Petrology and earth scientific analyses for basalts from the Indian Ridge.

本文研究了印度洋Rodriguez三联点附近新发现的蚀变玄武岩的主、次元素含量，以阐明大洋中脊热液蚀变和低温海底风化过程中这些岩石的元素行为。根据蚀变程度，KH 93 -3巡航10号站的疏浚样品分为三组：相对新鲜的玄武岩、带有棕色次生矿物的蚀变玄武岩和高度蚀变的绿泥石化岩石（玄武岩和粗玄岩）。全岩化学成分表明，新鲜玄武岩是洋中脊玄武岩（MORB），但只有Nb略有亏损。与新鲜玄武岩相比，含有棕色蚀变矿物的蚀变玄武岩富含K、Rb和U，表明该类型玄武岩经历了低温海底风化。另一方面，绿泥石化岩石显示出Mn、Mg、Na、Cu和Zn的富集和Ca、K、Co、Rb、Sr和Ba的亏损。这些地球化学特征很可能是由于热液中添加了贱金属，以及由于海水循环通过洋中脊的洋壳而发生的绿泥石化和钠长石化等热液蚀变。这些蚀变玄武岩的地球化学特征与IPOD/ODP 504 B孔枕状玄武岩带和向席状岩墙带过渡带的地球化学变化具有可比性，并对洋中脊体系和岛弧体系与弧后盆地的岩浆活动及其蚀变过程进行了对比研究。

Major and minor element concentrations of fresh to altered basaltic rocks dredged from near the Rodriguez Triple Junction in the Indian Ocean are studied in order to clarify the elemental behavior of these rocks during the hydrothermal alteration at mid-oceanic ridges and low-temperature seafloor weathering. Dredged samples from station #10 of the KH93-3 Cruise are classified into three groups based on the degree of alteration; relatively fresh basalt, altered basalt with a brown secondary mineral, and highly altered chloritized rocks (basalt and dolerite). Whole-rock chemical compositions indicate that the fresh basalt is mid-oceanic ridge basalt (MORB) although only Nb is slightly depleted. Compared to the fresh basalt, altered basalt containing a brown alteration minerals is enriched in K, Rb, and U, suggesting that this type of basalt is subjected to low-temperature seafloor weathering. On the other hand, chloritized rocks show an enrichment of Mn, Mg, Na, Cu, and Zn and a depletion of Ca, K, Co, Rb, Sr, and Ba. These geochemical signatures are likely to have been attributed to an addition of base metals from hydrothermal solutions and to the hydrothermal alteration such as chloritization and albitization by virtue of seawater cycling through oceanic crust at mid-oceanic ridges. Geochemical features of these altered basaltic rocks can be comparable with the geochemical variations of the pillow basalt zone and the transitional zone toward the sheeted dike zone in the IPOD/ODP Hole 504B.Comparative studies have been also done on the igneous activities and their alteration processes between mid-oceanic ridge systems and island-arc systems with back-arc basins.