P-T Stability of Phlogopite, K-Richterite and Phengite, as a Source of Potassium in the Mantle - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 368

Cpx+Gar+Ol+Phase X

14

12

Cpx+Gar+Ol+Amph

10

Ph+Cpx+Gar+Ol+Amph

L+Cpx+Gar+Ol

8

Ph+Cpx+Gar+Ol

6

AS

4

L+Ol+Cpx

Ph+Cpx

2

0

1000 1100 1200 1300 1400 1500 1600

0

Temperature ( C)

Fig. 12.16 Pressure-temperature projection of experimental results from this study (squares),
from Modreski and Boettcher (1972) (circles, solid symbols are subsoildus experiments, open
symbols are supersolidus experiments) and from Sudo and Tatsumi (1990) (triangles, differnet
symbols refer to the different phase assamblages as labelled on the figure). Stable phase
assamblages are noted on the diagram, and the inferred boundaries between the differing
subsolidus assamblages are drawn as dashed lines. The solidus is shown as a heavy solid line. Also
shown on this diagram is the average modern mantle adiabat (AMA) and a geotherm for an
Archean shield (AS) (after Luth 1997)

12.7 Investigation on the Assemblage Phlogopite-Diopside
up to 17 GPa

The experimental study of the system phlogopite-diopside from 3.5 to 17 GPa has
been made by Luth (1997, Fig. 12.16 ). On the basis of both natural samples and
experimental studies, clinopyroxene has been suggested as a potential reservoir for
potassium in the Earth s mantle (Harlow and Veblen 1991). The amount of K

partitioning into clinopyroxene depends on the phase assemblage present, the bulk
composition, pressure, and temperature. To investigate some of these dependencies,
subsolidus and melting phase relations in the system phlogopite-diopside was
studied by Luth (1997) to 17 GPa. In this system, phlogopite becomes unstable with
increasing pressure, breaking down to potassium richterite, which in turn yields
potassium-bearing hydrous phase (phase X), such that a potassium-rich phase
coexists with clinopyroxene to 17 GPa. Clinopyroxenes contain

'

1.3 wt% K 2 Oin
assemblage of phlogopite + clinopyroxene + olivine + liquid at 3

≤

5 GPa, phlog-

-

opite + clinopyroxene + garnet + olivine + liquid at 7 9 GPa, clinopyrox-

ene + garnet + olivine + potassium richterite + liquid at 11 GPa, and
clinopyroxene + olivine + garnet + phase X at 14 and 17 GPa.

-