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Continental Crust Beneath Southeast Iceland PNAS PLUS Continental crust beneath southeast Iceland PNAS PLUS Trond H. Torsvika,b,c,1, Hans E. F. Amundsend, Reidar G. Trønnesa,e, Pavel V. Doubrovinea, Carmen Gainaa, Nick J. Kusznirf, Bernhard Steinbergera,g, Fernando Corfua,h, Lewis D. Ashwalc, William L. Griffini, Stephanie C. Wernera, and Bjørn Jamtveitj aCentre for Earth Evolution and Dynamics, University of Oslo, N-0315 Oslo, Norway; bGeological Survey of Norway, Geodynamics, N-7491 Trondheim, Norway; cSchool of Geosciences, University of Witwatersrand, Wits 2050, South Africa; dVestfonna Geophysical, N-8310 Kabelvåg, Norway; eNatural History Museum, University of Oslo, N-0318 Oslo, Norway, fDepartment of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, United Kingdom; gHelmholtz Centre Potsdam, GeoForschungsZentrum, Section 2.5 Geodynamic Modelling, D-14473 Potsdam, Germany; hGeosciences, University of Oslo, N-0316 Oslo, Norway; iCore to Crust Fluid Systems/Geochemical Evolution and Metallogeny of Continents, Macquarie University, Sydney, NSW 2109, Australia; and jPhysics of Geological Processes, University of Oslo, N-0316 Oslo, Norway Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved March 11, 2015 (received for review December 4, 2014) Themagmaticactivity(0–16 Ma) in Iceland is linked to a deep mantle lites, is characterized by high 87Sr/86Sr, intermediate 206Pb/204Pb, plume that has been active for the past 62 My. Icelandic and north- as well as 207Pb/204Pb and 208Pb/204Pb ratios that lie well above east Atlantic basalts contain variable proportions of two enriched the Northern Hemisphere Reference Line (12) (Figs. 3 and 4). components, interpreted as recycled oceanic crust supplied by the These geochemical features, which also include uniformly high 18 plume, and subcontinental lithospheric mantle derived from the δ Obulk-rock of 4.8–5.9‰, have been attributed to an enriched- nearby continental margins. A restricted area in southeast Iceland— mantle (EM)-like mantle source (8, 13–15). We suggest here that and especially the Öræfajökull volcano—is characterized by a unique the unique geochemical signature is caused by contamination of enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and primitive basaltic melts with continental crust. Our modeling of 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb the Sr–Nd–Pb abundances and isotope ratios demonstrate that abundances and isotope ratios that the primitive Öræfajökull melts the Öræfajökull basalts could be derived from basaltic rift-zone could have assimilated 2–6% of underlying continental crust before magmas similar to those in the Mid-Icelandic Belt by assimila- differentiating to more evolved melts. From inversion of gravity tion of 2–6% of continental crust (Fig. 5). The basalts of the anomaly data (crustal thickness), analysis of regional magnetic data, Eastern Rift Zone and the Snæfell volcano (Fig. 2A) are com- and plate reconstructions, we propose that continental crust beneath positionally intermediate between the Mid-Icelandic Belt and EARTH, ATMOSPHERIC, southeast Iceland is part of ∼350-km-long and 70-km-wide extension Öræfajökull basalts, indicating 0–2% and 2–6% assimilation of AND PLANETARY SCIENCES of the Jan Mayen Microcontinent (JMM). The extended JMM was continental crust within the Eastern Rift Zone and Eastern marginal to East Greenland but detached in the Early Eocene (be- Flank Zone, respectively (Figs. 3 and 5). This geographically tween 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the restricted isotope array (Fig. 3) is unique in Iceland and the JMM permanently became part of the Eurasian plate following a northeast Atlantic. It may possibly support the arrival of distinct westward ridge jump in the direction of the Iceland plume. and volumetrically limited enriched mantle material, e.g., recy- cled terrigenous sediments as an EM2-component (16) in the geology | continental crust | geochemistry | plumes | plate reconstructions shallow melting zone of the plume. However, we favor a model with in situ continental crust contamination, and interpret the he North Atlantic Igneous Province covers vast areas in uniform O-isotope composition across the entire spectrum from TBaffin Island, Greenland, United Kingdom, Ireland, the basalt to rhyolite in the flank-zone volcanoes as indicating that Faroe Islands, and offshore regions (Fig. 1). Volcanic activity the intermediate and silicic magmas were chiefly generated by commenced ∼62 Mya (1) and is attributed to the impingement of fractional crystallization (17, 18). Assimilation of continental a mantle plume head on the lithosphere (2). Enriched and de- crust probably occurred at a deep level before the differentiation pleted geochemical signatures in the Paleogene to Recent basalts to evolved magmas, and we suggest that the source of continental from Iceland reflect a combination of plume-derived and shallow material beneath southeast Iceland is a sliver of the JMM. asthenospheric material, representing a classic case of plume– ridge interaction. The volcanic