Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba)

Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl-silicate complexes), in the agates confirm their contemporaneous supply with SiO2 and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO2 with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm−1 and moganite at 502 cm−1, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply

Ruthenium isotopes show the Chicxulub impactor was a carbonaceous-type asteroid

An impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth’s final stages of accretion

Detection and Quantification of Extraterrestrial Platinum Group Element Alloy Micronuggets from Archean Impactite Deposits by Low-Voltage Scanning Electron Microscopy/Energy-Dispersive X-ray Spectrometry

Abstract Rare, heterogeneously composed platinum group element alloy micronuggets (PGNs) occur in primitive meteorites, micrometeorites, and terrestrial impactite deposits. To gain insight into the nature of these phases, we developed a workflow for the characterization of PGNs using modern scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry at a low accelerating voltage of 6 kV. Automated feature analysis—a combination of morphological image analysis and elemental analysis with stage control—allowed us to detect PGNs down to 200 nm over a relatively large analysis area of 53 mm2 with a conventional silicon drift detector (SDD). Hyperspectral imaging with a high-sensitivity, annular SDD can be performed at low beam current (∼100 pA) which improves the SEM image resolution and minimizes hydrocarbon contamination. The severe overlapping peaks of the platinum group element L and M line families at 2–3 keV and the Fe and Ni L line families at <1 keV can be resolved by peak deconvolution. Quantitative elemental analysis can be performed at a spatial resolution of <80 nm; however, the results are affected by background subtraction errors for the Fe L line family. Furthermore, the inaccuracy of the matrix correction coefficients may influence standards-based quantification with pure element reference samples.Copyright © The Author(s) 2023. Published by Oxford University Press on behalf of the Microscopy Society of America. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.NHM Repositor

Nondestructive spectroscopic and petrochemical investigations of Paleoarchean spherule layers from the ICDP drill core BARB5, Barberton Mountain Land, South Africa

International audienceA Paleoarchean impact spherule‐bearing interval of the 763 m long International Continental Scientific Drilling Program (ICDP) drill core BARB5 from the lower Mapepe Formation of the Fig Tree Group, Barberton Mountain Land (South Africa) was investigated using nondestructive analytical techniques. The results of visual observation, infrared (IR) spectroscopic imaging, and micro‐X‐ray fluorescence (μXRF) of drill cores are presented. Petrographic and sedimentary features, as well as major and trace element compositions of lithologies from the micrometer to kilometer‐scale, assisted in the localization and characterization of eight spherule‐bearing intervals between 512.6 and 510.5 m depth. The spherule layers occur in a strongly deformed section between 517 and 503 m, and the rocks in the core above and below are clearly less disturbed. The μXRF element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in (1) sorting of two types of spherules and (2) occurrence of primary minerals (Ni‐Cr spinel and zircon). We favor a single impact scenario followed by postimpact reworking, and subsequent alteration. The spherule layers are Al2O3‐rich and can be distinguished from the Al2O3‐poor marine sediments by distinct Al‐OH absorption features in the short wave infrared (SWIR) region of the electromagnetic spectrum. Infrared images can cover tens to hundreds of square meters of lithologies and, thus, may be used to search for Al‐OH‐rich spherule layers in Al2O3‐poor sediments, such as Eoarchean metasediments, where the textural characteristics of the spherule layers are obscured by metamorphism

Ruthenium isotopes show the Chicxulub impactor was a carbonaceous-type asteroid

An impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth’s final stages of accretion.Editor’s summary The geologic boundary between the Cretaceous and Paleogene eras 66 million years ago is marked by worldwide deposits from an impact at modern Chicxulub, Mexico. The impact coincides with a mass extinction that wiped out non-avian dinosaurs and many other species. Fischer-Gödde et al . measured ruthenium isotopes in the impact deposits and compared them with multiple classes of meteorites, which represent potential impactor compositions. They found that the Chicxulub impactor was a carbonaceous asteroid that formed in the outer Solar System. Additional measurements of five other impacts showed that those were due to silicate asteroids that formed in the inner Solar System. —Keith T. SmithAn impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth’s final stages of accretion.Editor’s summary The geologic boundary between the Cretaceous and Paleogene eras 66 million years ago is marked by worldwide deposits from an impact at modern Chicxulub, Mexico. The impact coincides with a mass extinction that wiped out non-avian dinosaurs and many other species. Fischer-Gödde et al . measured ruthenium isotopes in the impact deposits and compared them with multiple classes of meteorites, which represent potential impactor compositions. They found that the Chicxulub impactor was a carbonaceous asteroid that formed in the outer Solar System. Additional measurements of five other impacts showed that those were due to silicate asteroids that formed in the inner Solar System. —Keith T. Smit