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Complex spectral interactions of different minerals and textures in Mars terrestrial analogues: Some examples
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Planetary and Space Science 52 (2004) 141 – 147
www.elsevier.com/locate/pss
Complex spectral interactions of di!erent minerals and textures in Mars terrestrial analogues: Some examples I. Longhi, M. Sgavetti∗ , S. Meli, L. Pompilio Dipartimento di Scienze della Terra, Parco Area delle Scienze, Universita’ di Parma, 157 Parma 43100, Italy Received 17 January 2003; received in revised form 16 June 2003; accepted 29 August 2003
Abstract Mineral interpretation of a planet surface using spectral libraries can be improved if the spectral variability, often characterizing the spectral data, can be explained as the result of well-de4ned geological processes. Re5ectance spectra of metamorphic rocks are analyzed. Selected examples point out systematic and non-systematic relationships between spectral parameters (band wavelength, shape, and re5ectance peaks) and geologically controlled variables (composition of minerals and mineral assemblages), thus establishing a conceptual framework that can facilitate the analysis of unknown spectra. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Re5ectance spectroscopy; Mineral chemistry; Grain size; Rocks; Visible-short wave infrared
1. Introduction The close deadline for future Mars missions using sophisticated spectrometers calls for the development of timely and reliable interpretation procedures for identifying surface mineralogy. The Mars spectroscopy community, gathered in Houston, June 4 – 6, 2002, recommended, among other things, the 4lling of existing gaps both in available spectral libraries and theoretical work, in order to settle “critical current uncertainties in detectability, uniqueness of identi4cation, abundance mapping” of minerals (Kirkland et al., 2002). When exploring the “unknown”, a common procedure is to compare the remotely acquired spectra with spectral libraries, including laboratory spectra of rocks and minerals from the Earth or meteorites. Our knowledge of Mars composition, provided by Earth-based telescopic spectroscopy, remote sensing spectroscopy from orbiting sensors and close-range spectroscopy from rovers (see an overview in Roush et al., 1993; Christensen et al., 2001), allows us to identify terrestrial analogues from which speci4c spectral libraries can be created. Even in this case however, the range of variability of diagnostic absorption features is too ∗ Corresponding author. Tel.: +39-0521-905361; fax: +39-0521905305. E-mail address: [email protected] (M. Sgavetti).
0032-0633/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2003.08.025
large for an all-inclusive spectral library. This variability depends on several factors, among which complex geologic processes are of primary importance. Thus, a viable approach, complementary to the use of spectral libraries, is to establish theoretical ties between spectroscopic variability and the e!ects of geologic processes. Starting from rock suites from a complex, pre-Paleozoic geologic context, we discuss problems related to absorption band modi4cations due to mineral composition variability and grain size variation: two variables a!ecting both the accuracy of rock and regolith identi4cation and mineral content quantitative determination (e.g., Clark and Roush, 1984; Mustard and Pieters, 1987; Clark, 1995). The discussion is based on examples of laboratory re5ectance spectroscopy of the rocks (Longhi, 1996; Longhi et al., 2000, 2001), with particular attention to the geologic processes that have been recognized to exert a signi4cant control on the variables a!ecting the spectroscopic behavior. The spectral region considered here is the already well studied 2.0 –2:5 m interval, where vibrational overtones and combinations of tones occur, highly sensitive to small compositional variations of the minerals. Improved knowledge of the mineral spectral behavior in this region can be useful for remote interpretation of Martian spectra and will provide the basis for subsequent modeling in the laboratory.
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2. Problems in the spectroscopy of geological materials The interaction of light with mineral electronic and molecular systems produces a set of absorption and scattering peaks associated with particular structural groups within the mineral. Frequency, or wavelength position, symmetry and relative intensity of observed peaks are sensitive, for given temperature and pressure values, to the mineral composition and crystal structure (e.g., Burns, 1970; Adams, 1974; Nash and Conel, 1974; King and Ridley, 1987; Cloutis and Ga!ey, 1991; Clark, 1995; Cloutis, 1996, and references therein). In nature, the composition of a mineral may vary owing to ionic substitution and the presence of impurities. In a rock, i.e., an association of di!erent minerals, the relative abundance of the components may vary, depending on several causes. Variations in composition may generate during mineral and rock genesis and subsequent alteration, according to well-de4ned geochemical processes in speci4c geologic settings, or may occur through unpredictable contamination or mixing in various geological environments. In the 4rst case, understanding the geological and geochemical processes responsible for systematic variations in mineral and rock composition can be a way to reduce the uncertainty in interpreting frequency variation in diagnostic spectral features. The second case mostly refers to mineral mixtures originated by sedimentary processes, and further analyses of how spectral convergence and interaction among di!erent minerals a!ect speci4c absorption bands still remains the best means to establish criteria for understanding distorted bands and unmixing composite bands. A second order of problems is related to radiation transfer within rocks and soils, controlled by grain size, shape and optical properties in such discontinuous and anisotropic media (e.g., Clark and Roush, 1984; Mustard and Pieters, 1987). A large number of papers provide theoretical treatments and the necessary analytical solutions (e.g., Hapke, 1981), but more laboratory and 4eld measurements are needed to increase the number of observational and experimental inputs for theoretical models. 3. Spectral variability in the VIS-SWIR region: examples and discussion 3.1. Spectral variability and mineral identi/cation Many of the minerals supposed to be components of the Mars surface were analyzed through laboratory and remote sensing spectroscopy of rocks exposed on the Earth surface, i.e., rocks whose origin and geologic history can be easily determined in many cases. Here, we show examples of the relationships between the spectroscopic properties of some of these minerals and the compositional characteristics they acquired through complex geologic processes or when they are associated with other minerals. In both cases, the shape and position of the absorption features are
Fig. 1. (a) Al–O–H absorption band in muscovite in a suite of quartzites and (b) relationship between the band position in a suite of quartzites (full diamonds) and micaschists (open circles), and AlVI abundance. The AlVI content is related to the rock metamorphic grade. Deviation from the curve of two micaschist samples (open triangles) is due to high AlVI content related to near-ideal phengite substitution. Equations of the regression curves and relative R2 are reported.
