To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand,
And Eternity in an hour.
—WILLIAM BLAKE, “AUGURIES OF INNOCENCE”
I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.
—ISAAC NEWTON
MORE PRECIOUS THAN DIAMONDS
If you flip channels and watch some of those home-shopping shows or infomercials on TV, sooner or later you see someone hawking gaudy jewelry made of “cubic zirconia crystals.” They ooh and ahh about the diamond-like appearance of the cubic zirconia crystals, for sale at a fraction of the cost of real diamonds—but who can tell? Likewise, you can find many vendors online touting their cubic zirconia jewelry with texts like this:
Ziamond is where discerning customers shop for the best high quality cubic zirconia jewelry exclusively set in precious metals like 14k gold, 18k gold and luxurious Platinum. Our cubic zirconia jewelry and gems are the epitome of the finest lab created man-made diamond simulants that the world has to offer and are backed with our comprehensive Lifetime Guarantee. Ziamond’s amazing cubic zirconia jewelry and cz stones are precisely cut then polished to fine diamond standards, ensuring our customers the true look and feel of a brilliant genuine diamond. You can wear Ziamond’s cubic zirconia jewelry daily with confidence and clean it just as if you would your fine diamond jewelry. With over a century of fine jewelry experience our staff has the technical and artistic skills to create and execute any cubic zirconia jewelry design while maintaining attention to details, design quality and craftsmanship expected with fine jewelry. Whether it’s diamond look cubic zirconia rings, cubic zirconia wedding rings, cubic zirconia engagement rings, cubic zirconia bracelets or cubic zirconia earrings, we look forward to providing you the Ziamond experience and see for yourself why we are recognized as a leader in our field as the best cubic zirconia jewelry company.
There’s nothing wrong with cubic zirconia if you want a cheap faux diamond to impress your friends. But it’s not a real diamond, and zirconium is not even that rare in nature. Cubic zirconia is just a synthetic gem made of zirconium dioxide (ZrO2). (In its natural mineral form, ZrO2 is known a baddeleyite, after Joseph Baddeley, a superintendent of a railroad project in Sri Lanka who first discovered it. Not a name you would hear the infomercial announcers using to sell jewelry.) Another mineral formed from the element zirconium is zirconium silicate (ZrSiO4), know as the mineral zircon (figure 12.1). Large crystals of zircons form an octahedron, a structure that looks like two pyramids glued together. They come in many different colors, including purple, yellow, pink, red, and clear, depending upon what impurities are in them and what has happened to their crystal structure.
Figure 12.1
A crystal of zircon. (Courtesy of Wikimedia Commons)
But although diamonds have great commercial value, the scientific information content of zircons is much greater, making them more precious scientifically than any diamond. Zircons are incredibly useful minerals to geologists. They form very hard, durable crystals, especially in granitic magmas during the last stage of cooling. Because zircon crystals have spaces for big atoms like zirconium, they also trap other large atoms that won’t fit in any other mineral and that were concentrated in the last stage of magma crystallization. These include extremely rare elements like uranium and thorium. Thus, you can take a crystal of zircon and analyze it for its uranium content. Zircons are widely used to produce dates in the uranium-lead or lead-lead method of dating and in another method known as fission-track dating.
Zircons are so durable and resistant to nearly everything that brute force methods must be used to extract them from granitic rocks. Typically, you use a crusher to reduce the original rock to a fine powder. Then you soak the powder in the most powerful acid on Earth, hydrofluoric acid (HF). It is so caustic you must work in a very good fume hood and wear lots of protection for your skin, eyes, and lungs. HF even dissolves many types of containers, so you have to keep it in special bottles. HF will break down nearly every other mineral in the rock except the zircons, so once the acid bath is finished, you rinse the residue with water, and you have concentrated zircons, ready for analysis.
Zircons are durable not only in the lab setting but in nature as well. When rocks weather into sand grains that go bumping and bashing in the sand along the bottom of a stream, zircons are among the most durable minerals. Even in the most heavily weathered sand, which is about 99 percent quartz (the most common and durable mineral on the earth’s surface), there will still be a small fraction of a percent in zircons as well. In fact, the presence of zircon (plus two other durable minerals in sand, tourmaline and rutile) has been used as a “ZTR index” to measure how extremely weathered and winnowed the sand or sandstone is. There are many geologists who specialize in zircons because they can be very powerful tools for all sorts of geological problems.
WHO’S ON FIRST?
Zircons are particularly useful, because geochronologists find that they are often the best mineral for dating very ancient rocks. The dates on some of the meteorites (chapter 10) and moon rocks (chapter 11) came from zircons, and zircons work for dating ancient Earth rocks as well. Many of the oldest rocks on Earth have been dated by applying not only uranium-lead methods but also lead-lead and rubidium-strontium dating to zircons.
