HR: 16:30h
AN: V54B-03    [Abstracts]
TI: Silicon Isotopic Composition of Isua BIF and Other Early Archean Supracrustal Rocks: a Tracer for Early Life?
AU: * Andr\'{e}, L
EM: luc.andre@africamuseum.be
AF: Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg,13, Tervuren, B-3080 Belgium
AU: Cardinal, D
EM: dcardinal@africamuseum.be
AF: Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg,13, Tervuren, B-3080 Belgium
AU: Alleman, L Y
EM: lalleman@africamuseum.be
AF: Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg,13, Tervuren, B-3080 Belgium
AU: Moorbath, S
EM: stephen.moorbath@earth.ox.ac.uk
AF: Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR United Kingdom
AB: High temperature vapor condensations and stardust forming stellar outflows strongly differentiate Si-isotopes (-650\permil$<$\delta$^{29}$Si$<$+200\permil). In contrast, on Earth, the major cause of slight Si isotope fractionation (-1.8\permil $<$\delta$^{29}$Si $<$+1.5\permil) is related to preferential biological uptake of $^{28}$Si by diatoms, radiolarian, sponges and plants in building their opaline frustules, spicules and phytoliths. As a consequence, modern waters, clays, soils and sediments are also fractionated: -0.9\permil$<$\delta$^{29}$Si$<$+1.7\permil, while Phanerozoic magmatic and metamorphic processes leave terrestrial crystalline rocks almost unfractionated: -0.4\permil $<$\delta$^{29}$Si $<$+0.2\permil. Spectacular claims for discovery of oldest evidence for terrestrial life in early Archean (ca 3.7-3.8 Ga) metamorphosed rocks from southern West Greenland have been challenged in a series of studies, and there is need for some independent tracer to gain new insights into the quest for primitive life. In order to unravel potentialities of silicon isotopes, we determined the Si-isotope composition of four groups of rocks from the Isua Greenstone Belt: magnetite-quartz "Banded Iron Formations" (BIF); mica-feldspar-quartz-(garnet) schists of potential pelitic origin; a series of tonalitic gneisses, metabasalt pillows, volcanogenic sediments and hydrothermal quartz veins and vesicles. Specimens were powdered using diamond-coated microdrills. Si was purified by TEA molybdate co-precipitation and measured on a Nu Plasma MC-ICP-MS using Mg external doping in dry plasma mode following Cardinal et al's (2003) methodology. Measured $^{29}$Si/$^{28}$Si are expressed with the \delta$^{29}$Si notation relative to the NBS28 quartz standard. Overall reproducibility assessed on 5 BIF duplicates is better than 0.08\permil. Merck Quartz aliquots processed in the same way as the Isua specimens are unfractionated, showing that our chemical procedure does not produce any isotopic artefacts. Metasediments yield an undifferentiated signature: -0.15\permil $<$\delta$^{29}$Si $<$+0.00\permil. Since they may reflect the average composition of pre-3.8Ga Archean crust, this points to a lack of significant "stellar dust" or "impact-related thermal volatilisation" imprints from pre-3.8Ga meteoritic infall on detrital Si-isotope compositions. The undifferentiated signature (-0.06\permil$<$\delta$^{29}$Si $<$+0.21\permil) of magmatic and hydrothermal derived rocks confirms that Archean crystalline rock-forming processes compare with their Phanerozoic counterparts in terms of their inefficacy to fractionate silicon isotopes. BIF's differ by their significant relative $^{29}$Si depletion, which is stronger for the well-preserved, finely-banded BIF (-1.28\permil$<$\delta$^{29}$Si$<$-1.11\permil) than for those with irregular or folded banding (-0.71\permil$<$\delta$^{29}$Si$<$-0.28\permil). Moreover, the finely banded BIF presents very homogeneous interlayer \delta$^{29}$Si. Considering the seawater-derived origin of Isua BIF (e.g.Bolhar et al, 2004), the $^{29}$Si depletion might either directly reflect the ambient early Archean ocean or seawater-derived hydrothermal fluid, or indicate that BIF precipitation was associated with some unknown organic or inorganic fractionation process. Because the observed isotopic differentiation fits in the range of modern biogically-mediated Si-isotope fractionations, the hypothesis of some potential links between BIF precipitation and early biological activity cannot be overlooked.
DE: 9619 Precambrian
DE: 8125 Evolution of the Earth
DE: 4267 Paleoceanography
DE: 1040 Isotopic composition/chemistry
DE: 0400 Biogeosciences
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting