Meteorite impacts can be catastrophic events in a planet’s history, melting rocks, changing atmospheric chemistry and wreaking general havoc.
However, influences may also have created the Earth’s continents, which supported ecological niches life startedeven developed metal ores.
In a new study published in Earth and Planetary Science Letters, we examined what remains of the world’s oldest known impact crater: the 2.29 billion-year-old site at Yarrabubba in Western Australia.
We found evidence that hot water was circulating in fractures in the rock after the impact, possibly because the impact melted some of the ice that covered much of the planet at the time. Hot water in fractured rock may have provided a niche for early life forms, and its presence also has implications for our understanding of how metal ore deposits form in the Earth’s crust.
Space rocks have been important players in Earth’s history
Meteorite impacts seem to come and go on a 200 million year cycle over the course of Earth’s history.
All over the planet, about 200 major impact sites have been documented. The oldest of these is in Yarrabubba in Western Australia.
More than two billion years ago, a space rock slammed into the continental crust at Yarrabubba. This ancient crust had formed some 2.65 billion years before the present and had been profoundly altered by the impact.
The result was one crater with an estimated diameter of about 70 km, which today has eroded into a pimple. The shock of the impact was so great that it even melted parts of the surrounding crust, which is made of granite – a common type of stone you might see on fancy kitchen countertops.
In our new study, we looked closely at what the impact did to the crust’s chemistry. The chemical effects of meteorite impacts are not commonly studied, but they can be important in understanding the full range of environmental consequences.
Geologists forensically study minerals trapped in rocks to investigate what is happening inside the Earth, in much the same way that crime scene investigators study materials on site to determine their origin.
One kind of clue that geologists are particularly excited about is isotopes. These are different forms of a particular element.
Different isotopes of an element all behave the same in chemical reactions, but they contain different numbers of neutrons in the atom. This makes some isotopes unstable: over time, they will radioactively decay into different elements.
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We can take advantage of this radioactive decay. For example, we can determine the age of the Yarrabubba crater and surrounding rocks by measuring the ratio of uranium to lead isotopes, which acts like a stopwatch that counts the time since a mineral has grown.
This tells us the age because uranium decays into lead over time, and we know the rate at which this decay occurs. So measuring the isotopes of both elements in a sample shows us how much decay has occurred, which helps us calculate the age of the mineral.
Another way to use isotopes is in certain minerals where these ratios remain fixed over time and do not change. The isotopic signatures then become a powerful tool for tracking where material comes from, just as a person’s last name can provide a clue as to their family’s ancestry.
Messengers in a crystal bottle
We analyzed the isotopic compositions of lead in mineral grains from the crust around the crater at Yarrabubba.
We looked at feldspar crystals, typically the pink-colored grains in our granite benchtop, because they naturally contain lead but no uranium.
This is important because the lead isotopes trapped in this mineral reflect the composition of the fluid from which the mineral originally grew.
We found a wide range of lead isotope compositions, as well as new uranium-containing minerals growing in fractures in the grains at the time of impact, starting new stopwatches.
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The only plausible explanation for these modifications of isotopic signatures is that the impact must have generated networks of circulating hot water that infiltrated damage zones through the bedrock. In the case of Yarrabubba, the water may have come from the meteor that hit an ice sheet, as ice covered much of the globe 2.29 billion years ago.
The effects of securities
Our documentation of the circulation of heated water produced by an impact is important from two very different perspectives.
First, hot fluid systems may have fueled early life. The impacts were much larger and more frequent on the early Earth, and in some ways these violent and disruptive events would have hindered the evolution of complex life.
Yet researchers have shown that microbial communities can flourish where heat, water and nutrients meet pulverized rock: the very conditions that can trigger effects. Some have even suggested that effects are a fundamental part of planetary evolution and are necessary for creating a habitable planet.
Second, we can understand how ore deposits form by seeing how impact-generated hot water can transport metals. Some of the first sources of metal for early humans were meteorites, from which they chopped bits of metal for tools and jewellery.
Yet impact sites can contain greater concentrations of metals than just from the meteorite itself, which often vaporizes. Ore deposits usually form when there is a geological structure, for example a fracture in a rock, in which metals can be moved by fluids.
Impacts clearly shatter the crust, but they also provide circulating hot water. If metal is present in the target rocks to begin with, this hot water can take those metals with it and concentrate them into a richer deposit.