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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

A series of synthetic Mid-Ocean Ridge Basalt (MORB) glasses with Fe3+/FeTOT from 0 to 1, determined previously by Mossbauer spectroscopy, were used to test methods for quantifying Fe3+/FeTOT by Raman spectroscopy. Six numerical data reduction methods were investigated, based on conventional approaches as well as supervised and unsupervised machine learning algorithms. For the set of MORB glass standards, with fixed composition, the precision of all methods was ≤±0.04 (1 St.dev.). However, Raman spectra recorded for 42 natural MORB glasses from a wide range of locations revealed a strong correlation between the spectra and composition, despite the latter varying only over a relatively limited range, such that the methods calibrated using the glass standards are not directly applicable to the natural samples. This compositional effect can be corrected by using a compositional term that links spectral variations to the Fe3+/FeTOT value of the glass. The resulting average Fe3+/FeTOT determined by Raman spectroscopy was 0.090 ± 0.067 (n = 42). This value agrees with the latest Fe K-edge XANES and wet-chemistry estimates of 0.10 ± 0.02. The larger uncertainty of the Raman determination reflects the sensitivity of Raman spectroscopy to small changes in the glass structure. While this sensitivity is detrimental for high precision Fe3+/FeTOT determinations, it allows the major element composition of natural MORB glasses to be determined within 1 mol% through the use of an artificial neural network. This suggests that Raman spectrometers may be used to determine the composition of samples in situ at difficult to access locations that are incompatible with X-ray spectrometry (e.g., mid-ocean ridges).

Aim Maintaining genetic connectivity is crucial for species that inhabit the disjunct and unstable deep‐sea hydrothermal vents. We aimed to re‐assess the connectivity of the annelid genus Hesiolyra distributed at hydrothermal vents along the East Pacific Rise (EPR). A previous study detected a major clade among five Hesiolyra populations spanning from 13°N to 21°S and a minor sympatric southern clade with ~1% divergence from the major clade. However, this study was based on a short locus of 366‐bp COI gene, which might not contain sufficient informative sites. Location East Pacific Rise. Methods We sequenced 188 specimens of Hesiolyra from five hydrothermal vent fields along the East Pacific Rise for six mitochondrial and two nuclear loci to infer their genetic divergence, population diversity and gene flow. Results We found a minor southern clade (Hesiolyra aff. bergi) which was genetically distinct from Hesiolyra bergi sensu stricto for all gene markers explored except 16S. We also found shared genotypes between H. bergi and H. aff. bergi likely resulted from incomplete lineage sorting. For H. bergi s.s., we found a low but fixed divergence between the north and south EPR populations. We estimated the northern and southern metapopulations of H. bergi s.s. split ~0.45 Mya (HPD: 0.27–0.74 Mya). The northern metapopulation had a higher haplotype diversity than the southern metapopulation, indicating historical gene flow and loss of genetic diversity in the southern clade. Main conclusions We confirmed that the equatorial dispersal filter observed previously on several other vent species also applies to H. bergi. Our results highlight the power of the multi‐locus approach in revealing the divergence and population genetic history of marine species with strong dispersal capabilities, indicating that the NEPR and the SEPR should be considered as separate biogeographic regions in environmental management and biological conservation.

