Spectral Characteristics of Pillow Lava Series Rock units in the
Near-IR (1300-2500nm) Range, Troodos Massif, Cyprus
K. T. U. S. De Silva
Geological Survey and Mines Bureau,
No: 4, Galle Road, Dehiwala, Sri Lanka
Abstract
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.
