Questions: Rb-Sr Isotope System

Question 1

Five mineral separates from a granite yield a well-defined Rb-Sr isochron with an age of 350 Ma and an initial 87Sr/86Sr of 0.712. What do these results indicate about the granite's source?

• A. The granite crystallized directly from mantle-derived basaltic magma
• B. The high initial 87Sr/86Sr (>>0.704) indicates significant incorporation of old continental crustal material with elevated Rb/Sr ratios that had evolved high 87Sr/86Sr over time; the granite likely formed by partial melting of or extensive assimilation of continental crust
• C. The granite formed at a mid-ocean ridge
• D. The high initial ratio indicates contamination by seawater

Answer: (B)

Depleted mantle has 87Sr/86Sr of ~0.7025-0.704. An initial ratio of 0.712 requires a source that accumulated radiogenic 87Sr from 87Rb decay over a long period -- characteristic of old continental crust. This granite either formed by partial melting of continental crust or by extensive assimilation of crustal material by a mantle-derived magma.

Question 2

An Rb-Sr isochron requires all samples to have had the same initial 87Sr/86Sr ratio when they formed.

• T. True
• F. False

Answer: True

The isochron method assumes all samples (minerals or whole rocks) were in isotopic equilibrium at time zero, sharing a common initial 87Sr/86Sr. After formation, each sample evolves independently along a trajectory determined by its Rb/Sr ratio. Samples with higher Rb/Sr develop higher 87Sr/86Sr over time, producing the sloped isochron. If initial ratios were not uniform (e.g., mixing of isotopically distinct magmas without homogenization), the samples will scatter rather than define a line, and the apparent age will be meaningless.

Question 3

Explain why the Rb-Sr system is readily reset by metamorphism while the Sm-Nd system is more resistant.

Model answer: Rb and Sr are relatively mobile during metamorphism because they substitute into minerals (micas, feldspars) that readily recrystallize at metamorphic conditions, and because both elements are moderately soluble in metamorphic fluids. Metamorphic recrystallization and fluid flow can re-homogenize Sr isotopes among minerals, resetting the isochron clock. Sm and Nd are rare earth elements with very similar geochemical behavior, high ionic potential, and low fluid mobility. They reside in refractory accessory minerals (garnet, monazite) that are more resistant to metamorphic resetting. This contrast makes Rb-Sr useful for dating the last metamorphic or thermal event, while Sm-Nd more often preserves the original igneous age.

The difference is fundamentally about element mobility: alkali/alkaline earth elements are mobile in crustal fluids; rare earth elements are not.