Abstract
Abstract. U–Pb geochronology of calcite by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is an emerging field with potential to solve a vast array of geologic problems. Because of low levels of U and Pb, measurement by more sensitive instruments, such as those with multiple collectors (MCs), is advantageous. However, whereas measurement of traditional geochronometers (e.g., zircon) by MC-ICPMS has been limited by detection of the daughter isotope, U–Pb dating of calcite can be limited by detection of the parent isotope if measured on a Faraday detector. The Nu P3D MC-ICPMS employs a new detector array to measure all isotopes of interest on Daly detectors. A new method, described herein, utilizes the low detection limit and high dynamic range of the Nu P3D for calcite U–Pb geochronology and compares it with traditional methods. Data from natural samples indicate that measurement of 238U by Daly is advantageous at count rates < 30 000; this includes samples low in U or those necessitating smaller spots. Age precision for samples run in this mode are limited by 207Pb counts and the maximum U ∕ Pbc. To explore these limits – i.e., the minimum U, Pb, and U ∕ Pb ratios that can be measured by LA-ICPMS – a model is created and discussed; these models are meant to serve as a guide to evaluate potential candidate materials for geochronology. As an example, for samples necessitating a < 1 Ma uncertainty, a minimum of ∼ 10 ppb U is needed at a spot size of 100 µm and rep rate of 10 Hz; absolute uncertainty scales roughly with U concentration.
Highlights
Studies focused on carbonates more likely to contain high concentrations of U, such as speleothems (e.g., Richards et al, 1998), because the method employed – thermal ionization mass spectrometry (TIMS) – required weeks to produce reliable ratios; samples with a low likelihood of success, that is, those with potentially low U contents, were ignored
The instrumentation used in the study consists of a Photon Machines Excite 193 nm excimer laser equipped with a HelEx cell, coupled to a Nu Instruments Plasma 3D (P3D) for standard LA-ICPMS analyses
While there is a clear advantage of using the new Daly-only detector setup on the P3D for LA-based calcite geochronology for some samples, the extent to which this advantage obtains for all samples is still somewhat ambiguous
Summary
Calcite U–Pb geochronology by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is a relatively new technique with untapped potential for solving numerous geochronologic problems from the timing of faulting (e.g., Roberts and Walker, 2016; Nuriel et al, 2017; Goodfellow et al, 2017) and the age of ore deposits (Burisch et al, 2017) to paleoclimate, sedimentation, and diagenesis (e.g., Mangenot et al, 2018; Rasbury et al, 1997; Hoff et al, 1995; Winter and Johnson, 1995; Wang et al, 1998; Rasbury et al, 1998). LA-ICPMS has the advantage of sampling smaller volumes of material; it can take advantage of the heterogenous nature of calcite with respect to U and Pb, using larger datasets to better constrain both the initial 207Pb / 206Pb compositions and the common Pbcorrected concordia ages. These isochron ages are calculated with ease on a Tera–Wasserburg diagram similar to other common-Pb-bearing mineral chronometers like titanite and apatite (e.g., Chew et al, 2014; Spencer et al, 2013), but cal-. Kylander-Clark: Expanding the limits of laser-ablation U–Pb calcite geochronology cite lends itself to a 208Pb-based correction, given that it usually contains low levels of Th (Parrish et al, 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
