Volatile (C, N, Ar) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge

Abstract

In order to better constrain the origin of volatile variability of mid ocean ridge basalt (MORB), we have performed crushing analyses on 33 fresh basaltic glasses. Most samples originate from the Southwest Indian Ridge (i.e. 78–49°E, EDUL cruise, August 97), the Southeast Indian Ridge, the Central Indian Ridge and the Rodriguez Triple Junction. δ13C and δ18O of CO2, δ15N of N2 together with C, N and Ar contents were determined. δ15N values vary between −5.9‰ and +2.1‰ while δ13C values range between −11.4 and −4.3‰. C/N2 ratios vary by one order of magnitude (316–3900). Most N2/Ar ratios fall within a narrow range of values (48–90) but four samples yield higher values up to 220. Overall, the data for δ13C, δ18O, δ15N, C/N2 and N2/Ar are within the range of those previously reported for Pacific and Atlantic oceans. No volatile DUPAL anomaly has been detected in the present study. The correlations between N2/Ar, C/Ar and C/N2 together with δ13C and δ15N show that major volatile signatures, including δ15N, are more influenced by degassing-induced fractionation than by mantle heterogeneity and/or late atmospheric contamination. A two stage degassing model (a closed-system degassing followed by a Rayleigh distillation) can be used to explain the data set. This model gives initial Indian MORB values similar to those for the Atlantic and Pacific oceanic basalts δ13C0∼−4.5‰, δ15N0∼−6.0‰, (C/N2)0∼130, (C/Ar)0∼14 000 and (N2/Ar)0∼110 and C0 between 1100 and 5000 ppm C. This large range of possible initial carbon concentrations (due to the lack of constraints on the extent of degassing under closed-system conditions) results in a large range of mantle flux estimates (i.e. from 0.4 to 1.8×1013 mol/yr). The correlations induced by these degassing processes permit an estimation of the relative solubilities of C, N and Ar: SC/SN2∼5 and SAr/SN2∼1.2 as well as an evaluation of the fractionation of nitrogen isotopes between the vesicles and the melt: ΔN=−1.6‰. The present study proposes a coherent data set for volatile mantle fluxes with φC∼1.3±0.1×102φN2∼1.4±0.1×104φAr (molar).