Systematic distribution of incompatible elements in mantle peridotite: importance of intra- and inter-granular melt-like components
Systematic distribution of incompatible elements in mantle peridotite: importance of intra- and inter-granular melt-like components
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地幔橄榄岩中不相容元素的系统分布：粒内和粒间熔体状成分的重要性
DOI：
10.1007/s00410-009-0375-8
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发表时间：
2009
影响因子：
3.5
通讯作者：
D. Kohlstedt
中科院分区：
文献类型：
--
作者：
T. Hiraga;D. Kohlstedt
本文根据全球5个地区20个地幔包体的数据，研究了地幔中不相容元素的分布。结合元素划分模型的结构模型构成了我们分析的基础。前者将大块橄榄岩分离成矿物晶格、界面（颗粒和相间边界）以及作为不相容元素位点的粒内和粒间包裹体。后者基于晶格应变理论将元素在这些位置之间的分布联系起来。通过将粒内和粒间包裹体视为熔体状相，组合模型成功地再现了矿物、干净岩石（由矿物组合物和矿物模式重构）和整个岩石中不相容元素的相对浓度。结果表明：（1）橄榄石、斜方辉石和单斜辉石晶格中的不相容元素达到化学平衡; (2)橄榄石、斜方辉石和单斜辉石的颗粒含有相似数量的颗粒内熔体状成分，可能是亚微米包裹体的形式，重量（体积）分数在5 × 10− 5和1 × 10−2之间。(3)所有的岩石都含有一种类似于颗粒间熔体的成分，其含量在10− 4到10−2之间，远高于预期的沿着界面储存的量。(4)粒间组分和粒内组分的分数呈正相关，表明它们来自同一过程。(5)颗粒间和颗粒内的熔融状相与其他结构组分化学平衡。根据地幔捕虏体的上涌速率，熔体状成分不太可能在上升过程中形成。相反，它的无处不在的外观，它的不可见的光学显微镜，其吸收的不相容的元素，即使在亚固相线条件下，以类似于熔融相的方式，都可能被解释为存在的无定形二氧化硅沉淀物，如以前观察到的自然发生和实验退火地幔复合材料。从矿物模式，粒度，矿物和全岩浓度的不相容元素，我们可以归因于捕虏体的化学签名，以实现化学平衡在地幔条件下，而不是一些不平衡（交代）的后果已经做了以前。虽然它应该通过额外的分析研究进行测试，我们的模型将有可能确定岩石是否在不相容元素的分布方面是化学平衡的，或者是否需要交代（不平衡）事件。
In this paper, we examine the distribution of incompatible elements in Earth’s mantle based on data reported for 20 mantle xenoliths collected from 5 localities worldwide. A structural model combined with an element partitioning model forms the basis for our analyses. The former separates a bulk peridotite into mineral crystal lattices, interfaces (grain and interphase boundaries), and intra- and inter-granular inclusions as sites for incompatible elements. The latter relates the distribution of elements among these sites based on lattice strain theory. By treating both intra- and inter-granular inclusions as a melt-like phase, the combined models successfully reproduce the relative concentrations of incompatible elements among minerals, clean rock (reconstituted from mineral compositions and mineral mode), and whole rock. The analyses reveal common signatures in the rocks: (1) incompatible elements in the crystal lattices of olivine, orthopyroxene and clinopyroxene achieved chemical equilibrium. (2) Olivine, orthopyroxene and clinopyroxene grains contain similar amounts of an intra-granular, melt-like component possibly in the form of sub-micron inclusions with weight (≈volume) fractions between 5 × 10−5and 1 × 10−2. (3) All rocks contain an inter-granular melt-like component with a fraction between 10−4and 10−2, well above the amount expected to be stored along interfaces. (4) Fractions of the inter- and intra-granular components are positively correlated, indicating that they were originated from the same process. (5) The inter- and intra-granular melt-like phases are chemically equilibrated with other structural components. Based on plausible upwelling rates for mantle xenoliths, it is unlikely that the melt-like component formed during ascent. Instead, its ubiquitous appearance, its invisibility to optical microscopy, and its absorption of the incompatible elements in a manner similar to a melt phase even at sub-solidus condition, all might be explained by the presence of amorphous silica precipitates such as those observed previously in naturally occurring and experimentally annealed mantle composites. From the mineral mode, grain size, and mineral plus whole rock concentrations of incompatible elements, we can ascribe the chemical signatures of xenoliths to achievement of chemical equilibrium at mantle conditions rather than to a consequence of some disequilibrium (metasomatic) effect as has been done previously. Although it should be tested by additional analytical studies, our model will make it possible to determine whether or not a rock is chemically equilibrated in terms of the distribution of incompatible elements or if a metasomatic (disequilibrium) event is required.
影响因子：
3.7
作者：
E. Takazawa;Toshie Okayasu;K. Satoh
通讯作者：
E. Takazawa;Toshie Okayasu;K. Satoh