Fig. 2. Spectra of granofels containing altered plagioclases, showing the CO2− absorption band. The intensity of the band is related to the 3 abundance of calcic plagioclase.
modi4ed, with signi4cant e!ects on the accuracy of mineral recognition. In the following, total re5ectance SWIR spectra are discussed, measured with a Perkin-Elmer lambda 19 spectrometer, using 60 mm integrating sphere and KBr standard. Examples of systematic spectral variations are shown in Figs. 1–3. In Fig. 1, the wavelength of the 2200 nm Al–O– H vibrational absorption band in muscovite shifts gradually towards shorter values (Fig. 1a). For a given rock bulk composition, this variation was observed in a suite of quartzites and micaschists from the Madagascar basement, and for each group of rocks the wavelength shift is inversely, linearly correlated with the content of Al in octahedral sites (AlVI ) of the mineral (Fig. 1b). The involved geochemical process is the ionic AlVI enrichment in muscovite, controlled by pressure and temperature conditions. In both groups of rocks, the increase of AlVI is signi4cantly correlated with a progressive decrease of phengite substitution and to an increase of paragonite substitution, respectively, and thus, for any 4xed rock bulk chemistry, it can be related to an increase of metamorphic grade (Duke, 1994; Longhi et al., 2000).
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Fig. 3. (a) Re5ectance variation at 605 nm, due to photoluminescence in a suite of impure marbles; (b) increasing re5ectance is related with increasing Mn/Fe ratio. Equation of the regression curve and relative R2 are reported.
In the case reported here, Fe3+ –Al exchange may control the slope of the regression lines in the 4gure, i.e. the rate of the spectral variation (Longhi et al., 2000). In other geologic settings, a variation of the Al/Fe ratio, in particular of the AlVI content, has been reported as due to hydrothermal rock alteration during 5uid/rock interactions, and also in this case it is closely correlated with the Al–O–H band position (Swayze et al., 1992). In a suite of uniformly coarse grained (500 m up to millimetric) granofels, consisting of more that 90% quartz and plagioclase, plagioclase alteration produces speci4c mineral associations. Increasing Ca content in plagioclase is positively correlated with a deeper absorption band at 2332–2335 nm (Fig. 2; Longhi et al., 2001). Plagioclase alteration is in5uenced by the interaction among di!erent factors, such as temperature, PPCO2 , pH. In particular, the temperature controls the formation of distinct mineral associations. At relatively high temperature, calcite + sericite + epidote develop through processes such as 5uid-present metamorphism and igneous deuteresis. At low temperature, surface weathering processes, requiring liquid water, originate calcite + kaolinite + smectite. In the samples considered here, calcite and epidote were observed in the most anorthitic sample, both contributing to the 2332–2335 nm band, whereas hydrated mineral bands were observed in the most albitic samples. A systematic enhancement in the re5ectance curve in the 590 –680 nm region was observed for a suite of impure marbles (Fig. 3; Longhi, 1996), and this anomalous spectral radiance was interpreted as due to photoluminescence. Luminescence in carbonates can be due to impurities in the mineral and crystal lattice defects (Machel et al., 1991). In the marbles considered here, Mn2+ substitutes for Ca2+ in calcite and Ca2+ and Mg2+ in dolomite. Iron acts as a quencher, so that the relative abundance of Mn and Fe can be indicative of the luminescence intensity. Fig. 3b shows an overall direct relationship between the Mn/Fe ratio and the percentage of total radiance at 605 nm for this suite of rocks. A non-systematic spectral variability is shown by carbonate rocks containing various amounts of di!erent
impurities (Longhi et al., 2001), due to 5uid/rock interaction. In a suite of calcitic to calcitic–dolomitic marbles, two distinct groups of samples can be recognized, as shown in Fig. 4. In one group of samples, calcite is predominant (full circles in Fig. 4b), with a grain size of 100 –500 m. Rare impurities, represented by phlogopite (with grain size of a few microns) produce an increase in left hand band asymmetry of the CO2− 3 band. In the samples of the second group (full diamonds in the 4gure) containing calcite and dolomite with comparable grain size of 100 –200 m, the presence of relatively abundant chlorite and anorthite tends to reduce the band asymmetry. In the examples discussed above, the active geochemical process is related to 5uid/rock interactions. Both mineral chemistry variation and enrichment of accessory minerals require adequate cation concentrations in the primary solution or diagenetic 5uid, and are controlled by P–T conditions and the areal extent of the 5uid–rock interface. When the 5uid/rock interactions produce variations in the crystal chemical composition of minerals, then the variation of the spectral properties is systematic. On the contrary, in the case of accessory mineral enrichment in a rock or sediment, the trend of the spectral variation is less predictable, as the water or 5uid can be almost randomly enriched in various elements. Nevertheless, even in this case the interpretation of the band distortion may contribute information about the geologic environment of rock genesis or alteration. 3.2. Spectral variability and mineral abundance determination The quantitative determination of the mineral content in a rock or sediment is sensitive to the same spectral feature modifying factors previously reported, as well as to other additional factors. Scattering e!ects in the VIS-SWIR are responsible for the spectral features of di!erent minerals in a mixture not combining linearly, so that the absorption intensity, often expressed by band depth and area, depends on the amount of the spectrally active mineral, and the grain size distribution in the mixture (see a review in Clark, 1995).
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Fig. 4. (a) Spectroscopic variability of CO2− band in a suite of impure marbles; (b) band asymmetry variation related to accessory minerals with 3 absorption bands at a wavelength close to that of the carbonatic band. Asymmetry is expressed as the ratio between the right and left portions, with respect to the band peak, of the continuum removed band area. Full circles: calcitic marbles; full diamonds: calcitic–dolomitic marbles. Open symbols: accessory minerals: Chl: chlorite bands at approximately 2340 nm; Phl: phlogopite band at approximately 2325 nm; An: anorthite.
