Lithic Clasts Research Articles

Roman Wall Paintings: Characterisation of Plaster Coats Made of Clay Mud

This paper reports on the mineralogical characterisation of samples of wall paintings from various Roman sites in Lombardy (Italy), revealing recurrent types of stratigraphy. One of the stratigraphic samples analysed was found to be a particular kind of plaster: a three-coat work featuring two coats made of clay mud, found in the site of Santa Maria alla Porta (area of the Imperial Palace of Milan—first century CE). The fragments were analysed using optical microscopy on thin sections, X-ray diffraction, scanning electron microscopy with an energy-dispersive spectrometer and infrared spectroscopy, also in non-invasive external reflection mode (7500–375 cm−1). The most interesting feature found was the finish coat made of clay mud (illite, chlorite, kaolinite and fine quartz) with a few coarse clasts and linear cavities. This clay coat was the first example ever detected in Roman Lombardy and was used in combination with a thin painted coat made of clay mud with coarse clasts together with a blue pigment (Egyptian blue) and a render coat made of lime associated with lithic clasts (sand). Our findings brought to light a particular construction technique, since in the historical sources clay is only recommended for daubing on reeds and as a render coat.

Recognizing subsurface breccias in Archean terranes: Implications for district scale metallogeny

Breccias are common in ancient and modern volcanic terranes, where they form at and below surface through volcanic, hydrovolcanic, magmatic, or tectonic mechanisms. They are critical for volcanic reconstruction as stratigraphic markers and indicators of geodynamic change and since they can be associated with mineralization their genesis is also important from an exploration perspective. Their origin can be difficult to ascertain in ancient terranes that have undergone polyphase deformation and associated metamorphism. The Joliet Breccia is a subeconomic Cu-Ag prospect within the Neoarchean Rouyn-Noranda mining district, in the Abitbi greenstone belt, Canada. It was previously interpreted as a phreatic breccia formed on the seafloor. This study presents new data indicating that the Joliet Breccia is a subsurface magmatic-hydrothermal breccia. Specifically, the recognition of gradational contacts with host rocks and between internal breccia domains, lack of sedimentary features and the spatial and temporal association with a tonalite intrusion strongly supports this interpretation. The angular, poorly sorted, lithic clasts derived from the immediate host rocks, hydrothermal cement, complete absence of a rock flour matrix, and presence of a radial and concentric fracture pattern extending into the host rocks, further support a subsurface magmatic-hydrothermal emplacement mechanism.A new TIMS U-Pb zircon age of 2698.0 ± 0.9 Ma (2σ) for a tonalite block within the breccia constrains the maximum age of brecciation. Four LA-ICP-MS U-Pb dates on hydrothermal monazite (2693.7 ± 1.0/8.9 Ma; 2696.9 ± 0.45/8.9 Ma; 2698.7 ± 1.4/8.9 Ma; 2701.2 ± 0.33/8.9 Ma; 2 s/2ssys) found in the cement of the Breccia indicate brecciation and mineralization occurred shortly after the emplacement of the tonalite. Similarly, the ca. 2697 Ma St. Jude intrusive breccia indicates a localized ca. 2699–2695 Ma magmatic-hydrothermal event superimposed on ca. 2704–2701 Ma strata. These breccias are temporally correlative with the youngest units of the Blake River Group strata and associated ca. 2698–2695 Ma volcanogenic massive sulfide (VMS) deposits, which suggests an indirect genetic link between these two hydrothermal systems. The VMS deposits may represent the near surface, distal manifestations of a deeper magmatic-hydrothermal system akin to the porphyry-intermediate sulfidation epithermal continuum in modern subaerial volcanic arcs.

