Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas

Re-Os isotopes have been measured on postorogenic potassic lavas from the T ibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Pl ateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinen tal lithospheric mantle peridotites typically have subchondritic Os-187/Os- 188; however, the postorogenic lavas are characterized by radiogenic Os-187 /Os-188 ratios (0.139-0.559). Simple modeling shows that only very large de grees of melt depletion (>25%-30%) can lower source Re/Os ratios sufficient ly to permit time-integrated development of subchondritic Os-187/Os-188 rat ios. Such processes may have been largely restricted to the older Precambri an, and the peridotite component of the postorogenic lavas source was proba bly depleted by <25%. The more radiogenic values may reflect increasing con tributions from metasomatic components or possibly crustal contamination. O ur findings imply the need for caution in the use of Os isotopes as a diagn ostic tracer of subcontinental lithospheric mantle contributions to lavas e rupted through younger Proterozoic and Phanerozoic lithosphere.