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NASA NTRS1972

Shock metamorphic effects in the Luna 16 soil sample from Mare Fecunditatis.

Diverse lunar soil fragments returned from the Luna 16 mission were examined for evidence in shock metamorphism in order to evaluate the role of meteorite impact in forming the lunar regolith at a new site relatively distant from the Apollo landing sites. It was found that shock metamorphic effects, characteristic of meteorite impact and virtually identical to those observed in Apollo samples, are common in the Luna 16 fragments. Two types of shock effects are present - i.e., a deformation and partial melting features in rock and mineral fragments, and heterogeneous glasses and glassy breccias produced by shock melting. It is concluded that these shock metamorphic effects indicate that regolith formation by meteorite impact has occurred in Mare Fecunditatis and is a general process over the entire moon.

French, B. M.↗

NASA NTRS1977

Carbonaceous chondrites of the Ornans type - A metamorphic sequence

The six identified carbonaceous chondrites classified as C3(O) (also known as the Ornans subtype or ornansites) are studied as a group. The Karoonda meteorite is also examined to determine whether it may be classified as a highly metamorphosed equivalent of the Ornans subtype. Petrographic and chemical similarities among these meteorites are discussed along with such systematic variations as textural, mineralogical, metallurgical, and chemical differences. Three metamorphic grades are delineated, chemical variations are correlated with the grades, and an attempt is made to determine why the Ornans meteorite belongs in a higher metamorphic grade on the basis of its chemical properties than on the basis of its petrographic characteristics. It is concluded that subtle differences among the meteorites may be related to small variations in the thermal metamorphism they have experienced.

Mcsween, H. Y., Jr.↗

NASA NTRS1985

Ubiquitous brecciation after metamorphism in equilibrated ordinary chondrites

Ten objects with aberrant Fe/(Fe + Mg) ratios have been found in apparently unbrecciated types 4-6 H and L chondrites. Since the Fe/(Fe + Mg) ratios of these objects are incompatible with the metamorphic history of the host chondrites, it is concluded that a high proportion of ordinary chondrites are breccias that were lithified after peak metamorphism. This is consistent with the results of Scott (1984), who concluded that most type three ordinary chondrites are breccias of materials with diverse thermal histories, even though they do not show prominent brecciation. It is found that the classification scheme of Van Schmus and Wood (1967) does not identify chondrites with similar thermal histories; the petrologic type of a chondrite is only a measure of the average thermal history of its ingredients. Chondrite and achondrite breccias are also compared in order to understand how brecciation of chondrites after metamorphism is so well camouflaged.

Scott, E. R. D.↗

NASA NTRS1987

Chemical and physical studies of type 3 chondrites. VIII - Thermoluminescence and metamorphism in the CO chondrites

A possible relationship between the thermoluminescence (TL) properties of CO chondrites and their metamorphic history was investigated by measuring the TL properties of seven normal CO chondrites and of the Colony and the Allan Hills A77307 (ALHA 77307) CO-related chondrites. With the exception of Colony and ALHA 77307, whose maximum induced TL emission is at approximately 350 C, the CO chondrites were found to exhibit two TL peaks: a 130 C and a 250 C peaks. Among the CO chondrites, the 130 C peak showed a 100-fold range in TL sensitivity and was found to correlate with various metamorphism-related phenomena, such as silicate heterogeneity, metal composition, and McSween's metamorphic subtypes. The peak at 250 did not show these correlations and, with exception of Colony, showed little variation.

Keck, Bradly D.↗

NASA NTRS1988

Planet Alsioff - A problem set for students of phase equilibria or metamorphic petrology

This paper presents a problem set that contains questions for students of phase equilibria or metamorphic petrology concerning a hypothetical planet Alsioff, for which incomplete data are given. On this panet, the SiF4 is the major volatile and Al, Si, O, and F are the only elements present. Progressive metamorphism on Alsioff mainly involves devolatilization of fluid SiF4. The problem set includes ten questions. Some of these are concerned with possible chemical reactions that should affect water, wollastonite, or Ca-SiO3 exposed to the atmosphere of Alsioff; the mechanism of controls of the O2 and F2 contents of the Alsioffian atmosphere; and the devolatilization reactions involving SiF4 with progressive thermal metamorphism.

