Charles H. Langmuir

The chemical composition of subducting sediment and its consequences for the crust and mantle

Subducted sediments play an important role in arc magmatism and crust-mantle recycling. Models of continental growth, continental composition, convergent margin magmatism and mantle heterogeneity all require a better understanding of the mass and chemical fluxes associated with subducting sediments. We have evaluated subducting sediments on a global basis in order to better define their chemical systematics and to determine both regional and global average compositions. We then use these compositions to assess the importance of sediments to arc volcanism and crust-mantle recycling, and to re-evaluate the chemical composition of the continental crust. The large variations in the chemical composition of marine . sediments are for the most part linked to the main lithological constituents. The alkali elements K, Rb and Cs and high . field strength elements Ti, Nb, Hf, Zr are closely linked to the detrital phase in marine sediments; Th is largely detrital but may be enriched in the hydrogenous Fe-Mn component of sediments; REE patterns are largely continental, but abundances are closely linked to fish debris phosphate; U is mostly detrital, but also dependent on the supply and burial rate of organic matter; Ba is linked to both biogenic barite and hydrothermal components; Sr is linked to carbonate phases. Thus, the important geochemical tracers follow the lithology of the sediments. Sediment lithologies are controlled in turn by a small . number of factors: proximity of detrital sources volcanic and continental ; biological productivity and preservation of carbonate and opal; and sedimentation rate. Because of the link with lithology and the wealth of lithological data routinely collected for ODP and DSDP drill cores, bulk geochemical averages can be calculated to better than 30% for most elements . from fewer than ten chemical analyses for a typical drill core 100-1000 m . Combining the geochemical systematics with convergence rate and other parameters permits calculation of regional compositional fluxes for subducting sediment. These regional fluxes can be compared to the compositions of arc volcanics to asses the importance of sediment subduction to arc volcanism. For the 70% of the trenches worldwide where estimates can be made, the regional fluxes also provide the basis . for a global subducting sediment GLOSS composition and flux. GLOSS is dominated by terrigenous material 76 wt% q. terrigenous, 7 wt% calcium carbonate, 10 wt% opal, 7 wt% mineral-bound H O , and therefore similar to upper 2 . continental crust UCC in composition. Exceptions include enrichment in Ba, Mn and the middle and heavy REE, and . depletions in detrital elements diluted by biogenic material alkalis, Th, Zr, Hf . Sr and Pb are identical in GLOSS and UCC as a result of a balance between dilution and enrichment by marine phases. GLOSS and the systematics of marine sediments provide an independent approach to the composition of the upper continental crust for detrital elements. Significant discrepancies of up to a factor of two exist between the marine sediment data and current upper crustal estimates for Cs, Nb, . . . . Ta and Ti. Suggested revisions to UCC include Cs 7.3 ppm , Nb 13.7 ppm , Ta 0.96 ppm and TiO 0.76 wt% . These 2

Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earths mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

An evaluation of the global variations in the major element chemistry of arc basalts

Arc volcanoes occur at convergent margins with a wide range in subduction parameters, and variations in these parameters might be expected to lead to variations in the chemistry of magmas parental to arcs. Major element analyses from approximately 100 volcanic centers within 30 arcs, normalized to 6% MgO to minimize the effects of crystal fractionation, display wide variations. Na2O and CaO at 6% MgO (Na6.0 and Ca6.0) correlate remarkably well with the thickness of the overlying crust. These systematics are consistent with two possible models. In the first model, the crust behaves as a chemical filter; where the crust is thick, magmas crystallize at higher pressure and interact more extensively with the arc crust. Modeling of high-pressure crystallization and assimilation, however, does not reproduce the associated variations in Na6.0 and Ca6.0 without calling upon complicated combinations of fractionating phases and assimilants. In the second model, crustal thickness determines the height of the mantle column available for melting beneath arc volcanoes. If melting begins beneath arcs at similar depths, then the column of mantle that undergoes decompression melting is much shorter beneath the thickest arc crust. The shorter mantle column for arcs built on thick crust will lead to smaller extents of melting in the mantle, and hence higher Na6.0 and lower Ca6.0 in the parental magmas. Modeling shows that variations in the extent of melting in the mantle can easily account for the associated variations in Ca6.0 and Na6.0. The abundances of the other major elements at 6% MgO do not correlate well with crustal thickness, or any other subduction parameter. Co-variation of some of these other major elements (e.g., Si6.0 and Fe6.0) within individual arcs suggests that they are strongly influenced by local crustal level processes that obscure partial melting systematics. Correction for the crustal processes improves the relationship between Na6.0 and Ca6.0 that is so readily explained by partial melting. The extents of melting in the mantle beneath arc volcanoes estimated from the ranges in Na6.0 and Ca6.0 are remarkably similar to those estimated beneath mid-ocean ridges. This observation provides further evidence that the mantle wedge, and not the slab, melts beneath arc volcanic fronts.

