Ca-rich Achondrites Research Articles

Uranium and thorium microdistributions in stony meteorites

Uranium distributions have been determined in seventeen meteorites using fission track techniques. In seven cases, Th was also determined by a new method using fast neutrons. The actinides are generally concentrated in phosphates, usually whitlockite and/or chlorapatite. Wherever whitlockite and chlorapatite coexist, chlorapatite is richer in uranium. U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also show large variations in the same meteorite. A clinopyroxene phase enriched in U (0.2–0.3 ppm) is usually found in Ca-rich achondrites. The Th U ratios of phosphates differ considerably from whole rock values indicating that these elements were fractionated during the meteorite formation.

Fission-track ages of four meteorites

A method for selective annealing of cosmic-ray tracks has been developed, permitting determination of fission-track ages in the presence of a large background of cosmic-ray tracks. The mesosiderite Bondoc contains 41 fission tracks/cm 2, of which about 75% are due to neutron-induced fission of U 235 during cosmic-ray exposure. Its net fission-track age is 140 ± 40 Myr, nearly identical to its cosmic-ray exposure age of 150 Myr. The mesosiderite Mincy has a fission-track age of 1500 ± 400 Myr. Nakhla (nakhlite) contains an excess of apparent fission tracks, which may be either genuine fission tracks from Pu 244 or etch pits mimicking fission tracks in length, thermal stability, random orientation, and other characteristics. On the assumption that they are fission tracks, the Pu 244/U 238 ratio at the onset of track retention in Nakhla was (3.1 ± 1.3) × 10 −3, nearly an order of magnitude lower than the initial solar system ratio. This may reflect a chemical fractionation of Pu and U, or a late impact or magmatic event. Different minerals of the Washougal howardite have different Pu 244/U 238 ratios, from (24 ± 7) × 10 −3 to (2.3 ± 0.7) × 10 −3. This may imply a succession of impacts over a period of time. Additionally, Pu and U may have been chemically fractionated from each other in this meteorite. Shocked meteorites show no consistent pattern in the retentivity of fission tracks and of fissiogenic or radiogenic noble gases. Some meteorites, e.g. Bondoc, Serra de Magé, and Mincy, retain gases more completely than tracks; others, e.g. Nakhla and Allende, retain them less completely. Uranium was determined in feldspar and/or pyroxene from 19 Ca-rich achondrites and mesosiderites. For most, only upper limits of 0.01–0.03 ppb were obtained. Apparently the uranium in these meteorites resides almost exclusively in minor phases, as in terrestrial and lunar rocks.

Curium-248 in the Early Solar System

PRIMITIVE meteorites such as carbonaceous chondrites (CC) and unequilibrated ordinary chondrites (UOC) have an anomalous fission xenon component1–3 which has a distinctly different fission spectrum from that of Ca-rich achondrites4,5. Although the latter is established as due to 244Pu spontaneous fission4, the origin of the former component is still debated. Further, the amount of fission xenon present in CC II–IV and some UOC is about two orders of magnitude6,7 higher than that of Ca-rich achondrites and even the maximum value of 244Pu could account only for 10% of CC fission xenon. Several mechanisms, such as carrier hypothesis8, mass fractionation9, superheavy elements6,7, and two-component trapped xenon10, have been proposed to explain the CC fission xenon. Here we attempt to determine whether some actinide isotopes with known properties could produce this excess fission xenon in the primitive chondrites.

Petrology and chemistry of mesosiderites—II. Silicate textures and compositions and metal-silicate relationships

The mineral compositions of all members of the mesosiderite group are similar, consisting of roughly equal parts by weight of Ni-Fe metal and silicates, the latter including orthopyroxene and calcic-plagioclase, with minor pigeonite and olivine and accessory tridymite. Heterogeneous mineral distribution and textural variation within individual mesosiderites are common. Silicate brecciation is characteristic, though cataclastic textures are variably obscured by partial recrystallization. Microprobe analyses reveal grain-to-grain silicate compositional variations indicating a variety of original environments and a general lack of equilibrium among phases. Metal-silicate textures indicate that kamacite-taenite structures formed in situ, after the presently observed physical relationships were established by mixing of solid silicates and metal. Metal structures and compositional inhomogeneities indicate extremely slow cooling (~0.1°C/ 10 6 yr) in the temperature range 500-350°C. New chemical analyses reveal a general similarity between silicate portions of mesosiderites and howardites. Previous theories of mesosiderite genesis involving pallasite-achondrite mixing are found to be untenable on the basis of olivine and metal compositions, although mesosiderites do appear to be genetically related to eucrites and howardites. The genetic history of mesosiderites is very complex and includes a significant change in cooling environment. An evolutionary model is described which includes magmatic differentiation, followed by brecciation, metal-silicate mixing, insulation by burial, metamorphism, and, ultimately, removal from the parent body. Mesosiderites are a distinct group, which, aside from the similarities and genetic relationship to Ca-rich achondrites, appear to have no affinities to other meteorite types.

