Mario Trieloff

Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry

Our Solar System formed ∼4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of <10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar–39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed ∼160 Myr to reach 390K.

Evolution of interstellar dust and stardust in the solar neighbourhood

Aims. We studied the evolution of the abundance in interstellar dust species that originate in stellar sources and from condensation in molecular clouds in the local interstellar medium of the Milky Way. We determined from this the input of dust material to the Solar System. Methods. A one-zone chemical evolution model of the Milky Way for the elemental composition of the disk combined with an evolution model for its interstellar dust component similar to that of Dwek (1998) is developed. The dust model considers dust-mass return from AGB stars as calculated from synthetic AGB models combined with models for dust condensation in stellar outflows. Supernova dust formation is included in a simple parametrised form that is gauged by observed abundances of presolar dust grains with a supernova origin. For dust growth in the ISM, a simple method is developed for coupling this with disk and dust evolution models. Results. A chemical evolution model of the solar neighbourhood in the Milky Way is calculated, which forms the basis for calculating a model of the evolution of the interstellar dust population at the galactocentric radius of the Milky Way. The model successfully passes all standard tests for the reliability of such models. In particular the abundance evolution of the important dust-forming elements is compared with observational results for the metallicity-dependent evolution of the abundances for G-type stars from the solar neighbourhood. It is found that the new tables of Nomoto et al. (2006) for the heavy element production give much better results for the abundance evolution of these important elements than the widely used tables of Woosley & Weaver (1995). The time evolution for the abundance of the following dust species is followed in the model: silicate, carbon, silicon carbide, and iron dust from AGB stars and from supernovae, as well as silicate, carbon, and iron dust grown in molecular clouds. It is shown that the interstellar dust population is dominated by dust accreted in molecular clouds; stardust only forms a minor fraction. Most of the dust material entering the Solar System at its formation does not show isotopic abundance anomalies of the refractory elements, i.e., inconspicuous isotopic abundances do not point to a Solar System origin for dust grains. The observed abundance ratios of presolar dust grains formed in supernova ejecta and in AGB star outflows requires that, for the ejecta from supernovae, the fraction of refractory elements condensed into dust is 0.15 for carbon dust and is quite small (∼10 −4 ) for other dust species.

Noble gas systematics of the Réunion mantle plume source and the origin of primordial noble gases in Earth’s mantle

New noble gas data of ultramafic xenoliths from Reunion Island, Indian Ocean, further constrain the characteristics of primordial and radiogenic noble gases in Earth’s mantle plume reservoirs. The mantle source excess of nucleogenic 21Ne is significantly higher than for the Hawaiian and Icelandic plume reservoirs, similar to excess of radiogenic 4He. 40Ar/36Ar of the Reunion mantle source can be constrained to range between 8000 and 12 000, significant 129Xe and fission Xe excess are present. Regarding the relative contribution of primordial and radiogenic rare gas nuclides, the Reunion mantle source is intermediate between Loihi- and MORB-type reservoirs. This confirms the compositional diversity of plume sources recognized in other radioisotope systematics. Another major result of this study is the identification of the same basic primordial component previously found for the Hawaiian and Icelandic mantle plumes and the MORB reservoir. It is a hybrid of solar-type He and Ne, and ‘atmosphere-like’ or ‘planetary’ Ar, Kr, Xe (Science 288 (2000) 1036). 20Ne/22Ne ratios extend to maximum values close to 12.5 (Ne-B), which is the typical signature of solar neon implanted as solar corpuscular radiation. This suggests that Earth’s solar-type noble gas inventory was acquired by small (less than km-sized) precursor planetesimals that were irradiated by an active early sun in the accretion disk after nebular gas dissipation, or, alternatively, that planetesimals incorporated constituents irradiated in transparent regions of the solar nebula. Previously, such an early irradiation scenario was suggested for carbonaceous chondrites which follow common volatile depletion trends in the sequence CI–CM–CV–Earth. In turn, CV chondrites closely match Earth’s mantle composition in 20Ne/22Ne, 36Ar/22Ne and 36Ar/38Ar. This indicates that mantle Ar could well be a planetary component inherited from precursor planetesimals. However, a corresponding conclusion for mantle Kr and Xe is less convincing yet, but this may be just due to the lack of appropriate ‘meteoritic’ building blocks matching terrestrial composition. Alternatively, heavy noble gases in Earth’s mantle could be due to admixing of severely fractionated air, but this effect must have affected all mantle sources to a very similar extent, e.g. by global subduction before the last homogenization of the mantle reservoirs.

