Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Formation and composition of H 2 O-undersaturated granitic melts

Abstract: Granites are a very common component of the continental crust, and granitic melts are the most important medium for the transport of silicon, alkali metals and some associated, less abundant, elements from lower to higher crustal levels. Granites have been the subject of numerous petrographic and experimental investigations. The first systematic experimental research was performed in the synthetic haplogranitic system Qz-Ab-Or-H2O (Tuttle & Bowen 1958, Luth, Jahns & Tuttle 1964). Other investigations with natural rocks as the starting material were performed by Winkler and co-workers (summarized in Winkler 1979). The results of these H2O-saturated experiments emphasized that granitic magmas can exist at relatively low temperatures within the continental crust, and that many crystalline rocks can be the source of granitic melts.

Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 1. Determination of mineralogy, chemistry, and classification strategies

Abstract: We have examined and applied existing classification schemes for volcanic rocks and developed new schemes using thermal emission spectra of terrestrial volcanic rocks. Laboratory thermal infrared spectra (5–25 μm, at 2 cm−1 spectral sampling), deconvolved modal mineralogies, and derived mineral and bulk rock chemistries were used to distinguish basalt, basaltic andesite, andesite, and dacite. Modal mineralogies derived from linear deconvolution of terrestrial volcanic rocks were compared to modes measured by an electron microprobe phase mapping technique to determine the accuracy of linear deconvolution in modeling specific mineral abundances. One σ standard deviations of the absolute differences between modeled and measured mineral abundances range from 2.4 to 12.2 vol %, with an average standard deviation of 4.8 vol % being in agreement with average uncertainties calculated in previous studies. Weighted average compositions of feldspars in the deconvolution generally overlap the measured ranges of plagioclase compositions and the presence of low-calcium and high-calcium pyroxenes was correctly identified. Bulk chemistries of volcanic rocks were derived with a relatively high degree of accuracy (1σ standard deviations ranging from 0.4 to 2.6 vol %) by combining the compositions of spectrally modeled phases in proportion to their relative abundances in a particular sample. These data were collectively used to examine existing and develop new volcanic rock classification schemes. However, no single classification scheme was effective in accurately classifying all samples. Multiple steps of classification were required to distinguish volcanic rocks, reflecting the mineralogic diversity and continuum of compositions that exists in volcanic rock types. In a companion paper [Hamilton et al., this issue] these schemes are applied to the classification of Martian surface compositions.

Sandstone Petrology, Evidence from Luminescence Petrography

Abstract: Using luminescence petrography equipment previously described in the literature, and modifications of this equipment reported here, one can usually distinguish secondary from primary quartz in sandstones. It is thus possible to distinguish solution and overgrowth phenomena even when usual petrographic criteria fail. By luminescence, examples of concavo-convex boundaries, long boundaries, and sutured contacts, all criteria of pressure solution, are revealed to result from optically continuous quartz overgrowths. The luminescence not only prevents ambiguity regarding mechanisms active in the sandstones, but reveals the detrital fabric so that size, shape, and degree of rounding of the detrital grains may be clearly seen. Features relating to prior cycles of sedimentation are also freque tly seen. Fractures and crushing events, long since healed by growth of secondary quartz, are also distinguishable by the luminescence technique; in some immature sandstones such events seem to be very common. Often, the crystal regions revealed by crossed nicols are related to, but are by no means identical with, the fractured fragments, giving evidence of recrystallization with mobility of grain boundaries. Crenulated boundaries also form in the fracturing-healing-recrystallization process resulting in polycrystalline grains showing metamorphic quartzite texture. It is suggested that many of the polycrystalline quartz grains found in sandstones originate in this way either in place or in precursor source rocks. All these features are well documented with photomicrographs.