HR: 0800h
AN: V41D-1491    [Abstracts]
TI: Lithium concentration and Li isotopic compositions of carbonatitic complexes
AU: * Halama, R
EM: rhalama@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: McDonough, W F
EM: mcdonoug@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Rudnick, R L
EM: rudnick@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Ash, R D
EM: rdash@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Keller, J
EM: Joerg.Keller@minpet.uni-freiburg.de
AF: Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universitaet Freiburg Albertstr. 23b, Freiburg, 79104 Germany
AU: Klaudius, J
EM: Jurgis.Klaudius@minpet.uni-freiburg.de
AF: Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universitaet Freiburg Albertstr. 23b, Freiburg, 79104 Germany
AU: Trumbull, R
EM: bobby@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AB: To evaluate the Li isotopic signatures of the mantle sources of carbonatites and the influence of magmatic differentiation and post-magmatic processes on δ⁷Li, we determined the Li concentrations and isotopic compositions of carbonatites and spatially associated silicate rocks, spanning a wide range in composition and age. Natrocarbonatites from Oldoinyo Lengai (1995 and 2000 eruptions) have high Li concentrations (211-292 ppm) and uniform Li isotopic signatures (δ⁷Li = +4.4 to +5.1 per mil). Associated silicate rocks (melilitite, nephelinite and phonolite) have lower Li concentrations (16-47 ppm) and trend towards lighter Li isotopic values (δ⁷Li = 0 to +3.5 per mil). Clinopyroxenes from these lavas are significantly lighter than the whole rocks by 1 to 6 per mil. Since the lavas appear to be fresh, this suggests fractionation of Li isotopes between minerals and whole rocks. In comparison to the modern natrocarbonatites, Proterozoic calciocarbonatites from Greenland (Grønnedal-Ika) and Cretaceous calciocarbonatites from Namibia (Kalkfeld) are poor in Li (< 2 ppm) and have more scattered Li isotopic compositions (δ⁷Li = -1 to +4 and -0.5 to +5 per mil, respectively). The lower δ⁷Li values may reflect contamination by crustal Li, since the low Li contents in the carbonatites make them susceptible to this. Silicate lavas from Kalkfeld have higher Li concentrations (11-12 ppm) than their associated carbonatites, but overlapping isotopic compositions (δ⁷Li = +4 to +6 per mil). At Grønnedal-Ika, clinopyroxene separates from nepheline syenites vary considerably in δ⁷Li from -6 to +5. Since Li is preferentially partitioned into fenitizing fluids [1] and an enrichment of light ⁶Li in fluids during degassing can be anticipated [2], the trend towards negative δ⁷Li can be interpreted as a result of variable interaction with metasomatizing fluids. However, fractionation of Li isotopes between minerals and melts may also have played a role. Our preliminary data suggest that the Li isotopic composition of the mantle source of modern to Proterozoic carbonatites is between δ⁷Li = +3 and +6, which is indistinguishable from that of fresh MORB (δ⁷Li = +3 to +5) and OIB (δ⁷Li = +3 to +7) [e.g. 3, 4]. This is consistent with earlier studies that demonstrated that mantle-derived magmas show only a limited range of Li isotopic compositions, despite highly variable compositions found in the mantle itself [5], and probably reflects homogenization of heterogeneous mantle regions by the melting process [6]. There is no indication of Li isotopic values significantly heavier than those of the oceanic mantle, which might be expected for subcontinental mantle lithosphere that has been enriched in ⁷Li by subduction zone processes. References: [1] Cooper, A.F., et al. (1995), Min. Mag. 59: 401-408. [2] Beck, P. et al. (2005), GCA 68: 2925-2933. [3] Chan, L.H., et al. (1992), EPSL 108: 151-160. [4] Ryan, J.G. and Kyle, P.R. (2004), Chem. Geol. 212: 125-142. [5] Nishio, Y. et al. (2004), EPSL 217: 245-261. [6] Kobayashi, K. et al. (2004), Chem. Geol. 212: 143-161.
DE: 1000 GEOCHEMISTRY
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005