A Novel Laser-Ablation Resonance-Ionization Mass Spectrometer for In Situ Dating of Lunar Rocks

In situ geochemical and geochronological measurements on the Moon are needed to test models of late-stage lunar volcanism and to interpret crater-retention anomalies on terrains appearing geologically young. The irregular mare patch Ina is a key case: its exceptionally low crater density has been interpreted as very recent volcanic activity, but non-chronologic explanations-unusual surface mechanical properties or resurfacing-remain plausible. Resolving these hypotheses requires radiometric ages that are co-registered with petrologic and geochemical context at the scale of individual rock and mineral domains under the mass-volume-power constraints of landed payloads. We report the architecture of the Chemistry, Organics and Dating Experiment (CODEX), a compact dual-mode time-of-flight mass spectrometer developed for NASA's DIMPLE payload targeting Ina via CLPS. CODEX combines laser-ablation mass spectrometry (LAMS) for major- and trace-element mapping with laser-ablation resonance-ionization mass spectrometry (LARIMS) for interference-free$\text{R b - S r}$isotope analysis in a single-reflection, off-axis reflectron analyzer. In LAMS mode, CODEX produces elemental maps over$\mathrm{m} / \mathrm{z} \mathrm{1}-\text{2 5 0}$with mass resolution$\mathrm{m} / \Delta \mathrm{m}$of about$300-400$(FWHM) to establish mineralogical context. In LARIMS mode, primary (prompt) ablation ions are suppressed and neutral$\text{R b}$and$\text{S r}$are selectively ionized, mitigating the${ }^{87} \text{Rb} /{ }^{87} \text{Sr}$isobar without requiring ultra-high mass resolution. Three-dimensional ion-optical modeling (SIMION) and a vented, shared ion-source geometry enable rapid electrostatic switching between modes. This two-in-one instrument is implemented in a flight-oriented opto-mechanical architecture compatible with GEVS environments and CLPS-class packaging. CODEX is designed to return co-registered elemental and isotope datasets that can constrain the timing of emplacement at Ina and inform interpretations of the Moon's late-stage thermo-magmatic evolution.