Seismic energy arriving before the compressional (P) wave passing through the core (PKP), called PKP precursors, have been detected for decades, but the origin of those arrivals is ambiguous. The largest amplitude arrivals are linked to scattering at small‐scale lowermost mantle structure, but because these arrivals traverse both source and receiver sides of the mantle, it is unknown which side of the path the energy is scattered from. To address this ambiguity, we apply a new seismic array method to analyze PKP waveforms from 58 earthquakes recorded in North America that allows localization of the origin of the PKP precursors at the core‐mantle boundary (CMB). We compare these measurements with high frequency 2.5‐D synthetic predictions showing that the PKP precursors are most likely associated with ultra‐low velocity zone structures beneath the western Pacific and North America. The most feasible scenario to generate ULVZs in both locations is through melting of mid‐ocean ridge basalt in subducted oceanic crust. Plain Language Summary When seismic wave energy created by earthquakes encounters features with abrupt changes in material properties some of that energy can be scattered and ultimately be recorded on seismometers. Some of the scattered energy comes from the lowermost mantle near the core‐mantle boundary (CMB), but locating where on the CMB the scattered energy comes from has been challenging. Using new state of the art measurement procedures and by analyzing supercomputer simulations of seismic wave scattering we are able to improve our ability to locate scatterers. Using seismic wave energy from earthquakes occurring in the New Guinea region and recorded on seismometers in North America we have identified regions of scattering at the CMB beneath both the western Pacific Ocean and western North America. The amplitude of the recorded scattered energy is sensitive to material contrasts and is most consistent with an origin from features known as ultra‐low velocity zones (ULVZs). These are thin regions sitting on top of the CMB in which seismic waves passing through them move at a much slower speed. A way to explain their existence in both settings is through partial melting of Mid‐Ocean Ridge Basalt that is in the oceanic crust of subducting slabs. Key Points 2.5‐D synthetic waveform predictions and advances in array analyses allow us to localize PKP scatterer locations Individual scatterers are located beneath North America and beneath the western Pacific Ocean which may be generated by ultra‐low velocity zones Mid‐ocean ridge basalt melting could explain ULVZ occurrence in both locations

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics'.

I present evidence, through an empirical pre‐whitening analysis, that periodic signals are embedded in otherwise aperiodic abyssal hills across paleo‐spreading rates ranging from 1 to 9 cm/yr (half rate). Data are comprised of archival trackline and multibeam bathymetry profiles in north and south Atlantic Oceans, Pacific‐Antarctic Ridge flanks, and north and south East Pacific Rise flanks. Detected periods vary widely, from the low tens of thousands to millions of years, and do not cluster at specific values. Up to half spreading rates of ∼4 cm/yr, periodicities are negatively correlated with spreading rate at high confidence, despite the strong scatter in results; there is no correlation at higher spreading rates. Both the strong scatter and spreading rate dependence indicate that periodicities are intrinsically generated. I hypothesize that periodic signals are controlled by volcanic processes, whereas aperiodic morphology is controlled by faulting.

Continental breakup represents the successful process of rifting and thinning of the continental lithosphere, leading to plate rupture and initiation of oceanic crust formation. Magmatism during breakup seems to follow a path of either excessive, transient magmatism (magma-rich margins) or of igneous starvation (magma-poor margins). The latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation. Discovery of magma-poor margins has raised fundamental questions about the onset of ocean-floor type magmatism, and has guided interpretation of seismic data across many rifted margins, including the highly extended northern South China Sea margin. Here we report International Ocean Discovery Program drilling data from the northern South China Sea margin, testing the magma-poor margin model outside the North Atlantic. Contrary to expectations, results show initiation of Mid-Ocean Ridge basalt type magmatism during breakup, with a narrow and rapid transition into igneous oceanic crust. Coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers. Asthenospheric upwelling yielding Mid-Ocean Ridge basalt-type magmatism from normal-temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre-rift lithosphere.

The Rodriguez Triple Junction (RTJ) marks the intersection of the Indian Ocean's three principal mid‐ocean ridges and is a type locality for Ridge‐Ridge‐Ridge triple junctions. A local seismic network was deployed for 8 months to monitor seismicity around the RTJ and the adjacent Central Indian Ridge (CIR). The resulting earthquake catalog reveals seismicity systematically displaced from the central axes of both the Southwest Indian Ridge and the CIR, producing an asymmetric distribution. This pattern indicates that the CIR is migrating south‐eastward, through sequential relocations of the ridge axis. These relocations accumulate over time, generating sufficient ridge offset to alter the triple junction's mode. Our observations provide direct seismological evidence that ridge migration is driven by axis relocation, complementing theoretical models of triple junction stability and highlighting the role of small‐scale boundary adjustments in the global plate tectonic system.