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

Can you spot a speck of space dust? NASAs Stardust spacecraft has been collecting cosmic dust: Aerogel tiles and aluminum foil sat for nearly 200 days in the interstellar dust stream before returning to Earth. Citizen scientists identified most of the 71 tracks where particles were caught in the aerogel, and scanning electron microscopy revealed 25 craterlike features where particles punched through the foil. By performing trajectory and composition analysis, Westphal et al. report that seven of the particles may have an interstellar origin. These dust particles have surprisingly diverse mineral content and structure as compared with models of interstellar dust based on previous astronomical observations. Science, this issue p. 786 Analysis of seven particles captured by aerogel and foil reveals diverse characteristics not conforming to a single model. Seven particles captured by the Stardust Interstellar Dust Collector and returned to Earth for laboratory analysis have features consistent with an origin in the contemporary interstellar dust stream. More than 50 spacecraft debris particles were also identified. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from any one representative model of interstellar dust inferred from astronomical observations and theory.

STARDUST FROM ASYMPTOTIC GIANT BRANCH STARS

The formation of dust in the outflows of low- and intermediate-mass stars on the first giant branch and asymptotic giant branch (AGB) is studied and the relative contributions of stars of different initial masses and metallicities to the interstellar medium (ISM) at the instant of solar system formation are derived. These predictions are compared with the characteristics of the parent stars of presolar dust grains found in primitive meteorites and interplanetary dust particles (IDPs) inferred from their isotopic compositions. For this purpose, model calculations for dust condensation in stellar outflows are combined with synthetic models of stellar evolution on the first giant branch and AGB and an evolution model of the Milky Way for the solar neighborhood. The dust components considered are olivine, pyroxene, carbon, SiC, and iron. The corresponding dust production rates are derived for the solar vicinity. From these rates and taking into account dust destruction by supernova shocks in the ISM, the contributions to the inventory of presolar dust grains in the solar system are derived for stars of different initial masses and metallicities. It is shown that stars on the first giant branch and the early AGB are not expected to form dust, in accord with astronomical observations. Dust formation is concentrated in the last phase of evolution, the thermally pulsing AGB. Due to the limited lifetime of dust grains in the ISM only parent stars from a narrow range of metallicities are expected to contribute to the population of presolar dust grains. Silicate and silicon carbide dust grains are predicted to come from parent stars with metallicities not less than about Z ≈ 0.008 (0.6 × solar). This metallicity limit is higher than that inferred from presolar SiC grain isotope data. The population of presolar carbon dust grains is predicted to originate from a wider range of metallicities, down to Z ≈ 0.004. Masses of AGB stars that produce C-rich dust are in the range ≈1.5-4 M ☉, in good agreement with what was inferred from the isotope data of presolar grains. The mass distribution of AGB stars that produce O-rich dust is essentially bimodal, with roughly equal contributions from stars in the ranges 1.3-2.5 M ☉ and ≈4-8 M ☉. These model predictions are in conflict with the O-isotope data of presolar grains that indicate contributions essentially only from 1 to 2.5 M ☉ AGB stars.

Neon isotopes in mantle rocks from the Red Sea region reveal large-scale plume–lithosphere interaction

Abstract Helium and neon isotopes are ideal tracers to quantify contributions of primitive mantle plumes, which are characterized by a higher proportion of primordial solar-type noble gases compared to lithospheric or asthenospheric mantle sources. This property was used to investigate the role of the Afar mantle plume (a high 3 He/ 4 He plume of up to 20 R A ; 1 R A =atmospheric composition) during continental breakup of the Red Sea rift. We analyzed ultramafic rocks from Zabargad Island and mantle xenoliths from the Quaternary volcanic fields of Al Birk and Jizan (Saudi Arabia) that are representative of the local subcontinental lithospheric mantle. 3 He/ 4 He ratios range from 6.1 to 8.3 R A , similar to results of worldwide lithospheric and asthenospheric mantle, and therefore are not indicative of a plume component. In contrast, we observe significant contributions of plume-derived neon, which is characterized by a higher proportion of primordial solar-type neon than typical mid-ocean ridge basalts (MORB). Considering both helium and neon isotope systematics reveals mixing of a deep mantle plume, a pre-rift MORB-like, and a more radiogenic pre-rift lithospheric mantle component. As the deep mantle plume component has a higher Ne/He ratio when compared to the shallow mantle components, it is more prominent in Ne than in He, with up to 57% plume-derived neon and 11% plume-derived helium present in the investigated samples. This further underlines the importance of high-precision neon measurements. It demonstrates that the Afar plume source contributed primordial noble gases to an intrinsically more radiogenic and nucleogenic lithospheric and asthenospheric component up to a distance of >1800 km (Zabargad), even in the early stages of continental rifting, ∼20 Ma ago. These results present the first unambiguous geochemical evidence suggesting an active role for the Afar mantle plume during the evolution of the Red Sea rift, supporting geochronological and geodynamic evidence.

