Martin Schmieder^1,2 et al. (>10)*
¹Lunar and Planetary Institute, Houston, Texas, USA
²NASA Solar System Exploration Research Virtual Institute (SSERVI)
*Find the extensive, full author and affiliation list on the publishers website

The two neighboring Suvasvesi North and South impact structures in central-east Finland have been discussed as a possible impact crater doublet produced by the impact of a binary asteroid. This study presents ⁴⁰Ar/³⁹Ar geochronologic data for impact melt rocks recovered from the drilling into the center of the Suvasvesi North impact structure and melt rock from glacially transported boulders linked to Suvasvesi South. ⁴⁰Ar/³⁹Ar step-heating analysis yielded two essentially flat age spectra indicating a Late Cretaceous age of ~85 Ma for the Suvasvesi North melt rock, whereas the Suvasvesi South melt sample gave a Neoproterozoic minimum (alteration) age of ~710 Ma. Although the statistical likelihood for two independent meteorite strikes in close proximity to each other is rather low, the remarkable difference in ⁴⁰Ar/³⁹Ar ages of >600 Myr for the two Suvasvesi impact melt samples is interpreted as evidence for two temporally separate, but geographically closely spaced, impacts into the Fennoscandian Shield. The Suvasvesi North and South impact structures are, thus, interpreted as a “false” crater doublet, similar to the larger East and West Clearwater Lake impact structures in Québec, Canada, recently shown to be unrelated. Our findings have implications for the reliable recognition of impact crater doublets and the apparent rate of binary asteroid impacts on Earth and other planetary bodies in the inner solar system.

Reference
Schmieder M (2016) The two Suvasvesi impact structures, Finland: Argon isotopic evidence for a “false” impact crater doublet. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12636]
Published by arrangement with John Wiley & Sons

Isabella Pignatelli^1,2, Yves Marrocchi^1,2, Lionel. G. Vacher^1,2, Rémi Delon^1,2 andMatthieu Gounelle^3,4
1Université de Lorraine, CRPG, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France
²CNRS, CRPG UMR 7358, Vandoeuvre-lès-Nancy, France
³IMPMC, MNHM, UPMC, UMR CNRS 7590, 75005 Paris, France
⁴Institut Universitaire de France, Maison des Universités, 75005 Paris, France

We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths (TCIs) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type-I TCIs consist of compact tochilinite/cronstedtite rims surrounding Fe-Ni metal beads, thus confirming kamacite as the precursor of type-I TCIs. In contrast, type-II TCIs are characterized by complex compositional zoning composed of three different Fe-bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type-II TCIs present well-developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors. The results demonstrate that type-II TCIs formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type-I and type-II TCIs. In addition, the complex chemical zoning observed within type-II TCIs suggests that the alteration conditions evolved dramatically over time. At least three stages of alteration can be proposed, characterized by alteration fluids with varying compositions (1) Fe- and S-rich fluids; (2) S-poor and Fe- and Si-rich fluids; and (3) S- and Si-poor, Fe-rich fluids. The presence of unaltered silicates in close association with euhedral type-II TCIs suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg-bearing secondary minerals in Paris TCIs suggests that the Mg content increases during the course of alteration.

Reference
Pignatelli I, Marrocchi Y, Vacher LG, Delon R and Gounelle M (2016) Multiple precursors of secondary mineralogical assemblages in CM chondrites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12625]
Published by arrangement with John Wiley & Sons

Sean S. Lindsay^1,2, Tasha L. Dunn³, Joshua P. Emery² and Neil E. Bowles^1
1Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK
²Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, USA
³Department of Geology, Colby College, Waterville, Maine, USA

Near-infrared reflectance spectra of S-type asteroids contain two absorptions at 1 and 2 μm (band I and II) that are diagnostic of mineralogy. A parameterization of these two bands is frequently employed to determine the mineralogy of S(IV) asteroids through the use of ordinary chondrite calibration equations that link the mineralogy to band parameters. The most widely used calibration study uses a Band II terminal wavelength point (red edge) at 2.50 μm. However, due to the limitations of the NIR detectors on prominent telescopes used in asteroid research, spectral data for asteroids are typically only reliable out to 2.45 μm. We refer to this discrepancy as “The Red Edge Problem.” In this report, we evaluate the associated errors for measured band area ratios (BAR = Area BII/BI) and calculated relative abundance measurements. We find that the Red Edge Problem is often not the dominant source of error for the observationally limited red edge set at 2.45 μm, but it frequently is for a red edge set at 2.40 μm. The error, however, is one sided and therefore systematic. As such, we provide equations to adjust measured BARs to values with a different red edge definition. We also provide new ol/(ol+px) calibration equations for red edges set at 2.40 and 2.45 μm.

Reference
Lindsay SS, Dunn TL, Emery JP and Bowles NE (2016) The Red Edge Problem in asteroid band parameter analysis. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12611]
Published by arrangement with John Wiley & Sons

John T. Wasson^1,2
1Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California, USA
²Department of Chemistry and Biochemistry, University of California, Los Angeles, California, USA

Treysa and Delegate have compositions closely similar to those of IIIAB irons but plot above the IIIAB field on Ir-Au diagrams; for this reason they are designated anomalous members of IIIAB. All refractory siderophiles share this anomaly. Wasson (1999) interpreted the large spread on IIIAB Ir-Au diagrams to result from melt-trapping and generated solid and liquid fractional crystallization tracks; almost all IIIAB irons fall between the tracks. In contrast, Treysa, Delegate, and three other irons (the Treysa quintet) plot beyond the liquid track. Ideal fractional crystallization cannot account for compositions that plot outside the region between the tracks. Possible explanations for the anomalous compositions of the Treysa quintet are that (1) these meteorites did not form in the IIIAB magma or (2) they formed by the mixing of early crystallized solids with a late liquid. The weight of the evidence including cosmic-ray ages favor the second explanation. Although this explanation can account for positions plotting above the liquid track, it requires special circumstances. The infalling blocks must be assimilated and the resulting melt must crystallize quickly into pockets small enough (<1 m) to allow igneous gradients to be leveled by subsequent diffusion. The Treysa quintet shares the region beyond the liquid track with most main-group pallasites (PMG), which may have also originated in the IIIAB body. It appears that Treysa, its relatives, and the PMG were formed in one or more impact events that mixed olivine and solid metal formed near the core-mantle boundary with nearby magma. It is then necessary to cool the melt rapidly; the best way to achieve rapid cooling is by heat exchange with cooler solids. That the Treysa quintet and the PMG can be explained by the same processes operating on late IIIAB magma supports the conclusion that PMG formed on the IIIAB parent asteroid.

Reference
Wasson JT (2016) Formation of the Treysa quintet and the main-group pallasites by impact-generated processes in the IIIAB asteroid. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12635]
Published by arrangement with John Wiley & Sons