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Compositional Trends of Icelandic Basalts: Implications for Short–Length Scale Lithological Heterogeneity in Mantle Plumes

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Compositional Trends of Icelandic Basalts: Implications for Short–Length Scale Lithological Heterogeneity in Mantle Plumes Article Volume 12, Number 11 18 November 2011 Q11008, doi:10.1029/2011GC003748 ISSN: 1525‐2027 Compositional trends of Icelandic basalts: Implications for short–length scale lithological heterogeneity in mantle plumes O. Shorttle and J. Maclennan Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK ([email protected]) [1] Lithological variations in the mantle source regions under mid‐ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we com- bine compositional observations from Icelandic basalts and modeling of melting of a bilithologic peridotite‐pyroxenite mantle to demonstrate that, while short–length scale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of end‐member Icelandic melts, we find enriched melts to be characterized by low SiO2 and CaO, but high FeO. We quantitatively com- pare end‐member compositions to experimental partial melts generated from a range of lithologies, pres- sures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilized by the addition of up to 40% mid‐ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be overrepresented in accu- mulated melts compared with its abundance in the source. Modeling of peridotite‐pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations. Components: 20,000 words, 14 figures, 7 tables. Keywords: Iceland; major elements; mantle heterogeneity; mantle plumes; melting model. Index Terms: 1009 Geochemistry: Geochemical modeling (3610, 8410); 1025 Geochemistry: Composition of the mantle; 1030 Geochemistry: Geochemical cycles (0330). Received 14 June 2011; Revised 11 October 2011; Accepted 12 October 2011; Published 18 November 2011. Shorttle, O., and J. Maclennan (2011), Compositional trends of Icelandic basalts: Implications for short–length scale lithological heterogeneity in mantle plumes, Geochem. Geophys. Geosyst., 12, Q11008, doi:10.1029/2011GC003748. 1. Introduction last 15 years that a significant role for recycled oceanic crustal material in oceanic island and mid‐ [2] Subduction has long been identified as a gen- ocean ridge basalt (OIB and MORB) genesis erator of mantle isotopic and trace element het- has become widely proposed [e.g., Hauri, 1996; erogeneity [Hofmann and White, 1982; White and Hirschmann and Stolper, 1996; Niu et al., 2008; Hofmann, 1982]. However, it is only during the Sobolev et al., 2005; Herzberg, 2006; Prytulak and Copyright 2011 by the American Geophysical Union 1 of 32 Geochemistry Geophysics 3 SHORTTLE AND MACLENNAN: COMPOSITIONAL TREND OF ICELANDIC BASALTS 10.1029/2011GC003748 Geosystems G Elliot, 2007; Sobolev et al., 2007; Dasgupta et al., demonstrated that small‐scale source variability is 2010; Herzberg, 2011]. In particular, these authors also present in the major elements. identify the recycled basaltic component to be [5] The aim of this work is to investigate the nature present as discrete pyroxenitic or eclogitic litholo- of lithological heterogeneity in the mantle under gies. Regions in the mantle of distinct mineralogy Iceland, and to constrain how such heterogeneity have important implications not only for models can influence melt flux in mid‐ocean ridge set- of melt generation, but also for mantle dynamics tings. In order to achieve this target, the study is and interpretation of geophysical data [e.g., Ita and divided into two parts. In the first part, we system- Stixrude, 1992; Van der Hilst and Kárason, 1999; atically assess major, trace element and isotopic Ritsema et al., 2009a, 2009b]. data from Icelandic basalts, to place constraints on [3] The identification of clear geochemical proxies the lithology of the sources producing the enriched for the involvement of varying lithologies in basalt and depleted melts. We find that while major ele- genesis remains a significant challenge. Part of the ment variability of the source beneath Iceland difficulty lies in the interpretation of basalt major is required, none of the geochemical observations element chemistry, which is both strongly affected necessitate the role of an olivine‐free lithology by low‐pressure crystallization, and the depth directly contributing to erupted melts. Instead, and extent of melting [Grove et al., 1992; Klein and variably enriched peridotites, refertilized by up to Langmuir, 1987; Langmuir