Can increased pressure induce cross-over of Fe partitioning in olivine under upper mantle P–T conditions?

• Published in: Journal of Earth System Science 2024-09, Vol.133 (4), p.194, Article 194
• Main Authors: Gholamimahmoodabadi, Zeinab, Mondal, Rabindranath, Chatterjee, Swastika, Saha-Dasgupta, Tanusri
• Format: Article
• Language: English
• Subjects: Approximation; Atoms & subatomic particles; Crystal structure; Discontinuity; Earth; Earth and Environmental Science; Earth mantle; Earth Sciences; Elasticity; Energy; Ferrous ions; First principles; High pressure; High temperature; Investigations; Iron; Molecular dynamics; Olivine; Physical properties; Pressure; Pressure effects; Simulation; Site preference (crystals); Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Structure-function relationships; Temperature; Temperature preferences; Thermoelastic properties; Upper mantle
• ISSN: 0973-774X; 0253-4126; 0973-774X
• DOI: 10.1007/s12040-024-02390-4

Olivine (M 2 SiO 4 ; M: Mg, Fe), constituting about 60% (by volume) of the upper mantle of our Earth, contains ~10% Fe (at the M-octahedral site). The Fe 2+ ion is known to preferentially occupy one of the two inequivalent M-octahedral sites, namely M1 and M2, in the crystal structure of olivine. The site preference of Fe in olivine as a function of increasing temperature has been a matter of great debate for over three decades. However, later first-principles studies found that Fe preferred the M1 site for all temperatures of interest. A more recent study Mandal et al. ( J . Geophys . Res . Solid Earth 117  B12, 2012, https://doi.org/10.1029/2012JB009225 ) incorporated the effect of pressure in addition to high temperature and found that increased pressure may bring on a cross-over of the site preference of Fe in olivine. This was proposed to be the cause behind the Hales discontinuity, which is observed at a depth of ~100 km deep inside the Earth. In a bid to understand the microscopic origin behind (i) the cross-over (if any) and (ii) the drastic change in elasticity of olivine as Fe migrates from M1 site to the M2 site at high pressure and temperature, we have re-addressed the problem using a combination of ab-initio molecular dynamics simulation (AIMD) and quasi-harmonic approximation (QHA). Our calculations find that, thermodynamically, Fe 2+ prefers the M1 site, and that there is no cross-over under high pressure and temperatures that are prevalent in the Earth’s upper mantle. Since the distribution of Fe in the mantle might not be homogeneous, the effect of increased Fe concentration (to capture local effects) has also been investigated. We find that increased Fe concentration does not have any effect on the site preferred by Fe 2+ in the olivine lattice. The calculated thermoelastic properties of olivine with Fe at M1 site and M2 site, respectively, indicate that even the hypothetical situation of cross-over of Fe 2+ from the M1 to M2 site, cannot account for the Hales discontinuity, as proposed previously (Mandal et al. 2012).