H. Palme

Cosmochemical Estimates of Mantle Composition

The composition of the primitive mantle derived here shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. These processes occurred at the initial stage of the solar system formation, under conditions thought to be present in the solar nebula. But the stable isotope record excludes chondritic meteorites as the ‘building blocks’ of Earth. Meteorites formed in local environments separated from that part of the inner solar system where much of the material forming the terrestrial planets was sourced.

The solar-system abundances of Nb, Ta, and Y, and the relative abundances of refractory lithophile elements in differentiated planetary bodies

Abstract Analytical data for Nb, Y, and Ho in 8 carbonaceous chondrites were obtained by spark source mass spectrometry (SSMS). In addition, three carbonaceous chondrites were analyzed for Ta by radiochemical neutron activation analysis (RNAA). From these data and earlier literature data on the C1 -chondrite Orgueil a consistent set of solar-system abundances is derived for Nb, Y, Zr, Ta, Hf and the REE. Ratios among these elements are constant within analytical uncertainties in all groups of carbonaceous chondrites. In particular we do not find a difference in Zr Hf ratios between C1 and C2 chondrites. The new abundances for C1-chondrites are: Nb (0.246 ppm), Y (1.57 ppm), Ta (0.014 ppm), or 0.696, 4.64, 0.020 atoms/106 Si atoms, respectively. Based on a large number of analytical data on oceanic basalts, it is argued that the relative abundances of these elements are chondritic in the bulk Earth. Ratios such as Zr Hf or Nb Ta are constant and chondritic in oceanic basalts and agree with estimates of the continental crust. The constant but non-chondritic Nb U ratio (47 vs. 30) in oceanic basalts is balanced by a lower Nb U ratio (~ 10) in the crust. The bulk Earth ratio may therefore be chondritic. The Zr Hf and Nb Ta ratios are correlated in lunar rocks. Both ratios vary within a factor of two, similar to the variability in terrestrial oceanic basalts. The Zr Nb and Hf Ta ratios, however, are much more constant on the Moon. The available evidence suggests that refractory lithophile elements in the Earth, the Moon and achondritic meteorites occur in the same proportions as in carbonaceous chondrites. Refractory elements have greatly different volatilities. The same pattern of refractory lithophile elements in chondrites and planets therefore indicates that protoplanetary materials have never been subject to high temperature processes that would fractionate refractory elements from each other. The same ratio of Zr Nb in the three types of carbonaceous chondrites, in the Earth, the Moon and in differentiated meteorites is a good example, since the condensation temperature for Zr is 177 K higher than that for Nb.

Collisional erosion and the non-chondritic composition of the terrestrial planets

The compositional variations among the chondrites inform us about cosmochemical fractionation processes during condensation and aggregation of solid matter from the solar nebula. These fractionations include: (i) variable Mg–Si–RLE ratios (RLE: refractory lithophile element), (ii) depletions in elements more volatile than Mg, (iii) a cosmochemical metal–silicate fractionation, and (iv) variations in oxidation state. Moon- to Mars-sized planetary bodies, formed by rapid accretion of chondrite-like planetesimals in local feeding zones within 106 years, may exhibit some of these chemical variations. However, the next stage of planetary accretion is the growth of the terrestrial planets from approximately 102 embryos sourced across wide heliocentric distances, involving energetic collisions, in which material may be lost from a growing planet as well as gained. While this may result in averaging out of the ‘chondritic’ fractionations, it introduces two non-chondritic chemical fractionation processes: post-nebular volatilization and preferential collisional erosion. In the latter, geochemically enriched crust formed previously is preferentially lost. That post-nebular volatilization was widespread is demonstrated by the non-chondritic Mn/Na ratio in all the small, differentiated, rocky bodies for which we have basaltic samples, including the Moon and Mars. The bulk silicate Earth (BSE) has chondritic Mn/Na, but shows several other compositional features in its pattern of depletion of volatile elements suggestive of non-chondritic fractionation. The whole-Earth Fe/Mg ratio is 2.1±0.1, significantly greater than the solar ratio of 1.9±0.1, implying net collisional erosion of approximately 10 per cent silicate relative to metal during the Earths accretion. If this collisional erosion preferentially removed differentiated crust, the assumption of chondritic ratios among all RLEs in the BSE would not be valid, with the BSE depleted in elements according to their geochemical incompatibility. In the extreme case, the Earth would only have half the chondritic abundances of the highly incompatible, heat-producing elements Th, U and K. Such an Earth model resolves several geochemical paradoxes: the depleted mantle occupies the whole mantle, is completely outgassed in 40Ar and produces the observed 4He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss.

