Vertical variation in the deuteric oxidation of titanomagnetites in an ignimbrite deposit: Kızılkaya Ignimbrite (Cappadocia, Turkey)

Journal of Volcanology and Geothermal Research 308 (2015) 10–18

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Vertical variation in the deuteric oxidation of titanomagnetites in an ignimbrite deposit: Kızılkaya Ignimbrite (Cappadocia, Turkey) H. Evren Çubukçu Hacettepe University Dept. Geological Engineering, 06800 Beytepe, Ankara, Turkey

a r t i c l e

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Article history: Received 2 July 2015 Accepted 9 October 2015 Available online 23 October 2015 Keywords: Titanomagnetite Oxidation Oxyexsolution Ignimbrite

a b s t r a c t Titanomagnetites are arguably the most important Fe–Ti oxide mineral in magmatic rocks because they are widely used in paleomagnetic, magnetic anisotropy, and petrologic studies. Elemental concentrations of Ti in titanomagnetites can change rapidly with varying temperature and oxygen fugacity, with oxidation producing “oxyexsolution” lamellae. Titanomagnetites of the ca. 5 Ma Kızılkaya ignimbrite deposit in Cappadocia exhibit systematic vertical variation of trellis-type oxyexsolution forms in three sampled sections. The basal zones of Kızılkaya ignimbrite contain homogeneous titanomagnetites, whereas middle and the upper zones of the deposit comprise increasingly oxidized titanomagnetites where the oxyexsolved Ti-rich lamellae get thicker and more populated towards the top. The existence of oxidized titanomagnetite grains with increasing volume of trellistype exsolution lamellae indicates that the middle and the upper zones of the deposit cooled down more slowly than the basal zones and were influenced by deuteric oxidation during cooling. The progressive increases in oxidation upsection are also correlated with increasing devitrification. Oxidation results in the decrease of magnetically effective grain sizes of ferromagnetic titanomagnetites, which are surrounded by paramagnetic Ti-rich oxyexsolution lamellae, consequently reducing the saturation magnetization Js. Vertical variation in titanomagnetite textures requires careful selection of the sampling position in an ignimbrite deposit if magnetic properties are to be determined. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Fe–Ti oxides are nearly ubiquitous accessory mineral components in a wide variety of igneous rocks. They are compositionally defined by a ternary system with FeO, Fe2O3 and TiO2 as the end-members. This ternary system contains three essential solid-solution (ss) series: 1) magnetite (Fe3O4) – ulvöspinel (Fe2TiO4); these are also known as titanomagnetites; b) hematite (αFe2O3) – ilmenite (FeTiO3); the titanohematites and c) pseudobrookite (Fe2TiO5) – ferropseudobrookite (FeTi2O5). Titanomagnetites represent the most important series in because their unique magnetic and mineralogical characteristics have many applications in volcanological and petrological studies, such as in determining the paleomagnetic field directions of volcanic rocks, in identifying the source of pyroclastic flow deposits via anisotropy of magnetic susceptibility (AMS) (e.g. Canon-Tapia and MendozaBorunda, 2014 and references therein), and in estimating the temperatures and oxygen fugacities of igneous rocks (e.g. Ghiorso and Evans, 2008; Sauerzapf et al., 2008). Titanomagnetites are also used for geochemical characterization and correlation of tephra layers (e.g. Marcaida et al., 2014 and references therein), for revealing eruption

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histories (Turner et al., 2008) and for (U-Th)/He dating of intermediate to mafic volcanic rocks (Blackburn et al., 2007). Titanomagnetites in igneous rocks tend to oxidize as rocks cool or when oxygen fugacity increases (Buddington and Lindsley, 1964). Oxidation of Fe+2 to Fe+3 results in vacancies in octahedral sites of the {111} crystal planes, into where Ti diffuses increasingly (Aragon et al., 1984; Turner et al., 2008). Subsolidus exsolution-oxidation of Fe–Ti oxides is also common in plutonic and metamorphic rocks (Bacon and Hirschmann, 1988). Due to the rapid diffusion in titanomagnetites, elemental concentrations of Ti can change significantly even under post-magmatic conditions where they adjust to temperature and oxygen fugacity in their surrounding medium (Buddington and Lindsley, 1964; Devine et al., 2003; Tomiya and Takahashi, 2005; Turner et al., 2011). In slowly cooled volcanic rocks, titanomagnetites are often oxidized and transformed into composite multiphase grains with distinct chemical compositions (Saito et al., 2004). During the cooling of igneous rocks, primary Fe–Ti oxides can be affected by solid state exsolution and/or deuteric oxidation, which alter compositions and the grain size of Fe–Ti oxides with profound effects on their magnetic properties (Butler, 1998). Microtextures of Fe–Ti oxides are very sensitive to changes in redox state (Lattard et al., 2012) and “oxyexsolution” lamellae of ilmenite – hematitess are often observed as a result of high temperature (Buddington and Lindsley, 1964) (or deuteric) oxidation (Ramdohr, 1955; Saito et al., 2004) of titanomagnetite. The appearance

