SMOW indicates the standard mean ocean water.

13. The polished sample was coated with 30 nm of gold film for SIMS analysis to eliminate the electrostatic charge on the sample surface. Oxygen isotope ratios were measured with a modified Cameca ims 1270 SIMS of TiTech with a high mass resolution technique. The primary ion beam was mass filtered positive Cs ions accelerated to 10 keV and the beam spot size was �3 �m in diameter. The primary current was adjusted for each measurement to obtain the count rate of negative 16O ions of �4 � 105 cps. A normal-incident electron gun was utilized for charge compensation of the analysis area. Negative secondary ions from the 16O tail, 16O, 17O, 16OH and 18O were analyzed at a mass resolution power of �6000, sufficient to completely eliminate hydride interference. Secondary ions were detected by an electro

10. N. Kita et al., inChondrites and the Protoplanetary Disk,

pp. 558–587. 11. Materials and methods are available as supporting material on Science Online. 12. Y. Amelin, A. N. Krot, I. D. Hutcheon, A. A. Ulyanov