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Revisiting Spectral Parameters of Silicate-Bearing Meteorites

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Revisiting Spectral Parameters of Silicate-Bearing Meteorites Meteoritics & Planetary Science 40, Nr 3, 445–459 (2005) Abstract available online at http://meteoritics.org Revisiting spectral parameters of silicate-bearing meteorites RenË DUFFARD,1* Daniela LAZZARO,1 and Julia DE LEµN2 1ObservatÕrio Nacional Rua Gal. JosË Cristino 77, Rio de Janeiro, 20921-400 RJ, Brazil 2Instituto de AstrofÏVLFDde Canarias, c/ Via LÃctea s/n, E38200, La Laguna, Tenerife, Spain *Corresponding author. E-mail: [email protected] (Received 16 July 2004; revision accepted 31 January 2005) Abstract–Visible and near-infrared reflectance spectra of a sample of silicate-bearing meteorites have been used to evaluate the spectral parameters space defined in the pioneering work of Gaffey et al. (1993). The studied sample consisted of 91 ordinary chondrites, 47 basaltic achondrites, and 21 different laboratory mixtures obtained from the RELAB database. Our results indicate that the spectral parameter space, in particular the BAR versus band I center, is not suitable enough to identify the mineralogy of meteorites and asteroids. The grain size of the sample also appears as a very sensitive parameter and can play an important role in locating an object in the spectral parameter space. Finally, the application of our study to the question of a genetic link between V-type asteroids and HED meteorites shows that these bodies plot in distinct regions in the BAR versus band I center space. This result further confirms that those spectral parameters cannot uniquely define the mineralogy of a sample. INTRODUCTION type asteroids and compared them to ordinary chondrites and basaltic achondrite meteorites using data from the combined Olivine and pyroxene are very common rock-forming ECAS (Zellner et al. 1985), the 52-channel data (Bell et al. minerals in the solar system. Several classes of meteorites, 1988), and the combined 24-color (Chapmann et al. 1979) such as the ordinary chondrite and the achondrites, are rich in surveys. In the space BAR versus band I center, the authors both of these minerals. The spectral signatures due to olivine defined an “Ol” rectangular region, which encompasses the and/or pyroxene are also detected when the surfaces of olivine assemblages, an “OC” polygonal region, which asteroids belonging to several taxonomic classes are remotely represents the mafic silicate components of ordinary probed. Since meteorites are believed to derive from chondrites, and a “BA” rectangular zone, which includes the asteroids, the comparison of remote spectrophotometry of pyroxene-dominated basaltic achondritic assemblage. The asteroids and laboratory reflectance spectra of meteorites can BA and OC regions were defined with 20 and 44 meteorites, help constrain genetic relations between these two respectively. However, no reference was given to the specific populations. meteorites used or to the grain size of the samples. The spectrum of olivine is dominated by a complex Apparently, this data was obtained from the work of Gaffey absorption band centered near 1.0 Pm. Pyroxene, on the other (1976), where an extensive analysis of meteorite spectra was hand, presents absorption features near 1.0 Pm (band I) and performed using the Beckmann DK2A ratio recording 2.0 Pm (band II) for low-calcium pyroxenes and a broad 1 Pm spectroreflectometer described by Adams et al. (1970). An feature with a double minimum for certain calcic pyroxenes. offset in wavelength was applied to this original data because In the spectra of olivine-pyroxene mixtures, the wavelength of systematic calibration errors, as reported by McFadden position of band II is only sensitive to pyroxene composition, et al. (1982) and Gaffey (1984). while the position of band I is a function of the relative The band area plots introduced by Gaffey et al. (1993) abundance and composition of olivine and pyroxene phases. have been used in several works aiming to characterize the Cloutis et al. (1986) have shown that the relative contrast of mineralogy of basaltic asteroids (Kelley et al. 2003; band I and band II (BII/BI area ratio, or BAR) may be useful Hardersen et al. 2004; Duffard et al. 2004), to analyze the in estimating the relative olivine and orthopyroxene spectral properties of 433 Eros (Murchie et al. 1996), and to abundances in a mixture. study the reflectance spectra of olivine-orthopyroxene- Aiming of establishing a mineralogical link between bearing assemblages at different temperatures (Moroz et al. meteorites and asteroids, Gaffey et al. (1993) analyzed 40 S- 2000). 