A Model of Fe Isotope Fractionation Within a High-Temperature Sulfide Structure from the Mothra Field, Juan de Fuca Ridge

Significant variations in δ56Fe and δ65Cu have been documented for seafloor hydrothermal sulfides, and may provide insights to the development of mid-ocean ridge sulfide structures and evolution of hydrothermal fluids. We present δ56Fe and δ65Cu data from concentric mineralogical zones within the wall of "Finn" - a high temperature sulfide structure recovered from the Juan de Fuca Ridge - and a model of Fe isotope fractionation within the chimney wall. The mineralogical zones range from a high temperature, chalcopyrite lined inner wall, to low temperature, oxidized rinds of the outer surface. We analyzed two samples from each of seven mineralogical zones, and samples from two transects (6 and 9 cm) that cross all zones. This suite is the most thorough sampling of a single structure for transition metal isotope analysis to date. δ56Fe values measured in Finn vary from 0‰ to -1.2‰ (relative to the IRMM-014 standard) and show strong negative correlations with some trace element abundances, namely As, Ag, Au and Sb. δ65Cu values range from 0.9‰ to -0.6‰ (relative to NIST-976). The lack of correlation between δ56Fe and δ65Cu suggests they are decoupled in this environment; the mechanism of Cu isotope fractionation remains unclear. Observed trace element patterns and δ56Fe variations in the two transects of Finn are reproduced with a simple equilibrium thermodynamic box model. The model simulates interactions between the sulfide matrix of the chimney wall and the percolating fluid by continuously introducing discrete fluid packets of a single isotopic composition into the walls of the chimney, which are divided into a series of reaction volumes. Each packet of fluid equilibrates with each volume of sulfide matrix according to the partition coefficient or fractionation factor, then the modified fluid packet moves into the adjacent volume. δ56Fe variations of the transects are reproduced using an initial hydrothermal fluid value of - 0.6‰ and fractionation factors from 1.0005 in high temperature regions to 1.0017 in the outer rind. These values are compatible with those of previous studies. A single mechanism in which Fe isotope fractionation occurs as fluids and sulfides interact within chimney walls can produce both 54Fe and trace element enrichments in the outer, cooler part of Finn chimney walls.