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and the development of these oxyexsolution textures can be regarded as tracers for the progress of high temperature (600–1000 °C) oxidation in titanomagnetites (Buddington and Lindsley, 1964: Lattard et al., 2012). Such textures have been produced in synthetic Fe–Ti oxides under controlled oxidation conditions over timescales ranging from a few seconds to a few hours (Lattard et al., 2012 and references therein). Despite their importance for the study of pyroclastic flow deposits, especially for widespread ignimbrites as deposits of hot pyroclastic density currents (e.g., Branney and Kokelaar, 2002), there have been no systematic studies that have investigated the effects of syn- and post-depositional oxidation in pyroclastic flow deposits with few exceptions. Saito et al. (2004) studied the oxidation state of Fe–Ti oxides in pyroclastic block-and-ash flows in order to assess deuteric oxidation processes in a lava dome. Turner et al. (2008) used exsolution textures of titanomagnetites in distal fall layer of Mount Taranaki in lake deposits to classify its eruption styles in the context of two simple end-member states: fast-ascent and slow-ascent eruptions. It is well documented that following emplacement and during cooling, the internal structure of ignimbrite deposits is affected by compactional welding, devitrification, vapor-phase crystallization, and fracturing (Keating, 2005). Chemical composition, thickness of the deposit and hence cooling rate, viscosity, and abundance of volatiles are potential factors that could control oxidation and magnetic mineral modification (Haggerty, 1976). Such processes inevitably alter not only the structural characteristics but also the overall mineralogical composition of the ignimbrite as a result of either secondary crystallization or deuteric transformations of the existing juvenile crystals of the ignimbrite body. The purpose of this study is to document the vertical variation in oxyexsolution textures of titanomagnetites in an ignimbrite deposit. In order to correlate deuteric alteration of titanomagnetites with the physical and mineralogical conditions, single cooling-unit Kızılkaya

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ignimbrite was sampled in three different localities (in the vicinity of Şahinefendi, Güzelöz and Kaymaklı towns; Fig. 1). A total of 18 samples (6 from each locality) were collected in order to represent basal, middle and upper zones of the flow. Both mineral separates of Fe–Ti oxides and respective bulk rock fragments were investigated using electron microscopy and spectrometry methods. Moreover, magnetic properties (saturation magnetization) of titanomagnetites have been measured for each sample. Through detailed textural and mineral-chemical analysis combined with quantifying the changes in magnetic properties of variably oxidized titanomagnetites, it is shown that careful sample selection is the key for the reliable use of titanomagnetite in volcanology and petrology. 2. Volcanological setting and Kızılkaya İgnimbrite The calc-alkaline Central Anatolian Volcanic Province (CAVP) comprises Miocene–Holocene eruption centers with associated effusive and eruptive products that collectively cover 20,000 km2 (Fig. 1). Explosive volcanism in the CAVP is mostly represented by extensive ignimbrite sequence comprising 10 major units (in stratigraphic order from old to young: Kavak, Zelve, Sarımadentepe, Sofular, Cemilköy, Tahar, Gördeles, Kızılkaya, Valibabatepe, and Kumtepe) (Le Pennec et al., 1994; Le Pennec et al., 2005; Aydar et al., 2012). Although the stratigraphy and the correlations between these major ignimbrite units have been a matter of debate, there is a broad consensus in both nomenclature and age of the Pliocene Kızılkaya ignimbrite. Apart from studies that covered CAVP ignimbrite volcanism in general, rhyolitic Kızılkaya ignimbrite has been the focus of several individual studies concerning its volcanological features (Schumacher and MuesSchumacher, 1996), magnetic fabric (Le Pennec et al., 1998; Agrò et al., 2015) and its probable source area using anisotropy of magnetic susceptibility (AMS) data (Le Pennec et al., 1998; Le Pennec, 2000). An

Fig. 1. Generalized geological map and the distribution of Kızılkaya ignimbrite in the study area. Sampling locations KK1, KK2 and KK3 are shown with stars (modified from Aydar et al., 2012).

