• Noble gas space exposure ages of individual interplanetary dust particles

      Kehm, K.; Flynn, G. J.; Sutton, S. R.; Hohenberg, C. M. (The Meteoritical Society, 2006-01-01)
      The He, Ne, and Ar compositions of 32 individual interplanetary dust particles (IDPs) were measured using low-blank laser probe gas extraction. These measurements reveal definitive evidence of space exposure. The Ne and Ar isotopic compositions in the IDPs are primarily a mixture between solar wind (SW) and an isotopically heavier component dubbed fractionated solar (FS), which could be implantation-fractionated solar wind or a distinct component of the solar corpuscular radiation previously identified as solar energetic particles (SEP). Space exposure ages based on the Ar content of individual IDPs are estimated for a subset of the grains that appear to have escaped significant volatile losses during atmosphere entry. Although model-dependent, most of the particles in this subset have ages that are roughly consistent with origin in the asteroid belt. A short (<1000 years) space exposure age is inferred for one particle, which is suggestive of cometary origin. Among the subset of grains that show some evidence for relatively high atmospheric entry heating, two possess elevated 21Ne/22Ne ratios generated by extended exposure to solar and galactic cosmic rays. The inferred cosmic ray exposure ages of these particles exceeds 107 years, which tends to rule out origin in the asteroid belt. A favorable possibility is that these 21Ne‐rich IDPs previously resided on a relatively stable regolith of an Edgeworth‐Kuiper belt or Oort cloud body and were introduced into the inner solar system by cometary activity. These results demonstrate the utility of noble gas measurements in constraining models for the origins of interplanetary dust particles.
    • Frontier Mountain 93001: A coarse-grained, enstatite-augite-oligoclase-rich, igneous rock from the acapulcoite-lodranite parent asteroid

      Folco, L.; D'Orazio, M.; Burroni, A. (The Meteoritical Society, 2006-01-01)
      The Frontier Mountain (FRO) 93001 meteorite is a 4.86 g fragment of an unshocked, medium-to coarse-grained rock from the acapulcoite-lodranite (AL) parent body. It consists of anhedral orthoenstatite (FS13.3 +/- .04 WO 3.1 +/- 0.2), augite (FS6.1 +/- 0.7 WO42.3 +/- 0.9; Cr2O3 = 1.54 +/- 0.03), and oligoclase (Ab80.5 +/- 3.3 Or 3.1 +/- 0.6) up to >1 cm in size enclosing polycrystalline aggregates of fine-grained olivine (average grain size: 460 +/- 210 micrometers) showing granoblastic textures, often associated with Fe,Ni metal, troilite, chromite (cr# = 0.91 +/- 0.03; fe# = 0.62 +/ 0.04), schreibersite, and phosphates. Such aggregates appear to have been corroded by a melt. They are interpreted as lodranitic xenoliths. After the igneous (the term "igneous" is used here strictly to describe rocks or minerals that solidified from molten material) lithology intruding an acapulcoite host in Lewis Cliff (LEW) 86220, FRO 93001 is the second-known silicate-rich melt from the AL parent asteroid. Despite some similarities, the silicate igneous component of FRO 930011 (i.e., the pyroxene-plagioclase mineral assemblage) differs in being coarser-grained and containing abundant enstatite. Melting-crystallization modeling suggests that FRO 93001 formed through high-degree partial melting (greater than or equal to 35 wt%; namely, greater than or equal to 15 wt% silicate melting and ~20 wt% metal melting) of an acapulcoite source rock, or its chondritic precursor, at temperatures greater than or equal to 1200 degrees C, under reducing conditions. The resulting magnesium-rich silicate melt then underwent equilibrium crystallization; prior to complete crystallization at ~1040 degrees C, it incorporated lodranitic xenoliths. FRO 93001 is the highest-temperature melt from the AL parent-body so far available in laboratory. The fact that FRO 93001 could form by partial melting and crystallization under equilibrium conditions, coupled with the lack of quench-textures and evidence for shock deformation in xenoliths, suggests that FRO 93001 is a magmatic rock produced by endogenic heating rather than impact melting.
    • Erratum

      The Meteoritical Society, 2006-01-01
    • Surface electronic states of meteoritic nanodiamonds

