『Abstract
A laser-ablation inductively-coupled plasma mass spectrometry
technique was developed to measure U, Th, and Ce zonation in polished
sections of apatite for assessing the consequences of parent zonation
for (U-Th)/He thermochronometry. The technique produces concentration
maps with an averaging length-scale of 〜20μm, comparable to the
α-stopping distance, and a precision of 〜5% down to few ppm concentration
levels. A model was developed to transform the measured concentration
distribution into a simplified representation for use in spherical-geometry
He production-diffusion models. To illustrate these methods, 30
sections of apatite from a single granite (GC863) were mapped.
Every analyzed apatite from GC863 is zoned, with most grains having
variable thickness rims and terminations that are enriched in
U and Th by about a factor of three over the grain cores.
Parent zonation has three independent effects on (U-Th)/He He
ages: it influences the α ejection correction, the 4He
concentration profile which governs diffusive loss, and, via radiation
damage trap accumulation, spatial variability of diffusively within
the crystal. If the observed zonation is typical of the apatite
population in GC863, use of the standard homogeneous α ejection
correction would cause He ages to be on average 3% too young,
and with a large amount of grain-to-grain variability (9% too
young in the most rim-enriched case to 6% too old in a core-enriched
case). Independence of the ejection correction, the concentration
profile modifies the effective closure temperature of the apatites
by placing more (or less) 4He near the grain edge.
The parent zonation in GC863 apatites causes closure temperatures
to range from four degrees lower (rim-enriched case) to two degrees
higher (core-enriched case) than applies in the homogeneous case.
Alpha ejection and concentration profile effects on He age are
additive and of the same sense. In the case of typical grains
in GC863 cooled between 1 and 10℃/Ma, the two effects are roughly
equal in magnitude. The effects of intracrystalline variations
in radiation damage trap accumulation become apparent at slow
cooling rates (1℃/Ma). For example, in rim-enriched GC863 grains
cooled at 1℃/Ma, preferential accumulation of radiation damage
traps near the grain rim almost compensates for the higher loss
rate expected of 4He also located preferentially near
the rim. Under some circumstances strong rim-enrichment may actually
increase the effective closure temperature of an apatite. Zonation
at the level observed in GC863 modifies the 4He/3He
spectra substantially from that expected from a uniform distribution.
Measured 4He/3He spectra are strikingly
similar to predictions based on the mapped eU distributions of
the very same crystals, supporting the overall validity of the
analytical and interpretive approach presented here.
The magnitude and style of U, Th zonation documented in GC863
is one possible source of frequently observed over-dispersion
of apatite (U-Th)/He ages as well as anomalous 4He/3He
spectra.』
1. Introduction
2. Samples and analytical methods
3. Converting a map view of concentration into an equivalent sphere
for simulation of He diffusion from a zoned crystal
4. Results from sample GC863
4.1. Concentration maps, spherical equivalents, and interelement
correlation
4.2. Consequences for the α ejection correction
4.3. Zoning effects on diffusive He loss
4.4. Comparison with (U-Th)/He ages and 4He/3He
spectra measured on sample GC863
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References