Magma ascent and crustal accretion at ultraslow-spreading ridges: Constraints from plagioclase ultraphyric basalts from the arctic mid-ocean ridge

Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclase phenocrysts were dredged in the rift valley and adjacent flanks of the ultraslow-spreading Mohns and Knipovich ridges. The PUBs show large variations in crystal morphologies and zoning. The large variations suggest that single basalt samples contain a mixture of plagioclase crystals that aggregated at different levels in the magma conduits. Resorbed crystals and repeated reverse zones suggest that the magma reservoirs were replenished and heated several times. Thin concentric zones with melt inclusions, and sharp reductions in the anorthite content of 3-7%, are common between the reverse zones. These zones, and skeletal crystals with distinctly lower anorthite contents than massive crystals, are interpreted to be the result of rapid crystalliztion during strong undercooling. The changes between short periods of cooling and longer periods with reheating are explained by multiple advances of crystal-rich magma into cool regions followed by longer periods of gradual magma inflow and temperature increase. The porphyritic basalts are characterizd by more depleted and more fractionated compositions than the aphyric basalts, with lower (La/Sm)(N), K2O and Mg-numbers. This relationship, and the observation that PUBs are sampled only close to segment centres along these ridges, suggests that the PUBs formed by higher degrees of melting and evolved in more long-lived magma reservoirs. We propose that the zoning patterns of plagioclase crystals and crystal morphologies of these PUBs reflect the development and flow of magma through a stacked sill complex-like conduit system, whereas the aphyric equivalents represent later flow of magma through the conduit. The formation of voluminous higher-degree melts may trigger the development of the magma conduits and explain the generally depleted compositions of PUB magmas.