Paper No. 33-2
Presentation Time: 1:50 PM
THE FIRST HIGH RESOLUTION GEOCHRONOLOGICAL CONSTRAINTS ON IRON FORMATION DEPOSITION AFTER THE GREAT OXIDATION EVENT
The oldest recognized proxies for low atmospheric oxygen are massive iron-rich deposits. Following the rise of oxygen during the Great Oxidation Event, massive iron formations largely disappear from the geologic record, only to reappear in a pulse ~1.88 Ga, which is also associated with distinctive fossil morphologies and unique biotic assemblages not documented in older strata. This resurgence of iron deposition has been attributed to changing ocean dynamics, atmospheric oxygen, or ocean chemistry triggered by intense volcanism. However, the lack of high-resolution age constraints have hampered questions about correlations between the various deposits and causation mechanisms. Here we report new U-Pb zircon LA-ICP-MS and CA-ID-TIMS eruptive ages from the Emperor Volcanics in the Gogebic Range near Lake Superior that both inter-finger with iron formation and overlie the Ironwood Iron formation. These new ages predate the previously considered regional time-calibration and allow for a revaluation of the link between tectonic setting and iron deposition, critical to assessing continuous vs. pulsed iron formation scenarios and determining mechanisms that trigger large changes in ocean redox and iron deposition. Furthermore, high-resolution eruptive ages reported here from units associated with post-GOE iron formations provide key tie-points to construct a refined temporal global framework to compare the Gunflint and Sokoman Iron formations from the Superior Craton and the Frere Iron Formation from Australia. By synthesizing this compilation with existing paleomagnetic datasets and global proxies for atmospheric and oceanic redox, we highlight the possible global paleogeographic context for massive iron deposition and implications for the co-evolution of life.