As a planetary geologist studying the surface of the Moon and Mercury, I think daily about old rocks—up to about 4.5 billion years old. These planetary bodies—smaller than our Earth—have ancient crusts, which have not been greatly disturbed or overhauled through their history. They do not have operating plate tectonic systems to create and destroy crust in long and regular cycles. Nor do they (currently, at least) have surrounding atmospheres with active climates to weather and erode surface materials. This means that the rocks on the Moon and Mercury are witnesses to the earliest part of the inner solar system's history—a record that has been almost completely erased from the surface of Earth.
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| The heavily cratered surfaces of the Moon (left) and Mercury (right) bear the scars of many meteoroid impacts that have occurred during their 4.5-billion-year lifetimes. Credit: NASA |
When I first started thinking about this blog and its central theme, it seemed to me that one of the most obvious rocks to best represent the Earth would be the oldest. But deciding on what is truly the oldest terrestrial rock is not as easy as it sounds. Rocks are made up of minerals, and over time, the rocks may be changed through the actions of heat, pressure, and/or chemistry so that the original rock gets broken down. At least some of the constituent minerals, however, can survive and become incorporated into new rocks.
Some of my colleagues at the Department of Terrestrial Magnetism work on the analyses of rocks that are amongst the most ancient found on Earth, but I want to leave those as a subject for another postcard and another day. Instead, I want to focus here on the oldest minerals that have yet been discovered.
New work published this week in Nature Geoscience provides an age for what is thought to be the oldest fragment of material from Earth's crust. This grain is a fragment of the mineral zircon. Zircons are found ubiquitously in all kinds of rocks—igneous, metamorphic, and sedimentary. They are hard (with a value of 7.5 on the Mohs hardness scale), which together with their chemical inertness, means they are difficult to destroy. A very ancient zircon grain can therefore have inhabited a number of different rocks during its lifetime. In the new work conducted by John Valley, from the University of Wisconsin, and coauthors, a zircon grain from a sandstone outcrop in the Jack Hills of Western Australia is shown to be 4.4 billion years old.
Valley and his team used a radiometric dating technique to find the age of this rare mineral fragment. Although zircon is composed almost entirely of the three elements zirconium, silicon, and oxygen, other elements can be incorporated into its mineral structure in very small (trace) amounts as it grows. In particular, they measured the numbers of uranium and lead atoms in the sample. Certain isotopes of uranium decay at fixed rates to form isotopes of lead. If a specific sample has remained a 'closed system', the number of these measured uranium and lead isotopes can be used as a chronometer to tell us the age of the sample.
This zircon fragment dates to the Hadean eon—Earth's earliest geologic period—that was characterized by hot and violent conditions. The grain, along with other slightly younger zircons, is evidence that a solid crust formed soon (geologically speaking) after Earth's formation (about 4.6 billion years ago) and the giant impact event that likely formed the Moon and created an Earth-wide expanse of molten material, known popularly as a 'magma ocean'.
The majority of Earth's surface may not be as old as what we see on some of our solar system neighbors, but these little zircon pieces from Australia are about as old as we are going to find. If they have survived this long, I think that they should definitely make the celestial trip to meet our alien planetary geologists. Perhaps our hypothetical friends are somewhere 4.4 billion light-years away and can even observe the Hadean Earth firsthand.
Some of my colleagues at the Department of Terrestrial Magnetism work on the analyses of rocks that are amongst the most ancient found on Earth, but I want to leave those as a subject for another postcard and another day. Instead, I want to focus here on the oldest minerals that have yet been discovered.
New work published this week in Nature Geoscience provides an age for what is thought to be the oldest fragment of material from Earth's crust. This grain is a fragment of the mineral zircon. Zircons are found ubiquitously in all kinds of rocks—igneous, metamorphic, and sedimentary. They are hard (with a value of 7.5 on the Mohs hardness scale), which together with their chemical inertness, means they are difficult to destroy. A very ancient zircon grain can therefore have inhabited a number of different rocks during its lifetime. In the new work conducted by John Valley, from the University of Wisconsin, and coauthors, a zircon grain from a sandstone outcrop in the Jack Hills of Western Australia is shown to be 4.4 billion years old.
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| Fragment of a 4.4 billion year-old zircon grain (about 0.5 mm in length). Credit: John Valley |
This zircon fragment dates to the Hadean eon—Earth's earliest geologic period—that was characterized by hot and violent conditions. The grain, along with other slightly younger zircons, is evidence that a solid crust formed soon (geologically speaking) after Earth's formation (about 4.6 billion years ago) and the giant impact event that likely formed the Moon and created an Earth-wide expanse of molten material, known popularly as a 'magma ocean'.
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| View on Earth during the Hadean eon? Credit: Mark Garlick - Space Art |
The majority of Earth's surface may not be as old as what we see on some of our solar system neighbors, but these little zircon pieces from Australia are about as old as we are going to find. If they have survived this long, I think that they should definitely make the celestial trip to meet our alien planetary geologists. Perhaps our hypothetical friends are somewhere 4.4 billion light-years away and can even observe the Hadean Earth firsthand.
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