Potassic fluids as metasomatic agens of K-rich rocks origin

Kogarko L.N.

GEOCHI RAS, Moscow, Russia

On the basis of data-base (about 6000 analyses ofš oceanic islands volcanics) we distinguished primary magmas characterized by high MgO and Ni (about 2000 compositions). In this data-base we determined unique potassium-rich melts with Na2O/K2O <1. This value is much lower than Na2O/K2O ratio in mantle rocks (average close to 12.5). The majority of authorsš consider the high-potassium rocks as result of partial melting of metasomatised mantle material. The question arises what was the mechanism of K-rich fluid in mantleš environment.

At present, most geochemists accept the chondritemodel of the Earth’s composition, its upper and lower mantle included. According to this model, the K content in the Earth’s primitive mantle is low (250 ppm) [1]. In the uppermost blocks of the mantle, K is entirely concentrated in plagioclases. With increase in depth (and pressure), the plagioclase disappears and potassium budget is determined mainly by clinopyroxene and, to a lesser extent, garnet with the K distribution coefficient equal to approximately 0.04–0.12 for clinopyroxene and 0.038 for garnet [2]. Moreover, this parameter in the pyroxene–melt equilibrium increases with depth (and pressure) [3]. Clinopyroxene with the Na distribution coefficient equal to 0.8 represents the main Na-concentrating mineral. Under further increase in the pressure, pyroxene and garnet react to form majorite with K and Na distribution coefficients equal to 0.015 and 0.39, respectively [2]. In the depth interval of 410–660 km, majorite is associated with wadsleyite (410–500 km) and ringwoodite (500–660 km),both practically lacking K and Na. At deeper levels of the mantle (below 660 km), the majorite–ringwoodite association is replaced by the ferripericlase–Mg-perovskite–Ca-perovskite paragenesis. The content of Ca perovskite in this mantle zone approximates 8%. The K distribution coefficient in Ca-perovskite is relatively high (0.39), and that of Na is even higher (2.0) [4].Thus, practically all K and Na are concentrated in Ca-perovskite from the upper zone of the lower mantle. When a mantle diapir ascends from a depth of approximately 660 km, Ca-perovskite turns into the unstablephase, which reacts with Mg-perovskite and ferripericlaseto produce majorite; ringwoodite; and, under further pressure decrease, wadsleyite. This process is accompanied by partial transition of K into majorite, since the K distribution coefficient in Ca-perovskite is 26 times higher as compared with that in majorite [4].The remaining K likely remains beyond crystalline lattices The released fluids should migrate to the upper structural stages of the mantle and carry out metasomatic alterations of host rocks

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