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unusually low aspect-ratio (1:7000–1:10000) characterizes the Kızılkaya ignimbrite which is the most widespread ignimbrite in the CAVP covering a surface area of ~8,500–10,600 km2 with an estimated volume of 180 km3 (Le Pennec et al., 1994; Schumacher and MuesSchumacher, 1996; Aydar et al., 2012). Its eruption age is geochronologically well constrained by 40Ar/39Ar plagioclase (5.19 ± 0.07 Ma) and U–Pb zircon (5.11 ± 0.37 Ma) ages for minerals separated from pumice clasts (Aydar et al., 2012). Paquette and Le Pennec (2012) used cores from partly to strongly welded Kızılkaya ignimbrite to extract zircon and documented the occurrence of Proterozoic and Archean zircon xenocrysts ranging from ca. 2.3 Ga to 3.8 Ga in age in the ignimbrite. Rhyolitic Kızılkaya ignimbrite is a reddish-to-brownish scarlet unit forming prominent flat surfaces usually on top of the pyroclastic sequence which crops out in the Nevşehir plateau region (Fig. 1). The standardized section of Kızılkaya ignimbrite comprises a 10–15 cm thick co-ignimbrite plinian fall layer at the base overlain by flow deposits with fairly constant thickness varying between 15 and 25 m except for channel fill in deep valleys where the maximum thickness is up to c. 85 m. Kızılkaya ignimbrite is a single cooling unit, where a sharp planar discontinuity separates two sub-units with different degrees of devitrification (Le Pennec et al., 1998). These sub-units, however, have been also interpreted as distinct flow units (Le Pennec et al., 1994; Schumacher and MuesSchumacher, 1996; Aydar et al., 2012). Columnar joints are continuous between the flow sub-units indicating that both belong to a single cooling unit. Welding and induration is low-tomoderate in the basal sub-unit below the planar discontinuity, and gradually increases towards the top. The uppermost zone is strongly indurated, probably due to the devitrification and secondary crystallization of ash matrix. Pumice clasts exhibit inverse grading with the largest clast sizes (up to 80 cm) at the top of the section. Pumice in upper zones is frequently devitrified and altered. 3. Materials and methods Whole rock samples were collected from selected outcrops where characteristic features of the Kızılkaya ignimbrite were completely exposed. In order to characterize vertical variability of the ignimbrite in terms of petrography and mineralogy, systematic sampling was carried out from the bottom to the top of the deposit in three distinct outcrops (sampling localities KK1, KK2 and KK3) (Figs. 1 and 2). In all of these outcrops, the unit was sampled at 6 equidistant different stratigraphical positions which correspond to the three main subunits of the deposit, namely basal (stratigraphical positions 01 and 02, below the planar discontinuity), middle (stratigraphical positions 03 and 04) and upper zones (stratigraphical positions 05 and 06). Hence, the naming convention for samples throughout this study will be as “KKx-y” where x and y represent the sampling locality and sample sets from different stratigraphical positions respectively. Rocks were crushed and sieved to a size fraction of 125–250 μm. Magnetic minerals were separated by using a hand magnet. Mineralogical and petrographical analyses were carried out in the laboratories of Hacettepe University, Department of Geological Engineering. For Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS) analyses, a Carl Zeiss EVO 50 EP SEM integrated with a Bruker Xflash 3001 SDD (Silicon Drift Detector) EDS was used. Fe–Ti oxides (titanomagnetites and ilmenites) and rock fragments were embedded in separate epoxy mounts which were polished for SEM-EDS analyses. A total of 707 polished mineral separates from all of the samples have been examined using SEM and classified according to their oxidation stages. Whole rock chips were mounted on a conductive adhesive surface and coated with carbon prior to SEM investigations. The SEM was operated with a 15–20 kV accelerating voltage at ~10 mm working distance. Beam currents were kept between 10 and 35 pA for secondary electron imaging of petrographic features, and raised to 2–5 nA for both backscattered electron (BSE) imaging and EDS analyses of mineral

Fig. 2. Field picture of Kızılkaya ignimbrite in KK1 sampling locality with relative positions of the sample sets. Internal structure of the ignimbrite including a Plinian fall deposit and basal, middle and upper zones of the flow deposit and respective sample sets are depicted.

