The composition of the Earth

Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an Fe---Ni core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an Fe---Ni alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence ( 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk Fe/Al of 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth ( 14%) and the core ( 86%). Chondritic meteorites display a range of Fe/Al ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.     

Oxidation state of iron and extensive redistribution of sulfur in thermally modified Stardust particles

Two representative thermally modified Stardust samples were investigated by analytical transmission electron microscopy in order to decipher their iron oxidation state after the strong thermal episode due to the capture in aerogel. Their dominant microstructure consists of evenly distributed rounded Fe-Ni-S nano-droplets within a silica-rich glassy matrix. The mineralogy and associated redox state of iron is assessed using a Fe-Mg-S ternary diagram on which ferromagnesian silicates, sulfides and metal can be represented and potentially compared with any other extraterrestrial material. In this diagram, all the data (bulk and local analysis of silicates, sulfide + metal) scatter along a mixing line between the Mg corner and the average composition of the iron-sulfide. There is an obvious genetic relationship between the different phases observed in such samples, further supported by the very low concentration of iron in the glassy matrix. Silicate glasses contain a significant concentration of dissolved sulfur probably present as MgS complexes. This chemical signature is typical of highly reduced environments. These secondary microstructures were established during the high temperature stage of the capture. A significant part of the Fe-droplets formed in situ by reduction at high temperature of ferromagnesian silicates (olivine and pyroxenes) during the impact. At this stage, the indigenous sulfides destabilized and sulfur readily volatilized as S₂, diffused into molten materials and condensed later onto the Fe-precipitates that formed in the silicate melt. This scenario is supported by the structure of Fe-Ni-S beads with a metal core and a sulfide rim. It will be difficult to derive reliable information on the redox state of 81P/Wild 2 particles based on bulk analyses of whole tracks because particles found along the walls of tracks suffered strong reduction reactions, contrary to terminal particles that may have preserved their pristine redox state. The capture effect must be taken into account for comparison of Wild 2 particles with other chondritic material.     

Deep mantle storage of the Earth’s missing niobium in late-stage residual melts from a magma ocean

The origin of the observed niobium deficit in the bulk silicate Earth (BSE) compared to chondritic meteorites constitutes a long-standing problem in geochemistry. The deficit requires a large-scale process fractionating niobium from tantalum, and a super-chondritic Nb/Ta reservoir hidden in the deep silicate Earth and/or in the metallic core. The only voluminous super-chondritic Nb/Ta silicate reservoir analysed to date is found in lunar basalts that assimilated highly evolved Fe-rich rocks associated with anorthosites in the lunar crust. These Fe-rich rocks, enriched in incompatible elements, are thought to represent the last fractions of melt remaining at the end of lunar magma ocean crystallization. Here we report high-precision Nb-Ta data for a Fe-rich, late-stage rock suite associated with a terrestrial anorthosite from the Proterozoic Bolangir complex in India. The geochemical characteristics of this rock suite resemble those expected for late-stage residual melts from a terrestrial magma ocean. Samples show extreme, super-chondritic Nb/Ta up to 31.1 and highly elevated Nb concentrations up to 338 ppm. We argue that formation of an early enriched crustal reservoir (EECR) with these characteristics (high Fe, high Nb, superchondritic Nb/Ta) is likely in the course of Hadean late-stage terrestrial magma ocean solidification. Subduction and subsequent permanent deep mantle storage in the D′′ layer of a minor amount (∼0.5% of the BSE mass) of this EECR can readily explain the terrestrial Nb deficit, without the need to invoke core Nb storage. Our model is consistent with short-lived ¹⁴²Nd and long-lived ¹⁷⁶Hf-¹⁴³Nd isotope models for early differentiation of the Earth’s crust. In addition, the inferred Lu/Hf of this EECR implies that this reservoir can also balance the offset of terrestrial Hf isotope ratios compared to the chondritic reservoir. As such, late-stage magma ocean residual melts may constitute the enigmatic parental reservoir of Hadean zircons with low time-integrated Hf isotope compositions.     

The chemical composition of metallic spheroids and metallic particles within impactite from Barringer Meteorite Crater,Arizona

Atomic absorption analyses of 25 metallic spheroids from Barringer Meteorite Crater have been carried out for Fe, Ni, Co and Cu. In addition, electron microprobe analyses of 58 impactite metallic particles have been carried out for Fe, Ni and Co from four different impactite samples. The normalized Ni, Co and Cu contents of the spheroids were from 13–22 per cent, 0.8–1.3 per cent and 260–430 μg/g, respectively. These figures represent enrichment factors of 2–3 in the spheroids compared to analyses of the bulk meteorite. The Co/Ni and Cu/Ni ratios in the spheroids are close to the respective ratios in the bulk meteorite. This suggests that the spheroids were formed by preferential removal of iron by oxidation from a chemically homogeneous liquid.The impactite metallic particles had Ni contents from 10 to 95 per cent and Co contents from 0·3 to 4 per cent. The Co contents of these particles showed a positive correlation with Ni up to 60 per cent Ni and a negative correlation beyond 60 per cent Ni. Reaction of the impactite metallic particles with SiO₂ of the target can explain these variations. Our findings show that extensive chemical reaction between projectile and target occurs at impact.     

