Paleoarchean Spherule Layers

Abstract

The Archean marks the earliest time in Earth history for which there is a decipherable record of surface events. As noted in Chap. 7, the end of the Archean Eon is formally defined chronologically as 2,500 Ma. The International Union of Geological Sciences (IUGS) provisionally recommended dividing the Archean into four Eras named the Neoarchean, Mesoarchean, Paleoarchean, and Eoarchean (Robb et al. 2004). They also suggested placing the Mesoarchean-Neoarchean boundary at 2,800 Ma, the Paleoarchean-Mesoarchean boundary at 3,200 Ma, and the Eoarchean-Paleoarchean boundary at 3,600 Ma. The time period from the start of Earth history to the beginning of the Archean is widely referred to as the Hadean, but there is no universal agreement on where to place the Hadean-Eoarchean boundary. Robb et al. (2004) say the age of the Hadean-Eoarchean boundary is ‘‘undetermined,’’ whereas Van Kranendonk et al. (2008) gave an age of ~4,000 Ma for the boundary but other values (e.g., 4,030 Ma) are under consideration. Cawood et al. (2006) put the boundary at 3.8 Ga, whereas Tanaka and Hartmann (2008) avoid the term Hadean altogether and extend the Eoarchean from 3,600 Ma back to the start of Earth history. In terms of the IUGS recommended subdivision, the spherule layers described in this chapter are all Paleoarchean in age. More significantly, they are ~600 million years or more older than the Neoarchean layers described in Chap. 8, and no distal ejecta or spherule layers of intermediate ages have been reported.