The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (delta(17)O = 0.95 * delta(18)O - 3.24; R(2) = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo(99.6-99.8); delta(18)O = -7.2 parts per thousand to -5.5 parts per thousand; delta(17)O = -9.6 parts per thousand to -8.2 parts per thousand) are more (16)O-rich than the isolated olivine grains (Fo(39.6-86.8); delta(18)O = 3.1 parts per thousand to 5.1 parts per thousand; delta(17)O = -0.3 parts per thousand to 2.2 parts per thousand), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo(97.2-99.8)) with oxygen-isotope compositions (delta(18)O = -5.2 parts per thousand to 5.9 parts per thousand; delta(17)O = -8.1 parts per thousand to 1.2 parts per thousand) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Delta(17)O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with (16)O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 +/- 0.1, R(2) = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM(slope) = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with (16)O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to Cl carbonaceous chondrites. (C) 2010 Elsevier Ltd. All rights reserved.