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Macpherson, G. J.

Publications and source records attributed to Macpherson, G. J..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Isotopically distinct reservoirs in the solar nebula: Isotope anomalies in Vigarano meteorite inclusions

The isotopic compositions of Mg, Ca, Ti, Cr, Zn, Sr, Ba, Nd, and Sm were measured in four relatively unaltered refractory inclusions from the Vigarano carbonaceous chondrite meteorite. Three of the inclusions (USNM 1623-2, 1623-3, and 1623-8) show similar Mg, Ca, Ti, and Cr isotopic compositions to those found in most inclusions in the Allende carbonaceous chondrite. This indicates that these Vigarano inclusions sampled the same isotopic reservoirs as the majority of the Allende inclusions that isotope signatures in the latter were not significantly modified by the secondary alteration that permeates most Allende inclusions. In contrast, inclusion 1623-5 has large deficits in Mg-26, Ca-48, and Ti-50 and small but distinct Cr-54, Zn-66, Sr-84, Ba-135, Ba-137, and Sm-144 anomalies. The magnitudes of these unusual anomalies in the refractory elements are within analytical uncertainty of those found in the Allende 'FUN" inclusion C1, yet 1623-5 has a very different bulk chemical composition from C1. The fact that 1623-5 and C1 have identical isotopic anomalies yet have significantly distinct major and trace element contents provide convincing evidence for the presence of isotopically distinct reservoirs in the early solar system.

Loss, R. D.↗
NASA NTRS1994

Chronology of chrondrule and CAI formation: Mg-Al isotopic evidence

Details of the chondrule and Ca-Al-rich inclusion (CAI) formation during the earliest history of the solar system are imperfectly known. Because CAI's are more 'refractory' than ferromagnesian chondrules and have the lowest recorded initial Sr-87/Sr-86 ratios of any solar system materials, the expectation is that CAI's formed earlier than chondrules. But it is not known, for example, if CAI formation had stopped by the time chondrule formation began. Conventional (absolute) age-dating techniques cannot adequately resolve small age differences (less than 10(exp 6) years) between objects of such antiquity. One approach has been to look at systematic differences in the daughter products of short-lived radionuclides such as Al-26 and I-129. Unfortunately, neither system appears to be 'well-behaved.' One possible reason for this circumstance is that later secondary events have partially reset the isotopic systems, but a viable alternative continues to be large-scale (nebular) heterogeneity in initial isotopic abundances, which would of course render the systems nearly useless as chronometers. In the past two years the nature of this problem has been redefined somewhat. Examination of the Al-Mg isotopic database for all CAI's suggests that the vast majority of inclusions originally had the same initial Al-26/Al-27 abundance ratio, and that the ill-behaved isotopic systematics now observed are the results of later partial reequilibration due to thermal processing. Isotopic heterogeneities did exist in the nebula, as demonstrated by the existence of so-called FUN inclusions in CV3 chondrites and isotopically anomalous hibonite grains in CM2 chondrites, which had little or no live Al-26 at the time of their formation. But, among the population of CV3 inclusions at least, FUN inclusions appear to have been a relatively minor nebular component.

Macpherson, G. J.↗
NASA NTRS1988

New type of primitive meteorite found in Antarcitca

A stony meteorite found in Antarctica in 1985, recently identified as a type of chondrite, probably represents material from a previously unsampled region of the solar system. Scientists are currently debating how the object formed. Preliminary results on Allan Hills 85085 (ALH85085) were presented.

Grossman, J. N.↗
NASA NTRS1985

Accretionary rims on inclusions in the Allende meteorite

The origin and composition of the rim sequence on the refractory inclusion in the Allende meteorite are studied. The different textures, mineralogy, and mineral-chemistry of the four layers of the rim are described. The layers are composed of: pyroxene, needles, olivine, hedenbergite, and andradite. Tables of the element and chemical compositions of the layers are presented. The data reveals that: (1) the layers are highly porous masses of euhedral crystals with no intergrowth; (2) layers contain highly disequilibrium mineral assemblages; and (3) the thickness of the layers varies with the underlying topography. These results support the theory that rim structures are accretionary aggregates formed from accretion of independently grown particles onto the surface of inclusions. The formation of the grains in the layers and matrix from nebular condensates is studied.

