High ammonium contents in the 3800 Ma Isua supracrustal rocks, central West Greenland

Abstract

Ammonium contents of whole-rock and biotite samples from the metasedimentary sequence of the 3800 Ma Isua supracrustal belt, central Greenland, have been determined and compared with the ammonium contents of volcanic, sedimentary, metamorphic, and granitic rocks from the Late Archaean-Early Proterozoic. A sample of the Amitsoq gneiss has an NH 4 + content (whole-rock: 9 ppm, biotite: 29 ppm) that is similar to many younger igneous rocks. Ammonium contents of the supracrustal rocks vary with chemistry and mineralogy of the rock and most likely were inherited from the original sediments. A series of carbonate-bearing and basic plagioclase-bearing biotite-rich schists from sequence A have varying NH 4 + contents (whole-rock: 6–24 ppm) that increase with an increasing chemical /clastic component and decreasing volcanogenic component. Biotites in these rocks, together with biotites in mica schists of the same sequence, have high NH 4 + contents which vary over a small range of 54–95 ppm, suggesting that the NH 4 + in these rocks was contained mostly in authigenic clays of the original sediments. All these data imply that clay minerals were the major sink of NH 4 + or other nitrogen compounds on the Earth's surface at 3800 Ma. Two samples of graphite-bearing garnet-staurolite-mica schist from sequence B have remarkably high NH 4 + contents (whole-rock: 68 and 86 ppm, biotite: 234 and 253 ppm), which are as high as, or even higher than, those of common Late Archaean-Early Proterozoic argillites. It appears almost certain that the original, thick, clay-rich bottom sediments contained a large amount of NH 4 + or other nitrogen compounds. The most likely major source of nitrogen in these NH 4 +-enriched, graphite-bearing mica schists might be organic nitrogen.