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At least 523 records · Page 29

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NASA NTRS1976

Rb, Sr and strontium isotopic composition, K/Ar age and large ion lithophile trace element abundances in rocks and glasses from the Wanapitei Lake impact structure

Shock metamorphosed rocks and shock-produced melt glasses from the Wanapitei Lake impact structure have been examined petrographically and by electron microprobe. Eleven clasts exhibiting varying degrees of shock metamorphism and eight impact-produced glasses have been analyzed for Rb, Sr and Sr isotopic composition. Five clasts and one glass have also been analyzed for large ion lithophile (LIL) trace element abundances including Li, Rb, Sr, and Ba and the REE's. The impact event forming the Wanapitei Lake structure occurred 37 m.y. ago based on K/Ar dating of glass and glassy whole-rock samples. Rb/Sr isotopic dating failed to provide a meaningful whole-rock or internal isochron. The isotopic composition of the glasses can be explained by impact-produced mixing and melting of metasediments.

Winzer, S. R.↗

NASA NTRS1975

Boulder 1, Station 2, Apollo 17 - Petrology and petrogenesis

The clasts and matrices of Boulder 1, Station 2 are investigated by petrographic and microprobe methods. The boulder is shown to consist of two separate entities: an older metamorphosed breccia containing a diverse lithic clast population, and a friable matrix containing KREEPy basalts. It is suggested that the friable matrix was created in the Serenitatis event in a cool upper portion of the ejecta blanket and that the clasts were created in an earlier impact event. These results are shown to indicate that the Serenitatis event is younger than 4.01 billion years and that the lunar crust had a complex and varied history of magmatism, metamorphism, and brecciation in the interval between the origin of the moon and the assembly of the boulder.

Ryder, G.↗

NASA NTRS1975

Implications of poikilitic textures in LL-group chondrites

Five lithic fragments with poikilitic textures in five LL-group chondrites are examined by microscope and electron microprobe to determine whether the textures have resulted from processes related to impact events, such as thermal metamorphism or partial melting. The bulk and pyroxene compositions of the fragments are determined. The compositional characteristics of minerals in certain fragments are found to indicate an apparent melt origin. It is concluded that impact processes produced the poikilitic textures and that complete melting, partial melting, and solid-state recrystallization all have had some part in producing the textures.

Fodor, R. V.↗

NASA NTRS1975

Pyroxenes in the Patwar mesosiderite

Major and minor elements were determined with an electron microprobe in pyroxene from Patwar, one of the least metamorphosed mesosiderites. All but two orthopyroxene crystal fragments fall in the range Wo(2.3)En(73) to Wo(3.9)En(59), while pigeonite from a basalt clast clusters around Wo(13)En(38). Absolute and relative amounts of Ti, Cr, and Al are distinct from those in pyroxenes in irons-with-silicate inclusions. Zoning within orthopyroxene grains suggests crystallization from an igneous melt in the presence of plagioclase and chromite, followed at some later time by brecciation and subsequent minor thermal metamorphism. Compositional data support petrographic evidence that the silicates in Patwar were derived from a heterogeneous source.

Weigand, P. W.↗

NASA NTRS1976

The frequency dependence of the viscous component of the magnetic susceptibility of lunar rock and soil samples

The susceptibility of two lunar samples (a soil and a low metamorphic grade breccia) has been measured in a weak field - 0.001 Oe - and as a function of frequency from 0.032 to 1.0 Hz. The measurements were made using a superconducting magnetometer. The results show that the susceptibility decreases linearly with the log of frequency. This observation is in agreement with a theoretical model for viscous decay based on the Neel theory of single-domain and superparamagnetic grains. The relation derived agrees with a model in which there is a uniform distribution of relaxation times.

Hanneken, J. W.↗

NASA NTRS1976

Geological applications of Nimbus radiation data in Middle East

Plateaus of Eocene limestone and exposed limestone escarpments, in Egypt and Saudi Arabia respectively, were indicated by cool brightness temperatures T sub B (less than 240 to 265 K) by the Nimbus 5 electrically scanning microwave radiometer (ESMR) over a 2-year period. Nubian sandstone, desert eolian sand and igneous metamorphic rocks were differentiated from these limestone areas by warm T sub B values (more than 265 to 300 K). Cool T sub B values in the form of a V were found over broad areas of the Nile Valley and in the western desert of central Egypt. Similar cool T sub B values were shown over limestone-dolomitic hills of the interior Homocline and the Hadramawt plateau of Saudi Arabia. Nimbus 5 and 6 ESMR T sub B values selectively identified intermediate dense rock types (limestone versus sandstone/granite) in the Lake Nasser region whose thermal inertia ranged from 0.035 to 0.06 cal cm to minus 2 C sec 1/2 to minus one half.

