The Bencubbin chondrite breccia and its relationship to CR chondrites and the ALH85085 chondrite

— Bencubbin is an unclassified meteorite breccia which consists mainly of host silicate (∼40 vol.%) and host metal (∼60%) components. Rare (< 1%) ordinary chondrite clasts and a dark xenolith (formerly called a carbonaceous chondrite clast) are also found. A petrologic study of the host silicates shows that they have textures, modes, mineralogy and bulk compositions that are essentially the same as that of barred olivine (BO) chondrules, and they are considered to be BO chondritic material. Bulk compositions of individual host silicate clasts are identical and differ only in their textures which are a continuum from coarsely barred, to finely barred, to feathery microcrystalline; these result from differing cooling rates. The host silicates differ from average BO chondrules only in being angular clasts rather than fluid droplet-shaped objects, and in being larger in size (up to 1 cm) than most chondrules; but large angular to droplet-shaped chondrules occur in many chondrites. Bencubbin host metallic FeNi clasts have a positive Ni-Co trend, which coincides with that of a calculated equilibrium nebular condensation path. This appears to indicate a chondritic, rather than impact, origin for this component as well. The rare ordinary chondrite clast and dark xenolith also contain FeNi metal with compositions similar to that of the host metal. Two scenarios are offered for the origin of the Bencubbin breccia. One is that the Bencubbin components are chondritic and were produced in the solar nebula. Later brecciation, reaggregation and minor melting of the chondritic material resulted in it becoming a monomict chondritic breccia. The alternative scenario is that the Bencubbin components formed as a result of major impact melting on a chondritic parent body; the silicate fragments were formed from an impact-induced lava flow and are analogous to the spinifex-textured rocks characteristic of terrestrial komatiites. Both scenarios have difficulties, but the petrologic, chemical and isotopic data are more consistent with Bencubbin being a brecciated chondrite. Bencubbin has a number of important chemical and isotopic characteristics in common with the major components in the CR (Renazzo-type) chondrites and the unique ALH85085 chondrite, which suggests that their major components may be related. These include: (1) Mafic silicates that are similarly Mg-rich and formed in similar reducing environments. (2) Similarly low volatiles; TiO2, Al2O3 and Cr2O3 contents are also similar. (3) Similar metallic FeNi compositions that sharply differ from those in other chondrites. (4) Remarkable enrichments in 15N. (5) Similar oxygen isotopic compositions that lie on the same mixing line. Thus, the major components of the Bencubbin breccia are highly similar to those of the ALH85085 and CR chondrites and they may have all formed in the same isotopic reservoir, under similar conditions, in the CR region of the solar nebula.

[1]  C. Pillinger,et al.  ALH 85085: Nitrogen isotope analysis of a highly unusual primitive chondrite , 1990 .

[2]  M. Prinz,et al.  Petrology of ALH85085: a chondrite with unique characteristics , 1988 .

[3]  A. Rubin,et al.  ALH85085: a unique volatile-poor carbonaceous chondrite with possible implications for nebular fractionation processes , 1988 .

[4]  M. Prinz,et al.  Chondrule Texture/Composition Relations Revisited: Constraints on the Thermal Conditions in the Chondrule Forming Region , 1988 .

[5]  E. Scott.  A new kind of primitive chondrite, Allan Hills 85085 , 1988 .

[6]  M. Weisberg.  Barred olivine chondrules in ordinary chondrites , 1987 .

[7]  M. Weisberg.  Barred Olivine Chondrules in Carbonaceous Chondrites , 1986 .

[8]  C. Pillinger,et al.  Heavy nitrogen in Bencubbin—a light-element isotopic anomaly in a stony-iron meteorite , 1986, Nature.

[9]  R. Clayton,et al.  A Striking Nitrogen Isotope Anomaly in the Bencubbin and Weatherford Meteorites , 1985, Science.

