Calcium Aluminum Inclusions Research Articles

SOLAR WIND IMPLANTATION MODEL FOR10Be IN CALCIUM–ALUMINUM INCLUSIONS

We propose a model for the incorporation of 10 Be within calcium–aluminum inclusions (CAIs) in primitive carbonaceous meteorites. In this model, 10 Be is produced by energetic particle reactions in the proto-solar atmosphere of a more active proto-Sun characterized by energetic particle fluxes higher than contemporary particle fluxes. This 10 Be is incorporated into the solar wind that is then implanted into CAI precursor material. This production mechanism is operational in the contemporary solar system implanting 10 Be in lunar materials. The contemporary production rate of 10 Be at the surface of the Sun is ∼0.1 10 Be cm −2 s −1 . Scaling up the contemporary 10 Be production in the protoSun by a factor of 10 5 would increase the production rate to 10 41 0 Be cm −2 s −1 . Using this enhanced production value in conjunction with refractory mass inflow rates at 0.06 AU from the proto-Sun we model 10 Be concentrations in CAI precursors. We calculate the content of solar-wind-implanted 10 Be would have been of the order of 10 12 10 Be g −1 in CAIs, consistent with initial 10 Be content found from boron–beryllium isotopic systematics in CAIs.

A HETEROGENEOUS CHEMICAL ORIGIN FOR THE 16 O-ENRICHED AND 16 O-DEPLETED RESERVOIRS OF THE EARLY SOLAR SYSTEM

The oldest solids formed in the solar system, calcium-aluminum inclusions, are 16O-enriched compared to chondrules, asteroids, Earth, and Mars. Based on the preliminary measurements of the solar wind by Genesis, the Sun also appears to be 16O-enriched. This distribution of oxygen isotopes in the solar system cannot be reconciled via conventional mass-dependent isotopic fractionation processes and instead require the existence and/or production of distinct 16O-enriched and 16O-depleted reservoirs in the early solar system. The origin of these distinct reservoirs is unknown, although several mechanisms have been proposed to date including the following: (1) the injection of pure 16O by a supernova into the protoplanetary disk or parent molecular cloud, (2) self-shielding of CO in the parent molecular cloud or protoplanetary disk, (3) symmetry-dependent chemical fractionation processes in the protoplanetary disk, and (4) Galactic chemical evolution. While some of these proposals have been ruled out, the validity of others is still open. Here I propose that the 16O-enriched and 16O-depleted reservoirs present in the early solar system originated in the parent molecular cloud via the heterogeneous chemical processes that form H2O, a significant oxygen reservoir, on the surface of interstellar (IS) dust grains in dense molecular clouds, the astrophysical setting where star formation is observed to occur. As a consequence, this model predicts that molecular cloud H2O and possibly other IS solids inherited from the molecular cloud were depleted in 16O compared to the bulk gas-phase O present, thus providing distinct 16O reservoirs at the earliest stages of planetary formation.

THE FORMATION OF GRAPHITE WHISKERS IN THE PRIMITIVE SOLAR NEBULA

It has been suggested that carbonaceous grains are efficiently destroyed in the interstellar medium and must either reform in situ at very low pressures and temperatures or in an alternative environment more conducive to grain growth. Graphite whiskers have been discovered associated with high-temperature phases in meteorites such as calcium aluminum inclusions and chondrules, and it has been suggested that the expulsion of such material from proto stellar nebulae could significantly affect the optical properties of the average interstellar grain population. We have experimentally studied the potential for Fischer-Tropsch and Haber-Bosch type reactions to produce organic materials in protostellar systems from the abundant H2, CO, and N2 reacting on the surfaces of available silicate grains. When graphite grains are repeatedly exposed to H2, CO, and N2 at 875 K abundant graphite whiskers are observed to form on or from the surfaces of the graphite grains. In a dense, turbulent nebula, such extended whiskers are very likely to be broken off, and fragments could be ejected either in polar jets or by photon pressure after transport to the outer reaches of the nebula.

