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The early Cambrian Takab iron ore deposit is situated in the northern part of the Sanandaj-Sirjan zone, western Iran. It consists of banded, nodular and disseminated magnetite hosted in folded micaschists. Trace element and Fe and O isotopic experiments reveal various hydrothermal precipitation environments under reduced to slightly oxidizing conditions. Disseminated magnetite has high Ti (945–1940 ppm) positively correlated with Mg + Al + Si, and heavy Fe (+0.76 to +1.86‰) and O (+1.0 to +4.07‰) isotopic compositions that support a magmatic/high-T hydrothermal origin. Banded magnetite has low Ti (15−200 ppm), V (≤100 ppm), Si and Mg (mostly ≤300 ppm) and variable Al. The ∂56Fe values vary from −0.2‰ to +1.12‰ but most values also support a magmatic/high-T hydrothermal origin. However, variable ∂18O (−2.52 to +1.22‰) values provide evidence of re-equilibration with lower-T fluid at ~200–300 °C. Nodular magnetite shows high Mn (≤1%), and mostly negative ∂56Fe values (average, −0.3‰) indicative of precipitation from an isotopically light hydrothermal fluid. Re-equilibration with carbonated rocks/fluids likely results in a negative Ce anomaly and higher ∂18O (average, +6.30‰). The Takab iron ore deposit has, thus, experienced a complex hydrothermal history.

International audience; It is well-established that lanthanide rare earth elements (REEs) have the potential to record the nature and source characteristics of their host magmas, in both whole-rock and their minerals. Accessory minerals that concentrate REEs are especially useful in crustal evolution studies, both for their elemental and isotopic information; the classic and unrivalled example being zircon. Approaches using a single radiogenic isotopic system, or one radiogenic and one stable isotope system (e.g., Hf and O) in one REE-bearing mineral (usually zircon) are common, but those involving multiple isotopes in several minerals remain scarce despite offering many advantages. Importantly, the latter approaches also allow comparing different techniques and provide evidence on whether isotopic systems were disturbed by secondary processes. This contribution documents several isotopic systems within the abundant accessory mineral of Caledonian high Ba-Sr granitoids from Northwest Scotland. We present a multi-isotope study of titanite, zircon and apatite from two localities (Strontian and Rogart), which were selected for their contrasting whole-rock isotopic signatures-the former deriving from a depleted mantle source, whereas the latter derived from a strongly enriched mantle source. New in-situ Sm-Nd in titanite and apatite and Hf in zircon isotope data are discussed and compared with in-situ 2 oxygen isotope data previously published for the same samples. An internal consistency is observed for Nd isotopes in apatite and titanite. Nd isotopes values for both minerals strongly correlate with Hf isotopes in zircon. Isotopic data at the mineral scale confirm the Strontian and Rogart source characteristics previously defined from whole-rock isotope data, with the Rogart having a more enriched signature than the Strontian source along the "Caledonian Parental Magma Array"(CPMA). In contrast, a significant discrepancy exists between whole-rock and REE-mineral oxygen isotope data as the latter can be affected by post emplacement alteration and hence can be misinterpreted. Importantly, the contribution of sediments in the CPMA source(s) can be estimated from a diagram combining Sm-Nd isotopic signatures and (La/Sm)N in apatite and titanite. Overall, we demonstrate that detailed petrogenetic records are not only available in zircons but also in magmatic titanite and apatite, and we suggest that integrated multi-mineral approaches have potential to maximise constraints from in-situ mineral isotope geochemistry.

