Dating the Martian meteorite Zagami by the ⁸⁷Rb‐⁸⁷Sr isochron method with a prototype in situ resonance ionization mass spectrometer

Abstract

RATIONALEThe geologic history of the Solar System builds on an extensive record of impact flux models, crater counts, and ∼270 kg of lunar samples analyzed in terrestrial laboratories. However, estimates of impactor flux may be biased by the fact that most of the dated Apollo samples were only tenuously connected to an assumed geologic context. Moreover, uncertainties in the modeled cratering rates are significant enough to lead to estimated errors for dates on Mars and the Moon of ∼1 Ga. Given the great cost of sample return missions, combined with the need to sample multiple terrains on multiple planets, we have developed a prototype instrument that can be used for in situ dating to better constrain the age of planetary samples.METHODSWe demonstrate the first use of laser ablation resonance ionization mass spectrometry for 87Rb-87Sr isochron dating of geological specimens. The demands of accuracy and precision have required us to meet challenges including regulation of the ambient temperature, measurement of appropriate backgrounds, sufficient ablation laser intensity, avoidance of the defocusing effect of the plasma created by ablation pulses, and shielding of our detector from atoms and ions of other elements.RESULTSTo test whether we could meaningfully date planetary materials, we have analyzed a piece of the Martian meteorite Zagami. In each of four separate measurements we obtained 87Rb-87Sr isochron ages for Zagami consistent with its published age, and, in both of two measurements that reached completion, we obtained better than 200 Ma precision. Combining all our data into a single isochron with 581 spot analyses gives an 87Rb-87Sr age for this specimen of 360 ±90 Ma.CONCLUSIONSOur analyses of the Zagami meteorite represent the first successful application of resonance ionization mass spectrometry to isochron geochronology. Furthermore, the technique is miniaturizable for spaceflight and in situ dating on other planetary bodies. © 2014 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.

Rb‐Sr resonance ionization geochronology of the Duluth Gabbro: A proof of concept forin situdating on the Moon

“…The best indication that the Duluth Gabbro is a useful analog for KREEP basalt in this sense may come from our own analyses: none of our 310 spot analyses found 87 Rb/ 86 Sr ratios higher than 0.6. By contrast, our previous analyses of the Boulder Creek Granite found 87 Rb/ 86 Sr ratios up to ~1000, and for Zagami we found 87 Rb/ 86 Sr ratios up to ~30 . Hence, based on the highest mineral 87 Rb/ 86 Sr ratios, which most strongly constrain the slope of an isochron line, the Duluth Gabbro is ~1000 times harder to date than Boulder Creek Granite, ~30 times more difficult than Zagami, and of similar difficulty to KREEP basalts.…”

“…We also observe a slow drift, over tens of minutes to hours, in the instrumental fractionation of Rb from Sr. [e.g., ] We presently account for this drift by interpolating between successive measurements of standards to estimate instrumental fractionations at the time that each spot analysis is made, but the interpolation introduces its own uncertainty of several percent. We stress, however, that in spite of this drift, we have achieved age precision on the Duluth Gabbro that meets the NASA Technology Roadmap benchmark…”

“…The method and instrumental approach have been fully described by Anderson et al ., and are only summarized here. We use laser ablation (~5 ns pulses at 213 nm into ~100 µm spots, ~0.6 GW/cm 2 ) to vaporize a small sample of the target rock, generating ions and neutral atoms.…”

“…We analyzed our sample of the Duluth Gabbro in much the same way as we analyzed the Martian meteorite Zagami . We calculated ratios of 87 Sr/ 86 Sr, 87 Rb/ 86 Sr, and their associated correlated errors, for every spot analysis in which 85 Rb, 86 Sr, 87 Sr, 87 Rb, and 88 Sr were all detected at greater than twice the background noise level.…”

“…In that paper, we presented repeated analyses of Zagami with ages in agreement with the TIMS age of 166±6 Ma and precision better than 200 Ma. A key finding of that work was that high‐power ablation reduced the matrix dependence of the fractionation of Rb from Sr . Here, we used LARIMS to determine a radioisotopic date for an analog of KREEP‐rich lunar rocks, which are thought to constitute approximately 32% of the nearside of the lunar surface …”