Lunar meteorite Northwest Africa 11962: A regolith breccia containing records of titanium‐rich lunar volcanism and the high alkali suite

AbstractNorthwest Africa (NWA) 11962 is a lunar regolith breccia composed of a wide range of different clasts. The lunar origin of this meteorite is confirmed by oxygen isotope analysis and the Fe/Mn ratio in pyroxene and olivine. In the present study, the clasts and the matrix of NWA 11962 are characterized by optical and electron microscopy along with electron microprobe analyses and micro‐Raman spectroscopy. The meteorite has a glassy impact melt matrix, which accounts for 35% of the surface area in the two thin sections examined, and which contains a very large variety of different lithic clasts, monomineralic clasts, and glass fragments. The presence of volcanic and impact‐related glass spherules led to the classification of this meteorite as a regolith breccia. Lithic clasts include numerous fragments of quartz monzogabbro and lunar felsite, which are quite rare lithologies in the lunar alkali suite. However, the most abundant components in the breccia are gabbroic clasts. The mineral chemistry of the pyroxenes in the gabbroic clasts and the chemistry of various types of glass fragments in the breccia indicate an origin of the regolith from an area with low‐Ti to high‐Ti mare basalt volcanism. In addition to the peculiar petrographic characteristics of NWA 11962, the possible pairing relationships with other lunar meteorites are discussed.

Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting

Granular neoblastic zircon (ZrSiO4) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200°C). It is widely agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with [110] axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO2) and amorphous silica melt. Tetragonal and cubic ZrO2 also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt.

Extending the range of optically stimulated luminescence dating using vein-quartz and quartzite sedimentary pebbles

The feasibility of applying optically stimulated luminescence (OSL) techniques to obtain estimates of the burial age of lithic clasts in the form of pebbles of quartzose lithology is investigated in this study. We applied OSL measurement procedures to vein-quartz and quartzite pebbles from contexts at five sites with fluvial and coastal beach contexts of depositional age ranging from 500 ka to the mid Holocene. As in previous OSL studies with lithics, measured depth-dose profiles were interpreted to identify regions associated with key stages in the burial history of the pebbles, including the extent of pre-burial optical resetting, primary burial and any subsequent secondary stages of exposure to sunlight. While some of the pebbles had not been fully optically reset before burial, none were in field saturation, as commonly encountered with larger lithic clasts such as cobbles. The relatively low concentrations of radionuclides within vein-quartz and quartzite pebbles simplified the assessment of the radiation dose rate. OSL age estimates consistent with independent dating evidence were obtained for the three sites sampled in the UK (Langford Quarry, Leet Hill Quarry, Swanscombe Skull Site and Barnfield Pit) and one of two sites tested in Portugal (Forte Cão), whereas evidence of secondary optical bleaching detected in pebbles from a second site (Vale de Atela) underestimated the independent age for primary burial. The oldest depositional age of the contexts tested was ~500 ka (Leet Hill Quarry), and for some of the pebbles tested there was the capacity to extend the potential dating range further as the age equivalence of the limiting value of D e was in excess of 1 Ma. The advantages of working with smaller, portable, clasts in the form of pebbles and with a lithology of higher transparency opens up a wide range of potential applications, whether the depositional processes are environmental or anthropogenic in origin or modification. • New approach to the dating of Quaternary sediments containing vein-quartz and quartzite pebbles. • OSL techniques applied to sliced pebble cores produce depth-dose profiles that are related to optical exposure history. • Successful comparison of OSL ages with independently dated MIS 12, MIS 11, MIS 5 and Holocene depositional contexts. • Low dose rates for vein-quartz and quartzite pebbles potentially enable further extension of the dating range.

Presolar Silicate and Oxide Grains Found in Lithic Clasts from Isheyevo and the Fine-grained Matrix of Northwest Africa 801

