Re-Os isotopes in the Horoman peridotite: Evidence for refertilization?

Re-Os isotopic data for 20 samples from a well-characterized 140 m section across a layered sequence, ranging from plagioclase lherzolite through lher zolite to harzburgite, of the Horoman peridotite show: (1) a range in Os-18 7/Os-188 ratios (from 0.1158 to 0.1283) similar to that reported for other peridotitic massifs, thereby suggesting that the processes responsible for the Re-Os isotopic variation at the meter-scale and the whole-massif scale are similar; (2) that the Os isotopic ratio is controlled by the Re content through radiogenic ingrowth over a period of similar to0.9 Gy. The ultrama fic and some of the mafic rocks (Type I layers)from the Horoman massif defi ne an 'apparent age' of 1.12 +/- 0.24 Ga in the Re-OS isochron diagram, wit hin error of the previously reported age of 833 +/- 78 Ma based on Sm-Nd is otopes. Although the Re-Os isotopic data do not define an isochron, the con sistency of the similar to 900 Ma age defined by both isotopic systems sugg ests that this age has a geologic meaning and that mafic (Type I layers) an d ultramafic rocks are genetically related, A plausible explanation for the genetic relationship between the mafic and ultramafic rocks, the meter-sca le compositional variations from lherzolite to plagioclase lheriolite, the suprachondritic Re-187/Os-188 ratios in some fertile peridotites, and the o ldest RE depletion model age of similar to1.86 Ga obtained for Horoman rock s is a refertilization process involving reaction of a mid-ocean ridge basa lt-like magma with depleted lithospheric mantle at similar to 900 Ma.