Reconstructing mantle volatile contents through the veil of degassing

Abstract:

The abundance of volatile elements in the mantle reveals critical information about the Earth’s origin and evolution such as the chemical constituents that built the Earth and material exchange between the mantle and exosphere. However, due to magmatic degassing, volatile element abundances measured in basalts usually do not represent those in undegassed magmas and hence in the mantle source of the basalts. While estimates of average mantle concentrations of some volatile species can be obtained, such as from the ³He flux into the oceans, volatile element variability within the mantle remains poorly constrained. Here, we use CO₂-He-Ne-Ar-Xe measurements in basalts and a new degassing model to reconstruct the initial volatile contents of 8 MORBs from the Mid-Atlantic Ridge and Southwest Indian Ridge that span a wide geochemical range from depleted to enriched MORBs.

We first show that equilibrium degassing (e.g. Rayleigh degassing), cannot simultaneously fit the measured CO₂-He-Ne-Ar-Xe compositions in MORBs and argue that kinetic fractionation between bubbles and melt lowers the dissolved ratios of light to heavy noble gas species in the melt from that expected at equilibrium. We present a degassing model (after Gonnermann and Mukhopadhyay, 2007) that explicitly accounts for diffusive fractionation between melt and bubbles. The model computes the degassed composition based on an initial volatile composition and a diffusive timescale. To reconstruct the undegassed volatile content of a sample, we find the initial composition and degassing timescale which minimize the misfit between predicted and measured degassed compositions.

Initial ³He contents calculated for the 8 MORB samples vary by a factor of ~7. We observe a correlation between initial ³He and CO₂ contents, indicating relatively constant CO₂/³He ratios despite the geochemical diversity and variable gas content in the basalts. Importantly, the gas-rich popping rock from the North Atlantic, as well as the average mantle ratio computed from the ridge ³He flux and independently estimated CO₂ content fall along the same correlation. This observation suggests that undegassed CO₂ and noble gas concentrations can be reconstructed in individual samples through measurement of noble gases and CO₂ in erupted basalts.