The iron-nickel-phosphorus system: Effects on the distribution of trace elements during the evolution of iron meteorites

To better understand the partitioning behavior of elements during the formation and evolution of iron meteorites, two sets of experiments were conducted at 1 atm in the Fe-Ni-P system. The first set examined the effect of P on solid metal/liquid metal partitioning behavior of 22 elements, while the other set explored the effect of the crystal structures of body-centered cubic (α)- and face-centered cubic (γ)-solid Fe alloys on partitioning behavior. Overall, the effect of P on the partition coefficients for the majority of the elements was minimal. As, Au, Ga, Ge, Ir, Os, Pt, Re, and Sb showed slightly increasing partition coefficients with increasing P-content of the metallic liquid. Co, Cu, Pd, and Sn showed constant partition coefficients. Rh, Ru, W, and Mo showed phosphorophile (P-loving) tendencies. Parameterization models were applied to solid metal/liquid metal results for 12 elements. As, Au, Pt, and Re failed to match previous parameterization models, requiring the determination of separate parameters for the Fe-Ni-S and Fe-Ni-P systems.Experiments with coexisting α and γ Fe alloy solids produced partitioning ratios close to unity, indicating that an α versus γ Fe alloy crystal structure has only a minor influence on the partitioning behaviors of the trace element studied. A simple relationship between an element’s natural crystal structure and its α/γ partitioning ratio was not observed. If an iron meteorite crystallizes from a single metallic liquid that contains both S and P, the effect of P on the distribution of elements between the crystallizing solids and the residual liquid will be minor in comparison to the effect of S. This indicates that to a first order, fractional crystallization models of the Fe-Ni-S-P system that do not take into account P are appropriate for interpreting the evolution of iron meteorites if the effects of S are appropriately included in the effort.     

The case against early melting of the bulk of the Moon

Recent data suggest that the source region of mare basalts became compositionally closed at 4.42 Gy. presumably some 170 My. after the Moon accreted. Thermal-history models indicate that the outer part of the Moon could not have cooled to temperatures low enough to cause closure unless only the outer few hundred kilometers were initially molten. A total early lunar differentiation is therefore prohibited. The bulk of the Moon was therefore pristine and undifferentiated at the time of mare basalt formation. bl     

Highly siderophile elements in the Earth, Moon and Mars: Update and implications for planetary accretion and differentiation

The highly siderophile elements (HSE) pose a challenge for planetary geochemistry. They are normally strongly partitioned into metal relative to silicate. Consequently, planetary core segregation might be expected to essentially quantitatively remove these elements from planetary mantles. Yet the abundances of these elements estimated for Earth's primitive upper mantle (PUM) and the martian mantle are broadly similar, and only about 200 times lower than those of chondritic meteorites. In contrast, although problematic to estimate, abundances in the lunar mantle may be more than twenty times lower than in the terrestrial PUM. The generally chondritic Os isotopic compositions estimated for the terrestrial, lunar and martian mantles require that their long-term Re/Os ratios were within the range of chondritic meteorites. Further, most HSE in the terrestrial PUM also appear to be present in chondritic relative abundances, although Ru/Ir and Pd/Ir ratios are slightly suprachondritic. Similarly suprachondritic Ru/Ir and Pd/Ir ratios have also been reported for some lunar impact melt breccias that were created via large basin forming events.Numerous hypotheses have been proposed to account for the HSE present in Earth's mantle. These hypotheses include inefficient core formation, lowered metal-silicate D values resulting from metal segregation at elevated temperatures and pressures (as may occur at the base of a deep magma ocean), and late accretion of materials with chondritic bulk compositions after the cessation of core segregation. Synthesis of the large database now available for HSE in the terrestrial mantle, lunar samples, and martian meteorites reveals that each of the main hypotheses has flaws. Most difficult to explain is the similarity between HSE in the Earth's PUM and estimates for the martian mantle, coupled with the striking differences between the PUM and estimates for the lunar mantle. More complex, hybrid models that may include aspects of inefficient core formation, HSE partitioning at elevated temperatures and pressures, and late accretion may ultimately be necessary to account for all of the observed HSE characteristics. Participation of aspects of each process may not be surprising as it is difficult to envision the growth of a planet, like Earth, without the involvement of each.     

