Siderophile Element Abundances Research Articles

The origin of the moon and the early history of the earth—A chemical model. Part 2: The earth

The geochemical implications for the Earth of a “giant impact” model for the origin of the Earth-Moon system ( O'Neill, 1991) are discussed, using a mass balance between three components: the proto-Earth, the Impactor, and a late veneer. It is argued that the proto-Earth (nearly 90% of the present Earth) accretes from material resembling a high temperature condensate from the solar nebula. Core formation takes place under very reducing conditions, resulting in the mantle of the proto-Earth being completely stripped of all elements more siderophile than Fe, and partly depleted in the barely siderophile elements V, Cr, and perhaps Si. The Impactor (11% of the present Earth), which is constrained to be an oxidised body of approximately chondritic (CI) composition, then collides with the proto-Earth, causing vaporisation of both the Impactor and a substantial portion of the Earth's mantle. Most of this material recondenses to the Earth, but some forms the Moon. The Impactor adds most of the complement of the siderophile elements of the present mantle (e.g., Ni, Co) in an oxidised form. The oxidation state of the mantle is set near to its present, oxidised level. The Impactor also adds volatile elements, including a small amount of S. This S forms a separate Ni-rich sulfide phase, which segregates to the core, taking with it all the highly siderophile elements (e.g., the noble metals), and depleting the mantle in the moderately siderophile elements according to their sulfide/silicate partition coefficients. Finally, the addition of a late veneer, of composition similar to that of the H-group ordinary chondrites, accounts for the complement of the highly siderophile elements of the present mantle. The model accounts at least semi-quantitatively for the siderophile element abundances of the present mantle; a more quantitative test would be possible if suitable partition coefficients, between silicate and Ni-rich sulfide under relatively oxidising conditions, were available. Implications for the composition of the Earth's core are discussed; the model predicts that neither S, O, nor Si should be present in sufficient quantities to provide the required light element in the core, whose identity, therefore, remains enigmatic.

Activation analysis as a geochemical tool: Statement of its capabilities for trace element studies in light of long term and routin investigation, and geochemical discussions

Sulfur is a potential light element in the liquid outer core of the Earth. Its presence in segregating metal may have had an influence in distribution of metal-loving (siderophile) elements during early accretion and core formation events in the Earth. The observed “excess” abundance of siderophile elements in the terrestrial mantle, relative to an abundance expected from simple core-mantle equilibrium at low temperature and pressure, may indicate a reduction in the iron-loving tendency of siderophile elements in the presence of sulfur in the metallic phase. The present experimental partitioning study between iron-carbon-sulfur-siderophile element bearing liquid metal and liquid silicate shows that for some siderophile elements this sulfur effect may be significant enough to even change their character to lithophile. Large and intricate variations in metal-silicate partition coefficients (Dmet/sil) have been observed for many elements, e.g., Ni, Co, Ge, W, P, Au, and Re as a function of sulfur content. Moderately siderophile elements Ge, P, and W show the most significant response (sulfur-avoidance) by an enhanced segregation into the associated sulfur-deficient phases. Highly siderophile elements Ir, Pt, and Re show a different style of sulfur-avoidance (alloy-preference) by segregating as sulfur-poor, siderophile element-rich alloys. Both groups are chalcophobic. Dmet/sil for Ni, Co, and Au moderately decreases with increasing sulfur-content in the liquid metal. Dmet/sil for chalcophile element, Cr, in contrast, increases with sulfur. Irrespective of the sulfur-content, in the presence of a carbon-saturated liquid metal, P is always lithophile. The general nonmetal-avoidance tendency of siderophile elements (and acceptance of chalcophile elements) in the liquid metal, postulated by Jones and Malvin (1990) in the FeNiS(sulfur)M (siderophile) system is found to be present in the metal-silicate system as well. A sulfur-bearning liquid metal segregation can potentially reduce the metal-loving nature of many elements to explain the excess paradox. Sulfur-bearing core segregation, however, might require an efficient draining of exsolved immiscible sulfide liquids from the molten silicate, or an increasing siderophility of sulfur at high pressure to reduce the mantle sulfur content to the observed (<300 ppm) value. Moreover, the chondritic relative abundance pattern of many moderately or highly siderophile elements in the upper mantle is not explained by the presence of sulfur in the segregating metals. Core formation is more complex and intricate than equilibrium segregation.

