Laurel L. Wilkening

Rare gases and fossil particle tracks in the Kenna ureilite

Fossil particle tracks and spallation-produced He and Ne in the Kenna ureilite indicate that it existed in space as a small object for 23 m.y. In our study of Kenna, we found no evidence of trapped He or Ne. Large amounts of heavy rare gases occur in Kenna in concentrations typical of ureilites. In a step-wise release of gases, the isotopic compositions of Kr and Xe were found to be constant above 600°C, revealing the presence of a single retentively sited component. The Xe isotopic abundances are characterized by 124:126:128:129:130:131:132:134:136 = 0.471:0.414:8.280:103.61: 16.296:81.92:100:37.70:31.23. This isotopic composition is distinct from AVCC (average carbonaceous chondritic), but similar to compositions known for some time in certain temperature fractions of Renazzo, Murray and Murchison. Kenna-type Xe appears to be one of the several components found in carbonaceous chondrites. Binz et al. (Geochim. Cosmochim. Acta39, 1576–1579, 1975) have recently found that many volatile trace elements are strongly depleted in ureilites. Thus, the relatively large amounts of heavy rare gases present in ureilites did not result from a mixture of a volatile-rich component with the ureilite host. It appears that some material rich in carbon and heavy rare gases was incorporated into a differentiated ureilite host. All current hypotheses which purport to explain the origin of trapped gases in meteorites encounter difficulty in accounting for trapped gases in ureilites in a straightforward manner.

A study of chondrule rims and chondrule irradiation records in unequilibrated ordinary chondrites

Abstract In order to investigate the possibility that chondrules may have had an independent existence in space, we have searched for unusual nuclear track densities in chondrules and studied the compositions of chondrule rims on chondrules from thirteen unequilibrated ordinary chondrites. Our search for unusual radiation features has been negative. Observed track densities can be explained in terms of cosmic ray exposure ages of the respective meteorites. Fine-grained rims that surround chondrules in unequilibrated ordinary chondrites are heterogeneous in composition consisting of varying proportions of iron sulfide and a poorly characterized silicate phase. The latter phase or phases are roughly chondritic in composition. Fine-grained rims of the kind seen in primitive type 3 ordinary chondrites are absent in higher petrographie grades; more crystalline, coarse-grained and lacy sulfide rims, however, are observed. Our observations can be explained by chondrules having had an independent existence in space during which they acquired rims either by condensation on their surfaces or by accretion of fine particles. However, accumulation of rims while chondrules resided on a meteorite parent body cannot be ruled out at this time. In any case, we do not propose that the chondrules themselves formed by condensation. Absence of a track record of space exposure of chondrules could be due to shielding by matter in space if, for example, chondrules were present in space in clouds made of dust, gas and/or chondrules.

Ice clathrate as a possible source of the atmospheres of the terrestrial planets

Abstract The presence and compositions of atmospheres on the terrestrial planets do not follow directly from condensation models which would have Earth accreting near 500°K. No single mechanism yet proposed adequately accounts for the abundances of noble gases and carbon and nitrogen in the atmospheres. We show that the composition of clathrates forming at low temperatures in cold regions of the nebula can be predicted. Addition of about 1 ppm clathrate material to the Earth can explain observed abundances of Ar, Kr, and Xe. Condensation and adsorption processes occuring at 400–500°K are necessary to explain the observed abundances of Ne, H 2 O, C, and N. Possible sources of clathrates could be cometary bodies formed in the outer solar system.

Carbonaceous Chondritic Material in the Solar System

Carbonaceous chondritic matrix material (CCMM) appears to be an important planet-forming unit in the mid-solar system, from the orbit of Mars to that of Uranus. The type specimen for CCMM is the lowtemperature (400–500 K) assemblage of clay minerals, organic polymer, magnetite, and Ni-rich iron sulfides which constitutes the black, fine-grained matrix of primitive carbonaceous chondrites. Solar-system objects which appear to be partly or wholly made of CCMM are the satellites of Mars, most asteroids, interplanetary dust, and, perhaps, comets, satellites of the outer planets and the rings of Uranus. CCMM constituents probably formed by low-temperature reactions of higher-temperature condensates with the ambient solar composition gas, or in the case of the organic polymer, by reactions of gaseous species catalyzed by solids.