26Al decay: Heat production and a revised age for Iapetus

We revisit the appropriate energies to be used for computing heat production from 26Al decay. Due to the complexity of the decay scheme of this radioisotope, previous geophysical studies have used values ranging from 1.2 to 4 MeV per decay. The upper bound corresponds to the difference in mass energy between the 26Al and 26Mg ground states. This includes energy carried away by neutrinos, which does not contribute to heating planetary material. The lower bound does not account for the heating caused by the absorption of the γ rays from the excited 26Mg, or for the annihilation energy deposited in the material if the decay occurs inside even small planetesimals. Based on the calculations described by Schramm et al. [Schramm, D., Tera, F., Wasserburg, G.J., 1970. The isotopic abundance of 26Mg and limits on 26Al in the early Solar System. Earth Planet. Sci. Lett. 10, 44–59] updated with the most recent nuclear constants, we recommend using a heat production value of 3.12 MeV per decay, which is the total energy of disintegration minus the energy carried off by the neutrinos. This heat production value is higher than the value used in the modeling of Iapetus by Castillo-Rogez et al. [Castillo-Rogez, J., Matson, D.L., Sotin, C., Johnson, T.V., Lunine, J.I., Thomas, P.C., 2007. Iapetus’ geophysics: Rotation rate, shape, and equatorial ridge. Icarus 190, 179–202] by about a factor 2.5. The resulting estimate of the time of formation of Iapetus is shifted by about 1 Myr, to between ∼3.4 and 5.4 Myr after the production of the calcium–aluminum inclusions (CAIs).

Ceres’ evolution and present state constrained by shape data

We model Ceres’ thermo-physical-chemical evolution by considering a large range of initial conditions as well as various evolutionary scenarios. Models are constrained by available shape measurements, which point to a differentiated interior for Ceres. We address the role played by hydrothermal activity in the long-term evolution of Ceres and especially the evolution of its hydrosphere. We suggest that models with times of formation shorter than about 5 My after the production of calcium–aluminum inclusions are more likely to undergo hydrothermal activity in their early history, which affects Ceres’ long-term thermal evolution. We evaluate the conditions for preserving liquid water inside Ceres, a possibility enhanced by its warm surface temperature and the enrichment of its hydrosphere in a variety of chemical species. However, thermal modeling of the hydrosphere needs to be further investigated. We show that shape data can help constrain the amount of hydrated silicate in the core, and thus the extent of hydrothermal activity in Ceres. We discuss the importance of these results for the Dawn mission’s arrival at Ceres in 2015.

Pb–Pb dating of chondrules from CV chondrites by progressive dissolution

To refine absolute Pb–Pb ages of chondrules, we have experimented with progressive dissolution procedures of chondrules in the CV chondrite Allende in an attempt to effectively remove terrestrial contamination and generate Pb fractions of sufficient size and spread in 207Pb– 206Pb space to adequately define meaningful isochrons. Samples are extricated from the matrix, ultrasonicated, abraded and repeatedly pre-cleaned by ultrasonicating in distilled ethanol, water and acetone. Subsequent steps of weak HCl and HBr acids combined with ultrasonic agitation and modest heating effectively removes terrestrial contamination in most samples we analyzed. Subsequent exposure to warm 4 M HNO 3 typically isolates the least-radiogenic component of the stepwise dissolution series. Continuing the dissolution steps with HCl of increasing molarity with longer and higher temperature heating steps returns more radiogenic Pb fractions. The most radiogenic Pb fraction typically corresponds to the first warm 1 M HF step with subsequent steps in stronger HF yielding progressively less radiogenic signatures. Importantly, the Pb components extracted in the dissolution of the final residues with 28 M HF + 14 M HNO 3 typically lie slightly below the isochron defined by previous steps, corresponding to a slightly younger 207Pb /206Pb model age than the inferred real age. The wide range of total Pb contents (20–> 2000 pg) in different fractions returned by this method requires analyses by TIMS using a high-efficiency Pb emitter, 202Pb/ 205Pb double spike and diligent monitoring of laboratory Pb blank. As expected, U and Pb are highly and variably fractionated during the procedure such that no U/Pb age information is recovered. A multi-chondrule fraction from the CV chondrite Allende yields an age of 4565.32 ± 0.81 Ma, which is statistically younger than calcium–aluminum inclusions from the CV chondrite Efremovka.

GRAPHITE WHISKERS FOUND IN METEORITES

WHISKERS MADE OF GRAPHITE have been discovered in meteorites, lending credence to a long-held idea that they may exist in outer space. This in turn may help explain why radiation from some astronomical objects appears dimmed. Astronomers have suggested that needle shapes—whether silicon, graphite, or iron—may be present in interstellar dust, absorbing visible light from some supernovas and infrared light from the centers of some galaxies. Now, for the first time, scientists have examples of graphite whiskers in hand, and “this should result in a better understanding of the basic physical workings of our universe,” Marc D. Fries says. Fries, along with astrobiologist Andrew Steele, at the Carnegie Institution of Washington, in Washington, D.C., examined several carbonaceous chondrite-type meteorites, the sort thought to be formed in the early solar system. Using electron microscopy and Raman spectroscopy on meteorite samples, they found calcium-aluminum inclusions that contained the needlelike graphite str...

