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Parent Body (parent + body)

Distribution by Scientific Domains

Physics and Astronomy	95%
2 Other Domains	5%

Selected Abstracts

Building Niobate Nanoparticles with Hexaniobate Lindqvist Ions

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2010
Hua Tong
Abstract We present a general method to fabricate niobate nanoparticles (NPs) by using hexaniobate Lindqvist ions as niobium source. First, soft-chemical synthesis was adopted to prepare amorphous compound NPs, which subsequently served as parent bodies, affording not only nanoscale size but also optimal compositions and elements for the final annealed nanocrystalline niobates. By this method, a series of nanocrystalline niobates, including MNbO4 (M = In, Ga, Fe), MNb2O6 (M = Ba, Sr, Ni, Cd, Pb), MxFe1,xNbO4 (M = In, Ga), MxNi1,xNb2O6 (M = Ba, Sr), and (AgNb)1,x(SrTi)xO3, were successfully prepared. Experimental results presented herein show that the compositions and components of niobate NPs can be adjusted as desired, markedly influencing the crystal phase, energy band structure, and photocatalytic performance of the niobate NPs. [source]

Ar-Ar ages and thermal histories of enstatite meteorites

METEORITICS & PLANETARY SCIENCE, Issue 5 2010
Donald D. BOGARD
In this study, we report 39Ar- 40Ar dating results for five EL chondrites: Khairpur, Pillistfer, Hvittis, Blithfield, and Forrest; five EH chondrites: Parsa, Saint Marks, Indarch, Bethune, and Reckling Peak 80259; three igneous-textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies: Zaklodzie, Queen Alexandra Range 97348, and Queen Alexandra Range 97289; and three aubrites, Norton County, Bishopville, and Cumberland Falls Several Ar-Ar age spectra show unusual 39Ar recoil effects, possibly the result of some of the K residing in unusual sulfide minerals, such as djerfisherite and rodderite, and other age spectra show 40Ar diffusion loss. Few additional Ar-Ar ages for enstatite meteorites are available in the literature. When all available Ar-Ar data on enstatite meteorites are considered, preferred ages of nine chondrites and one aubrite show a range of 4.50,4.54 Ga, whereas five other meteorites show only lower age limits over 4.35,4.46 Ga. Ar-Ar ages of several enstatite chondrites are as old or older as the oldest Ar-Ar ages of ordinary chondrites, which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies, or were metamorphosed to lower temperatures compared to other chondrite types. Many enstatite meteorites are brecciated and/or shocked, and some of the younger Ar-Ar ages may record these impact events. Although impact heating of ordinary chondrites within the last 1 Ga is relatively common for ordinary chondrites, only Bethune gives any significant evidence for such a young event. [source]

Sulfide-rich metallic impact melts from chondritic parent bodies

METEORITICS & PLANETARY SCIENCE, Issue 5 2010
Devin L. SCHRADER
We compare them with the H-metal meteorite, Lewis Cliff 88432. Phase diagram analyses suggest that SaW 005, MET 00428, and HOW 88403 were liquids at temperatures above 1350 °C. Tridymite in HOW 88403 constrains formation to a high-temperature and low-pressure environment. The morphology of their metal-troilite structures may suggest that MET 00428 cooled the slowest, SaW 005 cooled faster, and HOW 88403 cooled the quickest. SaW 005 and MET 00428 contain H-chondrite like silicates, and SaW 005 contains a chondrule-bearing inclusion that is texturally and compositionally similar to H4 chondrites. The compositional and morphological similarities of SaW 005 and MET 00428 suggest that they are likely the result of impact processing on the H-chondrite parent body. SaW 005 and MET 00428 are the first recognized iron- and sulfide-rich meteorites, which formed by impact on the H-chondrite parent body, which are distinct from the IIE-iron meteorite group. The morphological and chemical differences of HOW 88403 suggest that it is not from the H-chondrite body, although it likely formed during an impact on a chondritic parent body. [source]

