Meteoritics & Planetary Science, Volume 39 (2004)
ABOUT THIS COLLECTION
Meteoritics & Planetary Science is an international monthly journal of the Meteoritical Society—a scholarly organization promoting research and education in planetary science. Topics include the origin and history of the solar system, planets and natural satellites, interplanetary dust and interstellar medium, lunar samples, meteors and meteorites, asteroids, comets, craters, and tektites.
Meteoritics & Planetary Science was first published in 1935 under the title Contributions of the Society for Research on Meteorites. In 1947, the publication became known as Contributions of the Meteoritical Society and continued through 1951. From 1953 to 1995, the publication was known as Meteoritics, and in 1996, the journal's name was changed to Meteoritics & Planetary Science or MAPS. The journal was not published in 1952 and from 1957 to 1964.
This archive provides access to Meteoritics & Planetary Science Volumes 37-44 (2002-2009).
Visit Wiley Online Library for new and retrospective Meteoritics & Planetary Science content (1935-present).ISSN: 1086-9379
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Fluid inclusion evidence for impact-related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP-Chicxulub drill core Yax-1Fluid inclusions studies in quartz and calcite in samples from the ICDP-Chicxulub drill core Yaxcopoil-1 (Yax-1) have revealed compelling evidence for impact-induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 degrees C), highly saline, metal-rich, hydrocarbon-bearing brines. Hydrocarbons present in some inclusions in quartz are assumed to derive from cracking of pre-impact organic matter. The center of the crater is assumed to be the source of the hot quartz-forming brines. Fluid inclusions in abundant newly-formed calcite indicate lower cyrstallization temperatures (75100 degrees C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite-forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact-induced conductive heating of the sediments.
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Oxygen isotopic alteration in Ca‐Al‐rich inclusions from Efremovka: Nebular or parent body setting?In situ SIMS oxygen isotope data were collected from a coarse‐grained type B1 Ca‐Al‐rich inclusion (CAI) and an adjacent fine‐grained CAI in the reduced CV3 Efremovka to evaluate the timing of isotopic alteration of these two objects. The coarse‐grained CAI (CGI‐10) is a sub‐spherical object composed of elongate, euhedral, normally‐zoned melilite crystals ranging up to several hundreds of Pm in length, coarse‐grained anorthite and Al, Ti‐diopside (fassaite), all with finegrained (∼10 μm across) inclusions of spinel. Similar to many previously examined coarse‐grained CAIs from CV chondrites, spinel and fassaite are 16O‐rich and melilite is 16O‐poor, but in contrast to many previous results, anorthite is 16O‐rich. Isotopic composition does not vary with textural setting in the CAI: analyses of melilite from the core and mantle and analyses from a variety of major element compositions yield consistent 16O‐poor compositions. CGI‐10 originated in an 16O‐rich environment, and subsequent alteration resulted in complete isotopic exchange in melilite. The fine‐grained CAI (FGI‐12) also preserves evidence of a 1st‐generation origin in an 16O‐rich setting but underwent less severe isotopic alteration. FGI‐12 is composed of spinel ± melilite nodules linked by a mass of Al‐diopside and minor forsterite along the CAI rim. All minerals are very fine‐grained (<5 μm) with no apparent igneous textures or zoning. Spinel, Al‐diopside, and forsterite are 16O‐rich, while melilite is variably depleted in 16O (δ17,18O from ∼‐40‰ to −5‰). The contrast in isotopic distributions in CGI‐10 and FGI‐12 is opposite to the pattern that would result from simultaneous alteration: the object with finer‐grained melilite and a greater surface area/ volume has undergone less isotopic exchange than the coarser‐grained object. Thus, the two CAIs were altered in different settings. As the CAIs are adjacent to each other in the meteorite, isotopic exchange in CGI‐10 must have preceded incorporation of this CAI in the Efremovka parent body. This supports a nebular setting for isotopic alteration of the commonly observed 16O‐poor melilite in coarse‐grained CAIs from CV chondrites.
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Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundaryThe possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.
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"Comet-tail" ejecta streaks: A predicted cratering landform unique to TitanA model for an impact ejecta landform peculiar to Saturns moon Titan is presented. Expansion of the ejecta plume from moderate-sized craters is constrained by Titans thick atmosphere. Much of the plume is collimated along the incoming bolides trajectory, as was observed for plumes from impacts on Jupiter of P/Shoemaker-Levy-9, but is retained as a linear, diagonal ejecta cloud, unlike on Venus where the plume blows out. On Titan, the blowout is suppressed because the vertically-extended atmosphere requires a long wake to reach the vacuum of space, and the modest impact velocities mean plume expansion along the wake is slow enough to allow the wake to close off. Beyond the immediate ejecta blanket around the crater, distal ejecta is released into the atmosphere from an oblique line source: this material is winnowed by the zonal wind field to form streaks, with coarse radar-bright particles transported less far than fine radar-dark material. Thus, the ejecta form two distinct streaks faintly reminiscent of dual comet tails, a sharply W-E radar-dark one, and a less swept and sometimes comma-shaped radar-bright one.
