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Journal of Petrology Volume 42 Number 1 Pages 25-37 2001
© Oxford University Press 2001
Re–Os Isotopes in the Horoman Peridotite: Evidence for Refertilization?
1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA
2DEPARTMENT OF GEOLOGY, FACULTY OF SCIENCE, NIIGATA UNIVERSITY, 2-8050 IKARASHI, NIIGATA 950-2181, JAPAN
3DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
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 harzburgite, of the Horoman peridotite show: (1) a range in 187Os/188Os 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 
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 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.
KEY WORDS: Re–Os isotopes; Horoman peridotite
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