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Rationale  We report new  87 Rb‐ 87 Sr isochron data for the Duluth Gabbro, obtained with a laser ablation resonance ionization mass spectrometer that is a prototype spaceflight instrument. The gabbro has a Rb abundance and a range of Rb/Sr ratios that are similar to those of KREEP‐rich basalts found on the nearside of the Moon. Dating of previously un‐sampled young lunar basalts, which generally have a KREEP‐rich composition, is critical for understanding the bombardment history of the Moon since 3.5 Ga, which in turn informs the chronology of the solar system. Measurements of lunar analogs like the Duluth Gabbro are a proof of concept for  in situ  dating of rocks on the Moon to constrain lunar history.    Methods  Using the laser ablation resonance ionization mass spectrometer we ablated hundreds of locations on a sample, and at each one measured the relative abundances of the isotopes of Rb and Sr. A delay between the resonant photoionization processes separates the elements in time, eliminating the potential interference between  87 Rb and  87 Sr. This enables the determination of  87 Rb‐ 87 Sr isochron ages without sophisticated sample preparation that would be impractical in a spaceflight context.    Results  We successfully dated the Duluth Gabbro to 800 ± 300 Ma using traditional isochron methods like those used in our earlier analysis of the Martian meteorite Zagami. However, we were able to improve this to 1100 ± 200 Ma, an accuracy of <1σ, using a novel normalization approach. Both these results agree with the age determined by Faure  et al . in 1969, but our novel normalization improves our precision.    Conclusions  Demonstrating that this technique can be used for measurements at this level of difficulty makes ~32% of the lunar nearside amenable to  in situ  dating, which can complement or supplement a sample return program. Given these results and the scientific value of dating young lunar basalts, we have recently proposed a spaceflight mission called the  Moon Age and Regolith Explorer (MARE) . © 2015 The Authors and Southwest Research Institute.  Rapid Communications in Mass Spectrometry  published by John Wiley & Sons Ltd.

Rb‐Sr resonance ionization geochronology of the Duluth Gabbro: A proof of concept for in situ dating on the Moon