Allison, L. J.↗

NASA NTRS1977

Geological applications of Nimbus radiation data in the Middle East

Large plateaus of Eocene limestone and exposed limestone escarpments, in Egypt and Saudi Arabia, respectively, were indicated by cool brightness temperatures recorded by the Nimbus-5 electrically scanning microwave radiometer (ESMR), over a 2-year period. Nubian sandstone, desert eolian sand, and igneous-metamorphic rock of the Pliocene, Miocene, Oligocene, and Cretaceous periods were differentiated from these limestone areas by warm T sub B values. These brightness temperature differences are a result of seasonal in-situ ground temperatures and differential emissivity of limestone and sand, sandstone and granite, whose dielectric constants are 6 to 8.9 and 2.9 and 4.2 to 5.3, respectively, at 19.35 GHz.

Allison, L. J.↗

NASA NTRS1976

Lunar mare basalts - Conference summary

Compositionally, lunar mare basalts are similar to some very young subalkaline basalts from terrestrial mid-ocean ridges and to very old pods of basaltic material incorporated into ancient metamorphic rocks. Basalt flows in Mare Imbrium are considered, taking into account the results of orbital gamma ray spectroscopic studies. The results of the analyses of lunar samples obtained from the Apollo missions are evaluated and various models are discussed in connection with an interpretation of the observed conditions.

Merrill, B.↗

NASA NTRS1976

Mineralogy of the Ibitira eucrite and comparison with other eucrites and lunar samples

The mineralogy of the Ibitira meteorite is studied using single-crystal X-ray and energy-dispersive electron-probe techniques. It is shown that this strongly metamorphosed unbrecciated vesicular eucrite has a primary variolitic and secondary hornfelsic texture dominated by equant grains of pale-brown pyroxene (60%) set in an irregular matrix of plagioclase (30%). Minor phases are detected which indicate igneous crystallization, a prolonged metamorphic history, and vapor deposition. The following sequence of events is suggested as the simplest explanation for the mineralogical features observed: (1) crystallization of the meteorite in a lava flow; (2) prolonged annealing, reduction of primary spinel, and vapor deposition; (3) strong shock sufficient to modify the texture of the plagioclase grains; and (4) sufficient annealing to allow crystallization of the plagioclase as a subparallel mosaic and partial conversion to the low structural state. Evidence for this sequence is examined in detail, particularly in relation to the general features of eucrites, howardites, and mesosiderites

Steele, I. M.↗

NASA NTRS1976

On the age of Rhodesian greenstone belts

Isochron ages and initial Sr-87/Sr-86 ratios are derived from Rb-Sr isotopic analyses on metavolcanic rocks from some greenstone belts of the Rhodesian Archean craton. Three mineral isochron ages (about 2.4 billion years) are tentatively suggested to represent the time of an important low-grade metamorphic event.

Jahn, B.-M.↗

NASA NTRS1977

Comments on 'Extinct radioactivities: Trapped residuals of presolar grains'

It has recently been suggested that extinct I-129 and Pu-244 were trapped in primitive-solar-nebula ('presolar') grains and decayed into radiogenic Xe-129 and fission Xe before the grains were incorporated into meteorite bodies. This idea is reconsidered in light of the thermal and metamorphic history of meteorites. The criteria that parent and daughter species should never separate and that minerals or grains containing the anomalous xenon should not be subjected to temperatures exceeding 500 C are applied to iron meteorites, achondrites, and chondrites to determine whether presolar grains could be the carriers of rare-gas anomalies to meteorites. The results strongly indicate that the xenon anomaly could not have originated in presolar grains. Other difficulties with the presolar-grain model are discussed, including insufficiently small grain sizes, large variations in Xe-129/I-127 ratios in various meteorites, and apparently unrealistic meteorite formation times and locations.

Trivedi, B. M. P.↗

NASA NTRS1977

Some petrological aspects of Imbrium stratigraphy

Descriptions are given of the petrochemistry of two Apennine Front breccias, both ejected to the surface during excavation of Spur Crater. The first clast type is breccia number 15445, a spinel pyroxenite whose mineralogy and petrochemistry are consistent with the original rock type being a garnet pyroxenite. The second rock, breccia 15459, is plutonic norite, in which coarsely exsolved inverted pigeonite is associated with anorthitic plagioclase. Application of mineral geothermometers indicates crystallization of these rocks below 1100 C; hence their textures probably developed largely by solid state recrystallization during impact-metamorphism.

