FLUID MIXING AND ANHYDRITE PRECIPITATION WITHIN THE TAG MOUND

Reworking of sulfide material and zone-refining during lower temperature circulation within sulfide bodies has been identified as an important process in mound accretion at mid-ocean ridge spreading centers. One site, the Trans-Atlantic Geotraverse (TAG), at 26°N on the Mid-Atlantic Ridge has been studied in detail by submersible and by drilling. The importance of se ter entrainment into, and fluid circulation within, the mound has been recognized as a major control on the TAG mound ture and composition. Because anhydrite composition records the seawater-hydrothermal fluid mixing process, the nat consequences of seawater entrainment can be monitored through analysis of anhydrite recovered from the Ocean Drill gram (ODP) sampling. Sr, Ca, Mg, and the Sr-isotopic composition of selected anhy rite samples are used here as geochemic tracers of fluid mixing and evolution during mound circulation. Sr and Mg partitioning into anhydrite is largely controlled crystallographic controls imposed by lattice strain, though high partition coefficients are observed in surface anhydrite s e, which are inferred to be caused by the presence of a fine-grained Mg-bearing talc phase. Comparison of existing white fluid data to black smoker fluid data suggests that the Sr partition coefficient for anhydrite precipitation within the TAG m nd is ≤1, which results in fluid evolution to high Sr/Ca values with ongoing circulation and precipitation. Temperature varia through the mound have little effect on D Sr values, and fluid evolution arising from ongoing seawater entrainment and anh drite precipitation is the dominant control on solid phase geochemistry. These results give new insights into subsurfac circulation by evaluating seawater entrainment into the mound and fluid evolution during circulation. Extensive sea entrainment into the mound, coupled with conductive heating of the mixture, must be occurring to explain the distributio geochemistry of TAG anhydrite.

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