A laser desorption resonance ionization mass spectrometer for Rb-Sr geochronology: Sr isotope results

We are developing a portable laser desorption resonance ionization mass spectrometer (LDRIMS) for determining the radiometric age of rocks using the ⁸⁷Rb- ⁸⁷Sr isotope system, as well as constraining lithologic evolution and measuring chemical compositions. The bench-top prototype has been used to assess the capability of LDRIMS to measure ⁸⁷Sr/ ⁸⁶Sr. In this paper, we demonstrate that LDRIMS can measure the isotope ratios of a glass standard with 10 ppm net Sr to a precision of ±0.5% (1σ), with a sensitivity of 1:10 ¹⁰ in less than 1 minute. Increasing the measurement time to 15 minutes improves the precision to 0.1% (1σ). The speed of the LDRIMS measurement allows samples to be measured in significantly shorter periods of time than traditional methods, with little or no sample preparation. Models of the age error derived from isochron dating the SNC meteorites that would be obtained using 100-1000 LDRIMS measurements at ±0.1% (1σ) accuracy show that for ALH84001 and Zagami, which have ages ranging from 4.09 Ga to 165 Ma, dates with analytical uncertainties less than ±50 Ma are possible. These results were obtained using low laser powers (∼10 μJ for resonance, <1 mJ for photoionization), consistent with the potential for space flight to Mars.