Experimental petrology of the Mars Pathfinder rock composition: Constraints on the interpretation of Martian reflectance spectra

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007je002983, 2008 Experimental petrology of the Mars Pathfinder rock composition: Constraints on the interpretation of Martian reflectance spectra S. J. Singletary 1 and T. L. Grove 2 Received 8 August 2007; revised 18 July 2008; accepted 14 August 2008; published 26 November [1] Experiments to determine the crystallization sequence of the Mars Pathfinder (MPF) rock composition have been carried out to explore the range of possible mineral assemblages in this SiO 2 -rich (57.4 wt %) Martian igneous rock. Recent Mars Global Surveyor thermal emission spectrometer observations have suggested the presence of large volumes of andesitic lava covering the northern hemisphere of Mars, but this interpretation has been called into question. Experimentally produced mineral assemblages can be used to infer spectral characteristics that would exist in andesitic lavas similar in composition to the MPF rock. The inferred spectra could then be used with remotely sensed data to further constrain the identity of surface rock types on Mars. We performed a series of experiments at 0.1 MPa at the quartz-fayalite-magnetite and iron-quartz-fayalite oxygen buffers and 100 MPa at the Ni-NiO oxygen buffer using the calculated soil free MPF composition. Potential mineralogies for a lava with the MPF composition would be SiO 2 (57 wt %), FeO (18 wt %) rich glass, plagioclase, quartz, augite, and Fe-Ti oxides. A contoured pyroxene quadrilateral (with band I and II minima) was used to predict pyroxene composition on the basis of reflectance spectra. Using the quadrilateral in reverse, the augite produced in the experimental series should have band I and II minima at 0.97 mm and 2.15 mm, respectively. The absence of the 1-mm minima in the Imager for Mars Pathfinder (IMP) spectra could be due to masking phases, possibly the Fe-Ti oxides produced in our experiments at 1000 C. Citation: Singletary, S. J., and T. L. Grove (2008), Experimental petrology of the Mars Pathfinder rock composition: Constraints on the interpretation of Martian reflectance spectra, J. Geophys. Res., 113,, doi: /2007je Introduction [2] Remote sensing missions and subsequent analyses have mapped a variety of distinct surfaces and inferred mineral compositions on the surface of Mars. Observations from the Martian Global Surveyor Thermal Emission Spectrometer (MGS-TES) [Bandfield et al., 2000] have been used to suggest the presence of large volumes of andesitic lava covering the northern hemisphere of Mars. Bandfield [2002] mapped the mineralogy of the Martian surface, revealing high proportions of plagioclase and high calcium pyroxene. Recent results from the OMEGA instrument on board Mars Express have provided mineralogical maps of the Martian surface that also display areas of pyroxene as well as olivine, ferric oxides and hydrated minerals [Mustard et al., 2005; Poulet et al., 2007]. [3] Pyroxene is an important rock forming mineral that is ubiquitous in the inner solar system [Papike, 1998]. Pyroxene compositions span a wide range with a variety of cations 1 Department of Natural Sciences, Fayetteville State University, Fayetteville, North Carolina, USA. 2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. Copyright 2008 by the American Geophysical Union /08/2007JE capable of occupying the crystallographic sites. The nature of the cation in the pyroxene will influence mineral characteristics that can be detected remotely such as thermal emission spectra and reflectance spectra (Figure 1). These remotely sensed characteristics can then be used to infer the composition of surface mineralogies from orbit [Hamilton, 2000; Cloutis, 2002]. Sunshine et al. [1993] demonstrated that the use of reflectance spectra could accurately recover (within 5 10%) relative