Question

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?

• A. I
• B. II
• C. III
• D. IV

Hint:

When a phase crystallizes, the residual melt moves \emph{away} from the crystallized phase’s oxide components on a composition diagram: removal of Ca–Mg pyroxene (\(\mathrm{CaMgSi_2O_6}\)) drives the melt toward lower CaO and MgO.

Answer 1

The Correct Option is C

Step 1: Identify what crystallizes.
Diopside has the ideal composition \(\mathrm{CaMgSi_2O_6}\). Its crystallization removes \(\mathrm{CaO}\) and \(\mathrm{MgO}\) from the melt in roughly comparable proportions, while it does not incorporate \(\mathrm{Al_2O_3}\). Step 2: Predict melt evolution.
Because crystals take up Ca and Mg, the residual melt becomes depleted in both \(\mathrm{CaO}\) and \(\mathrm{MgO}\). Hence, starting from \(X\), the melt composition should move toward lower CaO (downward on the CaO axis) and lower MgO (leftward on the MgO axis). Step 3: Match with the arrows.
Among the indicated trends, only arrow III points down-left, i.e., to lower CaO and lower MgO, which correctly represents the residual melt evolution during diopside crystallization. \[ \boxed{\text{Arrow III (Option C)}} \]