      Garvie, Laurence A. J.; Buseck, Peter R. (The Meteoritical Society, 2006-01-01)
      The C K edge of Orgueil nanodiamonds (C-delta diamonds) was acquired by electron energy-loss spectroscopy (EELS), with an energy resolution of 300 meV. The spectra show peaks at 282.5, 284.7, and 286.4 eV, which occur in the band gap below the main diamond edge and are absent from the bulk diamond spectrum. These peaks are attributed to transitions from degrees C 1s surface core levels to unoccupied surface states, and arise from single and pi-bonded dangling bonds and C-H bonds. A shoulder to the main absorption edge at 287.8 eV may correspond to hydrocarbon adsorbates. These results can be used to further our understanding of C-delta diamond structure and may reveal the presence of a fullerene-like surface. The unique surface electronic states of the C-delta diamond surfaces are expected to affect their optical properties, which are dependent on features such as extent of H coverage, particle size, and surface structure.
    • Alkali-feldspathic material entrained in Fe,S-rich veins in a monomict ureilite

      Warren, Paul H.; Huber, Heinz; Ulff-Møller, Finn (The Meteoritical Society, 2006-01-01)
      The Elephant Moraine (EET) 96001 ureilite contains a remarkable diversity of feldspars, which occur as tiny (no more than 60 micrometers maximum dimension) grains within a few Fe,S-rich (now weathered to mostly Fe oxide) veins. Molar S:Fe ratio in the veins averages 0.08 0.02. The veins meander and feature large fluctuations in apparent width; they appear to have entered this monomict breccia by a gentle, percolative process, not by violent impact injection. The feldspars are accompanied by a diverse suite of K-rich (and generally also Ti-rich) feldspathic glasses, and also major proportions of silica and pyroxene, which is largely fassaitic. A rhnite-like phase is also found, and, as inclusions in one of the fassaites, a Cr-poor spinel-like phase. The feldspars mostly feature remarkably high K/Na compared to feldspars of comparable An from polymict ureilites. The EET 96001 feldspathic component was probably once part of a thin basaltic crust on a ureilite asteroid. The spinel included in one of the fassaites formed at remarkably high fO2 (apparent oxidation state of iron: 41 atom% Fe3+), suggesting that the parent magma possibly assimilated near-surface water (however, the Fe3+was not directly measured, and has conceivably been affected by terrestrial weathering; also, there is no assurance that this fassaite originated together with the typical feldspar). We speculate that the feldspathic component was mixed into the dense, Fe,S-rich vein material, and very soon thereafter the Fe,S-rich vein material was emplaced adjacent to the EET 96001 host ureilite, at an advanced stage in a chaotic catastrophic disruption and partial reassembly process that affected all ureilites. The high-K nature of the EET 96001 feldspathic component, including the feldspathic glasses, suggests that fractional fusion may not have been as common during ureilite anatexis as has been inferred from recent studies of clast assemblages in polymict ureilites.
    • Erratum

      The Meteoritical Society, 2006-01-01
      Shock waves-Phenomenology, experimental, and numerial simulationi
    • Macroscopic subdivision of silica aerogel collectors for sample return missions

      Ishii, H. A.; Bradley, J. P. (The Meteoritical Society, 2006-01-01)
      Silica aerogel collector tiles have been employed for the collection of particles in low Earth orbit and, more recently, for the capture of cometary particles by NASA's Stardust mission. Reliable, reproducible methods for cutting these and future collector tiles from sample return missions are necessary to maximize the science output from the extremely valuable embedded particles. We present a means of macroscopic subdivision of collector tiles by generating large-scale cuts over several centimeters in silica aerogel with almost no material loss. The cut surfaces are smooth and optically clear allowing visual location of particles for analysis and extraction. This capability is complementary to the smaller-scale cutting capabilities previously described (Westphal 2004; Ishii 2005a, 2005b) for removing individual impacts and particulate debris in tiny aerogel extractions. Macroscopic cuts enable division and storage or distribution of portions of aerogel tiles for immediate analysis of samples by certain techniques in situ or further extraction of samples suited for other methods of analysis. The capability has been implemented in the Stardust Laboratory at NASA's Johnson Space Center as one of a suite of aerogel cutting methods to be used in Stardust sample curation.
    • TOF-SIMS analysis of Allende projectiles shot into silica aerogel

