Browsing by Subject "RECYCLED CRUST"

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  • Heinonen, Jussi S.; Luttinen, Arto V.; Spera, Frank J.; Bohrson, Wendy A. (2019)
    Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from similar to 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., epsilon(Nd) from -16 to +2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (epsilon(Nd) up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (delta O-18 = 4.4-5.2%; Fo = 78-92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher delta O-18 (6.2-7.5%; Fo = 70-88 mol%); and one of the dike samples gives intermediate compositions (5.2-6.1%, Fo = 83-87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, delta O-18 correlates positively with enrichments in large-ion lithophile elements (especially K) and Os-187. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.
  • Heinonen, Jussi S.; Luttinen, Arto V.; Whitehouse, M.J. (2018)
    Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from similar to 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., epsilon(Nd) from -16 to +2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (epsilon(Nd) up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (delta O-18 = 4.4-5.2%; Fo = 78-92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher delta O-18 (6.2-7.5%; Fo = 70-88 mol%); and one of the dike samples gives intermediate compositions (5.2-6.1%, Fo = 83-87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, delta O-18 correlates positively with enrichments in large-ion lithophile elements (especially K) and Os-187. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.
  • Nikkola, Paavo; Gudfinnsson, Gudmundur H.; Bali, Eniko; Ramo, O. Tapani; Fusswinkel, Tobias; Thordarson, Thorvaldur (2019)
    We present new high-precision major and trace element data on olivine macrocrysts from various volcano-tectonic settings in Iceland and use these data as a proxy for mantle mode and melting conditions. Within individual sampling sites examined (seven lavas and one tephra) olivine-dominated fractional crystallization, magma mixing and diffusive re-equilibration control observed variability in olivine composition. High-pressure fractional crystallization of clinopyroxene may have lowered Ca and increased Fe/Mn in one olivine population and subsolidus diffusion of Ni and Fe-Mg affected the mantle-derived Ni/Fo ratio in some compositionally zoned olivine macrocrysts. Interestingly, magmas erupted at the southern tip of the Eastern Volcanic Zone (SEVZ), South Iceland, have olivines with elevated Ni and low Mn and Ca contents compared to olivines from elsewhere in Iceland, and some of the SEVZ olivines have relatively low Sc and V and high Cr, Ti, Zn, Cu and Li in comparison to depleted Iceland rift tholeiite. In these olivines, the high Ni and low Mn indicate relatively deep melting (P-final>1.4GPa,similar to 45km), Sc, Ti and V are compatible with low-degree melts of lherzolite mantle, and elevated Zn may suggest modal (low-olivine) or geochemical (high Zn) enrichment in the source. The SEVZ olivine macrocrysts probably crystallized from magmas derived from olivine-bearing but relatively deep, enriched and fertile parts of the sub-Icelandic mantle, and indicate swift ascent of magma through the SEVZ lithosphere.