Flow in the uppermost mantle during back-arc spreading revealed by Ichinomegata peridotite xenoliths, NE Japan
Flow in the uppermost mantle during back-arc spreading revealed by Ichinomegata peridotite xenoliths, NE Japan
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DOI：
10.1016/j.lithos.2013.10.035
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发表时间：
2014-02
期刊：
影响因子：
3.5
通讯作者：
T. Satsukawa;K. Michibayashi
中科院分区：
文献类型：
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作者：
T. Satsukawa;K. Michibayashi
产于日本东北部一目火山的尖晶石橄榄岩捕虏体具有明显的叶理和线理，前者由富橄榄石层和富辉石层之间的成分分层确定，后者由拉长的尖晶石颗粒确定。橄榄石的晶体学优选取向（CPO）与（010）[100]和{0 kl}[100]上的滑移一致。叶理和橄榄石滑移面之间的角度随着J指数（即CPO强度）值的增加而减小。橄榄岩捕虏体内部的这种复合平面关系可能是上地幔剪切作用的结果，因此剪切应变可以通过叶理与橄榄石滑动面之间的夹角用简单剪切应变（0.31-4.26）来估算。从这些观察，我们认为，一套橄榄岩捕虏体记录了一个罕见的快照上地幔流有关的弧后扩张在日本海的开放。橄榄岩捕虏体具有较高的J-指数（即较高的剪切应变）往往有略低的最低温度，可能定义的垂直应变梯度在最上地幔部分的火山喷发的时候。CPO数据已被用来计算的捕虏体在PT条件下获得的地热测压的地震性质，并从文献中的现场地球物理数据进行了比较。我们的研究结果是一致的，大致东西向最快的P波传播方向在日本弧东北部下的最上地幔。根据三种不同的结构参考框架计算平均样本;平行于1）叶理的水平面，2）包含橄榄石最大浓度的平面[100]，以及3）P波最大方向。由CPO推导出的横波各向异性要求各向异性层的合理厚度（24.1-26.6 km），构造参考架对厚度的估算没有显著影响。因此，Ichinomegata橄榄岩记录了漫长而复杂的构造历史，它们在弧后扩张过程中保持变形“冻结”，这种变形也对现今地幔流动产生影响。
Spinel peridotite xenoliths from the Ichinomegata Volcano (NE Japan) have distinct foliations defined by compositional layering between olivine-rich and pyroxene-rich layers as well as lineations defined by elongated spinel grains. Crystallographic preferred orientations (CPOs) of olivine are consistent with slip on (010)[100] and {0kl}[100]. The angles between the foliation and the olivine slip planes decrease with increasing values of theJ-index (i.e. CPO strength). Such composite planar relationships within the peridotite xenoliths could result from shearing in the uppermost mantle, so that shear strains can be estimated by the angles between the foliation and the olivine slip plane in terms of simple shear strain (0.31–4.26). From these observations, we argue that a suite of the peridotite xenoliths recorded a rare snapshot of uppermost-mantle flow related to back-arc spreading during the opening of the Japan Sea. The peridotite xenoliths with higherJ-indices (i.e. higher shear strain) tend to have slightly lower minimum temperatures, possibly defining a vertical strain gradient in the uppermost mantle section at the time of the volcano's eruption. The CPO data have been used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and are compared to field geophysical data from the literature. Our results are consistent with a roughly EW-oriented fastest P-wave propagation direction in the uppermost mantle beneath the northeast part of the Japan arc. Average samples are calculated based on three different structural reference frames; horizontal plane parallel to 1) foliation, 2) the plane containing the maximum concentration of olivine [100], and 3) P-wave maximum direction. S-wave anisotropy deduced from CPOs requires a reasonable thickness of the anisotropic layer (24.1–26.6 km), and the structural reference frame does not have significant effect on the estimation of thickness. Consequently, Ichinomegata peridotites record a long and complicated tectonic history; they preserve deformation ‘frozen in’ during back-arc spreading and this deformation also has an effect on present-day mantle flow.