Rubidium strontium dating half life
Can we find in rocks a natural clock that has been operating since they formed?It was discovered that some chemical elements, notably uranium and thorium, are strongly radioactive.
In some parent-daughter pairs, the daughter is still radioactive and subject to further decay to a new daughter.In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar method, although the two are not exactly alike. When an igneous rock is first formed, its minerals will contain varying concentrations of rubidium and strontium, with some minerals being high in rubidium and low in strontium, others being high in strontium and low in rubidium.We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: as we have noted, rubidium substitutes for potassium, and strontium for calcium.These elements occur naturally in nearly all rocks, and they account for the radioactivity you could observe with a Geiger counter.The radioactive decay process can be described simply as the transformation of an unstable radioactive atom (called the parent) to a new atom (called the daughter) that may differ in atomic number, atomic mass, or both.In other cases, decay yields a daughter that is non-radioactive (stable) and will remain unchanged for the rest of time.
The time interval it takes for the parent atoms to decay by half is always the same, no matter how much of the parent element remains.
So we have every reason to think that rocks when they form do incorporate strontium, and Sr in particular.
However, there is still a way to extract a date from the rock.
Concurrently, the crust will become enriched in Rb relative to Sr.
A brief introduction to the radiogenic relationship between Rb and Sr: Rubidium-87 decays to Strontium-87 by beta decay according to the following equation: The amount of 87Sr found in a sample at any time is determined by the decay constant of 87Rb, the initial amount of 87Sr in the sample, the time since the initial time and the ratio of Rb to Sr in the system. Where lambda is the decay constant and t is the age of the system.
As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotite, potassium feldspar, and hornblende.