445 © The Meteoritical Society, 2005. Printed in USA. 446 R. Duffard et al. Fig. 1. BAR versus band I center for ordinary chondrites and achondrites meteorites obtained from the RELAB database. This implies that a precise definition of where the samples were crushed into diverse particle sizes to obtain the different classes of meteorites plot is necessary in order to reflectance spectra. correctly interpret the results obtained for asteroids or The extraction of the spectral parameters was done laboratory mineral analysis. Considering the large dataset of following the procedure first outlined by Cloutis et al. (1986) spectral parameters for meteorites now available, we decided and used by Gaffey et al. (1993). Among the most important to repeat the calculations of Gaffey et al. (1993). The main parameters in characterizing the mineralogy associated with point of the present work, therefore, is that all meteorite the reflectance spectra of an asteroid or meteorite are the band spectra were obtained using the same instrumental minimum and the band centers. These are defined as the configuration and wavelength calibration. wavelength position of the point of lowest reflectance before The analysis of the data and extraction of the parameters and after the removal of the continuum, respectively. are presented in the Analysis of the Data section. The study of Using the meteorite reflectance spectra, we determined some laboratory mixtures and the effects of different grain the two maxima, near 0.7 and 1.4 Pm, and with the reflectance sizes are presented in the Laboratory Mixtures and Grain Size values and positions of the maxima, we computed a linear Effects sections. Finally, an application to V-type asteroids continuum in this region. A similar procedure was done in the and a discussion of the results are outlined in the last section. interval between 1.4 and 2.4 Pm. These continua were then extracted from the spectra by dividing each linear continuum ANALYSIS OF THE DATA in the corresponding region for each object. We note that, although the entire sample has spectra up to longer The reflectance spectra of meteorites in the range of 0.3 wavelengths than 2.4 Pm, this limit was used to compare our to 2.4 Pm at a 0.005 Pm sampling resolution were obtained results with other works. Moreover, all of the remote asteroid from the Reflectance Experiment Laboratory (RELAB) reflectance spectra obtained in the near-infrared region have a database (http://www.planetary.brown.edu/relab). The technical CCD limitation of 2.5 Pm. RELAB instrument is designed to simulate the diverse The spectra obtained after the removal of the continuum viewing geometries in remote telescopic measurements. A were used to compute the band centers, band areas, and band 30° incident angle and a 0° emission angle (measured from depths. The center determination was done by fitting a second the vertical) were the default parameters for the measured order polynomia in the region of the minimum. Following spectra. The details of RELAB and how the spectra were Cloutis et al. (1986), the band area is defined as the area obtained are described in Pieters (1983) and in the RELAB enclosed by the spectral curve and a straight-line tangent to users manual. As explained in that document, the meteorite the respective maxima. Band area I is between the maxima of Spectral parameters of silicate-bearing meteorites 447 Fig. 2. BAR versus band I center for the different achondrite meteorites: howardites, eucrites, and diogenites (HED), shergottites, and nakhlites. 0.7 and 1.4 Pm and band area II is between 1.4 and 2.4 Pm. plotted the band II center versus band I center for all the HED The ratio between band area II and band area I is defined as meteorites studied (9 diogenites, 25 eucrites, and 13 the BAR parameter. howardites) as done previously by Gaffey (1997) when We selected a large sample composed of 91 ordinary analyzing the surface composition of 4 Vesta. In Fig. 3, values chondrites, 47 basaltic achondrites, and 21 different for the howardites, diogenites, and eucrites are superimposed laboratory mixtures. The complete set of results indicating the on the regions for diogenites and eucrites from Gaffey (1997) RELAB file, reference, meteorite name, sample grain size, (dashed line). Note that Gaffey (1997) used values taken from centers of bands I and II, and BAR parameter is given in Gaffey (1976) and applied a wavelength correction of Appendix 1. +0.025 Pm as discussed in Gaffey (1984). The BAR versus band I center for all meteorites in our The difference between the two determinations of the work is given in Fig. 1, defining regions for the different eucrite and diogenite regions is a small displacement of both classes of meteorites. The regions defined by Gaffey et al. band centers to shorter wavelengths. As can be seen, the (1993) are given for comparison (dashed lines). We observe eucrite region plots at larger wavelength in both bands than that most of our HED meteorites are outside the region the howardites and diogenites, respectively. It is worth defined by Gaffey et al. (1993) as BA and the region defined recalling that the crust of the HED parent body is associated as OC are moved to smaller BAR and band I center values, with two different mineralogies: (i) a diogenite lower crust, maintaining its overall shape.
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