Baddeleyite SK10-3: A Natural Reference Material for Microbeam U-Pb Geochronology and Hf Isotopic Measurements

Abstract

Baddeleyite is an important U-Pb geochronometer and Hf isotope tracer that commonly occurs as an accessory phase in silica-undersaturated igneous rocks of terrestrial and extra-terrestrial origin. Currently, very few well-characterised, large sized reference materials are available for baddeleyite U-Pb geochronology and Hf isotope measurement. In this study, we document a baddeleyite reference material (SK10-3) of Cenozoic age. SK10-3 is inclusion-free and does not contain secondary alteration minerals. The baddeleyite has uniform U-Pb ages and Hf isotope ratios, within analytical uncertainty, as demonstrated by multiple LA-ICP-MS spot analyses (weighted mean ²⁰⁶Pb/²³⁸U age: 31.59 ± 0.11 Ma, MSWD = 0.7, n = 197) and LA-MC-ICP-MS analyses (arithmetic mean ¹⁷⁶Hf/¹⁷⁷Hf ratio: 0.282741 ± 59, 2s, n = 188). Seven ID-TIMS analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 31.592 ± 0.020/0.022/0.040 Ma (n = 7, 2s, MSWD = 2.2). Nine aliquots of MC-ICP-MS analyses yielded an arithmetic mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282742 ± 8 (2s). We further demonstrate that the method of shallow-pit (~ 2 μm depth) ablation substantially improves the precision and accuracy of baddeleyite U-Pb ages. SK10-3 has a relatively high ¹⁷⁶Yb/¹⁷⁷Hf ratio (~ 0.007) compared with most other baddeleyites, allowing the precise measurement of β_Yb and may be useful in generating the β_Yb-β_Hf relationship during LA-MC-ICP-MS Hf isotope measurement. SK10-3 may be a useful addition to previously distributed baddeleyite reference materials for microbeam-based U-Pb geochronology and Hf isotope measurements.