Ligi Marco, Bonatti Enrico, Cipriani Anna, Ottolini Luisa
Istituto Scienze Marine, Geologia Marina, CNR, Via Gobetti 101, 40129 Bologna, Italy.
Nature. 2005 Mar 3;434(7029):66-9. doi: 10.1038/nature03264.
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 has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by 'wet' mantle plumes. 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'.
尽管水在洋壳下地幔中仅以痕量存在，但人们认为它在影响地幔粘度、熔融以及大洋中脊地壳的形成方面发挥着重要作用。除了在热点附近采集的、由“湿”地幔柱产生的富水玄武岩外，大洋玄武岩中的水含量一直被观测到低于0.2重量%。然而，在此我们报告了赤道大西洋中洋脊系统一个寒冷区域的玄武玻璃中异常高的水含量。这些玄武岩富含钠，由地幔低度熔融产生，并且含有异常高的轻稀土元素与重稀土元素比率，这意味着熔融区域存在石榴石。我们推断，来自此类热极小值区域的富水玄武岩源自地幔深处超过60千米的低度“湿”熔融，同时受到较浅“干”熔融产生的熔体的少量稀释——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是富水的，在“冷点”也是如此。
