Water-rich basalts at mid-ocean-ridge cold spots
Water-rich basalts at mid-ocean-ridge cold spots
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DOI：
10.1038/nature03264
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发表时间：
2005-03-03
期刊：
影响因子：
64.8
通讯作者：
Ottolini, L
中科院分区：
文献类型：
--
作者：
Ligi, M;Bonatti, E;Ottolini, L
尽管水在次大洋上地幔中只以微量存在，但它被认为在影响地幔粘度、熔融和大洋中脊地壳形成方面发挥了重要作用。大洋玄武岩(1，2)中的水浓度一直保持在0.2wt%以下，但热点附近的富水玄武岩除外，它们是由“湿”地幔热柱(3-5)产生的。然而，在这里，我们报告了来自赤道大西洋大洋中脊系统寒冷地区的玄武岩玻璃中异常高的水分含量。这些玄武岩富钠，是由地幔低熔度产生的，含有异常高的轻稀土元素与重稀土元素的比率，这意味着熔融区存在石榴石。我们推断，来自这些热极小区的富水玄武岩来自于地幔深处大于60公里的低度“湿”熔融，并被较浅的“干”熔融产生的熔体轻微稀释--这一观点得到了数值模拟的支持。因此，我们得出结论，大洋玄武岩不仅在热点附近富水，而且在冷点也是富水的。
Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges. The concentration of water in oceanic basalts(1,2) has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by 'wet' mantle plumes(3-5). Here, however, we report unusually high water content in basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from such regions of thermal minima derive from low degrees of 'wet' melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower 'dry' melting - a view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hotspots, but also at 'cold spots'.