Paul H. Benoit

The classification and complex thermal history of the enstatite chondrites

We have carried out instrumental neutron activation analyses of 11 enstatite chondrites and electron microprobe analyses of 17 enstatite chondrites, most of which were previously little described. We report here the third known EH5 chondrite (LEW 88180) and an unusual EL6 chondrite (LEW 87119), new data on four EL3 chondrites (ALH 85119, EET 90299, PCA 91020, and MAC 88136, which is paired with MAC 88180 and MAC 88184), the second EL5 chondrite (TIL 91714), and an unusual metal-rich and sulfide-poor EL3 chondrite (LEW 87223). The often discussed differences in mineral composition displayed by the EH and EL chondrites are not as marked after the inclusion of the new samples in the database, and the two classes apparently experienced a similar range of equilibration temperatures. However, texturally the EL chondrites appear to have experienced much higher levels of metamorphic alteration than EH chondrites of similar equilibration temperatures. Most of the petrologic type criteria are not applicable to enstatite chondrites and, unlike the ordinary chondrites, texture and mineralogy reflect different aspects of the meteorite history. We therefore propose that the existing petrologic type scheme not be used for enstatite chondrites. We suggest that while “textural type” reflects peak metamorphic temperatures, the “mineralogical type” reflects equilibration during postmetamorphic (probably regolith) processes. Unlike the ordinary chondrites and EH chondrites, EL chondrites experienced an extensive low-temperature metamorphic episode. There are now a large number of enstatite meteorite breccias and impact melts, and apparently surface processes were important in determining the present nature of the enstatite chondrites.

The natural thermoluminescence of meteorites: II. Meteorite orbits and orbital evolution

Natural thermoluminescence (TL) data for 26 meteorites for which orbital elements have been estimated are reported. Calculations of equilibrium natural TL level in ordinary chondrites suggest that TL should be a sensitive indicator of perihelion. Meteorites with perihelia 0.85 AU should show a range of natural TL (>5 krad) with significant scatter as a result of slight variations in dose rate (shielding) and albedo. The data presented here generally agree with the theory and suggest an effective dose rate of ≈3 rad/year for most meteorites. Comparison with cosmic ray exposure ages indicates that natural TL level is also partly related to exposure age. Meteorites with high (>20 Ma) exposure ages generally have a lower and more constrained (10–30 krad) range of natural TL than meteorites with ages 90 krad). We suggest that this reflects orbital evolution, since mature meteorite orbits evolve to lower perihelia. The data presented here confirm the earlier observations that only a very small proportion of meteorites have experienced orbits with low perihelia within the last 105 years. The data suggest that <15% of ordinary chondrites have been in recent orbits with perihelia <0.85 AU. The reheating of these meteorites is independently confirmed by their generally low 3He/21Ne ratios.

CHEMICAL AND PHYSICAL STUDIES OF CHONDRITES. X: CATHODOLUMINESCENCE AND PHASE COMPOSITION STUDIES OF METAMORPHISM AND NEBULAR PROCESSES IN CHONDRULES OF TYPE 3 ORDINARY CHONDRITES

Abstract The cathodoluminescence (CL) properties of eight type 3 ordinary chondrites and one L5 chondrite have been determined, and phenocryst and mesostasis compositions have been analyzed in the chondrules of four of them (Semarkona, type 3.0; Krymka, 3.1; Allan Hills A77214, 3.5; and Dhajala, 3.8) in order to investigate their origins and metamorphic history. Two major classes of chondrule with eight subdivisions have been identified mainly on the basis of CL properties, and >95% of the chondrules can be assigned to these groups on the basis of phenocryst and mesostasis composition. Class A chondrules, consisting of those with plagioclase-normative mesostasis with bright CL, are subdivided into groups A1, A2, A3, A4, and A5. Class B chondrules, with little or no CL and having quartz-normative mesostases in the least metamorphosed chondrites which becomes feldspathic with metamorphism, are subdivided into groups B1, B2, and B3. Relationships between the eight chondrule groups can be deduced from their relative abundance in each of the nine chondrites. Groups A1, A2, B1, and A5 are present in Semarkona (15, 20, 60, 5% by number, respectively), group A5 chondrules in Semarkona being more heterogeneous than A5 chondrules in chondrites of higher petrologic type. Chondrule group A1 evolves into A3 then A4 and then A5 during metamorphism while A2 evolves into A4 and then A5. Chondrule group B1 evolves into B2, B3, and then A5 but higher levels of metamorphism are required to complete the series than for the A1–5 series. Conversion of a group to an adjacent group can be observed in Allan Hills A77214 (3.5), where several chondrules are group B3 in their central regions and group A5 in their outer regions. The present chondrule groups are essentially independent of texture. Since group A and group B chondrules differ in bulk composition, redox state, and possibly oxygen isotope systematics, their relative abundance might be a factor in the creation of the nine chondrite classes.

