bluuebunny/crossref_metadata_2025 huggingface dataset:
Ocean Drilling Project Hole 689B spherules and upper Eocene microtektite and clinopyroxene‐bearing spherule strewn fields
Authors: GLASS B. P. , KOEBERL C.
Published: February 2010

Abstract— Montanari et al. (1993) reported a positive Ir anomaly in the upper Eocene sediments from Ocean Drilling Program Hole 689B on the Maud Rise, Southern Ocean. Vonhof (1998) described microtektites and clinopyroxene‐bearing (cpx) spherules associated with the Ir anomaly in Hole 689B and suggested that they belong to the North American and equatorial Pacific cpx strewn fields, respectively. We searched a suite of 27 samples taken through the spherule layer from Hole 689B, and we recovered 386 microtektites and 667 cpx spherules. We studied the petrography of the microtektites and cpx spherules and determined the major element compositions of 31 microtektites and 14 cpx spherules using energy dispersive x‐ray analysis. We also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis. We found that the peak abundance of cpx spherules is ∼2 cm below the peak abundance of the microtektites (∼128.7 m below sea floor), which suggests that the cpx spherule layer may be slightly older (∼3–5 ka). The microtektites are mostly spherical and are generally transparent and colorless. They are similar to the North American microtektites in composition, the biggest differences being their generally lower Na2O and generally higher Zr, Ba, and Ir (up to 0.3 ppb) contents. We agree with Vonhof (1998) that the Hole 689B microtektites probably belong to the North American tektite strewn field. We calculate that the number of microtektites (>125 μm)/cm2 at Hole 689B is 52. This number is close to the concentration predicted by extrapolation of the trend of concentration vs. distance from the Chesapeake Bay structure, based on data from other North American microtektite‐bearing sites. Thus, the North American strewn field may be at least four times larger than previously mapped. The Hole 689B cpx spherules range from translucent yellow to opaque black, but most are opaque tan to dark brown. They are generally spherical in shape and all are < 125 μm in diameter. Some contain Ni‐rich spinels in addition to clinopyroxene microlites. The cpx spherules are petrographically and compositionally similar to cpx spherules previously found in the northwestern Atlantic Ocean, Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern Indian Ocean. The abundance and widespread geographic occurrence of these spherules suggest that the strewn field may be global in geographic extent. Assuming a global extent, we estimate that there may be at least 25 billion metric tons of cpx spherules in the strewn field. Based on age, size, and geographic location, we speculate that the 100 km diameter Popigai crater in northern Siberia may be the source of the cpx spherule layer.
Origin of polymict breccias on asteroids deduced from their pyroxene fragments
Authors: SAIKI KAZUTO , TAKEDA HIROSHI
Published: February 2010

Abstract— Cumulate eucrite, noncumulate eucrite, and diogenite meteorites are considered to have come from the crust of one (or similar) parent asteroid. Howardites are regarded as regolith breccias of eucrites and diogenites, and polymict eucrites are regarded as polymict breccias of eucrites.

These polymict breccias show many textural and chemical features. In order to gain a better understanding of the origin of polymict breccias and the origin of their components, we investigated four polymict breccias, Yamato (Y)‐791439, Y‐791192, Y‐82009, and Y‐82049 with a scanning electron microscope (SEM) equipped with a chemical mapping system, and by electron probe microanalysis (EPMA). We analyzed all pyroxene grains with chemical maps, classified them by chemical composition, and observed their chemistry and mineralogy in detail.

The characteristics of pyroxenes suggest that the polymict breccias were generated by gathering locally ordinary eucrites and cumulate eucrites. The chemical‐evolutionary features of the pyroxenes (such as homogenization, chemical zoning, and exsolution lamellae) suggest that there were at least two long annealing events and one short (or low‐temperature) annealing event, separated by mixing events. Local heterogeneity on the asteroidal crust is also suggested.
Atmospheric trajectory and orbit of the Omolon meteorite
Authors: Bronshten V. a. , Zharov V. e. , Khotinok R. l.
Published: February 2010

