Crustal Imaging and Characterization Team
Task Contact: John Aleinikoff
The objective of this Research and Developmen project is to develop the methodology to make geochronology of diagenetic xenotime a routine technique. We have begun a pilot study of rocks from the Belt Supergroup to show the potential of the method. Samples have been obtained from the bottom (Pritchard Formation) and top (Pilcher and Garnet Range Formations) of the Belt to determine both the span of time and the absolute ages of Belt sedimentation, a historically contentious issue. Xenotime from the Prichard Formation was dated in April 2003; the age agrees with independent age information, proving that the method works. We are applying this technique to dating the Neoproterozoic Ocoee Supergroup (TN-NC); excellent overgrowths have been found and extracted from polished thin sections. They were dated on SHRIMP (Sensitive High-Resolution Ion Microprobe) in 2004. We expect that there will be significant interest in this dating technique from many projects throughout the Geologic Discipline, including basin sediments in the Mid-Continent Rift , Middle Proterozoic metasedimentary rocks in Idaho (possible Belt correlatives), and middle Proterozoic quartzites in Wisconsin .
Task no longer active.
We have had great success finding large xenotime overgrowths on detrital zircons in three samples from the Belt Supergroup. Samples from the Prichard Formation, lower Belt Supergroup were dated on SHRIMP II in Australia in 2003. The ages agree with independent age evidence, providing proof that the method works. We dated 2 samples from the uppermost Belt in 2004. Based on poor results with ferruginous samples (zircon and xenotime are obscured in SEM backscatter view), future efforts will concentrate on well-sorted, clean (i.e. not iron-stained) medium-to coarse-grained samples. Another key finding is that samples can be screened using XRF analysis for yttrium before the costly step of fabrication of polished thin sections. Only samples with sufficient Y (greater than 20 ppm) will be used. This pre-screening will save both time and money.
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Mineral Resources |
Eastern / Central / Western / Alaska / Minerals Information |
| Crustal Imaging & Characterization / Spatial Data |
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://crustal.usgs.gov
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