Flinders petrie dating system
For an isochron to be valid, each sample tested must (1) have had the same initial ratio, (2) have been a closed system over geologic time, and (3) have the same age.
Well-preserved, unweathered rocks that crystallized rapidly and have not been subjected to major reheating events are most likely to give valid isochrons.
In most cases, the changes in the Sr ratio are so large that an initial value can be assumed without jeopardizing the accuracy of the results.
When minerals with a low-rubidium or a high-strontium content are analyzed, the isochron-diagram approach can be used to provide an evaluation of the data.
Dissolved strontium in the oceans today has a value of 0.709 that is dependent on the relative input from the continents and the ridges.
In the geologic past, changes in the activity of these two sources produced varying Sr ratios over time.
Using estimates of measurement precision, the crucial question of whether or not scatter outside of measurement error exists is addressed.
Potassium-bearing minerals including several varieties of mica, are ideal for rubidium–strontium dating as they have abundant parent rubidium and a low abundance of initial strontium.
In contrast, Earth’s most abundant lava rocks, which represent the mantle and make up the major oceanic ridges, have values between 0.703 and 0.705.
This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70,000, it is quite significant.
At the time of crystallization, this produces a wide range in the Rb/Sr ratio in rocks that have identical − 1).
In practice, rock samples weighing several kilograms each are collected from a suite of rocks that are believed to have been part of a single homogeneous liquid prior to solidification.