Abstract— The Omolon meteorite fell on 1981 May 15 at 17:10 U.T. to a point with the coordinates φ = 64°01′08″ N, λ = 161°48′30″ E. This is the fifth pallasite that was observed at the moment of its fall and the largest of the pallasites known worldwide (250 kg). The history of the observation, search, and finding of the meteorite is briefly described. From the size of the meteorite and the funnel that it produced, the velocity of its encounter with the ground is estimated by aerodynamic formulas to be 220 m/s. An attempt at estimating the meteorite's initial velocity and mass from its terminal values (which yielded the mass range of 390–490 kg that corresponds to the velocity range of 12–15 km/s) was successful for the mass but unsuccessful for the velocity and the incidence angle, because the problem was ill posed. The position of the radiant is determined from the available observations to be α = 176.4°, δ = +24.1° (Leo). The radiant was situated at an elongation of 29° from the antapex, which means that this was an overtaking meteorite and its entry velocity did not exceed 16 km/s. Three variants of the calculation of the orbital elements—for an entry velocity of 12, 14, and 16 km/s—are presented. In all the three cases, the meteoroid's orbit is close to the orbits of Apollo asteroids and to the orbits of iron meteoroids observed as fireballs with bright iron lines in their spectra. The Omolon meteorite was probably a fragment of an Apollo M‐type asteroid. This study is the first attempt at calculating the orbit of a pallasite.
Radiogenic iron
Authors: Clayton Donald d.
Published: February 2010

Abstract— This historical review focuses on the idea that this very abundant chemical element was overwhelmingly created and ejected from the stars not in its own chemical form but that of radioactive Ni progenitors. Iron in the universe outnumbers all of the common metals. Its thermonuclear origin provided the beginnings for the theory of nucleosynthesis in stars. Three of its isotopes (masses 54, 56, and 57) are counted among the most prominent isotopes of any element. Two of these isotopes (56Fe and 57Fe) are now known to have derived naturally from the radioactive decay of Ni isobars outside of exploding stars. Tension and numerous mistakes surrounding the discovery of its radiogenic origin are analyzed with historical accuracy. The radioactive origin is described as having been first overlooked and later resisted to considerable degree. But incontrovertible evidence, especially from supernova light curves and gamma‐ray‐line astronomy, has established its correctness. Radiogenic Fe thus remains the centerpiece for both the theory and the observations of nucleosynthesis in stars.
The Meteoritical Bulletin, No. 83, 1999 July
Authors: Grossman Jeffrey n.
Published: February 2010

Abstract—Meteoritical Bulletin No. 83 lists information for 898 newly described meteorites. These include 473 from Antarctica, 341 from the Sahara, and 22 from dry lakes in the southwestern United States. Seven of the meteorites are falls: Kunya‐Urgench (H5), Lohawat (howardite), Ourique (H4), Portales Valley (H6), San Pedro de Quiles (L6), Talampaya (eucrite), and Zag (H3‐6). Also included are a dozen new iron meteorites; several mesosiderites; a pallasite; several HED meteorites; several ureilites; a variety of CM, CO, CV, CR, and R chondrites; and numerous unequilibrated ordinary chondrites. All shock classifications are after Stöffleret al.(1991) and weathering grades are after Wlotzka (1993), except as noted. All italicized abbreviations refer to addresses tabulated at the end of this document.
An experimental study on kinetically‐driven precipitation of calcium‐magnesium‐iron carbonates from solution: Implications for the low‐temperature formation of carbonates in martian meteorite Allan Hills 84001
Authors: GOLDEN D. C. , MING D. W. , SCHWANDT C. S. , MORRIS R. V. , YANG S. V. , LOFGREN G. E.
Published: February 2010

Abstract— Spherical carbonate globules of similar composition, size, and radial Ca‐, Mg‐, and Fe‐zonation to those in martian meteorite Allan Hills (ALH) 84001 were precipitated from Mg‐rich, supersaturated solutions of Ca‐Mg‐Fe‐CO2‐H2O at 150 °C. The supersaturated solutions (pH ≅ 6–7) were prepared at room temperature and contained in TeflonTM‐lined stainless steel vessels, which were sealed and heated to 150 °C for 24 h. Experiments were also conducted at 25 °C and no globules comparable to those of ALH 84001 were precipitated. Instead, amorphous Fe‐rich carbonates were formed after 24 h and Mg‐Fe calcites formed after 96 h. These experiments suggest a possible low‐temperature inorganic origin for the carbonates in martian meteorite ALH 84001.
Noble gases in interplanetary dust particles, I: The excess helium‐3 problem and estimates of the relative fluxes of solar wind and solar energetic particles in interplanetary space
Authors: PEPIN R. O. , PALMA R. L. , SCHLUTTER D. J.
Published: February 2010