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Geochemistry of drill core samples from Yaxcopoil-1, Chicxulub impact crater, MexicoThe chemical composition of suevites, displaced Cretaceous target rocks, and impactgenerated dikes within these rocks from the Yaxcopoil-1 (Yax-1) drill core, Chicxulub impact crater, Mexico, is reported and compared with the data from the Yucatán 6 (Y6) samples. Within the six suevite subunits of Yax-1, four units with different chemical compositions can be distinguished: a) upper/lower sorted and upper suevite (depth of 795-846 m); b) middle suevite (depth of 846-861 m); degrees C) brecciated impact melt rock (depth of 861-885 m); and d) lower suevite (depth of 885-895 m). The suevite sequence (a), (b), and (d) display an increase of the CaO content and a decrease of the silicate basement component from top to bottom. In contrast, the suevite of Y6 shows an inverse trend. The different distances of the Yax-1 and Y6 drilling sites from the crater center (~60, and ~47 km, respectively) lead to different suevite sequences. Within the Cretaceous rocks of Yax-1, a suevitic dike (depth of ~916 m) does not display chemical differences when compared with the suevite, while an impact melt rock dike (depth of ~1348 m) is significantly enriched in immobile elements. A clastic breccia dike (depth of ~1316 m) is dominated by material derived locally from the host rock, while the silicate-rich component is similar to that found in the suevite. Significant enrichments of the K2O content were observed in the Yax-1 suevite and the impact-generated dikes. All impactites of Yax-1 and Y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover. An anhydrite component in the impactites is missing (Yax-1) or negligible (Y6).
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Origin and emplacement of the impact formations of Chicxulub, Mexico, as revealed by the ICDP deep drilling at Yaxcopoil-1 and by numerical modelingWe present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 m deep ICDP Yaxcopoil-1 (Yax-1) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax-1 and of the impact structure as a whole is derived. The lower part of Yax-1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite-type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact-induced units of Yax-1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous megablocks; 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall-back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall-back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak-ring impact basin that is at the transition to a multi-ring basin.
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Spectroscopic anatomy of a meteor trail cross section with the European Southern Observatory Very Large TelescopeA meteor spectrum was recorded serendipitously at the European Southern Obrervatory (ESO) Very Large Telescope (VLT) during a long exposure in long-slit spectroscopic mode with FORS1. The -8 magnitude fireball crossed the narrow 1Î x 7î slit during the observation of a high z supernova in normal service mode operation on May 12, 2002. The spectrum covered the range of 6371050 nm, where the meteors air plasma emissions from N2, N, and O dominate. Carbon atom emission was not detected in the relatively unexplored wavelength range above 900 nm, but the observed upper limit was only 3 sigma less than expected from the dissociation of atmospheric CO2. The meteor trail was resolved along the slit, and the emission had a Gaussian distribution with a dimension of FWHM = 7.0 (+/- 0.4) * sin(alpha) * H (km)/90 m, where alpha is the unknown angle between the orientation of the meteor path and slit and H the assumed altitude of the meteor in km. To our knowledge, this is the first observation of a spatially resolved spectrum across a meteor trail. Unlike model predictions, the plasma excitation temperature varied only from about 4,300 to 4,365 K across the trail, based on the ratio of atomic and molecular nitrogen emissions. Unfortunately, we conclude that this was because the meteor at 100 km altitude was out of focus.
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Annual Author IndexThe Meteoritical Society, 2004-01-01
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Annual Subject IndexThe Meteoritical Society, 2004-01-01
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Are high-temperature fractionations in the solar nebula preserved in highly siderophile element systematics of the Earth's mantle?The relative abundances of the highly siderophile elements (HSE) Os, Ir, Ru, Pt, Rh, and Pd in relatively pristine lherzolites differ from solar abundance ratios and are several orders of magnitude higher than predicted for equilibrium distribution between metal/silicate (core-mantle). The samples are characterized by a mean Ca/Al ratio of 1.18 +/- 0.09 sigma-M and a mean Ca/Si ratio of 0.10 +/- 0.01 sigma-M, overlapping with a mean Ca/Al of 1.069 +/- 0.044 sigma-M and a mean Ca/Si of 0.081 +/- 0.023 sigma-M found in chondrites (Wasson and Kallemeyn 1988). Interestingly, the CI-normalized abundance pattern shows decreasing solar system normalized abundances with increasing condensation temperatures. The abundance of the moderately volatile element Pd is about 2x higher than those in the most refractory siderophiles Ir and Os. Thus, the HSE systematics of upper mantle samples suggest that the late bombardment, which added these elements to the accreting Earth, more closely resembles materials of highly reduced EH or EL chondrites than carbonaceous chondrites. In fact, the HSE in the Earth mantle are even more fractionated than the enstatite chondritesan indication that some inner solar system materials were more highly fractionated than the latter.
