Lutz Hecht

Reconstruction of the Chicxulub ejecta plume from its deposits in drill core Yaxcopoil-1

Formation conditions of suevite-like impactites from an ∼100 m thick drill core sequence through the Cretaceous-Tertiary Chicxulub crater were reconstructed from empirical data obtained by petrologic and image analytical methods. The temporal evolution of the cratering process from the initial stage of excavation to the collapse of the ejecta plume is evidenced by the petrographic characteristics and modal composition of the suevitic rocks, including the size distribution and shape parameters of melt particles. Emplacement of the lowermost suevitic deposits likely started in the first minute after the impact by the passing ejecta curtain that interacted with the expanding ejecta plume. These ejecta deposits were capped by a tongue of coherent impact melt that was transported outward from the crater center during the collapse of the central uplift ∼5 min after impact. On top of this brecciated impact melt rock, the collapsing ejecta plume deposited air-fall suevites. The basal air-fall unit, Middle Suevite, may have been deposited due to a density current–like clumping of hot debris. With progressive cooling, regions of the ejecta plume were entrained in its collapse that produced vapor condensates, accretionary rims, and different oxygen fugacities. After cooling progressed, atmospheric conditions began to reestablish over the crater and turbulence decreased, supposedly after the first 10 min of initial ejecta plume collapse. This led to a winnowing out of fine matrix material and distinct sorting. However, due to aquatic reworking, only material that was deposited until ∼1 h after cessation of turbulent atmospheric conditions was retained.

Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was <2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic.

Density current origin of a melt-bearing impact ejecta blanket (Ries suevite, Germany)

Melt-bearing clastic deposits (suevites) at impact craters have traditionally been regarded as plume fallout deposits. We present new field, textural, and chemical evidence that the subcircular blanket of suevite at the type locality, the Ries impact crater, Germany, was emplaced by a radial, granular fluid–based particulate density current, analogous to those that form ignimbrites of volcanic origin. Newly mapped chemical zoning patterns in the blanket record the response of the current to changing topography during the earliest modification stages of impact crater formation. The eastern sector of the suevite blanket has a different high field strength element composition than the western sector. The crater-fill facies also shows vertical gradational zoning that records changes in the composition of suevite deposited with time. The lateral zoning is best explained by radial outflow of the density currents, but changes in the crater topography caused the flow directions of the melt-bearing density current to change (return flow). The later convergence of flow paths allowed more thorough mixing in the crater, and is recorded by the more uniform composition of the later deposited upper parts of the crater-fill suevite. Emplacement by density currents is indicated by (1) topography-influenced (ponded) thickness variations of the suevite sheet, (2) very poor sorting, (3) matrix support, (4) massive nature, (5) subtle coarse-tail grading, (6) abundant elutriation pipes, (7) abundance of broken and whole matrix-supported concentric-laminated accretionary lapilli in uppermost parts, and (8) an inverse-graded basal layer with low-angle cross-stratification. These are classic features of deposits from granular fluid–based density currents, such as ignimbrites deposited by pyroclastic density currents at explosive caldera volcanoes, but differ markedly from fallout deposits worldwide. INTRODUCTION Melt-bearing impact breccias (suevites) are one of the most important records of impact cratering, which is a fundamental geological process in the solar system. This rock type records various aspects of impact-induced rock comminution, different degrees of shock-metamorphism including melting, and the dynamics of crater formation (e.g., Stöffler et al., 2013). The way in which suevites (including the type example at Ries crater, Germany) are formed and deposited is not well understood, despite their petrogenetic importance (e.g., Meyer et al., 2011; Stöffler et al., 2013). Previous hypotheses for the origin and/or emplacement of the Ries suevite are (1) collapse of an ejecta plume (e.g., Engelhardt, 1997), (2) deposition via a density flow (Newsom et al., 1990) or lateral flow (Bringemeier, 1994; Meyer et al., 2011), (3) deposition from an impact melt flow (Osinski, 2004), and (4) collapse of post-impact phreatomagmatic plume or plumes caused by fuel-coolant interaction (FCI) of an impact melt sheet with water or an aquifer (Stöffler et al., 2013; Artemieva et al., 2013). This paper presents the results of a study to investigate how the Ries suevite was emplaced, using geochemistry and techniques adapted from physical volcanology. Deposit-scale chemical zoning patterns through the suevite blanket are documented, along with the depositional structures and particle textures. This new approach of combining geochemical zoning and field and textural data facilitates a new interpretation for the emplacement of the Ries suevite. This has implications for our understanding of impact deposits elsewhere. FIELD AND TEXTURAL CHARACTERIZATION OF THE RIES SUEVITE The ca. 15 Ma impact crater at Nördlingen (Ries) in Germany is 26 km in diameter and has features typical of moderate-sized craters (e.g., Wünnemann et al., 2005). The target rocks are a varied crystalline basement overlain by a sedimentary cover as much as 600 m thick. The impact ejecta comprises a lower layer of lithic breccia (the Bunte Breccia) sharply overlain by a clastic deposit (here termed suevite) that contains former melt particles and target-rock lithic clasts. The suevite blanket extends from within to beyond the crater (crater suevite to outer suevite; Stöffler et al., 2013; Fig. 1A). The lower part of the impact melt–bearing deposit in the crater is a coarser grained proximal facies (impact melt breccia) with a remnant vitrophyric matrix between large fluidal-shaped clasts (Reimold et al., 2013), interpreted here to be a welded, coarse-grained proximal facies of the suevite, similar to coarse welded scoria agglomerates in proximal facies of some large ignimbrite sheets (Branney and Kokelaar, 2002, their figure 5.4). The thickness of the suevite ranges from 10 to 400 m, according to the underlying inner ring and central crater basin topography (Pohl et al., 1977). Thickness variations of the outer suevite also correspond with the underlying topography, from 90 m in the megablock zone between the inner ring and the crater rim, and from 20 to 2 m beyond the crater rim. Most of the suevite is massive and nongraded, with former melt particles and angular rock fragments supported in a poorly sorted fine-grained matrix (Fig. 1B). There are local subtle vertical and lateral coarse-tail grading patterns (Branney and Kokelaar, 2002, their figure 5.6). Well-developed subvertical elutriation pipes are abundant in the upper parts (Fig. 1D; Engelhardt et al., 1995), and the lowest 4 cm are locally inverse graded and exhibit diffuse low-angle splay-and-fade cross-lamination (Fig. 1F). Abundant accretionary lapilli occur within the uppermost parts of crater suevite (Graup, 1981; Newsom et al., 1990). CHEMICAL ZONING PATTERNS OF BULK SUEVITE AND CONSTITUENT CLASTS We present the first detailed trace element study of Ries suevite and its various components (Item DR1 in the GSA Data Repository1). The major element composition of the suevite predominantly reflects the crystalline basement 1 GSA Data Repository item 2017285, Item DR1 (outer suevite chemical data), Item DR2 (Ce, Zr suevite whole-rock versus suevite components diagrams), Item DR3 (Th-Nb diagram analogous to Fig. 3), Item DR4 (crater suevite chemical data), Item DR5 (Ce, Zr histograms of Ries impact target lithologies), and Item DR6 (photo locations Fig. 1), is available online at http://www.geosociety.org /datarepository /2017/ or on request from editing@geosociety.org. GEOLOGY, September 2017; v. 45; no. 9; p. 855–858 | Data Repository item 2017285 | doi:10.1130/G39198.1 | Published online 26 July 2017

