ERB222 Chap.12 Geochronology and Igneous Timescales
Geochronology and Igneous Timescales
What a radiometric age actually measures
Radioactive isotopes decay to a different isotope at a constant rate, so the amount of parent left and daughter grown records elapsed time. The Week 10 lecture builds every method in this chapter on one law, N/No = e−λt, and its half-life form t1/2 = ln 2/λ.
Because isotopes of one element are not separated by partial melting or fractional crystallisation, the clock is not reset by ordinary magmatic chemistry.
Two methods you must be able to run by hand
The Rb-Sr isochron plots 87Sr/86Sr against 87Rb/86Sr for cogenetic samples; the slope equals eλt − 1 and the intercept is the initial ratio.
The U-Pb zircon method uses t = (1/λ) ln(Pb/U + 1), relying on zircon taking up U but almost no Pb.
Workshop 10 asks for both on Lachlan Fold Belt granites, and the geochronology workshop extends the zircon work to the Noosa quartz diorite used in the field report.
From ages to timescales
Ages date eruptions and intrusions, but the unit also uses them to constrain how long a magma chamber took to build, fractionate or cool.
Blocking temperature decides when each mineral starts recording, which explains inherited zircon cores and reset micas.
What this chapter covers
- 01
Radioactive decay law and half-life
- 02
Decay constants and useful age ranges
- 03
Rb-Sr isochron slope and initial ratio
- 04
U-Pb zircon ages from Pb/U
- 05
Inherited and discordant zircons
- 06
Blocking temperature
- 07
Timescales of magmatic processes
Worked example · free
Two-mineral Rb-Sr age and initial ratio
- 1Slope m = (0.72060 - 0.70460) / (12.00 - 0.150) = 0.01600 / 11.85 = 0.0013502.
- 1Intercept = 0.70460 - 0.0013502 x 0.150 = 0.70440, the initial 87Sr/86Sr.
- 1Set the slope equal to e^(lambda t) - 1, so e^(lambda t) = 1.0013502.
- 1Take logs: lambda t = ln(1.0013502) = 0.0013493.
- 1t = 0.0013493 / 1.42 x 10^-11 = 9.50 x 10^7 years, about 95 Ma.
Key terms
- Half-Life
- The time taken for half of the parent atoms in a radioactive system to decay, equal to ln 2 divided by the decay constant.
- Daughter Isotope
- The decay product that builds up as a parent isotope breaks down, such as 206Pb from 238U, whose growth over time is what an age measures.
- Initial Ratio
- The isotope ratio, such as 87Sr/86Sr, that a suite of rocks or minerals carried when the clock started, read from the intercept of the fitted line.
- Blocking Temperature
- The temperature below which a mineral's isotope system closes so the radiogenic daughter can no longer escape by diffusion.
- Inherited Zircon
- A zircon crystal or core older than its host magma, surviving melting because zircon has a high blocking temperature.
- Discordia
- A spread of zircon U-Pb analyses off the concordia curve, recording Pb loss during later melting, metamorphic or metasomatic events.
Geochronology and Igneous Timescales FAQ
Why is zircon used so often for U-Pb dating of granites?
Zircon is a common accessory mineral in intermediate to felsic rocks, hard and resistant to weathering. It accepts uranium into its structure but excludes lead, so lead measured in a fresh grain is treated as radiogenic. Microprobe analysis at 10 to 30 micrometre spots also lets you date single grains and their cores separately.
How do I know which dating method suits a sample?
Match the method to the expected age and the minerals present. Rb-Sr works from about 10 Ma upwards and needs Rb-bearing phases such as feldspar or mica, U-Pb on zircon works from about 1 Ma, while Sm-Nd and Lu-Hf suit rocks older than about 200 Ma. Combining two methods on one rock improves confidence.
Why might Rb-Sr and U-Pb ages from the same granite disagree?
Rb and Sr can be lost during metamorphism or weathering, and minerals with lower blocking temperatures may be reset after crystallisation. Zircon, with a high blocking temperature, can instead carry inherited older cores. Comparing the two ages, as the Lachlan Fold Belt workshop asks, therefore tells you about the rock's later history as well as its age.
Exam move
Memorise one equation, N/No = e^(-lambda t), and derive the rest: t1/2 = ln 2/lambda, Pb = U(e^(lambda t) - 1) and the isochron slope. Then rehearse the spreadsheet routine of plotting 87Sr/86Sr against 87Rb/86Sr, fitting the line and converting slope to age.
For zircon sets, practise sorting grains into a concordant cluster, inherited cores and discordant grains before you average anything, and always state lambda with its units.
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