Description: Oxygen isotope ratios were measured in zircon by laser fluorination and by SIMS from in over 50 silicic (dacite–rhyolite) volcanic rocks of Triassic to Cretaceous (230-98 Ma) age from the Sierra Nevada batholith, White-Inyo Mountains, and Mojave Desert. These data give broad geographic and temporal context of volcanism in the Mesozoic California arcs system that was previously unobtainable because of secondary hydrothermal exchange that routinely alters original magmatic δ18O values in these rocks (c.f., Sorensen et al. GSAB 1998). SIMS analysis of δ18O using 10 µm spot size, augmented by U-Pb dating by LA-ICP-MS, further allows appraisal of variability within zircon grains and grain populations as potential evidence of assimilation or magma mixing within caldera systems, including mixing during eruptions. Values of δ18O(zircon) in most rocks studied are 5.0–7.5‰, and overlap with values of δ18O in plutonic rocks (6.80±1.85‰, S.D., Lackey et al. 2005,2006,2008,2012). A subset of Late Jurassic (152-148 Ma) tuffs distributed from the Mojave Desert to Mt. Goddard pendant that contain zircons whose δ18O values commonly are 〈5.0‰; such low values are not found in coeval plutonic rocks or dikes of the ca. 148 Ma Independence Dike Swarm. Among these rocks, δ18O values are as low as 2.6‰ and average 4.4±0.8‰; porphyries associated with caldera complexes in the Mojave Desert are similarly low. The restriction of these lower values to volcanic rocks of Late Jurassic age suggests that caldera systems at this time interacted with low-δ18O surface water (meteoric or marine). Such infiltration into caldera environments has not been recognized before or after. The Late Jurassic corresponds to a pronounced but brief transtension event in the arc, which is hypothesized to have impelled mafic mantle melts high into the crust, and also encouraged low-δ18O surface water exchange with arc wall rocks (especially volcanic crust). These two factors thus created a period of low-δ18O magmatism in the upper arc crust that is strikingly mismatched with the δ18O record in coeval plutons. Moreover, the volcanic δ18O record may provide previously unrecognized information about tectonic stress regimes in arc systems as they respond to plate-tectonic reorganizations.

Description: The ability to constrain the petrogenesis of multiple serpentine generations recorded at the microscale is crucial for estimating the extent and conditions of modern versus fossil serpentinisation in ophiolites. To address matrix bias effects during oxygen isotope analysis by SIMS, we present the first investigation analysing antigorite in the compositional range Mg# = 77.5–99.5 mole %, using a CAMECA IMS‐1280 secondary ion mass spectrometer. Spot‐to‐spot homogeneity is ≤ 0.5‰ (2s) for the new antigorite reference materials. The relative bias between antigorite reference materials with different Mg/Fe ratios is described by a second‐order polynomial, and a maximum difference in bias of ~ 1.8‰ was measured for Mg# ~ 78 to 100. We observed a bias up to ~ 1.0‰ between lizardite and antigorite attributed to their different crystal structures. Orientation effects up to ~ 1‰ were observed in chrysotile. The new analytical protocol allowed the identification of oxygen isotope zoning up to ~ 7‰ in serpentine minerals from two serpentinites recovered from an area of active serpentinisation in the Samail ophiolite. Thus, in situ analysis is capable of resolving isotopic heterogeneity that may directly reflect changes in the physical and chemical conditions of multiple serpentinisation events in the Samail ophiolite.

Description: A c. 2.4 Ga microbialite reef complex within the Turee Creek Group (TCG) in Western Australia was deposited in the aftermath of the Great Oxidation Event (GOE). The diverse reef contains the first appearance of thrombolites, a complex deep water microfossil assemblage, and the oldest shallow water sedimentary phosphorous deposit [1, 2, 3]. Silica is present throughout the reef, as microcrystalline quartz in thrombolites, fine chert preserving the deep water microfossil assemblage, and as euhedral quartz crystals within phosphorous-rich peloids and pebbles [1, 2, 3]. Petrographic examination indicates some relatively early silica phases. Si isotope analysis will be used to evaluate the effect of re-equilibration by Proterozoic sea water and pore fluids on the cherts and quartz grains within this reef, to determine whether primary information (such as sea water temperature, pH, and salinity [4, 5]) can be retained. Here we present a wide range of recorded δ30Si from -2.8 to 4.1 ‰, which is typical of Precambrian cherts [4, 5, 6]. It was recently demonstrated that low temperature re-equilibration of Si isotopes between amorphous and aqueous states readily occurs in water with high ionic strength and that is supersaturated with Si [4]. Since these units were deposited before the advent of silicifers (e.g. diatoms), the ocean would have been supersaturated with silica [4, 5]. Re-equilbration is likely to have occurred, and it is possible that some isotope values reflect the sea water, or diagenesis, rather than the process that first precipitated the silica or the source of silica. By determing how much of an effect re-equilibration has had, we can try to determine what useful, primary information is retained and what the environment was like during the GOE.