phases. During EDS spectra acquisition, the spot diameter was ~1–2 μm and counting times were between 90 and 120 s. Room temperature magnetization measurements were carried out in laboratories of Hacettepe University Department of Physics Engineering using a Quantum Design Physical Property Measurement System (PPMS) with Vibrating Sample Magnetometer (VSM) option with the sample held at a temperature of 300 K and magnetic field strengths between -2 T and +2 T. 4. Petrographical and mineralogical features 4.1. Petrography On a regional scale, Kızılkaya ignimbrite exhibits some spatial variation of petrographical features due to different emplacement conditions and post-depositional processes. Systematic sampling strategies were performed at three different outcrops. Kızılkaya ignimbrite typically overlies a succession of fluvio-lacustrine and older pyroclastic rocks deposited between 7.2 Ma (Cemilköy ignimbrite) and 5.2 Ma (Kızılkaya ignimbrite; Aydar et al., 2012). In all of the sampling locations, Kızılkaya ignimbrite was directly emplaced onto a paleosoil horizon where its contact with pumice-rich pre-ignimbrite plinian fall deposit is clearly visible (Fig. 2). The fall deposit consists of dominantly sub-angular pumice (maximum clast size 2 cm) which is intercalated with several ash-rich layers. At two of the sampling localities, Kızılkaya ignimbrite is overlain by Early-Middle Pliocene Kışladağ limestone (Göz et al., 2014) indicating that lacustrine conditions have prevailed locally after the deposition of the Kızılkaya ignimbrite. Electron microscopy imaging using secondary electron (SE) signals reveals that glass shards are pristine in the basal zones (Sample sets

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01 and 02) (Fig. 3a) where they are mostly cuspate in shape with abundant surficial spherical cavities (Fig. 3b) ranging from tens of μm down to sub-μm scale. Glass is present without incipient crystallization, indicating the absence of devitrification in the basal zones (Fig. 3c). Sample sets 03 and to a lesser degree 04 mark the onset of devitrification in the middle zones (Fig. 3d). The petrographical observations conform to those in Le Pennec et al. (1998) who reported that the planar discontinuity in Kızılkaya ignimbrite represents the different degrees of devitrification in the basal and middle (and upper) sub-units of Kızılkaya. Although the overall shape of the glass shards is retained, incipient devitrification is indicated by axiolitic laths of silica phases and alkali feldspar (Fig. 3e, f). The uppermost zones of Kızılkaya ignimbrite (Sample sets 05 and 06) represent the highest degree of devitrification. The shapes of individual glass shards are rarely distinguishable (Fig. 3g) and glass has been replaced completely by alkali feldspar and euhedral quartz (Fig. 3h). In all of the samples showing devitrification with axiolitic crystallization, the growth directions of crystallites intersect at the centers of the nearest (sub)spherical cavity. This indicates that the crystallization initiated at the energetically unfavorable surface between glass and pore space. In the uppermost zone, radial axiolitic devitrification starting from the centers of spherical cavities has produced spherulites (Fig. 3i). 4.2. Mineralogy The primary modal mineralogy of the Kızılkaya ignimbrite is fairly constant throughout all subunits and comprises plagioclase, biotite, orthopyroxene, amphibole, quartz and Fe–Ti oxides; with apatite and zircon as frequent accessory phases. In addition, devitrification led to the formation of secondary minerals which are detailed below. 4.2.1. Primary silicate and accessory phases The most abundant primary silicate phase is represented by subhedral and seriate plagioclase feldspars (An29–47) which become increasingly altered showing well-developed clayey rims towards the uppermost zones of the flow. Biotite is subhedral with Mg# 0.64–0.68 with Fe-rich segregations developed along cleavage planes and rims,

