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 207Pb/206Pb 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.