International audience; The early Cambrian Takab iron ore deposit is situated in the northern part of the Sanandaj-Sirjan zone, western Iran. It consists of banded, nodular and disseminated magnetite hosted in folded micaschists. Trace element and Fe and O isotopic experiments reveal various hydrothermal precipitation environments under reduced to slightly oxidizing conditions. Disseminated magnetite has high Ti (945-1940 ppm) positively correlated with Mg + Al + Si, and heavy Fe (+0.76 to +1.86‰) and O (+1.0 to +4.07‰) isotopic compositions that support a magmatic/high-T hydrothermal origin. Banded magnetite has low Ti (15−200 ppm), V (≤100 ppm), Si and Mg (mostly ≤300 ppm) and variable Al. The ∂ 56 Fe values vary from −0.2‰ to +1.12‰ but most values also support a magmatic/high-T hydrothermal origin. However, variable ∂ 18 O (−2.52 to +1.22‰) values provide evidence of re-equilibration with lower-T fluid at ~200-300 • C. Nodular magnetite shows high Mn (≤1%), and mostly negative ∂ 56 Fe values (average, −0.3‰) indicative of precipitation from an isotopically light hydrothermal fluid. Re-equilibration with carbonated rocks/fluids likely results in a negative Ce anomaly and higher ∂ 18 O (average, +6.30‰). The Takab iron ore deposit has, thus, experienced a complex hydrothermal history.

The world-class Shimensi tungsten (W)-polymetallic deposit is located in Jiangnan Orogen, with an estimated reserve of 742.5 kt WO3 @ 0.195% W, 403.6 kt Cu and 28 kt Mo. In this paper, the trace elements and Pb-O isotopes of scheelite (the main ore mineral) are presented to study the ore-forming material source and ore-forming fluid evolution. The results show that the REE distribution in scheelite is mainly controlled by the substitution mechanism of 3Ca2+ = 2REE3+ + □Ca (where □Ca is the Ca-site vacancy). Oxygen isotope data indicate that the scheelite mineralization occurred under high-temperature oxygen isotope equilibrium conditions, and that the ore-forming fluid has a magmatic–hydrothermal origin. The variation in scheelite Eu anomalies and the wide range of scheelite Y/Ho ratio indicate that the ore-forming fluid evolves from reducing to oxidizing, and the early-stage and late-stage ore-forming fluid may have been relatively rich in F− and HCO3−, respectively. The significant Mo decrease in scheelite from the early to late stage that are opposite to the influence of fO2 variation may have resulted from the crystallization of molybdenite and Mo-rich scheelite. Lead isotopes of the ore minerals of scheelite, wolframite, molybdenite and chalcopyrite can be divided into three groups, similar to these of feldspars in different granites. Both the Mesozoic porphyritic and fine-grained biotite granites have Pb isotope ratios similar to the ores, which suggests that the former two are the main ore material source.

International audience; The isotopic composition of carbonate minerals provides a record of historical geochemical and environmental conditions, but the ability to interpret these compositions as paleo-proxies hinges on their preservation over thousand to million year timescales. At chemical equilibrium, alteration of initial isotopic compositions of calcite can occur in the presence of a fluid without visible changes in morphology at the submicron scale, complicating the interpretation of stable isotope compositions of carbonates. However, the rates and mechanisms of isotope exchange at chemical equilibrium are poorly understood. To evaluate the rates and processes by which C and O isotopes are exchanged between calcite and fluid, batch reactor experiments were conducted at chemical equilibrium between calcite and a fluid enriched in 13C and 18O relative to the solid at 25 °C. Both natural and synthetic calcite of different grain sizes were investigated to evaluate the impact of mineral surface area and size on C and O isotope exchange rates. Our experimental results indicate that rapid exchange of both C and O isotopes occurs within 72 h for all calcite grain sizes studied, likely indicative of exchange of surface species in combination with a backward reaction during dissolution and Ostwald ripening of high energy surface sites. After 72 h, C and O isotope exchange rates were slower but near constant for timescales of thousands of hours. Surface-area normalized C and O isotope exchange rates were similar for all calcite grain sizes studied, and O and C were exchanged in a ∼3:1 ratio consistent with exchange of CO32–. The rates of C and O exchange were ∼4 orders of magnitude lower than far-from-equilibrium calcite dissolution rates, suggesting exchange was controlled either by dissolution-precipitation of pre-existing reactive sites alone, or a combination of dissolution-precipitation and solid state/aqueous mediated diffusion. Overall, the results of this study suggest alteration of O and C isotope compositions of calcite at ambient temperatures can proceed readily over short time scales, though the extent to which this process continues to operate over geologic time scales is difficult to predict at present. The results of this study further highlight the importance of generating a mechanistic understanding of the process of isotope exchange at chemical equilibrium, and represent a step towards this understanding