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towards the top of the flow. Orthopyroxene is enstatite (~ En62Fs38) with scarce euhedral microcrystals. Amphibole is rare and classified as magnesiohornblende. Pyrogenic quartz is anhedral and frequently embayed due to reaction with the surrounding melt. Zircon and apatite are the dominant accessory phases; commonly occur as inclusions and in crystal clots with oxide phases. 4.2.2. Glass and devitrification assemblage Devitrification is defined as the crystallization of (metastable) volcanic glass at temperatures below the glass transition temperature (Tg) (Lofgren, 1971). Although the basal zones of Kızılkaya ignimbrite are devoid of devitrification (Fig. 3a, b, c), occasional alteration of glass to secondary smectite-type clay minerals with typical crenulated crystal morphology is present. In the middle zones, pectinate-textured (McArthur et al., 1998) axiolitic crystallization of glass shards produced (sub)spherical intergrowths of alkali feldspars and cristobalite (Fig. 3d, e). Alkali feldspars exhibit compositional zonation where crystals are Na-rich near the contact between glass and vesicles and increasingly K-rich further away from the glass wall (Fig. 3f). In the upper zones of Kızılkaya ignimbrite, devitrification crystals include euhedral alkali feldspars with coarser grain sizes than those in the middle zones and euhedral silica phases reaching up to 100 μm in size (Fig. 3g, h). Glass shards are completely altered into a texture resembling granophyre composed of alkali feldspars and euhedral quartz which often form spherulites (Fig. 3i). 4.2.3. Fe–Ti oxides Fe–Ti oxide mineralogy in Kızılkaya ignimbrite is represented by titanomagnetites and scarce ilmenites. Titanomagnetite crystals show varying proportions and morphologies of Ti-rich intergrown lamellae dependent of the vertical position of the sample within the Kızılkaya ignimbrite deposit (Fig. 4). In their pioneering study, Buddington and Lindsley (1964) classified ilmenitess intergrowths in titanomagnetite into three general textural forms as a) trellis, b) composite (granular) and c) sandwich types. Trellis types are the most abundant and are true oxidation products of magnetite – ulvöspinelss (Haggerty, 1976). Titanomagnetites of Kızılkaya ignimbrite exhibit vertical variation in trellis-type oxyexsolution forms and their relative abundances.

Fig. 3. The compilation of petrographical features of Kızılkaya ignimbrite with respect to stratigraphical positions of the samples obtained by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Pristine glass with unfilled vesicles indicates the absence of devitrification at basal levels (a, b, c). Incipient devitrification textures where glass shards still retain their original shapes (d) and axiolitic crystallization of glass into alkali feldspars (Af) and silica phases in middle zones (e, f). Glass is completely devitrified into alkali feldspars (g) and euhedral quartz (Q) in pore spaces (h) forming spherulites (i).

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Fig. 4. Average relative abundances of titanomagnetites with C1, C2 and C3 type oxidation stages (after Haggerty, 1976) with respect to the stratigraphical positions of the sample sets.

The variations in thickness and abundance of fine lamellae oriented in {111} networks are due to various stages of diffusive re-equilibration and hence qualitatively correlate with the intensity of oxidation (Buddington and Lindsley, 1964; Haggerty, 1976). Haggerty (1976) defines seven general oxyexsolution stages in cubic titanomagnetites as C1 to C7, where trellis type oxyexsolution forms are associated with the stages C2 and C3. The C1 stage corresponds to optically homogeneous ulvöspinel-rich magnetite solid solution without evident oxyexsolution patterns developed. The C2 stage is characterized by magnetite-enriched ulvöspinelss with a small number of ilmenitess lamellae parallel to {111} parting planes whereas the C3 stage consists of ulvöspinel-poor magnetitess with densely crowded Ti-rich exsolution lamellae. These stages represent progressively higher states of oxidation (Haggerty, 1976). The textural variation of oxyexsolution and intracrystal compositions within the titanomagnetites (including the host mineral, its rim domains, and Ti-rich exsolution lamellae) from different stratigraphical zones within the ignimbrite package were quantified as follows: 4.2.3.1. Basal zone. The basal zone of Kızılkaya ignimbrite (Samples KK101, KK1-02, KK2-01, KK2-02, KK3-01 and KK3-02) contains fairly homogeneous titanomagnetites without intracrystal Ti-rich exsolution lamellae (Fig. 4). The vast majority of the sampled grains in sample sets 01 and 02 are categorized as C1 stage. The dominant homogeneous domains contain 3.9–5.2 wt% TiO2 with XUsp [mole fraction of ulvöspinel: Ti/(Ti + Fe+ 3/2) after Evans et al. (2006)] ranging from 0.12 to 0.16. The rims of these crystals have 4.2–5.6 wt% TiO2 with XUsp between 0.13 and 0.17. Scarce ilmenites range between XIlm 0.65 and 0.72 [mole fraction of ilmenite: Fe+ 2/(Fe+ 2 + Fe+ 3/2) after Evans et al. (2006)] (Fig. 5, Table 1).

