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Weak B‐Type Olivine Fabric Induced by Fast Compaction of Crystal Mush in a Crustal Magma Reservoir
Author(s) -
Yao Zhuosen,
Qin Kezhang,
Wang Qin,
Xue Shengchao
Publication year - 2019
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2018jb016728
Subject(s) - geology , olivine , peridotite , ultramafic rock , dislocation creep , seismic anisotropy , mafic , petrology , compaction , mineralogy , geochemistry , mantle (geology) , dislocation , composite material , materials science , geomorphology
Understanding the deformation mechanisms of olivine has been considered as the crucial factor in tracing the dynamic processes from seismic anisotropy. Due to extensive overprinting of deformation and alteration, natural examples of original magmatic fabric of olivine are scarce. In this study we selected one wehrlite and three dunite samples from a deep borehole in the Poyi mafic‐ultramafic intrusion in the Tarim Craton. The fresh samples show adcumulate texture and plastic deformation of dry olivine (≤10 ppm H 2 O). Thermobarometer calculations yield crystallization conditions at 200–400 MPa and >1,124–1,275 °C. Olivine developed a B‐type fabric characterized by a concentration of [001] axes parallel to the interpreted lineation and that of [010] axes normal to the interpreted foliation. Activation of [001] and [100] dislocations is confirmed by microstructural observations. Simulation of gravity‐driven compaction process reveals extremely rapid compaction of kilometer‐scale crystal mush in the Poyi magmatic system. During the early compaction with high porosity, crystallographic habit of olivine may produce shape‐preferred orientation and the related B‐type fabric. In the final stage of compaction and crystallization, the initial shape‐preferred orientation was gradually overprinted, whereas the weak B‐type fabric has remained with dislocation creep and dislocation‐accommodated grain boundary sliding. Calculated seismic velocities of peridotite samples show low P and S wave anisotropies, with the fastest S wave polarization direction normal to the foliation. Hence, a mush‐dominated ultramafic intrusion exhibits weak seismic anisotropy, which has potential to better understand seismic anisotropy in the transcrustal magmatic systems.