Deep biospheric platinum
Platinum long ago gained precious-metal status, being used in jewellery for millennia and in industrial applications for more than half a century. Nuggets of platinum are greatly prized. Dense, shiny and globular, such nuggets are found in placer deposits across the world, but how do they form? Most past analyses agree that these nuggets derive from the weathering of mafic intrusive rocks. However, does this mechanism predominantly involve physical processes and the abrasion of initial (bedrock) agglomerations of platinum; or do chemical processes shape these grains, with platinum enrichment occurring during weathering, and possibly aided by biogenic reactions? Intriguingly, the answer may come from another element: selenium.
Platinum nuggets contain trace amounts of selenium, an essential element for organisms. Microbes preferentially incorporate 76Se relative to 82Se, so by analysing the selenium isotopes of a platinum nugget, it is possible to infer microbial processes and thus discriminate between an abiogenic or biogenic origin for the deposits.
Alexandre Raphael Cabral at the Universidade Federal de Minas Gerais, Brazil, and Stephan König and Benjamin Eickmann, then at the Universität Tübingen, Germany, and colleagues measure the selenium isotopes in platinum nuggets from a placer deposit in southeast Brazil. They find that the platinum nuggets, which are dated to around 180 million years old, are extremely depleted in 82Se – in fact, the authors report the “lightest” 82Se/76Se ratios ever recorded in geological samples. The researchers argue that these extreme ratios must reflect formation via biochemical processes, and specifically that the concentrated platinum grains were precipitated by anaerobic microbes circulating within fractured rocks hundreds of metres beneath Earth’s surface.
The results imply that bacteria could have been thriving in deep-seated groundwater during the Jurassic breakup of West Gondwana.
• Geology 49, 1327–1331 (2021); doi.org/10.1130/G49088.1
Prof Gerald (Jerry) Dickens, Head of the Geology Department at Trinity College Dublin and Chief Editor of Geology