@Article{Choi:2003, author = {B. Choi and J.T. Wasson}, title={Microscale oxygen isotopic exchange and formation of magnetite in the {N}ingqiang anomalous carbonaceous chondrite}, journal={Geochim. Cosmochim. Acta.}, year={2003}, volume={67}, number={}, month={Dec}, pages={4655--4660}, note={}, annote={}, keywords={}, url={http://sims.ess.ucla.edu/PDF/Choi_Wasson_GCA_2003.pdf}, doi={10.1016/S0016-7037(03)00390-9}, abstract={We report in situ measurements of O-isotopic compositions of magnetite, olivine and pyroxene in chondrules of the Ningqiang anomalous carbonaceous chondrite. The petrographic setting of Ningqiang magnetite is similar to those in oxidized-CV chondrites such as Allende, where magnetite is found together with Ni-rich metal and sulfide in opaque assemblages in chondrules. Both magnetite and silicate oxygen data fall close to the carbonaceous-chondrite-anhydrous-mineral line with relatively large ranges in $\delta$$^{18}$O in magnetite (-4.9 to -4.2$\permil$) and in silicates (-15.2 to -4.5$\permil$). Magnetite and silicates are not in O-isotopic equilibrium: the weighted average $\Delta$$^{17}$O (=$\delta$$^{17}$O - 0.52 $\times$ $\delta$$^{18}$O) values of magnetite are 1.7 to 3.6$\permil$ higher than those of the silicates in the same chondrules. The petrological characteristics and O-isotopic disequilibrium between magnetite and silicates suggest the formation of Ningqiang magnetite by the oxidation of preexisting metal grains by an aqueous fluid during parent body alteration. The weighted average $\Delta$$^{17}$O of -3.3 $\pm$ 0.3$\permil$ is the lowest magnetite value measured in unequilibrated chondrites and there is a positive correlation between $\Delta$$^{17}$O values of magnetite and silicates in each chondrule. These observations indicate that, during aqueous alteration in the Ningqiang parent asteroid, the water/rock ratio was relatively low and O-isotopic exchange between the fluid and chondrule silicates occurred on the scale of individual chondrules.} }