Microtektites from the Transantarctic Mountains

Microtektites are microscopic glass spherules produced by the vaporisation of material from the Earth's crust due to hypervelocity impacts by extraterrestrial bodies. They are spherical in shape because they are formed by the cooling of liquid silica within the atmosphere. These are found in many locations, but can be linked together into regions related to, but typically far from, impact sites, known as scattered fields. To date four such scattered fields have been found on Earth, known as the Australasian, Central European, Ivory Coast and North American fields, each of which is thought to relate to a different impact event.

In a paper published in the journal Geochemica et Cosmochemica Acta on 6 March 2018, Matthias Van Ginneken of the Department of Earth Science and Engineering at Imperial College London, the Department of Earth Science at The Natural History Museum, the Department of Analytical, Environmental and Geo-Chemistry at the Vrije Universiteit Brussel, and the Laboratoire G-Time, Université Libre de Bruxelles, Mathew Genge, also of the Department of Earth Science and Engineering at Imperial College London, and Ralph Harvey of the Department of Geological Sciences at Case Western Reserve University, describe the discovery of a new microtektite producing area in a glacial moraine near Larkman Nunatak in the Transantarctic Mountains of Antarctica.

The new site and comprises a moraine roughly 1.5 km in length and 700 m wide, orientated in an east-west direction. 250 g of material was removed from this site in 2006, and subsequently washed and sieved, producing 52 glass spherules between 107 and 388 μm in diameter and pale yellow in colour; one of these contains a bubble 10 μm in diameter.

Stereomicrograph of three Larkman Nunatak microtektites. The scalebar is 100 lm. A vesicle is arrowed in (c). Van Ginneken et al. (2018).

Chemically these spherules fall within the Australasian Scattered Field, which have previously been found in deep sea sediments of the Indian and Pacific Ocean. These are the youngest of the four scattered fields, thought to have originated about 800 000 years ago from an impact somewhere in Southeast Asia, probably Vietnam, from which they are spread out in a three-lobed shape. The Antarctic microtektites are the furthest yet known from the presumed location of the impact, about 11 000 km away. 

Sampling location of Larkman Nunatak microtektites. (a) Sketch map showing the sampling sites of Australasian microtektites and the current extension of the Australasian strewn field. (b) Regional map showing the locations where microtektites were found in the Transantarctic Mountains. (c) Panoramic view of Larkman Nunatak. Arrowed is the sampling area where glacial moraine was collected. (c) Detail of the sampling area. Van Ginneken et al. (2018).

The Antarctic microtektites are smaller than any previously found Austrolasian microtektites and more-or-less totally depleted in volatile elements. This fits with a consistent pattern within the Austrolasian Scattered Field, in which the further microtektites are found from the presumed impact site, the smaller and more depleted in volatiles they are. Van Ginneken et al. suggest that this is because the furthest material actually originates from closest to the impact site, and thus was thrown higher into the atmosphere, and therefore the microtektites are more altered the further they are found from this origin.

See also...

Follow Sciency Thoughts on Facebook.

Micrometerites from Late Cretaceous Chalk depostits from southern England.

Micrometeorites are tiny (less than 2 mm) fragments of asteroidal (or occasionally cometary) material that survive the descent through the Earth's atmosphere, and which are collected by planetary scientists from sites where input from terrestrial sedimentary sources is minimal, such as Antarctic ice- and snow-fields and oceanic basins. Some micrometeorites survive the journey to the Earth relatively impact, but most are melted by the temperatures caused by friction with the Earths atmosphere (which is greater than that caused by simply falling due to the high relative speeds at which these bodies are travelling prior to encountering the Earth), often reaching temperatures in excess of 2000°C,  which causes them to melt, forming spherical droplets due to surface tension, which recrystallise to form circular bodies called spherules.

These spherules do not retain the same chemistry as their parent bodies; lighter elements, such as sodium, sulphur, phosphorus, chlorine, and manganese tend to evaporate completely, while heavier elements such as iron and nickel separate internally, forming discrete layers. As these liquid spherules descend further they are quench cooled through contact with the denser lower atmosphere, causing them dendritic crystals to form. About 95% of such spherules have a silicate dominated composition, while about 4% are iron dominated and about 1% have a mixed composition. Silicate dominated spherules can have different mineralogies, which is thought to relate to the temperature to which they were heated, rather than their original composition, with porphyritic spherules forming at the lowest temperatures, then barred olivine spherules, then cryptocrystalline spherules, and finally vitreous (glassy) spherules at the highest temperatures. Iron dominated spherules are divided into metal-bearing spherules, which contain an iron-nickel bead surrounded by a layer of wüstite (an iron-oxide mineral), and oxidised sperules, which are composed of a mixture of wüstite and magnetite (another iron-oxide mineral).

