In addition to high-temperature fluid fluxes, many of the Kolumbo vents actively discharge gases (Fig. 5C). These gases are virtually pure CO2 with MORB-like 3He/4He signatures indicative of deep degassing of mantle-derived magmas (Carey et al. 2013; Rizzo et al. 2016).
MO4.? Comparison of magma series at different tectonic settings and associated major features.
So this builds a lot on the concepts we were talking about earlier in the course and also very recently. Both isotopes of helium gas are considered volatiles, which have a big impact on the ways that magmas behave before and during an eruption. In this case there is no eruption, but as we see in Figure 5C there is significant degassing of these volatiles in the hydrothermal field. They are similar to the degassing composition of mid-ocean ridge basalts, which come from magma degassing deeper in the mantle. The MORBs are mafic, while the magma here could be any combination of felsic to intermediate to mafic. The magma still in the mantle that is degassing here is likely mafic, as that silica concentration doesn't hold onto gases as much as a felsic melt would. Previous eruptions have generated felsic rocks, so it is possible that the degassing is just fighting its way through a felsic magma. But either way, this shows that the mantle in this area is rising similar to the way it would at a mid-ocean ridge. The melting process is different, but the upwelling is similar and contains similar helium isotope signatures.