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 ± 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.     

CLASSIFICATION OF THE ALLAN HILLS A77307 METEORITE

The Allan Hills A77307 meteorite has variously been described as a CO, CV, and a unique CO-CM related chondrite. We have found that its thermolum-inescence properties are very different from the established members of the CO chondrite class; it has a TL peak at 170 and a suggestion of a peak at 250°C, while CO chondrites have peaks at 91 ± 7 and 203 ± 11°C. Either the meteorite has suffered some form of alteration or it is not a normal CO chondrite. The latter is consistent with petrologic and compositional data which we interpret to indicate that although Allan Hills A77307 is related to CO chondrites it is not a normal member of that group.     

CALCIUM-ALUMINIUM-RICH CHONDRULES IN THE UNEQUILIBRATED ORDINARY CHONDRITES

Calcium-aluminium-rich chondrules were found in L-3 (ALH-77015) and LL-3 (ALH-77278) chondrites. They consist of spinel, olivine, fassaite and glassy groundmass enriched in anorthite component. The bulk chemical compositions of the two chondrules are nearly the same and resemble those of the calcium-aluminium-rich inclusions in the carbonaceous chondrites, but the former is more enriched in MgO and SiO₂ and depleted in Al₂O₃ and CaO. The calcium-aluminium-rich chondrules would have been primarily early condensates which existed at the place where the chondrules were formed, and heated up to 1400 °C or higher to have melted and cooled at the rate similar to those for other chondrules, or the products of intensive heating followed by reaction with gas.     

GOBABEB,A NEW CHONDRITE: THE COEXISTENCE OF EQUILIBRATED SILICATES AND UNEQUILIBRATED SPINELS

Gobabeb, an ordinary chondrite, was found near Gobabeb, South West Africa in 1969. Chemically and petrographically it belongs in the H4 group. But, in addition to almost homogeneous silicates and chromites, it contains rare, non-opaque spinels that vary greatly in composition from grain to grain. A similar association in an “almost equilibrated” portion of the Mezö-Madaras chondrite has been interpreted as evidence against the hypothesized metamorphic homogenization of ordinary chondrites. A comparison of the chromites and variable spinels from Mezö-Madaras and Gobabeb suggests, instead, that cation exchange is simply slower in the variable spinels than in the chromites. Based on the evidence to date, the survival of these highly variable spinels is not incompatible with a metamorphic episode for both these meteorites.     

SIZE-DISTRIBUTIONS OF CHONDRULE TYPES IN THE INMAN AND ALLAN HILLS A77011 L3 CHONDRITES

A petrographc study of 9 thin sections of Inman (L3) and 18 thin sections of ALHA77011 (L3) served to determine the size-distributions of different chondrule textural types. Inman chondrules are significantly larger than those in ALHA77011, but in each chondrite, there is no statistically significant difference between the size-distributions of barred olivine and radial pyroxene plus cryptocrystalline chondrules. In ALHA77011, barred olivine chondrules outnumber radial pyroxene plus cryptocrystalline chondrules, whereas in Inman, the reverse is true. Because compound and cratered chondrules were formed by the collision of similarly-sized objects, the dustball precursors of chondrules must have been size-sorted prior to chondrule formation. The region of dustball size-sorting in the solar nebula must have been very large, similarly affecting the physically-separated precursors of different chondrule types. Size-sorting was probably accomplished by aerodynamic particle-gas interactions. Zones of dustball melting (i.e., chondrule formation) were relatively small, generally affecting only dustballs of one compositional type and relatively uniform size. Different chondrule types were then mixed together in somewhat variable ratios. Within the region where chondrites of a particular compositional group agglomerated, there were sub-reservoirs that contained (roughly) uniformly large or uniformly small chondrules with different mixtures of textural types.     

