Geochemistry of hydrothermally altered basaltic rocks from the Southwest Indian Ridge near the Rodriguez Triple Junction
Geochemistry of hydrothermally altered basaltic rocks from the Southwest Indian Ridge near the Rodriguez Triple Junction
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DOI：
10.1016/j.margeo.2007.01.003
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发表时间：
2007-04
期刊：
影响因子：
2.9
通讯作者：
Kentaro Nakamura;Y. Kato;K. Tamaki;T. Ishii
中科院分区：
文献类型：
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作者：
Kentaro Nakamura;Y. Kato;K. Tamaki;T. Ishii
为了研究印度洋洋中脊（莫尔）热液蚀变过程中的水-岩相互作用过程，对西南印度洋海岭第一段Rodriguez三联点附近的玄武岩进行了岩相学和地球化学分析。在研究区，我们已经成功地恢复了一系列的上洋壳沿着陡峭的裂谷壁，这是上升和侵位沿着低角度的正断层暴露。在显微镜观察的基础上，将疏浚物样品分为三种类型：新鲜熔岩、低温蚀变岩和高温蚀变岩。新鲜熔岩的化学成分基本上与典型的N-MORB相同，但LILE和Nb分别略有富集和亏损。低温蚀变使K2 O、Rb和U富集，这是由于富K的绿松石和U吸附到Fe-羟基氧化物和粘土矿物上。绿泥石化、钠长石化以及高温热液蚀变作用对贱金属的加入导致MnO、MgO、Na_2 O、Cu和Zn的富集和CaO、K_2 O、Cr、Co、Ni、Rb、Sr和Ba的亏损。此外，铀在高温蚀变岩中的富集可能是由于还原性高温热液降低了铀矿的溶解度。这些岩石学和地球化学特征与DSDP/ODP 504 B孔中火山带到过渡带岩石的岩石学和地球化学特征相当，表明我们的样品是从大洋地壳的上1000 m部分回收的。在扩张轴附近采到的样品中，仅缺少与轴外蚀变有关的蚀变矿物。印度洋样品和太平洋504 B孔岩石之间蚀变的相似性表明，世界各大洋的莫尔热液系统可能是相似的。
Petrographic and geochemical analyses of basaltic rocks dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction have been completed in order to investigate water-rock interaction processes during mid-ocean ridge (MOR) hydrothermal alteration in the Indian Ocean. In the study area, we have successfully recovered a serial section of upper oceanic crust exposed along a steep rift valley wall which was uplifted and emplaced along a low angle normal fault. On the basis of microscopic observation, dredged samples are classified into three types: fresh lavas, low-temperature altered rocks, and high-temperature altered rocks. The fresh lavas have essentially the same chemical composition as typical N-MORB, although LILE and Nb are slightly enriched and depleted, respectively. Low temperature alteration brought about the enrichment of K2O, Rb, and U due to the presence of K-rich celadonite and U-adsorption onto Fe-oxyhydroxide and clay minerals. On the other hand, chloritization, albitization, and addition of base metals by high temperature hydrothermal alteration result in enrichments of MnO, MgO, Na2O, Cu, and Zn and depletions of CaO, K2O, Cr, Co, Ni, Rb, Sr, and Ba. In addition, U-enrichment is also observable in the high temperature altered rocks probably due to the decrease of uranite solubility in the reducing high-temperature hydrothermal solution. These petrological and geochemical features are comparable to those of the volcanic zone to transition zone rocks in the DSDP/ODP Hole 504B, indicating that our samples were recovered from the upper ∼1000 m section of the oceanic crust. Only the alteration minerals related to off-axis alteration are absent in our samples dredged from near the spreading axis. The similarity of alteration between our samples from the Indian Ocean and the Hole 504B rocks from the Pacific Ocean suggests that MOR hydrothermal systems are probably similar across all world oceans.