U and Th zonation in apatite observed by laser ablation ICPMS, and implications for the (U–Th)/He system

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 ~20 μm, 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 ^(4)He concentration profile which governs diffusive loss, and, via radiation damage trap accumulation, spatial variability of diffusivity within the crystal. If the observed zonation is typical of the apatite population in GC863, use of the standard homogenous 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). Independent of the ejection correction, the concentration profile modifies the effective closure temperature of the apatites by placing more (or less) ^(4)He 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 homogenous 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 °C/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 °C/Ma). For example, in rim-enriched GC863 grains cooled at 1 °C/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 ^(4)He/^(3)He spectra substantially from that expected from a uniform distribution. Measured ^(4)He/^(3)He 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 ^(4)He/^(3)He spectra.

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