FRAGMENTAL BRECCIAS AND THE COLLISIONAL EVOLUTION OF ORDINARY CHONDRITE PARENT BODIES

Our survey of type 4–6 ordinary chondrites indicates that gas-poor, melt-rock and/or exotic clast-bearing fragmental breccias constitute 5%, 22% and 23%, respectively, of H, L and LL chondrites. These abundances contrast with the percentages of solar-gas-rich regolith breccias among ordinary chondrites: H (14%), L (3%) and LL (8%) (Crabb and Schultz, 1981). Petrologic study of several melt-rock-clast-bearing fragmental breccias indicates that some acquired their clasts prior to breccia metamorphism and others acquired them after metamorphism of host material. In general, the melt-rock clasts in gas-poor H chondrite fragmental breccias were acquired after breccia metamorphism and were probably formed by impacts into boulders or exposed outcrops of H4-6 material in the H chondrite parent body regolith. In contrast, most of the melt-rock clasts in gas-poor L and LL fragmental breccias were acquired prior to breccia metamorphism. The low abundance of regolith breccias among L chondrites and evidence that at least two-thirds of the L chondrites suffered a major shock event 0.5 Gyr ago, suggest that the L parent body may have been disrupted by a major collision at that time and that the remaining parent body fragments were too small to develop substantial regoliths (e.g., Heymann, 1967; Crabb and Schultz, 1981). Such a disruption would have exposed a large amount of L chondrite bedrock, some of which would consist of fragmental breccias that acquired melt-rock clasts very early in solar system history, prior to metamorphism. The exposed bedrock would serve as a potential target for sporadic meteoroid impacts to produce a few fragmental breccias with unmetamorphosed melt-rock clasts. The high proportion of genomict brecciated LL chondrites reflects a complex collisional history, probably including several episodes of parent body disruption and gravitational reassembly. Differences in the abundances of different kinds of breccias among the ordinary chondrite groups are probably due to the stochastic nature of major asteroidal collisions.     

MORRO DO ROCIO: AN UNEQUILIBRATED H5 CHONDRITE*

Morro do Rocio (MdR) initially appeared to be a perfectly normal, ordinary H5 chondrite. Fortuitously, however, during the course of our study free silica and a few olivine grains with higher iron content than the majority Fa 17.8 ± 0.3 were observed. MdR has normal texture and is normal in chemical, modal and, generally, mineral composition. Further detailed studies revealed more “anomalies,” additional indications that MdR is not equilibrated. Because most, if not all, ordinary chondrites are breccias, it seems likely that they may have similar “anomalies” which then are normal and must be considered in attempts to understand the origin of chondrites.     

AN UNEQUILIBRATED INCLUSION IN THE ROMERO (H3–4) CHONDRITE

Within the chondritic matrix of Romero (H3–4) is a metal-poor inclusion consisting largely of olivines showing quench textures, minor low-Ca pyroxene and rare metal and sulphide. The olivines within this inclusion have the dominant composition Fa⁸ with a range Fa^{7, 5–17}. The matrix (Romero host) olivines are, in the main, equilibrated with an average composition of Fa¹⁸, but are lower in CaO and Cr²O³ than the inclusion olivines. The high chromium content of the inclusion olivines suggests that they formed under conditions different from those represented by the other olivines in Romero.     

PETROLOGY OF NINE ORDINARY CHONDRITE FALLS FROM CHINA

Nine twentieth-century ordinary chondrite falls from China are described and classified. They include: Nantong (H6), Zaoyang (H5), Zhaodong (L4), Qidong (L-LL5), Raoyang (L6), Sheyang (L6), Guangnan (L6), Suizhou (L6) and Nan Yang Pao (L6). Kamacite in Qidong is rare and contains much more Co (15 mg/g) than is characteristic of L-group chondrites; Qidong may be a member of a chondrite group intermediate in its properties between L and LL. Zhaodong, Qidong, Raoyang, Sheyang and Suizhou have several olivine and/or low-Ca pyroxene grains with aberrant Fel(Fe + Mg) ratios; it is probable that these five chondrites are fragmental breccias. The lack of correlation between shock facies and occurrence of aberrant silicate grains suggests that breccia lithification caused only minimal shock effects in many meteorites. Alternatively, postshock annealing may have resulted in the recrystallization of shock-indicating phases, leading to assignment of a shock facies that is lower than that present immediately after the shock event.     

THE CORREO AND SUWANEE SPRING METEORITES: TWO NEW ORDINARY CHONDRITE FINDS

Two new ordinary chondrites were found about 40 km west of Albuquerque, New Mexico. Correo is an H4 chondrite with distinct chondrules and major olivine (Fo_81.4), orthopyroxene (En_82.3) and plagioclase (An₁₂). Suwanee Spring is an L5 chondrite with few distinct chondrules and a highly recrystallized matrix. Major minerals are olivine (Fo_75.4), orthopyroxene (En_77.7) and plagioclase (An₉). The metallic Ni-Fe phases of both meteorites are typical of slowly-cooled ordinary chondrites.     

A DETAILED CHEMICAL STUDY ON THE BARWISE CHONDRITE

Six fragments of the Barwise meteorite were analyzed for REE and eleven other elements (Al, Ca, Mg, Mn, Na, K, Cr, Fe, Co, Ni and Ba). In addition, two fragments were analyzed for Si and Mg. The chondrite-normalized REE patterns of six fragments studied show interesting systematic variations. Three fragments with relatively high La abundances show a negative Ce anomaly. Since the meteorite in question is a find, it could be suspected that the observed REE fractionations are due to terrestrial contamination. To examine this point, a soil sample from the find site was also analyzed for REE and major chemical elements. It is considered that several facts, especially, the relationships between La and SiO₂ and between SiO₂ and MgO, suggest the pre-terrestrial fractionation rather than the terrestrial contamination. Unexpectedly, it is shown that the REE fractionation observed in the investigated fragments correlates with the metal-silicate and the Fe-Co-Ni fractionations. In this connection, large metal grains were investigated for Fe, Co and Ni contents. A suggestion is presented that this chondrite was formed through the melting of the surface of a planetesimal and the subsequent collision, although the possibility of terrestrial contamination might not be ruled out.     