compositions and abundances of pyroxene in Martian meteorites. [4] Cloutis and Gaffey [1991] developed a band I and II minima contoured pyroxene quadrilateral in order to estimate pyroxene composition on the basis of reflectance spectra and infer the chemical composition of minerals present on remotely sensed surfaces (Figure 2). The band 1-mm minima commonly associated with pyroxene has been observed from both telescopic and orbiter imaging [Mustard and Sunshine, 1995]. Results from the OMEGA instrument display a strong signal for nanophase ferric oxide at the MPF landing site, however the signal for pyroxene is absent [Poulet et al., 2007]. It is important to note that olivine also displays minima in the area of 1-mm but lacks the 2-mm minima displayed by pyroxene [Poulet et al., 2007]. [5] Direct measurements of rocks on the Martian surface were conducted by the Sojourner Rover during the Mars 1of9

2 Figure 1. Representative reflectance spectra of orthopyroxene and clinopyroxene demonstrating the variation between structural groups (opx versus cpx) and the range of minima within each type (see Cloutis and Gaffey [1991] for details). Pathfinder (MPF) mission. Analyses of the Mars Pathfinder (MPF) rocks composition revealed the presence of an extremely silica-rich lava at the MPF landing site (57.4 wt% SiO 2 )[McSween et al., 1999; Wanke et al., 2001] (Tables 1 and 2). A Cross-Iddings-Pierce-Washington (CIPW) normative calculation of the composition [Cross, 1902] reveals that a rock that crystallizes from that composition may have greater than 40% pyroxene as a crystalline phase. No band I minima was observed, however, in the spectra obtained by the Imager for Mars Pathfinder (IMP) taken of rocks and soils at the Pathfinder Site in the mm region [Smith et al., 1997] as would be expected if pyroxene was present in the rocks at the MPF landing site. The presence of high calcium pyroxene, which display minima >1 mm (Figure 1) is not precluded as the IMP filters only extend to mm. [6] In this study, we have carried out experiments using the mean elemental composition of soil free rocks at the MPF landing site (as reported by Wanke et al. [2001]; see Table 1) to determine the crystallization sequence of this silica-rich lava. Glass and mineral phases, along with their compositions, produced in crystallization experiments are then used to explore the possible mineral assemblages that would form under a range of conditions expected for magmatism on Mars. The crystallization products of Martian magmas may be sensitive to changes in oxygen fugacity and by the presence of H 2 O. We explore the influence of these variables on the crystallization of magma with the mean 2of9

3 Figure 2. Pyroxene quadrilateral with contours of band I and II minima (solid lines, numbers are in microns [adapted from Cloutis and Gaffey, 1991]). The dotted lines are isotherms (numbers are degrees Celsius) that indicate pyroxene composition at a given equilibrium temperature [Lindsley, 1983]. Diopside (Di), hedenbergite (Hd), enstatite (En), and ferrosilite (Fs) represent the four end-member pyroxene compositions. MPF rock composition. The conditions of Martian magmatism may then be able to be discerned from remotely sensed data. Ultimately, if sufficient spectral sensitivity and spatial resolution can be achieved in orbital missions, remotely sensed mineralogy of Martian volcanic units might provide valuable information on conditions of magma production in the Martian interior. 2. Experimental Procedure [7] The starting material used for the experiments in this study is a synthetic analog prepared by mixing high-purity Table 1. Mars Pathfinder Rocks Sulphur and Chlorine-Free Mean Bulk Composition a Weight % SiO TiO Al 2 O MnO 0.55 FeO MgO 1.52 CaO 8.15 Na 2 O 2.48 K 2 O 1.37 P 2 O Total a From Wanke et al. [2001]. oxides and Fe sponge as outlined by Grove and Bence [1977]. Experiments were performed at 0.1 MPa and 100 MPa over the temperature range C atthe quartz-fayalite-magnetite oxygen buffer (QFM), the nickelnickel oxide oxygen buffer (NNO) and in iron capsules held in evacuated silica glass tubes [Grove and Bence, 1977]. The oxygen fugacity of the evacuated silica glass tube technique is near that of the iron-quartz-fayalite (IQF) buffer [Grove et al., 2007]. For the 0.1 MPa QFM experiments, mg of the homogenized starting material was pressed into a pellet along with a binding agent. The pellet was then sintered onto a PtFe alloy wire loop ( diameter). The Table 2. CIPW Norm of Composition Reported in Table 1 Percent Quartz Plagioclase Orthoclase 8.07 Albite Anorthite Pyroxene Diopside Hypersthene Fe-Ti oxide Apatite 2.29 Ilmenite 1.31 Total of9

4 wire loop was preannealed with 7 9% Fe to reduce Fe loss from the silicate charge during the experiment [Grove, 1981]. The pellet and PtFe loop were then hung in the hot spot of a Deltech DT31VT vertical quenching furnace. Oxygen fugacity was buffered by fluxing a controlled CO 2 -H gas mixture through the furnace and monitored using azro 2 -CaO electrolyte cell. Experimental durations varied from 144 to 264 h. [8] One cooling rate experiment was conducted to explore the effect of rapid crystallization on the phase appearance sequence and on the compositions of pyroxene that would form under these conditions. The sample was run at QFM conditions and was cooled at 100 C/h from a liquidus temperature of 1200 C. Cooling at these rapid rates should reproduce the conditions of crystallization in the outer centimeter of a surface lava flow. [9] The 100 MPa experiments were carried out under H 2 O saturated conditions in ZHM (zirconium-hafniumcarbide-molybdenum) cold seal pressure vessels following the experimental procedures described by Sisson and Grove [1993a, 1993b]. Oxygen fugacity was controlled at the NNO buffer with buffer assemblages placed into two to three unsealed Pt capsules inside the Au outer capsule. The experimental charge was contained in a Au inner capsule as described by Sisson and Grove [1993a, 1993b]. The pressure vessel was positioned vertically in a Deltech DT31VT furnace and held at pressure and temperature of the experiment for its duration. Experiments were terminated by removing the vessel from the furnace, inverting it and rapping on the hot portion of the vessel with a wrench. The capsule dropped to the water-cooled pressure seal and quenched rapidly with no growth of quench crystals. [10] The experimentally produced phases were analyzed with the MIT JEOL 733 Electron microprobes using wavelength dispersive techniques. Crystalline phases were analyzed at an accelerating voltage of 15 kv and with a beam diameter of 2 mm. Glasses were analyzed with a 10 mm beam size to minimize Na loss during analysis. Data reduction used the CITZAF program of Armstrong [1995] with the atomic number correction of Duncomb and Reed, the Heinrichs tabulation of absorption coefficients and the fluorescence correction of Armstrong. Compositions of minerals and glasses produced in the experiments are reported in Table 3. The majority of the experiments were checked for element gain or loss by a materials balance calculation except where quantitative analysis was precluded by the size and intergrowth of phases. The results of the mass balance provide an estimate of the proportions of phases in each experiment (also reported in Table 3) as well as the amount of Fe gain or loss (expressed as % relative to the total amount in the starting composition). The sum of squared residuals (expressed as SSR in Table 3) indicates how well the experimental products represent the starting composition. 