Zircon
From EvoWiki
The mineral Zircon is a silicate (ZrSiO4). Zircons are very resistant to erosion and metamorphism. Thus very old zircon minerals have been found. The oldest zircon minerals on earth have been found in Western Australia ( Narryer Gneiss Terrane) with an age up to 4.4 billion years. This are the oldest minerals found on earth at all giving an lower limit of the age of the earth. The rocks in which these zircons were originally embedded when they crystallized were destroyed by erosion, and only the resistant zircon minerals survived. Thus the rocks in which they are embedded today are younger than the zircon minerals themselves.
Besides providing a limit on the age of the earth, zircon crystals have been used to deduct the conditions of the early earth. According to research by John W. Valley, the oldest zircon crystals were formed in a relatively cold environment, as indicated by the ratio of oxygen isotopes. Furthermore, the crystals show signs of abrasion, which requires transport by wind or water over solid materials, which again points to a solid (rather than molten) early earth crust. Thus the traditional view that it took 500 million years for the earth to cool down so water could condense may be wrong. This new view would push the time when life could have originated on earth further back.
Literature
- John W. Valley: A Cool Early Earth?, Scientific American October 2005.
