¹Norbert Kömle,²Craig Pitcher,²Yang Gao,³Lutz Richter
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.08.019]
¹Space Research Institute, Austrian Academy of Sciences, Schmiedlstraße 6, A-8042 Graz, Austria
²STAR Lab, Surrey Space Centre, University of Surrey, Guildford, GU2 7XH, UK
³OHB System AG, Manfred-Fuchs-Straße 1, 82234 Weßling – Oberpfaffenhofen, Germany
Copyright Elsevier

The Powdered Sample Dosing and Distribution System (PSDDS) of the ExoMars rover will be required to handle and contain samples of Mars regolith for long periods of time. Cementation of the regolith, caused by water and salts in the soil, results in clumpy material and a duricrust layer forming on the surface. It is therefore possible that material residing in the sampling system may cement, and could potentially hinder its operation. There has yet to be an investigation into the formation of duricrusts under simulated Martian conditions, or how this may affect the performance of sample handling mechanisms. Therefore experiments have been performed to create a duricrust and to explore the cementation of Mars analogues, before performing a series of tests on a qualification model of the PSDDS under simulated Martian conditions.

It was possible to create a consolidated crust of cemented material several millimetres deep, with the material below remaining powder-like. It was seen that due to the very low permeability of the Montmorillonite component material, diffusion of water through the material was quickly blocked, resulting in a sample with an inhomogeneous water content. Additionally, samples with a water mass content of 10% or higher would cement into a single solid piece. Finally, tests with the PSDDS revealed that samples with a water mass content of just 5% created small clumps with significant internal cohesion, blocking the sample funnels and preventing transportation of the material. These experiments have highlighted that the cementation of regolith in Martian conditions must be taken into consideration in the design of sample handling instruments.

¹M.B. Biren, ¹M.C. van Soest, ^1,2J.-A. Wartho, ¹K.V. Hodges, ³J.G. Spray
Earth and Planetary Science Letters 453, 56–66 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.07.053]
¹Group 18 Laboratories, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
²GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, D-24148 Kiel, Germany
³Planetary and Space Science Centre, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick E3B 5A3, Canada
Copyright Elsevier

The (U–Th)/He method has been applied to constrain the formation ages of the Clearwater West and East impact structures of Quebec, Canada. Zircons were separated from impact melt samples derived from a surface exposure at Clearwater West (32 km diameter), and from a drill core at Clearwater East (26 km diameter). The (U–Th)/He results indicate ages of 280±27 Ma280±27 Ma (2σ , n=7n=7) for Clearwater West, and 450±56 Ma450±56 Ma (2σ , n=8n=8) for Clearwater East. Our (U–Th)/He date for Clearwater West supports the findings of previous Rb–Sr (266±15 Ma266±15 Ma; 2σ) and 40Ar/39Ar (280±4 Ma280±4 Ma and 283.8±2.2 Ma283.8±2.2 Ma, 2σ) impact melt studies. Our (U–Th)/He date for Clearwater East also overlaps with previously published 40Ar/39Ar dating results, which yielded U-shaped spectra, with ‘maximum’ and ‘best-estimate’ dates of ∼ 460–470 Ma. Our results support the contention, previously based solely on 40Ar/39Ar data, that the Clearwater West and East impact structures do not comprise an impact doublet that formed coevally from a binary asteroid pair.

¹William C. Clyde, ²Jahandar Ramezani, ³Kirk R. Johnson, ²Samuel A. Bowring, ^1,4Matthew M. Jones
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.07.041]
¹Department of Earth Sciences, University of New Hampshire, 56 College Rd., Durham, NH 03824, United States
²Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
³National Museum of Natural History, Smithsonian Institution, MRC 106, P.O. Box 37012, Washington, DC, 20013, United States
⁴Department of Earth and Planetary Sciences, Northwestern University, Technological Institute, 2145 Sheridan Road, Evanston, IL 60208, United States
Copyright Elsevier

The Cretaceous–Paleogene (K–Pg) boundary is the best known and most widely recognized global time horizon in Earth history and coincides with one of the two largest known mass extinctions. We present a series of new high-precision uranium–lead (U–Pb) age determinations by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) method from volcanic ash deposits within a tightly constrained magnetobiostratigraphic framework across the K–Pg boundary in the Denver Basin, Colorado, USA. This new timeline provides a precise interpolated absolute age for the K–Pg boundary of 66.021±0.024/0.039/0.081 Ma66.021±0.024/0.039/0.081 Ma, constrains the ages of magnetic polarity Chrons C28 to C30, and offers a direct and independent test of early Paleogene astronomical and 40Ar/39Ar based timescales. Temporal calibration of paleontological and palynological data from the same deposits shows that the interval between the extinction of the dinosaurs and the appearance of earliest Cenozoic mammals in the Denver Basin lasted ∼185 ky (and no more than 570 ky) and the ‘fern spike’ lasted ∼1 ky (and no more than 71 ky) after the K–Pg boundary layer was deposited, indicating rapid rates of biotic extinction and initial recovery in the Denver Basin during this event.

