Origin of basalt fire-fountain eruptions on Earth versus the Moon

Fire-fountain eruptions of basaltic magma occur on Earth at centers such as Kilauea (Hawaii), and deposits from apparently similar eruptions have been found on the lunar surface. The driving force for terrestrial fire-fountain eruptions is the exsolution of dissolved CO(2) based on gases dissolved in melt inclusions trapped in olivine phenocrysts and the relatively high oxidation state of these magmas. Gases released at the vent show that SO(2), and eventually H(2)O, are partitioned into the CO(2)-rich gas, adding to the gas volume. In contrast, analytical and experimental studies of lunar samples indicate that the gas phase responsible for driving the lunar eruptions was CO-rich and produced by the oxidation of C (graphite) carried in the slowly ascending low-fO(2) basalt. The graphite oxidation occurs when the pressure in the ascending lunar magma reaches that of the graphite-gas reaction surface (40 +/- 1 MPa or similar to 8 km depth for the Apollo 17 orange-glass magma). As graphite is oxidized, some FeO is reduced, potentially forming a Fe-rich metal phase, and Fe-rich metal spherules are present in beads in lunar glass deposits. Other gas species such as S, Cl, and F partitioned variously into CO-rich lunar volcanic gas, and appear in surface coatings on the glass spherules. Modeling of the magma flow from 8 km depth to the lunar surface shows that the gas bubble volume fraction (assuming initial C at 50-500 ppm levels) ranges from 0.5 to 0.8 at the surface, the exit velocity ranges from 15 to 35 m/s, and the low-viscosity magma fragments only as it erupts at the lunar surface.