ERB222 Chap.11 Radiogenic Isotopes as Magma Tracers
Radiogenic Isotopes as Magma Tracers
Ratios that survive melting
Isotopes of one element share their chemistry, so partial melting and fractional crystallisation cannot separate them. A magma therefore carries the 87Sr/86Sr and 143Nd/144Nd of its source. Parent and daughter elements do separate, however: Rb leaves the mantle more readily than Sr, and Nd more readily than Sm.
Over long periods that splits the Earth into depleted reservoirs, sampled by MORB, and enriched ones such as continental crust, with ocean island basalts spreading toward enriched mantle.
The calculations the workshops use
Three operations recur in Workshop 11 and in Problem-Solving Task 3. Measured ratios are age-corrected to initial ratios by subtracting the ingrowth term, using the parent to daughter ratio and the decay factor for the pluton age.
Nd ratios are converted to epsilon units relative to the Chondritic Uniform Reservoir. Two-component mixing between a mantle-derived melt and crust is modelled with a concentration-weighted equation that produces a curved array.
From numbers to petrogenesis
The payoff is interpretation.
I-type granites carry initial 87Sr/86Sr of 0.707 or less, while S-types sit at 0.708 or more with negative epsilon Nd and abundant inherited zircon. In arc settings the same tools, together with 10Be and uranium-series nuclides, separate slab sediment, slab fluid and crustal assimilation during ascent.
What this chapter covers
- 01
Stable versus radioactive isotopes
- 02
Why chemical fractionation leaves isotope ratios intact
- 03
Depleted and enriched mantle reservoirs
- 04
Epsilon Nd relative to CHUR
- 05
Age correction to initial ratios
- 06
Two-component mixing between mantle and crust
- 07
Isotopic tests for slab input and crustal assimilation
Worked example · free
Model the Sr isotope ratio of a mantle and crust mixture
- 1Sr in the mixture: 0.6 x 300 + 0.4 x 200 = 180 + 80 = 260 ppm.
- 1Weighted numerator: 0.6 x 300 x 0.7040 + 0.4 x 200 x 0.7180 = 126.72 + 57.44 = 184.16.
- 1Divide by the mixture's Sr: 184.16 / 260 = 0.70831.
- 1Compare: 0.70831 is at or above 0.708, the S-type side of the I and S boundary, even though the mantle melt supplied 60% of the mass.
Key terms
- Radiogenic Isotope
- An isotope produced by radioactive decay of a parent, such as 87Sr from 87Rb or 143Nd from 147Sm.
- Epsilon Nd
- The deviation of a sample's 143Nd/144Nd from the chondritic reference value, expressed in parts per ten thousand.
- Chondritic Uniform Reservoir
- The reference composition representing the initial planetary composition, used as the zero point for epsilon Nd.
- Initial Isotope Ratio
- The isotope ratio a rock had when it formed, recovered by subtracting the radiogenic ingrowth accumulated since then.
- Mass Fractionation
- The small separation of isotopes of one element caused by their different masses, as when lighter oxygen evaporates more easily.
Radiogenic Isotopes as Magma Tracers FAQ
Why don't partial melting and crystallisation change Sr and Nd isotope ratios?
Those processes separate elements by their chemical behaviour, and isotopes of one element behave almost identically. Only mass fractionation, a tiny effect, and radioactive decay alter the ratios, so a fresh magma records the 87Sr/86Sr and 143Nd/144Nd of its source.
What does a negative epsilon Nd value tell you about a granite?
It means the source had a lower Sm/Nd ratio than the chondritic reference for a long time, typical of older continental crust. S-type granites always show negative values, but I-types can be negative too, so combine the value with initial Sr ratios and mineralogy.
Why do granite isotope ratios need to be age-corrected?
Rb keeps decaying to 87Sr after the granite crystallises, so a measured ratio mixes source composition with elapsed time. Subtracting the ingrowth gives the ratio at emplacement, allowing plutons of different ages to be compared fairly. Samples with high Rb/Sr need the largest corrections.
How can isotopes show that a magma assimilated crust on its way up?
Closed-system crystallisation leaves isotope ratios unchanged, so within one suite the ratios should stay constant as silica rises. If 87Sr/86Sr increases and epsilon Nd decreases with differentiation, or oxygen isotope values climb above mantle values, the magma has taken up crustal material during ascent.
Exam move
Rehearse the three calculations until they are automatic: age-correct a Sr ratio with the time in years, convert a Nd ratio to epsilon units with six decimals, and run the mixing equation at a few values of x. Then practise the interpretation side by sorting invented suites into source contamination, sediment input or crustal assimilation, and by placing granites on the I and S type boundaries.
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