Universität Tübingen
Experimental studies on phase relations in iron-rich peralkaline phonolitic melts
Abstract
The experimental investigation of phase equilibria is a powerful method to study the formation of magmatic rocks. This work presents phase equilibrium experiments investigating an iron-rich peralkaline phonolitic composition with variable fluorine and chlorine contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilímaussaq plutonic complex (South Greenland). The dyke composition is perfectly suited for performing phase equilibrium experiments since, in contrast to the Ilímaussaq plutonic rocks, no major cumulate formation processes occurred. Experiments were performed isobarically (100 MPa) at 1000 - 650 °C with variable H2O concentrations (nominally dry to H2O-saturated) and oxygen fugacities (Δ log FMQ - 4 to +1, where FMQ represents the fayalite-magnetite-quartz oxygen buffer). Experimental conditions were applied using gold capsules and graphite-lined gold capsules in an internally heated argon pressure vessel and rapid-quench cold seal pressure vessels. To cover this large T interval a two-step fractional crystallization strategy was applied where glasses, representing residual melt compositions at 800 °C, were synthesized as starting material for consecutive experiments at lower T. The observed mineral phases coexisting with residual melt are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (Na2Fe5TiSi6O20), alkali feldspar and nepheline ( ±native iron). Above 1.5 wt% F in the coexisting melt, fluorite (CaF2) and hiortdahlite (Ca6Zr2Si4O16F4) are stable in favor of clinopyroxene. Sodalite (Na8Al6Si6O24Cl2) and eudialyte (Na15Ca6Fe3Zr3Si26O73(OH)3Cl2) form at Cl melt concentrations of 0.2 - 0.5 wt% (depending on T) and ZrO2 melt concentrations higher than 0.7 wt%, are additionally needed to stabilize hiortdahlite and eudialyte. The phenocryst assemblage of the dyke rock was reproduced at 850 - 800 °C, nominally dry conditions and Δ log FMQ -2. Except amphibole, all major mineral phases of the groundmass assemblage were reproduced at < 750 °C in H2O-poor experiments at Δ log FMQ -1. Therefore, both the phenocryst assemblage and the groundmass assemblage of the dyke rock may crystallize from one peralkaline melt through fractional crystallization. This may indicate that the Ilímaussaq plutonic rocks evolved from one single magma batch. Geothermometry in such highly evolved peralkaline rocks is often complicated by the absence of Fe-Ti oxides, olivine and clinopyroxene. Erroneous results were obtained when testing several thermometers with experimental data from the present study. Therefore, these experiments were used to calibrate four new geothermometers based on the distribution of Mn between clinopyroxene, aenigmatite, eudialyte and melt, and Na/Ca partitioning between clinoyproxene and melt. Preliminary experiments with REE-doped (Nb, La, Ce, Y and Sr) starting compositions show that phase stabilities and the liquid line of descent are similar to experiments using the same starting composition without trace elements, supporting the significance of the experimental results using simplified starting compositions with respect to nature.
Author and committee
dc:creator, dc:contributor.*- Author
-
- Giehl, Christopher
Identifiers
dc:identifier.*- Identifier
- hdl:10900/52077