Julian Schilling

The compositional variability of eudialyte-group minerals

Abstract Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mössbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/∑Fe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

A fast and easy-to-use approach to cation site assignment for eudialyte-group minerals

Eudialyte-group minerals (EGM) are typical constituents of agpaitic varieties of peralkaline rocks. In their complex structure (N15–16M(1)6M(2)3Z3M(3)M(4)Si24O66–73(OH)0–9X2), many cations (e.g. Na+, Ca2+, Fe2+, Mn2+, REE3+, Zr4+, and Si4+) as well as different hydrogen-bearing species (H2O, OH–, H3O+) may occupy different structural sites. Also, two potentially vacancy bearing positions are present. Thus, various methods of calculation of mineral formulae for EGM in the literature are inconsistent and in some cases not charge-balanced. We present an extended and improved scheme for site assignment using IMA-approved end-members and taking into account the different structural units of EGM. This method is based on electron microprobe analyses alone not considering different valence states of Fe and Mn and undetermined H2O-contents. However, comparison with structural refinement data from the literature reveals major agreement and significant improvement compared to earlier proposed methods. The instruction given here can easily be transferred to a table calculation spread sheet (e.g. EXCEL©), which is available from the corresponding author on request.

Fe-Ti oxide-silicate (QUIlF-type) equilibria in feldspathoid-bearing systems

Abstract Silicate-oxide equilibria (abbreviated as QUIlF) have proven to be very powerful tools for reconstructing the temperature and oxygen fugacity evolution of magmatic systems containing magnetite and ilmenite with olivine, quartz, or pyroxenes. In this paper, we extend these QUIlF equilibria to include rocks where silica activity is controlled by equilibria between feldspars and feldspathoids. We present data on the orthomagmatic assemblage of titanomagnetite + ilmenite + feldspar + nepheline + compositionally variable olivine, which we call AUNIlF: The AUNIlF reference curve (with unit activities for albite, nepheline, and fayalite) is stable at oxygen fugacities ≥2 log units below the QUIlF surface at temperatures of about 700 to 800 °C, temperatures typical of late-magmatic stages. At temperatures > ~800 °C, the AUNIlF reference assemblage would only be stable at unrealistically low fO₂ conditions more than 5 log units below FMQ (where FMQ is the fayalite-magnetite-quartz buffer), which explains the rarity or absence of orthomagmatic AUNIlF assemblages. We determine the most reduced conditions indicated by displaced AUNIlF assemblage from Mont Saint-Hilaire (Quebec, Canada) to be ΔFMQ = -1.15 at ~800 °C (olivine is Fa67 and aSiO₂ = 0.41) and conclude that AUNIlF assemblages involving pure fayalite do not stably occur in terrestrial magmatic systems. The stability field of naturally occurring AUNIlF assemblages is a function of albite, nepheline, and olivine compositions and is controlled by the ratio of silica activity to fayalite activity (aSiO₂/ aFa). At values higher than ~0.77 for aSiO₂/aFa, olivine is Fa < ~70 when silica activity is buffered by the nepheline-albite equilibrium. In these situations, AUNIlF is stable at oxygen fugacities ≥ -1.15 (ΔFMQ). At values below aSiO₂/aFa ~0.77, the AUNIlF equilibrium is shifted to lower oxygen fugacities and ilmenite becomes unstable relative to ulvøspinel. Analogous to the construction and application of AUNIlF, a QUIlF-type reaction curve for potassic systems (KULIlF) involving leucite and alkali feldspar is presented and applied to naturally occurring assemblages. Potassic rocks invariably crystallize forsteritic olivine in the presence of ilmenite and magnetite, reflecting higher oxygen fugacities during crystallization than their sodic counterparts. As a result of low fayalite component in olivine, the aSiO₂/aFa ratio becomes ≥4 in assemblages of potassic systems consisting of alkali feldspar, magnetite, leucite, ilmenite, and olivine.