• Nature of the Martian uplands: Effect on Martian meteorite age distribution and secondary cratering

      Hartmann, William K.; Barlow, Nadine G. (The Meteoritical Society, 2006-01-01)
      Martian meteorites (MMs) have been launched from an estimated 5-9 sites on Mars within the last 20 Myr. Some 80-89% of these launch sites sampled igneous rock formations from only the last 29% of Martian time. We hypothesize that this imbalance arises not merely from poor statistics, but because the launch processes are dominated by two main phenomena: first, much of the older Martian surface is inefficient in launching rocks during impacts, and second, the volumetrically enormous reservoir of original cumulate crust enhances launch probability for 4.5 Gyr old rocks. There are four lines of evidence for the first point, not all of equal strength. First, impact theory implies that MM launch is favored by surface exposures of near-surface coherent rock (10^2 m deep), whereas Noachian surfaces generally should have 10^2 m of loose or weakly cemented regolith with high ice content, reducing efficiency of rock launch. Second, similarly, both Mars Exploration Rovers found sedimentary strata, 1-2 orders of magnitude weaker than Martian igneous rocks, favoring low launch efficiency among some fluvial-derived Hesperian and Noachian rocks. Even if launched, such rocks may be unrecognized as meteorites on Earth. Third, statistics of MM formation age versus cosmic-ray exposure (CRE) age weakly suggest that older surfaces may need larger, deeper craters to launch rocks. Fourth, in direct confirmation, one of us (N. G. B.) has found that older surfaces need larger craters to produce secondary impact crater fields (cf. Barlow and Block 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with prominent fields of secondaries have diameters of ~45 km, ~19 km, and ~10 km, respectively. Because 40% of Mars is Noachian, and 74% is either Noachian or Hesperian, the subsurface geologic characteristics of the older areas probably affect statistics of recognized MMs and production rates of secondary crater populations, and the MM and secondary crater statistics may give us clues to those properties.
    • New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation

      Wieland, F.; Reimold, W. U.; Gibson, R. L. (The Meteoritical Society, 2006-01-01)
      Shatter cones have been described from many meteorite impact structures and are widelyregarded as a diagnostic macroscopic recognition feature for impact. However, the origin of thismeso- to macroscopic striated fracture phenomenon has not yet been satisfactorily resolved, and thetiming of shatter cone formation in the cratering process still remains enigmatic. Here, previousresults from studies of shatter cones from the Vredefort impact structure and other impact structuresare discussed in the light of new field observations made in the Vredefort Dome. Contrary to earlier claims, Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated master cone concept. Simple back-rotation of impact-rotated strata to a horizontal pre-impact position also does not lead to a uniform centripetal-upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from the typically diverging pattern branching off the cone apex to subparallel-to parallel patterns on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the center of the dome, as alleged in the literature. Instead, a range of striation angles is measured on individual shatter cones from a specific outcrop. New observations on small-scale structures in the collar around the Vredefort Dome confirm the relationship of shatter cones with subparallel sets of curviplanar fractures (so-called multipli-striated joint sets, MSJS). Pervasive, meter-scale tensile fractures cross-cut shatter cones and appear to have formed after the closely spaced MSJ-type fractures. The results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon. Therefore, we favor a combination of aspects of different hypotheses that includes the interaction of elastic waves, as supported by numerical modeling results and which reasonably explains the variety of shatter cone shapes, the range of striation geometries and angles, and the relationship of closely spaced fracture systems with the striated surfaces. In the light of the currently available theoretical basis for the formation of shatter cones, the results of this investigation lead to the conclusion that shatter cones are tensile fractures and might have formed during shock unloading, after the passage of the shock wave through the target rocks.
    • 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.
    • Noble gases in the Martian meteorite Northwest Africa 2737: A new chassignite signature

      Marty, Bernard; Heber, Veronika S.; Grimberg, Ansgar; Wieler, Rainer; Barrat, Jean-Alix (The Meteoritical Society, 2006-01-01)
      We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic 3He, 21Ne, and 38Ar around 10-11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U-Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm-Nd signature comparable tothat of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 doesnot contain, in detectable amount, the solar-like xenon found in Chassigny and thought to characterizethe Martian mantle nor apparently fission xenon from 244Pu, which is abundant in Chassigny andsome of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped inamounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noblegases, together with the trapping of Martian atmospheric gases, could have occurred duringassimilation of Martian surface components, or more likely during shock metamorphism, which isrecorded in the petrology of this meteorite.
    • Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite?

