On the basis of both natural samples and experimental studies, clinopy
roxene is a potential reservoir for potassium in the Earth's mantle. T
he amount of K partitioning into clinopyroxene depends on the phase as
semblage present, the bulk composition, pressure, and temperature. To
investigate some of these dependencies, subsolidus and melting phase r
elations in the system phlogopite-diopside have been studied to 17 Cpa
. In this system, phlogopite becomes unstable with increasing pressure
, breaking down to potassium richterite, which in turn breaks down to
another K-bearing hydrous phase (phase X), such that a K-rich phase co
exists with clinopyroxene to 17 Cpa. Clinopyroxenes contain less than
or equal to 1.3 wt% K2O in assemblages of phlogopite + clinopyroxene /- olivine +/- liquid at 3-5 Gpa, phlogopite + clinopyroxene + garnet
+/- olivine +/- liquid at 7-9 GPa, clinopyroxene + garnet + olivine +/
- potassium richterite +/- liquid at 11 Gpa, and clinopyroxene + olivi
ne + garnet + phase X at 14 and 17 GPa. In these assemblages, K is par
titioned into hydrous phases or liquid, rather than into the clinopyro
xene. By inference, phlogopite (or its higher-pressure breakdown produ
cts) is the primary host for K in the mantle (if H2O is present), and
any coexisting clinopyroxene has very low concentrations of K. Convers
ely, the natural occurrence of clinopyroxene with >> 1 wt% K2O require
s that phlogopite, potassium richterite, or phase X is not stable in t
he local source environment of such samples.