CD002: Geochronology and initial conditions

Claim CD002:

Source:

Response:

1. Isochron methods do not assume that the initial parent or daughter concentrations are known. In basic radiometric dating, a parent isotope (call it P) decays to a daughter isotope (D) at a predictable rate. The age can be calculated from the ratio daughter isotope to parent isotope in a sample. However, this assumes that we know how much of the daughter isotope was in the sample initially. (It also assumes that neither isotope entered or left the sample.)
   
   With isochron dating, we also measure a different isotope of the same element as the daughter (call it D₂), and we take measurements of several different minerals that formed at the same time from the same pool of materials. Instead of assuming a known amount of daughter isotope, we only assume that D/D₂ is initially the same in all of the samples. Plotting P/D₂ on the x axis and D/D₂ on the y axis for several different samples gives a line that is initially horizontal. Over time, as P decays to D, the line remains straight, but its slope increases. The age of the sample can be calculated from the slope, and the initial concentration of the daughter element D is given by where the line meets the y axis. If D/D₂ is not initially the same in all samples, the data points tend to scatter on the isochron diagram, rather than falling on a straight line.
   
   
2. For some radiometric dating techniques, the assumed initial conditions are reasonable. For example:
   • K-Ar (potassium-argon) dating assumes that minerals form with no argon in them. Since argon is an inert gas, it will usually be excluded from forming crystals. This assumption can be tested by looking for argon in low-potassium minerals (such as quartz), which would not contain substantial argon daughter products. ⁴⁰Ar/³⁹Ar dating and K-Ar isochron dating can also identify the presence of initial excess argon.
   • The concordia method is used on minerals, mostly zircon, that reject lead as they crystalize.
   • Radiocarbon dating is based on the relative abundance of carbon-14 in the atmosphere when a plant or animal lived. This varies somewhat, but calibration with other techniques (such as dendrochronology) allows the variations to be corrected.
   • Fission-track dating assumes that newly solidified minerals will not have fission tracks in them.

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