Determine the correctness or otherwise of the following Assertion (a) and Reason (r). Assertion: Strontium concentration in a basic magma decreases with fractional crystallization of plagioclase.
Reason: Strontium is a compatible trace element in plagioclase during magmatic crystallization.
Understanding Trace Element Behavior in Magma
Trace elements are elements present in magma in low concentrations compared to major elements. Their behavior during magmatic processes like fractional crystallization depends on their chemical compatibility with the crystallizing minerals.
- A compatible trace element is one that readily incorporates into the crystal lattice of a crystallizing mineral. Its concentration in the remaining magma decreases as the mineral crystallizes.
- An incompatible trace element is one that does not readily fit into the crystal lattice. It tends to remain in the melt, increasing its concentration as minerals crystallize.
Evaluating Assertion (a): Strontium Concentration Decrease
The Assertion states that Strontium concentration decreases in a basic magma during fractional crystallization of plagioclase.
Plagioclase is a solid solution mineral series (e.g., anorthite-albite). Strontium (Sr$^{2+}$) has an ionic radius and charge similar to Calcium (Ca$^{2+}$), a major cation in the plagioclase structure (especially in the more calcic end-member, anorthite). Due to this similarity, Sr$^{2+}$ substitutes readily for Ca$^{2+}$ within the plagioclase crystal lattice.
Therefore, when plagioclase crystallizes from a basic magma, it preferentially incorporates Strontium. This process removes Strontium from the melt, causing its concentration in the remaining magma to decrease. Thus, the Assertion (a) is true.
Evaluating Reason (r): Strontium's Compatibility
The Reason states that Strontium is a compatible trace element in plagioclase during magmatic crystallization.
As explained above, Strontium's chemical properties (ionic size and charge) allow it to substitute effectively for Calcium in the plagioclase structure. This confirms that Strontium is indeed a compatible trace element in plagioclase.
Thus, the Reason (r) is true.
Connecting Reason to Assertion
The Reason directly explains the Assertion. Because Strontium is compatible in plagioclase (Reason is true), it gets removed from the magma as plagioclase crystallizes, leading to a decrease in Strontium concentration in the residual magma (Assertion is true).
Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.
Important Questions from Igneous Petrology (Trace Elements/Melting/Evolution)
- 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?
- Choose the explanation(s) for negative Eu anomalies in upper crustal rocks like granite and granodiorite.
- The modal abundance in an ultramafic rock and the partition coefficients of lutetium (Lu) in clinopyroxene, orthopyroxene, olivine and plagioclase are tabulated below. The bulk distribution coefficient of lutetium ($D_{Lu}$) in the ultramafic rock is ________.
Mineral Modal abundance (%) Partition coefficient Clinopyroxene 45 0.506 Orthopyroxene 40 0.42 Olivine 10 0.045 Plagioclase 05 0.019 - A source rock undergoes melting. Assuming batch melting, $5\%$ partial melting and bulk distribution coefficient of $0.045$, the enrichment factor ($C_L/C_o$) of Rb in the melt will be ________. (Round off to $2$ decimal places)
Choose the CORRECT combination for the following four statements.
Statement I: Anhydrous partial melting of peridotites produces basaltic magma.
Statement II: Hydrous melting of peridotites produces andesitic magma.
Statement III: Congruent melting of minerals produces liquids of compositions identical to the minerals.
Statement IV: Incongruent melting of minerals produces liquids of different compositions and new solids.