Fig. 5. Relationship between Al–O–H band intensity and muscovite content and grain size in a suite of quartzites: (a): relationship between muscovite abundance and grain size; (b,c): relationship between band depth and muscovite grain size and percentage, respectively. Equations of the regression curves and relative R2 are reported.
In Figs. 5 and 6 the two variables are examined for a suite of quartzites and micaschists. The quartzites almost entirely consist of quartz with large grain size (500 –3000 m), and 2%–10% of muscovite (grain size from 40 to 500 m). Biotite, with a grain size ranging between 40 and 300 m, has been observed in just one sample (#66). Tourmaline is present as accessory mineral and no opaque minerals have been found. In these rocks, the grain size is correlated with the abundance of muscovite (Fig. 5a). The intensity of the 2200 nm Al–O–H band appears to be directly and linearly correlated with both the grain size and the muscovite abundance (Fig. 5b,c), i.e., particle dimension and mineral abundance
of the spectroscopically active mineral act in the same sense in determining the absorption band intensity. The micaschists include quartz (grain size from 50 to 1000 m), biotite, muscovite (15 –25%), k-feldspar (rare, 4ne grained), tourmaline as accessory mineral, and no opaques. Muscovite and biotite have comparable grain size in individual samples, and vary from 10 to 900 m through the di!erent samples. Some of the coarsest samples also contain sillimanite (up to millimetric size) and plagioclase. In these rocks, mineral grain size and abundance are not correlated (Fig. 6a), and a very weak correlation exists also between Al–O–H absorption band depth and both muscovite grain size and abundance (Fig. 6b, c). The 4ne
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Fig. 6. Relationship between Al–O–H band intensity and muscovite content and grain size in a suite micaschists: (a): relationship between muscovite abundance and grain size; (b,c): relationship between band depth and muscovite grain size and percentage, respectively.
Fig. 7. Relationship between muscovite abundance and AlVI content (a), and between band intensity and AlVI content, in quartzites. Equations of the regression curves and relative R2 are reported.
grained samples, with muscovite grains below or equal to 50 m, are characterized by extremely variable absorption band depth. The coarse grained samples (above 200 m), with a relatively low muscovite content (between 15% and 20%), display invariant band depth values. In both cases, band depth variation is mostly irrespective of both muscovite abundance and grain size. The divide between the two groups of samples roughly corresponds to the grain size dimension above which the absorption intensity modi4es the overall albedo, with consequent band depth attenuation (Clark, 1995), but within each group the band depth variation is not consistent with that observed for pure mineral mixtures. Although in some cases the samples discussed above behave consistently with the results of experiments in the SWIR using synthetic mixtures of pure minerals, they often present signi4cant deviations from what is expected from laboratory simulations. One question that appears to be of particular interest is whether, and to what extent, the chemistry of the spectroscopically active mineral, responsible for
the absorption peak wavelength shift, may a!ect the band depth. This question arises from the observation that, in samples in which mineral content and grain size are correlated (Fig. 5a), the depth of the 2200 nm Al–O–H band of muscovite is also related with the mineral abundance (Fig. 5c), and mineral abundance, as shown in Fig. 7a, is in turn related to AlVI content. Hence, the abundance of the spectroscopically active AlVI –OH bond in muscovite directly correlates with the band depth, as shown in Fig. 7b. This is veri4ed for the quartzites discussed above, but in the case of micaschists, not only the muscovite abundance is not obviously related to the Al–O–H band intensity (Fig. 6c), due to the complex mineral assemblage and grain size, but also a roughly inverse, not clearly de4ned relationship exists between muscovite and AlVI abundance (Fig. 8a). Consequently, as shown in Fig. 8b, the relationship between band depth and AlVI abundance in muscovite has no signi4cant trend, even when considering coarse grained samples (larger than 200 m, full
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Fig. 8. Relationship between muscovite abundance and AlVI content (a), and between band intensity and AlVI content (b), in micaschists. Open symbols indicate very 4ne grained samples.
symbols in Fig. 8b), for which the grain size e!ect should be negligible. This means that additional minerals, such as the plagioclase found in the coarsest samples, probably contribute in this case to the 2200 nm band intensity. Further work should be therefore focused on understanding the reciprocal role of muscovite and plagioclase in band intensity determination, as well as the e!ects induced by sillimanite included, in some samples, within large muscovite grains. 4. Discussion and conclusions A large number of papers give the extent of the e!ort done to establish systematic relationships between spectral features and both compositional and grain size variations in minerals and mineral mixtures. However, people working with natural geological materials know that both compositional and textural variations of rocks and component minerals can be the result of well-de4ned geological, geochemical and sedimentary processes, systematically controlled by individual factors or a combination of speci4c factors. This paper reports examples of spectroscopic variations, the trend of which is related to speci4c geochemical processes at mineral or mineral association scale. As these processes may occur in di!erent geologic contexts, the spectroscopic variability observed in one geologic setting can give insight for process interpretation also in di!erent situations. However, since the geochemical processes represent only one category of controlling factors in the geologic history of a region, the 4nal interpretation of a geologic context will always require the integration of data of di!erent nature. Two types of problems are presented in this paper. One concerns systematic and non-systematic variations of absorption band wavelength and shape, as a response to mineral chemistry and rock composition variation generated by di!erent geologic processes. The second one includes a few topics for a discussion relative to mineral abundance determination in a rock or sediment, based on band depth evaluation. Rock genesis, diagenesis and alteration can produce systematic chemical and structural variations in individual minerals, with consequent variation of the spectral parameters.