3. Experimental Results: Phase Stability [11] The run products and phase compositions are tabulated in Table 3 and shown graphically in Figure 3. At the QFM buffer, the liquidus of the MPF composition lies between 1075 C and 1100 C. Plagioclase (An 75)(An = anorthite) and a silica phase appear simultaneously as liquidus phases at 1075 C and together represent only 6% of the experimental charge. An iron-rich augite (Wo 29, En 30)(Wo = wollastonite; En = enstatite) joins plagioclase (An 73) and silica in the crystallizing assemblage at 1050 C. At this temperature the silica phase comprises 4% of the charge, plagioclase 4% and pyroxene 1%. The same phases are present at 1025 C: augite (Wo 24, En26), plagioclase (An 72) and silica. The solid phases at 1025 C make up 23% of the experimental charge (silica 8%, plag 12% and augite 3%). At 1000 C the stable phases are plagioclase, augite, silica, liquid and Fe-Ti oxides (predominantly ilmenite). The size and intergrowth of some of the solid phases in this experiment precluded quantitative analysis and therefore no mass balance was attempted. The proportion of solid phases to liquid, however, is greater than that in the experiment at 1025 C. [12] Experiments performed in Fe capsules are estimated to be at the IQF buffer and contain a similar assemblage of phases and the order of phase appearance is identical to that of the QFM experiments. The liquidus is determined to exist between 1075 C and 1050 C, lower than that of the QFM experiments. The first phases to appear at 1050 C are plagioclase (An 78) and a silica phase, which constitute only 5% of the experimental charge. At 1025 C, augite (Wo 31, En 24, 6% of the charge) joins An 65 plagioclase (11% of the charge) and silica (10% of the charge). Fe- and Ti-rich oxides join the assemblage at 1005 C; however, the experiment was extremely fine grained and crystal intergrowth precluded obtaining quantitative chemical analyses. [13] The liquidus temperature for the mean MPF composition at 100 MPa at the NNO buffer is determined to be between 1040 C and 975 C. The order of phase appearance is reversed, with augite (Wo 37, En 27) and Fe- and Ti-rich oxides, representing 11% and 5% of the experimental charge respectively, appearing first at 975 C. Plagioclase (An 92), comprising 5% of the experimental charge, joins augite (Wo 35, En 24), which has now increased to 15% of the experimental charge, and Fe-Ti oxides, now at 7% of the charge, at 950 C. Silica, present in all 0.1 MPa runs was absent over the entire temperature range at 100 MPa. [14] The 100 C/h cooling rate experiment contains glass, pyroxene, oxides, plagioclase and silica. Figure 4 shows the texture of the cooling rate experiment. The crystals are 5 to 10 mm in size and abundant glass is present (>50 wt %). The compositions of the pyroxenes produced in this experiment are higher in Wo content (Wo 46) than the isothermal experiments. The pyroxenes also contain higher abundances of Al and Ti due to a kinetic effect caused by the departure of the crystal/liquid system from equilibrium. As cooling rate increases, the partition coefficients for elements like Ti and Al, which are< 1 under equilibrium conditions, approach unity. Crystallization during rapid cooling incorporates elements in proportion to their abundance in the melt phase. 4. Discussion 4.1. Pyroxenes [15] Pyroxenes are present in six of the fourteen experiments conducted in this study. Pyroxene modal proportions increase dramatically with pressure; however, varying fo 2 did not change modal proportions. The increase in pyroxene 4of9

5 Table 3. Experimental Conditions and Products a Experiment Duration (days) Temperature ( C) Pressure (MPa) Phase b Mode SiO2 c TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total SSR MPF Glass (25) (6) 0.7 (1) 11.5 (2) 16.0 (3) 0.58 (6) 1.70 (9) 8.4 (2) 1.3 (1) 0.97 (5) 0.6 (2) % MPF Glass (14) (2) 0.6 (1) 11.2 (1) 16.8 (3) 0.53 (6) 1.60 (8) 8.6 (1) 1.5 (1) 1.00 (6) 1.2 (2) % MPF Glass (55) (2) 0.72 (6) (7) 16.9 (3) 0.56 (4) 1.71 (4) 8.5 (1) 1.5 (1) 1.01 (3) 0.5 (1) % Plag (13) (7) 29.1 (3) 2.0 (1) 0.21 (1) 14.3 (3) 2.6 (1) 013 (1) Silica MPF Glass (43) (4) 0.8 (1) 10.4 (2) 18.2 (4) 0.66 (6) 1.87 (6) 8.6 (2) 1.4 (2) 1.23 (7) 1.2 (3) % Plag (15) (6) 29.3 (9) 2.1 (7) 0.22 (8) 14.3 (4) 2.8 (2) 0.21 (5) Augite (48) (5) 0.26 (4) 1.3 (3) 23 (2) 1.0 (7) 10.2 (4) 13 (2) 0.05 (7) 98.6 Silica MPF Glass (27) (6) 0.8 (1) 10.0 (2) 19.6 (5) 0.62 (6) 1.41 (6) 8.1 (2) 1.59 (9) 1.23 (7) 0.9 (3) % Augite (21) (5) 0.29 (4) 1.3 (3) 28.0 (2.2) 1.22 (9) 8.8 (4) 11.3 (2.3) 0.07 (5) Plag (6) (1.5) 29.1 (1.3) 2.5 (9) (6) 13.8 (9) 2.9 (5) Silica MPF Glass (6) 59.6 (8) 0.7 (2) 10.3 (2) 15.1 (7) 0.52 (3) 0.76 (4) 5.9 (3) 1.8 (2) 2.4 (1) 0.9 (4) 98 Augite (29) 47.0 (9) 0.26 (9) 2.2 (4) 24 (2) 1.4 (9) 5 (1) 19 (1) 0.3 (1) Fe, Ti oxides d Plag (7) 52.3 (8) 28 (1) 1.8 (4) (5) 13.2 (6) 3.4 (2) 0.32 (5) 99.2 Silica MPF Fe Glass (15) (4) 0.74 (6) 10.7 (2) 16.6 (2) 0.54 (8) 1.60 (6) 8.2 (1) 1.7 (2) 1.07 (2) 0.7 (2) 99.1 Plag (2) (7) Silica MPF Fe Glass (11) (6) 0.6 (2) 11.2 (2) 16.7 (4) 0.49 (8) 1.50 (6) 8.2 (2) 1.7 (2) 0.97 (6) 0.6 (2) 99.5 MPF Fe Glass (9) (8) 0.8 (1) 10.2 (2) 18.5 (4) 0.52 (8) 1.18 (4) 7.9 (1) 1.7 (2) 1.27 (4) 1.0 (2) 98.9 Augite (25) (6) 0.41 (8) 0.9 (4) 26 (2) 1.0 (2) 8.0 (4) 14 (2) 0.08 (4) Plag (6) (4) 29.1 (2) 1.1 (2) 0.09 (4) 12.9 (4) 3.8 (2) 0.25 (2) Silica MPF Fe-4 e Augite Fe, Ti oxides Plag Silica MPF Glass (10) (6) 0.7 (1) 11.1 (1) 15.3 (4) 0.46 (6) 1.48 (6) 8.0 (2) 1.3 (1) 0.8 (1) 0.9 (1) 96.0 MPF Glass (10) (4) 0.7 (1) 11.1 (6) 15 (1) 0.48 (8) 1.5 (2) 8.0 (1) 1.7 (2) 0.8 (1) 0.9 (2) 96.3 MPF Glass (10) (1) 0.28 (6) 13 (1) 9.8 (4) 0.5 (1) 0.96 (6) 7.4 (3) 2.0 (4) 1.3 (2) 0.5 (2) 95.6 Augite (20) (8) 0.22 (6) 1.6 (8) 21 (2) 1.0 (1) 8.9 (6) 17 (1) 0.10 (8) Fe, Ti oxides (9) (6) 5.8 (2) 2.6 (2) 82.5 (4) 0.6 (1) 0.66 (4) 0.28 (4) MPF Glass (8) (1) 0.23 (2) 12.3 (4) 7.5 (4) 0.4 (1) 0.62 (8) 6.2 (4) 2.0 (2) 1.32 (6) 0.3 (1) 96.0 Augite (29) (1) 0.19 (6) 1.7 (8) 23 (2) 1.3 (2) 8.2 (8) 16 (2) Plag (12) (2) 34 (2) 1.4 (4) 0.06 (4) 17.8 (4) 0.9 (2) 0.08 (6) Fe, Ti oxides (6) (1) 5.5 (8) 2.6 (2) 83.4 (6) 0.72 (6) 0.50 (6) 0.3 (1) a Analyses are reported in weight percent oxides. Dash indicates element not analyzed or below detectability limit. b Number in parentheses indicates the number of analyses used for average. c Number in parentheses is one sigma standard deviation in terms of least units cited. Therefore, 38.7(3) should be read as 38.7 ± 0.3 wt %. d The size and intergrowth of solid phases in this experiment precluded the obtaining of quality quantitative data. e The size and intergrowth of solid phases in this experiment precluded the obtaining of quality quantitative data (near solidus/fine grained). %Fe Loss 5of9

6 Figure 3. Graphical representation of the stable phases in all experiments conducted in this study. Crosses represent experiments, and the length of the bar represents appearance temperature of each phase. modal abundance is most likely due to the suppression of plagioclase crystallization at higher-pressure, water-saturated conditions [Sisson and Grove, 1993a]. Pyroxene compositions change with experimental conditions, moving from iron- and calcium-rich at higher temperatures to lower iron and calcium at lower temperature. Pyroxenes at pressure are much more calcium-rich than the 0.1 MPa experiments. [16] Pyroxenes display a pair of absorption bands in the neighborhood of 1 and 2 mm (band I and II) that vary between orthopyroxene and clinopyroxene (see Figure 1), as well as between intermediate members of the solid solution series [Adams, 1974]. Thus, remotely sensed pyroxene bearing rocks may display minima at positions that correspond to the composition of the pyroxene present in the rock (Figure 2). Therefore, the compositions of the experimentally produced pyroxenes could be used to predict the reflectance spectra of pyroxene-bearing rocks by using the band I and II minima contoured pyroxene quadrilateral developed by Cloutis and Gaffey [1991] in reverse (Figure 5). [17] The Mars Pathfinder rover, however, did not detect any minima in the 