Chondritic Silicate Research Articles

The Early Solar System Abundance of Iron-60: New Constraints from Chondritic Silicates

The abundance of iron-60 in the early solar system is important for planetary evolution models, and has been hotly debated. To put further constraints on the initial 60Fe/56Fe ratio of the solar system, here we present new iron-nickel isotope data, measured in situ by NanoSIMS, for 14 silicate chondrules from three carbonaceous and three unequilibrated ordinary chondrites. NanoSIMS measurements were performed at high spatial resolution (200–300 nm primary beam diameter), to avoid inclusion of unwanted phases in the analysis volume. The average initial 60Fe/56Fe ratios that can be estimated from our pooled chondrule data are 2.1 (±1.3) × 10−7 and 0.8 (±1.0) × 10−7 for carbonaceous and ordinary chondrites, respectively (1σ uncertainties). The estimated average initial 60Fe/56Fe ratio of all analyzed chondrules is 1.0 (±0.7) × 10−7. These results are inconsistent with initial 60Fe/56Fe ratios >2.4 × 10−7 (2σ upper limit of our entire data set) reported in the literature for some chondrule silicates based on in situ isotope data, and agree better with our previously published in situ data on chondritic troilites (0.10 ± 0.15 × 10−7), as well as with 60Fe/56Fe ratios estimated from isotope data of bulk meteorites and chondrules (0.10–0.75 × 10−7). Our isotope data hint at a possible difference between the initial 60Fe/56Fe ratios of the early solar system’s two major isotope reservoirs, with the carbonaceous chondritic reservoir having higher iron-60 abundance than the non-carbonaceous reservoir. Nevertheless, in light of similar hints in the literature, this possibility deserves further investigation.

Formation, cooling history and age of impact events on the IIE iron parent body: Evidence from the Miles meteorite

Most iron meteorites formed in planetary cores during differentiation, but the IIE iron meteorites have chemical and physical features that are inconsistent with this origin. By combining mineral chemistry, mineral modes and three-dimensional petrography, we reconstruct the bulk chemistry of the felsic silicate-bearing Miles IIE iron meteorite and demonstrate that the silicate inclusion compositions are similar to partial melts produced experimentally from an H chondrite composition. We use the reconstructed bulk composition, mineralogy and thermodynamic modelling to show that melting above ∼1200°C under reducing conditions formed metal (Fe-Ni alloy) and felsic silicate partial melts. Upon cooling, the melts crystallized Mg-rich pyroxenes, Na- and K-rich feldspars, and tridymite. Importantly, this mechanism enriches cosmochemically volatile elements (i.e., those with a 50% condensation temperature of ∼430–830°C, like Na and K) to the level found in the felsic silicate inclusions.The presence of crystallographically disordered srilankite (only stable above 1160°C) and an absence of Widmanstätten texture require both high peak temperatures and rapid cooling, which cannot be explained by core formation. Instead they point to small melt volumes, a transient heat pulse, and small thermal mass, and imply efficient physical segregation of silicate and metallic melts through buoyancy separation followed by rapid cooling that arrested the separation of metal and silicate liquid phases. In situ 207Pb/206Pb age of 4542.3 ± 4.0 Ma in Zr-oxide and phosphate minerals dates the melting event that formed the silicate inclusions. This age aligns with the earliest ages found in other IIE iron meteorite silicates and requires a heating event ∼25 million years after the solar system formed. We found 39Ar/40Ar ages of 3495 ± 52 Ma (low-T) and 4303 ± 7 Ma (high-T) in a K-feldspar grain, with the 3495 Ma age aligning with later thermal events recorded in other IIE iron meteorites. Dating reveals the complex petrogenetic and thermal history of Miles and the IIE iron meteorites. This is the first IIE iron meteorite found to record evidence of heating at 4.5 and 3.5 Ga likely from impact events.We propose that high-velocity impact(s) into an iron-rich, porous chondritic parent body at ∼4.54 Ga produced immiscible metal and silicate melts that cooled rapidly and trapped low density silicate inclusions within high density metal. Other IIE irons that formed at lower peak temperatures (900–1000°C) contain chondritic silicate inclusions and relict chondrules, supporting this conceptual model. This petrogenesis is consistent with thermodynamic modelling, experimental data and the wide range of peak temperatures and cooling rates observed in the IIE iron meteorites.