EXPERIMENTAL PETROCHEMISTRY OF SOME HIGHLY SIDEROPHILE ELEMENTS AT HIGH TEMPERATURES, AND SOME IMPLICATIONS FOR CORE FORMATION AND THE MANTLE'S EARLY HISTORY

The highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) and those elements with distribution coefficients between Fe-rich metal and silicate phases which exceed 104. The large magnitude of these distribution coefficients makes them exceedingly difficult to measure experimentally. We describe a new experimental campaign aimed at obtaining reliable values of DMmets/sil melt for selected HSEs indirectly, by measuring the solubilities of the pure metals (or simple HSE alloys) in haplobasaltic melts as a function of oxygen fugacity. n nPreliminary results for Pd, Au, Ir and Re indicate that the HSEs may dissolve in silicate melts in unusually low valence states, e.g., 2+ for Ir and 1+ for the others. These unusual valence states may be important in understanding the geochemical properties of the HSEs. Inferred values of DMmet/sil melt from the solubility data at 1400°C and IW −1 are ∼107 for Pd and Au, and 109−1012 for Ir. Metal/silicate partition coefficients are thus confirmed to be very large, and are also different for the different HSEs. n nA review of the abundance of the HSEs in the Earths upper mantle shows that they are all present at ∼0.8% of chondritic, i.e. they have the same relative abundance, and the ratios of their concentrations are chondritic (e.g., Re/Os). Both the low degree of depletion (compared to the high values of DMmet/sil melt) and the chondritic relative abundances support the idea that the mantles HSEs were added in a “late veneer” after the cessation of core formation. Sulfur is even more depleted in the mantle relative to CI chondrites than the HSEs: this implies a late veneer which was depleted in volatile elements, and which was added to a mantle stripped of S. Since considerable S dissolves in silicate melt, this further implies that core formation in the Earth either occurred under P−T conditions below the solicate solidus, or, if the process occurred over a range of temperatures in a cooling Earth, then the process continued down to conditions below the silicate solidus. n nThe chondritic relative abundances of the HSEs in the upper mantle argue for a chemically unstratified primitive mantle, unless the late veneer was mixed only into the upper mantle.

Evidence for oxidizing conditions in the solar nebula from Mo and W depletions in refractory inclusions in carbonaceous chondrites

Abstract Depletions of Mo and W relative to other refractory metals of similar volatility (Re, Os, Ir, Ru, Pt) are common in a suite of 16 Ca,Al-rich inclusions (CAIs) from 5 carbonaceous chondrites. Twelve of the 16 CAIs from Allende, Grosnaja, Leoville, and Ornans show Mo depletions; six of these 12 inclusions also show W depletions. The one CAI analyzed from the Essebi chondrite shows no depletions. The Mo and W depletions have a very characteristic pattern with Mo always more depleted than W. The same Mo and W depletion pattern occurs in calculated refractory metal alloy compositions formed at oxygen fugacities 10 3 to 10 4 times greater than the canonical solar nebula oxygen fugacity. We conclude that Mo and W depletions are common in CAIs from carbonaceous chondrites and that the depletions result from high temperature oxidation. The oxidation may have occurred during the evaporation of primitive dust into CAIs but is also consistent with condensation at high oxygen fugacities. No potential alternative processes appear to be capable of producing the observed Mo and W depletion patterns.

