Processes involved in calcite and aragonite precipitation during carbonate chimney formation on Conical Seamount, Mariana Forearc: Evidence from geochemistry and carbon, oxygen, and strontium isotopes

摘要

Samples were collected from two carbonate chimneys on the summit of Conical Seamount, a serpentine diapir in the Mariana Forearc, by JAMSTEC's Shinkai 6500 submersible. The chimneys are composed of calcite, aragonite and an amorphous phase. The calcite and aragonite were precipitated under conditions of carbonate saturation, but the processes are complex in detail. Stained thin sections from samples of both chimneys demonstrate that the aragonite formed during the initial stage of growth, mainly in the presence of seawater with relatively high concentrations of Mg, while the calcite formed later in fractures in the aragonite framework, where rising vent fluids effectively diluted any seawater present, thus reducing the amount of Mg available during carbonate growth. A staining technique was used to differentiate between calcite and aragonite prior to analysis of various isotopes (δ~(13)C, δ~(18)0, Δ~(14)C, and ~(87)Sr/~(86)Sr) in each mineral, which provide clues to the processes involved in their formation. Radiocarbon activities (Δ~(14)C) and strontium isotope ratios (~(87)Sr/~(86)Sr) are significantly different between the aragonite and calcite. These results suggest that the calcite and aragonite were precipitated at different stages in the formation of the chimney. Since the A14C in each mineral is strongly depleted compared with ambient seawater, both calcite and aragonite precipitation must have been affected by vent fluids (cold seepage) that contained "dead carbon" derived from the serpentinization that is taking place under Conical Seamount. These isotopic analyses (Δ~(14)C and ~(87)Sr/~(86)Sr) enabled us to estimate the amount of mixing of ambient seawater and vent fluids during precipitation of the calcite and aragonite. However, the calculated amounts of mixing for aragonite precipitation are not in good agreement, and this is probably due to the low concentrations of Sr in the vent fluid. The low abundance of Sr within the vent fluid suggests that the amount of mixing calculated with Δ~(14)C provides a better indication of the true mixing ratios of seawater to cold seepage during calcite and aragonite precipitation.