Anderson, R.

Blaney, D.

Bridges, J. C.

Dyar, M. D.

Edwards, P. H.

Filiberto, J.

Fisk, M.

Gasada, P.

Goetz, W.

Schwenzer, S. P.

Thompson, L.

Wiens, R. C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsIgneous differentiation on Mars: Trachybasalts in GaleCraterConference or Workshop ItemHow to cite:Bridges, J. C.; Edwards, P. H.; Anderson, R.; Dyar, M. D.; Fisk, M.; Thompson, L.; Gasada, P.; Schwenzer,S. P.; Goetz, W.; Blaney, D.; Filiberto, J. and Wiens, R. C. (2016). Igneous differentiation on Mars: Trachybasalts inGale Crater. In: 47th Lunar and Planetary Science Conference, 21-25 Mar 2016, Houston, Texas.For guidance on citations see FAQs.c© 2016 The AuthorsVersion: Version of RecordLink(s) to article on publisher’s website:http://www.hou.usra.edu/meetings/lpsc2016/pdf/2160.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukIGNEOUS DIFFERENTIATION ON MARS: TRACHYBASALTS IN GALE CRATER.  J. C. Bridges1, P. H. Edwards1, R. Anderson2, M. D. Dyar3, M. Fisk4, L. Thompson5, P. Gasda6, S. P. Schwenzer7, W. Goetz8, D. Blaney9, J. Filiberto10, R. C. Wiens6, 1Space Research Centre, University of Leicester UK j.bridges@le.ac.uk 2USGS Astro-geology Science Center, Flagstaff, AZ, 3Mt. Holyoke College, Ma, 4Oregon State Univ., 5Planetary and Space Sci-ence Centre, University of New Brunswick, Can., 6Los Alamos National Lab., NM, 7Open University, UK, 8Max Planck Inst., Göttingen, Gemany, 9JPL, Pasadena, Ca, 10Southern Illinois University Carbondale, Il.   Introduction:  Since landing in 2012 the Mars Sci-ence Laboratory (MSL) rover Curiosity has analysed a a large number of float rocks and conglomerate clasts. Many of these are of igneous origin emplaced through fluvial and probably impact processes postdating the formation of Gale Crater itself in the early Hesperian [1]. Here we use the ChemCam instrument on MSL [2] in order to determine the compositions of the igneous rocks.  This unique, large dataset enables representa-tive, average compositions to be identified for the dif-ferent igneous rock types, and comparisons with basal-tic shergottites, MER basalts [3-5] and Gale sediments to be made in order to better understand igneous differ-entiation of the Mars lithosphere.  Fig. 1. Mastcam and RMI of Gale basalts and trachybasalts showing their coarse feldspar-rich min-eralogy. Methods:  ChemCam contains a NIR laser and tel-escope within MSL’s mast and 3 spectrometers inside the body unit [2]. It remotely analyses targets by Laser Induced Breakdown Spectroscopy LIBS, with optimal performance at ≤4 m, and also has a Remote MicroIm-ager (RMI). Each time the laser is fired is termed a shot. Typically there are around 30-50 shots on a single observation point in a raster (e.g. 3x3), on a target. ChemCam  uses a combination of ICA and PLS to de-rive quantitative compositions [6]. The large number of shots  means that frequency and density plots are well suited to show the unnormalised average compositions of the rock types.  Igneous float rocks and conglomer-ate clasts were selected for LIBS in MastCam and RMI images.  Results:  Twenty five igneous float and conglome-ate rocks were identified over the first 530 sols (Fig. 1). A textural classification was initially described in [7]. One type of igneous rock is volcanic and includes dark, fine grained basaltic samples (e.g. Ashuanipi float rock). Coarse-grained  >500 µm [7], feldspar-rich samples are also notable e.g. Robin Hood and the Har-rison clast - the latter from the Hottah faces conglom-erate (sols 365, 514).    Fig. 2. Frequency plots of Gale igneous ChemCam shots (wt% SiO2, Al2O3, FeO). Red columns = av. MER Spirit basalts [3-4], black = av.  shergottite.  Feldspar was identified on the basis of the pale col-our and tabular, mm-size crystal forms, subsequent LIBS analyses supported this identification [8]. Igneous Rock Compositions.  The Fig. 2 frequency histograms and Fig. 3 density plots show differences in composition between the igneous float rock and sedi-2160.pdf47th Lunar and Planetary Science Conference (2016)mentary outcrop samples from the first 1000 sols.  The sedimentary outcrop analyses have a focal point of ~45 wt% SiO2 (Fig. 3) but the igneous analyses are less homogenous.  