Explore the vast range of titles available.

Deciphering the Earth's deep carbon cycle, from mantle plumes to mid‐ocean ridges, remains incompletely understood. In this study, we analyze the magnesium isotope composition of basalts collected from the South Mid‐Atlantic Ridge (SMAR), which have been influenced by the off‐axis Saint Helena plume originating from the core‐mantle boundary. The magnesium isotope composition of SMAR basalts falls within a similar range (−0.22 to −0.32‰; average −0.25‰ ± 0.03‰) to that of known global oceanic basalts. However, isotope mixing calculations suggest that the lighter magnesium isotope composition in the SMAR basalts is due to the incorporation of approximately 5%–10% recycled carbonate material carried by the Saint Helena plume into the SMAR asthenosphere. This finding not only highlights the interaction between ridges and off‐axis plumes but also proposes a comprehensive model for the Earth's deep carbon cycle, spanning from the subduction zone through the core‐mantle boundary to the mid‐ocean ridge system. Plain Language Summary The investigation of the Earth's deep carbon cycle is crucial for elucidating the processes of material transport within the Earth's interior and mantle convection. Despite significant advancements, understanding the complete carbon cycle still presents challenges, particularly in relation to the process of carbon transfer from subducted ancient oceanic crust to the generation of new oceanic crust. By exploring the interaction between mantle plumes and mid‐ocean ridges (MORs), it is possible to achieve a more comprehensive understanding of the intricate Earth's deep carbon cycle. In this study, we present precise Mg isotopic data obtained from mid‐ocean ridge basalts (MORBs) in the South Atlantic region. By integrating the Mg isotope and radiogenic isotopic compositions of basalts from the South Mid‐Atlantic Ridge (SMAR) and Saint Helena Island, we have determined that approximately 5%–10% of recycled carbonate material carried by the Saint Helena mantle plume has been transported into the asthenosphere beneath the SMAR system. Our findings contribute to the development of a coherent model of the Earth's deep carbon cycle, tracing the pathway from subduction zones to the core‐mantle boundary and ultimately return to MOR systems. This model provides valuable insights for geologists seeking to comprehend the material cycle of the Earth. Key Points The composition of Mg isotope in basalts suggests interaction of the Saint Helena plume and South Mid‐Atlantic Ridge system Subducted carbon from the Saint Helena plume has been transported to the South Mid‐Atlantic Ridge system Carbon derived from the subduction zone has the potential to transport to the core‐mantle boundary and return to the mid‐ocean ridge system

In addition to traditional isotopes, non-traditional isotopes have gradually been applied in the hydrosphere, such as lithium, strontium, magnesium, etc. Magnesium is one of the main elements in the earth’s crust and participates in most geophysical and chemical processes. In seawater, the δ 26 Mg value is -0.82‰ and relatively constant, which determines that the magnesium isotope composition of terrestrial water from seawater recharge is relatively consistent. This provides a constant initial isotope scale for studying water cycle processes and can be used as an ideal tracer element for groundwater research. Through comparative analysis of magnesium isotope composition characteristics and water cycle processes it is shown that the fractionation range of magnesium isotopes can reach 3.5‰, and they have different compositional characteristics in surface water and groundwater. In the process of groundwater rock interaction, magnesium isotopes undergo fractionation, providing valuable information for identifying mineral phase evolution. Magnesium isotopes follow the law of mass dependent fractionation. Therefore, in the process of groundwater infiltration and runoff, magnesium isotope fractionation caused by ion exchange and cohesive soil formation is easier for lighter magnesium isotopes to migrate to heavier magnesium isotopes. This provides the potential for magnesium isotopes to be used to study salt transport and the formation process of clay minerals.

We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ 57 Fe = −0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n  = 6); δ 26 Mg = − 0.28 to −0.25 with an average of −0.26 ± 0.03 (2SD, n  = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ 57 Fe ranging from 0.12 to 0.30. The δ 57 Fe values of minerals in these garnet-bearing pyroxenites also vary widely (−0.25 to 0.08 in olivines, −0.04 to 0.25 in orthopyroxenes, −0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ 26 Mg (−0.43) relative to the mantle. The δ 26 Mg of minerals in the garnet-bearing pyroxenite range from −0.35 for olivine and orthopyroxene, to −0.34 for clinopyroxene, 0.04 for spinel and −0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ 57 Fe (as high as 1.00) but the lightest δ 26 Mg (as low as −1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ 57 Fe and δ 26 Mg in the Hannuoba garnet-bearing pyroxenite and in the phlogopite clinopyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle.

