Engineering topics
Publications and source records attributed to Hewins, R. H..
A wide range of analytical and experimental techniques were used to determine the nature and origin of meteoritc materials, and its significance in the early history of the solar system. The results on four major topics are summarized: howardites, mesosiderites, diogenites, and chrondules.
The effectiveness of dynamic recrystallization experiments to reconstruct chondrule textures is evaluated. Consideration is given to the results of several experimental simulations of the silicate spheres of pyroxene-olivine chondrules, Si-rich chondrules, chondrites, droplet chondrules, and microporphyritic chondrules. It is shown that many chondrules have textures consistent with heating to 1400-1600 C, and cooling rates from a few degrees to several thousands of degrees per hour. It is shown that this physical constraint effectively rules out mechanisms for chondrule genesis which do not require instantaneous or very slow cooling. Analysis of the available textural, chemical, and isotopic data suggest that chondrules were generated from primordial particles, and several mechanisms might have assisted in chondrule heating as they settled down toward the median plane of the nebula. The most important of these mechanisms appear to be friction with the nebular gas and heating by flares.
Understanding the exact mechanism of origin of chondrules and the attendant physical conditions remains a key factor in understanding the early evolution of the solar system. The present investigation is concerned with an interpretation of the range of cooling histories needed to make chondrules. Chondrule-like spherules were formed in dynamic crystallization experiments, using gas-mixing facilities. It is found that textures in pyroxene-rich chondrules are well reproduced in the laboratory by cooling melts of chondrule composition from just above the liquidus. The range of pyroxene dendrite widths is slightly greater for natural chondrules than for droplets cooled from 50 C/hr to greater than 3000 C/hr. A blanketing medium of variable thickness explains the cooling rates lower than expected for radiative cooling. Experimental results, textures indicating incomplete melting, and isotopic disequilibrium are compatible with reheating of primitive material in the early solar nebula.
Diogenites contain equilibrated orthopyroxene-olivine assemblages. Mn is very regularly partitioned between olivine and orthopyroxene in pallasites, diogenites and synthetic eucrite melts, with an FeO/MnO partition ratio for olivine versus orthopyroxene of 1.6 by weight over a very wide range of FeO contents. In contrast to diogenites, Fe and Mn are not regularly partitioned between the olivine and orthopyroxene of mesosiderites and these minerals were not in equilibrium. Mesosiderite olivine differs from diogenite olivine in Fe/Mn and Ca/Mn ratios. Lack of olivine-orthopyroxene equilibrium suggests that olivine in mesosiderites was derived not from a pyroxenite component analogous to diogenites but from dunites.
The Manych (L-3) chondrite contains chondrules that have radically different thermal histories. The high-SiO2 composition cooled at about 1 K/min to a glassy state or more slowly to form a variety of fractions of crystals. The low-SiO2 composition cooled at about 3000 K/min to form a glassy coating or much more slowly to form microporphyry. The extreme range of cooling rates displayed by objects in the Manych chondrite is difficult to reconcile with a model involving droplet condensation at low pressure which provides a uniform temperature, a uniform cooling rate, and an ambient that is not necessarily cold. The experimental results presented here are interpreted to mean that the origin of the chondrules is likely a result of impact melting because of the range of cooling histories represented in the chondrite.
The abundance of orthopyroxene, inverted pigeonite, plagioclase, tridymite, kamacite, and tetrataenite, plus the whole rock analysis, indicates that ALHA 77219 is a mesosiderite. The presence of inverted pigeonite rims on orthopyroxene clasts plus the range of Fe/Mg and Fe/Mn ratios for pyroxene and olivine are characteristic of mesosiderites. All the petrographic and chemical data are consistent with classification of ALHA 77219 as a mesosiderite and, because the matrix is fine-grained and little recrystallized, as a subgroup I mesosiderite. The Fe/Mn trends in pyroxenes of mesosiderites such as ALHA 77219 can be explained by igneous fractionation of pyroxene along with metal and subsequent subsolidus reduction in the breccia.
The metal compositions of howardite breccias are examined and the significance of variations in metal content to meteorite origins is discussed. It is shown that metals in howardites are found primarily in Ni-poor components, associated with primary clasts and similar in composition to pristine lunar anorthosite metal, and in Ni-rich grains, associated with glassy melt rocks and possibly derived from the projectiles which caused the melting. Variations in primary clast metal compositions among the howardite and the identities of matrix and clast metal contents within many howardites suggest that each howardite is derived from a distinct source region. Evidence of thermal metamorphism in Petersburg is interpreted as the results of deeper burial than in other howardites.
The four mesosiderites studied in the present paper have samples similar to pyroxenitic and noritic rocks, but two of them (Morristown and Dyarrl Island) lack the very magnesian orthopyroxenes found in Estherville and Lowicz. Pigeonite crystallized at minimum temperatures of 1170 C in Estherville source rocks (before plagioclase crystallization), 1150 C in Lowicz and Morristown, and 1120 C in Dyarrl Island (after plagioclase crystallization).
The Bununu howardite is a polymict regolith breccia which contains impact melt that is largely restricted to a 1-cm thick intrusion containing residual glass. As in Malvern, the melt rock contains melt with meteoritic Ni-Co contents. The cooling rate, interpreted for forming glass from this composition, is a few tenths of a degree per minute. The intrusive melts rock, which is a feature unique to Bununu, may indicate that Bununu was consolidated at the time of impact melting.
