Is a cation ordering transition of the Mg‐Fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the Indian Craton?

Abstract

We use first‐principles molecular dynamics simulations to study the behavior of cation ordering in the non‐equivalent octahedral sites of Mg‐Fe olivine solid solutions. Our theoretical calculations confirm the previous experimental finding that Mg2+ and Fe2+ can invert their octahedral site occupancy at a critical temperature. Assuming that the site preference of Fe changes discontinuously between two states in which it is completely restricted to either M1 or M2 sites, we have calculated the transition temperature, Tt, between the two extreme states. Under ambient pressure Tt is calculated to be 520°C that agrees fairly with the experimental finding in which, however, the ordering state changed discontinuously over a much smaller range of the site occupancy of Fe. Tt is found to be pressure sensitive, showing an increase by 30 to 100°C per unit GPa, depending upon the iron content. Using the Indian continental geotherm, we estimate a depth of around 75 Km corresponding to the calculated transition pressure and temperature of cation ordering, which matches well with the depth for the Hales discontinuity marked by a jump of shear wave velocity by ∼4%. For olivine solid solutions with 12.5% iron, the ordering transition increases Vs from 4.5 to 4.7 Km/s. Both the inferences, viz. depth of discontinuity and magnitude of velocity increase find support from the modeling of teleseismic earthquake waveforms recorded over broadband seismographs on the Dharwar Craton. This leads us to infer that the cation ordering transition in ferromagnesian olivine might be a potential factor for the Hales discontinuity.