Silicon isotopes in 3.8 Ga West Greenland rocks as clues to the Eoarchaean supracrustal Si cycle

We report MC-ICP-MS Si-isotopic measurements (delta(29) Si recalculated as delta Si-30) on micro-subsamples (at 500 mu m scale) from several major rock types of the similar to 3.8-Ga-old Isua Greenstone Belt (IGB, southern West Greenland) and surrounding Eoarchaean terrains. With a large overall range of variations (-2.80 parts per thousand <delta Si-30 <+0.68 parts per thousand), they demonstrate strong involvement of surface fluids enriched with dissolved Si. The resistance of Si-isotopes to metamorphic resettings and metasomatic overprints is also established. Metabasaltic pillows and metasediments display similar Si-30-enriched signatures, suggesting that emergent surfaces of the Eoarchaean protocrust were composed of slightly weathered, hydrothermally altered, mafic-ultramafic bodies. Isua magnetite-quartz Banded Iron Formation (BIF) is strongly depleted in Si-30 relative to all coeval rocks. This depletion supports Rayleigh-controlled precipitation from seafloor-vented hydrothermal fluids. In contrast, banded quartz-pyroxene rocks (from Akilia Island, some 150 km southwest of the IGB), which some authors have identified as BIF-related, yield quartz with Si-isotopic composition (delta Si-30=-0.36 parts per thousand) similar to metamorphic-derived quartz (delta Si-30=-0.50 parts per thousand). This supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers. (c) 2006 Elsevier B.V. All rights reserved.