HR: 0800h
AN: V31C-1453    [Abstracts]
TI: Carbon flux and C/Nb ratios in the mantle in ridge context
AU: Pineau, F
EM: fpi@ipgp.jussieu.fr
AF: Lab. G‚ochimie des Isotopes Stables, IPG-Paris, T54-64, 1er etage, 4 place Jussieu, Paris Cedex 05, 75251 France
AU: * Cartigny, P
EM: cartigny@ipgp.jussieu.fr
AF: Lab. G‚ochimie des Isotopes Stables, IPG-Paris, T54-64, 1er etage, 4 place Jussieu, Paris Cedex 05, 75251 France
AU: Javoy, M
EM: mja@ipgp.jussieu.fr
AF: Lab. G‚ochimie des Isotopes Stables, IPG-Paris, T54-64, 1er etage, 4 place Jussieu, Paris Cedex 05, 75251 France
AB: The hypothesis that undegassed MORB would display constant CO$_{2}$/Nb-ratios of \sim 239 (ppm/ppm) obtained by Saal et al (2002) is not supported by available data. Incorrectely quoted by these authors, the MORB popping rock shows a CO$_{2}$/Nb-ratio higher by a factor of \sim 3 (Javoy and Pineau, 1991) than the ratio measured among their carbon under-saturated glass inclusions from the Siquieros transform fault. In order to better constrain the variability of CO$_{2}$/Nb-ratios among MORB, we have analysed and compiled volatile, trace and major element data from 14 \degN and 34 \degN basaltic glasses on the Mid Atlantic Ridge. The sample set includes N-, E-MORB and alkali basalts. Carbon (and water for alkali basalts) have been affected by degassing and initial carbon and water concentrations were reconstructed using methods described previously (e.g. Pineau et al. 2004). The variations in \delta D-values (-88 to -55 per mil) with water contents (1300 to 7700 ppm) show consistent variations with published data of worldwide mid-ocean ridges. The samples define rather constant regional ratios from 349 to 589 and from 524 to 1065 at 14 \deg and 34 \degN respectively. CO$_{2}$/Nb-ratios roughly correlate positively with enrichment indexes such K/Ti. It is worth noting that the little variable CO$_{2}$/Nb-ratios over a large range of CO$_{2}$, up to 44,000 ppm, (i.e. 12,000 ppmC) and Nb-contents, up to 41.3 ppm, confirm the accuracy of degassing corrections. From the broad inverse correlation between CO$_{2}$/Nb and TiO$_{2}$/H$_{2}$0 and the mean TiO$_{2}$/H$_{2}$0-ratio of worldwide MORB with 8 \pm1weight percent MgO, a MORB CO$_{2}$/Nb-ratio from 500 to 950 seems more appropriate than the value quoted by Saal et al. (2002). This corresponds to carbon flux from 4.0 to 7.6\times10$^{12}$ mol/y. However, in the light of the regional dependency of CO$_{2}$/Nb-ratios it is unclear yet whether it represents the best tool to constrain mantle carbon flux or should be more likely used to distinguish different mantle domains.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1247 Terrestrial reference systems
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting