Isotopic signatures from the weathering of ophiolitic massifs and volcanic deposits in the Zambales region, Philippines.

Lithium isotopes (d⁷Li) are a useful proxy to track silicate weathering, the fundamental process in which carbon is removed from Earth’s surface. Here we present d⁷Li and elemental data from 14 riverine localities in the Zambales region, Philippines. The warm, humid climate coupled with monolithic rivers draining ophiolitic massifs and volcanic deposits (from the major 1991 Pinatubo eruption) allows for the comparison of silicate weathering and riverine geochemistry across different lithologies. The most striking part of our dataset is that all rivers draining ophiolitic terranes have heavier d⁷Li_diss values (range from 22.8 to 37.1‰) than those draining Pinatubo volcanic deposits (range from 8.9 to 18.4‰). As all rivers feature similar topographic relief and hydrological conditions, this suggests a strong lithological influence on d⁷Li_diss values despite both bedrock lithologies being highly weatherable. We postulate that the mafic and ultramafic composition of the ophiolite terrain significantly enhances incongruent weathering and clay mineral formation, increasing Li fractionation, and leading to the heavier d⁷Li_diss values. Conversely, the lighter d⁷Li_diss values for the Pinatubo rivers could be explained by the more felsic composition and unconsolidated nature of the volcanic deposits leading to increased congruent weathering and low clay formation, and thus low Li fractionation. Notable differences in major element concentrations are also observed. The Mg²⁺ is the dominant cation in ophiolitic-draining rivers reflecting the weathering of Mg-rich mafic and ultramafic minerals. Meanwhile, Na⁺ and Ca²⁺ dominate in rivers flowing off the Pinatubo volcanic deposits. The ophiolitic-draining rivers also have total major cation concentrations ([Na⁺] + [Mg²⁺] + [K⁺] + [Ca²⁺]) almost 3.5 times lower than those draining the volcanic deposits (~1600 vs ~5600 µM). Despite the composition of the ophiolites consisting of highly weatherable mafic and ultramafic minerals, the volcanic deposits are seemingly even more favorably weathered. This is interpreted to be caused by the freshness of the material deposited from the recent eruption. Overall, our study shows that despite all rivers in this study draining highly weatherable lithologies, their intrinsic lithological differences can lead to significantly contrasting d⁷Li_diss and major cation signatures. New clay mineralogical data from riverine sediments will further inform us on silicate weathering processes in the region.