The Kalkadoon-Leichhardt Domain of the Mount Isa Inlier has been interpreted to represent the ‘basement’ of the larger inlier, onto which many of the younger, economically prospective sedimentary and volcanic units were deposited. The domain itself is dominated by 1860–1850 Ma granitic to volcanic Kalkadoon Supersuite rocks, but these units are interpreted to have been emplaced/erupted onto older units of the Kurbayia Metamorphic Complex. This study aims to provide insights into a number of geological questions: 1. What is the isotopic character of the pre-1860–1850 Ma rocks? 2. How do these vary laterally within the Kalkadoon-Leichhardt Domain? 3. What is the tectonic/stratigraphic relationship between the 1860–1850 Ma rocks of the Mount Isa Inlier and c. 1850 Ma rocks of the Tennant Creek region and Greater McArthur Basin basement? Detrital zircon U–Pb results indicate the presence of 2500 Ma detritus within the Kurbayia Metamorphic Complex, suggesting that the Kalkadoon-Leichhardt Domain was a sedimentary depocentre in the Paleoproterozoic and potentially had sources such as the Pine Creek Orogen, or, as some authors suggest, potential sources from cratons in northern North America. Existing Hf and Nd-isotopic data suggest that the ‘basement’ units of the Mount Isa Inlier have early Proterozoic model ages (TDM) of 2500–2000 Ma. Oxygen and Hf-isotopic studies on samples from this study will allow us to test these models, and provide further insights into the character and history of these ‘basement’ rocks within the Mount Isa Inlier, and northern Australia more broadly.

Evaporites are widely distributed within continental &ldquo, red beds&rdquo, in the Lanping&ndash, Simao Basin, west Yunnan, China. Sr (Strontium), S (Sulfur), and O (Oxygen) isotope compositions have been measured on 54 sulfate or/and sulfate-bearing samples collected from Lanping, Nuodeng, Jinggu, Mengyejing, Baozang throughout the Lanping&ndash, Simao Basin. The 87Sr/86Sr ratios of all samples (0.708081 to 0.710049) are higher than those of contemporaneous seawater, indicating a significant continental contribution to the drainage basin. Sulfates in the Lanping Basin have higher 87Sr/86Sr ratios (0.709406 to 0.710049) than those (0.708081 to 0.709548) in the Simao Basin. Nevertheless, the &delta, 34S values of gypsums (13.4&permil, to 17.6&permil, ) in Lanping and Baozang fall within the range of Cretaceous seawater. Gypsums from a single section in Baozang have trends of decreasing &delta, 34S values and increasing 87Sr/86Sr ratios from base to top, indicating continental input played an increasingly significant role with the evaporation of brines. High &delta, 34S values (20.5&permil, to 20.7&permil, ) of celestites in Lanping are probably caused by bacterial sulfate reduction (BSR) process in which 34S were enriched in residual sulfates and/or recycling of Triassic evaporites. The reduced &delta, 34S values of gypsums (9.5&permil, to 10.4&permil, ) in Nuodeng could have been caused by oxidation of sulfides weathered from Jinding Pb-Zn deposit. The complex O isotope compositions indicate that sulfates in the Lanping&ndash, Simao Basin had undergone sulfate reduction, re-oxidation, reservoir effects, etc. In conclusion, the formation of continental evaporites was likely derived from seawater due to marine transgression during the Cretaceous period. Meanwhile, non-marine inflows have contributed to the basin significantly.