Macpherson, G. J.↗
NASA NTRS1984

Petrographic studies of refractory inclusions from the Murchison meteorite

Textural and mineral-chemical data on freeze-thaw disaggregated refractory inclusions from the Murchison meteorite are reported. The data were obtained with neutron activation analysis, SEM, and spectroscopy, the study revealed corundum-bearing inclusions, spinel-hibonite aggregates and spherules, and spinel-pyroxene and elivine-pyroxene inclusions. One of the three spinel-, pyroxene-, forsterite-rich inclusions had an amoeba-shaped spinel-pyroxene core, implying vapor-to-solid condensation and therefore crystallization from a melt. It is concluded that the meteorite formation encompassed diverse nebular materials, and that further studies of the meteorite will enhance the data base on the planetary nebular processes.

Macpherson, G. J.↗
NASA NTRS1984

Two forsterite-bearing FUN inclusions in the Allende meteorite

Two forsterite-, fassaite-, spinel-rich inclusions in Allende which share common mineralogy and texture with three previously described inclusions are described. These inclusions were at least partially molten at temperatures over 1400 C, and their crystallization sequence was spinel, olivine, fassaite, and Mg-rich melilite. At least some of them experienced partial volatilization of MgO and SiO2 from their outer margins. At least one of the inclusions is highly enriched in MgO relative to CaO and Al2O3 compared to Allende coarse-grained inclusions, although it is just as strongly enriched in refractory trace elements as the latter, relative to C1 chondrites. Two of the objects are FUN inclusions on the basis of their oxygen, magnesium, and silicon isotopic compositions.

Clayton, R. N.↗
NASA NTRS1984

'Fluffy' Type A Ca-, Al-rich inclusions in the Allende meteorite

Inclusions called 'fluffy' Type A's or FTA's in the Allende meteorite are examined as possible candidates for relict vapor-solid condensate grains, remaining from the original solar nebula. Type A inclusions are characterized by abundant melilite and absence of primary anorthite and titaniferous pyroxene. Fluffy Type A's were probably loosely bound clumps of crystals drifting in the solar nebula, analogous to dustballs or snowflakes. Polished thin sections of all samples were studied optically and with a JEOL JSM-35 scanning electron microscope. It is reasonably clear that neither whole FTA's nor constituent nodules of the coarser grained ones were ever molten. Despite solid-state recrystallization which has affected these inclusions to varying degrees, the coarser grained material remaining in many of them is probably a relic of vapor-solid condensation in the solar nebula.

Macpherson, G. J.↗
NASA NTRS1984

The origin and significance of reverse zoning in melilite from Allende Type B inclusions

Many of Type B Allende inclusions are found to have reverse zoning over restricted portions of each crystal, in contrast to those in Type A's, which are reversely zoned throughout. Results of dynamic crystallization experiments on compositions similar to an average Type B inclusion are reported in which reversely zoned melilite rims were grown on normally zoned cores. The fact that these zoning patterns have been duplicated is interpreted as evidence that natural melilites having such zoning were formed by crystallization from a melt. Since reverse zoning could only be produced over a restricted range of experimental conditions, the presence in natural inclusions of the kinds of zoning patterns described here may place constraints on the conditions under which the inclusions formed.

Macpherson, G. J.↗
NASA NTRS1983

Refractory inclusions in the Murchison meteorite

Although melilite-rich and feldspathoid-bearing inclusions have been revealed in the Murchison C2 chondrite by mineralogical and petrographic studies, none of these is identical to the inclusions found to date in Allende. Each type of inclusion is characterized by a different suite of alteration products and/or rim layers from all the other types, indicating modification of the inclusions in a wide range of different physicochemical environments after their primary crystallization. All of these inclusions contain some iron-free rim phases which could not have formed by reaction of the inclusions with fluids in the Murchison parent body, because the latter would presumably have been rich in oxidized iron.

