MICRO-SCALE HETEROGENEITY AND REDISTRIBUTION OF IRON ISOTOPES IN EARTH'S OLDEST BANDED IRON FORMATION REVEALED BY SECONDARY ION MASS SPECTROMETRY

Deformed and metamorphosed rocks from the > 3.7 Ga Isua greenstone belt, southwest Greenland, represent Earth's oldest known supracrustal succession. The section is dominated by abundant mafic volcanic rocks, with less abundant sedimentary rocks such as BIF and conglomerate, sandstone, and mudstone of volcaniclastic origin. Here we present iron isotopic compositions determined by secondary ion mass spectrometry (SIMS) for magnetite crystals comprising the BIF and pyrite from younger, cross-cutting veins and clusters from BIF and conglomerate. In three lithologic variants of BIF (quartz+magnetite; quartz+magnetite+minor amphibole; amphibole+magnetite+quartz), magnetite crystals span a range of δ⁵⁶Fe from -0.7 to +2.7 ‰ over a sub-millimeter scale, although many values range between +1 and +2 ‰. Such values are unpredicted at such a small scale and potentially consistent with an abiotic origin for the BIF. Secondary pyrite veinlets in BIF and cubes of metamorphic pyrite from a conglomerate also show a similar distinct enrichment in heavy Fe isotopes, suggesting that such fractionations are not unique to the original environment as has been assumed, but can be redistributed during subsequent post-formational processes.