HELIUM-ISOTOPES AND MANTLE VOLATILES IN LOIHI SEAMOUNT AND HAWAIIAN ISLAND BASALTS AND XENOLITHS
HELIUM-ISOTOPES AND MANTLE VOLATILES IN LOIHI SEAMOUNT AND HAWAIIAN ISLAND BASALTS AND XENOLITHS
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DOI：
10.1016/0012-821x(83)90155-3
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发表时间：
1983-01-01
影响因子：
5.3
通讯作者：
CRAIG, H
中科院分区：
文献类型：
--
作者：
RISON, W;CRAIG, H
我们在本文中检查使用氦同位素比值的热点火山作用的研究沿着年龄递进的岛屿火山链。夏威夷群岛是原始的“高3 He”热点，3 He/4 He比高达32倍大气比;在太平洋，它们与周围的MORB海（相当均匀地为8倍大气比）形成鲜明对比，MORB充满了整个太平洋，除了南太平洋的麦克唐纳-梅赫西亚-萨摩亚轴。最近从美国地质调查局和我们自己的挖掘机上获得的各种碱性和拉斑玄武岩玻璃促使我们首先研究一座可能直接位于地幔柱顶部的新火山的同位素变化。因此，我们已经比较详细地研究了玻璃枕缘中的氦总量、封闭泡中的氦、拉斑玄武岩和碱性熔岩中玻璃本身中的氦，以及相关斑晶和捕虏体中的氦。测得的3 He/4 He比值范围从大气压到30 ×大气压，但我们看到明确的证据表明，高度泡状的熔岩遭受交换之间的薄玻璃壁的囊泡和周围的海水，使我们观察到一个喷发后的玻璃和气相之间的同位素不平衡。主要的影响是在喷发泡状作用期间初始He含量的大量损失，这导致环境He的少量增加产生相当大的同位素效应（每克玄武岩约0.02 μcc He;对应于0.5的“水/岩比”）。橄榄石中的Phenocrystic He证实了气相He不受约5%的囊泡的影响。碱性玄武岩He似乎是独立的泡状高达35%的价值观，这是他有点低的3 He/4 He的比例，但精确匹配相关的捕虏体He。然而，从目前的数据，我们不能排除的可能性，扩散交换与海水影响了He比碱性vesicle.On大规模，沿着10%的夏威夷链可用于陆上采样，我们发现高3 He/4 He比（24 ×大气）在5.5 × 106岁的熔岩在考艾岛。到目前为止，观察到的最大值的比例是在前侵蚀库拉玄武岩毛伊岛，证实了以前的结果Kaneoka和高冈。最初观测到这些高值的夏威夷，现在的范围从莫纳罗亚山的MORB比到基拉韦厄火山的只有15 × RAs。大多数捕虏体他到目前为止测量MORB他，但Loihi捕虏体有很高的价值，在这方面是相当不同的。最后，我们还讨论了氢和碳同位素的结果Loihi熔岩，并表明，这些元素类似MORB，似乎没有显示出独特的羽签名。
We examine in this paper the use of helium isotope ratios for the study of hotspot volcanism along age-progressive island volcanic chains. The Hawaiian Islands are the original “high3He” hotspot, with3He/4He ratios as high as 32 × the atmospheric ratio; in the Pacific they stand out against the surrounding sea of MORB (rather uniformly 8 × atmospheric) which fills the entire Pacific with the exception of the Macdonald-Mehetia-Samoa axis in the South Pacific. The recent availability of a variety of alkalic and tholeiitic glasses from the U.S. Geological Survey and our own dredge hauls has prompted us to look first at isotopic variability within asinglefresh and new volcano which is probably sitting directly atop a mantle plume. Thus we have looked in some detail at the total helium in glass pillow rims, at He in the enclosed vesicles, and at He in the glass itself, in both tholeiitic and alkalic lavas, and also at helium in associated phenocrysts and xenoliths. The measured3He/4He ratios range from atmospheric to 30 × atmospheric, but we see clear evidence that the highly vesiculated lavas suffer exchange of He between the thin glass walls of vesicles and ambient seawater, so that we observe a post-eruptive isotopic disequilibrium between glass and gas phases. The primary effect is the very large loss of initial He content during eruptive vesiculation, which results in quite large isotopic effects from small additions of ambient He (of the order of 0.02 μcc He per gram of basalt; corresponding to a “water/rock ratio” of 0.5). Phenocrystic He in olivines verifies that the gas-phase He is not affected by vesicularities up to about 5%. Alkali basalt He appears to be independent of vesicularity up to values as high as 35%; this He is somewhat lower in3He/4He ratio, but matches precisely the associated xenolithic He. However, from the present data we cannot exclude the possibility that diffusive exchange with seawater has affected the He ratio in alkalic vesicles.On the large scale, along the 10% of the Hawaiian chain available for subaerial sampling, we find high3He/4He ratios (24 × atmospheric) in 5.5 × 106-year-old lavas on Kauai. Maximum values of the ratio so far observed are in the pre-erosional Kula basalts on Maui, confirming the previous results of Kaneoka and Takaoka. Hawaii, where these high values were first observed is now seen to range from MORB ratios at Mauna Loa to only 15 ×RAat Kilauea fumaroles. Most xenolithic He so far measured is MORB He, but Loihi xenoliths have high values and are quite different in this respect. Finally, we discuss also the hydrogen and carbon isotope results on Loihi lavas, and show that these elements resemble MORB and appear not to show a distinctive plume signature.