Crystal settling and crystal growth of olivine in magmatic differentiation - the Murotomisaki Gabbroic Complex, Shikoku, Japan
Crystal settling and crystal growth of olivine in magmatic differentiation - the Murotomisaki Gabbroic Complex, Shikoku, Japan
复制标题
岩浆分异过程中橄榄石的晶体沉降和晶体生长 - 日本四国的室富岬辉长岩杂岩
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发表时间：
2006
期刊：
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通讯作者：
T. Akatsuka
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作者：
T. Hoshide;M. Obata;T. Akatsuka
室富崎辉长岩杂岩位于日本高知县室户角，是一个厚达220 m的岩槛状层状侵入体。岩体内部存在多个富橄榄石带，可能是橄榄石晶体堆积形成的。然而，到目前为止，还不清楚是否所有的橄榄石富集区都是以这种方式形成的。为了澄清这一点，我们重新调查的层状结构，通过收集一个一致的数据集的模态组成，晶体大小，和晶体数密度的橄榄石从整个入侵。结果发现，几乎所有的橄榄石晶体，在晶体数量方面，发生在基底富橄榄石带（侵入体底部的40米以内），和整个侵入体的橄榄石晶体数密度的平均值与橄榄石的晶体数密度的最外部分的下部和上部的冷却边缘相一致。因此，我们得出结论，大多数原生橄榄石斑晶的岩浆沉降重力的影响下，积累形成的基础橄榄石丰富的地带。中层区域（距侵入体底部40-100 m）内橄榄石的晶体数密度远小于初始值，如冷却边缘记录的值所示。证明该带内橄榄石众数的增加不是由于橄榄石晶体的堆积，而是由于橄榄石晶体尺寸的增大，晶体生长。通过这种方式，考虑到整个侵入体中橄榄石的模式、晶体尺寸和晶体数密度，侵入体内的橄榄石富集区可以根据它们的起源被分类为晶体聚集区（AC区）或晶体生长区（GR区）。GR带中橄榄石晶体的生长显然伴随着MgO、FeO和MnO浓度增加到远高于初始（即，冷却的边缘）值。这种富集表明，晶体生长发生在一个化学开放的系统中的意义上说，在GR区内的MgO含量的增加引起的边界层（GR区）和上覆岩浆之间的物质转移。
The Murotomisaki Gabbroic Complex is a sill-like layered intrusion of up to 220 m in thickness and is located at Cape Muroto, Kochi Prefecture, Japan. There are several olivine-rich zones within the intrusion, which may have been formed through accumulation of olivine crystals. However, up to now it has not been clear as to whether all of the olivine-rich zones formed in this way. To clarify this, we reinvestigated the layered structure by collecting a consistent data set of modal composition, crystal size, and crystal number density of olivine from throughout the intrusion. It was found out that nearly all of the olivine crystals, in terms of crystal numbers, occur in the basal olivine-rich zone (within 40 m of the base of the intrusion), and the average value of the crystal number density of olivine throughout the entire intrusion coincides with the crystal number density of olivine in the outermost parts of the lower and upper chilled margins. Hence, we conclude that most primary olivine phenocrysts within the magma settled under the influence of gravity and accumulated to form the basal olivine-rich zone. The crystal number density of olivine within the mid-level zones (40-100 m from the base of the intrusion) is much less than the initial values, as indicated by values recorded in the chilled margins. It is proved that the increase of the olivine mode within this zone is attributed not to the crystal accumulation of olivine but to the increase of the crystal size of olivine, i.e., the crystal growth. In this way, considering the mode, crystal size, and crystal number density of olivine throughout the intrusion, the olivine-rich zones within the intrusion can be classified, in terms of their origin, as either crystal accumulation zone (AC zone) or crystal growth zone (GR zone). The growth of olivine crystals in the GR zone was apparently accompanied by an increase in MgO, FeO, and MnO concentrations to levels well above initial (i.e., the chilled marginal) values. This enrichment suggests that crystal growth occurred within a chemically open system in the sense that the increase in MgO content within the GR zone arose from material transfer between the boundary layer (the GR zone) and the overlying magma.