MORB degassing: evolution of CO 2

Abstract

With the gas pressure model for the occurrence of volcanic eruptions of Jaupart, Vergniolle and Tait and the 13C/ 12C Rayleigh fractionation model of Pineau and Javoy we can explain, using the results of our previous calculations on the nucleation and growth of bubbles in MORB, the observed quantities and isotopic ratios of carbon present in different forms in oceanic ridge tholeiites. The processes associated with the generation of MORB in the upper mantle produce a basaltic liquid which is saturated with CO 2. During the migration of this liquid to a magma chamber just below the ridge crest, nucleation of carbon dioxide bubbles takes place. In the magma chamber MORB loses many of these primary bubbles while it degasses. A second generation of bubbles is nucleated during eruption. Our model explains the presence of relatively large primary bubbles rich in CO 2 with a 13C/ 12C ratio comparable to that of diamonds or carbonatites. The secondary bubbles, which are smaller than the primary ones, also contain predominantly CO 2, but this CO 2 is relatively enriched in 12C and is not greatly different in isotopic composition from the carbon remaining in solution when MORB is emplaced on the ocean floor. Contamination carbon, always present, tends to be even more enriched in 12C.