JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS, VOL. 119, 1–18, doi:10.1002/2013JE004481, 2014 Geochemical diversity in first rocks examined by the Curiosity Rover in Gale Crater: Evidence for and significance of an alkali and volatile-rich igneous source M. E. Schmidt,1 J. L. Campbell,2 R. Gellert,2 G. M. Perrett,2 A. H. Treiman,3 D. L. Blaney,4 A. Olilla,5 F. J. Calef III,4 L. Edgar,6 B. E. Elliott,7 J. Grotzinger,4,8 J. Hurowitz,9 P. L. King,10 M. E. Minitti,11 V. Sautter,12 K. Stack,8 J. A. Berger,13 J. C. Bridges,14 B. L. Ehlmann,4,8 O. Forni,15 L. A. Leshin,16 K. W. Lewis,17 S. M. McLennan,17 D. W. Ming,18 H. Newsom,5 I. Pradler,2 S. W. Squyres,19 E. M. Stolper,8 L. Thompson,7 S. VanBommel,2 and R. C. Wiens 20 Received 9 July 2013; revised 27 November 2013; accepted 3 December 2013. [1] The first four rocks examined by the Mars Science Laboratory Alpha Particle X-ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46–102). Compositions range from Na- and Al-rich mugearite Jake_Matijevic to Fe-, Mg-, and Zn-rich alkali-rich basalt/hawaiite Bathurst_Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt-rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al-rich and Fe-rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et_Then likely reflect secondary precipitation of Fe3+ oxides as a cement or rind. The K-rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle. Citation: Schmidt, M. E., et al. (2014), Geochemical diversity in first rocks examined by the Curiosity Rover in Gale Crater: Evidence for and significance of an alkali and volatile-rich igneous source, J. Geophys. Res. Planets, 119,doi:10.1002/ 2013JE004481. 1Department of Earth Sciences, Brock University, St. Catharines, Ontario, Fredericton, New Brunswick, Canada. Canada. 8California Institute of Technology, Pasadena, California, USA. 2Department of Physics, University of Guelph, Guelph, Ontario, Canada. 9Department of Geosciences, SUNY Stony Brook, Stony Brook, New 3Lunar Planetary Science Institute, Houston, Texas, USA. York, USA. 4Jet Propulsion Laboratory, Pasadena, California, USA. 10Research School of Earth Sciences, Australia National University, 5Institute of Meteoritics, University of New Mexico, Albuquerque, New Canberra, ACT, Australia. Mexico, USA. 11Applied Physics Laboratory, Johns Hopkins University, Laurel, 6Arizona State University, Tempe, Arizona, USA. Maryland, USA. 12 7Department of Earth Sciences, University of New Brunswick, Museum National D’Histoire Naturelle, Paris, France. 13Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada. 14 Corresponding author: M. E. Schmidt, Department of Earth Sciences, Space Research Centre, Department of Physics and Astronomy, University of Leicester, Leicester, Leicestershire, UK. Brock University, St. Catharines, Ontario, L2S 3A1, Canada. 15 ([email protected]) Institut de Recherch en Astrophysique et Planetologie, Université Paul Sabatier, UPS-OMP, Toulouse, France. ©2013. The Authors. 16School of Science, Rensselear Polytechnic Institute, Troy, New York, This is an open access article under the terms of the Creative Commons USA. Attribution-NonCommercial-NoDerivs License, which permits use and dis- 17Princeton University, Princeton, New Jersey, USA. tribution in any medium, provided the original work is properly cited, the 18NASA Johnson Space Center, Houston, Texas, USA. use is non-commercial and no modifications or adaptations are made. 19Department of Astronomy, Cornell University, Ithaca, New York, USA. 2169-9097/14/10.1002/2013JE004481 20Los Alamos National Laboratory, Los Alamos, New Mexico, USA. 1 SCHMIDT ET AL.: FIRST ROCKS EXAMINED BY THE CURIOSITY ROVER IN GALE CRATER 1. Introduction with regular bedding and no hydrated mineral signatures [Milliken et al., 2010]. The mineralogical and morphological [2] Each landing site on Mars is unique and contributes to diversity present at Mt. Sharp suggests a range of depositional our understanding of the diversity of Martian rocks and soils processes and environments that may span several billion and the wide range of geologic processes and conditions that years [Grotzinger and Milliken, 2012] and is the primary generated them [e.g., Squyres et al., 2004a; Squyres et al., target of the MSL mission. 