Chondritic Abundances Research Articles

Noble gases in presolar diamonds III: Implications of ion implantation experiments with synthetic nanodiamonds

Abstract— A series of experiments carried out by Koscheev et al. (1998, 2001, 2004, 2005) showed that the bimodal release of heavy noble gases from meteoritic nanodiamonds can be reproduced by a single implanted component. This paper investigates the implications of this result for interpreting the noble gas compositions of meteoritic nanodiamonds and for their origin and history. If the bimodal release exhibited by meteorite diamonds reflects release of the P3 noble gas component, then the composition inferred for the pure Xe‐HL end member changes slightly, the excesses of heavy krypton isotopes that define Kr‐H become less extreme, evidence appears for a Kr‐L component, and the nucleosynthetic contribution to argon becomes much smaller. After correction for cosmogenic neon inherited from the host meteorites, the neon in presolar diamonds shows evidence for pre‐irradiation, perhaps in interstellar space, and a nucleosynthetic component perhaps consistent with a supernova source. After a similar correction, helium also shows evidence for presolar irradiation and/or a nucleosynthetic component. For the case of presolar irradiation, due to the small size of the diamonds, a large entity must have been irradiated and recoiling product nuclei collected by the nanodiamonds. The high 3He/21Ne ratio (˜43) calls for a target with a (C + O)/heavier‐element ratio higher than in chondritic abundances. Bulk gas + dust (cosmic abundances) meet this criteria, as would solids enriched in carbonaceous material. The long recoil range of cosmogenic 3He argues against a specific phase. The excess 3He in presolar diamonds may represent trapped cosmic rays rather than cosmogenic 3He produced in the vicinity of the diamond crystals.

Modal abundances of CAIs: Implications for bulk chondrite element abundances and fractionations

Abstract— Modal abundances of Ca,Al‐rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. CAIs are Poisson distributed, and if only small areas (<1000 mm2) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang and Dar al Gani [DaG] 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas are studied, the data are probably not representative of the true CAI modal abundances, Carbonaceous chondrites have excess bulk Al concentrations when compared to the CI‐chondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ˜10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much 26Al to heat the chondrite parent body. Ordinary, enstatite, R and K chondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high‐T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins.

Accretion and core formation: constraints from metal–silicate partitioning

Experimental metal-silicate partitioning data for Ni, Co, V, Cr, Nb, Mn, Si and W were used to investigate the geochemical consequences of a range of models for accretion and core formation on Earth. The starting assumptions were chondritic ratios of refractory elements in the Earth and the segregation of metal at the bottom of a magma ocean, which deepened as the planet grew and which had, at its base, a temperature close to the liquidus of the silicate. The models examined were as follows. (i) Continuous segregation from a mantle which is chemically homogeneous and which has a fixed oxidation state, corresponding to 6.26 per cent oxidized Fe. Although Ni, Co and W partitioning is consistent with chondritic ratios, the current V content of the silicate Earth cannot be reconciled with core segregation under these conditions of fixed oxidation state. (ii) Continuous segregation from a mantle which is chemically homogeneous but in which the Earth became more oxidized as it grew. In this case, the Ni, Co, W, V, Cr and Nb contents of core and mantle are easily matched to those calculated from the chondritic ratios of refractory elements. The magma ocean is calculated to maintain a thickness approximately 35 per cent of the depth to the core-mantle boundary in the accreting Earth, yielding a maximum pressure of 44GPa. This model yields a Si content of the core of 5.7 per cent, in good agreement with cosmochemical estimates and with recent isotopic data. (iii) Continuous segregation from a mantle which is not homogeneous and in which the core equilibrates with a restricted volume of mantle at the base of the magma ocean. This is found to increase depth of the magma ocean by approximately 50 per cent. All of the other elements (except Mn) have partitioning consistent with chondritic abundances in the Earth, provided the Earth became, as before, progressively oxidized during accretion. (iv) Continuous segregation of metal from a crystal-melt mush. In this case, pressures decrease to a maximum of 31GPa and it is extremely difficult to match the calculated mantle contents of the highly incompatible elements Nb and W to those observed. Progressive oxidation is required to fit the observed mantle contents of vanadium. All of the scenarios discussed above point to progressive oxidation having occurred as the Earth grew. The Earth appears to be depleted in Mn relative to the chondritic reference.