Computer simulation of diffusion-controlled growth of the large Ni-rich grains of schreibersite found in the Emery mesosiderite indicates that exsolution from kamacite occurred during cooling at the rate of 0.1 C/Myr. This finding agrees with the mesosiderite cooling rate determined by Powell (1969) from taenite-kamacite data. The cooling rate is the lowest found for any meteorite group, and implications for the cooling history, with a possibility of reheating, are considered. The procedure for computing a family of cooling rate curves is based on Randich's (1975) method.
Malvern has been recognized as a breccia containing crystal clasts, clasts of parent rock types, and clasts of impact origin. The matrix consists primarily of small angular crystal fragments of pyroxene and plagioclase. Contrary to achondrites, the Malvern clasts of primary rocks and crystals are virtually devoid of metal, and the matrix contains no metal or primary achondrite composition. The melt rocks contain metal of meteoritic composition, which is also present in the matrix of Malvern.
A numerical model for diffusion-controlled phase growth has been applied to the exsolution of phosphide lamellae in lunar metal grains. Computer simulations reproduce observed composition profiles, and reveal the influence of cooling rate on dimensional and compositional parameters of phosphide and metal. At lower cooling rates, phosphide lamellae are larger and the concentration of P in the metal host close to the interface is lower. Cooling rates inferred for Apollo 16 samples, based on compositions and dimensions of the phosphide-metal grains, are mostly in the range 1-100 C/day. These rates correspond to burial depths of 5-0.5 m for melt rocks and 3-0.3 m for breccias. This is in good agreement with thicknesses of lunar cooling units determined by other techniques.
Carbides and the surrounding metal of eight particles from Apollo 14, 16, and 17 soils were analyzed for C, Fe, Ni, and Co with the electron microprobe. In addition, carbide-containing metal particles in two Apollo 17 rocks were studied. Carbide-metal assemblages were found in anorthositic hornfels, melt rock, and agglutinate, but not in any primary igneous rocks. Carbon measurements show that cohenite is the carbide phase present in all the samples. Cohenite exsolves from the carbon-rich metal phase during cooling of the lunar metal particle in the host rocks or soil fragments. Measurements of two-phase interface compositions indicate equilibration for carbides and metal during cooling below 600 C and possibly below 500 C.
The moderately magnetic fractions of soils from a bright ray of South Ray Crater and from the continuous ejecta of North Ray Crater are described. The clasts in these fractions are classified as agglutinate, ANT melt rocks, glassy melt rocks, and ANT hornfels/breccia. Determinations are made of the Ni-Co and P contents of single-phase metal grains in the clasts. It is found that most of these grains have compositions in the meteoritic range and that the compositions tend to cluster at about 6% Ni. A significant amount of Fe-rich metal is also detected in some clasts along with a clear P-enrichment in metal from one soil sample. Meteoritic contamination of lunar metal having compositions in or close to the meteoritic range is considered. A model involving nine separate factors is proposed as an explanation of the observed metal compositions.
Calculated compositions for metal crystallized from anorthositic melt rocks or feldspathic basalts such as 14310, 68415, and 77017 are (1) meteoritic in the initial stages or if metal is the only phase crystallized, and (2) higher in Co than meteoritic metal as a result of fractional crystallization of metals and silicates. Metal in coarse anorthositic rocks 15415, 15455, 60015, 64435, and 78238 is not of meteoritic composition and has Co contents from 0 to 9% with a Ni/Co ratio of about 1. The formation of anorthosites and the composition of the earliest metal are discussed with attention to Ni concentrations and the Ni/Co ratio. It is shown that anorthositic hornfels and breccias contain metal of a composition not found in their source material, coarse anorthosite; it is suggested that the hornfels includes metal grains probably derived with little change in composition from ancient meteorites.
Olivine crystals in mare basalts 12004,8 and 12022,12 are normally zoned with Cr-poor rims. The Ni content of rare 2- to 10-micron metal inclusions in olivine decreases markedly as Fe/Mg in their immediate olivine hosts increases. Each metal grain appears to have been enclosed by late olivine almost immediately after it crystallized. The fractionation trend for the olivine and metal contrasts with the subsolidus equilibration trend for pallasites. For the basalts, not even local equilibrium of Fe, Ni and Co at metal/olivine interfaces can be detected by microprobe. Ni and Co concentrations range from about 300 ppm in olivine cores to about 70 ppm in rims. The limits of detection, at 95% confidence, are 36 ppm (Ni) and 25 ppm (Co). The distribution of Ni and Co in olivine, like that of Mg and Cr, records the depletion of these elements in the melt.
A petrographic, metallographic, and electron probe study of particles from two Apollo 17 soils is reported. A mixing model for the formation of the Apollo 17 soils is proposed. It is found that equilibration temperatures for two-phase metal aggregates range from 375 to 475 C for metal-phosphide particles and from 480 to 630 C for alpha-gamma particles, and that the anorthositic soil particles contain metal of meteoritic Ni-Co content. The Camelot Crater and the Sculptured Hills soil compositions are discussed.