Question

A mantle source rock melts at a time to giving rise to melt (M) and residue (R). Which of the following statements is CORRECT about evolution of the \(( ^{143}Nd/^{144}Nd )\) and \(( ^{87}Sr/^{86}Sr )\) isotope ratio in M (that crystallized to form a rock) and R?

• A. The growth of Nd isotope ratio versus time is faster in R than M and the Sr isotope ratio grows slower in R than M.
• B. The growth of Nd isotope ratio versus time is slower in R than M and the Sr isotope ratio grows faster in R than M.
• C. Both the Nd and Sr isotope ratios grow at identical rates in R and M.
• D. The growth of Nd and Sr isotope ratio in M and R would depend on the initial concentrations of Sm and Rb in the mantle source rock.

Hint:

In partial melting, the Nd isotope ratio increases faster in the residue (R) than in the melt (M) due to the incompatibility of Nd, while the Sr ratio grows slower in the residue.

Answer 1

The Correct Option is A

When a mantle source rock melts, the resulting melt (M) and residue (R) will have different isotope ratios. The Nd and Sr isotopes behave differently during the process of partial melting.

Step 1: Behavior of Nd and Sr isotopes during melting.
- The Nd isotopic ratio (which is related to the ratio of \( ^{143}Nd/^{144}Nd \)) typically increases more rapidly in the residue (R) because Nd is incompatible with the melt. As the melt fraction increases, the concentration of Nd in the residual phase increases, resulting in a faster growth of the Nd isotope ratio in R than in M. - On the other hand, the Sr isotope ratio (which is related to the ratio of \( ^{87}Sr/^{86}Sr \)) is less sensitive to partitioning during partial melting, meaning the Sr ratio increases slower in the residue compared to the melt.

Step 2: Analyzing the options.
- (A) This is correct. The Nd ratio grows faster in R due to Nd's incompatibility with the melt, while the Sr ratio grows slower in R. - (B) This is incorrect because the Nd ratio grows faster in R, not slower. - (C) This is incorrect because the Nd and Sr isotope ratios do not grow at identical rates in R and M due to their different behaviors during melting. - (D) While the initial concentrations of Sm and Rb do affect the evolution of the isotope ratios, the key factor in the correct answer is the difference in how Nd and Sr behave during partial melting.

Step 3: Conclusion.
The correct answer is (A) because it accurately reflects the differing rates of isotope ratio growth in the melt and residue during partial melting.

Final Answer: (A) The growth of Nd isotope ratio versus time is faster in R than M and the Sr isotope ratio grows slower in R than M.