Implications of Os-187 isotopic heterogeneities in a mantle plume: Evidence from Gorgona Island and Curacao

Recent work has suggested that the mafic-ultramafic volcanism in evidence t hroughout portions of the Caribbean, Central America, and northern South Am erica, including the islands of Gorgona and Curacao, was generated as part of a middle-Cretaceous, large igneous province. New Re-Os isochron results for tholeiitic basalts from Gorgona and Curacao indicate crystallization ag es of 89.2 +/- 5.2 and 85.6 +/- 8.1 Ma, respectively, consistent with repor ted Ar ages. The Gorgona ultramafic suite shows a large range in initial Os isotopic com position, with gamma(Os) values ranging from -0.5 to + 12.4. This large ran ge reflects isotopic heterogeneities in the mantle source similar to those observed for modem ocean island basalts. In contrast to ocean island basalt s, however, Os isotopic compositions do not correlate with variations in Nd , Sr, or Pb isotopic compositions, which are within the range of depleted m id-ocean ridge basalts. The processes that produced these rocks evidently r esulted in the decoupling of Os isotopes from the Nd, Sr, and Pb isotopic s ystems. Picrites from Curacao have very uniform, chondritic initial Os isotopic com positions, with initial gamma(Os) values ranging only from -0.4 to +/-1.4. Basalts from Curacao, however, define an isochron with a Os-187-enriched in itial isotopic composition (gamma(Os) = -+9.5). In contrast to the Os-187-e nriched ultramafic rocks from Gorgona, the enrichment in these basalts coul d have resulted from lithospheric contamination. If the Gorgona and Curacao rocks were derived from the same plume, Os results, combined with Sr, Nd, and Pb data indicate a heterogeneous plume, with multiple compositionally a nd isotopically distinct domains. The Os isotopic results require derivatio n of Os from a minimum of two distinct reservoirs, one with a composition v ery similar to the chondritic average and one with long-term enriched Re/Os . Oceanic crustal recycling has been invoked to explain most of the Os-187 en richments that have been observed in ocean island basalt sources and could potentially apply to the Gorgona suite. Crustal recycling, however, require s large proportions of very ancient recycled basaltic crust in the sources of the Os-187-enriched ultramafic rocks to explain the magnitude of Os-187 enrichments observed. For example, addition of 20% oceanic crust to fertile mantle, and nearly 3 billion years are necessary to generate a reservoir w ith the Os isotopic composition of the most radiogenic komatiites. If the r ecycled oceanic crust was added to basalt-depleted mantle, as may be indica ted by epsilon(Nd) values for the komatiites averaging about +10, even larg er proportions of older crust are required. Large proportions of oceanic ma fic crust in the sources of the Os-187-enriched komatiites, although petrol ogically conceivable under certain melting conditions, is unlikely here giv en the limited trace element and lithophile isotope system variations. Thes e results raise questions about the efficacy of using Os isotopes to constr ain the proportion of recycled oceanic crust in other plumes. Other possibl e mechanisms for generating Os-187-enriched mantle include invoking the exi stence of a Os-187-enriched lower mantle, and minor outer core-lower mantle interactions. Copyright (C) 1999 Elsevier Science Ltd.