Ludovic Ferrière

Characterisation of ballen quartz and cristobalite in impact breccias: new observations and constraints on ballen formation

Ballen quartz and cristobalite in impactite samples from five impact structures (Bosumtwi, Chicxulub, Mien, Ries, and Rochechouart) were investigated by optical microscopy, scanning electron microscopy (SEM), cathodoluminescence (CL), transmission electron microscopy (TEM), and Raman spectroscopy to better understand ballen formation. The occurrence of so-called “ballen quartz” has been reported from about one in five of the known terrestrial impact structures, mostly from clasts in impact melt rock and, more rarely, in suevite. “Ballen silica”, with either α-quartz or α-cristobalite structure, occurs as independent clasts or within diaplectic quartz glass or lechatelierite inclusions. Ballen are more or less spheroidal, in some cases elongate (ovoid) bodies that range in size from 8 to 214 μm, and either intersect or penetrate each other or abut each other. Based mostly on optical microscopic observations and Raman spectroscopy, we distinguish five types of ballen silica: α-cristobalite ballen with homogeneous extinction (type I); ballen α-quartz with homogeneous extinction (type II), with heterogeneous extinction (type III), and with intraballen recrystallisation (type IV); chert-like recristallized ballen α-quartz (type V). For the first time, coesite has been identified within ballen silica – in the form of tiny inclusions and exclusively within ballen of type I. The formation of ballen involves an impact-triggered solid-solid transition from α-quartz to diaplectic quartz glass, followed by the formation at high temperature of ballen of β-cristobalite and/or β-quartz, and finally back-transformation to α-cristobalite and/or α-quartz; or a solid-liquid transition from quartz to lechatelierite followed by nucleation and crystal growth at high temperature. The different types of ballen silica are interpreted as the result of back-transformation of β-cristobalite and/or β-quartz to α-cristobalite and/or to α-quartz with time. In nature, ballen silica has not been found anywhere else but associated with impact structures and, thus, these features could be added to the list of impact-diagnostic criteria.

The formation of peak rings in large impact craters

Drilling into Chicxulubs formation The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces. Science, this issue p. 878; see also p. 836 Rock samples from IODP/ICDP Expedition 364 support the dynamic collapse model for the formation of the Chicxulub crater. Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Shock Metamorphism of Bosumtwi Impact Crater Rocks, Shock Attenuation, and Uplift Formation

Shock wave attenuation rate and formation of central uplifts are not precisely constrained for moderately sized complex impact structures. The distribution of shock metamorphism in drilled basement rocks from the 10.5-kilometer-diameter Bosumtwi crater, and results of numerical modeling of inelastic rock deformation and modification processes during uplift, constrained with petrographic data, allowed reconstruction of the pre-impact position of the drilled rocks and revealed a shock attenuation by ∼5 gigapascals in the uppermost 200 meters of the central uplift. The proportion of shocked quartz grains and the average number of planar deformation feature sets per grain provide a sensitive indication of minor changes in shock pressure. The results further imply that for moderately sized craters the rise of the central uplift is dominated by brittle failure.

The newly confirmed Luizi impact structure, Democratic Republic of Congo—Insights into central uplift formation and post-impact erosion

Rocks exposed within the uplifted central part of meteorite impact structures come from signifi cant stratigraphic depths, in some cases as much as several kilometers. On Earth, cen- tral uplifts are commonly the fi nal and only feature of an impact crater that remains after the rest of the structure is lost to erosion. However, the crater-forming process that results in the formation of intricate features such as central peak and peak rings is poorly understood. Much of our knowledge is based on extraterrestrial observations; as on Earth, there are very few unequivocal examples of impact craters with well-preserved peak and ring morphologies, because of erosion. In this study we describe the ~17-km-diameter Luizi structure (Katanga region, Democratic Republic of Congo), a moderate-sized complex crater, with an intermedi- ate ring (~5.2 km in diameter), and an ~2-km-wide circular central ring around a central depression. For the fi rst time, unique evidence of shock metamorphism, in the form of macro- scopic shatter cones and multiple sets of microscopic planar deformation features in quartz and feldspar grains, is described, confi rming the meteorite impact origin of the structure. Our observations at Luizi provide insights into the formation of mid-sized impact craters on Earth, adding to the evidence that, in the case of sedimentary target lithologies, structural ring struc- tures within the central uplift may form by the collapse of an unstable central peak. Given the preservation state of the Luizi crater, it cannot be excluded that structural rings may be a common feature for mid-size craters developed in layered target rocks.

Shatter cones: (Mis)understood?

Shatter cones can be used to confirm impact craters and determine their size. Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and “double” cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationship Dsc = 0.4 Da, where Dsc is the maximum spatial extent of in situ shatter cones, and Da is the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.

Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body

Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar systems asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years. (Less)

Chicxulub and the Exploration of Large Peak-Ring Impact Craters through Scientific Drilling

The Chicxulub crater is the only well-preserved peak-ring crater on Earth and linked, famously, to the K-T or K-Pg mass extinction event. For the first time, geologists have drilled into the peak ring of that crater in the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP-ICDP) Expedition 364. The Chicxulub impact event, the environmental calamity it produced, and the paleobiological consequences are among the most captivating topics being discussed in the geologic community. Here we focus attention on the geological processes that shaped the ~200-km-wide impact crater responsible for that discussion and the expedition’s first year results.

High pressure minerals in the Château-Renard (L6) ordinary chondrite: implications for collisions on its parent body

We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyropess, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteorite and the L-chondrites in general.

Rapid recovery of life at ground zero of the end-Cretaceous mass extinction

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth1,2. It was caused by the impact of an asteroid3,4 on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago5, forming the Chicxulub impact crater6,7. After the mass extinction, the recovery of the global marine ecosystem—measured as primary productivity—was geographically heterogeneous8; export production in the Gulf of Mexico and North Atlantic–western Tethys was slower than in most other regions8–11, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning12, on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors—trophic interactions13, species incumbency and competitive exclusion by opportunists14—and ‘chance’8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.Micro- and nannofossil, trace fossil and geochemical evidence from the Chicxulub impact crater demonstrates that proximity to the asteroid impact site did not determine rates of recovery of marine ecosystems after the end-Cretaceous mass extinction.