Engineering topics

Scott, E. R. D.

Publications and source records attributed to Scott, E. R. D..

At least 37 records · Page 2

NASA NTRS1988

Primitive material surviving in chondrites - Matrix

A logical place to search for surviving pristine nebular material is in the fine-grained matrices of ordinary and carbonaceous chondrites of petrographic type 3. Unfortunately, many of these chondrites have experienced brecciation, thermal metamorphism, and aqueous alteration, so that interpreting individual features in terms of specific nebular conditions and/or processes is difficult. It follows that the origin and evolutionary history of such matrix phases are controversial, and a consensus is difficult to define. In this chapter, therefore, after summarizing the salient mineralogical, petrographic, chemical, and isotopic features of matrix in apparently primitive chondrites, an attempt is made to provide an overview both of areas of agreement and of topics that are currently in dispute.

Scott, E. R. D.↗

NASA NTRS1986

Recovery and classification of thirty new meteorites from Roosevelt County, New Mexico

The discovery and classification of 30 new meteorites found in or close to Roosevelt County, NM, are reported, including two H3 chondrites and a ureilite; the others are equilibrated ordinary chondrites. Over 160 meteorites representing at least 100 different falls have been recovered from this region, mostly from wind blowout areas. As in Antarctica, small specimens predominate and irons, achondrites, and C and E chondrites are rare. Paired specimens are also very difficult to identify.

Scott, E. R. D.↗

NASA NTRS1986

Ragland - An LL3.4 chondrite find from New Mexico

The Ragland, New Mexico, chondrite was found in 1978. It consists of a single stone of 12.16 kg that broke into three pieces. The stone is moderately weathered and has a pronounced chondritic texture. Bulk composition favors an LL classification, and modal analysis and oxygen isotopic composition are consistent with this. The thermoluminescence sensitivity of 0.056 + or - 0.020 normalized to Dhajala, compositional variability of olivine (mean Fa 18.3, sigma = 10.1) and low-Ca pyroxene (mean Fs 14.6, sigma = 6.7), and Ca concentrations in olivine indicate metamorphic subtype 3.4 + or - 0.1. The isotopically heavy oxygen composition, which is characteristic of subtypes 3.0-3.1, may be a primary characteristic and not a result of weathering. Low concentrations of radiogenic Ar-40 and planetary Ar-36 suggest noble gas loss.

Recca, S. I.↗

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 NTRS1985

Petrology of types 4-6 carbonaceous chondrites

A comparative mineralogic study has been made of Coolidge, Karoonda, and new C4 chondrites in order to clarify the origin of C4-6 chondrites. It is shown that the properties of all C4-6 chondrites are consistent with an origin by metamorphism of C3-like precursors by processes analogous to those operating in ordinary chondrite parent bodies or planetesimals. However, type 4-6 material was not well mixed with type 3 material in the CV3 parent body. It is concluded that C4-6 chondrites may come from one or more separate parent bodies.

Scott, E. R. D.↗

NASA NTRS1985

Origin and evolution of ordinary chondrite meteorites

The effects of heating on the chemical composition, minerology, and texture of chondrite meteorites are discussed chondrite origin and evolution. Various asteroidal and nebular heating mechanisms are considered to account meteorite compositions.

Scott, E. R. D.↗

NASA NTRS1984

Matrix material in type 3 chondrites - Occurrence, heterogeneity and relationship with chondrules

Variations between mean matrix compositions of individual type 3 ordinary chondrites are nearly fivefold, and partly reflect systematic differences between H, L, and LL matrices. Such variations are probably the result of a nebular separation of feldspathic material and ferromagnesian silicates. While compositions of chondrules and their matrix rims are normally unrelated, rim compositions are correlated with those of matrix lumps inside chondrules. Matrix lumps are as heterogeneous as chondrules, but mean chondrule and matrix compositions differ. Since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation, the present matrix samples cannot represent typical chondrule precursor materials.

Scott, E. R. D.↗

NASA NTRS1984

Relict and other anomalous grains in chondrules - Implications for chondrule formation

Relict olivine and pyroxene grains have been identified in chondrules from ordinary and carbonaceous chondrites that probably did not crystallize in situ. Some of these olivines are clear, but others contain fine-grained Fe, Ni ('dusty olivines') and resemble previously described occurrences in ordinary chondrites. There are also chondrules in which all olivine is dusty. It is concluded that: (1) not all relict olivines are dusty, (2) not all dusty olivines crystallized outside the chondrule in which they occur, and (3) some dusty olivines were produced during chondrule formation by a reduction process that affected the whole chondrule. The occurrence of dusty olivines and relict pyroxenes and olivines in chondrules from carbonaceous as well as ordinary chondrites supports the argument that chondrules from all chondrites had similar origins and histories. It is proposed that chondrules and mineral fragments were transported across f(O2) gradients in the solar nebula while they were hot, or were reheated in an environment different from the one in which they formed. Partially molten chondrules sometimes incorporated mineral grains or chondrules with different redox states, producing compound chondrules and chondrules containing anomalous grains. Dusty olivines may also have formed when hot chondrules were transported to regions of lower oxygen fugacity.

