Cosmogenic 3He in igneous and fossil tooth enamel fluorapatite

Igneous fluorapatite samples from a suite of six granitic rocks from the Transantarctic Mountains have high He-3 concentrations (to 5x10(9) atoms g(-1)) and high He-3/He-4 ratios (to 9x10(-7)). These values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on Li-6. This He-3 is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation. Several samples of fossil tooth enamel fluorapatite from the Turkana Basin of Kenya are similarly rich in He-3, with up to 1x10(7) atoms He-3 g(-1) and He-3/He-4 ratios up to 4x10(-6). Again, this He-3 is most logically attributed to cosmic ray reactions. Provided that cosmogenic He-3, like radiogenic He-4, is quantitatively retained in fluorapatite under Earth surface conditions, routine He-3 exposure dating of this common phase may be possible. Based on its chemical composition, the He-3 production rate in fluorapatite is about 100 atoms g(-1) yr(-1) at sea level and high latitude. Using this rate the apatites from the Transantarctic Mountains have apparent exposure ages of 0.5-6.2 Myr, in agreement with values elsewhere in the range. The fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr. Such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the Earth's surface. He-3 exposure ages can provide insights to the depositional and reworking history of enamel-bearing fossils. At present the major limitations to He-3 exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of He-3 in the presence of large quantities of He-4. In addition, further work is necessary to establish the nucleogenic He-3 background in fluorapatite. (C) 2001 Elsevier Science B.V. All rights reserved.