4.2.3.3. Upper zone. Titanomagnetites in the uppermost samples of Kızılkaya ignimbrite (Samples KK1-05, KK1-06, KK2-05, KK2-06, KK305 and KK3-06) fall into C2 and C3 stages (Fig. 4). Homogeneous domains of titanomagnetites are observed to be significantly depleted in Ti with XUsp content varies between 0.19 and 0.01. Ulvöspinel mole fractions of the rims on the other hand, reach up to 0.32. Well-defined trellis-type lamellae are Ti-rich with XUsp reaching up to 0.86 (Fig. 5, Table 1). Ilmenite is scarce and XIlm varies between 0.70 and 0.78. 4.3. Magnetization measurements Room temperature magnetization measurements on titanomagnetite separates from Kızılkaya ignimbrite show that their saturation magnetization (Js) depends on the vertical position of the samples (Fig. 6). For any given magnetic field (H), the Js values of basal and middle zones of Kızılkaya ignimbrite vary between ~ + 0.060 and 0.075 Am2/kg and −0.055–−0.075 Am2/kg. By contrast, the saturation magnetization of titanomagnetite from the uppermost zones is considerably lower at b +0.04 and N0.04 Am2/kg. 4.4. Fe–Ti oxide geothermometer Pre-eruptive temperatures of Kızılkaya magma have been calculated using Fe–Ti two-oxide geothermometer on coexisting titanomagnetite and ilmenite pairs (Ghiorso and Evans, 2008). Since exsolved crystals could not behave as closed systems (Bacon and Hirschmann, 1988), only crystal pairs without any traces of exsolution have been selected from the basal zones of Kızılkaya ignimbrite. Fe–Ti oxide geothermometer (Ghiorso and Evans, 2008) yields pre-eruptive temperatures between 735 and 775 °C for Kızılkaya magmas (Table 2). 5. Discussion

4.2.3.2. Middle zone. The titanomagnetites from the middle zones of Kızılkaya ignimbrite (Samples KK1-03, KK1-04, KK2-03, KK2-04, KK3-03 and KK3-04) are classified as C1 and C2 stages. However, the amount of C2-stage crystals increases towards the top (Fig. 4). C2 titanomagnetites exhibit very fine trellis-type lamellae with widths b2 μm observed chiefly propagating from the rims. Optically homogeneous hosts have between 4.1 and 5.4 wt% TiO2 with XUsp between 0.13 and 0.17 (Fig. 5, Table 1). However, some titanomagnetites bear halos, determined by BSE imaging in SEM, with slight enrichment in Al2O3 and depletion in MgO and FeOTotal. Ilmenite is rare and compositionally similar to ilmenite in basal zones with XIlm between 0.67 and 0.72.

5.1. Vertical variation in deuteric oxidation state: implications on emplacement and cooling The chemical composition and pre-eruptive temperature and pressure of magmas profoundly control the eruption and emplacement dynamics and hence, the textural evolution of pyroclastic rocks during cooling. Pyroclastic eruptions which result in the formation of ignimbrites dominantly involve silica-rich felsic magmas. Emplacement temperatures may reach up to 950 °C, depending on the height of the eruption column, the amount of admixed ambient air and the path of the flow (Keating, 2005 and references therein). According to the

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Fig. 5. Stratigraphical variation of chemical composition of titanomagnetites expressed in FeO, Fe2O3 and TiO2 ternary plane. For the basal and middle zones, close-up views of the dashed area in the ternary plot are also shown for clarity.

titanomagnetite – ilmenite exchange (Ghiorso and Evans, 2008), preeruption equilibration temperatures of Kızılkaya ignimbrite were between 735 and 775 °C (Table 2). This is the maximum temperature interval for emplacement, but existing data are insufficient to determine the actual temperature of the flow during its initial contact with the underlying surface. Turner et al. (2008) show that rocks formed by slow-ascent eruptions (near-stagnant magma rising slowly at shallow levels) contain exsolved titanomagnetite, whereas fast-ascent eruptions (rapidly chilled magma in sub-Plinian eruptions) comprise fresh and homogeneous titanomagnetite. Although Kızılkaya ignimbrite is considered to be formed by a fast-ascent eruption as a whole, its basal zones that were in contact with the subsurface (i.e., a paleosoil initially at ambient temperature) can be reasonably assumed to cool down rapidly. This is consistent with the presence of unoxidized homogeneous titanomagnetite crystals in the basal zones of Kızılkaya ignimbrite. The existence of oxidized titanomagnetite grains with increasing volume of trellis-type exsolution lamellae towards the top indicates that the