Changes in stable chromium isotopes (denoted as &delta, 53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth&rsquo, s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of &delta, 53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and &delta, 53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, &delta, 53Cr &asymp, 0.99&permil, ) and Mono Lake (USA, &asymp, 0.78&permil, ) display significantly higher &delta, 53Cr values compared with ancient microbialites from the Andr&eacute, e Land Group in Greenland (720 Ma, &asymp, 0.36&permil, ) and the Bitter Springs Formation in Australia (800 Ma, &asymp, &minus, 0.12&permil, ). The &delta, 53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable &delta, 53Cr values in skeletal carbonates, do not induce variability in &delta, 53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, &delta, 53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and &delta, 53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the &delta, 53Cr values of the waters from which they precipitated. However, it remains unclear if these &delta, 53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.

In this contribution, we report the results for the characterization of the BB zircon, a newly developed zircon reference material from Sri Lanka, via secondary ion mass spectrometry (SIMS) and multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The focus of this work was to further investigate the applicability of the BB zircon as a reference material for micro-beam analysis, including Li, O, and Hf isotopes. The SIMS analyses reveal that BB zircon is characterized by significant localized variations in Li concentration and isotopic ratio, which makes it unsuitable as a lithium isotope reference material. The SIMS-determined &delta, 18O values are 13.81&permil, &plusmn, 0.39&permil, (2SD, BB16) and 13.61&permil, 0.40&permil, (2SD, BB40), which, combined with previous studies, indicates that there is no evidence of conspicuous O isotope heterogeneity within individual BB zircon megacrysts. The mean 176Hf/177Hf ratio of BB16 determined by solution MC-ICP-MS is 0.281669 &plusmn, 0.000012 (2SD, n = 29) indistinguishable from results achieved by laser ablation (LA)-MC-ICP-MS. Based on the SIMS and MC-ICP-MS data, BB zircon is proposed as a reference material for the O isotope and Hf isotope determination.

This study introduces a new zircon reference material, LKZ-1, for the in situ U&ndash, Pb dating and O&ndash, Hf isotopic and trace element analyses. The secondary ion mass spectrometric analyses for this gem-quality single-crystal zircon yielded a weighted mean 206Pb/238U age of 572.6 &plusmn, 2.0 Ma (2&sigma, n = 22, MSWD = 0.90), with moderately high U concentrations (619 &plusmn, 21 ppm, 1 SD), restricted Th/U ratios (0.146 &plusmn, 0.002, 1 SD), and negligible common Pb content (206Pbc &lt, 0.2%). A comparable 206Pb/238U age (570.0 &plusmn, 2.5 Ma, 2&sigma, ) was produced by the isotope dilution-thermal ionization mass spectrometry. The secondary ion mass spectrometric and laser ablation-assisted multiple collector inductively coupled plasma mass spectrometer analyses respectively showed that LKZ-1 had little variation in O (&delta, 18OV-SMOW = 10.65 &plusmn, 0.14&permil, laser fluorination value = 10.72 &plusmn, 0.02&permil, 1 SD) and Hf (176Hf/177Hf = 0.281794 &plusmn, 0.000016, 1 SD) isotopic compositions. LKZ-1 was also fairly homogeneous in its chemical composition (RSD of laser ablation ICPMS data &le, 10%), displaying a relatively uniform chondrite-normalized rare earth element pattern ((Lu/Gd)N = 31 &plusmn, 3, Eu/Eu* = 0.43 &plusmn, 0.17, Ce/Ce* = 44 &plusmn, 32, 1 SD). These consistencies suggest that the LKZ-1 zircon is a suitable working standard for geochronological and geochemical analyses.

