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Investigation of polymineralic melt inclusions preserved in garnet of eclogite-facies metapelites of the Usagaran belt, Tanzania, is of particular importance as these metapelites, intercalated in oceanic metabasites, document the rare case of partial melting at high temperatures in a subducted oceanic crust. With an age of 2 Ga the rocks represent one of the oldest oceanic crusts and confirm a subduction process already at Paleoproterozoic times. Partial melting probably was initiated by dehydration melting under the presence of a CO2-rich fluid phase. The melt is preserved in siliceous polymineralic inclusions, while CO2 locally reacted with the garnet host to form dolomite-quartz-kyanite inclusions. During this reaction, the REE spectrum of garnet is adopted by the dolomite. Furthermore, graphite inclusions in garnet must have precipitated from the CO2 fluid by reduction. The highly ordered graphite structure indicates a formation temperature of at least 700 °C. Rehomogenization experiments of the siliceous polymineralic inclusions yield a homogeneous melt of rhyolitic, peraluminous composition. Thermodynamic modelling enables to deduce a P–T path in accordance with high P–T conditions (minimum 2.0 GPa, 900 °C) where a partial melt formed due to phengite breakdown leading to the preserved peak mineral assemblage garnet, alkali feldspar, kyanite, quartz and rutile. A very fast uplift of the oceanic crustal rocks can be deduced from the occurrence of very finely exsolved metastable ternary feldspar and from the preserved prograde zoning in garnet. [ABSTRACT FROM AUTHOR]

Fluorine and chlorine affect phase stabilities in magmatic rocks. We present phase equilibrium experiments investigating a peralkaline and iron-rich phonolitic composition with variable F and Cl contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilímaussaq plutonic complex (South Greenland). Experiments were performed at 100 MPa, 1,000-650 °C and low oxygen fugacity adjusted with graphite-lined gold capsules in an internally heated argon pressure vessel and rapid quench cold seal pressure vessels. To cover this large T interval, we applied a two-step fractional crystallization strategy where glasses representing residual melt compositions at 800 °C were synthesized as starting material for consecutive experiments at lower T. In these experiments, oxidized starting glasses allocate oxygen by reduction of ferric iron and up to 1.2 wt% dissolved OH form through reaction with hydrogen provided by the pressure medium (HO) in initially dry experiments. For OH determination, hydrated super-liquidus experiments in Au capsules were performed to calibrate the extinction coefficient for the fundamental OH stretching vibration using infrared spectroscopy ( ε = 48 ± 3 L mol cm). Observed mineral phases in our experiments are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (NaFeTiSiO), alkali feldspar and nepheline (±native iron) coexisting with residual melt. Above 1.5 wt% F concentrations, fluorite (CaF) and hiortdahlite (CaZrSiOF) are stable in favor of Ca-rich clinopyroxene. Sodalite (NaAlSiOCl) and eudialyte (NaCaFeZrSiO(OH)Cl) form at Cl concentrations of 0.2-0.5 wt% (depending on T) and ZrO concentrations >0.7 wt% are additionally needed to stabilize eudialyte and hiortdahlite. Therefore, F and Cl may become compatible in such systems and have the potential to influence F/Cl melt ratios in evolving magmas. [ABSTRACT FROM AUTHOR]

We experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the Ilímaussaq 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 Ilímaussaq complex. This dyke rock is therefore perfectly suited for performing phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate formation 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. HO contents ranging from anhydrous to HO saturated (~5 wt% HO) and varying fO (~ΔlogFMQ −3 to +1; where FMQ represents the fayalite-magnetite-quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystallization of alkali feldspar, nepheline, hedenbergite-rich clinopyroxene, fayalite-rich olivine and minor amounts of ulvøspinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and HO-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 and aHO force the evolution of the residual melt toward decreasing SiO, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO and aHO produce residual melts with relatively low FeO*, high SiO and a relatively constant alkalinity index. We show that variations of aHO and fO 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 (NaFeTiSiO), coexisting with the most evolved melts at 750 °C. Contrary to what might be expected, experiments with high aHO and interlinked high fO exhibit higher liquidus T's compared with experiments performed at low aHO and fO for experiments where magnetite is liquidus phase. This is because ulvøspinel-poor magnetite crystallizes at higher fO and has a higher melting point than ulvøspinel-rich magnetite, which is favored at lower fO. [ABSTRACT FROM AUTHOR]