Kracher, A.↗

NASA NTRS1984

Composition and origin of enstatite in E chondrites

A petrologic and electron-probe study of enstatite in six EH3, 4, and 5 chondrites, including the previously undescribed Allan Hills A77156, provides no support for Leitch and Smith's (1982) model in which crystals and liquids from two chemically distinct, partially molten planetesimals are mixed. It is found that Cr and Mn concentrations in enstatite grains in matrix and chondrules are not uniquely defined by their cathodoluminescence color. Wide variations in enstatite compositions among chondrules may partly reflect heterogeneities in the precursor material. However, correlated variations of Cr, Mn, and Fe concentrations in enstatite are probably largely due to fractionation during chondrule crystallization. The inverse correlation of petrologic type with minor element concentrations in enstatite appears to be a metamorphic effect.

Mckinley, S. G.↗

NASA NTRS1984

Primitive meteorites: Where do they come from and how do they form?

The most primitive meteorites are called chondrites; which contain small spherules called chondrules. Chondrules and the other ingredients in chondrites appear to have formed long before there were any planets in the solar system. The rocks offer clues to the earliest history of the solar system and an opportunity to understand how some of the small planets were formed, how they were heated and how they were affected by impacts. The origin of asteroids, that supply the meteorites is examined and two factors are proposed: (1) meteorites come from many different bodies that have formed and evolved in slightly different ways, and (2) impacts provide samples of both the surfaces and deep interiors of these bodies.

Scott, E. R. D.↗

NASA NTRS1984

Fine-grained millimeter-sized objects in type 3 ordinary chondrites and their relation to chondrules and matrix

Fine-grained, opaque, silicate-rich matrix material, occurring as rims on chondrules and clasts, discrete clasts, and between chondrule and mineral fragments is a primitive component of chondrites and may resemble the material from which chondrules formed. To elucidate the relationship between chondrules and matrix and to help identify chondrule precursor material, the nimeralogy and bulk compositions of 30 mm-sized, fine-grained objects in Tieschitz (h3.6), Sharps (H3.4), Allan Hills A77299 (H3.7), Yamato 74191 (L3.6), Semarkona (LL3.0), and St. Mary's Co. (LL3.3) were studied. All are composed of submicron to micron sized material and may contain larger mineral fragments. Three texturally and mineralogically distinct types of objects were identified; all are minor components of the chondrites studied ( 1% except Sharps where melt-breccia-textured objects are approximately minus 1%). Bulk compositions of network and melt-breccia-textured objects define a rather narrow trend on a plot of Si/Al vs. Mg/Al, emphasizing a possible genetic relationship. Data show that Si/Mg is higher for network and melt-breccia-textured objects than for porphyritic and barred olivine chondrules. Melting in vaccuo of network-textured objects would cause a loss of Si relative to Mg (3), suggesting that material compositionally similar to fine grained objects could have been precursors to some chondrules.

Recca, S. I.↗

NASA NTRS1983

Chondrules and other components in C, O, and E chondrites Similarities in their properties and origins

Three types of chondrules are described that are common to H3, LL3, CM2, CO3, and CV3 chondrites. Low- and high-FeO, porphyritic olivine chondrules contain olivine with Fa0.3-8 and Fa5-50 respectively, and can easily be distinguished petrographically. Poikilitic pyroxene chondrules have 1-20 vol pct olivine (Fa0.2-8) enclosed by low-Ca pyroxene (Fs0.5-7), and also occur in E chondrites. These three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types. Sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups. Metallic Fe,Ni grains containing 0.1-1 percent Cr, Si, and P are present in low-FeO olivine chondrules from all type 2 and least metamorphosed type 3 chondrites. Metal compositions reflect reduction during chondrule formation in the nebula, not nebular condensation. Opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy, and probably have related origins. It is concluded that chondrules in all known chondrite groups share similar nebular origins.

