Trace elements (REE + Y) reveal marine, subaerial, and hydrothermal controls on an early Archean habitat for life: The 3.48 Ga volcanic-caldera system of the dresser formation, Pilbara Craton

Abstract

Host to some of Earth's earliest evidence of life, the fossiliferous lowermost chert-barite sequence (DFc1) in the ∼3.5 Ga Dresser Formation of the Pilbara Craton, Western Australia is interpreted to have deposited within a dynamic volcanic-hydrothermal caldera. To date, three end-member fluid compositions (marine, hydrothermal, and subaerial) have been identified using trace and rare earth elements. However, this data has been limited to a few, single rock units, providing only snapshots at various depositional time intervals, and does not represent a synthesis of habitat evolution over time. Here, we present a systematic trace and rare earth element study of 31 whole-rock samples from 10 lithofacies across the entire stratigraphic sequence. Samples include bedded cherts (silicified chemical and clastic sedimentary rocks), hot spring geyserite and sinter terracettes, jaspilitic chert, and silicified volcaniclastic siltstone. Four lithostratrigraphically-defined assemblages (A1-A4) in DFc1 illustrate a transition from marine (underlying pillowed North Star Basalts, deep marine cherts) to shallow water and subaerial conditions (evaporitic carbonate-chert couplets, hot spring geyserite and sinter terracettes, channelized conglomerates and basaltic-evolved volcaniclastics)(A1-A2), followed by a return to deep marine conditions (jaspilitic chert, volcaniclastic siltstone and sandstone)(A3-A4). Our trace and rare earth element data enhance the four-stage model (reflecting, A1-A4 respectively) as follows: stages 1–2, crustal inflation generated the emplacement of a deep-rooted magmatic chamber, leading to surface uplift and a transition from deep water to hydrothermally-influenced semi-restricted lakes, stromatolites, and subaerial hot springs. Trace and rare earth element patterns of A2 geyserite indicate the circulation of low-temperature near-surface, non-marine geothermal fluids through basaltic to more evolved volcaniclastic rocks. REE + Y patterns of sinter terracettes may indicate the formation of hot spring travertine in DFc1; stages 3–4, hydrothermally-influenced jaspilites and volcaniclastic sediments deposited in marine basins driven by caldera collapse. This stifled stromatolite growth and represents the end of subaerial exposure. This work contributes to ongoing efforts to reconstruct Earth's early life habitats, which inform the search for life on Mars and origin of life studies.