Alexander Deutsch

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

The Sudbury Structure' Controversial or Misunderstood?

The origins of the Sudbury Structure and associated Igneous Complex have been controversial. Most models call for a major impact event followed by impact-induced igneous activity, although totally igneous models are still being proposed. Much of the controversy is due, in our opinion, to a misunderstanding of the size of the original Sudbury Structure. By analogy with other terrestrial impact structures, the spatial distribution of shock features and Huronian cover rocks at the Sudbury Structure suggest that the transient cavity was ∼100 km in diameter, which places the original final structural rim diameter in the range of 150–200 km. Theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in ∼104 km3 of impact melt, more than sufficient to produce a melt body the size of the Igneous Complex (present volume 4–8 × 103 km3). For the Igneous Complex to be an impact melt sheet it must have a composition similar to that of the target rocks. Evidence for this has been presented previously for Sr and Nd isotopic data, which suggest a crustal origin. Here, we also present new evidence from least squares mixing models that the average composition of the Igneous Complex corresponds to a mix of Archean granite-greenstone terrain, with possibly a small component of Huronian cover rocks. This is a geologically reasonable mix, based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size. The Igneous Complex is differentiated, which is not a characteristic of previously studied terrestrial impact melt sheets. This can be ascribed, however, to its great thickness and slower cooling. That large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted. If this working hypothesis is accepted, namely, that both the Sudbury Structure and the Igneous Complex are impact in origin, then previous hybrid impact-igneous hypotheses can be discarded and the Sudbury Structure can be studied specifically for the constraints it provides to large-scale cratering and the formation of basin-sized (multiring?) impact structures.

Shock experiments on pre-heated α- and β-quartz: I. Optical and density data

Abstract Discs of single crystal quartz, unheated, and pre-heated to 275°C and 540°C (i.e., α-quartz) and 630°C (i.e., β-quartz) were experimentally shocked to pressures ranging from 20 to 40 GPa, with the shock front propagating parallel to either (10 1 0) or (0001). Refractive indices, density and the orientation of planar deformation features (PDFs) were determined on the recovered quartz samples. Refractive indices of pre-heated quartz are unaffected up to 25 GPa but density starts to decrease slightly up to this pressure. Above 25 GPa, pre-heating causes drastic variations: Refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously, until complete isotropization is reached at 35 GPa. In quartz shocked at 630°C, refractivity drops discontinuously in the interval from 25 to 26 GPa, and complete transformation to diaplectic glass is reached at 26 GPa. Density follows the trends demonstrated by the optical parameters, with higher pre-shock temperatures yielding lower density at a given shock pressure. These results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature-invariant, lying at 25 GPa, whereas the pressure limit for complete transformation decreases with increasing pre-shock temperature from ≈ 35 to ≈ 26 GPa. Quartz shocked parallel to (0001) always has a higher density and refractivity than that shocked parallel to (10 1 0), indicating a significant influence of the structural anisotropy. This is also evident from the distribution of PDF orientations. Pressures ⩾ 25 GPa cause, in quartz shocked parallel to (10 1 0), PDFs that are predominantly oriented parallel to {10 1 2}, while quartz shocked to the same pressures but parallel to (0001) contains almost exclusively PDFs parallel to {10 1 3}. PDF orientations in quartz shocked at ambient temperature parallel to (10 1 0) show the following characteristics: (i) The frequency of {10 1 3} is high at 20 GPa, declines with increasing pressure, and finally is totally absent at 32 GPa, (ii) {10 1 3} begins to occur at 25 GPa, increases, and finally is the only remaining orientation at 32 GPa, and (iii) {10 1 2} and {11 2 2} are only present at 20 and 25 GPa. In contrast, quartz shocked at 630°C parallel to (10 1 0) displays a broad PDF distribution with indistinct maxima, and {10 1 3} is totally absent. The small structural difference between α- and β-quartz, which is reflected in the PDF distribution, should allow, in principle, evaluation of the pre-shock temperature in naturally impacted metamorphic rocks. The results substantiate that formation of shock effects in quartz is dependent on (i) shock pressure, (ii) pre-shock temperature and (iii) shock wave direction. So far, the latter two parameters have not been considered in the application of experimental results to impact sites.

The Sudbury Structure (Ontario, Canada): a tectonically deformed multi-ring impact basin

The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.

Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique

Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 µm3, U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al–rich phase may have formed as a result of fluid activity at 500 Ma, and the phases amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data.

The phase diagram of CaCO3 in relation to shock compression and decomposition

Abstract The phase diagram for calcite (CaCO3) is re-evaluated in relation to dynamic compression and following release from shock. Available shock compression data on Hugoniot dynamic measurements, analysis of recovered samples, and observation at terrestrial impact sites are compared with theoretically derived equations of state (EOS) for CaCO3 and its decomposition products CaO and CO2. The study results in a refined phase diagram for CaCO3 in which the major change is the extension of the liquid field of CaCO3. A general outcome of this analysis is that release of CO2 from naturally shocked carbonates to the atmosphere is (grossly) overestimated if based on the calcite phase diagram constructed from thermodynamic equilibrium conditions.

Rb-Sr-analyses of apollo 16 melt rocks and a new age estimate for the imbrium basin: lunar basin chronology and the early heavy bombardment of the moon

Rb-Sr-model ages on 7 impact “glass-bombs” and internal Rb-Sr isochrons for two crystalline impact melt rocks from the Apollo 16 collection have been determined. The post-Cayley “glass-bombs” with model ages between 4.75 ± 0.45 AE and 3.97 ± 0.08 AE can be classified according to their calculated single stage (87Rb86Sr)I-ratios: 67728, 67946, and 67627.8 point to a KREEP-free precursor terrain—the Descartes highlands; whereas 63566, 67567, 67627.10 and 67629 are derived from the more heterogeneous Cayley plains. The very feldspar-rich impact melt rock 65795, which is compositionally similar to the group of feldspathic microporphyritic melt breccias (FM-suite), yields a crystallization age of 3.81 ± 0.04 AE (2σ; λ87Rb = 1.42−11yr−) and ISr of .69929 ± 3. In the subophitic impact melt rock 60635.5 two ages were recorded: One isochron with 3.87 ± 0.02AE and ISr of 0.69920 ± 3 gives the crystallization age of a fine-grained internal clast. At 3.75 ± 0.03 AE this fragment was incorporated into the host lithology of 60635 which is a member of the Anorthositic Noritic Melt Rock (ANMR) group. This group has a crystallization age of about 3.75 AE. The ANMR rocks and several samples of Apollo 14 crystalline impact melt rocks belong to a suite of “young crystalline impact melt rocks” ranging in age from 3.71 to 3.81 AE. These ages are in conflict with the canonical age of the Imbrium event (3.85 AE). It is argued that the young Apollo 14 and 16 melt rocks cannot represent solid exotic fragments or melt splashes ejected from post-Imbrium local or distant craters. Instead, these rocks are derived from coherent impact melt sheets of pre-Imbrian craters with diameters larger than 5–7 km. They are interpreted as clasts of the Fra Mauro and Cayley formation breccia deposits, and therefore their ages are used as an upper limit for the age of the Imbrium event. We suggest that the Imbrium basin and the related Fra Mauro and Cayley formations were formed 3.77 ± 0.02 AE ago and could be even as young as 3.75 AE. As a consequence, we adopt 3.92 ± 0.03 AE, 3.87 ± 0.03 AE, and 3.84 ± 0.04 AE as ages for the Nectaris, Serenitatis, and Crisium basins, respectively, in agreement with the relative crater densities measured on the ejecta blankets of these basins (Wilhelms, 1984). The proposed age sequence leads to an average formation interval for the observed 12–13 Nectarian basins of 7 to 14 m.y. leaving ~30 pre-Nectarian basins of unknown age. These facts suggest that there is no “late terminal lunar cataclysm” in the sense of a culmination of the lunar impact rate at ~3.8 AE ago. Rather, the observations are compatible with a steeply and steadily decreasing flux of impactors (Hartmann, 1980) in the sense of an “early heavy bombardment” which started at the time of the moons accretion and terminated around 3.75 AE ago.

