Jaime D. Barnes

Sulfur (S) and the halogens (F, Cl, Br, I) play key roles in geochemical processes that govern volatile cycling, redox evolution, phase reactions and ore formation in the Earth's crust and mantle. Quantifying these elements in situ requires well‐characterised reference materials for calibration and quality control, yet such materials for silicate matrices remain scarce. In this study, we present new data for a suite of gem‐quality crystals of the feldspathoid mineral afghanite and evaluate their potential as a silicate reference material for halogen and S microanalysis. Afghanite incorporates up to ~ 4.5% m/m Cl and S each within its silicate framework. Our results demonstrate that the investigated afghanite crystals are compositionally homogeneous and yield highly precise measurements with excellent agreement between multiple laboratories and analytical methods. LA‐ICP‐MS measurements calibrated with afghanite determine Cl mass fractions in silicate glass reference materials GSE‐1G and NIST SRM 610 within 5% of compiled literature values without any corrections applied. In contrast, when calibrated with scapolite, Cl mass fractions in afghanite and the same silicate glasses are systematically overestimated. To quantitatively evaluate this bias, we introduce the matrix sensitivity factor (MSF), which demonstrates that matrix effects affecting scapolite result in systematic Cl overestimations of 8–13% in other silicate matrices. Sulfur mass fractions in the scapolite control reference material Sc‐1 determined by LA‐ICP‐MS using afghanite as calibrator are within 5% of results obtained by EPMA and matrix‐matched calibrated LA‐ICP‐MS analyses. We propose afghanite as a robust silicate reference material for in situ determination of Cl and S for geochemical and petrological applications.

Serpentinization and magmatism weaken the lithosphere and facilitate mantle exhumation during magma-poor rifting and ultraslow seafloor spreading. However, the complex interplay and timing among these competing mechanisms along detachment faults remain poorly constrained. The International Ocean Discovery Program (IODP) Expedition 402 drilled an incipient oceanic basin in the Tyrrhenian Sea offshore Italy. Recovered cores consist of one sequence of variably deformed granitic intrusions intercalated with slivers of peridotites and another of primarily serpentinized peridotites with heterogeneous deformation and local granitic intrusions. Geochronological and geochemical data reveal that the granites crystallized at 4 million years ago at a depth of ~7 to 9 kilometers and rapidly exhumed within ~0.5 million years, requiring exhumation rates of ~2 centimeters per year. Structural observations show that these granites accommodated significant postcrystallization strain and enhanced localization of detachment faulting. Stable isotopes record serpentinization temperatures of ~200°C, suggesting that serpentinization occurred after the emplacement and deformation of granites. We conclude that weak felsic rocks may enhance strain localization along detachment faults at intermediate depths and aid continental breakup and mantle exhumation.

In order to examine the progressive chemical evolution of halogens (F, Cl, Br, I) in altered ocean crust (AOC) during prograde subduction, this study compares bulk and in situ halogen concentrations in mafic samples from three petrogenetically related exhumed terrains in the Western Alps (the Chenaillet ophiolite, the Queyras ophiolites of the Schistes Lustrés, and the Monviso ophiolite). Samples from the Chenaillet ophiolite represent oceanic crust unaffected by metamorphic halogen loss and define a protolith halogen content (122 μg/g F, 29 μg/g Cl, 82 ng/g Br, and 98 ng/g I). Samples from the Queyras ophiolites experienced blueschist facies conditions, undergoing recrystallization and halogen loss (74 μg/g F, 19 μg/g Cl, 70 ng/g Br, and 63 ng/g I). Eclogite facies samples from the Monviso meta‐ophiolite exhibit markedly reduced Cl (8 μg/g Cl) and Br (42 ng/g Br) contents relative to samples from Chenaillet and Queyras. Using electron probe microanalysis (EPMA), F and Cl host minerals (e.g., amphibole, chlorite, epidote) are identified and characterized in selected samples, showing a broad distribution of F and Cl, lending support to the view that halogen devolatilization in the subducting slab occurs continuously and is not dependent on the breakdown of a particular phase. In situ Cl concentrations decrease significantly between sub‐greenschist and blueschist assemblages. Fluorine is retained within subducting AOC and is decoupled from the heavy halogens (Cl, Br, I), which undergo continuous devolatilization during prograde metamorphism.

Spring waters from across the Costa Rica margin were analyzed for their Li and He isotope compositions to determine the utility of Li isotopes as a tracer of volatile sources in subduction zones. Li isotope ratios systematically decrease with increasing depth to the subducting slab: averaging +15.0‰ ± 9.2‰ in the outer forearc (80 km to the slab). In contrast, air-corrected ³He/⁴He values (reported relative to the ratio in air, R_A) range from 0.4 to 7.5 R_A and increase from predominantly crustal values near the trench to mantle values in the arc. Together, these data support progressive devolatilization of the subducting plate with slab-derived Li components sourced from shallowly expelled pore fluids in the outer forearc, sedimentary and/or altered oceanic crust contributing to the forearc, and limited slab input beneath the arc.