When one of these minerals is spectroscopically dominant in a rock suite, these variations can also be observed in the rocks. Each variation may be generated by geochemical processes occurring in distinct geologic contexts, such as magmatic or tectonic contexts, associated with high temperature or pressure conditions; or near surface contexts, that is, under low temperature and pressure conditions. One example is represented by the spectroscopic e!ects of AlVI content variation in Al–O–H group in muscovite. They were observed as related to the rock metamorphic grade in a suite of quartzites and micaschists (this paper), as well as they were reported as related to hydrothermal alteration in volcanic rocks (Swayze et al., 1992). Mineral alteration can also produce systematic absorption band deepening, as observed for the CO23 produced by the alteration of calcic plagioclase into calcite+epidote (see the second example above) although the exact nature of the relationship is not clearly established. The e!ects of cationic substitution were shown for a suite of impure marbles (third example above). Mn2+ substitution for Ca2+ and Mg2+ in calcite and dolomite, respectively, is responsible for increased re5ectance in the visible, interpreted as due to photoluminescence. Also in this case, re5ectance variability is systematically (directly and linearly) related to the luminescence activator and inhibitor ratio. Variation in the mineral assemblage in a rock or sediment produces non-systematic variations in the overall rock composition, and thus in the spectral response. One of the most common geochemical processes responsible for this behavior is the interaction between the rocks or sediments and the enriched permeating 5uid, and generally results into a new, unpredictable mineral association. In this case too, the geochemical process may occur in di!erent geologic settings. A sedimentary context was inferred in the example reported in this paper, where the distortion of the carbonate band in marbles is interpreted as due to the contiguity with absorption bands of other minerals, indicating the presence of a 5uid that had probably been in contact with a magma. Although the distortions are not predictable, further analysis may have important implications for carbonate recognition and interpretation of their genetic environment. Mineral abundance determination in a rock or sediment requires an accurate analysis of the complex interactions
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between mineral assemblage composition and grain size. In the examples discussed in this paper, the spectral behavior in some cases reproduces the results predicted by theoretical work, whereas in some other cases it displays a much higher complexity than expected, even from laboratory simulations. Paradoxically, the most interesting case is probably represented by quartzites for which the compositional and textural variables are each other highly correlated, suggesting that in nature simple cases may be present. From this end member case, it results that a quantitative mineral determination based on band intensity is reliable when the rock contains only one spectroscopically active mineral in a given spectral range, when mineral abundance and grain sizes are correlated, and when mineral abundance is related to the abundance of the spectroscopically active group in the mineral. In the micaschists, perhaps representing one of the most complex end member cases, mineral chemistry and structure, along with mineral assemblage composition, interact in determining the band intensity, over which the grain size e!ects are superimposed. As a result, the observed complexity is higher than that than ever simulated in laboratory experiments. The geochemical processes investigated in the examples above include rock genesis, diagenesis, and alteration, related to speci4c geologic context and evolution and to the climatic history of the Earth. Not all the examples are probably suitable as analogues to Mars, even if the analysis and understanding of the relationships between spectroscopic properties and terrestrial geologic processes can contribute to derive more general criteria, useful also for other planets exploration, facilitating data interpretation and reducing the number of end members needed for surface exploration. Acknowledgements The authors are indebted to Ted Roush and Fabrizio Capaccioni for the accurate reviews and the useful suggestions that signi4cantly improved the original manuscript. References Adams, J.B., 1974. Visible and near-infrared di!use re5ectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system. J. Geophys. Res. 79, 4829–4836. Burns, R.G., 1970. Crystal 4eld spectra and evidence of cation ordering in olivine minerals. Am. Mineral. 55, 1608–1633. Christensen, P.R., Band4eld, L.J., Hamilton, V.E., Ru!, S.W., Kie!er, H.H., Titus, T.N., Malin, M.C., Morris, R.V., Lane, M.D., Clark, R.L., Jakosky, B.M., Mellon, M.T., Pearl, J.C., Conrath, B.J.,
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Smith, M.D., Clancy, R.T., Kuzmin, R.O., Roush, T., Mehall, G.L., Gorelick, N., Bender, K., Murray, K., Dason, S., Greene, E., Silverman, S., Green4eld, M., 2001. Mars global surveyor thermal emission spectrometer experiment: investigation description and surface science results. J. Geophys. Res. 106, 23,823–23,871. Clark, R.N., 1995. Re5ectance spectra. In: Ahrens, T.J (Ed.), Rock Physics and Phase Relations, A Handbook of Physical Constant, AGU, Washington, D.C., Reference Shelf, Vol. 3, pp. 178–187. Clark, R.N., Roush, T.L., 1984. Re5ectance spectroscopy: quantitative analysis techniques for remote sensing applications. J. Geophys. Res. 89, 6329–6340. Cloutis, E.A., 1996. Hyperspectral geological remote sensing: evaluation of analytical techniques. Int. J. Remote Sensing 17, 2215–2242. Cloutis, E.A., Ga!ey, M.J., 1991. Pyroxene spectroscopy revisited: spectral-compositional correlations and relationship to geotherometry. J. Geophys. Res. 96, 22,809–22,826. Duke, F.E., 1994. Near infrared spectra of muscovite, Tschermak substitution, and metamorphic reaction progress: implications for remote sensing. Geology 22, 621–624. Hapke, B., 1981. Bidirectional re5ectance spectroscopy. 