Petrography

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

Early Pre-Cambrian Onverwacht Microstructures : Possibly the Oldest Fossils on Earth?

Abstract: Early Precambrian Onverwacht microstructures studied in petrographic thin sections and powdered preparations for possibility of oldest terrestrial fossils

Chemical variation of the large Apollo 15 olivine-normative mare basalt rock samples

Abstract: Most chemical analyses of Apollo 15 olivine-normative mare basalts have been conducted on subsamples of 4 g) to obtain greater whole-rock representivity. These subsamples were individually ground and homogenized, and splits were taken for analysis. Furthermore, we used both X-ray fluorescence and neutron activation techniques to analyze for a comprehensive set of elements suitable for petrogenetic interpretation. The analyses show that the samples form a single coherent suite with almost all of the variation corresponding with olivine control (15% range). A few of the coarser rocks are not quite represented even at this sampling size. The analyses show that the rocks are individually distinct and that analyses are not merely of unrepresentative pieces of a single rock, undifferentiated rock unit, or rocks differing only by short-range unmixing of residual fluids. The petrographic features, including the low abundance of olivine and its small size, and the vesicularity of even some of the coarser samples, show that the olivine that controlled the chemical variation is not accumulated in any of the rocks. The Apollo 15 olivine-normative mare basalts were extruded as a series of magmas from a shallow but not locally surficial, olivine-accumulating magma system and formed a sequence of thin flows. A greater understanding of the relationships within and among other mare basalt sequences would be obtained by obtaining comprehensive chemical analyses on splits taken from subsamples of 5 g of all rocks large enough to obtain such subsamples.

Mineralogical and geochemical aspects of impact craters

Abstract: The importance of impact cratering on terrestrial planets is obvious from the abundance of craters on their surfaces. On Earth, active geological processes rapidly obliterate the cratering record. To date only about 170 impact structures have been recognized on the Earth’s surface. Mineralogical, petrographic, and geochemical criteria are used to identify the impact origin of such structures or related ejecta layers. The two most important criteria are the presence of shock metamorphic effects in mineral and rock inclusions in breccias and melt rocks, as well as the demonstration, by geochemical techniques, that these rocks contain a minor extraterrestrial component. There is a variety of macroscopic and microscopic shock metamorphic effects. The most important ones include the presence of planar deformation features in rock-forming minerals, high-pressure polymorphs (e.g. of coesite and stishovite from quartz, or diamond from graphite), diaplectic glass, and rock and mineral melts. These features have been studied by traditional methods involving the petrographic microscope, and more recently with a variety of instrumental techniques, including transmission electron microscopy, Raman spectroscopy, cathodoluminescence imaging and spectroscopy, and high-resolution X-ray computed tomography. Geochemical methods to detect an extraterrestrial component include measurements of the concentrations of siderophile elements, mainly of the platinum-group elements (PGEs), and, more recently, chromium and osmium isotopic studies. The latter two methods can provide confirmation that these elements are actually of meteoritic origin. The Cr isotopic method is also capable of providing information on the meteorite type. In impact studies there is now a trend towards the use of interdisciplinary and multi-technique approaches to solve open questions.