The Breakup of a Meteorite Parent Body and the Delivery of Meteorites to Earth

Whether many of the 10,000 meteorites collected in the Antarctic are unlike those failing elsewhere is contentious. The Antarctic H chondrites, one of the major classes of stony meteorites, include a number of individuals with higher induced thermoluminescence peak temperatures than observed among non-Antarctic H chondrites. The proportion of such individuals decreases with the mean terrestrial age of the meteorites at the various ice fields. These H chondrites have cosmic-ray exposure ages of about 8 million years, experienced little cosmic-ray shielding, and suffered rapid postmetamorphic cooling. Breakup of the H chondrite parent body, 8 million years ago, may have produced two types of material with different size distributions and thermal histories. The smaller objects reached Earth more rapidly through more rapid orbital evolution.

Chondrule formation, metamorphism, brecciation, an important new primary chondrule group, and the classification of chondrules

Abstract The recently proposed compositional classification scheme for meteoritic chondrules divides the chondrules into groups depending on the composition of their two major phases, olivine (or pyroxene) and the mesostasis, both of which are genetically important. The scheme is here applied to discussions of three topics: the petrographic classification of Roosevelt County 075 (the least-metamorphosed H chondrite known), brecciation (an extremely important and ubiquitous process probably experienced by >40% of all unequilibrated ordinary chondrites), and the group A5 chondrules in the least metamorphosed ordinary chondrites which have many similarities to chondrules in the highly metamorphosed ‘equilibrated’ chondrites. Since composition provides insights into both primary formation properties of the chondrules and the effects of metamorphism on the entire assemblage it is possible to determine the petrographic type of RC075 as 3.1 with unique certainty. Similarly, the new scheme can be applied to individual chondrules without knowledge of the petrographic type of the host chondrite, which makes it especially suitable for studying breccias. Finally, the new scheme has revealed the existence of chondrules not identified by previous techniques and which appear to be extremely important. Like group A1 and A2 chondrules (but unlike group B1 chondrules) the primitive group A5 chondrules did not supercool during formation, but unlike group A1 and A2 chondrules (and like group B1 chondrules) they did not suffer volatile loss and reduction during formation. It is concluded that the compositional classification scheme provides important new insights into the formation and history of chondrules and chondrites which would be overlooked by previous schemes.

The natural thermoluminescence of meteorites 4. Ordinary chondrites at the Lewis Cliff Ice Field

Natural thermoluminescence (TL) measurements have been made on 302 meteorites from the vicinity of the Lewis Cliff in the Beardmore region of Antarctica. The data provide information on terrestrial age and unusual radiation and thermal histories, which, in turn, are helpful in identifying fragments of a single fall and in understanding ice sheet movements and the mechanisms by which meteorite concentration occurs at this site. The present data with data for induced TL, class, find location, hand-specimen descriptions and mineral composition have enabled 70 of the present samples to be assigned to 27 groups of “paired” meteorites, with between 5 and 2 meteorites in each group. The distribution of meteorites on the ice, the shape of the fields of “paired” meteorites, and trends in the natural TL data indicate that there is a western component to the movement of the ice at this location, as well as the previously supposed movement to the north. This western vector probably explains the concentration of meteorites along the western edge of the ice tongue. Meteorites at the northern end of the tongue (the Lower Ice Tongue), and Meteorite Moraine to the east, have relatively high natural TL, and therefore young terrestrial ages, while those on the upper tongue show a broad range of ages including a great many large ages. Details of the meteorite concentration are different on the Upper and Lower Tongue and it might be that these two parts of the ice sheet are unrelated. These new natural TL data identify several recent falls and several meteorites which probably had unusually small perihelia immediately prior to capture by the earth.

The induced thermoluminescence and thermal history of plagioclase feldspars

Abstract Feldspars are a common component in igneous and metamorphic rocks. Most feldspars exhibit luminescence, and this has proved useful in a number of mineralogical applications. In this paper, we concentrate on the thermoluminescence (TL) properties of feldspar, or the luminescence produced when a sample of feldspar is heated. We determined the induced TL properties of four feldspars of various compositions in their natural states, and after heating, and we compared the TL data with structural changes as determined by X-ray diffraction. The major TL peak at 120-240 °C in the TL glow curve, a plot of light intensity against temperature, varies significantly among feldsparbearing samples. Meteorites and lunar samples with slow cooling histories of ∼10 °C/My as determined by independent methods, have induced TL peak temperatures of ∼120 °C, while samples with fast cooling histories (∼100 °C/My) have induced TL peak temperatures of ∼220 °C. This variation in TL peak temperature can be reproduced by heating the present feldspar samples, meteorites and lunar samples prior to the TL measurement. Most of the present samples in their natural state had TL peak temperatures of ∼120 °C. Heating below 750 °C in the laboratory caused no change in TL peak temperatures or the structural disorder of the feldspar, while heating >750 °C caused TL peak temperatures to move to ∼220 °C and disordered the feldspar structure. We suggest that induced TL peak temperature in feldspar is influenced by the degree of Al-Si ordering in the feldspar. Thus, induced TL peak temperature can be used as an indicator of cooling rate for igneous and metamorphic rocks.