Experimental study of reaction between perovskite and molten iron to 146 GPa and implications for chemically distinct buoyant layer at the top of the core

Partitioning of oxygen and silicon between molten iron and (Mg,Fe)SiO₃ perovskite was investigated by a combination of laser-heated diamond-anvil cell (LHDAC) and analytical transmission electron microscope (TEM) to 146 GPa and 3,500 K. The chemical compositions of co-existing quenched molten iron and perovskite were determined quantitatively with energy-dispersive X-ray spectrometry (EDS) and electron energy loss spectroscopy (EELS). The results demonstrate that the quenched liquid iron in contact with perovskite contained substantial amounts of oxygen and silicon at such high pressure and temperature (P–T). The chemical equilibrium between perovskite, ferropericlase, and molten iron at the P–T conditions of the core–mantle boundary (CMB) was calculated in Mg–Fe–Si–O system from these experimental results and previous data on partitioning of oxygen between molten iron and ferropericlase. We found that molten iron should include oxygen and silicon more than required to account for the core density deficit (<10%) when co-existing with both perovskite and ferropericlase at the CMB. This suggests that the very bottom of the mantle may consist of either one of perovskite or ferropericlase. Alternatively, it is also possible that the bulk outer core liquid is not in direct contact with the mantle. Seismological observations of a small P-wave velocity reduction in the topmost core suggest the presence of chemically-distinct buoyant liquid layer. Such layer physically separates the mantle from the bulk outer core liquid, hindering the chemical reaction between them.     

Chemical composition of Luna 20 rocks and soil and Apollo 16 soils

The abundances of 24 major, minor and trace elements have been measured by INAA in Luna 20 metaigneous rocks 22006,1 and 22007,1, breccia 22004 and soil 22001,9 and in Apollo 16 soils 62281, 66041 and 66081. An additional 12 trace meteoritic and non-meteoritic elements have also been determined in 22001 and 62281 soils by RNAA. The bulk compositions of L 20 and Ap 16 rocks and soils show close similarity between the two highland sites. There are appreciable differences in bulk compositions between the L 20 highland and the L 16 mare site (120 km apart), suggesting little intermixing of rocks and soils from either site. Luna 20 rocks 22006 and 22007 are nearly identical in chemical composition to Ap 16 metaigneous rocks 61156 and 66095. Luna 20 rocks are feldspathic and are similar to low K-type Fra Mauro basalts. Such rocks and anorthositic gabbros appear to be the major components in highland soils. Luna 20 soil can be distinguished from Ap 16 soils by lower abundances of Al₂O₃, CaO and large ion lithophilic elements. Luna 20 breccia 22004 probably is compacted soil. All L 20 samples show negative Eu anomalies with $\text{Sm}\text{Eu}$ ratios of 5.8, 7.2, 3.9 and 3.3 for rocks 22006, 22007, breccia 22004 and soil 22001, respectively. Norite-KREEP is insignificant, ≤1 per cent, at the L 20 highland site. The derivation of the L 20 soil may be explained by ≈33 per cent of L 20 metaigneous rocks and ≈ 65 per cent anorthositic gabbroic breccia rocks like 15418 (with a positive Eu anomaly) and ≈ 2 per cent meteoritic contributions. Interelement correlations observed previously for maria are also found in highland samples. Luna 20 and Ap 16 soils are low in alkalis. Both soils show an apparent Cd-Zn rich component similar to that observed at the mare sites and high 11 abundances relative to mare sites. The Ap 16 (62281) soil contains a fractionated meteoritic component (probably ancient) of ≈ 1.5 per cent in addition to ≈ 1.9 per cent Cl like material. Luna 20 soil may simply contain 1.9 per cent Cl equivalent.     

Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar Petrogenesis Revisited

There is a conspicuous dichotomy in the conventional model oflunar petrogenesis between the total intra-crustal differentiationpostulated for the products of feldspathic volcanism in thelunar highlands and the near absence of differentiation postulatedfor the products of mare volcanism. Both the cumulate mantlemodel, and the selenotherm postulated to accompany genesis ofalleged ‘primary’ mare magmas by remelting of thosecumulates, imply supra-adiabatic thermal gradients in near-solidusmaterials throughout the lunar mantle 4·3–3·2Ga ago. This should have resulted in vigorous convective motion,which has not occurred. There is no positive europium anomalyin the average lunar highland crust. That crust cannot, therefore,have formed by plagioclase flotation from a lunar magma ocean,for which there is no other requirement. There is no negativeeuropium anomaly in the average mantle to be inherited by latermare basalts. Other rocky bodies of lunar size in the SolarSystem have accreted at rates that allowed incorporation ofplenty of volatiles and without forming global magma oceans.Partial melting in the presence of water, followed by near-surfacefractionation and volatile losses can explain the feldspathiccharacter, high incompatible element concentrations and lackof Eu anomaly in the lunar highlands. Volcanic eruption on theMoon must have been accompanied by selective volatilizationlosses of sodium, sulphur and other elements similar to theprocess seen on Io, which can account for the major differencesbetween terrestrial and lunar basalts. Siderophile element depletionin lunar lavas may reflect immiscible sulphide liquid and metalseparation, rather than global impoverishment in such elements,and large ore bodies may have formed close to the lunar surface.Mare basalt volcanism appears to have been a protracted, lowmagma productivity event with few similarities to terrestrialocean-floor, ocean-island, continental flood basalt or komatiitevolcanism. At low pressure the crystallization of plagioclasewell before pyroxene typifies those terrestrial mid-ocean ridgebasalt, ocean-island basalt and continental flood basalt magmas.A similar sequence is demanded of the postulated lunar primarymagmas. Mare basalt hand-specimen and pyroclastic glass beadcompositions do not, however, display the required crystallizationsequence and cannot represent the required primary melt compositions.The true erupted lava compositions which gave rise to the regolithcompositions across all the maria are much more feldspathicthan the majority of large hand specimens and, in common withbasalts on other planets, they are close to low-pressure plagioclase-saturatedcotectic residual liquids which have evolved by removal of gabbrosin crustal magma chambers, or perhaps in giant lava lakes akinto topless Bushveld complexes. Any further debate could be resolvedby a 100 m drill core in a few mare locations. Field provenanceof samples from Mars, a planet half covered by flood basaltsand products of central volcanoes, will be little better thanfor those from the Moon. It will be important to encourage multipleworking hypotheses, rather than to rush to a consensus. KEY WORDS: lunar; basalt; highland; magma ocean; europium     

Petrography and mineral chemistry of Ca-rich inclusions in the Allende meteorite

There are two types of white, coarse-grained, Ca-Al-rich inclusions in Allende. Type A inclusions contain 80–85 per cent melilite, 15–20 per cent spinel, 1–2 per cent perovskite and rare plagioclase, hibonite, wollastonite and grossularite. Clinopyroxene, if present, is restricted to thin rims around inclusions or cavities in their interiors. Type B inclusions contain 35–60 per cent pyroxene, 15–30 per cent spinel, 5–25 per cent plagioclase and 5–20 per cent melilite. The coarse pyroxene crystals in Type B's contain >15 per cent Al₂O₃ and >1.8 per cent Ti, some of which is trivalent. Type A pyroxenes contain <9 per cent Al₂O₃ and <0.7 per cent Ti.Electron microprobe analyses of 600 melilite, 39 pyroxene, 35 plagioelase, 33 spinel and 20 perovskite grains were performed in 16 Type A, 1 intermediate and 9 Type B inclusions in Allende and 1 Type A in Grosnaja. Melilite composition histograms from individual Type A inclusions are usually peaked between Ak10 and Ak30 and are 15–20 mole % wide while those from Type B inclusions are broader, unpeaked and displaced to higher åkermanite contents. Most pyroxenes contain < 1 per cent FeO. All plagioclase is An 98 to An 100. Spinel is almost pure MgAl₂O₄. Perovskite contains small (< 1 per cent) but significant amounts of Mg, Al, Fe, Y, Zr and Nb.Inferred bulk chemical compositions of Type A inclusions are rather close to those expected for high-temperature condensates. Those of Type B inclusions suggest slightly lower temperatures but their Ca/Al ratio seems less than the Type A's, indicating that the Type B's may not be their direct descendants. Some textural features suggest that the inclusions are primordial solid condensetes while others indicate that they may have been melted after condensation. Fragmentation and metamorphism may have also occurred after condensation.     

Sulfide Petrology of Spinel and Garnet Pyroxenite Layers from Mantle-derived Spinel Lherzolite Massifs of Arieg`, Northeastern Pyrenees, France

