Incompatible element

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

Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics

Abstract: The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in 87Sr /86Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma.

Characterization and origin of a compositionally zoned aluminous A-type granite from South India

Abstract: The Pan-African Ambalavayal granite intrudes the high-grade metamorphic terrain of northern Kerala, South India and is spatially associated with the Moyar and Calicut lineaments. The pluton was aligned nearly parallel to the northeast–southwest and east–west faults in the basement, consistent with magma ascent along pre-existing deep-crustal lineaments in an extensional tectonic regime. The pluton is characterized by the presence of iron-rich hydrous mafic minerals, primary magnetite, f O2 above the Ni–NiO buffer and high initial emplacement temperatures near 1000 °C. Modal and textural analyses reveal two probable compositional zones within the pluton: outer and inner. Major element variations support this zoning and point to a peralkaline to metaluminous outer zone and a metaluminous to slightly peraluminous inner zone. Both zones exhibit major and trace element characteristics of the A-type granites with the outer zone belonging to the A1 subtype and the inner zone to the A2 subtype of Eby. The trace element trends observed from outer zone to the inner zone suggests that crystal fractionation may have been the dominant process in the generation of high levels of the incompatible elements in the case of inner zone samples. The high initial 87Sr/86Sr ratio (0.7135) and high Y/Nb ratios (Y/Nb > 1.2) are in the range expected for rocks derived from crustal protoliths. A petrogenetic model involving partial melting of a charnockitic, mafic to intermediate lower crust followed by limited fractional crystallization of the magma in a high-level magma chamber is proposed. The enrichment of HFSE and REE (except Eu) in the inner zone is considered the ultimate product of crystal–melt and volatile activity during the final stage of crystallization in a highly silicic (SiO2 > 74 %) magma chamber.