A convective model of water flow in Mururoa basalts

Abstract

Even long after the end of volcanic activity, the background geothermal flux of Mururoa atoll (French Polynesia) maintains fluid convection. We present evidences that interstitial water is continuously renewed in the carbonate platform, as well as in the volcanic basement. In the carbonate rocks, the presence of a karst system allows convective fluxes high enough for the thermal equilibration of the formation with the ocean around. On the contrary, convection in the volcanic basement is, in most places, too slow to cause a measurable disturbance of temperature profiles. Thermal convection models indicate that the average permeability of the volcanic basement cannot be more than a few 10 mD (10 −14 m 2), implying a residence time of more than 10,000 years. The concentration of Sr in porewaters is used as an indicator of the rock/water ratio and of the residence time of the fluid. Considering the measured permeabilities and the estimated rates of reaction, residence times of more than 1 My, corresponding to average permeabilities of less than 10 −16 m 2, are unlikely in the studied upper kilometer of the volcano. However, the extrapolation of the rates of dissolution for basaltic glass measured in the laboratory to in situ conditions apparently leads to overestimate the rates of reaction. Chemically reactive surface area per volume of fluid is a critical parameter in this extrapolation and its value is dependent on the method used to measure it. Although it may not be the only explanation, the discrepancies can be caused by the presence of clays in conduits for fluid flow and as a replacement product of glass. Comparing our results with studies of Quaternary basalts in Iceland, the 10 Ma alteration history of the Mururoa basalt results in a decrease of the permeability of the aquifers by several orders of magnitude, but does not cause a large change of the chemically reactive surface area.