For many years, the oldest known rocks on Earth were the Amitsôq Gneisses (figure 12.2) from the Isua Supracrustal belt on the southwest coast of Greenland, which gave dates of 3.8 billion years. They represented some of the earliest crustal rocks formed, including small blocks of proto-continental crust (now metamorphosed into gneisses), plus slices of ancient proto-oceanic crust (known as greenstones), and even some pieces of the earliest mantle (peridotites). However, this was not the maximum age, because these rocks had been highly metamorphosed and altered, so it was always possible that their true age was quite a bit older. What they did tell us was that the earliest crust of the earth was made of very small blocks of continental crust (proto-continents) that floated in a very thin, hot oceanic crust made of lavas that erupted directly from the mantle. These weird lavas, known as komatiites, were made entirely of mantle minerals like the greenish silicate called olivine. They are even richer in magnesium and iron than the basaltic lavas that make up all the oceanic crust today. They tell us that the early earth was still very hot and that the crust was thin and highly mobile and easily remelted. It still had not differentiated into the mature types of oceanic and continental crust rocks we have today. In fact, the crustal blocks were so small and thin that true plate tectonics probably did not exist yet. Komatiite lavas could only form under these conditions and no longer erupt anywhere on Earth now that oceanic crust has matured and the temperature and chemistry of the upper mantle has changed. Today, only lavas that cool to form basalts erupt to form the ocean floor.
Figure 12.2
The Isua Supracrustals of southern Greenland, dated at 3.8 billion years old. (Courtesy of Wikimedia Commons)
Then, in 1999, another ancient rock, known as the Acasta Gneiss (figure 12.3), pushed the record for the earth’s oldest rock back from 3.8 billion years to 4.031 ± 0.003 billion years. The Acasta Gneiss is another piece of proto-continental crust, but it is from a block known as the Slave Terrane, which got its name from Great Slave Lake in the Northwest Territories of Canada. This rock was mentioned in all the textbooks and held the record for a number of years.
Figure 12.3
The Acasta Gneiss near Great Slave Lake, Canada, dated at 4.01 billion years old. (Courtesy of Wikimedia Commons)
Just as in athletics, however, records are meant to be broken. In 2008 the Nuvvuagittuq Greenstone belt in northwest Quebec, on the east shore of Hudson Bay, gave dates of 4.28 and 4.321 billion years. This was determined not by direct uranium-lead dating, but by samarium-neodymium dating of the lavas in the greenstone belt. However, this date is controversial. Many scientists think that the dates of 4.28 billion years and older are not the age of the rocks but the age of the parent material that was remelted to form these rocks. The oldest uranium-lead date on the zircons from the rocks themselves suggest that they are really only around 3.78 billion years old. Even if this is true, it is evidence of the oldest crustal formation around 4.28 to 4.32 billion years ago. Given the history of research, we can expect some geologist to find a rock even older before long.
Notice that the oldest Earth rocks are no older than 4.32 billion years, yet the oldest materials of the solar system (meteorites and moon rocks) are at least 4.55 billion years old. Why the difference? The answer is plate tectonics, and the deep weathering of the earth’s surface caused by water and wind. The earth’s surface is constantly being recycled and remodeled by the motion of plates melting and plunging into the mantle, then being born again. The moon, by contrast, has a dead surface with no plate tectonics, so some of its rocks date to 4.5 billion years ago when it formed. Meteorites that formed with the early solar system, like the carbonaceous chondrites discussed in chapter 10, have been unaltered since they cooled, so they give the oldest dates of all.
COOL EARTH
These are the dates for the oldest rocks on Earth, but they are not the oldest Earth materials known. That distinction goes to a handful of zircon sand grains (figure 12.4) from a much younger sandstone found in the Jack Hills of Western Australia. Each individual grain can be dated by uranium-lead methods, so they give a scatter of ages. But the oldest grains of all give an age of 4.404 billion years, at least 100 million years older than the 4.3-billion-year-old materials from Quebec. Thus, the current record holder for the oldest material from Earth (that is, not a meteorite or moon rock) is 4.4 billion years. These sand grain dates put us closer and closer to the age of moon rocks and meteorites, but we still have a gap of about 200 million years. This is about the same time span as the beginning of the Age of Dinosaurs (Late Triassic) until today, so it is not a trivial amount of time.
Figure 12.4
Microphotograph of a zircon grain from the Jack Hills of Australia, which gives a date of 4.4 Ga. It provides evidence that the earth was covered by liquid water at that early date. (Courtesy of J. Valley)
But those same tiny zircon sand grains held even more surprises. Not only did they give the oldest known dates, but when scientists analyzed the tiny bubbles of gases trapped inside them, they found evidence of the early atmosphere from over 4 billion years ago. These bubbles had oxygen isotopes in them that suggested the earth had liquid water on its surface as early as 4.4 billion years ago!
