Experimental determination of C, F, and H partitioning between mantle minerals and carbonated basalt, CO2/Ba and CO2/Nb systematics of partial melting, and the CO2 contents of basaltic source regions

To determine partitioning of C between upper mantle silicate minerals and basaltic melts, we executed 26 experiments between 0.8 and 3 GPa and 1250-1500 degrees C which yielded 37 mineral/glass pairs suitable for C analysis by secondary ion mass spectrometry (SIMS). To enhance detection limits, experiments were conducted with C-13-enriched bulk compositions. Independent measurements of C-13 and C-12 in coexisting phases produced two C partition coefficients for each mineral pair and allowed assessment of the approach to equilibrium during each experiment. Concentrations of C in olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and garnet (gt) range from 0.2 to 3.5 ppm, and resulting C partition coefficients for ol/melt, opx/melt, cpx/melt and gt/melt are, respectively, 0.0007 +/- 0.0004 (n = 2), 0.0003 +/- 0.0002 (n = 45), 0.0005 +/- 0.0004 (n = 17) and 0.0001 +/- 0.00007 (n = 5). The effective partition coefficient of C during partial melting of peridotite is 0.00055 +/- 0.00025, and therefore C is significantly more incompatible than Nb, slightly more compatible than Ba, and, among refractory trace elements, most similar in behavior to U or Th. Experiments also yielded partition coefficients for F and H between minerals and melts. Combining new and previous values of D-F(mineral/melt) yields bulk D-F(pendotite/melt) = 0.011 +/- 0.002, which suggests that F behaves similarly to La during partial melting of peridotite. Values of D-H(pyx/melt) correlate with tetrahedral Al along a trend consistent with previously published determinations. Small-degree partial melting of the mantle results in considerable CO2/Nb fractionation, which is likely the cause of high CO2/Nb evident in some Nb-rich oceanic basalts. CO2/Ba is much less easily fractionated, with incompatible-element-enriched partial melts having lower CO2/Ba than less enriched basalts. Comparison of calculated behavior of CO2, Nb, and Ba to systematics of oceanic basalts suggests that depleted (DMM-like) sources have 75 25 ppm CO2 (CO2/Nb = 505 168, CO2/Ba = 133 44), whereas enriched sources of intraplate basalts similar in concentrations to primitive mantle have 600 +/- 200 ppm CO2. If all mantle reservoirs are expressed in the current inventory of oceanic basalts for which nearly undegassed CO2 concentrations are available, then we estimate the likely range of mantle C concentrations to be 1.4-4.8 x 10(23) grams of C, or 1.5-5.2 times the mass of the current C surface reservoir. Depending on the assumed Ba and Nb contents of average oceanic crust, resulting ridge fluxes of C range from 7.2 x 10(13) to 2.9 x 10(14) g/yr. (C) 2014 Published by Elsevier B.V.