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Sulfides in Enstate Chondrites

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Sulfides in Enstate Chondrites Sulfides in Enstatite Chondrites: Indicators of Impact History Kristyn Hill1,2, Emma Bullock2, Cari Corrigan2, and Timothy McCoy2 1Lock Haven University of Pennsylvania, Lock Haven, PA 17745, USA 2National Museum of Natural History, Department of Mineral Sciences, Washington D.C. 20013, USA Introductton Results Discussion Enstatite chondrites are a class of meteorites. They are Determining the history of the parent body of a referred to as chondrites because of the spherical Impact Melt, Slowly Cooled chondritic meteorite often includes distinguishing whether chondrules found in the matrix of the meteorites. Enstatite aa bb cc d d or not the meteorite was impact melted, and the cooling chondrites are the most highly reduced meteorites and rate. contain iron-nickel metal and sulfide bearing minerals. The Impact melts are distinguishable by the texture of the matrix is made up of silicates, enstatite in particular. There metal and sulfide assemblages. A meteorite that was are usually no oxides found, which supports the idea that impact melted will contain a texture of euhedral to these formed in very oxygen poor environments. The subhedral silicates, like enstatite, protruding into the metal enstatite chondrites in this study are type 3, meaning they or sulfides (figure 2). Some textures will look shattered are unmetamorphosed and not affected by fluids. (figure 2b). Meteorites that contain the mineral keilite are Studying enstatite chondrites will help us determine the PCA 91125 ALHA 77156 PCA 91444 PCA 91085 also an indicator of impact melts (figure 3). Keilite only evolution of their parent bodies which formed at the occurs in enstatite chondrite impact-melt rocks that cooled beginning of our solar system. These meteorites show ee f f Figure 2. Evidence Minerals Found rapidly (Keil, 2006). similarities with the sulfur found on the surface of Mercury. Backscattered • Enstatite crystals • Troilite Cooling rates can be determined by the suite of minerals This suggests that Mercury may have formed from similar electron images of protruding into • Daubréelite present. Exsolution pairs occur when elements of one materials and/or in a similar region of the solar nebula to the EH3 impact melts. or intermixed • Kamacite mineral have time to exsolve out to form another mineral, enstatite meteorites. Images (a-f) display with metal or • Schreibersite indicating a slow cooling rate. For example, troilite (FeS) evidence of impact sulfides • Niningerite anddaubréelite (FeCr2S4) melts due to enstatite • Shattered • Perryite are exsolution pairs (figure 4). If they are Analyttcal Techniques protruding or appearance • Taenite PCA 91451 PCAPCA91254 91254 intermixed with metal • Exsolution pairs • Brezinaite both present in a • Samples studied are Antarctic Meteorites from the 1.2 mm or sulfide minerals. • Oldhamite sample, the meteorite Tro National Museum of Natural History Antarctic Meteorite cooled slowly. If Collection. Impact Melt, Quickly Cooled daubréelite is absent, Daub • Eight samples were examined in reflected light under a the meteorite cooled petrographic microscope. Reflected light images of quickly, not allowing sulfide mineral assemblages were taken to observe the Cr to separate from textural features using an OPELCO microscope. troilite. Therefore Exsolution Pairs • Images of the samples were stitched together using Kam Old meteorites that cooled Figure 4. Exsolution pairs troilite and mosaicking software written in the Department of Tro quickly will not have daubréelite in PCA 91258 (under Mineral Sciences. Kei exsolution pairs. reflected light) indicating slow cooling. • Textures were observed to determine if samples were Tro impact melted or unaltered. Kam Old • Compositions of minerals were obtained using a Scanning Ens KLE 98300 KLE 98300 Electron Microscope (SEM) and analyzed using Noran KLE 98300 Future Work System Six (NSS) software (Figure 2). Blue = Fe; Red = S; Green = Ca • Examine other enstatite chondrites (EH4-6, and EL3-6) Figure 3. Evidence Minerals Found as well as known enstatite impact melts Backscattered • Enstatite crystals • Keilite • Old Measure the sulfur isotopic compositions of individual Mineral Spectra electron images protruding into or • Troilite (Cr-rich) sulfide grains using the Cameca ims 1280 secondary ion and spectral intermixed with • Oldhamite mass spectrometer (SIMS) at the University of Hawai’i Kam imaging of KLE metal or sulfides • Kamacite in collaboration with Dr. Gary Huss 98300 displaying • Shattered • Schreibersite • Look for other indications of impact melt (abundance Kei keilite as evidence appearance • Graphite of presolar grains) using the NanoSIMS at the Carnegie for impact melt. • Presence of keilite Institution of Washington • Exsolution pairs Troilite KLE 98300 Blue = Fe; Red = S; Green = Mn absent References Unaltered by Impact Melt, Slowly Cooled Keil, K., 2006. Occurrence and origin of keilite, (Fe>0.5,Mg<0.5)S, in enstatite chondrite impact-melt Evidence Minerals Found Sch rocks and impact-melt breccias. Chem. Erde 67, 37-54. Tro Kam • Enstatite crystals • Troilite Daubréelite do not appear to be • Daubréelite Daub protruding into or • Kamacite Nin intermixed with • Schreibersite Acknowledgements • We would like to thank the National Per metal or sulfides Niningerite Nin Djer Kam • Shattered • Perryite Science Foundation for the support for appearance absent • Djerfisherite this project. We would also like to thank Tro Kamacite Liz Cottrell, Gene Hunt, and Virginia EET 87746 EET 87746 1.2 mm Power who coordinate the NHRE intern Figure 1. SEM spectra showing minerals found in various EH3 meteorites with reflective light images taken with a petrographic microscope to the right. program. .
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