RE-OS SYSTEMATICS OF EARLY PROTEROZOIC FERROPICRITES, PECHENGA COMPLEX, NORTHWESTERN RUSSIA - EVIDENCE FOR ANCIENT OS-187-ENRICHED PLUMES

The Re-Os isotopic systematics of various ferropicritic flows and sill s of the Pechenga Complex, Russia, have been examined. During crystall ization about 1.98 Ga ago, many of these bodies became highly differen tiated. In addition, some of the larger igneous units are associated w ith major Ni-Cu ore deposits. The melts that produced these rocks have been termed ferropicritic because of their high FeO and MgO contents. They are also enriched in light rare earth elements (LREEs), TiO2, Zr , and many other incompatible trace elements. Previous studies have co ncluded that the ferropicrites were most likely derived from an Fe-ric h mantle plume that had a complex history of long-term LREE depletion (initial epsilon(Nd) = +1.4), but that also experienced a LREE enrichm ent event within 200 Ma of the generation of the rocks. Whole rock sam ples believed to be most representative of primary melt compositions i ndicate that initial melt concentrations of rhenium and osmium were ap proximately 1.1 ppb and 0.5 ppb, respectively. The high primary melt c oncentrations presumably made the osmium contained in the melts relati vely immune to the effects of crustal contamination. Nonetheless, all ore-bearing intrusions examined show osmium isotopic evidence for crus tal contamination. For example, the initial gamma(Os) for some primary magmatic sulfides from the Pilgujarvi intrusion average +46. Other or e-bearing intrusions, such as the Kammikivi sill, appear to have been similarly contaminated by crustal osmium during the injection of magma , with initial gamma(Os) values as high as +251. The seemingly high le vels of crustal osmium may be attributed to the rapidly diminishing co ncentrations of osmium in the melts as the larger bodies differentiate d, combined with localized in situ assimilation of the metasedimentary rocks that comprise the country rocks. The Re-Os systematics of some whole rock samples of both mineralized and sulfide-poor intrusions wer e affected by post-magmatic events, especially the greenschist grade m etamorphism that impacted the rocks between about 1.7 and 1.8 Ga ago. The metamorphic effects are reflected in the recrystallization of many of the primary sulfides. As a consequence of this open-system behavio r in many whole rock samples, the primary igneous Re-Os systematics of these rocks are best examined via analysis of magmatic phases such as chromite, olivine, clinopyroxene, and primary sulfides. Chromite and ilmenite+sulfide separates from two sulfide-poor lava flows, the Lamma s and Keskitunturi, have characteristically low Re-187/Os-188 (<1), an d because of the limited age correction, precisely define the initial gamma(Os) of these systems to be +6.0+/-0.7. Because of the identical initial compositions of the two, spatially distinct lava flows, and th e fact that these flows were extruded onto only slightly older volcani c rocks, we conclude that the +6.0 value reflects the composition of t he mantle source and not minor crustal contamination. Although Os-187- enriched, plume-derived systems are common during the Phanerozoic, thi s is the earliest known evidence for the existence of long-term, Re-en riched mantle reservoirs. The most commonly invoked model to explain O s-187 enrichments in Phanerozoic systems, oceanic crustal recycling, i n this instance requires that very large proportions of oceanic crust were recycled into the mantle source and that the event was likely ver y ancient. Other options, such as core-mantle interaction and a strati fied mantle, are also discussed. Copyright (C) 1997 Elsevier Science L td.