Reflectance spectra of iron meteorites: Implications for spectral identification of their parent bodies

METEORITICS & PLANETARY SCIENCE, Issue 2 2010
Edward A. CLOUTIS
Powder spectra are invariably red-sloped over this wavelength interval and have a narrow range of visible albedos (approximately 10,15% at 0.56 ,m). Metal (Fe:Ni) compositional variations have no systematic effect on the powder spectra, increasing grain size results in more red-sloped spectra, and changes in viewing geometry have variable effects on overall reflectance and spectral slope. Roughened metal slab spectra have a wider, and higher, range of visible albedos than powders (22,74% at 0.56 ,m), and are also red-sloped. Smoother slabs exhibit greater differences from iron meteorite powder spectra, exhibiting wider variations in overall reflectance, spectral slopes, and spectral shapes. No unique spectral parameters exist that allow for powder and slab spectra to be fully separated in all cases. Spectral differences between slabs and powders can be used to constrain possible surface properties, and causes of rotational spectral variations, of M-asteroids. The magnitude of spectral variations between M-asteroids and rotational and spectral variability does not necessarily imply a dramatic change in surface properties, as the differences in albedo and/or spectral slope can be accommodated by modest changes in grain size (for powders), small changes in surface roughness (for slabs), or variations in viewing geometry. Since metal powders exhibit much less spectral variability than slabs, M-asteroid spectral variability requires larger changes in either powder properties or viewing geometry than for slabs for a given degree of spectral variation. [source]

Analysis of ordinary chondrites using powder X-ray diffraction: 2.

METEORITICS & PLANETARY SCIENCE, Issue 1 2010
Applications to ordinary chondrite parent-body processes
Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD-derived olivine and low-Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3,0.4 wt% H2O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies. [source]

Reclassification and thermal history of Trenzano chondrite

METEORITICS & PLANETARY SCIENCE, Issue 12 2007
A. M. FIORETTI
The quenched intracrystalline Fe2+ -Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature Tc, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene Tc of 522 ± 13 °C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this Tc is about 100 °C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two-pyroxene equilibrium temperature of Trenzano (824 ± 24 °C), calculated with QUILF95, is similar to the two-pyroxene temperature of 750,840 °C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999). [source]

Ernst Florens Friedrich Chladni (1756,1827) and the origins of modern meteorite research

METEORITICS & PLANETARY SCIENCE, Issue S9 2007
Ursula B. Marvin
These ideas violated two strongly held contemporary beliefs: 1) fragments of rock and metal do not fall from the sky, and 2) no small bodies exist in space beyond the Moon. From the beginning, Chladni was severely criticized for basing his hypotheses on historical eyewitness reports of falls, which others regarded as folk tales, and for taking gross liberties with the laws of physics. Ten years later, the study of fallen stones and irons was established as a valid field of investigation. Today, some scholars credit Chladni with founding meteoritics as a science; others regard his contributions as scarcely worthy of mention. Writings by his contemporaries suggest that Chladni's book alone would not have led to changes of prevailing theories; thus, he narrowly escaped the fate of those scientists who propose valid hypotheses prematurely. However, between 1794 and 1798, four falls of stones were witnessed and widely publicized. There followed a series of epoch-making analyses of fallen stones and "native irons" by the chemist Edward C. Howard and the mineralogist Jacques-Louis de Bournon. They showed that all the stones were much alike in texture and composition but significantly different from the Earth's known crustal rocks. Of primary importance was Howard's discovery of nickel in the irons and the metal grains of the stones. This linked the two as belonging to the same natural phenomenon. These chemical results, published in February 1802, persuaded some of the leading scientists in England, France, and Germany that bodies do fall from the sky. Within a few months, chemists in France reported similar results and a new field of study was inaugurated internationally, although opposition lingered on until April 1803, when nearly 3,000 stones fell at L'Aigle in Normandy and transformed the last skeptics into believers. Chladni immediately received full credit for his hypothesis of falls, but decades passed before his linking of falling bodies with fireballs received general acceptance. His hypothesis of their origin in cosmic space met with strong resistance from those who argued that stones formed within the Earth's atmosphere or were ejected by lunar volcanoes. After 1860, when both of these hypotheses were abandoned, there followed a century of debate between proponents of an interstellar versus a planetary origin. Not until the 1950s did conclusive evidence of their elliptical orbits establish meteorite parent bodies as members of the solar system. Thus, nearly 200 years passed before the questions of origin that Chladni raised finally were resolved. [source]