Pyroxenite layers in the orogenic spinel lherzolite massifsof Ari?ge are porphyroclastic textured and range in compositionfrom spinel websterite to garnet clinopyroxenite. Each pyroxenitetype forms individual layers or occurs as part of compositelayers in which the Opx/Cpx and Sp/Gt ratios decrease from marginsto core. They are interpreted as crystalline segregations separatedby flow crystallization from continental tholeiites en routeto the surface. The primary magmatic phases consist of Al-richpyroxenes, together with a minor amount of spinel, which startedto crystallize at 1400?C and 20–22 kb pressure; the pyroxeneshave locally survived plastic strains and subsolidus rccrystallizationsand now occur in the form of clinopyroxene and orthopyroxenemegacrysts displaying unmixing features. Although the differentiated silicate liquid was fully expelledduring the flow crystallization process, the layered pyroxeniteshave concentrated the highly incompatible elements S and Cuand locally display significant chalcophile platinum-group elementenrichment (Pd, Pt). Cu and S behave coherently over the wholerange of pyroxenite composition; their highest concentrationsare found in the thinnest websterite layers or at the marginof composite layers. Microscopic investigation of 214 polishedthin sections shows these elements to be present as accessoryCu-Fe-Ni sulfides interstitial among the silicate phase or formingdiscrete bodies included in the relic pyroxene megacrysts. Allthese features indicate the presence of a sulfide liquid, immisciblewith the silicate magma, during the crystallization of the layeredpyroxenites. Sulfide liquid immiscibility probably occurredin response to thermal contrast between the pyroxenites andthe cooler surrounding peridotites. It is proposed that the megacryst-hosted sulfide inclusionswere trapped as linear arrays arranged on host megacryst growthplanes. Due to the slow cooling and complex unmixing historyof the megacrysts, these arrays have been transformed into coarse,isolated sulfide inclusions by subsolidus migration and spheroidizationprocesses. They started to crystallize at T = 1200?C as monosulfidesolid solution (MSS), probably coexisting with a minor amountof Ni- and Cu-rich sulfide liquid down to r=900?C. The reconstructionof the bulk chemistry of each individual inclusion reveals significantbetween-inclusion variations of Cu/Ni+ Fe and Ni/Fe ratios,which would result from strain-induced immobilization of theseliquids. On cooling, the high-temperature MSS has decomposedbelow 230?C into Ni-rich pyrrhotite, nickeliferous pentlandite,chalcopyrite and minor pyrite. The post-magmatic history ofthe interstitial sulfide grains was not unlike that of the inclusions,except at near-surface temperatures where the primary sulfidesresulting from unmixing of MSS have been partly altered intosecondary sulfides by serpentinizing aqueous fluids. In spite of these post-magmatic alterations, the inclusionsand the interstitial sulfide phases are remarkably homogeneousas regards their bulk Ni/Cu ratio, which is close to 3. Thisvalue is characteristic of sulfide separated from primary ratherthan partially differentiated tholeiitic melts. It is thus concludedthat the continental tholeiite parent to the layered pyroxeniteswas saturated with sulfides when it left its mantle source regioaIn this aspect, it would not be different from MORBs which containsimilar sulfide compositions. In both cases, sulfide fractionationcannot be ignored in models for chalcophile trace element fractionationduring initial evolution of these magmas.     

Lunar Magma Ocean crystallization revisited: Bulk composition, early cumulate mineralogy, and the source regions of the highlands Mg-suite

Crystallization of the Lunar Magma Ocean (LMO) has been numerically modeled and its products inferred from sample observations, but it has never been fully tested experimentally. This study is a reexamination of the LMO hypothesis by means of the first experimental simulation of lunar differentiation. Two end-member bulk Moon compositions are considered: one enriched in refractory lithophile elements relative to Earth and one with no such enrichment. A “two-stage” model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean. An initially entirely molten Moon is assumed. Part 1 of this study, presented here, focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization, differences in mantle mineralogy resulting from different bulk Moon compositions, and implications for the source regions of the highlands Mg-suite.Refractory element enriched bulk Moon compositions produce a deep mantle that contains garnet and trace Cr-spinel in addition to low-Ca pyroxene and olivine. In contrast, compositions without refractory element enrichment produce a deep dunitic mantle with low-Ca pyroxene but without an aluminous phase. The differences in bulk composition are magnified in the residual melt; the residual LMO from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities. Both compositions produce Mg-rich early cumulate piles that extend from the core-mantle boundary to ∼355 km depth, if 50% equilibrium crystallization and whole Moon melting are assumed. These early LMO cumulates provide good fits for the source regions for a component of the high-Mg^∗, Ni- and Co-poor parental magmas of the Mg-suite cumulates, if certain conditions are called upon. The olivine in early LMO cumulates produced by either bulk Moon composition is far too rich in Cr to be reasonable for the source regions of the Mg-suite, meaning either core formation in the presence of S and/or C must be invoked to deplete the LMO and the crystallizing olivine in Cr, or that current estimates of the bulk lunar Cr content are too high. We infer that melts meeting the criteria of the Mg-suite parents could be produced from early LMO cumulates by solid state KREEP and plagioclase hybridization near the base of the crust and subsequent partial melting. Additionally, we propose a revised model for Mg-suite petrogenesis.     