1. Theory. J. Geophys. Res. 86, 3039–3054. King, T.V.V., Ridley, W.I., 1987. Relation of the spectroscopic re5ectance of olivine to mineral chemistry and some remote sensing implications. J. Geophys. Res. 94, 11,457–11,469. Kirkland, L., Mustard, J., McAfee, J., Hapke, B., Ramsay, M., 2002. Mars infrared spectroscopy: from theory and the laboratory to 4eld observations, 4 – 6 June 2002, LPI contribution 1148, www.lpi.usra,edu/science. Longhi, I., 1996. Studio dei principali litotipi aNoranti nell’area di Ambati4nandrahana Madagascar) con integrazione di dati multispettrali telerilevati e analisi petrogra4che e spettroscopiche di laboratorio. Ph.D. Thesis, UniversitPa di Padova, Italy. Longhi, I., Mazzoli, C., Sgavetti, M., 2000. Determination of metamorphic grade in siliceous muscovite-bearing rocks in Madagascar using re5ectance spectroscopy. Terra Nova 12, 21–27. Longhi, I., Sgavetti, M., Chiari, R., Mazzoli, C., 2001. Spectral analysis and classi4cation of metamorphic rocks from laboratory re5ectance spectra in the 0.4 –2:5 m interval: a tool for hyperspectral data interpretation. Int. J. Remote Sensing 22, 3763–3782. Machel, H.G., Mason, R.A., Mariano, A.N., Mucci, A., 1991. Causes and emission of luminescence in calcite and dolomite. In: Barker, C.E., Kopp, O.C. (Eds.), Luminescence Microscopy and Spectroscopy: Qualitative and Quantitative Applications, SEPM Short Course, Vol. 25. Dallas, TX, pp. 9 –15. Mustard, J.F., Pieters, C.M., 1987. Quantitative abundance estimates from bidirectional re5ectance measurements. J. Geophys. Res. 92, E617–E626. Nash, D.B., Conel, J.E., 1974. Spectral re5ectance systematics for mixtures of powdered hypersthene, labradorite, and illmenite. J. Geophys. Res. 79, 1615–1621. Roush, T.L., Blaney, D.L., Singer, R.B., 1993. The surface composition of Mars as inferred from spectroscopic observations. In: Pieters, C.M., Englert, P.A. (Eds.), Remote Geochemical Analysis: Elemental and Mineralogical Composition. Cambridge University Press, Cambridge, pp. 367–393. Swayze, G.A., Clark, R.N., Cruse, F., Sutley, S., Gallagher, A., 1992. Ground-truthing AVIRIS mineral mapping at Cuprite, Nevada. In: Green, R.O. (Ed.), Summaries of the Third Annual JPL Airborne Geoscience Workshop, Vol. 1. Jet Propulsion Laboratory Publication, Pasadena, C.A., pp. 92–14, 47– 49.
Planetary and Space Science 52 (2004) 141 – 147
www.elsevier.com/locate/pss
Complex spectral interactions of di!erent minerals and textures in Mars terrestrial analogues: Some examples I. Longhi, M. Sgavetti∗ , S. Meli, L. Pompilio Dipartimento di Scienze della Terra, Parco Area delle Scienze, Universita’ di Parma, 157 Parma 43100, Italy Received 17 January 2003; received in revised form 16 June 2003; accepted 29 August 2003
Abstract Mineral interpretation of a planet surface using spectral libraries can be improved if the spectral variability, often characterizing the spectral data, can be explained as the result of well-de4ned geological processes. Re5ectance spectra of metamorphic rocks are analyzed. Selected examples point out systematic and non-systematic relationships between spectral parameters (band wavelength, shape, and re5ectance peaks) and geologically controlled variables (composition of minerals and mineral assemblages), thus establishing a conceptual framework that can facilitate the analysis of unknown spectra. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Re5ectance spectroscopy; Mineral chemistry; Grain size; Rocks; Visible-short wave infrared
1. Introduction The close deadline for future Mars missions using sophisticated spectrometers calls for the development of timely and reliable interpretation procedures for identifying surface mineralogy. The Mars spectroscopy community, gathered in Houston, June 4 – 6, 2002, recommended, among other things, the 4lling of existing gaps both in available spectral libraries and theoretical work, in order to settle “critical current uncertainties in detectability, uniqueness of identi4cation, abundance mapping” of minerals (Kirkland et al., 2002). When exploring the “unknown”, a common procedure is to compare the remotely acquired spectra with spectral libraries, including laboratory spectra of rocks and minerals from the Earth or meteorites. Our knowledge of Mars composition, provided by Earth-based telescopic spectroscopy, remote sensing spectroscopy from orbiting sensors and close-range spectroscopy from rovers (see an overview in Roush et al., 1993; Christensen et al., 2001), allows us to identify terrestrial analogues from which speci4c spectral libraries can be created. Even in this case however, the range of variability of diagnostic absorption features is too ∗ Corresponding author. Tel.: +39-0521-905361; fax: +39-0521905305. E-mail address: [email protected] (M. Sgavetti).
0032-0633/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2003.08.025
large for an all-inclusive spectral library. This variability depends on several factors, among which complex geologic processes are of primary importance. Thus, a viable approach, complementary to the use of spectral libraries, is to establish theoretical ties between spectroscopic variability and the e!ects of geologic processes. Starting from rock suites from a complex, pre-Paleozoic geologic context, we discuss problems related to absorption band modi4cations due to mineral composition variability and grain size variation: two variables a!ecting both the accuracy of rock and regolith identi4cation and mineral content quantitative determination (e.g., Clark and Roush, 1984; Mustard and Pieters, 1987; Clark, 1995). The discussion is based on examples of laboratory re5ectance spectroscopy of the rocks (Longhi, 1996; Longhi et al., 2000, 2001), with particular attention to the geologic processes that have been recognized to exert a signi4cant control on the variables a!ecting the spectroscopic behavior. The spectral region considered here is the already well studied 2.0 –2:5 m interval, where vibrational overtones and combinations of tones occur, highly sensitive to small compositional variations of the minerals. Improved knowledge of the mineral spectral behavior in this region can be useful for remote interpretation of Martian spectra and will provide the basis for subsequent modeling in the laboratory.
