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

Whole rock serpentinites, disseminated oxides separated from serpentinites, and podiform chromitites, all from the mantle portions of the circa 1.95 G a Jormua ophiolite complex (JOC), were analyzed for their Re-Os systematics . The concentrations of Os in the serpentinites are generally consistent wi th concentrations assumed for the modern convecting upper mantle. Re abunda nces, however, are highly variable. Most samples have concentrations that a re lower than that assumed for the convecting upper mantle, although some a re enriched. The enrichments probably reflect recent additions of Re. Indee d, 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 behavio r and the major element compositions of the oxides, Because of their genera lly homogeneous calculated initial Os isotopic compositions and extremely l ow Re/Os ratios, the disseminated oxides from the Antinmaki block of the JO C probably reflect closed-system Re-Os isotopic behavior. These samples hav e an average calculated initial gamma(Os) of -5.1 +/- 0.8. In addition, sev eral whole rock and oxide samples from different tectonic blocks within the JOC have very depleted present-day Os-187/Os-188 Of < 0.110, requiring a m aximum gamma(Os) of -4 at 1.95 Ga. The presence of such Os-187-depleted mat erials in the JOC requires the contribution of Os from a mantle reservoir t hat evolved with a significantly subchondritic Re/Os for at least 1 billion years prior to the formation of the JOG. Today, such strongly negative gam ma(Os) values are observed only in ancient subcontinental lithospheric mant le. Consequently, the new results may indicate the incorporation of late Ar chean subcontinental lithospheric mantle during the formation of the JOG. I n contrast to the results for the oxides from the Antinmaki block, multiple samples from two chromitite boulders found in the central portion of the J OC have Re-Os systematics that are each consistent with closed-system behav ior and calculated initial gamma(Os) of approximately 0 and +3. Thus, it is possible that the JOC incorporated both ancient subcontinental lithospheri c mantle and a more MORE-like (chondritic) mantle. These results demonstrat e that large Os isotopic heterogeneities were well established in the upper mantle by ca. 2 Ga, and that these isotopically disparate reservoirs becam e intermingled during cratonic rifting (which is the inferred setting of th e JOG). (C) 2000 Elsevier Science B.V. All rights reserved.