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Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years. However, the isotopic composition of these calcitic tests can be substantially altered during diagenesis without discernible textural changes. Here, we investigate fluid-mediated isotopic exchange in pristine tests of three modern benthic foraminifera species (Ammonia sp., Haynesina germanica, and Amphistegina lessonii) following immersion into an 18 O-enriched artificial seawater at 90 °C for hours to days. Reacted tests remain texturally pristine but their bulk oxygen isotope compositions reveal rapid and species-dependent isotopic exchange with the water. NanoSIMS imaging reveals the 3-dimensional intra-test distributions of 18 O-enrichment that correlates with test ultra-structure and associated organic matter. Image analysis is used to quantify species level differences in test ultrastructure, which explains the observed species-dependent rates of isotopic exchange. Consequently, even tests considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange; critical paleo-temperature record re-examination is warranted., (© 2022. The Author(s).)

Successful application of diffusion chronometry is based on detailed knowledge of diffusion parameters obtained from laboratory experiments, which also need to be upscaled to be applied to natural samples. While generally applicable, several examples of disagreement with other dating techniques and discrepancies with geological observations highlight the importance of validating diffusion parameters and their use to geochronologic application by testing them on samples from well-characterized geologic environments. The incomplete dissolution and reprecipitation reaction of dolomite associated with infiltration along grain boundaries of a fluid of magmatic origin in a contact metamorphic environment is the starting point of this study. The reaction produces dolomite rims, which are characterized by lower δ18O values (about 16‰) and slightly higher Fe (∼1000 ppm) and Mn (∼200 ppm) concentrations. The cores, in comparison, have no Fe and Mn and a δ18O value of about 30‰. This compositional and isotopic gradient led to chemical re-equilibration through diffusion at the reaction interface. We acquired oxygen isotope profiles using a secondary ion mass spectrometer (SIMS), with spot sizes as small as 3 µm, and iron and manganese profiles with a NanoSIMS and a beam diameter of about 240 nm. These profiles were then compared against cumulative theoretical diffusion distances calculated for a modeled time – temperature path of the contact aureole using the experimentally determined diffusion parameters for O, Fe and Mn in dolomite. The oxygen isotope profiles yielded diffusion distances of ca. 10 µm, corresponding to a formation temperature of about 550 °C, which is concordant with phase petrology and a timing for fluid infiltration at the onset of cooling of the contact aureole (i.e., non-isothermal evolution of the dolomite marbles). The Fe and Mn profiles were very sharp, with a diffusion length of about 150 nm. Such short diffusion length would be achieved at near-surface temperature and corresponding cooling timescale are several orders of magnitude smaller than those derived from the oxygen diffusion profiles. From the mineralogy and textures, a decoupling of the oxygen isotopes and Fe–Mn reaction interface can be ruled out, suggesting that the published diffusion parameters are not fully representative, either due to the experimental conditions or by the assumed diffusion mechanism of natural processes. This case study is not the first example in the literature where the diffusion rate of Fe and Mn in natural dolomite seems to be substantially slower than predicted by experiments. Thus, there is a need to re-assess the experimental diffusivities before Fe and Mn diffusion in dolomite can be reliably used to determine duration, cooling or heating paths of metamorphic processes. [ABSTRACT FROM AUTHOR]

In situ oxygen isotope compositions of white mica and quartz have been used to characterize the interplay of metamorphism and fluid events between a metasomatic whiteschist and its granite protolith in the Monte Rosa nappe, Western Alps. New natural muscovite and phengite reference materials were calibrated for in situ Secondary Ion Mass Spectrometry (SIMS) oxygen isotope measurement. White mica and quartz oxygen isotope compositions were measured in situ in one whiteschist and two metagranites. Based on microtextural observation, phengite composition of white mica, and phase petrology modeling, it is possible to identify two events of fluid infiltration and one event of fluid expulsion, all of which were responsible for forming this unique whiteschist occurrence and for tracing its metamorphic evolution from late Permian intrusion to Alpine subduction and finally to the present day, exhumed whiteschist. Metagranite samples contain three generations of white mica: igneous, high-P metamorphic, and late Alpine, retrograde compositions. In the whiteschist samples, we distinguish two distinct Alpine white mica generations: (1) prograde to peak generation and (2) retrograde generation. The δ18OVSMOW values of white mica and quartz from a whiteschist of 5.3 to 7.3‰ and 9.1 to 10.6‰ are significantly lower than in the metagranites, with 9.1 to 10.8‰ and 13.2 to 14.6‰, respectively. This indicates a complete recrystallization of the whiteschist protolith during intense fluid-rock interaction. Subsequent Alpine metamorphism transformed the protolith into the whiteschist. The isotopic composition of the whiteschist, fine-grained, retrograde white mica (5.3 to 6‰) is lower than that of the high-pressure phengite (6.2 and 7.5‰). The low δ18O values could be explained by infiltration of external fluids with δ18O values of 2 to 6‰. Such fluids would carry the isotopic signature of the serpentinites of the Piemonte-Liguria Ocean by either equilibration of fluids with or dehydration of serpentinites. Another, more simple explanation would be the infiltration of very small quantities of fluids leading to the breakdown of chloritoid. Local inheritance of the oxygen composition would then hide the origin of the fluids. Isotope exchange temperatures calculated from high-P phengite-quartz pairs in whiteschist give an average temperature of 440 ± 50 °C. These are lower than the best T-estimates from phase petrology of 570 °C, at 2.2 GPa. Igneous muscovite-quartz pairs in the metagranite yield 400 ± 40 °C. Only one high-P phengite-quartz pair was analyzed, resulting in 350 ± 40 °C. Greenschist facies, low silica phengites give an average temperature of 310 ± 10 °C. Propagation of analytical uncertainty suggests large errors of 60 to 120 °C, due to the relatively small T-dependence of the quartz-white mica fractionation factor for oxygen isotopes. [ABSTRACT FROM AUTHOR]