A precise [formula omitted] age and initial [formula omitted] for the Colomera iron meteorite

Rb-Sr analyses of silicates from the Colomera iron meteorite show a wide range in Rb/Sr and give 87Sr 86Sr values ranging from 0.699 to 8.45. These analyses define a precise linear array on the Rb-Sr evolution diagram yielding an age of 4.61 ± 0.04 × 10 9 yr. A highprecision analysis of a whitlockite separate having very low Rb/Sr yields an initial ( 87Sr 86Sr ) I = 0.69940 ± 0.00004 . An upper limit of 48 ± 7 million years can be set for the time interval between dispersion of the silicate in the metal phase and final Sr isotopic equilibration, if we assume that Colomera silicate formed from a parent material which had the same initial 87Sr 86Sr as Ca-rich achondrites. The data are consistent with a simple evolutionary history in which Colomera differentiated from a parent material of chondritie Rb Sr ratio at a time 39 million years after the parent material had the initial 87Sr 86Sr of Ca-rich achondrites and 35 million years before the final Sr equilibration in the Guareña chondrite.

Isotopic analyses of krypton and xenon in fourteen stone meteorites

The concentrations and isotopic compositions of Kr and Xe in twelve chondrites and two achondrites have been determined. In addition to the trapped component, Kr and Xe produced by spallation and (n, γβ) reactions as well as fission and radiogenic Xe have been found. Because of the chemical uniformity of the ordinary chondrites the concentrations of spallation-produced Kr and Xe found in these meteorites correlate with the concentrations of the spallation-produced light rare gases. It is shown that variations in the Kr spallation spectrum are caused by differences in the hardness of the energy spectrum, which are also responsible for the variations in the spallation ratio Ne21/He3 in ordinary chondrites. Excesses of Kr80, Kr82, and Xe128 produced by capture of secondary neutrons in Br and I, respectively, are sensitive indicators for shielding and preatmospheric meteorite sizes. Neutron slowing down densities and minimal preatmospheric radii and masses are calculated on this basis for some of the investigated chondrites. Kr81 radiation ages are obtained for three chondrites and for the achondrites. For the chondrites and for a Ca-poor achondrite the Kr81 ages agree with the ages calculated from the He3 concentrations. However, the Ca-rich achondrites show Kr81 ages up to one order of magnitude higher, indicating He3 diffusion losses. A remarkable correlation between the concentrations of Br and trapped Ar, Kr, and Xe in chondrites has been found. The isotopic compositions of the trapped components in Kr- and Xe-rich ordinary chondrites are found to agree with those known for carbonaceous chondrites; however, the Khor Temiki achondrite seems to differ in this respect. For the Mezo Madaras chondrite a sample of unequilibrated host material and a sample of an inclusion of equilibrated material have been analyzed for all five rare gas elements. The spallation components of both samples are in excellent agreement, whereas the concentrations of trapped Ne, Ar, Kr, and Xe are about twenty times higher in the unequilibrated sample.

Determination of iron, nickel, cobalt, calcium, chromium and manganese in stony meteorites by X-ray fluorescence

The concentrations of total iron, and of nickel, cobalt, calcium, chromium and manganese have been determined by X-ray fluorescence in fifty-seven chondrites, seven carbonaceous chondrites, three enstatite chondrites, fifteen achondrites and in the silicate phase of two pallasites. The high-iron-group chondrites have an average of 27.36 % iron by wt., and the low-iron group has an average of 21.71 % iron by wt. Their nickel and cobalt contents show comparable differences. Significant differences in the concentrations of calcium and manganese are found between chondrites which belong to the low-iron group and those which belong to the high-iron group. The ratios Ca Mn are quite constant between the groups. The ratios Fe:Ca and Ca:Mn for the carbonaceous chondrites of Type II and Type III differ generally from the Fe:Ca and Ca:Mn ratios for the high-iron chondrites as a group. Among the achondrites examined, the Ca-poor achondrites are much more variable in composition than the Ca-rich achondrites.

