Michael A.W. Marks

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.

The influence of T, aSiO2, and fO2 on exsolution textures in Fe-Mg olivine: An example from augite syenites of the Ilimaussaq Intrusion, South Greenland

Abstract Two samples from the augite syenite unit of the 1.13 Ga alkaline to peralkaline Ilimaussaq intrusion in South Greenland contain olivine (Fa75-85) with micrometer-sized exsolution lamellae of intergrown augite and magnetite. The exsolved olivine contains only about 0.6 wt% CaO, whereas the reintegrated magmatic composition contained about 1.4% CaO corresponding to about 3 mol% of a larnite (La) component. The exsolved olivine bears no measurable Na, Al, or Ti, but the reintegrated magmatic olivine contained up to 0.4 wt% Na2O, 0.2 wt% Al2O3, and 0.3 wt% TiO2. Magmatic baddeleyite in the same samples indicates an extremely low silica activity of below 0.4 during the early stages of magmatic crystallization at > 900 °C and 1 kbar. According to the equilibrium Ca2SiO4 (La in Ol) + SiO2 = 2 CaSiO3 (Wo in Cpx), the low silica activity is thought to be responsible for the unusually Ca-rich compositions of the primary olivines, which, in these samples, formed at 875 ± 15 °C and at oxygen fugacities between 1.2 and 2.3 log units below the QFM buffer. The composite exsolution lamellae formed during cooling due to an overstep of the schematic reaction Fa + Kst (Ca-rich component in olivine) + 1/3 O2 = Hd + 2/3 Mt. Phase equilibria indicate temperatures between 300 and 750 °C and oxygen fugacities close to QFM for this exsolution process. The lamellae only developed in samples where the olivines were both especially high in Ca and presumably rapidly cooled (although not chilled!), as the lamella-bearing samples were collected at the outermost rim of the augite syenite unit and close to the top of the Ilimaussaq intrusion. Schreinemakers analysis shows that Ca-rich olivine of the kirschsteinite-monticellite solid solution series [Ca(Fe,Mg)SiO4] would exsolve from Ca-bearing olivine of the fayalite-forsterite series only at unusually low silica activities, whereas at higher silica activities, the observed exsolutions of clinopyroxene with magnetite are more stable. Hence, both cooling history and evolution of intensive variables during cooling are important for the formation of olivine exsolution textures.

Apatites from the Kaiserstuhl Volcanic Complex, Germany: new constraints on the relationship between carbonatite and associated silicate rocks

Apatites from carbonatites, related alkaline silicate rocks, a carbonate-bearing melilititic dyke rock (bergalite), and a diatreme breccia (containing both carbonate and silicate fragments) of the Miocene Kaiserstuhl Volcanic Complex, SW Germany, are used to reconstruct the petrogenetic relationship among these rocks. Apatites from carbonatites reach higher Sr and Nb contents but are generally lower in Fe, Mn, Th, U, Si, S, Cl and Br compared to apatites from associated silicate rocks, whilst Na, REE and F contents are overlapping. Apatites from bergalite show a systematic and discontinuous core-rim zonation, with the core being compositionally similar to apatites from silicate rocks and the rim corresponding to carbonatitic apatites. These observations imply that the bergalite apatites nucleated in a silicate melt and continued to crystallize from an evolving CO 2 -enriched melt probably with carbonatitic affinity. Apatites from a diatreme breccia comprise three populations: (1) similar to the apatites from silicate rocks, (2) similar to the carbonatitic apatites, and (3) resembling apatite population (1) partially replaced by apatite (2). We infer that apatite (1) was derived from silicate-rock fragments and apatite (2) crystallized from a later intruding carbonatitic melt, which metasomatized the silicate-rock fragments and caused the replacement textures as observed in apatite population (3). We conclude that apatites from the Kaiserstuhl complex preserve important information on the petrogenetic relationship between carbonatitic and silicate melts. The carbonatitic melts at the Kaiserstuhl complex are probably the products of protracted fractionation of a CO 2 -rich nephelinitic melt.

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.

The Ilímaussaq Alkaline Complex, South Greenland

The Ilimaussaq complex in South Greenland is a well-studied multiphase alkaline to peralkaline intrusion of MesoProterozoic age. Most of the Ilimaussaq rocks are extremely enriched in alkalis, iron, halogens, high-field-strength elements (HFSE) and other rare elements, forming one of the most differentiated peralkaline rock suites known.

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.

The onset and origin of differentiated Rhine Graben volcanism based on U-Pb ages and oxygen isotopic composition of zircon

