BULK OXYGEN-ISOTOPE COMPOSITIONS OF EQUILIBRATED ORDINARY CHONDRITE FALLS USING CO2 LASER-BrF5 FLUORINATION TECHNIQUE

We measured O-isotopic compositions of equilibrated ordinary chondrites (EOCs) using the CO 2 laser-BrF 5 fluorination technique at Korea Polar Research Institute. Fifty one EOC falls from UCLA collections were selected for this study. About 30-50 mg from each chondrite was gently crushed using agate mortar and pestle. About 2 mm-size fragment each was used to make polished thick sections. We observed terrestrial weathering products in 5 EOCs. Their O-isotope compositions differ (higher in δ 18 O and lower in Δ 17 O) from those of the others, also indicating that terrestrial contamination. Thus we discuss only the 46 fresh EOCs: 10 H, 17 L, 4 L/LL and 15 LL chondrites. Three or more samples, each about 2-mg of non-magnetic fractions washed with dilute acid were analyzed using the analytical procedures similar to those described in [1]. Results are generally similar to [2] but differ in details. The highlights of the observations follow: (1) our data define a much sharper mixing trend than those of [2]. Variation of δ 18 O at a given Δ 17 O is small in the low Δ 17 O end (H-group) and becomes larger in high Δ 17 O end (LL-group). It is plausible that the correlated changes in O-isotopic composition and degree of oxidation of OCs is related, possibly because the oxidizing agent and the high Δ 17 O materials were in the same water-bearing nebular component [3,4]. It seems possible that the spread in δ 18 O at constant Δ 17 O is related to the partial loss of this component during thermal metamorphism. If we ignore six type-4 LLs and two L/LLs with high δ 18 O (>5‰) the other 38 EOCs form a well defined linear array with a slope close to unity. The line intersects with CCAM line [5] at δ 18 O ≈ δ 17 O ≈-50‰. This is further evidence that the O-isotopic composition of chondritic matter was largely controlled by mixing of two reservoirs that fall along the slope-1 line differing greatly in their Δ 17 O values. (2) Our L and LL data are marginally separated but our L/LL data scatter widely ; we assume that the latter is a stochastic effect. (3) There are correlations between Δ 17 O values and some bulk volatiles and siderophiles [6]; e.g., positively with Ga and Zn, negatively with As, Au, Ir and Os. (4) A positive correlation …