Chemical and Textural Re-equilibration in the UG2 Chromitite Layer of the Bushveld Complex, South Africa

Variations of mineral chemistry and whole-rock compositions were studied in detail, at millimetre to centimetre intervals, in two vertical drill core profiles through the platiniferous UG2 chromitite layer in the western and eastern limbs of the Bushveld Complex, South Africa. Analytical methods included electron microprobe and LA-ICP-MS analyses of the main rock-forming minerals, orthopyroxene, plagioclase and interstitial clinopyroxene. One profile was also studied by synchrotronsource XRF. Statistical analysis of crystal size distribution of chromite was also performed at different levels in the chromitite layer and in adjacent silicate rocks. The results provide new evidence for chemical and textural late magmatic re-equilibration in the UG2 layer and in the silicate rocks at the contact zones. The chromite crystal size distributions imply extensive coarsening of that mineral within the main chromitite seam, which has erased any textural evidence of primary deposition features such as recharge or mechanical sorting of crystals, if those features originally existed. The mineral compositions in chromitite differ from those in adjacent silicate rocks, in general agreement with predictions of chemical re-equilibration with evolved, residual melt (the trapped liquid shift effect). In detail, the geochemical data imply, however, that the conventional trapped liquid shift model has shortcomings, due to the effects of material transport driven by chemical gradients between modally contrasting layers of crystal mush undergoing reequilibration reactions. In the presence of such gradients, selective open-system conditions may hold for alkalis and hydrogen because of their higher diffusion rates in silicate melts. Differential mobility of components in the interstitial melt can also sharpen the original modal layering by causing minerals to crystallise in one layer and dissolve in another. Detailed trace element profiles by synchrotron XRF reveal an uneven vertical distribution of incompatible elements which implies that the permeability of the chromitite layer may have been significant, even at the latest stages of interstitial crystallization.

Orthopyroxene oikocrysts in the MG1 chromitite layer of the Bushveld Complex: implications for cumulate formation and recrystallisation

Two typical mineral textures of the MG 1 chromitite of the Bushveld Complex, South Africa, were observed; one characterised by abundant orthopyroxene oikocrysts, and the other by coarse-grained granular chromitite with only minor amounts of interstitial material. Oikocrysts form elongate clusters of several crystals aligned parallel to the layering, and typically have subhedral, almost chromite-free, core zones containing remnants of olivine. The core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts. Chromite grains show no preferred crystal orientation, whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation. Oikocryst core zones have lower Mg# and higher concentrations of incompatible trace elements compared to their poikilitic aureoles. Core zones are relatively enriched in REE compared to a postulated parental magma (B1) and did not crystallise in equilibrium with the surrounding minerals, whereas the composition of the poikilitic orthopyroxene is consistent with growth from the B1 magma. These observations cannot be explained by the classic cumulus and post-cumulus models of oikocryst formation. Instead, we suggest that the oikocryst core zones in the MG1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards-percolating melt enriched in incompatible trace elements. Poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of B1 composition near the magma-crystal mush interface. Finally, adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite.

Correlating laser-generated melts with impact-generated melts: An integrated thermodynamic-petrologic approach: Laser Melting of Planetary Materials

Planetary collisions in the solar system typically induce melting and vaporization of the impactor and a certain volume of the target. To study the dynamics of quasi-instantaneous melting and subsequent quenching under postshock P-T conditions of impact melting, we used continuous-wave laser irradiation to melt and vaporize sandstone, iron meteorite, and basalt. Using high-speed imaging, temperature measurements, and petrologic investigations of the irradiation targets, we show that laser-generated melts exhibit typical characteristics of impact melts (particularly ballistic ejecta). We then calculate the entropy gains of the laser-generated melts and compare them with the entropy gains associated with the thermodynamic states produced in hypervelocity impacts at various velocities. In conclusion, our experiments extend currently attainable postshock temperatures in impact experiments to ranges commensurate with impacts in the velocity range of 4–20 km s–1 and allow to study timescales and magnitudes of petrogenetic processes in impact melts.