Compositional evidence regarding the origins of rims on Semarkona chondrules

The compositions of the interiors and abraded surfaces of 7 chondrules from Semarkona (LL3.0) were measured by neutron activation analysis. For nonvolatile elements, the lithophile and siderophile element abundance patterns in the surfaces are generally similar to those in the corresponding interiors. Siderophile and chalcophile concentrations are much higher in the surfaces, whereas lithophile concentrations are similar in both fractions. Most of the similarities in lithophile patterns and some of the similarities in siderophile patterns between surfaces and interiors may reflect incomplete separation of the fractions in the laboratory, but for 3 or 4 chondrules the siderophile resemblance is inherent, implying that the surface and interior metal formed from a single precursor assemblage. Metal and sulfide-rich chondrule rims probably formed when droplets of these phases that migrated to the chondrule surface during melting were reheated and incorporated into matrix-like material that had accreted onto the surface. The moderately-volatile to volatile elements K, As and Zn tend to be enriched in the surfaces compared with other elements of similar mineral affinity; both enrichments and depletions are observed for other moderately volatile elements. A small fraction of chondrules experienced fractional evaporation while they were molten.

ZHAMANSHIN AND AOUELLOUL: CRATERS PRODUCED BY IMPACT OF TEKTITE‐LIKE GLASSES?

It is shown that the enhanced abundance of siderophile elements and chromium in tektite‐like glasses from the two impact craters of Zhamanshin and Aouelloul cannot be explained as a result of contamination of the country rock by meteorites nor, probably, comets. The pattern is, however, like that found in certain Australasian tektites, and in Ivory Coast tektites. It is concluded, in agreement with earlier suggestions by Campbell‐Smith and Hey, that these craters were formed by the impact of large masses of tektite‐like glass, of which the glasses which were studied are fragments. It follows that it is necessary, in considering an impact crater, to bear in mind that the projectile may have been a glass.

The nature of the meteoritic components of Apollo 16 soil, as inferred from correlations of iron, cobalt, iridium, and gold with nickel

Concentrations of Co, Ni, Ir, and Au in small samples of submature and mature soil from Apollo 16 are highly variable. Correlations of Fe, Co, Ir, and Au with Ni indicate that the variation in siderophile element concentrations results from variation in the concentration of Fe‐Ni metal. From these correlations, the mean composition of the metal is inferred to be 5.6% Ni and 0.36% Co for a Ni/Co ratio of 15.5 ± 0.7, i.e., dissimilar to the chondritic ratio of 20–21. The Fe‐Ni metal is also characterized by low Ir/Ni and Ir/Au ratios compared to metal in ordinary chondrites. Metal with a similar composition is also found in metal‐rich, noritic impact melt breccias that occur as discrete rocks as well as small clasts in ancient regolith breccias at Apollo 16. Mass‐balance models using lithophile element concentrations indicate that about 35% of the soil is noritic impact melt. The Fe‐Ni metal associated with this component is sufficient to supply virtually all the ancient Fe‐Ni metal found in the typical Apollo 16 soil (0.4–0.5%), i.e., no other source is required. The soil also contains 1–2% carbonaceous chondrite. The concentration of this component appears to be relatively constant from sample to sample and does not contribute significantly to the variation in siderophile element abundances. These two meteoritic components contribute nearly equal concentrations of Ni to the typical Apollo 16 soil.

