinuities, particularly the Hales and Lehmann discontinuities. These investigations also help us decipher the complexity of lithospheric evolution by providing a comprehensive understanding of the processes observed during continental rifting. Starting with the Hales discontinuity, we examine one of the hypotheses which states that a pressure-induced cross-over in Fe²⁺ site preference between the M1 and M2 octahedral sites in olivine ((Fe, Mg)₂SiO₄) might generate the Hales discontinuity, which is generally observed at a depth of ∼100 km inside the Earth. Through our study, we show that Fe²⁺ remains thermodynamically stable at the M1 site across all upper-mantle conditions, with no evidence of cross-over even at elevated Fe concentrations. Moreover, the elastic contrasts associated with a hypothetical site exchange are insufficient to reproduce the seismic signature of the Hales discontinuity. Thus, we conclude that Fe partitioning in olivine cannot account for this feature. As we investigate potential compositional sources of the Hales discontinuity, we focus on the spinel-garnet phase transition and the role of key dopants (Fe, Cr, Ca) in shaping the phase boundary at lithospheric temperatures. Our results reveal that Cr (when placed at the Al sites) dramatically enhances the stability of the spinel phase to higher pressures and results in a negative Clapeyron slope across the entire pressure-temperature range, whereas Cr-free compositions exhibit a slope reversal at high temperatures. This compositional control on the spinel-garnet phase boundary, apart from potentially producing seismic discontinuities in the Earth’s upper mantle, also provides a mechanism for lithospheric delamination, which leads to a plausible explanation for the enhanced domal uplift followed by subsidence observed during continental rifting. Finally, we investigate the role played by Cr in modulating the thermoelastic properties of spinel and garnet. The fact that Cr shifts the spinel stability fields to higher pressures and results in a negative Clapeyron slope (dP/dT), as found in our study, makes it a potential candidate for being the “cause” behind the observed Lehmann discontinuity (L-discontinuity) in the deeper parts of the lithospheric mantle. Our first-principles calculations suggest that the sharp increase in P- and S-wave velocities and density, as observed by seismic studies at the L-discontinuity, is adequately replicated by the spinel-garnet phase transition in the Cr-rich limit. These results identify the transformation of Cr-doped spinel to garnet as a robust mineralogical mechanism for the Lehmann discontinuity. In addition to governing discrete phase transitions, the progressive transition from Al-rich to Cr-enriched mineralogical assemblages with increasing pressure leads to a systematic reduction in seismic wave velocities, providing a plausible explanation for the development of low-velocity zones (LVZs) in the upper mantle. In summary, this work offers new constraints on the chemical controls of phase transitions, provides physically consistent explanations for upper-mantle discontinuities, and advances our understanding of lithospheric stability and tectonic evolution.

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