Prior to this discovery geologists had always assumed that the earth took a long time to cool from its molten state at 4.55 billion years ago. Most thought that the earth took about 700 million years to cool down below the boiling point of water (100°C), because that was the age of the oldest sedimentary rocks that would have been produced by running water (the Isua Supracrustals from Greenland mentioned above, which are 3.8 billion years old). But the Jack Hills zircons turn that assumption inside out. If they truly indicate the presence of liquid water on the earth 4.4 billion years ago, then it took only 200 million years for the earth to cool from its molten state to a condition that was below the boiling point of water. This also suggests that there were not as many meteorite impacts during this time interval, or the oceans would have been vaporized over and over again. Taken together, these data suggest what is now called the “cool early Earth hypothesis.”
So where did this early Earth water come from? Traditionally, geologists thought that it was water trapped inside the earth’s mantle when it cooled, gradually escaping through volcanoes in a process called degassing. But lately, chemical analyses of extraterrestrial objects match the chemistry of the earth’s oceans (especially carbonaceous chondrite meteorites—see chapter 10). This suggests that there was a lot of water trapped in the debris of the early solar system (of which the chondrites are remnants). The same is true of moon rocks, which do not have much water in them today, but apparently were pretty wet when the solar system formed. If this is so, then the earth was born with its water already present as it cooled and condensed. It only required its surface temperature to drop below 100°C for that water to form the first oceans.
One thing we can rule out is comets. Although comets are often called “dirty snowballs” because they are made mostly of dust and water ice, chemical analyses of four comets now show that their geochemistry is very different from the earth’s water. Thus, the popular idea that comets impacted the early earth and melted to form its oceans can be dismissed.
Those tiny zircon sand grains from the Jack Hills had one more surprise in them. In 2015 a paper was published on the tiny crystals of graphite that were also trapped inside them. Graphite is more familiar to most people as a mineral form of crystalline carbon, the same mineral that makes your pencil “lead.” Amazingly, the geochemical data from that graphite was consistent with the isotope ratios of carbon found in life! These particular zircon grains gave dates of 4.1 billion years, so they were not as old as the oldest zircons with water chemistry in them at 4.4 billion years. Nevertheless, this is a startling piece of data. Previously, the oldest carbon that had the right chemistry to be produced by life, as well as possibly the oldest fossils, came from those Isua rocks from Greenland, dated at 3.8 billion years. The Jack Hills zircons are 300 million years older than the previous record holder. And the oldest strong fossil evidence of life comes from the Apex Chert in the Warrawoona Group of Western Australia, dated at 3.5 billion years, and the Fig Tree Group of South Africa, dated at 3.4 billion years. So this extends the origin of life much earlier than previously supposed and not that much later than the early oceans on the cool early earth.
Once more, this much older evidence of life, just like the evidence of early oceans from the same zircons, forces us to revise our ideas about the early earth. Based on the dates of the moon craters (which all cluster between 3.9 and 4.4 billion years in age), we assumed that the early earth must have also undergone intense bombardment of leftover debris from the solar system prior to 3.9 billion years ago. But the evidence of liquid water oceans at 4.4 billion, and possibly even life at 4.1 billion, makes it appear that the bombardment of Earth was much less intense than previously supposed.
By the time this book is published, the odds are good that even more amazing discoveries will be announced, and there may be a yet older candidate for the oldest rock on Earth. But that is a good thing. That is a sign that the science of early earth geology is an active and vibrant field, always yielding groundbreaking discoveries. For some, it may be frustrating to write a book that is out of date before it is published. But science marches on, and we are always learning new and more surprising things about the earth every time another critical analysis is published.
FOR FURTHER READING
Chambers, John, and Jacqueline Mitton. From Dust to Life: The Origin and Evolution of Our Solar System. Princeton, N.J.: Princeton University Press, 2013.
Gargaud, Muriel, Hervé Martin, Purificacíon López-García, Thierry Montmerle, and Robert Pascal. Young Sun, Early Earth, and the Origins of Life: Lessons for Astrobiology. Berlin: Springer, 2013.
Hazen, Robert M. The Story of the Earth: The First 4.5 Billion Years from Stardust to Living Planet. New York: Penguin, 2013.
Shaw, George H. Earth’s Early Atmosphere and Oceans, and the Origin of Life. Berlin: Springer, 2015.
Ward, Peter, and Joe Kirschvink. A New History of Life: The Radical New Discoveries About the Origin and Evolution of Life on Earth. New York: Bloomsbury, 2015.