Abstract— We report mass‐spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios—up to >40x the solar‐wind value—‐and found primarily but not exclusively in shards of some of the larger IDPs (“cluster particles”) that fragmented on impact with the collectors carried by high‐altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic‐ray‐induced spallation during estimated space residence times of IDPs, or by direct implantation of solar‐flare He. Minimum exposure ages inferred from the 3He excesses range from ∼50 Ma to an impossible >10 Ga, compared to Poynting‐Robertson drag lifetimes for low‐density 20–30 μm particles on the order of ∼0.1 Ma for an asteroidal source and ∼10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar‐energetic‐particle (SEP) gases, to the virtual exclusion of solar‐wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ∼1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10–100 above estimates for this ratio in low‐energy solar particle emission.
Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories
Authors: LEROUX Hugues , DOUKHAN Jean‐Claude , PERRON Claude
Published: February 2010

Abstract— This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe‐Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories.

In Saint Séverin, the microstructure of the Ni‐rich metal grains is due to slow cooling. It consists of a two‐phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni‐rich phase is tetrataenite (Ni = 47 wt%) and the Ni‐poor phase, with a composition of ∼25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni‐poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high‐Ni tetrataenite rim in contact with the cloudy zone displays high‐Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature.

Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high‐Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low‐Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni‐rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate‐matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev and 10–100 °C/year for Agen.
Classification of the Didwana‐Rajod meteorite: A Mössbauer spectroscopic study
Authors: PALIWAL B. S. , TRIPATHI R. P. , VERMA H. C. , SHARMA S. K.
Published: February 2010

Abstract— Mössbauer spectroscopic studies of the Didwana‐Rajod chondrite, which fell on 1991 August 12 in western Rajasthan, India, are presented. The results are compared with the Mössbauer data of several enstatite and ordinary chondrites including the Dhajala chondrite for which Mössbauer data were acquired during the present study. The Didwana‐Rajod chondrite's iron phases and its oxidation states strongly suggest that it should be classified as an H‐type ordinary chondrite instead of the earlier suggestion (based on petrographic studies) that it could be an enstatite chondrite. The present study demonstrates that Mössbauer spectroscopy is a very powerful technique for aiding in the classification of meteorites.
Charlevoix and Sudbury as gravity‐readjusted impact structures
Authors: RONDOT J.
Published: February 2010

Abstract— On the basis of detailed field investigations and the study of impact breccias at the Charlevoix impact structure (Québec), a structural analysis is given. This analysis shows that the annular arrangement of both topography and lithology can be applied to subdivide the two craters discussed in this review. Successive rings characterize the central uplift and its collapsed ring graben in the Charlevoix structure.

Fault breccias show an annular distribution and indicate diverse directions of movement suggesting progressive readjustment. They formed cohesionless products inside fault planes, named mylolisthenite, and are believed to have acted as lubricants during the modification stage of the formation of the structure: listric readjustment by gravity after the formation of the transient crater.

The Sudbury Structure (Ontario) shows fault breccias in a concentric arrangement similar to that of Charlevoix. It may be compared with the Charlevoix structure by applying the same mechanical readjustment model with deep listric faults. The only difference is the amount of the rise structural uplift, that is, the height of central uplift, which is less important in the larger impact structure.
The Bosumtwi meteorite impact structure, Ghana: A magnetic model
Authors: PLADO J. , PESONEN L. J. , KOEBERL C. , ELO S.
Published: February 2010

Abstract— A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (∼1.07 Ma) structure with a diameter of ∼10.5 km is exposed within early Proterozoic Birimian—Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high‐resolution, low altitude (∼70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, ∼12 km in diameter. The central‐north part of the lake reveals a central negative magnetic anomaly with smaller positive side‐anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact‐melt rock or melt‐rich suevite bodies.

Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.
Enstatite chemical composition and microstructures in the La Villa H4 chondrite
Authors: FOLCO L. , MELLINI M.
Published: February 2010

Abstract—La Villa is an unshocked H4 chondrite. Chemical compositions require crystallization at temperatures >1250 °C for enstatite and >1211 °C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to either ortho‐ or clinoenstatite or both on cooling. High‐resolution transmission electron microscopy shows a range of ortho‐clinoenstatite intergrowths: heavily faulted clinoenstatite in radial and poikilitic chondrules, almost regular orthoenstatite in a microgranular chondrule and in the matrix. In the former, the clinoenstatite lamellae are both even or odd multiples of the 9Å periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 Å. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by “U‐shaped” and “Z‐shaped” terminations. The variable microstructures suggest different cooling rates for the different chondrule types, soon after the liquidus‐to‐solidus transition (1200 to 1300 °C) but prior to accretion. In particular, clinoenstatite‐rich crystals from radial and poikilitic chondrules give cooling rates on the order of 100 and 10 °C/h. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe‐Mg cation ordering indicate a nonlinear cooling path from the high chondrule formation temperatures to a postaccretionary low‐temperature (340–480 °C) evolution.
Major and trace element compositions of georgiaites: Clues to the source of North American tektites
Authors: ALBIN Edward F. , NORMAN Marc D. , RODEN Michael
Published: February 2010

Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO2) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO2 favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.
Crystallization of magmatic iron meteorites: The effects of phosphorus and liquid immiscibility
Authors: CHABOT Nancy L. , DRAKE Michael J.
Published: February 2010

Abstract— Magmatic iron meteorites are commonly thought to have formed by fractional crystallization of the metallic cores of asteroid‐sized bodies. As fractional crystallization proceeds, light elements such as P and S become enriched in the molten portion of the core. The light element content of the metallic liquid influences the partitioning behavior of trace elements and may cause liquid immiscibility to occur. The elemental trends observed in magmatic iron meteorites may have been affected by both of these processes.

We have examined experimentally the effect of P on the solid‐metal‐liquid‐metal partitioning behavior of Ag and Pd, Re and Os, two element pairs used to date iron meteorite processes. Phosphorus has no effect on the partition coefficient of either Ag or Pd, which are incompatible and identical within experimental error. Compatible Re and Os also have identical partitioning behavior, within experimental error, and show increasing compatibility in the solid metal with increasing P content of the metallic liquid. Including the effects of both S and P on the partitioning behavior of Re and Os, simple fractional crystallization calculations can reproduce the large variation of Re and Os concentrations observed in four magmatic iron meteorite groups but have difficulty matching the later crystallizing portions of the trends.

We have also conducted experiments with three phases—solid metal and two immiscible metallic liquids—to determine the location of the liquid immiscibility field near conditions thought to be relevant to magmatic iron meteorites. Our results show a significantly smaller liquid immiscibility field as compared to the previously published Fe‐P‐S phase diagram. Our revised phase diagram suggests that liquid immiscibility was encountered during the crystallization of asteroidal cores, but much later during the crystallization process than predicted by the previously published diagram.
A new martian meteorite from the Sahara: The shergottite Dar al Gani 489
Authors: FOLCO L. , FRANCHI I. A. , D'ORAZIO M. , ROCCHI S. , SCHULTZ L.
Published: February 2010

Abstract— Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo79–59) set in a fine‐grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En75–57 Wo5–15) crystals and interstitial feldspathic glass (An67–56 Or0–1). Minor phases include enstatite (En82–71 Wo2–4), augite (En48–52 Wo29–32), chromite, Ti‐chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O‐isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn(bulk) = 42.1, Ni/Mg(bulk) = 0.002; δ17O = 2.89, δ18O = 4.98, and Δ17O = 0.305), belonging to the basaltic shergottite subgroup.

The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites.

Noble gases data show that DaG 489 has an ejection age of ∼1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites.

In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ∼25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall.

In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high‐degree partial melting of a lherzolite‐like material.
The lack of potassium‐isotopic fractionation in Bishunpur chondrules
Authors: ALEXANDER C. M. O'D. , GROSSMAN J. N. , WANG J. , ZANDA B. , BOUROT‐DENISE M. , HEWINS R. H.
Published: February 2010

Abstract— In a search for evidence of evaporation during chondrule formation, the mesostases of 11 Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K‐isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had δ41K compositions that were normal within error (typically ±3%, 2s̀). The six “anomalous” points are probably all artifacts. Experiments have shown that free evaporation of K leads to large 41K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3% enrichment in δ41K is produced by ∼12% loss of K. The range of L‐chondrite‐normalized K/Al ratios (a measure of the K‐elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L‐chondrite‐like K abundances and the K loss occurred via Rayleigh fractionation, the most K‐depleted chondrules would have had compositions of up to δ41K ≅ 200%. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of K/Al fractionation in chondrules does not explain the K‐isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K‐isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim‐matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K‐isotopic fractionation in them requires that the K‐isotopic exchange took place during chondrule formation, which would probably require gas‐chondrule exchange. Potassium evaporated from fine‐grained dust and chondrules during chondrule formation may have produced sufficient K‐vapor pressure for gas‐chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.
Petrology and geochemistry of the Elephant Moraine A79002 diogenite: A genomict breccia containing a magnesian harzburgite component
Authors: MITTLEFEHLDT David W.
Published: February 2010