Abstract We report on the discovery of 33 oxygen-anomalous grains from the CH3/CBb3 chondrite Isheyevo and the CR2 chondrite Northwest Africa (NWA) 801. Oxygen isotopic compositions indicate the origin of the majority grains in stellar outflows of low-mass (∼1.2 to ∼2.2 M ⊙), solar-metallicity red giant or asymptotic giant branch stars, while highly 17O-enriched grains probably have nova origins. Isotopic compositions of the eight 18O-rich grains, including an extremely 18O-rich grain (∼16 times solar 18O/16O ratio), are reproduced by zone mixing of SNe II ejecta. Close-to-normal silicon, magnesium, and calcium isotopic compositions of grains are consistent with the isotope exchange in the interstellar medium or the meteorite parent body, while two grains with Si isotopic anomalies and one grain with Mg isotopic anomalies reflect the Galactic chemical evolution. An Isheyevo clast showed several hot spots with moderate to high 15N enrichments, including a hot spot with an extreme 15N excess of (7225 ± 316)‰. However, no correlation between 15N enrichment and presolar oxygen-rich grain abundance is found. Grains with elliptical shapes probably indicate primary condensation feature. Two complex grains possibly display decoupling of the isotopic and elemental compositions in the grain formation environments. The low silicate-to-oxide abundance ratio for the fine-grained chondrule rims in NWA 801 likely reflects the preferential destruction of silicates due to terrestrial weathering. In NWA 801, the presolar O-rich grain abundance in fine-grained chondrule rims is higher than in the interchondrule matrix, similar to the trend observed for some aqueously altered chondrites of petrologic type 2.

X-ray computed microtomography of diamondiferous impact suevitic breccia and clast-poor melt rock from the Kara astrobleme (Pay-Khoy, Russia)

Abstract X-ray computed microtomography (CT) of impact rock varieties from the Kara astrobleme is used to test the method’s ability to identify the morphology and distribution of the rock components. Three types of suevitic breccias, clast-poor melt rock, and a melt clast from a suevite were studied with a spatial resolution of 24 µm to assess CT data values of 3D structure and components of the impactites. The purpose is first to reconstruct pore space, morphology, and distribution of all distinguishable crystallized melt, clastic components, and carbon products of impact metamorphism, including the impact glasses, after-coal diamonds, and other carbon phases. Second, the data are applied to analyze the morphology and distribution of aluminosilicate and sulfide components in the melt and suevitic breccias. The technical limitations of the CT measurements applied to the Kara impactites are discussed. Because of the similar chemical composition of the aluminosilicate matrix, glasses, and some lithic and crystal clasts, these components are hard to distinguish in tomograms. The carbonaceous matter has absorption characteristics close to air, so the pores and carbonaceous inclusions appear similar. However, X-ray microtomography could be used to prove the differences between the studied types of suevites from the Kara astrobleme using structural-textural features of the whole rock, porosity, and the distributions of carbonates and sulfides.

What lithic clasts and lithic-rich facies can tell us about diatreme processes: An example at Round Butte, Hopi Buttes volcanic field, Navajo Nation, Arizona

Round Butte (Hopi Buttes volcanic field, Arizona) exposes a diatreme 170–190 m across, 190 m below the pre-eruptive surface. The central part of the massif is 130–150 m in diameter, displaying 20–30 m-high subvertical cliffs. The well-known layer-cake stratigraphy of the sedimentary rocks of the Colorado Plateau permits identification of the largest lithic fragments preserved in the Round Butte diatreme. We define three main groups of pyroclastic facies: undisturbed beds, disturbed beds and non-bedded rocks. Two other minor facies groups were mapped: megablocks (blocks over 2 m in long axis), and small-volume debris avalanche deposits. Pyroclastic megablocks are finer grained and richer in lithic clasts than most diatreme rocks surrounding them. These pyroclastic megablocks are interpreted as subsided portions of the maar ejecta ring. Sedimentary megablocks originate either from above, or from the same level, relative to their current location, i.e. no megablock has a net upward displacement. Small-volume debris avalanche deposits are poorly sorted deposits resulting from gravitational destabilization of the surrounding country rocks into the syn-eruptive crater. Small-volume debris avalanches and individual megablock collapse are the main ways in which the crater grew in size laterally during the eruption.We combine the componentry of the disturbed bedded pyroclastic facies, the non-bedded pyroclastic facies and the pyroclastic megablocks with a series of conceptual models for country rock fragmentation. This exercise further allows us to estimate diatreme wall slopes of 70° below the Bidahochi Formation to approximately the depth of the root zone around 440 m below the pre-eruptive surface. Lithic fragments at the current level of exposure come from elevations up to 190 m above (i.e., up to the pre-eruptive surface) and up to 250 m below (i.e., down to the root zone) their current locations. Pyroclastic units displaying the richest content of lithic clasts with a deep origin are typically the non-bedded facies interpreted to have formed from debris jets during the eruption.

