G. J. Taylor

The matrices of unequilibrated ordinary chondrites: Implications for the origin and history of chondrites

Abstract The matrices of sixteen unequilibrated ordinary chondrites (all witnessed falls) were studied microscopically in transmitted and reflected light and analyzed by electron microprobe. Selected specimens were also studied by scanning electron microscopy. These studies indicate that the fine-grained, opaque, silicate matrix of type 3 unequilibrated chondrites is compositionally, mineralogically and texturally distinct from the chondrules and chondrule fragments and may be the low temperature condensate proposed by Larimer and Anders (1967, 1970). Examination of the matrices of unequilibrated chondrites also shows that each meteorite has been metamorphosed, with the alteration ranging in intensity from quite mild, where the matrix has been only slightly altered, to a more severe metamorphism that has completely recrystallized the opaque matrix. Most of the metamorphic changes in the matrix occurred without significant effects on the compositions or textures of the chondrules. The metamorphic alteration probably resulted from a combination of processes including thermal metamorphism and the passage of shock waves. The present appearance of each unequilibrated chondrite is a result of the particular temperature and pressure conditions under which it and its components formed, plus the subsequent metamorphic alteration it experienced.

The Shaw meteorite: History of a chondrite consisting of impact-melted and metamorphic lithologies

Abstract The Shaw L-group chondrite consists of three intermingled lithologies. One is light-colored and has a poikilitic texture, consisting of olivine (many skeletal and euhedral) and augite crystals surrounded by larger (up to 1 mm) orthopyroxene grains; plagioclase occurs between orthopyroxene crystals and rare, small ( c -axes of the olivines are aligned. The light-colored lithology also contains numerous vugs and vesicles: SEM studies reveal euhedral, possibly vapor-deposited, crystals of olivine and pyroxene in the vugs. A second lithologic type is dark-colored, contains remnant chondrules. and has a microgranular texture. Poikilitic orthopyroxene crystals, where present, are smaller (0.1–0.2mm) than they are in the light-colored lithology. Microgranular olivine crystals contain 2 slab shows, contrary to published opinions, that Shaw contains normal L-group chondrite abundances of metal and troilite. However, these phases are distributed irregularly throughout the meteorite. The light colored lithology is nearly devoid of metal and troilite and centimeter-sized metal-troilite globules occur between the three silicate lithologies. Wherever the metal occurs, it consists of nearly homogeneous martensite (13.9 wt% Ni) rimmed by kamacite (7.1 wt% Ni). These data indicate that Shaw is a partly-melted shock-breccia. The light-colored lithology must have been totally melted, as shown by the presence of aligned. CaO-rich, skeletal olivines; Si-K-rich residual material: and vugs and vesicles lined with euhedral crystals of mafic silicates. The dark areas appear to be unmelted target rock of L-group composition. Analysis of the growth of kamacite at the taenite (now martensite) borders indicates a cooling rate of ~ 3 C/10 3 yr. or one thousand times faster than most ordinary chondntes. The Shaw impact event probably formed a crater several kilometers in diameter on its meteorite parent body.