Mass yield spectrum of cosmic-ray-produced xenon

Results of a study of the isotopic composition of xenon have indicated that the two Ca-rich achondrites that comprise in toto the group of diopside-olivine achondritic meteorites (the nakhlites) contain only a very small amount of excess fission-produced xenon. Thus it appears possible to obtain a relatively ‘pure’ isotopic spectrum of xenon isotopes resulting from cosmic-ray spallation reactions. The mass yield spectrum of spallation xenon normalized to Xe126 is found to be Xe124:Xe126:Xe128:Xe130:Xe131:Xe132 = 0.57:1.00:1.53:0.86:2.5:0.60. The values obtained here are compared with current estimates by other workers. It is proposed on the basis of the xenon isotopic abundance that Nakhla and Lafayette are identical and do not represent independent falls.

Zirconium abundances in meteorites and implications to nucleosynthesis

The atomic abundance of Zr has been established at 16± 5 Zr atoms/10 6 Si atoms in seventeen ordinary chondrites, which agrees well with Aller's (1962) solar value of 14 Zr atoms/10 6 Si atoms. Apparently, fractionation of Zr has occurred within the chondritic family, since 22 ± 3 Zr atoms/10 6 Si atoms were observed in seven Type II and Type III carbonacceous chondrites. Zr has been enriched in four Ca-rich achondrites by a factor of five, which is consistent with other trace-element enrichments. Zr may be depleted in Ca-poor achondrites and pallasitic olivines. The low Zr abundance in chondrites, coupled with Rb, Sr, Y and Mo chondritic abundances, indicates ~50 per cent enhancement of the odd-A mass distribution at A = 89 due to the neutron-shell closure. Abundance values of Zr and other nearby elements indicate Fe 56 seed nuclei were exposed to different neutron fluxes. The atomic ratio Zr/Hf of 100 in meteorites compared with 106 in the terrestrial crust indicates no serious fractionation between these two very similar chemical elements.

Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter—II

Abundances and selected isotopic ratios of the 14 rare-earth elements (REE), yttrium, and scandium have been determined by neutron-activation analysis in 13 meteorites and 2 terrestrial specimens: 3 carbonaceous, 2 hypersthenic, and 1 bronzitic (troilite phase) chondrite; 2 calcium-rich and 2 nakhlitic achondrites; 1 mesosiderite; 2 pallasites; an Australian eclogite; and a Columbia Plateau basalt. This research is a continuation of recent REE work by Schmitt et al. (1963a). Absolute abundances of the REE and Sc in 2 Type I carbonaceous chondrites are about 33 per cent less compared with the Type II. Atomic ratios of Y 10 6 Si in carbonaceous chondrites remain approximately constant at 4.7 (Type I), 4.1 (Type II), and 4.8 (Type III), and ratios of the REE (La is representative) yielded La 10 6 Si at 0.36 (Type I), 0.53 (Type II), and 0.51 (Type III). The REE, Y, and Sc contents in the troilite-phase of a chondrite is ~0.3 of the content in the entire chondritic matrix. REE and Y abundances in Ca-rich achondrites and nakhlites are ~10 and ~5 times larger, respectively, than found in ordinary chondrites. No fractionation of the REE and Y distribution was observed in Ca-rich achondrites compared to chondritic REE and Y; fractionation in nakhlites is similar to terrestrial basalts. Fractionation of the REE and Y (Eu is enriched) in the mesosiderite Veramin and in two pallasites (Eu depleted in one pallasite) has been found. The observed fractionation of the REE and Y in Australian eclogite is opposite to fractionation in African eclogite, and fractionation in Columbia Plateau basalt was similar to Kilauea basalt.

The isotopic composition of strontium and the age of stone meteorites—I

New measurements of the Rb and Sr content and Sr isotope composition are reported for four Ca-rich achondrites and five bronzite or hypersthene chondrites. All the achondrites have identical 87Sr 86Sr ratios; one chondrite, Beardsley, is very much enriched in Rb relative to the other chondrites. Several alternative interpretations of the data are discussed; all agree on a period of chemical differentiation 4.3 to 4.7 Aeons ago (Aeon = 10 9 yrs.) in the parent body of the meteorites.