Magmas phase compositions

In summary, the composition of Fe-Ti oxides in magmatic rocks provides the pet-rologist with important information about the oxygen fugacity and temperature and also the silicon activity of the magma. It also has a strong effect on the magnetic properties of these phases (see Chap. 6 7). 

Crystallization of melts provides another way to fractionate chemical elements. Most crystals are denser than the melt from which they form, so crystal settling can separate the phases. Convection currents in hot magma can also entrain crystals and carry them to the bottom of a magma chamber. Accumulations of crystals are a common occurrence on the floors of terrestrial plutons, and many achondrites are cumulates formed in an analogous way in asteroids. Cumulates and the complementary melt have different elemental compositions. 

There have been tremendous strides made recently towards understanding how volatiles in general and water in particular is transported and stored in the mantle. This progress is based on research on a number of fronts studies of mantle-derived samples have provided insight into the nature and occurrence of hydrous phases such as amphibole, mica, and chlorite, and have provided constraints on the capacity of nominally anhydrous minerals (NAMs) such as olivine, pyroxenes, and garnet to contain water by a variety of substimtion mechanisms. Experimental studies on mantle-derived magmas have provided constraints on volatile contents in their source regions. Other studies have constrained the pressure, temperature, and composition conditions over which hydrous phases are stable in the mantle. 

---