New Sr–Nd–Pb–Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf–Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (∼95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ∼55 Ma ago. Thus, Hf–Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume [Thirlwall, J. Geol. Soc. London 152 (1995) 991–996; Hards et al., J. Geol. Soc. London 152 (1995) 1003–1009; Fitton et al., Earth Planet. Sci. Lett. 153 (1997) 197–208]. They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of ‘enriched’ streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive ΔNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower eNd and eHf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its eNd and eHf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative ΔNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower eHf for a given eNd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be ‘polluted’ not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting [Klein and Langmuir, J. Geophys. Res. 92 (1987) 8089–8115] along the MAR in the North Atlantic.

The Iceland plume in space and time: a Sr-Nd-Pb-Hf study of the North Atlantic rifted margin

The basaltic basement of the large igneous province formed by the Kerguelen Plateau and Broken Ridge in the southeastern Indian Ocean has been sampled by the three Ocean Drilling Program cruises (Legs 119, 120 and 183). Although the Cretaceous parts of this plateau formed in the embryonic Indian Ocean basin, presumably by melting associated with the Kerguelen plume, trace element abundances and isotopic ratios of Sr, Nd and Pb of Cretaceous basalt from several drill sites indicate that continental lithosphere was involved in their petrogenesis. On the basis of relative depletions in Nb, Ta and Th, and isotopic characteristics similar to those of EMI ocean island basalt, lavas from Leg 120 Site 747 in the Central Kerguelen Plateau contain a component derived from the lower continental crust. On the basis of relative abundances of Sr and Eu and EMI-like Pb isotopic ratios, the source of basalt from Leg 120 Site 750 in the northeastern part of the Southern Kerguelen Plateau also contained a component derived from lower continental crust; in this case, the crustal component formed as a plagioclase-rich, clinopyroxene-bearing cumulate. Basalts from Leg 120 Site 749 define two distinct isotopic (Sr, Nd and Pb) groups which differ from the isotopic fields for Site 747 and 750 basalts. Among Site 749 lavas, there is subtle evidence for a continental component, broadly similar (i.e. moderate 200Pb/201Pb ~ 18.0) to that expressed more obviously in basalt from Leg 119, Site 738 on the southern edge of the Southern Kerguelen Plateau and Leg 183 Site 1137 on Elan Bank. The continental components in the Kerguelen Plateau basalts may have resided in a heterogeneous mantle plume that was formed, in part, by deep recycling of crust. It is more likely, however, that slivers of Gondwana lithosphere reside within the lithosphere and athemosphere of the Indian Ocean mantle where they contaminate both plume-derived and mid-ocean ridge basaltic magmas.

Involvement of continental crust in the formation of the Cretaceous Kerguelen Plateau: New perspectives from ODP Leg 120 sites

Petrogenesis of the earliest Early Cretaceous mafic rocks from the Cona area of the eastern Tethyan Himalaya in south Tibet: Interaction between the incubating Kerguelen plume and the eastern Greater India lithosphere?

http://www.gm.univ-montp2.fr/IMG/pdf/2004_Lapierre_alLithos.pdf

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin

thick lithospheric cap, with He contributed from volatile-rich fluids We present elemental and isotopic (Sr–Nd–Pb–Hf–Os–He) data from elsewhere in the Icelandic plume. The exact nature of the recycled on primitive alkalic lavas from the Prinsen af Wales Bjerge, component is not yet resolved, although Hf isotope compositions rule East Greenland. Stratigraphical, compositional and Ar–Ar data out any significant role for recycled pelagic sediment, and the low indicate that this inland alkalic activity was contemporaneous with Os/Os limits the participation of recycled basaltic material the upper parts of the main tholeiitic plateau basalts and also postand argues instead for a contribution from the mantle section of the dated them. The alkalic rocks show a marked crustal influence, recycled slab. indicating establishment of new magmatic plumbing systems distinct from the long-lived coastal systems that fed the relatively uncontaminated plateau basalts. The least contaminated lavas have high He/He isotope ratios (R/RA 12·4–18·5), sub-chondritic Os/Osi (0·120–0·126), low Ndi (>+4) and Hfi

/pdf/the-prinsen-af-wales-bjerge-formation-lavas-east-greenland-4brmf3af25.pdf

The Prinsen af Wales Bjerge Formation lavas, East Greenland: the transition from tholeiitic to alkalic magmatism during Palaeogene continental break-up

The southeast Greenland seaward-dipping reflector sequence (SDRS) is composed of Paleocene to Eocene volcanic rocks erupted during continental breakup. Volcanic rocks recovered from a transect across the SDRS at 63°N, during Ocean Drilling Program Leg 152, range in composition from picrite to dacite and represent all the magmatic phases in the development of the continental margin. The earliest magmas, represented by the pre-breakup succession at Site 917 (Lower and Middle Series), were strongly contaminated with continental crust, but the degree of contamination declined rapidly during the late stages of breakup (Site 917 Upper Series). Very low concentrations of incompatible elements in the uncontaminated primitive magmas made them extremely sensitive to the isotopic effects of crustal contamination. Basaltic rocks from the most seaward part of th e transect (Site 918) were erupted after breakup and show no signs of contamination with continental crust. Two distinct crustal contaminants can be recognized: (1) lower crustal basic granulite with unradiogenic Nd, Sr, and Pb; and (2) upper crustal amphibolite-facies gneiss with unradiogenic Nd but highly radiogenic Sr and high 208 Pb/ 204 Pb. The first contaminant affected only the earliest magmas, represented by the lower volcanic units in the Lower Series at Site 917. Later continental magmas were affected by the second contaminant, suggesting storage of magmas at progressively shallower levels in the crust as lithospheric extension proceeded toward continental breakup. The nature and degree of contamination are strikingly similar to those observed in the Hebridean Tertiary igneous province, which would have been adjacent to southeast Greenland during continental breakup.

http://www-odp.tamu.edu/publications/152_SR/VOLUME/CHAP_29.PDF

29. Sr-, Nd-, AND Pb-ISOTOPIC COMPOSITION OF VOLCANIC ROCKS FROM THE SOUTHEAST GREENLAND MARGIN AT 63°N: TEMPORAL VARIATION IN CRUSTAL CONTAMINATION DURING CONTINENTAL BREAKUP 1

B. S. Hardarson

Papers

29. Sr-, Nd-, AND Pb-ISOTOPIC COMPOSITION OF VOLCANIC ROCKS FROM THE SOUTHEAST GREENLAND MARGIN AT 63°N: TEMPORAL VARIATION IN CRUSTAL CONTAMINATION DURING CONTINENTAL BREAKUP 1