Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite crater

High temperature impact melt breccias from the Rochechouart (France) meteorite crater record magnetization component with antipodal, normal and reverse polarities. The corresponding paleomagnetic pole for this component lies between the 220 Ma and 210 Ma reference poles on the Eurasian apparent polar wander path, consistent with the 214 $\pm$ 8 Ma 40Ar/39Ar age of the crater. Late Triassic tectonic reconstructions of the Eurasian and North American plates place this pole within 95% confidence limits of the paleomagnetic pole from the Manicouagan (Canada) meteorite impact crater, which is dated at 214 $\pm$ 1 Ma. Together, these observations reinforce the hypothesis of a Late Triassic, multiple meteorite impact event on Earth

Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies

A number of distinct methodologies are available for determining the oxygen isotope composition of minerals and rocks, these include laser-assisted fluorination, secondary ion mass spectrometry (SIMS)and UV laser ablation. In this review we focus on laser-assisted fluorination, which currently achieves the highest levels of precision available for oxygen isotope analysis. In particular, we examine how results using this method have furthered our understanding of early-formed differentiated meteorites. Due to its rapid reaction times and low blank levels, laser-assisted fluorination has now largely superseded the conventional externally-heated Ni “bomb” technique for bulk analysis. Unlike UV laser ablation and SIMS analysis, laser-assisted fluorination is not capable of focused spot analysis. While laser fluorination is now a mature technology, further analytical improvements are possible via refinements to the construction of sample chambers, clean-up lines and the use of ultra-high resolution mass spectrometers. High-precision oxygen isotope analysis has proved to be a particularly powerful technique for investigating the formation and evolution of early-formed differentiated asteroids and has provided unique insights into the interrelationships between various groups of achondrites. A clear example of this is seenin samples that lie close to the terrestrial fractionation line (TFL). Based on the data from conventional oxygen isotope analysis, it was suggested that the main-group pallasites, the howardite eucrite diogenite suite (HEDs) and mesosiderites could all be derived from a single common parent body. However,high precision analysis demonstrates that main-group pallasites have a Δ17O composition that is fully resolvable from that of the HEDs and mesosiderites, indicating the involvement of at least two parent bodies. The range of Δ17O values exhibited by an achondrite group provides a useful means of assessing the extent to which their parent body underwent melting and isotopic homogenization. Oxygen isotope analysis can also highlight relationships between ungrouped achondrites and the more well-populated groups. A clear example of this is the proposed link between the evolved GRA 06128/9 meteorites and the brachinites. The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples from approximately 110 asteroidal parent bodies (∼60 irons, ∼35 achondrites and stony-iron, and ∼15 chondrites) in our global meteorite collection. However, compared to the likely size of the original protoplanetary asteroid population, this is an extremely low value. In addition, almost all of the differentiated samples (achondrites, stony-iron and irons) are derived from parent bodies that were highly disrupted early in their evolution. High-precision oxygen isotope analysis of achondrites provides some important insights into the origin of mass-independent variation in the early Solar System. In particular, the evidence from various primitive achondrite groups indicates that both the slope 1 (Y&R) and CCAM lines are of primordial significance. Δ17O differences between water ice and silicate-rich solids were probably the initial source of the slope 1 anomaly. These phases most likely acquired their isotopic composition as a result of UV photo-dissociation of CO that took place either in the early solar nebula or precursor giant molecular cloud. Such small-scale isotopic heterogeneities were propagated into larger-sized bodies, such as asteroids and planets, as a result of early Solar System processes, including dehydration, aqueous alteration,melting and collisional interactions

Long-lived magnetism from solidification-driven convection on the pallasite parent body.

