A carbon, nitrogen, and multi-isotope study of basalt glasses near 14°N on the Mid-Atlantic Ridge. Part A: Degassing processes

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14°N on the Mid-Atlantic ridge (MAR) is one of only a few locations worldwide where volatile-saturated, geochemically enriched mid-ocean ridge basalts (E-MORBs) have been recovered. These basaltic glasses are so gas-rich that CO2-filled bubbles may “pop” when brought to the surface, due to the pressure and temperature change. Although these “popping rocks” have long been regarded as representative samples of un-degassed magmas sourced from the upper mantle, uncertainties regarding both their generation mechanism(s) and the potential effects of gas loss/accumulation processes have hampered unambiguous quantification of the upper mantle volatile element (water, carbon, nitrogen, noble gas) inventory. Fortunately, the extent and consequences of gas loss/accumulation processes can be tested by studying characteristic changes in volatile elements compositions, including 4He/40Ar* (where 40Ar* is 40Ar corrected for atmospheric contamination). To document the mechanism of popping rock generation and potential effects of degassing and gas accumulation processes on MORB volatile systematics, we present a comprehensive volatile characterization (carbon, nitrogen and noble gas systematics) of popping rocks and associated MORBs (n = 19) recently sampled at 14°N on the MAR, including 2 normal MORBs (N-MORB) from an oceanic core complex (OCC) and 17 E-MORBs. In line with previous studies, we find that PR exhibit the lowest 4He/40Ar* (1.08 ± 0.04) among all MORB samples, lower than the conventional mantle production ratio of 3 ± 1. Such low 4He/40Ar* could either (i) derive from accumulation of first-generated bubbles originating from open-system degassing of underlying magmas, or (ii) represent the actual upper mantle production ratio. We summarize the arguments in favor of each of these two scenarios (including the required accumulation times for radiogenic noble gases accumulation, the K/U and 232Th/238U of the upper mantle, popping rock vesicle size distributions, and physical considerations for vesicle growth and upwelling through a basaltic magma), and discuss their implications for the volatile composition of un-degassed magmas from the upper mantle. We find homogenous N isotope compositions (average δ15N of −4.49 ± 1.40 ‰ at 14°N) but variable δ13C (from −11 ‰ to −3.4 ‰), potentially compatible with the expectations for residual dissolved gas after Rayleigh fractionation. However, explaining the light δ13C signatures of PR via this process appears incompatible with any of the two scenarios proposed for explaining their low 4He/40Ar*, which would predict (i) 13C-enrichements and (ii) no fractionation relative to an initial composition at δ13C ∼ -5 ‰, respectively. After correction for solubility-controlled degassing fractionation and potential gas accumulation processes using 4He/40Ar* systematics, we find relatively homogeneous C/3He ((2.65 ± 0.51) × 109) but variable C/N (from 125 up to 4578), whose potential origins are discussed as part of a companion paper (Part B: Source effects).

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Geochimica et Cosmochimica Acta