 | Residual forearc peridotites recording the early magmatic stage of subduction initiation Abstract: Subduction initiation is fundamental to plate tectonics. Early basaltic magmatism preceding boninitic magmatism is key to understanding the onset and mechanism of subduction initiation. However, the mantle processes during the early basaltic magmatism remain poorly understood due to the scarcity of corresponding residual peridotites. To find the residual peridotites from the early stage of subduction initiation, we compiled spinel data and examined peridotites from the Izu-Bonin-Mariana and Tonga forearc regions that contain some of the lowest spinel Cr/Al ratios. These samples present small amounts of pargasitic amphibole and light rare earth elements depleted patterns in their clinopyroxenes and amphiboles. These features differ from depleted forearc peridotites associated with boninitic magma, abyssal peridotites, and backarc peridotites, but resemble lherzolites from the Oman ophiolite. Melting models can reproduce the compositions of clinopyroxene and the corresponding early basalts for each region. We suggest that the studied forearc peridotite samples are residues after early basaltic magma extractions, which potentially involved a common process of near fractional melting under moist conditions. |
 | Mixed paragenesis diamond inclusions form during metasomatic fluid evolution Abstract: Mineral inclusions in diamond provide the context for diamond formation, yet some diamonds contain disequilibrium ‘mixed paragenesis’ mineral inclusions. Mixed paragenesis diamonds are inferred to somehow record growth of a single diamond in both mafic eclogitic and peridotitic substrates. Here we use thermodynamic modelling with the Extended Deep Earth Water (DEW) model to show that mixed paragenesis mineral inclusions can form during a single episode of diamond growth via fluid-rock metasomatism in the lithospheric mantle. Simulations of fluid infiltration and reactions predict sequential mineral precipitation, with early eclogitic minerals (omphacite and garnet) followed by more peridotitic minerals (e.g., olivine and Mg-rich garnet) during continued diamond growth. Concurrently, modelled fluids evolve from silicic toward carbonatitic compositions and reproduce the compositional range of high density fluids observed in fibrous diamonds. These results demonstrate that mixed paragenesis inclusions and the diversity of diamond-forming fluids can arise from progressive metasomatic reactions during a single diamond-forming event, rather than requiring a diamond to grow first in eclogite and then in peridotite. |
 | Is Hg recycling into the Earth’s mantle negligible? Abstract: Mercury (Hg), with its ultra-trace abundance in rocks and high volatility, challenges the investigation of its distribution and mobilisation in geological systems. Based on similar Hg isotope mass independent fractionation (i.e. Δ199Hg) values between marine and terrestrial sediments and magmatic rocks, it is commonly suggested that Hg has been continuously exchanged between the Earth’s mantle and the atmosphere through subduction recycling. However, a review of Hg concentrations in ophiolites and orogenic peridotites, mantle xenoliths, mantle derived basalts and mafic rocks (Mg# ≥60), and variably metamorphosed collision and subduction related rocks suggests otherwise. Ultra-trace Hg concentrations in mantle rocks and mantle derived magmas are inconsistent with Hg recycling into the mantle. In fact, collision and subduction related rocks progressively lose Hg with increasing metamorphic grade, indicating that only ≤1 ng/g of Hg is transferred to the sub-arc mantle melting regions. These observations also suggest that Hg recycling was inhibited during the Archean. Here we warn about the risk of using Hg isotopes alone to interpret the Hg cycle on Earth and urge the need for new and accurate Hg concentrations in crystalline rocks from different geological settings. |
 | Can CO2 outgassing and carbonate precipitation explain the Lomagundi Excursion? Abstract: The Lomagundi-Jatuli event (2.3–2.0 Ga) is one of the striking carbon isotopic (δ13Ccarbonate) excursion events in the Earth’s history, marked by anomalously high δ13Ccarbonate reaching up to +30 ‰. The conventional explanation attributes it to enhanced organic carbon burial. However, the lack of organic-rich strata synchronous with the excursion demands the reconsideration of alternative biogeochemical processes to explain this isotopic anomaly. Moreover, the excursion is observed only in the evaporitic and near shore carbonates, with no evidence from the open ocean, demanding a facies based biogeochemical explanation. Here, we propose a depositional framework for Lomagundi successions and explore the possibility of CO2 outgassing and carbonate precipitation as potential drivers responsible for this excursion, as these processes remain the least explored among the proposed hypotheses. Through sedimentological evidence from previous studies and Rayleigh fractionation calculations, we argue that dominant loss of dissolved inorganic carbon (DIC) through CO2 outgassing in the evaporitic facies and carbonate precipitation in the near shore facies along with a well mixed DIC reservoir in the open ocean explains the observed Lomagundi Excursion. |
 | Revisiting K-Pg boundary with lithium isotopes: divergent marine and continental responses Abstract: Seawater lithium isotope records (δ7Li) are powerful tools for investigating long term climate change and its relationship with continental silicate weathering. While most past ocean δ7Li reconstructions rely on foraminifera-rich carbonates, we introduce a novel approach using marine authigenic clays. Our findings demonstrate that clay authigenesis is an abiotic process that fractionates Li isotopes consistently in laboratory settings and across marine sediments. We apply this method to the Cretaceous-Paleogene (K-Pg) boundary, a critical interval marked by one of Earth’s five largest mass extinctions, and present a new clay derived seawater δ7Li record (64–69 Ma). Our results, compared with those of the carbonate record, uncover a dual environmental disturbance. The clay record indicates a protracted perturbation initiated at 69 Ma, driven by gradual shifts in soils, fluvial systems, and continental weathering, which remained largely unaffected by Deccan volcanism or the meteorite impact. In contrast, abrupt fluctuations are recorded in marine carbonates at 66 ± 0.3 Ma, signaling short lived yet significant changes of oceanic carbon chemistry. These findings highlight the asynchronous behaviour of terrestrial and marine systems during this major ecological transition. |
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