Along the proto-Pacific margin of Gondwana, from Venezuela to northeastern Patagonia, the Early–Middle Ordovician Famatinian orogeny was the first orogenic event following assembly of the supercontinent. Previous isotope studies of the igneous and (meta-)sedimentary rocks of southwestern Gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin. To interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern South America, including the 700 × 600 km type sector of the orogen in the Sierras Pampeanas (27°–33°S), the Precordillera, and northeastern Patagonia. Previous geological, geochemical and geochronological results are reviewed together with new U—Pb SHRIMP crystallization ages, 177Hf/176Hf and 18O/16O data for dated zircon, and whole-rock Sr and Nd isotope compositions., Four geological domains are recognized in the Sierras Pampeanas (Western, Central, Eastern and Foreland Famatinian domains). Magmatism is mostly restricted to the interval 463 ± 4 to 486 ± 7 Ma, with the most intense period of emplacement between 468 and 472 Ma constituting a magmatic flare-up. Granitoid emplacement in both northeastern Patagonia and the Cordon de Lila (Puna Altiplano, Chile) was effectively synchronous with that in the Sierras Pampeanas, defining a continuous belt. Combined geochemical and isotopic data (whole-rock Sr, Nd; Hf, O in zircon) indicate that the source of calcic metaluminous suites is the subcontinental lithosphere – both mantle and mafic lower crust – with variable contamination by the Early Paleozoic metasedimentary country rocks. The lithospheric mantle involved is assumed to underlie the outcropping 1330–1030 Ma age basement of the Western Domain, which exhibits tectonic characteristics of active continental margin in the north and oceanic arc-back arc in the south. The latter sector is the potential source of some minor Famatinian igneous rocks with less evolved isotopic compositions, although a restricted asthenospheric addition cannot be discarded in this case. Minor peraluminous granites are spatially associated with the metaluminous sequence, but major highly-peraluminous batholiths occur on the eastern flank of the Central Domain. Field relations and geochemical/isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post-early Cambrian metasedimentary sequence comprising the host country rocks., Episodic tectono-magmatic evolution of the Famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector: • ca. 474–486? Ma, roll-back stage. This is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle; full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly-peraluminous batholiths in the Central and Eastern Famatinian domains. Trondhjemite plutons with an adakitic signature were emplaced in the Foreland Domain • ca. 468–472 Ma, slab break-off stage. Steepening of the oceanic slab and arc migration to the southwest ended with slab break-off due to subduction of continental crust during continental collision with the Precordillera terrane. This stage produced voluminous metaluminous magmatism at the western edge of the Central Domain (the flare-up episode), K-bentonites in the Precordillera, leucogranites in the Western Domain and scattered metaluminous and peraluminous plutons in all Famatinian domains. Both slab roll-back and break-off stages developed during a high-T regime typical of hot orogens. Although asthenospheric mantle was a necessary heat source for lithospheric melting, its material contribution to the growth of Early Paleozoic crust was apparently very minor. Recycling of Mesoproterozoic lithosphere, including the subcontinental mantle, coupled with crustal melting of Early Paleozoic metasedimentary sequences, accounts for most Famatinian magmatism. Comparable results from the Central Andes and East Antarctica confirm that the early stages of the Terra Australis orogen in SW Gondwana were dominated by lithospheric reworking processes., Financial support for this paper was provided by Argentine public grants CONICET PIP0229, FONCYT PICT 2013-0472 and Spanish grants CGL2009-07984, GR58/08 UCM-Santander and CGL2016-76439-P.