Some studies of an unusual eucrite: Ibitira

The Ibitira eucrite is remarkable both for its vesicles and its unbrecciated nature. It consists of ~63 vol. % pyroxene (Wo14En38Fs48), 31% plagioclase (An95–96), ~2% of nickel-iron, troilite, ilmenite, titanian chromite, and 4% of a silica polymorph. It has a mean track density of 1.8 ± 0.3 × 106 cm−2, mainly due to cosmic rays. Its pre-atmospheric radius must have been at least 10 cm. The absence of complex radiation effects and presence of vesicles place constraints on the thickness of the Ibitira basalt flow. From the freezing time calculations of Provost and Bottinga, it appears that Ibitira came from a flow no less than 2.5 m and probably no more than 10 or 20 m thick. However, this estimate depends strongly on the viscosity of the melt, which is not well known.

Evidence for aqueous alteration in a carbonaceous xenolith from the Plainview (H5) chondrite

Abstract Within a CM-like clast in the Plainview (H5) chondrite are two inclusions which have the distinctive morphologies of an Allende-like, coarse-grained CAI and an amoeboid olivine inclusion respectively. The compositions of the mineral components within the inclusions were ascertained in this microprobe study. The major constituents of the altered inclusions are calcite, Mg-Fe-rich phyllosilicates, Fe-Ti oxides, and an unusually Al-rich (21–32 wt% A1 2 O 3 ) phyllosilicate. Assuming the starting compositions for these inclusions suggested by their morphologies, namely, Ca-Al refractory-rich oxides and silicates, the alteration process would have required transport of Na, Cl, H 2 O, “CO 2 ” and “FeO”. Because significant quantities of iron are required to produce the mineral assemblages now present from the inferred starting materials, and because of the presence of hydrous phases, it seems that liquid water was probably the medium in which the alteration reactions took place. The two possible sources of liquid water in meteorite parent bodies are primordially formed clay minerals and water ice. As yet neither source can be ruled out.

SOLAR RARE GASES AND THE ABUNDANCES OF THE ELEMENTS.

Gas rich meteorites and lunar materials solar rare gases component observed and predicted relative abundance agreement indicating absence of fractionation in solar nebula formation

Trace elements in rims and interiors of Chainpur chondrules

Trace elements were measured in the rims and interiors of nine chondrules separated from the Chainpur LL-3 chondrite. Whole rock samples of Chainpur and samples of separated rims were also measured. Chondrule rims are moderately enriched in siderophile and volatile elements relative to the chondrule interiors. The enriched volatile elements include the lithophilic volatile element Zn. The moderate enrichment of volatiles in chondrule rims and the lack of severe depletion in chondrules can account for the complete volatile inventory in Chainpur. These results support a three-component model of chondrite formation in which metal plus sulfide, chondrules plus rims and matrix silicates are mixed to form chondrites.

Missions to Comets: The Perspective in 1980

The impending return of Comet Halley to the inner solar system in 1985–1986 has prompted several countries to design missions to take advantage of this rare opportunity. The European Space Agency, Soviet Union and Japan have missions to study Comet Halley. The United States has not committed itself to a Halley mission, but a rendezvous mission with a short period comet remains among the National Aeronautics and Space Administration’s high priorities for the 1980’s.

Asteroid missions: Goals for elemental abundance analysis

Abstract Since it is known that there is a diversity of surface types among asteroids and assumed that asteroids represent several different bulk compositions and stages in planetary evolution, a first mission to the asteroid belt must study and compare several asteroids of differing types. Both very primitive and highly evolved asteroids should be studied. Identifications of any asteroid with a known type of meteorite will permit the attachment of a large body of accurate data to a known location, and thereby secure many commonly made assumptions as facts. Thus, it is essential that remote analysis of asteroids be able to distinguish among the compositions of known meteorites. Determination of the absolute abundances of Mg, Al, Si, Ca, Fe, Ni, and S will permit meteorite types to be distinguished. Analysis of additional elements such as C and H and other trace elements will permit more certainty in identification. Remote sensing of primitive asteroids should permit the detection of water on or outgassing from asteroid surfaces. An important goal will be to determine the degree to which remote observations of surfaces reflect real differences in interior compositions; hence, accurate determinations of densities will be essential. High-resolution photography of asteroidal surfaces may yield information on the heterogeneity of the surfaces.