Geology topics

E. C. T. Chao

Publications and source records attributed to E. C. T. Chao.

28 records · Page 2Linked to original sources

USGS1975

Origin of lunar light plains

Most Cayley-type Imbrian-age plains deposits and adjacent mantled slopes, including those at the Apollo 16 site, may be composed at least near the surface of ejecta from the Orientale basin, the youngest multiringed impact basin on the Moon. The distribution and apparent age of the plains deposits and preliminary data on the highly feldspathic breccias collected by the Apollo 16 crew indicate that these surficial materials are neither locally derived nor part of the Imbrium ejecta. Stratigraphic relations, crater size-frequency distributions, and dating by erosional morphology of superposed craters have established Cayley light-plains deposits as younger than the Imbrium basin and older than mare material. All such crater-dated Cayley-type plains on both the near and far sides of the Moon are contemporaneous within the limits of the technique. Furthermore, comparisons of the crater size-frequency distributions of the Hevelius Formation (Orientale ejecta) and the plains show that the Cayley and Hevelius Formations are indistinguishable in age. The surface and near-surface materials of the Apollo 16 plains, therefore, are contemporaneous with Orientale basin ejecta not with Imbrium ejecta, in which both crater densities and crater degradation are greater than in the light plains. Imbrium ejecta may, however, be present on the surface at the Apollo 16 site where excavated by craters from depth. Limited age data now available on the Apollo 16 samples are consistent with this interpretation. We conclude that rocks of Orientale provenance may be predominant on and near the surface at the Apollo 16 site. This hypothesis implies that the catastrophic Orientale impact struck a highland area underlain by highly feldspathic material and spread it over much of the Moon. Thus, much of the lunar highlands crust need not consist of anorthositic materials to any significant depth. These conclusions apply to the plains at the Apollo 16 site and most of the other Cayley-type plains, but we do not exclude the possibility that, moonwide, the plains may be polygenetic.

Journal of Research of the U.S. Geological Survey↗

USGS1974

Preliminary petrographic description and geologic implications of the Apollo 17 Station 7 boulder consortium samples

Preliminary petrographic description and mineral composition of four hand samples (77135, 77115, 77075 and 77215) are presented. 77135, 77115, and 77075 all crystallized from fragment-laden melts; they are similar in textures but differ in grain size. 77135 and 77115 are pigeonite feldspathic basalts. On the basis of geologic and petrographic evidence, 77115 and 77075 are related; they formed, cooled, and consolidated before being engulfed in the vesicular 77135. The impact or igneous origin of the melts from which these rocks crystallized cannot be determined. 77215 is a shocked, strongly sheared and granulated microbreccia consisting of three major lithologies dominated by mineral clasts of orthopyroxene and calcic plagioclase. The orthopyroxene clasts contain coarse exsolved blebs of augite, suggesting a deep-seated origin. The major, minor, and trace element compositions of 77135, 77115, and 77075 are in general similar. They represent a major highland rock type, perhaps more important than anorthosites.

Earth and Planetary Science Letters↗

USGS1973

Geologic implications of the Apollo 14 Fra Mauro breccias and comparison with ejecta from the Ries Crater, Germany

On the basis of petrographic and laboratory and active seismic data for the Fra Mauro breccias, and by comparison with the nature and distribution of the ejecta from the Ries crater, Germany, some tentative conclusions regarding the geologic significance of the Fra Mauro Formation on the moon can be drawn. The Fra Mauro Formation, as a whole, consists of unwcldcd, porous ejecta, slightly less porous than the regolith. It contains hand-specimen and larger size clasts of strongly annealed complex breccias, partly to slightly annealed breccias, basalts, and perhaps spherule-rich breccias. These clasts are embedded in a matrix of porous aggregate dominated by mineral and breccia fragments and probably largely free of undevitrified glass. All strongly annealed hand-specimen-size breccias are clasts in the Fra Mauro Formation. To account for the porous, unwelded state of the Fra Mauro Formation, the ejecta must have been deposited at a temperature below that required for welding and annealing. Large boulders probably compacted by the Cone crater event occur near the rim of the crater. They probably consist of a similar suite of fragments, but are probably less porous than the formation. The geochronologic clocks of fragments in the Fra Mauro Formation, with textures ranging from unannealed to strongly annealed, were not reset or strongly modified by the Imbrian event. Strongly annealed breccia clasts and basalt clasts are pre-Imbrian, and probably existed as ejecta mixed with basalt flows in the Imbrium Basin prior to the Imbrian event. The Imbrian event probably occurred between 3.90 or 3.88 and 3.65 b.y. ago.

Ries Crater↗

USGS1970

Petrology of unshocked crystalline rocks and shock effects in lunar rocks and minerals

On the basis of rock modes, textures, and mineralogy, unshocked crystalline rocks are classified into a dominant ilmenite-rich suite (subdivided into intersertal, ophitic, and hornfels types) and a subordinate feldspar-rich suite (subdivided into poikilitic and granular types). Weakly to moderately shocked rocks show high strain-rate deformation and solid-state transformation of minerals to glasses; intensely shocked rocks are converted to rock glasses. Data on an unknown calcium-bearing iron metasilicate are presented.

