Abiotic oxidation of pyrite by Fe(III) in acidic media and its implications for sulfur isotope measurements of lattice-bound sulfate in sediments

Abstract

We have evaluated the relative importance of Fe sup(3+) to dissolved oxygen in pyrite oxidation, and its implications for measuring an accurate sulfur isotope composition (delta Sup(34) S) in trace sulfates extracted from sedimentary rocks. Results from our pure pyrite oxidation experiments show that in a solution of acidic pH, the amount of sulfate formed is very similar under both oxygenated and oxygen-free experimental conditions which is suggestive of the dominant role of Fe sup(3+) in pyrite oxidation compared to dissolved oxygen. One implication of this study is the influence of artifact sulfate on determination of sulfur isotopic composition of lattice-bound sulfate in phosphorite and carbonate sediments. Delta Sup(34) S values of trace sulfates in some lesser Himalayan phosphorites studied in this work show no significant influence of dissolved oxygen during sample dissolution. We have attempted to explore the possible pathways, which can cause depleted sulfur isotopic composition of trace sulfate in some of our phosphorite samples accompanied by relatively enriched Fe sup(3+) concentrations. Although artifact sulfate generation by pyrite oxidation during sample dissolution seems to be a probable mechanism, however several diagenetic pathways can also result in sup(34) S depleted lattice sulfate accompanied by Fe sup(3+) enrichment. The degree of influence of laboratory artifact sulfate generation on determination of delta sup(34) S of trace sulfates is governed by combination of several factors like the amount of pyrite, concentration of ferric iron, concentration of lattice-bound sulfate etc. We propose mass balance calculations to derive the delta sup(34) S value of the uncontaminated trace sulfate.