The Lithium Isotopic Signature of Hawaiian Basalts

Recycling of oceanic crust and sediment is a common mechanism to account for the presence of chemical heterogeneities observed in oceanic island basalts (OIBs). On Hawai';i, a mantle plume-sourced OIB with a high mass flux, sampling of deep mantle heterogeneities accounts for the presence of two unique geochemical and geographical trends called the Loa and Kea trends. The Loa trend overlaps the Pacific large low shear velocity province and is distinctly more enriched [1] than the Kea trend with average Pacific mantle compositions [2]. Because of the sizeable fractionation of lithium isotopes in low temperature environments, lithium serves as a tracer for the presence of recycled material in OIB sources, including Hawai'i. In this study, we analyzed 87 samples of Hawaiian basalt from the pre-shield, shield, post-shield, and rejuvenated volcanic stages and 10 samples of altered oceanic crust from ODP Site 843 for lithium isotopes using a multi-collector inductively coupled plasma mass spectrometer. Correlations of lithium isotopes with the radiogenic isotopes Pb, Hf, Nd, and Sr indicate lithium isotopes may be used to trace components in mantle plumes such as Hawai';i. The measured range of lithium isotopes for shield stage lavas is δ⁷Li = 1.8 - 5.7‰ and for post-shield lavas is δ⁷Li = 0.8 - 4.7‰. Pre-shield stage lavas (Lo'ihi volcano only) and rejuvenated lavas are the least and most homogeneous volcanic stages, respectively, in lithium isotopes. The Loa and Kea geochemical trends have different lithium isotopic signatures, with Loa trend shield volcanoes exhibiting lighter lithium isotopic signatures (δ⁷Li = 3.5‰ [N=43]) than Kea trend shield volcanoes (δ⁷Li = 4.0‰ [N=31]) [3]. Similarly, post-shield lavas have systematically lighter δ⁷Li than shield lavas. The presence of systematic differences in lithium isotopic signatures may indicate: 1) the sampling of distinct components in the deep source, to account for variations between Kea and Loa trend shield stage volcanoes or amongst individual volcanoes; 2) differences in degree of mantle melt and spatial structure of the mantle heterogeneity melting in the case of pre-shield, shield, and post-shield differences. In Hawaiian basalts, lithium isotopes help distinguish between 'enriched' Loa source components: Ko';olau Makapu';u shield stage lavas may have between 4-10% of a carbonate input and Hualalai post-shield and shield lavas may reflect incorporation of subduction eroded lower continental crust. Comparison of this dataset with worldwide OIB published lithium isotopic data indicates that the lithium isotopic system behaves systematically on a mantle-wide scale. Hawai'i is generally characterized as EM-I like, with Hualalai post-shield lavas exhibiting both the lightest lithium isotopic signature and the most extreme EM-I characteristics. Lithium isotopes thus provide an additional insight into the nature of EM-I type mantle. [1] Weis, D. et al. (2011) Nature Geoscience 4, doi:10.1038/NGEO1328. [2] Nobre Silva, I.G. et al. (2013) Geochem. Geophy. Geosys. 14(3), doi: doi:10.1002/ggge.20047. [3] Chan, L.H., and Frey, F.A. (2003) Geochem. Geophy. Geosys. 4(3), doi: 10.1029/2002GC000365.