The following diagram shows phase relations in a system consisting of components A and B at 1 bar pressure. If the initial composition of liquid is R, during cooling and crystallization of magma, which of the following statement(s) is/are CORRECT? 
The diagram represents a binary eutectic system consisting of components A and B at 1 bar pressure. The horizontal axis shows composition in wt.% of B, and the vertical axis shows temperature. Point E is the eutectic point. The initial composition of the magma is R, which lies on the B-rich side of the diagram.
We are asked to determine which of the given statements are correct during cooling and crystallization starting from composition R.
Interpretation of the phase diagram
Composition R is at about 75 wt.% B.
On cooling from R, the system first enters the B + Liquid field.
Therefore, mineral B crystallizes first, not A.
As cooling proceeds, the liquid composition moves along the liquidus curve toward the eutectic point E.
At the eutectic temperature, both A and B crystallize together until the liquid is exhausted.
Evaluation of statements
Statement 1
“On complete crystallization of magma, the final composition (in wt.%) of rock consists of 25 of mineral A and 75 of mineral B.”
This is correct.
In a closed system, the bulk composition of the final rock equals the initial bulk composition of the magma. Since the starting composition is 75 wt.% B, the final rock must contain 75 wt.% B and 25 wt.% A.
Statement 2
“On cooling of magma, mineral A is the first mineral to crystallize.”
This is incorrect.
From composition R, cooling first enters the B + Liquid field, so mineral B crystallizes first, not A.
Statement 3
“At point Q, the weight percentages of crystal and liquid are 37.5 and 62.5, respectively.”
This is correct.
At point Q, the system lies in the B + Liquid field. Using the lever rule between the solid B composition (100 wt.% B) and the liquid composition at Q, the relative proportions evaluate to 37.5% solid (crystal) and 62.5% liquid.
Statement 4
“The composition (in wt.%) of liquid at point E is 40 A and 60 B.”
This is incorrect.
Point E is the eutectic composition, which from the diagram is approximately 40 wt.% B and 60 wt.% A, not the reverse.
Final Answer
The correct statements are Option 1 and Option 3.
Important Questions from Igneous Petrology (Binary and Ternary Phase Diagrams)
If 'X' represents the initial composition of a melt, which one of the trends indicated by arrows in the schematic diagram corresponds to the evolution of the residual melt composition during crystallization of diopside?
The diagram given below shows phase relations between components P and Q at 1 bar pressure. If ‘X' represents the initial liquid composition, which of the following statements is/are CORRECT during equilibrium crystallization?
- Choose the CORRECT statement regarding crystallization of a single feldspar of composition $Or_{50}Ab_{50}$ in the Albite-Orthoclase system.
The figure below represents an isobaric binary liquidus phase diagram, with the solid phases A, B and C. What are the degrees of freedom associated with equilibrium phase assemblages represented by the bulk compositions w, x, y and z, in the fields indicated in the figure?
The given T-X diagram shows the phase relations in olivine solid solution at 1 bar pressure. If 'P' is the initial position of melt, the proportion of melt at $1500^\circ$C is __________ %.
