Early Archaean Am�tsoq tonalites and granites of the Isukasia area, southern West Greenland: development of the oldest-known sial

Nutman, Allen P.; Bridgwater, D.

doi:10.1007/bf00592931

Cited by 95 publications

(37 citation statements)

References 32 publications

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“…There is no significant difference in the REE distributions between nodules, their matrix and samples of layered schist (Fig. 6C), and there is no significant fractionation compared with Eoarchaean plutonic granites and tonalites (data in Nutman and Bridgwater, 1986;Nutman et al, 1999). This demonstrates that the process that added potassium or that which caused addition of carbonate did not disturb the magmatic REE chemistry to a degree detectable beyond the likely 10% uncertainties in the REE abundances.…”

Section: Alterationmentioning

confidence: 82%

“…The inclusion that has an apatite habit is connected to the grain exterior by a microfracture. Nutman and Bridgwater (1986) and Nutman et al (1994Nutman et al ( , 1999. ( shown compared with ~3800 Ma tonalites and quartz diorites (Nutman et al, 1999) and a non-silicified ~3.46 Ga Kittys Gap volcanic rock from the Pilbara craton (Smithies et al, 2007).…”

Section: Discussionmentioning

confidence: 94%

“…These are augmented with a large number of analyses undertaken by XRF in the 1970s at the Geological Survey of Greenland, two of which were analysed for the REE by O'Nions and Pankhurst (1974; data in online supplementary Table 2 and see footnote of Table 2 in Nutman and Bridgwater (1986) for analytical methods on all these samples). The modern and old data are congruent, and some of the 1970s data are particularly useful because CO 2 was analysed, permitting chemical changes related to carbonation to be identified.…”

Section: Methodsmentioning

confidence: 99%

“…Nutman 1986;Nutman et al, 2004). This means that the compositional layering in the volcanic and sedimentary rocks is mostly of transposed tectonic origin (Nutman et al, 1986(Nutman et al, , 1996(Nutman et al, , 2002(Nutman et al, , 2007Myers, 2001). Thus primary volcanic and sedimentary structures have survived in only a few places.…”

Section: Geology Of the Isua Supracrustal Beltmentioning

confidence: 99%

“…All these rocks have fresh Archaean amphibolite facies mineral assemblages (Nutman, 1986) and +7.8‰ respectively (Fig. 4).…”

Section: Interpretation Of Isua Whole-rock Oxygen Isotopic Signaturesmentioning

confidence: 99%

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

Nutman

Bennett

Chivas

et al. 2015

Precambrian Research

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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.

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Section: Alterationmentioning

confidence: 82%