An Investigation of Jarosite and Associated Alteration Mineralogy in Martian Meteorite Roberts Massif 04262 Using Micro-Raman Spectroscopy

47th Lunar and Planetary Science Conference (2016) 1311.pdf

An investigation of jarosite and associated alteration mineralogy in Martian Meteorite Roberts Massif 04262 using Micro-Raman Spectroscopy. B. E. McKeeby1, S. Mahmood1, M. Lowe1, and J. P. Greenwood1. 1Dept. of Earth & Environmental Sciences, Wes- leyan University, 265 Church St., Middletown, CT 06459

Introduction: Sulfate minerals have been discov- the RRuff™ spectral database, a combination of natro- ered at various locations on the Martian surface [1] as jarosite (Na-rich), hydronium-jarosite, and jarosite (K- well as in Martian meteorites; MIL 03346[2], EETA rich) (Fig. 1B) was identified [7][8]. Raman mapping 79001[3], and RBT 04262[4]. RBT 04262 has been of the region detected predominantly K-jarosite, how- identified as a lherzolitic shergottite [5]. Primary min- ever point spectra identified pockets of natrojarosite. eralogy is composed of coarse grained olivine, pyrox- Additionally in two of the Raman spectra collected, a ene, and maskelynite with minor amounts of oxides, minor manganese phosphorus phase was found inter- and phosphates [6]. Weathering of Martian basalts to mixed with jarosite. Using spectral matching this was form calcium sulfates such as gypsum and iron sulfate identified as frondelite. Previously EDS spectra were minerals like jarosite has been identified as one possi- collected for this region showing enrichment in sulfur ble formation mechanism for sulfates found in Martian and iron, with lesser amounts of sodium, phosphorus meteorites. This suggests the presence of an acidic oxi- and potassium (Fig. 1A). These assemblages support dizing fluid present within the meteorite. Although the Raman conclusion of a combination of natrojarosite weathering of RBT 04262 has likely transpired in Ant- and jarosite. arctica [4], it has some interesting implications for Three other weathered sulfate grains were investi- Mars. It might be expected that a similar weathering gated for presence of jarosite alteration products. Ra- process of iron sulfides leads to increasingly acidic man spectroscopy showed that vein fill was instead fluids, enhancing the dissolution of calcium phosphate composed almost exclusively of hematite. Although minerals. unexpected, this could be a result of laser induced oxi- Micro-Raman spectroscopy was used to investigate dation and requires more study. sulfates minerals found in RBT 04262. Raman spec- Discussion: Four weathered pyrrohite grains were troscopy of minerals produces clear spectral features studied. Grains are extensively crosscut by vein net- allowing for an easier identification of minerals espe- works and appear to be composed of iron oxide altera- cially in fine-grained mixtures. Unlike visible, near- tion products of varying crystalline structure. We hy- infrared and thermal spectroscopy, there is minimal pothesize that some veins are composed of poorly crys- overlap of spectral features [6]. Micro-Raman allows talline iron oxides and hydroxides that were trans- the same spectral resolution on a micron spatial scale. formed into hematite upon excitation by the Raman Sample and Methods: Martian thin section RBT laser. Crystalline regions appear to be composed of 04262,30 was studied using a Wi-TEC alpha300R multiple phases of jarosite most notably natrojarosite, confocal Raman microscope system equipped with a and hydronium-jarosite. 50 mW frequency doubled 532 nm Nd:YAG exci- The manganese phosphate frondelite was detected tation laser at the Stony Brook University Vibrational by Raman spectroscopy as a minor component within Spectroscopy Laboratory. Point spectra were collected two of the regions of jarosite alteration (Fig. 1C). using 100x objective with sub-micron spatial resolu- Frondelite, if correctly identified would be the first iron tion. Spectra were collected between 0-3700 cm-1. phosphate identified for a Martian meteorite. Frondeli- Micro-Raman mapping was also completed on select te is described as a hydroxylated phosphate containing sulfate grains. Fe2+ and Mn2+. On Earth they are formed as alteration RBT 04262,30 was also imaged using the Wesley- products of iron-manganese phosphates in pegmatites an JEOL JSM6390LV/LGS SEM with BSE and EDAX [9]. Genesis and hyperspectral EDS mapping. The presence of jarosite as vein fill indicates aque- Results: In sample RBT 04262,30 micro-Raman ous alteration after the shock event. Jarosite requires spectroscopy confirms the presence of jarosite initially acidic conditions for formation and is common in acid- identified by SEM. Figure 1 shows the initial grain ic environments around Earth. Hydrogen isotopes sug- studied. Layered vein fill, approximately 10µm in gest jarosite formation likely occurring in Antarctica width, can be seen crosscutting pyrrohite grains. Multi- [4]. ple spectra were collected around the bottom left edge Summary and Future Work: Vein material of the grain using micro-Raman spectroscopy. Using around sulfate grains was studied using SEM micros- Crystal Sleuth® spectral matching software paired with 47th Lunar and Planetary Science Conference (2016) 1311.pdf

copy and Raman Spectroscopy. Jarosite and iron oxide were both detected by Raman spectroscopy. A new iron-manganese phosphate may have been observed, however additional Raman Spectroscopy will be needed to confirm its presence. Acknowledgements: We would like to thank Dr. Timothy Glotch and Steven Jaret at Stony Brook Uni- versity with their help acquiring and interpreting Ra- man Spectroscopic data. Also the Meteorite Working Group for loan of sample.

References: [1] Squyres, S. W., & Knoll, A. H. (2005). EPSL, 240 (1), 1–10. [2] Vicenzi E. P. et al. (2007) LPSC XXXVIII, Abstract #2335. [3] Gooding J. L. et al. (1988) GCA, 52, 909-915. [4] Greenwood J. P. (2008) LPSC XXXIX, Abstract # 2011 [5] Usui, T., et al. (2010). GCA, 74(24), 7283–7306. [6] Angel, S. et al. (2012). Appl. Spec., 66, 137–150. [7] Laetsch T, Downs (2006) GMEMA XIX. [8] Lafuente B et al. (2015). Highlights Min. Cyrst. Pp 1- 30. [9] Frye, K. (1981). Min. SE - 48 (p. 162).

1 1A

Jar

Figure 1. BSE image of pyrrho- tite grain cut by jarosite. Jar: Jarosite, Po: Pyrrohite. 1A: EDS spectra showing enrichment in Iron and Sulfur with lesser amounts of Phosphorus, Potassium and Sodium. 1B: Raman Shift cm-1 Micro-Raman spectra showing observed spectra in black and K-jarosite spectra in blue. 1C: 1C Micro-Raman spectra showing observed spectra in black, hydronium-jarosite in green and frondelite in blue.

Raman Shift cm-1