Grain scale processes recorded by oxygen isotopes in olivine-hosted melt inclusions from two MORB samples

International audienceAlthough olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy formantle composition, these melt inclusions generally show larger elemental and isotopic compositional variationthan their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotopedata from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions fromdifferent crystals within the same sample show>2.5‰ δ18O variation within each sample, which is nearly eighttimes 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 thehost olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions fromeach sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higherolivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here wediscuss several potential processes that could contribute to these observations, but none satisfactorily explain theolivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear thatthe 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 interpretedwith caution

Toward a better understanding of olivine-hosted melt inclusion compositional variability

Olivine hosted melt inclusions are widely used to constrain the primitive melt composition, unaffected by superficial processes. However, melt inclusions always show larger compositional variations than lavas or glasses. Moreover, the idea of how olivine grows lias been largely revisited since the discovery of phosphorus (P) zonation in otherwise compo- sitionally homogenous olivine. Understanding the process of melt inclusion entrapment is thus essential to better constrain what melt inclusion composition represents. Coupling structural observations (elemental X-ray maps, especially from P and NanoSIMS profiles) with in-situ compositional data (major, trace, volatiles, P contents and oxygen isotope ratios) of melt inclusions and olivine from two différent samples from the Atlantic ridge, we show that olivine might be polyphasé modified by dissolution and re-precipitation after their formation and melt inclusion entrapment and prior to the éruption. This process is probably due to adiabatic décompression melting. Melting of olivine, which encompass a rapid growth event and lias thus entrapped boundary layers enriched in incompatible elements, can lead to melt inclusions with a modified composition. The new melt inclusion will tlius have a composition, at least for P, that is not représentative of the primary melt. NanoSIMS profiles in olivine allows calculating their residence time within the magmatic system, which is in the order of a few days to a few weeks. Melt inclusions from the studied MORB samples have large oxygen isotope variations (>2.5%o) uncorrelated with other geochemical indicators. The origin of these variations is not totally understood yet, but they can represent local, grain scale processes rather than mantle source heterogene- ity. In subduction zones, these processes might be overprinted by the influence of slab fluids. As chlorine isotopes do not fractionate at magmatic temperatures, they should retain the source signature and can thus help to identify the respective contribution(s) of différent lithologies to the Cl enrichment of the mantle. We developed a method and a set of standards to measure chlorine isotope in glasses with SIMS and analysed melt inclusion from 3 différent volcanic arcs (Vanuatu, Aeolian and Lesser Antilles). We show that the différent arcs have a différent $37C1 signature, which is possibly due to a différent geometry in the subduction zone (e.g. différent dip angle of the subducting plate), causing différent depth of fluids release and thus différent origins of fluids (sediments, serpentinites or marine pore fluids for example). -- Les inclusions vitreuses piégées dans les olivines sont couramment utilisées pour contraindre la composition primitive du magma, non affectée par les processus superficiels. Cependant, les inclusions vitreuses montrent une gamme de composition beaucoup plus large que les laves et verres volcaniques. La mise en évidence de zonation en phosphore (P) dans des olivines homogènes pour tous les autres éléments a fait largement évoluer la pensée du mécanisme de croissance des olivines. Comprendre les processus de piégeage des inclusions est donc essentiel afin de déterminer ce que représente réellement la composition de ces inclusions. En couplant des observations structurelles (carte (microsonde électronique) et profiles (NanoSIMS)) de la concentration en P avec des données in-situ de la composition (concentrations en éléments majeurs, P, traces et volatils, et isotopes de l'oxygène) des inclusions et des olivines hôtes de deux différents échantillons de la ride médio océanique atlantique, nous montrons que les olivines et les inclusions vitreuses peuvent subir un épisode de dissolution et de re-précipitation entre le moment de leur formation et l'éruption du magma qui les charrie. Ce processus est causé par la décompression adiabatique. La dissolution d'une olivine ayant subi une croissance rapide va générer un magma localement enrichi en P ainsi que peut-être aussi en certains autres éléments incompatibles. Les inclusions piégées au contact de ces zones vont avoir une composition modifiée, au moins pour P. Le temps de résidence dans le système magmatique calculé pour les olivines étudiées est de quelques jours à quelques semaines. Ces inclusions vitreuses montrent de grandes variations en isotopie de l'oxygène qui ne sont pas corrélées avec d'autres indicateurs géochimiques. L'origine de ces variations ne sont pas encore totalement comprises mais elles pourraient être liées au processus de dissolution et re-précipitation à l'échelle du grain. La composition des inclusions vitreuses de MORB n'est donc probablement pas représentative de la composition du manteau mais reflète plutôt les différents processus affectant la composition des magmas et des minéraux entre leur formation et leur éruption. Cependant, dans les zones de subduction, ces processus sont masqués par l'influence des fluides provenant de la déshydratation du slab. Etant donné que les isotopes du chlore ($37C1 ) ne fractionnent pas à température magmatique, ces derniers vont retenir la signature de la source et sont alors utiles pour identifier les fluides responsables de l'enrichissement en Cl du magma dans les zones de subduction, Nous avons développé une méthode d'analyse ainsi que des standards afin de pouvoir mesurer, in-situ (SIMS), les isotopes du chlore dans les verres. Nous avons ensuite analysé des inclusions de 3 différents arcs volcaniques (Vanuatu, Eolien et Petites Antilles). Les premiers résultats montrent que les différents arcs étudiés sont caractérisés par une signature en <537C1 différente, qui pourrait être due à une géométrie distincte au niveau de la zone de subduction. Cette différence va alors influencer le type de fluides relâchés par la plaque subductante (sédiments, serpentinites ou marine pore fluides)

