SYNTHESIS OF BASALTS ANALOGUE TO GUSEV CRATER

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Elemental analyses by ICP

Seven samples were selected to follow the chemical evolution (1) along the groove, with the samples S1-1, S2-1, S3-1 and S5-1, and (2) perpendicularly to the groove, with the samples S3-1 to S3-4 in site 3; the sample S3-1 is located at the intersection of the two series (Fig. 4.3). The geochemical analyses are reported in the Appendice C.
◄Figure 4.4: Total Alkali-Silica (TAS) diagram showing the compositions of fresh basalts from Cyprus ophiolite (red stars; after (1) Bear, 1960, (2) Bear, 1961, (3) Moores and Vine, 1971and (4) Dill et al., 2007. the large red star is mean composition of the fresh basalts. The red dots represent the samples taken along the groove of flowing water. Basalts from Gusev crater, meteorites (after Mc Sween et al., 2009) and artificial basalts are also reported for comparison (see Chapter V and Bost et al., 2012).
No fresh basalt could be sampled in our sampling site to obtain the initial basaltic composition, but chemical analyses of these unaltered basalts in Cyprus are available (Bear 1960, 1961 in Moores et Vine, 1971; Dill et al., 2007). These data are plotted in the TAS diagram in Figure. 4.4 (red stars), showing high variability in composition, in particular on the total alkali parameter Na2O+K2O. Thus, the average composition of the 6 analyses was chosen as a reference (large red star). This average composition is similar to that of the Martian Gusev basalts and to the artificial samples described later (McSween et al., 2009; Bost et al., 2012 and Chapter V), although the Fe- and Mg-contents are very different.
As suggested by Harnois (1988), the weathering corresponds to a depletion of the mobile components with respect to the immobile components. During basalt weathering, Si, Mg, Ca and Na are generally leached but Al and Ti remain essentially in the system and accumulate in the residue (e.g., laterites). Fe and K have more complicated behaviours. The proportion of Fe remaining in the residue is partially a function of the redox conditions of the system, and if K is generally leached during soil formation, clays particles absorb and retain K+. The redox conditions for the host rock in the NW Phoenix pit are reducing (green colour) and become, at the surface, oxidising because of the acidic alteration as shown by the red-orange colour. The chemical index of weathering (CIW), proposed by Harnois (1988), can be applied to soils and to silicate rocks of felsic to mafic composition. It is given by:
with Al2O3, CaO, and Na2O being the mole percent of the corresponding oxide in the material. In this index, Al2O3 represents the immobile component and CaO and Na2O the mobile components. This index increases with weathering. For fresh basalts, the CIW varies around 40 mole% whereas the index of highly weathered basalt is below 80 mole%. Fresh basalts from Cyprus have a low CIW (36.70%; Dill et al., 2007) while altered samples have a CIW between 47 to 99 %, the most altered sample being S5-1 (Table 4.2).
There is no apparent correlation between the CIW and the position of the samples along the groove in Table 4.2. However, the elemental contents in mole% versus the CIW index plotted in Figure 4.5 show that SiO2, CaO, Na2O and K2O decrease, Al2O3, TiO2 and MnO are constant, and Fe2O3, MgO and Cu increase, with an increase in CIW. These results are in good accordance with the Goldschmidt classification (Goldschmidt, 1926) of insoluble precipitating elements (Fe3+, Ti, Mn and Al) and soluble elements (K, Ca, Na, Fe2+, Mg and Si). Even though Mg is a soluble cation, MgO shows a similar behaviour to that of TiO2, which can be explained by its proximity to the insoluble field in the Goldschmidt diagram.
There is no correlation between the CIW with respect to the vertical sampling sequence (along the groove) but there is good correlation with the horizontal sequence: alteration increases closer to the groove. The geochemical analyses shows global alteration associated with tardi-magmatic/deuteritic processes but this does not allow any conclusion to be drawn regarding the different stages of the alteration process.Further mineralogical study is therefore necessary.

Mineralogical characterisation

The mineralogy of the sampled rocks was characterized using XRD analysis, optical observation and Raman spectroscopy. The samples are described using the 3 main alteration facies observed: (1) quartz-chlorite facies, and intermediate facies, (2) smectite facies, and (3) smectite + sulphate facies. Finally, the waste dump mineralogy is also described.

Quartz-chlorite facies.

The massive rocks were originally basalts from the upper pillow basalt lavas (UPL) described in part IV.4.3. The first alteration process which they were submitted to was the hydrothermal and deuteritic weathering, producing a typical quartz-chlorite facies. This mineralogy is observed in the lower level of the open pit (site 5) and corresponds to samples S5-1, S5-2 and S5-3 (locations and macroscopic views are displayed in Fig. 4.6).
XRD analyses (Fig. 4.7) indicates that clays are well organised, and could be chamosite, (Fe,Al,Mg)6(Si,Al)4O10(OH)8. Some traces of gypsum, natroalunite (zeolite) and feldspars are also observed.

Waste dump mineralogy

The waste dump, resulting from enhanced acid leaching to extract residual Cu, was also sampled in order to be compared with materials outcropping along the groove. . Three samples were collected: 11CY05, 11CY06 and 11CY07. Sample 11CY07 contains only a quartz-chlorite assemblage, whereas sample 11CY06 contains only gypsum associated with small quartz grains, natrojarosite and traces of hexahydrite, alunogene and chlorite. At the top of the waste dump, the materials are essentially composed of sulphates. The most abundant one is a hydrated sulphate with a chemical composition falling in the wupatkiite-pickeringite solid solution: ((Co,Mg)Al2(SO4)4,22H2O and MgAl2(SO4)4,22H2O, respectively). Hexahydrite (MgSO4, 6H2O) is the second most abundant hydrated sulphate is . The only phyllosilicates present in the waste dump are highly crystallized chlorite and chamosite.
►Figure 4.14: a- X-ray diffractograms of samples S3-1, S4-1 and S4-2. b- X-ray diffraction patterns of an oriented preparation of sample S3-1. The main mineral phase identified is smectite (Sm) with very low amount of F-feldspar (FK) and quartz (Q). Analyses made on preparation that were air dried(AD), treated with ethylene glycol (EG), and heatied to 350°C and 550°C.