Burt, Donald M.↗

NASA NTRS1988

Thermal metamorphism

Most chondrites have experienced thermal metamorphism, resulting in changes in texture, mineralogy and possibly chemical composition. The physical conditions for metamorphism range from approximately 400 to 1000 C at low lithostatic pressure. Metamorphism may have resulted from decay of short-lived radionuclides, electromagnetic induction or accretion of hot materials. Several thermal models for chondrite parent bodies have been proposed. The least metamorphosed type-3 chondrites probably carry the most information about the early solar system, but even these have been affected to some degree by thermal processing.

Mcsween, Harry Y., Jr.↗

NASA NTRS1993

Oxidation during metamorphism of the ordinary chondrites

It is suggested that some current concepts about the conditions of metamorphism in ordinary chondrites may be flawed. These meteorites display small systematic variations in the oxidation state of Fe. Evidence is presented that oxidation of Fe is linked to metamorphic grade in types 4-6 ordinary chondrites. This conclusion is at variance with a commonly accepted model for chondrite metamorphism that assumes Fe reduction by graphite.

Mcsween, Harry Y., Jr.↗

NASA NTRS1998

Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

This handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures emphasizes terrestrial impact structures, field geology, and particularly the recognition and petrographic study of shock-metamorphic effects in terrestrial rocks. Individual chapters include: 1) Landscapes with Craters: Meteorite Impacts, Earth, and the Solar System; 2) Target Earth: Present, Past and Future; 3) Formation of Impact Craters; 4) Shock-Metamorphic Effects in Rocks and Minerals; 5) Shock-Metamorphosed Rocks (Impactities) in Impact Structures; 6) Impact Melts; 7) How to Find Impact Structures; and 8) What Next? Current Problems and Future Investigations.

French, Bevan M.↗

NASA NTRS1972

Metamorphism of Apollo 14 breccias.

All Apollo 14 breccias for which thin sections are available (27 rocks) were studied petrographically. From one sample in each of six groups, electron microprobe analyses were performed on matrix (less than 25 microns) plagioclase, pyroxene, and olivine and of rims and cores of plagioclase and pyroxene clasts (50 to 300 microns). Matrix plagioclase, pyroxene, and olivine in the high-grade rocks display a narrow range of compositions, suggesting an equilibrated or metamorphic origin. However, in the low-grade rocks a wide range of compositions, suggesting a detrital or unequilibrated origin, is observed. Partially equilibrated states are observed in medium-grade samples. It is concluded that the Apollo 14 breccias formed as part of the Imbrium Basin ejecta blanket - the Fra Mauro formation. The continuous nature of the metamorphic series suggests that the medium- and high-grade breccias were derived from unmetamorphosed equivalents by autometamorphism in the thick, hot Fra Mauro formation at the Apollo 14 site.

Warner, J. L.↗

NASA NTRS1973

The age and petrography of two Luna 20 fragments and inferences for widespread lunar metamorphism.

Ages were determined by the Ar-40/Ar-39 method on two metaclastic rocks returned from the lunar highlands north of Mare Fecunditatis by the Luna 20 probe. Both samples gave very well-defined argon retention ages of 3.90 plus or minus 0.04 AE which are indistinguishable from each other within a resolution of 0.02 AE. Both fragments, 22006 and 22007, are highly recrystallized polymict breccias; there is no evidence of radiogenic Ar-40, and the age almost surely dates the time of recrystallization. The cosmic ray exposure ages of these fragments are similar and high: 900 million years for 22006, 1300 million years for 22007. 22007 also contains substantial trapped argon with a high Ar-40/Ar-36. The Luna 20 results greatly extend the area of the moon's surface exhibiting a well-defined record of metamorphism at 3.9 AE. So far, lunar history in the interval from 4.6 to 3.9 AE is not preserved in the ages of surface rocks. This obliteration suggests lunar-wide metamorphic conditions occurring or terminating at this time as a result of major impacts.