The igneous data range from 6-25 wt% Al2O3 with local maxima at 11 and 22 wt % (Fig. 2). The Gale igneous groups have much less MgO than the average shergottites (16 wt%). The shergottites have considerably less alkalis than either the sedimentary or igneous samples which are both very similar for K2O but have much higher Na2O contents in the igneous samples. The MER Spirit samples are broadly similar to the sedimentary targets, particularly SiO2 (Fig. 3) but also Al2O3, CaO, Na2O and K2O though FeO is lower in the MER basalts with correspondingly higher MgO.  The feldspar-rich igneous group has relatively high SiO2 (av. ~53 wt%), high Al2O3 and alkalis and is low in FeO and MgO – similar in composition to terrestrial trachybasalts (Fig. 2, 3). In contrast, the dark, basaltic group is much more similar to the Gale sedimentary rocks and terrestrial basalts – high in FeO and MgO whilst lower in SiO2, (av. on Fig. 3. ~45 wt%),  Al2O3, Na2O and K2O oxides.   Fig. 3. Density contours for Gale igneous and sed-imentary samples from first 1000 sols. Each contour encloses an equal amount of data (total 33200 shots). Terrestrial trachybasalts, shergottites, MER Spirit, MSL APXS [3-5,9] also plotted. Crystal fractionation line shows differentiation from Spirit basalt composi-tion.  Discussion:  Igneous Rock Types. The silica con-tents of the Gale igneous samples, e.g. much of the data focused around 45-55 wt%, show that a significant amount of differentiation has taken place to form the high SiO2 samples.  The dark, fine grained basalts are similar in composition to MER basalts whereas the feldspar-rich samples can be classified texturally and compositionally as trachybasalts.  The CIPW composi-tions (Fig. 4) are olivine and hypersthene normative for the representative basalt and trachybasalt compositions derived from the density plots.  Thus the great majority of these igneous rocks have tholeiitic affinities. How-ever, the presence of alkali-rich feldspars in some of the sediments e.g. [10,11] may indicate the local pres-ence of alkaline igneous rocks as well [9]. Crystal Fractionation.  The differentiation trend e.g. increasing silica, was modelled assuming crystal fractionation dominated by olivine with lesser pyrox-ene. Starting from a MER Spirit Humphrey_RAT composition [3] 20% augite was subtracted and olivine subtraction in divisions of 10% (Fig. 3). The fractiona-tion line is close to the two densest points on the igne-ous distributions, this means that olivine being re-moved from a MER-like basalt through crystal frac-tionation can readily explain the compositional trends. Gale Samples and Differentiation of the Mars Lithosphere: The Gale and MER basalts have high Fe, Al and alkali abundances suggesting they were derived from similar mantle source regions beneath the ancient highlands. In contrast, the shergottites - more likely derived from the northern plains and Tharsis - have a more Al, alkali-poor composition which probably re-flects differences in their mantle source composition. The Gale igneous samples’ compositional variation, from basalt to trachybasalt, can be understood as the result of crystal fractionation within localized crustal magma chambers rather than regional differentiation of the basaltic Mars crust.  This is likely to be anhydrous fractionation as no signs of amphibole have yet been detected by CheMin e.g. [9] in the sediments which may share a source region with them.   References: [1] Grotzinger J. P. et al. (2015). Sci-ence, 10.1126/science.aac7575 [2] Wiens R.C. et al. (2012) Space Sci. Rev 170, 167-227; Maurice S. et al. (2012) LPS, XXXXIII, #2899 [3] McSween H. Y. et al. (2006) JGR, 111, doi:10.1029/2006JE002698; pds.nasa.gov [4] McSween H. Y., et al. (2009) Science, 324, 736-739. [5] Gellert R., et al. (2009) LPS XXXX #2364 [6] Clegg et al. (2016) Spectrochim Acta, subm. [7] Sautter V. et al. (2014) JGR,119, 30–46. [8] Gasda P. et al. (2016) LPS, [9] Schmidt M. E. et al. (2014) JGR, 119, 64–81. [10] Anderson R. et al (2014) Ica-rus, 249, 2-21. [11] Vaniman D. T. et al. (2014). Sci-ence, 10.1126/science.1243480.    Fig. 4. Basalt tet-rahedron with CIPW normative compositions of av. Gale basalt and trachybasalts, av. shergottite, MER Bounce and Wishstone [3,4]. 2160.pdf47th Lunar and Planetary Science Conference (2016)

Igneous differentiation on Mars: Trachybasalts in Gale Crater

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Igneous differentiation on Mars: Trachybasalts in Gale Crater

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