Abstract— The Elephant Moraine A79002 (EETA79002) diogenite is a fragmental breccia with a subtle lightdark structure. It is composed of orthopyroxene, with minor olivine, chromite, and ubiquitous, inhomogeneously distributed, approximately 5–500 μm sized troilite and metal grains. These latter are present in the matrix, and as inclusions in and as symplectic intergrowths with orthopyroxene and olivine. Trace amounts of silica and diopside are also present. Most orthopyroxene compositions (typical orthopyroxenes) are in the narrow range Wo2.1–2.7En74.1–75.6Fs22.2–23.8 like those of most diogenites. A few magnesian orthopyroxenes are present with compositions of Wo1.7‐2.5En77.5–80.2 Fs18.2–20.3. These are among the most magnesian orthopyroxenes known from diogenites. A few ferroan orthopyroxenes have compositions of Wo2.1–2.9En71.7–73.7Fs24.2–25.5. Differences in Al2O3, TiO2, and Cr2O3 between the different orthopyroxene groups are inconsistent with a simple igneous fractionation relationship between them. Olivine compositions are Fo75.0–76.9. The olivines could be in equilibrium with the magnesian orthopyroxenes, but not with the typical or ferroan orthopyroxenes that form the bulk of EETA79002. Metal grains exhibit a range of Ni and Co contents and Ni/Co ratios; their compositions indicate that they are primary igneous metal. Metal and troilite grains are more prevalent in the dark samples.

The trace incompatible lithophile element contents of 16 samples are remarkably uniform. Their Yb concentrations are all within their 2s̀ analytical uncertainties of the mean. The uniformity and low content of light rare earth elements in EETA79002 indicate that negligible amounts of a trapped liquid component, or foreign material mixed in the breccia, could be present. The siderophile and chalcophile element data show that the light‐dark structure is due to the distribution of metal and troilite grains; dark samples contain higher Ni, Co, and Se compared to light samples.

Meteorite EETA79002 appears to contain material from three or more related plutons, a magnesian harzburgite, and two orthopyroxenites, and is a genomict breccia.
Organosilane occurrence in irghizite samples from the Zhamanshin impact crater, Kazakhstan
Authors: ZBIK Marek , JASIENIAK Marek , SMART Roger St.C.
Published: February 2010

Abstract— The composition of surface deposits on vesicle walls in irghizites (i.e., impact glasses at site) from the Zhamanshin meteorite crater were studied using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The cavity walls are unique interfaces for condensation of gases from the superheated, high‐silica melt during the impact. Initially, signals from the cavity wall are dominated by hydrocarbon fragments whereas the glass fracture face surrounding the cavity gave only signals corresponding to glass components. After 12 h in ultra high vacuum (UHV), signals from the cavity wall are dominated by peaks corresponding to fragments normally measured from organosilanes and organosiloxanes with the majority of the hydrocarbon signals markedly reduced. Characteristic hydrocarbon fragments are now observed on the glass fracture surface next to the cavity in an annulus around the cavity perimeter. There are also minor signals in this region from organosilanes and organosiloxanes. In contrast, four tektites (Australites) (i.e., glassy distal ejecta) gave no organosilane or organosiloxane signals after the same preparation and vacuum evaporation procedure. These species appear to be formed only at the impact site where higher levels of organic material are likely to be present in soil and are trapped before evaporation. This appears to be the first report of naturally occurring silicon‐organic compounds.
Evidence for common breakup events of the acapulcoites‐lodranites and chondrites
Authors: TERRIBILINI Dario , EUGSTER Otto , HERZOG Gregory F. , SCHNABEL Christoph
Published: February 2010

