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SPECTRAL CHARACTERIZATION of the ANCIENT SHERGOTTITES NORTHWEST AFRICA 7034 and 8159. KJ Orr1, LV Forman1, GK Benedix1

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SPECTRAL CHARACTERIZATION of the ANCIENT SHERGOTTITES NORTHWEST AFRICA 7034 and 8159. KJ Orr1, LV Forman1, GK Benedix1 Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6177.pdf SPECTRAL CHARACTERIZATION OF THE ANCIENT SHERGOTTITES NORTHWEST AFRICA 7034 AND 8159. K. J. Orr1, L. V. Forman1, G. K. Benedix1, M. J. Hackett2, V. E. Hamilton3, and A. R. Santos. 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia ([email protected]), 2School of Molecular and Life Sciences, Curtin University, Perth, Western Australia, Australia, 3Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302 USA. 4USRA, 7178 Columbia Gateway Dr., Columbia, MD 21046. Introduction: Thermal infrared (TIR) spectros- Ga), the oldest confirmed shergottites recovered so far copy is a powerful remote sensing tool used to unravel [5]. As the only shergottites of Early Amazonian to No- the surface compositions of a target body. This tech- achian in age, they provide an invaluable opportunity to nique has been widely used in space missions, because understanding Mars’ early history. of its ability to detect and determine modal mineralogy Methods: Both samples (~0.5g chips) were ac- of the surface geology. It has been instrumental in de- quired from UNM and were made into epoxy mounts. veloping our understanding of Mars, as the majority of NWA 8159 was analyzed with a Tescan Integrated Min- missions sent to Mars have included an infrared spec- eral Analyzer (TIMA) to determine modal mineral trometer. These spectrometers can operate either in the abundancies and produce high-resolution mineral maps. visible (VIS) to near-infrared (NIR) or in the mid-infra- NWA 8159 was also analyzed using EBSD to charac- red (MIR). The results presented here primarily cover terize the crystallographic orientation of the grains. Data 5-15µm, which overlaps the THEMIS spectral range. were collected at a step size of 1 µm and an accelerating In the MIR, the bulk spectrum of the surface ac- voltage of 20 keV, using a Tescan Mira3 scanning elec- quired from a satellite can be linearly deconvolved to tron microscope (SEM) and the Oxford Instrument reproduce the modal mineralogy [1]. Deconvolution re- Symmetry CMOS detector. NWA 7034 will also be an- lies on the use of spectral end-members to accurately alyzed using TIMA and EBSD. Both samples will be deconstruct the acquired bulk spectrum. These end- analyzed a Nicolet iN10MX Infrared Imaging Micro- members are derived in the laboratory from particulate scope. They will be analyzed using a nitrogen-cooled samples of either bulk rocks and/or minerals. The ma- MCT/A (Mercury Cadmium Telluride) detector with a jority of end-members used in the deconvolution are ter- wavelength range of ~5-15µm. Though a shorter wave- restrial. Though Martian meteorite bulk spectra have length range than the MCT/B or DTGS (Deutered Trigl- also been used in deconvolutions, they don’t provide a cyerine Sulfate) detector’s, the signal to noise ratio is good match to the Martian surface geology [2], most significantly improved. This allows a much smaller spot likely because the spatial resolution of the instruments size to be viably used. Increasing the spatial resolution was too low. While the use of terrestrial end-members of the analysis will allow smaller and more complicated has significantly advanced our understanding of the ge- grains to be accurately analyzed. A 100µm spot/step ology of Mars, the use of Martian meteorite mineral size will be used to create large spectral maps of the sec- end-members may be able to provide additional infor- tions, followed by a 25µm spot/step size to analyze spe- mation and accuracy to mapping the surface geology. cific areas of interest at high resolution. Due to the rarity of Martian meteorites, the destruc- Results: Initial results indicate NWA 8159 is pri- tive technique of acquiring mineral spectra from a par- marily composed of pyroxene and plagioclase, both ticulate is not feasible or desired. However, by analyz- ~43% (Fig. 1). The pyroxene is zoned, with high-Ca au- ing thin-sections using non-destructive µ-FTIR (micro- gite cores and low-Ca pigeonite rims. The plagioclase Fourier Transform Infrared) spectroscopy, we can ac- has not been fully shocked to form maskelynite. Mag- quire spectra from specific Martian minerals using thin- netite and