A lunar differentiation model in light of new chemical data on Luna 20 and Apollo 16 soils

Fines from a Luna 20 soil sample and from three Apollo 16 deep drill core samples have been analyzed for major-minor element abundances by a combined, semi-micro atomic absorption spectrophotometric and colorimetric method. Both the major element and large ion lithophile trace element abundances in these soils, the first from interior highland sites, are greatly influenced by the very high normative plagioclase content, being distinctly richer in Al and Ca, and poorer in K, P, Cr, Mn, Fe, and Ti, than most bulk soil samples from previous lunar missions. The relatively large compositional variations in the Apollo 16 core can be ascribed almost entirely to decreasing plagioclase with increasing depth. The chemical composition of the Luna 20 soil indicates less plagioclase and less KREEP than in the Apollo 16 soils. A lunar differentiation model is presented in which is made the suggestion that KREEP is the result of a second fusion event in a lunar crust consisting of early feldspathic cumulates and primary aluminous ‘liquid’.     

Feldspathic lunar meteorites and their implications for compositional remote sensing of the lunar surface and the composition of the lunar crust

We present new compositional data for six feldspathic lunar meteorites, two from cold deserts (Yamato 791197 and 82192) and four from hot deserts (Dhofar 025, Northwest Africa 482, and Dar al Gani 262 and 400). The concentrations of FeO (or Al₂O₃) and Th (or any other incompatible element) together provide first-order compositional information about lunar polymict samples (breccias and regoliths) and regions of the lunar surface observed from orbit. Concentrations of both elements on the lunar surface have been determined from data acquired by orbiting spacecraft, although the derived concentrations have large uncertainties and some systematic errors compared to sample data. Within the uncertainties and errors in the concentrations derived from orbital data, the distribution of FeO and Th concentrations among lunar meteorites, which represent ∼18 source regions on the lunar surface, is consistent with that of 18 random samples from the surface. Approximately 11 of the lunar meteorites are low-FeO and low-Th breccias, consistent with large regions of the lunar surface, particularly the northern farside highlands. Almost all regoliths from Apollo sites, on the other hand, have larger concentrations of both elements because they contain Fe-rich volcanic lithologies from the nearside maria and Th-rich lithologies from the high-Th anomaly in the northwestern nearside. The feldspathic lunar meteorites thus offer our best estimate of the composition of the surface of the feldspathic highlands, and we provide such an estimate based on the eight most well-characterized feldspathic lunar meteorites. The variable but high (on average) Mg/Fe ratio of the feldspathic lunar meteorites compared to ferroan anorthosites confirms a hypothesis that much of the plagioclase at the surface of the feldspathic highlands is associated with high-Mg/Fe feldspathic rocks such as magnesian granulitic breccia, not ferroan anorthosite. Geochemically, the high-Mg/Fe breccias appear to be unrelated to the mafic magnesian-suite rocks of the Apollo collection. Models for the formation of the upper lunar crust as a simple flotation cumulate composed mainly of ferroan anorthosite do not account for the complexity of the crust as inferred from the feldspathic lunar meteorites.     

Petrogenesis of Basalt Glasses from the Tamayo Region, East Pacific Rise

Samples collected by the Tamayo scientific team both by dredgingand by submersible along 75 km of the East Pacific Rise (EPR)up to its intersection with the Tamayo fracture zone allow explorationof the systematics of ocean ridge basalt petrogenesis in thevicinity of a major transform fault. To investigate these systematicstwenty-nine samples of hand-picked glasses have been analyzedfor major elements, REE and other trace elements. There are distinct chemical differences between samples collectedfar from the Tamayo transform and close to the transform. Samplesfurthest from the transform, where the EPR is characterizedby a broad swell, are, for the same MgO content, lower in abundancesof all incompatible elements than samples near the transformwhere the ridge morphology is characterized by a rift. We callsuch chemical systematics the transform fault effect (TFE).Possible models for the transform fault effect include: (1)fractional crystallization at low or high pressure; (2) partialmelting; and (3) open system fractionation and mixing. Thesemodels have been evaluated using accurately calculated liquidlines of descent for the major elements, and an inversion techniquefor the trace elements. Although low and possibly high pressure fractionation are important,partial melting accounts best for the variations among the Tamayoparental magmas. Basalts erupted in close proximity (< 16km) to the transform (‘rift’ samples) are derivedfrom melts generated by smaller extents of melting than thoseerupted farther (45–75 km) from the transform (‘swell’samples). Inversion of the Tamayo trace element data shows thatbatch melting can account quantitatively for the trace elementvariations, provided that the extents of melting are very small(1–5 per cent). Continuous melting seems more physicallyrealistic and allows slightly higher extents of melting althoughthe total amount of melt removed from the mantle would be ofthe order of 5 per cent. Both melting models account for thedata better if the swell samples are derived from magmas formedat greater depths where garnet is a stable phase while the riftsamples are derived at shallower levels where garnet is notstable. This suggests that melting may occur near the garnet/spinelphase transition in the mantle. The lesser extents of meltingnear the transform could be caused directly by the cooling effectsof ridge truncation, or by perturbation of mantle flow and magmadynamics at ridge/transform intersections. Examination of theTFE at many other ridge/transform intersections is necessaryto distinguish among these possibilities.     

