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

Abstract

We report MC-ICP-MS Si-isotopic measurements ( δ 29Si recalculated as δ 30Si) on micro-subsamples (at 500 μm scale) from several major rock types of the ∼3.8-Ga-old Isua Greenstone Belt (IGB, southern West Greenland) and surrounding Eoarchaean terrains. With a large overall range of variations (− 2.80‰ < δ 30Si < + 0.68‰), 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 30Si-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 30Si 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 ( δ 30Si = − 0.36‰) similar to metamorphic-derived quartz ( δ 30Si = − 0.50‰). 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.