2004b; Smith et al., 2009]. Gale Crater, the site selected for [6]Thefloor of Gale Crater contains a range of rock units the Mars Science Laboratory (MSL) Curiosity rover, is no identified from HiRISE High Resolution Imaging Science exception [Grotzinger et al., 2012]. An important component Experiment (surface morphology) and CRISM (Compact of the MSL payload is the Alpha Particle X-ray spectrometer ’ Reconnaissance Imaging Spectrometer for Mars; visible-near (APXS) on the rover s arm, which measures elemental infrared spectra) data [Anderson and Bell, 2010; Calef et al., compositions of ~1.7 cm diameter spots on rock surfaces 2013; Rice et al., 2013; Sumner et al., 2013] that are inferred and soils [Gellert et al., 2013]. The MSL APXS is the fourth to be largely sedimentary and variably affected by impact pro- such instrument to have landed on Mars and allows direct cesses. These sedimentary units may be sourced from volcanic comparison with rocks and soils from previous Mars landing and/or igneous rock exposed at or near the Gale Crater rim, sites [e.g., Rieder et al., 2004; Gellert et al., 2006; Brückner which is 5 km tall relative to the crater floor, although a more et al., 2003]. fi ’ local volcanic source from within Gale Crater is possible, as [3]The rst sign of Gale s unique geochemistry came from are distal, large centers of volcanism, such as Elysium Mons, the ChemCam instrument, which uses laser-induced break- which is located ~1500 km directly north of Gale. Also, within down spectroscopy (LIBS) to measure chemical compositions – μ 20 km of Gale Crater, there are outcrops of possible pyroclas- of 200 400 m spots [Maurice et al., 2012]. ChemCam tic ashfall tuff or ash flows from the Medusae Fossae analyses are complementary to those done by the APXS Formation [Kerber et al., 2011; Zimbelman and Scheidt, and give an indication of the degree of geochemical heteroge- 2012]. These rocks have been proposed to make up units neity, grain size, and mineral components. The compositions within Mt. Sharp [Scott and Chapman,1995;Thomson of soils and rocks examined by the ChemCam early along ’ et al., 2011; Zimbelman and Scheidt, 2012] and may also con- the rover s traverse are variable and include spots with high tribute material to the region near the Curiosity landing site. Si, Al, and K, consistent with feldspar-rich components [7] In August of 2012, the Curiosity rover touched down in [Meslin et al., 2013]. Gale Crater at Bradbury Landing, several kilometers north of [4] During Curiosity’s eastward traverse from the landing ’ Mt. Sharp and ~14 km south of the apex of the Peace Vallis site that is Bradbury Landing (All of the small-scale features fan, a large, low gradient fan in Gale Crater [Palucis et al., names in this paper are informal and are not approved by the 2013; Sumner et al., 2013; Williams et al., 2013]. This alluvial International Astronomical Union) toward Glenelg, a contact fan is fed by Peace Vallis, a ~32 km long, ~40 m wide channel between three distinct geomorphic units (sols 46–102; that incises the northwestern rim of Gale Crater and empties Figure 1), the APXS examined four rocks and one soil. We, onto a fan defined primarily by convex contours within an hereafter, refer to the four analyzed rocks as the “Bradbury enclosed basin that includes Bradbury Landing and Glenelg. assemblage,” a classification without implied genetic connec- The gully system feeding the fan is estimated to have removed tion, and present their chemical compositions, make inferences ~500(106)m3 of material, and the current fan covers an area of about their petrologic origin, and place them in a broader geo- ~80km2 [Palucis et al., 2013; Sumner et al., 2013]. logic context within Gale Crater. We present evidence that the [8] After arriving at Bradbury Landing, the Curiosity rover Bradbury assemblage is composed of volcanic, volcaniclastic, drove east (Figure 1) toward an area informally named and/or relatively immature sedimentary rocks. Compared to Glenelg, which represents the juxtaposition of three geomorphic other Martian rocks, the Bradbury assemblage is chemically units: a dark-toned hummocky plains unit (HP), a dark-cratered diverse and expands the range of known Martian rock compo- unit, and a light-toned, bedded, fractured unit (BF) [Grotzinger sitions to include high alkalis (up to 2.9 wt % K2O), high Fe et al., 2013]. The eastward traverse took Curiosity downhill (up to 26.3 wt % FeO*; all Fe reported as FeO*), and high across a geomorphic unit (HP) characterized in HiRISE Mn (up to 0.8 wt % MnO).