Description: Serpentinization plays an important role in fluid and mass transfer between the ocean, the crust, and the mantle, in biogeochemical processes, and CO2 sequestration within oceanic and continental settings. The physical-chemical conditions of serpentinization, such as temperature and fluid source, are often investigated using oxygen isotopes. However, the ability to precisely constrain such parameters is limited by the accuracy of calibrations for oxygen isotope fractionation between serpentine and water – i.e. 1000 lnα(Srp-w) – which disagree by up to 20‰ when extrapolated to T 〈 200 °C [1-5]. In this study, we present a new empirical calibration of 1000 lnα(Srp-w) aiming to improve applications of oxygen isotope thermometry to very low-T serpentinization (T 〈 100 °C). We used the high-spatial resolution capabilities of Secondary Ion Mass Spectrometry (SIMS) to analyze oxygen isotope ratios in mineral pairs of calcite+serpentine, quartz+serpentine and talc+serpentine co-crystallized at scales ≤ 50 μm in six serpentinite samples from the Samail ophiolite (Oman). SIMS analysis shows that the mineral pairs are relatively homogeneous in oxygen isotope ratios with variability in δ18O values ≤ 2‰ (2s). Clumped isotope thermometry and petrological constraints indicate crystallization temperatures from ~20 to 90 °C for the investigated samples [6,7]. These independent constraints on temperature allowed us to derive 1000 lnα(Srp-w) by combining mineral-serpentine oxygen isotope fractionations measured by SIMS with published mineral-water oxygen isotope fractionations. Our empirical calibration of 1000 lnα(Srp-w) = 1.12±0.42 × 106/T2 (T in K), from T = 20 to 90 °C, is within uncertainty of former high-temperature empirical calibrations [1,4] extrapolated to T 〈 100 °C. The new 1000 lnα(Srp-w) calibration enables more accurate reconstructions of fluid-rock interactions occurring during low-temperature serpentinization processes in various tectonic settings.

Description: The study of serpentinites and ophicarbonates from ophiolitic terrains provides a three-dimensional perspective on the hydration and carbonation processes affecting modern oceanic lithosphere. The Chenaillet ophiolite (western Alps) is interpreted as a fragment of an oceanic core complex that resembles a modern slow spreading center, and it was weakly affected by Alpine metamorphism. Ophicarbonates from the Chenaillet ophiolite were targeted in this study for in situ analysis by Secondary Ion Mass Spectrometry (SIMS) of oxygen and carbon isotopes in serpentine, calcite, dolomite and magnetite. The high spatial resolution of SIMS allowed us to target different serpentine, carbonate and magnetite generations intergrown at scales ≤ 50 μm, and reveal systematic zoning in δ18O with a range of 5.8‰ in serpentine (from 3.0 to 8.8‰, V-SMOW), 21.2‰ in carbonate (9.4 to 30.6‰), and 5.6‰ in magnetite (–5.0 to –10.6‰). Coupled analysis of oxygen isotopes in seven different touching-pairs of co-crystallized serpentine+carbonate and serpentine+magnetite provides independent constraints on both the temperatures and δ18O(water) values during serpentinization and carbonation responsible for the formation of the Chenaillet ophicarbonates. The new stable isotope data and thermometric estimates can be directly linked to textural and petrographic observations. These new results identify at least four different stages of hydrothermal alteration in the Chenaillet ophicarbonates: (1) peridotite hydration during seafloor exhumation at temperatures down to 200-130 °C and water δ18O values varying from 5 to 2‰, as documented by serpentine+magnetite in mesh textures; (2) carbonation during exhumation near the seafloor at temperatures as low as 10 °C assuming water δ18O values of –1‰, as documented by the highest oxygen isotope ratios in texturally older calcite; (3) serpentinization and carbonation at temperatures up to 240 °C and water δ18O values of 2-3‰, as documented by serpentine+magnetite in veins crosscutting mesh textures (T = 192±66 °C, δ18O(water) = 2±1‰, 2 standard deviation), serpentine+magnetite (T = 182±32 °C, δ18O(water) = 2±1‰) and serpentine+dolomite (T = 243±79 °C, δ18O(water) = 3±2‰) in recrystallized hourglass domains within serpentinite clasts, serpentine+dolomite (T = 229±50 °C, δ18O(water) = 3±1‰) and serpentine+calcite (T = 208±40 °C, δ18O(water) = 2±1‰) within the fine-grained calcite matrix surrounding serpentinite clasts; (4) late stage carbonation at temperatures down to 70-40 °C assuming water δ18O values of 3 to –1‰, as documented by the highest oxygen isotope ratios in a large calcite vein crosscutting both serpentinite clasts and fine-grained carbonate matrix. We suggest that the textural and isotopic observations are consistent with a protracted serpentinization and carbonation of the lithospheric mantle that started during progressive exhumation to the seafloor and continued due to interaction with hot and isotopically shifted seawater, which circulated at depth in the oceanic crust and was then discharged near the seafloor, similar to modern mid-ocean ridge venting systems.

