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-Ag 2 S 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 (Ag 2 S), a superionic-conducting material. Stable Ag isotope analysis of wire silvers provides a means to observe the geochemical effects of SSIC in Ag 2 S. 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, 109 Ag enrichment was amplified by an order of magnitude in synthetic samples grown at high temperature (>450 °C), which had a median δ 109 Ag 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.

The Isotopic Composition of Silver in Ore Minerals

“…With a precision of ~1–2‰, SIMS results are imprecise compared to those measured with current multicollector inductively coupled plasma mass spectrometric (MC‐ICP‐MS) techniques, and are not discussed further. Since 2000, eight peer‐reviewed articles (Anderson et al, 2019; Argapadmi et al, 2018; Chugaev & Chernyshev, 2012; Desaulty et al, 2011; Fujii & Albarède, 2018; Hauri & Carlson, 2000; Voisey et al, 2019; Woodland et al, 2005) and two extended conference abstracts (Fukuyama & Lee, 2010; Tessalina et al, 2015) present Ag isotopic measurements from ore deposits. This list excludes the data in Mathur et al (2018), as they form part of this same research and they are interpreted here together as a complete package.…”

“…Altogether, these previous studies report details of 126 individual samples, including 36 samples from Au deposits (native Au) and 95 samples of Ag deposits. With the exception of 15 acanthite samples in Anderson et al (2019), no Ag sulfide or sulfosalts or Ag‐halides were analyzed. The combined results of these published studies show a limited total range of δ 109 Ag values, −0.83‰ to 0.66‰.…”

On thermal stability of nanocrystalline Ag–Cu-S powders

“…The length of silver wires reaches about 7 mm. The individual wires seem to be single crystals (pure Ag) in contrary to finding in natural silver wires (Boellinghaus et al 2018), synthetic silver wires above acanthite (Anderson et al 2019) or at silver corrosion (whisker formation) (Chudnovsky et al 2002).…”

Deciphering the origin of small metal artefacts from Castillo de Huarmey (Peru) with Pb, Cu, and Ag isotopes

“…Both Cu and Ag isotope analysis was completed on the Neptune Multicollector ICP-MS at Pennsylvania State University. Instrumentation, data collection, and error analysis for Cu and Ag are described in Mathur, Titley, Hart, et al (2009) and Mathur, Titley, Barra, et al (2009), and in Mathur et al (2018) and Anderson et al (2019), respectively. The techniques are briefly described here.…”