A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth (XMg > 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline (XMg = 0.25, δ18O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with XMg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing whiteschists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism. [ABSTRACT FROM AUTHOR]

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ18O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ18O = ~15‰) to the rim (δ18O = ~11‰) suggests that the 18O/16O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the 18O/16O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ18O value of the garnet rim, together with the previously reported low δ18O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ18O fluid/melt possibly derived from meta‐mafic to ultramafic rocks. [ABSTRACT FROM AUTHOR]

Talc + olivine in metaperidotites result from the serpentinite breakdown due to increasing temperature in the Bergell contact aureole. Jack‐straw olivine textures are present in close proximity to the serpentine breakdown reaction. As the intrusion is approached, the number of olivine crystals increases while the size of the crystals decreases; this feature documents increased overstepping with increased heating rates. Talc veinlets are observed in the outer parts of the talc–olivine zone and are interpreted to be pathways of fluid produced during devolatilization of serpentine. None of the talc–olivine oxygen isotope pairs analysed are in isotopic equilibrium with respect to the peak contact temperature. This implies that escaping fluids cannot be in equilibrium with both phases. Hence, the fluid produced by serpentine reaction does not directly reflect the protolith composition, and attention must be given to the reaction mechanism before interpreting fluid isotope composition. [ABSTRACT FROM AUTHOR]

Two quartz samples of igneous origin, UNIL-Q1 (Torres del Paine Intrusion, Chile) and BGI-Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL-Q1 and BGI-Q1 were evaluated for homogeneity using SIMS. Their reference δ18O values were determined by CO2 laser fluorination. The average δ18O value found for UNIL-Q1 is 9.8 ± 0.06‰ and that for BGI-Q1 is 7.7 ± 0.11‰ (1 s). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32-0.41‰ (2 s; UNIL-Q1) and 0.40-0.48‰ (2 s; BGI-Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI-Q1 is also suitable for SIMS trace element measurements. [ABSTRACT FROM AUTHOR]

Extrapolation of reaction paths and rates of metamorphic mineral growth from experimental to natural systems is complicated by a number of factors. Many of these factors are difficult to evaluate for natural systems. A combination of textural modeling and stable isotope analysis allows for a distinction between several possible reaction paths for olivine growth in a siliceous dolomite contact aureole. It is suggested that olivine forms directly from dolomite and quartz. The formation of olivine from this meta- stable reaction implies metamorphic crystallization far from equilibrium. Stable and metastable reaction paths predict the textures observed (calcite haloes around bladed olivine crystals) well. It is possible to discriminate between individual reaction paths only on the basis of the oxygen isotope compositions of the minerals involved. Products were found to be in stable isotope equilibrium, but in disequilibrium with the reactants. Only the metastable overall reaction dolomite + quartz → olivine + calcite + CO2 produces no dolomite by local reactions, and hence agrees with the oxygen isotope data. Thus, significant mineral growth occurred far from equilibrium with respect to the thermodynamically stable reactions of the system. This amazing finding implies that metamorphism of contact aureoles has to be reinterpreted in a more complex, dynamic fashion, involving metastable reactions and metastable equilibria as well. The spatial distribution of metamorphic mineral assemblages in a contact aureole cannot be interpreted as a proxy for the temporal evolution of a single rock specimen, because each rock undergoes a different reaction path, depending on temperature, heating rate, and fluid-infiltration rate. [ABSTRACT FROM AUTHOR]

