New Shock Recovery Experiments on Lunar-Composition Plagioclase

Introduction: The Moon has undoubtedly been transformed by impact cratering, with much of the lunar surface consisting of an impact gardened regolith. The micro-scale processes of lunar space weathering –a combination of both irradiation and impacts –has been the focus of many recent studies, but largely centered on micrometeorite irradiation. Here we present new spectroscopic analyses of shock recovery experiments on plagioclase with a composition (An92) consistent with what is found in the lunar samples. This is important because previous shock studies on feldspar have been conducted on less-calcic plagioclase and primarily used for X-Ray and optical analyses. Our work is particularly relevant to recent and upcoming remote sensing missions which use visible and infrared spectroscopy.

Methods: We conducted shock experiments on the Stillwater Complex, Montana, anorthosite is a medium grained (~2-6 mm) anorthosite comprised of highly calcic plagioclase grains (An92). Starting material consisted of two suites: slices of the rock which preserve mineral fabrics, and powders designed to simulate the porous nature of the lunar regolith. Samples were cut into circular slices roughly 4 mm in radius and ~1 mm thick to comply with the experimental set-up. The particle size making up the powders is approximately 30 µm. Experiments are ongoing, using the flat-plate accelerator at the Experimental Impact Laboratory at NASA JSC. Ultimately, a range of shock pressures between 3 and 70 GPa will be covered. As of the time of abstract submission, we have recovered 11 samples: 6 from the powders and 5 from the slices.

Results: The powdered samples show an overall difference in spectral shape, which is expected when switching between fine particulate and solid/slab samples (as the post-shock samples are now compressed into slabs). These effects include the flattening of the region from 2000-1300 cm-1, the increase in band depth for the reststrahlen band region (~1200-900 cm-1), and the loss of the transparency feature near 850 cm-1.

After accounting for the changes due to particle size/slab, we can see some of the changes caused by the shock pressures. With increasing pressure, there is a loss of the distinct peaks seen at 1850, 1800 and 1600 cm-1, a shift in the Christiansen feature position from ~1300 to ~1250 cm-1, and a shift from 2 reststrahlen band features to a broader 1 in both the 1200 and 500 cm-1 regions. The observed changes in spectral features are what we would expect to see as the crystalline structure is disrupted with increasing pressures.

For shocked slices, the post-shock spectrum shows a shift in both position to longer wavenumbers (shorter wavelengths) and band depth (weaker and broader, though still two peaks) of the major features. The increase in slope in the 2000-1300 cm-1 region is also detectable at this (relatively) low shock pressure of 10 GPa.