Secondary Minerals Drive Extreme Lithium Isotope Fractionation During Tropical Weathering

Lithium isotopes are used to trace weathering intensity, but little is known about the processes that fractionate them in highly weathered settings, where secondary minerals play a dominant role reactions. To help fill this gap our knowledge of Li isotope systematics, we investigated fractionation at an andesitic catchment Puerto Rico, highest rates silicate on Earth have been documented. We found lowest δ7Li values published date for porewater (−27‰) and bulk regolith (−38‰), representing apparent fractionations relative parent rock −31‰ −42‰, respectively. also lower exchangeable fraction than or porewater, opposite expected from mineral precipitation. interpret these large isotopic offsets unusual relationships between pools as resulting two distinct different depths regolith. At bedrock-regolith transition (9.3–8.5 m depth), precipitation preferentially retains lighter 6Li isotope. These then dissolve further up profile, leaching solid, with total variation +50‰ within attributable primarily clay dissolution. Importantly, streamwater (about +35‰) divorced entirely processes, instead reflecting deeper reactions (>9.3 m). Our work thus shows waters draining catchments may reflect bedrock mineralogy hydrology, rather intensity covering catchment.