      Goodrich, Cyrena Anne; Wlotzka, Frank; Ross, D. Kent; Bartoschewitz, Rainer (The Meteoritical Society, 2006-01-01)
      Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (100-500 micrometers) olivine (95-96%), augite (2-3%), and chromite (0.6-1.6%) in an equilibrated texture. Plagioclase (0.7-1.8%) occurs as poikilitic grains (up to 3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65-73, with a strong peak at Fo 68-69. Most grains are reverse-zoned, and also have ~10-30 micrometer reduction rims.In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo is greater than or equal to 75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite-bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800-1000 degrees C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.
    • Olivine zoning and retrograde olivine-orthopyroxene-metal equilibration in H5 and H6 chondrites

      Reisener, R. J.; Goldstein, J. I.; Petaev, M. I. (The Meteoritical Society, 2006-01-01)
      Electron microprobe studies of several H5 and H6 chondrites reveal that olivine crystals exhibit systematic Fe-Mg zoning near olivine-metal interfaces. Olivine Fa concentrations decrease by up to 2 mol% toward zoned taenite + kamacite particles (formed after relatively small amounts of taenite undercooling) and increase by up to 2 mol% toward zoneless plessite particles (formed after ~200 degrees C of taenite undercooling). The olivine zoning can be understood in terms of localized olivine-orthopyroxene-metal reactions during cooling from the peak metamorphic temperature. The silicate-metal reactions were influenced by solid-state metal phase transformations, and the two types of olivine zoning profiles resulted from variable amounts of taenite undercooling at temperatures <700 degrees C. The relevant silicate-metal reactions are modeled using chemical thermodynamics. Systematic olivine Fe-Mg zoning adjacent to metal is an expected consequence of retrograde silicate-metal reactions, and the presence of such zoning provides strong evidence that the silicate and metallic minerals evolved in situ during cooling from the peak metamorphic temperature.
    • Oxide-bearing and FeO-rich clasts in aubrites

      Rosenshein, E. B.; Ivanova, M. A.; Dickinson, T. L.; McCoy, T. J.; Lauretta, D. S.; Guan, Y.; Leshin, L. A.; Benedix, G. K. (The Meteoritical Society, 2006-01-01)
      We report the occurrence of an oxide-bearing clast and an FeO-rich clast from aubrites. The FeO-rich clast in Pesyanoe is dominated by olivine and pyroxene phenocrysts with mineral compositions slightly less FeO-rich than is typical for H chondrites. In Allan Hills (ALH) 84008, the oxide-bearing clast consists of a single forsterite grain rimmed by an array of sulfides, oxides, and phosphides. We consider a number of possible origins. We can exclude formation by melting of oxide-bearing chondrules and CAIs formed in enstatite chondrites. The Pesyanoe clast may have formed in a more oxidized region of the aubrite parent body or, more likely, is a foreign clast from a more oxidized parent body. The ALH 84008 clast likely formed by reaction between sulfides and silicates as a result of cooling, oxidation, or de-sulfidization. This clast appears to be the first oxide-bearing clast from an aubritic breccia that formed on the aubrite parent body. Identification of additional oxide-bearing clasts in aubrites could shed light on whether this was a widespread phenomenon and the origin of these enigmatic objects.
    • Oxygen isotope and 26Al-26Mg systematics of aluminum-rich chondrules from unequilibrated enstatite chondrites

      Guan, Yunbin; Huss, Gary R.; Leshin, Laurie A.; MacPherson, Glenn J.; McKeegan, Kevin D. (The Meteoritical Society, 2006-01-01)
      Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum-rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum-rich chondrules and two plagioclase fragments measured for 26Al-26Mg systematics, only one aluminum-rich chondrule contains excess 26Mg from the in situ decay of 26Al; the inferred initial ratio (26Al/27Al)o = (6.8 +/- 2.4) x 10^(-6) is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites.The oxygen isotopic compositions of five aluminum-rich chondrules and one plagioclase fragment define a line of slope ~0.6 +/- 0.1 on a three-oxygen-isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16O-rich compositions with a range in delta-18O of about ~12 ppm. Based on their oxygen isotopic compositions, aluminum-rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium-aluminum-rich inclusions (CAIs).Relative to their counterparts from unequilibrated ordinary chondrites, aluminum-rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26Mg from the in situ decay of 26Al, probably resulting from higher degrees of equilibration and isotopic exchange during post-crystallization metamorphism. However, the presence of 26Al-bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26Al was at least approximately homogeneously distributed across the chondrite-forming region.
    • 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.
    • Petrographic comparison of refractory inclusions from different chemical groups of chondrites

