Re–Os isotopic systematics of the 1.95 Ga Jormua Ophiolite Complex, northeastern Finland

Abstract

Whole rock serpentinites, disseminated oxides separated from serpentinites, and podiform chromitites, all from the mantle portions of the circa 1.95 Ga Jormua ophiolite complex (JOC), were analyzed for their Re–Os systematics. The concentrations of Os in the serpentinites are generally consistent with concentrations assumed for the modern convecting upper mantle. Re abundances, however, are highly variable. Most samples have concentrations that are lower than that assumed for the convecting upper mantle, although some are enriched. The enrichments probably reflect recent additions of Re. Indeed, most of the whole rock samples and some of the oxide separates examined show clear evidence of open-system isotopic systematics dominated by recent additions of Re. There is no apparent relation between open-system behavior and the major element compositions of the oxides. Because of their generally homogeneous calculated initial Os isotopic compositions and extremely low Re/Os ratios, the disseminated oxides from the Antinmäki block of the JOC probably reflect closed-system Re–Os isotopic behavior. These samples have an average calculated initial γ Os of −5.1±0.8. In addition, several whole rock and oxide samples from different tectonic blocks within the JOC have very depleted present-day 187 Os/ 188 Os of <0.110, requiring a maximum γ Os of −4 at 1.95 Ga. The presence of such 187 Os -depleted materials in the JOC requires the contribution of Os from a mantle reservoir that evolved with a significantly subchondritic Re/Os for at least 1 billion years prior to the formation of the JOC. Today, such strongly negative γ Os values are observed only in ancient subcontinental lithospheric mantle. Consequently, the new results may indicate the incorporation of late Archean subcontinental lithospheric mantle during the formation of the JOC. In contrast to the results for the oxides from the Antinmäki block, multiple samples from two chromitite boulders found in the central portion of the JOC have Re–Os systematics that are each consistent with closed-system behavior and calculated initial γ Os of approximately 0 and +3. Thus, it is possible that the JOC incorporated both ancient subcontinental lithospheric mantle and a more MORB-like (chondritic) mantle. These results demonstrate that large Os isotopic heterogeneities were well established in the upper mantle by ca. 2 Ga, and that these isotopically disparate reservoirs became intermingled during cratonic rifting (which is the inferred setting of the JOC).