Development of A-type olivine fabric in water-rich deep upper mantle

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• 作者：Tomohiro Ohuchi ; ^{ohuchi@sci.ehime-u.ac.jp} ; Tetsuo Irifune
• 关键词：olivine ; crystallographic preferred orientation ; water ; deep upper mantle ; seismic anisotropy
• 刊名：Earth and Planetary Science Letters
• 出版年：2013
• 出版时间：15 January, 2013
• 年：2013
• 卷：362
• 期：Complete
• 页码：20-30
• 全文大小：1485 K

文摘

Water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation (i.e., fabric) in mantle rocks. The A-type olivine fabric is the most commonly observed olivine fabric in natural peridotites. Development of A-type olivine fabric (developed by the (010)[100] slip system) is known to be limited to the water-poor conditions of the shallow upper mantle (<200 km depth). We have performed simple-shear deformation experiments of olivine at 7.2-11.1 GPa and 1400-1770  K. Here we show that A-type olivine fabric was developed under water-rich conditions (>2130 ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) was observed under moderately wet conditions (750-2130 ppm H/Si). Developments of C-type (by the (100)[001] slip system) fabric was limited to water-poor conditions (<220 ppm H/Si). We found that monotonic decrease in the seismic anisotropy V_SH/V_SV (the ratio of horizontally and vertically polarized shear waves) with depth in the global one-dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water-poor mantle. Only A-type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of V_SH/V_SV<1 in the deep upper mantle (>200 km depth). A strong anomaly of V_SH/V_SV<1 observed in the deep upper mantle beneath the East Pacific Rise is well explained by the distribution of water-rich regions (in which A-type olivine fabric is dominantly developed) in the deep upper mantle and upwelling mantle flows.