      Lin, Y.; Kimura, M.; Miao, B.; Dai, D.; Monoi, A. (The Meteoritical Society, 2006-01-01)
      Twenty-four refractory inclusions (40-230 micrometers, with average of 86 +/- 40 micrometers) were found by X-ray mapping of 18 ordinary chondrites. All inclusions are heavily altered, consisting of finegrained feldspathoids, spinel, and Ca-pyroxene with minor ilmenite. The presence of feldspathoids and lack of melilite are due to alteration that took place under oxidizing conditions as indicated by FeO-ZnO-rich spinel and ilmenite. The pre-altered mineral assemblages are dominated by two types: one rich in melilite, referred to as type A-like, and the other rich in spinel, referred to as spinel-pyroxene inclusions. This study and previous data show similar type and size distributions of refractory inclusions in ordinary and enstatite chondrites. A survey of refractory inclusions was also conducted on Allende and Murchison in order to make unbiased comparison with their counterparts in other chondrites. The predominant inclusions are type A and spinel-pyroxene, with average sizes of 170 +/- 130 micrometers (except for two mm-sized inclusions) in Allende and 150 +/- 100 micrometers in Murchison. The relatively larger sizes are partially due to common conglomerating of smaller nodules in both chondrites. The survey reveals closely similar type and size distributions of refractory inclusions in various chondrites, consistent with our previous data of other carbonaceous chondrites. The petrographic observations suggest that refractory inclusions in various groups of chondrites had primarily formed under similar processes and conditions, and were transported to different chondrite-accreting regions. Heterogeneous abundance and distinct alteration assemblages of refractory inclusions from various chondrites could be contributed to transporting processes and secondary reactions under different conditions.
    • Petrographic studies of "fallout" suevite from outside the Bosumtwi impact structure, Ghana

      Boamah, D.; Koeberl, C. (The Meteoritical Society, 2006-01-01)
      Field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ~1.5 km^2 area; its thickness is less than or equal to 15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits. Here we discuss the petrographic characteristics of drill core samples of melt-rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock-melted material that is indicative of shock pressures up to ~60 GPa.
    • Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post-basin deposition of basaltic material at the site

      Zeigler, Ryan A.; Korotev, Randy L.; Haskin, Larry A.; Jolliff, Bradley L.; Gillis, Jeffrey J. (The Meteoritical Society, 2006-01-01)
      We present the petrography and geochemistry of five 2-4 mm basalt fragments from the Apollo 16 regolith. These fragments are 1) a high-Ti vitrophyric basalt compositionally similar to Apollo 17 high-Ti mare basalts, 2) a very high-Ti vitrophyric basalt compositionally similar to Apollos 12 and 14 red-black pyroclastic glass, 3) a coarsely crystalline high-Al basalt compositionally similar to group 5 Apollo 14 high-Al mare basalts, 4) a very low-Ti (VLT) crystalline basalt compositionally similar to Luna 24 VLT basalts, and 5) a VLT basaltic glass fragment compositionally similar to Apollo 17 VLT basalts. High-Ti basalt has been reported previously at the Apollo 16 site; the other basalt types have not been reported previously. As there are no known cryptomaria or pyroclastic deposits in the highlands near the Apollo 16 site (ruling out a local origin), and scant evidence for basaltic material in the Apollo 16 ancient regolith breccias or Apollo 16 soils collected near North Ray Crater (ruling out a basin ejecta origin), we infer that the basaltic material in the Apollo 16 regolith originated in maria near the Apollo 16 site and was transported laterally to the site by small- to medium-sized post-basin impacts. On the basis of TiO2 concentrations derived from the Clementine UVVIS data, Mare Tranquillitatis (~300 km north) is the most likely source for the high-Ti basaltic material at the Apollo 16 site (craters Ross, Arago, Dionysius, Maskelyne, Moltke, Sosigenes, Schmidt), Mare Nectaris/Sinus Asperitatis (~220 km east) is the most likely source for the low-Ti and VLT basaltic material (craters Theophilus, Madler, Torricelli), and a large regional pyroclastic deposit near Mare Vaporum (600 km northwest) is the most likely source region for pyroclastic material (although no source craters are apparent in the region).
    • Petrology and chemistry of MIL 03346 and its significance in understanding the petrogenesis of nakhlites on Mars