Zircon occurs as a minor constituent in most differentiated magmatic rocks. Its robustness to later modification means that its isotopic and chemical composition generally records conditions prevailing when it formed, and the systematic changes in the oxygen isotope record of zircon through geological time have been used to trace the temporal evolution of crust–mantle interaction and intra-crustal recycling. Here we present U–Pb, Hf, and oxygen isotopic compositions for high grade metamorphic Archean rocks from the Dniester-Bug Series, western Ukrainian Shield. Zircon from a quartz-dominated rock is up to 3.8 Ga old, and enriched in 18 O compared to most previously reported values from Archean zircon. Similar values are recorded in zircon cores, which exhibit a variety of internal textures including magmatic-style oscillatory zonation, and rims. If this rock is metasedimentary and the isotope signatures in cores are primary, the zircon sources were characterized by heavier oxygen isotopic compositions than any known major area of Archean crust. Alternatively the O isotope compositions have been modified. We show that a large fraction of the analyzed zircon appear not to be modified by radiation damage, and speculate that O exchange may have taken place by diffusion during extreme metasomatic alteration of the host rock. The possibility that igneous-looking, apparently unaltered zircon may not preserve a primary oxygen isotope signature has implications for its use in the interpretation of crustal evolution, including early terrestrial geodynamics.

In this study we present U–Pb and Hf isotope data combined with O isotopes in zircon from Neoarchean granitoids and gneisses of the southern São Francisco craton in Brazil. The basement rocks record three distinct magmatic events: Rio das Velhas I (2920–2850 Ma), Rio das Velhas II (2800–2760 Ma) and Mamona (2750–2680 Ma). The three sampled metamorphic complexes (Bação, Bonfim and Belo Horizonte) have distinct εHf vs. time arrays, indicating that they grew as separate terranes. Paleoarchean crust is identified as a source which has been incorporated into younger magmatic rocks via melting and mixing with younger juvenile material, assimilation and/or source contamination processes. The continental crust in the southern São Francisco craton underwent a change in magmatic composition from medium- to high-K granitoids in the latest stages, indicating a progressive HFSE enrichment of the sources that underwent anatexis in the different stages and possibly shallowing of the melting depth. Oxygen isotope data shows a secular trend towards high δ18O (up to 7.79‰) indicating the involvement of metasediments in the petrogenesis of the high potassium granitoids during the Mamona event. In addition, low δ18O values (down to 2.50‰) throughout the Meso- and Neoarchean emphasize the importance of meteoritic fluids in intra-crustal magmatism. We used hafnium isotope modelling from a compilation of detrital zircon compositions to constrain crustal growth rates and geodynamics from 3.50 to 2.65 Ga. The modelling points to a change in geodynamic process in the southern São Francisco craton at 2.9 Ga, from a regime dominated by net crustal growth in the Paleoarchean to a Neoarchean regime marked by crustal reworking. The reworking processes account for the wide variety of granitoid magmatism and are attributed to the onset of continental collision. Fil: Albert, Capucine. Universidade Federal de Ouro Preto; Brasil Fil: Farina, Federico. Universidad de Ginebra; Suiza Fil: Lana, Cristiano. Universidade Federal de Ouro Preto; Brasil Fil: Stevens, Gary. Stellenbosch University; Sudáfrica Fil: Storey, Craig. University of Portsmouth; Reino Unido Fil: Gerdes, Axel. Goethe Universitat Frankfurt; Alemania Fil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotopica; Argentina

The oxygen (O) and neodymium (Nd) isotopic composition of monazite provides an ideal tracer of metamorphism and hydrothermal activity. Calibration of the matrix effect and monitoring of the external precision of monazite O&ndash, Nd isotopes with microbeam techniques, such as secondary ion mass spectrometry (SIMS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICPMS), require well-characterized natural monazite standards for precise microbeam measurements. However, the limited number of standards available is impeding the application of monazite O&ndash, Nd isotopes. Here, we report on the RW-1 monazite as a potential new working reference material for microbeam analysis of O&ndash, Nd isotopes. Microbeam measurements by electron probe microanalysis (EPMA), SIMS, and LA-MC-ICPMS at 10&ndash, 24 &micro, m scales have confirmed that it is homogeneous in both elemental and O&ndash, Nd isotopic compositions. SIMS measurements yield &delta, 18O values consistent, within errors, with those obtained by laser fluorination techniques. Precise analyses of Nd isotope by thermal ionization mass spectrometry (TIMS) are consistent with mean results of LA-MC-ICPMS analyses. We recommend &delta, 18O = 6.30&permil, &plusmn, 0.16&permil, (2SD) and 143Nd/144Nd = 0.512282 &plusmn, 0.000011 (2SD) as being the reference values for the RW-1 monazite.