Ion microprobe U-Pb zircon ages for late (phase II) syenite dikes of the Katzenbuckel (SW Germany) average 69.6 ± 1.9 Ma (mean square of weighted deviates MSWD =0.36; number of spot analyses n =9; 2σ error) and represent a minimum age for the emplacement of earlier, volumetrically dominant phonolites and nepheline syenites (phase I). This age is ~ 13 and 6 Ma older than previously published K-Ar whole-rock and bulk biotite ages for phase I rocks, respectively. Because of close chemical affinities in magma composition and the small volume of the Katzenbuckel subvolcanic intrusion, zircon crystallization in phase II syenite dikes presumably only shortly postdates phase I magmatism. Trachytes from the Northern Upper Rhine Graben (Sprendlinger Horst, Wetterau) yielded similar U-Pb zircon ages of 68.1 ± 1.4 Ma (MSWD =2.2) and 70.3 ± 1.6 Ma (MSWD =0.4), respectively. Zircon oxygen isotopic ratios of Katzenbuckel indicate primitive mantle-like compositions, whereas Sprendlinger Horst and Wetterau trachytes zircons show minor crustal contamination. The investigated differentiates rank among the oldest known post-Permian igneous rocks in the Northern foreland of the Alpine collisional belt, and are contemporaneous with recently dated lamprophyres in the Northern Rhine Graben area. This suggests that a short (<1–2 Ma) but regionally widespread magmatic pulse occurred along the E and NE flanks of the nascent Rhine Graben rift as early as Late Maastrichtian.

The sulfur speciation in S-bearing minerals: New constraints by a combination of electron microprobe analysis and DFT calculations with special reference to sodalite-group minerals

Abstract In this work, we present an improved method for the semi-quantitative determination of sulfur species in S-bearing minerals by electron microprobe analysis. For calibration, we analyzed several sulfate and sulfide standard minerals such as baryte, celestine, chalcopyrite, and pyrite, and correlated the results with theoretical calculations retrieved from density functional theory (DFT). We applied this method to natural sodalite-group minerals from various localities. In addition, we applied the more common Raman spectroscopy to some samples and show that this method cannot be applied to sodalite-group minerals to determine their sulfur speciation. We show that even though sodalite-group minerals have a complex crystal structure and are sensitive to the electron beam, electron microprobe analysis is a reliable tool for the analysis of their sulfur speciation. The natural sodalite-group minerals show systematic variations in sulfur speciation. These variations can be correlated with the independently determined oxidation state of the parental magmas thus making S-bearing sodalitegroup minerals good redox proxies, although we show that the electron microprobe analysis of the sulfur speciation is matrix-dependent, and the sulfur speciation itself depends on crystal chemistry and structure, and not only on fO₂.

Layered intrusions and traffic jams

A wide range of explanations has been proposed for the origin of repetitive layering in mafic-ultramafic and in (per)alkaline intrusions. Here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions, without the need to invoke extrinsic factors. Similar to traffic jams on a motorway, small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals, respectively. These “traffic jams” separate layers of magma from the rest of the magma chamber. The magma in the individual layers further evolves as a largely independent subsystem, with gravitational sorting organizing the mineral distribution within each layer. Layering can develop in the intermediate range between full mineral separation in low-viscosity or slowly cooling magma chambers and homogeneous crystallization in high-viscosity or fast-cooling chambers. This self-organization mechanism provides a novel explanation for the formation of rhythmic layering in low-viscosity magmas, for example in the Ilimaussaq igneous complex in southwest Greenland.

Major, minor, and trace element composition of pyromorphite-group minerals as recorder of supergene weathering processes from the Schwarzwald mining district, SW Germany

Abstract More than 150 samples of pyromorphite, mimetite, vanadinite, and minerals of the hedyphane-group (which collectively are summarized here under the term PyGM for pyromorphite-group minerals) from the Schwarzwald mining district, southwest Germany, have been analyzed by electron microprobe and LA-ICP-MS. In this largest study of its kind, the relations of PyGM composition to host rock and fluid compositions and the amount of solid solution between the various end-members were investigated. In addition, we report the colors of the many analyzed mineral compositions. Here is a list of the most important results. • Pyromorphite and mimetite are completely miscible. • At conditions of the oxidation zone in ore deposits, the solvus between pyromorphite and vanadinite appears to asymmetrical with up to about 3 mol% of vanadinite component in pyromorphite and up to about 39 mol% of pyromorphite component in vanadinite in cases where both minerals coexist. • Due to a lack of suitable samples, the solvus between vanadinite and mimetite could not be completely constrained, but we report vanadinite analyses with up to 12 mol% mimetite and 8 mol% pyromorphite component. • There is complete miscibility between pyromorphite and phosphohedyphane and between mimetite and hedyphane. • F-rich varieties appear only to exist in hedyphanes and phosphohedyphanes, while pyromorphites, mimetites, and vanadinites are Cl- or OH-dominated. • We report for the first time analyses suggesting the occurrence of an OH-end-member corresponding to mimetite. • While pyromorphites are preferably green and mimetites yellow, this is not at all a diagnostic feature, as many exceptions exist; major elements are not correlated at all with the color of a specific crystal. • Most PyGM are strongly zoned and display, for example, significant variations in Ca that are likely to be related to variations of the compositions of various fluid pulses from which the crystals formed. • PyGM composition is generally uncorrelated with host rock composition, but PyGM enrich other metals like REE, Cr, Sb, Bi, or U up to a factor of 106; therefore, they can be regarded as very sensitive recorders of the metal inventory of an oxidation zone and they even record metals only present as traces in the primary ore deposit very reliably. • REE patterns of PyGM show significant variability even at one location; this may suggest that each zone of a PyGM crystal records the REE pattern of a single fluid pulse or it may indicate fractionation of the REE during PyGM growth. In the absence of conclusive data, the former possibility appears the more likely one. • PyGM are extremely efficient filters for heavy metals from supergene solutions in and in the vicinity of ore deposits.