EARTH'S EARLIEST ICE AGES: NEW ZIRCON GEOCHRONOLOGY OF ARCHEAN GLACIAL DIAMICTITES FROM MONTANA

The timing, duration, and location of Earth's earliest glaciations are poorly understood. To date, the oldest well-established glacial deposists are found in the Pongola Supergroup of South Africa. These units are ~2.9 Ga. Here, we revisit purported glacial units in the Beartooth Mountains in Montana, USA. Initial zircon dating from the 1970s suggested an age of 3.0 Ga. Our work both supports a glacial origin, through identification of dropstones and till, and a new detrital zircon age of ~2.9 Ga. This implies that, on at least two crustal blocks, there was extensive glaciation during the Neoarchean. This finding has important ramifications for Archean climate, suggesting that, at least for periods, it was cold enough to support active glaciation.