Penciling in Details of the Hadean

Penciling in details of the Hadean

Author:

House

Abstract:

Some truly remarkable graphite is described by Bell et al. in PNAS (1). Graphite is, of course, the same material as found in pencil tips or in the anode of lithium ion batteries. Graphite is, however, also a very common material in Earth Science, and is often the form of carbon found in very old fossils that have been subjected to substantial heat. The graphite described in the Bell et al. article is remarkable because it is exceptionally old, dating to the Hadean eon. Officially, the Hadean is defined as the time period from the formation of the Earth until 4 billion y ago. Until recently, this has been a seemingly convenient definition, leaving it as the geological eon without a rock record on Earth. Over the past quarter century, however, the discovery and exploration of detrital zircon minerals from the Jack Hills conglomerates of Western Australia (2) have provided a new window into this early time. Jack Hills zircons crystallized in magma chambers at various times as far back as 4.4 billion y ago (3, 4). So far, these zircons and their inclusions are currently our only tangible record of the first half a billion years of Earth history. Based in part on the extreme age of some of these zircon minerals, along with similarly old age dates for a Martian meteorite (5), the Planetary Science and Earth Science communities now appreciate that planets form and cool rather quickly (6). No longer is the Hadean just a placeholder on our timelines between the formation of the Earth and the oldest known rocks