Their chemical composition says they didn’t erupt directly from the mantle, but were instead formed from pre-existing basalts that were pulled below the surface, heated, and then recycled back to the surface to form the granites we see today.
In the process, the physical remnants of the older rocks were destroyed, but their isotopic signatures remain.
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Shape and colour of the fragments also offered clues about their origins.“The spherules often have surface pits and in some cases microcraters,” the researchers write, “indicating relative velocities high enough to fracture the spherules on impact with one another, or other objects, after solidification.”Not everyone's convinced, though.
But the next step, according to the research team, is to uncover spherules in more locations and start to figure out how far the debris spread.
Now, the possibility of a meteorite impact may be thrown into the mix.
To draw clues from the spherules, the researchers analysed the size, structure, layout and abundance of the particles they had uncovered, and compared the data to evidence of other impact sites.
The find is important in part because on most of the planet’s surface, geological processes have long ago erased visible traces of the Earth’s primitive crust.
There are a few places with rocks believed to be at least four billion years old, and in Western Australia, geologists have found crystals, called zircons, that might have formed 4.4 billion years ago, only 150 million years or so after the Earth’s formation.
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Robert Allsopp Robert Allsopp has worked in film, theatre and television as a specialist costumier for the last 30 years.