Tracing of Cl input into the sub-arc mantle through the combined analysis of B, O and Cl isotopes in melt inclusions

co-auteur étrangerInternational audienceThe 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 δ37Cl of primary magmas, since they are unaffected by near surface processes. In this study, δ37Cl were coupled with δ11B and δ18O 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 δ37Cl variation in melt inclusions from a single sample. OHMIs from the Vulcano (Aeolian arc) and Sukumoyama (Izu-Bonin arc) samples have similar δ37Cl (−2.5 ±0.5and −2.6 ±0.8, respectively). These are different from δ37Cl in OHMIs from the other three localities (δ37Cl of −0.7 ±0.6for Aoba (Vanuatu arc) and St. Vincent (Lesser Antilles arc), −1 ±0.9for Iwate (NE Japan)). Vulcano OHMIs also have statistically different B and O isotope compositions compared to those from the other locations: average δ11B of −5.1 ±2.9for Vulcano OHMIs, compared to 2.5 ±3.7, 5.2 ±1.4, 7.0 ±2.2, 3.8 ±7.5for Sukumoyama, Iwate, Aoba and St. Vincent OHMIs, respectively. All OHMIs have δ18O between 4.0 and 7.4, except for those from Vulcano, which are significantly different, with δ18O from 7.2 to 9.1. Combining these three stable isotope systems suggests that the large variation (>2) of δ37Cl 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

Source and fractionation controls on subduction-related plutons and dike swarms in southern Patagonia (Torres del Paine area) and the low Nb/Ta of upper crustal igneous rocks

The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~ 51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U–Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U–Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29–30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes ~ 17–16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5 ± 0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO2, for both the 17–16 Ma calc-alkaline and the 12–13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust

Weekly to monthly time scale of melt inclusion entrapment prior to eruption recorded by phosphorus distribution in olivine from mid-ocean ridges

co-auteur étrangerInternational audienceMelt inclusions (MIs) hosted in euhedral olivine have been proposed to represent droplets of primary melt, protected from processes occurring near Earth’s surface during eruption. The complex zoning of phosphorus (P) in some olivines and the presence of a P-depleted zone around MIs indicate a complex history for the host-MI system. We analyzed P in olivine and MIs from two mid-oceanic ridge basalt (MORB) samples from the Mid-Atlantic Ridge (MAR) by electron probe microanalyzer, secondary ion mass spectrometry (SIMS), and NanoSIMS. Phosphorus dendrites in olivine suggest an initial fast olivine growth followed by a stage of slower growth. Dissolution textures around some MIs were identified and were probably caused by adiabatic decompression melting. Based on diffusion modeling of P in olivine, we infer that olivine beneath the MAR remains in the system (1) for days to weeks after crystallization of P-rich lamellae, and (2) for a few hours after recrystallization of dissolved olivine. Dissolution and reprecipitation of olivine containing boundary layers suggests that most MIs might be affected by late post-entrapment processes

SIMS chlorine isotope analyses in melt inclusions from arc settings

International audienceThe relative contribution of volatiles from altered oceanic crust, serpentinized mantle, and subducted sediments on the composition of primary arc magmas is poorly constrained. We measured the chlorine stable isotope composition in olivine-hosted melt inclusions in order to provide a first order constraint on the δ37Cl values of primary magmas, since melt inclusions are not or only little affected by near surface processes. Chlorine isotope analyses were obtained with a CAMECA IMS 1280-HR at the University of Lausanne. A series of six Cl-bearing glass standards with δ37Cl values of − 1.1 to + 1.7‰, SiO2 between 50 and 76 wt% and Cl concentration between 0.15 and 3.25 wt% were used for calibration. We determined that SiO2, Al2O3 and K2O in glasses are primarily responsible for the observed variations in instrumental mass fractionation.We obtained a δ37Cl reproducibility on the standard glasses typically better than 0.3‰ (2SD) and a total uncertainty, corresponding to the propagated error of SIMS and Pyro-IRMS uncertainty, of 0.4‰ (2SD). Melt inclusions from Lesser Antilles, Vanuatu and Aeolian arcs display ranges of δ37Cl values from − 1.9 to + 0.6‰, − 1.7 to + 0.4‰ and − 3.4 to − 1.4‰ respectively. Combined with Cl/K2O ratios, these data suggest that Cl addition mainly originates from serpentinites (lithospheric serpentinites or serpentinized mantle wedge) in the Lesser Antilles and Vanuatu arcs. In contrast, Cl added to the mantle wedge beneath the Aeolian Islands may record a higher proportion of fluids derived from the subducting sediments. The observed differences of δ37Cl values in melt inclusions from the Lesser Antilles and Vanuatu arcs and the Aeolian Islands may directly reflect the different subduction geometries present