Halogen ratios in crustal fluids through time—Proxies for the emergence of aerobic life?

Abstract Halogens (Cl, Br, I) are exceptional provenance tracers in crustal fluids because their ratios are not strongly altered during most fluid-rock interaction processes. The halogen systematics of metamorphic fluids are of particular interest because such fluids are key drivers of crustal-scale element fluxes and ore formation in orogenic belts, but they remain poorly studied due to analytical challenges. We present novel triple-halogen laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) fluid-inclusion data from metamorphic systems ranging in age from Archean to Phanerozoic. Our results show that the halogen signatures in Phanerozoic metamorphic fluids are controlled by variable degrees of organic-matter interaction in their source rocks, leading to increased I/Cl and decreased Br/I ratios relative to seawater. By contrast, Archean metamorphic fluids from organic matter–rich source rocks have low I/Cl and very high Br/I ratios, distinctly different from any known fluid source signature. We propose that these signatures nevertheless are consistent with organic-matter interaction because dominantly prokaryotic Archean lifeforms did not yet produce iodine-bearing metabolites. This prevented biosequestration and accumulation of iodine-rich organic matter in sediments and imposed halogen signatures onto Archean metamorphic fluids entirely unlike those in younger fluids.