Mantle source and melting processes beneath Iceland’s Flank and Rift
Zones: Forward Modelling of Heterogeneous Mantle Melting
Abstract
The Icelandic mantle contains a range of lithologies associated with the
depleted upper mantle, a mantle plume, and recycled oceanic lithosphere
but the precise nature of depleted and enriched components in the mantle
and their relative contributions to melt production remain poorly
constrained. In this study, we collect new olivine- and
plagioclase-hosted melt inclusion data and compile this with existing
literature data to investigate the relative contributions from different
mantle lithologies to basaltic magmas erupted in Icelandic flank zones
and neovolcanic zones by modelling the melting of a heterogeneous mantle
and subsequent mixing of derived melts. We find that observed melt
inclusion compositions from off-axis flank zones are best explained as
homogenized mixtures of pyroxenite- and lherzolite-derived melts
produced at depths around 80-93 km, by which point lherzolite has only
experienced a low degree of melting whereas the pyroxenite lithology has
melted extensively. These melts represent the onset of channelization in
the mantle and are transported rapidly to the surface without input from
shallower melts. Melt compositions from the on-axis neovolcanic zones
and off-axis Öræfajökull, are produced by mixing this deep melt
component with higher degree lherzolite melts produced at shallower
depths, between 57-93 km. Proportions of shallow lherzolite-derived
melts and deep homogenized melt vary, but the lowest contribution from
the deep homogenized melt is seen in the Northern Volcanic Zone.
Ourresults support a model whereby deep melts mix until melt
channelization starts in the mantle, after which binary mixing between
the homogenized deep melt and shallower fractional melts occurs.