2. DEVELOPMENT OF TOOLS FOR THE DETECTION OF CORE-MANTLE INTERACTIONS.

Probably the most sensitive suite of elements for geochemically identifying possible core contributions to plumes are the highly siderophile (iron loving) elements (HSE = Pt, Re, Os, Ir, Pd, Ru, Rh, Au), and certain moderately siderophile elements (MSE) such as Ag and W. The HSE are so termed because of their extreme preference for metal relative to silicate. Because of this characteristic, formation of the core sequestered most of Earth’s HSE and MSE, and much higher concentrations prevail in the core than silicate Earth. For these elements, mass balance is potentially optimal for detection of core additions to the mantle. The high affinity for iron also means that the relative abundances of these elements in the bulk core are probably little fractionated compared to chondritic (primitive) meteorites, or the bulk earth. Formation of the inner core, however, may have fractionated the HSE, as occurred in differentiates of asteroidal cores sampled by iron meteorites.

Discovering a unique isotopic fingerprint of the outer core using HSE and MSE is a promising means of identifying a core component in rocks collected from Earth’s surface. The coupled ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os long-lived isotope systems (¹⁸⁷Re t_½ = 42 b.y.; ¹⁹⁰Pt t_½ = 430 b.y.) may be useful in identifying the presence of an evolved outer core component in mantle-derived rocks (Walker et al., 1995). This hypothesis is based on conclusions from the study of asteroidal core crystallization and both low and high-pressure experimental studies indicating that Pt/Os and Re/Os ratios may be substantially higher in the outer core than in chondritic meteorites or the bulk silicate mantle, as a consequence of inner core crystallization.

If an inner core with substantial mass formed within ~2 b.y. of planetary formation, the outer core could exhibit coupled ¹⁸⁷Os and ¹⁸⁶Os enrichments of >7% and >0.01%, respectively, relative to the other isotopes of Os, as compared to chondrites. The degree of isotopic enrichment would reflect both the cumulative formation age of the inner core, and the magnitude of partitioning of these elements between solid and liquid metal. It is now possible to measure Pt-Re-Os partitioning between liquid and solid Fe metal alloys at relatively high pressures. Work at 100 kbars indicates that the relative and absolute partitioning characteristics of these elements are not sensitive to pressure (Dave Walker, 2000). The magnitude of partitioning, however, is strongly correlated with S and P content. Variable mixing between Os contained in an outer core component and “normal” Os present in a mantle plume could lead to formation of a suite of rocks that define a linear trend on a plot of ¹⁸⁷Os/¹⁸⁸Os versus ¹⁸⁶Os/¹⁸⁸Os.

Predicted effects on evolution of ¹⁸⁶Os/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os in chondrites and the outer core. Trend for outer core is based on iron meteorite analogy and assuming early growth of inner core. Variations in ¹⁸⁶Os/¹⁸⁸Os relative to initial ¹⁸⁷Os/¹⁸⁸Os for volcanic and intrusive rocks from Hawaii, Siberian Traps and Gorgona Island, Colombia.

Coupled enrichments in ¹⁸⁶Os-¹⁸⁷Os similar to those predicted have been detected in putative plume-derived rocks related to the 251 Ma Siberian flood basalt event, the 89 Ma Caribbean Large Igneous Province (e.g Gorgona Island komatiites), and the Hawaiian hotspot (e.g. Brandon et al., 2003). We have tentatively interpreted these results as indicative of core-mantle interaction, with minor portions of these plumes recording chemical or isotopic exchange with the outer core.

Implicit for this interpretation is the assumption that similar isotopic trends do not develop elsewhere in the Earth, such as in the upper mantle. As a result of this concern, we recently completed an examination of the ¹⁸⁶Os-¹⁸⁷Os systematics of Os-Ir-Ru grains presumably derived from the upper mantle. Our work was spurred by recent studies of reported coupled ¹⁸⁶Os-¹⁸⁷Os isotope systematics of alloy grains from Port Orford, Oregon and spatially associated northern California Os-Ir-Ru alloy occurrences (Bird et al., 1999; Meibom and Frei, 2002; Meibom et al., 2004). These studies have reported supra-chondritic ¹⁸⁶Os/¹⁸⁸Os for most of the samples analyzed. The ¹⁸⁶Os-¹⁸⁷Os relations reported by these studies have been argued to be consistent with: a) an outer core origin (Bird et al., 1999), b) derivation from an “ancient” reservoir (Meibom and Frei, 2002), or c) derivation from upper mantle sources that are variably enriched in pyroxene (Meibom et al., 2004).