A major unanswered question in solar system formation is the origin of the oxygen isotopic dichotomy between the Sun and the planets. Individual Calcium–Aluminum-rich inclusions (CAIs) from CV chondrites exhibit almost the full isotopic range, but how their composition evolved is still unclear, which prevents robust astrochemical conclusions. A key issue is notably the yet unsolved origin of the 16 O-rich isotopic composition of pyroxene in type B CAIs. Here, I report an in-situ oxygen isotope study of the archetypal type B CAI USNM-3529-Z from Allende with emphasis on the isotopic composition of pyroxene and its isotopic and petrographic relationships with other major minerals. The O isotopic composition of pyroxene is correlated with indicators of magmatic growth, indicating that the pyroxene evolved from a 16 O-poor composition and became progressively enriched in 16 O during its crystallization, contrary to the long held assumption that pyroxene was initially 16 O-rich. This variation is well explained by isotopic exchange between a 16 O-poor partial melt having the isotopic composition of melilite and a 16 O-rich gas having the isotopic composition of spinel, during pyroxene crystallization. The isotopic evolution of 3529-Z is consistent with formation in an initially 16 O-rich environment where spinel and gehlenitic melilite crystallized, followed by a 16 O-depletion associated with melilite partial melting and recrystallization and finally a return to the initial 16 O-rich environment before pyroxene crystallization. This strongly suggests that the environment of CAI formation was globally 16 O-rich, with local 16 O-depletions systematically associated with high temperature events. The Al/Mg isotopic systematics of 3529-Z further indicates that this suite of isotopic changes occurred in the first 150 000 yr of the solar system, during the main CAI formation period. A new astrophysical setting is proposed, where the 16 O-depletion occurs in an optically thin surface layer of the disk and may have originated by evaporation of 16 O-poor interstellar dust or non-mass-dependant isotopic fractionation.

International audience; The combination of U–Pb, Lu–Hf and O isotopic analyses in global zircon databases has recently been used to constrain continental crustal growth and evolution. To identify crust-forming events, these studies rely on the assumption that new crust is formed from depleted mantle sources. In contrast, this work suggests that post-collisional mafic magmas and their derivatives represent a non-negligible contribution to crustal growth, despite having zircons with “crust-like” Hf–O isotopic characteristics. We address this paradox and its implications for crustal evolution on the basis of a case study from the Variscan French Massif Central (FMC). The late stages of continental collisions are systematically marked by the emplacement of peculiar mafic magmas, rich in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K2O, HFSE, LREE) and displaying crust-like trace element patterns. This dual signature is best explained by melting of phlogopite- (and/or amphibole-) bearing peridotite, formed by contamination of the mantle by limited amounts (10–20%) of crustal material during continental subduction shortly preceding collision. Mass balance constraints show that in melts derived from such a hybrid source, 62–85% of the bulk mass is provided by the mantle component, whereas incompatible trace elements are dominantly crustal in origin. Thereby, post-collisional mafic magmas represent significant additions to the crust, whilst their zircons have “crustal” isotope signatures (e.g. −2