Geochemistry of bentonite clays from the Eastern Rhodopes (SE Bulgaria): preliminary results

In this work we report data from LA-ICP-MS analyses of 7 bentonite samples from the Eastern Rhodopes as three of the samples come from two bentonite deposits – Zlatna Livada and Propast-Dobrovolets. The obtained results show no significant accumulation of trace elements in the studied samples. Their overall chemical composition is strongly influenced by the volcanic glass chemistry, the presence and abundance of magmatic minerals and lithic clasts and to lesser extent by secondary authigenic mineral associations. Respectively, their trace element signature is similar to that of the unaltered extension-related orogenic volcanic rocks in the Eastern Rhodopes. No correlation between any of the trace elements measured and the relative montmorillonite abundance (in samples A-ZL and I-PD) have been established. Nevertheless, these data complete the knowledge of bentonite clays from the Eastern Rhodopes; they can shed more light on the connection between volcanic precursors and alteration products and can provide deeper insight into the process of glass transformation.

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence (µXRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65°/{112}) are also found inside the vein, and twinned monazite (180°/[101]) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.

Geological evidence of high-energy marine flooding events on the Tangier coastal plain, Morocco

The coastal sedimentary record is currently being studied to investigate evidence of past extreme coastal flooding events. Extreme marine inundations along Morocco's coasts are described in historical documents. In this work, evidence of high-energy marine flooding events on the Tangier coastal plain (the Ain Guemout area) was obtained by studying the size and mineralogy of quartz grains, performing exoscopy of the grains, and reconstructing their possible source sediments. In Ain Guemout, the inundation had run-up heights of more than 10 m and deposited distinct metric boulders, broken shells, sand, and lithic clasts 1 km inland. These unusual deposits typically underwent abrupt basal contact with a paleosoil. The deposits were found to thin and fine landward and present major structural and significant lateral variation. Textural analysis showed that clay and coarse sand are the most abundant grain populations in the sediments. The sand fraction is moderately or poorly sorted with a few well-sorted sandy deposits. In contrast to possible source sediments (coastal dune and beach sand), microtextural analysis indicated the recurrent presence of fresh surfaces and percussion marks. The deposits showed similarities to beach and coastal dune materials in their assemblages of heavy minerals, and presented a decreasing trend in such assemblages up-unit. The altitude reached by the sand layers, the increased boulder size inland, and the abundance of rip-up clasts in the sediments demonstrate the tsunamigenic origin of the Ain Guemout deposits. These results improve our understanding of marine floods, and should aid the development of strategies to protect coastal environments.

The old, unique C1 chondrite Flensburg – Insight into the first processes of aqueous alteration, brecciation, and the diversity of water-bearing parent bodies and lithologies

