Spectral Characteristics of
Pillow Lava Series Rock units in the Near-IR (1300-2500nm) Range, Troodos
Massif, Cyprus K. T. U. S. De
Silva Abstract Geological Survey and Mines Bureau, No: 4, Galle Road, Dehiwala, Sri Lanka Spectral characteristics of rocks depend on the constituent minerals, their composition and molecular structures with which they are bonded. Most of the diagnostic absorption features of the various mineral types fall within the near-IR (1300-2500 nm) region, in which the 2000-2500nm range is predominant. These absorption features are hard to be identified by analyzing the TM spectra, mainly due to its broad spectral resolution. Therefore with the intentin of resolving spectral absorption features that are observed in pillow lava series rock units and their weathered products in the Troodos massif in Cyprus, the high resolution Portable Infrared Mineral Analyzer (PIMA-II field spectrometer) was used in this study. Spectral analytical studies of the pillow lava series rock and soil samples revealed that some characteristic absorption features can be observed in Upper Pillow Lava (UPL) and Lower Pillow Lava (LPL) soil and rock units. Similarly to reflectance and absorption features of both UPL rock and soil samples indicate that the soils developed on UPL reflect the spectral signature of the parent UPL materials and their weathered products. Spectral features of the pink inter pillow material indicate that weathered products. Spectral features of the pink inter pillow materials. It is clear that when the pinkish inter pillow materials are absent in the UPL samples, the absorption are influenced mainly by carbonate ions and the spectrum is identical to that of LPL. Spectral studies further reveal that the absorption features of LPL. Spectrum of chalcedony rich LPL rocks is very similar to spectrum of pinkish gray UPL rocks and soils, both in reflectance and absorption features. The most characteristic mineral identified in the LPL series is the green coloured celadonite. Celadonite occur as stains and open space fillings in lavas and in some of the dikes. Spectral analysis of pure celadonite indicates that its spectral signature is similar to glauconites. The occurrence of celadonite in LPL and its absence in UPL can be used to explain that the two lava units have erupted in different phases under submarine conditions. Introduction Rocks are assemblages of minerals and so their spectra are composites of each of their constituents. Minerals in turn comprise of various proportions of different elements, held together as molecules by different kinds of bonds. When the minerals are sufficiently abundant and posses prominent spectral features, they may be detected, subject to their structure and composition. The spectra of minerals are dominated by the effects of ions and the molecular structures in which they are bonded. Characteristic energy levels of isolated elements are changed when they are combined in minerals because of the valence states of their ions, the type of bonding and their relationship to other ions. Spectral characteristics of minerals indicate the most of diagnostic absorption features of the various mineral types fall within the visible and near IR region, in which the 2000-2500 nm range is predominant. The wave bands of Landsat TM 7 (2080-2350nm) also lie within the above range. Landsat TM sensor system is adequate to discriminate Pillow Lava series rock units which have different spectral characteristics such as overall reflectance. However, the absorption features are hard to be identified by analyzing the TM spectra mainly due to its broad spectral resolution. Therefore with the intention of resolving spectral absorptin features that are observed in pillow lava series rock unit and their weathered products, the high resolution Portable Infrared Mineral Analyzer (PIMA - II field spectrometer) was used in this study. It is a dual field of view handheld spectrometer, which covers the region of 1300-2500 nm wavelength with a 7-10 nm spectral resolution and a 2.5 nm sample interval. Out of the complete set of spectral band observed in the Landsat TM sensor system, only bands 5 and 7 fall within the wavelength region covered by the PIMA-II spectrometer. Outline of the Geology of Troodos Pillow Lava Series Troodos pillow lava series, also known as the extrusive series, in the name given to the volcanic rocks associated with the Troodos ophiolite complex in Cyprus. It is extending as an irregular, incomplete ring around the periphery of the massif in a belt of metamorphosed pillow lavas and dikes, that form the upper most 1.5 -2 km of the ophiolite complex. On the basis of changing field appearance, the series is divided into the Upper Pillow Lavas (UPL), Lower Pillow Lavas (LPL) and Basal Group (BG) (Wilson, 1959). Out of these three units, UPL & LPL are alike, and at