Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 mu g) with min or amounts of adhering matrix were separated from the Allende meteorite. Af ter bulk chemical analysis by instrumental neutron activation analysis (INA A), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well-defined unique com position, different from average Allende matrix in having higher Cr and low er Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory ele ments Ca, Al, Sc, and rare-earth elements (REE), although some of these ele ments, such as Al, do not quantitatively reside in forsterite; whereas othe rs (e.g., Ca) are intrinsic to forsterite. The chondritic refractory elemen t ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca-Al-Ti-rich glass inclusions suggest a gene tic relationship of refractory condensates with forsteritic olivine. The Ca -Al-Ti-rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chon drules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forst erite of 0.2-0.4 wt% imposes an upper limit of similar to 1 wt% of FeO on t he parental melt, too low for ordinary and carbonaceous chondrule melts. (c ) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivin es in all classes of meteorites. The observed Mn contents of refractory for sterite are much too low for equilibrium between olivine and melts of chond rule composition. (d) As shown earlier, refractory forsterites have O-isoto pic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in c hondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 199 2). As refractory forsterite cannot have formed by crystallization from cho ndrule melts, we conclude that refractory forsterite fi om chondrules are r elic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of ref ractory forsterite by condensation from an oxidized nebular gas.