HR: 17:15h
AN: V22G-06 INVITED     [PDF]
TI: Problematic Samples for Apatite (U-Th)/He Dating: Some Possible Causes and Solutions
AU: * Farley, K A
EM: farley@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech MS 170-25, Pasadena, CA 91125 United States
AB: Seven years of experience with (U-Th)/He dating reveals a subset of granites and gneisses (perhaps 10%) which yield apatites problematic for the method. Here I document these challenging samples and provide possible explanations and solutions. Rocks from some areas (e.g., Himalayas and Taiwan) tend to yield apatites in which every grain is broken, precluding accurate $\alpha$ emission correction. We developed a protocol in which the grain fragments are abraded to eliminate the $\alpha$-ejection-affected surfaces. The now opaque fragments are immersed in appropriate refractive index oil, rendering inclusions remarkably visible for hand-picking. By eliminating the need to simultaneously have good morphology and freedom from inclusions, this technique greatly increases the population of datable grains. Results of a case study from Taiwan will be presented, as will modeling which shows how removal of outer edges must bias He ages, especially when cooling is slow. \\More problematic are seemingly good apatites that yield irreproducible and anomalously old He ages. The following observations apply to these rocks: strong geographic control, with problem samples common in some areas (e.g., Transantarctic Mtns) but absent elsewhere (e.g., Coast Mtns); highly variable U,Th among apatite grains (up to 2-3x); REEs (now measured on every dated apatite) have LREE depletion, compared with LREE enrichment in most non-problematic apatites. These observations are consistent with problematic apatites occurring preferentially in S-type granites, which have precipitated monazite (Sha and Chappell, 1999). Flux melting indicates that inclusions cannot explain the aberrant ages, and modeling and ion probe measurements suggest that U,Th zonation is not a likely cause either. A possible role for implantation of He from neighboring monazites is consistent with abrasion experiments yielding younger cores than obtained from whole grains. If this explanation is correct, it may be impossible to obtain meaningful He ages on such samples. In any case it seems prudent to reproduce all He age determinations to insure data quality.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting