Questions: Sm-Nd Isotope System

Question 1

A mid-ocean ridge basalt has epsilon-Nd of +10, while a continental granite has epsilon-Nd of -15. What fundamental process creates this difference?

• A. Radioactive decay rates differ between oceanic and continental settings
• B. Extraction of continental crust from the mantle fractionated Sm/Nd: the crust acquired low Sm/Nd, evolving to low 143Nd/144Nd (negative epsilon-Nd) over time, while the residual depleted mantle retained high Sm/Nd, evolving to high 143Nd/144Nd (positive epsilon-Nd)
• C. Seawater contamination of oceanic basalts increases their epsilon-Nd
• D. Continental granites form at higher temperatures, shifting epsilon-Nd

Answer: (B)

Partial melting of the mantle to form continental crust concentrates Nd (light REE) preferentially over Sm in the melt. The crust accumulates lower Sm/Nd over time, so its 143Nd/144Nd grows more slowly than chondrite (negative epsilon-Nd). The complementary depleted mantle retains elevated Sm/Nd, growing 143Nd/144Nd faster than chondrite (positive epsilon-Nd). After billions of years, this produces the observed ~25 epsilon units difference between depleted mantle and old continental crust.

Question 2

A Sm-Nd depleted mantle model age (T-DM) of 2.8 Ga for a granite means the granite crystallized 2.8 billion years ago.

• T. True
• F. False

Answer: False

A T-DM age estimates when the granite's source material was extracted from the depleted mantle, not when the granite itself crystallized. If the granite formed at 350 Ma by melting 2.8 Ga crust, the T-DM would be 2.8 Ga while the crystallization age (from U-Pb zircon or Rb-Sr) would be 350 Ma. T-DM provides a crustal residence age -- how long the source material has been part of the continental crust.

Question 3

Explain why the Sm-Nd system is more robust against metamorphic resetting than the Rb-Sr system.

Model answer: Sm and Nd are both rare earth elements (REE) with similar ionic radii, high charge, and very similar geochemical behavior. They are strongly bonded in refractory minerals (garnet, zircon, monazite) and have very low mobility in metamorphic fluids. Because metamorphic resetting requires redistribution of parent and daughter isotopes, and Sm-Nd redistribution requires either very high temperatures or complete recrystallization of host minerals, the system is resistant to all but the highest-grade metamorphism. Rb and Sr, being alkali/alkaline-earth metals in fluid-mobile phases (micas, feldspars), are much more easily redistributed during metamorphism and fluid flow.

Robustness correlates inversely with element mobility: the similar, refractory REE Sm and Nd resist mobilization while the dissimilar, fluid-mobile Rb and Sr do not.