      Stephan, Thomas; Butterworth, Anna L.; Hörz, Friedrich; Snead, Christopher J.; Westphal, Andrew J. (The Meteoritical Society, 2006-01-01)
      Powdered Allende projectiles were fired into silica aerogel at 6.1 km/sec in order to evaluate particle retrieval and analysis techniques for samples from the Stardust mission. Since particles may disintegrate and ablate along the penetration paths in a high-porosity aerogel, TOF-SIMS analysis may be a suitable method to determine the distribution of such materials along the tracks as well as potential compositional modifications. Therefore, two 350 micrometer-sized tracks, residing at the surface of a keystone specimen that was flattened between two silicon chips, were analyzed. TOF-SIMS allows for a detailed study of the chemical composition of particles that survived the impact mostly intact and of fine-grained material from disintegrated projectiles. In the investigated keystone, material from light gas gun debris dominated. Besides the two tracks, a continuous, 40-micrometer-thick surface layer of implanted material - probably gun residue - was found. One of the two analyzed tracks is compositionally distinct from this surface layer and is likely to contain residual material of an Allende projectile. The analyses clearly demonstrate that tracks, resulting from impactors in the 5-10 micrometer size range, can be successfully analyzed with TOF-SIMS.
    • Annual Author Index

      The Meteoritical Society, 2006-01-01
    • Annual Subject Index

      The Meteoritical Society, 2006-01-01
    • Differentiation and evolution of the IVA meteorite parent body: Clues from pyroxene geochemistry in the Steinbach stony-iron meteorite

      Ruzicka, A.; Hutson, M. (The Meteoritical Society, 2006-01-01)
      We analyzed the Steinbach IVA stony-iron meteorite using scanning electron microscopy (SEM), electron microprobe analysis (EMPA), laser ablation inductively-coupled-plasma mass spectroscopy (LA-ICP-MS), and modeling techniques. Different and sometimes adjacent low-Ca pyroxene grains have distinct compositions and evidently crystallized at different stages in a chemically evolving system prior to the solidification of metal and troilite. Early crystallizing pyroxene shows evidence for disequilibrium and formation under conditions of rapid cooling, producing clinobronzite and type 1 pyroxene rich in troilite and other inclusions. Subsequently, type 2 pyroxene crystallized over an extensive fractionation interval. Steinbach probably formed as a cumulate produced by extensive crystal fractionation (~60-70% fractional crystallization) from a high-temperature (~1450-1490 degrees C) silicate-metallic magma. The inferred composition of the precursor magma is best modeled as having formed by greater than or equal to 30-50% silicate partial melting of a chondritic protolith. If this protolith was similar to an LL chondrite (as implied by O-isotopic data), then olivine must have separated from the partial melt, and a substantial amount (~53-56%) of FeO must have been reduced in the silicate magma. A model of simultaneous endogenic heating and collisional disruption appears best able to explain the data for Steinbach and other IVA meteorites. Impact disruption occurred while the parent body was substantially molten, causing liquids to separate from solids and oxygen-bearing gas to vent to space, leading to a molten metal-rich body that was smaller than the original parent body and that solidified from the outside in. This model can simultaneously explain the characteristics of both stony-iron and iron IVA meteorites, including the apparent correlation between metal composition and metallographic cooling rate observed for metal.
    • Hydrocode modeling of the Sierra Madera impact structure

      Goldin, T. J.; Wünnemann, K.; Melosh, H. J.; Collins, G. S. (The Meteoritical Society, 2006-01-01)
      We present the first hydrocode simulations of the formation of the Sierra Madera structure (west Texas, USA), which was caused by an impact into a thick sedimentary target sequence. We modeled Sierra Madera using the iSALE hydrocode, and here we present two best-fit models: 1) a crater with a rim (final crater) diameter of ~12 km, in agreement with previous authors interpretations of the original structure, and 2) a crater ~16 km in diameter with increased postimpact erosion. Both models fit some of the geologic observational data, but discrepancies with estimates of peak shock pressure, extent of deformation, and stratigraphic displacement remain. This study suggests that Sierra Madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies. As many terrestrial craters possess some amount of sedimentary rocks in the target sequence, numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation.
    • Mobile element analysis by secondary ion mass spectrometry (SIMS) of impactite matrix samples from the Yaxcopoil-1 drill core in the Chicxulub impact structure

