Deformation and Fluid–Rock Interaction in the Supra-subduction Mantle: Microstructures and Water Contents in Peridotite Xenoliths from the Avacha Volcano, Kamchatka
Deformation and Fluid–Rock Interaction in the Supra-subduction Mantle: Microstructures and Water Contents in Peridotite Xenoliths from the Avacha Volcano, Kamchatka
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DOI：
10.1093/petrology/egp085
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发表时间：
2010
影响因子：
3.9
通讯作者：
V. Soustelle;A. Tommasi;S. Demouchy;D. Ionov
中科院分区：
文献类型：
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作者：
V. Soustelle;A. Tommasi;S. Demouchy;D. Ionov
俯冲板块上方的地幔是变形、部分熔融、流体迁移和岩浆搬运等复杂相互作用的场所。为了限制这些相互作用及其对橄榄石变形的影响，我们分析了堪察加弧南部阿瓦恰火山安山岩夹带的橄榄岩包体的微观结构、晶体优选取向和含水量。这些包体为难熔尖晶石尖刺石，晶粒粗大，亚晶界间距宽，橄榄石中晶界弯曲，与低偏应力（< 13 MPa）下位错蠕变变形一致，并有明显的扩散作用。晶体择优取向（CPO）分析表明，橄榄石中的高温、低应力{0kl}[100]滑移体系和正辉石中的（100）[001]滑移体系起主导作用。在大多数样品中，与线理平行拉长的粗晶石晶体包裹着小橄榄石晶粒，与邻近的晶体形成晶体连续性，表明在高温变形的同时，富硅流体的反应性渗透以橄榄石为代价，导致了正辉石的结晶。间隙正辉石的二次结晶在局部形成了与叶理平行的富opx透镜体，其特征是橄榄石晶粒尺寸减小，橄榄石CPO分散，但未改变主导滑移体系。一半的样品还显示针状的正辉石聚集体，与之相关的是由圆形无应变橄榄石、正辉石、尖晶石和罕见的角闪石晶体组成的细粒基质。这种基体沿晶界普遍存在或形成毫米级的不规则透镜和吻合的细脉，横切粗晶及其韧性变形结构。针状正辉石和细粒基质都被解释为在静态条件下，可能是岩石圈地幔中富氢流体反应性输运的结果。红外分析表明橄榄石含有1-8个中心点，按重量计为6ppm的水。这些低含水量与其他俯冲带的尖晶石橄榄岩相似，可能记录了低压下水在橄榄石中的低溶解度和捕虏体挖掘过程中的脱水。正辉石中的含水量变化很大（25-506 ppm H2O），可能记录了所研究样品中与流体或熔体的空间非均质相互作用和成分不平衡。从多孔渗流到压裂的主要渗流机制的变化表明了冷却作用，这与辉石测温估计的低温（< 800-900℃）相一致。因此，阿瓦查捕虏体记录了软流圈条件下地幔区域的普遍变形，随后它向岩石圈底部吸积，这可能是地幔楔冷却的结果。在各个阶段均记录了富硅流体或含水熔体的渗透；这可能增强了扩散，降低了韧性变形过程中的偏应力，但并未改变橄榄石从[100]到[001]的主要滑移方向。
The mantle above a subducting slab is the site of complex interactions between deformation, partial melting, fluid migration and magma transport. To constrain these interactions and their effects on olivine deformation, we analyze microstructures, crystal preferred orientations, and water contents of peridotite xenoliths entrained by andesites of the Avacha volcano, southern Kamchatka arc. These xenoliths are refractory spinel harzburgites that have coarse-grained microstructures with widely spaced subgrain boundaries and sinuous grain boundaries in olivine, consistent with deformation by dislocation creep under low deviatoric stress (< 13 MPa) and with a significant contribution from diffusional processes. Analysis of crystal preferred orientations (CPO) indicates dominant activation of high-temperature, low-stress {0kl}[100] slip systems in olivine and of(100)[001] in orthopyroxene. In most samples, coarse opx crystals, elongated parallel to the lineation, enclose small olivine grains in crystallographic continuity with neighbouring crystals, indicating crystallization of orthopyroxene at the expense of olivine as a result of reactive percolation of Si-rich fluids coeval with the high-temperature deformation. Secondary crystallization of interstitial orthopyroxene led locally to development of opx-rich lenses parallel to the foliation, characterized by a decrease in olivine grain size and dispersion of the olivine CPO without changing the dominant slip systems. Half of the samples also show acicular orthopyroxene aggregates with which is associated a fine-grained matrix composed of rounded strain-free olivine, orthopyroxene, spinel, and rare amphibole crystals. This matrix occurs pervasively along grain boundaries or forms millimeter-scale irregular lenses and anastomosing veinlets that crosscut the coarse crystals and their ductile deformation structures. Both acicular orthopyroxene and the fine-grained matrix are interpreted as resulting from reactive transport of H2O-rich fluids under static conditions, probably in the lithospheric mantle. Infrared analyses show that olivine contains 1-8 center dot 6 ppm by weight of water. These low water contents are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at low pressure and dehydration during exhumation of the xenoliths. Water contents in orthopyroxene are highly variable (25-506 ppm H2O), probably recording spatially heterogeneous interaction with fluids or melts and compositional disequilibrium in the studied samples. Change in the dominant percolation mechanism from porous flow to fracturing suggests cooling, consistent with the low temperatures estimated from pyroxene thermometry (< 800-900 degrees C). The Avacha xenoliths therefore record pervasive deformation of a region of the mantle under asthenospheric conditions, followed by its accretion to the base of the lithosphere, probably as a result of cooling of the mantle wedge. Percolation of Si-rich fluids or hydrous melts is recorded at all stages; this probably enhanced diffusion and lowered deviatoric stresses during ductile deformation, but did not change the dominant slip direction in olivine from [100] to [001].