The Importance of Differentiation During the Ascent of Primitive Magmas

Magmas that are in chemical equilibrium with the mantle residues of partial melting are exceedingly rare. Even many basalts containing mantle xenoliths have Mg-numbers that are low for primary magmas, implying differentiation at mantle depths. We argue that this differentiation can take place rapidly during ascent (over hours to days) rather than slowly during storage in magma chambers (centuries to millennia, or longer). Our evidence comes from small, compositionally zoned alkali basalt centres in the Auckland Volcanic Field, New Zealand. In the case study here, the Crater Hill eruption (0.01 cubic km) produced a sequence of pyroclastic deposits and lava that changed systematically in Mg-number from 59 to 67. The geochemical variations are explained by fractional crystallization at high pressures (with cpx±gt) of a parent magma generated at 2.5- 3 GPa. The similar densities of the magmas, their orderly eruption sequence, the lack of magma mixing, and the small total eruption volume indicate that the magmas did not become zoned in a magma chamber. Instead, we model the compositions by differentiation processes in the magma while it rises in a dyke. On leaving its source region, the first parcel of magma encounters mantle that is slightly cooler than the magma's liquidus temperature. The small thermal contrast is insufficient to quench the magma but causes liquidus phases to crystallise on the dyke walls, leaving liquid that is the product of a small amount of high pressure fractional crystallization to ascend farther. As the parcel of magma traverses still-cooler mantle, it evolves along a polybaric liquid line of descent until it encounters lithosphere that is cold enough to chill the dyke margin, rather than crystallize liquidus phases. Above this point the magma rises without changing chemical composition, although intermediate to low pressure phenocrysts may grow with cooling and gas-loss. The passage of the earliest erupted magmas heats the wall rocks, so later magmas reach the surface having undergone less evolution than earlier ones, resulting in compositionally zoned products. This thermal limitation on the eruption of primary magmas should apply in all tectonic settings because of the geometries of the geotherm and the magma's liquidus.