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2. Problems in the spectroscopy of geological materials The interaction of light with mineral electronic and molecular systems produces a set of absorption and scattering peaks associated with particular structural groups within the mineral. Frequency, or wavelength position, symmetry and relative intensity of observed peaks are sensitive, for given temperature and pressure values, to the mineral composition and crystal structure (e.g., Burns, 1970; Adams, 1974; Nash and Conel, 1974; King and Ridley, 1987; Cloutis and Ga!ey, 1991; Clark, 1995; Cloutis, 1996, and references therein). In nature, the composition of a mineral may vary owing to ionic substitution and the presence of impurities. In a rock, i.e., an association of di!erent minerals, the relative abundance of the components may vary, depending on several causes. Variations in composition may generate during mineral and rock genesis and subsequent alteration, according to well-de4ned geochemical processes in speci4c geologic settings, or may occur through unpredictable contamination or mixing in various geological environments. In the 4rst case, understanding the geological and geochemical processes responsible for systematic variations in mineral and rock composition can be a way to reduce the uncertainty in interpreting frequency variation in diagnostic spectral features. The second case mostly refers to mineral mixtures originated by sedimentary processes, and further analyses of how spectral convergence and interaction among di!erent minerals a!ect speci4c absorption bands still remains the best means to establish criteria for understanding distorted bands and unmixing composite bands. A second order of problems is related to radiation transfer within rocks and soils, controlled by grain size, shape and optical properties in such discontinuous and anisotropic media (e.g., Clark and Roush, 1984; Mustard and Pieters, 1987). A large number of papers provide theoretical treatments and the necessary analytical solutions (e.g., Hapke, 1981), but more laboratory and 4eld measurements are needed to increase the number of observational and experimental inputs for theoretical models. 3. Spectral variability in the VIS-SWIR region: examples and discussion 3.1. Spectral variability and mineral identi/cation Many of the minerals supposed to be components of the Mars surface were analyzed through laboratory and remote sensing spectroscopy of rocks exposed on the Earth surface, i.e., rocks whose origin and geologic history can be easily determined in many cases. Here, we show examples of the relationships between the spectroscopic properties of some of these minerals and the compositional characteristics they acquired through complex geologic processes or when they are associated with other minerals. In both cases, the shape and position of the absorption features are
Fig. 1. (a) Al–O–H absorption band in muscovite in a suite of quartzites and (b) relationship between the band position in a suite of quartzites (full diamonds) and micaschists (open circles), and AlVI abundance. The AlVI content is related to the rock metamorphic grade. Deviation from the curve of two micaschist samples (open triangles) is due to high AlVI content related to near-ideal phengite substitution. Equations of the regression curves and relative R2 are reported.
Fig. 2. Spectra of granofels containing altered plagioclases, showing the CO2− absorption band. The intensity of the band is related to the 3 abundance of calcic plagioclase.
modi4ed, with signi4cant e!ects on the accuracy of mineral recognition. In the following, total re5ectance SWIR spectra are discussed, measured with a Perkin-Elmer lambda 19 spectrometer, using 60 mm integrating sphere and KBr standard. Examples of systematic spectral variations are shown in Figs. 1–3. In Fig. 1, the wavelength of the 2200 nm Al–O– H vibrational absorption band in muscovite shifts gradually towards shorter values (Fig. 1a). For a given rock bulk composition, this variation was observed in a suite of quartzites and micaschists from the Madagascar basement, and for each group of rocks the wavelength shift is inversely, linearly correlated with the content of Al in octahedral sites (AlVI ) of the mineral (Fig. 1b). The involved geochemical process is the ionic AlVI enrichment in muscovite, controlled by pressure and temperature conditions. In both groups of rocks, the increase of AlVI is signi4cantly correlated with a progressive decrease of phengite substitution and to an increase of paragonite substitution, respectively, and thus, for any 4xed rock bulk chemistry, it can be related to an increase of metamorphic grade (Duke, 1994; Longhi et al., 2000).
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Fig. 3. (a) Re5ectance variation at 605 nm, due to photoluminescence in a suite of impure marbles; (b) increasing re5ectance is related with increasing Mn/Fe ratio. Equation of the regression curve and relative R2 are reported.
In the case reported here, Fe3+ –Al exchange may control the slope of the regression lines in the 4gure, i.e. the rate of the spectral variation (Longhi et al., 2000). In other geologic settings, a variation of the Al/Fe ratio, in particular of the AlVI content, has been reported as due to hydrothermal rock alteration during 5uid/rock interactions, and also in this case it is closely correlated with the Al–O–H band position (Swayze et al., 1992). In a suite of uniformly coarse grained (500 m up to millimetric) granofels, consisting of more that 90% quartz and plagioclase, plagioclase alteration produces speci4c mineral associations. Increasing Ca content in plagioclase is positively correlated with a deeper absorption band at 2332–2335 nm (Fig. 2; Longhi et al., 2001). Plagioclase alteration is in5uenced by the interaction among di!erent factors, such as temperature, PPCO2 , pH. In particular, the temperature controls the formation of distinct mineral associations. At relatively high temperature, calcite + sericite + epidote develop through processes such as 5uid-present metamorphism and igneous deuteresis. At low temperature, surface weathering processes, requiring liquid water, originate calcite + kaolinite + smectite. In the samples considered here, calcite and epidote were observed in the most anorthitic sample, both contributing to the 2332–2335 nm band, whereas hydrated mineral bands were observed in the most albitic samples. A systematic enhancement in the re5ectance curve in the 590 –680 nm region was observed for a suite of impure marbles (Fig. 3; Longhi, 1996), and this anomalous spectral radiance was interpreted as due to photoluminescence. Luminescence in carbonates can be due to impurities in the mineral and crystal lattice defects (Machel et al., 1991). In the marbles considered here, Mn2+ substitutes for Ca2+ in calcite and Ca2+ and Mg2+ in dolomite. Iron acts as a quencher, so that the relative abundance of Mn and Fe can be indicative of the luminescence intensity. Fig. 3b shows an overall direct relationship between the Mn/Fe ratio and the percentage of total radiance at 605 nm for this suite of rocks. A non-systematic spectral variability is shown by carbonate rocks containing various amounts of di!erent
impurities (Longhi et al., 2001), due to 5uid/rock interaction. In a suite of calcitic to calcitic–dolomitic marbles, two distinct groups of samples can be recognized, as shown in Fig. 4. In one group of samples, calcite is predominant (full circles in Fig. 4b), with a grain size of 100 –500 m. Rare impurities, represented by phlogopite (with grain size of a few microns) produce an increase in left hand band asymmetry of the CO2− 3 band. In the samples of the second group (full diamonds in the 4gure) containing calcite and dolomite with comparable grain size of 100 –200 m, the presence of relatively abundant chlorite and anorthite tends to reduce the band asymmetry. In the examples discussed above, the active geochemical process is related to 5uid/rock interactions. Both mineral chemistry variation and enrichment of accessory minerals require adequate cation concentrations in the primary solution or diagenetic 5uid, and are controlled by P–T conditions and the areal extent of the 5uid–rock interface. When the 5uid/rock interactions produce variations in the crystal chemical composition of minerals, then the variation of the spectral properties is systematic. On the contrary, in the case of accessory mineral enrichment in a rock or sediment, the trend of the spectral variation is less predictable, as the water or 5uid can be almost randomly enriched in various elements. Nevertheless, even in this case the interpretation of the band distortion may contribute information about the geologic environment of rock genesis or alteration. 3.2. Spectral variability and mineral abundance determination The quantitative determination of the mineral content in a rock or sediment is sensitive to the same spectral feature modifying factors previously reported, as well as to other additional factors. Scattering e!ects in the VIS-SWIR are responsible for the spectral features of di!erent minerals in a mixture not combining linearly, so that the absorption intensity, often expressed by band depth and area, depends on the amount of the spectrally active mineral, and the grain size distribution in the mixture (see a review in Clark, 1995).