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 Earth's 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 (40)Ar and produces the observed (4)He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss.

Primary melting sequence of a deep (> 250 km) lithospheric mantle as recorded in the geochemistry of kimberlite–carbonatite assemblages, Snap Lake dyke system, Canada

Geochemistry of kimberlites and associated carbonatites from the Snap Lake dyke system, Slave craton, Canada, shows that kimberlites are similar, but not identical to Group I kimberlites from South Africa and Siberia. Snap Lake kimberlites are enriched in most incompatible elements such as U, Th, Nb and LREE (up to 1400 times chondritic abundances), and are weakly differentiated relative to primitive mantle in their Sr and Nd isotope compositions. The Snap Lake carbonatites and kimberlites are enriched or depleted in certain trace elements relative to each other. Carbonatites are depleted in Cs, Rb, K, Ta and Ti but enriched in U, Sr, P, Zr, Hf, middle and heavy REEs relative to kimberlites. The observed element distributions are not consistent with experimental data on distribution of trace elements between immiscible carbonate and silicate liquids. However, the chemistry of these rocks shows a continuous systematic change from carbonatites to kimberlites, suggesting their genetic relationship. The range of major element concentrations from low SiO 2 (3 wt.%) magmas to typical kimberlites are in excellent agreement with experimental data for melting of carbonated lherzolite at high pressure. Carbonatites have significantly superchondritic Nb/Ta (22–81) and Zr/Hf (57–128) ratios, which systematically decrease with increase of SiO 2. Modeling of partial melting requires that the source for Snap Lake dyke rocks was enriched in incompatible elements but depleted in HREE relative to the primitive mantle. The most appropriate source rocks are metasomatically enriched peridotites of the deep lithospheric mantle roots. In the melting sequence of this source, the incipient magma was carbonatitic in composition but subsequently became kimberlitic as the degree of partial melting increased to 1%. Rocks of the Snap Lake dyke represent a natural example of primary melting process within a deep (> 250 km) lithospheric mantle.

The Formation Conditions of Chondrules and Chondrites

Chondrules, which are roughly millimeter-sized silicate-rich spherules, dominate the most primitive meteorites, the chondrites. They formed as molten droplets and, judging from their abundances in chondrites, are the products of one of the most energetic processes that operated in the early inner solar system. The conditions and mechanism of chondrule formation remain poorly understood. Here we show that the abundance of the volatile element sodium remained relatively constant during chondrule formation. Prevention of the evaporation of sodium requires that chondrules formed in regions with much higher solid densities than predicted by known nebular concentration mechanisms. These regions would probably have been self-gravitating. Our model explains many other chemical characteristics of chondrules and also implies that chondrule and planetesimal formation were linked.

A revision of the solar manganese abundance using new and remeasured laboratory oscillator strengths

Context.The solar photospheric element abundances are generally in good agreement with the meteoritic CI chondrite abundances, with the exception of a small number of elements including manganese. The solar photospheric abundances, determined using model atmospheres, include laboratory oscillator strengths where available. However, the current laboratory database for Mn i oscillator strengths is derived from several different laboratory observations determined from several different laboratory techniques. The uncertainty in the solar photospheric manganese abundance and the difference between it and the meteoritic CI chondrite abundance may just be an artefact of inaccurate laboratory data.

