Noble gas systematics of submarine alkalic lavas near the Hawaiian hotspot
Noble gas systematics of submarine alkalic lavas near the Hawaiian hotspot
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DOI：
10.1016/j.chemgeo.2004.08.051
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发表时间：
2005-01
期刊：
影响因子：
3.9
通讯作者：
T. Hanyu;D. Clague;I. Kaneoka;T. Dunai;G. Davies
中科院分区：
文献类型：
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作者：
T. Hanyu;D. Clague;I. Kaneoka;T. Dunai;G. Davies
对分布在夏威夷群岛周围的海底碱性火山岩进行了稀有气体同位素比率测定，以限制这种碱性火山作用的起源，从而了解夏威夷地下地幔上升的细节。样品是通过疏浚或使用潜水器从考艾岛和考艾岛之间的考艾河海峡、莫洛凯岛以北、尼豪岛西北部、瓦胡岛西南部、南拱门和北拱门火山田采集的。位于热点中心下游的站点的3He/4He比值接近MORB，约为8Ra(Ra；大气比)。北拱样品在21Ne/22Ne-20Ne/22Ne图中具有位于MORB阵列上的Ne同位素比值。稀有气体同位素证据表明，在这些地点喷发的岩浆具有最小的地幔热柱挥发物贡献。相比之下，南拱门位于夏威夷拱门热点的上游，其3He/4He比值在17-21Ra之间，表明有很强的羽流影响。热区下游和上游碱性火山活动稀有气体同位素特征的差异表明，上游火山活动含有上升地幔热柱的初始熔体，具有3He/4He原生元素。结合亲石元素同位素数据，我们得出结论：上游岩浆作用最有可能的来源是亏损的软流圈地幔，它已被地幔热柱的初始熔体交代。在为屏蔽阶段生产岩浆的过程中，从地幔热柱中主要熔体提取后，热柱物质在稀有气体中高度贫化，在亲石元素中中度贫化。来自挥发性贫化热柱来源的熔体浸染的贫化地幔部分熔融，可以解释下游碱性岩浆作用的同位素特征。西南部瓦胡岛火山岩的3He/4He比值中等，约为10Ra，这表明与下游碱性火山作用相反，一些熔体是从地幔热柱输送到火山源区的。
Noble gas isotopic ratios were determined for submarine alkalic volcanic rocks distributed around the Hawaiian islands to constrain the origin of such alkalic volcanism and hence understand the details of mantle upwelling beneath Hawaii. Samples were collected by dredging or using submersibles from the Kauai Channel between Oahu and Kauai, north of Molokai, northwest of Niihau, Southwest Oahu, South Arch, and North Arch volcanic fields. Sites located downstream from the center of the hotspot have3He/4He ratios close to MORB at about 8 Ra (Ra; atmospheric ratio). North Arch samples have neon isotope ratios that lie on the MORB array in a21Ne/22Ne–20Ne/22Ne diagram. The noble gas isotope evidence demonstrates that the magmas erupted at these sites had minimum contribution of volatiles from a mantle plume. In contrast, South Arch located upstream of the hotspot on the Hawaiian Arch has3He/4He ratios between 17 and 21 Ra, indicating a strong plume influence. Differences in noble gas isotopic characteristics between alkalic volcanism downstream and upstream of the hotspot imply that upstream volcanism contains incipient melts from an upwelling mantle plume, having primitive3He/4He. In combination with lithophile element isotopic data, we conclude that the most likely source of the upstream magmatism is depleted asthenospheric mantle that has been metasomatised by incipient melt from a mantle plume. After major melt extraction from the mantle plume during production of magmas for the shield stage, the plume material is highly depleted in noble gases and moderately depleted in lithophile elements. Partial melting of the depleted mantle impregnated by melts derived from this volatile depleted plume source may explain the isotopic characteristics of the downstream alkalic magmatism. Lavas from the Southwest Oahu volcanic field have intermediate3He/4He ratios of about 10 Ra, which suggests that some melt was fed to the source region of the volcano from a mantle plume, in contrast with downstream side alkalic volcanism.