The variation in upper mantle mineral assemblages with temperature and pressure can be determined experimentally. But there is uncertainty about composition. Because a lot of observed mantle material is not primary, but has had a liquid (basalt) fraction removed by partial melting, Ringwood coined the term 'PYROLITE' for primitive fertile mantle - in effect dunite with basalt put back in!
PYROLITE = 3 parts DUNITE + 1 part BASALT
Mantle mineralogy varies mainly on account of the nature of the aluminous phase, which is P-T dependent, i.e.
Olivine (MgFe)2SiO4 + Orthopyroxene (MgFe)2Si2O6 as major phases, plus:
Plagioclase CaAl2Si2O8 or
Spinel (MgFe)Al2O4 or
Clinopyroxene (NaCa)2(AlSi)2O6 or
Garnet Mg3Al2Si3O12 or
Phase diagram (Fig. 7), though rather complicated, shows that plagioclase peridotite can exist only at very shallow depths where the geothermal gradient is high; spinel- and pyroxene peridotites have a larger stability field in the upper mantle; but garnet peridotite will occur at deeper levels (hence common as nodules in kimberlite pipes).
|Fig. 7. Summary of phase relations in pyrolite (after Green & Ringwood) appropriate to upper mantle conditions. Wet solidus for small amount of hornblende breakdown. Note that only oceanic geotherm intersects this wet solidus.|