Cosmic-ray exposure age and heliocentric distance of the parent bodies of enstatite chondrites ALH 85119 and MAC 88136

METEORITICS & PLANETARY SCIENCE, Issue 6 2006
D. Nakashima
These two meteorites contain solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated heliocentric distances, parent body exposure ages, and space exposure ages of the two meteorites. The parent body exposure ages are longer than 6.7 Ma for ALH 85119 and longer than 8.7 Ma for MAC 88136. The space exposure ages are shorter than 2.2 Ma for ALH 85119 and shorter than 3.9 Ma for MAC 88136. The estimated heliocentric distances are more than 1.1 AU for ALH 85119 and 1.3 AU for MAC 88136. Derived heliocentric distances indicate the locations of parent bodies in the past when constituents of the meteorites were exposed to the Sun. From the mineralogy and chemistry of E chondrites, it is believed that E chondrites formed in regions within 1.4 AU from the Sun. The heliocentric distances of the two E chondrite parent bodies are not different from the formation regions of E chondrites. This may imply that heliocentric distances of E chondrites have been relatively constant from their formation stage to the stage of exposure to the solar wind. [source]

Halite and stable chlorine isotopes in the Zag H3,6 breccia

METEORITICS & PLANETARY SCIENCE, Issue 5 2004
J. C. Bridges
The purity of the associated NaCl-H2O brine is implied by freezing characteristics of fluid inclusions in the halite and EPMA analyses together with a lack of other evaporite-like phases in the Zag H3,6 component. This is inconsistent with multi-stage evolution of the fluid involving scavenging of cations in the Zag region of the parent body. We suggest that the halite grains are clastic and did not crystallize in situ. Halite and water-soluble extracts from Zag have light Cl isotopic compositions, ,37Cl = ,1.4 to ,2.8%. Previously reported bulk carbonaceous chondrite values are approximately ,37Cl = +3 to +4%. This difference is too great to be the result of fractionation during evaporation, and instead, we suggest that Cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates. Extraterrestrial carbon released at 600 °C from the H3,4 matrix has ,13C = ,20%, consistent with poorly graphitized material being introduced into the matrix rather than indigenous carbonate derived from a brine. We have also examined 28 other H chondrite falls to ascertain how widespread halite or evaporite-like mineral assemblages are in ordinary chondrites. We did not find any more to add to Zag (H3-6) and Monahans (H5), which suggests that such highly soluble phases were not usually preserved on the parent bodies. [source]

The role of sticky interstellar organic material in the formation of asteroids

METEORITICS & PLANETARY SCIENCE, Issue 12 2002
T. Kudo
The organic material was found to be stickiest at a radius of between 2.3 and 3.0 AU, with a maximum sticking velocity of 5 m s,1 for millimeter-size organic grains. This stickiness is considered to have resulted in the very rapid coagulation of organic grain aggregates and subsequent formation of planetesimals in the early stage of the turbulent accretion disk. The planetesimals formed in this region appear to be represent achondrite parent bodies. In contrast, the formation of planetesimals at <2.1 and >3.0 AU begins with the establishment of a passive disk because silicate and ice grains are not as sticky as organic grains. [source]

Linking meteorites with their parent bodies

METEORITICS & PLANETARY SCIENCE, Issue 2 2001
Dale P. Cruikshank
[source]

