Symmetrical Helium isotope distribution on the Cameroon Volcanic Line, West Africa
Symmetrical Helium isotope distribution on the Cameroon Volcanic Line, West Africa
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DOI：
10.1016/j.chemgeo.2003.10.003
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发表时间：
2004-02-16
期刊：
影响因子：
3.9
通讯作者：
Hell, J
中科院分区：
文献类型：
--
作者：
Aka, FT;Nagao, K;Hell, J
我们报告了从西非喀麦隆板内火山线 12 个主要火山中心中的 10 个分离出来的玄武岩和超镁铁质捕虏体全岩石和矿物中镁铁相的 38 个新的 He-3/He-4 比率。样本的喷发年龄从大约 6 Ma 至今。这构成了 1600 公里长的主要碱性火山链的最大氦气数据集。对于像洛伊希和冰岛这样的“高 He-3/He-4”热点地区，He-3/He-4 比率均低于预期，这表明没有一个火山中心直接从为这些热点供应的地幔源中获取物质。 He-3/He-4 比率的分布存在地理控制，从海洋（8.31R(a)，其中 R-a 是大气比率 1.4 x 10(-6)）和大陆（7.85R(a)）末端的类 MORB 值降低到海洋/大陆边界区域的高 mu OIB 类值（4-6R(a)）。这种变化不可能是由于喷发后或岩浆室改造过程造成的，而是被认为是喀麦隆火山线系统的固有特征。之前的工作表明，喀麦隆线的高μ氦特征不能用将回收材料注入地幔来解释。用类似 MORB 的 U-238/He-3 比率分离地幔源存在一个问题，即所需的时间尺度比所需的时间尺度高五倍以上。或者，有人认为，火山线下方的地幔在空间上获得了较高的 (U + Th)/He 比率，这是由于新奥克世晚期（类似于 130-128 Ma 前）原始地幔柱衍生熔体的尖峰或污染。受污染影响最严重的地幔区域（例如海洋/大陆边界带）的 He-3/He-4 比率因放射源 He-4 向内生长而降低，而那些受影响最小的区域（例如现在为更远的火山产生熔体的区域）仍然继承了其最初的 He-3/He-4 特征。这种模型与地质证据一致，也与 He-3/He-4 与亲石元素 (Pb-206、Pb-208/Pb-204) 同位素比率的共同变化一致。结果表明，整个喀麦隆线地幔的 He-3/He-4 比率可能并不像之前认为的那样均匀。 (C) 2003 Elsevier B.V. 保留所有权利。
We report 38 new He-3/He-4 ratios for mafic phases in basalts and ultramafic xenolith whole rock and mineral separates from 10 of the 12 main volcanic centers of the intraplate Cameroon Volcanic Line in West Africa. Samples range in eruption age from similar to 6 Ma to the present. This constitutes yet the largest helium data set for the 1600-km long mainly alkaline volcanic chain. He-3/He-4 ratios are all lower than expected for 'high He-3/He-4' hotspots like Loihi and Iceland, demonstrating that none of the volcanic centers is tapping material directly from a mantle source supplying such hotspots. There is a geographic control on the distribution of He-3/He-4 ratios that decrease from MORB-like values in the oceanic (8.31R(a), where R-a is the atmospheric ratio of 1.4 x 10(-6)) and continental (7.85R(a)) terminals of the line to high-mu OIB-like values (4-6R(a)) in the ocean/continent boundary zone. The variation cannot be due to post-eruptive or magma chamber modification processes and is considered to be an intrinsic characteristic of the Cameroon Volcanic Line system. Previous work has shown that the high-mu helium signature of the Cameroon Line cannot be explained by injection of recycled materials into the mantle. Isolation of a mantle source with MORB-like U-238/He-3 ratios has the problem that necessary time scales are more than five times higher than required. Alternatively, it is suggested that the mantle underlying the volcanic line spatially acquired high (U + Th)/He ratios from,spiking or contamination' by primitive plume-derived melts during the Lower Neocomian (similar to 130-128 Ma ago). Areas of the mantle like the ocean/continent boundary zone that were most affected by the contamination had their He-3/He-4 ratios reduced by radiogenic He-4 in-growth while those that were least affected like the ones now generating melts for the more distant volcanoes still inherit their initial He-3/He-4 characteristics. Such a model is consistent with geologic evidence and also with the co-variation of He-3/He-4 with lithophile element (Pb-206,Pb-208/Pb-204) isotopic ratios. The results suggest that the whole Cameroon Line mantle may not be as homogenous in its He-3/He-4 ratios as was previously thought. (C) 2003 Elsevier B.V. All rights reserved.