Abstract Although olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy for mantle composition, these melt inclusions generally show larger elemental and isotopic compositional variation than their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotope data from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions from different crystals within the same sample show >2.5‰ δ18O variation within each sample, which is nearly eight times the analytical error of 0.3‰ (2 standard deviations) and five times the δ18O range in unaltered MORB. Measured δ18O in melt inclusions do not correlate with common magmatic tracers, and δ18O measured in the host olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions from each sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higher olivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here we discuss several potential processes that could contribute to these observations, but none satisfactorily explain the olivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear that the variability of δ18O in melt inclusion from two MORB samples do not record only common magmatic process(es), but rather a localized grain scale process. Any δ18O variation in melt inclusions should thus be interpreted with caution. [ABSTRACT FROM AUTHOR]

Abstract Syntectonic veins commonly have been used to assess the composition and source of fluids involved in fault zone activity. Such veins also provide information on the ambient stress conditions during deformation and mineralization. Based on bulk sampling and bulk O- and C-isotope analysis, combined with fluid inclusion microthermometry, many studies have demonstrated that syntectonic veins provide snapshots of fluid composition and stress conditions over the course of fault history. This is widely acknowledged for mode I extension veins that develop in the damage zones of faults. However, an important and unanswered question is the extent to which such veins record a more detailed fault history at the micron scale. In this study, we present new detailed in-situ micron-scale δ 18 O data, measured using Secondary Ion Mass Spectrometry (SIMS), combined with detailed fluid inclusion microthermometry and Δ 47 clumped isotope thermometry to document the fluid and temperature conditions during mode I vein growth related to deformation along the Cotiella thrust fault (Pyrenees). All the studied veins show three distinct episodes of vein opening, recording a complex history of varying fluid composition and temperature. Results show that the studied fault portion passed from a hydrological rock-buffered system, in which formation waters were in isotopic equilibrium with the host sediments, to a fluid-buffered system involving meteoric water. However, such information is only achievable at the micron scale using Secondary Ion Mass Spectrometry. This study demonstrates the potential of these new micro-beam techniques for investigating fault behavior in more detail than previously available, particularly with respect to the nature of the fluids involved and the P–T conditions extant during fault activity. Highlights • Extension veins from the Cotiella thrust record 3 micron-scale opening fractures. • In-situ O-isotope are coupled with fluid inclusion and Δ 47 analysis. • Extension veins record the P – T conditions of the 3 stages of vein openings. • We propose the paleohydrological history of the Cotiella thrust fault. [ABSTRACT FROM AUTHOR]

Monazite can record several episodes of fluid/rock interaction within a single grain. Coupled in situ determination of age and oxygen isotope composition by SIMS (secondary ion mass spectrometer) may characterize such events. Yet, monazite reference materials for oxygen isotope analyses are scarce. Ion probe (CAMECA IMS 1280) and laser fluorination measurements of δ 18 O values from four monazites are presented. Monazites (YREEPO 4 ) with variable cheralite [CaTh(PO 4 ) 2 ] and huttonite (ThSiO 4 ) substitutions, which span a wide compositional range covering most magmatic and metamorphic, as well as hydrothermal monazites were selected. Three monazites, Moacyr, Manangoutry and UNIL-Mnz1, have a homogeneous chemical (determined by electron microprobe analysis and back-scattered imaging) and oxygen isotopic composition with uncertainties (2 SD) in the range of 0.3 to 0.4‰ for ion probe measurements. These three samples are thus potentially suitable as reference materials, and sample UNIL-Mnz1 is available for inter-laboratory comparison. Sample UNIL-Mnz2 is less homogeneous, but it is still useful for internal monitoring. Uncertainties (2 SD) on the ion probe data are in the range of 0.5 to 0.6‰. Using the investigated monazites as well as literature data, a new calibration curve has been established to account for instrumental mass fractionation (IMF) due to the solid solution in natural monazites. The data illustrate that the IMF is primarily a function of the YREEPO 4 content, while the influence of the Th-content seems to be less important. The δ 18 O values for the four studied monazites have been determined by laser fluorination (LF). They are as follows (1 SD): 1.5 ± 0.1‰ for Moacyr, 10.2 ± 0.1‰ for Manangoutry, 8.5 ± 0.2‰ for UNIL-Mnz1, and 9.5 ± 0.3‰ for the slightly less homogenous UNIL-Mnz2. The analytical protocol adopted here results in consistently high yields (> 90%) and reproducible values, and hence it is argued that LF is a viable method for the calibration of phosphate reference materials. [ABSTRACT FROM AUTHOR]

