Ray Macdonald

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al2O3 contents as low as 2.3xa0wt.%, FeO* contents up to 17xa0wt.% and SiO2 contents ~65xa0wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5xa0wt.% Al2O3, 13xa0wt.% FeO* and 66u2009±u20092xa0wt.% SiO2. The dominant fractionating assemblage is alkali feldsparu2009+u2009fayaliteu2009+u2009hedenbergiteu2009+u2009oxidesu2009±u2009quartz. Feldspar – melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.

Low-temperature zircon growth related to hydrothermal alteration of siderite concretions in Mississippian shales, Scotland

Zircon occurs in voids and cracks in phosphatic coprolites enclosed in siderite concretions in Mississippian shales near Edinburgh, Scotland. The zircon formed during hydrothermal alteration of early-diagenetic concretions and occurs as spherical aggregates of prismatic crystals, sometimes radiating. Vitrinite reflectance measurements indicate temperatures of ~270°C for the zircon-bearing concretions and the host shales. Molecular parameter values based on dibenzothiophene and phenanthrene distribution and occurrence of di- and tetra-hydro-products of polycyclic aromatic compounds suggest that the rocks experienced relatively high-temperature aqueous conditions related to hydrothermal fluids, perhaps associated with neighboring mafic intrusions. The zircon was dissolved from the concretions, transported in fluids, and reprecipitated in voids. This is the first record of the precipitation of authigenic zircon in sedimentary rock as a new phase, not as outgrowths.

Distribution of germanium between phenocrysts and melt in peralkaline rhyolites from the Kenya Rift Valley

Abstract Germanium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glass matrices from a metaluminous trachyte and four peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley, Africa. Abundances (in ppm) are: sanidine 0.45-0.61; fayalite 4.8-11.7; hedenbergite 5.1-9.0; titanomagnetite 2.7; ilmenite 0.48; amphibole 8.3-8.9; biotite 7.0; chevkinite-(Ce) 309; trachyte glass 3.0; rhyolitic glasses 2.3-3.9. These values are generally greater than those recorded for silicic rocks in the literature, whilst the chevkinite-(Ce) value is the largest yet found in a magmatic mineral. Apparent partition coefficients range from 0.15-0.26 in sanidine to 124 in chevkinite-(Ce). Those for fayalite and hedenbergite increase with whole-rock peralkalinity and Fe content. The possibility of a role for accessory phases in influencing Ge distribution in rock-forming minerals is also raised.

Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

Abstract The Kûngnât Complex (1275±1.8 Ma) in the Gardar Alkaline Province, South Greenland, cuts Archaean gneisses and comprises two intersecting syenitic stocks and a gabbroic ring-dyke. The magmas, with increasingly more primitive compositions, were emplaced successively by ring-faulting and roof stoping. The syenites are orthocumulates (cumulus alkali feldspar, olivine, pyroxene, titanomagnetite and apatite; intercumulus phases include alkali amphibole, biotite, quartz and calcite). In the well dissected earlier stock, a 2.2 km-thick layered sequence displays graded modal layering, feldspar lamination and cryptic layering. Modal layering in both stocks is directed mainly inwards at 35°−50°. Heterogeneous nucleation of the cumulus assemblage, close to steep thermal boundary layers, is inferred. The modal layering is ascribed primarily to gravitational sorting aided by the large density differential between a) feldspar and b) Fe-rich silicates and oxides. Episodic collapse of cumulus + melt slurries contributed to inward-dipping crystal pediments on the chamber floors. The Ring-Dyke (up to 100 m wide) is nearly continuous through 360°. Kûngnât exhibits a compositional nearcontinuum from olivine gabbro through syenite intermediaries to alkali granite, ascribed to protracted assimilation/fractional crystallization processes. The most radiogenic Nd isotope data from Kûngnât (ƐNdi values between −3.3 and −1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment in LREE by crustal contamination of the magmas.

Peralkaline felsic magmatism at the Nemrut volcano, Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

AbstractnNemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000xa0years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750xa0°C, at fO2 close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78xa0wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas.

Multiphase magmatic activity in the Variscan Kłodzko–Złoty Stok intrusion, Polish Sudetes: evidence from SHRIMP U–Pb zircon ages

The Kłodzko–Złoty Stok intrusion (KZSI), located in the NE part of the Bohemian Massif of Central Europe, has preserved records of the magmatic and tectonic activity of the Variscan orogeny in the Sudetes. KZSI is extraordinarily complex texturally, and shows a very wide range of chemical, isotopic and mineralogical compositions which indicates a complex, multiphase history of intrusion. New SHRIMP U–Pb zircon ages provide evidence of two magmatic episodes with a distinctive hiatus between them at ~u2009326–308xa0Ma. The youngest dated zircons (303.8u2009±u20093.8/4.2xa0Ma) are from the Laski leucocratic hornblende–biotite monzogranite and mark a short interval of post-collisional reactivation of the magma system. Zircons from a biotite–hornblende diorite sheet from the northern part of KZSI yielded an age of 349u2009±u20093.4/3.7xa0Ma, interpreted as indicating the initial phase of an early Carboniferous, multi-pulse episode. The evolution of the magmatic system can be tied to a 334.4u2009±u20092.9/3.3xa0Ma zircon age for melanocratic syenites from Podzamek which represent lamprophyric melt. Based on textural and compositional data and field relationships we interpret the intrusion as having been tilted post-emplacement towards the NW. This, together with the new dating results, ties the early Carboniferous episode of KZSI activity and the Jawornik granitoids (5xa0km to the SE) to the same magmatic system and arguably to bentonites of the Paprotnia beds (5xa0km to the NW), all representing different levels of such a system. The newly estimated late Carboniferous age from Laski ties part of the KZSI to other late Carboniferous Sudetic intrusions and late Carboniferous–Permian volcanism.

