Experimental insights into the effects of mantle-derived chemical variability on oceanic basalt petrogenesis

Abstract:

Volcanic eruptions offer crucial insights into the chemical structure of Earth’s convecting mantle. However, the true complexity of deep chemical variability is often erased from erupted records by mixing and crystallisation during magma ascent in important but partially understood ways. Here we discuss how mantle-derived chemical variability affects not only the phase equilibria of primitive oceanic basalts but also the ways in which these basalts record this variability. We performed high-pressure (300 MPa), high-temperature (1140–1260°C) crystallisation experiments on synthetic analogues of chemically extreme primitive basalts from the Reykjanes Peninsula of Iceland. These experiments show how magmas from enriched, recycled sources maintain higher melt fractions during crystallisation in the crust than those from ambient sources of fertile lherzolite. This is because plagioclase saturates at lower temperatures in Na- and H₂O-rich melts from recycled sources than Ca- and Al-rich melts from lherzolitic ones. Erupted records of mantle chemistry may thus be systematically biased towards melts from recycled sources that are predisposed to survive crustal processing. By performing magma-magma reaction experiments under similar conditions, we then investigated how chemically distinct primitive basalts respond to magma mixing at depth. Experimental products show that melts diffusively re-equilibrate, with different elements responding at rates commensurate with their different diffusivities. This re-equilibration appears little affected by the presence of crystals, yet exerts a major control on their stability, with a wave of plagioclase dissolution permeating initially plagioclase-saturated portions of the samples. The dissolution of plagioclase in response to recharge by enriched (as opposed to hot) melts may thus constitute a widespread trigger for crystal mush disaggregation and melt inclusion formation within basaltic plumbing systems in oceanic settings.