Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study

Abstract

We experimentally investigated the phase rela- tions of a peralkaline phonolitic dyke rock associated with the Ilimaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilimaussaq com- plex. This dyke rock is therefore perfectly suited for per- forming phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate for- mation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950-750 C. H2O contents ranging from anhy- drous to H2O saturated (*5 wt% H2O) and varying fO2 (*DlogFMQ - 3t o?1; where FMQ represents the fayalite-magnetite-quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystalliza- tion of alkali feldspar, nepheline, hedenbergite-rich clino- pyroxene, fayalite-rich olivine and minor amounts of ulvospinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H2O-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO2 and aH2O force the evolution of the residual melt toward decreasing SiO2, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO2 and aH2O produce residual melts with relatively low FeO*, high SiO2 and a relatively constant alkalinity index. We show that variations of aH2O and fO2 lead to contrasting trends regarding the liquid lines of descent of iron-rich silica- undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (Na2Fe5TiSi6O20), coexisting with the most evolved melts at 750 C. Contrary to what might be expected, experiments with high aH2O and interlinked high fO2 exhibit higher liquidus T's compared with exper- iments performed at low aH2O and fO2 for experiments where magnetite is liquidus phase. This is because ulvospinel-poor magnetite crystallizes at higher fO2 and has a higher melting point than ulvospinel-rich magnetite, which is favored at lower fO2.