      Day, James M. D.; Taylor, Lawrence A.; Floss, Christine; McSween, Harry Y. (The Meteoritical Society, 2006-01-01)
      Antarctic meteorite Miller Range (MIL) 03346 is a nakhlite composed of 79% clinopyroxene, ~1% olivine, and 20% vitrophyric intercumulus material. We have performed a petrological and geochemical study of MIL 03346, demonstrating a petrogenetic history similar to previously discovered nakhlites. Quantitative textural study of MIL 03346 indicates long (>1 x 10^1 yr) residence times for the cumulus augite, whereas the skeletal Fe-Ti oxide, fayalite, and sulfide in the vitrophyric intercumulus matrix suggest rapid cooling, probably as a lava flow. From the relatively high forsterite contents of olivine (up to Fo43) compared with other nakhlites and compositions of augite cores (Wo38-42En35-40Fs22-28) and their hedenbergite rims, we suggest that MIL 03346 is part of the same or a similar Martian cumulate-rich lava flow as other nakhlites. However, MIL 03346 has experienced less equilibration and faster cooling than other nakhlites discovered to date. Calculated trace element concentrations based upon modal abundances of MIL 03346 and its constituent minerals are identical to whole rock trace element abundances. Parental melts for augite have REE patterns that are approximately parallel with whole rock and intercumulus melt using experimentally defined partition coefficients. This parallelism reflects closed-system crystallization for MIL 03346, where the only significant petrogenetic process between formation of augite and eruption and emplacement of the nakhlite flow has been fractional crystallization. A model for the petrogenesis of MIL 03346 and the nakhlites (Nakhla, Governador Valadares, Lafayette, Yamato-000593, Northwest Africa (NWA) 817, NWA 998) would include: 1) partial melting and ascent of melt generated from a long-term LREE depleted mantle source, 2) crystallization of cumulus augite (olivine, magnetite) in a shallow-level Martian magma chamber, 3) eruption of the crystal-laden nakhlite magma onto the surface of Mars, 4) cooling, crystal settling, overgrowth, and partial equilibration to different extents within the flow, 5) secondary alteration through hydrothermal processes, possibly immediately succeeding or during emplacement of the flow. This model might apply to single - or multiple - flow models for the nakhlites. Ultimately, MIL 03346 and the other nakhlites preserve a record of magmatic processes in volcanic rocks on Mars with analogous petrogenetic histories to pyroxene-rich terrestrial lava flows and to komatiites.
    • 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.
    • Petrology and geochemistry of the fine-grained, unbrecciated diogenite Northwest Africa 4215

