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 of 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 authors 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 mantle environment.

At present, most geochemists accept the chondritemodel of the Earths composition, its upper and lower mantle included. According to this model, the K content in the Earths 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 0.040.12 for clinopyroxene and 0.038 for garnet [2]. Moreover, this parameter in the pyroxenemelt 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 410660 km, majorite is associated with wadsleyite (410500 km) and ringwoodite (500660 km),both practically lacking K and Na. At deeper levels of the mantle (below 660 km), the majoriteringwoodite association is replaced by the ferripericlaseMg-perovskiteCa-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





  1. Sun S., McDonough W. // Geol. Soc. Amer. Spec. Publ. 1989. V. 42. P. 313 342.
  2. Corgne A., Wood B. J. // Earth and Planet. Inter. 2004. V. 48. P. 143 144.
  3. Safonov O. G., Perchuk L. L., Litvin Yu. A. // Earth and Planet. Sci. Lett. 2006. V. 10. P. 1016 1031.
  4. Corgne A., Libeske C., Wood B. J. et al, // Geochim. et cosmochim. acta. 2005. V. 69. 2. P. 485 496.

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