Low-temperature alteration of basalt by interstitial seawater, Mururoa, French Polynesia

Chemical and isotopic analysis of interstitial fluids and secondary phases in two drill cores from the volcanic pile of the Mururoa atoll (French Polynesia) permits documentation of the lowtemperature alteration by seawater of basalt and secondary minerals as a function of rock/water ratio (R/W) up to 110 g/L. As R/W increases in this system up to ~70 g/L, the interstitial waters exhibit an almost total removal of Mg and K, a net decrease of pH and Eh, a marked enrichment in Ca, Sr, and Si, and depletion of 18O and D. With further increases in R/W above ~70 g/L, a reversal of Mg-Ca exchange is observed; Mg accumulates in the fluid while Ca is removed. In addition, the enrichment of 18O and D is also observed in the fluids at these high R/W. These chemical and isotopic reversals are shown to result from the transformation of early precipitated Mg-rich saponite into Ca-Na saponite. Chemical and isotopic reactions take place at two separate solid/solution interfaces: 1.(1) an interface at the surface of the fresh basalt that leads to the formation of a hydrated amorphous layer which progressively crystallizes into clay and2.(2) a second interface between the interstitial bulk fluid and the resultant clay layer. Consequently, geochemical codes which fail to account for both aqueous diffusion in altered layers and the existence of multiple reaction interfaces cannot accurately describe alteration at high rock/water ratios.