Collaborative Research: Petrologic and Geochemical Studies of a Layered Peridotite, Horoman, Japan: Implications for Melt Segregation and Migration in Arc-Related...

9406914清水8x10千米霍罗曼橄榄岩(日本北海道)是一种层状上地幔橄榄岩，它包含了熔体分离、迁移和反应过程的记录，这些过程造成了相当大的地球化学不均一性。在与日本同事的合作下，F.Frey(麻省理工学院)、研究生E.Takazawa(麻省理工学院)和N.Shimizu(WHOI)正在确定霍罗曼岩石及其组成单斜辉石的地球化学特征。这些数据被用来确定：(A)主要岩石类型斜长二辉橄榄岩是大洋中脊玄武岩的合适源岩，但(B)相对贫CaO和Al_2O_3的无斜长二辉橄榄岩和方辉橄榄岩与不相容的富元素熔体发生了反应。这种不相容元素的富集，表现为高的轻稀土/重稀土丰度比，通常被称为“地幔交代”。通过研究熔体-橄榄岩反应的三维空间范围(这是研究地幔包体无法达到的目标)，正在确定这一过程的机制和规模。这项研究的结果将确定在弧形和大洋构造背景下形成不均匀上地幔的事件和过程的顺序。

9406914 Shimizu The 8 x 10 Km Horoman Peridotite (Hokkaido, Japan) is a layered upper mantle peridotite that contains a record of melt segregation, migration and reaction processes that have created considerable geochemical heterogeneity. In a collaborative effort with Japanese colleagues, F. Frey (MIT), graduate student E. Takazawa (MIT) and N. Shimizu (WHOI) are determining the geochemical characteristics of Horoman rocks and their constituent clinopyroxene. These data have been used to establish that: (a) the main rock type, plagioclase lherzolite, is a suitable source rock for mid-ocean ridge basalts but that (b) the plagioclase-free lherzolites and harzburgites which are relatively depleted in CaO and Al2O3 have reacted with an incompatible element-rich melt. This enrichment of incompatible elements, expressed by high light REE/heavy REE abundance ratios, is commonly known as "mantle metasomatism". By studying the spatial extent of the melt- peridotite reaction in three dimensions (an objective that cannot be attained by studying mantle xenoliths), the mechanism and scale of this process is being determined. The results of this study will identify the sequences of events and processes that create heterogeneous upper mantle in an arc and oceanic tectonic settings.