Siderophile elements in planetary mantles and the origin of the Moon

The origin of the moon is examined in the context of theories of planetary accretion and of siderophile element abundances inferred for the upper mantles of the earth, moon, and shergottite parent body (SPB=Mars?). The lunar origin hypotheses examined are collisional ejection in a giant impact, and coaccretion from a circumterrestrial disk of metal‐depleted material.If the moon originated in a giant impact, the impactor must have contained free metal and was most probably differentiated with siderophile elements segregated into a core, in order to account for the low absolute and generally nonchondritic relative abundances of moderately and noble siderophile elements in the lunar mantle. If impactor metal was accreted to the earth's upper mantle during the impact and if it perfectly scavenged siderophile elements from the upper mantle into the core, then it is necessary to postulate a late‐stage chondritic “veneer” in order to account for the high absolute and chondritic relative abundances of noble siderophile elements in the present‐day upper mantle of the earth. These elements are less abundant in the mantles of the moon and SPB than in the earth, and they may not be present in chondritic ratios. If they are present in chondritic ratios (the scatter in noble siderophile element data permits this possibility), they are present at approximately 10−3 and 10−4 × CI concentrations in the SPB and moon, respectively. In either case, the noble siderophile elements present a problem for the chondritic “veneer” version of the collisional ejection hypothesis because they are not present in any reservoir in the moon at the required abundance levels.If the impactor metal that was accreted to the earth imperfectly scavenged siderophile elements from the earth's mantle, it is not necessary to postulate a late stage “veneer.” However, the giant impact would substantially melt or vaporize the earth's mantle, yet there is no evidence in samples derived from the upper mantle for large‐scale vertical compositional zoning that might be expected to result from crystallization in a gravity field, by analogy with the moon and with terrestrial layered intrusions. Mantle nodules from all over the world are homogeneous with respect to compatible siderophile elements such as Ni, Co, and Ir at the hand specimen scale.Coaccretion hypotheses of lunar origin are inherently less testable using siderophile elements than collisional ejection hypotheses because abundances of these elements in the mantles of the earth and SPB are less useful as constraints. At present, none of the hypotheses of lunar origin satisfactorily accounts for siderophile element concentrations in the mantles of the earth, moon, and SPB.

Enstatite chondrites and enstatite achondrites (aubrites) were not derived from the same parent body

Enstatite achondrites (aubrites) were not derived from known enstatite chondrites by melting and fractionation on one and the same parent body, for these and other reasons: (1) There is no satisfactory mechanism for fractionating metal plus troilite in enstatite chondrites to form these phases in different proportions and with different Ti contents in aubrites. (2) Many enstatite chondrites and aubrites are regolith or fragmental breccias, but clasts of one within the other have not been found. (3) Cosmic ray exposure ages of the two groups are difficult to explain if they are from the same parent body, but are easy to explain if they are from different parent bodies. Siderophile element abundances in metal from the Mt. Egerton meteorite, which consists of enstatite and metallic Fe,Ni, preclude it from being a complementary differentiate of the aubrites. Rather, it appears that Mt. Egerton was formed from the same source material as enstatite chondrites, but the components were mixed in different proportions.

Chemical and physical studies of type 3 chondrites—VI: Siderophile elements in ordinary chondrites

The abundances of Fe, Ni, Co, Au, Ir, Ga, As and Mg have been determined by instrumental neutron activation analysis in 38 type 3 ordinary chondrites (10 of which may be paired) and 15 equilibrated chondrites. Classification of type 3 ordinary chondrites into the H, L and LL classes using oxygen isotopes and parameters which reflect oxidation state (Fa and Fs in the olivine and pyroxene and Co in kamacite) is difficult or impossible. Bulk compositional parameters, based on the equilibrated chondrites, have therefore been used to classify the type 3 chondrites. The distribution of the type 3 ordinary chondrites over the classes is very different from that of the equilibrated chondrites, the LL chondrites being more heavily represented. The type 3 ordinary chondrites contain 5 to 15 percent lower abundances of siderophile elements and a compilation of the present data and literature data indicates a small, systematic decrease in siderophile element concentration with decreasing petrologic type. The type 3 ordinary chondrites have, like the equilibrated ordinary chondrites, suffered a fractionation of their siderophile elements, but the loss of Ni in comparison with Au and Ir is greater for the type 3 chondrites. These siderophile element trends were established at the nebula phase of chondritic history and the co-variation with petrologic type implies onion-shell structures for the ordinary chondrite parent bodies. It is also clear that the relationship between the type 3 and the equilibrated ordinary chondrites involves more than simple, closed-system metamorphism.

