DO IRON ISOTOPES REQUIRE A BIF ORIGIN FOR QUARTZ-PYROXENE ROCKS ON AKILIA, SW GREENLAND?

The age and origin of a sequence of quartz-pyroxene rocks from the island of Akilia, southern West Greenland, has been the subject of intense debate since publication of C-isotope data, which suggested the presence of Earth's earliest biological activity. Not only is the claim for biogenic relicts debateable, but also the possibility that the rocks might represent some of the Earth's earliest water lain sediments and, hence, a suitable repository for life, remains an open question. We focus on the recent conclusion that iron isotopes provide a diagnostic test of a banded iron formation (BIF) origin for the quartz pyroxene rock. The primary line of evidence to propose a BIF protolith is that quartz-pyroxene rocks on Akilia are characterised by isotopically heavy iron (δ⁵⁶Fe ≤ +1 ‰) that is similar to that from undisputed >3.7 Ga quartz-magnetite bearing BIFs of the nearby Isua Greenstone Belt, while their host ultramafic and tonalitic rocks are typically unfractionated. We have undertaken a detailed high spatial resolution Fe isotope study of magnetite and pyrrhotite from a series of distinct lithologic units in the 5 m wide quartz-pyroxene zone on Akilia. Our SIMS analyses of magnetites and pyrrhotites from coarse and fine grained variants show them to be dominantly isotopically heavy (δ⁵⁶Fe = -0.5 to +2.5 ‰) as would be expected in a sedimentary environment. However, a pyroxenite boudin, which is considered to have an igneous origin, contained within the quartz-pyroxene lithology also possesses isotopically heavy magnetite (δ⁵⁶Fe = +0.5 to +3.0 ‰) and pyrrhotite (δ⁵⁶Fe = 0 to +1.0 ‰). Furthermore, veins of pyroxenite that cross cut the main banding, and therefore are younger, contain magnetites that are also isotopically heavy. Such results indicate that fractionated iron can be transported and included into potentially much younger secondary minerals, and so the mere presence of isotopically heavy iron in a mineral cannot uniquely identify a sedimentary origin. In the case of Akilia, our results support the possibility that at least some of the iron-bearing phases must have formed during geologic events post-dating the inferred time of primary rock emplacement.