many places studies like mineralogical and petrological analysis are necessary to discriminate these two units. Upper Pillow Lavas are generally undersaturated, often oliving bearing basalts with more basic varieties (limburgites and picrities), occurring at the top of the sequence. Dikes form less than 10% by volume, silica and celadonite are absent, and calcite and analcime are common. Lower Pillow Lavas are mainly ovesaturated basalsts, often intensely silicified and celadonite is common. Dikes, sills and massive flows forming between 30-60% of the ourcrop. Spectral Characteristics of Pillow Lava Series Rocks and Soils Spectral curves of the soil and rock samples collected from Troodos pillow lava series rock units were recorded using the PIMA-II spectrometer. When re cording the spectral curves, several precautionary measures were taken to minimize the errors due to secondary sources and to get smooth curves. However, some of the resultant curves were slightly noisy as a result of surface roughness of the sample. The resultant spectral curves were analyzed using the PIMAVIEW software package, available with the instrument. Spectral Features of Upper Pillow Lavas (UPL) Spectra of UPL rocks and soils are displayed in Figures 1 & 2 respectively. All the samples which were used to record the above spectra are composted of a mixture of both pinkish and grayish portions of the UPL. The reflectance values of spectra are occupied in a very narrow zone which indicates the similar reflectance behaviour in all samples. In addition, they show identical absorption features having equal depths. The broad absorption bands at 1400 and 1900 nm positions are due to the presence of poorly ordered molecular water in all samples. These bands, especially the one at 1900 nm, are sharply enhanced due to the weathering effect. A characteristic weak absorption feature is observed in the 2170-2270 nm range and it appears to be a result of combination vibrations involving the OH stretch and Al-OH bend of smectites (montmorillinite) or molecular water absorption of zeolites (natrolites). Apart from that, a very weak absorption band is observed at 2270-2330 nm in most of the samples and is due to CO32- absorption to calcites, associated with vesicular fillings and inter pillow materials. In some samples, especially one at win 4-4, this feature is somewhat clear due to presence of pseudomorphs of olivine phenocrysts which are now replaced by iron stained calcites. Similarity of reflectance and absorption features of both UPL rock and soil units indicate that the soils developed on Upper Pillow Lavas reflect the spectral signature of the parent UPL materials and their weathered products. In some areas, UPL do not display any pinkish appearance and are solely comprised of gray coloured UPL. By overlapping the digitized geological boundaries of the published maps on the processed images, it is observed that some parts of the UPL succession appear similar in colour as LPL units. With the intention of resolving these features, spectra were recorded separately from pinkish inter pillow material, grayish pillow fragments and limey fragments of the inter pillow material of UPL, collected near the Agrokipia mine. These graphs are displayed in Fig. 3. In the sample of pink inter pillow material, carbonates also occur at a minor constituent. The spectrum of pink inter pillow material exhibits two absorption bands. Out of these, the one at 2170-2265 nm range is broad and it may be due to bending of the Al-OH bond in montmorillonite or molecular water absorption of zeolites (natrolites). The other is a very weak which is observed at 2270-2370 nm wavelength region. It occurs as a result of CO32- ions, associated with the pink inter pillow material. The reflectance values and absorption features of the spectrum of pink inter pillow material is almost identical to that of all UPL rock and soil units (Fig. 1 and 2). This indicates that the spectrum of UPL unit is influenced by the pink inter pillow materials. The spectrum of limey materials was made by choosing a cream coloured lime fragment in the inter pillow materials. In this sample, apart from the carbonates, a scattered pink staining is also visible in lesser amounts. The spectrum shows considerable high overall reflectance due to its bright colour. Absorption bands of this sample are dominated by planner CO32- ions, which results a sharp feature at 2275-2380 nm region. A very weak absorption at 2175-2260 nm region is either due to Al-OH bend of smectite or molecular water absorption of zeolites (natrolites) present in the pink stainings of the fragments. However, except for vesicular fillings, this type of white limey fragments are not widespreaded in the UPL sequence and are restricted to minor associations within the pink inter pillow materials. Therefore overall absorption features