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Lofgren, G. E.

Publications and source records attributed to Lofgren, G. E..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

The Mn-Fe negative correlation in olivines in ALHA 77257 ureilite

An electron probe microanalyzer is used to measure the Mn, Fe, and oxygen zoning profiles of olivines in the ALHA 77257 ureilite. This is done to study the effects of reduction on the Mn-Fe value, as ureilite olivines exhibit thin reduced rims. Since the Mn content gradually increases toward the rim of ureilite olivines, while the Fa (= 100 x Fe/(Mg + Fe), mol percent) component decreases, the Mn-Fe content of olivine is likely related to redox conditions. The results of melting experiments suggest that the Mn-Fe positive correlation is related to temperature and that the negative correlation of Mn-Fe in olivine and low-Ca pyroxene is related to reduction.

Miyamoto, M.↗
NASA NTRS1993

An experimental study of trace element partitioning between olivine, orthopyroxene and melt in chondrules - Equilibrium values and kinetic effects

Mineral/melt partition coefficients were measured using an ion microprobe for 32 elements in orthopyroxene and olivine in equilibrium and dynamic crystallization experiments on compositions corresponding to chondrules. The mineral/melt partition coefficients calculated from the measured concentrations for both olivine and orthopyroxene show very little change between equilibrium experiments and dynamic experiments with cooling rates of up to 100 C/h. The results provide a self-consistent set of partition coefficients that can be used in thermodynamic models of equilibrium and kinetic partitioning between olivine, orthopyroxene, and melt. These data can be used in models of partial melting and crystal fractionation in olivine- and orthopyroxene-rich systems, such as chondrules. The results may also be applicable to mantle peridotites, komatiitic and picritic lavas, and ultramafic intrusions.

Kennedy, A. K.↗
NASA NTRS1993

An experimental study of trace element partitioning between perovskite, hibonite and melt: Equilibrium values

The presence of perovskite (CATiO3) and hibonite (Ca Al12O19) within different regions of Calcium-, Aluminum-rich Inclusions (CAI) and the trace element concentrations of these minerals in each circumstance, constrain models of precursor formation, nebular condensation, the thermal history of inclusions with relict perovskite and hibonite, and the formation of the Wark-Lovering rim. At present mineral/melt partition coefficient data for hibonite are limited to a few elements in simple experimental systems, or to those derived from hibonite-glass pairs in hibonite/glass microspherules. Similarly, there is only limited data on perovskite D that are applicable to meteorite compositions. Apart from the importance of partitioning studies to meteorite research, D values also are invaluable in the development of thermodynamic models, especially when data is available for a large number of elements that have different ionic charge and radii. In addition, study of the effect of rapid cooling on partitioning is crucial to our understanding of meteorite inclusions. To expand our knowledge of mineral/melt D for perovskite and hibonite, a study was instituted where D values are obtained in both equilibrium and dynamic cooling experiments. As an initial phase of this study mineral/melt D was measured for major elements (Ca, Mg, Al, Ti, and Si), 15 rare earth elements (La-Lu) and 8 other elements (Ba, Sr, U, Th, Nb, Zr, Hf, and Ge) in perovskite and hibonite grown under equilibrium conditions, in bulk compositions that are respectively similar to Compact Type A (CTA) CAI and to a hibonite/glass microspherule. Experimental mixes were doped with REE at 20-50x chondritic (ch) abundances, Ba at 50 ppm, Sr, Hf, Nb, and Zr at 100 ppm and, U and Th at 200 ppm. Trace element abundances were measured with the PANURGE ion microprobe. Major element compositions were obtained by electron microprobe analysis.

Kennedy, A. K.↗
NASA NTRS1992

An experiment-based model for the petrogenesis of high-alumina basalts

To understand magmatism at convergent margins, one must know the origin of their characteristic plagioclase-rich high-alumina basalts (HABs). Wet melting experiments on basalts at 3 kilobars yielded high-alumina liquids and a coexisting mineral assemblage with plagioclase crystallization in these melts, while mafic minerals underwent limited crystallization or even resorption. These results suggest that hydrous HAB liquids, presumably formed by fractionation of a hydrous basalt at depth, precipitate voluminous plagioclase as pressure drops during ascent and eruption. Plagioclase accumulation is not necessarily required to explain the petrogenesis of plagioclase-rich HAB.

