Mantle Sources of Icelandic Flank and Rift Zone Magmas: Forward Modelling of a Heterogeneous Mantle to Recreate Trace Element Signatures

The Icelandic mantle is lithologically and chemically heterogeneous. Depleted anhydrous peridotite, residual harzburgite, pyroxenite and primitive mantle material are all components of the Icelandic mantle. The pyroxenite component is generally thought to be recycled oceanic lithosphere but the spatial distribution of this component remains uncertain [1].

Icelandic basalts vary between tholeiitic in the active rift zones and intermediate to alkalic in the flank zones. We use melt inclusions from Snæfellsjökull and Oræfajökull volcanoes in Iceland's flank zones and Miðfell in the Western Volcanic Zone to investigate mantle heterogeneity across the Iceland mantle plume.

We use REEBOX Pro software [2] to model the melting of a three-component upper mantle beneath Iceland. We then model further mixing between these synthetic melts and deep melts of a primitive mantle source. We then match trace element signatures from each sample location with our modelled melt compositions, to infer the lithologies involved in their melt production.

Melt inclusions from Miðfell fall into two distinct compositional groups. Melt inclusions with the most incompatible trace element (ITE)-depleted signatures are best modelled by melting of anhydrous peridotite. Moderately ITE-enriched melts are best modelled by mixing melts of the same anhydrous peridotite (80%) with smaller contributions of pyroxenite melts (10-15%) produced at greater depths and primitive plume-derived melts (5-10%).

ITE-enriched melts from Oræfajökull are similar to those of Miðfell, but require a greater contribution from pyroxenitic (30-35%) and plume-like (15-25%) mantle components. Snæfellsjökull melt inclusions are highly ITE-enriched and derive from pooled melts of pyroxenite and peridotite sources from the same mantle depth, with no contribution from primitive mantle. The pooled melts are dominated by pyroxenite-derived melts.

Our results show that pyroxenite is present in the mantle sources of both plume-influenced Icelandic melts (Miðfell and Oræfajökull) and non-plume-influenced melts (Snæfellsjökull). We investigate the origin of and potential differences between these pyroxenite lithologies.

[1] Shorttle et al (2014) EPSL 395, 24-40

[2] Brown and Lesher (2016) G³ 17, 3929-3968