(Note: reprints /PDF files of all most cited UMCP papers are currently available upon request)

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

Theory and seismic evidence has suggested that some mantle plumes may rise from core-mantle boundary, yet there has been no definitive geochemical evidence for the derivation of plume derived materials from the deepest mantle. This may be because plumes may simply not rise for such depths. Alternately, the geochemical tools (theory and analysis) may not be yet permit identification of core-mantle interaction. Arguably the most sensitive suite of elements for geochemically identifying possible core contributions to plumes are certain moderately siderophile (iron loving) elements (MSE: including W, Ag and Mo), and highly siderophile elements (HSE: including Re, Os, Ir, Ru, Pt, Rh, Pd, Au). 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 ~1.5 b.y. of planetary formation, the outer core could exhibit coupled ¹⁸⁷Os and ¹⁸⁶Os enrichments, 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.

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., 1999; 2003). These results have been interpreted to be indicative of core-mantle interaction, with minor portions of these plumes recording chemical or isotopic exchange with the outer core (e.g., Brandon et al., 1999), perhaps via infiltration of metal into the lower mantle source of plumes (below).

Schematic diagram showing one possible mechanism (metallic liquid infiltration) for transferring an isotopic signature from the core to a rising mantle plume (figure from Brandon and Walker, 2005).

If metal infiltration into plume sources occurs, this should have a collateral effect on both absolute and relative abundances of HSE in the plume source of lavas contaminated by a core component. Towards this end, we have analyzed a large number of Hawaiian picrites for HSE abundances and Os isotopic compositions. Via extrapolations to presumed primitive melt compositions. Results are shown below. In brief, the HSE patterns (chondrite normalized) are neither abnormally enriched in HSE, nor are the HSE abnormally fractionated relative to expected effects resulting from melting a source with generally chondritic relative abundances. Both observations are inconsistent with significant metal infiltration.

Chondrite normalized plot of estimated HSE abundances for Hawaiian picritic lavas. Except for Re, HSE abundances lie between average Pacific crust and pattern typical of higher degrees of mantle melting (komatiite). Figure is from Ireland et al. (2009; Chemical Geology).

It has also been suggested that core-mantle interaction may be recorded in a rising plume via isotopic exchange between metal and silicate that is more rapid than elemental equilibration. If so, this could explain the lack of concentrational evidence for exchange in the Hawaiian lavas. To consider this option further, we have collaborated with experimentalist David Walker (Lamont-Doherty Earth Observatory) to examine the mechanisms of metal-silicate elemental and isotopic exchange for Os. Our results suggest that isotopic exchange may be considerably slower than elemental exchange (below), and that transfer of an Os isotopic signature from the core to the mantle may be more difficult than originally anticipated. These results are published in Yokoyama et al. (2009).

Plot of ¹⁸⁷Os/ ¹⁸⁸Os versus 1/ ¹⁸⁸Os (in ppb -1) for silicates equilibrated with metal at high temperature and pressure. The starting silicate and metal were isotopically distinct. The fact that the measured trends are shallower than the trend for simple two component mixing suggests suggests that microdroplets of metal from the main mass scavenged Os from silicate at elevated temperature, but did not isotopically equilibrate with the silicate during the scavenging.

As a MSE, the element W may also be a useful tracer for core mantle interaction. The early solar system fractionation of Hf from W (where ¹⁸²Hf → ¹⁸²W + β^- ; and t_½ = 8.9 x 10⁶ yr) has evidently led to an isotopic contrast of approximately 2 epsilon units (parts per 10,000) between the silicate Earth and the core. Thus, an outer core W signature could potentially be recognized in silicates at the surface by resolution of a small depletion in ¹⁸²Hf. Such depletions have been sought but not identified in Hawaiian lavas. To examine this issue further, we measured W abundances in Hawaiian picritic lavas, and via projection methods determined the W abundances in the associated primitive melts. These results (below) suggest that all Hawaiian sources are enriched in W relative to the convecting upper mantle. The majority of this excess W cannot be from the core, and instead likely derives from recycled oceanic crust. This crustal addition makes the quest for a core signature more difficult in Hawaiian lavas. The techniques developed, however, may allow us to choose better (less crustally contaminated) samples for isotopic analysis.

Abundances of W (in ng/g) estimated for various Hawaiian volcanic centers. Note that all sources are elevated relative to estimates for the convecting upper mantle (DMM). Figure is from Ireland et al. (2009; GCA).

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.

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.

Ireland T., Walker R.J. and Garcia M.O. (2009) Highly siderophile element and ¹⁸⁷Os isotopes systematics of Hawaiian picrites: Implications for parental melt composition and source heterogeneity. Chem. Geol. 260, 112-128.

Pitcher L., Helz R.T., Walker R.J. and Piccoli P.M. (2009) Fractionation of the platinum-group elements and Re during crystallization of basalt in Kilauea Iki lava lake, Hawaii. Chem. Geol. 260, 196-210.

Yokoyama D., Walker D. and Walker R.J. (2009) Low osmium solubility in silicate at high pressures and temperatures. Earth Plan. Sci. Lett. 279, 165-173.

Ireland T.J., Arevalo R.D., Walker R.J. and McDonough W.F. (2009) Tungsten in Hawaiian picrites: a compositional model for the source of Hawaiian lavas. Geochim. Cosmochim. Acta 73, 4517-4530.

Last Revised July 2009.