Xenotime dating

McNaughton, N.J., Rasmussen B. Fletcher I.R. (1999) SHRIMP uranium-lead dating of diagenetic xenotime in siliciclastic sedimentary rocks. Science 285, 78-80. 

Fletcher, I.R., Rasmussen, B. McNaughton, N.J. (2000) SHRIMP U-Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins. Australian Journal of Earth Sciences 47, 845-859. 

Mineral inclusions. In our experience at Caltech the single biggest difficulty in He dating of apatite is the presence of small U-Th-rich inclusions within the dated grains. This difficulty was first noted by Lippolt et al. (1994) and was further described by House et al. (1997). The most common inclusions are zircon and monazite, but we have occasionally encountered xenotime and allanite. We have also observed inclusions of quartz, feldspar, pyrite and graphite( ), but these are unlikely to carry sufficient U and Th to be a problem. 

Background. The phosphate minerals apatite, monazite and xenotime have strongly variable retention properties for Pb under crustal conditions. The mechanisms by which the daughter product can be lost include dissolution/reprecipitation reactions, recrystallization, and diffusive loss. The latter mechanism is likely a common source of discrepancy between a mineral date and the age of the rock from which it formed. 

The diffusion data presented above indicate that all three phosphates quantitatively retain He at Earth surface temperatures (see Wolf et al. 1998 for a discussion of the minimal affects of diurnal heating and forest fires on He diffusion from apatite). As a result, all three minerals may be used to accurately date the formation of quickly cooled rocks, such as volcanics. Apatite has already been used for this purpose (Stockli, in preparation Farley et al. 2002). Because these phases are rare in volcanic rocks, the more likely application of phosphate He chronometry is to assessment of cooling histories. In slowly cooled rocks, the quantity t computed from the age equation (Eqn. 4) is an apparent age, which may or may not correspond to a specific geologic event. The simplest way to interpret such an apparent age is to associate it with cooling through a particular closure temperature, computed from the diffusion parameters (Dodson 1973). The diffusion arrays in Figure 5 indicate closure temperatures of 115°C and 220°C for xenotime, and monazite, respectively. For apatite, in which diffusivity is known to scale with inverse square of grain radius, the diffusivity data indicate a closure temperature of 73°C for a prism half-width of 75 pm. Half-widths of 50 and 100 pm yield closure temperatures of 68 and 77°C respectively. 

At this date, the principal ore minerals for the REEs are monazite, bastnaesite, and xenotime. The first REE mineral to be used was gadolinite, and from this mineral, several of the REE were first isolated, but it was not applied on an industrial scale. The first REE ore mineral from which REE were extracted for industrial use was monazite. 

Xenotime (YPO4). Another phosphate of some dating interest is xenotime (Ivantishin et al, 1962 Komlev et al, 1961 Lyakhovich, 1961). 

---