海洋リソスフェアの熱水変質に伴う元素移動

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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°C). Low-temperature alteration (< 150°C) 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 hydrothermal 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 footwalls of detachment faults, from the Central Indian Ridge. Less deformed serpentinized and gabbros were recovered from the ridge-facing slope, whereas highly deformed schist-mylonites of a mixture of these rocks were recovered from the top of the surface (i.e., detachment fault). Efficient localization of strain was probably due to the formation of secondary minerals (e.g., talc, chlorite, serpentine) onto large, discrete shear zones where fluid was introduced locally. In-situ microanalysis of trace elements of the primary minerals and their secondary minerals revealed that selective elements, such as Rb, Sr, Ba, Pb and U, are enriched in the secondary minerals. Although oceanic core complexes are places that allow cross-sectional samplings of deep-seated rocks (i.e., gabbros and peridotites) in the oceanic lithosphere, we should keep in mind the implications of the results for the normal oceanic lithosphere. To understand the nature of the oceanic lithosphere, a close linkage between the ophiolite study and a number of deep holes in the oceanic lithosphere, including a deep hole through the crust-mantle boundary, is required.
2008-02-25

海洋リソスフェアの熱水変質に伴う元素移動

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