Beard, J. S.↗
NASA NTRS1992

Dynamic crystallization experiments on the Angra dos Reis achondritic meteorite

The types of cooling histories necessary to produce the porphyritic texture and the observed mineral assemblage in Angra dos Reis (ADOR) achondritic meteorite, which, on the basis of its trace element characteristics was considered to be a cumulate with a recrystallized texture, while a recent model suggested that ADOR is a porphyry. Dynamic crystallization experiments were conducted with nucleation conditions varied by melting the starting material at different degrees of superheat. The results show that, at low pressure, a volcanic or hypabyssal history of ADOR is possible. The most likely history involved cooling from a slightly subliquidus temperature with a relatively high and irregular nucleation density resulting in a granular texture with the poikilitic fassaite growing late; accumulation of some crystal is tought to be probable.

Lofgren, G. E.↗
NASA NTRS1986

The thermoluminescence sensitivity-metamorphism relationship in ordinary chondrites - Experimental data on the mechanism and implications for terrestrial systems

Hydrothermal annealing experiments have been performed on samples of the Sharps meteorite in order to investigate the mechanism responsible for the metamorphism-related, 10-to-the-5th-fold range in the thermoluminescence (TL) sensitivity in ordinary chondrites. Duplicate 50 mg samples of meteorite were annealed under the following conditions: (1) 168 h at 785 C and 1 kbar; (2) the same time, temperature and pressure, but with 2 wt pct water; (3) 174 h at 855 C and 0.77 kbar with 2 wt pct water and 2 molal sodium disilicate (NadiSi); (4) the same time, temperature and pressure as the preceding samples, but with 10 wt pct H2O and 2 molal NadiSi. Samples annealed under the first three sets of conditions showed little or no change in their TL sensitivities, however the samples annealed with 10 wt pct water and 2 molal NadiSi showed a three-fold to 10-fold increase in TL sensitivity, and the temperature of the TL peak was suggestive of feldspar in the high-temperature form. It is suggested that these data are consistent with the TL sensitivity-metamorphism relationship in ordinary chondrites being due to the formation of the TL phosphor, feldspar, by the crystallization of chondrule glass.

Guimon, R. K.↗
NASA NTRS1983

An analytical and experimental study of zoning in plagioclase

A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals.

Smith, R. K.↗
NASA NTRS1982

Electron microprobe study of lunar and planetary zoned plagioclase feldspars: An analytical and experimental study of zoning in plagioclase

Natural and experimentally grown zoned plagioclase feldspars were examined by electron microprobe. The analyses revealed discontinuous, sector, and oscillary chemical zoning superimposed on continuous normal or reverse zoning trends. Postulated mechanisms for the origin of zoning are based on either physical changes external to the magma (P, T, H2O saturation) or kinetic changes internal to the magma (diffusion, supersaturation, growth rate). Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments show that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from local kinetic control on the growth of individual crystals.

Smith, R. K.↗
NASA NTRS1979

One atmosphere melting experiments on ilmenite basalt 12008

An evaluation of a crystal-fractionation model for Apollo 12 ilmenite basalts with melting experiments under controlled oxygen fugacities is reported. The crystallization sequence including olivine, chromium spinel, and pigeonite phases was determined, showing that the changes in melt composition are dominated by olivine crystallization and the decrease in MgO with a corresponding increase in CaO, Al2O3, and TiO2. It is concluded that the bulk composition of the ilmenite basalts was established by crystallization of olivine and minor spinel prior to the onset of pyroxene and plagioclase.