Tectonic significance of Late Triassic post-collisional lamprophyre dykes from the Qinling Mountains (China)

The Qinling–Dabie orogen in central China is one of the major orogenic belts in East Asia. In the eastern Dabie–Sulu region, mafic lamprophyres show the enriched signatures of old sub-continental lithospheric mantle. However, little is known about the mafic igneous rocks and their lithospheric mantle sources in the western Qinling Range. New 40Ar–39Ar age dating, major- and trace-element data, and isotopic analyses of Qinling lamprophyres reveal their differences from the Dabie Sulu lamprophyres. Biotite 40Ar–39Ar dating yielded a plateau age of 219±2 Ma, identical to the ages of rapakivi-textured granitoids in the area. The association of lamprophyre dykes and rapakivi-textured granitoids indicates that the Qinling region was a post-collisional setting at c. 220 Ma. The Qinling lamprophyres are calc-alkaline, and rich in large ion lithophile elements (e.g. Ba, K), but depleted in Nb, Ta and Ti. They show highly fractionated REE patterns with LaN>100 and HREE <10 times chondrite abundances. εNd (219 Ma) values range from −0.5 to −3.3 and initial Sr isotope values from 0.7036 to 0.7058. These features suggest generation of the lamprophyre by partial melting of a metasomatized, garnet peridotite mantle source. The Qinling lamprophyres are distinct from the Dabie–Sulu lamprophyres in emplacement age (c. 135 Ma for Dabie–Sulu) and isotopic composition, suggesting that the nature of the lithospheric mantle and geodynamic evolution of the Qinling region contrasts with that of the Dabie–Sulu region.

A priori estimates of anomalously high 140, 142Ce and 148, 152, 154Sm abundances and anomalously low 141Pr and 151, 153Eu abundances in chondrites

A priori estimates of anomalously high 140, 142Ce and 148, 152, 154Sm abundances and anomalously low 141Pr and 151, 153Eu abundances in chondrites

Quantitative meteor spectroscopy: Elemental abundances

Procedures are outlined to derive from a meteor spectrum the elemental abundances of its meteoroid, with particular application to observations obtained by an unintensified cooled-CCD slit-less spectrograph. Results are given for two Leonid meteors observed during the 2001 encounter of Earth with dust ejected by comet 55P/Tempel-Tuttle in 1767. The spectra contain air plasma lines of N and O, and meteoric metal atom lines of Fe, Mg, Na, Ca, Si, Mn, Al, and Cr. Excitation conditions are investigated from the relative line intensity of Fe and N + lines. The elemental abundances, normalized to solar system abundances, show a striking correlation with condensation temperature, defined as the temperature at which 50% of elements in a cooling gas mixture with chondritic abundance have condensed into a solid phase. Iron is depleted by a factor of 3, magnesium, calcium, and aluminum by a factor of 8. I conclude that rapid evaporation keeps the mineral surface temperature at ∼1150 K. Much of the refractory elements in these fast 71.6 km/s Leonid meteoroids are deposited in the form of solid meteoric debris rich in Mg, Ca, and Al.

Petrogenesis of the ultrapotassic Lincoln Syenite, Maine: Late Silurian-Early Devonian melting of a source region modified by subduction driven metasomatism

The Late Silurian-Early Devonian (418 ± 1 Ma) Lincoln syenite, exposed discontinuously over a distance of 75 kilometers in south-central and mid-coastal Maine, represents one of the most unusual igneous rocks in the Appalachian orogen. Detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post-magmatic recrystallization. However, a small proportion of the syenites in south-central Maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis. The syenites are characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar + clinopyroxene + orthopyroxene + biotite + oxides. The rocks are intermediate in terms of SiO2 content (55–59 wt. %), yet are ultrapotassic (6–7 wt. % K2O) and relatively primitive in terms of their MgO (Mg# > 67), Cr (> 400 ppm), and Ni (> 150 ppm) concentrations. Additionally, the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements (200–300 chondritic abundances). The syenites show limited radiogenic isotopic variability with relatively high initial 87Sr/86Sr values (0.707 to 0.710), relatively low initial eNd values (−4.8 to −6.7), and initial Pb isotopic compositions characteristic of igneous rocks in central Maine (for example, 207Pb/204Pb = 15.577 to 15.639). The geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids. Crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization, facilitated the crystallization of Mg-rich biotite, and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts. Correlation with the geochemically similar Parks Pond pluton in eastern Maine extends the distribution of Late Silurian-Early Devonian ultrapotassic magmatism in Maine to an orogen-parallel distance of over 150 km. Comparisons with rocks of similar composition in known tectonic environments suggest the Lincoln syenite (and by association the Parks Pond pluton) formed as a result of lithospheric extension superimposed on a sub-continental lithospheric mantle source region previously modified by subduction-driven metasomatism. The timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab-break off or the convective removal of lithospheric mantle (delamination) in association with Acadian convergence. Both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes.

