Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average La<i>_N</i>/Sm<i>_N </i>= 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.

本研究针对西南印度洋中脊（Southwest Indian Ridge, SWIR）53°E无岩浆段的深海橄榄岩（abyssal peridotites）开展常量与微量元素地球化学研究，旨在厘清熔体抽取过程中基性熔体对地幔橄榄岩（mantle peridotites）的改造机制。研究结果显示，该长约90 km的洋脊段地球化学特征存在显著不均一性，地幔熔融程度介于4%至24%之间。超慢速扩张洋脊中高度亏损型橄榄岩的产出，可通过一期古老熔融事件予以合理解释。53°E段橄榄岩表现出轻稀土元素（light rare earth elements, LREEs）富集特征，其球粒陨石标准化La_N/Sm_N平均值为1.87；且相对于原始地幔（primitive mantle, PM）标准化后，U与Pb呈现显著正异常。轻稀土元素（La、Ce、Pr、Nd）与高场强元素（high field strength elements, HFSEs，如Nb、Zr）之间的正相关关系，表明轻稀土元素的富集源于熔体再富集（melt refertilization）作用，该结论亦得到橄榄岩中普遍发育的岩浆微观结构（magmatic microstructures）的佐证。熔体再富集模型显示，向橄榄岩中添加0.02%~2.7%的玄武质熔体（basaltic melts），即可解释轻稀土元素的富集现象。我们认为，U的正异常大概率归因于流体蚀变（fluid alteration）作用，而Pb的富集则可能同时由熔体再富集与流体蚀变共同导致。53°E段橄榄岩中的熔体再富集作用，系熔体-岩石反应（melt–rock reaction）与捕获于橄榄岩内的熔体结晶共同作用的结果。上述过程的强度或得到双重提升：一是古老熔融作用形成的难熔橄榄岩提升了地幔熔体渗透率；二是53°E洋脊段寒冷且厚密的岩石圈（lithosphere）中熔体抽取效率降低。熔体再富集现象的存在，意味着洋脊地幔中的熔体抽取并不完全，这或许是超慢速扩张洋脊地壳厚度（crustal thickness）极薄且变化极不规则的重要原因之一。