Oxidation states of mid-ocean ridge basalt glasses

Abstract Precise new analyses of ferrous and total iron for 78 hand-picked mid-ocean ridge basalt (MORB) glasses constrain the redox states of MORB magmas, and the systematics of those redox states with respect to geography and chemistry. The data indicate that MORB magmas at the point of eruption include some of the most reduced terrestrial lavas yet analyzed. Irrespective of their tectonic setting, geographic setting or chemical type, quench glasses from the outer surfaces of MORB lavas have Fe3+/ΣFe ratios of 0.07 ± 0.03 (2σ), equivalent to relative oxygen fugacities 1–2 log10 units below the fayalite-magnetite-quartz buffer (FMQ). These reduced values contrast markedly with those of cogenetic whole rocks, even for samples from the same pillow. Pillow cores (including virtually all published whole rock analyses) typically have Fe3+/ΣFe close to 0.15, equivalent to oxygen fugacities close to FMQ. The post-eruptive oxidation responsible for this contrast apparently occurs rapidly, while the lava is still partially molten, most likely as a result of hydrogen loss. The striking contrast between glass and whole rock data suggests that two common assumptions with respect to treatment of MORB are inappropriate: (1) assumption of Fe3+/ΣFe close to 0.15 for calculating norms and magnesium numbers; and (2) assumption of FMQ conditions for experimental studies of MORB crystallization. Oxygen fugacities in the mantle source regions of MORB have also been assumed to be close to FMQ. It is likely that, redox states determined from wholly or partially crystalline lavas are generally not representative of magmatic values. The new data reported herein define a new and rigidupper limit for the oxidation state of the sub-oceanic mantle.

Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean

A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

The importance of water to oceanic mantle melting regimes

The formation of basaltic crust at mid-ocean ridges and ocean islands provides a window into the compositional and thermal state of the Earths upper mantle. But the interpretation of geochemical and crustal-thickness data in terms of magma source parameters depends on our understanding of the melting, melt-extraction and differentiation processes that intervene between the magma source and the crust. Much of the quantitative theory developed to model these processes has neglected the role of water in the mantle and in magma, despite the observed presence of water in ocean-floor basalts. Here we extend two quantitative models of ridge melting, mixing and fractionation to show that the addition of water can cause an increase in total melt production and crustal thickness while causing a decrease in mean extent of melting. This may help to resolve several enigmatic observations in the major- and trace-element chemistry of both normal and hotspot-affected ridge basalts.

A hydrous melting and fractionation model for mid‐ocean ridge basalts: Application to the Mid‐Atlantic Ridge near the Azores

The major element, trace element, and isotopic composition of mid-ocean ridge basalt glasses affected by the Azores hotspot are strongly correlated with H2O content of the glass. Distinguishing the relative importance of source chemistry and potential temperature in ridge-hotspot interaction therefore requires a comprehensive model that accounts for the effect of H2O in the source on melting behavior and for the effect of H2O in primitive liquids on the fractionation path. We develop such a model by coupling the latest version of the MELTS algorithm to a model for partitioning of water among silicate melts and nominally anhydrous minerals. We find that much of the variation in all major oxides except TiO2 and a significant fraction of the crustal thickness anomaly at the Azores platform are explained by the combined effects on melting and fractionation of up to ~700 ppm H2O in the source with only a small thermal anomaly, particularly if there is a small component of buoyantly driven active flow associated with the more H2O-rich melting regimes. An on-axis thermal anomaly of ~35°C in potential temperature explains the full crustal thickness increase of ~4 km approaching the Azores platform, whereas a ≥75°C thermal anomaly would be required in the absence of water or active flow. The polybaric hydrous melting and fractionation model allows us to solve for the TiO2, trace element and isotopic composition of the H2O-rich component in a way that self-consistently accounts for the changes in the melting and fractionation regimes resulting from enrichment, although the presence and concentration in the enriched component of elements more compatible than Dy cannot be resolved.