Hydration of Nebular Minerals through the Implantation–Diffusion Process

Abstract Recent studies have detected structurally bound water in the refractory silicate minerals present in ordinary and enstatite chondrite meteorites. The mechanism for the incorporation of the hydrogen is not well defined. In this paper we quantitatively examine a two-fold process involving the implantation and diffusion of nebular hydrogen ions that is responsible for the hydration of the chondritic minerals. Our simulations show that depending on critical parameters, including the flux of the protons in nebular plasma, retention coefficient, temperature of the silicate minerals, and desorption rate of implanted hydrogen, the implantation of low-energy hydrogen ions can result in equivalent water contents of ∼0.1 wt% in chondritic silicates within 10 years. Thus, this novel mechanism operating in the nebula at 10−3 bar pressure and <650 K temperatures can efficiently hydrate the free-floating chondritic minerals prior to the rapid formation of planetesimals inside the snow line, and agree well with the wet accretion scenario for the inner solar system objects.

Uranium and thorium partitioning in the bulk silicate Earth and the oxygen content of Earth’s core

This study investigates the partitioning of U and Th between molten metal and silicate liquid (DU and DTh) during Earth’s core-mantle differentiation. We report new Th partition coefficients between chondritic silicate melt and various Fe-rich alloys in the system Fe-Ni-C-S-Si as determined by experiments in a multi-anvil apparatus at 3–8 GPa, 2073–2373 K, and oxygen fugacities from 1.5 to 5 log units below the iron-wüstite (IW) buffer. By compiling all existing data on molten metal-silicate liquid partitioning of U and Th, we develop global models of U and Th partitioning between the mantle and core throughout Earth’s accretion. The calculated concentrations in the Bulk Silicate Earth (BSE) are in agreement with previous studies (UBSE = 11.42 ± 0.45 ppb and ThBSE = 43.20 ± 1.73 ppb), whereas the contents of these radioactive elements in the Earth’s core remain negligible. Compared to recent geochemical estimations, the calculated (Th/U)BSE supports EL rather than EH enstatite chondrites as the reduced building blocks of the Earth. Furthermore, we demonstrate that Th is much more sensitive than U to the oxygen content of the metallic phase. To reproduce the Th/U ratio of the BSE within its uncertainties, the oxygen content of the Earth’s core must be lower than 4.0 wt%. By combining other existing constraints, this suggests that the core contains 2.0–4.0 wt% O. The calculations of U and Th concentrations and Th/U in the BSE developed herein can be used as new constraints for determining the concentrations of other refractory lithophile elements in the BSE as soon as their metal-silicate partition coefficients are well constrained over the conditions of core segregation.

Ceres’s internal evolution: The view after Dawn

AbstractAs the Dawn mission approaches a successful conclusion at Ceres, it seems time to assess how its findings have sharpened the picture of Ceres’s evolution. Before Dawn, we inferred from Ceres's bulk density of about 2100 kg m−3 that Ceres contained about 25% water by mass. Thermodynamic modeling of the interior evolution suggested that the original accreted ice had to melt even if only long‐lived radionuclides were present, leading to the aqueous alteration of the original chondritic silicates and differentiation of the altered silicates from any remaining water, consistent with telescopic detection of aqueously altered silicates (serpentine and clay minerals) on Ceres’s surface. Earth‐based observations of Ceres’s shape were not accurate enough to constrain the extent of differentiation of its interior. Dawn's results confirm these early findings and extend them dramatically to reveal an evolved and active small planet, probably even today, due to water/ice‐driven processes. A nearly uniform global distribution of surface mineralogy, which includes Mg‐serpentines, ammoniated clays, and salts including carbonates, suggests extensive, endogenous, planet‐wide aqueous alteration. Local exceptions show salt‐rich deposits of varied composition, which suggests subsurface heterogeneities. Concentration of Fe below carbonaceous chondrite levels suggests chemical fractionation, leading to Ceres being chemically differentiated. The high spatial uniformity of element abundance measurements of equatorial regolith also indicates that some ice‐rock fractionation occurred on a global scale. Even some local exposures of ice are seen, especially in higher latitudes and in low‐illumination regions that must be very young, as surface water ice is unstable on time scales of 1–1000 years under Ceres’s surface temperatures. Subsurface ice is also likely in abundance at higher latitudes in at least the upper few meters of the surface, as suggested by near‐surface H‐rich polar deposits. Observations of bright ice deposits in permanently shadowed regions suggest cold‐trapping of migrating H2O across the surface. Gravity field measurements indicate a concentration of mass toward the center and near isostatic equilibrium, consistent with at least some mass differentiation driven by water‐related processes. Abundant small and midsize craters but relaxed or missing large craters suggest a stiff upper crust with water abundance lower than 30 vol%. A sharp decrease in viscosity at ~40 km depth suggests the occurrence of a small fraction of liquid, consistent with earlier thermophysical models. Surface cryogenic features, such as flows, extrusions, and domes, some geologically very recent, are evidence of active water/ice‐driven subsurface processes. Ceres experienced extensive water‐related processes and at least some mass and chemical fractionation and is probably active today, consistent with previous moderate heating thermodynamic models. Clearly, Ceres is a “wet,” evolved planet at the edge of the inner solar system, as described in this special issue. We conclude with a list of questions suggested by the Dawn findings; they especially regard the state and fate of water and its role in driving past and possibly current chemical and physical activity in this dwarf planet.

