Thorsberg – Not Your Average Meteorite

How to Fossilise a Space Rock

A while ago I wrote about the marvelous Murchison meteorite.? It lit up northern Victorian skies in 1969 and, with a range of organic compounds in the makeup of its collected fragments, contributed to conjecture about life on Earth being seeded from space.

Meteorites are relatively common, but in rare cases these space travelers have interacted with the petrification of long-dead marine organisms. In the same way as relics of ancient life forms are encased and preserved in sedimentary rock, some meteorites fall in locations conducive to fossilisation. The principal cluster of such specimens is in southern Sweden, in a working limestone quarry. Here the stone is a compressed calcareous residue of Ordovician-era marine life; cephalopods and trilobites.

The Ordovician was a 42-million-year transition classification that bridged the Cambrian and Silurian periods, and was book-ended by conspicuous extinction events.?During this time, starting 485 million years ago, the Gondwana supercontinent dominated the antipodes. North of the equator was virtually all ocean, and a steady deposition of arthropod exoskeletons on the ocean floor laid down, very slowly, what would become the limestone beds of the Thorsberg quarry.

The Ordovician also saw Earth bombarded by chondritic meteoroids.* This heightened flux is thought to originate in a collision between two bodies of the Asteroid Belt 470 million years ago. One of these is the source of the Thorsberg meteorite fossils: astronomical objects that journeyed to Earth, fell into the northern ocean, sunk to the sea floor, and were subsumed by sedimentary rock-making action. They form a fascinating palimpsest; antique vestiges of the primitive Solar System, fossilised twice over.

Discovered in 1952, the meteorites present in the Thorsberg stone are mixed with cephalopod fossils, often the long tubular shells of the Orthoceras – think of an ice-cream cone with tentacles – so common that the limestone is called orthoceratite.?

The sedimentation that formed the beds ran at a pace of one or two millimetres per thousand years. The trilobite shells and cephalopod casings gradually underwent a mineralogical transformation to become stone, and the chondritic meteorites in their midst also sustained an alteration to their chemical makeup; a process called diagenesis. Traces of the iron compound hematite in limestone gives the rock a reddish hue, and a reaction can occur with the embedded meteorites so that the iron is reduced, and a halo of gray limestone surrounds the specimen. In the early days the quarry labourers thought these to be blemishes in their product, and discarded many of the plates.

To add to the peculiarity of these interpolated Thorsberg samples, they have meteoric siblings elsewhere in the world. In March 2003, a fiery meteor shower lit up the Chicago suburb of Park Forest. The collected meteorites were identified as coming from the same asteroid collision as those found at Thorsberg. How do we know? Because they share similar signatures of radiogenic gases - minute traces of cosmogenic isotopes of helium and argon.?

There is one more twist. Cosmogenic isotopes are created by the effect of cosmic rays when an asteroid is broken up in collision and its interior is exposed to the radiation. Once a meteor enters Earth’s protective atmosphere the process ceases. This means the amounts of these isotopes measured in a meteorite can help discern the period it spent travelling in space.?

Intriguingly, the oldest of Thorsberg’s fossil meteorites — those found in the deepest layers of limestone — reflect a shorter space-travel time than that of the site’s samples with a younger terrestrial age. Jupiter’s gravitational puppeteering may well be the cause. An enormous impact, deep in space, pelted rocks at Earth via a cosmic conveyor belt for two million years, and continues to do so today, albeit at a reduced rate. Its first arrivals (Thorsberg), took about 50,000 years to make the journey, possibly hastened by a timely Jovian resonance** close to the massive collision; the later visitors (Park Forest), approximately 470 million years.?

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* A meteoroid not modified by melting, so still in possession of its constituent chondrules – tiny grains of gas and dust particles from the very early Solar System.?

** Asteroids and meteoroids orbit the Sun. When such a body's orbital period is a simple fraction of Jupiter’s, it is said to be in resonance.?