ORIGINAL PAPER Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study Christopher Giehl • Michael Marks • Marcus Nowak Received: 2 May 2012 / Accepted: 30 August 2012 Ó Springer-Verlag 2012 Abstract We experimentally investigated the phase rela- tions of a peralkaline phonolitic dyke rock associated with the Ilı ´maussaq 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 Ilı ´maussaq 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. H 2 O contents ranging from anhy- drous to H 2 O saturated (*5 wt% H 2 O) and varying fO 2 (*DlogFMQ -3 to ?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 ulvøspinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H 2 O-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 fO 2 and aH 2 O force the evolution of the residual melt toward decreasing SiO 2 , increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO 2 and aH 2 O produce residual melts with relatively low FeO*, high SiO 2 and a relatively constant alkalinity index. We show that variations of aH 2 O and fO 2 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 (Na 2 Fe 5 TiSi 6 O 20 ), coexisting with the most evolved melts at 750 °C. Contrary to what might be expected, experiments with high aH 2 O and interlinked high fO 2 exhibit higher liquidus T’s compared with exper- iments performed at low aH 2 O and fO 2 for experiments where magnetite is liquidus phase. This is because ulvøspinel-poor magnetite crystallizes at higher fO 2 and has a higher melting point than ulvøspinel-rich magnetite, which is favored at lower fO 2 . Keywords Crystallization experiments  Phase relations  Liquid line of descent  Peralkaline  Phonolite  Ilı ´maussaq Introduction The investigation of phase equilibria is a powerful method to elucidate crystallization, differentiation and fraction- ation processes in magmatic systems. Previous experi- mental work on phase relations in igneous systems covers a range of whole-rock compositions like granites (Tuttle and Bowen 1958), alkali and tholeiitic basalts (Villiger et al. 2004; Berndt et al. 2005; Freise et al. 2009), ferro basalts (Toplis and Carroll 1995; Botcharnikov et al. 2008), Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi:10.1007/s00410-012-0809-6) contains supplementary material, which is available to authorized users. C. Giehl (&)  M. Marks  M. Nowak Eberhard Karls Universita ¨t Tu ¨bingen, Wilhelmstr. 56, 72074 Tu ¨bingen, Germany e-mail: christopher.giehl@uni-tuebingen.de URL: http://www.geo.uni-tuebingen.de/index.php?id=2179 123 Contrib Mineral Petrol DOI 10.1007/s00410-012-0809-6