Spherules, unlike unaltered micrometeorites, are distinctive enough to be recognised in the rock record and recovered from ancient sediments, with examples having been recovered from a wide range of sedimentary rocks, from Archean limestones in Australia to Eocene marine sediments in Barbados, as well as ancient granites in China. However, such spherules do not simply enter the rock record and remain unchanged waiting to be discovered, they are altered by chemical processes going on within the rock, causing changes referred to as taphonomy. This taphonomy can take a number of forms, including alteration of minerals, hydration or dissolution of anhydrous minerals and metals, and encrustation with halite (salt), calcite (carbonate) or other materials.

In a paper published in the journal Earth and Planetary Science Letters on 1 September 2017, Martin Suttle and Mathew Genge of the Impacts and Astromaterials Research Centre at Imperial College London and the Department of Earth Science at the Natural History Museum, describe the discovery of spherule micrometeorites in Late Cretaceous chalk deposits obtained from a road cutting at Ranmore Common in Surrey, southern England.

Suttle and Genge were able to obtain 76 spherules from their rock sample, including 60 iron oxide spherules, 13 iron/silica spherules and 3 silica spherules, ranging in size from 10 to 165 μm. The majority of the iron oxide spherules were comprised of magnetite, with small amounts of aluminium, silicon and manganese, and trace amounts of other minerals but no nickel, while one was comprised of a mixture of wüstite and magnetite, with a significant proportion of nickel. The iron/silica spherules also lacked any nickel, but did contain small amounts of chromium and manganese. The silica spherules contained small amounts of iron, aluminium and manganese.

External and internal textures of Fe-oxide spherules, interpreted as fossilised cosmic spherules. Particle (D) (C16-0003) is the single unaltered nickel-bearing iron oxide spherule, composed of wüstite, while the remaining spherules are composed of maganese-bearing magnetite. Surface dendrites and residual chalk sediment, including fragmented coccolithophore tests can be seen coating external surfaces of spherules (A)–(C). In spherule (D) and (F) sub-circular cavities are present, representing the loss of an iron–nickel metal bead by corrosion during residence on the Cretaceous seafloor, these spherules can therefore, be identified as metal-bearing iron oxide spherules. In contrast, spherule (G) contains isolated irregular small cavities, representing vesicles formed by residual gas trapped during inward crystallisation and is therefore an oxidised iron spherule. Suttle & Genge (2017).

The iron oxide and iron silica spherules were either homogeneous throughout or showed dendritic crystal formation, with many of the spherules with dendritic crystals also having cavities within, probably indicative of dissolution of minerals. One of the silica spherules has an olivine mineralogy, with the other two being porphyritic.

External and internal textures of iron-silicide spherules, most probably composed of suessite. These spherules are interpreted as fossilised cosmic spherules. Replacement by silicides imperfectly pseudomorphs the original texture, leading to changes in volume, accounting for the presence of micron sized voids seen in (F) and protrusions (D), protecting from the particle’s surface. Despite preservation artifacts, original textures can be discerned, allowing their identification as cosmic spherules. Dendritic crystals are observable in all particles and attest to a rapid cooling history as molten droplets. Particle C16-0009 (A) and (B) preserves only a single phase (most likely wüstite) while particle C16-0010 (D)–(F) preserves both the original magnetite and wüstite as different silicide minerals. Cavities in (B) are a result of volatile gases released during atmospheric entry. Suttle & Genge (2017).

Four of the spherules, the three silica spherules and the nickel-bearing wüstite spherule, are essentially identical to modern spherules obtained from Antarctic snowfields, leaving little doubt as to their meteoric origin. The presence of dendritic crystals in many of the magnetite spherules suggests being heated to a temperature of over 1350°C. The cavities within many of the spherules are probably due to the dissolution of soluble crystals as the spherules lay upon the sea-floor.

