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A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near‐fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America‐Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n = 0.002–0.05); depletion of Ti (300–1600 ppm), Zr (0.1–10 ppm), and Sr (0.1–10 ppm); and fractionation of Zr relative to Ti (Ti/Zr = 250–4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in ≤0.1% increments to a total of 5–25% melting, a process very close to Rayleigh (fractional) melting; batch melting of a LREE‐depleted source cannot account for the observed trace element concentrations in abyssal peridotites. The shapes of some REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Trace element concentrations in calculated integrated fractional liquids approximate the composition of primitive ocean floor basalts, consistent with postsegregation aggregation of small increment melts produced over a depth and melting interval.

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DefinitionsD VSMOW is the difference between the measured D/H ratio of a sample and the D/H ratio of Vienna Standard Mean Ocean Water (VSMOW) expressed in per mil (‰):

Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane-and sulfurcycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host delta C-13-and delta S-34-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic delta C-13 signatures locally attenuated by heterotrophic metabolism. Main text Subseafloor basaltic crust represents the largest habitable zone by volume on Earth (1). Chemical reactions of basalt with seawater flowing through fractures release energy that may support chemosynthetic communities. Microbes exploiting these reactions are known from basalt exposed at the seafloor, where the oxidation of reduced sulfur (S) and iron (Fe) from basalt with dissolved oxygen and nitrate from seawater supports high microbial biomass and diversity (2, 3). Multiple lines of indirect evidence that include textural alterations (4), depletions in δ 34 S-pyrite (FeS 2) (5) and δ 13 C-dissolved inorganic carbon (DIC) (6), and DNA sequences from

The origin of bulk silicate Earth carbon inventory is unknown and the fate of carbon during the early Earth differentiation and core formation is a missing link in the evolution of the terrestrial carbon cycle. Here we present high pressure (P)-temperature (T) experiments that offer new constraints upon the partitioning of carbon between metallic and silicate melt in a shallow magma ocean. Experiments were performed at 1-5 GPa, 1600-2100°C on mixtures of synthetic or natural silicates (tholeiitic basalt/alkali basalt/komatiite/fertile peridotite) and Fe-Ni-C ± Co ± S contained in graphite or MgO capsules. All the experiments produced immiscible Fe-rich metallic and silicate melts at oxygen fugacity (fO 2) between $IW-1.5 and IW-1.9. Carbon and hydrogen concentrations of basaltic glasses and non-glassy quenched silicate melts were determined using secondary ionization mass spectrometry (SIMS) and speciation of dissolved CO -H volatiles in silicate glasses was studied using Raman spectroscopy. Carbon contents of metallic melts were determined using both electron microprobe and SIMS. Our experiments indicate that at core-forming, reduced conditions, carbon in deep mafic-ultramafic magmas may dissolve primarily as various hydrogenated species but the total carbon storage capacity, although is significantly higher than solubility of CO 2 under similar conditions, remains low (<500 ppm). The total carbon content in our reduced melts at graphite saturation increases with increasing melt depolymerization (NBO/T), consistent with recent spectroscopic studies, and modestly with increasing hydration. Carbon behaves as a metal-loving element during core-mantle separation and our experimental D metal=silicate C varies between $4750 and P150 and increases with increasing pressure and decreases with increasing temperature and melt NBO/T. Our data suggest that if only a trace amount of carbon ($730 ppm C) was available during early Earth differentiation, most of it was partitioned to the core (with 0.20-0.25 wt.% C) and no more than $10-30% of the present-day mantle carbon budget (50-200 ppm CO 2) could be derived from a magma ocean residual to core formation. With equilibrium core formation removing most of the carbon initially retained in the terrestrial magma ocean, explanation of the modern bulk silicate Earth carbon inventory requires a later replenishment mechanism. Partial entrapment of metal melt in solid silicate matrix, carbon ingassing by magma ocean-atmosphere interaction, and carbon outgassing from the core aided by reaction of core metal and deeply subducted water are some of the viable mechanisms.

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We review physical and chemical constraints on the mechanisms of melt extraction from the mantle beneath mid-ocean ridges. Compositional constraints from MORB and abyssal peridotite are summarized, followed by observations of melt extraction features in the mantle, and constraints from the physical properties of partially molten peridotite. We address two main issues. ( 1) To what extent is melting 'nearfractional', with low porosities in the source and chemical isolation of ascending melt? To what extent are other processes, loosely termed reactive flow, important in MORB genesis? (2) Where chemically isolated melt extraction is required, does this occur mainly in melt-filled fractures or in conduits of focused porous flow?Reactive flow plays an important role, but somewhere in the upwelling mantle melting must be 'near fractional', with intergranular porosities less than 1%, and most melt extraction must be in isolated conduits. Two porosity models provide the best paradigm for this type of process. Field relationships and geochemical data show that replacive dunites mark conduits for focused, chemically isolated, porous flow of mid-ocean ridge basalt (MORB) in the upwelling mantle. By contrast, pyroxenite and gabbro dikes are lithospheric features; they do not represent conduits for melt extraction from the upwelling mantle. Thus, preserved melt extraction features do not require hydrofracture in the melting region. However, field evidence does not rule out hydrofracture.Predicted porous flow velocities satisfy 230 Th excess constraints (ca. 1 m yr −1 ), provided melt extraction occurs in porous conduits rather than by diffuse flow, and melt-free, solid viscosity is less than ca. 10 20 Pa s. Melt velocities of ca. 50 m yr −1 are inferred from patterns of post-glacial volcanism in Iceland and from 226 Ra excess. If these inferences are correct, minimum conditions for hydrofracture may be reached in the shallowest part of melting region beneath ridges. However, necessary high porosities can only be attained within pre-existing conduits for focused porous flow. Alternatively, the requirement for high melt velocity could be satisfied in melt-filled tubes formed by dissolution or mechanical instabilities.Melt-filled fractures or tubes, if they form, are probably closed at the top and bottom, limited in size by the supply of melt. Therefore, to satisfy the requirements for geochemical isolation from surrounding peridotite, melt-filled conduits may be surrounded by a dunite zone. Furthermore, individual melt-filled voids probably contain too little melt to form sufficient dunite by reaction, suggesting that dunite zones must be present before melt extraction in fractures or tubes.