In Situ U-Th-Pb Geochronology
Recent developments and advances in LA-HR-ICPMS have led to widespread use of in situ U-Th-Pb dating of accessory minerals (zircon, monazite, baddeleyite, sphene) for studies of sedimentary provenance using separated detrital grains and of magmatic and metamorphic crystallization using grain mounts or thin sections.
The good spatial resolution of LA-ICPMS is a particularly useful tool for resolving ages in grains with inherited cores and metamorphic overgrowths in complex populations.
As well, in situ analyses of accessory minerals allows correlation of textural and fabric relationships with temporal events.
The techniques used in our laboratory are described in detailed by Kosler and Sylvester (2003).
Instrumental mass bias is corrected using a solution containing 203 Tl, 205Tl, 209Bi, 237Np and 233U that is aspirated simultaneously with laser ablation.
U/Pb fractionation is limited by rastering – the intercept method is used to correct for any residual fractionation.
Clients may prepare their own grain mounts and images of minerals. Alternatively this work can be done by MAF-IIC stafff on a fee-for-service basis.
Characterization of grains using backscattered electron (BSE) imaging by scanning electron microscopy (link to SEM page) and/or cathodoluminescence (CL) imaging is very beneficial in determining the presence of zoning, overgrowths and inclusions that may affect the location of spot analyses. About 30 grains can be imaged in 1 hour.
Analyses may also be made in rock sections, normally cut to 100-micron thickness and polished. Small monazites (>20 microns) can be analyzed in section. While it is possible to analyse <40 micron zircons in section, larger zircons (≥100 microns) are required for best results.
More detailed information is available from Mike Tubrett (firstname.lastname@example.org).