Petrography

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

Zoning in highly alkaline magma bodies

Abstract: We present chemical data on magmatically heterogeneous pyroclastic deposits of late Quaternary age erupted from zoned magma systems underlying Tenerife (Canary Islands), Sao Miguel and Faial (Azores), and Vesuvius. The most fractionated magmas present at each centre are respectively Na-rich phonolite, trachyte, and K-rich phonolite. Within any one deposit, chemical variation is either accompanied by changes in the phenocryst assemblage (petrographic zonation) or is largely manifested in trace element abundances, unaccompanied by any petrographic change (occult zonation). Zoning is analogous to that in calc-alkaline systems where the most fractionated products are high-silica rhyolites. When a range of magma types are considered, a correlation emerges between roofward depletion of trace elements (especially REE) in the zoned system and compatability of those same trace elements in the accessory phenocryst phases present. Thus, allanite- or chevkinite-bearing rhyolitic systems are light-REE depleted roofwards, the sphene-bearing Tenerife system is middle-REE depleted roofwards, the melanite-bearing Vesuvius system is heavy-REE depleted roofwards, while the Azores systems, which lack these phases, display roofward REE enrichment. Therefore, the behaviour of trace elements may in each case be explained by fractionation of observed phenocryst assemblages. The resemblance between features of zoned magma systems and published work on the dynamic consequences of cooling saturated aqueous solutions prompts us to suggest that sidewall crystallization and consequent boundary-layer uprise to form a capping layer at top of the system may be a plausible mechanism for the generation of both petrographic and occult zonation. Reverse zoning occurs among the first-erupted tephra of some deposits, demonstrating that the most highly differentiated magma available is not always the first to be tapped during an eruption from a zoned system.

Composition and evolution of the lunar crust in the Descartes highlands, Apollo 16.

Abstract: Samples from the North Ray crater ejecta blanket, Apollo 16, were investigated by petrographic microscope, electron microprobe, instrumental neutron activation and Xray fluorescence analyses, and 40Ar-39Ar and Rb-Sr dating techniques. Nine major groups of monomict and polymict breccias were defined on the basis of microscopic texture and these were further subdivided into chemical subgroups on the basis of characteristic elements such as Al, Mg, Fe, Cr, REE, Ni, and Co. The polymict breccias — fragmental breccias, granulitic breccias, and impact melt breccias — are the result of multiple impact-induced mechanical mixing and melting, and of thermal and impact metamorphism of rock and mineral clasts derived from primordial igneous crustal rocks. For calculations of mixing models it was found that end-members consisting of the pristine igneous rock components present as discrete samples at the Apollo 16 site and supplemented by KREEP, dunite, and a meteoritic component yield the best fits for the composition of polymict breccias. The end-member rocks a re: ferroan anorthosite, various magnesian gabbronorites including “sodic ferrogabbro” and “feldspathic lherzolite,” and spinei troctolite. The following model is proposed for the composition and stratigraphy of the target for North Ray crater. The lower section of the stratigraphy is composed of a megabreccia with clasts of highly feldspathic polymict breccias (KREEP-free “Old Eastern Highland Rock Suite”) interpreted as Nectaris ejecta (Descartes formation). The top section contains KREEP-bearing polymict breccias (KREEP-bearing “Young Western Highland Rock Suite”) and appears to be similar to the lithologies found in the Cayley plains. This material interpreted as Cayley formation may be a distant facies of Imbrium basin ejecta deposits of the Imbrium basin. The petrographic differences between these two major selenographic units (the Descartes and the Cayley formations) in the Apollo 16 area are more distinct than the chemical differences. The petrographic and chemical composition of the primordial igneous upper crust in the regions of the Nectaris and Imbrium basins has been calculated by subtraction of the KREEP and meteoritic components from the bulk composition of the Descartes and Cayley materials. The Nectaris, crust which is better constrained, consists of 86-87% a northosite, 4% sodic ferrogabbro, 0.5-1.3% feldspathic lherzolite, 6-8% regular magnesian gabbronorite, 1.8-2.8% dunite, and 0.1% spinel troctolite. A model for the evolution of the upper lunar crust in the Descartes highlands is proposed on the basis of isotope ages and clast-matrix relationships of polymict breccias. Essential features of this model in sequenial order are: (1) development of multiple layers of KREEP-free “early feldspathic fragmental megabreccias” and impact melt sheets on the primordial crust in the time period from 4.4 aeons to the time of the Nectaris impact, which could have occurred as late as 3.85 aeons ago, (2) excavation of these megabreccias by the Nectaris event and deposition of a “Nectarian feldspathic fragmental megabreccia” in the Descartes area, (3) introduction of KREEP-basaltic material into the upper crust, mainly west and northwest of the Descartes site and formation of KREEPy impact melt sheets (most probably from 4.05 to 3.80 aeons ago), (4) Imbrium impact at 3.8 aeons ago and deposition of KREEP-bearing “Imbrian feldspathic fragmental breccias” at the Descartes site which form the Cayley formation, and (5) redeposition , mixing, and melting of all preexisting breccias yielding KREEP-bearing and KREEP-poor post-Cayley fragmental breccias and melt ejecta (“glass bombs”) as the result of small impacts (craters < 1.5 km in diameter) in the Cayley plains and the Descartes mountains, respectively.

Sem Petrography of Eastern Mediterranean Sapropels: Analogue Data For Assessing Organic Matter In Oil and Gas Shales

Abstract: Examination of organic-matter-hosted pore systems in unconventional reservoir rocks has drawn attention to the appearance of sedimentary organic matter (OM) as seen in high-resolution SEM images. Field-emission SEM imaging of eastern Mediterranean sapropels (Pliocene–Pleistocene age from Ocean Drilling Program drill sites) was performed on samples prepared by Ar-ion cross-section polishing. The immaturity of marine kerogen in these rocks allows inspection of petrographic textures without overprints related to thermal maturation (hydrocarbon generation). Kerogen in these samples includes discrete particles that in some cases contain primary intragranular pores. The most abundant organic matter is in the form of microns-thick flaky or stringy material, largely nonporous and internally amorphous, and lacking the well-defined shapes of discrete particles. This dominant OM has behaved in a highly ductile manner in compaction and is now highly pervaded into intergranular spaces between silt- and clay-size grains. A portion of the larger mineral-hosted pores remain open, unfilled by ductile OM. Minor nanometer-scale pores of uncertain origin occur in the dominant OM. Silt- and clay-size mineral crystals are distributed in organic matter of most types, and it is difficult to fully ascertain if the OM/mineral mixing has occurred during compaction versus during aggregation during gravity settling (marine-snow sedimentation) or through sediment accumulation in OM-rich microbial mats. Results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss in early burial. These observations of marine detrital organic matter in its early diagenetic state provide a useful baseline for interpretation of petrographic features in more mature OM-rich mudrocks. It is clear that petrographic discrimination of diagenetically generated organic matter (bitumen and related OM types) from ductile dispersed amorphous detrital OM (kerogen) using SEM data will be challenging in the absence of compositional information.