Page 69 - Laboratory Manual in Physical Geology

peridotite is made of olivine, pyroxene, amphibole, and case (see FIGURE 2.8 ), there are stability fields for solid,
garnet mineral crystals. Therefore, this diagram also solid + liquid, and liquid.
shows the combined effects of pressure and tempera- Notice that lines for the continental and oceanic
ture on a rock made of several different minerals. At geothermal gradients are also plotted on FIGURE 2.8 .
P-T points below (to the left of) the solidus, all mineral They show how temperature normally varies accord-
crystals in the rock remain solid. At P-T points above ing to depth below the continents and ocean basins.
(to the right of) the liquidus, all mineral crystals in the Temperatures along both of these geothermal gradients
rock melt to liquid. At P-T points between the solidus are not great enough to begin melting peridotite. Both
and liquidus, the rock undergoes partial melting—one gradients occur along temperatures below (to the left of)
kind of mineral at a time. Therefore, a P-T diagram also the peridotite solidus.
reveals stability fields for states (phases) of matter. In this

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54 ■ L ABOR ATORY 2