Re–Os and S systematics of spinel peridotite xenoliths from east central China: Evidence for contrasting effects of melt percolation

Abstract

Abstract Os isotopic compositions and Re and Os concentrations were determined for Cenozoic basalt-borne ultramafic xenoliths from the Subei Basin of east central China. Re–Os analyses were coupled with whole rock major and trace element and S abundance determinations, and with characterization of rock textures, modal phase proportions and sulfide petrography. The two main sampling areas, Lianshan and Panshishan, separated by only 6 km, have similar textures and major and moderately incompatible lithophile trace element compositions. The Os isotopic ratios of these two areas plot on the same trends relating 187Os/188Os to indices of melt extraction such as whole rock Yb content. These Os isotopic systematics suggest that both areas were affected by an early Proterozoic (∼1.8 Ga) melt extraction event. Thus the two areas apparently shared the same long term lithospheric history. Nevertheless, the sulfide abundances and whole rock S, Os and Re concentrations are strikingly lower in Lianshan than in Panshishan, and the two localities have different incompatible lithophile trace element signatures. These differences resulted from contrasting melt percolation styles between the two areas. Lianshan was affected by extensive percolation of sulfur undersaturated melts that removed Re, Os and S, while Panshishan experienced interaction with low degree or highly evolved melts that added Re, Cu and S, but had no affect on Os abundances. The lack of correlation between 187Os/188Os and 187Re/188Os, compared with the good correlation between 187Os/188Os and Yb, indicates that the perturbation of the Re and Os concentrations was fairly recent, and perhaps related to Mesozoic or Cenozoic lithospheric thinning in eastern China. The Lianshan Os concentrations are typical of those of off-cratonic mantle xenoliths, while the Panshishan Os concentrations are closer to those of orogenic peridotites. This suggests that the low Os concentrations, and by extension, the low concentrations of all of the highly siderophile elements (HSE) typically observed in ultramafic mantle xenoliths, may result from recent melt percolation processes, probably directly or indirectly related to the magmatism that brought the xenoliths to the surface. Thus ultramafic xenoliths may not provide reliable estimates of the HSE contents of the upper mantle, and variations in HSE abundances between xenolith localities should not be used to define global scale processes. Os and other HSE abundances may prove to be sensitive indicators of melt percolation, and may provide information about the degree of sulfur saturation of the melts. Despite the loss of Os during recent percolation, the Os isotopic ratios of most samples are nearly unmodified by this process, confirming the utility of this system for dating ancient melt extraction events.