Multi-step TIMS and CA-TIMS monazite U–Pb geochronology

• Published in: Chemical geology 2012-06, Vol.312-313, p.58-73
• Main Authors: Peterman, Emily M., Mattinson, James M., Hacker, Bradley R.
• Format: Article
• Language: English
• Subjects: Abrasion; Abrasion resistance; Age; Annealing; Chemical abrasion; chemical analysis; Dissolution; Geochronology; ionization; mass spectrometry; Monazite; Mountains; plateaus; rare earth elements; solubility; temperature; U–Pb
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2012.04.006

Multi-step chemical abrasion thermal ionization mass spectrometry (CA-TIMS) methods were developed for monazite using six samples that vary in composition and age—Amelia, Jefferson County, Burke, Elk Mountain, 554 and Madagascar. To evaluate whether the multi-step CA-TIMS approach reveals complexities in either age or composition that might be masked by single-step analysis, this study presents a side-by-side comparison of monazite samples dated via multi-step TIMS (not annealed) and CA-TIMS (annealed), and measurement of rare earth element ratios for each dissolution step. The data demonstrate three important contributions. First, annealing reduces solubility—after one partial dissolution step, not-annealed fractions were 3.3–4.1 times more digested than the annealed fractions. The difference in solubility suggests that monazite does not fully self-anneal at low temperatures. Second, multi-step TIMS analyses yielded high-precision U–Pb plateau ages for the Burke and Amelia monazites; CA-TIMS analyses yielded high-precision U–Pb plateau ages for Madagascar and Amelia monazites. Jefferson County, Elk Mountain and 554 yielded more complex results and no U–Pb plateau ages. Third, chemical analysis of partial-dissolution steps reveals heterogeneous age and compositional data for annealed samples. Because not-annealed samples yielded more consistent age and compositional data, high-temperature annealing is not recommended for monazite. Instead, optimal TIMS results are provided by slow, partial dissolution of monazite in weak acid. ► High temperature annealing reduces the solubility of monazite. ► Multi-step CA-TIMS approach yields high-precision plateau ages. ► Chemical analysis shows complex age and composition data for annealed samples. ► Not-annealed samples yield more consistent age and compositional data.