AFM and SNOM characterization of ordinary chondrites: A contribution to solving the problem of asteroid reddening

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2010
Giuliano Pompeo
Abstract Space weathering (SW) is an ensemble of processes that act on a body exposed to the space environment. Typically, the exposure to SW results in the accumulation, at the surface, of nanoparticles, that are thought to be produced through a vaporization and subsequent cooling of the metallo-silicaceous components exposed to the space environment. The presence of such nanoparticles is responsible for the so-called reddening of the asteroids' reflectance spectra (i.e., the increase in Vis,NIR reflectance with increase in wavelength) observed by remote-sensing measurements. To investigate the mechanism of formation of these nanoparticles, we have employed atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) to morphologically and optically characterize ordinary chondrites (OC), the most abundant class of meteorites collected on Earth and whose parent bodies are the S-type asteroids. The AFM study reveals the occurrence of a diffuse nanophase (martensite) in the meteorite's metal inclusions. Since the same areas show a reddening of the reflectivity spectra, this suggests that such spectral modification is based on a shock-induced phase transformation of the metal components of the extraterrestrial body. To gain more insight into this nanophase and on its role in the SW of the asteroids, an optical characterization by SNOM has been performed on OCs. In this work we exploited the peculiarity of this technique to search for a correlation between the topography on the nanoscale and the spectral characteristics, at different wavelengths in the red-NIR range, of the observed nanophase. Indeed, a high-resolution mapping of the optical properties of the meteorite provides an interesting method to discriminate between martensite-based and Fe-silicaceous nanoparticles. [source]

Sulfide-rich metallic impact melts from chondritic parent bodies

Analysis of ordinary chondrites using powder X-ray diffraction: 2.

The Fountain Hills unique CB chondrite: Insights into thermal processes on the CB parent body

METEORITICS & PLANETARY SCIENCE, Issue 6 2009
Dante S. LAURETTA
This meteorite is closely related to the CBa class. Mineral compositions and O-isotopic ratios are indistinguishable from other members of this group. However, many features of Fountain Hills are distinct from the other CB chondrites. Fountain Hills contains 23 volume percent metal, significantly lower than other members of this class. In addition, Fountain Hills contains porphyritic chondrules, which are extremely rare in other CBa chondrites. Fountain Hills does not appear to have experienced the extensive shock seen in other CB chondrites. The chondrule textures and lack of fine-grained matrix suggests that Fountain Hills formed in a dust-poor region of the early solar system by melting of solid precursors. Refractory siderophiles and lithophile elements are present in near-CI abundances (within a factor of two, related to the enhancement of metal). Moderately volatile and highly volatile elements are significantly depleted in Fountain Hills. The abundances of refractory siderophile trace elements in metal grains are consistent with condensation from a gas that is reduced relative to solar composition and at relatively high pressures (10,3bars). Fountain Hills experienced significant thermal metamorphism on its parent asteroid. Combining results from the chemical gradients in an isolated spinel grain with olivine-spinel geothermometry suggests a peak temperature of metamorphism between 535 °C and 878 °C, similar to type-4 ordinary chondrites. [source]

Stardust in Antarctic micrometeorites

METEORITICS & PLANETARY SCIENCE, Issue 8 2008
Toru YADA
The oxygen isotopic compositions of the eighteen presolar silicate (and one oxide) grains found are similar those observed previously in primitive meteorites and interplanetary dust particles, and indicate origins in oxygen-rich red giant or asymptotic giant branch stars, or in supernovae. Four grains with anomalous C isotopic compositions were also detected. 12C/13C as well as Si ratios are similar to those of mainstream SiC grains; the N isotopic composition of one grain is also consistent with a mainstream SiC classification. Presolar silicate grains were found in three of the seven AMMs studied, and are heterogeneously distributed within these micrometeorites. Fourteen of the 18 presolar silicate grains and 3 of the 4 C-anomalous grains were found within one AMM, T98G8. Presolar silicate-bearing micrometeorites contain crystalline silicates that give sharp X-ray diffractions and do not contain magnesiowüstite, which forms mainly through the decomposition of phyllosilicates and carbonates. The occurrence of this mineral in AMMs without presolar silicates suggests that secondary parent body processes probably determine the presence or absence of presolar silicates in Antarctic micrometeorites. [source]