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core

We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earth's core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earth's primitive mantle. Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle (≅ 0.036) is too low to represent the primitive value (≅ 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.

Trace elements in natural metallic iron from Disko Island, Greenland

The largest occurrence of natural metallic iron on Earth is on the island of Disko, Greenland. Metallic iron is found there in a variety of different types, from small metal particles in basalts to large meter-sized blocks. We have studied three different types of metallic iron: small metal spherules (< 300 μm) in basaltic magma; larger metal grains (300 μm-3 mm), often composed of aggregates of smaller particles, in similar host rocks; and massive iron lumps (up to several tons). Analytical data for 13 siderophile elements in samples from these three types are presented. All metals analysed have a distinctly crustal pattern of siderophile elements. High Co/Ni, Re/Ir or W/Ir ratios clearly demonstrate that a meteoritic origin for the metallic iron must be excluded. Since the Co/Ni and Re/Ir ratios are approximately chondritic in the upper mantle of the Earth, a mantle origin for the Disko metals can also be ruled out. This supports earlier petrological and geological evidence that the metallic iron was formed through reduction of basaltic magma by carbon derived from Tertiary shales and coals. Significant differences in absolute and relative abundances of siderophile elements occur among the three kinds of metals. The strongly siderophile elements (e.g. Ir, Re, Ni) increase in concentration from the small metal spherules through the larger grains to the massive iron lumps. The contents of less strongly siderophile elements (P, W, Ga) decrease in the same sequence. Evidence is presented that the small metal spherules are formed by in situ reduction. Larger iron metal grains and massive iron lumps are composed of small spherules, accumulated by gravitational settling in a magma reservoir. These metal cumulates have extracted highly siderophile elements from a larger volume of basaltic melt.

Core formation in the Earth and Shergottite Parent Body (SPB): Chemical evidence from basalts

Constraints on processes of core formation in terrestrial planets may be inferred from the abundances of siderophile and chalcophile elements in their mantles. Of particular interest is a comparison of processes of core formation in the Earth and in the Shergottite Parent Body (SPB), a terrestrial planet tentatively inferred to be Mars. To this end, we (i) present new INAA and RNAA analyses of the non-Antarctic SNC meteorites, (ii) infer the composition of the SPB mantle from the compositions of the SNC meteorites, (iii) infer the composition of the Earth's mantle from the compositions of terrestrial basalts, and (iv) deconvolve the effects of volatile depletion, core formation, and mineral/melt fractionation on the abundances of siderophile and chalcophile elements in the SPB and the Earth. Element abundances in the mantles of the SPB and the Earth are estimated from element/element correlations observed among basalt samples. In basalts from the SPB (the SNC meteorites), four groups of covariant elements are observed: highly incompatible, moderately incompatible, indifferent and compatible. From correlations within these groups, the SPB mantle is found to be depleted in volatile elements, and strongly depleted in siderophile and chalcophile elements. For the Earth, the element/element correlation method gives a mantle composition similar to previous estimates. Compared to the Earth, the SPB mantle is richer in moderately siderophile elements (e.g., W, P), consistent with its inferred higher oxidation state. Chalcophile elements in the SPB mantle are more depleted than in the Earth's mantle, particularly when compared to estimates of the original abundances of volatile chalcophile elements in the two planets. In the SPB mantle, the NiCo ratio is nonchondritic, in contrast to the chondritic ratio in the Earth's mantle. Abundances of siderophile and chalcophile elements in the SPB mantle may be modelled by equilibrium with solid metal and metallic sulfide liquid, with some metal and sulfide trapped in the mantle (i.e., homogeneous accretion and inefficient core formation). Neither this model nor a heterogeneous accretion model is satisfactory in explaining element abundances in the Earth's mantle, particularly the abundances of Ni, Co, Mo, and W. Nevertheless it appears that core formation in the SPB and the Earth left quite different chemical signatures in their planetary mantles.

