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 fiammelike structures, all supporting a volcanic protolith. The nodules and surrounding schistose matrix contain abundant, 100-50 μm, euhedral, oscillatory zoned 3806 Ma 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 ɛHf initial values of ∼0 and δ18OVSMOW 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 δ18OVSMOW values are elevated at +14.7 to +16.2‰, indicative of superimposed lowtemperature alteration processes. The nodules and matrix schists have non-igneous bulk compositions, exemplified by strong enrichment in K2O and depletion in Na2O. They are depleted in Sr, have no negative Eu anomalies, but have high Rb/Sr, with an Rb-Sr age of 3760 ± 140 Ma ( 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 3806 Ma felsic volcanic rocks underwent Eoarchaean lowtemperature 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 CO2 into rocks made porous by the weathering.

A non–plate tectonic model for the Eoarchean Isua supracrustal belt

“…3.8 Ga belt is robust. This is illustrated via a compilation of SHRIMP U-Pb zircon spot ages from meta-tonalites and felsic metavolcanics in the Isua area from Nutman et al (2013); see Figure 3A. The distribution of ages clearly shows peaks at ca.…”

“…The results appear to fit well within the broad distribution of the ca. 3.7 Ga age peak from the Nutman et al (2013) compilation (Fig. 3A).…”

“…Three deformation events have been proposed in time windows that overlap with the ca. 3.69-3.66 Ga period in which north-dipping subduction and collision juxtaposing the two main sequences are posited (e.g., Nutman et al, 2013Nutman et al, , 2015b. First, in the northwestern part of the Isua supracrustal belt, a panel of metamorphosed banded iron formation interlayered with felsic schists, all of which are sheared, is interpreted as being tectonically emplaced (Nutman et al, 2015b).…”

“…All age constraints fit within the ca. 3.7 Ga age peak of Nutman et al (2013;see Figs. 3A and 3D herein).…”

“…3.61 Ga has been proposed based on U-Pb zircon and titanite geochronology of meter-scale granitic intrusions (e.g., Crowley et al, 2002;Nutman et al, 2013). The intrusions are largely pre-and/or syndeformational (Crowley et al, 2002;Nutman et al, 1996Nutman et al, , 2000Nutman et al, , 2002Nutman et al, , 2013, with proposed postdeformational sheets having relatively young ages (Crowley et al, 2002;Friend et al, 2002;Nutman et al, 2002). The interpretation that deformation was extensional RESEARCH is based on sparse reports (without orientation information) of outcrop-scale structures such as "sigmoid shapes" and fracture networks (Nutman et al, 2013).…”