Showing papers by "Smail Mostefaoui published in 2005"

60Fe: A Heat Source for Planetary Differentiation from a Nearby Supernova Explosion

Abstract: From a sample of the Semarkona (LL 3.0) ordinary chondrite we report the in situ discovery of 60Ni isotopic anomalies attributable to the decay of short-lived 60Fe (half-life 1.5 Myr) in the mineral phases troilite (FeS) and magnetite (Fe3O4). The troilite shows a 60Ni excesses of up to ~100 parts per thousand (‰) relative to its solar isotopic abundance. A positive correlation between 60Ni excesses and 56Fe/58Ni ratios provides evidence for live 60Fe in the early solar system. The inferred 60Fe/56Fe ratio of (0.92 ± 0.24) × 10-6 is the highest measured in any meteorite sample so far. This ratio is higher than predictions for production within asymptotic giant branch stars, but falls within the range expected for a Type II supernova source. This result is strongly suggestive of injection of freshly synthesized 60Fe into the nascent solar nebula by a nearby supernova explosion. Such a high abundance of 60Fe will exclude irradiation with solar energetic particles as the sole mechanism responsible for the production of short-lived radionuclides. It further shows that the decay of 60Fe was an important heat source for early planetary melting and differentiation and for keeping asteroids thermally active for much longer than would be possible from the decay of 26Al alone.

Timescales of shock processes in chondritic and martian meteorites.

Abstract: About 35 of the thousands of meteorites so far found on Earth are recognized as being from Mars, probably thrown up by the impacts of large bodies such as asteroids on the martian surface. The trace-element distribution between high-pressure minerals formed by intense shock in these meteorites is a measure of the duration of the events that formed them, and the brief (10 ms) duration suggests that the impacting bodies were of the order of 100 metres in diameter. In contrast, stony meteorites (chondrites) formed by collisions much earlier in the life of the Solar System record the presence of much larger colliding bodies, around 5 km in size and causing a 1-second shock on impact. The accretion of the terrestrial planets from asteroid collisions and the delivery to the Earth of martian and lunar meteorites has been modelled extensively1,2. Meteorites that have experienced shock waves from such collisions can potentially be used to reveal the accretion process at different stages of evolution within the Solar System. Here we have determined the peak pressure experienced and the duration of impact in a chondrite and a martian meteorite, and have combined the data with impact scaling laws to infer the sizes of the impactors and the associated craters on the meteorite parent bodies. The duration of shock events is inferred from trace element distributions between coexisting high-pressure minerals in the shear melt veins of the meteorites. The shock duration and the associated sizes of the impactor are found to be much greater in the chondrite (∼1 s and 5 km, respectively) than in the martian meteorite (∼10 ms and 100 m). The latter result compares well with numerical modelling studies of cratering on Mars, and we suggest that martian meteorites with similar, recent ejection ages (105 to 107 years ago)3 may have originated from the same few square kilometres on Mars.

Trace element distribution between orthopyroxene and clinopyroxene in peridotites from the Gakkel Ridge: a SIMS and NanoSIMS study

Abstract: Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14 plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800°C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle melting models proposed thus far for mid-ocean ridges.

In situ survey of graphite in unequilibrated chondrites: Morphologies, C, N, O, and H isotopic ratios

Abstract: We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezˆ-Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine-grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ 15 N and δ 13 C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15 N excesses are found, with δ 15 Nmax ∼+955‰, confirming previous results. Excesses in 15 N are also detected in fine-grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ 15 N ranging up to +440‰. The 15 N excesses are attributed to ion-molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ 13 C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small- and large-scale carbon isotopic variations in the solar nebula.

NanoSIMS oxygen- and sulfur-isotope imaging of primitive solar system materials

Abstract: Introduction: Primitive meteorites and interplanetary dust particles (IDPs) contain nmto μm-sized presolar dust grains that formed in the winds of evolved stars or in the ejecta of supernova and nova explosions [1-3]. Silicates are the major constituent of O-rich dust around young stars and in outflows from evolved red giant stars [4]. Although the first presolar minerals, namely diamond and SiC, were identified already in 1987 in carbonaceous meteorites, presolar silicates were discovered only recently, first in IDPs and later also in primitive meteorites [5-8]. The identification of presolar silicates was based on the application of improved measurement techniques and the invention of the NanoSIMS ion microprobe with its superior lateral resolution (<100 nm) and capability for the search of in-situ presolar dust in slices of IDPs and meteorites has played a key role in this respect. In a previous study we reported the discovery of abundant in-situ presolar silicate and spinel grains in the matrix of the Acfer 094 meteorite [8]. The finding of these grains was based on O-isotope mapping of several matrix areas with the NanoSIMS ion microprobe at MPI for Chemistry. In an attempt to further characterize presolar minerals in IDPs and meteorites we report here results from an Oand S-isotope imaging survey of the Acfer 094 meteorite and of two IDPs. Experimental: The O-isotopic measurements were performed on a polished thin section of Acfer 094 and on 11 microtome sections of IDPs U2071J2 and U2071C9 using the NanoSIMS at MPI for Chemistry. For the O-isotopic measurements in Acfer 094 we selected 40 different matrix areas. A focussed Cs ion beam (<100 nm) of ~0.5 pA was rastered over the selected areas, each 9×9 μm in size. Negative secondary ions of the three O isotopes, Si, and AlO were simultaneously measured in multi-collection and 256×256 pixel image sequences (total integration time of ~30 minutes to ~1 hour per image set) were acquired. Microtome sections of the two non-cluster IDPs, 9×5 μm (U2071J2) and 12×8 μm (U2071C9) in size, were prepared. These particles are part of a comprehensive consortium study of particles from collection surface U2071 [9,10]. The O-isotope imaging was done on four (U2071J2) and seven (U2071C9) microtome sections using similar analytical conditions as for Acfer 094, except that raster sizes varied from 8×8 to 14×14 μm and that AlO was not included in the measurements. The S-isotopic measurements were done on three areas in the matrix of Acfer 094 which were studied for O-isotopic compositions before. A focused Cs ion beam of ~0.5 pA was rastered over the three areas, 8x8 to 10×10 μm in size. Negative secondary ions of O, S, S, S, and S were simultaneously measured in multi-collection and 256×256 pixel image sequences (total integration time of ~1 to ~1.5 hours per image set) were acquired.

Trace Element Distribution Between Orthopyroxene and Clinopyroxene in Peridotites From the Gakkel Ridge: a SIMS and NanoSIMS Study

Abstract: Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14 plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800°C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle melting models proposed thus far for mid-ocean ridges.