On September 12, 2019 at 12:49:48 (UT) a bolide was observed by hundreds of eye-witnesses from the Netherlands, Germany, Belgium, Denmark and the UK. One day later a small meteorite stone was found by accident in Flensburg. The presence of short-lived cosmogenic radionuclides with half-lives as short as 16 days proves the recent exposure of the found object to cosmic rays in space linking it clearly to the bolide event. An exceptionally short exposure time of ∼5000 years was determined. The 24.5 g stone has a fresh black fusion crust, a low density of <2 g/cm3, and a magnetic susceptibility of logχ = 4.35 (χ in 10−9 m3/kg). The rock consists of relict chondrules and clusters of sulfide and magnetite grains set in a fine-grained matrix. The most abundant phases are phyllosilicates. Carbonates (∼3.9 vol.%) occur as calcites, dolomites, and a Na-rich phase. The relict chondrules (often surrounded by sulfide laths) are free of anhydrous silicates and contain abundant serpentine. Lithic clasts are also surrounded by similar sulfide laths partly intergrown with carbonates. 53Mn-53Cr ages of carbonates in Flensburg indicate that brecciation and contemporaneous formation of the pyrrhotite-carbonate intergrowths by hydrothermal activities occurred no later than 4564.6 ± 1.0 Ma (using the angrite D'Orbigny as the Mn-Cr age anchor). This corresponds to 2.6 ± 1.0 or 3.4 ± 1.0 Ma after formation of CAIs, depending on the exact absolute age of CAIs. This is the oldest dated evidence for brecciation and carbonate formation, which likely occurred during parent body growth and incipient heating due to decay of 26Al.In the three oxygen isotope diagram, Flensburg plots at the 16O-rich end of the CM chondrite field and in the transition field to CV-CK-CR chondrites. The mass-dependent Te isotopic composition of Flensburg is slightly different from mean CM chondrites and is most similar to those of the ungrouped C2 chondrite Tagish Lake. On the other hand, 50Ti and 54Cr isotope anomalies indicate that Flensburg is similar to CM chondrites, as do the ∼10 wt.% H2O of the bulk material. Yet, the bulk Zn, Cu, and Pb concentrations are about 30% lower than those of mean CM chondrites. The He, Ne, and Ar isotopes of Flensburg show no solar wind contribution; its trapped noble gas signature is similar to that of CMs with a slightly lower concentration of 20Netr.Based on the bulk H, C, and N elemental abundances and isotopic compositions, Flensburg is unique among chondrites, because it has the lightest bulk H and N isotopic compositions of any type 1 or 2 chondrite investigated so far. Moreover, the number of soluble organic compounds in Flensburg is even lower than that of the brecciated CI chondrite Orgueil.The extraordinary significance of Flensburg is evident from the observation that it represents the oldest chondrite sample in which the contemporaneous episodes of aqueous alteration and brecciation have been preserved. The characterization of a large variety of carbonaceous chondrites with different alteration histories is important for interpreting returned samples from the OSIRIS-REx and Hayabusa 2 missions.

The Bigrlyi Tabular Sandstone-Hosted Uranium–Vanadium Deposit, Ngalia Basin, Central Australia

The Bigrlyi deposit is a tabular, sandstone-hosted, uranium–vanadium deposit of Carboniferous age located in the Ngalia Basin of central Australia. The deposit is similar to the continental, fluvial Saltwash-type of sandstone-hosted U-V deposits which are well known from the Colorado Plateau, USA. Most mineralization at Bigrlyi occurs as thin, multiple-stacked, stratiform lenses at the base of fluvial channels near the contact between a grey sandstone succession and a hematitic, purple–red sandstone succession. A larger halo of lower grade vanadium mineralization extends beyond the main U-V-mineralized zone. The host is an immature, feldspathic sandstone, grading into arkose and lithic-rich variants. Lithic ‘rip-up’ clasts of clay-rich sediments are common in the basal parts of fluvial channels, and are frequently the focus of, and have acted as sites for, U-V mineralization. Coffinite and uraninite are the main uranium minerals, with the former dominant. Vanadium is mainly hosted by Fe-V-bearing clays and chlorite, including roscoelite, grading into vanadian illite, the interlayer mineral corrensite, and altered detrital biotite. The V-Fe–oxyhydroxide minerals montroseite, haggite and doloresite, and altered detrital Fe-Ti oxides, are minor V-hosts. Mineralized zones correlate with enrichments in Se, Li, Ba, Be, Mo, Mg and Fe, and elevated Se/S ratios are characteristic of U-mineralized zones. Petrographic studies show that a heterogeneous mixture of variably mineralized lithic clasts is present; in the same rock, some clasts are Fe-rich and only weakly U-V-mineralized, while other clasts are strongly V- and/or U-mineralized. These observations point to mineralization processes that did not take place in-situ in the host sandstone at the site of deposition as required by conventional groundwater models. Lead isotope results provide evidence of the open-system mobility of radiogenic elements in parts of the deposit. In V-bearing zones, radiogenic Pb contents were found to be unsupported by current U levels, suggesting that over time U has been mobilized from these zones and redistributed, resulting in U-enrichment in other parts of the deposit. Mobility pathways were likely open over time from early in the history of the Bigrlyi deposit. A hybrid mineralization model, involving an interplay between solution-precipitation processes, detrital transport and post-depositional U remobilization, is proposed for Bigrlyi. Ferrous-ion-bearing clay minerals and pyrite are considered to be the most likely primary reductants/adsorbents, while the deposit is lacking carbonaceous matter.

