斜方辉石筛状反应边的成因机制及其对岩石圈地幔性质转变的意义

The reaction mechanism of sieve-textured orthopyroxene:implications for lithospheric mantle rejuvenation

本文利用电子背散射衍射(EBSD)技术对河北张家口大麻坪汉诺坝玄武岩橄榄岩捕虏体中斜方辉石与玄武岩反应结构的显微构造进行了详细研究。研究结果表明:1其反应边具有复杂的多层筛状结构特征,包括玄武岩层、富橄榄石层以及橄榄石和单斜辉石交生层;o由外至内,橄榄石的M g值逐渐升高,而单斜辉石的M g值变化不大;反应边结构中的单斜辉石与斜方辉石残斑之间存在很好的结晶学拓扑关系,即(100)opx//(100)cpx、(010)opx//(010)cpx、(001)opx//(001)cpx;橄榄石的结晶学取向无序且与斜方辉石和单斜辉石均无关。这个反应过程可以用贫硅熔体先与斜方辉石反应形成单斜辉石,并导致熔体富硅,从而橄榄石从反应熔体中结晶出来合理地解释。相对富硅熔体与橄榄岩的反应,贫硅熔体与橄榄岩的反应过程由于筛状反应边的存在可能更容易进行,筛状结构反应边可以为熔体在橄榄岩中沿颗粒边界运移提供通道,并使橄榄岩主要组成矿物发生细粒化和岩石结构变得松散,可以有效保证熔体上升过程中对橄榄岩的持续侵蚀和破坏,从而可能导致岩石圈地幔性质的迅速转变。

An electronic backscattered diffraction （EBSD） study of the sieve-textured reaction rim between or- thopyroxene and basalt in the Hannuoba mantle xenoliths was conducted in Damaping area, Hebei Province. Some conclusions have been rearched： （1） The reaction rim has a complex multi-layer structure composed of a basalt layer, an olivine-rich layer and an intergrowth layer of olivine and clinopyroxene; （2） The Mg# of the o- livine grains in the reaction rim increases toward the orthopyroxene grain while the Mg# of the clinopyroxene grains in the reaction rim remains stable; （3） There is a well-defined crystallographic topotactic relationship be- tween orthopyroxene and clinopyroxene, i.e., （100）opx//（100）cpx, （010）opx//（010）cpx, （001）opx//（001）cpx; （4） The crystallographic distribution of olivine grains is nearly at random and shows no relationship with orthopy- roxene and clinopyroxene. This sieve-textured reaction rim can be interpreted as the result of a multistage reac- tion process. In the first step, the orthopyroxene reacts with the melt to form clinopyroxene which made the re- action melt more silicic. As the melt gets saturated with silica, olivine precipitates from the reaction melt. Com- paring to the reaction between a silica-saturated melt and a peridotite, the reaction between a silica-tmderstaturated melt and a peridotite is probably easier to proceed due to the formation of sieve-textured reaction rims. The sieve-textured reaction rims provide a feasible channel for the migration of the melt along otherwise closed grain boundaries at high pressure, weaken the surrounding peridotite through the reduction of mineral grain size, and maintain a continuous erosion and destruction of the peridotite during the upward migration of the melt. This process may lead to a quick rejuvenation of lithospheric mantle.