Relevance for Mars exploration

Martian basalts have been altered by different acidic processes during the Noachian and Hesperian periods. Acid-sulphate weathering of basalts is a likely explanation for the sulphate-rich outcrops observed by the MERs Spirit and Opportunity (Golden et al., 2005). For example, the jarosite occurrence (associated with other hydrated sulphates) in Eagle, Fram and Endurance crateroutcrops in Meridiani Planum (Squyres et al., 2004; Clark et al., 2005) could only have been formed by aqueous processes under very acidic conditions (van Breeman, 1980 and Bigham and Nordstron, 2000 in Golden et al., 2005). Phyllosilicates (clays) have been interpreted to be the result of aqueous weathering (e.g. Bibring et al., 2006) but some of them may also be associated with deuteritic processes (Ehlman et al., 2011 and Meunier et al., 2012). The latter phenomenon corresponds to deep-seated alteration of magmatic rocks during the later stages of their formation and is the direct result of magma consolidation. The phyllosilicates precipitate from fluids representing the residual liquid present at the end of the crystallization. Thus the precipitation of phyllosilicates in these conditions can be considered as a sort of hydrothermal phenomenon.
The rocks in the Skouriotissa mine exhibit alteration that is due to both natural processes and human activity. Two types of alteration have occurred. First, a pseudo-deuteritic alteration that resulted in the quartz-chlorite facies, followed by a second weathering process (smectite facies) that is associated with alteration by acidic waters flowing down the groove from the sulphide ore deposit and resulting in the formation of zeolites and sulphates.

Mg enrichment

The Mg-content of the basalts is the discrimant parameter between Martian and terrestrial basalts (cf. Chapters II and III). The Upper Pillow basalt series in the Troodos ophiolite has Mg- and Fe-contents typical of terrestrial basalts and therefore strongly depleted in Mg and Fe with respect to Martian basalts.
The ternary diagram in Fig. 4.16 shows the evolution of the composition of basaltic rocks during weathering. Itcompares typical terrestrial basaltic weathering (Nesbitt and Wilson, 1992), Martian basalt weathering (as suggested by Hurowitz et al., 2006 and Ehlmann et al., 2011), and the weathering observed in Cyprus. The classical terrestrial weathering is associated with an increase in Al and a decrease in Ca, Na, K, and especially in Fe and Mg. The Martian weathering sequence is more difficult to explain. The measurements in Gusev Crater (yellow and green squares) show different evolutions: increase of Ca, Na, K, Al and decrease of Fe and Mg, as well as the contrary. ►Figure 4.16: FM-A-CNK ternary diagram (mole %). FM= total Fe as FeO* + MgO ; A = Al2O3; CNK=CaO + Na2O + K2O (). The diagram shows the chemical evolution of basaltic rocks during terrestrial weathering (grey arrows: Baynton basalt, Australia, described by Nesbitt and Wilson, 1992) compared to the clays analyzed on Mars (white hexagons after Ehlmann et al., 2011). Note the different directions of alteration of the rocks in Gusev crater (green squares for RAT and brown squares for surface; after Hurowitz et al., 2006). The samples from this study correspond to the red dots and exhibit typical terrestrial weathering sequences. This alteration favours the concentration in Fe and Mg, in particular in sample S5-1, shown by the dotted line, and could be similar to a Martian process.

Table of contents :

CHAPTER I THE INTERNATIONAL SPACE ANALOGUE ROCKSTORE
I.1 The Project
I.2. Which analyses?
I.3. Nomenclature and relationship between samples
I.4 The ISAR database
CHAPTER II MARS
II.1. Geological setting
II.2 Life on Mars
II.3. Earth versus Mars
CHAPTER III SELECTION AND ANALYSIS OF THE FIRST GROUP OF MARS-ANALOGUE SAMPLES
III.1. Analogue sample selection
III.2 Sample description
III.3. Results and discussion
III.4. Mineral selection
III.5 Conclusion
CHAPTER IV A NEW MARS ANALOGUE SITE IN CYPRUS
IV.1 Introduction
IV.2. Location
IV.3. Geological Background
IV.4. Sampling
IV.5. Methods
IV.6. Results
IV.7. Discussion
IV.8 Conclusion
CHAPTER V SYNTHESIS OF BASALTS ANALOGUE TO GUSEV CRATER
V.1 Introduction
V.2 Materials and Methods
V.3 Results
V.4 Discussion
V.5 Conclusion
CONCLUSIONS
REFERENCES
SCIENTIFIC PRODUCTION
APPENDICES
APPENDICE A THE CATHODOLUMINESCENCE INSTRUMENT
A.1. Physical principle.
A.2 Instrument optimization
A.3 Cathodoluminescence imagery test
A.5. Conclusions
APPENDICE B EXAMPLE OF DETAILED SAMPLES CHARACTERIZATION FOR THE ISAR: SAMPLES FROM SVALBARD
B.1. Geological Background
B.2 : 09SV15 analysis
B.3: 09SV05 analysis
APPENDICE C: GEOCHEMICAL DATA

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