[10]  Jeffrey N. Grossman,et al.  Refractory precursor components of Semarkona chondrules and the fractionation of refractory elements among chondrites , 1983 .

[11]  E. Scott,et al.  Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules , 1982 .

[12]  J. Wasson,et al.  Evidence for primitive nebular components in chondrules from the Chainpur chondrite , 1982 .

[13]  F. Robert,et al.  The concentration and isotopic composition of hydrogen, carbon and nitrogen in carbonaceous meteorites☆ , 1982 .

[14]  J. Wood.  The interstellar dust as a precursor of Ca, Al-rich inclusions in carbonaceous chondrites , 1981 .

[15]  K. Keil,et al.  Chondrules in H3 chondrites: textures, compositions and origins , 1981 .

[16]  K. Keil,et al.  RELATIVE ABUNDANCES OF CHONDRULE PRIMARY TEXTURAL TYPES IN ORDINARY CHONDRITES AND THEIR BEARING ON CONDITIONS OF CHONDRULE FORMATION , 1981 .

[17]  K. Keil,et al.  Elemental abundances in chondrules from unequilibrated chondrites: Evidence for chondrule origin by melting of pre-existing materials , 1980 .

[18]  A. Tsuchiyama,et al.  Experimental reproduction of textures of chondrules , 1980 .

[19]  J. Wasson,et al.  Composition of the metal phases in ordinary chondrites - Implications regarding classification and metamorphism , 1980 .

[20]  J. Delaney,et al.  Modal studies of mesosiderites and related achondrites, including the new mesosiderite ALHA 77219 , 1980 .

[21]  J. Lattimer,et al.  Silicon in Carbonaceous Chondrite Metal: Relic of High-Temperature Condensation , 1979, Science.

[22]  H. Newsom,et al.  The origin of metal clasts in the Bencubbin meteoritic breccia , 1979 .

[23]  W. Boynton,et al.  Formation of the Bencubbin polymict meteoritic breccia , 1978 .

[24]  R. Clayton,et al.  Nitrogen abundances and isotopic compositions in stony meteorites , 1978 .

[25]  H. McSween.  Chemical and petrographic constraints on the origin of chondrules and inclusions in carbonaceous chondrites , 1977 .

[26]  H. McSween.  Petrographic variations among carbonaceous chondrites of the Vigarano type , 1977 .

[27]  S. Saxena,et al.  Formation of Fe–Ni–Si planetary cores , 1977, Nature.

[28]  A. J. Naldrett,et al.  Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario , 1977 .

[29]  E. Roedder,et al.  Barred olivine 'chondrules' in lunar spinel troctolite 62295 , 1977 .

[30]  H. Palme,et al.  A metal particle from a Ca,Al-rich inclusion from the meteorite Allende, and the condensation of refractory siderophile elements , 1976 .

[31]  C. Donaldson.  An experimental investigation of olivine morphology , 1976 .

[32]  C. Donaldson.  Olivine Crystal Types in Harrisitic Rocks of the Rhum Pluton and in Archean Spinifex Rocks , 1974 .

[33]  E. Olsen,et al.  Origin of the high-temperature fraction of C2 chondrites , 1974 .

[34]  A. Albee,et al.  Empirical Correction Factors for the Electron Microanalysis of Silicates and Oxides , 1968, The Journal of Geology.

[35]  J. Nelen,et al.  The weatherford meteorite , 1968 .

[36]  G. Mccall.  The Bencubbin Meteorite: Further Details, Including Microscopic Character of Host Material and Two Chondrite Enclaves , 1968 .

[37]  R. Binns.  An Exceptionally Large Chondrule in the Parnallee Meteorite , 1967 .

[38]  J. Wood.  Chondrites - Their metallic minerals, thermal histories, and parent planets. , 1967 .

[39]  J. Lovering,et al.  The analysis of chondritic meteorites , 1963 .

[40]  E. Simpson.  A New Siderolite from Bencubbin, Western Australia , 1932 .