The natural thermoluminescence of meteorites. III - Lunar and basaltic meteorites

Natural thermoluminescence (TL) data have been obtained for the lunar meteorite MacAlpine Hills 88104/5 and for 65 eucrites, howardites, diogenites, and mesosiderites in order to investigate their recent thermal and radiation histories. All these meteorites have low levels of natural TL compared to chondrites, which is primarily because they display “anomalous fading” (i.e., fading by non-classical mechanisms). However, some have especially low natural TL (<5 krad at 250°C in the glow curve) which cannot be attributed to anomalous fading or thermal fading over especially large terrestrial ages, and which must reflect heating within the last 105–106 y. In some cases, this heating may have been associated with shock (e.g., LEW85303) or regolith processes (Kapoeta), but in most cases (Bununu, Lowicz, the diogenites ALHA77256, ALHA84001, EET79002, and maybe others) solar heating at perihelia <0.8 AU may be responsible for the low TL values. The fraction of basaltic meteorites thought to have had small perihelia (about 20%) is comparable to the fraction of chondrites with low natural TL and to the fraction of observed falls and fireballs with small perihelia. This may imply ejection from the asteroid belt via similar mechanisms. Assuming plausible values for cosmic ray dose rate, and that the natural TL of MAC88104/5 was totally drained by ejection from the moon, the parameters for TL decay determined in the present study suggest that the Moon-Earth transit times for MAC88104 and MAC88105 were 2,000 and 1,800 y, respectively, compared with 19,000 and 2,500 y for Y791197 and ALHA81005, respectively. Although they are clearly not paired, the possibility that MAC88104/5 and ALHA81005 were ejected from the moon by the same event should be considered, since diverse rock types are found in close proximity on the lunar surface. The natural TL data confirm most previous published pairings among basaltic meteorites and suggest others.

The natural thermoluminescence of meteorites: 5. Ordinary chondrites at the Allan Hills Ice Fields

Natural thermoluminescence (TL) data have been obtained for 167 ordinary chondrites from the ice fields in the vicinity of the Allan Hills in Victoria Land, Antarctica, in order to investigate their thermal and radiation history, pairing, terrestrial age, and concentration mechanisms. Using fairly conservative criteria (including natural and induced TL, find location, and petrographic data), the 167 meteorite fragments are thought to represent a maximum of 129 separate meteorites. Natural TL values for meteorites from the Main ice field are fairly low (typically 5–30 krad, indicative of terrestrial ages of ∼400 ka), while the Farwestern field shows a spread with many values 30–80 krad, suggestive of < 150-ka terrestrial ages. There appear to be trends in TL levels within individual ice fields which are suggestive of directions of ice movement at these sites during the period of meteorite concentration. These directions seem to be confirmed by the orientations of elongation preserved in meteorite pairing groups. The proportion of meteorites with very low natural TL levels (<5 krad) at each field is comparable to that observed at the Lewis Cliff site and for modern non-Antarctic falls and is also similar to the fraction of small perihelia (<0.85 AU) orbits calculated from fireball and fall observations. Induced TL data for meteorites from the Allan Hills confirm trends observed for meteorites collected during the 1977/1978 and 1978/1979 field seasons which show that a select group of H chondrites from the Antarctic experienced a different extraterrestrial thermal history to that of non-Antarctic H chondrites,

A recent meteorite shower in Antarctica with an unusual orbital history

The Antarctic meteorite collection has proved to be a source of many important discoveries, including a number of previously unknown or very rare meteorite types. A thermoluminescence (TL) survey of meteorite samples recovered by the 1988/89 European expedition and pre-1988 American expeditions to the Allan Hills Main blue ice field resulted in the discovery of 15 meteorites with very high TL levels ( > 100 krad at 250°C in the glow curvc). It is likely that these samples are fragments of a single meteoroid body which: (1) fell very recently and (2) experienced a decrease in orbital perihelia from ⩾ 1.1 AU to 1 AU within the last 105 yr. Carbon-14 data for two of the samples confirm their young terrestrial age compared to most Antarctic meteorites. Studies of the cosmogenic isotopes in at least one non-Antarctic meteorite which also has very high natural TL, Jilin, indicate that the meteorite experienced a multi-stage irradiation history, the most recent stage being 0.4 Ma in duration following a major break-up of the object. These meteorites, and the few equivalent modern falls, are the only documented samples from bodies which were recently in Earth-approaching (Amor) orbits (i.e., with perihelion > 1.0 AU), as opposed to the Earth-crossing (Apollo) orbits which are the source of most other meteorites. Their rarity indicates that such rapid orbit changes are unusual for meteoroid bodies and may be the result of isolated, large break-up events.