Iron isotopes in hot springs along the Juan de Fuca Ridge

Abstract

We present Fe isotope data for high temperature hydrothermal fluids, sulfides, and basalts from the Juan de Fuca Ridge, USGS basalt standard BCR-2 and the Canyon Diablo iron meteorite. The Fe isotopic compositions of all samples fall within a range of ∼1‰. However, there is a small but statistically significant isotopic variation between Fe dissolved in hydrothermal fluids and Fe in Canyon Diablo. Specifically, the hydrothermal fluids are lighter compared to Canyon Diablo, yielding δ56Fe (=[(56Fe/54Fe)sample/(56Fe/54Fe)Canyon Diablo−1]×1000) values from −0.30‰ to −0.77‰. In contrast, the BCR-2 basalt and Canyon Diablo are isotopically indistinguishable. Measurements of δ56Fe in Juan de Fuca Ridge basalts and sulfides suggest that Fe isotopes are fractionated in hydrothermal systems. However, the data do not exclude the possibility of small δ56Fe differences between the rocks from which fluid Fe is derived and other igneous Fe, in which case fractionation in igneous systems is indicated. Regardless, these results are evidence of non-biological mass fractionation of Fe isotopes in nature. Further, venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light Fe to the deep oceans, contributing to Fe isotopic variations observed in deep-sea sediments.