The Shallowater aubrite: Evidence for origin by planetesimal impacts

Shallowater is the only known unbrecciated, igneous aubrite (enstatite achondrite). It is a coarse-grained orthopyroxenite consisting of (in vol%) poikilitic orthoenstatite crystals (80) up to 4.5 cm in size which contain, as inclusions and in the interstices, xenoliths of an assemblage of twinned low-Ca clinoenstatite (1), forsterite (2.9), plagioclase (2.5), metallic Fe,Ni (3.3), troilite (2.9), schreibersite (0.4), weathered opaques (8), and traces of niningerite and oldhamite. Conclusions: 1. Shallowater is of igneous origin. 2. Shallowater could not have formed by internal, igneous, nor by impact melting processes, either on the H or L enstatite chondrite or aubrite parent bodies. Instead, Shallowater appears to be a sample of yet a fourth enstatite-type asteroid. 3. Shallowater experienced an extraordinarily complex and unusual, three-stage cooling history, as determined by mineral stability considerations and modelling of micro-structural evolution as a function of cooling rate. Stage 1: Very fast cooling from ≥1,580°C to somewhere above 712°C, at a rate of ≥100°C/hour through 1000°C; above 1000°C, the rate was probably much faster. Stage 2: Very slow cooling (annealing), from 712 to 680°C at a rate of ≤7.5°C/106 y. Stage 3: Fast cooling, from 680 to 600°C at a rate of ≥0.5°C/day and from 600 to 300°C at a rate of ≥0.4°C/day. 4. This complex cooling history suggests an equally unusual origin for Shallowater, as follows: A completely or partly molten asteroid of nearly pure enstatite composition was broken up by low-velocity impact with a solid, E-like object. The melt cooled very fast (stage 1), due to incorporation of cold (≥20%) projectile debris (the xenoliths). Fragments reassembled into a rubble-pile body while T was ≥712°C, and deeply buried materials (like the object that is now Shallowater) cooled very slowly from 712 to 680°C (stage 2). The anneal of stage 2 requires burial at a depth of 40 km on a 100 km diameter asteroid. Excavation from this depth by impact to account for the fast cooling of stage 3 can be accounted for in two scenarios. Scenario 1: During catastrophic impact, the Shallowater object could have been by chance near the surface (within 10 m) of a very large fragment, thus accounting for the fast cooling and for surviving for 4.5 × 109 y. Scenario 2: The Shallowater object may have been excavated by break-up and subsequently buried near the surface of a gravitationally reassembled body. This would require yet a second break-up and reassembly episode. 5. The xenoliths represent an enstatite-like meteorite type unknown as an individual fall, not unexpected for a rock type from an asteroid that appears to have been largely destroyed early in the history of the solar system.

Martian parent craters for the SNC meteorites

The young ages (∼1.3 Ga) and the basaltic to ultramafic compositions of the shergottites, nakhlites, and chassignites meteorites severely restrict their potential source regions on Mars. We have used this age and compositional information, together with geologic data derived from Viking Orbiter images, to identify 25 candidate impact craters in the Tharsis region of Mars that could be the source crater for these meteorites. None of these craters are close to the size (∼100 km diameter) implied by the dynamical study of SNC ejection developed by Vickery and Melosh (1987). The craters in our study were selected because they are >10 km in diameter, have morphologies indicative of young craters, and satisfy both the petrologic criteria of the SNCs and the proposed 1.3 Ga crystallization ages. Of these 25 craters, only nine are found on geologic units believed to be young (crater density is less than 570 craters of greater than 1 km diameter per 106 km2). No crater exists to satisfy well the criteria of sampling both a 1.3 Ga surface (nakhlites and Chassigny) and a 180 Ma surface (shergottites) without at the same time imposing significant constraints on the chronology of Mars as inferred from the cumulative crater curves. The relatively young age (based on their inferred position in the stratigraphic column of Tharsis (Scott et al., 1981)) of the SNCs implies that volcanic activity on the plains of the Tharsis region extended well past 1.3 Ga.

Impact melting of the Cachari eucrite 3.0 Gy ago

Both the host phase and glass veins of the Cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their 39Ar-40Ar gas retention ages. Cachari is chemically similar to other non-cumulate eucrites. The vesicular glass veins vary from pure glass, to devitrified glass, to areas that are substantially crystalline. The glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of Cachari, but some differences, probably due to preferential dissolution, occur along melt contacts. The glass formed by shock melting of Cachari host or of rock identical to it. 39Ar-40Ar data for the host and glass suggest distinctly different ages of 3.04 ±.07 Gy and 3.47 ±.04 Gy, respectively. The time of glass formation, which may also be the time of brecciation, is most likely given by the 3.0 Gy age of the host. The higher age for the glass is interpreted to represent incomplete Ar degassing during the 3.0 Gy event due to the greater resistance to Ar diffusion shown by the glass compared to the host. Event ages significantly younger than 4.5 Gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body.