Characterizing Effects of Shock Experiments on Plagioclase in the Infrared

CHARACTERIZING EFFECTS OF SHOCK EXPERIMENTS ON PLAGIOCLASE IN THE INFRARED. K. A. Shirley1, S. J. Jaret2, K. L. Donaldson Hanna3, C. J. Cline4, M. J. Cintala4, M. D. Mouser4, K. D. Burgess5, M E Gemma6, 1University of Oxford, Oxford UK, ([email protected]), 2Kingsborough Community College, City University of New York, NY, USA. 3University of Central Florida, Orlando, FL, USA, 4NASA JSC, Houston, TX, USA, 5U.S. Naval Research Laboratory, Washington, D.C., USA, 6Stony Brook University, Stony Brook, NY, USA. Introduction: The lunar landscape has been transformed due to space weathering, encompassing both irradiation and impact effects. It can be difficult to disentangle these comingled processes, however, understanding the contribution from impacts is important throughout the Solar System. The effect of impacts has been studied in several cases [e.g. 1,2], but they have not fully characterized the effects of shock on both the VNIR and MIR on highly calcic plagioclase, the composition of plagioclase thought to dominate the lunar highlands. Here we will examine the effects of shock on an anorthosite sample from the Stillwater Igneous Complex (Montana, USA), often used as a lunar analogue. To specifically investigate the effects of impacts, we will carry out a series of shock experiments on both rock slices and powdered (< 30 m) samples. We will characterize the anorthosite slices and powders before and after shock experiments to see how spectral features are affected with increasing degrees of shock. While we will be unable to reconstruct the original position and orientations of the pre-shock sample mineralogy, we will be able to compare each sample to its pre-shock counterpart. These in-depth analyses will allow us to draw general conclusions about the transformations undergone during shock in a true rock sample with multiple crystals with boundaries and various orientations as we would expect in a lunar rock. In this abstract we describe the spectral changes observed within the samples from the first series of shock experiments. Methods: The Stillwater Complex anorthosite is a medium grained (~2-6 mm) anorthosite comprised of highly calcic plagioclase grains (An92) [3,4]. Samples were cut into circular slices roughly 4 mm in radius and ~1 mm thick to comply with the experimental set-up. The powders are roughly on the order of 30 µm particle size. Samples were sent to the flat-plate accelerator at the Experimental Impact Laboratory at NASA Johnson Space Center (JSC) to be prepared and shocked to a range of pressures between 3 and 70 GPa. Experiments are ongoing, but as of the time of abstract submission, we have recovered 11 samples: 6 from the powders and 5 from the slices. Samples were returned in their metal casings, mostly as cohesive discs within; however, at least 2 of those from the powders broke apart when removed from the cases. We note that all data presented here are from cohesive samples (aside from the two that broke) and slice vs powder designations refer to their pre-impact state. Prior to shock experiments, the slices were imaged using Back Scatter Electron (BSE) as well as measured using visible and infrared microscopy and diffuse reflectance spectroscopy [5]. Post-shock, the samples were again characterized with visible to near infrared spectroscopy (both diffuse and microspectroscopy) using the Planetary Spectroscopy Facility at University of Oxford. Infrared diffuse reflectance spectroscopy and microscopy was carried out using a Bruker Vertex v70 spectrometer and a Hyperion 2000 microscope. The diffuse measurements use a wide-range beamsplitter coupled with a DTGS detector to measure reflectance from ∼6,000–100 cm−1 (1.6-25 µm) at 4 cm-1 resolution calibrated to a diffuse gold standard. The microscopy measurements take visible images and IR reflectance measurements through a 15x Schwartzchild Objective using a cooled MCT detector with wide-range beamsplitter and are calibrated to the internal gold standard to measure over 0.8-25 µm at 4 cm-1 resolution. Figure 1. Diffuse reflectance spectra of the unshocked powder (black) compared to powder shocked at increasing pressures (colors with estimated shock pressures shown). Results: The powdered sample spectra show a marked difference across the full spectral range that 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 at ~850 cm-1 (Fig. 1). After accounting for this change in spectral features, we can see some of the changes caused by shock at increasing levels. With increasing pressure, there is a loss of the distinct peaks observed 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 single broader feature in both the 1200 and 500 cm-1 regions. These losses in distinct features are on par with what we would expect to see as the crystalline structure is disrupted with increasing pressures. Figure 2. Diffuse reflectance spectra of the slices before (black, slice A5) and after shock (colors with associated pressures and pre-shock slice names). We do not have to account for the differences in powder vs slab for the slices, so the pre-shock material looks more consistent spectrally (Fig. 2). However, each slice had slightly different starting compositional constituents than the homogenized powders; while they were all majority plagioclase, there was a measureable (and variable) amount of quartz in each. We do know the characterized slice for the majority of these samples, so preand postcomparisons can be made (Fig. 3). In general, we see a loss of two separate Reststrahlen band features (~1175 & 950 cm-1), but not necessarily trending with increasing shock pressure. Interestingly, there does seem to be a change in slope of the ~2000-1300 cm-1 region with increasing shock pressure that is also present in the powder samples. Figure 3 shows a more direct comparison between the ‘C1’ slice preand post-shock. The preshock data are averaged from microspectroscopy maps shown in [5] for both sides of the slice. In the post-shock spectrum we see 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 observed at this (relatively) low shock pressure of 10 GPa. Figure 3. Comparison of the same slice pre- (colors) and post-shock (black). Summary: The careful characterization of the variations in our samples prior to subjecting them to shock experiments has provided a rich base for spectral analysis, and in this initial set of samples we observe variations trending with shock pressures. Our continued analyes of these samples with microFTIR spectroscopy will provide further evidence for the effect of shock pressure on spectral features, and will allow a better understanding of the variation in spectral effects/shock experienced within the same small (~1 cm diameter) sample. These analyses will help disentangle the effects of space weathering due to shock versus those due to irradiation, and allow better interpretation of remote sensing data like that from M3, Diviner, and upcoming infrared landed instrumentation. References: [1] Johnson et al., (2002) JGR 107, 5037 [2] Pernet-Fisher et al., (2016) Sci Reports 7 [3] McCallum, I. S., Raedeke, I. D., & Mathez, E. A. (1980) AJS 280A, 59-87 [4] Czamanske, M. L. & Zientek, G. K., (1985) Montana Bureau of Mines and Geology Special Publication 92 [5] Shirley et al. (2024) ELS