Various rocks have been recovered near axial discontinuities along mid-ocean ridges, which record a complex evolution of oceanic crust. However, the petrogenesis of these rocks and its relation with axial discontinuities remains hotly debated. Here we report whole-rock elemental compositions, together with MoSrNdPbHf isotopes, and mineral compositions for a suite of rocks from axial discontinuities along the Southwest Indian Ridge (SWIR, 42–53°E). The suite comprises greenstones, basaltic glasses and basaltic andesites. The greenstones include albite+quartz+chlorite+epidote, with olivine+plagioclase assemblages in basalts and basaltic andesites. The suite has a large variation in major element compositions and their trace elemental patterns are between enriched (E-) and normal mid-ocean ridge basalt (N-MORB) endmembers. The studied rocks show an elevated ¹⁴³Nd/¹⁴⁴Nd (0.51313–0.51321) and ¹⁷⁶Hf/¹⁷⁷Hf (0.28332–0.28344), and low ⁸⁷Sr/⁸⁶Sr (0.70301–0.70374) (except for the greenstones that have relatively high ⁸⁷Sr/⁸⁶Sr), and low δ^98/95Mo (−0.76 to 0.11‰) values. The ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf of the basaltic glasses are lower than those of the basaltic andesites and greenstones, although there is no distinct difference in Pb isotopes. Results suggest that the greenstones are generated by the metasomatism of MORB by fluids near hydrothermal vents. The basaltic glasses probably result from low-degree focused melting of the garnet-free pyroxene-poor (harzburgite) mantle source without any contribution from a mantle plume. The basaltic andesites can be generated by a combination of crystal fractionation of a mantle-derived basaltic magma and assimilation of crustal rocks. We speculate that the presence of axial discontinuities along mid-ocean ridges is a key factor in the formation of heterogenous oceanic crust of the SWIR.

沿大洋中脊的轴向断层附近发现了各种岩石，记录了海洋地壳的复杂演化。然而，这些岩石的岩石成因及其与轴向不连续性的关系仍然存在激烈争论。在这里，我们报告了全岩元素组成，以及 Mo Sr Nd Pb沿西南印度洋脊（SWIR，42-53°E）轴向不连续的一组岩石的 Hf 同位素和矿物成分。套房包括绿岩、玄武玻璃和玄武安山岩。绿岩包括钠长石+石英+绿泥石+绿帘石，玄武岩和玄武安山岩中橄榄石+斜长石组合。该套件的主要元素组成差异很大，其微量元素模式介于富集 (E-) 和正常洋中脊玄武岩 (N-MORB) 端元之间。研究的岩石显示¹⁴³ Nd/ ¹⁴⁴ Nd (0.51313–0.51321) 和¹⁷⁶ Hf/ ¹⁷⁷ Hf (0.28332–0.28344) 升高，以及⁸⁷ Sr/ ⁸⁶Sr (0.70301–0.70374)（⁸⁷ Sr/ ⁸⁶ Sr相对较高的绿岩除外），δ ^98/95 Mo (-0.76 至 0.11‰) 值较低。在¹⁴³的Nd / ¹⁴⁴ Nd和¹⁷⁶ HF / ¹⁷⁷玄武岩玻璃的 Hf 低于玄武安山岩和绿岩，但 Pb 同位素没有明显差异。结果表明，绿岩是由热液喷口附近的流体对 MORB 的交代作用产生的。玄武岩玻璃可能来自不含石榴石的贫辉石（方辉石）地幔源的低度聚焦熔化，而没有地幔柱的任何贡献。玄武质安山岩可以通过地幔来源玄武质岩浆的晶体分馏和地壳岩石的同化作用形成。我们推测，沿大洋中脊存在的轴向不连续性是 SWIR 异质洋壳形成的关键因素。