Photo of Os-Ir-Ru “lamellae” grain from Port Orford, Oregon. We have examined the major element, ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os systematics of a large number of these unusual grains (Walker et al., 2005).

Our data for lamellae and matrix samples for Os-Ir-Ru grains from Port Orford, Oregon did not reveal a trend similar to that of the plume suites or those of the previous studies. If our new data above are representative of the Port Orford grains, and Os in the upper mantle in general, then there is no evidence that positive ¹⁸⁶Os-¹⁸⁷Os correlations can be produced via mixing of Os reservoirs within the upper mantle.

Our data for lamellae and matrix samples for Os-Ir-Ru grains from Port Orford, Oregon are shown, above. A datum for a chromitite from the presumed source of the grains, the Josephine ophiolite (JOS-1), is also shown for comparison. Data from Meibom and Frei (2002) and Meibom et al. (2004) (above) show coupled, positive ¹⁸⁶Os-¹⁸⁷Os correlations for Os-Ir-Ru alloy grains from NW California and SW Oregon, consistent with mixing between an ancient reservoir that is depleted relative to chondrites in both ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios, and a reservoir that is conversely enriched. Although our ¹⁸⁷Os/¹⁸⁸Os ratios were a good match to the earlier studies, variations in the ¹⁸⁶Os/¹⁸⁸Os ratios were not confirmed. From Walker et al. (2005).

To learn more about this work, please refer to:

Walker R.J., Morgan J.W. and Horan M.F. (1995) ¹⁸⁷Os enrichment in some mantle plume sources: Evidence for core-mantle interaction? Science 269, 819-822.

Walker R.J., Morgan J.W., Beary E., Smoliar M.I., Czamanske G.K. and Horan M.F. (1997) Applications of the ¹⁹⁰Pt-¹⁸⁶Os isotope system to geochemistry and cosmochemistry. Geochim. Cosmochim. Acta 61, 4799-4808.

Brandon A., Walker R.J., Morgan J.W., Norman M.D. and Prichard H.M.(1998) Coupled ¹⁸⁶Os and ¹⁸⁷Os evidence for core-mantle interaction. Science 280, 1570-1573.

Brandon A.D., Norman M.D., Walker R.J. and Morgan J.W. (1999) ¹⁸⁶Os-¹⁸⁷Os systematics of Hawaiian picrites. Earth Planet. Sci. Lett. 172, 25-42.

Brandon A.D., Snow J.E., Walker R.J., Morgan J.W. and Mock T.D. (2000) ¹⁹⁰Pt-¹⁸⁶Os and ¹⁸⁷Re-¹⁸⁷Os systematics of abyssal peridotites. Earth Planet. Sci. Lett. 177, 319-335.

Morgan J.W., Walker R.J., Horan M.F. and Beary E.S. (2002) ¹⁹⁰Pt-¹⁸⁶Os and ¹⁸⁷Re-¹⁸⁷Os systematics of the Sudbury Igneous Complex, Ontario. Geochim. Cosmochim. Acta 66, 273-290.

Brandon A.D., Walker R.J., Puchtel I.S., Becker H, Humayun M. and Revillon S. (2003) ¹⁸⁶Os-¹⁸⁷Os systematics of Gorgona Island komatiites: implications for early growth of the inner core. Earth Planet. Sci. Lett. 206, 411-426.

Walker R. J., Brandon A.D., Bird J.M., Piccoli P.M., McDonough W.F. and Ash R.D. (2005) ¹⁸⁶Os-¹⁸⁷Os systematics of Os-Ir-Ru alloy grains, southwestern Oregon. Earth Planet. Sci. Lett 230, 211-226.

Brandon A. D. and Walker R.J. (2005) The debate over core-mantle interaction. Earth Planet. Sci. Lett. Frontiers 232, 211-225.

Walker R.J. and Walker D. (2005) Does the core leak? EOS 86, 237-242.

Puchtel I.S., Brandon A.D., Humayun M., and Walker, R.J. (2005) Evidence for the early differentiation of the core from Pt-Re-Os isotope systematics of 2.8-Ga komatiites. Earth Planet. Sci. Lett. 237, 118-134.

Brandon A.D., Walker R.J. and Puchtel I.S. (2006) Platinum-Os isotope evolution of the Earth’s mantle: constraints from chondrites and Os-rich alloys. Geochim. Cosmochim. Acta 70, 2093-2103.

Last Revised July 2007