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 in weathering reactions. To help fill this gap in our knowledge of Li isotope systematics, we investigated Li isotope fractionation at an andesitic catchment in Puerto Rico, where the highest rates of silicate weathering on Earth have been documented. We found the lowest delta Li-7 values published to date for porewater (-27 parts per thousand) and bulk regolith (-38 parts per thousand), representing apparent fractionations relative to parent rock of -31 parts per thousand and -42 parts per thousand, respectively. We also found delta Li-7 values that are lower in the exchangeable fraction than in the bulk regolith or porewater, the opposite than expected from secondary mineral precipitation. We interpret these large isotopic offsets and the unusual relationships between Li pools as resulting from two distinct weathering processes at different depths in the regolith. At the bedrock-regolith transition (9.3-8.5 m depth), secondary mineral precipitation preferentially retains the lighter Li-6 isotope. These minerals then dissolve further up the profile, leaching Li-6 from the bulk solid, with a total variation of about +50 parts per thousand within the profile, attributable primarily to clay dissolution. Importantly, streamwater delta Li-7 (about +35 parts per thousand) is divorced entirely from these regolith weathering processes, instead reflecting deeper weathering reactions (>9.3 m). Our work thus shows that the delta Li-7 of waters draining highly weathered catchments may reflect bedrock mineralogy and hydrology, rather than weathering intensity in the regolith covering the catchment.