Lunar-A mission: Outline and current status

The scientific objective of the Lunar-A, Japanese Penetrator Mission, is to explore the lunar interior by seismic and heat-flow experiments. Two penetrators containing two seismometers (horizontal and vertical components) and heat-flow probes will be deployed from a spacecraft onto the lunar surface, one on the near-side and the other on the far-side of the moon. The data obtained by the penetrators will be transmitted to the earth station via the Lunar-A mother spacecraft orbiting at an altitude of about 200 km. The spacecraft of a cylindrical shape, 2.2 m in maximum diameter and 1.7 m in height, is designed to be spin-stabilized. The spacecraft will be inserted into an elliptic lunar orbit, after about a half-year cruise during which complex manoeuvering is made using the lunar-solar gravity assist. After lunar orbit insertion, two penetrators will be separated from the spacecraft near perilune, one by one, and will be landed on the lunar surface. The final impact velocity of the penetrator will be about 285 m/sec; it will encounter a shock of about 8000 G at impact on the lunar surface. According to numerous experimental impact tests using model penetrators and a lunar-regolith analog target, each penetrator is predicted to penetrate to a depth between l and 3 m, depending on the hardness and/or particle-size distribution of the lunar regolith. The penetration depth is important for ensuring the temperature stability of the instruments in the penetrator and heat flow measurements. According to the results of the Apollo heat flow experiment, an insulating regolith blanket of only 30 cm is sufficient to dampen out about 280 K lunar surface temperature fluctuation to < 3 K variation. The seismic observations are expected to provide key data on the size of the lunar core, as well as data on deep lunar mantle structure. The heat flow measurements at two penetrator-landing sites will also provide important data on the thermal structure and bulk concentrations of heat-generating elements in the Moon. These data will provide much stronger geophysical constraints on the origin and evolution of the Moon than has been obtained so far. Currently, the Lunar-A system is being reviewed and a more robust system for communication between the penetrators and spacecraft is being implemented according to the lessons learned from Beagle-2 and DS-2 failures. More impact tests for penetrators onto a lunar regolith analogue target will be undertaken before its launch.     

High-pressure phase equilibria and element partitioning experiments on Apollo 15 green C picritic glass: Implications for the role of garnet in the deep lunar interior

We report results of nominally anhydrous near-liquidus experiments on a synthetic analog to very low-titanium Apollo 15 green C lunar picritic glass from ∼2 to 5 GPa. Apollo 15 green C glass (A15C) is saturated with garnet and pyroxene on the liquidus at ∼3 GPa. However, such an assemblage is unlikely to represent the lunar-mantle source region for this glass, and instead an olivine + orthopyroxene-dominated source is favored, in accord with earlier lower-pressure experiments on A15C. Near-liquidus garnet has a slight but significant majorite component at ∼5 GPa in this iron-rich bulk composition, as expected from our previous work in ordinary-chondritic bulk compositions. Ion microprobe measurements of partitioning of Sr, Ba, Sc, Nd, Sm, Dy, Yb, Y, Zr, Hf, and Th between garnet and coexisting melt in these experiments are the first garnet partition coefficients (D values) available that are directly relevant to lunar compositions. D values for these garnets differ significantly compared to D values for garnets grown in more magnesian, terrestrial bulk compositions, which until now are all that have been available in modeling the possible role of garnet in the lunar interior. For example, D values for heavy rare earth elements are lower than are those from terrestrial basaltic systems. These partitioning values are well-described by the lattice-strain partitioning model, but predictive relationships for garnet partitioning using that model fail to match the measured values, as was the case in our earlier work on chondritic compositions. Using our new D values in place of the “terrestrial” values in a variety of models of lunar petrogenesis, we suggest that garnet is unlikely to be present in the source regions for very titanium-poor lunar liquids despite its appearance on the liquidus of A15C.     

