Melting of a Two-component Source beneath Iceland

Abstract

New trace element and Hf^Nd isotope data on post-glacial basalts from Iceland' s main rift zones are used in conjunction with literature data to evaluate the relative importance of source heterogeneity and the melting process for the final melt composition. Correlations between Hf and Nd isotope compositions and trace element ratios indicate that at least two source components are sampled systematically as a function of the degree and pressure of melting beneath Iceland. Strong depletion in Rb, Ba, U and Th and enrichment in Nb and Ta compared with La in the most enriched samples from the Reykjanes Peninsula and Western Rift Zone suggests that the enriched source component is similar to ancient recycled enriched mid-ocean ridge basalt (E-MORB) crust. Highly incompatible trace element ratios such as Nb/La and Nb/U and Pb isotope ratios are variable across Iceland. This observation suggests that either the enriched component is intrinsically heterogeneous, or that there is a larger proportion of the enriched source component beneath the Southwestern Rift Zone compared with the Northern Rift Zone. The relative effect of source heterogeneity and melting on the final melt composition was evaluated with a one-dimensional polybaric melt mixing model in which accumulated melts from a depleted MORB mantle and a recycled E-MORB crust are mixed in different ways. Two styles of melt mixing were simulated: (1) complete mixing of melts with variable proportions of the depleted mantle and recycled E-MORB components; (2) incomplete mixing with a fixed initial proportion of the two source components. Calculated pressure-dependent compositional changes using these simple two-component melting models can explain the observed trends in trace element ratio and isotope ratio diagrams for Icelandic basalts, even in cases where conventional binary mixing models would require more than two source components. The example of Iceland demonstrates that melt mixing during extraction from the mantle is a key process for controlling the trace element and isotope variability observed in basaltic lavas and must be evaluated before inferring the presence of multiple source components.

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

“…1). Koornneef et al (2012) previously reported their major and trace element and Hf and Nd isotope composition. In addition to the samples from the Reykjanes Peninsula (RP, n = 10), the Western Volcanic Zone (WV, n = 7), and the Northern Volcanic Zone (NV, n = 8), we re-analysed four samples from Theistareykir, a small area in the Northern Volcanic Zone, that were previously analysed by Stracke et al (2003aStracke et al ( , 2006 for their U, Th, Pa and Ra concentrations and isotope ratios.…”

“…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). The inferred lithological source heterogeneity may therefore affect the U-series disequilibria in young Icelandic lavas.…”

“…Here, we present new 226 Th-238 U and 231 Pa-235 U disequilibria data on 25 geochemically well characterised post-glacial tholeiites from Iceland's main rift areas (Koornneef et al, 2012) and replicate analyses of four lavas from Theistareykir previously analysed by Stracke et al (2006Stracke et al ( , 2003a. In addition to 230 Th-238 U disequilibria, the aim is to use ( 231 Pa/ 235 U) ratios, which are more sensitive to variability in melting rates compared to ( 230 Th/ 238 U) ratios, to evaluate the potential effects of source heterogeneity and the inferred variations in regional upwelling velocity on the U-series disequilibria.…”

“…Even though variations in mantle potential temperature have no resolvable influence on the major, trace element, and long-lived isotope systematics (Koornneef et al, 2012), our ( 230 Th/ 238 U) data demonstrate that systematic variation in regional mantle upwelling velocity across Iceland is required (Kokfelt et al, 2006;Bourdon et al, 2006). Variability of the ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) ratios on a local scale and the observed correlations with highly incompatible trace elements reveal an important role of source heterogeneity for establishing the U-series disequilibria in Icelandic rift zone lavas.…”