      Newsom, H. E.; Nelson, M. J.; Shearer, C. K.; Dressler, B. O. (The Meteoritical Society, 2006-01-01)
      The concentrations of the fluid mobile trace elements lithium, beryllium, boron, and barium were measured in samples of the altered matrix of several impactite breccias of the Yaxcopoil-1 drill core using secondary ion mass spectrometry (SIMS) to determine the extent of transport due to aqueous or hydrothermal processes. Three of the elements, Li, Be, and B, have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3.5, 2.2, and 1.5, respectively. Lithium and B are the most enriched elements up section, and appear to have had the greatest mobility. The similar fractionation of Li and B is consistent with fluid transport and alteration under low-temperature conditions of less than 150 degrees C based on published experimental studies. In contrast to Li, Be, and B, the concentration of Ba in the altered matrix materials decreases upward in the section, and the concentration of Ba in the matrix is an order of magnitude less than the bulk concentrations, likely due to the presence of barite. The origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units. A different protolith in the altered matrix is suggested by the Mg-rich composition of the lower units versus the Al-rich composition of the upper units, which largely correlates with the mobile element variations. The possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system. However, the mobile elements in the sediments do not have to originate from the underlying impactites. In conclusion, our data suggests that the impactites at this location did not experience extensive high-temperature hydrothermal processing, and that only limited transport of some elements, including Li, Be, and B, occurred.
    • Petrology and geochemistry of a silicate clast from the Mount Padbury mesosiderite: Implications for metal-silicate mixing events of mesosiderite

      Tamaki, M.; Yamaguchi, A.; Misawa, K.; Ebihara, M.; Takeda, H. (The Meteoritical Society, 2006-01-01)
      Petrological and bulk geochemical studies were performed on a large silicate clast from the Mount Padbury mesosiderite. The silicate clast is composed mainly of pyroxene and plagioclase with minor amounts of ilmenite, spinel, and other accessory minerals, and it shows subophitic texture. Pyroxenes in the clast are similar to those in type 5 eucrites and could have experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. Ilmenite and spinel vary chemically, indicating growth under disequilibrium conditions. The clast seems to have experienced an episode of rapid reheating and cooling, possibly as a result of metal-silicate mixing. Abundances of siderophile elements are obviously higher than in eucrites, although the clast is also extremely depleted in highly siderophile elements. The fractionated pattern can be explained by injection of Fe- FeS melts generated by partial melting of metallic portions during metal-silicate mixing. The silicate clast had a complex petrogenesis that could have included: 1) rapid crystallization from magma in a lava flow or a shallow intrusion; 2) prolonged thermal metamorphism to equilibrate the mineral compositions of pyroxene and plagioclase after primary crystallization; 3) metal-silicate mixing probably caused by the impact of solid metal bodies on the surface of the mesosiderite parent body; and 4) partial melting of metal and sulfide portions (and silicate in some cases) caused by the collisional heating, which produced Fe-FeS melts with highly fractionated siderophile elements that were injected into silicate portions along cracks and fractures.
    • Estimating shock pressures based on high-pressure minerals in shock-induced melt veins of L chondrites

