Sr isotope variations in vent fluids from 9°46′-9°54′N East Pacific Rise: evidence of a non-zero-Mg fluid component

Abstract

Sr isotopic variations in vent fluids from six high-temperature hydrothermal vents from between 9°46′N and 9°54′N on the East Pacific Rise are reported. The Mg/Sr- 87Sr/ 86Sr systematics of one vent, Biovent, require the presence of a fluid component that has experienced significant Mg depletion (≈50%), but has undergone very little Sr exchange with local basalts. This fluid component accounts for 3% to 10% of the Biovent samples by volume. Similarities between the composition of the inferred fluid component and that of bore-hole fluids sampled from Hole 504B suggest that this fluid component is modified seawater, partially reacted with basalt at temperatures less than 150°C, and subsequently entrained by high-temperature vent fluids. Previously reported vent fluid data from 12°50′N East Pacific Rise suggest that the presence of the additional lower temperature fluid component admixed in Biovent samples may not be unique among seafloor hydrothermal systems. If such a fluid component is common in seafloor hydrothermal systems it will need to be considered in estimates of axial hydrothermal fluxes. If not accounted for properly, the presence of an additional fluid component can bias calculated end-member 87Sr/ 86Sr ratios. After correction for the influence of the additional fluid component on the Biovent samples, the calculated end-member 87Sr/ 86Sr ratio of this vent is 0.7041. Other nearby vents, P vent and Bio9′, are slightly more radiogenic, 0.7042. A and L vents from the southern portion of the study area have end-member 87Sr/ 86Sr ratios that are less radiogenic, 0.7039 and 0.7037, respectively. The reason for the greater influence of seawater-derived Sr in the more northern vents is not known, but it may be related to entrainment of seawater in the upflow zone.