Laboratory experiments have shown that thermal gradients in silicate melts can lead to isotopic fractionation; this is known as the Richter effect. However, it is perplexing that the Richter effect has not been documented in natural samples as thermal gradients commonly exist within natural igneous systems. To resolve this discrepancy, theoretical analysis and calculations were undertaken. We found that the Richter effect, commonly seen in experiments with wholly molten silicates, cannot be applied to natural systems because natural igneous samples are more likely to be formed out of partially molten magma and the presence of minerals adds complexity to the behaviour of the isotope. In this study, we consider two related diffusion-rate kinetic isotope effects that originate from chemical diffusion, which are absent from experiments with wholly molten samples. We performed detailed calculations for magnesium isotopes, and the results indicated that the Richter effect for magnesium isotopes is buffered by kinetic isotope effects and the total value of magnesium isotope fractionation can be zero or even undetectable. Our study provides a new understanding of isotopic behaviour during the processes of cooling and solidification in natural magmatic systems.

Seafloor hydrothermal systems play a significant role in the oceanic Mg cycle due to ubiquitous deposits of secondary Mg-rich clays during the strong fluid-rock reactions. However, the magnitude of net Mg enrichment and Mg isotopic fractionation, particularly within the medium-high temperature hydrothermal systems in felsic-hosted settings, are not well studied yet. Here we report elemental and isotopic compositions of Mg in hydrothermal chlorite-rich sediments, volcanic materials, and terrigenous sediments collected during the IODP Expedition 331 drilled to the thick sediment-covered and felsic-hosted middle Okinawa Trough (Iheya North Knoll) in the West Pacific. We investigate the sources of Mg in chlorite and Mg isotopic behavior at medium-high temperature hydrothermal alteration. After 1 mol/L HCl leaching, Mg isotopic compositions of chlorite-rich sediments present overall similar values in the residual fractions and bulk samples albeit with slightly higher values in the leachates. Mineralogical differentiation primarily determines the Mg isotopic compositions, showing that siliciclastic residues have slightly higher δ 26 Mg values than the leachates dominated by carbonates and oxides/hydroxides. Significant Mg isotopic fractionation happened in the medium-high temperature (∼150°C to 260°C) felsic-hosted hydrothermal system, with Δ 26 Mg Chl-SW ranging from 0.15‰ to 0.71‰ and yielding a negative correlation with temperature. This observation suggests the preferential incorporation of heavy Mg isotopes by the secondary chlorite precipitation. We infer that the medium-high temperature hydrothermal systems can take up about 8–14% of riverine input of Mg in the arc and back-arc regions. Incomplete removal of aqueous Mg in porewater and vent fluids by the medium-high temperature hydrothermal alterations in the arc and back-arc basins provides constraints on the Mg budget and isotopic composition of seawater.

The measurement of magnesium isotope ratios at improved accuracy suggests that planetary compositions result from fractionation between liquid and vapour, followed by vapour escape during accretionary growth. Earth's volatile origins In comparison to primitive, chondritic meteorites, which are widely thought to be the building blocks of Earth, Earth and other differentiated planetary bodies are chemically fractionated, with Earth's crust and mantle—the 'silicate Earth'—being strongly depleted in moderately volatile elements (such as lead, zinc, indium and the alkali metals). Two papers in this week's issue suggest that this difference in composition between chondritic meteorites and Earth could be a natural consequence of vapour loss from magma on the surface of growing planetesimals. Ashley Norris and Bernard Wood examined the melting processes that would have occurred during accretion on Earth and its precursor bodies and performed vaporization experiments under conditions of fixed temperature and oxygen partial pressure. They found that the pattern of volatile-element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. Remco Hin and co-authors show that differentiated planetary bodies have isotopically heavier magnesium compositions compared to chondritic meteorites, and conclude that this could be due to the isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals. It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion 1 or inherited from prior nebular fractionation 2 . The isotopic compositions of the main constituents of planetary bodies can contribute to this debate 3 , 4 , 5 , 6 . Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism.

