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

Brandon, A.D., Walker, R.J., Puchtel, I.S., Becker, H., Humayun, M. and Revillon, S. (2003) 186 Os-187 Os systematics of Gorgona Island komatiites: implications for early growth of the inner core. Earth and Planetary Science Letters, 206 (3-4), 411-426. (doi:10.1016/S0012-821X(02)01101-9).

Abstract

The presence of coupled enrichments in 186Os/188Os and 187Os/188Os 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 186Os and 187Os, with 186Os/188Os ranging between 0.1198397±22 and 0.1198470±38, and with ?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 modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186Os/188Os of approximately 0.119870 and ?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 ? Os. The 187Os- and 186Os-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 186Os/188Os and ?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 paleo-intensity measurements as data sources that constrain models based on secular cooling of the Earth.

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