OPAL AS A U-PB GEOCHRONOMETER: SEARCH FOR A STANDARD

Abstract

We report structural, geochemical, and TIMS U-Th-Pb and U-series isotopic data for several samples of precious and common opal, which we tested as potential standards for ion microprobe and LA-ICPMS isotopic studies. The precious opals are low in U, contain high concentration of common Pb, and are not suitable for U-Pb age determinations. In contrast, all studied samples of common opal are enriched in uranium and contain sufficiently radiogenic Pb for precise calculations of 206Pb/238U and 207Pb/235U dates. Complications in the U-Pb systematics are due to the presence of ubiquitous unsupported 206Pb, probably derived from initial excess 234U and possibly other intermediate products of 238U decay, elevated concentrations and possibly anomalous isotopic composition of initial Pb in some of the opals, and the presence of two opal generations of different ages in one of the samples. Despite these imperfections, two out of five studied common opals are acceptable as U-Pb and U-series standards, at least in the interim. The data obtained in this study can be used to guide further search for a better opal standard. © 2006 Elsevier B.V. All rights reserved.