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

Abstract

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.