0.9 to 1.0 mm region using the on board IMP even though the normative modal calculation predicts greater than 40% pyroxene should be present and pyroxene is present in the experimental charges using the same composition. The absence of minima could be due to dust coatings that are ubiquitous on rocks imaged by Pathfinder. Palagonitic dust is known to dampen the mafic silicate bands [Cloutis and Bell, 2004; Singer and Rousch, 1983]. Grain size could also play a role in the lack of distinct bands. Whole rocks and rocks with large grain sizes display weak absorption bands [Adams and Filice, 1967; Hunt and Salisbury, 1970]. [18] The results of this study present several alternate explanations for this major discrepancy. The first possibility is that if the rocks were erupted above the temperature at which pyroxene is stable, in excess of 1050 C at QFM and 1025 C at IQF, and cooled rapidly to a glassy andesite, then pyroxene may not be present in the rocks analyzed by the rover. The presence of the band I and II minima in the global OMEGA data indicate that surface rocks have Figure 4. Backscattered electron (BSE) images of two experiments conducted in this study. (a) Equilibrium experiment (MPF 1) conducted at the QFM buffer displaying the intergrowth of pyroxene, plagioclase, silica, liquid, and Fe-Ti bearing oxides. (b) Cooling rate experiment displaying the fine-grained nature of the crystal intergrowths. 6of9

7 Figure 5. Band I and II contoured pyroxene quadrilateral (see text and Figure 2). Average experimental pyroxene compositions are plotted along with the range of compositions observed in each experiment. Shergottite and nahklite pyroxene compositions are from McSween and Treiman [1998]. variable mineral assemblages, with pyroxene present in many lithologies but locally absent at the MPF landing site. In this case, it may be possible to map eruption temperatures if the presence or absence of pyroxene can be confirmed by in situ microscopic observations on Mars and combining experimentally produced phase assemblages with locally observed spectra. [19] A second possible scenario is that pyroxene is being masked by other phases. If the rocks analyzed by the Sojourner rover cooled and crystallized completely on the surface, the resulting rock would contain abundant Fe-Ti oxides, plagioclase and silica in addition to pyroxene. The relative abundances of the phases in the 0.1 MPa experiments are dominated by plagioclase, silica and oxides. The size and intergrowth of all these phases could prevent detection of the pyroxene absorption signature. It is important to note, that OMEGA results indicate the presence of nanophase ferric oxides in the vicinity of the MPF landing site [Poulet et al., 2007]. If the magma were generated at depth, pyroxene would dominate the signature. Therefore, the presence of pyroxene in the rock, but absence of the pyroxene signature could be used to estimate the depths of magma generation on the Martian surface. Again, however, the mineralogy of the rock would have to be confirmed by in situ microscopic observations. [20] A third possibility presents itself upon examining a plot of experimentally produced pyroxene compositions on the band I and II minima contoured pyroxene quadrilateral (Figure 5). A sharp absorption signature would be produced by pyroxene compositions that array themselves parallel to the minima contour lines. The range of compositions of the experimentally produced pyroxenes however, cut across the contours and would produce broad and diffuse minima. This is true for the isothermal crystallization experiments, but compositional variations would be even more pronounced in a rock that cooled and crystallized at the surface, because compositional zoning would be expected to occur in the pyroxene through fractional crystallization in the cooling flow Cooling Rate Experiment [21] Under conditions of rapid cooling, pyroxene still crystallizes in the MPF composition and would exhibit a distinctive absorption at 1.10 mm in band I and 2.2 mm in band II. The Sojourner rover did not carry any type of rock 7of9