Sectioned images of silicate cosmic spherules. Spherules (A) and (B) are ancient, unaltered chondritic silica-type spherules, classified as micro-porphyritic (A) and barred olivine (B) subtypes. For comparison particle (C) is a modern, barred olivine spherule recovered from Larkman Nunatak, Antarctica. Suttle & Genge (2017).

The presence of manganese in many of the spherules requires more explanation, as manganese is extremely rare in modern micrometeorites. Suttle and Genge suggest that this is probably not indicative of the original composition of the meteorites, but rather re-crystallisation of the spherules on the seafloor. They suggest that the spherules probably had a nickle-bearing wüstite composition, which was recrystallised to magnetite on the seafloor, in the process losing their nickel content, but gaining manganese (which is often abundant in marine sediments).

See also...

Follow Sciency Thoughts on Facebook.

Determining the origin of scoriaceous micrometeorites.

Micrometeorites are particles of extra-terrestrial material less than 2 mm across. These have been collected from a number of environments, including Antarctic blue ice and snow, deep sea sediments and even rooftops, and are the most abundant form of meteorite material available to scientists. Scoriaceous micrometeorites are micrometeorites dominated by micron-sized equant iron-rich olivine crystals within a glassy mesostasis, which is thought to be indicative of having undergone caused by superheating from friction with the Earth's atmosphere due to the orbital momentum of the asteroid, which is greater than that caused by simply falling.

In a paper published in the journal Geology on 17 August 2017, Matthew Genge and Martin Suttle of the Impact and Astromaterials Research Centre at Imperial College London, and the Earth Sciences Department at The Natural History Museum, and Matthias Van Ginneken of Earth System Science at the Vrije Universiteit Brussel, present the results of a study in which they examined scoriaceous micrometeorites in order to attempt to determine their origin and the stresses that they been subjected to.

Scoriaceous micrometeorites contain clusters of clusters of forsterite and enstatite crystals, which Genge et al. believe to have been formed by the fragmentation of larger crystals due to superheating, as other micrometeorites lack these, suggesting that the the original crystals cannot be fractured in this way by shockwaves passing through the minerals as the parent bodies fragment in the atmosphere, impact the ground, or undergo some process in space unrelated to their eventual decent to Earth. They suggest that the most likely cause of such fragmentation is thermal stress, as different minerals within the micrometeorite expand at different rates when heated.

Backscattered electron images of micrometeorites. (A) Highly vesicular scoriaceous micrometeorite containing a cluster of forsterite (FO) relicts and eskoliate (CR). (B) A scoriaceous micrometeorite with a well developed external magnetite rim and clusters of forsterite (FO). Small triangular shards of forsterite are present close to larger crystals. This particle is similar to a micro-porphyritic olivine cosmic spherule. An expanded inset shows a cluster of small enstatite crystals. (C) A scoriaceous micrometeorite containing abundant vesicles and a magnetite rim. Two areas of relicts occur, one enstatite (FE) exhibits abundant fractures partially infilled with mesostasis (MV). (D) A scoriaceous micrometeorite containing two clusters of forsterites (FO) consisting of numerous individual crystals. (E) An unmelted finegrained micrometeorite with an external igneous rim (IR) surrounding an unmelted core (UC). A forsterite relict (FO) is present that truncates the igneous rim ((f) shows expanded view) and contains numerous fractures partially infilled with melt and is surrounded by a magnetite rim (MR). Fracturing within the crystal is most abundant in the part closest to the surface of the particle. Scale bars are 50 μm, except in (f) where it is 5 μm. Genge et al. (2017).

Genge et al. were able to construct a model of the rate at which forsterite expands due to heating, and from this determine the temperature to which these minerals had been raised, and the difference in temperature across the mineral grain; suggesting that in some cases this temperature difference may be as much as 4000 K per μm, resulting in a high degree of shear stress, caused by different parts of the crystal expanding at different rates, and causing the crystal to shatter.

Such a high temperature difference within a micrometeorite requires some explanation, as most minerals conduct heat fairly well. Genge et al. theorise that this may have been caused by the parent bodies having been comprises of at least 5% phyllosilicates (sheet minerals such as micas, chlorite, serpentine, talc, and the clays), which conduct heat poorly, and which would have been destroyed by the very high temperatures implied. 