olivine are also present at ~6% and ~5%, re- sections. This however, introduces orientation effects spectively. Accessory phases include silica and apatite. on the spectra, which is avoided when using particu- Pyroxene and magnetite occur on the rims of the olivine lates, as they produce a randomly orientated spectra. grains, most likely due to a replacement reaction [4]. These effects can be quantified and accounted for using NWA 8159 is very fine grained at <200µm though phe- Electron Back-Scattered Diffraction (EBSD). nocrysts can be up to ~500µm (Fig. 2). There is a vary- Here we investigate MIR spectral characterization ing texture across the sample, with areas of coarser of the shergottites Northwest Africa (NWA) 7034, a grains (200-400µm) and areas of much finer grains polymict breccia, and NWA 8159, an augite basalt. (<100µm). A large shock melt vein (~600µm wide) cuts NWA 7034 contains ancient Martian crust as old as 4.39 through the sample. Maskelynite is present both within ± 0.04 Ga [3], while NWA 8159 crystallized 2.37 ± 0.25 and in close-proximity to the vein. Micro-garnets are Ga [4]. This makes them, and NWA 7635 (2.403 ± 0.14 Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6177.pdf also present inside the vein. Ca-rich veins, possibly car- Further petrological and MIR characterization of bonate ate, occur through the sample, most likely due to these meteorites and their constituent components will terrestrial weathering [6]. be presented at the meeting. Mineral orientation and spectral observations will be presented at the meeting. Discussion: NWA 7034 and NWA 8159 represent unique samples of the Martian crust. NWA 7034 is a polymict breccia, the only sedimentary Martian meteor- ite (and its pairs) so far confirmed. The igneous clasts in NWA 7034 are composed of a wide variety of igneous lithologies, representing a broad compositional range of the Martian crust [7]. In addition, there are a number of sedimentary clasts, lithic fragments of previously lithi- fied Martian regolith [7]. Both sets of clasts represent not previously sampled parts of the Martian crust and surface. NWA 8159, is classed as an ‘augite basalt’ due Figure 2. Mineral map of NWA 8159 showing the varying to the predominant clinopyroxene phase, augite, in the texture and the large shock-melt vein cutting through the meteorite. This makes it unique, as the other shergottites section. usually have a higher abundance of low-Ca pigeonite References: [1] Ramsey M. S. and Christensen P. R. than high-Ca augite. Furthermore, plagioclase that has (1998) J. Geophys. Res. 103, 577-596. [2] Hamilton V. E. et not been fully converted into maskelynite is also pre- al. (2003) Meteoritics & Planet. Sci., 38, 871-885. [3] sent, providing a constraint on the impact shock history McCubbin F. M. et al. (2016) JGR Planets, 121, 2120-2149. of this meteorite. [4] Herd C. D. K. et al. (2017) GCA, 218, 1-26. [5] Lapen T. Both these meteorites provide an opportunity to ex- J. et al. (2017) Sci. Adv. 3, 1-6. [6] Crozaz G. and Wadhwa M. (2001) GCA, 65, 971-978. [7] Santos A. R. et al. (2015) GCA, pand the Martian spectral library into compositionally 157, 56-85. [8] Cannon K. M. et al. (2015) Icarus 252, 150- diverse and more ancient fragments of the Martian crust. 153. [9] Mikouchi T. (2001) Ant. Met. Res. 14, 1-20. [10] While previous studies have spectrally analyzed these McCoy T. J. et al. (1992) GCA, 56, 3571-3582. [11] Usui T. meteorites [4,8,14], there has been limited MIR spectral et al. (2010) GCA, 74, 7283-7306. [12] Mikouchi T. and Miya- analysis of targeted regions inside these meteorites. moto M. (2001) LPS XXXII, Abstract #1644. [13] Irving A. J. Spectrally analyzing targeted areas, such as sedimentary et al. (2012) LPS XLIII, Abstract #2496. [14] Hamilton V. E. and Santos A. R. (2017) Meteoritics & Planet. Sci. 52, A6266. clasts, will enable their use in the deconvolution pro- cess, thereby providing a new avenue in mapping the surface geology of Mars. 100 90 80 Other Silica 70 60 Opaques 50 Sulphides 40 Chromite 30 Phosphates Mineral Abundance (%) 20 Plagioclase 10 Olivine 0 Pyroxene NWA 8159 NWA 8159 Los Angeles Zagami RBT 04262 Dhofar 019 NWA 7032 [4] Figure 1. Modal mineralogy of NWA 8159 compared to previously reported mineralogy [4], and typical mineralogy of shergottites from different sub-groups: Diabasic – Los Angeles [9], Fine-Grained – Zagami [10], Poikilitic – RBT 04262 [11], Olivine-Phyric – Dhofar 019 [12], Gabbroic – NWA 7032 [13]. .
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