      Xie, Z.; Sharp, T. G.; De Carli, P. S. (The Meteoritical Society, 2006-01-01)
      Here we report the transmission electron microscopy (TEM) observations of the mineral assemblages and textures in shock-induced melt veins from seven L chondrites of shock stages ranging from S3 to S6. The mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures, which can be used to constrain shock pressure in some cases. Thick melt veins in the TenhamL6 chondrite contain majorite and magnesiowstite in the center, and ringwoodite, akimotoite, vitrified silicate-perovskite, and majorite in the edge of the vein, indicating crystallization pressure of ~25 GPa. However, very thin melt veins (5-30 micrometers wide) in Tenham contain glass, olivine, clinopyroxene, and ringwoodite, suggesting crystallization during transient low-pressure excursions as the shock pressure equilibrated to a continuum level. Melt veins of Umbarger include ringwoodite, akimotoite, and clinopyroxene in the vein matrix, and Fe2SiO4-spinel and stishovite in SiO2-FeO-rich melt, indicating a crystallization pressure of ~18 GPa. The silicate melt veins in Roy contain majorite plus ringwoodite, indicating pressure of ~20 GPa. Melt veins of Ramsdorf and Nakhon Pathon contain olivine and clinoenstatite, indicating pressure of less than 15 GPa. Melt veins of Kunashak and La Lande include albite and olivine, indicating crystallization at less than 2.5 GPa. Based upon the assemblages observed, crystallization of shock veins can occur before, during, or after pressure release. When the assemblage consists of high-pressure minerals and that assemblage is constant across a larger melt vein or pocket, the crystallization pressure represents the equilibrium shock pressure.
    • A study of Mg and K isotopes in Allende CAIs: Implications to the time scale for the multiple heating processes

      Ito, M.; Nagasawa, H.; Yurimoto, H. (The Meteoritical Society, 2006-01-01)
      The measurements of magnesium and potassium isotopic compositions of refractory minerals in Allende calcium-aluminum-rich inclusions (CAIs), 7R-19-1, HN3-1, and EGG3 were taken by secondary ion mass spectrometry (SIMS). The 7R-19-1 contains 16O-rich and 16O-poor melilite grains and define a single isochron corresponding to an initial 26Al/27Al ratio of (6.6 +/- 1.3) x 10^(-5). The Al-Mg isochron, O isotope measurements and petrography of melilite in 7R-19-1 indicate that 16O-poor melilite crystallized within 0.4 Myr after crystallization of 16O-rich melilite, suggesting that oxygen isotopic composition of the CAI-forming region changed from 16O-rich to 16O-poor within this time interval. The 16O-poor melilite is highly depleted in K compared to the adjacent 16Orich melilite, indicating evaporation during remelting of 7R-19-1. We determined the isochron for 41Ca-41K isotopic systematics in EGG3 pyroxene with (4.1 +/- 2.0) x 10^(-9) (2-sigma) as an initial ratio of 41Ca/40Ca, which is at least two times smaller than the previous result (Sahijipal et al. 2000). The ratio of 41Ca/40Ca in the EGG3 pyroxene grain agrees within error with the value obtained by Hutcheon et al. (1984). No evidence for the presence of 41K excess (decay product of a short-lived radionuclide 41Ca) was found in 7R-19-1 and HN3-1. We infer that the CAI had at least an order of magnitude lower than canonical 41Ca/40Ca ratio at the time of the CAI formation.
    • Carbon and nitrogen in carbonaceous chondrites: Elemental abundances and stable isotopic compositions

      Pearson, V. K.; Sephton, M. A.; Franchi, I. A.; Gibson, J. M.; Gilmour, I. (The Meteoritical Society, 2006-01-01)
      We have undertaken a comprehensive study of carbon and nitrogen elemental abundances and isotopic compositions of bulk carbonaceous chondrites. A strategy of multiple analyses has enabled the investigation of hitherto unconstrained small-scale heterogeneities. No systematic differences are observed between meteorite falls and finds, suggesting that terrestrial processing has a minimal effect on bulk carbon and nitrogen chemistry. The changes in elemental abundance and isotopic composition over the petrologic range may reflect variations in primary accreted materials, but strong evidence exists of the alteration of components during secondary thermal and aqueous processing. These changes are reflected within the CM2 and CO3 groups and follow the published alteration scales for those groups. The nitrogen isotope system appears to be controlled by an organic host, which loses a 15N-rich component with progressive alteration. This study recommends caution, however, over the use of bulk carbon and nitrogen information for classification purposes; variance in relative abundance of different components in carbonaceous chondrites is significant and reflects intrameteorite heterogeneities.
    • Paleomagnetism and petrophysics of the Jänisjärvi impact structure, Russian Karelia

