Melting-relations in the system NaA1Si 3 O 8 –NaA1SiO 4 –NaFeSi 2 O 6 –CaMgSi 2 O 6 –H 2 O, and their bearing on the genesis of alkaline undersaturated rocks

Abstract

Experimental data are presented that constitute a basis for a more detailed treatment at present under investigation, involving the addition of the alkali pyroxene end-members acmite (NaFeSi 2 O 6 ), diopside (CaMgSi 2 O 6 ), and hedenbergite (CaFeSi 2 O 6 ) to the silica- and potash-poor portion of ‘petrogeny’s residua system’. Phase relations have been determined in the systems NaAlSi 3 O 8 –NaFeSi 2 O 6 , NaAlSiO 4 –NaFeSi 2 O 6 , NaAlSi 3 O 8 –NaAlSiO 4 –NaFeSi 2 O 6 at 1000 kg/cm 2 water vapour pressure; the partial pressure of oxygen was controlled in most of the experiments by a nickel–nickel oxide buffer assemblage. The composition of the ‘low melting-point’ in the system NaAlSi 3 O 8 –NaAlSiO 4 –NaFeSi 2 O 6 is located at Ab 15 Ne 30 Ac 55 . Little agreement was found between this point and the density distribution plot of normative Ab+Or, Ne+Ks, Ac+Di in a total of 256 volcanic and plutonic alkaline undersaturated rocks. However, with the addition of known amounts of diopside to the system NaAlSi 3 O 8 –NaAlSiO 4 –NaFeSi 2 O 6 , the determined phase relations show a close agreement with the normative plots for both alkaline and peralkaline undersaturated rocks. The possible genesis of alkaline rocks by processes involving crystal ⇋ liquid equilibrium is discussed in the light of this evidence.