Age of a lunar anorthosite

The crystallization age of an Apollo 15 anorthosite rock, 15415,9, returned from the lunar highlands has been measured to be (4.09 +/- 0.19) x 10(9) years. The primitive lunar crust must have been formed in the first 300 to 400 x 10(6) years. The results give some credence to the hypothesis that the primitive lunar surface was molten and large-scale fractional crystallization occurred in the early history of the moon.     

Mineralogic and petrologic implications of viking geochemical results from Mars: interim report

Chemical results from four samples of martian fines delivered to Viking landers 1 and 2 are remarkably similar in that they all have high iron; moderate magnesium, calcium, and sulfur; low aluminum; and apparently very low alkalies and trace elements. This composition is best interpreted as representing the weathering products of mafic igneous rocks. A mineralogic model, derived from computer mixing studies and laboratory analog preparations, suggests that Mars fines could be an intimate mixture of about 80 percent iron-rich clay, about 10 percent magnesium sulfate (kieserite?), about 5 percent carbonate (calcite), and about 5 percent iron oxides (hematite, magnetite, maghemite, goethite?). The mafic nature of the present fines (distributed globally) and their probable source rocks seems to preclude large-scale planetary differentiation of a terrestrial nature.     

Spinel troctolite and anorthosite in apollo 16 samples

A spinel troctolite and an anorthosite from the Apollo 16 landing site represent contrasting types of "primitive" lunar cumulates. The two rock types probably formed from the same parent magma type, a high-alumina magnesian basalt, with the troctolite forming earlier by crystal settling, and the anorthosite later, possibly by flotation.     

High-voltage transmission electron microscopy study of lunar surface material

The internal substructures of a type B sample have been examined at high magnification and compared with terrestrial rocks. Selected ultrathin sections were prepared from these multiphase materials by an ion-thinning technique and examined in a 1-Mev electron microscope, with complementary optical analyses. The structures in the ilmenite and plagioclase indicate that the lunar material has undergone plastic deformation by dislocation movement and possibly microtwinning, with subsequent recovery. The pyroxene exhibits complex lamellar structures of submicron spacing. These various observations are consistent with the optical microscopy evidence for distortion and recovery and identify the processes involved.     

Anorthosite belts, continental drift, and the anorthosite event

Most anorthosites lie in two principal belts when plotted on a predrift continental reconstruction. Anorthosite ages in the belts cluster around 1300 +/- 200 million years and range from 1100 to 1700 million years. This suggests that anorthosites are the product of a unique cataclysmic event or a thermal event that was normal only during the earth's early history.     

Bistatic-radar detection of lunar scattering centers with lunar orbiter I

Continuous-wave signals transmitted from Lunar Orbiter I have been received on Earth after they have been reflected from the surface of the moon. The frequency spectrum of the reflected signals is used to locate discrete, heterogeneous, scattering centers on the lunar surface. The scattering centers are probably distinguished from the surrounding terrain by a higher surface reflectivity. Continuous-wave bistatic radar could provide an important new method for the study and mapping of planetary surfaces.     

Lunar anorthosite 15415: texture, mineralogy, and metamorphic history

Lunar anorthosite 15415 consists almost entirely of anorthite (homogeneous anorthite 96.6 molecule percent), with accessory diopsidic augite and traces of hypersthene, ilmenite, and a silica mineral. The rock has had a complex metamorphic history. The texture reflects at least two episodes of shearing (followed by intense and partial recrystallization, respectively), one episode of cataclastic deformation, and one or more episodes of shattering and fragmentation.     

Thermal structure of the lithosphere: a petrologic model

A preliminary evaluation of the thermal history of the upper mantle as determined by petrologic techniques indicates a general correspondence with theoretically derived models. The petrologic data supply direct information which may be used as an independent calibration of calculated models, serve as a base for evaluating the assumptions of the theoretical approach, and allow more careful selection of the variables describing mantle thermal properties and processes. Like the theoretical counterpart, the petrological approach indicates that the lithosphere is dominated by two thermal regimes: first, there is a continental regime which cools at rates of the order of 10(9) years and represents the longterm cooling of the earth. Secondly, superimposed on the continental evolution is the thermal event associated with the formation of an oceanic basin, and which may be thought of as a 10(8) year convective perturbation on the continental cycle. Of special interest is petrologic evidence for a sudden steepening of the thermal gradients across the lithosphere-asthenosphere boundary not seen in the theoretical models. The unexpected change of slope points to the need for a critical reevaluation of the thermal processes and properties extant in the asthenosphere. The potential of the petrologic contribution has yet to be fully realized. For a start, this article points to an important body of independent evidence critical to our understanding of the earth's thermal history.     

Indium variations in a petrologic suite of L-group chondrites

Indium concentrations have been determined by neutron activation in four members of each of the L3, L4, L5, and L6 chondritic meteorite classes. The range of concentrations is found to be from 0.14 to 22 parts per billion, with the highest values in L3 chondrites and the lowest values in the L5 and L6 classes. Plots of indium concentration versus relative mean deviation of pyroxene iron content, total carbon concentration, and primordial argon-36 concentration show positive correlations to varying degrees. Indium concentration appears to be a valuable parameter relating to variable formation conditions of the chondritic meteorites during the early history of the solar system.     

