No slab‐derived CO 2 in Mariana Trough back‐arc basalts: Implications for carbon subduction and for temporary storage of CO 2 beneath slow spreading ridges

The Southern Mariana Trough is particularly well suited to study mass balance in subduction zones because the flux of material recycled from the subducted slab has been shown to diminish to negligible levels in the southernmost part of the area. We present new He and Ar concentration and isotopic data for 16 back‐arc basaltic glasses and combine these with previously published CO 2 and H 2 O concentration and δ 13 C data to explore the recycling of carbon and light noble gases in the Mariana back arc. Degassing has affected all samples and is particularly extensive in more water‐rich samples, i.e., those containing the largest recycled component. The degassing history features three stages: (1) deep degassing which commenced when the melt reached saturation of CO 2 and noble gases in the mantle, (2) preeruptive degassing during storage in the crust‐mantle transition zone which involved addition of extraneous CO 2 to the vapor phase, and (3) eruption. CO 2 released during stage 1 was, at least partially, incorporated into wall rock and subsequently remobilized during stage 2 degassing of later magma batches. Reconstructed parental values for 3 He/ 4 He, δ 13 C, CO 2 / 3 He, and CO 2 / 40 Ar* are indistinguishable from those of mid‐ocean ridge basalt. This implies that there is negligible recycling of subducted carbon, helium, or argon into the source of Mariana Trough basalt.