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Ophiolite obductions in the tropics are coeval with Phanerozoic glaciations. The exposure of mafic and ultramafic rocks is thought to trigger cooling by increasing global weatherability. However, each Palaeozoic icehouse also coincides with a δ 13 C increase of 3−5‰, interpreted as an increase in organic carbon burial, not weatherability. Here we provide a framework that explains the tectonic forces behind Palaeozoic glaciations through increased organic carbon burial caused by the weathering of mafic and ultramafic lithologies in ophiolites. To evaluate the leverage ophiolite obduction has over organic carbon burial, we couple a mineral weathering model with a carbon box model. We show that the weathering of (ultra)mafic rocks can substantially enhance the preservation of organic carbon through the formation of high-surface-area smectite clays. The heightened organic carbon burial induced by an idealized ophiolite obduction causes ocean δ 13 C to increase by ~3.7‰. The temporal evolution and magnitude of our modelled δ 13 C excursion approximates Palaeozoic records. We present an analysis of shale geochemistry, which shows a correlation between ultramafic provenance and total organic carbon. Our results indicate that high-surface-area clays, formed during weathering of (ultra)mafic lithologies, exert a major control over Earth’s long-term carbon cycle. Weathering of mafic and ultramafic lithologies in ophiolites can enhance the preservation of organic carbon through the formation of smectite clays and modulate Earth’s climate, according to a coupled mineral weathering and carbon box model.

The accretion of volatile-rich material from the outer Solar System represents a crucial prerequisite for Earth to develop oceans and become a habitable planet 1 – 4 . However, the timing of this accretion remains controversial 5 – 8 . It has been proposed that volatile elements were added to Earth by the late accretion of a late veneer consisting of carbonaceous-chondrite-like material after core formation had ceased 6 , 9 , 10 . This view could not be reconciled with the ruthenium (Ru) isotope composition of carbonaceous chondrites 5 , 11 , which is distinct from that of the modern mantle 12 , or of any known meteorite group 5 . As a possible solution, Earth’s pre-late-veneer mantle could already have contained a fraction of Ru that was not fully extracted by core formation 13 . The presence of such pre-late-veneer Ru can only be established if its isotope composition is distinct from that of the modern mantle. Here we report the first high-precision, mass-independent Ru isotope compositions for Eoarchaean ultramafic rocks from southwest Greenland, which display a relative 100 Ru excess of 22 parts per million compared with the modern mantle value. This 100 Ru excess indicates that the source of the Eoarchaean rocks already contained a substantial fraction of Ru before the accretion of the late veneer. By 3.7 billion years ago, the mantle beneath southwest Greenland had not yet fully equilibrated with late accreted material. Otherwise, no Ru isotopic difference relative to the modern mantle would be observed. If constraints from other highly siderophile elements besides Ru are also considered 14 , the composition of the modern mantle can only be reconciled if the late veneer contained substantial amounts of carbonaceous-chondrite-like materials with their characteristic 100 Ru deficits. These data therefore relax previous constraints on the late veneer and are consistent with volatile-rich material from the outer Solar System being delivered to Earth during late accretion. Ruthenium isotope compositions of the oldest preserved mantle rocks from Greenland imply that volatile-rich outer Solar System material was not delivered to Earth until very late in the planet’s formation.

The effect of damage on the brittle deformation of mafic and ultramafic rocks has been investigated by performing triaxial deformation experiments on thermally cracked and intact rock samples. The investigation was performed by recording the axial and lateral strains during deformation while simultaneously capturing the ultrasonic velocity, and electrical resistivity. While the peak strength is presumably controlled by the stiff intrinsic fractures, the crack opening mode also showed critical effects on the attained peak strength. The pore pressure distribution showed an apparent control over the dynamic Young's modulus as the ratio between the dynamic and static modulus of thermally cracked rocks is significantly higher than that of intact rocks. The compliant nature and the higher inelastic volumetric strain of the thermally cracked samples further indicated a possible explanation to the steep dipping plates and the taller topographic heights at the trench outer rise systems of old subduction zones. Plain Language Summary The presence of fractures strongly influences the physical properties of rocks and not many experiments have been performed to understand the effect those fractures have on the deformation of mafic and ultramafic rocks. We find that, to address the deformation of the subduction zones, it is essential to perform experiments on such rocks under fully saturated conditions. As the incoming, relatively intact, oceanic plate sinks at the convergent boundaries, the plate bends significantly and creates a large number of faults. Therefore, the physical properties of the rock are subject to change and the way that rock deforms changes compared to an undamaged rock. However, so far, no research has been conducted to understand this change fully. Therefore, by performing experiments in the laboratory, we have attempted to understand how damage affects the physical properties and the deformation. By such experiments, we have identified that when fractures are present, the oceanic rocks show strong volume expansions before break by fracture and alter the physical properties. The results we have obtained here by gathering velocity, electrical resistivity, and strain, at the same time, can be related to the geophysical and topographic observations of the trench outer‐rise systems. Key Points The effect of damage on brittle deformation of mafic and ultramafic rocks has been addressed via experiments The ultrasonic velocity, electrical resistivity, and strain are measured simultaneously during deformation of rocks Presence of cracks strongly influence deformation and physical properties which are comparable to outer rise geophysical observations