Petrology and chemistry of hyperferroan anorthosites and other clasts from lunar meteorite ALHA81005

Thirteen previously undescribed clasts from lunar meteorite Allan Hills A81005 include one that is a cataclastic, “hyperferroan” noritic anorthosite. Mafic minerals in this clast (low‐Ca pyroxene with mg′ 52 and olivine with Fo 41) have very low mg compared to most lunar anorthosites described before the discovery of 81005. Siderophile element concentrations are low and indicate that the clast is compositionally pristine. Rare earth elements (REE) are 2.5–10 times more abundant and slightly more fractionated than in previously described ferroan anorthosites. Another probably pristine clast is a granulitic gabbroic norite that has low‐Ca pyroxene with mg′ 53. Hyperferroan anorthosites are a significant component of 81005. Extreme iron enrichment and high REE abundances indicate that these rocks formed late in the evolution of the magma from which the ferroan anorthosite suite formed. Plagioclase was not enriched in Na even at these late stages. Another clast is a cataclastic ferroan anorthosite (Fo 66) that might be pristine. Of the remaining 10 clasts, nine are granular to cataclastic rocks (primarily anorthositic troctolites or troctolitic anorthosites), and two are impact melt breccias, all of which probably represent mixtures of pristine rock types. The former have REE that are essentially unfractionated relative to chondrites and range in abundance from 1.5–3×Cl, and the latter are similar to bulk 81005 (REE 5–8×Cl). Siderophile element abundances are high and indicate the presence of a significant meteoritic component. Our data indicate that 81005 has only a small KREEP (potassium, rare earth elements, phosphorus) component. This is consistent with previous results.

The Bakerian Lecture, 1983 - The Earth’s core: its composition, formation and bearing upon the origin of the Earth

The density of the outer core is about 3 % smaller than pure iron, which implies that the core contains a substantial amount of one or more low atomic mass elements. Candidates which have been suggested on various grounds include S, H, C, O, Si, and Mg. Plausible models of accretion of the Earth encounter difficulties in trapping sufficient S, H and C to explain the density deficit. On the other hand, entry of Si and Mg is not favoured by thermodynamic arguments. Oxygen is the most abundant element in the Earth and would be a prime candidate if it could be shown to be extensively soluble in molten iron at core temperatures and pressures. New experimental data on the solubility of FeO in molten iron are reviewed. They demonstrate that at atmospheric pressure, FeO is extensively soluble in iron at 2500 °C and that complete miscibility probably occurs above 2800 °C. Moreover, liquid iron in equilibrium with magnesiowüstite (Mg0.8Fe0.2)O also dissolves large quantities of FeO above 2800 °C. The solubility of FeO in molten iron is considerably increased by high pressures, because of the small partial molar volume of FeO in the Fe─FeO melt. If the core formed by segregation of metal originally dispersed throughout the Earth, it seems inevitable that it would have dissolved large amounts of FeO. The density of the outer core can be matched if it contains about 35 mol % FeO, a quantity that is readily explained by the new experimental data. Solution of FeO in iron causes the melting point of the metal phase to be depressed below the solidus temperature of the silicate phase assemblages in the mantle. A model for the formation of the core is described, based upon Fe-FeO phase relations at high temperatures and pressures. The model implies the presence of a high content of FeO in the Bulk Earth. This can be explained if the Earth accreted from a mixture of two components: A, a highly reduced, metal-rich devolatilized assemblage and B, a highly oxidized, volatile-rich assemblage similar to C1 chondrites. The formation of these components in the solar nebula is discussed. The large amount of FeO now inferred to be present in the Earth was mainly produced during accretion by oxidation of metallic iron from component A by water from component B. This two-component mixing model also provides an attractive explanation of some aspects of the chemistry of the Earth’s mantle including the abundances of siderophile and volatile elements.

