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°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 ¹³C-enriched bulk compositions. Independent measurements of ¹³C and ¹²C 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 DFmineral/melt yields bulk DFperidotite/melt=0.011±0.002, which suggests that F behaves similarly to La during partial melting of peridotite. Values of DHpyx/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 1.4-4.8×10²³ 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 7.2×10¹³ to 2.9×10¹⁴ g/yr.