Ridley, W. I.↗

NASA NTRS1976

Microcracks, micropores, and their petrologic interpretation for 72415 and 15418

Lunar samples 72415 and 15418 have complex microstructures that indicate a series of fracturing and healing events. Both samples contain relatively few open microcracks but many sealed and healed microcracks. Dunite 72415 contains abundant healed cracks that formed tectonically, symplectic intergrowths spatially and probably genetically related to microcracks, and a cataclastic matrix that has been extensively sintered. Metamorphosed breccia 15418 contains many post-metamorphic healed cracks, large shock induced cracks that have been sealed with glass, and a few younger, thin, open shock induced cracks.

Richter, D.↗

NASA NTRS1977

Lithic clasts in the Supuhee chondrite

Bulk chemical analyses of three foreign lithic clasts from Supuhee, a shocked and brecciated H6 chondrite, were carried out in an attempt to locate 'mysterite', a volatile-rich material postulated to be present. Clast 1 bears some similarities to C1 and C2 chondrites, but its relatively low (less than 9.4%) volatile content suggests a different set of physicochemical conditions during low-temperature condensation. Clast 2 was apparently derived from H-group chondrite material by loss of metal and sulfide. Clast 3 is an unusual meteorite type whose chemical composition is reminiscent of the unique chondrite Kakangari (Graham, et al., 1977). These chemically distinct bodies were apparently not present during the metamorphic event which affected Supuhee. Although the presence of mysterite was not confirmed or refuted, the present results indicate that conditions were favorable for the formation, incorporation and preservation of volatile-rich materials.

Leitch, C. A.↗

NASA NTRS1977

The composition of carbonaceous chondrite matrix

Matrix compositions of 32 of the approximately 40 known carbonaceous chondrites were analyzed using an electron probe defocussed-beam technique. Except in those chondrites that show evidence of metamorphism, matrices are compositionally similar and have correlation coefficients of +0.96 or greater. The implications of these analyses for the chemistry of individual matrix phases and for primitive solar system material are discussed.

Mcsween, H. Y., Jr.↗

NASA NTRS1977

Volatile/mobile trace elements in Karoonda /C4/ chondrite

Concentrations of ten volatile/mobile trace elements and of nonvolatile Co in the Karoonda (C4) meteorite were determined, and the atomic abundances relative to C1 are compared with values for the Murchison (C2) and Allende (C3) meteorites. Empirical Bi, In, and Tl data for Karoonda and heated Allende and Murchison are compared with theoretical curves for condensation from a gas of cosmic composition at low pressures. It is suggested that Karoonda might derive from low-temperature open-system metamorphism of pristine C3-like material.

Matza, S. D.↗

NASA NTRS1977

Imaging natural materials with a quasi-microscope

A Viking lander camera with auxilliary optics mounted inside the dust post was evaluated to determine its capability for imaging the inorganic properties of granular materials. During mission operations, prepared samples would be delivered to a plate positioned within the camera's field of view and depth of focus. The auxiliary optics would then allow soil samples to be imaged with an 11 pm pixel size in the broad band (high resolution, black and white) mode, and a 33 pm pixel size in the multispectral mode. The equipment will be used to characterize: (1) the size distribution of grains produced by igneous (intrusive and extrusive) processes or by shock metamorphism, (2) the size distribution resulting from crushing, chemical alteration, or by hydraulic or aerodynamic sorting; (3) the shape and degree of grain roundness and surface texture induced by mechanical and chemical alteration; and (4) the mineralogy and chemistry of grains.

Bragg, S.↗

NASA NTRS1977

A chemical model for lunar non-mare rocks

Nearly all rocks returned from the moon are readily divided into three broad categories on the basis of their chemical compositions: (1) mare basalts, (2) non-mare rocks of basaltic composition (KREEP, VHA), and (3) anorthositic rocks. Only mare basalts may unambiguously be considered to have original igneous textures and are widely understood to have an igneous origin. Nearly all other lunar rocks have lost their original textures during metamorphic and impact processes. For these rocks one must work primarily with chemical data in order to recognize and define rock groups and their possible modes of origin. Non-mare rocks of basaltic composition have chemical compositions consistent with an origin by partial melting of the lunar interior. The simplest origin for rocks of anorthositic chemical composition is the crystallization and removal of ferromagnesian minerals. It is proposed that the rock groups of anorthositic and non-mare basaltic chemical composition could have been generated from a single series of original, but not necessarily primitive, lunar materials.

Hubbard, N. J.↗