Re-Os and Pt-Os Isotopic Systematics of 3.81 Ga Metamorphosed Pillow Basalts and Layered Mafic/Ultramafic Units in the Isua Supracrustals Belt (W Greenland) and Adjoining Gneiss Areas.

Re-Os isotopic data from basalts formed at different times in Earth's evolution yield a fairly clear picture of how upper mantle magma sources have evolved through time. However, significant uncertainty remains with regard to mantle evolution in the Pt-Os system. Because of the different siderophile/chalcophile properties of daughter and parents, these two isotope systems provide sensitive information about mantle processes involving metal-silicate or sulfide-silicate reactions. The presence of coupled enrichments in ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os in some mantle-derived materials reflect long-term elevation of Pt/Os and Re/Os relative to the primitive upper mantle. In one scenario, such coupled enrichments are thought to derive from interaction between outer core and lower mantle material that subsequently is transported to the upper mantle by deep-rooted plume systems. Within the framework of this model, existing data from young (recent) as well as ancient (mid to late Archean) plume-derived magmas imply that crystallization of the inner core is likely to have happened fast and to have been completed already in the Hadean period. In order to precisely discern different core crystallization schemes, it is of vital importance to obtain coupled Os isotopic data on very early Archean mantle-derived rocks. Re-Os isotope data of the least altered pillow basalts from the NE section of the Isua Supracrustals Belt (ISB; W Greenland) yield an average initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1042 \pm 0.0008 (2\sigma; n=7) at 3.81 Ga, which is the minimum extrusion age of these basalts. This corresponds to a γOs[T=3.81 Ga] value of + 3.4 \pm 0.8, which is broadly consistent with a value of +1.3 \pm 2.5 for chromites from layered mafic/ultramafic units ~20 km to the S of the ISB and from the ISB itself (Frei and Jensen, in press). These data suggest that parts of the early Archean mantle was characterized by a slightly suprachondritic 187Os/188Os ratio. Two high-precision unspiked Os isotopic analyses of chromites of the units S of the ISB yielded suprachondritic ¹⁸⁶Os/¹⁸⁸Os ratios of 0.119838 \pm 0.00007 (2\sigma_pop) and 0.119837 \pm 0.000008 (2\sigma_pop). In situ correction of these ratios with appropriately obtained Pt and Os concentrations from spiked aliquots lead to consistent slightly suprachondritic ¹⁸⁶Os/¹⁸⁸Os ratios of 0.119831 \pm 0.000008 at 3.81 Ga, which at this stage cannot discriminate between a chondritic (or primitive upper) mantle evolution and inner core crystallization models that involve rapid, delayed, or constant growth schemes along with their possible effects on the PGE budget of the early Earth's mantle. Frei, R., and Kastbjerg Jensen, B., in press, Chemical Geology.