Page 8 - gt1512
P. 8

A                                     1                                       C 3,8 D
                                                        3
                                     2                                                                                                      1
                                                                  4
                                                                                                                            4
                                           5                                                                                                2
                                                                                                                                                  Reflectance (offset for clarity)
                               87       6                                                                                        53                                                 Atmospheric absorptions
                                                                                                     1m
                           B                                     1
                                                                                                                                                 4
                                     2                  3
                                                                  4                                                                              5

                                                                                                         Fe-, Mg-, and                                 6

                                                     5                                                   Al-bearing clay (5)
                           87 6
                                                                                                         Hydrated silica +

                                                                                                         minor clay? (4)                               7
                                                                                                         Calcite (1)

                                                                                                         Hydrated silica (6?)                          8
                                                                                                         Chlorite (7)

                                                                                                     1m  Epidote (2)                              0.5     1.0 1.5 2.0                                        2.5
                                                                                                         Hydrated silica +
                                                                                                                                                          Wavelength (μm)
                                                                                                         calcite or illite (3,8)

                           Figure 5. (A) Color composite image approximating true color of an outcrop along Mills Creek imaged with UCIS. (B) Spectral parameter map.
                           Red is the band depth of a spectral feature at 2.21 µm (BD2210) due to Si-OH and/or Al-OH combination absorptions. Green is the band depth
                           of a feature at 2.31–2.32 µm (BD2310-20) likely due to Mg-OH combinations. Blue is the band depth of a feature at 2.34–2.35 µm (BD2340-50)
                           due to either a C-O combination in calcite or Fe-OH combinations in a mineral such as chlorite or epidote (Clark et al., 1990). (C) Schematic
                           lithostratigraphic section from area outlined in yellow boxes in (A) and (B) with colors corresponding to the colors in (B). Dark green layers have
                           weak Fe/Mg/Al clay features. Other colors correspond with minerals names under the stratigraphic section. (D) Example spectra of different
                           units with colors corresponding to (B) and (C). Locations of each spectrum are numbered on both images. Dashed lines are, from left to right, at
                           2.21, 2.31, and 2.34 µm, corresponding with the positions of spectral features mapped in (B). Spectral regions with terrestrial atmosphere are
                           removed and covered with light gray boxes.

GSA TODAY | DECEMBER 2015    similar areas are compositionally quite variable. VSWIR micro-          whereas the float rocks to the right are more similar composition-
                             images acquired in minutes quickly map distributions of mafic           ally to the exposed layered outcrop.
                             and altered phases while highlighting key compositional varia-
                             tions, allowing investigators to zero-in on sample locations of         CONCLUSIONS
                             highest interest for more accurate but time-consuming instru-
                             mental techniques such as electron microprobe, transmission               Imaging spectroscopy of samples and outcrops enables new
                             electron microscopy, or NanoSIMS analyses.                              science investigations and yields novel or unexpected mineralogic
                                                                                                     and petrologic insights into a variety of geological processes
                             EXAMPLE 4: MAPPING OF SEDIMENTARY ROCKS EXPOSED                         (Greenberger, 2015). This technique rapidly and non-destructively
                             AT MILLS CREEK                                                          maps mineralogies with little sample preparation, showing mineral
                                                                                                     associations that might not otherwise be apparent and guiding
                                We imaged an outcrop of sedimentary rocks exposed along              further investigations using other, more time-consuming tech-
                             Mills Creek near Mono Lake, California, with UCIS in field              niques. Other investigations have used imaging spectroscopy at
                             mapping mode (Fig. 5; 0.5–2.5 µm; instantaneous field of view           similar scales to find economically viable deposits of minerals
                             1.35 mrad, 12 nm spectral resolution). Using spectral parameters,       (Kruse et al., 2012), map carbonate lithologies (Baissa et al., 2011;
                             we mapped specific mineral phases present at the site, including        Kurz et al., 2012), characterize alteration of basaltic samples from an
                             hydrated silica (opal A), clays (Fe-, Mg-, and Al-bearing), calcite,    impact structure (Yokoyama et al., 2015) and in a cold and arid envi-
                             illite/muscovite, and epidote (Fig. 5B). From these results, the        ronment (Cannon et al., 2015b), quantify the abundances of iron in
                             stratigraphy of the outcrop lithologies can be inferred (Fig. 5C),      a mine wall (Murphy and Monteiro, 2013), map clay layers (Murphy
                             showing variations in the depths of absorption features related to      et al., 2014), and study the Black Beauty meteorite from Mars
                             hydrated silica (red or orange) and clay mineral (green) contents       (Cannon et al., 2015a). These types of studies also aid in preparing
                             of the sedimentary layers. The float rocks above the outcrop and        for imaging spectrometers on future landed planetary missions.
                             along the creek bed at the bottom of the image have diverse             Imaging spectrometers used herein are currently employed in labo-
                             compositions. Based on the higher proportions of rocks with             ratories such as at Brown University, the California Institute of
                             significant calcite (cyan) and illite (magenta), we infer the likely    Technology, and the University of Winnipeg.
                             presence of calcite- and illite-rich units higher in the stratigraphy
                             that are not exposed at this outcrop. Much of this calcite- and         ACKNOWLEDGMENTS
                             illite-rich talus is located on the left portion of the outcrop near a
                             small fan and likely is sourced from above the layered section,            We would like to thank Headwall Photonics, Inc., especially David Bannon
                                                                                                     and Kwok Wong, for use of their hyperspectral imagers to acquire the images
                         8
   3   4   5   6   7   8   9   10   11   12   13