Graphite Whiskers in CV3 Meteorites

Graphite whiskers (GWs), an allotrope of carbon that has been proposed to occur in space, have been discovered in three CV-type carbonaceous chondrites via Raman imaging and electron microscopy. The GWs are associated with high-temperature calcium-aluminum inclusion (CAI) rims and interiors, with the rim of a dark inclusion, and within an inclusion inside an unusual chondrule that bears mineralogy and texture indicative of high-temperature processing. Current understanding of CAI formation places their condensation, and that of associated GWs, relatively close to the Sun and early in the condensation sequence of protoplanetary disk materials. If this is the case, then it is a possibility that GWs are expelled from any young solar system early in its history, thus populating interstellar space with diffuse GWs. Graphite whiskers have been postulated to play a role in the near-infrared (near-IR) dimming of type Ia supernovae, as well as in the thermalization of both the cosmic IR and microwave background and in galactic center dimming between 3 and 9 micrometers. Our observations, along with the further possibility that GWs could be manufactured during supernovae, suggest that GWs may have substantial effects in observational astronomy.

Chronology of the Solar System's Oldest Solids

Determining the origins of our solar system and, by proxy, other planetary systems, depends on knowing accurately and precisely the timing and tempo of the transformation of the disk of gas and dust to the solids that formed the planets. Relative ages based on the short-lived nuclide 26Al indicate that high-temperature calcium-aluminum inclusions (CAIs) formed before lower temperature chondrules but these ages are heavily dependant on a model of homogeneous distribution of 26Al within the protoplanetary disk. The competing X-wind model argues for heterogeneous distribution of 26Al due to its formation by intra-solar system irradiation such that this system would have no chronological significance. We report a207Pb-206Pb isochron age of 4565.45 ± 0.45 Myr for chondrules from the CV chondrite Allende, an age that is 1.66 ± 0.48 Myr younger than the accepted Pb-Pb age for CAIs from this chondrite group. This age offset is in excellent agreement with the relative ages determined using the 26Al-26Mg system, an observation that supports a supernova origin for 26Al and, importantly, the chronological significance of the 26Al-26Mg system in general. This is consistent with an early and brief CAI-forming event followed by recurrent chondrule formation throughout the life span of the protoplanetary disk. The paucity of old chondrules in chondrite meteorites may reflect their early incorporation into the parent bodies of differentiated meteorites after CAIs were effectively removed from the innermost regions of the protoplanetary disk. Lastly, the agreement between the absolute and relative chronology of CAIs and chondrules requires a solar system age younger than ~4567.5 Myr

Micro-Raman spectroscopy characterization of selected meteorites

Raman spectroscopy and scanning electron microscopy (SEM–EDS) have been used to determine some mineral components of four meteorites: Allende, ALHA77 182, ALHA78 045 and ALHA81 002. These meteorites are all chondrites and are believed to have undergone some secondary processes: aqueous alterations on the parent body or weathering processes on the Earth. Chondrites are stony meteorites composed of chondrules, spherical mineral aggregates of olivines and pyroxenes, often with excentroradial structure, set within a rocky groundmass. As these are not present in terrestrial rocks, this structure is very characteristic of metorites. Typical minerals present in the ordinary chondrites are olivine, pyroxenes, kamacite, taenite, plagioclase and troilite. In the carbonaceous chondrites spinel (MgAl 2O 4), pentlandite, phyllosilicates and amoeboid olivine aggregates (AOAs), calcium–aluminum inclusions (CAIs), melts and organic matter were found. Pore spaces and the presence of characteristic secondary minerals found in the Allende and ALHA81 002 meteorites allow to conclude that these meteorites could had been altered by aqueous solutions on their parent bodies. The main mineral products of the weathering processes found in the ALHA77 182 and ALHA78 045 meteorites – goethite and lepidocrocite, are evidence for the secondary processes on Earth. These non-destructive analytical methods are very useful in meteorite studies.

