Recycled Crustal Components of the Iceland Plume Centre: Ultra-Dehydrated Crust and Oxidised Water-Bearing Fertile Mantle

Abstract

AbstractPrimitive basalts erupted mainly above the proposed centre of the Iceland plume have been analysed geochemically. The data shed light on the distribution of plume heterogeneities and constrain their nature and origin. Low-Ca (down to 1420 ppm) olivine macrocrysts and chromite inclusions in Snæfell rocks of the eastern flank zone indicate that the magmas were hydrous (c. 2 wt% H2O) and relatively oxidised (fO2 = c. ΔQMF+1). Al-in-olivine crystallisation temperatures indicate that magmas from central Iceland magmas are not hotter than from other parts of Iceland. Some Gæsafjöll rocks with high Nb/U = 110–120 from the northern rift zone (NRZ) represent magmas widespread in Iceland that are also characterised by low Ba/La and high La/U, La/Th, Nb/Th and Ce/Pb. They reflect a mantle component with Nb/U ≈ 200 present in many basalts of all rift zones. This mantle apparently consists of peridotite refertilised by ultra-dehydrated recycled oceanic crust (ROC). This component is in clear contrast to the main source of the Snæfell magmas, which is modelled to be peridotite strongly refertilised by enriched ROC more hydrous than Mid-Ocean Ridge Basalt. In contrast to more-depleted rift zone melts, which are extracted from depths just below the thin rifted lithosphere, some rift zone basalts are extracted deeper and produced by intermediate degrees of melting of the two types of refertilised mantle. Snæfell-type melts are common in the NRZ and eastern rift zones (ERZ) and most strongly present at Upptyppingar. Mixing of magmas is indicated at crustal levels. Mantle melting may be modelled for homogeneous sources of peridotite or olivine websterite. Double-spike Pb isotope analyses suggest that the depleted mantle sources of the ERZ basalts together with Snæfell and high-Nb/U sources, both with 206Pb/204Pb = 18.3–18.6, Δ207Pb ≈ 0 and Δ208Pb = 20–30, explain both ERZ and NRZ magmas, except for Theistareykir. They are distinct from magmas of the western rift zone, which is characterised by melt contributions from a differently enriched mantle with more radiogenic lead, negative Δ207Pb, Δ208Pb ≈ 0. This difference in type of enriched mantle suggests that the Iceland mantle plume is laterally zoned.