Isotope evolution in the HIMU reservoir beneath St. Helena: Implications for the mantle recycling of U and Th

HIMU (high-μ; 238U/204Pb) is a mantle reservoir that has been thought to form by subduction and subsequent storage of ancient oceanic crust and lithosphere in the mantle. In order to constrain the processes that acted on subducted materials over several billion years, we present precise Pb–Sr–Nd–Hf–He isotopic data together with 40Ar/39Ar and K/Ar ages of HIMU lavas from St. Helena in the Atlantic. Clinopyroxene separates were analyzed together with whole-rock samples to better describe the geochemical characteristics of the HIMU component. Although isotopic variations are small in the St. Helena lavas (20.6–21.0 for 206Pb/204Pb) between 12 and 8Ma, the younger lavas have more HIMU-like isotopic compositions than the older lavas. The mixing arrays defined by these lavas are remarkably similar to those observed in HIMU lavas from Austral Islands in the Pacific, suggesting that the two HIMU reservoirs located in different mantle domains are characterized by similar isotopic compositions with radiogenic 206Pb/204Pb and 208Pb/204Pb, enriched Nd and Hf isotopes, depleted Sr isotopes, and radiogenic 3He/4He. However, there is a significant difference between the St. Helena and Austral Islands lavas in 207Pb/204Pb. The St. Helena lavas show systematically higher 207Pb/204Pb for a given 206Pb/204Pb. Lead isotope evolution models suggest that both HIMU reservoirs formed around 2Ga; however, the HIMU reservoir for St. Helena is about 0.3Ga older than that for Austral Islands. The relation between 206Pb/204Pb and 208Pb/204Pb could reflect the time-integrated κ (232Th/238U) in the components. The HIMU components for St. Helena and Austral Islands have κ values between 3.3 and 3.7, which are intermediate between the present-day fresh mid-ocean ridge basalts (MORB; 2.6–3.2) and the chondritic silicate Earth (∼4). This is consistent with the model that the HIMU precursor is subducted oceanic crust created around 2Ga from depleted upper mantle, in which κ monotonously decreased from the chondritic to the present-day values since late Archean or early Proterozoic, because of enhanced U recycling from the Earth’s surface back to the mantle in response to the increasing oxygen levels in the hydrosphere. Moreover, the fact that the HIMU components have much higher κ than the present-day hydrothermally altered MORB (0.2–2) suggests that either the HIMU precursor was an unaltered ancient oceanic crust, or more likely, an altered oceanic crust with minimal U enrichment by hydrothermal fluids in the less oxic marine environment of the late Archean or early Proterozoic. The unradiogenic 87Sr/86Sr of the HIMU components also suggests formation of ancient oceanic crust altered with hydrothermal fluids having much lower 87Sr/86Sr in that eon than at present, followed by removal of Rb from it by subduction dehydration.

