186Os-187Os systematics of Gorgona Island komatiites:: implications for early growth of the inner core

The presence of coupled enrichments in Os-186/Os-188 and Os-187/Os-188 in some mantle-derived materials reflects long-term elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in Os-186 and Os-187, with Os-186/Os-188 ranging between 0.1198397 +/- 22 and 0.1198470 +/- 38, and with gamma(Os) correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modem Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a Os-186/ Os-188 of approximately 0.119870 and gamma(Os) of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth's interior that mixes with a variety of different mantle compositions distinguished by variations in yo, The Os-187- and 1116 Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic Os-186/Os-188 and gamma(Os) in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal-liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleointensity measurements as data sources that constrain models based on secular cooling of the Earth. Published by Elsevier Science B.V.