Tungsten and hafnium distribution in calcium–aluminum inclusions (CAIs) from Allende and Efremovka

Recent 182Hf– 182W age determinations on Allende Ca-, Al-rich refractory inclusions (CAIs) and on iron meteorites indicate that CAIs have initial ε 182W (−3.47 ± 0.20, 2 σ) identical to that of magmatic iron meteorites after correction of cosmogenic 182W burn-out (−3.47 ± 0.35, 2 σ). Either the Allende CAIs were isotopically disturbed or the differentiation of magmatic irons (groups IIAB, IID, IIIAB, and IVB) all occurred <1 m.y. after CAI formation. To assess the extent of isotopic disturbance, we have analyzed the elemental distribution of Hf and W in two CAIs, Ef2 from Efremovka (CV3 reduced), and Golfball from Allende (CV3 oxidized). Fassaite is the sole host of Hf (10–25 ppm) and, therefore, of radiogenic W in CAIs, with 180Hf/ 184W > 10 3, which is lowered by the ubiquitous presence of metal inclusions to 180Hf/ 184W > 10 in bulk fassaite. Metal alloy (Ni ∼ 50%) is the sole host of W (∼500 ppm) in Ef2, while opaque assemblages (OAs) and secondary veins are the hosts of W in Golfball. A large metal alloy grain from Ef2, EM2, has 180Hf/ 184W < 0.006. Melilite has both Hf and W below detection limits (<0.01 ppm), but the presence of numerous metallic inclusions or OAs makes melilite a carrier for W, with 180Hf/ 184W < 1 in bulk melilite. Secondary processes had little impact on the 182Hf– 182W systematics of Ef2, but a vein cross-cutting fassaite in Golfball has >100 ppm W with no detectable Pt or S. This vein provides evidence for transport of oxidized W in the CAI. Because of the ubiquitous distribution of OAs, interpretations of the 182Hf– 182W isochron reported for Allende CAIs include: (i) all W in the OAs was derived by alteration of CAI metal, or (ii) at least some of the W in OAs may have been equilibrated with radiogenic W during metamorphism of Allende. Since (ii) cannot be ruled out, new 182Hf– 182W determinations on CAIs from reduced CV3 chondrites are needed to firmly establish the initial W isotopic composition of the solar system.

Iapetus' geophysics: Rotation rate, shape, and equatorial ridge

Iapetus has preserved evidence that constrains the modeling of its geophysical history from the time of its accretion until now. The evidence is (a) its present 79.33-day rotation or spin rate, (b) its shape that corresponds to the equilibrium figure for a hydrostatic body rotating with a period of ∼16 h, and (c) its high, equatorial ridge, which is unique in the Solar System. This paper reports the results of an investigation into the coupling between Iapetus' thermal and orbital evolution for a wide range of conditions including the spatial distributions with time of composition, porosity, short-lived radioactive isotopes (SLRI), and temperature. The thermal model uses conductive heat transfer with temperature-dependent conductivity. Only models with a thick lithosphere and an interior viscosity in the range of about the water ice melting point can explain the observed shape. Short-lived radioactive isotopes provide the heat needed to decrease porosity in Iapetus' early history. This increases thermal conductivity and allows the development of the strong lithosphere that is required to preserve the 16-h rotational shape and the high vertical relief of the topography. Long-lived radioactive isotopes and SLRI raise internal temperatures high enough that significant tidal dissipation can start, and despin Iapetus to synchronous rotation. This occurred several hundred million years after Iapetus formed. The models also constrain the time when Iapetus formed because the successful models are critically dependent upon having just the right amount of heat added by SLRI decay in this early period. The amount of heat available from short-lived radioactivity is not a free parameter but is fixed by the time when Iapetus accreted, by the canonical concentration of 26Al, and, to a lesser extent, by the concentration of 60Fe. The needed amount of heat is available only if Iapetus accreted between 2.5 and 5.0 Myr after the formation of the calcium aluminum inclusions as found in meteorites. Models with these features allow us to explain Iapetus' present synchronous rotation, its fossil 16-h shape, and the context within which the equatorial ridge arose.