Abstract— Acapulcoites and lodranites are believed to originate on a common parent body and to represent some of the earliest events in the differentiation of the chondritic asteroids. We have conducted isotopic studies of the noble gases He, Ne, Ar, Kr, and Xe, and determinations of the concentrations of the major elements and of the radionuclides 10Be, 26Al, and 36Cl in an attempt to constrain the cosmic‐ray exposure history of two members of the acapulcoite‐lodranite clan recovered in Antarctica: Frontier Mountain (FRO) 95029 and Graves Nunataks (GRA) 95209. From cosmic‐ray‐produced 3He, 21Ne, and 38Ar and appropriate production rates, we derive parent‐body breakup times of 4.59 ± 0.60 and 6.82 ± 0.60 Ma for FOR 95029 and GRA 95209, respectively. These times are consistent with those obtained from the pairs 10Be‐21Ne and 26Al‐21Ne; whereas the times inferred from the pair 36Cl‐36Ar are slightly longer, perhaps because the 36Cl activities decreased as a result of decay on Earth. Terrestrial ages up to ∼50 ka for the two meteorites are consistent with the measured 36Cl activities of the metal phases. All acapulcoites and lodranites dated until now show cosmic‐ray exposure ages in the range of 4–10 Ma. This is the same range as that found for the major exposure age cluster of the H chondrites. As a common parent body is improbable on the basis of the O‐isotopic systematics, a common set of impactors might have affected the asteroid belt 4–10 Ma ago.
The age of the Kaalijärv meteorite craters
Authors: RASMUSSEN Kaare L. , AABY Bent , GWOZDZ Raymond
Published: February 2010

Abstract— Precise radiometric age determination of the Kaalijärv meteorite craters on the island of Saaremaa in Estonia have so far proved inconclusive. Here we present trace element analyses of peat cores taken several kilometers away from the Kaalijärv craters that reveal a distinct Ir‐enriched layer produced by the meteorite impact. By radiocarbon dating the peat cores, we have determined for the first time the precise age of the impact that generated the Kaalijärv craters. The calibrated date of the impact is 400–370 B.C. at ± 1σ.
Complexities on the acapulcoite‐lodranite parent body: Evidence from trace element distributions in silicate minerals
Authors: FLOSS C.
Published: February 2010

Abstract— The trace element distributions of individual minerals from seven acapulcoites and lodranites have been studied. Systematic differences are evident between some members of the two groups. Specifically, pyroxenes from the lodranites MacAlpine Hills (MAC) 88177 and Lewis Cliff (LEW) 88280 exhibit depletions of the rare earth elements (REE) and other incompatible trace elements (Ti, Zr, Y), relative to acapulcoite (Acapulco, Allan Hills (ALH) A81261) pyroxenes, that are consistent with the formation and removal of 15% or more silicate partial melts from these meteorites. Phosphate REE patterns in these lodranites also support this scenario. However, other members of the acapulcoite‐lodranite clan exhibit more complex trace element variations. Elephant Moraine (EET) 84302, which has been classified as transitional between the acapulcoites and lodranites, generally has trace element characteristics similar to the acapulcoites. However, its plagioclase REE compositions suggest a somewhat greater degree of metamorphism than that experienced by acapulcoites such as Acapulco and ALHA81261. Similar and elevated REE abundances in the silicate phases from acapulcoite ALHA81187 and lodranite Graves Nunataks (GRA) 95209 suggest that these two meteorites, in fact, experienced similar thermal histories. This probably included some silicate partial melting, although little melt appears to have been lost from the samples.

The observed variations in the trace element abundances of these samples from the acapulcoite‐lodranite clan emphasize the complex and varied processes that have acted on their parent body. The simple bimodal classification of these meteorites based primarily on petrographic criteria, which has been used to date, appears to be inadequate to describe this diverse group of samples, as they represent a range of degrees of partial melting, both with and without accompanying melt migration. In some instances, secondary processes on the parent body, such as cryptic metasomatism, have further modified sample compositions.
A petrologic study of the IAB iron meteorites: Constraints on the formation of the IAB‐Winonaite parent body
Authors: BENEDIX G. K. , MCCOY T. J. , KEIL K. , LOVE S. G.
Published: February 2010

Abstract—We studied 26 IAB iron meteorites containing silicate‐bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. 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.
Chondrule formation by the ablation of small planetesimals
Authors: GENGE M. J.
Published: February 2010

