79th Annual Meeting of the Meteoritical Society (2016) 6391.pdf

BASALT-TRACHYBASALT FRACTIONATION IN GALE CRATER, MARS. J. C. Bridges1, P. H. Edwards1, J. Filiberto2, S. P. Schwenzer3, P. Gasda4 and R. Wiens4 1Space Research Centre, University of Leicester, UK [email protected], 2Southern Illinois University, Carbondale, Il. 62901, 3Open Univer- sity, UK, 4Los Alamos National Lab., NM.

Introduction: Mars Science Laboratory analysed a unique suite of igneous float rocks in the first 530 sols of the mission. These are basaltic and feldspar-rich extrusive rocks [1]. Here we use ChemCam LIBS data to determine their composition, the associated igneous fractionation processes and magmatic affinities. This dataset enables representa- tive, average compositions to be identified for the different igneous rock types, and comparisons with basaltic sher- gottites, MER basalts [2-4] and Gale sediments to be made in order to better understand igneous differentiation of the Mars lithosphere and the dominant magmatic types in the crater catchment, whether tholeiitic or alkaline. Methods: ChemCam contains a NIR laser and telescope, with 3 spectrometers inside Curiosity [5]. It remotely analyses targets by Laser Induced Breakdown Spectroscopy LIBS. Combined ICA and PLS are used to derive quan- titative compositions [6]. The large number of shots means that frequency and density plots are suited to show the unnormalised average compositions of the rock types and highlight the dominant versus minor end members. Results and Discussion: The silica contents of the Gale igneous samples, e.g. most of the data focused around 43-55 wt% SiO2, show that a significant amount of differentiation has taken place. The dark, fine grained basalts are similar in composition to MER basalts whereas the feldspar-rich samples can be classified texturally and composi- tionally as extrusive trachybasalts. The trachybasalt group has relatively high SiO2 (av. ~53 ±5 wt%), high Al2O3 and alkalis and is low in FeO and MgO – analogous in composition to terrestrial trachybasalts (Fig. 1). The sediments have a focal point of ~43-45 ±5 wt% SiO2 similar to the low SiO2 part of the igneous array (Fig. 1) showing that this basalt is the dominant compositional signature in Gale. The similarity of Gale and MER basalt compositions e.g. [2] suggests they were derived from similar mantle source regions beneath the ancient highlands. Crystal Fractionation. The Gale differentiation trend was modelled assuming crystal fractionation dominated by olivine with lesser pyroxene. Starting from a MER Spirit Humphrey_RAT composition [2] 10% augite was subtracted and olivine subtraction in divisions of 10% (Fig. 1). The fractionation line is close to the two maxima on the igneous distributions, so olivine being removed from a MER-like basalt can readily explain the compositional trend from basalt to trachybasalt seen in the Gale float rocks of the first 530 sols. This can be understood as the result of crystal frac- tionation within localized crustal magma chambers. It is likely to be anhydrous as no signs of hydrated mica or amphibole have yet been detected by CheMin e.g. [7] in the sediments which may share a source region with them. Tholeiitic or Alkaline? The mineralogy of aeolian deposits determined by CheMin is basaltic [8] and, we suggest, tholeiitic. This is likely to be the dominant magmatic differentiation series in Gale and the results here are compatible with subalkaline, tholeiitic differentiation. For instance the trachybasalt end member is silica and hypersthene nor- mative. However, there is also evidence for lesser amounts of alkaline sediment source material [9] and so Gale has sampled a mixture of tholeiitic with akaline igneous material. This is consistent with Oceanic Island terrestrial ana- logues where successive tholeiitic and alkaline magma- tism can be juxtaposed [10].

Figure 1. LIBS data density contours for Gale igneous and sedimentary samples from first 1000 sols. Each con- tour encloses an equal amount of data (total 33200 spec- tra). Terrestrial trachybasalts, shergottites, MER Spirit, MSL APXS for 1000 sols [11] also plotted. Crystal frac- tionation line shows differentiation from Spirit basalt composition. Ocean Island Basaltic compositions (OIB) [12] and trachybasalts [13] References: [1] Sautter V. et al. 2014 JGR,119, 30– 46. [2] McSween H. Y. et al. 2006 JGR, 111, doi:10.1029/2006JE002698; pds.nasa.gov [3] McSween H. Y. et al. 2009 Science, 324, 736-739. [4] Gellert R., et al. 2009 Abstract #2364 40th Lunar & Planetary Science Conference. [5] Wiens R. C. et al. 2012 Space Sci. Rev. 170, 167-227. [6] Clegg S. et al. 2016 Spectrochim Acta, (in press). [7] Vaniman D. T. et al. 2014. Science, 10.1126/sci- ence.1243480. [8] Achilles C.N. et al. 2016 #2532 47th Lunar & Planetary Science Conference. [9] Schmidt M. E. et al. 2014 JGR, 119, 64–81. [10] Floyd P. A. et al. 2007 Terra Nova 3, 257-265. [11] Gellert R. et al. 2016 Meteoritics and Planetary Science (this volume); pds.nasa.gov [12] Humphreys E. R. and Niu Y. 2009 Lithos, 112, 118-136. [13] Edwards P. et al. 2016 Meteoritics and Planetary Science (in prep).