Tin stable isotopes in magmatic-affected coal deposits: Insights in the geochemical behavior of tin

Abstract

Tin (Sn) is a redox-sensitive element and geochemically incompatible that accumulates in evolved magmas and associated hydrothermal fluids. Here, a Sn stable isotope approach is used to characterize sources and geochemical behaviors of Sn in magmatic intruded coal deposits by combining with petrographic and elemental analysis. Sn isotope ratios of twelve coal samples with various magmatic-affected degrees show a δ120/118Sn variation from −0.06 to 0.22‰ (2σ of 0.07‰, relative to NIST 3161a). Basing on Sn isotope data and geochemical characteristics of the samples, we suggest binary source mixing and redox state related isotope shift as two mechanisms accounting for the isotope variations. The first mechanism is an admixture of magmatic-hydrothermal fluids, characterized by high δ120/118Sn, into coal deposits with low δ120/118Sn. A binary mixing model is used to estimate Sn contributions of the two end-members. The second process results in a positive isotope shift caused by an oxidation of sample states when they are affected by magmatic intrusions. In fact, magmatic intrusions could trigger the oxidation of Sn, resulting in heavy Sn isotope enrichment in the magmatic affected samples. The change in coal redox-state is supported by a concomitant change of redox proxies, such as V/(V + Ni), Mn/Cr and V/Cr. The enrichment of heavy Sn isotopes in samples impacted by magmatic hydrothermal fluids compared to normally-deposited coals could be explained by the conversion of dominant Sn species from Sn (II)–S to Sn (IV)–O under the influence of oxidative magma, which favors the enrichment of heavier Sn isotopes in Sn (IV)–O due to its shorter bond length. This study shows that Sn isotope ratios are useful tools for investigating Sn geochemical behavior in different Earth reservoirs and during redox sensitive processes.