8 abrasion tool and the imaging capabilities of the rover s camera were incapable of resolving any crystal grains in the rocks that were examined. Terrestrial extrusive rocks may form glassy chill margins that can mask the mineralogy present in the interior of the rocks. One would expect from the broken and irregular shapes of the boulders at the Pathfinder site that there might be glassy as well as crystalline surfaces exposed, but textural variations could not be detected Martian Meteorite Pyroxenes [22] Martian meteorites are known to contain pyroxene [McSween and Treiman, 1998]. The compositions of pyroxenes from shergottites and nahklites are plotted in Figure 5. The compositions should generate well defined band I and II minima. The distinctive signatures would allow pyroxene from these meteorites to be distinguished from the compositions of pyroxene generated in our experiments. Rocks similar to those present at the MPF landing site should be distinguishable from the Martian meteorites by remote sensing techniques. As discussed by McSween [1985] the number of possible surface exposures of the lavas parental to Martian meteorites is limited by the young age of the samples. So far, no suitable candidate lavas have been found. Nevertheless, the spectral characteristics of pyroxene in the mean MPF composition and the source flows of Martian meteorites should be distinguishable. 5. Conclusion [23] On the basis of the experiments performed in this study, pyroxene may be absent in the rock if it were erupted above 1050 C and cooled rapidly to a glassy rock. Variations in eruption temperature or cooling rate could explain why a pyroxene signature is present in global imaging but not at the MPF landing site. If the rock was erupted at 1000 C or lower, abundant Fe- and Ti-rich oxides, feldspar, silica and glass could be masking the pyroxene signature. It is assumed that if pyroxene is present in the rocks at the MPF landing site, well defined, narrow minima should be observed in the IMP spectra. This will be true if the pyroxene compositions are in a restricted neighborhood about the average or lay along the band I and II minima contours depicted in Figures 2 and 5. Our experiments show that this may not be the case. Inspection of Figure 5 shows that the range of equilibrium pyroxene compositions cuts across the band I and II contours rather than paralleling them and there is a continual variation in the Wo content of the experimentally produced equilibrium pyroxenes from Wo 37 to Wo 21. These two factors will contribute to a broadening and diffusing of the band I and II minima. Such minima could be present in the spectra but have remained undetected and the absence of narrow minima may not necessarily indicate the absence of pyroxene in the rocks at the MPF landing site. Masking of the pyroxene by opaque minerals remains the simplest way to resolve the observed spectra and experimental results. [24] Acknowledgments. The authors wish to acknowledge the thorough and thoughtful reviews of Ed Cloutis and Robert Carlson. The work was supported by NASA grant NAG from the Mars Data Analysis program to T.L.G. References Adams, J. B. (1974), Visible and near infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system, J. Geophys. Res., 79, , doi: /jb079i032p Adams, J. B., and A. L. Filice (1967), Spectral reflectance 0.4 to 2.0 microns of silicate rock powders, J. Geophys. Res., 72, , doi: / JZ072i022p Armstrong, J. T. (1995), CITZAF A package of correction programs for the quantitative electron micro-beam analysis of thick polished materials, thin films and particles, Microbeam Anal., 4, Bandfield, J. L. (2002), Global mineral distributions on Mars, J. Geophys. 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