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Fig. 4. (a) Spectroscopic variability of CO2− band in a suite of impure marbles; (b) band asymmetry variation related to accessory minerals with 3 absorption bands at a wavelength close to that of the carbonatic band. Asymmetry is expressed as the ratio between the right and left portions, with respect to the band peak, of the continuum removed band area. Full circles: calcitic marbles; full diamonds: calcitic–dolomitic marbles. Open symbols: accessory minerals: Chl: chlorite bands at approximately 2340 nm; Phl: phlogopite band at approximately 2325 nm; An: anorthite.
Fig. 5. Relationship between Al–O–H band intensity and muscovite content and grain size in a suite of quartzites: (a): relationship between muscovite abundance and grain size; (b,c): relationship between band depth and muscovite grain size and percentage, respectively. Equations of the regression curves and relative R2 are reported.
In Figs. 5 and 6 the two variables are examined for a suite of quartzites and micaschists. The quartzites almost entirely consist of quartz with large grain size (500 –3000 m), and 2%–10% of muscovite (grain size from 40 to 500 m). Biotite, with a grain size ranging between 40 and 300 m, has been observed in just one sample (#66). Tourmaline is present as accessory mineral and no opaque minerals have been found. In these rocks, the grain size is correlated with the abundance of muscovite (Fig. 5a). The intensity of the 2200 nm Al–O–H band appears to be directly and linearly correlated with both the grain size and the muscovite abundance (Fig. 5b,c), i.e., particle dimension and mineral abundance
of the spectroscopically active mineral act in the same sense in determining the absorption band intensity. The micaschists include quartz (grain size from 50 to 1000 m), biotite, muscovite (15 –25%), k-feldspar (rare, 4ne grained), tourmaline as accessory mineral, and no opaques. Muscovite and biotite have comparable grain size in individual samples, and vary from 10 to 900 m through the di!erent samples. Some of the coarsest samples also contain sillimanite (up to millimetric size) and plagioclase. In these rocks, mineral grain size and abundance are not correlated (Fig. 6a), and a very weak correlation exists also between Al–O–H absorption band depth and both muscovite grain size and abundance (Fig. 6b, c). The 4ne
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Fig. 6. Relationship between Al–O–H band intensity and muscovite content and grain size in a suite micaschists: (a): relationship between muscovite abundance and grain size; (b,c): relationship between band depth and muscovite grain size and percentage, respectively.
Fig. 7. Relationship between muscovite abundance and AlVI content (a), and between band intensity and AlVI content, in quartzites. Equations of the regression curves and relative R2 are reported.
grained samples, with muscovite grains below or equal to 50 m, are characterized by extremely variable absorption band depth. The coarse grained samples (above 200 m), with a relatively low muscovite content (between 15% and 20%), display invariant band depth values. In both cases, band depth variation is mostly irrespective of both muscovite abundance and grain size. The divide between the two groups of samples roughly corresponds to the grain size dimension above which the absorption intensity modi4es the overall albedo, with consequent band depth attenuation (Clark, 1995), but within each group the band depth variation is not consistent with that observed for pure mineral mixtures. Although in some cases the samples discussed above behave consistently with the results of experiments in the SWIR using synthetic mixtures of pure minerals, they often present signi4cant deviations from what is expected from laboratory simulations. One question that appears to be of particular interest is whether, and to what extent, the chemistry of the spectroscopically active mineral, responsible for
the absorption peak wavelength shift, may a!ect the band depth. This question arises from the observation that, in samples in which mineral content and grain size are correlated (Fig. 5a), the depth of the 2200 nm Al–O–H band of muscovite is also related with the mineral abundance (Fig. 5c), and mineral abundance, as shown in Fig. 7a, is in turn related to AlVI content. Hence, the abundance of the spectroscopically active AlVI –OH bond in muscovite directly correlates with the band depth, as shown in Fig. 7b. This is veri4ed for the quartzites discussed above, but in the case of micaschists, not only the muscovite abundance is not obviously related to the Al–O–H band intensity (Fig. 6c), due to the complex mineral assemblage and grain size, but also a roughly inverse, not clearly de4ned relationship exists between muscovite and AlVI abundance (Fig. 8a). Consequently, as shown in Fig. 8b, the relationship between band depth and AlVI abundance in muscovite has no signi4cant trend, even when considering coarse grained samples (larger than 200 m, full
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Fig. 8. Relationship between muscovite abundance and AlVI content (a), and between band intensity and AlVI content (b), in micaschists. Open symbols indicate very 4ne grained samples.
