Origin and significance of Archean quartzose rocks at Akilia, Greenland.

Archean Quartzose Rocks at Akilia, Greenland Fedo and Whitehouse (1) presented data to support their hypothesis that granulite grade quartz magnetite rocks on the island of Akilia, southwest Greenland—which are conformably interposed with pyroxenite and hornblendite units that date from before 3830 million years ago (Ma) (2–4)—had a metasomatic ultramafic igneous origin. They offered that interpretation despite the fact that a key sample in earlier studies of this area, Akilia quartzite G91-26, which has been investigated for carbon isotope evidence for early life (5) and described in previous studies as a metamorphosed marine sediment (2–9), is a 70 to 90% quartz unit containing magnetite (2, 5, 6). Fedo and Whitehouse (1) made no apparent distinction between the petrogenesis of a structurally coherent quartzite magnetite unit and that of cross-cutting pyroxenite veins, nor did they explain the origin and nature of the remarkable metasomatic fluid that could have precipitated quartzmagnetite mineral assemblages hosting isotopically light carbon while removing Cr, Ni, Mg and platinoid (Pt, Ir) elements (7) typical of the mafic and ultramafic rocks from which they argue the quartzite was derived. We show here that all the data presented by Fedo and Whitehouse (1) are actually consistent with a primary sedimentary origin for the quartz-magnetite rocks on Akilia. Indeed, a more complete evaluation of geochemical evidence from previous studies (2, 5–10) demonstrates that the invasive pyroxenite veins are of no significance to the origin of the quartzite unit on Akilia and that these silica-saturated rocks could not be derived from metasomatism of ultramafic mineral assemblages. Fedo and Whitehouse (1) compared rare earth element (REE) patterns from quartz-rich lithologies with patterns from pyroxenite (AK 38), adjacent ultramafic rocks (AK 2 to 5), and pyroxenite vein-rich samples (AK 9, 10, 31, 36, 38, 44, and 46) on a chondrite-normalized plot. This geochemical relation, however, constitutes