Isotope systematics and shock-wave metamorphism: I. U-Pb in zircon, titanite and monazite, shocked experimentally up to 59 GPa

This study reports the first U-Pb isotope analyses on experimentally shocked zircon, titanite, and monazite extracted from Proterozoic granitoid rocks. In all three types of minerals, shock-waves produce drastic changes in the crystal lattices, causing strong lowering of birefringence, turbidization, and decolorization of the individual grains. Moreover, X-ray patterns indicate transition of the crystals into polycrystalline aggregates of <10−5 mm block-size. Precisely dated grains with concordant or nearly concordant ages were embedded in KBr and shocked at 35, 47.5 and 59 GPa. U-Pb isotope analyses on these grains show that shock metamorphism does not fractionate Pb isotopes within the analytical precision of ± 0.1%. As far as chemical fractionation is concerned, there is no difference in degree of concordancy between shocked and unshocked monazite, and small degrees (< 2%) of relative UPb fractionation in shocked zircon and titanite are due to time-integrated Pb-loss and not to the shock experiment. In consequence, the data document that shock-wave metamorphism alone does not measurably effect the U-Pb chronometer, questioning the view that lower intercept ages of discordant U-Pb data reflect shock-induced re-equilibration of the chronometer in moderately to highly shocked, rapidly cooling rocks.

Young Alpine dykes south of the Tauern Window (Austria): a K-Ar and Sr isotope study

Fortyfive new K-Ar ages and Sr isotope data on amphiboles, biotites, clinopyroxenes and whole rock samples from subvolcanic dykes south of the Tauern Window establish, that alkalibasaltic dykes were intruded 30 m.y. ago and shoshonitic volcanism occured between 30 and 24 m.y. ago. Two calc-alkaline rocks of high-potassium composition yielded ages of 40 and 26 m.y. resp., a spread which may or may not be real. Calc-alkaline dykes with medium and low potassium contain excess argon and are hence undatable. Alkalibasaltic dykes have 87Sr/86Sr ratios of 0.7056–0.7070, shoshonitic rocks 0.7075–0.7133, potassium rich calc-alkaline dykes 0.7077–0.7100. 87Sr/86Sr of all other calc-alkaline rocks scatter between 0.7074 and 0.7150. Sr data indicate that dykes studied do not represent closed Sr systems, but that Sr characteristics result from selective strontium assimilation en route to surface. Primary Sr isotopic ratios of alkalibasaltic dykes point to an origin of these rocks in enriched sub-continental upper mantle. The source region of shoshonitic and high-potassium calcalkaline rocks could have 87Sr/86Sr around 0.707, which is assigned to the input of a component rich in alkalies, LREE and LIL elements. Genetic relationships with other Tertiary magmatites of similar geotectonic position are explained in terms of plate tectonic models of the Eastern Alps.

Shock recovery experiments on dolomite and thermodynamical calculations of impact induced decarbonation

We have studied experimentally shocked dolomites and calcites by scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), X ray diffractometry (XRD) using Rietveld refinements, and mass spectrometry analysis of the abundance ratios of the stable isotopes of carbon and oxygen. The shock-recovery experiments have been perfomed by the multiple reverberation technique on natural dolomite rocks at 60 GPa, using steel devices and highexplosive driver-flyer plates as plane shock wave generators. Modified assemblies with grooves and holes were built in order to facilitate the escape of CO 2 , in case the conditions of breakdown of the carbonates into oxides and CO 2 would be reached during the shock or postshock history of the samples. In contrast with the results from previous studies, almost no evidence for outgassing, expressed, for example, by the identification of CaO or MgO, could be observed. Consequently, no isotope fractionation occured in the shocked samples. This result is consistent with the calculations of peak-shock and postshock temperatures, as well as with the examination of outgassing conditions of carbonates, calculated in this study up to 80 GPa. We have shown that in a direct shock, outgassing in air of nonporous dolomites and calcites should occur in impacts at 55-65 GPa and 35-45 GPa, respectively. The effect of porosity, which strongly lowers these values, has been estimated. The experimental setup for shock-recovery experiments is shown to be an important parameter : reverberated shocks lead to lower peak-shock and postshock temperatures than direct shocks at the same pressure ; differences among experimental setups might explain part of the discrepancies between previous studies. In this study, the only surviving shock-induced phenomenon is pulverization leading to grain sizes smaller than in the starting material. The decrease in grain size has been quantified via a structure refinement by Rietveld analysis of X ray powder patterns, which also allows the estimation of the lattice strains. A future pressure scale of shock effects in carbonates could probably be based on such parameters.