Tracing the Garnet Signature in MORB from Ultra-Slow Spreading Centers: A Lu-Hf Isotopic Study of the Southwest Indian Ridge

在大洋中脊玄武岩(MORB)的形成过程中，石榴石的作用一直是一个悬而未决的重大问题。MORB中的微量元素丰度表明，在大多数大洋中脊，熔融是在石榴石存在的情况下发生的，但如果熔融确实是在石榴石稳定区(深度75公里)内开始的，大多数熔融模型预测的熔体产量远远超过产生7公里厚的典型洋壳所需的熔体产量。这一悖论在超慢扩张海脊(1毫米/年半速率)上问题更大，因为来自这些海脊的熔岩显示出在MORB中发现的最强烈的石榴石特征，但产生的地壳明显比其他大洋中脊产生的地壳薄。我们打算通过对西南印度海脊(SWIR)的MORB进行Lu-Hf同位素研究，来探索该海脊石榴石特征的起源。特别是，我们希望检验石榴石特征可能是由于SWIR之下上地幔中的石榴石辉石岩脉被敲击所致的假设。这些辉石岩脉可能代表了在冈瓦瓦兰德裂解过程中引入西南印度洋地幔的古老的非均一大陆或大洋岩石圈物质，并可能是观测到的大量锶、钕和铅同位素异常的原因。Lu-Hf系统对石榴石存在下熔融的敏感性使该同位素系统成为检验石榴石辉石岩假说的理想选择。

The role of garnet remains a major unresolved question in the generation of mid-ocean ridge basalt (MORB). Trace element abundances in MORB suggest that melting occurs in the presence of garnet at most mid-ocean ridges, but if melting does begin within the garnet stability field (75 km depth) most melting models predict significantly more melt production than is needed to generate the 7 km thickness of typical oceanic crust. This paradox is even more problematic at ultra-slow spreading ridges (1 mm/y half-rate), as lavas from these ridges display the strongest garnet signatures found in MORB, yet the crust produced is significantly thinner than that generated at other mid-ocean ridges. We intend to probe the origin of the garnet signature in the ultra-slow spreading Southwest Indian Ridge (SWIR) by conducting a Lu-Hf isotopic investigation of MORB from this ridge. In particular we wish to test the hypothesis that the garnet signature may be due to the tapping of garnet pyroxenite veins in the upper mantle beneath the SWIR. These pyroxenite veins may represent ancient unhomogenized continental or oceanic lithospheric materials that were introduced into the southwestern Indian Ocean mantle during the breakup of Gondwanaland, and may be responsible for large observed Sr, Nd and Pb isotopic anomalies. The sensitivity of the Lu-Hf system to melting in the presence of garnet makes this isotope system ideal for testing the garnet pyroxenite hypothesis.