Probing redox states in the ancient and modern crust and possible biosphere-lithosphere interactions

Abstract:

The oxidation states of modern-day terrestrial environments are broadly constrained, though we are at the earliest stages of directly quantifying redox states during the first 700 million years. Redox states are constrainable through a combination of high temperature laboratory experiments, analyses of detrital Hadean zircon, and younger well-studied zircons and their host rocks. The redox state of the solid earth and the biosphere are believed to be interwoven: how might we exploit this to probe for the existence of possible biosphere-lithosphere interactions on the early Earth? Some insight comes from the investigation of "modern-day" zircon-bearing rocks and the new application of techniques that allow us to directly probe element valence as a proxy for magma redox state (XANES). Other insights are possible through the study of young ~400 Ma (S)edimentary- and (I)gneous-type granitoids from Lachlan Fold Belt (LFB), where the magma chemical properties in the former may be influenced by the assimilation of sedimentary material containing organic matter. We observe that zircons from LFB S-type granitoids formed under more reducing conditions when compared to LFB zircon formed in I-type granitoids. This observation, while reflecting 9 granitoids and 289 analyses of zircons where over 400 different plutons have been identified, is consistent with the incorporation of (reduced) organic matter in the former and highlights one possible manner in which life may influence the composition of igneous minerals. The chemical properties of rocks or igneous minerals such as zircon may extend the search for ancient biological activity to the earliest period of known igneous activity, which dates back to ~4.4 billion years ago. If organic matter was incorporated into Hadean sediments that were then buried and melted, then these biological remnants could imprint a chemical signature within the subsequent melt and the resulting crystal assemblage, including zircon.