Noble gases from oceanic island basalts do not require an undepleted mantle source
Noble gases from oceanic island basalts do not require an undepleted mantle source
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来自海洋岛屿玄武岩的稀有气体不需要未耗尽的地幔源
作者：
D. E. Fisher
基于相对稀土丰度和锶、铅和钕同位素地球化学的几条证据线表明，地幔可能大致分为挥发性成分枯竭的区域，为大洋中脊玄武岩（MORB’s）提供营养，而较少枯竭（“未枯竭”）的区域则为热点地区提供营养，因此也为冰岛和夏威夷等海洋岛屿玄武岩（OIB’s）提供营养。地幔脱气和大气演化的模型是基于在样品中测量的氩和氙同位素比率的差异，这些样品可能捕获了这两个储层中的稀有气体[1 - 7]。然而，最初作为未枯竭地幔样本提出的超基性氩和氙受到空气污染和扩散损失的影响，因此实验结果并不确定。来自夏威夷和冰岛的火山玻璃的重稀有气体新数据与MORB玻璃数据更直接地进行了比较，并且据说显示了未枯竭地幔的同位素模式2,5。然而，正如我在这里报告的那样，经过质量分异效应校正的绝对丰度和元素比表明，相对于假定的未枯竭地幔组分，枯竭地幔组分的部分损失加上大气中Ar和Xe组分的加入为这些重气体的同位素差异提供了更好的理论依据。
Several lines of evidence based on relative rare earth abundances and on strontium, lead and neodymium isotopic geochemistry have led to the suggestion that the Earth's mantle may be divided roughly into a region depleted in volatile components, feeding the mid-ocean ridge basalts (MORB's), and a less depleted (‘undepleted’) region which feeds hotspots and thus oceanic island basalts (OIB's) such as Iceland and Hawaii. Models of mantle degassing and atmospheric evolution have been based on differences in argon and xenon isotope ratios measured in samples which supposedly trap rare gases from these two reservoirs1–7. However, the ultra-mafic Ar and Xe that were first suggested as samples of the undepleted mantle were subject to air contamination and diffusion loss, so that the experimental results are not definitive8. New heavy rare gas data on volcanic glasses from Hawaii and Iceland are more directly comparable with the MORB glass data, and supposedly show the isotope pattern expected from the undepleted mantle2,5. As I report here, however, absolute abundances and elemental ratios corrected for mass fractionation effects indicate that partial loss of the depleted mantle component plus addition of an atmospheric Ar and Xe component provide a better rationale for the isotopic differences in these heavy gases than does the postulated undepleted mantle component.