Iron‐rich rocks undergoing oxidation in the presence of water are promising sources of geologic hydrogen (H2). Yet, such reactions are often considered self‐limiting due to passivation and reduced permeability if volume expansion occurs. This study investigates hydrogen generation during a flow‐through experiment on fractured iron‐rich cores packed with proppants, simulating a stimulated subsurface environment. Hydrogen generation initially declined over time. Introducing a thermochemical fluid to accelerate the reaction via thermal enhancement did not yield measurable hydrogen gains. Subsequent acid treatment for 2 hr led to a 30‐fold increase in hydrogen generation, which was sustained for 144 hr before the onset of a decline. Post‐acidizing hydrogen generation remained larger than pre‐acidizing hydrogen. Post‐experiment characterizations confirmed the development of preferential flow paths, mineral precipitation along fracture surfaces, and fluid‐induced surface alteration. These results demonstrate that periodic chemical stimulation can significantly enhance and prolong hydrogen generation by reactivating mineral‐fluid interfaces.

In the last two centuries, since the dawn of modern geology, heavy minerals have been used to investigate sediment provenance and for many other scientific or practical applications. Not always, however, with the correct approach. Difficulties are diverse, not just technical and related to the identification of tiny grains, but also procedural and conceptual. Even the definition of “heavy minerals” is elusive, and possibly impossible. Sampling is critical. In many environments (e.g., beaches), both absolute and relative heavy mineral abundances invariably increase or decrease locally to different degrees owing to hydraulic-sorting processes, so that samples close to \"neutral composition\" are hard to obtain. Several widely shared opinions are misleading. Choosing a narrow size-window for analysis leads to increased bias, not to increased accuracy or precision. Only point-counting provides real volume percentages, whereas grain-counting distorts results in favor of smaller minerals. This paper also briefly reviews the heavy mineral associations typically found in diverse plate-tectonic settings. A mineralogical assemblage, however, only reproduces the mineralogy of source rocks, which does not correlate univocally with the geodynamic setting in which those source rocks were formed and assembled. Moreover, it is affected by environmental bias, and by diagenetic bias on top in the case of ancient sandstones. One fruitful way to extract information on both provenance and sedimentological processes is to look for anomalies in mineralogical–textural relationships (e.g., denser minerals bigger than lower-density minerals; harder minerals better rounded than softer minerals; less durable minerals increasing with stratal age and stratigraphic depth). To minimize mistakes, it is necessary to invariably combine heavy mineral investigations with the petrographic analysis of bulk sand. Analysis of thin sections allows us to see also those source rocks that do not shed significant amounts of heavy minerals, such as limestone or granite, and helps us to assess heavy mineral concentration, the “outer” message carrying the key to decipher the “inner message” contained in the heavy mineral suite. The task becomes thorny indeed when dealing with samples with strong diagenetic overprint, which is, unfortunately, the case of most ancient sandstones. Diagenesis is the Moloch that devours all grains that are not chemically resistant, leaving a meager residue difficult or even impossible to interpret when diagenetic effects accumulate through multiple sedimentary cycles. We have conceived this friendly little handbook to help the student facing these problems, hoping that it may serve the purpose.

Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes. Chromian-spinel from mafic-ultramafic rocks is used as a reliable geotectonic and mantle melting indicator. Here, the authors argue that this only works partially – it can be used to assess information on mantle metasomatic processes but not petrogenesis.

