LITHIUM, BERYLLIUM AND BORON CONCENTRATIONS OF 3.6-3.8 GA METASEDIMENTARY AND METAVOLCANOGENIC ROCKS IN THE ISUA SUPRACRUSTAL BELT, WEST GREENLAND

Li, Be and B are among the least abundant elements lighter than Fe in the solar system, 1.45 μg/g Li, 0.0219 μg/g Be, and 0.775 μg/g B in CI chondrite, respectively, but after 4.5 Myr of Earth history, their concentrations in Earth’s upper continental crust have reached 24 μg/g Li, 2.1 μg/g Be, and 17 μg/g B i.e., these are quintessential crustal elements. The oldest reported occurrence of B minerals in the geologic record is the 3.6-3.8 Ga Isua supracrustal belt in which tourmaline is widespread, whereas the oldest reported occurrences for Li and Be minerals are pegmatites associated with 3.0-3.1 Ga granitic intrusives in southern Africa, e.g., beryl, fluor-elbaite and lepidolite with the Sinceni pluton, Swaziland, which is interpreted to be derived from partial melting of metasediments in mature continental crust at ca. 3000 Ma. In order to evaluate whether Isua belt metamorphic rocks can also be considered mature crust, we have analyzed 24 samples of tourmaline bearing, quartz±plagioclase-muscovite-biotite-garnet mica schist and felsic gneiss, several of which contain epidote or staurolite±kyanite; a few samples lack muscovite and/or contain hornblende. Results gave the following average concentrations and ranges 27±19 (6-83) μg/g Li, 1.4±1.0 (0.5-4.2) μg/g Be, and 114±111 (<8-543) μg/g B. Compared to the Post Archean Australian Shale (PAAS) composite, B concentration is the same, whereas Li is significantly lower (35%), as are elements that tend to be enriched in pegmatites (25-60% of PAAS), e.g. 96±44 μg/g Rb, 4.8±2.6 μg/g Nb, 3.7±3.4 μg/g Cs, 4.2±2.8 μg/g Th and 0.87±0.46 μg/g U. Other elements in concentrations only 43-60% of PAAS are 80±75 μg/g V, 85±71 μg/g Sr, 14±6 μg/g Y, 127±52 μg/g Zr and 361±220 μg/g Ba. In contrast, average Cr and Ni concentrations exceed 150% of PAAS. Comparison with the North American shale composite gives similar results except depletions are less for Cs and Sr. The localized basin characteristics inferred to have led to B enrichment apparently did not result in comparable enrichments of many other elements, including those characteristic of pegmatites, i.e., the Isua belt metamorphic rocks were not fully mature. That formation of Li-Cs-Ta pegmatites depends on chemical maturation of source sediments could explain why such pegmatites have not yet been reported in complexes older the 3.1 Ga.