Low δ18O in the Icelandic mantle and its origins: Evidence from Reykjanes Ridge and Icelandic lavas

186Os and 187Os enrichments and high-3He/4He sources in the Earth’s mantle: Evidence from Icelandic picrites

“…These Os isotope systematics for the Iceland picrites are consistent with additional geochemical evidence for ancient recycled slab components in the mantle sources for Iceland magmas (Hemond et al, l993;Hanan and Schilling, l997;Taylor et al, l997;Stecher et al, l999;Hanan et al, 2000;Chauvel and Hemond, 2000;Kempton et al, 2000;Thirlwall et al, 2004;Macpherson et al, 2005;Thirlwall et al, 2006). Ancient melt enrichment processes and/or additions of recycled crust to the mantle source generate radiogenic 187 Os/ 188 Os observed at other mantle plume localities.…”

“…This should lead to 4 He accumulation in the source resulting from decay of 238 U, 235 U, and 232 Th, with consequent low 3 He/ 4 He that is in contrast to the high 3 He/ 4 He observed for the lavas that have the strongest signals for enrichment (Figure 5,6). In addition to the correlation between 187 Os/ 188 Os, 3 He/ 4 He and the trace element ratios observed here, the high 3 He/ 4 He source for neovolcanic zone Iceland lavas may also have a low δ 18 O relative to MORB mantle (Thirlwall et al, 2006).…”

“…The high 3 He/ 4 He source therefore cannot have a direct physical relationship with the convecting MORB mantle over extended timescales and must exist in a region where recycled lithosphere/crustal material can be mixed in without contribution from the MORB mantle source. A similar inference was made based on the negative correlation of low δ 18 O with high 3 He/ 4 He observed in lavas from the neovocalanic zones in Iceland and the Reykjanes ridge (Thirlwall et al, 2006). Further support that the geochemical variability of Iceland magmatism reflects an origin from a deep mantle plume may be inferred from the primordial Ne isotopic compositions in neovolcanic zone lavas (Dixon et al, 2000;Moreira et al, 2001).…”

“…These noble gas characteristics likely require a deep mantle source that is distinct from the convecting upper mantle source for MORB (Kurz et al, l982;l985;Graham et al, l998;Harrison et al, l999;Hilton et al, l999;Breddam et al, 2000;Dixon et al, 2000;Moreira and Sarda, 2000;Moreira et al, 2001;Stuart et al, 2003;Graham, 2005;Ballentine et al, 2005;Macpherson et al, 2005). In addition, there is evidence in some Icelandic lavas, from both noble gases and lithophile elements, for varying contributions from the upper mantle MORB source as well as from several ancient crustal and/or mafic components within the mantle (Hemond et al, l993;Hanan and Schilling, l997;Fitton et al, l997;Taylor et al, l997;Stecher et al, l999;Hanan et al, 2000;Breddam et al, 2000;Chauvel and Hemond, 2000;Kempton et al, 2000;Thirlwall et al, 2004;Macpherson et al, 2005;Thirlwall et al, 2006;Kokfelt et al, 2006). These relationships are consistent with melting and mixing that accompanies the physical setting of the present-day Iceland plume beneath the spreading Mid-Atlantic Ridge system.…”

No direct contribution of recycled crust in Icelandic basalts

“…It is widely accepted that the compositional spectrum of Icelandic lavas requires the presence of at least two components in the mantle source (e.g., Thirlwall et al, 2004Thirlwall et al, , 2006Sobolev et al, 2007;Brown and Lesher, 2014). However, the nature of these components is still debated and two end-member models have been suggested: while trace element and isotopic analyses of Icelandic lavas seem to suggest the presence of two distinct lithologies in the form of pyroxenite and peridotite (e.g., Kokfelt et al, 2006;Koornneef et al, 2012;Brown and Lesher, 2014), major-element compositions and olivine chemistry suggest a peridotite mantle composed of more or less re-fertilised domains by reaction with the recycling material (Shorttle and Maclennan, 2011;Herzberg et al, 2016).…”