Shocked Quartz Grains Research Articles

New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary

THE 200-km-diameter Chicxulub structure1–3 in northern Yucatan, Mexico has emerged as the prime candidate for the Cretaceous/Tertiary (K/T) boundary impact crater3–6. Concentric geophysical anomalies associated with enigmatic occurrences of Upper Cretaceous breccias and andesitic rocks led Penfield and Camargo1 to suspect that this structure was a buried impact basin. More recently, the discovery of shocked quartz grains in a Chicxulub breccia3, and chemical similarities between Chicxulub rocks and K/T tektite-like glasses3–6 have been advanced as evidence that the Chicxulub structure is a K/T impact site. Here we present evidence from core samples that Chicxulub is indeed a K/T source crater, and can apparently account for all the evidence of impact distributed globally at the K/T boundary without the need for simultaneous multiple impacts or comet showers. Shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the K/T boundary ejecta layer7,8. The Chicxulub melt rocks that we studied contain anomalously high levels of iridium (up to 13.5 parts per 109), also consistent with the iridium-enriched K/T boundary layer9. Our best estimate of the crystallization age of these melt rocks, as determined by 40Ar/<39Ar analyses, is 65.2 ±0.4 (1σ) Myr, in good agreement with the mean plateau age of 64.98 ± 0.05 Myr recently reported10. Furthermore, these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity. The only such episode consistent with40Ar/<39Ar constraints is chron 29R, which includes the K/T boundary.

Impacts, Tsunamis, and the Haitian Cretaceous-Tertiary Boundary Layer

The marker bed at the Cretaceous-Tertiary boundary of the Beloc Formation (southern Haiti) contains abundant coarse-grained microtektites and minor amounts of shocked quartz grains in the basal part. The upper part is composed of medium-grained marl with amalgamated microtektite lenses and finer-grained marl lenses disseminated throughout. Field and petrographic observations, and the distribution of planktonic foraminifera suggest that the bed formed from a complex sequence of events. A bolide impact nearby produced microtektites that sett1led to form a nearly pure layer at the base. Vaporized materials with anomalously high extraterrestrial components settled last, along with carbonate sediments. The entire bed became sparsely consolidated. Subsequently, another major disruptive event, perhaps a giant tsunami, partly reworked the initial deposit. Cohesive fragments of the original marker bed mixed with exotic materials were redeposited as lenticular bodies. This process also may have caused further mixing of Cretaceous and Tertiary microfossils, as observed at Beloc and elsewhere.

Evidence for shock wave effect of meteoritic impact

There is material evidence for the existence of shock waves generated by meteoritic impact. This is demonstrated by the pressure and temperature dependent formation processes that exist in the memory of constituent minerals. Stishovite of high-pressure type silica mineral can be observed to be a relict of the compression stage of shock impact. Almost all of the silica minerals evolved from shock impact phenomena are α-quartz crystals of a low-temperature type silica polymorph. By using precise measurements of cell parameters and the corresponding calculated density of the quartz crystal grains collected by an ultrasonic cutter method, shocked quartz grains can be interpreted as the principle relict of meteoritic impact from natural impact craters, artificial impact craters and Cretaceous-Tertiary (K/T) boundary samples.

Cretaceous/Tertiary boundary of DSDP Site 596, South Pacific

Sediments from the Cretaceous/Tertiary (K/T) boundary at Deep Sea Drilling Project Site 596 record accumulation of pelagic clays at about lat 45°S,in the center of the ancestral South Pacific. The K/T boundary in this region is characterized by oxidized, thoroughly bioturbated sediments containing large amounts of Ir and relict high-pressure and high-temperature mineral phases. The net Ir fluence (320 ng cm -2 ) is among the largest on Earth,and the number of shocked quartz grains >30 μm in size (∼1800 cm -2 ) exceeds that in all localities outside of North America.

Shock-induced microdeformations in quartz and other mineralogical indications of an impact event at the Cretaceous-Tertiary boundary

