Middle and Upper Ordovician carbon isotope chemostratigraphy in Baltoscandia: A correlation standard and clues to environmental history

The Great Ordovician Biodiversification Event coincides ~470 million years ago with the break-up of a large asteroid and the resultant frequent bombardment of Earth with asteroid fragments.

/pdf/asteroid-breakup-linked-to-the-great-ordovician-5ba0u4kloh.pdf

Asteroid breakup linked to the Great Ordovician Biodiversification Event

Meteorites are samples of dozens of small planetary bodies that formed in the early Solar System. They exhibit great petrologic diversity, ranging from primordial accretional aggregates (chondrites), to partially melted residues (primitive achondrites), to once fully molten magmas (achondrites). It has long been thought that no single parent body could be the source of more than one of these three meteorite lithologies. This view is now being challenged by a variety of new measurements and theoretical models, including the discovery of primitive achondrites, paleomagnetic analyses of chondrites, thermal modeling of planetesimals, the discoveries of new metamorphosed chondrites and achondrites with affinities to some chondrite groups, and the possible identification of extant partially differentiated asteroids. These developments collectively suggest that some chondrites could in fact be samples of the outer, unmelted crusts of otherwise differentiated planetesimals with silicate mantles and metallic cores...

Differentiated Planetesimals and the Parent Bodies of Chondrites

A number of distinct methodologies are available for determining the oxygen isotope composition of minerals and rocks, these include laser-assisted fluorination, secondary ion mass spectrometry (SIMS) and UV laser ablation. In this review we focus on laser-assisted fluorination, which currently achieves the highest levels of precision available for oxygen isotope analysis. In particular, we examine how results using this method have furthered our understanding of early-formed differentiated meteorites. Due to its rapid reaction times and low blank levels, laser-assisted fluorination has now largely superseded the conventional externally-heated Ni “bomb” technique for bulk analysis. Unlike UV laser ablation and SIMS analysis, laser-assisted fluorination is not capable of focused spot analysis. While laser fluorination is now a mature technology, further analytical improvements are possible via refinements to the construction of sample chambers, clean-up lines and the use of ultra-high resolution mass spectrometers. High-precision oxygen isotope analysis has proved to be a particularly powerful technique for investigating the formation and evolution of early-formed differentiated asteroids and has provided unique insights into the interrelationships between various groups of achondrites. A clear example of this is seen in samples that lie close to the terrestrial fractionation line (TFL). Based on the data from conventional oxygen isotope analysis, it was suggested that the main-group pallasites, the howardite eucrite diogenite suite (HEDs) and mesosiderites could all be derived from a single common parent body. However, high precision analysis demonstrates that main-group pallasites have a Δ 17 O composition that is fully resolvable from that of the HEDs and mesosiderites, indicating the involvement of at least two parent bodies. The range of Δ 17 O values exhibited by an achondrite group provides a useful means of assessing the extent to which their parent body underwent melting and isotopic homogenization. Oxygen isotope analysis can also highlight relationships between ungrouped achondrites and the more well-populated groups. A clear example of this is the proposed link between the evolved GRA 06128/9 meteorites and the brachinites. The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples from approximately 110 asteroidal parent bodies (∼60 irons, ∼35 achondrites and stony-iron, and  ∼15 chondrites) in our global meteorite collection. However, compared to the likely size of the original protoplanetary asteroid population, this is an extremely low value. In addition, almost all of the differentiated samples (achondrites, stony-iron and irons) are derived from parent bodies that were highly disrupted early in their evolution. High-precision oxygen isotope analysis of achondrites provides some important insights into the origin of mass-independent variation in the early Solar System. In particular, the evidence from various primitive achondrite groups indicates that both the slope 1 (Y&R) and CCAM lines are of primordial significance. Δ 17 O differences between water ice and silicate-rich solids were probably the initial source of the slope 1 anomaly. These phases most likely acquired their isotopic composition as a result of UV photo-dissociation of CO that took place either in the early solar nebula or precursor giant molecular cloud. Such small-scale isotopic heterogeneities were propagated into larger-sized bodies, such as asteroids and planets, as a result of early Solar System processes, including dehydration, aqueous alteration, melting and collisional interactions. There is increasing evidence that chondritic parent bodies accreted relatively late compared to achondritic asteroids. This may account for the fact that apart from a few notable exceptions’ such as the aubrite-enstatite chondrite association, known chondrite groups could not have been the parents to the main achondrite groups.

Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies

This article presents a current (as of September 2019) list of recommended ages for proven terrestrial impact structures (n = 200) and deposits (n = 46) sourced from the primary literature. High-pr...

Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits.

Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ~480 Myr ago. An association with the L-chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ±  30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40Ar-39Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L-chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.

/pdf/l-chondrite-asteroid-breakup-tied-to-ordovician-meteorite-3fxp6r2nie.pdf

L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating

A relic impact structure was recognized within the strewn field of the Agoudal iron meteorite. The heavily eroded structure has preserved shatter cones in a limestone basement, and remnants of autochthonous and allochthonous breccias. Fragments of iron incorporated into the allochthonous breccia have a chemical composition (Ni = 5.16 wt%, Ir = 0.019 ppm) similar to that of the Agoudal meteorite, supporting a syngenetic origin of the strewn field and the impact structure. The total recovered mass of Agoudal meteorite fragments is estimated at approximately 500 kg. The estimated size of the SE–NW-oriented strewn field is 6 × 2 km. Model calculations with minimal preatmospheric size show that a similar meteorite strewn field plus one small crater with observed shock effects could be formed by fragmentation of a meteoroid approximately 1.4 m in diameter with an impact angle of approximately 60° from the horizontal. However, the most probable is an impact of a larger, 3–4 m diameter meteoroid, resulting a strewn field with approximately 10 craters, 10–30 m in diameter each, plus numerous meteorite fragments. The calculated scattering area of meteorite shrapnel ejected from these impact craters could completely cover the observed strewn field of the Agoudal meteorite.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12406

Formation of a small impact structure discovered within the Agoudal meteorite strewn field, Morocco

A new approach is developed to explain the nature of oriented pyroxene-spinel symplectites and magnetite-hematite inclusions in olivines. Symplectites are considered the products of dehydrogenation and solid-state substitution of OH-bearing olivine-like precursor phases occured as lamellar inclusions in olivine. According to the dehydrogenation model, the precursor of the oriented symplectic pyroxene-spinel intergrowths in olivine hydrous olivine [3(Mg,Fe)2SiO4] · (MgH2SiO4), while the appearance of hematite-magnetite intergrowths in olivine is related to the dehydrogenation and dehydration of humite-like interlayers [n(Mg,Fe)2SiO4] · [Mg(OH2)]. The dehydrogenation reactions are accompanied by the transformation of divalent transition metals into a trivalent state and interdiffusion of Fe, Cr, Ca, Mg, and Al between precursor phase and host olivine under the long-term decompression at temperatures >800°C. The transformation of a pre-existing phase into symplectite was driven through the topotaxic reaction, fulfilling the following conditions: inheritance of crystallographic orientation of precursor phase by symplectite; preservation of volume during reaction, i.e. tendency to zero volume effect of the reaction1; the mass conservation, which is consistent with a solid state reaction in a closed system without external input of chemical elements in olivine.

Dehydrogenation as the mechanism of formation of the oriented spinel-pyroxene symplectites and magnetite-hematite inclusions in terrestrial and extraterrestrial olivines

Mesosiderites are thermal metamorphic breccias consisting of fragments of pyroxene-plagioclase rocks and FeNi metal. The silicate constituent of mesosiderites has a chemical and oxygen isotopic composition analogous to those of meteorites of the HED group: howardites, eucrites, and diogenites. The hypothesis currently most widely accepted for the genesis of mesosiderites is the impact mixing of the material of a differentiated asteroid and an iron meteorite. In contrast to many other classes of meteorites, mesosiderites exhibit no traces of metasomatic processes. The Budulan mesosiderite is the first meteorite of this type in which traces of metasomatism under the effect of an anhydrous fluid were detected. The metasomatic alterations are manifested as chemical zoning of olivine, aggregates of secondary minerals, and the mobilization and redeposition of iron and nickel in the form of metals and sulfides. These alterations were most probably caused by a reaction of olivine with S- and/or CO-bearing gases of endogenic or supergenic provenance. Traces of such metasomatic alterations were previously found in some meteorites and lunar rocks, and these processes could likely play a certain role in the differentiation of chondritic bodies.

Metasomatic alterations of olivine inclusions in the Budulan mesosiderite

Aubrite Pesyanoe: Clues to composition and evolution of the enstatite achondrite parent body

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