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80 km Myr–1 (refs 1–4). These faults can accommodate extension for up to 1–3 Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of ∼2,500 km of the Mid-Atlantic Ridge between 12.5 and 35° N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults in oceanic lithospheric accretion.

Effects of the melting regime on the composition of the oceanic crust

The physical form of the melting regime and the mechanisms of melt extraction influence the composition of magmas erupted at ocean ridges. We investigate aspects of this relationship, beginning with the assumption that melts can be extracted from the melting regime without significant reequilibration during their passage to the surface. The ocean crust thus represents a mixture of the individual melts. Many melting regimes lead to the same “residual mantle column” (RMC), defined as a vertical section through the mantle external to the melting regime. The RMC is the integrated result of melt extraction and is useful in evaluating the geochemical effects of many different types of melting regimes. Consideration of the RMC shows that the “shape” of the melting regime is not necessarily an important parameter in affecting the composition of the ocean crust. The important parameters are the way mantle flows through the melting regime and the relationship between melt fraction and pressure during adiabatic melting. Calculating the volume and composition of the ocean crust can be reduced to a simple mixing problem. Virtually all ridge models predict continuous mixing of melts from the solidus to the maximum extent of melting. Given these boundary conditions, even complex melting regimes lead to geochemical results that are similar to those produced by batch melting. Thus batch melting may approximate the net effects of the melting process remarkably well. An important exception to these generalizations is binary mixing between melts of very different composition. This is possible beneath ocean ridges if very low degree melts, formed at the volatile-present solidus, mix with higher-degree melts formed directly beneath the ridge. There are limitations to the effectiveness of such a mixing process because the source volume for the low-degree melts is constrained by the finite pressure interval between the dry and volatile-present solidi of the mantle. These constraints place an upper limit of a factor of 5 on the incompatible element enrichment that can be explained by such mixing. This is a small factor relative to the global variability of mid-ocean ridge basalts. A few local regions, however, show major and trace element covariations that may be consistent with this type of mixing. Adequate data sets to fully test the possibility are lacking. If such mixing occurs, there must be a physical mechanism to focus the lowest degree melts from the furthest reaches of the melting regime into the mantle directly beneath the ridge axis. The physical difficulties associated with horizontal transport of low-degree melts over tens to hundreds of kilometers are imposing and suggest that alternative models should be seriously considered.

The systematics of lithium abundances in young volcanic rocks

Abstract Lithium is a moderately incompatible trace element in magmatic systems. High precision analyses for lithium conducted on well characterized suites of MORB and ocean island basalts suggest a bulk distribution coefficient of 0.25−0.35 and behavior which is similar to Yb during low pressure fractionation and V during melting, as long as garnet is not an important residual phase. Data for peridotites and basalts suggest a mantle lithium content of about 1.9 ppm and show that significant concentrations of lithium reside in olivine and orthopyroxene, resulting in unusual inter-mineral partitioning of Li and complex relationships between lithium and other incompatible trace elements. The lithium abundances of arc basalts are similar to those of MORB, but their Li/Yb ratios are considerably higher. The high Li/Yb suggests the addition of a Li-rich component to arc sources; relatively low Yb abundances are consistent with the derivation of some arc magmas by larger extents of melting or from a more depleted source than MORB. Although Li is enriched at arcs, K is enriched more, leading to elevated K/Li ratios in arc volcanics. The high K/Li and relatively low La/Yb of primitive arc basalts requires either incorporation of altered ocean crust into arc magma sources, or selective removal of K and Li from subducted sediments. Bulk incorporation of sediments alone does not explain the Li systematics. Data from primitive MORB indicate a relatively low (3–4 ppm) Li content for new oceanic crust. Thus, the Li flux from the ocean crust is probably 11 g/yr, and the oceanic crust may not be an important net source in the oceanic budget of lithium.