A metal particle from a Ca,Al-rich inclusion from the meteorite Allende, and the condensation of refractory siderophile elements

A small particle (ca. 10−6 g) was magnetically separated from a Ca,Al-rich inclusion of the Allende meteorite. By using instrumental neutron activation analysis it was found that the elements Os, W, Re, Ir, Mo, Ru and Pt were enriched by a mean factor of about 7000 relative to Cl chondrites. n nA polished section of the grain showed that it consisted mainly of silicates, with a rounded particle of metal and sulfide (20 μm across) attached to it. n nConcentrations of up to 11% Pt were determined with the microprobe in the Ni-Fe center of the particle. Furthermore, Rh was for the first time measured in an Allende inclusion. It is enriched in about the same degree as Pt, Ir and W. The Ni-Fe center was surrounded by troilite. Mo was concentrated in the sulfide, while Os and Ru were inhomogeneously distributed over the metal + sulfide phases. The particle is interpreted as direct product of metal condensation of the solar nebula. The sulfide phase formed at lower temperatures and caused redistribution of the refractory siderophile elements. Condensation calculations for a metal alloy show that Fe and Ni are expected to be already present at higher temperatures than the condensation temperatures of pure Fe. Pt and Rh, having lower condensation temperatures than pure Fe should also be completely condensed above the condensation temperature of pure Fe. Kinetic considerations show that minimum times to grow this kind of particle should be of the order of 500 years at 10−3 atm.

Experimental determination of metal/silicate partition coefficients for P, Co, Ni, Cu, Ga, Ge, Mo, and W and some implications for the early evolution of the Earth

Metal/silicate partition coefficients were determined at 1600°C for P, Ga, Ge and W and at 1300°C for P, Fe, Co, Ni, Cu, Ga, Ge, Mo and W. Experiments span a range of two orders-of-magnitude in oxygen fugacity. Good correlations between logƒO2 and the log of the partition coefficients were observed for all elements. The slopes of these correlations reflect the number of oxygen atoms associated with the metal oxides in the silicate phase. The composition of the silicates was basaltic, but variable with respect to FeO; from about 4% FeO at the most reducing experiments to 45% FeO at the most oxidizing experiments. Within these variations no dependence of the partition coefficients on silicate composition was observed, except for Ni and Co. The unusual slopes in the log DNi and log DCo vs. logƒO2 correlations can be explained by a strong dependence of the NiO and CoO activities on the liquid silicate composition. n nReasonable agreement is found with literature data, except for a major discrepancy in the Mo-partition coefficient. Since many of the literature data were obtained from experiments with concentration levels in the percent range and since the experiments reported here contained the metals only in the ppm range, the present data are in agreement with Henrys law. n nThe distribution of siderophile elements in the upper mantle can be explained by accretion of increasingly oxidizing material. After accretion of some 80 to 90% of the Earth the oxygen fugacity is sufficiently high that Fe, Ni, Co and some other siderophile elements will be quantitatively retained in the upper mantle. The small amount of metal still segregating to the core will suffice to extract highly siderophile elements. The final 1% of accreting material is so oxidizing that metal segregation is completely inhibited. The pattern of siderophile elements observed in the upper mantle is broadly consistent with results from calculations based on this model and utilizing metalsilicate partition coefficients reported here.

On the chemistry of the Allende inclusions and their origin as high temperature condensates

Abstract The abundances of 45 elements have been determined in a Ca,Al-rich chondrule from the Allende meteorite. The analytical techniques used were instrumental neutron and gamma activation analyses together with radiochemical neutron activation analysis. The refractory lithophile elements Al, Ca, Ti, Sc, V, Sr, Y, Zr, Nb, REE, Hf, Ta, (W) and U and the refractory siderophile elements Re, Os, Ir, Ru and Pt show a mean enrichment factor of 20 relative to C1-chondrites. Most of the other elements, which have been determined, are depleted relative to C1-chondrites. These measurements confirm the suggestion that non-magmatic processes are responsible for these enrichments. Especially the agreement of the measured abundances with those from the predictions of condensation calculations is excellent. Furthermore, by comparing the concentrations of some elements in a surface piece of the chondrule to a central part, a steep concentration gradient was found for the elements Fe, Mn, Na, K, Cl and Br. This suggests that these elements are of secondary origin, having entered the chondrule through the surface.

