Does MORB reflect upper mantle diversity?

It is often stated that MORB provides a window into the composition of the earth's upper mantle. Although MORB displays a range of compositions, its spatial scale tends to be much longer than that of oceanic hotspots or mantle plumes, which also display greater compositional heterogeneity. Hence it is tempting to conclude that oceanic upper mantle is more homogeneous than mantle involved in hotspots and plumes. Observations from the interaction between Iceland and the adjacent Reykjanes Ridge offer a chance to test this view. A remarkable feature of this interaction is the rapid diminution of heterogeneity in basaltic lavas from onshore Iceland to off-shore along the adjacent mid-ocean spreading ridge (the Reykjanes Ridge). Young volcanic rocks on Iceland exhibit a wide range of trace-element and isotopic compositions reflecting a diversity of composition within the plume mantle beneath Iceland. The neovolcanic axis of the adjacent spreading ridge is also affected by the Iceland plume: the presence of a large diameter bathymetric swell, V-shaped ridges migrating out from Iceland, and associated enriched geochemical compositions all point to a ~1000-km diameter regional plume influence. Despite this, the diversity of lava composition decreases rapidly along the Reykanes Ridge away from Iceland. This decrease is unlikely to be an artefact of sampling as the Reykjanes Ridge has a very high density of samples acquired from dredge stations located every 2-3 km, each of which recovered a number of individual lavas. Collapsing the diversity of young lava compositions on Iceland produces a mean composition that lies on a mixing line between lavas from the northern Reykjanes Ridge and the highly enriched peripheral Icelandic volcano of Snaefjelsness. We argue that this decrease in heterogeneity is real and is evidence for either a mantle process that homogenises the outflowing Icelandic plume as it flows away from Iceland beneath the Reykjanes Ridge, or alternatively a crustal phenomena related to the spreading ridge process. If the former then it requires a process, possibly partial mantle melting, in which the heterogeneity in the upwelling plume beneath Iceland is reduced but not exhausted. As such, the Reykjanes Ridge basalts genuinely reflect a relatively homogeneous mantle source. Alternatively, there is a spreading ridge crustal processes that homogenises diverse melts from a heterogeneous mantle source. Such a process is counter-intuitive and paradoxical since the degree of homogenisation increases as the crustal thickness decreases. Hence although the melt flux and, presumably, the residence time in magma chambers decreases, simultaneously the degree of magma mixing and homogenisation increases. In this case, mid-ocean spreading ridge processes mask the scale of upper mantle heterogeneity, giving us a false impression of mantle composition and evolution.