The solid Earth - Geochemistry: An Introduction - page 222

clinopyroxene

1485

1320

Common
mantle
compositions

Spinel (MgAlO 4 )

Plagioclase Olivine

Figure 11.4 Change in the stability field of different minerals in basaltic melts with pressure. At ambient

pressure, the minimum melt of mantle rocks (triple point at 1320 ◦ C) is saturated in olivine,
spinel, and plagioclase. Clinopyroxene therefore dissolves into the liquid. In contrast, at 1 GPa
( z 30 km), the minimum melt (1485 ◦ C) is saturated in olivine, spinel and clinopyroxene.

Plagioclase now dissolves into the liquid. After Presnall et al. (1978 ).

≈

melts. In arc (orogenic) magmas, the relatively high water content suppresses again
early plagioclase saturation: Al, Ti, and Fe are controlled by clinopyroxene and olivine
fractionation.
The fate of the different species of iron Fe 2 + and Fe 3 + is germane to what petrol-
ogists refer to as changes in oxygen fugacity. Since silicates are electrical insulators,
changes of the Fe 3 + /Fe 2 + ratio cannot be effected by simple electron transfer, but require
incorporation of one or other species into minerals and are therefore controlled by the
crystallizing mineral assemblages. Ferrous Fe 2 + is compatible with olivine and pyroxene
structure, whereas Fe 3 + is incompatible until saturation of magnetite Fe 3 O 4 is attained.
We can divide the familiar Rayleigh distillation equation (2.29) for Fe 3 + by that for Fe 2 +
in residual magmas (res) to obtain the Fe 3 + /Fe 2 + ratio as:
Fe 3 +
Fe 2 +

Fe 3 +
Fe 2 +

f D Fe 3 + − D Fe 2 +

res =

(11.2)

0

in which the 0 subscript refers to the undifferentiated magma, f is the fraction of residual
liquid (res), and D stands for bulk solid-liquid partition coefficients. In all cases, D Fe 3 +

is