Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Amphibolites of the Ashe and Alligator Back Formations, North Carolina: Samples of Late Proterozoic-early Paleozoic oceanic crust

Abstract: The amphibolites of the Late Proterozoic-early Paleozoic Ashe and Alligator Back Formations in western North Carolina represent metamorphosed tholeiitic basalts, unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the Grenville-age basement rocks. On the basis of their geochemistry, the amphibolites are divisible into three compositional groups (I, II, and III), the protoliths of which appear to have evolved from different parental magmas at a spreading center. Group II (intermediate-Ti) amphibolite, the predominant variety in the study area, constitutes a highly fractionated suite (Zr ≃ 40-200 ppm, TiO2 ≃ 0.7-2.3 wt.%) with geochemical characteristics similar to those of N-type MORB formed from a heterogeneous mantle source. Group III (high-Ti) amphibolite is enriched in high-field-strength incompatible elements (Zr, Nb, Ti, Y, REE's), especially Ti (TiO2 = 2.8-3.4 wt.%), and is interpreted as T-type MORB on the basis of its Zr/Nb and Y/Nb ratios. Group I (low-Ti) amphibolite, restricted to the Ashe Formation, is characterized by extremely low abundances of incompatible elements, including Ti (TiO2 < 0.45 wt.%), and U-shaped REE profiles, requiring the mixing of a depleted MORB-type mantle source with a fluid phase enriched in LREE's. The juxtaposition of depleted, low-Ti basalt (group I amphibolite) and MORB-like basalts (group II and group III amphibolites) is suggestive of a back-arc basin setting, as has been postulated for many ophiolites, but it can also be accounted for by multi-stage melting of an upper-mantle source adjacent to a mantle plume in a mid-oceanic-ridge environment. In either case, the Ashe and Alligator Back amphibolites represent metamorphosed oceanic crust material generated at a spreading center.

The Prince of Wales Formation - post-flood basalt alkali volcanism in the Tertiary of East Greenland

Abstract: In the nunataks of the Prince of Wales Mountains the tholeiite flood basalts of the East Greenland Tertiary Province are unconformably overlain by alkaline lavas. The majority of the alkaline lavas are strongly porphyritic picrites, ankaramites and hawaiites. These rocks have lower 143Nd/144Nd and higher 87Sr/86Sr than the tholeiitic flood basalts and are isotopically akin to ocean island basalts. The alkaline lavas also have high concentrations of incompatible elements which on normalised plots have a pattern which is similar in shape to that of enriched oceanic island basalts. The isotopic and chemical characteristics of these late-stage representatives of the East Greenland volcanic activity are attributed to their derivation from the peripheral regions of the East Greenland plume, the axial region of which was moving progressively eastwards relative to the westwards drift of the Greenland plate. It is proposed that the incompatible element contents of the magmas so produced were dominated by small degree melts formed beneath a cap of continental lithosphere in the marginal regions of the plume.

Mildly alkaline basalts from Pavagadh Hill, India; Deccan flood basalts with an asthenospheric origin

Abstract: Of twelve flows at Pavagadh Hill, the two three-phenocryst-basalt flows with Mg#∼0.70 and Ni/MgO∼33 are the most primitive and perhaps as primitive as any basalts in the Deccan province. Scatter on variation diagrams and the occurrence of primitive flows at two different levels in the volcanic sequence implies that most rocks are probably not, strictly speaking, comagmatic. Nevertheless, mass balance calculations indicate a generalized differentiation scheme from primitive basalt to hawaiite that involved removal of olivine, augite, plagioclase and Fe-Ti oxides in the proportions 40:33:22:5 with ∼ 50% of the magma remaining. Crustal assimilation had a minimal effect on evolution of the basalts but rhyolites at the top of the volcanic sequence may have been produced by crustal melting following prolonged heat release from alkali basalt pooled along fault zones in the continental crust. Major element based calculations indicate that the most primitive basalts were generated by 7 to 10% melting of mantle peridotite. These low percentages of melting, typical of alkali basalts, are consistent with the steep slopes on chondrite-normalized REE diagrams. Low heavy REE concentrations point to residual garnet in the source region. Incompatible element concentrations (e.g. Rb, Ba, Zr, La) in Pavagadh basalts exceed those in Deccan tholeiitic basalts but are substantially lower than those reported for some other Deccan alkali basalts. Obviously Pavagadh basalts do not reflect the lowest percentages of melting and greatest amount of source region metasomatic enrichment attained in the Deccan province. Deccan tholeiitic and alkali basalts are largely characterized by low La/Nb ratios and high La/Ba ratios similar to those in oceanic island basalts. This indicates minimal involvement of the subcontinental lithospheric mantle in their petrogenesis. Comparison with continental mafic magma provinces where a subcontinental lithospheric mantle imprint is common indicates long periods of extension and/or melting of mantle lithosphere still hot from pre-extension subduction are more likely to produce magmas bearing the lithospheric imprint.