Kistufell: Primitive melt from the Iceland mantle plume

This paper presents new, geochemical data from Kistufell (64degrees 48'N, 17degrees13'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 1270degreesC), and eruption temperatures (1240degreesC) 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 (Sr-87/Sr-86 0.70304 0.70308, Nd-143/Nd-144 0.513058-0.513099, Pb-206/Pb-204 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 Pb-207/Pb-204 (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 invokes lower oceanic crustal gabbros. The delta O-18 values (4.2-4.7 parts per thousand), which are lower than those observed in mantle peridotites but similar to those observed in ophiolites and in situ oceanic gabbros, are consistent with this interpretation. The,elevated He-3/He-4 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.