40Ar/39Ar age of plagioclase from Acapulco meteorite and the problem of systematic errors in cosmochronology

The Os isotopic and highly siderophile element (HSE) concentration systematics of the upper mantle have been the focus of much recent interest. However, little systematic study has addressed the combined HSE and Os isotopes in mantle rocks from MOR ophiolites. The Ligurian ophiolites in northern Italy represent an important class of ophiolites representing, like Zabargad or the Galicia margin, crust with clear ocean ridge affinity floored by older mantle with arguably more continental affinities [Rampone et al., J. Petrol. 36, 18¯105, 1995; Rampone et al., Contrib. Mineral. Petrol. 123, 61¯67, 1996]. We have studied a suite of 15 geochemically and geologically well characterized mantle peridotites from this ophiolite. The samples have Os isotopic and HSE concentration characteristics in agreement with similar measurements made in abyssal peridotites [Snow and Schmidt, Nature 391, 166¯169, 1998]. The observations are consistent with the notion that the depleted mantle shows a HSE signature consistent with the influence of core-derived metal. Robust age information could not be obtained for either body, however the data from the External unit are consistent with a Proterozoic melting age similar to the Ronda ultramafic massif. Os¯Nd isotopic correlations observed are not, however, consistent with the evolution of variably depleted mantle. Such Os¯Nd correlations could be modeled by addition of recycled sediment to depleted mantle, however this disagrees with current notions of the origin of the depleted mantle. Variable 187Os/188Os with essentially constant Al2O3 in mantle rocks can be explained by recent remelting of ancient mantle.