Macpherson, G. J.↗
NASA NTRS1982

A corundum-rich inclusion in the Murchison carbonaceous chondrite

Although thermodynamic calculations predict corundum to be the first condensate from a cooling gas of solar composition, a corundum-hibonite inclusion, BB-5, has for the first time been found in the Murchison carbonaceous chondrite. Ion microprobe measurements of Mg isotopic compositions yield the result, unexpected in such an early condensate, that Mg-26 excesses are small despite large Al-27/Mg-24 ratios. The extreme temperature required to melt this inclusion makes a liquid origin unlikely, except by the hypervelocity impact of refractory bodies. Alternatively, B-5 is a direct gas-solid condensate, and its uniform Mg-26 enrichment must be a characteristic of the reservoir from which it condensed. Nebular heterogeneity in magnesium isotopic composition is the preferred explanation for the formation of such a reservoir.

Bar-Matthews, M.↗
NASA NTRS1982

Origin of rims on coarse-grained inclusions in the Allende meteorite

Coarse-grained, calcium-rich inclusions in the Allende meteorite are enclosed by sequences of very thin, mineralogically distinct rim layers. A number of questions arise in connection with the presence of rims. The present investigation presents new observations bearing on all these questions and provides a model for the formation of rims. A zoned alteration vein is considered along with a sinuous inclusion, a spinel-perovskite-melilite band (SP), layers overlying the melilite-rich side of SP, a layer overlying the spinel-rich side of SP, and aspects of rim composition and the degree of alteration in coarse-grained inclusions. It is proposed that rims on coarse-grained inclusions formed in a nonequilibrium process involving those reactions between coarse-grained melilite and spinel and a nebular gas phase which also produced the secondary alteration products in the interiors of these inclusions.

Macpherson, G. J.↗
NASA NTRS1981

The Antarctic achondrite ALHA 76005 - A polymict eucrite

ALHA 76005 is a basaltic achondrite containing few, if any, orthopyroxenes. Its bulk major and trace element composition is like that of a non-cumulate eucrite, and unlike that of a howardite. It contains a variety of igneous clasts which differ in their textures, pyroxene/plagioclase ratios and pyroxene and plagioclase compositions. One clast, No. 4, was found to have the REE pattern of a cumulate eucrite and an oxygen isotopic composition different from that of the bulk meteorite. Both the chemical and oxygen isotopic composition of clast No. 4 suggest that it was derived from a source different from its host. These observations lead to the conclusion that ALHA 76005 is a polymict eucrite.

Grossman, L.↗
NASA NTRS1981

A once-molten, coarse-grained, Ca-rich inclusion in Allende

A compact, spheroidal Type B inclusion in Allende contains melilite laths that project radially inward from the inclusion edge which show interference growth textures. The combined textural and chemical features of this object cannot be explained by independent vapor-solid condensation of grains in space, followed by random aggregation of these grains into an inclusion. Instead, it probably formed from a once-molten droplet that crystallized in response to radiative cooling from its outer surface. The crystallization sequence in this and another similar inclusion in which oxygen isotopes have been measured is melilite-spinel-anorthite-fassaite; this sequence shows that oxygen isotopic heterogeneities in coarse-grained inclusions were formed after complete solidification of these objects by partial exchange with a less O-16-rich gas, and not during or before a melting event.

Macpherson, G. J.↗
NASA NTRS1980

Electron microprobe study of a 'mysterite'-bearing inclusion from the Krymka LL-chrondrite

The black inclusion from the Krymka LL3 chondrite previously found to contain 'mysterite' by Lewis et al. (1979) belongs to a hitherto unknown class of carbonaceous chondrites. Its olivine and pyroxene compositions, Fo 97-99 and En 96, respectively, are characteristic of carbonaceous chondrites and its plagioclase composition. An 100 is characteristic of C3's. It contains a peculiar group of Co-, Cr-rich metal grains whose compositions are similar, but not identical, to those in C2 chondrites. Its weight ratios of total Fe/SiO2 and SiO2/MgO are 0.74 and 1.43, respectively, and its atomic ratio of Si/Al is 10.7, exactly the same as in carbonaceous chondrites. Its bulk chemical composition is very close to that of the Murchison C2 chondrite. The association of mysterite with a special type of carbonaceous chondrite material suggests that mysterite formed by low-temperature condensation in a different region of the nebula from other carbonaceous chondrites.

Grossman, L.↗