PROTEROZOIC DEPLETION OF THE LITHOSPHERE RECORDED IN MANTLE XENOLITHS FROM INNER-MONGOLIA

MANTLE xenoliths entrained in alkali basalt provide direct samples from a cross-section of the lithosphere. In most cases, the chemistry and isotope systematics of these objects have suggested a complex history of chemical change by processes such as metasomatism and partial melting (see, for example, ref. 1). Here, however, we present evidence from a suite of xenoliths from alkali basalts in the Chinese province of Inner Mongolia, suggesting that the lithosphere in this region has remained stable and closed to chemical modification since experiencing partial melting in the Proterozoic. Clinopyroxene separates from these xenoliths yield an approximate isochron in the Nd-143/Nd-144 versus Sm-147/Nd-144 diagram, suggesting an episode of depletion approximately 1.6 Gyr ago. Although other explanations for this correlation, such as mixing, cannot be ruled out definitively, further evidence for the long-term undisturbed nature of the lithosphere in this region comes from neodymium model ages of the clinopyroxenes. In particular, one highly depleted sample with very high Sm-147/Nd-144 provides a model age of 1.6 Gyr, consistent with the isochron result.