Minerals Point to a Hot Origin for Icy Comets

HOUSTON, TEXAS— Scientists analyzing the first samples returned from a comet announced startling news this week. They are finding not the unprocessed “stardust” thought to have glommed together in the frigid fringes of the early solar system, but bits of rock forged in white-hot heat. The discovery may mean that the disk of dust and gas from which all planetary bodies formed was far more violently mixed than previously thought. At the Lunar and Planetary Science Conference here, leaders of the 150-strong Stardust science team told how team members on four continents have been slicing, dicing, and analyzing 10-micrometer particles collected by the Stardust spacecraft. It swept by comet Wild 2 two years ago and returned its samples to Earth on 15 January. Working first on the larger particles snared in the Stardust collectors, analysts are finding mineral crystals such as forsterite, pyroxene, anorthosite, spinel, and titanium nitride. These “are all minerals that formed at moderately high to extremely high temperatures,” Stardust principal investigator Donald Brownlee of the University of Washington, Seattle, later told a press conference at NASA's nearby Johnson Space Center. “These are hot minerals from the coldest place in the solar system,” the comet-forming region beyond Neptune. ![Figure][1] A hot one. This 2-micrometer bit of comet Wild 2—a magnesium-rich olivine called forsterite—formed at a high temperature, perhaps near the young sun. CREDIT: NASA/JPL-CALTECH/UNIVERSITY OF WASHINGTON The minerals must have formed at 1400 K or hotter, Brownlee said, especially a couple of particles resembling the so-called calcium-aluminum inclusions (CAIs) known from meteorites. In contrast, the dust the analysts expected to find in comets would be submicrometer in size and lacking in any crystalline structure. That's the form they would have taken as they condensed from vapor in deep space after being blown off other stars. Brownlee offered two possible solutions to the hot-and-cold conundrum. The crystals “could have come from the innermost region of the [still-forming] solar system,” he said. Astrophysicist Frank Shu of National Tsing Hua University in Taiwan has advanced that idea to explain CAIs and once-molten droplets called chondrules that dominate the most common type of meteorite coming from the asteroid belt ( Science , 20 June 1997, p. [1789][2]). Shu argues that the young, violently active sun would have blasted nearby solids to their melting points and magnetically flung them—including CAI and chondrule particles—out over the disk as far as the comet-forming region. Alternatively, says Brownlee, the Stardust minerals may have crystallized from melts near other stars and reached the forming solar system by some unspecified means. “If this were astronomy, we'd stop there,” Brownlee told his colleagues. Astronomers have nothing to go on but the electromagnetic spectrum, which would yield no further information in this case. “But we have samples; that will solve this mystery.” The key will be isotopes, he said. The mix of isotopes in solar system material is wildly different from that of other stars, he noted, as evidenced in rare bits of interstellar material long known from meteorites. “We'll know in weeks or months,” says Brownlee. [1]: pending:yes [2]: /lookup/volpage/276/1789

Hf–W evidence for rapid differentiation of iron meteorite parent bodies

Abstract New, high-precision W isotope data on iron meteorites are presented that provide important constraints on the timing of silicate–metal segregation in planetesimals. Magmatic iron meteorites all have e 182 W within error or less radiogenic than initial e 182 W estimated by studies of chondritic meteorites. At face value this implies that iron meteorites are as old and older than refractory calcium–aluminium rich inclusions (CAI), which are widely thought to be the oldest solar system objects. Moreover, different meteorites from the same magmatic groups, believed to be derived from the same planetissimal core, display a range of e 182 W. We suggest that the paradoxical e 182 W values more negative than initial Solar System Initial (SSI) are most readily explained as a result of secondary, spallation reactions with cosmic rays during transit between parent body and the earth. This is supported by the most negative e 182 W being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions. On the other hand, it is also striking that none of the magmatic iron group meteorites have e 182 W analyses, outside error, more radiogenic than the estimated solar system initial ratio. This suggests that core formation in parent bodies of magmatic iron meteorites occurred ≤ 1.5 Myr after the formation age of CAI [Y. Amelin, A.N. Krot, I.D. Hutcheon, and A.A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum inclusions, Science 297, 1678–1683, 2002]. This extremely early metal–silicate differentiation is coeval with the first chondrules [M. Bizzarro, J.A. Baker, and H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431, 275–278, 2004, A.N. Krot, Y. Amelin, P. Cassen, and A. Meibom, Young chondrules in CB chondrites from a giant impact in the early Solar System, Nature 436, 989–992, 2005]. Formation of later chondrules, and hence the parent bodies of some chondritic meteorites, must therefore have occurred in the presence of planetesimals large enough to possess iron cores. We conclude that early planetary accretion and differentiation was sufficiently fast for 26 Al-decay to be an important heat source. Non-magmatic iron meteorites, however, display more radiogenic and varied W isotope signatures. This is in keeping with them being generated later, by impact melting during which the metal (partially) re-equilibrated with the then more radiogenic silicate fraction.

