Garnet-pyroxene barometry for the assemblages with Na-bearing majoritic garnet

Abstract

The majority of diamonds from kimberlite and lamproite pipes were formed within a quite narrow range of 140‐220 km, as evident from calculations of pressures and temperatures for the formation of their silicate inclusions [1]. The upper boundary of this range corresponds to the graphite‐diamond transition in the Earth’s mantle. The lower boundary is limited by the maximal lithosphere thickness that is typical only for the old parts of continents (cratons). Mineral inclusions in ultradeep diamonds comprising majoritic garnet, magnesiowustite, and phases of CaSiO 3 and MgSiO 3 compositions (presumably, with a perovskite structure), as well as tetragonal phase of pyrope-almandine composition (TAPP) [2] are direct samples from mantle depths, which allow us to test models of the Earth’s deep structure based on geophysical and experimental investigations. Of special interest among all the ultrahigh-pressure phases is majoritic garnet, because its composition and parageneses are most indicative for estimation of physicochemical conditions of the formation of deep mineral assemblages. According to the experimental data, pyroxene component starts to dissolve in garnet at a depth of ~250 km, resulting in the increase of Si and Na and the decrease of Al concentrations in this mineral according to the scheme Na + + Si 4+ = Mg 2+ + Al 3+ [3]. Experiments performed for mantle eclogite systems in a wide PT range using a high-pressure multianvil apparatus [4‐8] confirmed the influence of pressure on the composition of garnets (mainly Na admixture and excess Si contents) obtained in experiments (Fig. 1). In our study, we have analyzed the experimental data in the multicomponent Na 2 O–CaO–MgO–FeO–Al 2 O 3 – SiO 2 system and suggested a model of the garnet‐clinopyroxene barometer based on the change of Na concentration in garnet. In the opinion of Haggerty and Sauter [9], jadeiterich pyroxene lamellae in garnet are related to the decomposition of high-pressure Na-bearing garnets. The increase of pressure may lead to the opposite process of pyroxene microlite dissolution in garnet. This fact provides an explanation for the increase in the Si content up to 3.3 f.u. and a decrease in the Al content down to 1.3 f.u., as observed in garnets from the Monastery kimberlite pipe. According to the experimental data, such garnets were formed at a pressure of >10 GPa. Therefore, Na incorporation in garnet may be considered as a result of jadeite dissolution accompanied by Na and Si ingress according to the reaction [1]