Question

Choose the explanation(s) for negative Eu anomalies in upper crustal rocks like granite and granodiorite.

• A. These rocks are end-products of magmatic differentiation
• B. These rocks were formed by melting of the mantle, which was already depleted in Eu
• C. Most of the Eu was incorporated in other minerals
• D. The melt residues contain plagioclase which are enriched in Eu

Hint:

Negative Eu anomalies are key indicators of magmatic processes and can be used to infer the history of magmatic differentiation and crystallization.

Answer 1

The Correct Option is A, D

Step 1: Explanation of negative Eu anomalies
Negative europium (Eu) anomalies in upper crustal rocks like granite and granodiorite are primarily due to the depletion of Eu during the magmatic process:
(A) These rocks are end-products of magmatic differentiation: This is correct because during magmatic differentiation, Eu is preferentially incorporated into plagioclase, leading to a depletion of Eu in the melt.
(B) These rocks were formed by melting of the mantle, which was already depleted in Eu: This is incorrect because the depletion of Eu in granite and granodiorite is due to crystallization processes, not because the mantle was initially depleted in Eu.
(C) Most of the Eu was incorporated in other minerals: This is partially correct, but the main reason for the Eu anomaly is the preference of Eu for plagioclase over other minerals.
(D) The melt residues contain plagioclase which are enriched in Eu: This is correct. Plagioclase crystallizes early in the process and incorporates Eu, which causes the negative Eu anomaly in the residual melt.
Step 2: Conclusion
The correct explanations for negative Eu anomalies in rocks like granite and granodiorite are (A) and (D).