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

印度洋脊玄武岩的岩石学和地球科学分析。

基本信息

批准号：
10640460
负责人：
ISHII Teruaki
金额：
$ 2.11万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1998
资助国家：
日本
起止时间：
1998 至 1999
项目状态：
已结题

项目摘要

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.

本文研究了印度洋罗德里格斯三合点附近新鲜和蚀变玄武岩的主、次元素含量，以阐明这些岩石在洋中脊热液蚀变和海底低温风化过程中的元素行为。根据改造程度将KH93-3号邮轮10号站的疏浚样本分为三组；相对新鲜的玄武岩、蚀变玄武岩（含棕色次生矿物）和高度蚀变的绿岩化岩石（玄武岩和玄武岩）。全岩化学成分表明新鲜玄武岩为洋中脊玄武岩（MORB），但只有微量的Nb亏缺。与新鲜玄武岩相比，蚀变玄武岩中含有一种褐色蚀变矿物，富含K、Rb和U，表明该类型玄武岩受海底低温风化作用。另一方面，氯化岩中Mn、Mg、Na、Cu、Zn富集，Ca、K、Co、Rb、Sr、Ba富集。这些地球化学特征可能是由于热液溶液中添加了贱金属，以及由于海水在洋中脊通过海洋地壳循环而产生的绿泥石化和钠长石化等热液蚀变所致。这些蚀变玄武岩的地球化学特征可与IPOD/ODP 504B孔枕状玄武岩带和向片状脉带过渡带的地球化学变化相比较。对比研究了洋中脊体系与弧后盆地岛弧体系的火成岩活动及其蚀变过程。