226Ra and 222Rn contents of Galapagos Rift hydrothermal waters —the importance of low-temperature interactions with crustal rocks

Abstract

Hydrothermal waters collected by “Alvin” from the Galapagos Spreading Center are enriched in 222Rn by factors of 50–200 over bottom waters. The 226Ra in the same samples, however, is enriched by less than a factor of four over bottom waters. Enrichments of 222Rn result primarily from α-recoil from rock surfaces while 226Ra enrichments are dominantly produced by high-temperature alteration of cooling ridge volcanics. The abundances of both nuclides exhibit positive correlations with temperature. The data extrapolate to bottom water temperatures and compositions, demonstrating the importance of seawater mixing. Different vents, however, have different mixing lines, and vents with high 222Rn have low 226Ra. We propose these patterns result from variations in the extent of low-temperature crustal interaction with the hydrothermal fluids. Low-temperature crustal waters can maintain high steady state 222Rn contents due to the α-recoil additions to the fluids. The 226Ra, however, is strongly adsorbed at low-temperatures resulting in low concentrations of this nuclide in low-temperature crustal waters. Thus, physical mixing of a crustal water component with hydrothermal waters or variable crustal path lengths of the hydrothermal fluids can account for the variable mixing lines and 222Rn/ 226Ra values of the hot springs. The 222Rn/ 226Ra value appears to be a sensitive indicator of low-temperature crustal interaction. Values > 100 have experienced extensive crustal interaction and are indicative of diffuse hydrothermal flow. Values between 1 and 10 are indicative of primary hydrothermal fluids which have not experienced significant interaction with the crust. Values of 222Rn/ 226Ra between 10 3 and 10 4 are indicative of interaction of the hydrothermal fluids with sediments. Such values are observed in water samples from the Galapagos hydrothermal mounds.