Palaeomagnetic measurements of meteorites suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields. Convection on these bodies is currently thought to have been thermally driven, implying that magnetic activity would have been short-lived. Here we report a time-series palaeomagnetic record derived from nanomagnetic imaging of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core is consistent with our measured magnetic field characteristics and cooling rates. Solidification-driven convection was probably common among small body cores, and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System.The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement No. 320750, the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 312284, the Natural Environment Research Council, Fundación ARAID and the Spanish MINECO MAT2011-23791.This is the accepted manuscript. The final version is available from Nature at http://www.nature.com/nature/journal/v517/n7535/full/nature14114.html

Long-lived magnetism on chondrite parent bodies

publisher: Elsevier articletitle: Long-lived magnetism on chondrite parent bodies journaltitle: Earth and Planetary Science Letters articlelink: http://dx.doi.org/10.1016/j.epsl.2017.07.035 content_type: article copyright: © 2017 The Authors. Published by Elsevier B.V.© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). The attached file is the published version of the article

Origin and implications of two Verwey transitions in the basement rocks of the Vredefort meteorite crater, South Africa

International audienceTwo populations of magnetite exist in the shocked basement rocks of the Vredefort meteorite impact crater: one associated with original crustal genesis and metamorphism around 3.0 Ga, and the other related to the impact itself at 2.02 Ga. Pre-impact magnetite is mostly micron to millimeter in size, lying within the multidomain to pseudo-single domain range. The second population of magnetite is less than 10 μm in size and formed within the interstices of planar deformation features or within the reaction rims of biotite, both of which were created during impact. Our study shows that each of these populations possesses specific Verwey transition temperatures: one around 124 K associated with pre-impact magnetite and the other around 102 K associated with impact-related magnetite. The high temperature Verwey transition is attributed to stoichiometric magnetite while the low temperature Verwey transition to non-stoichiometric magnetite. Pre-impact rocks containing both Verwey transitions are ubiquitous throughout the crater. Pseudotachylites formed during impact have a single Verwey transition spanning temperatures from 94 to 111 K. Heating the basement rocks above not, vert, similar 550–600 °C for 3 min or above not, vert, similar 500 °C for 1 h irreversibly modifies the 124 K Verwey transition by shifting it to lower temperatures. Based on these findings, it is possible that no wholesale heating of the crater occurred above 550–600 °C for 3 min or above 500 °C for 1 h during or since the time of impact, although some places of more localized heating are identified. An unresolved problem remains to reconcile these data with temperatures thought to exist in the crust during and after impact

Non-rigid Eurasian plate during the Tertiary evidenced by paleomagnetic data from effusive and sedimentary rocks from Central Asia, and clues on the Inclination anomaly

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Magnetic imaging of the Vredefort impact crater, South Africa

International audienceWhile most impact craters are characterised by negative magnetic anomalies over their central regions, aeromagnetic surveys over the Vredefort meteorite impact crater reveal multiple concentric magnetic patterns with no significant anomaly at its centre. We performed ground magnetic surveys across a portion of a prominent negative magnetic anomaly that extends in a broad semicircular belt about half way into the basement floor of the crater. Magnetic anomalies defined by our data are most often negative and occur over a wide range of wavelengths. The longest wavelength negative anomaly coincides well with aeromagnetic data. We find that this feature is centred over the amphibolite to granulite metamorphic facies transition exposed in the basement floor. The transition zone is analogous to the Conrad discontinuity, observed at depths of about 20 km elsewhere in the Kaapvaal craton. Petrographic studies show a marked increase in the intensity of the impact-related thermal and shock metamorphism at this transition, which we explain by the focusing and defocusing of shock waves at a rheologic interface during impact. We therefore suggest that the magnetic signature at this boundary is caused by a combination of both thermal and shock effects related to the impact event. A numerical model of the long wavelength anomaly suggests that it is underlain by a body of coherently magnetised rock whose direction and intensity are similar to those found in pseudotachylites and impact melts that formed during impact. On the other hand, negative anomalies occurring over smaller (100 to 20 m) wavelengths often do not coincide with the surface geology. These features cannot be modeled using the same criteria as that for the long wavelength anomaly