Diamond potential and origin ofÁ kimberlites.
Vernadsky Institute of Geochemistry and Analytical Chemistry,
Alkaline and carbonatitic Ámagmatism appeared on Arhean-Proterozoic border- 2.5-2.7 b y The activity of alkaline and carbonatitic magmatism increased continuously throughout the Earth's history (Fig. 1). Moreover its intensityÁ increased throughout geologic time. Kimberlites appeared later (2000-1400 Ma) and also their activity increased (Fig 2) At the same time the diamond potential of kimberlites continuously dropped.(fig 3). We develop (Kogarko 2005) the conception of gradualÁ oxidation of Earth mantle as result of subduction of oxidized oceanic crustal material.
With the increasing of oxygen fugasity in mantle theÁ stability field of diamond decrease and they are burned out resulting in the decreasing of diamond potential of kimberlites.
At this moment the hypotesis of metasomatic origin of diamonds is widely accepted.
The question arisesÁ as to the
genesis of high potassium melts-fluids in the mantle.WhenÁ mantle diapir ascends
from a depth approximately Á660km Ca-perovskite became unstable and react with Mg-perovskite, ferripericlase Áto produce majorite,
ringwoodite,and under further pressure decrease, wadsleite. 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 (Corgne and Wood,2004). The remaining K likely remains beyond crystalline lattices of minerals
in this mantle zone. Taking into considerationvalues
of the K distribution coefficient in Caperovskiteand majorite, it can be confidently stated that the
thermodynamic activity of K2O in the system increases by more than an order of
magnitude with thetransition of the Mg-perovskiteãCa-perovskiteãferripericlase association into the majoriteãringwoodite paragenesis. This results in generation of conditionsfavorable
for transition of K into the melt or fluid at thelowerãupper
mantle boundary (approximately
1. »Àgarko L.N.(2005). Geology and Geophysic, 46,12, 1234-1245
2. CorgneÁ A. and Wood B.,(2004) Earth Planet. Inter.48, 143
Distribution of alkaline rocks, kimberlites, and diamond potential of kimberlites in the Earthôs geological history. (a) Alkaline rocks;(b) kimberlites; (c) diamond potential (ratio of the numbers of diamond-free and diamond-bearing pipes).