Nebular thermal evolution and the properties of primitive planetary materials

Abstract— Models of the solar nebula are constructed to investigate the hypothesis that surviving planetary objects began to form as the nebula cooled from an early, hot epoch. The imprint of such an epoch might be retained in the spatial distribution of planetary material, the systematic deviations of its elemental composition from that of the Sun, chemical indicators of primordial oxidation state, and variations in oxygen and other isotopic compositions. Our method of investigation is to calculate the time‐dependent, two‐dimensional temperature distributions within model nebulas of prescribed dynamical evolution, and to deduce the consequences of the calculated thermal histories for coagulated solid material. The models are defined by parameters which characterize nebular initial states (mass and angular momentum), mass accretion histories, and coagulation rates and efficiencies. It is demonstrated that coagulation during the cooling of the nebula from a hot state is expected to produce systematic heterogeneities which affect the chemical and isotopic compositions of planetary material. The radial thermal gradient at the midplane results in delayed coagulation of the more volatile elements. Vertical thermal gradients isolate the most refractory material and concentrate evaporated heavy elements in the gas phase. It is concluded that these effects could be responsible for the distribution of terrestrial planetary masses, the systematic depletion patterns of the moderately volatile elements in chondritic meteorites and the Earth, the range of oxygen isotopic compositions exhibited by calcium‐aluminum‐rich inclusions (CAIs) and other refractory inclusions, and some geochemical evidence for a moderately enhanced oxidation state. However, nebular fractionations on a global scale are unlikely to account for the more oxidizing conditions inferred for some CAIs and chondritic silicates, which require dust enhancements greater than a few hundred. This conclusion, along with the well‐established evidence from studies of chondrules and CAIs for thermal excursions of short duration, make it likely that local environments, unrelated to nebular thermal evolution, were also important.

The ferroan‐anorthositic suite, the extent of primordial lunar melting, and the bulk composition of the Moon

The geochemical bimodality among compositionally pristine lunar rocks is strong evidence that a major component of the Moon's crust is fundamentally unrelated to the plagioclase flotation crust that accumulated over the primordial magma ocean. The ferroan anorthositic suite (FAS) has all the characteristics expected for a series of magma ocean plagioclase flotation cumulates, but we estimate, based on mass balance for Al2O3, that ∼ 30 wt% of the crust, and 20 wt% of the crust's total Al2O3 (or roughly 15% of the bulk‐Moon Al2O3 content, assumed to be 3–4 wt%), was emplaced by postmagma ocean, Mg‐suite “serial” magmatism. Remelting of plagioclase‐bearing magma ocean cumulates cannot plausibly account for the “late” emplacement of Al2O3, because the only plagioclase‐rich cumulates produced by a deep, ultramafic magma ocean would have mg ratios too low to engender the Mg‐suite. Conceivably, remelting of garnet‐rich magma ocean cumulates supplied the late Al2O3, but only if the bulk‐Moon Al2O3 content is substantially higher than generally assumed. High pressures in the magma ocean acted to enhance the py (pyroxene/(pyroxene+olivine)) ratio of the MO cumulates, and the Al2O3 content of whatever pyroxene formed. Remelting of pyroxene‐rich MO cumulates could conceivably have supplied the late Mg‐suite Al2O3 to the crust. The source region would have to encompass roughly 40 wt% of the mantle. Another possible source for late Al2O3 addition is a large portion of the deepest mantle that avoided more than a slight degree of partial melting, and thus remained Al‐rich, during formation of the magma ocean. The source region would have to encompass 20–30 wt% of the mantle. Ascent of diapiric masses from the lowermost mantle would likely coincide with (trigger, or be triggered by) a large‐scale overturn of the gravitationally unstable MO cumulate pile, with concomitant pressure‐release melting, in which case the Mg‐suite parental melts would be derived from a mix of deepest MO cumulates and the even deeper primitive mantle. The popular giant impact hypothesis of lunar origin implies an extremely hot beginning for the Moon, suggesting that the deep interior was either molten or depleted in Al2O3 (by differentiation in hot protomoons) as it formed. Origin of ∼ 30 wt% of the Moon's crust by postmagma ocean, serial magmatism is more consistent with coaccretion models that form the Moon without a giant, hot impact. In any scenario with a large fraction of the crust originating by partial remelting of deep magma ocean cumulates, mass balance for Al2O3 implies that the bulk‐Moon normative py ratio must be greater than roughly 0.5, i.e., significantly higher than generally estimated. In this respect, the Moon appears more compositionally similar to chondritic silicates than to the Earth's upper mantle.

