Distribution et comportement du bore dans la croûte océanique

Abstract

The behaviour of boron involved in alteration of oceanic crust by sea water was investigated by means of prompt gamma neutron activation analysis of basalt and serpentinite samples from in situ oceanic crust and ophiolites. The samples were grouped in the following categories: fresh basalts, low-temperature altered basalts, high-temperature altered basalts, and serpentinites.The average B content in fresh basalts is 2.2 ± 0.6 ppm. The low-temperature altered basalts and the serpentinites are 2–24 times richer in B than the fresh basalts. The alteration of the crust at low temperature and its serpentinization imply B extraction from sea water. The rate of B removal for this specific low-temperature alteration of basalts is estimated to be 10.5 × 1010 g/year, on the basis of the development of altered zones in the crust and of the B contents of these zones before and after their alteration.The high-temperature altered basalts having a low water/rock ratio show an average B content (2.3 ± 0.7 ppm) similar to that of fresh basalts, indicating that the alteration of the crust at high temperature has not affected the B content of those rocks under conditions of limited fluid access to the system. However, if high-temperature alteration is associated with high water/rock ratios, a solubilization of B in the hydrothermal fluids migrating through the crustal zones is to be expected. Our data and previously published results cannot provide a reliable estimate of B flows related to high-temperature alteration of the crust and to serpentinization, significantly constraining the establishment of the oceanic B budget. [Journal Translation]