middle and the upper zones of the deposit cooled down more slowly than the basal zones because the interior of the ignimbrite was isolated from the cold substrate by the basal parts of the flow, and thus retained its temperature for longer time compared to the basal zones. Indeed, the oxyexsolution of titanomagnetite into two (Fe and Ti-rich) phases most likely takes place during relatively slow cooling of interiors of welded tuffs when a vapor phase is present (Bacon and Hirschmann, 1988). McClelland et al. (2004) note for Taupo ignimbrite deposits that high temperature in-situ oxidation led to the breakdown of the original titanomagnetite into lamellar intergrowths of low-titanium magnetite and ilmenite. They report the alteration of the magnetic structure, where the deposits were emplaced at relatively high temperatures (N350 °C), whereas magmatic titanomagnetite compositions were retained where the emplacement occurred under relatively cool temperatures (b 300 °C), and the deposit has cooled relatively rapidly. Deuteric oxidation takes place during the primary cooling of a magmatic body. Vapor released during cooling of an ignimbrite rises through the deposit causing the deposition of vapor-phase crystals

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Table 1 Representative chemical compositions of Fe-Ti oxides in Kızılkaya ignimbrite acquired by EDS. Crystal ID (KKX_0Y_CZ) is the identification number of the analysis where X, Y and Z represent sampling locality, stratigraphical position and crystal analyzed respectively. The same Crystal IDs in Domain analyses refer to the same crystal. XUsp = ulvöspinel mole fraction, XIlm = ilmenite mole fraction. Stratigraphical position Titanomagnetites BASAL 1

2

MIDDLE 3

4

UPPER 5

6

Ilmenites 1 1 2 2 3 3 4 4 5 5

Sampling locality

Crystal ID

Domain

MgO

TiO2

FeOt

MnO

Al2O3

Total

Fe2O3

FeO

XUsp

KK1 KK1 KK2 KK2 KK3 KK3 KK1 KK1 KK2 KK2 KK3 KK3

KK1-01_C3 KK1-01_C3 KK2-01_C12 KK2-01_C12 KK3-01_C21 KK3-01_C21 KK1-02_C9 KK1-02_C9 KK2-02_C14 KK2-02_C14 KK3-02_C5 KK3-02_C5

host rim host rim host rim host rim host rim host rim

1.04 1.17 1.06 1.07 1.15 1.19 1.17 1.04 0.94 1.04 0.83 1.21

4.36 4.74 4.32 4.69 4.98 5.05 4.55 4.90 4.68 4.95 4.93 5.62

83.08 82.45 82.69 82.30 82.11 81.67 82.57 82.54 82.75 82.21 82.95 80.98

0.74 0.72 0.61 0.84 0.69 0.59 0.71 0.65 0.91 0.56 0.61 0.33

1.25 1.35 1.44 1.41 1.39 1.56 1.40 1.42 1.22 1.38 1.11 1.51

90.47 90.43 90.12 90.31 90.32 90.06 90.40 90.55 90.50 90.14 90.43 89.65

56.84 55.83 56.29 55.48 55.20 54.08 56.01 55.37 56.15 54.33 55.88 51.96

31.93 32.21 32.04 32.38 32.44 33.01 32.17 32.72 32.23 33.33 32.67 34.23

0.13 0.14 0.13 0.14 0.15 0.16 0.14 0.15 0.14 0.15 0.15 0.17

KK1 KK1 KK2 KK2 KK3 KK3 KK1 KK1 KK2 KK2 KK3 KK3

KK1-03_C11 KK1-03_C11 KK2-03_C7 KK2-03_C7 KK3-03_C6 KK3-03_C6 KK1-04_C38 KK1-04_C38 KK2-04_C26 KK2-04_C26 KK3-04_C10 KK3-04_C10

