The Li isotopic composition of Oldoinyo Lengai: Nature of the mantle sources and lack of isotopic fractionation during carbonatite petrogenesis

摘要

Lithium concentrations and Li isotope compositions are reported for natrocarbonatites and silicate lavas from Oldoinyo Lengai, Tanzania. Natrocarbonatites are characterized by very high Li contents (211-294 ppm) and a narrow range of 67 Li values between +3.3 and +5.1. These Li isotope compositions overlap with those reported for MORE, and OIB and suggest that the natrocarbonatites reflect the Li isotopic composition of their mantle source. Co-genetic silicate lavas, covering a wide compositional spectrum, show no obvious isotopic fractionation as a function of igneous diflerentiation or liquid immiscibility. Primitive olivine melilitites (Mg#=58-70), considered to be parental magmas, contain 14-23 ppm Li and have (5 Li values of +2.4 to +4.4. A highly differentiated, peralkaline nephelinite (Mg#=12), likely to be related to the natrocarbonatites by liquid immiscibility, has about twice as much Li as the melilitite (57 ppm), but a similar isotopic composition (delta Li-7=+3). In contrast, a plionolite with 15 ppm Li has a lighter Li isotope composition (delta Li-7=-0.4), which may reflect assimilation of isotopically light lower crustal mafic granulites, a conclusion supported by radiogenic isotope data. Clinopyroxene and olivine separates from the silicate lavas have uniformly lower Li concentrations (3-15 ppm) and lower delta Li-7 values (delta Li-7=-2.9 to -0.5) than the respective whole-rocks, with between 1.4 and 6.3. This difference between whole-rock and mineral data is interpreted to reflect diffusion-driven isotopic fractionation. (c) 2006 Elsevier B.V All rights reserved.