Penetration of molten iron alloy into the lower mantle phase

The core–mantle boundary is the only interface where the metallic core and the silicate mantle interact physically and chemically. Many geophysical anomalies such as low shear velocity and high electrical conductivity have been observed at the bottom of the mantle. Perturbations in the Earth's rotation rate at decadal time periods require the existence of a thin conductive layer with a conductance of 10⁸ S. Substantial additions of molten iron from the outer core into the mantle may produce these geophysical anomalies. Although iron enrichment by penetration has only been observed in (Mg,Fe)O, the second dominant mineral in the lower mantle, the penetration process leading to iron enrichment in the silicate mantle has not been experimentally confirmed. In this study, high-pressure and high-temperature experiments were conducted to investigate the penetration of molten iron alloy into lower mantle phases; postspinel, polycrystalline bridgmanite and polycrystalline (Mg,Fe)O. At the interface between (Mg,Fe)O aggregate and molten iron alloy, liquid metal penetrated the (Mg,Fe)O aggregate along grain boundaries and formed a thin layer containing metal-rich blobs. In contrast, no penetration of molten iron alloy was observed at the interface between molten iron alloy and silicate phases. Penetration of liquid iron alloy into the (Mg,Fe)O aggregate is caused by the capillarity phenomenon or Mullins–Sekerka instability. Neither mechanism occurs at the boundary of pure polycrystalline MgO, indicating that the FeO in (Mg,Fe)O plays an essential role in this phenomenon. Infiltration of molten iron alloy along grain boundaries (capillarity phenomenon) is the dominant process and precedes penetration due to the Mullins–Sekerka instability. The capillarity phenomenon is governed by the balance of forces between surface tension and gravity. In the case where the ultralow velocity zone (ULVZ) with a low shear velocity is composed of Fe-rich (Mg,Fe)O, the maximum penetration distance of molten iron alloy by capillary rise is limited to 20 m. The addition of iron-rich melt to the base of the mantle is therefore unlikely to be the main cause of the high conductance of the CMB region predicted from decadal variation of the length of day. Furthermore, the absence of molten iron alloy penetration into silicate phases does not allow an extensive modification of the chemical composition of the mantle by core–mantle interaction.     

Moon and Earth : compositional differences inferred from siderophiles,volatiles, and alkalis in basalts

We have compared RNAA analyses of 18 trace elements in 25 low-Ti lunar and 10 terrestrial oceanic basalts. According to Ringwood and Kesson, the abundance ratio in basalts for most of these elements approximates the ratio in the two planets.Volatiles (Ag, Bi, Br, Cd, In, Sb, Sn, Tl, Zn) are depleted in lunar basalts by a nearly constant factor of 0.026 ± 0.013, relative to terrestrial basalts. Given the differences in volatility among these elements, this constancy is not consistent with models that derive the Moon's volatiles from partial recondensation of the Earth's mantle or from partial degassing of a captured body. It is consistent with models that derive planetary volatiles from a thin veneer (or a residuum) of C-chondrite material; apparently the Moon received only 2.6% of the Earth's endowment of such material per unit mass.Chalcogens (Se and Te) have virtually constant and identical abundances in lunar and terrestrial basalts, probably reflecting saturation with Fe(S, Se, Te) in the source regions.Siderophiles show diverse trends. Ni is relatively abundant in lunar basalts (4 × 10^?3 × Cl-chondrites), whereas Ir, Re, Ge, Au are depleted to 10^?4?10^?5× Cl. Except for Ir, these elements are consistently enriched in terrestrial basalts: Ni 3 × , Re 370 ×, Ge 330 × , Au 9 × . This difference apparently reflects the presence of nickel-iron phase in the lunar mantle, which sequesters these metals. On Earth, where such metal is absent, these elements partition into the crust to a greater degree. Though no lunar mantle rock is known, an analogue is provided by the siderophile-rich dunite 72417 (~0.1% metal) and the complementary, siderophile-poor troctolite 76535. The implied metal-siderophile distribution coefficients range from 10⁴ to 10⁶, and are consistent with available laboratory data.The evidence does not support the alternative explanation advanced by Ringwood—that Re was volatilized during the Moon's formation, and is an incompatible element (like La or W⁴⁺) in igneous processes. Re is much more depleted than elements of far greater volatility: (Re/U)_Cl～- 4 × 10^?6 vs (T1/U)_Cl = 1.3 × 10^?4, and Re does not correlate with La or other incompatibles.Heavy alkalis (K, Rb, Cs) show increasing depletion with atomic number. Cs/Rb ratios in lunar basalts, eucrites, and shergottites are 0.44, 0.36, and 0.65 × Cl, whereas the value for the bulk Earth is 0.15–0.26. These ratios fall within the range observed in LL and E6 chondrites. supporting the suggestion that the alkali depletion in planets, as in chondrites, was caused by localized remelting of nebular dust (= chondrule formation). Indeed, the small fractionation of K, Rb and Cs, despite their great differences in volatility, suggests that the planets, like the chondrites, formed from a mixture of depleted and undepleted material, not from a single, partially devolatilized material.