The event terminating the Cretaceous period and the Mesozoic era caused massive extinctions of flora and fauna worldwide. Theories of the nature of this event can be classed as endogenic (volcanic, climatic, etc.) or exogenic (extraterrestrial causes). Mineralogical evidence from the boundary clays and claystones strongly favor the impact of an extraterrestrial body as the cause of this event. Nonmarine KT boundary claystones are comprised of two separate layers—an upper layer composed of high-angle ejecta material (shocked quartz, altered glass and spinel) and a basal kaolinitic layer containing spherules, clasts, and altered glass, together with some shocked grains. Recognition of this dual-layered nature of the boundary clay is important for the determination of the timing and processes involved in the impact event and in the assignment and interpretation of geochemical signatures. Multiple sets of shock-induced microdeformations (planar features) in quartz grains separated from KT boundary clays provide compelling evidence of an impact event. This mineralogical manifestation of shock metamorphism is associated worldwide with a large positive anomaly of iridium in these boundary clays, which has also been considered indicative of the impact of a large extraterrestrial body. Global distributions of maximum sizes of shocked quartz grains from the boundary clays and the mineralogy of the ejecta components favor an impact on or near the North American continent. Spinel crystals (magnesioferrite) occur in the boundary clays as micrometer-sized octahedra or skeletal forms. Their composition differs from that of spinels found in terrestrial oceanic basalts. Magnesioferrite crystals are restricted to the high-angle ejecta layer of the boundary clays and their small size and skeletal morphology suggest that they are condensation products of a vaporized bolide. Hollow spherules ranging up to 1 mm in size are ubiquitously associated with the boundary clays. In nonmarine sections, where a high-angle ejecta layer and an underlying kaolinitic layer can be distinguished, the spherules are found only in the kaolinitic layer. The morphologies and surface features of these spherules suggest that they are original forms, and not secondary growths or algal bodies. These impact spherules closely resemble microtektites in size and shape. All of these features of the boundary clay are uniquely associated with impact, and cannot have been formed by volcanic or other terrestrial processes.

Impact production of C02 by the Cretaceous/Tertiary extinction bolide and the resultant heating of the Earth

Evidence at the Cretaceous/Tertiary boundary suggests that the proposed 'extinction' bolide struck a continental or shallow marine terrane. This evidence includes: shocked quartz and feldspar grains found in the boundary layer inherited from a range of rock types; a high ^(87)Sr/^(86)Sr ratio in some planktonic fossils which could reflect continental-derived Sr(ref.5); and evidence that the platinum-group-element-rich clay layer is underlain (at some localities) by a deposit of possible tsunamic origin. These observations and data demonstrate that sea level at the end of the Cretaceous was ~150-200 m higher than at present, suggesting the possibility that the extinction bolide struck a shallow marine carbonate-rich sedimentary section. Here we show that the impact of such a bolide (~5km in radius) onto a carbonate-rich terrane would increase the CO_2 content of the atmosphere by a factor of two to ten. Additional dissolution of CO_2 from the ocean's photic zone could release much larger quantities of CO_2. The impact induced release of CO_2, by itself, would enhance atmospheric greenhouse heating and give rise to a worldwide increase in temperature from 2 K to 10 K for periods of 10^4 to 10^5 years.

Shocked quartz grains at the Cretaceous-Tertiary boundary

Shocked quartz grains at the Cretaceous-Tertiary boundary

Analytical chemists and dinosaurs

The role of the analytical chemist in the development of the extraterrestrial impact theory for mass extinctions at the terminal Cretaceous Period is reviewed. High iridium concentrations in Cretaceous/Tertiary boundary clays have been linked to a terrestrial impact from an iridium-rich asteroid or large meteorite some 65 million years ago. Other evidence in favour of the occurrence of such an impact has been provided by the detection of shocked quartz grains originating from impact and of amorphous carbon particles similar to soot, derived presumably from wordwide wildfires at the terminal Cretaceous. Further evidence provided by the analytical chemist involves the determination of isotopic ratios such as 144Nd/143Nd, 187Os/186Os, and 87Sr/86Sr. Countervailing arguments put forward by the gradualist school (mainly palaeontological) as opposed to the catastrophists (mainly chemists and geochemists) are also presented and discussed.

Studies on the lattice distortion and substructures of shocked lamellae in naturally shocked quartz

One unshocked and 9 naturally shocked single quartz crystal grains with 1–6 sets of shock lamellae from the Ries, West Germany, and the Lake Lappajarvi, Finland, covering a range from unshocked quartz withNo = 1.544 to nearly completely glassy quartz withNo = 1.461 have been used for X-ray precession and Laue investigations. Four of the shocked grains have preliminarily been studied under a transmission electron microscope. It is found that quartz havingNo less than 1.539 shows intensive anisotropic cell expansion and lattice disordering which gradually increase asNo decreases. Shock-induced lattice distortion of quartz is clearly shown on both precession and Laue photographs. For the weakly shocked quartz (p < 200 kb) slight to pronounced spreading of spots is observed. When the pressure reaches 200 kb, both concentric spreading of spots having long ‘tails’ and concentric rings (powder pattern) are revealed on the same photograph, which means that besides a part of single crystal there also exist randomly oriented tiny ‘fragments’ of quartz in this shocked quartz grain. As pressure increases from 230 to 315 kb, more and more crystalline puases in the quartz grains have transformed from solid state into silica glass, and the concentric rings and the long ‘tails’ disappear and the spot spreading becomes slight again, but reflection intensities become much lower in comparison with those of weakly shocked quartz.

