3806Ma Isua rhyolites and dacites affected by low temperature Eoarchaean surficial alteration: Earth's earliest weathering

Abstract

This paper reports evidence for Earth's oldest-recognised low temperature alteration, at ~3800. Ma. Potassic felsic schists with a protolith age of 3806 ± 2. Ma form a ~30. km long unit in the amphibolite facies, deformed, Isua supracrustal belt (West Greenland). At a single locality, boudinaged layers (nodules) within the schists are low strain zones: they are fine-grained, weakly feldspar-phyric, contain quartz amygdules and have fiamme-like structures, all supporting a volcanic protolith.The nodules and surrounding schistose matrix contain abundant, 100-50μm, euhedral, oscillatory zoned 3806Ma zircons. The rare earth patterns of the zircons indicate crystallisation was magmatic. Some zircons contain axial lobate voids indicating that they grew at low pressure as the magma exsolved a fluid. Ti-in-zircon thermometry indicates crystallisation temperatures of 750-650°C. Taken together, these zircon features indicates growth at eutectic temperatures in a hypabyssal chamber as the magma was exsolving a fluid phase. The magmatic zircons have e{open}<inf>Hf</inf> initial values of ~0 and δ18O<inf>VSMOW</inf> of +5.0‰ (Hiess et al., 2009), showing that the source of the volcanic rocks was devoid of assimilated markedly older or weathered crustal material, and probably essentially juvenile. In contrast, the whole rock δ18O<inf>VSMOW</inf> values are elevated at +14.7 to +16.2‰, indicative of superimposed low-temperature alteration processes.The nodules and matrix schists have non-igneous bulk compositions, exemplified by strong enrichment in K<inf>2</inf>O and depletion in Na<inf>2</inf>O. They are depleted in Sr, have no negative Eu anomalies, but have high Rb/Sr, with an Rb-Sr age of 3760±140Ma (Jacobsen and Dymek, 1988). This indicates that the alteration involving strong degradation of plagioclase occurred in the Eoarchaean. Trace element compositions and establishment of alteration vectors suggest the protoliths were likely rhyolitic and dacitic in composition.The strongest-modified matrix schist compositions contain biotite±calcite±dolomite with increase in MgO relative to the nodules, which indicates early magnesian carbonate growth. The whole-rock chemistry, decoupling of the igneous zircon and whole-rock oxygen isotope signatures and the Rb-Sr dating indicate that after eruption, the 3806Ma felsic volcanic rocks underwent Eoarchaean low-temperature potassic alteration with weathering and groundwater circulation the most likely process. The geochemistry of the Isua felsic schists is compared with that of better-preserved volcanic rocks where the alteration conditions are known. This suggests a subaerial environment. The carbonatisation of the Isua felsic schists demonstrates drawdown of atmospheric CO<inf>2</inf> into rocks made porous by the weathering.