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50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1228.pdf

IRON GRAINS OBSERVED BY CHEMCAM ON THE VERA RUBIN RIDGE, AT GALE CRATER, . G. David1, A. Cousin1, O. Forni1, P-Y. Meslin1, J. R. Johnson2, J. L’Haridon3, P. Beck4, S. Po- tin4, E. Dehouck5, A. M. Ollila6, A, A. Fraeman7 , S. Le Mouélic3, N. Mangold3, B. Chide1, O. Gasnault 1, R. C. Wiens8 , S. Maurice 1, J.F. Fronton9, P. Pinet1, M. Salvatore10 ,E. A. Cloutis11 ; 1Institut de Recherche en Astrophy- sique et Planétologie, Toulouse, France, 2JHU/APL, Maryland, USA, 3LPG, Nantes, France, 4IPAG, Grenoble, France, 5 LGL, Lyon, France, 6 EPS, Albuquerque, USA, 7JPL, Pasadena, California, USA, 8LANL, Los Alamos, USA, 9CNES, Toulouse, France,10 NAU, Arizona, USA, 11UOW, Manitoba, Canada; [[email protected]]

Introduction: After more than ~1800 sols (Mar- Methods: We prepared different mechanical mix- tian days), the Curiosity rover reached Vera Rubin tures with powders of (Ø < 5 μm), goethite (Ø Ridge (VRR), located ~300 m up Mount Sharp. Spec- < 50 μm) and magnetite (Ø < 50 μm). These iron ox- tral reflectance observations of VRR by both orbital ides were then mixed at different concentrations with remote sensing with CRISM data [1,2,3], and in situ by basaltic materials, simulating the bedrock surrounding Curiosity [4,5] show that the ridge’s spectral signature the dark-tone features. Four different matrices were is strongly dominated by crystalline hematite used: 1) pristine JSC martian simulant [13] and 2) a (Fe2O3). However, none of the chemistry instruments mixture of JSC martian simulant with onboard Curiosity (APXS or ChemCam [6,7]) record- (FeTiO3) in order to increase the iron content, to mar- ed any significant iron enrichment in the ridge com- tian abundances (~20 wt% FeOT). 3) Magnesi- pared to previous terrains [8,9]. Within VRR, only um sulfate (kieserite) and 4) calcium sulfate (mixture sporadic Fe-rich targets have been observed in the gray of bassanite, anhydrite and gypsum) were also used as rocks of the Jura member. Specifically, mm-scale, matrix from 0 up to 100 wt%. Mixtures were then dark-toned nodular concretions within or in close asso- pressed and analyzed with the ChemCam setup [14] in ciation with light-toned calcium sulfate veins (see a martian chamber, allowing similar martian conditions fig.1) revealed >40 wt% FeOT from ChemCam data. in terms of atmospheric pressure (~ 7 mbar) and com- However, this is beyond the range of our current FeO positions (mainly CO2 with 1.6 % Ar and 2.7% N). calibration. Comparison to observations of iron mete- Each pellet was probed at 5 observation points (of 30 orites on Mars [10,11] suggest the grains are nearly laser shots each). Data were processed the same way as pure iron . As LIBS plasma mixes atmospheric martian data [15]. Then, in order to improve the in- with the sample constituents, consequently it strumental response correction for these extreme com- does not distinguish oxidized from non-oxidized mete- position, the Pearson correlation factor was calculated oritic iron. Nevertheless, the spectral reflectance pat- for each laboratory sample between its spectrum and terns are inconsistent with red crystalline hematite the spectrum of the Mont Dieu iron meteorite [16], [11]. Constraining the nature of iron oxides is im- acquired with our terrestrial setup. In the same way, for portant as they are geochemical markers of past envi- flight data, the Pearson correlation factor was comput- ronments during their formation, such as condi- ed from the Aeolis Mons 001 iron meteorite ChemCam tions. The aim of our study is to analyze martian ana- spectra (from Sol 1505 [10]). This allowed Pearson logs in the laboratory with a ChemCam like setup, in correlation factors for laboratory and ChemCam data order to confirm iron oxide composition and better to be directly compared. Finally, reflectance spectra of constrain the mineralogy of dark-toned features. these samples were characterized by a spectrophotogo- niometer [17] for comparison to passive ChemCam observations (400-840 nm [4,7]). Point #3 Results: The iron correlation factor as a function of FeOT content of our laboratory samples displayed a monotonic trend for both basaltic and sulfate matrices Point #9 (fig.2). This experimental calibration curve was suc-

Point #2 cessfully validated with ChemCam basaltic calibration targets [18]. When applying this model to dark-toned targets, the iron correlation factors were close to that of pure iron oxides, which confirms that these dark-toned concretions and vein fills were composed of pristine iron oxides. Regarding reflectance observations, Fig.3 Figure 1: Illustration of dark-toned features located displays the band absorption depth at 535 nm (sensi- within light-toned veins (Sol 2218 Grange target, left) tive to the presence of crystalline ferric oxides), versus and close to a vein (Sol 1938 Loch_Maree_ccam target, the 750-840 nm slope (indicative of the strength of iron right), in grey Jura member, VRR. Fe-rich ChemCam observation points are noted (Point #3 for Grange target absorptions), for several Jura bedrocks [11]. Rocks and point #9 for Loch_Maree_ccam target). from red Jura (red dots) have spectral signatures con- sistent with ferric phases, possibly red crystalline hem- atite, whereas dark-toned features and their surround- 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1228.pdf

ing bedrocks (grey dots) are featureless in the spectral domain covered by ChemCam.

