Geochemical signatures of soapstones from the Nuuk area, southern West Greenland – their use for fingerprinting of archaeological artefacts

Soapstone was mined and traded by the Palaeo- and Neo-Eskimo cultures (Saqqaq, Dorset, and Thule) as well as the Norse and Moravian cultures living in Greenland through time. We collected soapstone from outcrops around Nuuk, which hold large quantities of Greenland's high-quality soapstone, and analysed their whole-rock geochemistry, Sm–Nd and Rb–Sr isotopes for fingerprinting geological and archaeological purposes. Fieldwork showed that the quality of soapstone in the Nuuk area varies widely; however, a correlation exists with the age and metamorphic history of the rocks. Soapstone in the Nuuk area is derived from Archaean ultramafic rocks and yield high Mg, Cr, Ni and low Fe, V, K concentrations and are depleted in Ca. Soapstones from Nuuk have very high Rb concentrations and average to low Sr concentrations compared to other Archaean ultramafic rocks, leading to very high and irregular ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios. A very poorly constrained Rb–Sr errorchron for unmetasomatised samples from the Nuuk area has a slope corresponding to an age of 3.05 ± 0.33 Ga. Soapstone samples from Nuuk, both affected and unaffected by metasomatism give a¹⁴⁷Sm/¹⁴⁴Nd isochron age of 2.95 ± 0.06 Ga. This is regarded as the age of metsomatism, and tectono-metamorphic overprinting, which affected the precursors to the soapstone. The best quality soapstones occur in the areas dominated by Eoarchaean gneisses. The ca. 2.95 Ga overprinting is uncommonly old for soapstones worldwide. Soapstone, and hence archaeological artefacts created from this soapstone, may likely be fingerprinted to the Nuuk area.