A new geochemical model for the Earth's mantle inferred from 146Sm– 142Nd systematics

New measurements of 142Nd/ 144Nd in kimberlites, carbonatites, komatiites, ocean island basalts from Pitcairn, and mid-ocean ridge basalts from the Pacific and Indian Oceans show no deviation from the terrestrial Nd standard within an external reproducibility of 8 ppm. Measurements of mafic rocks from Isua Greenland confirm previous discoveries of an excess of between 0 and 17 ppm in the 142Nd/ 144Nd of these rocks. Since the terrestrial Nd standard has a 142Nd/ 144Nd approximately 20 ppm higher than measured in chondrites and eucrites, all terrestrial rocks so far analyzed have superchondritic 142Nd/ 144Nd. This requires either that the bulk silicate earth has a superchondritic Sm/Nd ratio or that the Earth experienced a global differentiation event within a couple of hundred million years of planet formation that created complementary incompatible element enriched and depleted mantle reservoirs. If that portion of the mantle involved in the production of continental crust was originally characterized by incompatible element depletion (i.e. high Sm/Nd) rather than chondritic relative abundances of refractory lithophile elements, as is commonly assumed, the mass of mantle affected by continent formation may constitute as much as 80% or more of the mantle. The abundances of K, U and Th in this early-formed depleted mantle are 60–70% of those calculated for a “chondritic” mantle, which removes the inference that half of the 40Ar produced over Earth history must still reside in the mantle and explains why the mantle component characterized by high 3He/ 4He has superchondritic Nd and Hf isotopic composition. If these concentrations characterize the whole mantle, this implies a bulk-earth Urey ratio of ∼ 0.3 and hence that much of the heat escaping the Earth's interior is primordial and not the result of radioactive decay over Earth history. If the bulk silicate earth is presumed to have chondritic abundances of refractory lithophile elements, there must be an incompatible element enriched reservoir buried deep enough in to mantle to preclude its involvement in mantle convection and its contribution to surface volcanism because no terrestrial rock with 142Nd/ 144Nd less than chondritic has been identified.

Spitzer Spectral Observations of the Deep Impact Ejecta

Spitzer Space Telescope imaging spectrometer observations of comet 9P/Tempel 1 during the Deep Impact encounter returned detailed, highly structured, 5- to 35-micrometer spectra of the ejecta. Emission signatures due to amorphous and crystalline silicates, amorphous carbon, carbonates, phyllosilicates, polycyclic aromatic hydrocarbons, water gas and ice, and sulfides were found. Good agreement is seen between the ejecta spectra and the material emitted from comet C/1995 O1 (Hale-Bopp) and the circumstellar material around the young stellar object HD100546. The atomic abundance of the observed material is consistent with solar and C1 chondritic abundances, and the dust-to-gas ratio was determined to be greater than or equal to 1.3. The presence of the observed mix of materials requires efficient methods of annealing amorphous silicates and mixing of high- and low-temperature phases over large distances in the early protosolar nebula.

