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.
Statement I: Anhydrous Melting of Peridotite
Peridotite, the primary rock of the Earth's mantle, undergoes partial melting. When this melting process occurs without water (anhydrous), the resulting magma composition is predominantly basaltic. This is a key mechanism for magma generation beneath mid-ocean ridges and hot spots.
Statement I is correct.
Statement II: Hydrous Melting of Peridotite
The presence of water significantly influences mantle melting. Hydrous melting of peridotite, common in subduction zones where water is released from sinking oceanic plates, leads to lower melting temperatures.
This process generates magma that is typically more silica-rich than basalt, commonly resulting in an andesitic composition.
Statement II is correct.
Statement III: Congruent Mineral Melting
Congruent melting describes a process where a mineral melts to form a liquid that has the exact same chemical composition as the original solid mineral.
Statement III is correct.
Statement IV: Incongruent Mineral Melting
Incongruent melting involves a mineral melting to produce a liquid of a different composition than the original solid, along with the simultaneous formation of one or more new solid phases.
Statement IV is correct.
Conclusion: Correct Combination of Statements
Based on the analysis, all four statements regarding magma generation (anhydrous and hydrous melting of peridotite) and mineral melting (congruent and incongruent) are accurate descriptions of geological processes.
Therefore, the combination where all statements I, II, III, and IV are correct is the accurate choice.
Important Questions from Igneous Petrology (Trace Elements/Melting/Evolution)
- Choose the explanation(s) for negative Eu anomalies in upper crustal rocks like granite and granodiorite.
- 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)
- A basaltic magma has an initial nickel concentration of 300 ppm. Olivine crystallizes from this magma by equilibrium crystallization (Case I) or fractional crystallization (Case II). Then, the absolute value of the difference between the nickel concentrations of the liquids remaining after 25% crystallization in these two cases is ________.
(Use $K_{D,Ni}$ olivine/melt = 10). - 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 hypothetical garnet peridotite composed of 60% olivine, 25% orthopyroxene, 10% clinopyroxene and 5% garnet undergoes 10% batch melting described by $\frac{C_L}{C_o} = \frac{1}{F+D-F*D}$ where F is degree of melting and D is bulk partition coefficient. The ratio of Ce in the melt to the original rock will be ___________ (round off to 2 decimal places).
(The $K_D$ values of Ce for olivine, orthopyroxene, clinopyroxene and garnet are 0.001, 0.003, 0.10 and 0.02, respectively)
