Kistufell: Primitive Melt from the Iceland Mantle Plume

• Published in: Journal of Petrology
• Main Author: BREDDAM, KRESTEN
• Format: Text
• Language: English
• Published: Oxford University Press 2002
• Subjects: ARTICLES; Kistufell; Table Mountain; Iceland; North Atlantic
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/43/2/345; https://doi.org/10.1093/petrology/43.2.345

This paper presents new geochemical data from Kistufell (64°48′N, 17°13′W), a monogenetic table mountain situated directly above the inferred locus of the Iceland mantle plume. Kistufell is composed of the most primitive olivine tholeiitic glasses found in central Iceland (MgO 10·56 wt %, olivine Fo 89·7 ). The glasses are interpreted as near-primary, high-degree plume melts derived from a heterogeneous mantle source. Mineral, glass and bulk-rock (glass + minerals) chemistry indicates a low average melting pressure (15 kbar), high initial crystallization pressures and temperatures (10–15 kbar and 1270°C), and eruption temperatures (1240°C) that are among the highest observed in Iceland. The glasses have trace element signatures (La n /Yb n <1, Ba n /Zr n 0·55–0·58) indicative of a trace element depleted source, and the Sr–Nd–Pb isotopic ratios ( 87 Sr/ 86 Sr 0·70304–0·70308, 143 Nd/ 144 Nd 0·513058–0·513099, 206 Pb/ 204 Pb 18·343–18·361) further suggest a long-term trace element depletion relative to primordial mantle. High He isotopic ratios (15·3–16·8 R/R a ) combined with low 207 Pb/ 204 Pb (15·42–15·43) suggest that the mantle source of the magma is different from that of North Atlantic mid-ocean ridge basalt. Negative Pb anomalies, and positive Nb and Ta anomalies indicate that the source includes a recycled, subducted oceanic crustal or mantle component. Positive Sr anomalies (Sr n /Nd n = 1·39–1·50) further suggest that this recycled source component involves lower oceanic crustal gabbros. The δ 18 O values (4·2–4·7‰), which are lower than those observed in mantle peridotites but similar to those observed in ophiolites and <rm>in situ</rm> oceanic gabbros, are consistent with this interpretation. The elevated 3 He/ 4 He ratios are primarily attributed to a primitive, relatively undegassed component in the Iceland mantle plume, which dominates the He isotope signature as a result of long-term depletion of U, Th and He in the recycled gabbroic component.