symbols in Fig. 8b), for which the grain size e!ect should be negligible. This means that additional minerals, such as the plagioclase found in the coarsest samples, probably contribute in this case to the 2200 nm band intensity. Further work should be therefore focused on understanding the reciprocal role of muscovite and plagioclase in band intensity determination, as well as the e!ects induced by sillimanite included, in some samples, within large muscovite grains. 4. Discussion and conclusions A large number of papers give the extent of the e!ort done to establish systematic relationships between spectral features and both compositional and grain size variations in minerals and mineral mixtures. However, people working with natural geological materials know that both compositional and textural variations of rocks and component minerals can be the result of well-de4ned geological, geochemical and sedimentary processes, systematically controlled by individual factors or a combination of speci4c factors. This paper reports examples of spectroscopic variations, the trend of which is related to speci4c geochemical processes at mineral or mineral association scale. As these processes may occur in di!erent geologic contexts, the spectroscopic variability observed in one geologic setting can give insight for process interpretation also in di!erent situations. However, since the geochemical processes represent only one category of controlling factors in the geologic history of a region, the 4nal interpretation of a geologic context will always require the integration of data of di!erent nature. Two types of problems are presented in this paper. One concerns systematic and non-systematic variations of absorption band wavelength and shape, as a response to mineral chemistry and rock composition variation generated by di!erent geologic processes. The second one includes a few topics for a discussion relative to mineral abundance determination in a rock or sediment, based on band depth evaluation. Rock genesis, diagenesis and alteration can produce systematic chemical and structural variations in individual minerals, with consequent variation of the spectral parameters.
When one of these minerals is spectroscopically dominant in a rock suite, these variations can also be observed in the rocks. Each variation may be generated by geochemical processes occurring in distinct geologic contexts, such as magmatic or tectonic contexts, associated with high temperature or pressure conditions; or near surface contexts, that is, under low temperature and pressure conditions. One example is represented by the spectroscopic e!ects of AlVI content variation in Al–O–H group in muscovite. They were observed as related to the rock metamorphic grade in a suite of quartzites and micaschists (this paper), as well as they were reported as related to hydrothermal alteration in volcanic rocks (Swayze et al., 1992). Mineral alteration can also produce systematic absorption band deepening, as observed for the CO23 produced by the alteration of calcic plagioclase into calcite+epidote (see the second example above) although the exact nature of the relationship is not clearly established. The e!ects of cationic substitution were shown for a suite of impure marbles (third example above). Mn2+ substitution for Ca2+ and Mg2+ in calcite and dolomite, respectively, is responsible for increased re5ectance in the visible, interpreted as due to photoluminescence. Also in this case, re5ectance variability is systematically (directly and linearly) related to the luminescence activator and inhibitor ratio. Variation in the mineral assemblage in a rock or sediment produces non-systematic variations in the overall rock composition, and thus in the spectral response. One of the most common geochemical processes responsible for this behavior is the interaction between the rocks or sediments and the enriched permeating 5uid, and generally results into a new, unpredictable mineral association. In this case too, the geochemical process may occur in di!erent geologic settings. A sedimentary context was inferred in the example reported in this paper, where the distortion of the carbonate band in marbles is interpreted as due to the contiguity with absorption bands of other minerals, indicating the presence of a 5uid that had probably been in contact with a magma. Although the distortions are not predictable, further analysis may have important implications for carbonate recognition and interpretation of their genetic environment. Mineral abundance determination in a rock or sediment requires an accurate analysis of the complex interactions
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between mineral assemblage composition and grain size. In the examples discussed in this paper, the spectral behavior in some cases reproduces the results predicted by theoretical work, whereas in some other cases it displays a much higher complexity than expected, even from laboratory simulations. Paradoxically, the most interesting case is probably represented by quartzites for which the compositional and textural variables are each other highly correlated, suggesting that in nature simple cases may be present. From this end member case, it results that a quantitative mineral determination based on band intensity is reliable when the rock contains only one spectroscopically active mineral in a given spectral range, when mineral abundance and grain sizes are correlated, and when mineral abundance is related to the abundance of the spectroscopically active group in the mineral. In the micaschists, perhaps representing one of the most complex end member cases, mineral chemistry and structure, along with mineral assemblage composition, interact in determining the band intensity, over which the grain size e!ects are superimposed. As a result, the observed complexity is higher than that than ever simulated in laboratory experiments. The geochemical processes investigated in the examples above include rock genesis, diagenesis, and alteration, related to speci4c geologic context and evolution and to the climatic history of the Earth. Not all the examples are probably suitable as analogues to Mars, even if the analysis and understanding of the relationships between spectroscopic properties and terrestrial geologic processes can contribute to derive more general criteria, useful also for other planets exploration, facilitating data interpretation and reducing the number of end members needed for surface exploration. Acknowledgements The authors are indebted to Ted Roush and Fabrizio Capaccioni for the accurate reviews and the useful suggestions that signi4cantly improved the original manuscript. References Adams, J.B., 1974. Visible and near-infrared di!use re5ectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system. J. Geophys. Res. 79, 4829–4836. Burns, R.G., 1970. Crystal 4eld spectra and evidence of cation ordering in olivine minerals. Am. Mineral. 55, 1608–1633. Christensen, P.R., Band4eld, L.J., Hamilton, V.E., Ru!, S.W., Kie!er, H.H., Titus, T.N., Malin, M.C., Morris, R.V., Lane, M.D., Clark, R.L., Jakosky, B.M., Mellon, M.T., Pearl, J.C., Conrath, B.J.,
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