Geological Dating | The Canadian Encyclopedia
Precision dating of a single rock resolves lingering uncertainties about the Red Planet's history landers set down on the Red Planet has carried the necessary equipment. Some specimens out of the more than 60, meteorites in radioactive isotopes within them, because the isotopes decay into. Radiometric dating is used to estimate the age of rocks and other objects based So, we start out with two isotopes of uranium that are unstable and radioactive. In fact, this form of dating has been used to date the age of rocks brought back . Radiometric dating--the process of determining the age of rocks from the decay . by the scraping action of a glacier, the chipping of a meteorite off of an asteroid , In fact, tens of thousands of uranium-series dates have been performed on.
With most igneous rocks, which begin life as molten material, calculating the ratio of a long-lived isotope, such as uraniumto its decay product, leadyields a very good estimate of just how old that rock is—how long ago its isotopes became locked in minerals crystallizing out from a molten mass.
The trouble is that different isotopic tracers yield wildly different dates for the most common variety of Martian meteorites, hunks of igneous rock called shergottites.
Solved: The Mystery of the Martian Meteorites - Scientific American
Grind up a whole shergottite, and the ratio of lead isotopes in the powder will suggest the rock is about four billion years old. If you instead look at various isotopes isolated within microscopic mineral grains inside the shergottite, you will conclude the rock is relatively youthful—only hundreds of millions of years old.
This conundrum has flummoxed researchers for years, leaving them divided about the timing and duration of Martian volcanic activity, or when the consolidation of the Martian core and mantle occurred. Now, however, the matter seems to be settled: In a report appearing in the July 25 edition of Nature, a team of scientists lead by Desmond Moser of the University of Western Ontario has presented substantial new evidence that shergottites are young.
We first looked very carefully at the minerals, scanning grain by grain so that we could really understand everything in context Every rock has a story to tell. Interpreting clues to uncover that story is where we come in.
An anonymous Moroccan middleman purchased the rock from the nomad and eventually sold it for an undisclosed sum to David Gregory, a Canadian physician who later donated it to the ROM. He also noted the presence of micron-scale grains of baddeleyite, a highly durable zirconium-rich mineral that is often used in uranium—lead dating.
Using a sophisticated scanning electron microscope, Moser and his colleagues first mapped the baddeleyite grains within a thin cross section of NWAthen closely studied each one for clues about its past. Many of the grains bore concentric bands of material indicative of gradual growth, which in turn suggested they formed in slow-cooling magma. Subsequent uranium—lead radiogenic isotope dating showed their age to be approximately million years.
The only remaining crystalline minerals, in fact, were vanishingly thin rims of silica-rich zircon that had flowed around and flash-frozen onto the baddeleyite grains—a sign of shock melting followed by cooling in the frigid vacuum of space. Uranium and lead isotopes extracted from grains with those rims yielded an estimated age of less than 22 million years.
After correcting for sources of lead contamination originating on Earth, such as the burning of leaded gasoline, measurements of lead isotopes from the bulk mineral matrix around the baddeleyite grains yielded a date in excess of four billion years.
Geological Dating For centuries people have argued about the age of the Earth; only recently has it been possible to come close to achieving reliable estimates. In the 19th century some geologists realized that the vast thicknesses of sedimentary rocks meant that the Earth must be at least hundreds of millions of years old.
On the other hand, the great physicist Lord Kelvin vehemently objected and suggested that the Earth might only be a few tens of millions of years old, based on his calculations of its cooling history. These discussions were rendered obsolete by the discovery of radioactivity in by the French physicist Henri Becquerel.
The existence of radioactivities of various kinds in rocks has enabled earth scientists to determine the age of the Earth, the moon, meteorites, mountain chains and ocean basins, and to draw up a reasonably accurate time scale of evolution.
It has even been possible to work out a time scale of the reversals of the Earth's magnetic field. Radioactive Clock The vast majority of atoms each composed of a nucleus surrounded by electrons are stable. Essentially, they will exist forever.
A critical few, however, are unstable. Their nuclei tend to emit particles spontaneously - ie, they are radioactive. Because of this particle emission, the original radioactive parent atom changes its identity, becoming a different, stable daughter atom.
This change takes place at a known rate determined by the half-life; ie, the time required for one-half of the original number of radioactive atoms to convert to the stable daughter product. The remaining number of radioactive atoms is halved every half-life.
Radioactive elements of use in geological dating have relatively long half-lives. A good example is rubidium, which changes to strontium at a rate of one-half every 50 billion years. Therefore, a rock can be dated by measuring how much of its original rubidium content has changed into strontium. The other key dating techniques involve uranium transforming to lead at a rate of one-half every million years, uranium becoming lead at one-half every 4.
These radioactive processes present a set of natural clocks which reveal when the rock was formed, or when it was last heated severely. The well-known carbon method involves the conversion of radioactive carbon to stable nitrogen at a rate of one-half about every years. Sinceradiometric methods have been developed to a very sophisticated level in several countries, including Canada.
Solved: The Mystery of the Martian Meteorites
It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age. This uniformity demonstrates that the principle is reliable. When disturbed rocks are studied, the different techniques may give different readings, and much research has been carried out on how to interpret such results. It often proves possible to date even severely disturbed rocks. Only the tiniest fraction of the Earth, the crust, is accessible.
Two approaches have been developed to circumvent these problems. The first involves sampling as much of the Earth's crust as possible and dating these rocks. The Earth certainly must be older than the oldest terrestrial rocks found. Samuel Bowring, now of the Massachussetts Institute of Technology, and his coworkers Ian Williams and William Compston of the Australian National University at Canberra have shown that a small area of metamorphic rock in northern Canada, known as the Acasta gneiss, is the oldest known intact solid piece of the Earth's crust.
Using the uranium-lead technique they dated zircon crystals from the gneiss located southeast of Great Bear Lake in the NWT and showed that it was formed almost 4 billion years ago.
Therefore it is clear that the Earth is over 4 billion years old.