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

Abstract

Re-Os isotopic data for 20 samples from a well-characterized 140 m section across a layered sequence, ranging from plagioclase lherzolite through lherzolite, to to harzburgite, of the Horoman peridotile show: (1) a range in 1870s/1880s 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 Ο 0.9 Gy. The ultramafic and some of the mafic rocks (Type I layers) from the Horoman massif define an 'apparent age' of 1.12 ± 0.24 Ga in the Re-Os isochron diagram, within error of the previously reported age of 833 ± 78 Ma based on Sm-Nd isotopes. Although the Re-Os isotopic data do not define an isochron, the consistency of the Ο 900 Ma age defined by both isotopic systems suggests that this age has a geologic meaning and that mafic (Type I layers) and ultramafic rocks are genetically related . A plausible explanation for the genetic relationship between the mafic and ultramafic rocks, the meter-scale compositional variations from lherzolite to plagioclase lherzolite, the suprachondritic 187Re/188Os ratios in some fertile peridotites, and the oldest Re depletion model age of 1.86 Ga obtained for Horoman rocks is a refertilization process involving reaction of a mid-ocean ridge basalt-like magma with depleted lithospheric mantle at 900 Ma.