EPSC201 Lecture 21 Notes.doc

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 - Lecture 21 Notes Radiometric dating is based on the decay of unstable isotopes. Uranium is unstable and will de- cay at a constant rate. Uranium-238 will eventually form lead-106. Uranium-235 will decay down and form lead-107. The key is that they decay by emitting alpha radiation, which consisted of two protons and two neutrons. If there are a large number of uranium atoms in the sample, we can measure the constant decay rate to determine how old the rock is. If we know the rate of decay, concentration of uranium and lead present, we can determine how old the rock sample. Only trace amounts of uranium (ppm) need to be present for this to be applicable. It can be Uranium-238 or 235. In reality, uranium breaks down into many different atoms long its path to lead. So we can measure all the inter- mediates to help determine the original concentration of uranium, and therefore the age of the rock. Natural lead has a mass of 207, which is different then radioactively produced lead. So we can distinguish between the two types of lead. There is beta plus and beta minus decay. Beta minus is the loss of an electron. Beta plus is the loss of a positron. Key point – if you know how much parent and daughter isotope is present, you know how much parent isotope was originally present If you have a sedimentary rock con- taining zircon crystal, you can mea- sure the age of the zircon crystal. So the dating will only tell you the formation of the zircon crystal, not the age of the sedimentary rock from which it was extracted. If magma produced from sedimen- tary rock melting, and some of the zircon crystal remains, then your dating process will give you an age older then the age of the igneous rocks. This is called inherited zir- con, which never actually formed at the time of igneous rock formation, but at some earlier time. If you have a neutral atom, and it undergoes beta minus decay, then it loses a minus charge, and will be left with a positive charge. N = present amount of parent isotope measured N not = the initial amount of parent Isotope = sum of present parent isotope + daughter isotope Half-life is the amount of time it takes for the original amount of parent isotope to decay to half its original value. Different isotopes have different half-life’s. Note that half-life’s are independent of amount of isotope present. Using carbon dating is good for very recent samples, since the half-life is on the scale of thou- sands of years. Very old rocks use uranium dating, since it has the longest half-life. We are not restricted to measuring the half-life. We can measure the half-life multiple times, which adds up to be much longer then individual half life. For relatively young samples, we use carbon-14 dating. Our atmostphere is largely composed of nitrogen. We have radiation from the sun incoming to the earth (neutrons), which occasionally re- act with nitrogen, which forms carbon-14. The plants absorb carbon dioxide, so our plant life takes up the carbon-14. The carbon-14 is unstable and will decay. Animals eat the plants, and take up the carbon-14. When these animals die, the organic material contains carbon-14, and flows throughout the food chain. After the organic material gets buried, no more carbon-14 is consumed. The carbon-14 will decay through beta minus radiation. This allows the organic sample to be dated, through carbon-14 de- cay analysis. Prior to the Pre-Cambri- an era, there was no shellfish. The fist fossils come in around 700 mil- lion years ago, and this is where fossil dating be- gins. Anything older then 700 mya must be radioactive- ly dated. Understand the concept of geological time, but don’t memorize any of the names. The next section of the course involves the ori- gin and evolution of life. We will specu- late this from the view of a geologist. We think the oldest fossilized bacteria is about 3.5 billion years. There are a few examples that are 3.8 billion years old. The example is chert, which is a silica precipitate (SiO2 fine grained pre- cipitated in the ocean). If it forms a nice mud, and might be able to pre- serve bacteria as a fossil. These shapes on the left look similar to mod- ern bacteria. Two thirds of the paleon- tological community believe this to be an early life form, but nobody has proven this. They do look familiar to modern bacteria. So lets say, we know life started at least 3.5 billion years ago. Magnetobacteria – a magnetic field will cause them to be pulled in a certain direction. There are fossils of 850 million years ago. Other examples of 1.2 billion years old. Nobody disputes that the fossils are bacteria. There are algal mud colonies, that form layers of algae with layers of sand. There is only one area on earth where these algae still flourish (western Australia – shark bay – at the tidal inter- face). They build up these little shapes, called stromatolites. We can see stramatolites fossilized from 1.5 billion years ago. These Stromatolites survived depths of up to 20 kilometers, because they were preserved in sili- ca, surrounded by marble. The marble flowed, but the silica stayed stationary, and was preserved. So there was definitely life up to 1.5 billion years old. The oldest reliable record is 3.8 billion years ago, but is under debate still. This photograph on the left was taken in 2004, from a C1- carbonaceous chondrite (carbon rich meteorite). Carbonaneous chondrites are a very rare type of mete- orite. There is some similarities between the carbon structures on the left, and the modern cyanobacteria on the right. In this chondrite, there were many different amino acids present, which are the building blocks of life. These chondrites have very complex carbon make ups, which often are the same molecules that make up life. Not necessarily evidence, but it’s a start. With meteorites, we have to be careful about conamination from bacteria and molecules on Earth. The first map of Mars was made in 1888 by Schiaparelli. Lowell made a great ef- fort to map mars, mapping canals and river valleys. The low resolution picture shown below looks like a face. It was taken by a passing space explo- ration craft. People thought it was a sculpture, but after further analysis, it tu
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