Textures and compositions of minerals can be used to infer the physiochemical conditions present within magmatic systems. Given that plagioclase is an abundant phase in many magmatic systems, understanding the link between texture and process is vital. Here, we present a database of textural and compositional data for > 1800 plagioclase crystals in mid-ocean ridge basalt from the Gakkel Ridge (Arctic Ocean) to investigate the physiochemical conditions and processes that govern the formation of plagioclase textures and compositions. The Gakkel basalts have high modal crystal contents (up to 50%). The crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. The most common types of zoning are reverse and patchy; we attribute patchy zoning to infilling following either skeletal growth or resorption. Resorption is abundant, with multiple resorption events commonly present in a single crystal, and results from both magmatic recharge and decompression. Periods of strong undercooling, distinct to quench crystallisation, are indicated by matured skeletal crystals and thin normally zoned melt inclusion-rich bands following resorption. Individual samples often contain diverse textural and compositional plagioclase groups. Furthermore, most plagioclase is not in equilibrium with its host melt. Finally, the porous open structures of some glomerocrysts suggest that they represent pieces of entrained disaggregated mush. We interpret this to indicate that the crystal cargo is not generally phenocrystic in origin. Instead, plagioclase crystals that formed in different parts of a mush-dominated plumbing system were entrained into ascending melts. The textures of individual crystals are a function of their respective histories of (under)cooling, magma mixing and decompression. The morphologies of melt inclusion trapped in the plagioclase crystals are associated with specific host crystal textures, suggesting a link between plagioclase crystallisation processes and melt inclusion entrapment. The database of plagioclase presented herein may serve as a template for the interpretation of plagioclase textures in magmatic systems elsewhere.

Oceanic detachment fault systems are characteristic of slow‐spreading mid‐ocean ridges, where reduced magma supply leads to increased extension by faulting and exhumation of oceanic core complexes (OCCs). OCCs have complicated structure reflecting the interplay between magmatic, hydrothermal, and tectonic processes. We use microearthquake data from a 9‐month ocean bottom seismometer deployment to image deformation structures in the Rainbow massif on the Mid‐Atlantic Ridge. Using a machine‐learning enabled workflow to obtain an earthquake catalog containing >${ >} $ 68,000 events, we find seismicity occurred in distinct clusters that correlate with previously imaged velocity anomalies and dipping subsurface reflections. Our results are consistent with a dipping alteration front within the massif overlying late‐stage intrusions and suggest a transpressional fault accommodates a non‐transform offset north of the massif. Our results demonstrate OCCs continue to deform in a complex way after a detachment fault has been abandoned due to combined effects of tectonic stresses, magmatism, and alteration. Plain Language Summary New seafloor is created at mid‐ocean ridges where the tectonic plates spread apart. When ridges spread relatively quickly, considerable volumes of lava are erupted onto the seafloor to accommodate plate motion. When spreading is slower, less magma is available, and plate motion can instead occur by slip on kilometer‐scale faults called detachments. These detachment fault systems give rise to many tiny earthquakes whose distribution changes during the life‐cycle of detachment initiation, long‐lived slip, and abandonment. However, difficulty in detecting small‐magnitude earthquakes far from land means that relatively little is known about the later stages of this detachment life cycle. Rainbow massif on the Mid‐Atlantic Ridge is an abandoned detachment fault system that has created a dome which rises up ∼${\\sim} $ 1 km above the surrounding seafloor, and was surveyed and monitored for earthquakes in 2013–2014. Using machine learning techniques, we pinpointed >${ >} $ 68,000 earthquake locations from this experiment, and find that Rainbow massif dome is being sheared apart by ongoing plate spreading after the detachment has stopped slipping. We also find that magma and hydrothermal fluids are likely altering the rocks deep inside the dome. These results help us to understand how seafloor is formed under slow‐spreading conditions globally. Key Points Machine learning‐enabled methods yield new earthquake catalog of 68,326 microearthquakes at Rainbow massif Seismicity resolved into distinct clusters that correlate with seismic imagery Rainbow massif is undergoing extension with a seismicity pattern controlled by hydrothermal and magmatic processes