Ordinary and Carlisle Lakes-like Chondrite Clasts in the Weatherford Chondrite Breccia

Details Ordinary and Carlisle Lakes-like Chondrite Clasts in the Weatherford Chondrite Breccia Ordinary and Carlisle Lakes-like Chondrite Clasts in the Weatherford Chondrite Breccia

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.419p&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 419

Mathematics

Logic

1

Chondrites, Carlisle Lakes-Type, Chondrules, Clasts, Ordinary Chondrites, Oxygen Isotopes

Scientific paper

Weatherford is similar to Bencubbin [1], which was shown to be a highly unusual chondrite breccia [2]. These meteorites have similar oxygen and nitrogen isotope compositions [3,4] and are members of the CR clan [5], which includes ALH 85085, Acfer 182, LEW 85332, and CR chondrites. Both meteorites contain various chondritic clasts, but ordinary chondrite xenoliths have previously been found only in Bencubbin [6]. We have now found an ordinary chondrite xenolith, as well as a Carlisle Lakes-like [7,8] clast in Weatherford. Herein we discuss the findings of our petrologic and oxygen isotope study of these clasts and their significance. Ordinary Chondrite Clast: A 1.7 x 1.0-cm clast, in AMNH sample 4713, has sharp contacts with the host chondrite and one edge extends off the edge of the sample. It has about 85% chondrules, 1% matrix, and 14% metal and troilite. The average size of chondrules and chondrule fragments is 0.4 mm; H- chondrite chondrules are estimated at 0.3 mm, and L chondrites at 0.6-0.8 mm [9]. Modally, the chondrite has (in vol%) 39 olivine 36.9 opx, 4.1 cpx, 6.2 plag, 0.3 chromite, 7.6 FeNi, and 5.9 FeS, very similar to that of H chondrites [10]. Mineralogically, olivine is homogeneous at Fa(sub)17, orthopyroxene is zoned from Wo(sub)0.5-0.8 Fs(sub)7-16, clinopyroxene (often rimming opx) is Wo(sub)31 Fs(sub)20, plagioclase is glassy to devitrified and albitic, and FeNi is homogeneous, with 10.5% Ni, 0.45 Co, <0.02 P. The oxygen isotope composition of the clast is delta ^18O = 3.40, delta ^17O = 1.86, Delta ^17O = 0.09, giving it lighter oxygen than any other ordinary chondrite (Fig. 1). The clast is classified as an H>3.5 ordinary chondrite with unusual oxygen isotopic composition. Carlisle Lakes-like Clast: Small clasts of Fe-rich, olivine-rich, material are found in the Mg-rich Weatherford host chondrite. Clasts range from 50 micrometers to 2.5 mm are completely recrystallized and equilibrated, and no chondrules were observed. Modally, they have (in vol%) 74.4 olivine, 5.2 clinopyroxene, 13.5 plagioclase, 0.7 chromite, and 6.2 pyrrhotite/pentlandite; no orthopyroxene was found. Olivine is Fa(sub)41, clinopyroxene is Wo(sub)43 Fs(sub)14, plagioclase is albitic, and chromite has high TiO2 (5.5%) and low Al2O3 (6%). Petrologically, clasts are very similar to Carlisle Lake-type chondrites, except they are highly equilibrated (type 6) and contain no chondrules. They could not be derived from the ordinary chondrite clast (with olivine at Fa(sub)17) or the host chondrite (with Fa(sub)3). Clasts large enough for oxygen isotopic analysis have not yet been found. Conclusions: Weatherford is a remarkable chondrite breccia. The ordinary chondrite clast in Weatherford differs from the one in Bencubbin in petrologic grade (Bencubbin clast is H<3.5) as well as oxygen isotope composition (Fig. 1). Both clasts have unusual oxygen that differs from any other ordinary chondrite. Carlisle Lakes-like clasts have not yet been found in Bencubbin. The presence of three kinds of unusual chondritic materials in Bencubbin/Weatherford improves our knowledge of primitive chondritic materials by allowing us to study meteorites not represented in our current sampling. References: [1] Mason B. and Nelen J. (1968) GCA, 32, 661-664. [2] Weisberg M. K. et al. (1990) Meteoritics, 25, 269-279. [3] Clayton R. N. and Mayeda T. K. (1978) GCA, 41, 1777-1790. [4] Prombo C. A. and Clayton R. N. (1985) Science, 230, 935-937. [5] Prinz M. et al. (1993) LPS XXIV, 1185-1186. [6] Lovering J. F. (1962) In Researches on Meteorites (C. B. Moore, ed.), 179-197. [7] Rubin A. E. and Kallemeyn G. W. (1989) GCA, 53, 3035-3044. [8] Weisberg M. K. et al. (1991) GCA, 55, 2657-2669. [9] Grossman J. N. et al. (1988) In Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), 619-659. [10] Van Schmus W. R. (1969) Earth Sci. Rev., 5, 145-184. Fig. 1 appears here in the hard copy.

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