      Barrat, J. A.; Beck, P.; Bohn, M.; Cotten, J.; Gillet, P. H.; Greenwood, R. C.; Franchi, I. A. (The Meteoritical Society, 2006-01-01)
      We report on the petrology and geochemistry of Northwest Africa (NWA) 4215, an unbrecciated diogenite recovered in the Sahara. This single stone, weighing 46.4 g, displays a well-preserved cumulative texture. It consists of zoned xenomorphic orthopyroxene grains on the order of 500 micrometers in size, along with a few large chromite crystals (5 vol%, up to 3 mm). Accessory olivine and scarce diopside grains occur within the groundmass, usually around the chromite crystals. Minor phases are cristobalite, troilite, and metal. Unlike other diogenites, orthopyroxenes (En76.2Wo1.1Fs22.7 to En68.6Wo5.5Fs25.9), olivines (Fo76 to Fo71), and chromites (Mg# = 14.3 44.0, Cr# = 42.2-86.5) are chemically zoned. The minor element behavior in orthopyroxenes and the intricate chemical profiles obtained in chromites indicate that the zonings do not mirror the evolution of the parental melt. We suggest that they resulted from reaction of the crystals with intercumulus melt. In order to preserve the observed zoning profiles, NWA 4215 clearly cooled significantly faster than other diogenites. Indeed, the cooling rate determined from the diffusion of Cr in olivine abutting chromite is in the order of 10-50 degrees C/a, suggesting that NWA 4215 formed within a small, shallow intrusion.The bulk composition of NWA 4215 has been determined for major and trace elements. This meteorite is weathered and its fractures are filled with calcite, limonite, and gypsum, typical of hot desert alteration. In particular, the FeO, CaO abundances and most of the trace element concentrations (Sr, Ba, Pb, and REE among others) are high and indicate a significant contribution from the secondary minerals. To remove the terrestrial contribution, we have leached with HCl a subsample of the meteorite. The residue, made essentially of orthopyroxene and chromite, has similar major and trace element abundances to diogenites as shown by the shape of its REE pattern or by its high Al/Ga ratio. The connection of NWA 4215 with diogenites is confirmed by its O-isotopic composition (delta-17O = 1.431 +/- 0.102 ppm, delta-18O = 3.203 +/- 0.205 ppm, Delta-17O = -0.248 +/- 0.005 ppm).
    • 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.
    • Polygonal impact craters in the Argyre region, Mars: Evidence for influence of target structure on the final crater morphology

      Öhman, T.; Aittola, M.; Kostama, V.-P.; Hyvärinen, M.; Raitala, J. (The Meteoritical Society, 2006-01-01)
      Impact craters that in plan view are distinctly polygonal rather than circular or elliptical are common on Mars and other planets (hman et al. 2005). Their actual formation mechanism, however, is somewhat debatable. We studied the polygonal craters of different degradational stages in the region of the Argyre impact basin, Mars. The results show that in the same areas, heavily degraded, moderately degraded, and fresh polygonal craters display statistically similar strike distributions of the straight rim segments. The fact that the strike distributions are not dependent on lighting conditions was verified by using two data sets (Viking and MOC-WA) having different illumination geometries but similar resolutions. In addition, there are no significant differences in the amount of polygonality of craters in different degradational stages. These results clearly imply that large-scale polygonality is not caused by degradation, but originates from the cratering process itself, concurring with the findings regarding lunar craters by Eppler et al. (1983). The straight rims of polygonal craters apparently reflect areal fracture patterns that prevail for a geologically long time.
    • Polyhedral serpentine grains in CM chondrites

      Zega, Thomas J.; Garvie, Laurence A. J.; Dódony, István; Friedrich, Heiner; Stroud, Rhonda M.; Buseck, Peter R. (The Meteoritical Society, 2006-01-01)
      We used high-resolution transmission electron microscopy (HRTEM), electron tomography, electron energy-loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS) to investigate the structure and composition of polyhedral serpentine grains that occur in the matrices and fine-grained rims of the Murchison, Mighei, and Cold Bokkeveld CM chondrites. 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.
    • Proceedings of the Workshop on the Role of Volatiles and Atmospheres on Martian Impact Craters

      Barlow, Nadine G.; Stewart, Sarah; Barnouin-Jha, Olivier S. (The Meteoritical Society, 2006-01-01)
    • Ries and Chicxulub: Impact craters on Earth provide insights for Martian ejecta blankets

      Kenkmann, T.; Schönian, F. (The Meteoritical Society, 2006-01-01)
      Terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks. Here we discuss two examples that may yield implications for Martian craters: 1. Recent field analysis of the Ries crater has revealed the existence of subhorizontal shear planes (detachments) in the periphery of the crater beneath the ejecta blanket at 0.9-1.8 crater radii distance. Their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain. Both processes are enhanced in rheologically layered targets and in the presence of fluids. Detachment faulting may also occur in the periphery of Martian impacts and could be responsible for the formation of lobe-parallel ridges and furrows in the inner layer of double-layer and multiple-layer ejecta craters. 2. The ejecta blanket of the Chicxulub crater was identified on the southeastern Yucatán Peninsula at distances of 3.0-5.0 crater radii from the impact center. Abundance of glide planes within the ejecta and particle abrasion both rise with crater distance, which implies a ground-hugging, erosive, and cohesive secondary ejecta flow. Systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief. In analogy with Martian fluidized ejecta blankets, it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes, and was influenced by eroded bedrock lithologies. It is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress.