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AbstractCarbon contents and isotopic compositions were compared in the basalt groundmass of the oceanic crust of different age in the zone of the East Pacific Rise In samples the basalt groundmass of the ancient oceanic crust 270 Ma ODP Site 801C in which a carbonate phase was found the isotopic composition of the oxidized carbon 13C 1 5 indicates that this carbon was formed by the precipitation of seawater dissolved inorganic carbon DIC In the samples in which no carbonate phase was identified the low concentration 0 1 wt CO2 of oxidized dispersed carbon and its isotopic composition 13C 7 are in the range of values typical of carbon dissolved in basalt glasses without crystallinity This makes it possible to relate the oxidized dispersed carbon to residual carbon dissolved in the magmatic melt after CO2 degassing The precipitation of DIC results in a positive correlation between the concentration of total carbon and its 13C values along with the formation of a carbonate phase The application of this criterion to basalt groundmass samples of the young crust 15 Ma ODP Site 1256D showed that oxidized dispersed carbon in the young oceanic crust groundmass was not formed by the precipitation of DIC contradicting the generally accepted paradigm Constant concentration and 13C values of the reduced dispersed carbon in the basalt groundmass of the young and ancient oceanic crusts including lithological zones where microbial activity has not been recorded indicate that the most probable model is high-temperature abiogenic generation of reduced dispersed carbon near the ridge axis The Fischer Tropsch synthesis and or Bell Boudouard reaction provide a possible basis for the abiogenic model The Bell Boudouard reaction 2CO C CO2 leads to the formation of an adsorbed layer of elemental carbon on the fresh surfaces of minerals during background alteration of the oceanic basalt crust The CO2 CO gas-phase equilibrium maintains the necessary depletion of the newly formed elemental carbon in the 13C isotope to 13C 20 Abiogenic models for the origin of the isotopically light reduced dispersed carbon in the basalt groundmass do not assume the presence of carbon depleted in the heavy 13C isotope in the magmatic melt

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Dispersed Carbon in Basalts of the Altered Oceanic Crust: Isotope Composition and Mechanisms of Formation

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with basalts of altered oceanic crust as keyword