Some Nucleosynthesis Effects Associated withr‐Process Jets

In a recent work, I have discussed some of the consequences of including rotation and magnetic fields in a model of a core-collapse supernova. Rotation produces equatorial flattening in the resultant neutron star and also produces a surrounding accretion disk. Magnetic fields wrap into a toroid that extrudes material in the equatorial plane. The combination of these effects is an accretion-extrusion disk that feeds material into bipolar jets rooted in the disk where the inward and outward flows meet. The extrusion flow is the site of an r-process in which the seed nuclei embedded in the neutron Fermi sea in the outer neutron star envelope capture the neutrons in which they are embedded. At high latitudes on the neutron star a large neutrino-antineutrino flux drives a neutron wind of the type previously discussed for spherical models, and this is a separate site for a less efficient r-process. In the high-temperature accretion disk, nuclear statistical equilibrium exists, with an abundance peak near mass number 90. The jets are expected to have a velocity of about 0.5c (140 MeV nucleon-1) and to blast holes in the expanding envelope of the supernova, causing large secondary explosions and extensive spallation of both the envelope and jet nuclei. The general purpose of this paper is to discuss nuclear processes and effects associated with this rather complicated scenario and to identify features in the nuclidic abundances in solar system materials that are probably associated with these effects. One discussion tentatively identifies the fission point for r-process fission recycling with the nuclide 297Bh and argues for fission peaks around mass numbers 132 and 165, identifying the latter with an r-process abundance hump in the rare earth region. The equilibrium abundance peak at mass number 90 should be involved in creating the abundance peak in the p-process in that region, previously not understood. Spallation of r-process nuclei near the mass number 132 peak can be identified with an unusual Xe component in interstellar nanodiamonds called Xe-HL. A discussion is given of other isotopic anomalies in meteoritic components that imply mixing and turbulence in the expanding supernova envelope, and it identifies the macroscopic calcium-aluminum inclusions in meteorites as originating in the envelope of a supernova responsible for triggering formation of the primitive solar nebula. Spallation can also be expected to produce DLiBeB fragments in the CO and He layers of the envelope as a primary process, in contrast to cosmic-ray production of such fragments in secondary processes.

Complexly zoned chromium‐aluminum spinel found in situ in the Allende meteorite

Abstract— In addition to the Mg‐, Al‐, 16O‐rich spinels that are known to occur in refractory inclusions, the Murchison meteorite contains Cr‐rich, 16O‐poor spinels, most of whose sources are unknown because they are rarely found in situ. Here we report the in situ occurrence in Allende of Cr‐rich spinels, found in 13 chondrules and 4 “olivine‐rich objects”. The Allende spinels exhibit major and minor element contents, isotopic compositions, and zoning of Cr2O3 contents like those of the Cr‐spinels from Murchison. Some chondrules contain patchy‐zoned spinel (Simon et al., 1994), which suggests that such grains did not form by sintering but perhaps by formation of overgrowths on relic grains. Unlike the olivine‐rich objects, phases in all three chondrules that were analyzed by ion microprobe have uniform, near‐normal O‐isotopic compositions. One olivine‐rich object, ALSP1, has a huge (1 mm) fragment of chevron‐zoned spinel. This spinel has near‐normal O‐isotopic compositions that are quite distinct from those of adjacent forsteritic olivine, which are relatively 16O‐rich and plot on the calcium‐aluminum‐inclusion (CAI) line, like some isolated forsterite grains found in Allende. The spinel and olivine in this object are therefore not genetically related to each other. Another olivine‐rich object, ALSP11A, contains a rectangular, 150 ×s 100 μm, homogeneous spinel grain with 50 wt% Cr2O3 and 23 wt% FeO in a vuggy aggregate of finer‐grained (5–90 μm), FeO‐rich (Fo47–55) olivine. The magnesian core of one olivine grain has a somewhat 16O‐rich isotopic composition like that of the large spinel, whereas the FeO‐rich olivine is relatively 16O‐poor. The composition of the spinel in ALSP11A plots on the CAI line, the first Cr‐rich spinel found to do so. Chevron‐zoned spinel has not been observed in chondrules, and it is unlikely that either ALSP1 or ALSP11A were ever molten. Calculations show that a spinel with the composition of that in ALSP1 can condense at 1780 K at a Ptot of 10−3 atm and a dust/gas ratio of 100 relative to solar. The Cr‐rich spinel in ALSP11A could condense at ∼1420 K, but this would require a dust/gas enrichment of 1000 relative to solar. The data presented here confirm that, as in Murchison, the coarse Cr‐rich spinels in Allende are relatively 16O‐depleted and are isotopically distinct from the 16O‐enriched MgAl2O4 from CAIs. Sample ALSP11A may represent a third population, one that is Cr‐rich and plots on the CAI line. That the O‐isotopic composition of ALSP1 is like those of Cr‐rich spinels from chondrules indicates that O‐isotopic compositions cannot be used to distinguish whether grains from such unequilibrated objects are condensates or are fragments from a previous generation of chondrules.