Trace element compositions of magnetite of alkaline mafic–ultramafic rocks were not previously studied but are potentially useful to constrain their petrogenesis and related metallogenesis. In the Permian Emeishan large igneous province, subalkaline mafic–ultramafic intrusions are widely distributed, but sparse alkaline counterparts are recognized, including the Mouding intrusion. The alkaline intrusions are mainly composed of clinopyroxenite, melteigite, jacupirangite, gabbro, syenogabbro, monzogabbro and monzonite. All these lithologies contain magnetite with composite, sandwich and trellis types of ilmenite intergrowths due to heterogeneous oxy-exsolution. Chromium contents of magnetite grains in the magnetite clinopyroxenite unit of the Mouding intrusion decrease from 1257 to 41 ppm within an 80 m interval, which can best be explained by the diffusion-controlled in situ crystallization in a crystal-liquid framework. Notably, magnetite grains from these alkaline intrusions are rich in Nb, Ta, Zr and Hf relative to those from subalkaline intrusions and iron oxide–apatite (IOA), iron oxide copper gold (IOCG) and porphyry deposits. This enrichment demonstrates that the high field strength elements (HFSE) are rich in the parental magmas of alkaline intrusions and have high partition coefficients between magnetite and alkaline magma. Our study demonstrates that magnetite grains in alkaline mafic–ultramafic intrusions have distinctively different trace element compositions from those in subalkaline intrusions, being useful tools in discriminating magma series and understanding the crystallization processes.

The Rustenburg Layered Suite of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. It has long been regarded as a textbook result of fractional crystallization from a melt-dominated magma chamber. Here, we show that most units of the Rustenburg Layered Suite can be derived with thermodynamic models of crustal assimilation by komatiitic magma to form magmatic mushes without requiring the existence of a magma chamber. Ultramafic and mafic cumulate layers below the Upper and Upper Main Zone represent multiple crystal slurries produced by assimilation-batch crystallization in the upper and middle crust, whereas the chilled marginal rocks represent complementary supernatant liquids. Only the uppermost third formed via lower-crustal assimilation–fractional crystallization and evolved by fractional crystallization within a melt-rich pocket. Layered intrusions need not form in open magma chambers. Mineral deposits hitherto attributed to magma chamber processes might form in smaller intrusions of any geometric form, from mushy systems entirely lacking melt-dominated magma chambers. The Rustenburg Layered Suite of the Bushveld Complex, South Africa, has long been regarded as a textbook result of fractional crystallization from a melt-dominated magma chamber. Here, the authors find that the Rustenburg Layered Suite can be derived from crustal assimilation by komatiitic magma to form magmatic mushes without requiring the existence of a magma chamber by using thermodynamic models.

Ultramafic rocks of the Yarlung-Tsangpo ophiolites (lherzolites, harzburgites, dunites and pyroxenites) have been interpreted as lithospheric mantle relics of the Cretaceous Neo-Tethys Ocean (NTO). Studying these rocks could provide key constraints on the NTO’s evolution and invaluable information on sub-ridge mantle heterogeneity. The Xiugugabu ophiolite crops out in the western segment of the Yarlung-Tsangpo Suture Zone (YTSZ) and is dominantly composed of mantle components with minor crustal material. Here we report a comprehensive field, petrological and geochemical study of the ultramafic rocks from this ophiolitic massif. The lherzolites and harzburgites are less refractory than the Xigaze ultramafic rocks in the middle part of the YTSZ. Petrology and simple models are consistent with 10–20% fractional melting from an anhydrous, depleted mantle source, within the spinel and garnet stability fields. Part of dunites (Dunite-II) and pyroxenites formed during pervasive melt impregnation into host peridotites and record the modal and geochemical effects of post-melting melt–rock interactions. The discrepancy of light rare earth element (LREE) contents between whole rocks and their two pyroxene components in peridotites indicates that they have trapped interstitial melts along grain boundaries at a post-melting stage. These melts were potentially derived from shallow sources in spinel domains. Bulk rock osmium isotopic compositions suggest that the pyroxenites and part of the dunites are highly radiogenic. These enriched Os signatures are likely imposed by percolating melts derived from mantle sources hybridized by ancient recycled crustal materials. When comparing with results from previous studies of other YTSZ peridotites with ultra-depleted nature and unradiogenic Os isotopic compositions, we conclude that the sub-ridge mantle beneath the Neo-Tethyan Ocean was heterogeneous, with not only unradiogenic but also highly radiogenic mantle domains.

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip. Infiltration of carbon-rich fluids promotes the formation of carbonate minerals and weak minerals in the forearc mantle and influences the down-dip limit of seismic zones, according to thermodynamic calculations between fluid and mantle rocks.