The Diamantina Monazite: A New Low‐Th Reference Material for Microanalysis

Most monazite reference materials (RMs) for insitu U-Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix-matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low-U and low-Th Diamantina monazite from the Espinhaco Range, SE Brazil. It has a U-Pb ID-TIMS weighted mean Pb-207*/U-235 ratio of 0.62913 +/- 0.00079, Pb-206*/U-238 of 0.079861 +/- 0.000088 and Pb-207*/Pb-206* of 0.057130 +/- 0.000031, yielding a weighted mean Pb-206*/U-238 date of 495.26 +/- 0.54Ma (95% c.l.). In situ dates acquired with different methods (LA-(Q, SF, MC)-ICP-MS and SIMS) are within uncertainty of the ID-TIMS data. U-Pb LA-(Q, MC)-ICP-MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA-MC-ICP-MS analyses yielded homogeneous Sm-Nd isotopic compositions (Nd-143/Nd-144 = 0.511427 +/- 23, 2s; Sm-147/Nd-144 = 0.1177 +/- 13, 2s) and epsilon Nd-(495 (Ma)) of -18.7 +/- 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than +/- 0.3 parts per thousand (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng.