K. Fredriksson

Lonar Lake, India: An Impact Crater in Basalt

Discovery of shock-metamorphosed material establishes the impact origin of Lonar Crater. Coarse breccia with shatter coning and microbreccia with moderately shocked fragments containing maskelynite were found in drill holes through the crater floor. Trenches on the rim yield strongly shocked fragments in which plagioclase has melted and vesiculated, and bombs and spherules of homogeneous rock melt. As the only known terrestrial impact crater in basalt, Lonar Crater provides unique opportunities for comparison with lunar craters. In particular, microbreccias and glass spherules from Lonar Crater have close analogs among the Apollo specimens.

Morphology of Lonar Crater, India: Comparisons and implications

Lonar Crater is a young meteorite impact crater emplaced in Deccan basalt. Data from 5 drillholes, a gravity network, and field mapping are used to reconstruct its original dimensions, delineate the nature of the pre-impact target rocks, and interpret the emplacement mode of the ejecta. Our estimates of the pre-erosion dimensions are: average diameter of 1710 m; average rim height of 40 m (30–35 m of rim rock uplift, 5–10 m of ejected debris); depth of 230–245 m (from rim crest to crater floor). The craters circularity index is 0.9 and is unlikely to have been lower in the past. There are minor irregularities in the original crater floor (present sediment-breccia boundary) possibly due to incipient rebound effects. A continuous ejecta blanket extends an average of 1410 m beyond the pre-erosion rim crest.In general, ‘fresh’ terrestrial craters, less than 10 km in diameter, have smaller depth/diameter and larger rim height/diameter ratios than their lunar counterparts. Both ratios are intermediate for Mercurian craters, suggesting that crater shape is gravity dependent, all else being equal. Lonar demonstrates that all else is not always equal. Its depth/diameter ratio is normal but, because of less rim rock uplift, its rim height/diameter ratio is much smaller than both ‘fresh’ terrestrial and lunar impact craters. The target rock column at Lonar consists of one or more layers of weathered, soft basalt capped by fresh, dense flows. Plastic deformation and/or compaction of this lower, incompetent material probably absorbed much of the energy normally available in the cratering process for rim rock uplift.A variety of features within the ejecta blanket and the immediately underlying substrate, plus the broad extent of the blanket boundaries, suggest that a fluidized debris surge was the dominant mechanism of ejecta transportation and deposition at Lonar. In these aspects, Lonar should be a good analog for the ‘fluidized craters’ of Mars.

The LL-Group Chondrites

Abstract Electron probe analysis of olivine and pyroxene in twenty-five LL-group chondrites show that this group can be clearly distinguished. The LL-group olivines contain from 19.3 to 22.4 wt.% iron corresponding to 27–32 mol% fayalite as compared to 22–25 in the L-group and 16–20 in the H-group chondrites. The orthopyroxenes show a corresponding grouping. The relationship between the commonly well-crystallized LL-group chondrites and some unequilibrated stones is discussed, particularly with regard to recent suggestions that thermal metamorphism could be responsible for the constant olivine and pyroxene composition in the ordinary chondrites.

Phosphates in iron and pallasite meteorites

Abstract The ferrous (manganous) orthophosphates, sarcopside and graftonite (both calcium-free) have been found in four fine octahedrites (Bella Roca, Chupaderos, Sams Valley, Verkhne Dnieprovsk). They coexist with schreibersite, troilite and metal. Calculations of P ( O 2 ) − T for reactions relating these phases show that the presence of these phosphates presents no inconsistency with respect to the degree of reduction encountered in iron meteorites. Similar calculations for magnesium orthophosphate (farringtonite) indicate that the pallasite in which it occurs is somewhat more reduced than the octahedrites. These phosphate-bearing irons suggest a degree of oxidation close to that of common chondrites.

The Sharps chondrite - New evidence on the origin of chondrules and chondrites.

Chemical, petrographic and mineralogical studies of the Sharps chondrite reveal that this meteorite is a complex agglomerate of chondrules and lithic fragments in a matrix similar in composition to carbonaceous chondrites. One type of fragment resembles Type II carbonaceous chondrites in composition, but contains igneous glass and has been depleted in sulphur (0.5%) and enriched in carbon (6%). The change in carbon and sulphur contents is ascribed to heating after the agglomeration as is also indicated by the textures. The heating episode has also indurated the whole rock but not equilibrated the major silicate phases. Among the chondrules are a few conspicuous ones with two generations of olivine. Evidence is given to show that pre-existing chondritic, relatively iron-rich (7–15% Fe) and nickel-poor (<0.01% Ni) olivine was caught in a second chondrule- forming process producing reduced silicates, iron-poor olivines (1–2% Fe) and ‘glass’ which reacted with the earlier olivine. Thus, several cycles of igneous events and deposition can be recognized. The secondary chondrules provide strong arguments against direct condensation of chondrules. The evidence, rather, favors the so-called ‘ignimbritic’ model of chondrule and chondrite formation. Textural relations, e.g., welding of different components, are also consistent with the latter alternative.

Alkali differentiation in LL-chondrites

The LL-group chondrites Krahenberg (Krbg) and Bhola are heterogeneous agglomerates containing a variety of lithic fragments and chondrules as well as crystal fragments. The FeFe + Mg content of most olivine grains is uniform (Fa28), although a few with distinctly lower Fe contents were found (Fa19). Both meteorites contain large, cm-sized, fragments with high enrichments of K (~12×), Rb (~45×) and Cs (~70×) relative to LL-chondrites, while the REE concentrations are normal (except for a negative Eu anomaly); Na and Sr are depleted (~0.5×) and the NaK weight ratio is 0.33 compared to 11 in the host. However, there is no difference in the sum of Na + K atoms. Also, the major elements, Si, Al, Mg, Ca and Fe, are nearly the same in fragments as in the host material. The K-rich igneous lithic fragments have a microporphyritic texture of euhedral to skeletal olivines in a partly devitrified glass with ~4% K2O. The main pans of both Krbg and Bhola contain mesostasis glasses in porphyritic chondrules and lithic fragments with varying K content (0.1–8.6% K2O) and NaK ratios (0.2–100). Crystalline plagioclase is depleted in K with an average NaK ratio of 22, i.e. higher than that for ordinary chondritic plagioclase, 8.4. Olivines in the large, K-rich fragments and in the host meteorites have the same iron content (Fa28), indicating that both formed under the same oxygen fugacity and probably on the same parent body. Conceivable mechanisms for the formation of the K-rich rocks from normal LL-chondrite parent material are: 1, magmatic differentiation: 2. Na-K exchange via a vapor phase; 3. silicate liquid immiscibility; 4. volatilization and condensation in impact events. Process 2 appears most feasible for forming a rock enriched only in K and heavier alkalies and depleted in Na without noticeably changing other elements including the REE.

Primitive ultrafine matrix in ordinary chondrites

Abstract Ultrafine matrix material has been concentrated by sieving and filtering disaggregated samples of six ordinary chondrites of different classes. This component(s), “Holy Smoke” (HS), is enriched in both volatile, e.g. Na, K, Zn, Sb, and Pb, as well as refractory elements, e.g. W and REE; however, the element ratios vary greatly among the different chondrites. SEM studies show that HS contains fragile crystals, differing in composition, and apparently in gross disequilibrium not only among themselves but also with the major mineral phases and consequently thermodynamic equilibration did not occur. Thus HS must have originated from impacting bodies and/or was inherent in the “primitive” regolith. Subsequent impact brecciation and reheating appears to have altered, to varying degrees, the original composition of this ultrafine matrix material. Recent “cosmic dust” studies may indicate that HS still exists in the solar system. Survival of such delicate material must be considered in all theories for the origin of chondrites.

A Chondrule in the Chainpur Meteorite

The occurrence of glass as a major constituent in a chondrule from the Chainpur meteorite provides evidence that the chondrules formed by rapid cooling of liquid droplets. The virtual absence of nickel in the silicates suggests that it segregated into the metal phase in the molten stage, prior to crystallization of the silicates.

Celtic vitrified forts: Implications of a chemical-petrological study of glasses and source rocks

The term “vitrified forts” refers to remnants of stony fortifications of early Iron Age in which constituent rock fragments and boulders have been melted and/or welded together by heat, apparently in situ. The methods used are a matter of controversy, and even the question whether the firing was constructive or destructive is disputed. In this study we present bulk analyses of some twenty source rocks and of fifty glasses (in petrographic thin sections) from eleven sites and attempt to relate these data to the processes (and purposes) producing the observed sintering, welding and melting. The results are consistent with progressive partial, sometimes total, melting of the rocks used. In agreement with most previous investigators we conclude that no exotic rocks (although limited amounts of clays or organic debris cannot be excluded) were used as flux nor were easily fusible materials selected. On the other hand, simply burning an ordinary (of the later murus gallicus type) timber-laced wall cannot account for the evidence of strongly reducing conditions and sustained high temperatures, probably for days and in some cases well above 1000 °C. Rather, the fires seem to have been contained, perhaps by filling the space between the rocks in the timber framework with soil, clay and combustible materials, e.g. peat, brushwood and domestic debris. We cannot conclude whether the firing was accidental, or set by enemies or by the builders. In any case, the builders appear to have used more sophisticated techniques than previously believed to ensure the durability of their constructions.

Olivine and pyroxene in the Orgueil meteorite

Abstract The Orgueil meteorite contains angular unaltered grains of dominantly low-iron olivine and pyroxene. Their presence in a matrix of hydrous low-temperature silicates is evidence that Orgueil is a low-temperature mechanical mixture.