Figure 4: ChemCam passive spectra of Loch_Maree_ccam #2 (on bedrock) and Loch_Maree_ccam #9 (on concretion). Labora- tory spectra of sample mixtures are also represented for compar- ison (blue and red lines).

Discussion: From our experiment, the dark-toned features observed at VRR could correspond to at least 3 mineralogies: grey hematite, or mixture of Figure 2: Mean Fe correlation factor as a function of FeOT wt% of our laboratory samples. Black and brown dots correspond red hematite with opaque . Coarse grey hema- respectively to iron oxide mixtures with JSC martian simulant tite is strongly supported by theoretical calculation of with and without ilmenite. Grey and white dots represent respec- iron oxides stability fields, in Ca-S rich fluid [12]. On tively magnesium sulfate and calcium sulfate mixtures with JSC martian simulant. Fe correlation scores obtained for Grange #3 the other hand, pure magnetite is expected to produce and Loch_Maree_ccam #2 and #9 are also represented. spectra with very low reflectance intensity in contrast to red bedrocks, which is not observed (fig.4, [11]). Among reflectance spectra of pure iron oxide minerals, Possible dust contamination could be responsible for only magnetite and crystalline coarse gray hematite the higher albedo. Finally, mixtures of red crystalline have passive observations which could be consistent hematite with opaque minerals such as (FeS2) or with these martian dark-toned features. ilmenite (FeTiO3) can be discarded because no sulfur n or enrichment has been observed in Chem- Cam LIBS spectra. Therefore, magnetite appears to be a serious opaque mineral candidate for these dark- toned features mineralogy, although additional mix- tures of red and grey hematite need to be studied fur- ther. Association of hematite and magnetite has been observed at previous drill sites with CheMin. As an example, ChemCam passive spectra of the Telegraph Peak and Windjana drill tailings, which hold substan- tial magnetite contents (respectively 8.2 and 12 wt% [18, 19]), displayed very flat spectra [4], similar to that of dark-toned features, whereas CheMin results show Figure 3: 750-840 nm slope versus band depth at 535 nm, calcu- lated for some red Jura targets (red dots) and grey rocks/dark- no other opaque crystalline mineral in these samples toned features (grey dots), of the Jura member. Target names [18, 19]. But more work will need to be done to under- reference are shown on the right. Spectra for pure magnetite stand the iron mineralogy of these interesting features. (HS78.1B, Ø = 74-250 μm) and grey hematite (GDS69.a, Ø = References : [1] Milliken R. E. et al., (2010) GRL, 37, 150-250 μm) from the USGS spectral library are shown respec- L16202. [2] Murchie S. et al., (2007) JGR, 112. [3] Fraeman tively with black and grey stars. A. et al., (2013) Geology, 41, 1103 [4] Johnson J. R. et al., However, in addition to pure magnetite and (2016) American Mineralogist, 101. [5] Wellington D. F. et al., (2017) American Mineralogist, 102, 1202-1217. [6] crystalline grey hematite candidates, our experiments Maurice S. et al., (2012), SSR, 170,95. [7] Wiens R.C. et al., suggest a third possibility. Basaltic powder mixed with (2012), SSR, 170, 167. [8] Thompson L. et al., (2018) LPSC 40% red hematite and 10% magnetite shows a reflec- XLIX, [9] Frydenvang J. et al., (2018), LPSC XLIX. [10] tance spectrum that reproduces quite well the reflec- Meslin. P. Y. et al., (2017) LPSC XLVIII. [11] L’haridon J. tance pattern of dark-toned concretions, in particular et al., (2018), in prep, JGR. [12] L’haridon J. et al., (2018), the flat slopes break between 610 nm and 730 nm this meeting. [13] Allen C. C. et al., (1998) LPSC XXIX. (fig.4). For comparison, a mixture of 90 wt% basalt, 8 [14] Rapin W. et al., (2016), and Planetary Science wt% red hematite and 2 wt% magnetite, produces an Letters 452. [15] Wiens R.C. et al., (2013) Spectrochimica increase of the slope between 610 nm and 730 nm, Acta 82. [16] Grossman J. N. et al., (1997), The Meteoritical Bulletin, 81.[17] Potin S. et al., (2018), Applied Optics, 27. similar to the passive spectrum observed on the sur- [18] Fabre C. et al., (2011), Spectrochimica Acta 66. [19] rounding bedrock directly adjacent to the concretion. Rampe E. B. et al., (2017), Earth and Planetary Science Letters 471. [20] Treiman A. H. et al., (2016), Journal of geophysical research: Planets 121. [21] Cornell R. M. et al., (2003), Wiley.