Winnie X. Fan

Subduction zones govern Earth’s deep volatile cycle. The slab-mantle interface is a key locus of volatile exchange between the subducting crust and the overlying mantle wedge, yet whether and how chemical reactions at this interface affect arc magmatism remains poorly constrained. Arc magmas typically lack a geochemical hallmark of garnet fractionation, leading to the prevailing interpretation that they derive predominantly from the shallower, garnet-free mantle wedge. Here, we challenge this view using Mg isotopic data from Nicaragua, the South Sandwich Islands, and the central-eastern Aleutian arc, integrated with a global arc magma compilation. We show that heavy Mg isotopic signatures in mafic arc magmas vary with indicators of volatile enrichment, pointing to a volatile-rich source, most likely the heavily metasomatized base of the mantle wedge. The heaviest Mg isotopic compositions occur in arcs underlain by the most steeply dipping subducting slab, suggesting a focusing effect by the geometry of the downgoing slab. Monte Carlo isotope mixing models indicate that up to 50% of the Mg budget in some arc magma sources derives from deep, water-saturated melting near the slab-mantle interface. These findings highlight the base of the mantle wedge as a volumetrically and geochemically significant contributor to arc magmatism and the water cycle.

Rationale: Lithium isotope geochemistry is an important tool in the studies of Earth and planetary materials. In situ Li isotope analyses are typically performed using secondary ion mass spectrometry (SIMS) or laser ablation multicollector inductively coupled plasma mass spectrometry (LA‐MC‐ICPMS), but these instruments are not widely accessible. Here, the capability of laser ablation quadrupole ICPMS for conducting Li isotopic analyses is evaluated. Methods: An array of MPI‐DING and USGS silicate glass reference materials was analyzed repeatedly over the course of 6 months. These materials range from komatiite to rhyolite in terms of silica content (45.5–75.6 wt%) with 9–45 ppm Li. Their Li isotope compositions have been previously characterized so that matrix effects could be tested with these reference materials. Analyses were conducted using an NWR193 laser ablation system coupled to an Agilent 7900 ICPMS system. Results: Analytical precision is primarily limited by Li concentration in the samples. For samples with ~9 ppm Li, the internal precision is 6‰ (2 SD, 150 μm spot diameter), whereas that for a sample with ~45 ppm Li is 4‰ (2 SD, 120 μm spot diameter). The technique is somewhat sensitive to sample matrix: samples with SiO ₂ content that deviates from the bracketing standard display fractionated δ ⁷ Li, necessitating correction using a session‐specific matrix correction curve. Conclusion: Lithium isotope analysis by ns‐LA‐QICPMS is worthwhile for samples with high Li concentrations and when a matrix‐matched standard can be obtained. Although the precision of this method is not as high as those achievable with SIMS and LA‐MC‐ICPMS, it remains adequate for resolving large isotope fractionations found in natural and laboratory settings.