Compositions of group IVB iron meteorites and their parent melt

The concentrations of P, V, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the group IVB iron meteorites Cape of Good Hope, Hoba, Skookum, Santa Clara, Tawallah Valley, Tlacotepec, and Warburton Range have been measured by laser ablation inductively coupled plasma mass spectrometry. The data were fitted to a model of fractional crystallization of the IVB parent body core, from which the composition of the parent melt and metal/melt distribution coefficients for each element in the system were determined, for a chosen value of D(Ni). Relative to Ni and chondritic abundances, the parent melt was enriched in refractory siderophiles, with greatest enrichment of 5× chondritic in the most refractory elements, and was strongly volatile-depleted, down to 0.00014× chondritic in Ge. Comparison to an equilibrium condensation sequence from a gas of solar composition indicates that no single temperature satisfactorily explains the volatility trend in the IVB parent melt; a small (<1%) complement of ultrarefractory components added to metal that is volatile-depleted but otherwise has nearly chondritic abundances (for Fe, Co and Ni) best explains the volatility trend. In addition to this volatility processing, which probably occurred in a nebular setting, there was substantial oxidation of the metal in the IVB parent body, leading to loss of Fe and other moderately siderophile elements such as Cr, Ga, and W, and producing the high Ni contents that are observed in the IVB irons. By assuming that the entire IVB parent body underwent a similar chemical history as its core, the composition of the silicate that is complementary to the IVB parent melt was also estimated, and appears to be similar to that of the angrite parent.

Compositions of unzoned and zoned metal in the CBbchondrites Hammadah al Hamra 237 and Queen Alexandra Range 94627

Abstract— The CBb chondrites are rare, primitive, metal-rich meteorites that contain several features, including zoned metal, that have previously been interpreted as evidence for origins in the solar nebula. We have measured concentrations of Ni, Cu, Ga, Ru, Pd, Ir, and Au within both zoned and unzoned metal grains in the CBb chondrites Hammadah al Hamra (HaH) 237 and Queen Alexandra Range (QUE) 94627 using laser ablation inductively coupled plasma mass spectrometry. The refractory elements Ni, Ru, and Ir are enriched in the grain cores, relative to the rims, in the zoned metal. All refractory elements are uniform across the unzoned metal grains, at concentrations that are highly variable between grains. The volatile elements Cu, Ga, and Au are usually depleted relative to chondritic abundances and are most often uniform within the grains but are sometimes slightly elevated at the outermost rim. The Pd abundances are nearly uniform, at close to chondritic abundances, in all of the metal grains. A condensation origin is inferred for both types of metal. The data support a model in which the zoned metal formed at high temperatures, in a relatively rapidly cooling nebular gas, and the unzoned metal formed at lower temperatures and at a lower cooling rate. The CBb metal appears to have formed by a process very similar to that of the CH chondrites, but the CBb meteorite components experienced even less thermal alteration following their formation and are among the most primitive materials known to have formed in the solar nebula.

TEXTURAL AND MINERALOGICAL CONSTRAINTS ON CHROMITITE GENESIS: EVIDENCES FROM THE CUMULATE SEQUENCE OF NURALI MASSIF (SOUTHERN URALS, RUSSIA)

TEXTURAL AND MINERALOGICAL CONSTRAINTS ON CHROMITITE GENESIS: EVIDENCES FROM THE CUMULATE SEQUENCE OF NURALI MASSIF (SOUTHERN URALS, RUSSIA)

Thermal Metamorphism of Primitive Meteorites—XII. The Enstatite Chondrites Revisited

Environmental Context.The first Solar System material condensed 4.567 billion years ago, rapidly forming planetesimals—solid bodies that might combine to form planets (accretion) or survive as asteroidal meteorites. Earth’s main accretion ended within the next 30 million years, but subsequent high temperatures essentially erased evidence of this history. However, heating in these early episodes produced effects uniquely recorded by 14 volatile trace elements. The volatile element composition of chondritic meteorites, whose parent material formed closest to Earth, may thus provide important information about early planetesimal evolution. Abstract.We report data for 14 trace and ultratrace elements—Au, Co, Sb, Ga, Rb, Ag, Cs, Te, Zn, Cd, Bi, Tl, In (ordered by increasing putative nebular volatility)—in 13 enstatite (E) chondrites recovered from Antarctica and two E inclusions in the Kaidun polymict breccia that fell in 1980. These data, determined by radiochemical neutron activation analysis (RNAA), essentially double the amount of information known for E chondrites, whose parent materials formed closest to the Sun in the chondrite-forming nebular region. We discuss here the data for all 29 samples studied. The meteoritic suite studied here includes both representatives of previously rare types—like high-iron EH3 and EH5 individuals—but also unique individuals and previously unknown low-iron, EL3, chondrites. Prior hypothetical assertions by others are corrected by the new data. Volatile element contents of EL3 and EH3 chondrites are variable, but comparable, like those of type 3 ordinary chondrites (i.e. H3, L3, and LL3). Volatile element contents of EH4 chondrites are at least as high as those of the E3 types, in contrast to the lower contents of H4, L4, and LL4 types. Compositionally, E3,4 chondrites reflect only nebular condensation and/or accretion processes. Volatiles in E5 and E6 chondrites—whether of EH, EL or unique ones—are depleted relative to cosmic (i.e. CI1) or E3,4 chondrite abundances. The evidence indicates that E5,6 chondrites compositionally reflect vaporization and loss of volatiles during open-system, thermal metamorphism of their parent(s); this may have been the terrestrial environment during Earth’s formation from early planetesimals. Compositional differences between Antarctic E5,6 chondrites and contemporary falls probably do not reflect weathering during the long residence of these chondrites in Antarctica. They might reflect differences in the starting compositions and/or metamorphic conditions in the parent(s).