Chemical equilibration of the Earth's core and upper mantle

The oxygen fugacity (fO 2) of the Earth's upper mantle appears to lie somewhat above that of the iron-wüstite buffer, its fO 2 is assumed to have been similar to the present value at the time of core formation. In the upper mantle, the Fe-rich liquid protocore that would form under such conditions of fO 2 at elevated temperatures would lie predominantly in the system Fe-S-O. Distribution coefficients for Co, Cu, Ni, Ir, Au, Ir, W, Re, Mo, Ag and Ga between such liquids and basalt are known and minimum values are known for Ge. From these coefficients, upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole Earth and equilibrium between the Fe-S-O protocore and upper mantle. These calculated abundances are surprisingly close to presently known upper mantle abundances; agreements are within a factor of 5, except for Cu, W, and Mo. Therefore, siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late-stage meteoritic bombardment, and a protocore formed from the upper mantle containing S and O seems likely. As upper mantle abundances fit a local equilibrium model, then either the upper mantle has not been mixed with the rest of the mantle since core formation, or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of P, T, and fO 2. The latter possibility seems unlikely over such a P-T range.

The lunar core and the origin of the Moon

Recent data on the concentrations of the refractory siderophile (metal‐loving) elements molybdenum and rhenium confirm that most siderophile elements in lunar crustal rocks and mare basalts have significantly lower concentrations than in the earth's mantle and much lower concentrations compared to chondrite meteorites. The observed depletion of siderophile elements in lunar samples suggests the existence of a metal core. The amount of metal in the core depends on the conditions under which metal segregation occurred. If the moon accreted directly from the solar nebula, with chrondritic abundances of siderophile elements, then 2% to 5.5% metal must have segregated within the moon to form a core. Geophysical techniques constrain the radius of the lunar core to 500 km (5.5 wt%) or less. If the moon formed after the earth, from material out of the earth's mantle, segregation of 0.1% to 1% metal within the moon is required. Less metal is needed for a terrestrial origin because the earth's mantle was already partially depleted in siderophile elements due to formation of the earth's core. The siderophile elements therefore provide strong geochemical evidence for a lunar metal core, but do not rule out either formation of the moon from the earth's mantle or an independent origin by accretion from the solar nebula.

The Lappajärvi meteorite crater, Finland: petrography, Rb-Sr, major and trace element geochemistry of the impact melt and basement rocks

Impact melt lithologies of the 77 m.y. old Finnish meteorite crater Lappajärvi as well as the Precambrian target rocks have been studied in detail, to identify and characterize different impact melt types (clast-poor, clast-rich, suevitic melt) and to study their chemical (major and trace elements) and isotopic (Rb-Sr) compositions in comparison to the composition of the target rocks. The Rb-Sr system of the whole melt body—including the suevitic melt—is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks. Chemical interactions (exchange of alkali elements, 87Sr-redistribution) between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt. Exchange reactions between clasts and melt are determined by thermal activation, but the degree of shock metamorphism in the clasts plays an important role, too. Major and trace element distributions in impact melt and basement rocks indicate that the Lappajärvi melt body chemically is extremely homogeneous. Even volatile elements (such as Zn and Cu) were not strongly fractionated. Comparison of the abundances of siderophile elements in the impact melt ( e.g., 118–177 ppm Cr, 195–340 ppm Ni, 6–12 ppb Ir) and calculated target rock mixture (79% mica schist, 11% granite-pegmatite, 10% amphibolite) ( e.g., 85.6 ppm Cr, 54.8 ppm Ni, 0.5 ppb Ir) revealed the chondritic nature (C or H chondritic) of the meteoritic projectile. Less than 2% of the meteorite can be detected in the coherent melt, whereas the suevitic melt is uncontaminated by the projectile.