Recent progress in the study of accessory minerals

Accessory minerals are a common species in igneous and metamorphic rocks that are not considered characteristic of the host rock and hence do not affect its root name. Accessories tend to be complex in terms of their chemical and isotopic composition and their structural state. In spite of not being major rock constituents, they are, however, of enormous petrologic interest as they contain a record of the formation and post-formation history of their host rock. The study of accessory minerals hence has increased continuously during the past years, and still increases (Fig. 1). Recent progress is driven by new analytical opportunities of (in situ) micro-techniques. More and more the internal textures, that is, elemental, isotopic, and/or structural distribution patterns within individual grains, have come into the focus of researchers; a few examples are compiled in Fig. 2. The present special issue aims at providing an overview of recent progress in the study of accessory minerals. It was compiled on the occasion of CAM–2017, the Conference on Accessory Minerals (September 13–17, 2017, Vienna, Austria). This conference was a follow-up of the Workshop on Accessory Minerals (September 25–26, 2014, Warsaw, Poland), organised by Ray Macdonald and Bogusław Bagiński. The continuing interest in accessory mineral research is also underlined by the fact that Mineralogy & Petrology released a special issue “Accessory minerals in igneous and metamorphic rocks” just six years ago (vol. 102; October, 2011). The present special issue, devoted to the very same topic, contains 13 research articles. Even though zircon seems

Structure of Sr-Zr-bearing perrierite-(Ce) from the Burpala Massif, Russia

Abstract Sr- and Zr-bearing perrierite-(Ce) occurring in aegirinized syenite pegmatites of the Burpala massif, Russia, is compositionally intermediate between perrierite-(Ce) and hezuolinite and occupies a compositional gap in minerals of the chevkinite group. Its crystal structure has been determined using a single-crystal diffractometer fitted with a CCD detector and MoKa X-ray radiation. The mineral is monoclinic; a = 13.815(1), b = 5.668(1), c = 11.842(1) Å, β = 113.843(3)°, V = 848.18(4) Å3, space group C2/m, Z = 2. The crystal structure was refined with the occupancies [(Ce1.2La1.0Nd0.15) (Sr1.0Ca0.5Na0.15)]4(Zr0.5Fe0.3Mn0.2)(Ti1.3Fe0.7)2Ti2(Si2O7)2O8 on the basis of chemical composition although the allocation of cations to particular sites was performed on the basis of the number of refined electrons in each unique site. The dominance of Zr in the B site links the Burpala perrierite-(Ce) to more Sr-Zr-rich members of the chevkinite group, such as hezuolinite and rengeite. As in all of the perrierite members, there is a distortion of the D site octahedra, which is interpreted as due to the packing of the REE ions.

Niobium-rich chevkinite-(Ce) – structural investigations

The minerals belonging to the chevkinite group (CGM) are recognised as accessory phases in a wide range of igneous and metamorphic rocks [1, 2]. The geochemical importance of this group is associated with a high REE-concentration; the total REE2O3 contents are up to 50 wt%. They can be the dominant REE-bearing phases in any given rock. The REE are increasingly used in green technologies, such as the production of novel wind turbines, low-energy light bulbs also mobile phones. Additionally neodymium, one of the most common REE, is a key part of neodymium-iron-boron magnets used in hyperefficient motors and generators [3]. We will present the results of electron microprobe analyses and structural determination of a Nbbearing mineral from Biraya, Russia, which has the Nb2O5 content of 10.19 wt.% and the Ti/Nb ratio of 1.1:1. A special attention in the study has been paid to the role of Nb and its valency in the structure. The refinement of X-ray data based on microprobe analysis leads to the following formula for this phase: ( C e 2 L a 1 . 2 5 N d 0 . 4 P r 0 . 2 N a 0 . 1 5 ) (Fe0.7Ca0.15Sr0.15)(Fe1.3Mg0.15Nb0.55) (Ti1.2Nb0.55Al0.15V0.1) Si4O22. In order to determine which atoms occupy particular sites, the Bond Valence (BV) Model [4], together with an analysis of the ionic radii and volumes at each site within the first coordination sphere polyhedra, was used. There is some uncertainty in the literature as to which space group (or groups) does CGM belong to. P21/a was favoured in some publications ([5],[6],[7]), whereas in the other ones ([8],[9],[10], [11]) the C2/m space group was prefered. On the basis of our new structural data for niobian chevkinite-(Ce), one can rationalise a possible relationship between the P21/a and C2/m space groups. The same single crystal which was investigated using X-ray radiation was annealed at 750oC for 24 hours, and then rapidly cooled to room temperature (within 1 hour). After this process, we collected the X-ray scattering data on our single crystal X-ray diffractometer. The analysis of the reconstructed reciprocal lattice layers indicates a significant decrease of symmetry for the niobian chevkinite(Ce) from the C2/m to P21/a space group. The observed phase transition is in a good agreement with the group theory. The possibe space groups can be presented as the Bärnighausen tree [12]. This scheme is showing transition pathway from the supergroup (C2/m in this case) to one of the subgroups (P21/a). A possible explanation of the phase transition that occured for niobian chevkinite-(Ce) will be presented by authors.