The trace element composition of silicate inclusions in diamonds: a review

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE N to MREE N, fairly flat MREE N–HREE N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks. Eclogitic garnet inclusions show “normal” REE N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE. Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE N–HREE N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.

Trace element partitioning between majoritic garnet and silicate melt at 25 GPa

We have determined the partitioning of 39 trace elements between majorite-rich garnet and silicate melt at 25 GPa and ∼2300 °C. Concentrations in crystalline and quenched melt phases were determined by both SIMS and laser ablation-ICPMS techniques with, in general, very good agreement in partition coefficients. As anticipated from size and charge considerations, D Lu (where D Lu=[Lu] Majorite/[Lu] Melt=0.77) is much greater than D La (=0.02) and K, U and Th are strongly incompatible. Partition coefficients for isovalent cations entering either the X- or Y-site exhibit a near-parabolic dependence on the radius of the incorporated cations as predicted from the lattice strain model. This underlines the important contribution made by the crystal structure to the control of trace element partitioning. Our data indicate that anhydrous melting at high pressures in the presence of residual garnet could lead to the generation of Al-depleted komatiites. The partitioning data have also been used to assess the potential effects of majoritic garnet fractionation into the deep mantle during a putative magma ocean event in early earth history. Crystallisation of a maximum of 14% majoritic garnet would have slightly depleted the primitive upper mantle in Si and Al without disturbing the chondritic abundance patterns of other refractory elements. The resultant reservoir would have a high Lu/Hf ratio, but a chondritic Sm/Nd ratio and would not be enriched in heat producing elements.

Presolar diamond, silicon carbide, and graphite in carbonaceous chondrites: implications for thermal processing in the solar nebula

We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3 ox), Axtell (CV3 ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites. After accounting for parent-body metamorphism, abundances and characteristics of presolar components still show large variations across the classes of carbonaceous chondrites. These variations correlate with the bulk compositions of the host meteorites and imply that the same thermal processing that was responsible for generating the compositional differences between the various chondrite groups also modified the initial presolar-grain assemblages. The CI chondrites and CM2 matrix have the least fractionated bulk compositions relative to the sun and the highest abundances of most types of presolar material, particularly the most fragile types, and thus are probably most representative of the material inherited from the sun's parent molecular cloud. The other classes can be understood as the products of various degrees of heating of bulk molecular cloud material in the solar nebula, removing the volatile elements and destroying the most fragile presolar components, followed by chondrule formation, metal-silicate fractionation in some cases, further nebula processing in some cases, accretion, and parent body processing. If the bulk compositions and the characteristics of the presolar-grain assemblages in various chondrite classes reflect the same processes, as seems likely, then differential condensation from a nebula of solar composition is ruled out as the mechanism for producing the chondrite classes. Presolar grains would have been destroyed if the nebula had been completely vaporized. Our analysis shows that carbonaceous chondrites reflect all stages of nebular processing and thus are no more closely related to one another than they are to ordinary and enstatite chondrites.