Figure 2.
The Plate Model. High R basalts contain a component from a high 3He/U reservoir rather than a high 3He-undegassed reservoir. Magma degasses CO2 and He at shallow depth that are partially trapped in fluid inclusions or vugs in refractory (U, Th-poor) shallow mantle, (high 3He/U). The 3He/4He ratio in the gas is the same as, and the Ar/He and Ne/He ratios are higher than, in the melt. With time, the 3He/4He ratio in the newly formed crust and the original MORB reservoir (low 3He/U) decline, but it stays high in the fluid inclusions. Midplate or new rift magmatism interacts with the shallow mantle, picking up large-ion lithophile elements, from recycling, and old trapped melts, and seawater contamination (implied by heavy noble gas abundances) and CO2 with “frozen-in” high 3He/4He ratios. Recycling returns large-ion lithophiles to the shallow mantle (perisphere) by dehydration reactions and returns depleted slab material to the deep mantle (bicycling). High 3He/4He ratios are due to low U and low 4He abundances, compared with high U and Th crust and mantle source regions. Midplate magmatism involves near-surface interaction and/or recycled material. High 3He/4He ratios may have been MORB values when the shallow refractory mantle, or recycled lithosphere, formed. 3He/4He ratios of crust, lithosphere, and mantle source diverge at the time of magma emplacement, outgassing, and formation of fluid-filled inclusions. Eventually, the lithosphere in the vicinity of the conduit will be degassed but the volcano load may initiate a new crack, and the degassing process of a new section of lithosphere can start anew. Temporal variations are due to magmatism induced changes in the plate (depletion, degassing). Shallow material is incorporated into midplate volcanoes by wall-rock reactions, melting, magma-mixing, degassing, and near-surface contamination. In a propagating magma-filled crack, the magma runs out of pressure before it makes it to the very narrow crack tip, and there is a gap between the tip and the magma front. The tip cavity will therefore be filled with volatiles exsolving from the magma or pore fluid from the host rock. Gases can therefore exsolve from magma at greater depth than usually assumed, and wall rock gases can enter new cracks.