Siderophile element abundances in the upper mantle: evidence for a sulfide signature and equilibrium with the core

Measurements of the intrinsic oxygen fugacities of samples from the Earth's upper mantle have shown that the characteristic oxidation state ranges from values close to Fe-Ni metal saturation to more oxidized values. Tectonic processes can account for the progressive and variable oxidation of a mantle that was originally in equilibrium with metal, so that the oxygen fugacity data give strong support to the possibility of protocore -protomantle equilibrium. The data for apparently overabundant siderophile elements in the mantle are reviewed and 2 groups (Group A = Ge, Au, Pd, Pt, Ru, Rh, Ir, Re, Os; Group B = Sn, Sb, Ag, Ga, Cu, As, Co, Ni, W, Mo) are recognized. Group A abundances result from an overprinting, by a late-accreting chondritic component, of a mantle that had suffered major extraction of the indigenous complement by core-forming metal ( Chou, 1978). Group B, however, are ≈- 10 × more abundant than Group A and may be distinguished as a group by their greater chalcophile tendencies. This is highlighted, for example, by the large Cl-normalized Sn/ Ge ratio of the mantle, which is unexpected as a result of metal-silicate fractionation alone, but can be accounted for if some Sn. together with sulfur was retained in the mantle during core separation. Industrial experience in the equilibrium of molten Fe and silicate slags shows that sulfur is not overwhelmingly partitioned into metal so that the concept of partial (≈-20% of original accreting component) retention of sulfur and Group B elements in the mantle has experimental and theoretical support. Following core separation, progressive loss of sulfide from the mantle is indicated by the anomalously high Ni (at a given Mg-number) in Archaean basalts and siliceous, Mg-poor sediments. It is also a feature of oceanic and continental basalts that some U/Pb fractionation must have occurred in the mantle source regions following core separation, and this is most satisfactorily achieved if Pb is partially removed from U in a sulfide phase.

Petrology and origin of the shergottite meteorites

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components. The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.

Mineralogy of the planets: a voyage in space and time

The mineralogy of the planets must be considered in the context of chemical and physical differentiation of the solar nebula. Astronomical observations reveal warm dust clouds around young stars and evidence for violent processes in stellar nebulae including rapid expansion, strong rotation, high magnetic field, very powerful solar wind and big mass loss. Theoretical analyses are extremely difficult and the results depend greatly on the initial assumptions. An orthodox interpretation using a hot, massive nebula involves: local instability of several solar masses in galactic cloud; contraction to give Sun; temperature rising to over 2000 K at centre to a few degrees at 100 AU; progressive condensation to give dust; aggregation by several mechanisms including magnetic attraction; clearing and rapid cooling by radiation loss; acceleration to mid-plane of nebula and gravitational instability; multiple collisions producing streamline rotation; growth of planetesimals and ultimately planets; early melting; stunting of Mercury and Mars because of increased velocity of planetesimals near the Sun and Jupiter; inhibition of planet in asteroid region; formation and destruction of moons; intense early bombardment of proto-crust by decaying population of planetesimals; lucky preservation of residual debris as asteroids; ‘clean-up’ of inner region and removal of atmospheres of terrestrial planets by intense solar wind. Many features should be applicable to models based on a cooler, less massive nebula.

Trace elements in shergottite meteorites: Implications for the origins of planets

The average concentrations of 19 siderophile and volatile elements in shergottite meteorites differ from those in terrestrial basalts by less than a factor of ten. This observation undermines claims that the abundances of siderophile and volatile elements in the Earth's upper mantle are uniquely terrestrial. Claims that similarities in the Moon's siderophile element pattern imply a terrestrial origin for the Moon are also weakened. The implication that basalt source regions on the asteroidal parent body of the shergottites resembled the terrestrial upper mantle constrains models of planetary formation and evolution. Heterogeneous accretion models may explain many of the similarities between these planets. Alternatively, separation of sulfide from basaltic magmas or their source regions on the Earth and the shergottite parent body may explain some of these similarities.