Theistareykir revisited

Abstract

[1] Postglacial basalts from Theistareykir (younger than 10,000 years), northern Iceland, define the depleted end of the spectrum of chemical and isotopic compositions observed in Icelandic volcanics but extend to some of the most enriched chemical and isotopic compositions found in Icelandic tholeiites. A spectacular feature of these basalts is the impressive correlations observed between radiogenic isotope ratios (Sr, Nd, Hf, and Pb) and almost the entire spectrum of major and trace element concentrations and ratios. The radiogenic isotope and major and trace element compositions are little affected by crystal fractionation and are essentially unaffected by interaction with the preexisting crust. The Theistareykir basalts must therefore be relatively close in composition to primary melts from the mantle. Consequently, their chemical and isotopic compositions provide a unique opportunity to investigate the nature of melting beneath Iceland and the geochemical character and origin of the mantle source. Large variations in incompatible element abundances require source heterogeneity, as well as variable extents of melting, to be important factors in determining the final chemical composition of the melts. Melting integrates over a large pressure range and is dominated by melting a depleted peridotite similar to the ambient depleted North Atlantic mantle. The isotopically enriched component is of relatively minor abundance and probably has a lower solidus temperature compared to the depleted component. More than one isotopically enriched component must be involved, but it is difficult to identify the end-member compositions using those of the lavas because of preeruptive averaging and damping of the enriched isotopic signals by mixing with the ambient depleted mantle or melts thereof, suggesting that the isotopic signals in Icelandic melts represent a somewhat muted isotopic signal of the enriched component(s) in the Icelandic source mantle. Comparison of the isotopic arrays of Icelandic basalts with those of global OIB suggests that the dominant enriched

High-3He/4He, depleted mantle and low-δ18O, recycled oceanic lithosphere in the source of central Iceland magmatism

“…A similar conclusion has been reached for depleted lavas from Theistareykir in the NRZ [40]. Although the importance of AFC at Theistareykir has been disputed [47], the oxygen isotope data for Group 1 support the conclusions that depleted lavas have interacted with the crust and that depleted Icelandic mantle possesses δ 18 O which is within the range of the source of MORB [40]. However, more extensive differentiation of depleted magma by AFC cannot lower δ 18 O and enrich trace elements to the extent observed in Group 2 and 3 glasses (curve B, Fig.…”

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

“…In Icelandic rocks, correlations between major elements and trace element ratios and long-lived isotopes suggest that melting of at least two components, one isotopically depleted and one isotopically enriched, is required to explain the observed trends (Wood, 1981;Elliott et al, 1991;Maclennan et al, 2003;Stracke et al, 2003b;Kokfelt et al, 2006;Maclennan, 2008a;Stracke and Bourdon, 2009;Peate et al, 2010;Koornneef et al, 2012). Although the nature of the enriched Icelandic source component remains controversial, most previous studies favoured ancient recycled oceanic crust, present in form of small-scale mafic components (e.g., Chauvel and Hémond, 2000;Skovgaard et al, 2001;Stracke et al, 2003b;Kokfelt et al, 2006;Peate et al, 2010).…”

“…Although the nature of the enriched Icelandic source component remains controversial, most previous studies favoured ancient recycled oceanic crust, present in form of small-scale mafic components (e.g., Chauvel and Hémond, 2000;Skovgaard et al, 2001;Stracke et al, 2003b;Kokfelt et al, 2006;Peate et al, 2010). Enrichments in Nb/La and Nb/U ratios combined with higher 206 Pb/ 204 Pb in samples from the Western Rift Zone and the Reykjanes Peninsula suggest that the abundance of this enriched component is larger beneath these main rift areas compared to the Northern Rift Zone (Hanan et al, 2000;Koornneef et al, 2012).…”

Geochemical variability in MORB controlled by concurrent mixing and crystallisation

“…Magma mixing has been recognised in the local whole rock and glass records from mid-ocean ridge and ocean island settings using major and minor elements (Rhodes et al, 1979;Rhodes, 1983;Perfit and Chadwick, 1998;Rubin et al, 2001), trace elements (Bergmanis et al, 2007), uranium series systematics (Lundstrom et al, 1995) and radiogenic isotopes (Stracke et al, 2003). The pioneering work of Sobolev and Shimizu (1993) and Sobolev (1996) showed how high forsterite olivine-hosted melt inclusions can record greater chemical variability than their carrier liquids, and thus that some mixing must be occurring simultaneous to crystallisation in the crust or shallow mantle.…”

“…The essential feature of this model is that diverse primary melts are supplied from the mantle, both as an inevitable consequence of polybaric fractional melting (in the case of trace elements), and the isotopic and lithological heterogeneity intrinsic to the source region (e.g. Stracke et al, 2003;Shorttle and Maclennan, 2011;Shorttle et al, 2014). This initial variability is then destroyed as magma experiences mixing whilst resident in successive magma chambers.…”