[1] The mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ALL MORB” is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D-MORB the depleted end-member. ALL MORB and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petrogenesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of all morb and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.

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The mean composition of ocean ridge basalts

Why do mafic arc magmas contain ∼4wt% water on average?

Based on all available isotopic and trace element data for SNCs we obtain a mean age of formation of the crust on Mars of 34 +/- 6 Ga Additional information is contained in the original extended abstract

The Mean Age of Mantle and Crustal Reservoirs for the Planet Mars

Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting planet. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs. Many natural, 'superdeep' diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt-peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

/pdf/slab-melting-as-a-barrier-to-deep-carbon-subduction-1i5bz2ommq.pdf

Slab melting as a barrier to deep carbon subduction

Global variations in abyssal peridotite compositions

The temperature and composition of Earth’s mantle control fundamental planetary properties, including the vigor of mantle convection and the depths of the ocean basins. Seismic wave velocities, ocean ridge depths, and the composition of mid-ocean ridge basalts can all be used to determine variations in mantle temperature and composition, yet are typically considered in isolation. We show that correlations among these three data sets are consistent with 250°C variation extending to depths >400 kilometers and are inconsistent with variations in mantle composition at constant temperature. Anomalously hot ridge segments are located near hot spots, confirming a deep mantle-plume origin for hot spot volcanism. Chemical heterogeneity may contribute to scatter about the global trend. The coherent temperature signal provides a thermal calibration scale for interpreting seismic velocities located distant from ridges.

https://science.sciencemag.org/content/sci/344/6179/80.full.pdf

Geophysical and Geochemical Evidence for Deep Temperature Variations Beneath Mid-Ocean Ridges

Petrology of the unbrecciated eucrites

Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments.Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(MgþFe) in equilibrium with Fo90 olivine. Both sets of corrected compositions show large variations in Na2O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al90 and Fe90.These new data are not in agreement with the presentation of Niu & O’Hara (Journal of Petrology 49, 633^664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for highand low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 2008C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slowspreading ridges display a characteristic chemical signature of elevated Na90 and Al90 and lowered Si90 relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si90 should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle^melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K90. Such variations do not correlate with the other major elements, showing that major element and trace

/pdf/the-global-systematics-of-ocean-ridge-basalts-and-their-30wjec2tv3.pdf

The Global Systematics of Ocean Ridge Basalts and their Origin

Most published studies of olivine-hosted melt inclusions from mid-ocean ridges have been based on a single sample. Here we present a comprehensive melt inclusion study of major and trace elements from a single ocean ridge segment, the FAMOUS segment of the Mid-Atlantic Ridge. The melt inclusion dataset includes 312 olivine-hosted (Mg-number 85^92) melt inclusions from 14 samples distributed along the segment.This permits a more comprehensive assessment of the variability within melt inclusions from a single region, and of the relationship between melt inclusion and lava compositions. One recent question has been the extent to which melt inclusions truly preserve the original melt compositions, or instead are modified by late-stage processes occurring at shallow levels. In the FAMOUS inclusions, major elements have been affected by post-entrapment processes, but trace elements show no evidence of such processes, suggesting that diffusion coefficients for incompatible elements are small. Melt inclusions can be divided into three groups. (1) High-Mg inclusions are the most primitive and may potentially constrain the composition of the parental magmas that contribute to other melt inclusion and lava compositions. Although their trace element contents range from highly depleted to almost as enriched as the FAMOUS segment lavas, they are on average more depleted and the melts appear to be derived by greater extents of melting than the lavas. (2) Low-Al inclusions occur in the lower Mg-number olivines, and their major and trace element characteristics reflect mixing between high-Mg melt inclusion and lava compositions. (3) High-Al melt inclusions display Al2O3 contents as high as 18·4 wt %, SiO2 as low as 46·6 wt %, a strong depletion in the most incompatible elements and distinctively low middle to heavy rare earth element (MREE/HREE) ratios.The high Al2O3 and low SiO2 contents, as well as positive Sr anomalies in some of the high-Al melt inclusions, are best explained by assimilation of plagioclase-bearing cumulates. The trace element variability in the high-Mg melt inclusions is not consistent with a simple continuous melting column and requires pooling of near-fractional melts within the melting regime and a variable mantle source composition. Because the mean composition of these melt inclusions reflects greater extents of melting than the lavas, we propose that the melt inclusions come from the upper portions of the melting regime. Lavas, in contrast, sample the entire melting regime, including low-degree melts from the wings of the regime that are transported more directly to the surface along high-porosity channels. The high-Al, trace element ultra-depleted, low MREE/HREE melt inclusions derive from melting of a residual mantle source formed by previous melt extraction in the garnet stability field.There is a marked lack of correspondence between major and trace element variations in the melt inclusions. This may reflect a combination of processes, such as cumulate assimilation and re-equilibration of the magmas during ascent, which can reset major elements while having little effect on the trace element variations. The melt inclusions are not simply representative unpooled melts from the melting regime and they do not fully reflect the range of melt compositions contributing to the lavas.Their compositions reflect source heterogeneity as well as melting processes, and major and trace element indicators of depth of origin do not correspond. Combined comprehensive studies of lavas and melt inclusions have much more to reveal than studies based on either data source alone.

http://porklips.org/~mu/J.%20Petrology-2012-Laubier-665-98.pdf

A. Gale

Papers

The mean composition of ocean ridge basalts

Geophysical and Geochemical Evidence for Deep Temperature Variations Beneath Mid-Ocean Ridges

Petrology of the unbrecciated eucrites

The Global Systematics of Ocean Ridge Basalts and their Origin

Melting and Crustal Processes at the FAMOUS Segment (Mid-Atlantic Ridge): New Insights from Olivine-hosted Melt Inclusions from Multiple Samples