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

.[1] 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 Mg 2+ and Fe 2+ 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, T t , between the two extreme states. Under ambient pressure T t 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. T t 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 V s 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.Citation: Mandal, N., K. H. Chakravarty, K. Borah, and S. S. Rai (2012), 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?,

Hales Discontinuity in the Southern Indian Continental Lithosphere: Seismological and Petrological Models

“…Based on observations of SCLM discontinuities from other continental regions, this discontinuity at a depth of ∼75 km was labeled as the Hales discontinuity. On the other hand, Mandal et al (2012) used marginally higher frequency P-RFs (f max = 0.29 Hz) but ignored intracrustal layers while forward modeling the Moho and the upper mantle discontinuities. This led to a similar outcome of modeling the intracrustal positive amplitude reverberation as the Hales discontinuity.…”

“…The explanations for the Hales discontinuity beneath the EDC was either the spinel‐to‐garnet mineral transformation in mantle peridotite (Jagadeesh & Rai, 2008) or cation ordering transition in ferromagnesian olivine (Mandal et al., 2012). For all possible upper mantle compositions, Ziberna et al.…”

“…Jagadeesh and Rai (2008) qualitatively interpreted the observed Hales discontinuity V s increase as a spinel peridotite to garnet peridotite transition. Mandal et al (2012) reanalyzed the P-RFs of Jagadeesh and Rai (2008) from the Dharwar Craton and used first-principles calculations and molecular dynamic simulations to show that the V s increase at the Hales discontinuity may be caused by intracrystalline cation ordering in olivine for preference of Fe 2+ in the M1 site (normal olivine) to M2 site (inverse olivine). However, all these studies were either performed using P-RFs at low frequency (f max = 0.18 Hz) or the forward modeling of Hales discontinuity P h s phase ignored reverberations from the midcrustal discontinuity, otherwise observed and modeled in crustal studies from these seismograph stations (Julia et al, 2009;Rai et al, 2003).…”

Mineralogy of the Earth: Phase Transitions and Mineralogy of the Upper Mantle

“…Thus, in continental regions with relatively hot geotherms, such as the Variscan orogen in the SW Iberian Peninsula (Palomeras et al, 2011), the Hales gradient zone is only 10-20 km thick and lies at around 70 km depth (Ayarza et al, 2010). Note that there are alternative interpretations of the Hales discontinuity, ranging from seismic anisotropy (Bostock, 1998;Fuchs, 1983;Levin and Park, 2000) to pervasive partial melts (Thybo and Perchuc, 1997) and cation ordering in mantle olivine (Mandal et al, 2012). Ziberna et al (2013) further predict that bulk composition may control the extension of the Hales gradient zone in cold, cratonic settings, but its influence will progressively decrease at higher increasing geothermal gradients.…”

Upper-mantle anisotropy beneath the south Indian Shield: Influenced by ancient and recent Earth processes

“…Presence of an extremely high-velocity shallow layer is argued as indicative of mineralogy/ lithology (e.g., dunite that contains olivine and has high seismic anisotropy). Evidence of a layered lithosphere in the south Indian Shield has also been presented through the analysis of receiver function observations (Jagadeesh and Rai, 2008;Mandal et al, 2012).…”