Mineral grains far older than the Solar System

If a geologist with broad interests was asked, ‘what are the oldest materials on Earth?’ she or he would probably say the Acasta Gneiss from Canada’s North West Territories at 4.03 billion years (Ga) (see: At last, 4.0 Ga barrier broken, November 2008. A specialist in the Archaean Eon might say the Nuvvuagittuq Greenstone Belt on the eastern shore of Hudson Bay (see: Archaean continents derived from Hadean oceanic crust, March 2017); arguably 4.28 Ga old. An isotope geochemist would refer to a tiny 4.4 Ga zircon grain that had been washed into the much younger Mount Narryer quartz sandstone in Western Australia (see: Pushing back the “vestige of a beginning”, January 2001). A real smarty pants would cite a 4.5 Ga old sample of feldspar-rich Lunar Highland anorthosite in the Apollo Mission archive in Houston, USA. The last is less than 100 Ma younger than the formation of the Solar System itself at 4.568 Ga. Yet there are meteorites that have fallen to Earth, which contain minute mineral  grains that were incorporated into the initial dust from which the planets formed. Until recently, the best known were white inclusions in a 2 tonne meteorite that fell near Allende in Mexico; the largest carbonaceous chondrite ever found. This class of meteorite represents the most primitive material in orbit around the Sun. Its tiny inclusions contain proportions of isotopes of a variety of elements that are otherwise unknown in any other material from the Solar System and they are older. The conclusion is that these dust-sized, presolar grains originated elsewhere in the galaxy, perhaps from supernovas or red-giant stars.

A presolar grain from the Murchison meteorite made up of silicon carbide crystals (credit: Janaína N. Ávila)

Carbonaceous chondrites, as their name suggests, contain a huge variety of carbon-based compounds and they have been closely examined as possible suppliers of the precursor chemicals for the origin of life. Another large example of this class fell near the town of Murchison in Victoria, Australia in 1969. The first people to locate fragments of the 100 kg body noted a distinct smell of methylated spirits and steam rising from it: when crushed half a century later it still smells like rotting peanut butter. The Murchison meteorite has yielded signs of 14 thousand organic compounds, including 70 amino acids. It has also been a target for extracting possible presolar grains. This entails grinding small fragments and then dissolving out the carbonaceous and silicate material using various reagent to leave the more or less inert silicon carbide grains. The residue contains the most durable grains: despite being described as ‘large’ they are of the order of only 10 micrometres across. Some are made of silicon carbide; the same as the well-known abrasive carborundum. Throughout their lifetime in interstellar space the grains have been bombarded by high-energy protons and helium nuclei which move through space at nearly the speed of light – generally known as ‘cosmic rays’. When interacting with other matter they behave much like the particles in the Large Hadron Collider, being able to transmute natural isotopes into others. Measuring the relative proportions of these isotopes in material that has been bombarded by cosmic rays enables their exposure time to be estimated. In the case of the Murchison presolar grains the isotopes of choice are those of the noble gas neon (Heck, P.R. and 9 others 2020. Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide. Proceedings of the National Academy of Sciences, 201904573; DOI: 10.1073/pnas.1904573117). Analyses of 40 such grains yielded ages from 4.6 to 7.5 Ga, i.e. up to 3 billion years before the Solar System formed. They are, indeed, exotic. The highest age exceeds that of the oldest from such previously measured by 1.5 billion years

Investigations up to now suggest that dusts amount to about 1 % of interstellar matter, the rest being gases, mainly hydrogen and helium. With the formation of the planets and the parent bodies of asteroids a high proportion of presolar grains would have accreted to them to be mixed with other, more common stuff. What Heck and colleagues have discovered puts the Solar System into a broad framework of time and space. The grains must have formed at some stage in the evolution of stars older and larger than the Sun, to be blown out into the interstellar medium of the Milky Way galaxy. One possibility is that about 7 billion years ago there was a burst of star formation in a nearby sector of the galaxy. How the resulting dust made its way to the concentration of interstellar matter that eventually formed the Sun and Solar System is yet to be commented on.

See also: Bennett, J.  2020 Meteorite Grains Are the Oldest Known Solid Material on Earth.  Smithsonian Magazine(online)  13 January 2020.

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