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Laser ablation tandem mass spectrometry is a burgeoning field forin situRb‐Sr geochronology. Here, we determined simultaneous isotope ratios of⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr in metamorphic biotite from western Maine, using an ESL™ imageGEO™193 excimer laser ablation system coupled to a Thermo Scientific™ Neoma™ MC‐ICP‐MS/MS. Measurements were made on Faraday cups with Rb⁺at mass 87; Sr isotopes were reacted with SF₆gas and measured as SrF⁺at masses 103–107. Twenty‐two laser spots in biotite from a single sample yield a traditional Rb‐Sr isochron date of 289 ± 6 Ma. Time‐resolved signals reveal significant zoning in⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr within single spot analyses, which were used to construct single spot isochrons. Individual laser spots contain multiple isochronous subpopulations; some spots contain up to three distinct Rb‐Sr isochrons that are decoupled from variations in Rb/Sr. Thirty‐five isochron dates were determined using this sub‐spot approach, with⁸⁷Sr/⁸⁶Sr intercepts that systematically vary with Rb‐Sr date; two‐point isochrons were calculated for individual integrations (n= 780) based on these variable intercepts. Both methods yield age peaks at 303, 270 and 240 Ma. These data suggest that the Rb‐Sr system has the potential to record multiple heating, cooling or fluid‐alteration events spanning ~ 100 My within small domains in single biotite crystals.

A large volcanic sulfate increase observed in ice core records around 1450 C.E. has been attributed in previous studies to a volcanic eruption from the submarine Kuwae caldera in Vanuatu. Both EPMA–WDS (electron microprobe analysis using a wavelength dispersive spectrometer) and SEM–EDS (scanning electron microscopy analysis using an energy dispersive spectrometer) analyses of five microscopic volcanic ash (cryptotephra) particles extracted from the ice interval associated with a rise in sulfate ca. 1458 C.E. in the South Pole ice core (SPICEcore) indicate that the tephra deposits are chemically distinct from those erupted from the Kuwae caldera. Recognizing that the sulfate peak is not associated with the Kuwae volcano, and likely not a large stratospheric tropical eruption, requires revision of the stratospheric sulfate injection mass that is used for parameterization of paleoclimate models. Future work is needed to confirm that a volcanic eruption from Mt. Reclus is one of the possible sources of the 1458 C.E. sulfate anomaly in Antarctic ice cores.

Subduction is a key component of Earth's long‐term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution‐precipitation reactions, and inferring fluid sources. To investigate these processes, we report δ³⁴S values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high‐pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed‐system (re)crystallization from protolith‐inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The δ³⁴S values for metamorphic sulfides tend to reflect their precursor compositions: −4.3 ‰ to +13.5 ‰ for metabasic rocks, and −32.4 ‰ to −11.0 ‰ for metasediments. Metasomatic sulfides exhibit a range of δ³⁴S from −21.7 ‰ to +13.9 ‰. We suggest that sluggish sulfur self‐diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of −11 ‰ to +8 ‰ for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate‐bearing fluids, and an evolving fluid composition during reactive transport may account for the entire ~36 ‰ range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab‐derived fluids.