Scott, E. R. D.↗

NASA NTRS1983

Nature of the H chondrite parent body regolith - Evidence from the Dimmitt breccia

Meteorite regolith breccias are clastic rocks which formed by lithification of fragmental regolith material that once resided at the surface of a meteorite parent body. A study is reported of the matrix and 21 clasts of various sizes (0.2-24 mm) in the Dimmitt H chondrite regolith breccia using petrographic and electron microprobe techniques. In addition, oxygen isotope studies of three clasts and instrumental neutron activation analysis (INAA) and Ar-39/Ar-40 age dating of one clast are reported. The Dimmitt meteorite was found about 1942 near Dimmitt, Texas. Attention is given to analytical procedures, the clastic matrix, equilibrated clasts, poikilitic melt-rock clast, clasts of different chondrite groups, graphite-magnetite aggregates, the origin of exotic clasts, and the complexity of parent body surfaces processes.

Rubin, A. E.↗

NASA NTRS1983

Cosmic setting for chondrule formation

Chondrules are igneous-textured, millimeter-sized, spherical to irregularly-shaped silicate objects which constitute the major component of most chondrites. There is agreement that chondrules were once molten. Models for chondrule origin can be divided into two categories. One involves a 'planetary' setting, which envisages chondrules forming on the surfaces of parent bodies. Melting mechanisms include impact and volcanism. The other category is concerned with a cosmic setting in the solar nebula, prior to nebula formation. Aspects regarding the impact on planetary surfaces are considered, taking into account chondrule abundances, the abundancy of agglutinates on the moon, comminution, hypervelocity impact pits, questions of age, and chondrule compositions. Attention is also given to collisions during accretion, collisions between molten planetesimals, volcanism, and virtues of a nebular setting.

Taylor, G. J.↗

NASA NTRS1982

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite - A new kind of type 3 chondrite and its relevance to the history of chondrules

Electron microprobe, scanning electron microscope, and petrographic analyses of the microchondritic clast of the Piancaldoli LL3 chondrite are reported and compared with other type three chondrites. The clast, like other type three chondrites, has a fine-grained Fe-rich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si and Cr-bearing metallic Fe, Ni. However, the very high model matrix abundance, unique characteristics of the chondrules, and absence of microscopically observable olivine indicate that the clast is a new type of type three chondrite. It is concluded that the microchondrules were formed by the same process that formed normal-sized chondrules in type three chondrites: melting of preexisting dustballs. It is suggested that dust grains were mineralogically sorted in the nebula before aggregating into dustballs.

Rubin, A. E.↗

NASA NTRS1982

A new LL3 chondrite, Allan Hills A79003, and observations on matrices in ordinary chondrites

Allan Hills A79003 is an LL3 chondrite with a petrologic subtype of 3.4 + or - 0.2. Contrary to previous suggestions, it is not paired with other Allan Hills specimens. It contains haxonite, (Fe,Ni)23C6; shock-melted, 'fizzed' metal-troilite intergrowths; and translucent, glassy-looking Huss matrix (fine-grained, Fe-rich silicate matrix), in addition to the normal opaque and recrystallized varieties of Huss matrix. Some chondrules are partly coated with opaque matrix, others with translucent matrix. Translucent matrix is more uniform in composition and contains less S, CaO and FeO and more MgO than the opaque variety. Both kinds of matrix rimmed chondrules before consolidation of the meteorite.

Scott, E. R. D.↗

NASA NTRS1982

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Quantitative evidence is adduced by cooling rate calculations for the impact melting, on the surface of a parent body, of troilite and metallic Fe, Ni inclusions found on 12 ordinary chondrites rather than their formation at depth from an internally derived melt. In the case of the Shaw and Rose City breccias of unmelted and melted material, their melted metal need not have cooled through 1000 C within a few meters of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite form in situ in many irons and some chondrites by rapid solidification at cooling rates greater than 100,000 C/sec, so that their kamacite and taenite compositions may result from annealing of metallic glass or very fine quench products at about 250 C.

Scott, E. R. D.↗

NASA NTRS1982

A unique type 3 ordinary chondrite containing graphite-magnetite aggregates - Allan Hills A77011

ALHA 77011, which is the object of study in the present investigation, is a chondrite of the 1977 meteorite collection from Allan Hills, Antarctica. It contains an opaque and recrystallized silicate matrix (Huss matrix) and numerous aggregates consisting of micron- and submicron-sized graphite and magnetite. It is pointed out that no abundant graphite-magnetite aggregates could be observed in other type 3 ordinary chondrites, except for Sharps. Attention is given to the results of a modal analysis, relations between ALHA 77011 and other type 3 ordinary chondrites, and the association of graphite-magnetite and metallic Fe, Ni. The discovery of graphite-magnetite aggregates in type 3 ordinary chondrites is found to suggest that this material may have been an important component in the formation of ordinary chondrites.

Mckinley, S. G.↗