Fractionation models discussed so far are based on one distribution coefficient for one trace element and one mineral. Because several minerals are likely to be present during fractional crystallization or partial melting, the contributions of those several minerals must be included in our fractionation models. This is done by calculating a

**weighted average distribution coefficient**for element

**i**, D̅

_{i}, using the distribution coefficient D

_{i}

^{φ}and the mass fraction (X

_{φ}) for each mineral (φ) as follows:

(5) D̅

_{i}= Σφ X

_{φ}* D

_{i}

^{φ}

Once the bulk distribution coefficient (D̅

_{i}) is determined, a fractionation calculation can proceed in the same manner as for a single mineral and magma model.

Rayleigh and Batch Partial Melting models are programmed into Figure 9.05. Click on the figure to see the interactive version. After you have seen Figure 07, use it to answer the following questions.