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).
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