Is the transition impact to post‐impact rock complete? Some remarks based on XRF scanning,electron microprobe,and thin section analyses of the Yaxcopoil‐1 core in the Chicxulub crater

Abstract The transition from impact to post‐impact rocks in the Yaxcopoil‐1 (Yax‐1) core is marked by a 2 cm‐thick clay layer characterized by dissolution features. The clay overlies a 9 cm‐thick hardground, overlying a 66 cm‐thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro‐conglomerate layers with litho‐ and bioclasts and the altered remains of impact glass, now smectite. The micro‐conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex‐zoned, idiomorphic overgrowth, with Mn‐rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K‐feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni‐rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax‐1 core that can be used to argue against the synchroneity of the end‐Cretaceous mass‐extinctions and the Chicxulub crater.     

Offshore granulites from the Bay of Biscay margins: fission tracks constrain a Proterozoic to Tertiary thermal history

Zircon and apatite fission track ages were determined on granulites dredged along the Bay of Biscay margins. A sample from Ortegal Spur (Iberia margin) yielded 725 ± 67 Ma (zircon). A sample from Le Danois Bank (Iberia margin) yielded 284 ± 58 Ma (zircon), indicating post‐Variscan cooling. Apatite from this sample gave 52 ± 2 Ma, interpreted as final cooling after ‘Pyrenean’ thrust imbrication. Two other samples from Le Danois Bank have Early Cretaceous apatite ages (138 ± 7 and 120 ± 8 Ma), interpreted to result from exhumation during rifting. Finally, a granulite from Goban Spur (Armorican margin) gave 212 ± 10 Ma (apatite), coinciding with a precursory rifting phase. Together with published radiometric results, these data indicate a Precambrian high‐grade terrane at the site of the current margins. The distribution of the granulites on the seafloor reflects tectonic and erosional processes related to (a) Mesozoic rifting and (b) Early Tertiary incipient subduction of the Bay of Biscay beneath Iberia.     

Detailed <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of geologic events associated with the Mantos Blancos copper deposit,northern Chile

The ⁴⁰Ar/³⁹Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit. Because of the superposition of several magmatic and hydrothermal events, the obtained ⁴⁰Ar/³⁹Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma. Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in the metallogenic belt of the Coastal Cordillera of northern Chile. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Coupled redox transformations of catechol and cerium at the surface of a cerium(III) phosphate mineral

Highly insoluble Ce-bearing phosphate minerals form by weathering of apatite [Ca₅(PO₄)₃.(OH,F,Cl)], and are important phosphorous repositories in soils. Although these phases can be dissolved via biologically-mediated pathways, the dissolution mechanisms are poorly understood. In this paper we report spectroscopic evidence to support coupling of redox transformations of organic carbon and cerium during the reaction of rhabdophane (CePO₄·H₂O) and catechol, a ubiquitous biogenic compound, at pH 5. Results show that the oxic–anoxic conditions influence the mineral dissolution behavior. Under anoxic conditions, the release of P and Ce occurs stoichiometrically. In contrast, under oxic conditions, the mineral dissolution behavior is incongruent, with dissolving Ce³⁺ ions oxidizing to CeO₂. Reaction product analysis shows the formation of CO₂, polymeric C, and oxalate and malate. The presence of more complex forms of organic carbon was also confirmed. Near edge X-ray absorption fine structure spectroscopy measurements at Ce-M_4,5 and C-K absorption edges on reacted CePO₄·H₂O samples in the absence or presence of catechol and dissolved oxygen confirm that (1) the mineral surface converts to the oxide during this reaction, while full oxidation is limited to the near-surface region only; (2) the Ce valence remains unchanged when the reaction between CePO₄·H₂O and O₂ but in the absence of catechol. Carbon K-edge spectra acquired from rhabdophane reacted with catechol under oxic conditions show spectral features before and after reaction that are considerably different from catechol, indicating the formation of more complex organic molecules. Decreases in intensity of characteristic catechol peaks are accompanied by the appearance of new π^* resonances due to carbon in carboxyl (ca. 288.5 eV) and carbonyl (ca. 289.3 eV) groups, and the development of broad structure in the σ^* region characteristic of aliphatic carbon. Evolution of the C K-edge spectra is consistent with aromatic-ring cleavage and polymerization. These results further substantiate that the presence of catechol, O₂ (aq) causes both the oxidation of structural Ce³⁺ and the transformation of catechol to more complex organic molecules. Scanning Transmission X-Ray Microscopy measurements at the C K and Ce M_4,5 edges indicate three dominant organic species, varying in complexity and association with the inorganic phase. Untransformed catechol is loosely associated with CeO₂, whereas more complex organic molecules that exhibit lower aromaticity and stronger CO π^* resonances of carboxyl-C and carbonyl-C groups are only found in association with the grains. These results further serve as basis to postulate that, in the presence of O₂, CeO₂ can mediate the oxidative polymerization of catechol to form higher molecular weight polymers. The present work provides evidence for a pathway of biologically-induced, non-enzymatic oxidation of cerium and formation of small CeO₂ particles at room temperature. These findings may have implications for carbon cycling in natural and cerium-contaminated soils and aqueous environments.     

