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Reflectance spectroscopy (0.3–2.5 µm) at various scales for bulk-rock identification

¹ Università degli Studi di Parma, Dipartimento di Scienze della Terra, Parco Area delle Scienze, 157/A, 43100 Parma, Italy

View larger version (10K): [in this window] [in a new window]   Figure 1. (A) Average spectrum of 10 laboratory measurements randomly distributed on the slab surface of the sample (Mount Etna recent lava); gray area represents the standard deviation. (B) Average spectrum of 5 in situ measurements acquired on a target (Mount Etna pyroclastics); gray area represents the total uncertainty of measurements relative to the target.

View larger version (17K): [in this window] [in a new window]   Figure 2. (A) Inverse linear relationship between the Al-OH band wavelength and the VIAl abundance (apfu: atoms per formula unit), relative to muscovite in the quartzites-micaschists and graywackes suites. (B) Relationship between Al-OH band wavelength and VIAl abundance in muscovite times modal muscovite in quartzites; r is the coefficient of correlation. (C) Plot of Al-OH band wavelength and VIAl abundance in muscovite times modal muscovite in micaschists, showing the lack of any relationship. See discussion in the text.

View larger version (9K): [in this window] [in a new window]   Figure 3. (A) Direct linear relationship between the divalent iron CF band minima and Fe2+/M2 sites abundance (apfu) in pyroxene. (B) Inverse relationship between the iron CF band and Fe2+/M2 sites abundance times modal orthopyroxene. (C) Inverse relationship between Fe2+/M2 sites abundance and modal pyroxene. Data are from the suite of cumulates belonging to the Bjerkreim-Sokndal Layered Intrusion. See text for discussion.

View larger version (11K): [in this window] [in a new window]   Figure 3. (A) Direct linear relationship between the divalent iron CF band minima and Fe2+/M2 sites abundance (apfu) in pyroxene. (B) Inverse relationship between the iron CF band and Fe2+/M2 sites abundance times modal orthopyroxene. (C) Inverse relationship between Fe2+/M2 sites abundance and modal pyroxene. Data are from the suite of cumulates belonging to the Bjerkreim-Sokndal Layered Intrusion. See text for discussion.

View larger version (11K): [in this window] [in a new window]   Figure 4. Example of spectral patterns and relative spectrofacies "MA HE FE F" in a bulk-rock spectrum (see Table 2 for symbol description). The spectrofacies is an association of spectral patterns; it spectrally characterizes the rock. The spectrum of this example was acquired on a gneiss from the metamorphic basement of central Madagascar.

View larger version (164K): [in this window] [in a new window]   Figure 5. Thematic Mapper (TM) image of a portion of the central Madagascar crystalline basement; color composite of 741 bands (2.215, 0.830, 0.485 µm). The color units in the image correlate with the first-order classes identified in the set of spectra measured on rock samples collected in the area and previous geologic map (Moine, 1974). CA, AM, HF, and MA—predominant spectral patterns (see Table 2), identifying first order classes (see Table 3).

View larger version (23K): [in this window] [in a new window]   Figure 6. Laboratory reflectance spectra of Mount Etna basalts, representative of the spectral classes shown in Table 5. E29: ancient lava (100 ka); E6, E19: historic lava (1766 and 1970 flows, respectively); E11 and E25: 2002–2003 distal lava flow, south and northeast Mount Etna flanks, respectively; PC7 and E28: 2002–2003 proximal and intermediate lavas, respectively, northeast Mount Etna flank; PC5: pyroclastic deposits, northeast Mount Etna flank.

View larger version (145K): [in this window] [in a new window]   Figure 7. Advanced spaceborne thermal emission and reflection radiometer (ASTER) image of the northeast portion of Mount Etna, acquired on 19 July 2003; RGB (red-green-blue) color composite of 217 bands (0.660, 0.560, 2260 µm) with decorrelation stretching. 1: summit cone complex; 2: 2002–2003 lava flows; A, B, C, D: location of the field sites of Figure 8; thick black line: location of the 2002 fracture, source of the recent flows; box: location of Figure 9A–B.

