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  • 1
    Publication Date: 2011-08-24
    Description: As a prelude to determinations of the content of total iron as FeO(T) in melts in equilibrium with calcic anorthosites, the partition coefficients (Ds) for FeO(T) between calcic plagioclase and basaltic melt were determined, as a function of oxygen fugacity (f(O2)), for a basaltic composition that occurs as matrices for plagioclase megacrysts. Results showed that, at the liquidus conditions, the value of D for FeO(T) between calcic plagioclase and tholeiitic basalt changed little (from 0.030 to 0.044) between the very low f(O2) of the iron-wustite buffer and that of the quartz-fayalite-magnetite (QFM) buffer. At fugacities above QFM, the value for D increased rapidly to 0.14 at the magnetite-hematite buffer and to 0.33 in air. The increase in D results from the fact that, at f(O2) below QFM, nearly all of the Fe is in the Fe(2+) state; above QFM, the Fe(3+)/Fe(2+) ratio in the melt increases rapidly, causing more Fe to enter the plagioclase which accepts Fe(3+) more readily than Fe(2+).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 56; 5, Ma
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  • 2
    Publication Date: 2011-08-18
    Keywords: GEOPHYSICS
    Type: Geological Society of America Bulletin (ISSN 0016-7606); 94; Feb. 198
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  • 3
    Publication Date: 2016-06-07
    Description: Anorthositic complexes with large equidimensional plagioclase grains of highly calcic composition occur in nearly all Archean cratons. Similar plagioclase occur as megacrysts in many Archean sills, dikes, and volcanic flows. In the Canadian Shield these units occur throughout the Archean portions of the entire shield and are particularly common as dikes over an area of a few 100,000 sq km in Ontario and Manitoba during a period of at least 100 m.y. in many different rock types and metamorphic grades. The plagioclase generally occurs in three modes: as inclusions in mafic intrusions at various stages of fractionation, as crystal segregations in anorthosite complexes, or as megacrysts in fractionated sills, dikes, and flows. Most occurrences suggest that the plagioclase was formed elsewhere before being transported to its present location. The evidence seems to be quite clear that occurrences of these types of calcic plagioclase require: (1) ponding of a relatively undifferentiated Archean tholeiitic melt at some depth; (2) isothermal crystallization of large, equidimensional homogeneous plagioclase crystals; (3) separation of the plagioclase crystals from any other crystalline phases; (4) further fractionation of melt; (5)transport of various combinations of individual plagioclase crystals and clusters of crystals by variously fractionated melts; and (6) emplacement as various types of igneous intrusions or flows.
    Keywords: GEOPHYSICS
    Type: Lunar and Planetary Inst. Workshop on Early Crustal Genesis: The World's Oldest Rocks; p 90-91
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  • 4
    Publication Date: 2013-08-31
    Description: The combined evidence from terrestrial anorthosites and experimental laboratory studies strongly implies that lunar anorthosites have been subjected to high-grade metamorphic events that have erased the igneous signatures of FeO and MgO in their plagioclases. Arguments to the contrary have, to this point, been more hopeful than rigorous.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Inst., The Twenty-Fifth Lunar and Planetary Science Conference. Part 3: P-Z; p 1081-1082
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  • 5
    Publication Date: 2011-08-17
    Description: The paper reviews the physical and chemical characteristics of lunar surface materials noting the thickness of the regolith, grain size distribution, regolith petrography, and the chemical composition of the regolith. A comprehensive design of a processing system for the production of structural metals, oxygen, silicon, glass, and ceramic materials is presented. Specifications regarding the location of the processing plant, and its mass, volume, and power requirements are reviewed.
    Keywords: ASTRONAUTICS (GENERAL)
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  • 6
    Publication Date: 2013-08-31
    Description: The occurrences of megacrystic anorthosite and basalt in a variety of geologic settings were reviewed and it was found that these rock types occur in a variety of tectonic settings. Anorthosites and megacrystic basalts are petrogenetically related and are found in oceanic volcanic crust, cratons, and shelf environments. Although megacrystic basalts are most common in Archean terranes, similar occurrences are observed in rocks of early Proterozoic age, and even in young terranes such as the Galapagos hotspot. Based on inferences from experimental petrology, all of the occurrences are apparently associated with similar parental melts that are relatively Fe-rich tholeiites. The megacrystic rocks exhibit a two- (or more)-stage development of plagioclase, with the megacrysts having relatively uniform composition produced under nearly isothermal and isochemical conditions over substantial periods of time. The anorthosites appear to have intruded various crustal levels from very deep to very shallow. The petrogenetic indicators, however, suggest that conditions of formation of the Precambrian examples were different from Phanerozoic occurrences.
