Lithium Isotope Systematics in Azores Basalts
Abstract
Basalts from the Azores archipelago and MORB from the nearby Azores Platform exhibit extreme chemical and isotopic variations attributed to the influence of a heterogeneous mantle plume, with compositions ranging from depleted mantle (DMM) to strong HIMU, EMI and EMII signatures. In order to assess the utility of Li isotopes as a mantle source tracer and to better constrain the origin of heterogeneous mantle beneath the Azores, we have analyzed Li isotopes in a suite of young, fresh, MgO-rich basalts from São Miguel and three Central Group islands including Pico, Faial and Terceira. Despite large variations in radiogenic isotope signatures (e.g. 206Pb/204Pb = 19.3 to 20.1), δ7Li varies only slightly (3.1-4.7‰), and is within the range for global and North Atlantic MORB [1, 2]. More extreme δ7Li values such as those reported previously for some EMII, EMI and HIMU ocean island basalts (-17‰ to +10‰; [3-5]) were not observed. Nevertheless, basalts from the Central Group islands with EMI-type signatures are, on average, slightly heavier in δ7Li than the São Miguel samples, and they exhibit positive correlations with 87Sr/86Sr and negative correlations with 206Pb/204Pb, Nd, and Hf isotopes. Li isotopes do not correlate with indices of fractionation such as MgO, suggesting that the δ7Li correlations with radiogenic isotopes may represent subtle variations in mantle source signatures. Positive and negative correlations of δ7Li with 87Sr/86Sr and 206Pb/204Pb, respectively, and relatively unradiogenic Os (187Os/188Os = 0.1244-0.1269), may reflect old, slab-fluid metasomatized mantle beneath the Central Group islands. In contrast, δ7Li signatures in the São Miguel basalts do not correlate with radiogenic isotopes. Rather, δ7Li is essentially constant despite extremely high 87Sr/86Sr and 206Pb/204Pb and low ΔɛHf signatures that have been attributed to 3.5 Ga recycled E-MORB or evolved oceanic crust [6; 7]. This suggests either that the São Miguel source does not contain recycled oceanic crust or that recycled oceanic crust may not always result in anomalous Li isotope signatures in the mantle, as previously predicted [8-10]. [1] Tomascak et al. (2008), GCA 72, 1626-1637; [2] Simons et al. (2008), AGU abs; [3] Nishio et al. (2004), EPSL 217, 245-261; [4] Nishio et al. (2005), Geochem.J. 39, 91-103; [5] Ryan and Kyle (2004), Chem Geol. 212, 125-142; [6] Elliot et al. (2007), GCA 71, 219-240; [7] Beier et al. (2007), EPSL 259, 186-199, [8] Zack et al. (2003), EPSL 208, 279-290; [9] Marschall et al. (2007), EPSL 262, 563-580; [10] Chan et al. (2009), EPSL 277, 433-442.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.V23A..02Y
- Keywords:
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- 1025 Composition of the mantle;
- 1041 Stable isotope geochemistry (0454;
- 4870)