New IR mineralogy and Rb-Sr geochronology measurements of Martian meteorites

Abstract:

We are using coordinated IR spectroscopy and Rb-Sr geochronology measurements to map the mineralogy and assess the history of Martian meteorites. By integrating these techniques, we can identify primary and biased isochrons, and place them in mineralogical context, which is especially valuable for complex brecciated meteorites like NWA 7034. Our measurements are composed of hundreds of IR and Rb-Sr measurements made across a sample. The IR reflectance measurements are made with a Thermo Scientific iN10 Fourier transform infrared microscope (μFTIR) to map the dominant mineral signatures, with spot sizes that can range from 10 - 300 µm and making maps that can have spatial resolutions from 10 - 300 µm. Rb and Sr isotopes are measured with a prototype laser ablation resonance ionization mass spectrometer using 70 μm spots and 250 μm spacing. Using these techniques for well-behaved samples like Zagami, we have been able to determine an age of 280±100 Ma (vs 166 Ma via traditional methods). For some samples that have experienced multiple alteration events, we are able to identify multiple isochrons, each associated with a specific mineralogy. NWA 7034 is known to host multiple ages and lithologies, and hence is ideally suited to high-resolution Rb-Sr and mineralogy mapping analysis. We have completed the IR measurements of NWA 7034, and will report on these results synthesized with Rb-Sr isotope measurements. In addition to aiding the geochronology measurements, the IR spectra also provide new, phase-isolated data for interpreting orbital and landed datasets. These spectra are particularly useful for representing phases such as pigeonite that are difficult to isolate physically and vary compositionally from their terrestrial counterparts.