Evolution of the winonaite parent body: Clues from silicate mineral trace element distributions

METEORITICS & PLANETARY SCIENCE, Issue 4 2008
Christine FLOSS
Textural evidence in these meteorites, including the presence of a plagioclase/clinopyroxene-rich lithology and coarse-grained olivine lithologies, suggests that they may have experienced some silicate partial melting. However, trace element distributions in these lithologies do not show any clear signatures for such an event. Pyroxene trace element compositions do exhibit systematic trends, with abundances generally lowest in Pontlyfni and highest in Winona. The fact that the same trends are present for both incompatible and compatible trace elements suggests, however, that the systematics are more likely the result of equilibration of minerals with initially heterogeneous and distinct compositions, rather than partial melting of a compositionally homogeneous precursor. The winonaites have experienced brecciation and mixing of lithologies, followed by varying degrees of thermal metamorphism on their parent body. These factors probably account for the variable bulk rare earth element (REE) patterns noted for these meteorites and may have led to re-equilibration of trace elements in different lithologies. [source]

182Hf- 182W chronometry and the early evolution history in the acapulcoite-lodranite parent body

METEORITICS & PLANETARY SCIENCE, Issue 4 2008
Der-Chuen LEE
Unlike the more evolved achondrites originating from differentiated asteroids,e.g., eucrites and angrites,bulk rock acapulcoites and lodranite are characterized by distinct 182W deficits relative to the terrestrial W, as well as to the undifferentiated chondrites, ,w varies from ,2.7 to ,2.4. This suggests that live- 182Hf was present during the formation of acapulcoites and lodranites, and their parent body probably had never experienced a global melting event. Due to the large uncertainties associated with the isochron for each sample, the bulk isochron that regressed through the mineral separates from all 3 samples has provided the best estimate to date for the timing of metamorphism in the acapulcoite-lodranite parent body, 5 (+6/-5) Myr after the onset of the solar system. It is thus inconclusive whether acapulcoites and lodranites have shared the same petrogenetic origin, based on the Hf-W data of this study. Nevertheless, the formation of acapulcoite-lodranite clan appears to have post-dated the metal-silicate segregation in differentiated asteroids. This can be explained by a slower accretion rate for the acapulcoite-lodranite parent body, or that it had never accreted to a critical mass that could allow the metal-silicate segregation to occur naturally. [source]

The GRO 95577 CR1 chondrite and hydration of the CR parent body

METEORITICS & PLANETARY SCIENCE, Issue 9 2007
Michael K. Weisberg
GRO 95577 has many petrological similarities to the CR chondrites. Although the INAA data show patterns indicative of terrestrial weathering, some of the elemental abundances are consistent with a relationship to CR chondrites. The oxygen isotopic composition of GRO 95577 plots close to the Renazzo CR chondrite on the three-isotope diagram. However, GRO 95577 is remarkable in that the chondrules are completely hydrated, consisting almost entirely of phyllosilicates, magnetite, and sulfides. Although GRO 95577 is completely hydrated, the initial chondrule textures are perfectly preserved. The chondrules are in sharp contact with the matrix, their fine-grained rims are clearly visible, and the boundaries of the dark inclusions can be easily discerned. Many chondrules in GRO 95577 have textures suggestive of type I chondrules, but the phenocrysts have undergone perfect pseudomorphic replacement by yellow to brownish serpentine-rich phyllosilicate, with sharp original crystal outlines preserved. The chondrule mesostasis is a green aluminous chlorite-rich material, most likely a hydration product of the feldspathic mesostasis commonly found in anhydrous type I chondrules. Some chondrules contain magnetite spheres, most likely formed by oxidation of metal. We propose that GRO 95577 be classified as a CR1 chondrite, making it the first known CR1 chondrite and expanding the range of alteration conditions on the CR parent body. [source]