Rhodes, J. M.↗
NASA NTRS1979

Comparison of dynamic crystallization techniques on Apollo 15 quartz normative basalts

The paper compares dynamic crystallization techniques used on Apollo 15 quartz normative basalts which show differences related to their method. The variation of pyroxene core major element chemistry with cooling rate is confirmed, and it is shown that the Fe loss from liquid is shifted toward the En-Di joint; it is suggested that the nucleation temperature and breakdown of equilibrium partitioning as a function of cooling rate dictate the core composition. It is concluded that the different crystallization behavior that results from the Fe-capsule and Fe-loop container methods is related to the nucleation kinetics.

Lofgren, G. E.↗
NASA NTRS1978

Nucleation and growth of plagioclase and the development of textures in a high-alumina basaltic melt

Detailed experiments on the nucleation and growth of plagioclase from an igneous melt are carried out to characterize the magma prior to cooling. It is shown that the nature of the melt directly preceding cooling has a strong effect on texture development, regardless of the origin of the melt. The number, size, and shape of the crystals present in a melt upon extrusion can have a drastic effect on the final textures present in the resulting rocks. Conditions necessary for the formation of poikilitic, intersertal, and porphyritic textures in basaltic rocks are defined.

Nabelek, P. I.↗
NASA NTRS1978

Dynamic crystallization and kinetic melting of the lunar soil

Dynamic crystallization and kinetic melting tests have been conducted for a lunar soil that resembles the bulk composition of the matrices of melt-rock breccias. The melting tests indicate that plagioclase and olivine can exist for finite times above their equilibrium liquidus temperatures; within 20-30 C below its liquidus temperature the sample is choked with crystals. A range of textures from radiate to porphyritic pyroxene-plagioclase granophyric to subophitic to intergranular to intersertal to very fine-grained poikilitic is found in the dynamic crystallization experiments. The number of nuclei, as deduced from the textures, increases as the melting temperature deceases from above the liquidus to below.

Lofgren, G. E.↗
NASA NTRS1978

Dynamic crystallization of a eucrite basalt

The textural features produced in Stannern, a non-porpyritic representative of the eucrite basaltic achondrite class of meteorite, at differing cooling rates and various degrees of initial superheating were studied. Textures produced from mildly superheated melts were found to be fasciculate rather than porphyritic as the result of the cosaturated bulk chemistry of Stannern. The qualitative type of texture apparently depends mainly on the degree of initial superheating, whereas cooling rate exerts a strong influence on the coarseness of texture. Increasing the degree of superheating produces textures from intergranular/subophitic to fasciculate/porphyritic. With initial superheating to 1200 deg C the transition to quasi-porphyritic is controlled by cooling rate, but the development of phenocrysts is merely an overprint on the fasciculate background texture of the groundmass. The suppression of fasciculate texture is completed by a decrease of the degree of initial superheating below the plagioclast entry and suppression of quasi-porphyritic texture is completed by decrease of the degree of initial superheating below pyroxene entry; these qualitative changes do not seem to be produced by changes of cooling rate. A grain size/cooling rate dependence has been used to deduce the cooling rate of fasciculate-textured Stannern clasts (10.1 to 100 deg C/hr).

Walker, D.↗
NASA NTRS1977

Dynamic crystallization experiments bearing on the origin of textures in impact-generated liquids

Dynamic crystallization experiments on a synthetic glass of 14310 composition show that the density and distribution of crystals present at the initiation of cooling can drastically affect the texture of the crystallized product. Experiments were performed in one-atmosphere gas-mixing furnaces at an oxygen fugacity slightly below iron-wustite, and precooling crystallization, cooling history, and melt history were systematically varied. When cooled from a complete liquid, the experimental charges were porphyritic with 1-2 mm phenocrysts set in a very fine-grained matrix (less than 0.1 mm). This result suggests that 14310 did not cool from a complete melt. When cooled from a crystal-liquid mixture, the experimental charges had widely varying textures depending on the density and distribution of crystals that act as nucleation sites. It is suggested that the close spatial association of widely divergent textures in the matrices of some breccias and rocks might arise in this manner; local variations in cooling rates are not required.

Lofgren, G. E.↗
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.↗