Obtaining and Maintaining the Distribution of Natural Reference Materials for U-Pb Geochronology by LA-ICP-MS: Protocols and Future Perspectives

Over the past few decades, the great increase in the use of high-spatial resolution techniques, such as Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), for geochronology had a great impact in constraining the timing and rates of geological processes. As most of these techniques, especially LA-ICP-MS, are destructive by nature, they require a constant supply of well-characterized reference materials (RMs) that are used for both calibration and quality control. As general features, an RM should be isotopically homogeneous, have moderate to high U contents (tens to hundreds of ppm), negligible initial (non-radiogenic) Pb contents (Pbc), and be available in large quantities of coarse-grained, gemquality crystals for distribution. It is also desirable to have a range of RMs of a given mineral with different chemical compositions (e.g., Th, REE) in order to facilitate the correction of matrix-dependant isotopic fractionation. In this sense, some of the Brazilian Ediacaran-Cambrian orogens (such as the Araua and Borborema), related to Gondwana amalgamation, have proven to be ideal suppliers of such natural megacrysts due to their great number of gem-producing pegmatites and quartz veins. In the past, a number of RMs from this region have been proposed, both from pegmatites and hydrothermal quartz-veins. These include the Bananeira monazite [1], which has a 207 Pb/ 235 U age of 507.7 1.3 Ma (2s) and moderate Th contents (~6.0 wt%), the hydrothermal Diamantina monazite [2], with a 206 Pb/ 238 U age of 495.26 0.56 Ma (2s) and low-Th (~0.11 wt%), the alluvial/hydrothermal Datas xenotime [3], with 206 Pb/ 238 U ages of 513.4 0.9 and 515.4 0.8 Ma (2s) and a total REE (oxide) of ~19.0 Wt.%, and the hydrothermal Antonio Pereira rutile [4], with 206 Pb/ 238 U age of 499.4 1.1 Ma and U contents of ~ 45 ppm.