Microstructure and thermal history of metal particles in CH chondrites

METEORITICS & PLANETARY SCIENCE, Issue 6 2007
J. I. GOLDSTEIN
Four types of metal particles are common in all of these chondrites. Zoned and unzoned particles probably formed as condensates from a gas of chondritic composition in a monotonic cooling regime, as has been shown previously. We have demonstrated that these particles were cooled rapidly to temperatures below 500 K after they formed, and that condensation effectively closed around 700 K. Zoned and unzoned particles with exsolution precipitates, predominantly high-Ni taenite, have considerably more complex thermal histories. Precipitates grew in reheating episodes, but the details of the heating events vary among individual grains. Reheating temperatures are typically in the range 800,1000 K. Reheating could have been the result of impact events on the CH parent body. Some particles with precipitates may have been incorporated into chondrules, with further brief heating episodes taking place during chondrule formation. In addition to the four dominant types of metal particles, rare Ni-rich metal particles and Si-rich metal particles indicate that the metal assemblage in CH chondrites was a mixture of material that formed at different redox conditions. Metal in CH chondrites consists of a mechanical mixture of particles that underwent a variety of thermal histories prior to being assembled into the existing brecciated meteorites. [source]

Polyhedral serpentine grains in CM chondrites

METEORITICS & PLANETARY SCIENCE, Issue 5 2006
Thomas J. ZEGA
The structure of these grains is similar to terrestrial polygonal serpentine, but the data show that some have spherical or subspherical, rather than cylindrical morphologies. We therefore propose that the term polyhedral rather than polygonal be used to describe this material. EDS shows that the polyhedral grains are rich in Mg with up to 8 atom% Fe. EELS indicates that 70% of the Fe occurs as Fe3+. Alteration of cronstedtite on the meteorite parent body under relatively oxidizing conditions is one probable pathway by which the polyhedral material formed. The polyhedral grains are the end-member serpentine in a mineralogic alteration sequence for the CM chondrites. [source]

Northwest Africa 011: A "eucritic" basalt from a non-eucrite parent body

METEORITICS & PLANETARY SCIENCE, Issue 3 2005
Christine Floss
This meteorite bears many similarities to the eucrites it was initially identified with, although oxygen isotopic compositions rule out a genetic relationship. Like many eucrites, NWA 011 crystallized from a source with approximately chondritic proportions of REE, although a slightly LREE-enriched bulk composition with a small positive Eu anomaly, as well as highly fractionated Fe/Mg ratios and depleted Sc abundances (Korotchantseva et al. 2003), suggest that the NWA 011 source experienced some pyroxene and/or olivine fractionation. Thermal metamorphism resulted in homogenization of REE abundances within grains, but NWA 011 did not experience the intergrain REE redistribution seen in some highly metamorphosed eucrites. Despite a similarity in oxygen isotopic compositions, NWA 011 does not represent a basaltic partial melt from the acapulcoite/lodranite parent body. The material from which NWA 011 originated may have been like some CH or CB chondrites, members of the CR chondrite clan, which are all related through oxygen isotopic compositions. The NWA 011 parent body is probably of asteroidal origin, possibly the basaltic asteroid 1459 Magnya. [source]

Oxygen isotopic alteration in Ca-Al-rich inclusions from Efremovka: Nebular or parent body setting?