    Keywords: GEOPHYSICS
    Type: Lunar and Planetary Inst., Workshop on the Deep Continental Crust of South India; p 135-137
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  • 7
    Publication Date: 2013-08-31
    Description: The petrogenetic significance of plagioclase megacryst-bearing Archean rocks was considered. It was suggested that these developed in mid-to upper-crustal magma chambers that were repeatedly replenished. Crystallization of megacrysts from a primitive liquid that evolves to an Fe-rich tholeiite (with LREE enrichment) is nearly isothermal and is an equilibrium process. Cumulates probably form near the margins of the chambers and liquids with megacrysts are periodically extracted and can appear as volcanics. Some flows and intrusives are found in arc-like settings in greenstone belts. Megacrystic dikes represent large volumes of melt and dike swarms such as the Metachawan swarm of Ontario suggest multiple sources of similar compositions. A complex series of melt ponding and migration are probable and involve large amounts of liquid.
    Keywords: GEOPHYSICS
    Type: Lunar and Planetary Inst., Workshop on the Deep Continental Crust of South India; p 112-114
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  • 8
    Publication Date: 2011-08-17
    Description: An interdisciplinary study of the 65-km diameter Manicouagan ring structure in Quebec was undertaken to gain insight into the integrated effects of impacts on terrestrial planetary bodies. The paper serves as an introduction to several companion papers and summarizes major conclusions. Studies of the 214 m.y. old structure show that the melt rocks are texturally inhomogeneous but chemically homogeneous and can be modeled as a mixture of target lithologies. The melt rocks have a Sr-87/Sr-86 ratio compatible with the melting of crustal rocks. The melt had a two-stage cooling history with clast-melt interactions indicative of mixing superheated silicate liquid with cold clasts. Grain size, clast reactions, and thermal equilibration of clasts and melts are described.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 83; June 10
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  • 9
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    In:  Other Sources
    Publication Date: 2011-08-18
    Description: An examination is undertaken of the sources of data for the study of terrestrial crust and mantle evolution. The two interrelated approaches to this task involve, on the one hand, the interpretation of data from available materials, and on the other the modeling of expected processes and their products from experimental and theoretical arguments. The former is presently considered in light of the evidence for additions to the crust noted in the study of igneous activity and, possibly, of metasomatism, in the evidence found for fractionation in the mantle through the study of igneous activity and mantle xenoliths, in the evidence for recycling of material both within the crust and through return of crust to the mantle, and in the evidence provided by other planetary bodies.
    Keywords: GEOPHYSICS
    Type: Reviews of Geophysics and Space Physics (ISSN 0034-6853); 21; July 198
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  • 10
    Publication Date: 2011-08-19
    Description: In most Archean cratons, cumulates of equant plagioclase megacrysts form anorthositic complexes, including those at Bad Vermilion Lake (Ontario). In this paper, partition coefficients (Ds) of REEs between natural high-Ca plagioclase megacrysts and their basaltic matrices were determined, using a multiple aliquot techique, and megacrystic plagioclases occurring in anorthosites were analyzed for the same components which, in conjunction with their Ds, were applied to calculations of melts in equilibrium with anorthosites. The REE's Ds were found to agree well with experimentally determined values and to predict equilibrium melts for Archean anorthosites that agree well with coeval basaltic flows and dikes. The Ds also appear to be valid for both the tholeiitic and alkali basalts over a wide range of mg numbers and REE concentrations. It is suggested that the moderately Fe-rich tholeiites that are hosts to plagioclase megacrysts in greenstone belts form the parental melts for megacrysts which make up the Bad Vermilion Lake Archean anorthositic complex.
    Keywords: GEOPHYSICS
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 1639-165
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