MELT-PX 2.0: EFFECT OF THE MELTING REGIME ON THE MAGMATIC PRODUCTIVITY OF A HETEROGENEOUS MANTLE

In the last breakup phase of the supercontinent Pangea, voluminous magmatism off the coast of Norway contributed to the formation of the North Atlantic Igneous Province. Elucidating the causes of this excess magmatism was one of the main goals of the IODP Expedition 396 [1]. Three main processes have been proposed: (1) a thermal anomaly attributed to the proximal Iceland mantle plume, (2) small-scale convection at the base of the lithosphere, or (3) heterogeneity of the mantle source.

Here, we focus on the effect a heterogeneous mantle source has on magmatic productivity using the parameterization Melt-PX [2]. Melt-PX was initially designed to model the melting of a bilithological (pyroxenite and peridotite) mantle in a passive melting regime. We present a new version (Melt-PX 2.0) that allows for modelling both passive and active melting regimes, as well as the presence of a refractory and light component (harzburgite) in the mantle source.

We show that the melting regime strongly influences the magmatic production: assuming a pure lherzolitic mantle and a lithosphere lid of 20 km, the generated mafic crustal thickness is multiplied by ~2 and ~3.3 from a passive to an active melting regime, with a thermal anomaly of 100°C and 200°C, respectively. However, the addition of a pyroxenite component in the source does not necessarily increase the bulk magmatic productivity, highlighting the importance of characterizing the composition, phase assemblage, and density of the lithologies present in the source when modeling partial melting of a heterogeneous mantle.

Mantle plume buoyancy is controlled by the thermal anomaly and the density contrast between the various lithologies present in the source. Increasing the fraction of harzburgite in the mantle source is shown to sustain the buoyancy, and consequently, the active regime for larger proportions of pyroxenite in the source. As a result, even when harzburgite is not contributing to melting, its presence in the mantle source can result in increased magmatic productivity. Our results show that a mantle containing lherzolite, pyroxenite, and harzburgite can produce the crustal thickness observed in Iceland (18-20km) even for small (100°C) thermal anomaly.

[1] Planke et al. (2023) doi: 10.14379/iodp.proc.396.2023; [2] Lambart et al. (2016) doi: 10.1002/2015JB012762