The distribution of mantle and atmospheric argon in oceanic basalt glasses

Argon analyses by both high-resolution stepheating and stepcrushing of MORB and Loihi basalt glasses were performed to separate pristine mantle-derived Ar and contaminating atmospheric Ar. In high-vesicularity glasses (> 0.8% vesicles), most of the mantle argon resides in vesicles, from which it is released by crushing or stepheating between 600 and 900 °C. By contrast, in low vesicularity glasses (< permil vesicularity), most mantle argon is dissolved in the glass matrix, as inferred from the correlation with neutron-induced, glass-dissolved argon isotopes (39Ar, 37Ar, 38Ar from K, Ca, Cl). The distribution of mantle Ar between vesicles and glass matrix is well explained by melt-gas equilibrium partitioning at eruption according to Henry’s law, which is compatible with previously determined Henry constants of ∼(5–10) × 10−5ccSTP 40Ar mantle/g bar. Atmospheric Ar is heterogeneously distributed in all samples. Only a very minor part is dissolved in the glass matrix; a significant part correlates with vesicularity and is released by crushing, most probably from a rather small fraction of vesicles or microcracks that equilibrated with unfractionated air. Other carriers of atmospheric argon are pyroxene microlites and minor phases decomposing at intermediate temperatures that were probably contaminated upon eruption by fractionated atmospheric rare gases. Our high-resolution stepheating and stepcrushing analyses of low vesicularity samples with extraordinary high solar-like 20Ne/22Ne indicate successful discrimination of unfractionated air as a contamination source and suggest an upper mantle 40Ar/36Ar of 32,000 ± 4000 and a Hawaiian mantle plume source 40Ar/36Ar ratio close to 8000.

Thermal evolution and sintering of chondritic planetesimals

Aims. Radiometric ages for chondritic meteorites and their components provide information on the accretion timescale of chondrite parent bodies, and on cooling paths within certain areas of these bodies. However, to use this age information for constraining the internal structure, and the accretion and cooling history of the chondrite parent bodies, the empirical cooling paths obtained by dating chondrites must be combined with theoretical models of the thermal evolution of planetesimals. Important parameters in such thermal models include the initial abundances of heat-producing short-lived radionuclides ( 26 Al and 60 Fe), which are determined by the accretion timescale and the terminal size, chemical composition and physical properties of the chondritic planetesimals. The major aim of this study is to assess the effects of sintering of initially porous material on the thermal evolution of planetesimals, and to constrain the values of basic parameters that determined the structure and evolution of the H chondrite parent body. Methods. We present a new code for modelling the thermal evolution of ordinary chondrite parent bodies that initially are highly porous and undergo sintering by hot pressing as they are heated by decay of radioactive nuclei. The pressure and temperature stratification in the interior of the bodies was calculated by solving the equations of hydrostatic equilibrium and energy transport. The decrease of porosity of the granular material by hot pressing due to self-gravity was followed by solving a set of equations for the sintering of powder materials. For the heat-conductivity of granular material we combined recently measured data for highly porous powder materials, relevant for the surface layers of planetesimals, with data for heat-conductivity of chondrite material, relevant for the strongly sintered material in deeper layers. Results. Our new model demonstrates that in initially porous planetesimals heating to central temperatures sufficient for melting can occur for bodies a few km in size, that is, a factor of ≈10 smaller than for compact bodies. Furthermore, for high initial 60 Fe abundances small bodies may differentiate even when they had formed as late as 3−4 Ma after CAI formation. To demonstrate the capability of our new model, the thermal evolution of the H chondrite parent body was reconstructed. The model starts with a porous body that is later compacted first by “cold pressing” at low temperatures and then by “hot pressing” for temperatures above ≈700 K, i.e., the threshold temperature for sintering of silicates. The thermal model was fitted to the well-constrained cooling histories of the two H chondrites Kernouve (H6) and Richardton (H5). The best fit was obtained for a parent body with a radius of 100 km that accreted at t = 2.3 Ma after CAI formation, and had an initial 60 Fe/ 56 Fe = 4.1 × 10 −7 . Burial depths of 8.3 km and 36 km for Richardton and Kernouve were able to reproduce their empirically determined cooling history. These burial depths are shallower than those derived in previous models. This reflects the strong insulating effect of the residual powder surface layer, which is characterised by a low thermal conductivity.

PHOTOPHORETIC SEPARATION OF METALS AND SILICATES: THE FORMATION OF MERCURY-LIKE PLANETS AND METAL DEPLETION IN CHONDRITES

Mercurys high uncompressed mass density suggests that the planet is largely composed of iron, either bound within metal (mainly Fe-Ni) or iron sulfide. Recent results from the MESSENGER mission to Mercury imply a low temperature history of the planet which questions the standard formation models of impact mantle stripping or evaporation to explain the high metal content. Like Mercury, the two smallest extrasolar rocky planets with mass and size determination, CoRoT-7b and Kepler-10b, were found to be of high density. As they orbit close to their host stars, this indicates that iron-rich inner planets might not be a nuisance of the solar system but be part of a general scheme of planet formation. From undifferentiated chondrites, it is also known that the metal to silicate ratio is highly variable, which must be ascribed to preplanetary fractionation processes. Due to this fractionation, most chondritic parent bodies—most of them originated in the asteroid belt—are depleted in iron relative to average solar system abundances. The astrophysical processes leading to metal silicate fractionation in the solar nebula are essentially unknown. Here, we consider photophoretic forces. As these forces particularly act on irradiated solids, they might play a significant role in the composition of planetesimals forming at the inner edge of protoplanetary disks. Photophoresis can separate high thermal conductivity materials (iron) from lower thermal conductivity solids (silicate). We suggest that the silicates are preferentially pushed into the optically thick disk. Subsequent planetesimal formation at the edge moving outward leads to metal-rich planetesimals close to the star and metal depleted planetesimals farther out in the nebula.

Noble gases, their carrier phases, and argon chronology of upper mantle rocks from Zabargad Island, Red Sea

Abstract Three ultramafic bodies on Zabargad Island contain fresh peridotites with mostly unfractionated primitive bulk major and trace element abundances and mostly monomineralic vein rocks (pyroxenites, olivinites, homblendites, etc.). We analyzed a set of coarse grained vein rocks with the 40Ar-39Ar technique applying high resolution stepheating. Neutron induced argon isotopes derived from Ca, K, and Cl, and the specific degassing behaviour of major and accessory minerals enabled us to separate and identify different trapped and radiogenic argon components and their hosts. Within two clinopyroxenites trapped argon is present in (1) low temperature, low 40 Ar 36 Ar phases (serpentine and/or fluid inclusions), (2) pyroxene-related Cl-rich carriers (pyroxene and/or associated microinclusions) and (3) amphiboles which are intimately and nonseparably intergrown with pyroxene. The amphiboles, which can texturally, chemically, and isotopically be divided into different generations, formed by interaction of spinels and pyroxenes with mantle fluids during different stages of diapiric uplift (Agrinier et al., 1993). Formation of these amphiboles and microinclusions in pyroxenes, along with incorporation of isotopically distinct Ar with 40 Ar 36 Ar ratios up to 8000, can be related to recent mantle metasomatism also evident in Arabian xenoliths (Henjes-Kunst et al., 1990) and must have been induced by a variety of mantle fluids. For a homblendite, in situ radiogenic and excess argon components could be separated: the plateau age of 18.7 ± 1.3 Ma is in perfect agreement with a zircon Pb/Pb age of 18.4 ± 1.0 Ma (Oberli et al., 1987) interpreted as the age of crustal intrusion. Obviously, the formation of the hornblendite occurred during the final stage of uplift, most probably by interaction with seawater, as suggested by strontium, oxygen, and hydrogen isotopic data (Agrinier et al., 1993) and the low 40 Ar 36 Ar ratio (305) of the trapped argon. 4He, 20Ne, 40Ar, and 36Ar were measured in the orthopyroxenite vein rock Z31 by stepwise crushing and subsequent total fusion. Isotopic ratios show a well defined correlation with crushing step that indicates the presence of two different types or generations of inclusions, which were subjected to different degrees of contamination by atmosphere type noble gases. As inclusions were trapped before tje main deformation of the peridotite complex (Kurat et al., 1993), argon with relatively low 40 Ar 36 Ar ratios ( ≤ 1500) was trapped in the mantle, which requires an admixture of argon of atmospheric composition to the source region of the peridotites. Radiogenic isotopes (4He, 40Ar) are dominated by the mantle source, however, the 4 He 40 Ar ratio (∼0.16) is much lower than expected from long term decay of radioactive parent nuclides U, Th, and K. Such low ratios, which have previously been observed also in mantle xenoliths, obviously reflect the indigenous peridotitic source and are most probably due to fractionation processes in the mantle.