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At subduction zones, crustal material enters the mantle. Some of this material, however, is returned to the overriding plate through volcanic and plutonic activity. Magmas erupted above subduction zones show a characteristic range of compositions that reflect mixing in the magma source region between three components: hydrous fluids derived from the subducted oceanic crust, components of the thin veneer of subducted sediments and peridotite mantle rocks. The mechanism for mixing and transport of these components has been enigmatic. A combination of results from the fields of petrology, numerical modelling, geophysics and geochemistry suggests a two-step process. First, intensely mixed metamorphic rock formations—mélanges—form along the interface between the subducted slab and the mantle. As the mélange contains the characteristic three-component geochemical pattern of subduction-zone magmas, we suggest that mélange formation provides the physical mixing process. Then, blobs of low-density mélange material—diapirs—rise buoyantly from the surface of the subducting slab and transport the well-mixed mélange material into the mantle beneath the volcanoes. Magma erupted at subduction-zone volcanoes contains mantle rocks and a mixture of fluids and sediments derived from the subducted slab. A synthesis of work over past years provides an integrated physico-chemical framework for subduction zones with mixing at the slab–mantle interface and transport towards the surface volcanoes by buoyant diapirs.

Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth’s isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event.

Inner Solar System bodies are depleted in volatile elements relative to chondrite meteorites, yet the source(s) and mechanism(s) of volatile-element depletion and/or enrichment are poorly constrained. The timing, mechanisms and quantities of volatile elements present in the early inner Solar System have vast implications for diverse processes, from planetary differentiation to the emergence of life. We report major, trace and volatile-element contents of a glass bead derived from the D'Orbigny angrite, the hydrogen isotopic composition of this glass bead and that of coexisting olivine and silicophosphates, and the 207Pb–206Pb age of the silicophosphates, 4568 ± 20 Ma. We use volatile saturation models to demonstrate that the angrite parent body must have been a major body in the early inner Solar System. We further show via mixing calculations that all inner Solar System bodies accreted volatile elements with carbonaceous chondrite H and N isotope signatures extremely early in Solar System history. Only a small portion (if any) of comets and gaseous nebular H species contributed to the volatile content of the inner Solar System bodies. This article is part of the themed issue ‘The origin, history and role of water in the evolution of the inner Solar System’.

Artificial intelligence (AI) is being increasingly applied in the geosciences, particularly in fields like mineralogy, where it supports tasks such as mineral classification, automated thin‐section image analysis, or mineral exploration targeting. Such tasks require large structured and standardized data sets, which are currently not available. We build two databases to fill this gap: (i) MRMinerals contains a list of the 400 most common and geologically significant minerals, including major rock‐forming minerals, key accessory minerals, and economically important ore minerals with machine‐readable formulas as the key feature. (ii) MineralTD contains a large training data set with 10,000+ compositions for each of the 400 minerals in MRMinerals. MineralTD is split into two subdatasets: MineralTDMeasured and MineralTDSynthetic. MineralTDMeasured contains approximately 140,000 mineral compositions from the open‐access geochemical databases and repositories GEOROC, Pangaea, PetDB, RRUFF, and ESMD. MineralTDSynthetic contains synthetic mineral compositions, generated using machine‐readable formulas from MRMinerals, with at least 10,000 compositions per mineral. MineralTD is annotated with metadata, such as mineral frequency, rock classification, data source, and methods used to provide a full understanding of the individual data set. The MRMinerals and MineralTD are ready‐to‐use open access data sets that enable scalable, data‐driven research in mineralogy, e.g., ML applications. We present MRMinerals and MineralTD, two open‐access, ML‐ready mineral data sets. Together, they provide machine‐readable formulas and 10,000+ compositions per mineral, enabling scalable, data‐driven approaches for applying ML to mineral data analysis.

The understanding of silicate weathering and its role as a sink for atmospheric CO2 is important to get a better insight into how the Earth shifts from warm to cool climates. The lithium isotope composition (δ7Li) of marine carbonates can be used as a proxy to track the past chemical weathering of silicates. A high‐resolution δ7Li record would be helpful to evaluate the role of silicate weathering during the late Cretaceous climate cooling. Here, we assess chalk as a potential archive for reconstructing Late Cretaceous seawater Li isotope composition by comparing Maastrichtian chalk from Northern Germany (Hemmoor, Kronsmoor) to a Quaternary coccolith ooze from the Manihiki Plateau (Pacific Ocean) as a lithological analog to modern conditions. We observe a negative offset of 3.9 ± 0.6‰ for the coccolith ooze relative to the modern seawater Li isotope composition (+31.1 ± 0.3‰; 2SE; n = 54), a value that falls in the range of published offsets for modern core‐top samples and for brachiopod calcite. Further, the negative offset between the Li isotope compositions of Manihiki coccolith ooze and modern planktonic foraminifera is 2.3 ± 0.6‰. Although chalk represents a diagenetically altered modification of pelagic nannofossil ooze, manifested by changes in the composition of trace elements, we observe a consistent offset of Li isotope data between Maastrichtian chalk and Maastrichtian planktonic foraminiferal data (−1.4 ± 0. 5‰) that lies within the uncertainty of modern values. We therefore suggest that chalk can be used as a reliable archive for δ7Li reconstructions. Key Points Chalk is a reliable archive for the Li isotope composition of seawater Coccolith ooze has a negative offset of 3.9 ± 0.6‰ from modern seawater for Li isotope ratios The estimated mean value for the late Maastrichtian seawater Li isotope composition is +27.5 ± 1.0‰