The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate–silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

Incorporation of subducted slab in arc volcanism plays an important role in producing the geochemical and isotopic variations in arc lavas. The mechanism and process by which the slab materials are incorporated, however, are still uncertain. Here, we report, to our knowledge, the first set of Mg isotopic data for a suite of arc lava samples from Martinique Island in the Lesser Antilles arc, which displays one of the most extreme geochemical and isotopic ranges, although the origin of this variability is still highly debated. We find the δ26Mg of the Martinique Island lavas varies from −0.25 to −0.10, in contrast to the narrow range that characterizes the mantle (−0.25 ± 0.04, 2 SD). These high δ26Mg values suggest the incorporation of isotopically heavy Mg from the subducted slab. The large contrast in MgO content between peridotite, basalt, and sediment makes direct mixing between sediment and peridotite, or assimilation by arc crust sediment, unlikely to be the main mechanism to modify Mg isotopes. Instead, the heavy Mg isotopic signature of the Martinique arc lavas requires that the overall composition of the mantle wedge is buffered and modified by the preferential addition of heavy Mg isotopes from fluids released from the altered subducted slab during fluid–mantle interaction. This, in turn, suggests transfer of a large amount of fluidmobile elements from the subducting slab to the mantle wedge and makes Mg isotopes an excellent tracer of deep fluid migration.

The origin of titanium-rich basaltic magmatism on the Moon remains enigmatic. Ilmenite-bearing cumulates in the lunar mantle are often credited as the source, but their partial melts are not a compositional match and are too dense to enable eruption. Here we use petrological reaction experiments to show that partial melts of ilmenite-bearing cumulates react with olivine and orthopyroxene in the lunar mantle, shifting the melt composition to that of the high-Ti suite. New high-precision Mg isotope data confirm that high-Ti basalts have variable and isotopically light Mg isotope compositions that are inconsistent with equilibrium partial melting. We employ a diffusion model to demonstrate that kinetic isotope fractionation during reactive flow of partial melts derived from ilmenite-bearing cumulates can explain these anomalously light Mg isotope compositions, as well as the isotope composition of other elements such as Fe, Ca and Ti. Although this model does not fully replicate lunar melt–solid interaction, we suggest that titanium-rich magmas erupted on the surface of the Moon can be derived through partial melting of ilmenite-bearing cumulates, but melts undergo extensive modification of their elemental and isotopic composition through reactive flow in the lunar mantle. Reactive flow may therefore be the critical process that decreases melt density and allows high-Ti melts to erupt on the lunar surface. Petrological reaction experiments and magnesium isotope data suggest that reactive flow with mantle cumulates can explain the composition of Ti-rich basaltic magmas.

The oceanic magnesium budget is important to our understanding of Earth’s carbon cycle, because similar processes control both (e.g., weathering, volcanism, and carbonate precipitation). However, dolomite sedimentation and low-temperature hydrothermal circulation remain enigmatic oceanic Mg sinks. In recent years, magnesium isotopes (δ 26 Mg) have provided new constraints on the Mg cycle, but the lack of data for the low-temperature hydrothermal isotope fractionation has hindered this approach. Here we present new δ 26 Mg data for low-temperature hydrothermal fluids, demonstrating preferential 26 Mg incorporation into the oceanic crust, on average by ε solid-fluid  ≈ 1.6‰. These new data, along with the constant seawater δ 26 Mg over the past ~20 Myr, require a significant dolomitic sink (estimated to be 1.5–2.9 Tmol yr −1 ; 40–60% of the oceanic Mg outputs). This estimate argues strongly against the conventional view that dolomite formation has been negligible in the Neogene and points to the existence of significant hidden dolomite formation. Earth’s carbon cycle and oceanic magnesium cycle are controlled by processes such as weathering, volcanism and precipitation of carbonates, such as dolomite. Here, the authors contradict the view that modern dolomite formation is rare and suggest instead that dolomite accounts for ~40–60% of the global oceanic Mg output in the last 20 Ma.