Parman Stephen W, Kurz Mark D, Hart Stanley R, Grove Timothy L
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Building 54, Cambridge, Massachusetts 02139, USA.
Nature. 2005 Oct 20;437(7062):1140-3. doi: 10.1038/nature04215.
High 3He/4He ratios found in ocean island basalts are the main evidence for the existence of an undegassed mantle reservoir. However, models of helium isotope evolution depend critically on the chemical behaviour of helium during mantle melting. It is generally assumed that helium is strongly enriched in mantle melts relative to uranium and thorium, yet estimates of helium partitioning in mantle minerals have produced conflicting results. Here we present experimental measurements of helium solubility in olivine at atmospheric pressure. Natural and synthetic olivines were equilibrated with a 50% helium atmosphere and analysed by crushing in vacuo followed by melting, and yield a minimum olivine-melt partition coefficient of 0.0025 +/- 0.0005 (s.d.) and a maximum of 0.0060 +/- 0.0007 (s.d.). The results indicate that helium might be more compatible than uranium and thorium during mantle melting and that high 3He/4He ratios can be preserved in depleted residues of melting. A depleted source for high 3He/4He ocean island basalts would resolve the apparent discrepancy in the relative helium concentrations of ocean island and mid-ocean-ridge basalts.
在大洋岛玄武岩中发现的高3He/4He比值是存在未脱气地幔储库的主要证据。然而，氦同位素演化模型严重依赖于地幔熔融过程中氦的化学行为。通常认为，相对于铀和钍，氦在地幔熔体中强烈富集，但地幔矿物中氦分配的估计结果却相互矛盾。在此，我们展示了在大气压力下氦在橄榄石中溶解度的实验测量结果。天然和合成橄榄石与50%氦气氛达到平衡，通过在真空中破碎然后熔融进行分析，得到的最小橄榄石 - 熔体分配系数为0.0025±0.0005（标准差），最大为0.0060±0.0007（标准差）。结果表明，在地幔熔融过程中氦可能比铀和钍更具相容性，并且高3He/4He比值可以保存在熔融的亏损残余物中。高3He/4He大洋岛玄武岩的亏损源将解决大洋岛和中洋脊玄武岩相对氦浓度的明显差异。
