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

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2019-01, Vol.20 (1), p.183-207
• Main Authors: Demouchy, S., Tommasi, A., Ionov, D., Higgie, K., Carlson, R. W.
• Format: Article
• Language: English
• Subjects: Anisotropy; Anomalies; Basalt; Cenozoic; CPO; Deformation; Earth Sciences; Elastic waves; Evolution; Garnet; Geochemistry; Geophysics; Hangay Dome; High temperature; Hydrogen; Lithosphere; lithospheric mantle; Magma; Mantle; Olivine; peridotite; Physical properties; Sciences of the Universe; Seismic activity; seismic anisotropy; Seismic properties; Seismic velocities; Wave velocity
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2018GC007931

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. Key Points Tariat mantle rocks are chemically and texturally homogeneous, well annealed, and hydrogen poor There is no evidence for recent major viscoplastic deformation, abnormal thermal gradient, or partial melting in the lithospheric mantle The samples have moderate seismic anisotropy with fast S wave polarization marking flow directions in the mantle