In situ Rb–Sr geochronology and geochemistry to constrain lunar volcanism

The Chemistry, Organics and Dating Experiment (CODEX) is a compact, dual-mode laser-ablation time-of-flight mass spectrometer developed for the DIMPLE payload (CLPS CP-32) to provide co-registered geochemical context and in situ Rb–Sr chronometry on the lunar surface. DIMPLE targets Ina, among the largest irregular mare patches (IMPs), to test whether IMPs record geologically recent volcanism or instead reflect ancient, highly vesicular basaltic deposits with poor small-crater preservation. Absolute ages tied to measured composition are required because morphology and crater statistics alone are ambiguous for these terrains. The CODEX architecture couples 266 nm UV laser-ablation mass spectrometry (LAMS) for major and trace-element mapping (m/z 1–250) with laser-ablation resonance-ionization mass spectrometry (LARIMS) for selective, interference-free Rb and Sr isotope measurements that mitigate the ⁸⁷Rb/⁸⁷Sr isobar without relying on extreme mass resolving power. We are currently commissioning the CODEX Engineering Development Unit (EDU). First LAMS measurements on calibration samples show m/Δm ≈ 300–400 (FWHM) across the targeted range and clear isotopic structure (e.g., resolved Fe and Pb isotopes), indicating robust transmission and margin for compositional mapping. Ongoing work is extending these EDU results toward resonance-ionization operation to validate the end-to-end Rb–Sr measurement chain and quantify isotope performance under representative conditions.