Volcanic aerosols and lunar eclipses

The moon is visible during total lunar eclipses due to sunlight refracted into the earth's shadow by the atmosphere. Stratospheric aerosols can profoundly affect the brightness of the eclipsed moon. Observed brightnesses of 21 lunar eclipses during 1960-1982 are compared with theoretical calculations based on refraction by an aerosol-free atmosphere to yield globally averaged aerosol optical depths. Results indicate the global aerosol loading from the 1982 eruption of El Chichón is similar in magnitude to that from the 1963 Agung eruption.     

Mineralogy and petrography of lunar samples

The lunar samples consist largely of augite, calcic plagioclase, and ilmenite. Olivine is a minor constituent of some rocks, as is cristobalite. Other minerals present in small amounts include tridymite, chromite, kamacite, taenite, and troilite. The principal rock types can be broadly grouped into ilmenite basalts and breccias. Except for their high ilmenite content, the lunar rocks resemble the calcium-rich achondritic meteorites (eucrites and howardites) in composition and structure. Evidence of a meteoritic increment in the lunar soil is provided by the presence of nickel-iron particles in glass and breccia, and the occurrence of metal-troilite spheroids; the breccias contain occasional silicate aggregates that resemble meteoritic chondrules. The lunar fines contain 325 parts of watersoluble calcium per million.     

Thermoluminescence of lunar samples

Appreciable natural thermoluminescence with glow curve peaks at about 350 degrees centigrade for lunar fines and breccias and above 400 degrees centigrade for crystalline rocks has been recognized in lunar samples. Plagioclase has been identified as the principal carrier of thermoluminescence, and the diference in peak temperatures indicates compositional or structural differences between the feldspars of the different rock types. The present thermoluminescence in the lunar samples is probably the result of a dynamic equilibrium between acquisition from radiation and loss in the lunar thermal environment. A progressive change in the glow curves of core samples with depth below the surface suggests the use of thermoluminescence disequilibrium to detect surfaces buried by recent surface activity, and it also indicates that the lunar diurnal temperature variation penetrates to at least 10.5 centimeters.     

Mechanism of lunar polarization

A theoretical model to explain the negative polarization of moonlight at small lunar phase angles is developed. The model is based on the polarization of light in the diffraction region bordering the geometric shadow of an opaque dielectric obstacle.     

Viscosity of lunar lavas

The viscosity of a synthetic silicate liquid with the composition of a lunar rock has been determined experimentally and found to be lower than that of any previously studied volcanic rock on earth. Course Crystallinity can be a feature of rapidly cooled surface lavas.     

Rare gases in lunar samples: study of distribution and variafions by a microprobe technique

The rare gas distribution in lunar soil, breccias, and rocks was studied with a micro-helium-probe. Gases are concentrated in grain surfaces and originate from solar wind. Helium-4 concentrations of different mineral components vary by more than a factor of 10 apart from individual fluctuations for each type. Also grains with no detectable helium-4 exist. Titanium-rich components have the highest, calcium-rich minerals the lowest concentrations. The solar wind was redistributed by diffusion. Mean gas layer thicknesses are 10, 6, and 5 microm for helium, neon, and argon respectively. Lithic fragments in breccias contain no solar gases. Glass pitted surfaces of crystalline rocks contain about 10(-2) cubic centimeter of helium-4 per square centimeter. Etched dust grains clearly show spallogenic and radiogenic components. The apparent mean exposure age of dust is approximately 500 x 10(6) years, its potassium-argon age is approximately 3.5 x 10(9) yerars. Cavities of crystalline rocks contain helium-4, radiogenic argon, H(2), and N(2).     

Short-lived oxygen diffusion during hot, deep-seated meteoric alteration of anorthosite

Heterogeneous oxygen isotope compositions of plagioclase from the Boehls Butte anorthosite include some of the most oxygen-18-depleted values (to -16 per mil) reported for plagioclase in meta-igneous rocks and indicate high-temperature (T > 500 degrees C) isotopic exchange between plagioclase and nearly pristine meteoric fluid. Retrograde reaction-enhanced permeability assisted influx of meteoric-hydrothermal fluids into the deep-seated anorthosite. Isotopic gradients of about 14 per mil over 600 micrometers in single crystals require short-lived (about 10(4) years) diffusional exchange of oxygen and locally large effective water:rock ratios, followed by rapid loss of water and cessation of oxygen diffusion in the anorthosite.     

Lunar and martian surfaces: petrologic significance of absorption bands in the near-infrared

A reflection minimum at 1 micron, reported for Moon and for Mars, indicates olivine or iron- and calcium-bearing clinopyroxene, or both-major constituents of many basaltic rocks. If the 1-micron absorption features are real, both chondritic and acidic rocks are ruled out as primary constituents of the surfaces of the bodies. The reflectance spectrum of Mars matches closely that of an oxidized basalt.