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High-pressure phases in shock-induced melt veins of the Umbarger L6 chondrite: Constraints of shock pressureWe report a previously undocumented set of high-pressure minerals in shock-induced melt veins of the Umbarger L6 chondrite. High-pressure minerals were identified with transmission electron microscopy (TEM) using selected area electron diffraction and energy-dispersive X-ray spectroscopy. Ringwoodite (Fa30), akimotoite (En11Fs89), and augite (En42Wo33Fs25) were found in the silicate matrix of the melt vein, representing the crystallization from a silicate melt during the shock pulse. Ringwoodite (Fa27) and hollandite-structured plagioclase were also found as polycrystalline aggregates in the melt vein, representing solid state transformation or melting with subsequent crystallization of entrained host rock fragments in the vein. In addition, Fe2SiO4-spinel (Fa66-Fa99) and stishovite crystallized from a FeO-SiO2-rich zone in the melt vein, which formed by shock melting of FeO-SiO2-rich material that had been altered and metasomatized before shock. Based on the pressure stabilities of the high-pressure minerals, ringwoodite, akimotoite, and Ca-clinopyroxene, the melt vein crystallized at approximately 18 GPa. The Fe2SiO4-spinel + stishovite assemblage in the FeO-SiO2- rich melts is consistent with crystallization of the melt vein matrix at the pressure up to 18 GPa. The crystallization pressure of ~18 GPa is much lower than the 4590 GPa pressure one would conclude from the S6 shock effects in melt veins (Stöffler et al. 1991) and somewhat less than the 25-30 GPa inferred from S5 shock effects (Schmitt 2000) found in the bulk rock.
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Ion microprobe U-Th-Pb dating and REE analyses of phosphates in the nakhlites Lafayette and Yamato-000593/000749U, Th, and Pb isotopes and rare earth elements (REEs) in individual phosphate grains from martian meteorites Lafayette and Yamato-000593/000749 were measured using a sensitive highresolution ion microprobe (SHRIMP). Observed U-Pb data of 12 apatite grains from Yamato (Y-) 000593/000749 are well represented by linear regressions in both conventional 2D isochron plots and the 3D U-Pb plot (total Pb/U isochron), indicating that the formation age of this meteorite is 1.53 +/- 0.46 Ga (2-sigma). On the other hand, the data of nine apatite grains from Lafayette are well represented by planar regression rather than linear regression, indicating that its formation age is 1.15 +/- 0.34 Ga (2-sigma) and that a secondary alteration process slightly disturbed its U-Pb systematics as discussed in the literature regarding Nakhla. The observed REE abundance patterns of the apatites in Lafayette and Yamato-000749, normalized to CI chondrites, are characterized by a progressive depletion of heavy REEs (HREEs), a negative Eu anomaly, similarity to each other, and consistency with previously reported data for Nakhla. Considering the extensive data from other radiometric systems such as Sm- Nd, Rb-Sr, Ar-Ar, and trace elements, our results suggest that the parent magmas of the nakhlites, including the newly found Y-000593/000749, are similar and that their crystallization ages are ~1.3 Ga.
