Controls on the Sr and Nd isotopic compositions of hydrothermally altered rocks from the Pindos Ophiolite, Greece

Abstract

The Sr and Nd isotopic behaviour of hydrothermally altered basaltic rocks from the Pindos ophiolite (NW Greece) has been investigated. Samples were obtained mainly from rocks affected by intense hydrothermal alteration, in field areas believed to represent hydrothermal discharge zones formed at a Mesozoic seafloor spreading centre. 87 Sr 86 Sr ratios in the altered rocks show a narrow range of values, intermediate between those of seawater and fresh rock, regardless of the degree of alteration. This has been interpreted as the result of high overall water/rock ratios, which were sufficient to produce a homogeneous overprint throughout most of the hydrothermal cell. A Sr shift index which uses the Sr isotopic composition and Sr concentration of both rock and fluid has been derived, to express the degree of Sr exchange between rock and seawater. It has been used to investigate the variations in water/rock interaction between known hydrothermal systems, and to compare ophiolite-hosted with modern oceanic systems. The results indicate that even when compared on such a uniform basis, all the known present-day hydrothermal systems are generally more rock-dominated than those of Pindos and other ophiolites. 143 Nd 14 Nd ratios in the altered rocks are much less markedly affected, but may show a shift from original magmatic values. The results presented here suggest that this effect cannot simply be attributed to exchange with seawater Nd. These shifts must result from seawater transporting basaltic Nd of a different isotopic composition (from elsewhere in the volcanic pile) into the zones of intense alteration. This study has shown that Sr and Nd isotopes vary independently, as a result of their different original distribution, as well as their contrasting geochemical mobility.