Microstructures, Water Contents, and Seismic Properties of the Mantle Lithosphere Beneath the Northern Limit of the Hangay Dome, Mongolia

Abstract

To constrain the deformation, thermal evolution, and seismic properties of the mantle lithosphere beneath the Hangay Dome, we have analyzed the microstructures, crystal preferred orientations (CPO), and hydrogen concentrations of olivine and pyroxenes of 50 mantle xenoliths carried up by Cenozoic basalts from Zala, Haer, and Shavaryn‐Tsaram from Tariat, Mongolia. Most xenoliths are medium‐ to coarse‐grained spinel‐lherzolites, but four contain garnet + spinel. Coarse granular, highly annealed microstructures predominate. The microstructures are associated with well‐developed CPO, typical of deformation under high temperature, moderate pressure, and dry conditions. The hydrogen concentrations in olivine, orthopyroxene, and clinopyroxene are low and range around 5, 75, and 147 ppm H2O wt, respectively. Together, microstructures and CPO indicate that ductile deformation was followed by static recrystallization, which has annealed the microstructures but preserved the CPO and, hence, the anisotropy of physical properties. Lack of correlation between annealing and equilibrium temperatures suggest that the annealing is due to a long quiescence episode since the last deformation episode. Here, there is not evidence that the formation of Hangay Dome is associated with recent deformation in the lithospheric mantle. Calculated seismic properties show moderate seismic anisotropy, with fast propagation of P waves and polarization of S waves parallel to the flow direction and low birefringence for S waves propagating obliquely to the flow plane. The results are consistent with weak P wave anomalies but not with the strong low S wave velocity anomalies predicted by some tomographic models or with the high conductivity inferred from magnetotelluric data for the lithospheric mantle beneath the Hangay Dome.

Textural and Compositional Changes in the Lithospheric Mantle Atop the Hawaiian Plume: Consequences for Seismic Properties

“…The recrystallized fraction is quantified by the area fraction composed by grains with low intragranular misorientation (grain orientation spread, GOS < 1.5°). Dashed line in (b) indicates the shape factor of a well‐equilibrated synthetic olivine aggregate composed by polygonal crystals (Demouchy et al, 2019). Data presented in Table 2.…”

Textural and compositional changes in the lithospheric mantle atop the Hawaiian plume: Consequences to seismic properties

“…The recrystallized fraction is quantified by the area fraction composed by grains with low intragranular misorientation (grain orientation spread, GOS < 1.5°). Dashed line in (b) indicates the shape factor of a well-equilibrated synthetic olivine aggregate composed by polygonal crystals (Demouchy et al, 2019). Data presented in Table 2.…”

Defects in olivine

“…(2) dislocation-assisted grain boundary sliding as proposed by Hirth and Kohlstedt (1995) (see also Hansen et al, 2011Hansen et al, , 2012, which is then grain size dependent; (3) diffusionaccommodated grain boundary sliding (Miyazaki et al, 2013), which is limited to nanoscale grains of olivine (± pyroxenes) and thus is unlikely to be efficient in mantle olivinerich rocks with grain sizes of 0.1-10 mm and under high lithostatic pressure (> 1 GPa); (4) elastically accommodated grain boundary sliding (Jackson et al, 2013), which also requires very fine grained polycrystalline olivine of constant grain size, in which the grain boundary network is considered equivalent to a very low viscosity layer; however, this contradicts the crystalline sintering observed at high pressure and temperature (Hiraga et al, 2004;Burnard et al, 2015); (5) ionic diffusion operating only at grain boundaries as proposed by Detrez et al (2015), but they also showed that this mechanism cannot sustain a non-linear rheology in an anisotropic solid such as olivine; (6) disclination-assisted dislocation creep proposed by Cordier et al (2014), which involves rotational defects located near grain boundaries (see Sect. 7); or (7) the potential existence of disconnections located at grain boundaries such as the one observed in aluminum metal (see Sect.…”