Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v34b..07c&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V34B-07
Other
1025 Composition Of The Mantle, 1030 Geochemical Cycles (0330), 1038 Mantle Processes (3621), 1041 Stable Isotope Geochemistry (0454, 4870), 3037 Oceanic Hotspots And Intraplate Volcanism
Scientific paper
We have determined lithium isotopic compositions of seventy nearly unaltered basaltic lavas from the Samoan and Cook-Austral volcanic chains, and Pitcairn Island. In addition, the Li isotope record of Mauna Kea has been extended from 3.1 km to 3.3 km using the newly recovered deep drill core of Hawaii Scientific Drilling Project (HSDP-2). From the results of the Hawaii and South Pacific hotspots, we examine the source components of mantle plumes and consider the distribution of Li isotopes among Earth's major reservoirs. The total δ^{7}Li range observed in South Pacific and Hawaiian islands is 2.5 to 7.5‰, suggesting considerable heterogeneity in the deep mantle. The Hawaiian plume occupies the lower range, 2.5 to 5.7‰. Cook-Austral samples depict mixing between HIMU and depleted mantle (DM) components. The DM endmember has δ^{7}Li values of 3.2 to 4.2‰, identical to MORB. HIMU type lavas are isotopically heavier than MORB, reflecting the influence of recycled oceanic crust. The most extreme HIMU signature was observed at Mangaia (7.4‰; Nishio et al., 2005). The EM1 composition shown by Pitcairn is relatively light (3.3 to 4.1‰) and resembles the global average of subducting sediments (~3‰). Malumalu seamount of the Samoan chain defines the most extreme composition of the EM2 mantle; δ^{7}Li range from 4.5 to 5.6‰. High 3He/4He samples of Ofu Island (R/Ra = 24 to 33.7) suggest that the least degassed mantle has δ^{7}Li of about 3‰. In addition to these classic mantle endmembers, we observe an additional component that elevates Samoan lavas above the typical mantle baseline (~3‰) up to 7.5‰. The source of this enrichment is likely mantle wedge material that has been metasomatized by 7Li-rich slab fluids (Elliott et al., 2004). On the other hand, recent studies suggest that a large portion of subducted Li can be retained in high-pressure metamorphosed slab (Marschall et al., in press). This implies that deeply subducted slab need not be isotopically light but can be the origin of elevated δ^{7}Li of HIMU lavas. The global ocean island basalts yield an average δ^{7}Li value of 4.7 ± 0.9‰ (1σ). Thus the deep mantle is isotopically heavier than the upper mantle (3 to 4‰), volcanic arcs (3.5 ± 1.5‰), and the upper continental crust (0 ± 2‰). The implication is that heavy Li that enters the subduction zone is not significantly recycled to the crust but largely returned to the deep mantle via the slab-modified mantle wedge and possibly the residual slab. Unusually light Li does not appear to to be common in the deep mantle although it has been implicated by orogenic eclogites and subcontinental mantle xenoliths. References: Elliott T., A. Jeffcoate, and C. Bouman (2004) Earth Planet. Sci. Lett. 220, 231-245. Marschall H.R., R. Altherr, T. Ludwig et al. (in press) Geochim. Cosmochim. Acta. Nishio N., S. Nakai, T. Kogiso, and H.G. Barsczus (2005)Geochem. J. 39, 91-103.
Blusztajn Jerzy S.
Chan Leunglung
Frey Frederick A.
Hart Stan R.
Hauri Erik H.
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