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NWA 1152 and Sahara 00182: New primitive carbonaceous chondrites with affinities to the CR and CV groupsWe have investigated the mineralogy, petrography, bulk chemistry, and light element isotope composition of the ungrouped chondrites North West Africa (NWA) 1152 and Sahara 00182. NWA 1152 contains predominantly type 1 porphyritic olivine (PO) and porphyritic olivinepyroxene (POP) chondrules. Chondrule silicates are magnesium-rich (Fo98.8 +/- 1.2, n = 36; Fs2.3 +/- 2.1 Wo1.2 +/- 0.3, n = 23). Matrix comprises ~40 vol% of the sample and is composed of a micron sized silicate groundmass with larger silicate, sulfide, magnetite, and Fe-Ni metal (Ni ~50 wt%) grains. Phyllosilicates were not observed in the matrix. Refractory inclusions are rare (0.3 vol%) and are spinel pyroxene aggregates or amoeboid olivine aggregates; melilite is absent from the refractory inclusions. Sahara 00182 contains predominantly type 1 PO chondrules, POP chondrules are less common. Most chondrules contain blebs of, and are often rimmed with, Fe-Ni metal and sulfide. Chondrule phenocrysts are magnesium-rich (Fo92.2 +/- 0.6, n = 129; Fs4.4 +/- 1.8 Wo1.3 +/- 1.1, n = 16). Matrix comprises ~30 vol% of the meteorite and is predominantly sub-micron silicates, with rare larger silicate gains. Matrix Fe-Ni metal (mean Ni = 5.8 wt%) and sulfide grains are up to mm scale. No phyllosilicates were observed in the matrix. Refractory inclusions are rare (1.1 vol%) and melilite is absent. The oxygen isotope composition of NWA 1152 falls within the range of the CV chondrites with delta-17O = -3.43 ppm delta-18O = 0.70 ppm and is similar to Sahara 00182, delta-17O = -3.89 ppm, delta-18O = -0.19 ppm (Grossman and Zipfel 2001). Based on mineralogical and petrographic characteristics, we suggest NWA 1152 and Sahara 00182 show many similarities with the CR chondrites, however, oxygen isotopes suggest affinity with the CVs. Thus, neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics. Both samples demonstrate the complexity of inter- and intra-group relationships of the carbonaceous chondrites. Whatever their classification, NWA 1152 and Sahara 00182 represent a source of relatively pristine solar system material.
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Planetary accretion, oxygen isotopes, and the central limit theoremThe accumulation of presolar dust into increasingly larger aggregates such as calciumaluminum- rich inclusions (CAIs) and chondrules, asteroids, and planets should result in a drastic reduction in the numerical spread in oxygen isotopic composition between bodies of similar size, in accord with the central limit theorem. Observed variations in oxygen isotopic composition are many orders of magnitude larger than would be predicted by a simple, random accumulation model that begins in a well-mixed nebula, no matter what size objects are used as the beginning or end points of the calculation. This discrepancy implies either that some as yet unspecified but relatively long-lived process acted on the solids in the solar nebula to increase the spread in oxygen isotopic composition during each and every stage of accumulation, or that the nebula was heterogeneous (at least in oxygen) and maintained this heterogeneity throughout most of its nebular history. Depending on its origin, large-scale nebular heterogeneity could have significant consequences for many areas of cosmochemistry, including the application of well-known isotopic systems to the dating of nebular events and the prediction of bulk compositions of planetary bodies on the basis of a uniform cosmic abundance. The evidence supports a scenario wherein the oxygen isotopic composition of nebular solids becomes progressively depleted in 16O with time due to chemical processing within the nebula, rather than a scenario where 16O-rich dust and other materials are injected into the nebula, possibly causing its initial collapse.
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Martian xenon components in Shergotty mineral separates: Locations, sources, and trapping mechanismsIsotopic signatures and concentrations of xenon have been measured in Shergotty mineral separates by laser step heating. Martian atmosphere and martian interior xenon are present, as is a spallation component. Martian atmospheric xenon is 5-10 times more concentrated in opaque minerals (magnetite, ilmenite, and pyrrhotite) and maskelynite than in pyroxenes, perhaps reflecting grain size variation. This is shown to be consistent with shock incorporation. A component consisting of solar xenon with a fission contribution, similar to components previously identified in martian meteorites and associated with the martian interior, is best defined in the pyroxene-dominated separates. This component exhibits a consistent 129Xe (129Xe/132Xe ~1.2) excess over solar/planetary (129Xe/132Xe ~1.04). We suggest that gas present in the melt, perhaps a mixture of interior xenon and martian atmosphere, was incorporated into the pyroxenes in Shergotty as the minerals crystallized.
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Radiogenic isotope investigation of the St-Robert H5 fallThe St-Robert H5 chondrite yields a mineral/whole-rock Pb-Pb age of 4565 +/- 23 Ma (2-sigma) comparable to the accepted age of most chondrites. The regression of chondrule data give a similar age of 4566 +/- 7 Ma (2-sigma). These results imply that no major perturbation affected the Pb-Pb systematics of this meteorites parent body within the first few billion years following its accretion. Re and Os concentrations along with Os isotopic compositions of whole-rock fragments, surface fusion crusts and metal phases are also reported. The whole rock measurements for this ordinary chondrite are characterized by high Re/Os ratio coupled with relatively high 187Os/188Os (compared to average ordinary chondrites), that we interpret as a long term Re enrichment. As for most chondrites, no precise geochronological information could be extracted from the Re/Os systematics, although most data plot near the IIIAB reference isochron (Smoliar et al. 1996). From the fusion crust results, we rule out the possibility that atmospheric entry caused the perturbations in the Re-Os system, since melted crust analysis yields among the most concordant data points. Evidence from metal phases suggests that a very recent process perturbed the isochron, relocating Re from kamacite toward troilite.