Constraints on density as a function of pressure, temperature, and
composition are crucial to understand isostatic movements during
geodynamic processes. Here, we provide a systematic series of density
diagrams extracted from thermodynamic calculations for a variety of
crustal compositions within a wide P-T range. We quantify systematic
density changes in collisional settings for relevant compositional
variations and attempt to simplify the density-composition
relationships. Rock densities depend strongly on pressure, temperature,
and composition. Densities at some selected pressure-temperature
conditions increase linearly with increasing Al(2)O(3) as well as
MgO/FeO contents in pelitic rocks. Al- and Fe-rich pelites yield the
highest densities, which is mostly due to the formation of garnet but
also depends on other minerals and changes of reactions. The effect of
loading on densities is investigated, and we show that for deep burial,
a meta-pelite rich in Fe and Mg yields much larger density changes than
a dry basalt and that the burial of such a rock with a composition close
to typical lower crust may result in significant negative buoyancy.
Metamorphism of hydrous lower crust due to pressurization and heating
thus leads to densification of thickened lower crust, while heating of
dry crust leads to a decrease in density. Hence, water-loaded isostatic
subsidence due to metamorphism of water-saturated lower crust is
substantial and increases with the thickness and depth of the reacting
layer, while dry compositions show much less or only transient
densification and subsidence. The density change due to thermal
expansion, an extensively used concept in geodynamic models, predicts
uplift under the same P-T conditions and is an order of magnitude
smaller than the density variation calculated from petrologically
consistent diagrams.