HEMATITE FORMATION BY OXYGENATED GROUNDWATER AT 2.76 GA AND BY OXYGENATED SEAWATER AT 3.46 GA

A currently popular model for Earth's atmospheric evolution postulates an anoxic atmosphere prior to about 2.35 Ga. Here we report the discovery of two generations of abundant hematite in a deep drill core of the Towers Formation (3.46 Ga), a succession of volcanic rock and chert/jasper in the Pilbara Craton, Western Australia. The first generation of hematite occurs as the major component (≈6wt%) in the ferruginous chert and the second generation occurs as patches and veins in the volcanic rock.

We obtained a rhenium-osmium (Re-Os) isotopic age of 2.762 ± 0.018 Gs for pyrite veinlets that clearly cross-cut (i.e., are younger than) hematized volcanic rock. This isotopic age, together with mineralogical and geochemical data, suggests that reactions with oxygenated groundwater formed the hematite in the volcanic rocks prior to 2.76 Ga. Hematite in the chert displays an extremely small grain size (<1µm), an association with fine grained euhedral pyrite, and a correlation with indicators of hydrothermal activity (e.g. Eu*, Cu+Zn). These data suggest hematite in the chert formed during deposition at 3.46 Ga through rapid mixing of metal and sulfide rich hydrothermal fluids (≈250°C) with oxygenated deep (>500m) seawater.

Therefore, an oxygenated atmosphere most likely developed more than 400 million and possibly as early as 2.11 billion years prior to the currently accepted 2.35 Ga date, although it is uncertain whether the appearance of an oxygenated atmosphere was episodic or continuous.