Pb‐Pb Dating of Terrestrial and Extraterrestrial Samples Using Resonance Ionization Mass Spectrometry

Abstract

We are developing an in situ, rock-dating spectrometer for spaceflight called the Chemistry, Organics, and Dating EXperiment (CODEX). CODEX will measure Rb-Sr compositions and determine ages of samples on the Moon or Mars and can be augmented to obtain Pb-Pb ages. Coupling Rb-Sr and Pb-Pb measurements broadens the suite of samples that can be dated and could provide tests of concordance. Here we assess whether geochronologically meaningful Pb-Pb data could be measured in situ by tuning the prototype CODEX to acquire Pb-Pb data from a suite of well-characterized specimens from the Earth, Moon, and Mars. For Keuhl Lake Zircon 91500 our 207 Pb/ 206 Pb age of 1,090 ± 40 Ma is indistinguishable from the accepted age. In each of the Martian meteorites we studied, we could not resolve more than a single component of Pb and could not uniquely determine ages; nevertheless, our isotopic measurements were consistent with most previous analyses. On the other hand, we uniquely determined ages for three lunar meteorites. Our age for MIL 05035 is 3,550 ± 170 Ma, within 2σ of published ages for this specimen, in spite of it having <1 ppm Pb. LAP 02205 was contaminated by terrestrial Pb, but by filtering our data to exclude the most contaminated spots, we obtained an age of 3,010 ± 70 Ma, coincident with published values. Finally, our age for NWA 032 is nearly 1,000 Ma older than its age determined from other isotopic systems and is supported by additional Pb measurements made after chemical leaching.

Dating Granites Using CODEX, with Application to In Situ Dating on the Moon

“…For unaltered rocks, Equation (1) predicts linear variation between measured 87 Rb/ 86 Sr ratios and 87 Sr/ 86 Sr ratios among the minerals in a rock crystallized at a single time; the slope of the line relating these measurements is the factor e λ t − 1, and so determines the age t. A key advantage of isochron analysis over single measurements of the radioactive isotope and its daughter product is that differential alteration of rock can be diagnosed if the measurements do not obey the predicted linear relationship. In addition to our work in the Rb/Sr system, we have also demonstrated the use of our breadboard CODEX for Pb-Pb isochron dating with a precision of 50 Myr (Anderson et al 2020a).…”

“…However, there remain many important questions about the history of the Moon that the Apollo samples cannot address (e.g., Tartèse et al 2019), because sampling was sparse, most collected rocks are unprovenanced, and most landing sites were affected by the Imbrium impact (e.g., Spudis et al 2011). We (Anderson et al 2020a) and other authors (Zellner 2017;Cohen et al 2021) have recently outlined the most important outstanding questions of lunar chronology, so we present only a brief summary here, and we call out for particular mention the questions that concern granitic rocks. In this section, we discuss the unknowns of lunar dating in the context of the major stratigraphic units recognized on the Moon (Shoemaker & Hackman 1962;Wilhelms et al 1987), and we calculate the dating precision necessary to answer those key questions.…”

“…If the latter, a dynamical mechanism is required for suddenly delivering a large number of giant meteoroids to the Moon long after most planetary accretion had concluded, but if Nectaris is older, no such "lunar cataclysm" (Tera et al 1974) may have occurred. To distinguish with 95% confidence among an "old" Nectaris that formed 4250 Ma, a Nectaris that is 300 Myr younger, or an intermediate age, we have argued (Anderson et al 2020a) that its melt sheet or ejecta would need to be dated with ∼75 Myr precision (all uncertainties herein are 1σ). This precision requirement is stronger than that proposed by Cohen et al (2021) Seddio et al (2014) of lunar granophyres found that 11 out of 14 were older than 3900 Ma.…”

“…Addressing this question will require additional age measurements from Eratosthenian terranes. Given the magnitude of the discrepancy between the proposed chronologies, the exquisite precision afforded by a sample-return mission, as demonstrated by Li et al (2021), is not required; rather, it is sufficient to achieve dating precision of a few hundred million years, so in situ dating experiments potentially have much to contribute (Anderson et al 2020a).…”