of pinkish UPL are appeared to be influenced by the absorption bands due to clay minerals (smectites) or zeolites (natrolites) occurring within the pink inter pillow materials. However, a minor features at 2270-2380 nm region occurring in most of the samples in due to the presence of lesser amount of carbonates. The other spectrum was recorded by selecting only the grayish coloured potion of upper Pillow Lavas. In this samples, considerable amounts of calcites occur as vesticular fillings. In addition, weathered olivine phenocrysts which are now replaced by iron stained calcites are also presents in considerable amounts. Therefore, the band at 2265-2365 nm of this material is solely due to the combination and overtone bands of the CO32- ions occurring in the near IR region. It is clear that when the pinkish inter pillow materials are absent in the UPL samples, the absorptions are influenced only by CO32- ions. When comparing this spectrum with that of the LPL, it shows that both are similar in reflectance as well as in absorption features. So, when the pinkish inter pillow materials are absent in UPL, the discrimination of UPL and LPL is somewhat difficult. This may be the reason for UPL to appear in the same colour as in LPL in some parts of the processed images. Spectral Features of Lower Pillow Lavas (LPL) Most characteristic mineral identified in the LPL series is green coloured celadonite. Spectral behaviour of pure cleadonite and celadonite bearing LPL rocks are shown in Figure 4. The overall reflectance of pure celadonite is much higher than that of the other rock units in the Troodos pillow lava series. Its spectrum shows a prominent triplet absorption band, centered on 2256, 2300 and 2346 nm positions. This feature is due to the combination of the OH-stretching fundamentals with Mg-OH bonding mode in the celadonite structure. In addition to the triplet band, absorptions at 1400 and 1900 nm indicate the presence of molecular water in the samples. Out of these two features, one at 1900 nm is very sharp. The weathering effect contributed much for the enhancement of sharpness of this feature. Another very weak band which is observed only under the hull option at 2450 nm, appears to be result of Fe-OH band. Many previous workers have classified the mineral celadonite as a secondary product (clay-illite) of hydrothermal alternation (e.g.:- Hall and Yang, 1990). Howeve, spectral analysis of pure cleadonite indicates that it is more similar to galuconites. It is generally accepted that galuconites are formed from a variety of starting materials by marine diagenesis in shallow water during periods of slow sedimentation. According to Hadding (1932), glauconites are formed in sublittoral areas of shallow marine waters. They are never formed in a highly oxygenated environment. Takashashi (1939) mention that "glauconitization is on eof the processes of submarine metamorphism that gives rise to the mineral galuconite. The phenomenon is known only in marine sediments that are formed under anaerobic or rducing conditions". The formation of glauconite in anaerobic sedimentary sequences during diagenesis is similar to the formation of celadonite during submarine alteration of pillow lavas. In figure 4, two spectra of celadonite bearing LPL rocks are also displayed. The sample win 4-3 is greenish in colour due to the occurrence of a thin coating of celadonite on all surfaces. As a result it gives a higher overall spectral reflectance. In contrast, the sample win 2-3 has only scattered fillings of celadonite and its colour appers as greenish gray which result in a low reflectance spectra. Although the amount of celadonite is less in sample win 2-3, its celadonite triplet is still more prominent than that of the sample win 4-3. The reason for this is due to the occurrence of a high percentage of celadonites in the scattered fillings whereas only a thin coating of celadonite is present in sample win 4-3. When considering the absorption features, both spectra show identical absorptions as in the pure celadonite sample (triplet at 2256, 2300 and 2346 nm due to Mg-OH bending; single absorptions at 1400 and 1900 nm as a result of the presence of molecular water; very weak single absorption at 2462 due to Fe-OH bending). The steepness and sharpness of the water absorption bands of win 4-3 is higher than that of pure celadonite. However, the same band of win 2-3 is weak which indicates that it has a lesser amount of water molecules. Recorded spectra of normal LPL rocks and soils are shown in Fig. 5 and 6 respectively. In these spectra, similar to all the other previous samples, molecular water is present which results the absorptions at 1400 and 1900 nm. A characteristic feature of these samples, is the presence of an absorption at 2260-2360 nm region and the absence of any band around 2170-2260 nm region. As mentioned