Geochemistry

Prismatic Filters of Seawater

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Science  06 Sep 2002:
Vol. 297, Issue 5587, pp. 1613
DOI: 10.1126/science.297.5587.1613a

Seawater can be exchanged between the mantle and the ocean through subduction zones. The Nankai trough, offshore of Japan, is where the Philippine Sea plate subducts beneath the Eurasian plate. A prism of sediments has developed in front of the volcanic arc by partial scraping of about 700 meters of sediment from the Philippine Sea plate.

Spivack et al. have assessed how efficiently seawater is transported by measuring the Cl chemistry at three drill holes that transect the accretionary prism of the Nankai trough subduction zone. The Cl abundance and isotopic composition of the fluids vary from the top to the bottom of each drill hole but are the same at the same depth across the trio. The vertical changes reflect Cl isotopic fractionation by low-temperature dehydration reactions near the surface and by high-temperature hydration reactions near the crust-mantle boundary. The lateral similarities indicate episodic and channel-like horizontal fluid flow. The mineral/fluid reactions and the horizontal flow indicate that most of the seawater is efficiently recycled back to the surface. Thus, the observations explain the Cl isotopic fractionation between the ocean and the mantle and fit well with models in which the entire ocean volume is recycled through accretionary prisms in 300 million years. — LR

Geophys. Res. Lett.29, 10.1029/2001GL014122 (2002).

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