Podiform chromitites in ophiolite usually are interpreted as the crystallization products of mafic magmas, contemporaneous with the generation of the ophiolite at mid-oceanic ridges or back-arc spreading centres. However, the real ages of the chromitites can rarely be determined directly, because their chemistry provides few opportunities for geochronology. Here we employ the 187Re- 188Os decay system (N-TIMS) to date the chromitite and a cross-cutting troctolite in the Sartohay ophiolite, northwestern China, and we have dated zircons separated from the troctolite by SHRIMP U-Pb methods. Inherited zircons from the troctolite yield a lower intercept age of 0.48±0.08Ga and an upper intercept age of 2.28±0.11Ga; two concordant grains give ages >2.4Ga. Whole-rock Re-Os data for the troctolites and chromites plot between 2.45Ga and 0.43Ga reference isochrons. Plots of T MA vs 187Os/ 188Os suggest mixing between ultra-depleted chromitite and suprachondritic troctolite, mainly affecting the 187Os/ 188Os of the troctolites; subsets of troctolite samples scatter around reference isochrons with ages of 0.4-0.5Ga. The zircon data and the Re/Os data are consistent with published Sm-Nd evidence that the troctolites, and hence the Sartohay ophiolite, were formed in Palaeozoic time. However, the 187Os/ 188Os compositions of the chromites (0.1109±4 to 0.1249±5), give Neo-Proterozoic to Paleo-Archean model ages (T MA=0.8-3.5Ga), indicating extraction from the primitive mantle as early as 3.5Ga, some 3Ga before the formation of the Sartohay ophiolite. A Re-Os apparent isochron age (2.45Ga), the old T Ma model ages and the ancient zircon ages (>2.4Ga) all are consistent with part formation of the Sartohay chromitite in Archean time, and then reworked in the Palaeozoic time. We suggest that a volume of early Archean depleted mantle remained within the Siberian lithospheric mantle for 2.5-3.0Ga; it later became portions of the mantle wedge above the Paleozoic subduction zone of the Paleo-Asian Ocean, and ultimately was incorporated into the overthrust ophiolite.