GEOCHRONOLOGY HONOURS 2008

Lecture 02The Rubidium – Strontium System

The Rb-Sr system

Strontium has four naturally occurring isotopes all of which are stable

–38Sr88, 38Sr87, 38Sr86, 38Sr84

Their isotopic abundances are approximately

– 82.53%, 7.04%, 9.87%, and 0.56% However the isotopic abundances of strontium

isotopes varies because of the formation of radiogenic Sr87 from the decay of naturally occurring Rb87

Therefore the precise isotopic composition of strontium in a rock or mineral depends on the age and Rb/Sr ratio of that rock or mineral.

Rb-Sr Isochrons

If we are trying to date a rock using the Rb/Sr system then the basic decay equation we derived earlier has the form

Sr87 = Sr87i + Rb87(et –1) In practice, it is a lot easier to measure the ratio

of isotopes in a sample of rock or a mineral, rather than their absolute abundances. Therefore we can divide the above equation through by the number of Sr86 atoms which is constant because this isotope is stable and not produced by decay of a naturally occurring isotope of another element.

Rb-Sr Isochrons This gives us the equation

87Sr/86Sr = (87Sr/86Sr)i + 87Rb/86Sr(et – 1)

To solve this equation, the concentrations of Rb and Sr and the 87Sr/86Sr ratio must be measured.

The Sr isotope ratio is measured on a mass spectrometer whilst the concentrations of Rb and Sr are normally determined by XRF or ICPMS.

Rb-Sr Isochrons

The concentrations of Rb and Sr are converted to the 87Rb/86Sr ratio by the following equation.

87Rb/86Sr = (Rb/Sr) x (Ab87Rb x WSr)/(Ab86Sr x WRb), where Ab is the isotopic abundance and W is the atomic weight.

The abundance of 86Sr (Ab86Sr) and the atomic weight of Sr (WSr) depend on the abundance of 87Sr and therefore must be calculated for each sample.

What can we learn from this?

1. After each period of time, the 87Rb in each rock decays to 87Sr producing a new line

2. This line is still linear but is steeper than the previous line.

3. We can use this to tell us two important things

• The age of the rock• The initial 87Sr/86Sr isotope ratio

Determining the Age of a Rock

Determining the Age of a Rock

Let’s look now at the initial ratio

The Fitting of Isochrons

After the 87Sr/86Sr and 87Rb/86Sr ratios of the samples or minerals have been determined and have been plotted on an isochron, the problem arises of fitting the ‘best’ straight line to the data points.

The fit of data points to a straight line is complicated by the errors that are associated with each of the analyses

The Fitting of Isochrons

Equations for Calculating the Best Slope and Intercepts of a Straight Line

The initial ratio How do we know if a series of rocks are co-

genetic?

For rocks to be co-genetic, implies that they are derived from the same parent material.

This parent material would have had a single 87Sr/86Sr isotope value, ie the initial isotope ratio

Therefore, all samples derived from the same parent magma should all have the same 87Sr/86Sr isotope ratio

If they don’t, it implies that they are derived from a different parent source.

Errorchrons and MSWD Values – A brief view A line fitted to a set of data that display a

scatter about this line in excess of the experimental error is not an isochron.

The sum of the squares of miss-fits of each point to the regression line, may be divided by the number of degrees of freedom (number of data points minus two) to yield the Mean Squared Weighted Deviates (MSWD).

MSWD values give an indication of scatter and can therefore be used to indicate whether an errorchron or isochron is indicated by the data.

MSWD values should be near unity to be indicative of an isochron. Values over 2.5 are definitely errorchrons.