Podosek, F. A.↗

NASA NTRS1977

Effects of thermal metamorphism on compositions of lunar spinels

The reported study represents an attempt to evaluate experimentally the compositional and textural changes that are likely to be observed in the Fe-Ti-Cr spinels of lunar igneous rocks by subsequent thermal metamorphism. The Apollo 12 igneous rock, 12018,43, was chosen for this investigation because an earlier study of another fraction of this rock by El Goresy et al. (1971) has reported an almost continuous trend of spinel compositions between Cr-Ulvoespinel and Ti-chromite. The nature of the compositional changes in the heated spinels (and ilmenites) is found to be such that the changes cannot be explained by intragranular adjustments alone. In the heated sample, pyroxene grains adjacent to the high-Ti spinels show a decrease in FeO, and an increase in MgO and Al2O3 at the interface. This may account for the MgO depletion and a part of the FeO enrichment in the high-Ti spinels. It is believed that the heating experiment demonstrates that thermal metamorphism of lunar basalts is likely to modify the compositions of their preexisting spinels (and ilmenites).

Misra, K. C.↗

NASA NTRS1978

Metamorphism, argon depletion, heat flow and stress on the Alpine fault

The Alpine fault of New Zealand is a major continental transform fault which was uplifted on its southeast side 4 to 11 km within the last 5 m.y. This uplift has exposed the Haast schists, which were metamorphosed from the adjacent Torlesse graywackes. The Haast schists increase in metamorphic grade from prehnite-pumpellyite facies 9-12 km from the fault through the chlorite and biotite zones of the greenschist facies to the garnet-oligoclase zone amphibolite facies within 4 km of the fault. These metamorphic zone boundaries are subparallel to the fault for 350 km along the strike. The K-Ar and Rb-Sr ages of the schists increase with distance from the fault: from 4 m.y. within 3 km of the fault to approximately 110 m.y. 20 km from the fault. Field relations show that the source of heat that produced the argon depletion aureole was the fault itself.

Scholz, C. H.↗

NASA NTRS1978

Thermal metamorphism of primitive meteorites. VII - Mineralogy-petrology of heated Murchison /C2/ and alteration of C30 and other chondrites

Alterations caused by week-long heating of Murchison in a low-pressure environment at 400-1400 C are of two types: thermodynamically favored kinetically controlled or thermodynamically controlled rapid processes. Kinetically controlled changes pertinent to chondritic evolution and which vary progressively with temperature in heated Murchison include: chondrule blurring; matrix coarsening; increasing mean Fa and Fs contents of ferromagnesian silicates; equilibration of olivine; increasing Mg/Si, Ca/Si, Al/Si, and Cr/Si and decreasing Fe/Si, Ni/Si, and S/Si in matrix; Cr loss from kamacite; and homogenization and Ni-zoning in taenite at high temperatures. Low-temperature thermodynamically controlled changes include: transformation of high-Ni troilite to low-Ni and formation of Ni- and Co-rich metal from pentlandite. High-temperature changes include formation of Cr-rich magnetite and formation of a Ni-rich sulfide similar to that found in highly altered chondrites. Trends resulting from processes of both kinds in Murchison are consistent with characteristics of a postulated C30 metamorphic suite, while those changes caused by reactions of the second kind are similar to those in heavily shock-heated ordinary chondrites and the heavily metamorphosed C5-6 chondrite, Mulga West. Either the simulations support the metamorphic origin of the C30 suite and other thermally induced changes or the natural alterations support the utility of laboratory simulations in studying meteoritic evolution.