Boron abundances and B isotopic compositions of well-characterized blueschists and eclogites from the Raspas Complex (Ecuador) were analyzed to improve the use of boron as a tracer for recycling at convergent margins. The MORB-type eclogite interacted with internally-derived fluids released from metabasalt during the transition from blueschist to eclogite, with input from sediments. During metasomatism, B was gradually leached from the MORB-type eclogites (decrease from 6 g/g to 1.5 g/g), and their B isotopic composition was driven to isotopically heavier values in the range of 7.4 to 3.4 . The B isotopic composition of the metasomatic fluid is estimated between and +1 . The isotopic composition of the least metasomatized MORB-type eclogite samples ( ) is considered close to the B isotopic composition of the dehydrated AOC in the case of Raspas at the stage of deepest subduction and most extensive dehydration. This constitutes a decrease in of approximately 10 from its likely pre-subduction AOC protolith. The blueschist experienced a type of high-pressure metasomatism that is distinct from the one that affected the MORB-type eclogites. The metasomatic fluids were internally-derived and released by metabasalt as well, but with more input from sediments. The metasomatic fluid had a B isotope signature of approximately 5.2 . The zoisite eclogite samples show a very distinct mineralogical and geochemical composition that records the highest degree of high-pressure metasomatic overprint. Their elemental and isotopic composition was thereby set to and . As demonstrated in previous studies, the high-pressure metasomatic fluid that caused the metasomatic overprint was mainly derived from– or interacted with– serpentinite, but had admixed components from metabasalts and metasediments. The B isotopic composition of the respective fluid is estimated at , which overlaps with the composition of most volcanic arc basalts. This study, therefore shows, that metasomatic fluids that migrated through the Raspas slab at a depth of 50–70km had a B isotopic composition between to +1 and were, thus, significantly heavier than that of the mantle.

The extensive P-T stability and the high chemical variability of tourmaline (Tur) together with its common occurrence in metasediments proves its high potential for petrological and (isotope) geochemical studies on fluid-rock interaction in subduction- and collision-related rocks. This paper reviews the occurrence, major element chemistry and boron isotopic composition of Tur in high- and ultrahigh-pressure metamorphic (UHPM) rocks. In addition, it presents a new discovery of coesite-bearing Tur (schorl) from the Erzgebirge (Germany), as well as Tur (dravite) related to the retrograde history of coesite- and diamond-bearing rocks from the Erzgebirge and the Kokchetav Massif (Kazakhstan). The scarce data on worldwide occurrences of (U)HPM Tur reveal a high occupation of the crystallographic X-site (dominated by Na) and the possible presence of excess B, with little further distinctiveness in its major element composition when compared with Tur from medium-grade rocks. High K2O contents in Tur are probably not related to UHP growth or equilibration. The B isotopic composition of (U)HPM Tur ranges in δ11B from -16 to +1 per mil, with many samples in or below the range of continental crust. In contrast, Tur formed during retrograde fluid influx typically shows high δ11B values (up to +28 per mil), suggesting heavy-B fluids infiltrating the exhuming (U)HPM units. Coesite inclusions in Tur, characterized by Raman spectroscopy, are regarded as the best indicator for its UHP stability.

The boron isotopic composition of zoned tourmaline in two metasediments from the island of Syros, determined by secondary-ion mass spectrometry (SIMS), reflects the sedimentary and metamorphic record of the rocks. Tourmaline from a silicate-bearing marble contains small (≤20 μm) detrital cores with highly variable δ 11 B values (−10.7 to +3.6‰), pointing to a heterogeneous protolith derived from multiple sources. The sedimentary B isotopic record survived the entire metamorphic cycle with peak temperatures of ∼500°C. Prograde to peak metamorphic rims are homogeneous and similar among all analysed grains ( δ 11 B ≈ +0.9‰). The varying δ 11 B values of detrital cores in the siliceous marble demonstrate that in situ B isotope analysis of tourmaline by SIMS is a potentially powerful tool for provenance studies not only in sediments but also in metasediments. A meta-tuffitic blueschist bears abundant tourmaline with dravitic cores of detrital or authigenic origin ( δ 11 B ≈ −3.3‰), and prograde to peak metamorphic overgrowth zones (−1.6‰). Fe-rich rims, formed during influx of B-bearing fluids under retrograde conditions, show strongly increasing δ 11 B values (up to  +7.7‰) towards the margins of the grains. The δ 11 B values of metamorphic tourmaline from Syros, formed in mixed terrigenous–marine sediments, reflect the B signal blended from these two different sources, and was probably not altered by dehydration during subduction.

