Petrography

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

Visible and near‐infrared spectra of carbonate rocks: Reflectance variations related to petrographic texture and impurities

Abstract: Grain size statistics derived from thin section point measurements were used to classify carbonate rock textures and to identify related spectral characteristics in the visible and near-infrared (0.4–2.5 μm) wavelength range. Although the statistics did not permit all the carbonate textural varieties generally recognized by petrologists to be distinguished, three broad textural classes of pure carbonate material were found that produced distinctive spectra. Further refinement of the statistical technique may permit other textural classes to be identified. However, internal scattering in carbonate rocks is a complex phenomenon involving the grain size distribution, as well as the nature and abundance of inclusions and internal grain defects. To yield an optimum textural classification for spectroscopic studies, all such scattering contributors would have to be quantified. The spectral effects of iron oxide, clay, silica, and organic matter impurities were examined by comparing texturally similar pure and impure carbonate rock samples. Of these impurities, organic matter had the greatest spectral influence, and amounts as low as 0.01 wt % commonly caused marked reductions in sample brightness and absorption band intensity. In contrast, the spectral effects of rock texture were subtle and could only be identified in samples that were nearly free of organic matter impurities. The pronounced organic matter spectral effects imply that the organic matter in most carbonates is very finely disseminated. Samples collected from different regions usually showed different degrees of organic spectral quenching. However, no correlations were evident between sample reflectance and contained kerogen H/C ratios or kerogen amounts. Additional analytical work is needed to examine organic matter compositional variation and associated spectral quenching effects in detail. From the present study, it appears likely that contained organic matter will be a dominant factor in the visible and near-infrared spectral expression for most carbonate rocks, and perhaps for other sedimentary lithologies. Spectral quenching by organic matter will likely complicate remote sensing and laboratory spectral determinations of carbonate composition. On the other hand, variations in quenching may prove to be useful for distinguishing rocks that have experienced unusual diagenetic, thermal, or other conditions affecting the organic matter contents.

Petrology of kamafugites and kimberlites from the Alto Paranaíba Alkaline Province, Minas Gerais, Brazil

Abstract: Mafic rocks representative of the alkaline magmatism of the Alto Paranaiba Province in southwestern Minas Gerais, Brazil were studied by means of petrography, mineral, whole-rock and isotope geochemistry with the objective of better understanding this Cretaceous magmatism and the characteristics of the magma sources. Because of the variety and complexity of lithotypes examined in this research and the paucity of detailed studies of these Brazilian rocks in the literature, this study also attempts to establish parameters that allow for a clear distinction between kimberlite and kamafugite. Fifty-two occurrences are described and classified as kimberlite or kamafugite. Among the kamafugites, both ugandite (characterized by the presence of leucite) and mafurite (with kalsilite) end members have been characterized. Mineral compositions were found to be efficient in distinguishing between kimberlites, mafurites and ugandites in the province, primarily on the basis of phlogopite composition. The Re-Os isotope systematics permitted a better understanding of the relation of the sublithospheric mantle source to the magmatism in the region. Kimberlites, mafurites and ugandites have different 187Os/188Os ratios (0.117 to 0.129, 0.127 to 0.145 and 0.142 to 0.147, respectively). The Rb-Sr and Sm-Nd isotope systematics failed to indicate first-order differences between kamafugites and kimberlites, whilst 206Pb/204Pb ratios for the kimberlites are higher than those for the other rock types. Kimberlite and kamafugite isotopic compositions appear to be related to the mixture of at least two dominant mantle components: one with an isotopic signature similar to that of lithospheric peridotites, i.e., with 187Os/188Os ratios of the order of 0.118, similar to those observed in mantle-derived xenoliths entrained in kimberlites intruded in the Kaapvaal, Wyoming, and Siberian cratons; another with higher 187Os/188Os ratios of the order of 0.135, within the range of ratios reported for pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting depths of heterogeneous lithospheric sources by a mantle plume are suggested to explain the isotopic characteristics of the Alto Paranaiba Alkaline Province alkaline rocks.