Publication Date:
2017-09-24
Description:
Most monazite reference materials (RMs) for in situ 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 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 Espinhaço Range, SE Brazil. It has a U-Pb ID-TIMS weighted mean 207 Pb*/ 235 U ratio of 0.62913 ± 0.00079, 206 Pb*/ 238 U of 0.079861 ± 0.000088 and 207 Pb*/ 206 Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206 Pb*/ 238 U date of 495.26 ± 0.54 Ma (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 less than 1% deviation. The LA-MC-ICP-MS analyses yielded homogeneous Sm-Nd isotopic compositions ( 143 Nd/ 144 Nd = 0.511427 ± 23, 2 s ; 147 Sm/ 144 Nd = 0.1177 ± 13, 2 s ) and εNd (495 Ma) of -18.7 ± 0.5 (2 s ). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2 s ) confirming Diamantina's relative homogeneity at test portion masses below 1 ng. This article is protected by copyright. All rights reserved.
Print ISSN:
1639-4488
Electronic ISSN:
1751-908X
Topics:
Geosciences
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