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Recent [sup.142]Nd isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a 'hidden' reservoir in the Earth (1,2) or to loss of an early-formed terrestrial crust by impact ablation (3). Since removal of crust by ablation would also remove the heat-producing elements--potassium, uranium and thorium--such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow (3). In the 'hidden' reservoir alternative, a complementary low-Sm/Nd layer is usually considered to reside unobserved in the silicate lower mantle. We have previously shown, however, that the core is a likely reservoir for some lithophile elements such as niobium (4). We therefore address the question of whether core formation could have fractionated Nd from Sm and also acted as a sink for heat-producing elements. We show here that addition of a reduced Mercury-like body (or, alternatively, an enstatite-chondrite-like body) rich in sulfur to the early Earth would generate a superchondritic Sm/Nd in the mantle and an [sup.142]Nd/[sup.144]Nd anomaly of approximately +14 parts per million relative to chondrite. In addition, the sulfur-rich core would partition uranium strongly and thorium slightly, supplying a substantial part of the 'missing' heat source for the geodynamo., Terrestrial rocks were recently found to have higher ratios of radiogenic [sup.142]Nd to nonradiogenic [sup.144]Nd than do the chondritic meteorites generally supposed to be representative of the material from which [...]

Ian Carmichael spent 45 years thinking about and working on the activities of components in silicate melts and their use to estimate physicochemical conditions at eruption and in the source regions of igneous rocks. These interests, principally in major components such as Si[O.sub.2], led us to think about possible ways of determining the complementary activity coefficients of trace components in silicate melts. While investigating the conditions of accretion and differentiation of the Earth, a number of authors have determined the partitioning of trace elements such as Co, Ni, Mo and W between liquid Fe metal and liquid silicate. These data have the potential to provide activity information for a large number of trace components in silicate melts. In order to turn the partitioning measurements into activities, however, we need to know the activity coefficient of FeO, [γ.sub.FeO] in the silicate. We obtained [γ.sub.FeO] as a function of melt composition by fitting a simple model to 83 experimental data for which the authors had measured the FeO content of the silicate melt in equilibrium with metal (Fe-bearing alloy) at known [f.sub.O2]. The compositional dependence of [γ.sub.FeO] is weak, but, when calculated in the system Diopside-Anorthite-Forsterite, it decreases towards the Forsterite apex. A similar approach for Ni, for which twice as many data are available, leads to similar composition dependence of activity coefficient and confirms the suggestion that [γ.sub.NiO]/[γ.sub.FeO] is almost constant over a wide range of silicate melt composition. The activity coefficients for FeO were used in conjunction with measured Mo and W partitioning between Fe-rich metal and silicate melt to estimate activity coefficients for trace Mo[O.sub.2] and W[O.sub.3] dissolved in silicate melt. When combined with data on Mo- and W-saturated silicate melts a strong dependence of activity coefficient is observed. Calculated in the system Diopside-Anorthite-Forsterite, both Mo[O.sub.2] and W[O.sub.3] exhibit similar behaviour to FeO and NiO in that activity coefficients decrease as Forsterite content increases. The effect is much larger for Mo and W, however, [γ.sub.MoO2] and [γ.sub.WO3] varying by factors of 20 and nearly 100, respectively, in this system. In order to illustrate the potential applications of the metal-silicate partitioning approach to determine the activity coefficients of volatile elements, we used it to determine activity coefficients of PbO, Cu[O.sub.0.5] and In[O.sub.1.5] in a silica-saturated melt at 1,650°C. We find values of 0.22, 3.5 and 0.02, respectively, indicating a strong dependence on cation charge. The value for Cu[O.sub.0.5] is in excellent agreement with experimental data of Holzheid and Lodders (Geochim Cosmochim Acta 65:1933-1951, 2001), which shows that the method is viable. When combined with thermodynamic data on the gas species, we find that Pb is the most volatile of the 3 elements under 'normal' terrestrial conditions of oxygen fugacity but that In should become the most volatile under strongly reducing conditions such as those of the solar nebula. The oxygen fugacity dependence of volatility has implications for the high relative abundance of In in silicate Earth. We conclude that metal-silicate partitioning experiments are a viable means for determining activities of trace components in silicate melts and are particularly useful if the metal of the element is unstable or volatile at igneous temperatures. Keywords Activities * Trace elements * Silicate melts * Volatilities, Introduction Ian Carmichael broke new ground in igneous petrology with his use of the compositions of groundmass phases to estimate the activities of major oxide components in magmatic liquids. The [...]

Abstract Four-field anthropology departments are no longer in a 'steady state'. Their integrity is threatened by a combination of internal and external developments. It remains to be seen whether they, [...]