Revised model calculations for the thermal histories of ordinary chondrite parent bodies

Abstract— Recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model (Miyamoto et al., 1981) for the thermal evolution and internal structure of ordinary chondrite parent bodies. Model calculations assumed a heat source term derived from the decay of 26Al (justification is provided). Differences from the previous model include: varying the thermal diffusivity parameter with increasing temperature (and decreasing porosity), using variable physical and thermal parameters to provide end member models, and incorporating a shortened thermal history of 60 Ma (obtained from new Pb‐Pb chronology of phosphates) rather than 100 Ma. Times of isotopic closure in chondrite phosphates overlap the thermal model estimates, and postmetamorphic cooling rates from the model approximately coincide, in both trend and magnitude, with metallographic and fission track cooling rate data. Model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1.4 to 3.1 Ma after calcium‐aluminum inclusion (CAI) formation. Model calculations also predict that both the H and the L chondrite parent asteroids consisted of ∼80% equilibrated and 20% unequilibrated chondritic material and that their original radii ranged from 80 to 95 km.

Light Isotopes, Extinct Radioisotopes, and Gamma-Ray Lines from Low-Energy Cosmic-Ray Interactions

view Abstract Citations (135) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Light Isotopes, Extinct Radioisotopes, and Gamma-Ray Lines from Low-Energy Cosmic-Ray Interactions Ramaty, Reuven ; Kozlovsky, Benzion ; Lingenfelter, Richard E. Abstract We have carried out detailed calculations of gamma-ray line, light isotope, and extinct radioactive isotope production by low-energy cosmic rays for a broad range of accelerated particle abundances and energy spectra. We first apply these calculations to the gamma-ray line emission observed from the Orion molecular cloud complex. These gamma rays have provided the most direct evidence for the presence of large fluxes of low-energy cosmic rays in the Galaxy. Next we show that the bulk of the Galactic boron, beryllium, and 6Li could have been produced over the age of the Galaxy by interactions of low-energy cosmic rays having typical particle energies around 30 MeV nucleon-1 Such cosmic-ray interactions can account for the observed meteoritic 11B/10B which is higher than the isotopic ratio predicted by relativistic cosmic-ray interactions. Low- energy cosmic rays of similar energies could have also made a significant contribution to the production of 41Ca in meteoritic calcium-aluminum inclusions (CAIs) formed in the early solar system, provided that the cosmic-ray irradiation and CAl formation periods were essentially cotemporal. However, the production of significant amounts of 26Al by low-energy cosmic rays in the early solar system is unlikely because it would overproduce 9Be and 6Li. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176677 Bibcode: 1996ApJ...456..525R Keywords: ACCELERATION OF PARTICLES; ISM: COSMIC RAYS; GALAXY: ABUNDANCES; GAMMA RAYS: THEORY; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; ABUNDANCES; SOLAR SYSTEM: FORMATION; STARS: SUPERNOVAE: GENERAL full text sources ADS |