View larger version (58K): [in this window] [in a new window]   Figure 8. Reflectance spectra measured in the field using an ASD FieldSpec Pro FR (left side), and pictures of the measured surfaces (right side). Locations are in Figure 7. Missing values in the spectra correspond to detector change and atmospheric water bands. The spectra were smoothed using Fourier transform methods using 20 points as algorithm input. The spectra are representative of four characteristic surfaces: (A) "altered lava": lava with iron oxides and clay minerals alteration, both coarse and fine surface textures; (B) "pyroclastics": pyroclastic deposits consisting of ash and centimetric scoriae; (C) "lava": slightly altered, blocky recent lava; (D) "oxidized lava": lava with predominant iron-oxide alteration, mainly occurring very close to the effusive centers. See text for explanations.

View larger version (102K): [in this window] [in a new window]   Figure 9. (A) Hyperion image of the northeast portion of Mount Etna, acquired on 19 July 2003; see location in Figure 7; RGB (red-green-blue) color composite of 122 11 21 bands (1.750, 0.538, 0.630 µm), with decorrelation stretching; vegetation is masked. Inset: detail of a lateral cone. (B) Spectral Angle Mapper (SAM) classification of the same area as in A, with 0.18 rad spectral angle. (C) End-member spectra used for the SAM classification: "pyroclastics," "lava," and "oxidized" correspond to the spectra of Figure 8B–D, respectively; "altered": average of Figure 8A spectra; "lava/altered_1" and "lava/altered_2": linear combinations of predominant "lava" and subordinate "altered lava" spectra, with different albedos; "altered/lava": linear combination of predominant "altered lava" and subordinate " lava" spectra; "pyrocl/lava/oxid": linear combination of "pyroclastics," "lava," and "oxidized lava" spectra, in almost equal proportions; "lava/oxidized": linear combination of "lava" and "oxidized lava" in equal proportions. Large and small boxes in B are locations of Figures 10A and 11A, respectively.

View larger version (45K): [in this window] [in a new window]   Figure 10. (A) Detail of Figure 9B (large box) showing the 2002–2003 lava flows. (B–D) Spectral profiles (thin lines) of pixels from Spectral Angle Mapper (SAM) classes, average spectral profile (thick color line), and field end member (thick black line). Pixel spectral profiles were classified with angles 0.1 rad. The classes are: (B) lava, (C) pyroclastics, (D) pyrocl/lava/oxidized. See Figure 9C for end-member descriptions and Table 8 for the accuracy of spectral matches.

View larger version (44K): [in this window] [in a new window]   Figure 11. (A) Detail of Figure 9B (small box) showing the Spectral Angle Mapper (SAM) classification of a minor, lateral cone. Variously altered volcanic materials form concentric aprons around the crater on the downdip slope with respect to the Mount Etna northeast flank, downwind of the plume. (B–E) Spectral profiles (thin lines) of pixels from SAM classes, average spectral profile (thick color lines), and field spectrum (thick black lines). Pixel spectral profiles classified with angles 0.1 rad. SAM classes are: (B) oxidized, (C) altered/lava, (D) lava/oxidized, (E) lava/altered_1. See Figure 9C for end-member descriptions. Except for the "lava/oxidized" class (D), average spectra of the other classes best compare with field spectra linear combinations not coinciding exactly with those of the relative class end members. Mixed spectra reported here are: (B) oxidized lava and altered lava in almost equal proportions; (C) oxidized lava, altered lava, and lava in order of decreasing proportions; (E) predominant lava and subordinate oxidized lava and altered lava. See Table 8 for the accuracy of spectral matches.

View larger version (20K): [in this window] [in a new window]   Figure 12. Comparison between laboratory and field spectra. (A) Slightly oxidized distal lava; and (B) pyroclastic deposits. Lava spectrum is 15% offset for a better comparison. (C) and (D): Continuum-removed spectra of the VIS (visible) range of A and B, respectively. The absorption intensity is variable. WS, CFM, SFM, and SFO indicate the spectral patterns (see Table 4 for explanation). See discussion in text.

View larger version (16K): [in this window] [in a new window]   Figure 13. (A) Comparison between laboratory and field spectra of oxidized lava, proximal to the effusive centers, showing the Fe3+ absorption features characteristic of hematite. (B) Spectrum of hematite with <45 µm size, separated via filtering from oxidized lava samples. (C) and (D) Continuum-removed VIS (visible) spectra from A, showing individual absorption bands of the CFH spectral pattern. CFH and WM—spectral patterns (see Table 4 for explanation). See discussion in the text.