• A database of chondrite analyses including platinum group elements, Ni, Co, Au, and Cr: Implications for the identification of chondritic projectiles

      Tagle, R.; Berlin, J. (The Meteoritical Society, 2008-01-01)
      Siderophile elements have been used to constrain projectile compositions in terrestrial and lunar impact melt rocks. To obtain a better knowledge of compositional differences between potential chondritic projectile types, meteorite analyses of the elements Ru, Rh, Pd, Os, Ir, Pt, Cr, Co, Ni, and Au were gathered into a database. The presented compilation comprises 806 analyses of 278 chondrites including new ICP-MS analyses of Allende and two ordinary chondrites. Each data set was evaluated by comparing element ratios of meteorites from the same chondrite group. Characteristic element abundances and ratios were determined for each group. Features observed in the element abundance patterns can be linked directly to the presence of certain components, such as the abundance of refractory elements Os, Ir, and Ru correlating with the occurrence of refractory inclusions in CV, CO, CK, and CM chondrites. The refined characteristic element ratios appear to be representative not only for meteorites, but also for related asteroidal bodies. Chondrite element ratios were compared to previously published values from impact melt rocks of the Popigai and Morokweng impact structures confirming that an identification of the specific type of projectile (L and LL chondrite, respectively) is possible. The assessment for Morokweng is supported by the recent discovery of an LL chondrite fragment in the impact melt rocks. Ultimately, the database provides valuable information for understanding processes in the solar nebula as they are recorded in chondrites. A new type of complementarity between element patterns of CK and EH chondrites is suggested to be the result of condensation, redox, and transportation processes in the solar nebula.
    • Geochemical identification of projectiles in impact rocks

      Tagle, R.; Hecht, L. (The Meteoritical Society, 2006-01-01)
      The three major geochemical methods for impactor identification are evaluated with respect to their potential and limitations with regards to the precise detection and identification of meteoritic material in impactites. The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even <<0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle-derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr-isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post-impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition,especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the “projectile elemental ratio” derived from the slope of the mixing line (target-projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination.
    • New impact-melt rock from the Roter Kamm impact structure, Namibia: Further constraints on impact age, melt rock chemistry, and projectile composition

      Hecht, L.; Reimold, W. U.; Sherlock, S.; Tagle, R.; Koeberl, C.; Schmitt, R.-T. (The Meteoritical Society, 2008-01-01)
      A new locality of in situ massive impact-melt rock was discovered on the southsouthwestern rim of the Roter Kamm impact structure. While the sub-samples from this new locality are relatively homogeneous at the hand specimen scale, and despite being from a nearby location, they do not have the same composition of the only previously analyzed impact-melt rock sample from Roter Kamm. Both Roter Kamm impact-melt rock samples analyzed to date, as well as several suevite samples, exhibit a granitic-granodioritic precursor composition. Micro-chemical analyses of glassy matrix and Al-rich orthopyroxene microphenocrysts demonstrate rapid cooling and chemical disequilibrium at small scales. Platinum-group element abundances and ratios indicate an ordinary chondritic composition for the Roter Kamm impactor. Laser argon dating of two sub-samples did not reproduce the previously obtained age of 3.7 +/- 0.3 (1-sigma) for this impact event, based on 40Ar/39Ar dating of a single vesicular impact-melt rock. Instead, we obtained ages between 3.9 and 6.3 Ma, with an inverse isochron age of 4.7 +/- 0.3 Ma for one analyzed sub-sample and 5.1 +/- 0.4 Ma for the other. Clearly a post-5 Ma impact at Roter Kamm remains indicated, but further analytical work is required to better constrain the currently best estimate of 4-5 Ma. Both impactor and age constraints are clearly obstructed by the inherent microscopic heterogeneity and disequilibrium melting and cooling processes demonstrated in the present study.
    • Platinum group elements in impactites of the ICDP Chicxulub drill core Yaxcopoil-1: Are there traces of projectile?

      Tagle, R.; Erzinger, J.; Hecht, L.; Schmitt, R. T.; Stöffler, D.; Claeys, P. (The Meteoritical Society, 2004-01-01)
      This study presents results of platinum group element (PGE) analyses of impactites from the Yaxcopoil-1 (Yax-1) and Yucatán 6 drill cores of the 180 km-diameter Chicxulub crater. These are the main elements used for projectile identification. They were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry. The concentration of PGE in the samples are low. The concentration patterns of the suevite samples resemble the pattern of the continental crust. We conclude that any meteoritic fraction in these samples is below 0.05%. A synand post-impact modification of the PGE pattern from meteoritic toward a continental crust pattern is very unlikely. The globally distributed fallout at the Cretaceous-Tertiary (K/T) boundary, however, has high PGE concentrations. Therefore, the lack of a significant meteoritic PGE signature in the crater is not an argument for a PGE-poor impactor. Taking the results of three-dimensional numerical simulations of the Chicxulub event into account, the following conclusions are drawn: 1) The main fraction of the impactor was ejected into and beyond the stratosphere, distributed globally, and deposited in the K/T boundary clay; and 2) the low amount of projectile contamination in the Yax-1 lithologies may reflect an oblique impact. However, the role of volatiles in the mixing process between projectile and target is not well-understood and may also have played a fundamental role.
    • Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana

      Goderis, S.; Tagle, R.; Schmitt, R. T.; Erzinger, J.; Claeys, Ph. (The Meteoritical Society, 2007-01-01)
      In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasmamass spectrometry (ICP-MS). The majority of the samples come from the impactite material recovered by cores LB-07A and LB-08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB-05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result.
    • Traces of an H chondrite in the impact-melt rocks from the Lappajärvi impact structure, Finland

      Tagle, R.; Öhman, T.; Schmitt, R. T.; Erzinger, J.; Claeys, Ph. (The Meteoritical Society, 2007-01-01)
      Here we present the results of a geochemical study of the projectile component in impact melt rocks from the Lappajrvi impact structure, Finland. Main- and trace-element analyses, including platinum group elements (PGEs), were carried out on twenty impact-melt rock samples from different locations and on two shocked granite fragments. The results clearly illustrate that all the impact melt rocks are contaminated with an extraterrestrial component. An identification of the projectile type was performed by determining the projectile elemental ratios and comparing the corresponding element ratios in chondrites. The projectile elemental ratios suggest an H chondrite as the most likely projectile type for the Lappajrvi impact structure. The PGE composition of the highly diluted projectile component (~0.05 and 0.7 wt% in the impact-melt rocks) is similar to the recent meteorite population of H chondrites reaching Earth. The relative abundance of ordinary chondrites, including H, L, and LL chondrites, as projectiles at terrestrial impact structures is most likely related to the position of their parent bodies relative to the main resonance positions. This relative abundance of ordinary chondrites suggests a strong bias of the impactor population toward inner Main Belt objects.