Description: In order to resolve inter- and intracrystalline oxygen isotopic heterogeneities in olivine crystals encountered in mantle peridotites, basaltic lavas, chondritic meteorites and metamorphic rocks, in situ techniques such as ion microprobes are needed. Accurate ion microprobe analysis requires not only well-characterised reference materials, but also calibration of the matrix bias for compositional variations within a given mineral. We investigated matrix bias effects related to Mg/Fe variations in olivine during in situ analysis of oxygen isotopes with sensitive high-resolution ion microprobe (SHRIMP) by analysing chemically homogenous olivine samples with forsterite contents in the range Fo74–Fo100. The isotopic measurements were calibrated against San Carlos olivine (SCO; Fo91). The repeatability achieved for all samples was ±0.21–0.50‰ (standard deviation, SD, at 95% confidence level, c.l.) comparable to that of San Carlos olivine (±0.31–0.48‰, SD at 95% c.l.). A matrix bias up to ~−2.0‰ was observed in olivine with forsterite content above 92 mol%, conversely to what has been reported for Cameca instruments. The relationship between the magnitude of matrix bias and fayalite content (mol%) is described by the quadratic function: The correction scheme for the matrix bias was applied to chemically zoned olivine crystals from a partly serpentinised dunite from the Archean Nuasahi massif (eastern India). Olivine cores (Fo92) preserve their typical mantle-like signature with a δ18O value of 5.16 ± 0.30‰ (σ at 95% c.l.). During a low temperature stage of serpentinisation, olivine transformed to lizardite1 + brucite + magnetite. Olivine rims (Fo98; δ18O = 1.92 ± 0.60‰, σ at 95% c.l.) and the surrounding lizardite2 (4.87 ± 0.53‰, σ at 95% c.l.), formed during a later stage of rock-fluid interaction, are in isotopic equilibrium at ~405–430 °C, with a fluid having a δ18O of ~5.3–6.9‰. Evolved seawater enriched in 18O by isotopic exchange during infiltration could have been responsible for this later serpentinisation stage observed in the Nuasahi massif. The concomitant analysis of oxygen isotopes at the microscale in both olivine and serpentine represents a powerful tool to constrain the nature and source(s) of serpentinising fluid(s) as well as the temperature of serpentinisation.

Description: We present the first investigation of in situ oxygen isotopes in serpentine minerals by sensitive high‐resolution ion microprobe (SHRIMP). Chemically homogeneous samples of antigorite (δ18O = 8.30 ± 0.12‰), chrysotile (δ18O = 4.37 ± 0.02‰) and lizardite (δ18O = 5.26 ± 0.20‰) analysed by laser fluorination are identified as potential reference materials. They were analysed by SHRIMP to assess their homogeneity compared with the San Carlos olivine, as well as for potential matrix bias and crystal orientation effects. The reproducibility achieved for all samples was ± 0.30–0.55‰ (95% confidence level). Matrix bias between antigorite/olivine and antigorite/lizardite was up to ~ +3‰ and −1‰, respectively. Crystal orientation effects were identified only in chrysotile, and no matrix bias was observed over the investigated compositional range within each serpentine mineral. The new reference materials were used to measure the oxygen isotope composition of serpentines in an ultrahigh‐pressure metamorphic belt from Tianshan (China). By combining oxygen isotopes and trace element microanalyses, several stages of serpentinisation were recognised: (a) seafloor alteration, (b) recycling of internal metamorphic fluids during isothermal decompression and (c) shallow interaction with meteoric water during exhumation. No interaction with fluids derived from the surrounding metapelites during subduction was identified.

Description: Subduction zones provide a key link between the surficial biogenic, atmospheric and hydrospheric geochemical cycles with the Earth’s internal reservoirs. Sediment compaction and dehydration of variably altered oceanic lithosphere during subduction release volatile species (containing e.g., S, H, C, N) to the overlying mantle wedge. In particular, sulfur plays a key role in the formation of porphyry ore deposits and has a major control on redox processes in subduction zones, given it occurs in variable oxidation states from oxidized sulfate (S6+) to reduced sulfide (S2-). Here we studied samples from a contact between serpentinite and partly metasomatized eclogitic metagabbros in the Voltri Massif (Italy). We determined the bulk rock and in situ sulfur isotope composition of pyrite grains and combined this with detailed mineralogic and petrologic investigations. Along the serpentinite-metagabbro contact, the metagabbros are metasomatized to actinolite-chlorite schists and metagabbros rich in epidote and Na- and Na-Ca amphiboles. The serpentinites as well as the actinolite-chlorite schists along the serpentinite-metagabbro contact have very low sulfide contents and provide evidence for the oxidation of sulfides, including formation of Fe-oxides. Sulfur input from the serpentinite-metagabbro contact towards the less metasomatized eclogitic metagabbros is observed. This sulfur input is reflected by bulk rock δ34S values that increase from initially around +1.5‰ in samples distant from the contact to +7.3 to +12.5‰ in samples near the contact. This trend correlates with a general increase in the in situ δ34S values from core to rim of individual pyrite grains. Distinct Co and Ni growth zones in pyrite and variations in the in situ δ34S values indicate multiple phases of pyrite growth during subduction and exhumation of these rocks, with the last stage of pyrite growth clearly related to Mg-metasomatism along the serpentinite-metagabbro contact. Thus, this study provides new insight into processes of sulfur migration during metasomatism of gabbroic rocks within the subducting slab and at the slab–mantle interface.

Description: The capacity of garnet to preserve successive growth stages over the P–T evolution of the host rock remains unsurpassed. The distributions of major elements, trace elements and oxygen isotopes, can be mapped at high spatial resolution to decode this information. The combination of experimental studies and investigation of natural samples is needed to determine the systematics of garnet compositional zoning and translate it into petrological information. Trace element mapping of garnet from different metamorphic settings reveals that different categories of elements record distinct mineral reactions and that trace elements zoning in garnet is related to growth conditions (Rubatto et al. 2020). During sub-solidus growth of garnet, Y+REE zoning is mainly controlled by Rayleigh fractionation with the sporadic breakdown of accessory phases producing annuli. However, additional processes overprinting equilibrium growth can be recognised. Fluid-induced garnet replacement can decouple major elements from compatible trace elements, whereby only the major elements are subject to replacement along veinlets. Trace element zoning can also reveal inheritance from precursor and neighbouring phases, such as epidote, lawsonite and biotite. At higher temperature, partial melting results in enrichment of V and Cr in garnet due to mica consumption, as well as Zr, Y and HREE from dissolution of zircon and monazite. In situ oxygen isotope analyses of garnet are particularly suitable to retrieve information on fluid-rock interaction. In eclogite facies rocks that underwent relatively low T conditions (〈600°C), the different isotopic compositions of garnet growth zones within and across samples is preserved and can assist in determining the pervasive or localized nature of fluid flow. In different metamorphic units, garnet is instrumental in recognising high-pressure fluid-rock interaction versus inherited alteration from previous stages (Vho et al. 2020, Bovay et al. 2021). Supported by thermodynamic and geochemical modelling, the oxygen isotopic composition of garnet can be translated into time-integrated fluid fluxes at specific stages of the P–T path. At higher temperatures, diffusion of oxygen isotopes has to be considered, but remains poorly constrained. The results of a comprehensive experimental study (Scicchitano et al. 2022) show that the diffusivity of oxygen is similar to Fe-Mn diffusivity at 1000-1100 °C. However, the activation energy for O diffusion is larger, leading to lower diffusivities at P–T conditions characterizing crustal metamorphism. Therefore, original oxygen isotopic signatures can be retained in garnet showing Fe-Mn element zoning partially re-equilibrated by diffusion.

Description: The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo91-92 olivine with δ18OVSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo90.5. The northern lens ‘B’ contains coarser-grained Fo96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60◦-120◦ conjugate intersections. The Fo96-98 olivine has δ18OVSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals. We interpret that the Fo90-92 versus the Fo96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18OVSMOW + 5.4‰ olivine. The Fo96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18OVSMOW Fo96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism. In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth’s history.