Abstract— Numerous models have been proposed to explain the formation of chondrules, but none can be reconciled with the highly diverse properties of these objects. Here the formation of chondrules by the surface melting and ablation of small planetesimals in nebula shock waves is investigated using a numerical model. It is shown that bodies between ∼1 mm and 500 m in diameter would have produced molten droplets by ablation during gas drag in nebula shocks stronger than ∼2.0 Mach. The properties of chondrules produced by ablation are estimated by comparison with meteorite fusion crusts and through consideration of the environment within the bow shock envelope of ablating planetesimals. It is suggested that most ablation chondrules will have broadly chondritic compositions with depletions in siderophile and chalcophile elements and relatively high volatile contents and textures that are mainly porphyritic. The formation of chondrules by ablation of planetesimals in shock waves was probably most important at a late stage in nebula history and occurred at the same time as chondrules formed by the melting of dust particles. The high abundance of dust particles relative to larger bodies at all stages of accretion implies that only a proportion of chondrules may have been formed by ablation and that genetic groups of chondrules with very different origins may coexist in meteorites.
Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5‐1.5 μm
Authors: HOPPE Peter , STREBEL Roger , EBERHARDT Peter , AMARI Sachiko , LEWIS Roy S.
Published: February 2010

Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have 12C/13C ratios between 18 and 6800, 14N/15n ratios from 13 to 200, δ29Si/28Si between −750 and +60%0, δ30Si/28Si from −770 to −10%0, and 54Fe/56Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic 26Mg (from the radioactive decay of 26Al, half‐life ≅ 7 times 105 years) and radiogenic 44Ca (from the radioactive decay of 44Ti, half‐life = 60 years). While all X grains but one have radiogenic 26Mg, only ∼20% of them have detectable amounts of radiogenic 44Ca. Initial 26Al/27Al ratios of up to 0.36 and initial 44Ti/48Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct 44Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for 26Al/27Al ratios, except for the highest 26Al/27Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed 29Si/30Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured 14n/15n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 Modot; type II SN if rotational mixing, preferential trapping of N, or both from 15n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial 26Al/27Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.
Experimental constraints on alkali condensation in chondrule formation
Authors: GEORGES P. , LIBOUREL G. , DELOULE E.
Published: February 2010

Abstract— To assess whether the alkali behavior observed in chondrules of primitive meteorites is attributable to volatilization from the raw materials of chondrules during chondrule formation events or attributable to condensation processes from the nebular gas, we set up a new experimental device able to expose silicate melt samples to a controlled alkali partial pressure at high temperature under fixed O fugacity. Using a mixture of potassium carbonate (K2CO3) and graphite (C) as the source of the K gas (Kg), we studied the condensation kinetics of K and its solubility in CaO‐MgO‐Al2O3‐SiO2 silicate melts, according to the reaction 2 K (g) + 1/2 (g) = K2O (melt) From these results, we show that alkali entering in chondrules from the nebular gas is a viable mechanism to explain the chondrules alkali contents and their δ41K‐isotopic signatures, at timescales relevant to chondrule formation. Finally, we also suggest that chondrules may have formed in non‐canonical nebular environments and that the flash‐heating scenario is not a prerequisite to chondrule formation.
Ferrous silicate spherules with euhedral iron‐nickel metal grains from CH carbonaceous chondrites: Evidence for supercooling and condensation under oxidizing conditions
Authors: KROT A. N. , MEIBOM A. , PETAEV M. I. , KEIL K. , ZOLENSKY M. E. , SAITO A. , MUKAI M. , OHSUMI K.
Published: February 2010

Abstract— The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg) ≅ 0.1‐0.4) silicate spherules (chondrules), about 15–30 μm in apparent diameter, composed of cryptocrystalline olivinepyroxene normative material, ±SiO2‐rich glass, and rounded‐to‐euhedral Fe, Ni metal grains. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al2O3, and TiO2 <0.04 wt%) and enriched in FeO, MnO, Cr2O3, and Na2O relative to the dominant, volatile‐poor, magnesian chondrules from CH chondrites. The Fe/(Fe + Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal grains, which suggests that this correlation is not due to oxidation, reduction, or both of iron (FeOsil ⇄ Femet) during melting of metal‐silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: about 8–18 wt% Ni, <0.09 wt% Cr, and a sub‐solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded‐to‐euhedral metal grains, which has resulted in a Co/Ni ratio significantly below solar. The liquidus temperatures of the euhedral Fe, Ni metal grains are lower than those of the coexisting ferrous silicates, and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite, which suggests fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400–500 K.