Determination of Age of Minerals and Rocks using Rubidium-87/Strontium Method Calculator

Age Calculation Formula:

\[ t = \frac{1}{\lambda} \times \frac{(^{87}Sr/^{86}Sr)_t - (^{87}Sr/^{86}Sr)_0}{(^{87}Rb/^{86}Sr)_t} \]

Decay Constant (λ):

1/year

(87Sr/86Sr) at Time t:

Initial (87Sr/86Sr):

(87Rb/86Sr) at Time t:

Calculated Age:

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1. What is the Rubidium-87/Strontium Dating Method?

Definition: This is a radiometric dating technique used to determine the age of rocks and minerals based on the radioactive decay of rubidium-87 to strontium-87.

Purpose: It helps geologists and geochronologists determine the absolute age of rocks, particularly useful for ancient igneous and metamorphic rocks.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ t = \frac{1}{\lambda} \times \frac{(^{87}Sr/^{86}Sr)_t - (^{87}Sr/^{86}Sr)_0}{(^{87}Rb/^{86}Sr)_t} \]

Where:

\( t \) — Age of the sample (years)
\( \lambda \) — Decay constant of Rb-87 (1/year)
\( (^{87}Sr/^{86}Sr)_t \) — Current ratio of Sr isotopes
\( (^{87}Sr/^{86}Sr)_0 \) — Initial ratio of Sr isotopes
\( (^{87}Rb/^{86}Sr)_t \) — Current ratio of Rb to Sr

Explanation: The method compares the amount of strontium-87 produced by radioactive decay of rubidium-87 to the remaining amount of rubidium-87.

3. Importance of Rb-Sr Dating

Details: This method is particularly valuable for dating very old rocks (billions of years) and can provide information about the source materials of igneous rocks.

4. Using the Calculator

Tips: Enter the decay constant (default 4.4998 × 10⁻¹⁹ 1/year), current Sr ratio, initial Sr ratio, and current Rb/Sr ratio. All values must be valid positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical decay constant for Rb-87?
A: The accepted value is approximately 1.42 × 10⁻¹¹ per year (or 4.4998 × 10⁻¹⁹ per second).

Q2: How do we determine the initial Sr ratio?
A: It's typically determined from minerals with very low Rb/Sr ratios (like plagioclase) or from the intercept of an isochron plot.

Q3: What types of rocks are best suited for this method?
A: The method works best with igneous and metamorphic rocks, especially those containing potassium feldspar, biotite, or muscovite.

Q4: What's the time range this method can date?
A: The Rb-Sr method is most useful for rocks older than about 10 million years, and can date materials up to billions of years old.

Q5: Why use Sr-86 as the reference isotope?
A: Sr-86 is stable and not produced by radioactive decay, making it a good reference for normalization.