The robust nature of the mineral zircon, combined with our analytical ability to readily acquire in-situ uranium-lead (U-Pb), lutetium-hafnium (Lu-Hf) and oxygen (O) isotopic data, has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through Earth history. In such studies there has been a strong focus on developing methods to determine the timing and/or proportion of juvenile magmatic addition to the continental crust. One widespread approach to determine the timing of crustal growth has been the construction or fitting of 'reworking arrays' to regional Hf isotopic datasets. Simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on-going dilution and refertilisation of ancient crust, consistent with "Hot Zone" models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism. A new compilation of magmatic rock zircon Lu-Hf and O isotope data is used to demonstrate that the use of mantle-like O isotope data as a screening tool for "meaningful" Hf model ages is also unlikely to be reliable, with independently constrained data indicating that as few as 14% of Hf model ages provide a meaningful indicator of the timing of crustal growth. The limitations of Hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts. In an effort to address some of the limitations within existing models, we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions. Refereed/Peer-reviewed

The core of the Tauern tectonic window (Eastern Alps) consists of pre-Alpine granitoids (∼295 Ma) variably deformed during Alpine (∼30Ma) amphibolite-facies metamorphism. Episyenites occur as local alteration haloes (as wide as a few meters) surrounding steeply dipping, strike-slip faults, with offsets

The 618 Ma Curral de Cima tonalite and 577 Ma Lourenco monzodiorite, northeastern Brazil, are magmatic epidote-bearing plutons that carry ferrohornblende, biotite, titanite, and epidote. Major, trace, and isotope chemistry suggests that the major magmas of the two plutons followed similar differentiation trends but derived from source rocks that differed in age and isotopic composition. The mineral phases of the Curral de Cima tonalite, the presence of amphibole-rich clots, and juvenile component (average eNd = −3.55) point to an I-type source for these rocks. These data and high calculated δ 18 O(w.r.) (10.0‰) for the tonalite and high δ 18 O value for a clot (9.3‰) argue that the clots are fragments of a metabasaltic source rock that has been hydrothermally altered at a low temperature. In contrast, average calculated δ 18 O(w.r.) for the Lourenco monzodiorite = 7.8‰, 87 Sr/ 86 Sr = 0.7083, eNd = −14.6, and T DM = 1.92 Ga. These data are compatible with a long crustal residence time of lower crust amphibolites source. Epidote in the Curral de Cima pluton crystallized close to the NNO buffer, and hornblende chemistry, due to Al reequilibration, yielded sub-solidus temperature and pressure. In contrast, in the Lourenco pluton epidote crystallized close to the HM buffer and Al-in-hornblende points to near-solidus solidification (685 °C) around 4.4 Kbar. This study confirms that magmatic epidote in granitic plutons can crystallize at pressures lower than 5.5 Kbar under higher f O 2 as experimentally foreseen. Rapid magma transportation through hot continental crust during the peak of metamorphism in early stages of an orogenic cycle prevents epidote dissolution.

International audience; Mylonites display petrographical and geochemical characteristics that can be related to syn-deformation fluid circulation. The South Armorican Shear Zone, a major structural feature of the Armorican Massif (France), is outlined by the presence of mylonitic rocks cropping out mostly in open quarries. These mylonites were essentially formed at the expense of peraluminous granitic bodies. Deformation occurred from ductile conditions in the biotite stability field (>400 ◦C) down to lower greenschist cataclasis and brecciation, where carbonation developed. U-Pb analyses on zircon and monazite define a minimum duration of 15 Ma for the deformation and hydrothermal history, between 315 Ma and 300 Ma. Fluid circulations are well documented, by way of petrographic observation (chlorite and carbonate crystallization), mineralogical composition analysis (muscovite chemistry), erratic mobility behavior of some elements (As, Sn, U for instance), and stable isotope composition analysis of the infiltrated rocks. High temperature deformation is not accompanied by alteration of the O isotope system, which implies either low fluid/rock ratio and/or the involvement of 18O crustal fluids with a composition similar to that of the rocks. On the other hand, some low temperature mylonites show a drastic decrease in the 18O values, which has to be related to the influx of surface derived waters. The heat source necessary for this crustal scale downward infiltration of fluids followed by upward motion was likely provided by the exhumation of lower crustal units in the South Armorican domain.