Radiometric dating – The right tool for the right job Not just U-Pb… There are 40 different dating techniques and each one has an appropriate use –uranium-lead (between 1 million – 4.5 billion years) –radiocarbon (between present – 45,000 years) Multiple techniques can be used on the same sample to get more reliable dates –(e.g. use Uranium-Lead and Potassium-Argon). More than one mineral can be tested from one sample to get a more reliable date.
Activity Instructions Groups of 2-3 people Materials – data sheet and graph, bag of M&M’s (NOT for eating… yet) and a writing utensil We will go through six half-lives, measuring how many parent nuclides are left after each round –Starting at T=0 with 100 parent nuclides –“M” side up = parent nuclide
Graph three lines Your group’s data The class average data A “perfect” half life –T=1 50 –T=2 25 –T=3 12.5 –Etc…
What do we know from radiometric dating? Oldest known rocks on earth are 4.03 billion years old Meteorites are all around 4.56 billion years old and give us our best handle on the limits of the age of our solar system, and the Earth But wait, there’s more old stuff!
Jack Hills Metaconglomerate 3.0 billion years old (part of the Narryer Gneiss Terrane in Western Australia) Conglomerates are a sedimentary rock, made of bits of other rocks and minerals that get cemented together. Think concrete. Metaconglomerates are conglomerates that have been metamorphosed.
Why zircons? Zircons are everywhere –They form in igneous and metamorphic rocks and then erode and are incorporated into sedimentary rocks Zircons are tough –They are hard and can survive intact as the rest of the rock around it erodes. Then they can be transported and incorporated into sedimentary rocks. Zircons are good for radiometric dating –When they form in igneous and metamorphic rocks, they pull radioactive parent nuclides (isotopes) into their crystal structure like Uranium 238, Uranium 235 and Thorium 232
3. Isolate zircon crystals 1. Crush sample 2. Separate zircons from other minerals How to date a zircon