^1,2Paolo A. Sossi, ^1,3Oliver Nebel, ⁴John Foden
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.07.032]
¹Research School of Earth Sciences, The Australian National University, 2601 Acton, ACT, Australia
²Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France
³School of Earth, Atmosphere and Environment, Monash University, 3084 Clayton, VIC, Australia
⁴Department of Earth Sciences, The University of Adelaide, 5005 North Terrace, SA, Australia
Copyright Elsevier

Iron is the only polyvalent major element, and controls reduction–oxidation (redox) reactions in a host of geologic processes and reservoirs, from the mineral- to planetary-scale, on Earth and in space. Mass transfer of Fe is often accompanied by changes in bonding environment, meaning the resultant variation in bond-strength in crystals, liquids and gases induces stable isotope fractionation, even at high temperatures. In the absence of iron exchange, electron transfer can also affect iron’s valence state and calculated oxygen fugacity (fO2fO2), however its isotope composition remains unchanged. Thus, iron isotopes are a powerful tool to investigate processes that involve mass transfer, redox reactions and changes in bonding environment in planetary systems. Primitive chondritic meteorites show remarkable isotopic homogeneity, δ57Fe=−0.01±0.01‰δ57Fe=−0.01±0.01‰ (2SE), over a wide range of Fe/Mg vs Ni/Mg, a proxy for fO2fO2 in the solar nebula. In chondrites, there are iron isotope differences between metal and silicates that become more pronounced at higher metamorphic grades. However, on a planetary scale, Mars and Vesta overlap with chondrites, preserving no trace of core formation or volatile depletion on these bodies. Upon assessment of pristine lherzolites, the Bulk Silicate Earth is heavier than chondrites (δ57Fe=+0.05±0.01‰δ57Fe=+0.05±0.01‰; 2SE), and similar to or slightly lighter than the Moon. That the mantles of some differentiated inner solar system bodies extend to heavier compositions (+0.2‰+0.2‰) than chondrites may principally result from volatile depletion either at a nebular or late accretion stage. Within terrestrial silicate reservoirs, iron isotopes provide insight into petrogenetic and geodynamic processes. Partial melting of the upper mantle produces basalts that are heavier than their sources, scaling with degree of melting and driving the increasingly refractory peridotite to lighter compositions. Mid-Ocean Ridge Basalts (MORBs) are homogeneous to δ57Fe=0.10±0.01‰δ57Fe=0.10±0.01‰ (2SE) after correction to primary magmas, and can be produced from single stage melt extraction. Conversely, iron isotopes in arc basalts are more varied (View the MathML source−0.2<δ57Fe(‰)<+0.2) than can be produced from partial melting. Their iron isotope compositions are significantly lighter, suggesting they form from mantle re-enriched in light Fe and/or more depleted than Depleted MORB Mantle (DMM). If arc sources are more oxidised, an agent other than iron is required. Magmatic differentiation drives enrichment in heavy isotopes by partial melting of crustal rocks, fluid exsolution and crystallisation. Iron isotope trajectories in evolving magmas depend on their initial fO2fO2 and whether the system is closed or open to oxygen and/or mass exchange. Granite end-members carry signatures diagnostic of their tectonic setting, where reduced, anorogenic A-type granites (δ57Fe=+0.4‰δ57Fe=+0.4‰) are heavier than more oxidised I-types (δ57Fe=+0.2‰δ57Fe=+0.2‰).

¹Jonathan A. Lewis,^1,2Rhian H. Jones
Meteoritics&Planetary Sciences (in Press) Link to Article [DOI: 10.1111/maps.12719]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, USA
²School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
Published by arrangement with John Wiley & Sons