earlier, the band at 2170-2260 nm is prominent in UPL. In some of the samples (eg:-win 3-7 and win 4-3), the occurrence of celadonite is shown by the presence of a triplet band, centered on 2300 nm. The triplet is clearly seen under the hull option. It is interesting to mention that although celadonites are invisible to the naked eyes in LPL sample win 3-7, the presence of it can be identified from the spectral studies. In addition, all the other spectra show single absorption features at 2260-2360 nm. These include a dike sample from LPL unit (win 3-1 b), its absorption features are identical to that of the other leva flows in LPL. As calcites are almost absent in LPL rocks, the above features may not account for the usual carbonate absorption. Therefore it should be due to the bending of Fe-OH or Mg-OH bond of black glass or diopsides. When viewing the overall reflectance of LPL rock and soil spectra, sample of win 3-7 displays a high reflectance due to its light gray appearance which is contrasting to low reflectances of dark gray coloured other LPL samples. Since the inter pillow materials of LPL rocks comprise mostly of black glass it is worthwhile to study its spectra (Fig. 7). Similar to usual LPL rock and soil units, its absorptions are limited to 2260-2360 nm range, in addition to the normal water absorption at 1400 and 1900 nm. The absorption at 2260-2362 range appears to be due to bending of either Mg-OH or Fe-OH bonds in black glass. Its reflectance is also very low throughout the wavelength range measured by the PIMA-II spectrometer, due to its black colour. Therefore the low overall reflectance of LPL unit is influenced by its black inter pillow materials. Another characteristic mineral in LPL series is the secondary silica, chalcedony, occurring as a minor constituent. However, at some places chalcedony rich LPL rocks are encountered. Both pure chalcedony and chalcedony rich LPL rock samples were collected and their recorded spectra are displayed in Fig. 8. In the pure chalcedony spectrum, apart from the two usual water absorptions, it shows a prominent band, centered at 2200 nm region due to the stretching of Al-OH bond in chalcedony. In the spectrum of chalcedony rich LPL rock unit, the above absorption band at 2200 nm is still seen, indicating the influence of Al-OH bond of chalcedony for the absorptions of the rock unit. However, the feature is not as steep as that in pure chalcedony. The reflectance of both samples is also somewhat higher than that of the normal LPL rocks, mainly due to the presence of chalcedony. In addition, spectra of chalcedony rich LPL units show very weak absorption at 2260-2360 nm range. As mentioned above, this feature may be due to the bending of Fe-OH or Mg-OH bonds of black glass or diopsides. The above spectrum of chalcedony rich LPL rocks is very similar to spectra of UPL rocks and soils, both in reflectance as well as in absorption features. This may be the reason for similar appearance of chalcedony rich LPL rocks and UPL successions in processed images. Conclusions Spectral analytical studies of the pillow lava series rock and soil samples using the PIMA-II spectrometer revealed that the characteristic absorption features can be observed in UPL and LPL rock and soil units. Spectral reflectance and absorption features of the pink inter pillow material indicate that the spectrum of pinkish gray UPL units are mainly influenced by the pink inter pillow material. When the pink inter pillow materials are absent in the UPL units, the spectrum of UPL is almost similar to that of LPL Spectrum of pinkish gray UPL and that of chalcedony rich LPL rocks are alike, both in reflectance and absorption features. Therefore they appear in similar colour in the processed images. Similarity of spectral features of black glassy inter pillow material and normal LPL rock units show that the spectral features of LPL rock units are characteristically dependent on the black glassy inter pillow materials. Spectral analysis of pure celadonite indicate that it is more similar to galuconites which are formed in sublittoral environments of shallow marine waters. The occurrence of celadonite in LPL and its absence in UPL can be used to explain that the two lave units have erupted in different phases under sub marine conditions. Acknowledgments The material presented in this paper is from the writers Msc thesis presented at the International Institute of Aerospace Survey and Earth Sciences (ITC), The Netherlands. The study was supported by the Netherlands Fellowship Programme (NFP). The writer is indebted to Drs. P. M. Vandijk and F. Vander Meer for their valuable guidance and advices during the period of study. The author also wishes to thank Miss Uddhika Perera for providing assistance in preparing the manuscript. References
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