Shocked Quartz in the Cretaceous-Tertiary Boundary Clays: Evidence for a Global Distribution

Shocked quartz grains displaying planar features were isolated from Cretaceous- Tertiary boundary clays at five sites in Europe, a core from the north-central Pacific Ocean, and a site in New Zealand. At all of these sites, the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice. The grains display streaking indicative of shock in x-ray diffraction photographs and also show reduced refractive indices. These characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the Cretaceous-Tertiary boundary distributed ejecta products in an earth-girdling dust cloud, as postulated by the Alvarez impact hypothesis.

Mineralogy and Petrology of the Cretaceous-Tertiary Boundary Clay Bed and Adjacent Clay-Rich Rocks, Raton Basin, New Mexico and Colorado

ABSTRACT The Cretaceous-Tertiary (K-T) is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (Late Cretaceous and Paleocene) at several sites in the east-central part of the Raton Basin. The K-T occurs at the top of a kaolinitic claystone commonly referred to as the boundary clay layer, in an interval of coal and carbonaceous shale. The is defined by the disappearance of certain fossil-pollen taxa. The clay layer also contains shocked quartz grains and abundance anomalies of iridium, chromium, and other elements. Each of these characteristics support the hypothesis of an asteroid impact at the end of the Cretaceous. Small goyazite spheres were also found in the clay bed; however, their origin is ncertain. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thin-section petrography have shown that the clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay-rich beds in coal-bearing rocks of the Raton Basin. The clay bed is composed primarily of well-crystallized kaolinite but also contains subordinate, randomly interstratified, mixed-layer illite/smectite, both of which appear to have been formed by the alteration of vitreous material in a coal-swamp environment. The bulk mineralogy of the clay bed is similar to that of other kaolinite-rich clay beds (tonsteins) that altered in coal swamps from airfall volcanic ash. The alteration process and diagenetic products of the host material in the cl y bed, however, differed from those of the tonsteins, resulting in a unique petrographic fabric. The clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards, bubbles, and spheres. The original fallout material of the clay bed first altered to cabbage-like microspherules, indicating a halloysite or allophane precursor, and then to well-crystallized kaolinite; the well-crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions. In comparison, the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite. Alteration of the original fallout material in the bed was probably rapid, leading to hydrated, disordered, fine-grained clay minerals that formed into curled spherical or tubular shapes. The facto s that produced these unique fabrics in the clay bed existed at all K-T sites in the Raton Basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins. The distinctive character of the clay may be due to the unique nature and chemistry of the fallout material and (or) physical or chemical conditions existing shortly after its deposition.

The variation range of optical constants and distribution characteristics of shock lamellae in shock-metamorphosed quartz

Two moderately shocked rock samples collected from the Ries Crater, West Germany (granite—gneiss sample RC-647-29 and biotite-granite sample RP-627-55) and two weakly shocked pegmatite samples (Lj-711-12 and Lj-711-5) taken from Lake Lappajarvi, Finland, have been optically studied to establish the variation range of optical constants and distribution characteristics of shock lamellae in shocked quartz.It has been found that sample RC-647-29 contains shocked quartz grains with the average refractive index ranging from 1.4612 to 1.5331, and sample RP-627-55 from 1.5002 to 1.4669, i.e., they cover a wide range of shock pressures. As for the larger quartz grains in samples Lj-711-12 and Lj-711-5, the variation range of the average refractive indices are smaller than those of samples from the Ries Crater. Hence the estimation of degree of shock must est with the investigation of a set of representative shocked quartz crystals from a single shocked rock sample.The optical data on shocked quartz indicate that the degree of shock is highly independent of the number of shock lamellae sets and their orientations; the most sensitive optical indicator is the index of refraction.On the basis of TEM investigations of single crystal grains of shocked quartz differing in refractive index, three mechanisms of formation of shock lamellae have been established: host quartz crystals with lamellae having closely spaced dislocations; host quartz crystals with lamellae of randomly oriented fine grains of quartz; and host quartz crystals or their residual fragments with lamellae of silica glass.

Volcanos and Extinctions: Erupting of the Impact Idea?

Volcanos and Extinctions: Erupting of the Impact Idea?

Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary

A thin claystone layer found in nonmarine rocks at the palynological Cretaceous-Tertiary boundary in eastern Montana contains an anomalously high value of iridium. The nonclay fraction is mostly quartz with minor feldspar, and some of these grains display planar features. These planar features are related to specific crystallographic directions in the quartz lattice. The shocked quartz grains also exhibit asterism and have lowered refractive indices. All these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth. The shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile.

Geomedical research in Ernst Rodenwaldt's scientific work

Many craters in the Südliche Frankenalb between Eichstätt and Regensburg have been morphologically identified. The impact of a cluster of meteoritic material is concluded from the abundant breccias found in connection with the craters. These breccias contain shocked quartz grains. Some of the craters are filled with Uppermiocene sediments. This and other similarities make it probable that the craters and the brecciation synchronize with the Ries event. The primarily different rocks from Frankenalb offer new facts for the interpretation of the impact mechanism. On the other hand, they bring many new questions which can be answered only by further investigations.