Abstract The effect that recycling crust and sediments have on the composition of the mantle wedge, in particular in terms of volatiles, is still debated. Chlorine, an important fluid mobile element that has stable isotopes with different concentrations in the terrestrial reservoirs, has the potential to be used to trace slab-derived fluids. Olivine-hosted melt inclusions (OHMIs) provide a first order constraint on the δ 37 Cl of primary magmas, since they are unaffected by near surface processes. In this study, δ 37 Cl were coupled with δ 11 B and δ 18 O analyses in samples from the Lesser Antilles, Vanuatu, Aeolian, NE Japan and Izu-Bonin arcs. This unique dataset is used to better understand the large δ 37 Cl variation in melt inclusions from a single sample. OHMIs from the Vulcano (Aeolian arc) and Sukumoyama (Izu-Bonin arc) samples have similar δ 37 Cl (−2.5 ± 0.5‰ and −2.6 ± 0.8‰, respectively). These are different from δ 37 Cl in OHMIs from the other three localities (δ 37 Cl of −0.7 ± 0.6‰ for Aoba (Vanuatu arc) and St. Vincent (Lesser Antilles arc), −1 ± 0.9‰ for Iwate (NE Japan)). Vulcano OHMIs also have statistically different B and O isotope compositions compared to those from the other locations: average δ 11 B of −5.1 ± 2.9‰ for Vulcano OHMIs, compared to 2.5 ± 3.7‰, 5.2 ± 1.4‰, 7.0 ± 2.2‰, 3.8 ± 7.5‰ for Sukumoyama, Iwate, Aoba and St. Vincent OHMIs, respectively. All OHMIs have δ 18 O between 4.0 and 7.4‰, except for those from Vulcano, which are significantly different, with δ 18 O from 7.2 to 9.1‰. Combining these three stable isotope systems suggests that the large variation (>2‰) of δ 37 Cl in OHMIs from a sample reflects inputs from different sources of Cl rather than heterogeneities in a single main source. Variability between arcs might reflect different major sources of Cl. Comparing OHMIs Cl isotope data from the Aeolian and Izu-Bonin arcs with existing bulk rock Cl isotope data suggest that OHMIs preserve the source signature of Cl input whereas this signal can be lost in whole rocks as a result of Cl isotope diffusive fractionation during Cl degassing. SIMS measurements of Cl isotopes in OHMIs could thus help refine models of Cl cycles in the mantle. Highlights • Chlorine isotopes in melt inclusions from arc samples show large variation. • Combination of B, O and Cl isotopes allow identification of different sources of Cl. • Melt inclusion can record lower δ 37 Cl compare to bulk rock. • Bulk rock might lose pristine δ 37 Cl due to kinetic fractionation during degassing. [ABSTRACT FROM AUTHOR]

• In situ δ 18 O- δ 11 B analyses of tourmaline from the San Rafael Sn (–Cu) deposit, Peru. • Late-magmatic and pre-ore hydrothermal tourmaline has similar δ 18 O- δ 11 B variations. • Syn-ore tourmaline records mixing of Sn-rich magmatic brine with meteoric waters. • Powerful approach for understanding dynamic magmatic-hydrothermal systems. We present a high-resolution in situ study of oxygen and boron isotopes measured in tourmaline from the world-class San Rafael Sn (–Cu) deposit (Central Andean tin belt, Peru) aiming to trace major fluid processes at the magmatic-hydrothermal transition leading to the precipitation of cassiterite. Our results show that late-magmatic and pre-ore hydrothermal tourmaline has similar values of δ 18 O (from 10.6‰ to 14.1‰) and δ 11 B (from −11.5‰ to −6.9‰). The observed δ 18 O and δ 11 B variations are dominantly driven by Rayleigh fractionation, reflecting tourmaline crystallization in a continuously evolving magmatic-hydrothermal system. In contrast, syn-ore hydrothermal tourmaline intergrown with cassiterite has lower δ 18 O values (from 4.9‰ to 10.2‰) and in part higher δ 11 B values (from −9.9‰ to −5.4‰) than late-magmatic and pre-ore hydrothermal tourmaline, indicating important contribution of meteoric groundwater to the hydrothermal system during ore deposition. Quantitative geochemical modeling demonstrates that the δ 18 O- δ 11 B composition of syn-ore tourmaline records variable degrees of mixing of a hot Sn-rich magmatic brine with meteoric waters that partially exchanged with the host rocks. These results provide thus direct in situ isotopic evidence of fluid mixing as a major mechanism triggering cassiterite deposition. Further, this work shows that combined in situ δ 18 O and δ 11 B analyses of tourmaline is a powerful approach for understanding fluid processes in dynamic magmatic-hydrothermal environments. [ABSTRACT FROM AUTHOR]