METEORITICS & PLANETARY SCIENCE, Issue 8 2004
T. J. Fagan
The coarse-grained CAI (CGI-10) is a sub-spherical object composed of elongate, euhedral, normally-zoned melilite crystals ranging up to several hundreds of Pm in length, coarse-grained anorthite and Al, Ti-diopside (fassaite), all with finegrained (,10 ,m across) inclusions of spinel. Similar to many previously examined coarse-grained CAIs from CV chondrites, spinel and fassaite are 16O-rich and melilite is 16O-poor, but in contrast to many previous results, anorthite is 16O-rich. Isotopic composition does not vary with textural setting in the CAI: analyses of melilite from the core and mantle and analyses from a variety of major element compositions yield consistent 16O-poor compositions. CGI-10 originated in an 16O-rich environment, and subsequent alteration resulted in complete isotopic exchange in melilite. The fine-grained CAI (FGI-12) also preserves evidence of a 1st-generation origin in an 16O-rich setting but underwent less severe isotopic alteration. FGI-12 is composed of spinel ± melilite nodules linked by a mass of Al-diopside and minor forsterite along the CAI rim. All minerals are very fine-grained (<5 ,m) with no apparent igneous textures or zoning. Spinel, Al-diopside, and forsterite are 16O-rich, while melilite is variably depleted in 16O (,17,18O from ,-40, to ,5,). The contrast in isotopic distributions in CGI-10 and FGI-12 is opposite to the pattern that would result from simultaneous alteration: the object with finer-grained melilite and a greater surface area/ volume has undergone less isotopic exchange than the coarser-grained object. Thus, the two CAIs were altered in different settings. As the CAIs are adjacent to each other in the meteorite, isotopic exchange in CGI-10 must have preceded incorporation of this CAI in the Efremovka parent body. This supports a nebular setting for isotopic alteration of the commonly observed 16O-poor melilite in coarse-grained CAIs from CV chondrites. [source]

Halite and stable chlorine isotopes in the Zag H3,6 breccia

Importance of the accretion process in asteroid thermal evolution: 6 Hebe as an example

METEORITICS & PLANETARY SCIENCE, Issue 5 2003
Amitabha Ghosh
Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of 26Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high-grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories. [source]

39Ar- 40Ar chronology of R chondrites

METEORITICS & PLANETARY SCIENCE, Issue 3 2003
Eleanor T. DIXON
The 39Ar- 40Ar ages were determined on whole-rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak 39Ar- 40Ar ages of the four chondrites are 4.35, ,4.47 ± 0.02, 4.30 ± 0.07 Ga, and 4.37 Ga, respectively. These ages are similar to Ar-Ar ages of relatively unshocked ordinary chondrites (4.52,4.38 Ga) and are older than Ar-Ar ages of most shocked ordinary chondrites («4.2 Ga). Because the meteorites with the oldest (Rumuruti, ,4.47 Ga) and the youngest (Acfer 217, ,4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end-member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ,170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact-induced thick regolith layer, or within a rubble pile type parent body following parent body re-assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (,4.47 Ga) impact events and breccia formation on the R chondrite parent body. [source]

26Mg excess in hibonites of the Rumuruti chondrite Hughes 030

METEORITICS & PLANETARY SCIENCE, Issue 1 2003
A. Bischoff
Most samples of this group are gas-rich regolith breccias showing the typical light/dark structure and consist of abundant fragments of various parent body lithologies embedded in a fine-grained, olivine-rich matrix. Most R chondrites contain the typical components of primitive chondrites including chondrules, chondrule and mineral fragments, sulfides, and rare calcium-aluminum-rich inclusions (CAIs). In Hughes 030, an interesting CAI consisting of abundant hibonite and spinel was found. Mg isotopic analyses revealed excess 26Mg in components of R chondrites for the first time. The hibonite grains with high Al/Mg values (,1500 to 2600) show resolved 26Mg excess. The slope of the correlation line yields an initial 26Al/ 27Al = (1.4 ± 0.3) × 10,6, which is ,40 times lower than the initial value measured in CAIs from primitive meteorites. The inferred difference in 26Al abundance implies a time difference of ,4 million years for the closure of the Al-Mg system between CAIs from primitive chondrites and the Hughes 030 CAI. Based on mineralogy and the petrographic setting of the hibonite-rich CAI, it is suggested that 4 million years reflect the time interval between the formation of the CAI and the end of its secondary alteration. It is also suggested that most of this alteration may have occurred in the nebula (e.g. Zn- and Fe-incorporation in spinels). However, the CAI could not have survived in the nebula as a free floating object for a long period of time. Therefore, the possibility of storage in a precursor planetesimal for a few million years, resetting the magnesium-aluminum isotopic system, prior to impact brecciation, excavation, and accretion of the final R chondrite parent body cannot be ruled out. [source]

