Natural solid-state ion conduction induces metal isotope fractionation

Abstract

Solid-state ion conduction (SSIC) occurs in the formation of natural and synthetic wire silver and causes consistent isotope fractionation of the mobile ion, favoring the heavy isotope. Textural analysis of natural and synthetic samples revealed that wire silver is a mosaic-like polycrystalline aggregate with superimposed striations, consistent with very rapid basal addition of Ag atoms constrained within a lateral growth footprint at the Ag-Ag2S interface. Growth experiments demonstrate that this process is fundamentally dependent not on the chemical environment, but only on the SSIC ability of the substrate, readily provided in this case by argentite (Ag2S), a superionic-conducting material. Stable Ag isotope analysis of wire silvers provides a means to observe the geochemical effects of SSIC in Ag2S. Natural samples were found to be enriched in the heavy isotope with a median (interquartile range) of +0.283‰ (+0.145‰ to +0.453‰). Furthermore, 109Ag enrichment was amplified by an order of magnitude in synthetic samples grown at high temperature (> 450 °C), which had a median δ109Ag of +2.788‰ (+1.829‰ to +3.689‰). Known isotope fractionation mechanisms would indicate that SSIC products should have negative δ isotope values because normal reaction kinetics are more likely to mobilize the lighter isotope. This indicates a previously unrecognized isotope fractionation mechanism associated with SSIC in nature, and has important implications for the geochemistry of ore deposits where SSIC phases are present.