Ren Zhong-Yuan, Ingle Stephanie, Takahashi Eiichi, Hirano Naoto, Hirata Takafumi
Department of Earth and Planetary Sciences, Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku, 152-8551, Japan.
Nature. 2005 Aug 11;436(7052):837-40. doi: 10.1038/nature03907.
The Hawaiian-Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes--even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume.
夏威夷-帝王岛火山链和海山链通常归因于位于太平洋岩石圈下方的一个热地幔柱，该热地幔柱将来自地幔深处的物质输送到地表。夏威夷群岛的盾状火山呈两条曲线状平行分布趋势（称为“莫纳克亚”和“莫纳罗亚”），其岩石具有普遍的地球化学差异。这导致有人提出，夏威夷火山采样自下地幔柱中成分不同、呈同心带状分布的区域。熔体包裹体，即结晶过程中在橄榄石斑晶中“冻结”的局部岩浆样本，可能记录了地幔源区的复杂性，从而能更好地洞察地幔柱的化学结构。在此我们报告，在单个盾状阶段的夏威夷火山的橄榄石寄主熔体包裹体中发现了类似莫纳克亚和莫纳罗亚的主量和微量元素组成——甚至在单个岩石样本中也是如此。我们从这些数据推断，一种地幔源成分可能主导单一熔岩流，但两种地幔源成分在所有熔岩中都始终在一定程度上存在，无论火山的具体地理位置如何。因此我们认为，夏威夷地幔柱不太可能在成分上呈同心带状分布。相反，观测到的化学变化可能受地幔柱热结构控制。