Brittle Deformation of Carbonated Peridotite—Insights From Listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)

AbstractHole BT1B of the Oman Drilling Project provides a continuous sampling from listvenite into the metamorphic sole that preserves the deformation, hydration, and carbonation processes of oceanic mantle peridotite at the base of the Samail ophiolite, Oman. We present evidence of multistage brittle deformation in listvenites and serpentinites based on field observations, visual core logging and petrography. About 10 vol% of listvenite and serpentinite in Hole BT1B is composed of cataclasite bands. Cataclasites contain lithic clasts of listvenite with spheroidal, zoned magnesite and quartz, and fragments of chalcedony‐carbonate veins that elsewhere crosscut listvenite—showing that cataclasis postdates listvenite formation. Locally the cataclasites are reworked and cut by thin, sharp faults, pointing to repeated reactivation of brittle structures. SEM‐EDS mapping shows that cataclasis was related to dissolution of carbonate and/or silica cementation. Dolomite veins crosscut cataclasites and breccias, suggesting that part of the Ca gain in BT1B is related to late fluids after listvenite formation. These results indicate a multistage tectonic overprint after peridotite carbonation and listvenite formation, which may be related to the tectonic history of the deformed continental margin under the ophiolite. These relatively late brittle structures should be excluded when trying to understand the carbonation of peridotite to listvenite.

New insights into source and dispersal of Mediterranean S1 tephra, an early Holocene marker horizon erupted at Mt. Erciyes (Turkey)

Deposition of early Holocene Eastern Mediterranean S1 tephra and a Black Sea cryptotephra coincides with cultural transitions in the Fertile Crescent termed the Neolithic Revolution as well as sapropel formation during climate variability of the African humid period, classifying them as paramount regional marker horizons for archaeology as well as paleoclimatology. Their correlations with specific eruptions of the Mt. Erciyes stratovolcanic complex (Central Anatolia) remained inconclusive though. Here, we use zircon double-dating by (U–Th)/He and U–Th disequilibrium methods, major and trace element tephra glass geochemistry, and probabilistic modeling of tephra dispersal in an attempt to characterize all major late Quaternary proximal tephras of Mt. Erciyes, and to correlate them with distal deposits. Furthermore, we discuss contrasting proximal and distal tephra dispersal.Three nearly-coeval rhyolitic satellite domes (Dikkartın, Perikartın, and Karagüllü) erupted at Mt. Erciyes in the early Holocene, and their dome extrusions were all preceded by explosive phases producing pyroclastic material that formed tephra fall and pyroclastic flow deposits. The new eruption age of 9.03 ± 0.55 ka (1σ uncertainty here and elsewhere) for proximal Dikkartın pumice is consistent with 14C-based S1 tephra chronologies in distal locations averaging 8.92 ± 0.03 cal ka BP. Perikartın pyroclastic flow deposits predate S1 tephra by ca. 0.8 ka according to a pair of published 14C ages, and stratigraphically overlie Karagüllü fall-out, here dated to 8.2 ± 1.8 ka. Previously undated proximal tephras of Mt. Erciyes erupted in the Late (85.2 ± 4.9 ka) and Middle Pleistocene (154.5 ± 5.3 ka). S1 tephra glass is chemically similar to that of Dikkartın fall-out, but also indistinguishable from that of Perikartın fall-out. Karagüllü pumice is characterized by a distinct glass chemical composition, which correlates with that of unnamed cryptotephra reported for the southeastern Black Sea instead, where these results call for a re-evaluation of existing age models.Maximum lithic clast size isopleths for proximal Dikkartın fall-out indicate eastward dispersal of a 20 ± 5 km high eruption plume by stratospheric winds, in agreement with results of probabilistic tephra dispersal modeling. This azimuth contrasts with the known distribution of S1 tephra at distal locations that are all south of Mt. Erciyes. Significant tephra occurrences at up to 1300 km distance and orthogonal to prevalent stratospheric wind directions either result from very atypical wind conditions (probability ≪10 %), or are caused by tephra transport by prevailing low altitude winds. Two scenarios are proposed for low altitude transport: eolian reworking of primary fall-out (more likely from the more widespread Dikkartın deposits), or co-ignimbrite ash cloud dispersal (more likely from the Perikartın eruption which predominantly produced pyroclastic flows). Because S1 tephra is chemically indistinguishable from both Dikkartın and Perikartın by major and trace element glass compositions, its exact source and dispersal mechanism remain ambiguous, although existing 14C ages for Perikartın predating those for S1 tephra favor Dikkartın as its source.