CaAl ratio and composition of the Earth's upper mantle

Undifferentiated meteorites (chondrites) have the same relative abundances of refractory lithophile elements (Ca, Al, Ti, Sc, REE, etc.), despite variable absolute concentrations. The reasonable assumption of chondritic ratios among refractory elements in the bulk Earth is used to constrain the chemical composition of the upper mantle in the following way: Correlations of the compatible refractory elements Ca, Al, Ti, Sc and Yb with MgO are worldwide very similar in suites of spinel-lherzolite xenoliths from basaltic rocks. Such suites represent upper mantle material depleted to differing degrees by extraction of partial melts. From these refractory elements vs. MgO correlations, ratios of pairs of refractory elements were calculated at various MgO contents. Chondritic AlTi and ScTi ratios were only obtained for MgO contents below 36%. A chrondritic ScYb ratio requires an MgO content above 35%. We therefore accept 35.5% as the most reasonable MgO content of undepleted upper mantle. This MgO content is slightly below the spinel-lherzolite with the lowest measured MgO content (36.22%). The corresponding Al2O3 content of 4.75% is higher than in previous estimates of upper mantle composition. The concentrations of other elements were obtained from similar correlations at a MgO content of 35.5%. The resulting present upper mantle composition is enriched in refractory elements by a factor of 1.49 relative to Si and Cl and by a factor of 1.12 for Mg relative to Si and Cl. These enrichments are in the same range as those for the Vigarano type carbonaceous chondrites. The Mg/Mg + Fe ratio of 89 is slightly lower than previous estimates. n nThe CaAl ratio in spinel lherzolite suites is, however, uniformly higher worldwide than the chondritic ratio by about 15%. Orogenic peridotites as well as komatiites appear to have similar non-chondritic CaAl ratios. It is therefore suggested that this non-chondritic CaAl ratio is a characteristic of the upper mantle, possibly since the Archean. A minor fractionation of about 4% of garnet in an early, global melting event (deep magma ocean?) is presented as the most likely cause for the high CaAl-ratio. In this case the addition of 4% of such a garnet component to the undepleted present upper mantle would be required to obtain the composition of the primordial upper mantle. The CaAl-ratio of this primordial mantle would be 15% higher than that of the undepleted present upper mantle, resulting in an enrichment of refractory elements of 1.70 (AlSi relative to Cl) for the primordial upper mantle.

Solubility of palladium in silicate melts: Implications for core formation in the Earth

Abstract Palladium solubilities in silicate melts of anorthite-diopside-eutectic composition were determined at a wide range of oxygen fugacities, from pure O 2 to f o2 slightly below the iron-wustite buffer and at temperatures ranging from 1343 to 1472°C. Experiments were performed by heating palladiumloops with silicates inside a gas controlled furnace. Palladium concentrations were determined by neutron activation analysis. Repeated analyses of the glasses after removal of the outer layers and several reversed experiments with initially high Pd in the glass showed that equilibrium was attained in the experiments. At 1350°C concentrations of Pd in silicate melts range from 428 ppm to 1.2 ppm with decreasing palladium content at decreasing oxygen fugacities. The dependence of log Pd on log f o 2 indicates a change in valence of the dominant palladium species in the silicate melt. The data can be explained by the presence of complexes containing Pd 2+ and Pd 0 . Alternatively, a good fit is obtained by assuming mixtures of Pd 2+ , Pd 1+ and Pd 0 in the melt with increasing contributions of the lower valence species at increasingly reducing conditions. Solubilities increase with temperature at fixed oxygen fugacities independent of the absolute fugacity. This is an unexpected result. From the solubility data, metal/silicate partition coefficients were calculated using known activity coefficients of Pd in Fe-metal. Extrapolations were made to higher temperatures and lower oxygen fugacities. A palladium metal/silicate partition coefficient of 1.6 · 10 7 is inferred for 1623 K and IW-2. Extrapolation to 3500 K leads to a partition coefficient of 3.8 · 10 3 . From earlier data on Ir solubilites, a metal/silicate partition coefficient of 2 · 10 8 was estimated for the same conditions. The high absolute metal/silicate partition coefficients for Pd and Ir and the large difference between the two partition coefficients are not compatible with a global core/mantle equilibrium as a source of the highly siderophile elements in the Earth mantle. The data favour models invoking the accretion of a late chondritic veneer after core formation without further metal segregation.