Accumulation of which one of the following minerals can explain positive Eu and positive Sr anomalies in the primitive mantle-normalized multielement pattern of an igneous rock?
Show Hint
- Orthoclase
- Apatite
- Orthopyroxene
- Olivine
The Correct Option is B
Solution and Explanation
On a primitive mantle normalised multielement plot, europium (Eu) and strontium (Sr) stand out from their neighbouring elements when a mineral in the rock strongly favours them over the rest of the pattern. A positive spike at Eu and Sr together points to a mineral whose crystal structure specially welcomes both elements, since Eu can exist as \(Eu^{2+}\) (behaving like \(Ca^{2+}\)) under many igneous conditions, and Sr is also a close size match for \(Ca^{2+}\).
Step 2: Check option (A), orthoclase.
Orthoclase is a K-feldspar. Its large alkali site holds \(K^{+}\) and some Ba, but it takes in Sr and \(Eu^{2+}\) far less readily than a calcium rich mineral does, so accumulating orthoclase does not give a strong positive Sr and Eu spike.
Step 3: Check option (B), apatite.
Apatite, \(Ca_5(PO_4)_3(F,Cl,OH)\), has calcium sitting at the centre of its structure. This Ca site readily swaps in \(Sr^{2+}\), which is why apatite is known to strongly enrich Sr, and it can also take up \(Eu^{2+}\) in the same site, producing a positive Eu spike alongside the Sr spike when apatite builds up in a rock.
Step 4: Check option (C), orthopyroxene.
Orthopyroxene is built around Mg and Fe sites with only a small, poorly fitting space for Ca, so it holds very little Sr or Eu. Accumulating it would not create these anomalies.
Step 5: Check option (D), olivine.
Olivine is dominated by Mg and Fe with almost no calcium in its structure, so it strongly excludes both Sr and Eu. Accumulating olivine would not explain the observed spikes.
Step 6: Final conclusion.
Among the four choices, only apatite has a calcium site well suited to hosting both \(Sr^{2+}\) and \(Eu^{2+}\), so its accumulation best explains the positive Eu and Sr anomalies.
\[ \boxed{\text{Apatite}} \]
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