MONAZITE STANDARD ASSESSMENT BY LA-ICP-MS

Three monazite samples—‘Iveland', ‘Elk Mountain', and ‘Jefferson County' (supplied by Mike Bersch)—were compared against monazite standard ‘44069' (supplied by John Aleinikoff) to assess their suitability as geochronology standards. Back-scattered electron images of the grains are homogeneous down to the micron scale. U, Th, and Pb isotopic measurements were made with a New Wave (Merchantek) 193 nm ArF excimer laser operated in 20-sec long bursts at 4 Hz, 32 mJ, and a spot size of 10 ěm, coupled to a GV Isoprobe multiple-collector inductively coupled plasma mass spectrometer. The ablated material was transported using a mixture of Ar and He carrier gases where it underwent ‘dry' nebulization. The carrier gases have sufficiently low Hg abundance that any Hg in the 204 mass position was subtracted by measuring on-peak backgrounds prior to sample analysis. Mass 204 was measured using an ion-counting channel whereas masses 206, 207, 208, 232 and 238 were measured with Faraday detectors; all analyses were conducted in static mode. Each blank and unknown measurement consisted of 20 separate 1-second counts. Isotopic ratios corresponding to a ‘zero pit depth' were determined by regression of the ratio-vs.-time data. Further data reduction and plotting were completed with Isoplot [Ludwig, 2001]. All ages are reported at the 95% C.I.

Standard 44069 yielded weighted mean 232Th/208Pb, 206Pb/238U, and 207Pb/206Pb ages with precisions of 1.2%, 1.3%, and 33%, respectively, that were used to determine the relative ages of the unknown monazites. 232Th/208Pb spot ages from the Iveland monazite are heterogeneous, ranging from 813.6 ± 20 Ma to 908.4 ± 19 Ma, rendering it unfit as a standard. The Elk Mountain monazite has ~49000 ppm Th, ~1500 ppm U, and gave equivalent weighted mean 232Th/208Pb (1386 ± 27 Ma) and 206Pb/238U ages (1444 ± 64 Ma). Weighted mean 232Th/208Pb and 206Pb/238U ages for the Jefferson County monazite were also equivalent at 357.5 ± 5.3 Ma and 354.6 ± 6.1 Ma, respectively with ~28000 ppm Th and ~4000 ppm U. The single populations of spot ages for the Elk Mountain and the Jefferson County monazites indicate that both can be used as standards.