Inferring the Behavior, Concentration and Flux of CO2 from the Suboceanic Mantle from Undegassed Ocean Ridge and Ocean Island Basalts

Abstract:

We determined glass and vesicle CO₂ contents, plus trace element contents for fifty-one ultradepleted mid-ocean ridge basalt (MORB) glasses distributed globally. Sixteen had no vesicles and were volatile undersaturated. Thirty-five had vesicles and were slightly oversaturated. If this latter group lost bubbles during emplacement, then CO₂/Ba calculated for the undersaturated group alone is the most reliable and uniform ratio at 98±10, and CO₂/Nb is 283±32. If they did not lose bubbles, then CO₂/Nb is the most uniform ratio for the entire suite of ultradepleted MORBs at 291±132, while CO₂/Ba decreases with incompatible element enrichment.

For a wider range of compositions, we used published estimates of CO₂ in enriched basalts that retained vesicles e.g., “popping rocks”, and from melt inclusions in normal MORBs. As incompatible element enrichment increases, CO₂/Nb increases from 283±32 in ultradepleted MORBs to 603±69 in depleted melt inclusions to 936±132 in enriched basalts. In contrast, CO₂/Ba is nearly constant at 98±10, 106±24 and 111±11 respectively. This suggests that Ba is the best proxy for estimating CO₂ contents of MORBs, with an overall average CO₂/Ba = 105±9. Atlantic, Pacific and Indian basalts have similar values. Gakkel ridge has anomalously high Ba and low CO₂/Ba.

Using the CO₂/Ba ratio and an average MORB composition, the CO₂ concentration of a primary, average MORB is 2085⁺⁴⁷³/_-427 ppm while primary NMORB has 1840ppm CO₂. The annual flux of CO₂ from mid-ocean ridges is 1.25±0.16 x 10¹⁴ g/yr (0.93 - 1.61 x 10¹⁴ g/yr is possible): higher than published estimates that use CO₂/³He in MORB and the abyssal ocean ³He flux. This may be accounted for by a CO₂/³He ratio that is higher than the commonly accepted MORB ratio of 2x10⁹ due to leverage by more enriched basalts. NMORB mantle has 183 ppm CO₂ based on simple melting models. More realistic estimates of depleted mantle composition yield lower estimates of ~60-130ppm, with large uncertainties that depend more on melting models than on CO₂/Ba. CO₂/Ba is not correlated with isotopic or trace element ratios.