A SIMPLIFIED VIEW OF THE GEOCHEMICAL DIVERSITY SURROUNDING HOME PLATE. A. S. Yen1, R. V. Morris2, B. C. Clark3, R. Gellert4, and Athena Science Team. 1Jet Propulsion Laboratory, Califor- nia Institute of Technology, Pasadena, CA 91109 ([email protected]), 2NASA Johnson Space Center, Hous- ton, TX 77058, 3Lockheed Martin Corporation, Littleton, CO 80127, 4University of Guelph, Guelph, ON, Canada. Introduction: The Home Plate feature (Fig. 1) Geochemical Components: The chemical com- within the Inner Basin of the Columbia Hills consists postions of the clastic rocks (Fig. 2) which form Home of layered rocks and has been interpreted as an accu- Plate and surrounding deposits can be represented by mulation of pyroclastic deposits [1]. Samples analyzed mixtures of a small number of distinct components: by the Alpha Particle X-ray Spectrometer within ~25 Wishstone/Watchtower. APXS analyses of samples meters of the eastern margin of Home Plate exhibit a at the summit and on the northwest flank of Husband strikingly diverse range of geochemical compositions, Hill are characterized by elevated Ti and P with de- including the highest levels of Mg, Si, K, Zn, and Ni pleted Cr. This signature is clearly reflected in analy- measured at Gusev Crater. This wide range of chemi- ses of Home Plate and its surroundings (Fig. 3). cal variability across the 40+ samples analyzed on and Algonquin Class. Several of the samples near near Home Plate can be represented by contributions Home Plate exhibit the ultramafic signature (high Mg, from only six primary components. This reconstruction low Al, and elevated Cr) of Algonquin Class rocks is not reflected in the Mössbauer mineralogy suggest- found on the southern slopes of Husband Hill (Fig. 4). ing that significant alteration of the contributing com- Alkali-rich basalt. Samples analyzed in the vicinty ponents has occurred. of Home Plate are rich in Na and K, suggesting that alkali-rich basalts are likely components. Silica. The highest concentration of SiO (>90 2 wt%) measured at the surface of Mars is located in the valley immediately east of Home Plate. Other samples in this area also exhibit elevated Si. Hydrothermal processes are likely responsible for these enhance- ments [2, 3]. Martian dust. Due to surface roughness, dust and sand distributed by aeolian processes are difficult to completely remove from the analyzed samples by brushing with the Rock Abration Tool. Fortunately, the composition of martian dust is relatively uniform [4] and can be used in this mixing model. K, Ni, and Zn. Significant enhancements in K, Ni, and Zn are found in certain samples surrounding Home Plate. In several cases, there is a clear association with Fig. 1. HiRISE image of Home Plate. Cl, suggesting that these elements may have been mo- bilized in fluids or through high-temperature volcanic emissions. These listed constituents of the samples surround- ing Home Plate are not true endmembers, as several are themselves composed of multiple components. However, organizing the wide compositional diversity into mixtures of several known samples (Table I) re- sults in a simpler view of the geochemistry. Results: Samples on Home Plate are well repre- sented by a dust-like composition, an alkali basalt, and a combination of Watchtower and Algonquin class rocks. The Mg-rich samples, which may represent the lowest stratigraphic unit sampled near Home Plate, are Fig. 2. Example Microsopic Imager image of a sample largely dominated by the ultramafic composition of in the vicinity of Home Plate (granules are ~1 mm Algonquin class rocks. An intermediate stratigraphic dia.) unit between the platy deposits of Home Plate itself and the Mg-rich samples in the valley floor exhibits a strong K, Ni, Zn signature. This layer is tracable from 2.5 the edge of Home Plate to Mitcheltree and Low Ridges to the east (across the valley). Finally, the silica-rich deposits do not appear as a contiguous layer in the 2 stratigraphy and may be a product of hydrothermal processes which occurred more recently. 1.5 Fe-Mineralogy: This model of chemical compo- Ti nents does not reliably extend to the iron mineralogy 1 established by the Mössbauer spectrometer. The Mg- rich samples, for example, are dominated by magnet- 0.5 ite, while the ultramafic rocks identified as chemically- Home Plate Rocks related are dominated by olivine. One possible sce- Wishstone/Watchtower Rocks 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 nario for reconciling this discrepancy involves the P aqueous alteration of olivine to silica and magnetite. Fig. 3. Ti versus P for Home Plate rocks (red) and for The available data, however, does not provide an indi- the related Ti- and P-rich Wishstone/Watchtower class cation of phyllosilicates (e.g., serpentine) that would rocks (green). have resulted in this process. Nonetheless, Mg-rich weathering products (talc, chrysotile, etc.) could be 35 present but remain undetectable by the Mössbauer spectrometer. 30 Conclusions: Samples analyzed on and near Home 25 Plate can be represented by a small number of distinct components. These samples can be grouped into a de- 20 g fined statigraphy consisting of the light-toned deposits M 15 of Home Plate itself, an intermediate layer rich in vol- canic volatiles, and a lower layer which is consistent 10 with aqueously altered ultramafic rocks. 5 Mg-Rich near HP References: [1] Squyres, S. W. et al (2007) Sci- Algonquin Class Rocks 0 0 1 2 3 4 5 6 7 8 9 ence, 316, 738-742. [2] Morris, R. V. et al (2008) 39th Al LPSC. [3] Ruff, S. et al (2008) 39th LPSC. [4] Yen, A. Fig. 4. Mg versus Al for the Mg-rich samples near S. et al. (2005), Nature, 436, 49-54. Home Plate (blue) and the related Algonquin Class rocks (black). Table I. Mixing model results indicating that chemically diverse samples on and near Home Plate can be represented by a small number of distinct components. Wish/Watch Algonquin Class Alkali Basalt Silica Martian Dust K, Ni, Zn Residual Sample (Watchtower) (Palomino) (Humbolt Peak) (Kenosha Comets) (average of 20) (Torquas) (Montalva) (wt%) On Home Plate Barnhill (NW) 0 4 1 0 79 0 15 1.8 Posey (NW) 12 5 12 1 68 0 0 1.6 Papa Bell (NW) 24 6 49 3 18 0 0 1.3 Pesapallo (East) 21 3 31 3 42 0 0 0.4 Superpesis (East) 2 1 0 0 95 0 2 0.4 June Emerson (East) 4 6 42 1 48 0 0 0.5 Elizabeth Emery (East) 0 3 18 0 74 0 5 0.7 Texas Chili (South) 1 3 13 0 74 7 0 1.4 Pecan Pie (West) 1 7 25 1 64 0 3 1.3 Chanute (North) 0 0 0 0 89 0 9 2.0 Intermediate Madeline English 15 0 18 7 41 20 0 0.9 Layer King George Island 17 0 7 6 53 17 0 1.0 Riquelme 8 0 0 2 70 5 14 0.6 Mg-Rich Everett 1 51 1 5 3 39 0 0.6 Slide 0 76 0 10 0 6 6 1.7 GoodQuestion 0 18 0 20 43 0 18 1.2 Eileen Dean 11 36 6 13 9 24 0 1.1 Silica-Rich Nancy Warren 6 0 0 61 32 0 0 1.6 Elizabeth Mahon 0 1 0 60 31 7 0 1.2 Innocent Bystander 0 36 0 41 0 0 24 1.4 Nora Luker 0 6 0 53 39 2 0 0.6