Elemental Mobilizations during Hydrothermal Alteration of Oceanic Lithosphere

Abstract

The hydrothermal circulation of seawater in the oceanic lithosphere is an important factor controlling seawater chemistry, compositions of subducted materials returned to the mantle and microbial activity. We summarize the results of hydrothermally altered rocks taken directly from the ocean floor in terms of major and trace elements combined with petrographic descriptions. Hydrothermal circulation starts at the spreading axis where magmatic heat from a basaltic crustal formation is available（high temperature of ＞ 350℃）. Low-temperature alteration（＜ 150℃）may persist for ＞ a million of years through the ridge flanks. Due to ridge flanks occupying large regions of the seafloor, changes in chemistry, mineralogy and physical properties of the oceanic lithosphere are accompanied by geochemical fluxes that may be even larger than those at the ridge axis. Two deep drill holes, DSDP/ODP 504B and 1256D, allow an examination of downhole variations of hydrothermal alteration in basaltic rocks, and dolerite in the extrusive and sheeted dike sequence. Recent direct sampling from the ocean floor reveals that gabbros and peridotites crop out in significant amounts on the ocean floor, particularly in the slow-spreading ridges. The chemical behavior of these originally deep-seated rocks during hydrother-mal circulation thus has a large impact on global mass budgets for many elements. Previous studies on the ocean floor have been mainly conducted in the Atlantic Ocean and the Pacific Ocean. We present our results on hydrothermally altered basaltic rocks, gabbros and peridotites recovered from the Indian Ocean. Basaltic samples dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction are classified into three types-a fresh lavas, low-temperature altered rocks and high-temperature altered rocks. Petrological and geochemical features of these rocks are basically comparable to those of the basaltic rocks in DSDP/ODP Hole 504B, which suggests generalities in alteration processes and chemical exchange fluxes during hydrothermal activity across all world oceans. Gabbros and peridotites were sampled from an oceanic core complex, which was composed of tectonically exposed foot-＊ 金沢大学フロンティアサイエンス機構 ＊＊ （独）海洋研究開発機構地球内部変動研究センター ＊＊＊ 湘北短期大学情報メディア学科 ＊＊＊＊ 金沢大学大学院自然科学研究科 ＊ Frontier Science Organization, Kanazawa University