The first ten million years in the solar nebula

Abstract— The 26Al/27Al ratio in a large number of calcium‐aluminum inclusions (CAIs) is a rather uniform 5 × 10−5, whereas in chondrules the ratio is either undetectable or has a much lower value; the simplest interpretation of this is that there was an interval of a few million years between the times that these two meteoritic constituents formed stable solids. The present investigation was undertaken as an exploration of the physics of the processes in the solar nebula during and after the accumulation of the Sun. Understanding the time scales of events in this nebular model, to see if this would cast light on this apparent CAI to chondrule time interval, was the major motivation for the exploration.There were four stages in the history of the solar nebula; in stage 1, a fragment of an interstellar molecular cloud collapsed to form the Sun and solar nebula; in stage 2, the nebula was in approximate steady state balance between infall from the cloud and accretion onto the Sun and was in its FU Orionis accumulation stage; in stage 3, the Sun had been mainly accumulated and there was a slow residual mass flow into the Sun while it was in its classical T Tauri stage; and in stage 4, the nebula had finished accreting material onto the Sun (now a weak‐lined T Tauri star) and was in a static condition with no significant dissipation or motions, other than removal at the inner edge due to the T Tauri solar wind and photoevaporation beyond 9 astronomical units (AU). It is found that the energy source keeping the nebula warm during stages 3 and 4 is recombination of ionized H in the ionized bipolar jets and the T Tauri coronal expansion solar wind. The parameters of the heating model were adjusted to locate the ice sublimation line at 5.2 AU. In this work, a nebular model is used with a surface density of 4.25 × 103 gm/cm2 at 1 AU and a variation with radial distance as the inverse first power.Under normal conditions in the nebula, there is a negative pressure gradient that provides partial radial support for the gas, which thus circles the Sun more slowly than large solid objects do. Large objects undergo a slow inward spiral due to the gas drag; very small objects move essentially with the gas but have a slow inward drift; and intermediate objects (e.g., 1 m) have a fairly large inward drift velocity that traverses the full radial extent of the nebula in considerably less than the CAI to chondrule time interval. Such objects are thus lost unless they can grow rapidly to larger sizes. Near the inner edge (bow) of the nebula during stage 4, the pressure gradient becomes positive, creating a narrow zone of zero gas drag toward which solids drift from both directions, facilitating planetesimal formation in the inner solar nebula.Recent theoretical and experimental results on sticking probabilities of solids show that icy surfaces have the best sticking properties, but icy interstellar grains can only stick together when subjected to impact velocities of less than 2000 cm/sec. However, if the solid objects are very underdense, then a collision leads to interpenetration and many points at which the small constituent grains can adhere to one another, and thus coagulation becomes possible for such underdense objects. Simulations were made of such coagulation in the outer solar nebula, and it was found that the central plane of the nebula quickly becomes filled with meter‐sized and larger bodies that rapidly accumulated near the top of the nebula and rapidly descended; in a few thousand years this quickly leads to gravitational instabilities that can form planetesimals.These processes led to the rapid formation of Jupiter in the nebula (and the slightly less rapid formation of the other giant planets). The early formation of Jupiter opens an annular gap in the nebula, and thus a second region is created in the nebula with zero gas drag. It is concluded that CAIs were formed at the end of stage 2 of the nebula history and moved out into the nebula for long‐term storage, and that most chondrules were formed by magnetic reconnection flares in the bow region of the nebula during stage 4, several million years later. Carbonaceous meteorites should be formed on the far side of the Jovian gap, with the chondrules being heated by flares on the early Jupiter irradiating materials in the nearby zone of zero gas drag, and they should have essentially the same 26Al ages as the CAIs (this will be very hard to confirm owing to scarcity of Al mineral phases in these chondrules).

The Constituent Minerals in Calcium-Aluminum Inclusions: Spectral Reflectance Properties and Implications for CO Carbonaceous Chondrites and Asteroids

The reflectance spectra of the major constituent minerals of calcium aluminum inclusions (CAIs) have been measured to determine which minerals contribute to CAI reflectance spectra. Spinel appears to be the most significant phase because of its widespread presence in different classes of inclusions and chondrules and its higher ferrous iron content compared to other inclusion phases. Spectra of CAI phases such as perovskite and grossular are nearly featureless spectrally, while melilite, fassaite, and plagioclase feldspar are more weakly featured than spinel. Measured reflectance spectra of CAIs indicate a widespread and generally intense absorption feature in the 2-μm region attributable to minerals containing ferrous iron in tetrahedrally coordinated sites, primarily spinel. CO carbonaceous chondrite reflectance spectra exhibit the clearest spectral evidence for CAI absorption features, suggesting that asteroids with CO type surface mineralogies (or ∼10% CAIs on their surfaces) may be spectrally detectable if the CAIs are optically isolated from the dark matrix. Telescopic reflectance spectra of the K class asteroid 221 Eos show no clear evidence for CAI absorption features. Telescopic reflectance spectra of the anomalous S class asteroids 387 Aquitania and 980 Anacostia can be interpreted as indicating the presence of a chondritic assemblage enriched in minerals which constitute CAIs, such as spinel, or as achondritic chromite-rich assemblages similar to some unique meteorites.