et al., 1992]. As a result, 40% MORB material, are capable of generating all major element signatures inherited from the miner- the geochemical patterns observed, including the alogy of the source are difficult to uniquely dis- Ni content of olivines from four eruptions repre- tinguish. Therefore, while the trace element and sentative of the range of Icelandic melt composi- isotopic heterogeneity of the mantle has long tions. Enriched and depleted major element signals been recognized [Hart, 1971; Schilling, 1973], this exist within similar aged eruptions from a single diversity was for a long time incorporated into the volcanic system, indicating the major element classic peridotite model of Ringwood [1962]. variability in the Icelandic mantle source must be present on a short length scale. The nature of [4] Recently, Herzberg [2006, 2011] has inferred this source enrichment means the enriched lithol- the majority of the shield building phase on Hawaii ogy will be more fusible than the depleted source, to be composed of 100% pyroxenite melts, the aggregate melts will therefore be biased toward basis for this interpretation being the lower CaO melts from the deep melting, productive, enriched content of Hawaiian basalts than predicted from lithology. In the second part of this paper we partial melting of peridotite [Walter, 1998]. Simi- combine the constraints on mantle heterogeneity larly, the high Ni content of olivines found as from the first part, with the development of a new phenocrysts in Hawaiian and Icelandic basalts has model of melting a bilithological mantle, in order been taken by Sobolev et al. [2005, 2007, 2008] to to investigate the contribution of mantle composi- indicate the role of olivine‐free lithologies in their tional heterogeneity to excess melt generation mantle source regions. Focusing on Iceland, pre- around Iceland. Even allowing for melting of the vious workers have inferred the presence of recy- highly productive G2 pyroxenite [Pertermann and cled basalt in the source from incompatible trace Hirschmann, 2003b], oceanic crustal thickness element, radiogenic isotope and stable isotope variations observed cannot be produced by source compositions of Icelandic basalts [Fitton et al., enrichment alone. Mantle potential temperature (T ) 1997; Chauvel and Hémond, 2000; Thirlwall et al., p variations of at least +100°C are required, coupled 2004; McKenzie et al., 2004; Kokfelt et al., 2006; with a component of plume‐driven upwelling, to Peate et al., 2010]. The question remains, however, generate the thickest oceanic crust observed in to what extent these geochemical observations the North Atlantic. require a lithologically distinct basalt component in the mantle, as opposed to the material having [6] For 400 km of its ∼20,000 km length the Mid‐ reacted with and refertilized depleted peridotites. Atlantic Ridge becomes subaerial on Iceland. This Equally important is the scale of this heterogeneity; exposure provides a unique opportunity to gather trace elements and isotopes have been used to a high‐resolution spatial and temporal record of establish the presence of short–length scale het- mantle melting at a divergent plate boundary. We erogeneity in the Icelandic mantle [Zindler et al., have compiled a comprehensive major, trace and 1979; Slater et al., 2001; Stracke et al., 2003; isotopic data set from both onland and adjacent Maclennan, 2008a], however it remains to be submarine ridges, to characterize the geochemi- 2of32 Geochemistry Geophysics 3 SHORTTLE AND MACLENNAN: COMPOSITIONAL TREND OF ICELANDIC BASALTS 10.1029/2011GC003748 Geosystems G Figure 1. Map of Iceland showing the geographical distribution of data used in this study, with the neovolcanic zones marked in dark grey. The eruptions from which new olivine data are presented are marked: Thp, Theistareykir picrite; Gae, Gaesafjöll; Hal, Háleyjabunga; Stp, Stapafell. Inset shows Iceland in relation to the North Atlantic and the along‐ridge distribution of compiled literature data. Data sources listed in Table 6. cal variations associated with the plume and depleted melts erupting on Iceland, and determining explore their implications for lithological variation if nonperidotitic lithologies are necessary to match in the North Atlantic mantle. The list of sources the end‐member compositions. used in the Icelandic geochemical data set can be found in Appendix A, and in Figure 1 the sample distribution is mapped. 2. Characterizing the Icelandic End‐Member Melt Compositions [7] In Figure
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