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Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle. MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems. These variations define a spectrum between 'depleted' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir, or any of a number of processes of intramantle differentiations. Variations in (18)O/(16)O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that (18)O/(16)O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.

[1] Analysis of the compositions of crystals and melt inclusions from a suite of 40 gabbroic and wehrlitic nodules in a single eruptive body provides a record of concurrent mixing and crystallization of melts under NE Iceland. The crystals in the nodules have a similar range of compositions to those found as phenocrysts in the flow, and many of the nodules may have been generated by crystallization of a magma with a similar composition to that of the host flow. While plagioclase is only present in nodules where the average forsterite content of olivines is 0.8 GPa and is in agreement with estimates of crystallization pressures for the host basalt. The relationship between the compositional variability of melt inclusions and the forsterite content of the host olivine is revealed by REE analyses of over 120 melt inclusions. The degree of variability in REE concentrations and REE/Yb ratios decreases with falling forsterite content of the host olivine, as expected if melt mixing and fractional crystallization are operating together. The standard deviation of the REEs falls by a factor of $4 between Fo 90 and Fo 87 . This change in olivine composition can be produced by crystallization of 20% which occurs on cooling of $50°C. The relative rates of mixing, cooling and crystallization may provide constraints upon the dynamics of magma bodies. The oxygen isotopic composition of olivines from the nodules and phenocrysts is highly variable (d 18 O from 3.3-5.2 per mil) and shows little correlation with the forsterite content of the olivine. The full range of oxygen isotope variation is present in olivines with Fo 89 -90 , and the low d 18O signal is associated with melts of high Mg# and La/Yb. Such geochemical relationships cannot be produced by assimilation of low Mg# crustal materials alone, and may reflect oxygen isotopic variation within the mantle source. The geochemistry of the melt inclusions and their host crystals can be accounted for by fractional melting of a mantle source with variable composition, followed by concurrent mixing and crystallization beneath the Moho.

Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670" C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675" C, and indicate that regional metamorphic temperatures were up to 75" C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen ef al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670-780" C, in general agreement with cation and solvus thermometry.Data for amphibolite and granulite grade marbles show a 12% range in 8'3Cc,I and 8'3C8r. A strong correlation between carbon isotopic composition and the abundance of graphite (C8r/Crmk) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in 8°C vs. C8,/C,m, and 8I3C,, vs. 8I3C, could not have been preserved if significant amounts of C0,-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of 8I3C vs. C, / C, , indicates a biogenic origin for Adirondack graphites, even though low 8I3C values are not preserved in marble.Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher 8I3C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located <2km from anorthosite show a decrease of 0.5-0.8% in the outer 100pm of the grains, while samples at distances over 8 k m show smaller core-to-rim decreases of c. 0.2"~. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860-890" C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (A(cnl-gr) < 3.2%) are not included, then the remaining 38 granulite facies samples define the relation A'3C(cal-8r) = 3.56 X 106T-' (K).

Highlights:-We present a review and an expanded dataset of methane clumped isotope measurements.-Methane clumped isotope values often indicate equilibrium formation temperature.-Kinetic effects during or after methane production can affect clumped isotope values.-The wide variability in clumped isotope values suggests it will be a useful tracer. AbstractThe isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric 3 greenhouse gas concentrations, the global carbon cycle, and life in extreme environments.Recent analytical developments focusing on multiply substituted isotopologues ('clumped isotopes') are opening a valuable new window into methane geochemistry.When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions.We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism.

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