Science↗

USGS1967

Shock effects in certain rock-forming minerals

Shock effects in quartz, plagioclase, biotite, amphibole, and some accessory minerals have been observed in rocks subjected to various degrees of meta morphism by meteoritic impact. The shock features described are unique; they are never observed in rocks from normal geologic environments. Such features are described: 1) Multiple sets of closely spaced planar microstructures occur in quartz, plagioclase, and other rock-forming minerals. Those characteristic of shock consist of alternating platelets, with a range of reduced mean index of re fraction and birefringence; they con sist of platelets that have been partially or completely transformed to an amor phous phase. 2) Quartz and plagioclase are selec tively and completely transformed to silica glass and plagioclase glass in the solid state, whereas the associated mafic minerals remained crystalline. There is no reaction between adjacent minerals. 3) High-pressure polymorphs occur, such as coesite or stishovite. Coesite oc Curs exclusively within silica glass; it has not been observed as a reaction or breakdown product. 4) Nickel-iron spherules occur in the fused glass or impactites. 5) The occurrence of droplets of ilmenite, rutile, pseudobrookite, and baddaleyite in impactites indicates a temperature of formation exceeding 150°C. 6) Dense glass occurs, similar in composition to bulk rock, in which iron oxide, such as fine particles of mag netite, is completely dissolved. All these features are characteristic of a process involving the rapid rise and fall of extremiiely high pressures and temperatures. Minerals and mineral as semblages experiencing such high strain rates and sudden changes of pressures and temperatures react and change in dependently to the bulk chemical com position, under nonequilibrium condi tions. Many aspects of shock features re quire careful study. Kink bands in biotite and deformation lamellae in quartz occur in tectonically deformed rocks. These features should be studied with great care in order to determine whether reduction in mean index of refraction and total birefringence along the planar structures have resulted from vitrification or phase transition; their presence is additional evidence in favor of a shock mechanism. Vitreous phases or glasses formed by shock also have many unique prop erties; they have not been studied by such methods as thermoluminescence, electron spin resonance, low-angle x ray diffraction, or infrared spectroscopy. Shock-fused glass of high density needs to be studied in detail in carefully con trolled laboratory conditions. Experimental shock-wave studies of the equation-of-state of single minerals and mineral assemblages, under care fully controlled conditions, must pre cede estimates of peak pressures and peak and residual temperatures of shocked natural mineral assemblages. Detailed petrographic and mineralogic studies, however, have provided useful and definitive criteria for characteriza tion of impact events. Such data should be of paramount importance in the study of samples brought back from Moon.

Science↗

USGS1966

Nickel-iron spherules from Aouelloul glass

Nickel-iron spherules, ranging from less than 0.2 to 50 microns in diameter and containing 1.7 to 9.0 percent Ni by weight, occur in glass associated with the Aouelloul crater. They occur in discrete bands of siliceous glass enriched in dissolved iron. Their discovery is significant tangible evidence that both crater and glass originated from terrestrial impact.

Aouelloul crater↗

USGS1964

Spalled, aerodynamically modified moldavite from Slavice, Moravia, Czechoslovakia

A Czechoslovakian tektite or moldavite shows clear, indirect evidence of aerodynamic ablation. This large tektite has the shape of a teardrop, with a strongly convex, deeply corroded, but clearly identifiable front and a planoconvex, relatively smooth, posterior surface. In spite of much erosion and corrosion, demarcation of the posterior and the anterior part of the specimen (the keel) is clearly preserved locally. This specimen provides the first tangible evidence that moldavites entered the atmosphere cold, probably at a velocity exceeding 5 kilometers per second; the result was selective heating of the anterior face and perhaps ablation during the second melting. This provides evidence of the extraterrestial origin of moldavites.

Science↗

USGS1962

Metallic spherules in tektites from Isabela, Philippine Islands

Iron-nickel spherules, as much as 0.5 mm in diameter, have been found completely embedded in some philippinites. The spherules consist mainly of kamacite with unidentified pink inclusions. The meteoritic origin of these spherules seems reasonable, suggesting that the tektites containing them were formed by asteroidal or meteoritic impact.

Isabela↗

USGS1961

Coesite from Wabar crater, near Al Hadida, Arabia

The third natural occurrence of coesite, the high pressure polymorph of silica, is found at the Wabar meteorite crater, Arabia. The Wabar crater is about 300 feet in diameter and about 40 feet deep. It is the smallest of three craters where coesite has been found.

Al Hadida, Ar-Rub' Al-Khali (Empty Quarter)↗

USGS1960

First natural occurrence of coesite

Coesite, the high-pressure polymorph of SiO 2 , hitherto known only as a synthetic compound, is identified as an abundant mineral in sheared Coconino sandstone at Meteor Crater, Arizona. This natural occurrence has important bearing on the recognition of meteorite impact craters in quartz-bearing geologic formations.

Arizona↗