Matza, S. D.↗

NASA NTRS1978

Volatile elements in chondrites - Metamorphism or nebular fractionation

Three of the most highly metamorphosed meteorites of their respective classes, Shaw (LL7), Karoonda (C5), and Coolidge (C4), were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Comparison with data by Lipschutz and coworkers (1977) on artificially heated primitive meteorites shows that the natural metamorphism of meteorites cannot have taken place in a system open to volatiles. Shaw, metamorphosed at 1300 C for more than 1 million yr, is less depleted in In, Bi, Ag, Te, Zn, and Tl than Krymka heated at 1000 C for 1 week. Karoonda, metamorphosed at 600 C for many millennia, is less depleted in Bi and Tl than Allende heated at 600 C for 1 week. Data on primordial noble gases also show that the volatile-element patterns of ordinary and carbonaceous chondrites were established by nebular condensation and changed little, if at all, during metamorphism. For enstatite chondrites, the evidence is still incomplete but seems to favor a nebular origin of the volatile pattern.

Takahashi, H.↗

NASA NTRS1978

Metamorphic effects in experimentally heated Krymka /L3/ chondrite

Experimental charges of the Krymka unequilibrated ordinary chondrite heated from 500-1000 C have been examined petrographically for evidence of metamorphism. Of the petrologic criteria commonly used to distinguish types 4-6 chondrites, only changes in opaque mineral compositions are observed. Chemical and textural observations indicate development of a fine-grained intergrowth of taenite + troilite beginning at 700 C due to melting within the metal-rich portion of the Fe-Ni-S system, and minor reduction of troilite to metal, possibly through sulfur loss at higher temperatures. Overall textural integration, glass devitrification, and significant Fe-enrichment of ferromagnesian minerals are not observed because the short duration of these experiments was not sufficient for the development of other changes normally attributed to metamorphism in ordinary chondritic meteorites.

Mcsween, H. Y., Jr.↗

NASA NTRS1980

Composition of the metal phases in ordinary chondrites - Implications regarding classification and metamorphism

The paper examines the composition of metal phases and metamorphism in ordinary chondrites. It is shown that below 550 C increasing Co decreases the equilibrium kamacite Ni concentration of an alpha to gamma system, and that the equilibrated L chondrites have kamacite and taenite Co concentrations in the L-group range. Metal-phase studies of petrologic type-3 ordinary chondrites with highly unequilibrated silicates showed a wide range in the degree of matrix kamacite equilibration; in the three most unequilibrated chondrites most taenite is clear, and the high Ni content shows that metamorphic temperatures were lower than 400 C in these meteorites.

Afiattalab, F.↗

NASA NTRS1980

Carbonic metamorphism, granulites and crustal growth

Stabilization of early crust against melting by high radioactivity and against resorption into the mantle by fast convective overturn requires that water and heat producers were flushed upwards within 50 Myr of accretion. Creation of a refractory base of granulite by metamorphism associated with CO2 vapour explains CO2-rich fluid inclusions in ancient high-grade rocks, minor-element depletions and local phenomena of arrested development of charnockite in Precambrian terrains. The hot-spot and plate-tectonic models of Precambrian crustal evolution lead to different schemes for CO2 delivery to continental roots. New tectonic concepts may be needed to explain carbonic metamorphism and other features of early crustal evolution.

Newton, R. C.↗

NASA NTRS1983

Replacement textures in CAI and implications regarding planetary metamorphism

Formation by a secondary metamorphic event, rather than primary crystallization from a melt or a sequential nebular condensation, is indicated by textural and chemical features of five coarse grained, Ca- and Al-rich inclusions (CAI) from the Allende meteorite which contain embayed pyroxene surrounded by melilite. It is suggested that the most probable environment for a metamorphic process (requiring the addition of Ca derived from calcite or from the introduction of a fluid phase) is that of a small planetary body, rather than the solar nebula. These results are compatible with O isotopic heterogeneities within CAI, and offer a mechanism for the production of lower temperature alteration phases, together with the rim phases found in these inclusions.

Meeker, G. P.↗