      Salminen, J.; Donadini, F.; Pesonen, L. J.; Masaitis, V. L.; Naumov, M. V. (The Meteoritical Society, 2006-01-01)
      Paleomagnetic, rock magnetic, and petrophysical results are presented for impactites and target rocks from the Lake Jänisjärvi impact structure, Russian Karelia. The impactites (tagamites, suevites, and lithic breccias) are characterized by increased porosity and magnetization, which is in agreement with observations performed at other impact structures. Thermomagnetic, hysteresis, and scanning electron microscope (SEM) analysis document the presence of primary multidomain titanomagnetite with additional secondary titanomaghemite and ilmenohematite. The characteristic impact-related remanent magnetization (ChRM) direction (D = 101.5 degrees, I = 73.1 degrees, alpha-95 = 6.2 degrees) yields a pole (Lat. = 45.0mdegrees N, Long. = 76.9 degrees E, dp = 9.9 degrees, dm = 11.0 degrees). Additionally, the same component is observed as an overprint on some rocks located in the vicinity of the structure, which provides proofs of its primary origin. An attempt was made to determine the ancient geomagnetic field intensity. Seven reliable results were obtained, yielding an ancient intensity of 68.7 +/- 7.6 micro-T (corresponding to VDM of 10.3 +/- 1.1 x 10^22 Am^2). The intensity, however, appears to be biased toward high values mainly because of the concave shape of the Arai diagrams. The new paleomagnetic data and published isotopic ages for the structure are in disagreement. According to well-defined paleomagnetic data, two possible ages for magnetization of Jänisjärvi rocks exist: 1) Late Sveconorwegian age (900-850 Myr) or 2) Late Cambrian age (~500 Myr). However, published isotopic ages are 718 +/- 5 Myr (K-Ar) and 698 +/- 22 Myr (39Ar-40Ar), but such isotopic dating methods are often ambiguous for the impactites.
    • Petrology of silicate inclusions in the Sombrerete ungrouped iron meteorite: Implications for the origins of IIE-type silicate-bearing irons

      Ruzicka, A.; Hutson, M.; Floss, C. (The Meteoritical Society, 2006-01-01)
      The petrography and mineral and bulk chemistries of silicate inclusions in Sombrerete, an ungrouped iron that is one of the most phosphate-rich meteorites known, was studied using optical, scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and secondary ion mass spectrometry (SIMS) techniques. Inclusions contain variable proportions of alkalic siliceous glass (~69 vol% of inclusions on average), aluminous orthopyroxene (~9%, Wo1-4Fs2535, up to ~3 wt% Al), plagioclase (~8%, mainly An7092), Cl-apatite (~7%), chromite (~4%), yagiite (~1%), phosphaterich segregations (~1%), ilmenite, and merrillite. Ytterbium and Sm anomalies are sometimes present in various phases (positive anomalies for phosphates, negative for glass and orthopyroxene), which possibly reflect phosphate-melt-gas partitioning under transient, reducing conditions at high temperatures. Phosphate-rich segregations and different alkalic glasses (K-rich and Na-rich) formed by two types of liquid immiscibility. Yagiite, a K-Mg silicate previously found in the Colomera (IIE) iron, appears to have formed as a late-stage crystallization product, possibly aided by Na-K liquid unmixing. Trace-element phase compositions reflect fractional crystallization of a single liquid composition that originated by low-degree (~48%) equilibrium partial melting of a chondritic precursor. Compositional differences between inclusions appear to have originated as a result of a filter-press differentiation process, in which liquidus crystals of Cl-apatite and orthopyroxene were less able than silicate melt to flow through the metallic host between inclusions. This process enabled a phosphoran basaltic andesite precursor liquid to differentiate within the metallic host, yielding a dacite composition for some inclusions. Solidification was relatively rapid, but not so fast as to prevent flow and immiscibility phenomena. Sombrerete originated near a cooling surface in the parent body during rapid, probably impact-induced, mixing of metallic and silicate liquids. We suggest that Sombrerete formed when a planetesimal undergoing endogenic differentiation was collisionally disrupted, possibly in a breakup and reassembly event. Simultaneous endogenic heating and impact processes may have widely affected silicate-bearing irons and other solar system matter.
    • Erratum

      The Meteoritical Society, 2006-01-01