Undegassed Carbon Content from a Highly Depleted Segment of the Mid-Atlantic Ridge (1-5°S): Evidence from Melt Inclusions

Abstract:

As carbon solubility is low in basalts, MORB contain little dissolved CO₂ (189±61 ppm, 1σ, n=600 [1]). A global negative correlation between CO₂/Nb (proxy for amount of CO₂ lost by degassing) and Ba/La (proxy for source enrichment) indicates that depleted MORB are less affected by degassing than enriched MORB: CO₂/Nb ratios range from 0-100 for samples with Ba/La>4, while CO₂/Nb range from 50 to 400 for samples with Ba/La<4 [1]. To assess the CO₂ content of undegassed MORB, we analyzed the volatile content of 70 olivine-hosted, glassy melt inclusions (MIs) from four basalts dredged along MAR 1-5°S, a ridge segment that produces highly depleted MORB in terms of trace element enrichment and radiogenic isotopes [2, 3]. MIs contain CO₂ contents (180-1420 ppm) that are higher than their respective matrix glasses (130-220 ppm, typical for vapor-saturated melts erupted at 3-5 km b.s.l.).

One of the four dredges (EN061 5D-3Ag) contains MIs that do not exhibit shrinkage bubbles. For this sample only, we find a positive correlation between the CO₂ content (240-770 ppm) and the Cl content (6-20 ppm) of the MIs that is not found in the matrix glasses (see Fig.). We infer that the correlation between CO₂ and Cl, both highly incompatible in silicate minerals during fractional crystallization, is strong evidence for vapor-undersaturation, as any CO₂ degassing would have erased the correlation. Together with MIs from the Siqueiros Fracture Zone [4] and from northern Iceland [5], the MIs from EN061 5D-3Ag may represent another occurrence of carbon-undersaturated MORB. Our results will be used to model the primary carbon content of MORB and of the depleted upper mantle. Note that the average Cl content of the matrix glasses (20±1 ppm) is higher than those of the MIs (12±3 ppm, see Fig.). This indicates either that the matrix glasses assimilated a small amount of seawater Cl, or that the MIs are all more depleted than the matrix glass. We will acquire major and trace element data in order to further investigate the origin of the low Cl and high CO₂ contents in these MIs.

Ref. [1] Le Voyer et al. 2014 Goldschmidt abstr. [2] Schilling et al. 1994 JGR 99 [3] Kelley et al. 2013 G³ [4] Saal et al. 2002 Nature 419 [5] Hauri and Saal 2009 EOS Fall suppl. AGU abstr.