Compositional range in the Canyon Diablo meteoroid

The Ir distribution in the IAB Canyon Diablo meteorites associated with the formation of Meteor Crater, Arizona, ranges from 2.1 to 2.5 μg/g with peaks at 2.17 and 2.34 μg/g. Only Ir, Ni, and Cu show appreciably more variance in the large set of specimens than observed within a single specimen. The Ir peaks may reflect random sampling of the large (40–100 m), fractionated meteoroid or the presence of two distinct metallic regions differing in composition. None of the other elements we determined show strong correlations with Ir; the Au range is strikingly small (1.5–1.6 μg/g). The presence of chondritic silicates and high contents of planetary-type noble gases in IAB indicates that these solidified rapidly following melting, as expected if they originated as pools of impact-generated melt on a chondritic body. The absence of fractional crystallization trends is consistent with such a model. That 14 of 15 Ir contents fall into two “peaks” suggests the possibility that the meteoroid included two “pools”. The alternative that the distribution is continuous can be tested by the study of additional specimens; those from the crater rim are particularly important since these are largely shrapnel spalled from the trailing hemisphere of the meteoroid. Our studies show that the irons named Helt Township, Idaho, Las Vegas, Mamaroneck, Moab, and Pulaski County are probably mislabelled Canyon Diablo specimens; Jenny's Creek and Jenkins are also compositionally indistinguishable. Alexander County, Allan Hills A77283, Ashfork, Fairfield, and Rifle are compositionally distinct, independent falls.

I-Xe and 40Ar- 39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar- 39Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic 40Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred Pu U ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most 16O-rich meteorite known, gives a complex 40Ar- 39Ar age pattern which suggests a recent (≲0.85 Ga) severe thermal disturbance. The absence of excess 129Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between 40Ar- 39Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.

The compositional classification of chondrites: II The enstatite chondrite groups

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis. Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurböle, I-Xe formation intervals are −1.3 ± 0.6 Ma for EH chondrites and 2.9 ± 0.5 Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun. The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.

Trace element distribution between magnetic and non-magnetic portions of ordinary chondrites

The contents of As, Au, Ni, Co, Ir, Os, Re and Pt were determined in the metal and silicates-plus-troilite portions of 10 H-, 7 L-, and 5 LL-group chondrites. All elements are strongly siderophile and concentrated in the metal phase. The different values of the As/Ni, Au/Ni and Co/Ni ratios in the magnetic and non-magnetic portions of the three chemical groups indicate that the contents of these elements in the latter fractions cannot be entirely attributed to metal contamination. Analyses of minerals separated from L-group chondrites reveal relatively high concentrations of Ni and Co in troilite and of Au in chromite, feldspar and phosphate. Iridium, Os, Re and Pt are enriched with respect to Ni in the non-magnetic fractions of ordinary chondrites, and the proportion of refractory metals, in these fractions, increases from H- to LL-group chondrites, and, within each group, in the order Ir ≳ Os ≳ Pt ≫ Re. The Ir/Re ratio in ordinary chondritic metal is about 35% below C1 ratios, but higher than C1 in the non-magnetic fractions. The Ir-Re fractionation appears to have taken place in the nebula before condensation of Fe-Ni. The Ir-rich inclusions within chondritic silicates may represent a sample of the material that was lost during the agglomeration of ordinary chondrites.

Bitburg—a group-IB iron meteorite with silicate inclusions

SummaryChemical and mineralogical investigations show that the Bitburg meteorite is not a pallasite, but a group-IB iron meteorite. Like other members of this group it contains chondritic silicates.

Origin of Be 10 and Al 26 in tektites

An attempt is made to show that the recently reported amounts of Be 10 and Al 26 in tektites can possibly be explained in terms of Urey's “comet” hypothesis for tektite origin. On the basis of our calculations we obtain a similar level of Be 10 activity to that reported by Ehmann and Kohman (1958) but a somewhat lower figure for Al 26 activity. It is shown that by mixing one part of chondritic silicate, which is assumed to represent the non-volatile comet material and to contain the radioactive nuclides, with thirteen parts of arkose-like material, the overall tektite composition can be accounted for rather satisfactorarily.