THE TAMBAKWATU CHONDRITE

According to its petrography, uniform olivine, Fa_23.8, and pyroxene, Fs_20.4, a total iron content of 22.9 wt % Fe, 16.4 wt % FeO and an FeO/FeO + MgO ratio of 24.7 mol %, the Tambakwatu is a veined, intermediate hypersthene (Cia) or L6 chondrite.     

THE PARSA ENSTATITE CHONDRITE

Two meteoritic stones weighing roughly 600 and 200 g fell on 14 April 1942 near Parsa, Bihar, India. The meteorite is a high-Fe (EH) enstatite chondrite on the basis of its large abundance of chondrules, its low concentrations of refractory elements, the Si content of its metal (25–30 mg/g), and its enstatite composition Mg_0.975Ca_0.007Fe_0.018. The high contents of Zn, Cd and In suggest that Parsa is petrologic type 4. A unique feature is an irregular nodule of coarse enstatite, several cm long which is chemically different in its Ca and Fe content compared to the matrix. We have increased the elemental concentrations by 10% to allow for terrestrial oxidation and hydration. The revised siderophile and moderately volatile element concentrations fall within the range observed in EH chondrites and mostly outside the range found in EL chondrites. Terrestrial alteration is indicated by the presence of limonite and other hydrated minerals as well as the morphologies revealed by scanning electron microscopy. The ²⁶Al activity is 51 ± 6 dpm/kg consistent with the calculated production rate. Cosmogenic track densities combined with the ²¹Ne, ³⁸Ar exposure age of 17 Myr indicate 4–10 cm ablation loss, or a preatmospheric mass of about 40 kg.     

A NOTE ON THE MINERALOGY AND CLASSIFICATION OF THE VILNA METEORITE

Electron microprobe analyses of olivine, orthopyroxene, clinopyroxene, plagioclase feldspar, chromite, whitlockite, apatite, troilite and metals indicate that the Vilna meteorite is a hypersthene chondrite belonging to the L5 or L6 subgroups.     

A REFRACTORY GLASS CHONDRULE IN THE VIGARANO CHONDRITE

Vigarano, a type 3 carbonaceous chondrite, contains a chondrule composed of highly refractory Ca and Al rich glass with minor spinel. The chondrule formed from material similar to the Ca, Al, Ti-rich aggregates that are common in Vigarano and other type 3 chondrites and formation of these refractory aggregates must predate formation of some Vigarano chondrules. Experiments with synthetic analogues and a comparison with studies in the system CaO-MgO-Al₂O₃-SiO₂ indicate a temperature for formation of the chondrule at or above 1700 °C followed by very rapid cooling.     

THE CLASSIFICATION OF THE BENARES CHONDRITE

The Benares meteorite is an LL4 chondrite, not LL6 as recorded in the literature. Some specimens labelled Benares are misidentified.     

THE ATLANTA ENSTATITE CHONDRITE BRECCIA

Atlanta is the fifth known brecciated enstatite chondrite. It contains a centimeter-sized troilite-rich clast, similar to those that occur in Blithfield. All of these clasts probably formed in the solar nebula under high pS₂/pO₂ conditions in a gas of non-cosmic composition. The absence of ordinary or carbonaceous chondrite clasts in any of the enstatite chondrite breccias and absence of enstatite chondrite clasts or materials formed at high pS₂/pO₂ ratios in ordinary and carbonaceous chondrite breccias support the model that enstatite chondrites were formed at a location distant from those of the other chondritic groups.     

THE IJOPEGA CHONDRITE: A NEW H6 FALL

The Ijopega (Papua New Guinea) meteorite is a new H6 group chondrite fall which contains olivine (Fa 19.9 mole %), bronzite (Fs 17.8 mole %), plagioclase (An 12.1 Or 6.3 Ab 81.6 mole %), diopside, kamacite, taenite, troilite, chromite and whitlockite. The meteorite is extensively recrystallized and brecciated, and shows evidence of moderate shock deformation. Examination of Fe²⁺ and Mg partitioning between ortho- and clinopyroxene indicates a high equilibration temperature (940° or 880 °C). Chemical analysis shows the meteorite to be rich in S, containing about twice the average H-group abundance. Trace elements, including REE, are in accord with established H-group chondrite abundances.