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Abstract During the subduction of an oceanic plate, fluids are released from metabasaltic crust, metasediment, and serpentinite under high‐pressure/low‐temperature conditions. Although some fluids may eventually leave the slab, some participate in metamorphic reactions within the slab during subduction and exhumation. To identify fluid sources and other controls influencing mineral composition, we report the in situ‐measured δ¹⁸O of lawsonite and garnet in blueschist‐ to eclogite‐facies rocks from 10 subduction zones that represent various field settings, including mélanges, structurally coherent terranes, and an eclogite xenolith derived from a subducted plate. Lawsonite records distinct δ¹⁸O depending on the host rock type and other rock types that were fluid sources during lawsonite growth. In general, lawsonite in metabasalt (7.6 ± 0.2–14.8 ± 1.1‰) is isotopically lighter than in metasediment (20.6 ± 1.4–24.1 ± 1.3‰) but heavier than in metagabbro (4.0 ± 0.4–7.9 ± 0.3‰). The extent of δ¹⁸O fractionation was evaluated for lawsonite–fluid and lawsonite–garnet pairs as a function of temperature (T). Results demonstrate that variations of >1.7‰ in lawsonite and >0.9‰ in garnet are not related to changingT. More likely, the relative contributions of fluids derived from isotopically heavier lithologies (e.g., sediments) versus lighter lithologies (e.g., ultramafic rocks) are the major control. Monte Carlo simulations were performed to investigate the sources of metasomatic fluids and the water/rock ratio that formed lawsonite‐bearing metasomatite. Results indicate that δ¹⁸O_Lwsand δ¹⁸O_Grtrecord interactions with fluids sourced from diverse lithologies (sediment, serpentinite), further supporting that δ¹⁸O_Lwsis a useful indicator of subduction fluid‐rock interactions. 

Zirconium (Zr) stable isotopes recently emerged as potential tracers of magmatic processes and, as a result, their behavior in high-temperature environments have been the focus of extensive characterization. In contrast, few studies have focused on Zr behavior and isotopic fractionation in low temperature or aqueous environments. Here, we describe a new analytical routine for highly precise and accurate analysis of Zr isotopes of water samples, using a combination of double-spike and iron co-precipitation methods. To assess the impact of potential systematic biases a series of experiments were conducted on natural and synthetic water samples. Our results show that the spike-to-sample ratio, matrix composition, and high field-strength element (HFSE) concentration have negligible effects on measured seawater Zr isotopic compositions, and that the Fe co-precipitation method used yields accurate and precise Zr isotope data. We thus apply this method to natural seawater samples collected from a water column profile in the Pacific Ocean off the coast of California, with depths ranging from 5 to 711 m. We find that the natural seawater samples are highly fractionated relative to solid-Earth values and display marked variability in δ94/90Zr as a function of depth, ranging from ∼ +0.650 ‰ near the surface, to + 1.530 ‰ near the profile bottom, with an analytical uncertainty of ± ∼0.045 ‰ (2 SE, external reproducibility). The δ94/90Zr value of seawater is much higher than that of Earth’s mantle and continental crust, which has a δ94/90Zr value near zero, indicating the presence of processes in the hydrosphere capable of inducing large mass-dependent fractionation. Furthermore, the seawater δ94/90Zr value exhibits systematic variations with respect to water depth and salinity, suggesting that Zr isotopic compositions may be sensitive to seawater chemical properties and source highlighting its potential utility as a tracer of biogeochemical processes within the ocean. 

Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we report δ94/90ZrNIST of whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts have δ94/90ZrNIST values ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites have δ94/90ZrNIST values ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positive δ94/90ZrNIST values, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recent ab initio calculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variable δ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering. 

As the field of zirconium (Zr) stable isotopes is rapidly expanding from the study of mass-independent to that of mass-dependent isotope effects, a variety of Zr standards have appeared in the literature. While several of these standards have been proposed as the ideal isotope reference material (iRM) against which all data should be reported, none of them have been shown to meet the compositional and/or conflict-of-interest-free distribution requirements put forth by the community. To remedy this situation, we report on a community-led effort to develop and calibrate a scale defining iRM for Zr isotopes: NIST RM 8299. Developed in partnership with the National Institute of Standards and Technology (NIST) from the widely used SRM 3169 Zirconium Standard Solution (certified for mass fraction), the candidate RM 8299 was calibrated through an inter-laboratory study involving three laboratories. Our data show that candidate RM 8299 meets all requirements of an ideal iRM. It is an isotopically homogeneous, high-purity reference material, that is free of isotope anomalies, and whose composition is identical to that of a major geological reservoir (Ocean Island Basalts). Furthermore, RM 8299 will be curated and distributed by NIST, a neutral, conflict-of-interest free organization, and was produced in sufficient quantities to last multiple decades. We recommend that all Zr isotope data be reported against RM 8299. Our results also show that SRM 3169 lots #130920 and #071226 have indistinguishable composition compared to candidate RM 8299. Therefore, using RM 8299 as the scale defining iRM will enable direct comparison of all future data with the vast majority of the existing literature data, both for mass-independent and mass-dependent isotope effects. To facilitate conversion of δ94/90Zr values reported against other Zr standards, we provide high-precision conversion factors to the RM 8299 scale obtained using the double-spike method. 

Titanium and Fe isotopic compositions of lavas from a calc-alkaline differentiation suite and corresponding mineral separates from the Rindjani Volcano, Indonesia show that Fe and Ti isotopic fractionations between minerals and melts are lower than those recorded in other suites at all stages of differentiation. The limited isotopic fractionation for Ti is likely due to low-Ti magnetite and clinopyroxene being the dominant carriers of Ti in Rindjani lavas, as these minerals are thought to have limited equilibrium Ti isotopic fractionation relative to silicate magmas. Other magmatic differentiation suites controlled by removal of Ti-rich magnetite and characterized by a lesser role of clinopyroxene have larger Ti isotopic fractionations. This effect is an indirect consequence of the elevated Fe3+/Fe2+ ratio of calc-alkaline magmas such as Rindjani, which promotes Fe3+ incorporation into magnetite at the expense of Fe2+-Ti4+ pairs, such that increased oxygen fugacity will subdue Ti isotopic fractionation in global magmatic series. Similarly, we find negligible Fe isotopic fractionation in Rindjani bulk rocks and mineral separates, unlike previous studies. This is also likely due to the oxidized nature of the Rindjani differentiation suite, which leads to similar Fe3+/Fe2+ ratios in melt and minerals and decreases overall mineral-melt Fe fractionation factors. Paired Ti and Fe isotopic analyses may therefore represent a powerful tool to assess oxygen fugacity during differentiation, independent from Fe 3+ determinations of erupted samples. 

We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ94/90Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution. 

Recent studies of zirconium isotopes in igneous systems have revealed significant mass dependent variability, the origin of which remains intensely debated. While magmatic zircon crystallisation could potentially drive equilibrium isotope fractionation, given that Zr4+ undergoes a shift in coordination as zircon precipitates from a silicic melt, ab initio calculations predict only limited equilibrium fractionation between zircon and melt at magmatic temperatures. To resolve this debate, we determined the isotopic fractionation between co-existing zircon and silicic melt using controlled zircon growth experiments. Our experimental results indicate that zircon has a lower δ94/90Zr relative to co-existing melt by ∼0.045 ‰ at magmatic conditions, which is in excellent agreement with ab initio predictions. Our results imply that, for most natural systems studied to date, the observed variability is predominantly a result of non-equilibrium rather than equilibrium isotope fractionation during zircon crystallisation.