Posted: Nov 26, 2013 8:36 am
Blackadder wrote:cbm1203 wrote:I'm trying to understand how radiometric dating accounts for the time lag between the synthesis of radioisotopes and their subsequent deposition on proto-Earth. Assuming that radioisotopes begin decaying soon after they are formed in supernovae, then it would seem that radiometric dating indicates when the isotopes began decaying rather than the age of Earth in which are found today. If all radiometric dating on Earth points to a common starting point, then that could simply mean they all came from the same source supernova. If the time between radioisotope synthesis and the formation of our solar system is very small, then the age of isotopes is a reasonable estimate of the age of Earth. But if time between the synthesis of the isotopes and the formation of Earth is a significant fraction of the age of the isotopes, then Earth would be younger. But how this lag would be determined is not obvious. Please clarify my understanding.
Radiometric dating works on the principle of RATIOS of parent isotopes and daughter products (caused by the decay of the parent) to be found in a particular sample. When the parent isotope was first formed is not relevant to this exercise. What is relevant is how much of the daughter element(s) may have been present in the sample at the time of its formation and whether any subsequent contamination by external daughter elements may have taken place. The former is addressed by using multiple isotopic tests upon various minerals within the same sample to establish how much of the daughter products would have been initally present, in order for that variable to be fixed in the age equation.
There is a very good paper (actually written for Christians would you believe) which explains this very well in simple language. I refer you to page 4 of this paper:
http://www.asa3.org/ASA/resources/wiens.html#page%201
Yes. To add a small thing, one of the key findings is that when a specific isotope is captured in solid rock, it usually stays there because it can't get out. But the point about solid rock is important, because usually what you measure is the age when the rock formed. It could have been part of a lava flow, which means melted liquid rock, from which gaseous isotopes can easily escape. But once the rock solidifies, the radio isotope clock starts ticking.
There can still be challenges involved, because the kinds of minerals the rock contains depends on how hot it once was before it solidified. If the rock barely melted, chances are it contains various forms of crystals that didn't melt, and which therefore contain different isotope radios from the main rock body. Therefore to do a proper radiometric analysis you need make sure your rock doesn't contain these kinds of xenoliths. You'll not be surprised to find out that creationists often include the xenoliths in their dating attempts on purpose in order to give false pictures about the reliability of radiometric dating. In fact, an often cited example by creationists is the solidified lava flows from Mount St. Helens which erupted in the 1980's, but was subsequently dated to be millions of years old. That's because the lava flows contained xenoliths that hadn't completely melted in the lava, and therefore had skewed isotope ratios compared to the rest of the lava. Suffice it to say, scientists know how to look for and find xenoliths in their sample and therefore know when a sample is suitable for radiometric dating or not. Creationists won't tell you this.
Anyway you have molten rock, inside it there's a small piece of a radiactive isotope that slowly decays over time and escapes from the liquid rock. After some time the rock cools and solidifies, so there will be a certain amount of the starting isotope and a certain amount of decayproduct. If the rock is solid, the decayproduct can't escape and therefore you can work back through the ratios and calculate how much of the parent isotope there was in the rock when it first formed. From this you can then calculate the age using the decay-law of course.