products of the Iceland plume Significance have signatures of recycled oceanic crust and primordial-like 3 4 material with high He/ He ratios (up to 50 RA,whereRA is the The Iceland hotspot is widely thought to be the surface 3He/4He ratio of air), spanning a range of refractory to fertile expressionofadeepmantleplumefromthecore–mantle compositions (3, 4). Additional shallow-level contributions in- boundary that can be traced back in time at least 62 My. clude the depleted mid-ocean ridge basalt–type asthenosphere, However, some lavas contain continental material, which has variably mixed with subcontinental lithospheric mantle material previously been proposed to have been recycled through the (5–8). The proximity of Iceland to the Jan Mayen Micro- plume. Here, we argue that the plume split off a sliver of continent (JMM) (Fig. 1) raises the question of whether Iceland continent from Greenland in the Early Eocene. This sliver is includes underlying fragments of continental crust (9, 10). There now located beneath southeast Iceland where it locally con- are unconfirmed reports of both Precambrian and Mesozoic taminates some of the plume-derived magmas. zircons in young basalts from Iceland, but only in abstract form (9, 11). The recovery of a 1.8-Ga-old grain from the most primitive Author contributions: T.H.T. and H.E.F.A. designed research; T.H.T., H.E.F.A., R.G.T., and A C.G. performed research; T.H.T., H.E.F.A., R.G.T., C.G., and N.J.K. contributed new Öræfajökull basalts (9) in southeast Iceland (Figs. 1 and 2 )is reagents/analytic tools; T.H.T., H.E.F.A., R.G.T., P.V.D., C.G., N.J.K., B.S., F.C., L.D.A., W.L.G., potentially very interesting, although two Jurassic zircon grains S.C.W., and B.J. analyzed data; and T.H.T., H.E.F.A., R.G.T., P.V.D., C.G., N.J.K., B.S., F.C., (160 Ma) uncovered in the same separation are now suspected to L.D.A., W.L.G., S.C.W., and B.J. wrote the paper. be due to laboratory contamination. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Evidence for Assimilation of Underlying Continental Crust in Freely available online through the PNAS open access option. Southeast Iceland 1To whom correspondence should be addressed. Email: [email protected]. In comparison with other Icelandic and northeast Atlantic rocks, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the entire Öræfajökull rock suite, ranging from basalts to rhyo- 1073/pnas.1423099112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1423099112 PNAS Early Edition | 1of10 Downloaded by guest on September 28, 2021 40 30°W 10°W 30 Jan Mayen Mohns Ridge 70°N 50 Blosseville 60 20 40 Kolbeinsey Haltenbanken Ridge 10 Greenland Crustal thickness (km) 30 JMM 0 Vøring COB 20 Ægir Ridge 10 JMM-E Norway 60°N Öræfajökull Faroes COB Reykjanes Ridge Scotland Fig. 1. Crustal thickness map based on gravity inversion and revised location of the Iceland plume (white star symbols in 10-My intervals) relative to Greenland back to 60 Ma (see Fig. 6A for details). Transition between continental and oceanic crust (COB, black lines), plate boundaries (blue lines), site locations for dated North Atlantic Igneous Province magmatism (yellow circles with black fill), and the estimated size of the “classic” Jan Mayen Micro- continent (JMM) and the extended JMM (JMM-E) are also shown. The classic JMM is ∼500 km long (200 km at its widest and shown with four continental basement ridges), and crustal thicknesses are about 18–20 km (scale at Upper Left), which suggests considerable continental stretching before drifting off Greenland. We extend JMM 350 km southwestward (∼70 km wide) beneath southeast Iceland where we calculate maximum crustal thicknesses of ∼32 km (Fig. 2B). The size of JMM-E is a conservative estimate and could be as large as the white-stippled area. Öræfajökull location is shown as a yellow star. Geochemical Modeling under southeast Iceland. The proposed Continental Crustal The Sr–Nd–Pb isotope composition of Öræfajökull volcano has Contamination Trend (CCCT) (Fig. 3) differs from the other been interpreted to fall on a trend from a depleted mantle trends involving mantle source mixing and progressive melting in composition toward an enriched-mantle component, interpreted Iceland, and along the northeast Atlantic and Arctic spreading as EM1- or EM2-like (8, 13–15). Fig. 3 shows that the Sr–Nd–Pb ridges (Fig. S1). isotopic mixing trajectories from depleted tholeiites of the Mid- Sr–Pb–Nd isotope arrays (Fig. 3), including basalts from Iceland Belt to the enriched Öræfajökull basalts largely point Iceland, Spitsbergen and the Reykjanes, Kolbeinsey (west of toward the EM2 component. One of the reasons that previous Spitsbergen), and Gakkel (High Arctic) Ridges, demonstrate studies (13) have concluded that this unique isotope composition that the CCCT is—unique and geographically limited—extend- could be related to an EM-like mantle component is that the ing from the Mid-Iceland Belt via the Eastern Rift Zone and the entire compositional range from basalts to rhyolites has uniform Snæfell central volcano to the Öræfajökull central volcano.
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