This in turn suggests that the original material from which these micrometeorites were derived was similar in composition to that of a CI1 or CM2 carbonaceous chondrite, meteorites with a high composition of phyllosilicates. CM2 chondrites have previously been shown to lose phyllosilicates due to dehydration (the loss of hydrogen and oxygen from the mineral as water) at high presures, suggesting that scoriaceous micrometeorites may be formed specifically from the fragmentation of such chondrites, rather than being the products of any meteorite raised to the correct temperature.

About half of all micrometeorite-sized particles entering the Earth's atmosphere are thought to be derived from members of the Veritas Asteroid Family (a group of asteroids in the Outer Main Belt thought to have formed about 8.5 million years ago by the break-up of a large parent-body), with much of the remaining material derived from the Koronis Family Asteroids (a group of asteroids in the Central Main Belt thought to have formed by the collision of two large bodies about two billion years ago), and a small contribution from the Themis Family (a group of asteroids in the Outer Main Belt thought to be the main source of carbonaceous chondrites).

The Koronis Family Asteroids is thought to produce only ordinary chondritic material, while both the Veritas and Themis families are thought to produce carbonaceous chondritic material. Genge et al. suggest that the likely carbonaceous origin of scoriaceous micrometeorites implies that these are most likely to have originated from the Veritas and Themis asteroid groups.

See also...

Follow Sciency Thoughts on Facebook.

Micrometeorites from urban environments.

Micrometeorites are dust-sized particles of extraterrestrial origin, that are able to survive the fall through our atmosphere without being destroyed by friction-induced heating due to their low mass (the largest micrometeorites do undergo significant heating and melting, and are known as spherules due to their shape). Micrometeorites can be derived from asteroids or comets, and the study of these bodies is considered to provide significant insights into the evolution of small planetary bodies in our Solar System. However distinguishing micrometeorites from similarly sized mineral particles of terrestrial origin is difficult, and planetary scientists go to great lengths to collect such particles from environments where input from terrestrial sedimentary sources is minimal, such as Antarctic ice- and snow-fields and oceanic basins. Many amateur astronomers have suggested that such particles could also be collected from urban environments, particularly rooftops, but this suggestion has, to date, gained little traction with professional scientists.

In a paper published in the journal Geology on 5 December 2016, Matthew Genge of the Department of Earth Sciences and Engineering at Imperial College London and the Department of Earth Science at the Natural History Museum, Jon Larsen of Project Stardust, Matthias Van Ginneken of the Département des Géosciences at the Université Libre de Bruxelles and Martin Suttle, also of the Department of Earth Sciences and Engineering at Imperial College London and the Department of Earth Science at the Natural History Museum, describe the results of a study of micrometeorites collected from sediments accumulated on rooftops and gutters in Oslo and Paris.

Genge et al. were able to isolate 500 particles identified as micrometeorites from 300 kg of sedimentary material collected from a surface area of ~30,000 m² of rooftops in Olso and Paris (499 micrometeorites came from Oslo and one from Paris). All of these micrometeorites were between 300 and 400 μm in diameter, and were identified as micrometeorites on the basis of mineralogical similarities to other meteorites.

Backscattered electron images of urban cosmic spherules. REL—relict grain, SUL— sulphide, MET—metal, MGR—magnetite rim, OLR—olivine rim. A–C: Porphyritic spherules. D–F: Barred olivine spherules. G, H: Cryptocrystalline spherules. Genge et al. (2016).

These particles included porphyritic olivine spherules dominated by phenocrysts of olivine within glassy mesostasis, barred olivine spherules dominated by parallel growth dendrites of olivine with interstitial glass and cryptocrystalline spherules dominated by radiating clusters of fine olivine dendrites within glass. These compositions are consistent with S-type spherules derived from Antarctic and deep sea sediments, supporting the idea that these urban particles are of extra-terrestrial origin.

As well as providing a source of micrometeorites that does not involve research expeditions to remote locations, this study provides as source of very young material that has not been previously available. Most studies of micrometeorites have relied on sources that time-average material over long periods (thousands of years), enabling them to be found in sufficient numbers to justify collection. However the majority of the rooftop material is thought to have fallen in the last six years, as it is collected from gutteiring which people periodically clean, and all of the material to be less than 50 years old, the age of the oldest building from which it was collected.

See also...

Follow Sciency Thoughts on Facebook.