HIMU EM - THE FRENCH-POLYNESIAN CONNECTION
HIMU EM - THE FRENCH-POLYNESIAN CONNECTION
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DOI：
10.1016/0012-821x(92)90042-t
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发表时间：
1992-05-01
影响因子：
5.3
通讯作者：
VIDAL, P
中科院分区：
文献类型：
--
作者：
CHAUVEL, C;HOFMANN, AW;VIDAL, P
HIMU、EM I和Em II是形成洋岛玄武岩的三种主要地球化学地幔组分[1]。它们代表了在板内火山岩上测量的产生极端同位素组成的端元。在法属波利尼西亚，所有三种地幔成分都以火山岩为代表。在Tubuai、Mangaia和Rurutu发现了特征性的HIMU签名，EM I存在于拉罗汤加和皮特凯恩火山岩的来源中，EM II主导了大多数社会群岛的组成。在大多数岛屿上，三个端元之间的中间值被发现。我们认为，这三个组成部分不是独立的，但在地幔物理相关。HIMU组分被认为是再循环的洋壳，在俯冲之前和俯冲期间通过热液过程失去了部分Pb。EM I和EM II的同位素和微量元素特征是通过与洋壳一起俯冲的沉积物的夹带而获得的，HIMU熔岩的微量元素模式和同位素组成可以用约25%的老的再循环MORB地壳和75%的地幔橄榄岩的混合物来定量模拟。极端的铅成分建模假设铅失去了从洋壳时，在脊的热液蚀变浸出铅从玄武岩再沉积为硫化物的顶部和整个地壳，然后在俯冲带的脱水过程中的硫化物的优先溶解。这些过程导致了俯冲物质的U/Pb比值急剧增加，其演化超过2 Ga至非常放射成因的Pb同位素组成。铅同位素组成类似的EM I和EM II的假设，平均地壳铅同位素组成的沉积物俯冲和回收到地幔与下伏MORB洋壳。远洋沉积物（亩约5和卡帕约6）占铅同位素组成的EM I，而陆源沉积物（亩约10和卡帕约4.5）的EM II端员的发展。在再循环的地壳-沉积物混合物中，百分之几的沉积物将破坏Himu组分特有的Pb同位素特征。这一点，再加上孤立洋壳在地幔中为大于或等于2 Ga的低概率，解释了为什么极端的HIMU组成，如图布艾岛和圣赫勒拿岛，是如此罕见的海洋火山作用采样。
HIMU, EM I and Em II are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteriStiC HIMU signature is found in Tubuai, Mangaia and Rurutu, EM I is present in the source of Rarotonga and Pitcairn volcanics and EM II dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. The HIMU component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction. EM I and EM II are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition of HIMU lavas can be quantitatively modelled using a mixture of approximately 25% old recycled MORB Crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of the U/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those Of Em I and Em II are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlying MORB oceanic crust. Pelagic sediments (mu approximately 5 and kappa approximately 6) account for the Pb isotopic composition Of EM I whereas terrigenous sediments (mu approximately 10 and kappa approximately 4.5) evolve towards the EM II end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of the Himu component. This, together with the low probability of isolating oceanic crust in the mantle for greater-than-or-equal-to 2 Ga, explains why the extreme HIMU composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.