Improvements to an In-Situ Geochronology, Geolocation, and Nuclear Forensics LARIMS Instrument, 15-R9859

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Principal Investigators
F. Scott Anderson
Chris Anderson

Inclusive Dates:  08/01/08 – 09/01/09

Background - SwRI is developing a portable, real-time laser desorption resonance ionization mass spectrometer (LDRIMS) to enable in-situ, real time studies of 1) geochronology, 2) geolocation of chemical and nuclear materials, and 3) the interaction between light and matter. The LDRIMS instrument is capable of making extremely sensitive measurements of a wide range of difficult-to-measure isotopes in a portable and real-time mode. SwRI is currently focused on precise measurement of strontium (Sr), as precise measurement of the isotopic ratio of Sr enable the dating of rocks on Mars and other planets is a high priority goal for NASA.

Approach - The goals of this project were:

  • Task 1: Develop a multi-bounce resonance ionization stage
  • Task 2: Determine minimum power levels for efficient resonance ionization
  • Task 3: Characterize and optimize the geometry of laser desorption
  • Task 4: Characterize and optimize removal of prompt laser desorption ions
  • Task 5: Determine the optimal wavelength for neutral desorption
  • Task 6: Study the formation of ablation pits
  • Task 7: Determine the optimal approach to reducing Rb-Sr interferences
  • Task 8: Develop data reduction techniques and automated software control
  • Task 9: Install multi-bounce time-of-flight (MBTOF) mass spectrometer in LDRIMS design

Accomplishments - SwRI's prototype LDRIMS has had excellent success exciting Sr with 461 (<10µJ), 554 or 496 nm (<10µJ), and 1064 nm light (<1mJ), and Rb with 776, 780, and 1,064 nm light. Laser desorption can cause broadened, overlapping peaks; however, the MBTOF easily separates them with a resolution of approximately 5,000 after 2 to 5 bounces. Accuracy and precision of up to ±0.5 percent are repeatably achieved for 10 ppm net Sr samples for measurements with 1,000 shots on a single grain/spot, with SNR typically exceeding 1,000 on a 50 ppb 84Sr peak in under one minute of analysis. The results show that for 1,000 spots, at the current precision and accuracy (±0.5%), we could obtain ±50Ma dates for the Martian meteorite Zagami, and ±90Ma ALH84001. The power requirements of the lasers required to produce these results are an order of magnitude lower than anticipated, dramatically reducing the complexity of the laser subsystem, and requiring <30W in total. Traditional designs to produce these low-power, tunable systems consistent with space flight are possible; SwRI is currently seeking funding for this development. Furthermore, the requirements for the mass spectrometer and desorption laser system have largely been met by existing systems developed for space flight. And finally, the LDRIMS Rb-Sr technique is consistent with the largely basaltic surface materials found throughout the terrestrial planets, and specifically the Mars-derived SNC meteorites, without suffering from 40Ar contamination issues in primary igneous rocks or volcanically derived sediments like the K-Ar approach, and has in fact been applied under constrained circumstances to metamorphic and sedimentary materials as well. In summary, the astonishing new results for strontium are enabling for in-situ geochronology on Mars.

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