Metasomatic Alteration and Element Mobility in the 3.8 Ga Felsic-Metavolcanic Rocks of the Isua Supracrustal Belt

The Isua supracrustal belt (ISB) is one of the earth’s oldest preserved rock sequences and provides crucial insights into early Archean evolution. Polyphase metamorphism and variable degree of metasomatism have significantly obscured primary signatures requiring an understanding of metasomatic processes to reconstruct the evolution of the ISB.

A local metasomatic reaction zone in the western 3.8 Ga felsic-metavolcanic zone has been sampled, covering a traverse with decreasing alteration from the centre to the rim to identify the protolith, quantify element fluxes and reconstruct metamorphic conditions. Progression of the metasomatic front is characterised by four mineral assemblages: (1) talc-chlorite, (2) talc-chlorite-quartz, (3) chlorite-cordierite-biotite-quartz±kyanite, (4) cordierite-biotite-quartz±kyanite. While high Mg#: > 90 in ferromagnesian minerals appear to be constant across the outcrop, a systematic decrease in whole rock magnesium and nickel accompanied by an increase in silica, potassium, sodium, calcium and phosphorus can be observed. We interpret the element fluxes to be the consequence of infiltration by a fluid previously equilibrated with ultramafic rocks. Thermodynamic calculations indicate P-T conditions of 600-650°C and 0.5-0.7 GPa. The protolith has been constrained as dacitic to rhyolitic. In addition, the comparison of the least altered sample with a calculated average composition of the 3.8 felsic-metavolcanic rocks lead to the conclusion that they originate from the same protolith and had the same whole-rock chemistry before metasomatism. Further research is needed to constrain the origin of the fluid and the temporal relationship between metasomatism and metamorphism.