Chondrule thermal history from unequilibrated H chondrites: A transmission and analytical electron microscopy study

METEORITICS & PLANETARY SCIENCE, Issue 10 2002
C. Ferraris
Nanotextural and nanochemical data indicate similar thermal evolution for chondrules of the same textural groups; minor, yet meaningful differences occur among the different groups. Olivine is the earliest phase formed and crystallizes between 1500 and 1400 °C. Protoenstatite crystallizes at temperatures higher than 1350,1200 °C; it later inverts to clinoenstatite in the 1250,1200 °C range. Enstatite is surrounded by pigeonitic or (less frequently) augitic rims; the minimal crystallization temperature for the rims is 1000 °C; high pigeonite later inverts to low pigeonite, between 935 and 845 °C. The outer pigeonitic or augitic rims are constantly exsolved, producing sigmoidal augite or enstatite precipitates; sigmoidal precipitates record exsolution temperatures between 1000 and 640 °C. Cooling rate (determined using the speedometer based upon ortho-clinoenstatite intergrowth) was in the order of 50,3000 °C/h at the clinoenstatite-orthoenstatite transition temperature (close to 1250,1200 °C), but decreased to 5,10 °C/h or slower at the exsolution temperature (between 1000 and 650 °C), thus revealing nonlinear cooling paths. Nanoscale observations indicate that the individual chondrules formed and cooled separately from 1500 °C down to at least 650 °C. Accretion into chondritic parent body occurred at temperatures lower than 650 °C. [source]

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

METEORITICS & PLANETARY SCIENCE, Issue 4 2001
Yukio IKEDA
Type I ureilite clasts (olivine-pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component-enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields. DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene-metal and pyroxene-plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene-metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene-plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body. The felsic clasts may have formed mainly from albite component-rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali-bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt. [source]

A petrologic study of the IAB iron meteorites: Constraints on the formation of the IAB-Winonaite parent body

METEORITICS & PLANETARY SCIENCE, Issue 6 2000
G. K. BENEDIX
These meteorites contain inclusions that fall broadly into five types: (1) sulfide-rich, composed primarily of troilite and containing abundant embedded silicates; (2) nonchondritic, silicate-rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate-rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite-rich assemblages that sometimes contain silicates; and (5) phosphate-bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and O-isotopic compositions suggest that IAB iron and winonaite meteorites are from the same parent body. We propose a hypothesis for the origin of IAB iron meteorites that combines some aspects of previous formation models for these meteorites. We suggest that the precursor parent body was chondritic, although unlike any known chondrite group. Metamorphism, partial melting, and incomplete differentiation (i.e., incomplete separation of melt from residue) produced metallic, sulfide-rich and silicate partial melts (portions of which may have crystallized prior to the mixing event), as well as metamorphosed chondritic materials and residues. Catastrophic impact breakup and reassembly of the debris while near the peak temperature mixed materials from various depths into the re-accreted parent body. Thus, molten metal from depth was mixed with near-surface silicate rock, resulting in the formation of silicate-rich IAB iron and winonaite meteorites. Results of smoothed particle hydrodynamic model calculations support the feasibility of such a mixing mechanism. Not all of the metal melt bodies were mixed with silicate materials during this impact and reaccretion event, and these are now represented by silicate-free IAB iron meteorites. Ages of silicate inclusions and winonaites of 4.40-4.54 Ga indicate this entire process occurred early in solar system history. [source]