| Summary: | <p>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 <span class="inline-formula"><sup>238</sup></span>U by Daly is advantageous at count rates <span class="inline-formula"><i><</i></span> 30 000; this includes samples low in U or those necessitating
smaller spots. Age precision for samples run in this mode are limited by
<span class="inline-formula"><sup>207</sup></span>Pb counts and the maximum U <span class="inline-formula">∕</span> Pb<span class="inline-formula"><sub>c</sub></span>. To explore these
limits – i.e., the minimum U, Pb, and U <span class="inline-formula">∕</span> 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 <span class="inline-formula"><i>a</i></span> <span class="inline-formula"><i><</i></span> 1 Ma uncertainty, a
minimum of <span class="inline-formula">∼</span> 10 ppb U is needed at a spot size of 100 <span class="inline-formula">µ</span>m and rep rate of 10 Hz; absolute uncertainty scales roughly with U
concentration.</p>
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