Experimental study of phlogopite-forming reactions in the system orthopyroxene+garnet in presence of the H2O-KCl fluids.

Phlogopite is accepted as a major mineral indicator of the modal metasomatism in the upper mantle within a very wide P-T range and fluid/melt compositions. It extensively forms in mantle peridotites transforming initial harzburgites and lherzolites to phlogopite wehrlites both in garnet and spinel-facies. A reaction 5En + Grt + [K₂O + 2H₂O in fluid] = Phl + Di (Grt – pyrope-grossular garnet CaMg₂Al₂Si₃O₁₂) is considered as the major mechanism for phlogopite formation in garnet-facies peridotites. This reaction is commonly accompanied by regular compositional changes of primary garnet and pyroxenes. In order to illustrate the regularities, we report result of experimental study of the phlogopite-forming reactions in the model systems pyrope-enstatite, grossular-pyrope-enstatite and knorringite-pyrope-enstatite systems in presence of a H₂O-KCl fluid at pressure 3 and 5 GPa and temperatures of 900 and 1000°C. The experiments were aimed at the tracing of variations of grossular and knorringite contents in garnet, as well as Al content of pyroxenes, with variations of the KCl content in the fluid.

The increase of X_KCl in the fluid is accompanied by gradual decomposition of garnet and Al-bearing enstatite in all systems. The Al₂O₃ content in orthopyroxene decreases in the pyrope – enstatite system at 5 GPa and 900°C. In the system enstatite-pyrope-grossular at 5 GPa and 1000°C phlogopite forms at the KCl content 10 mol. % in the fluid. Further increase of the KCl content in the fluid results in gradual disappearance of garnet and orthopyroxene and stronger domination of phlogopite and clinopyroxene. Grossular content in garnet increases with the KCl concetration in the fluid up to 10 mol. %, but further increase of the KCl concentration to 20 mol. % results in decrease of the grossular content in garnet. In the system enstatite-pyrope-knorringite at the KCl content in the fluid 0 – 10 mol. %, garnet contains 8-9 mol. % of knorringite. Cr-bearing phlogopite (about 2 wt. % Cr₂O₃) appears in this system at 10 mol. % KCl in the fluid, and its formation results in a slight increase of the knorringite content in garnet. Because of relatively high SiO₂ bulk content in comparison to the typical peridotite, Cr-bearing kyanite (not spinel) forms at 20 mol. % KCl in the fluid resulting in a decrease of the knorringite content in garnet down to 3-5 mol. %. The Cr₂O₃ content in the coexisting phlogopite concomitantly decreases by about 1 wt. %.

The above experiments reproduced some characteristic regularities in variations of garnet and pyroxene compositions in the course of phlogopite formation in mantle peridotites. The applicability of the experimental results is illustrated by examples from peridotite xenoliths from kimberlites. These effects can be applied for the quantitative and qualitative estimates of variations in K activity during the modal mantle metasomatism.