The origin of accretionary lapilli

Experimental investigations in a recirculating wind tunnel of the mechanisms of formation of accretionary lapilli have demonstrated that growth is controlled by collision of liquid-coated particles, due to differences in fall velocities, and binding as a result of surface tension forces and secondary mineral growth. The liquids present on particle surfaces in eruption plumes are acid solutions stable at 100% relative humidity, from which secondary minerals, e.g. calcium sulphate and sodium chloride, precipitate prior to impact of accretionary lapilli with the ground. Concentric grain-size zones within accretionary lapilli build up due to differences in the supply of particular particle sizes during aggregate growth. Accretionary lapilli do not evolve by scavenging of particles by liquid drops followed by evaporation — a process which, in wind tunnel experiments, generates horizontally layered hemispherical aggregates. Size analysis of particles in the wind tunnel air stream and particles adhering to growing aggregates demonstrate that the aggregation coefficient is highly grain-size dependent. Theoretical simulation of accretionary lapilli growth in eruption plumes predicts maximum sizes in the range 0.7–20 mm for ash cloud thicknesses of 0.5–10 km respectively.     

Thermobarometry, Geochronology and the Interpretation of P-T-t Data in the Britt Domain, Ontario Grenville Orogen, Canada

Textural evidence, thermobarometry, and geochronology were usedto constrain the pressure-temperature-time (P—T—t)history of the southern portion of the Britt domain in the CentralGneiss Belt, Ontario Grenville Province. Typical metapeliticassemblages are quartz+plagioclase+ biotite + garnet + kyanite alkali feldspar sillimanite rutile ilmenite staurolite gahnite muscovite. Metatonalitic assemblages have quartz+ plagioclase + garnet biotite + hornblende + rutile + ilmenite.Metagabbroic rocks contain plagioclase + garnet + clinopyroxene+ biotite + ilmenite hornblende rutile quartz. Notabletextural features include overgrowths of sillimanite on kyaniteand of spinel on staurolite. The spinel overgrowths can be modeledby the breakdown of staurolite via the reaction Fe-staurolite= hercynite +kyanite + quartz + H₂O. The decomposition of stauroliteto her-cynite has a steep dP/dT slope and constrains the lateprograde path of a staurolite metapelite. Garnet—Al₂SiO₅—plagioclase—quartz(GASP) barometry applied to metapelitic garnets that preservecalcium zoning reveals a pressure decrease from 11 to 6 kbat an assumed temperature of 700 C. Garnet—plagioclase—ilmenite—rutile—quartzand garnet—clinopyroxene—plagioclase—quartzbarometry is in good agreement with pressures obtained withthe GASP barometer. Geochronologic data from garnet, allanite,and monazite in metapelitic rocks give ages that fall into twogroups, 1–4 Ga and 1.1 Ga, suggesting the presence ofat least two metamorphic events in the area. It is most reasonableto assign the 1.4 Ga age to the high-pressure data and the 1.1Ga age to the lower-pressure data. Collectively the P—T—tdata indicate a complex and protracted history rather than asingle cycle of burial and uplift for this part of the GrenvilleProvince.     

MEASURED UNDERWATER NEAR-FIELD E-PATTERNS OF A PULSED,HORIZONTAL DIPOLE ANTENNA IN AIR: COMPARISON WITH THE THEORY OF THE CONTINUOUS WAVE,INFINITESIMAL ELECTRIC DIPOLE1

At Delft Geotechnics the technique of ground-penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well-defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half-wave dipole which is used as a transmitting antenna (TA). It has a carrier-frequency of about 160 MHz and a pulse repetition-frequency of about 50 kHz. A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal E_φ,- and E_θ-components of the pulses as a function of probe-position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier-transformed, the transverse electric fields E_φ and E_θ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that:

• 1 Far-field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths.
• 2 The attenuation constant of the tapwater used, as deduced from E-field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1).
• 3 E _φ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves.
• 4 In accordance with the theory of the point-dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases.
• 5 The normalized underwater pulse-spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.