Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v22a1003m&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V22A-1003
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1065 Trace Elements (3670), 3640 Igneous Petrology, 3665 Mineral Occurrences And Deposits, 3670 Minor And Trace Element Composition
Scientific paper
Paleogene basalts ( ~55Ma) derived from the ancestral Iceland mantle plume and extruded during continental rifting are exposed along the Blosseville Kyst in central East Greenland. These basalts comprise three intercalated series, viz: a low-Ti, high-Ti and a very high-Ti series. The two Ti-rich series are interpreted to represent continental flood basalts formed by low degrees of partial melting (degree of melting F=3-9%) while the low-Ti series are believed to have formed by higher degrees of partial melting (F:15-25%). All three of the East Greenland basalt series are enriched in the PGE, relative to normal MORB. During differentiation of the low-Ti series, Pd increase from 11 to 24 ppb whereas Pt and Ir decrease from 12 and 0.6 ppb to 3 and <0.05 ppb respectively. The primitive basalts (molar Mg#60) of the dominant high-Ti series contain ~6-10 ppb Pd, ~7-10 ppb Pt and ~0.2 ppb Ir whereas the most evolved basalts (Mg#43) contain 25 ppb Pd, 5 ppb Pt and <0.05 ppb Ir. The PGE-rich nature of these basalts is surprising because low degree partial melts are generally S-saturated and hence strongly depleted in the PGE (cf, Keays, 1995). However, our data indicates that all of the East Greenland magmas were S-undersaturated and as they underwent differentiation, Pd behaved incompatibly while Ir and Pt behaved compatibly. Primitive Holocene Icelandic olivine tholeiites contain 120 ppm Cu, 6 ppb Pd, 4 ppb Pt and 0.2 ppb Ir while their picritic counterparts contain 74 ppm Cu, 17 ppb Pd, 7 ppb Pt and 0.3 ppb Ir. Both the olivine tholeiites and the picrites are believed to have formed by high degrees of partial melting (15-25%) which would have exhausted all of the sulphides in the mantle source region and produced S-undersaturated magmas. In Icelandic samples with 10-14wt% MgO, Cu and the PGEs vary systematically between the primitive picrite and olivine tholeiite compositions given above i.e there is an inverse correlation between Cu and the PGEs. This is best explained by mixing between parental olivine tholeiite and picrite magmas. The low Cu/Pd ratio in the most primitive picrite probably reflect derivation from a depleted mantle where Cu was less efficiently retained in sulphides compared to Pd during previous melt extraction episodes. Whithin the analysed suite of olivine tholeiites, Ir decreases from 0.15 to 0.06 ppb, Pd increases from ~6 to ~15 ppb and Pt/Pd ratio decreases from 0.8-0.2 during differentiation (7-4wt% MgO); these variations provide further evidence that the olivine tholeiite magmas remained S-undersaturated throughout their differentiation. To summarize, (1) Continental flood basalts and low-Ti tholeiites in the Paleogene East Greenland flood basalt sequence, as well as Holocene Icelandic olivine tholeiites are PGE-rich relative to normal MORB. (2) Their PGE-contents vary as a function of S-undersaturated differentiation. (3) Cu-PGE variations in Icelandic samples with 10-14 wt% MgO suggest that they represent mixtures between distinct tholeiitic (Cu/Pd: 20000) and depleted picritic (Cu/Pd: 4400) parental liquids. Reference: Keays RR (1995) The role of komatiitic magmatism and S-saturation in the formation of ore deposits. Lithos 34:1-18.
Brooks Kate
Grönvold Karl
Keays Reid R.
Momme P.
Oskarsson Niels
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