Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.

<p>The volcanic–plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic–plutonic connection is to quantify the amount and rates of melt segregation within a crystallising plutonic body, and to compare the volumes and rates with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello (WA) pluton. The WA tonalite (WAT) is part of the southern Alps and represents an intrusive body emplaced at 2.5 kbar in ~1.2 Myr (Floess and Baumgartner, 2015; Schaltegger <em>et al.</em>, 2019). The WAT exhibits a coarse-grained, equigranular texture and is composed of hornblende partially replaced by biotite, plagioclase, quartz, K-feldspar, apatite, zircon, and secondary epidote. K-feldspar, quartz and albite-rich plagioclase (An<sub>25-40</sub>) are late and occur as interstitial phases. Several types of igneous structures, constituting <0.5 vol.% of the WA, are found, comprising: (i) hornblende and biotite accumulations (0.1–30 m) with interstitial K-feldspar, quartz and albite-rich plagioclase (An<sub>25-40</sub>) representing 25–45 vol.% of the rock; (ii) plagioclase (An<sub>40-70</sub>) accumulations with 40–50 vol.% of the same interstitial assemblage; and (iii) quartz-, albite- and K-feldspar-rich domains (0.1–10 m) containing WAT-derived plagioclase phenocrysts which form either zoned aplitic to pegmatitic dikes or schlieren-shaped bodies probably representing <em>in situ</em> melt segregations. The latter are spatially associated with the accumulation zones. Hornblende, biotite, and plagioclase phenocrysts have essentially the same compositional range in accumulations and segregations. This observation indicates that deformation-driven crystal–crystal and crystal–melt segregation operated within the host tonalite. Quantitative modal compositions and mass balance calculations indicate that the hornblende–biotite accumulations lost 60–90 vol.% of their plagioclase phenocrysts and 20–55 vol.% interstitial melt, whereas the plagioclase accumulations lost up to 15 vol.% melt. Such calculations place the maximum efficiency of crystal–melt segregation to 40–55 % in the WAT, as most of the melt remains trapped within the crystal framework. Based on phase relationships and major element modelling, it is proposed that the peritectic relationship hornblende + melt<sub>1</sub> = biotite + quartz + melt<sub>2</sub> and the efficiency of plagioclase–melt separation are linked to the variable composition of the felsic dikes. Such a reaction is known from experimentally derived phase relationships of tonalite (Marxer and Ulmer, 2019) and probably plays a fundamental role linking pluton solidification and extraction of interstitial liquid.</p>

Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Tremadocian igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneisses in the Cévennes parautochton domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to grauwackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represent former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer-grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1±5.5 Ma and 483.0±5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Tremadocian volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wustite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Tremadocian igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an extensional environment and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.