Noble gas and CO2 systematics of petit-spot magma reveal low-degree melting of the upper mantle

Petit-spot volcanoes erupt on old oceanic plates far from mid-ocean ridges. Their magmas are characterised by exceptionally high initial CO 2 concentrations of up to ∼10 wt%. Although petit-spot magmas have been inferred to originate from CO 2 -rich melts at the lithosphere–asthenosphere boundary, their mantle source and melting mechanism remain debated. Here, we determined the noble gas and CO 2 compositions of vesicle gases in a glassy petit-spot basalt, 6K#1466R3–​003 (1466R3), from the western Pacific plate using stepwise crushing and laser ablation. We then estimated the initial noble gas concentrations of the 1466R3 magma. The highest measured 20 Ne/ 22 Ne and 40 Ar/ 36 Ar ratios are 12.56 ± 0.11 and 24,100 ± 1100, respectively. The estimated mantle 21 Ne/ 22 Ne ratio of 0.0563 ± 0.019 is similar to those of mid-ocean ridge basalts (MORBs). The inferred noble gas elemental and isotopic ratios of the 1466R3 magma are similar to those of typical MORB magma. The δ 13 C values of CO 2 (−4 to −7‰) are consistent with those of MORBs. The CO 2 / 22 Ne ratio in the 1466R3 magma is approximately twice that of average MORB magma, suggesting a heterogeneous carbon distribution in the upper mantle. The initial noble gas concentrations in the 1466R3 magma are >10-fold higher than those of average MORB magma. The extremely high noble gas concentrations suggest low-degree partial melting of the source mantle, with a melt fraction of ≤1%. These results provide evidence for the generation of CO 2 - and noble-gas-rich 1466R3 petit-spot magma by low-degree partial melting of the upper mantle.