# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 1 Introduction  
The term \"mantle metasomatism\" [1; 2; 3; 4; 5] is suggested to describe the processes of transformation of mantle rocks under the influence of external fluids and/or melts, regardless of their origin and composition. Melts acting during the mantle metasomatism vary from hydrous silicic to carbonatite varieties, whereas the C-O-H fluids contain various salt components [5]. Relics of these melts and fluids are preserved as inclusions in mantle minerals, including diamonds [5; 6; 7; 8]. The oceanic crustal material subducted to the mantle depths is traditionally considered as a major source for metasomatizing melts and fluids, enriched in H\({}_{2}\)O, CO\({}_{2}\), alkalis, halogens, etc. [9; 10; 11; 12; 13]. Their interaction with the upper mantle rocks is responsible not only for wide variability of parageneses in the mantle rocks but also for formation of specific magmas, such as kimberlites, carbonatites, lamproites, alkali basalts, kamafugites and others [14; 15]. The type of mantle metasomatism producing phases, which are atypical of the periodtites and eclogites, were called by [PERSON] [16] the \"modal mantle metasomatism\". This process is commonly expressed in the formation of amphiboles, phlogopite, apatite, carbonates, sulfides, titanite, ilmenite, rutile, as well as a number of specific rare mineral phases enriched in Large Ion Lithophile Elements (LILE), High Field Strength Elements (HFSE), Light Rare Earth Elements (LREE), U and Th, such as hollandite (priderite), magnetoblumbite (yimengite-hawthorneite) and crichtonite (mathiasite-lindleyite) group minerals and others.  
Among these phases, phlogopite is widely 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 [5]. In fact, pure phlogopite is stable up to pressure above 9 GPa and temperatures up to 1400 \({}^{\circ}\)C [17; 18]. InK-bearing sub-alkaline peridotite assemblages, it is stable to pressures to 6 GPa and temperatures up to 1100 \({}^{\circ}\)C, but phylogenite persists up to 10 GPa and higher temperatures in the peralkaline portion of the model system K\({}_{2}\)O-N\({}_{2}\)O-CaO-MgO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\)-H\({}_{2}\)O [19]. The assemblage diopside + phlogopite produces the K-richterite-bearing assemblages (which involve clinopyroxene containing about 1 wt. % K\({}_{2}\)O) at pressure between 8 and 14 GPa at 1200-1350 \({}^{\circ}\)C [20]. Isomorphic substitutions of Ti and F notably extend phlogopite P-T stability (e.g., [21]). Because of wide P-T stability range, phlogopite is a common mineral in mantle xenoliths, especially peridotitic and pyroxenitic ones, whereas individual phlogopite inclusions are present both in eclogitic and peridotitic diamonds worldwide [22; 23; 24; 25; 26]. Phlogopite is found as daughter phase in high-Mg carbonatitic fluid/melt inclusions in fibrous diamonds [27] suggesting phlogopite to be an important component of the diamond growth media.  
The present paper reviews data on major phlogopite-forming reactions in upper-mantle peridotites as mineral indicators of potassic modal metasomatism. The review includes both descriptions of naturally occurring reactions and results of experiments that model some of these reactions. The paper expands the approach of thermodynamic modeling of the phlogopite-forming reactions for various mantle rocks using the Gibbs free energy minimization (pseudosection method) suggested by us earlier [28].