We performed experiments with thin film diffusion couples to simultaneously measure diffusion coefficients of Zr, Hf, Nb and Ta parallel to the a- and c-axes of synthetic rutile in a gas mixing furnace at controlled oxygen fugacity at temperatures between 800 and 1100 ∘ C . Depth profiles of the diffusion couples were measured using secondary-ion mass spectrometry. Some of the diffusion profiles show a concentration dependence, which indicates different diffusion mechanisms above and below a particular trace-element concentration level ( ∼ 1000 μ g / g ). The diffusion coefficients for the mechanism dominant at high-concentration levels are approximately two orders of magnitude smaller than for the low-concentration mechanism. Below the critical concentration the diffusion coefficient is constant, as consistently shown in all of the experiments. For this diffusion coefficient we have found that D Zr ∼ D Nb > D Hf > > D Ta , and diffusion is isotropic for the four elements at all investigated T and f O 2 conditions. At 1000 ∘ C for log f O 2 < FMQ+1, the diffusion coefficients decrease with increasing oxygen fugacity where D is proportional to f O 2 n with exponents n ≈ - 0.25 for Zr and Hf and n ≈ - 0.30 for Nb and Ta. Diffusivites of Nb and Ta strongly differ from each other at all investigated conditions, thus providing the potential to fractionate these geochemical twins, as suggested earlier. The present data and literature data for Zr and Ti self diffusion are interpreted and predicted based on published quantitative point defect models. Two end-member diffusion mechanisms were identified for impurity diffusion of Zr: (i) an interstitialcy mechanism involving Ti 3 + on interstitial sites, which is dominant at approximately log f O 2 < FMQ+2; (ii) a vacancy mechanism involving Ti vacancies, which is dominant at approximately log f O 2 > FMQ+2. The point defect calculations also explain the observed effects of heterovalent substitutions, such as Nb 5 + for Ti 4 + at high concentration levels for changes in the diffusion mechanism and hence diffusion rates. In the case of rutile, this concentration effect becomes much more sensitive to the substitution level at lower temperature. In natural rutile penta- and hexavalent cations may largely be charge balanced by mono-, di- and trivalent cations, such that the doping effect on diffusion may be reduced or may even be reversed. The Arrhenius relationships established here may therefore not be directly applicable to natural rutile. We obtained the following Arrhenius relationships (with diffusion coefficients D in m 2 / s , f O 2 in Pascal and T in Kelvin), which are only applicable for log f O 2 < FMQ+2: log D Zr = ( - 0.40 ± 0.47 ) + ( - 0.253 ± 0.019 ) log f O 2 10 - 7 - 414 ± 11 kJ/mol R T ln 10 log D Hf = ( - 0.08 ± 0.63 ) + ( - 0.266 ± 0.023 ) log f O 2 10 - 7 - 428 ± 15 kJ/mol R T ln 10 log D Nb = ( - 0.19 ± 0.36 ) + ( - 0.294 ± 0.014 ) log f O 2 10 - 7 - 421 ± 9 kJ/mol R T ln 10 log D Ta = ( 0.45 ± 0.73 ) + ( - 0.304 ± 0.015 ) log f O 2 10 - 7 - 463 ± 18 kJ/mol R T ln 10

High-pressure metamorphic (HPM) rocks (derived from igneous protoliths) and their metasomatised rinds from the island of Syros (Greece) were analysed for their B and Cl whole-rock abundances and their H 2 O content by prompt-gamma neutron-activation analysis (PGNAA) and for their Li and Be whole-rock abundances by ICP-OES. In the HPM rocks, B /Be and Cl /Be ratios correlate with H 2 O contents and appear to be controlled by extraction of B and Cl during dehydration and prograde metamorphism. In contrast, samples of the metasomatised rinds show no such correlation. B /Be ratios in the rinds are solely governed by the presence or absence of tourmaline, and Cl /Be ratios vary significantly, possibly related to fluid inclusions. Li/Be ratios do not correlate with H 2 O contents in the HPM rocks, which may in part be explained by a conservative behaviour of Li during dehydration. However, Li abundances exceed the vast majority of published values for Li abundances in fresh, altered, or differentiated oceanic igneous rocks and presumably result from metasomatic enrichment of Li. High Li concentrations and highly elevated Li/Be ratios in most metasomatised samples demonstrate an enrichment of Li in the Syros HP mélange during fluid infiltration. This study suggests that B and Cl abundances of HPM meta-igneous rocks can be used to trace prograde dehydration, while Li concentrations seem to be more sensitive for retrograde metasomatic processes in such lithologies.