The sulfur budget and sulfur isotopic composition of Martian regolith breccia NWA 7533

AbstractThe sulfur isotope budget of Martian regolith breccia (NWA 7533) has been addressed from conventional fluorination bulk rock analyses and ion microprobe in situ analyses. The bulk rock analyses yield 865 ± 50 ppm S in agreement with LA‐ICP‐MS analyses. These new data support previous estimates of 80% S loss resulting from terrestrial weathering of NWA 7533 pyrite. Pyrite is by far the major S host. Apatite and Fe oxyhydroxides are negligible S carriers, as are the few tiny igneous pyrrhotite–pentlandite sulfide grains included in lithic clasts so far identified. A small nonzero Δ33S (−0.029 ± 0.010‰) signal is clearly resolved at the 2σ level in the bulk rock analyses, coupled with negative CDT‐normalized δ34S (−2.54 ± 0.10‰), and near‐zero Δ36S (0.002 ± 0.09‰). In situ analyses also yield negative Δ33S values (−0.05 to −0.30‰) with only a few positive Δ33S up to +0.38‰. The slight discrepancy compared to the bulk rock results is attributed to a possible sampling bias. The occurrence of mass‐independent fractionation (MIF) supports a model of NWA 7533 pyrite formation from surface sulfur that experienced photochemical reaction(s). The driving force that recycled crustal S in NWA 7533 lithologies—magmatic intrusions or impact‐induced heat—is presently unclear. However, in situ analyses also gave negative δ34S values (+1 to −5.8‰). Such negative values in the hydrothermal setting of NWA 7533 are reflective of hydrothermal sulfides precipitated from H2S/HS‐ aqueous fluid produced via open‐system thermochemical reduction of sulfates at high temperatures (&gt;300 °C).

Identifying Gaps in the Investigation of the Vredefort Granophyre Dikes: A Systematic Literature Review

The Vredefort impact structure is among the oldest and largest impact structures preserved on Earth. An understanding of its key features can serve as a guide for learning about the development of basin-sized impact structures on Earth and other planetary bodies. One of these features is the so-called Vredefort granophyre dikes, which formed when molten material from the impact melt sheet was emplaced below the crater floor. The importance of these dikes has been recognized since the earliest studies of the Vredefort structure, nearly 100 years ago. The present study is a systematic literature review to determine the extent to which peer-reviewed scientific publications have generated unique data regarding the granophyre dikes and to investigate how scientific methods used to investigate the granophyre have changed over time. In total, 33 unique studies have been identified. Of those, more studies have been performed into the core-collar dikes than the core dikes. The majority of the studies have focused on field analyses, bulk geochemistry, and the studies of mineral components. The granophyre has long been recognized as a product of post-deformational processes and thus has been a target of age dating to constrain the minimum age of the impact event. In the last 25 years, studies of stable isotopes and shock deformation of minerals in lithic clasts within the dikes have taken place. A small number of geophysical studies relevant to the granophyre dikes have also been undertaken. Overall, there has been a relatively small number of studies on this important rock type, and the studies that have taken place tend to focus on two particular dikes. Several of the dikes have only been investigated by regional studies and have not been specifically targeted. The use of modern techniques has been lacking. More fieldwork, as well as geophysical, isotopic, microstructural studies, and application of novel techniques, are necessary for the granophyre dikes to be truly understood.

The Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction measurements

Abstract “Cr-Zr-Ca armalcolite” is a mineral originally found in Apollo samples five decades ago. However, no structural information has been obtained for this mineral. In this study, we report a new occurrence of “Cr-Zr-Ca armalcolite” and its associated mineral assemblage in an Mg-suite lithic clast (Clast-20) from the brecciated lunar meteorite Northwest Africa 8182. In this lithic clast, plagioclase (An = 88–91), pyroxene (Mg#[Mg/(Mg+Fe)] = 0.87–0.91) and olivine (Mg# = 0.86–0.87) are the major rock-forming minerals. Armalcolite and “Cr-Zr-Ca armalcolite” are observed with other minor phases including ilmenite, chromite, rutile, fluorapatite, merrillite, monazite, FeNi metal, and Fe-sulfide. Based on 38 oxygen atoms, the chemical formula of “Cr-Zr-Ca armalcolite” is (Ca0.99Na0.01)Σ1.00(Ti14.22Fe2.06Cr2.01 Mg1.20Zr0.54Al0.49Ca0.21Y0.05Mn0.04Ce0.03Si0.03La0.01Nd0.01Dy0.01)Σ20.91O38. Electron backscatter diffraction (EBSD) results reveal that the “Cr-Zr-Ca armalcolite” has a loveringite R3 structure, differing from the armalcolite Bbmm structure. The estimated hexagonal cell parameters a and c of “Cr-Zr-Ca armalcolite” are 10.55 and 20.85 Å, respectively. These structural and compositional features indicate that “Cr-Zr-Ca armalcolite” is loveringite, not belonging to the armalcolite family. Comparison with “Cr-Zr-Ca armalcolite” and loveringite of other occurrences implies that loveringite might be an important carrier of rare earth elements in lunar Mg-suite rocks. The compositional features of plagioclase and mafic silicate minerals in Clast-20 differ from those in other Mg-suite lithic clasts from Apollo samples and lunar meteorites, indicating that Clast-20 represents a new example of diverse lunar Mg-suite lithic clasts.

Material mixing in shock-induced pseudotachylites, Vredefort impact structure, South Africa

Impact-generated pseudotachylites (PTs) have been extensively studied in the Vredefort Impact Structure, South Africa, and have been shown to correspond chemically and isotopically to their host rocks. Such melts are frequently described as forming strictly in situ with lithic clasts derived from the immediate wall rocks. Mixed compositions are rare, where PTs are observed directly along a lithological contact. In this study, we document the presence of vast numbers of granitic clasts within dolerite-hosted PT veins within the Vredefort impact structure. The groundmasses of dolerite-hosted PT veins have two distinct geochemical compositions: (1) mafic, and (2) intermediate. The two compositional melt phases have a sharp contact, indicating immiscibility and/or mingling. Shocked zircon and monazite within the granite clasts enclosed in a dolerite-hosted PT vein indicate shock deformation prior to clast transport into the dolerite. In monazite, we report a new shock microtwin geometry 180°/[104].The mechanisms by which granitic clasts were emplaced in the dolerite-hosted veins are complex. Previous experimental work has suggested that varying shock velocity in different lithologies may be responsible for material mixing. In addition, the thermal pulse accompanying the shock wave would have caused quartz in the granite to transform into volumetrically larger high-temperature polymorphs, significantly increasing the granite volume compared to the adjacent dolerite. Together with fluctuation of stresses (post-impact reverberations) after the passage of the main shock wave, this volume adjustment initiated brittle fracturing of the dolerite. A negative pressure gradient was generated within the dolerite that allowed granitic material to be injected into the dolerite-hosted melt veins.

Classification of CM chondrite breccias—Implications for the evaluation of samples from the OSIRIS‐REx and Hayabusa 2 missions

AbstractCM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM‐like clasts in three achondrites by chemical analysis of the tochilinite‐cronstedtite‐intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2‐2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0‐CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4‐CM2.6) or less altered fragments (CM2.7‐CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO2 ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO2 ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0‐CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS‐REx and Hayabusa 2 missions in the future.