In ordinary chondrites (OCs), phosphates and feldspar are secondary minerals known to be the products of parent-body metamorphism. Both minerals provide evidence that metasomatic fluids played a role during metamorphism. We studied the petrology and chemistry of phosphates and feldspar in petrologic type 4–6 L chondrites, to examine the role of metasomatic fluids, and to compare metamorphic conditions across all three OC groups. Apatite in L chondrites is Cl-rich, similar to H chondrites, whereas apatite in LL chondrites has lower Cl/F ratios. Merrillite has similar compositions among the three chondrite groups. Feldspar in L chondrites shows a similar equilibration trend to LL chondrites, from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6. This contrasts with H chondrites which have homogeneous albitic plagioclase in petrologic types 4–6. Alkali- and halogen-rich and likely hydrous metasomatic fluids acted during prograde metamorphism on OC parent bodies, resulting in albitization reactions and development of phosphate minerals. Fluid compositions transitioned to a more anhydrous, Cl-rich composition after the asteroid began to cool. Differences in secondary minerals between H and L, LL chondrites can be explained by differences in fluid abundance, duration, or timing of fluid release. Phosphate minerals in the regolith breccia, Kendleton, show lithology-dependent apatite compositions. Bulk Cl/F ratios for OCs inferred from apatite compositions are higher than measured bulk chondrite values, suggesting that bulk F abundances are overestimated and that bulk Cl/F ratios in OCs are similar to CI.

¹J.N. Connelly,¹M. Bizzarro
Earth and Planetary Science Letters 452,36–43 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.07.010]
¹Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K, Denm
Copyright Elsevier

A model of a giant impact between two planetary bodies is widely accepted to account for the Earth–Moon system. Despite the importance of this event for understanding early Earth evolution and the inventory of Earth’s volatiles critical to life, the timing of the impact is poorly constrained. We explore a data-based, two-stage Pb isotope evolution model in which the timing of the loss of volatile Pb relative to refractory U in the aftermath of the giant impact is faithfully recorded in the Pb isotopes of bulk silicate Earth. Constraining the first stage Pb isotopic evolution permits calculating an age range of 4.426–4.417 Ga for the inflection in the U/Pb ratio related to the giant impact. This model is supported by Pb isotope data for angrite meteorites that we use to demonstrate volatility-driven, planetary-scale Pb loss was an efficient process during the early Solar System. The revised age is ∼100 Myr younger than most current estimates for the age of the Moon but fully consistent with recent ages for lunar ferroan anorthosite and the timing of Earth’s first crust inferred from the terrestrial zircon record. The estimated loss of ∼98% of terrestrial Pb relative to the Solar System bulk composition by the end of the Moon-forming process implies that the current inventory of Earth’s most volatile elements, including water, arrived during post-impact veneering by volatile-rich bodies.

^1,2Mohit Melwani Daswani, ³Susanne P. Schwenzer, ⁴Mark H. Reed, ¹Ian P. Wright, ¹Monica M. Grady
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12713]
¹Department of Physical Sciences, The Open University, Milton Keynes, UK
²Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
³Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK
⁴Department of Geological Sciences, University of Oregon, Eugene, Oregon, USA
Published by arrangement with John Wiley & Sons

Clay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low-temperature (20 °C) one-dimensional (1-D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low-temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica-rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO2) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H2. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread.

^1,2A.Lamali et al. (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12715]
¹CRAAG, Alger, Algeria
²FSTGAT, Alger, Algeria
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley and Sons

The Maâdna structure is located approximately 400 km south of Algiers (33°19′ N, 4°19′ E) and emplaced in Upper-Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock-diagnostic features such as planar deformation features (PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene-Pliocene part of the African Apparent Polar Wander Path (APWP). Consequently, we estimate the formation of the Maâdna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer.

¹George G. Managadze et al. (>10)*
Planetary and Space Science (in Press) Link to Article [http://dx.doi.org/10.1016/j.pss.2016.07.005]
¹Space Research Institute, Profsoyuznaya, st. 84/32, Moscow 117997, Russia
*Find the extensive, full author and affiliation list on the publishers website

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^1,2Yusuke Kawabata, ¹Hiroko Nagahara
Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2016.08.005]
¹Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
²Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
Copyright Elsevier

Despite its small size, the asteroid 4 Vesta has been completely differentiated to core and mantle. Its composition is similar to howardite-eucrite-diogenite (HED) meteorites of which the detailed petrology is known. Therefore, 4 Vesta is a good target for understanding the differentiation of terrestrial planets. A new differentiation model for crust formation has been developed by taking magma ocean fluid dynamics, chemical equilibrium, the presence of 26Al, and cooling into consideration with a special focus on crystal separation. The role of crystal size, thickness of the conductive lid, and fO2 are evaluated as parameters. The results show that large crystals of at least 1 cm settled and formed a kilometer-thick cumulate layer of orthopyroxene with Mg## of 0.70–0.90 in ∼20 thousand years, which almost agrees with the Mg ## of diogenites. Smaller grain sizes formed thinner layers.