host rim host rim host rim host rim host rim host rim

1.13 0.85 1.05 0.83 1.27 1.07 0.67 0.87 0.92 0.63 1.00 0.26

4.49 4.92 4.45 4.84 4.27 5.12 4.16 5.23 4.38 5.64 4.49 5.24

82.60 82.61 82.74 82.75 82.49 82.25 83.29 81.88 82.75 81.93 82.94 83.63

0.61 0.36 0.66 0.26 0.84 0.56 0.25 0.50 0.67 0.54 0.42 0.00

1.49 1.49 1.41 1.57 1.40 1.43 1.41 1.58 1.53 1.59 1.49 1.40

90.32 90.23 90.31 90.25 90.27 90.43 89.78 90.06 90.25 90.33 90.34 90.53

56.03 54.70 56.15 54.79 56.47 54.79 55.90 53.75 55.96 53.25 56.04 54.59

32.18 33.39 32.21 33.44 31.68 32.95 32.99 33.52 32.39 34.01 32.52 34.51

0.14 0.15 0.14 0.15 0.13 0.16 0.13 0.16 0.13 0.17 0.14 0.16

KK1 KK1 KK1 KK2 KK2 KK3 KK3 KK3 KK1 KK1 KK1 KK1 KK1 KK1 KK1 KK2 KK2 KK2 KK3 KK3 KK3 KK3 KK3

KK1-05_C22 KK1-05_C22 KK1-05_C22 KK2-05_C14 KK2-05_C14 KK3-05_C9 KK3-05_C9 KK3-05_C9 KK1-06_C42 KK1-06_C42 KK1-06_C42 KK1-06_C42 KK1-06_C42 KK1-06_C42 KK1-06_C42 KK2-06_C1 KK2-06_C1 KK2-06_C1 KK3-06_C19 KK3-06_C19 KK3-06_C19 KK3-06_C19 KK3-06_C19

host rim lamella host lamella host lamella rim host rim lamella lamella host rim lamella host rim lamella host host lamella lamella lamella

1.12 0.62 0.46 0.95 0.71 1.01 0.59 1.09 0.50 0.74 0.36 0.75 2.08 0.74 0.41 1.84 0.69 0.71 1.84 0.32 0.26 0.52 0.62

3.96 6.69 18.20 3.97 14.42 3.97 10.05 7.97 3.28 9.02 22.29 12.57 1.38 9.02 23.22 0.83 6.80 10.08 0.61 1.16 28.52 25.87 15.39

83.11 81.36 71.30 82.96 70.99 83.09 76.91 79.52 83.88 79.91 69.17 76.79 82.39 79.91 67.89 83.57 81.86 78.80 83.77 87.23 63.41 64.71 74.49

0.73 0.29 0.46 0.92 0.44 0.76 0.46 0.72 0.28 0.00 0.00 0.00 1.45 0.00 0.33 1.41 0.05 0.36 1.63 0.00 0.10 0.04 0.26

1.25 1.39 1.27 1.03 1.18 1.21 1.61 1.03 0.73 0.65 0.39 0.68 2.34 0.65 0.48 1.98 0.75 0.65 1.86 0.95 0.37 0.60 0.49

90.17 90.35 91.69 89.83 87.74 90.04 89.62 90.33 88.67 90.32 92.21 90.79 89.64 90.32 92.33 89.63 90.15 90.60 89.71 89.66 92.66 91.74 91.25

57.36 51.21 28.54 56.91 33.09 57.04 41.39 48.96 55.52 47.45 21.81 40.17 61.32 47.45 19.78 62.71 51.49 45.26 63.68 62.65 8.67 13.38 34.86

31.50 35.28 45.62 31.76 41.22 31.76 39.67 35.47 33.92 37.22 49.55 40.64 27.22 37.22 50.09 27.15 35.53 38.07 26.47 30.85 55.60 52.68 43.12

0.12 0.21 0.56 0.12 0.46 0.12 0.31 0.24 0.10 0.27 0.67 0.38 0.04 0.27 0.70 0.03 0.21 0.31 0.02 0.04 0.86 0.79 0.46

KK1 KK2 KK1 KK3 KK3 KK2 KK1 KK2 KK2 KK3

KK1-01_C10 KK2-01_C13 KK1-02_C15 KK3-02_C11 KK3-03_C20 KK2-03_C18 KK1-04_C2 KK2-04_C4 KK2-05_C12 KK3-05_C5

host host host host host host host host host host

2.84 2.78 1.96 2.05 2.07 1.86 1.13 1.15 1.18 1.85

38.56 36.56 37.73 35.04 38.22 37.84 34.70 36.59 38.97 37.17

50.46 52.58 53.08 54.99 52.40 53.30 56.39 54.87 49.90 52.80

1.04 0.93 0.87 1.18 0.99 0.68 1.06 0.82 1.30 1.92

0.49 0.41 0.25 0.26 0.29 0.23 0.24 0.25 0.26 0.05

94.38 94.13 94.09 93.89 94.17 94.07 93.71 93.89 94.46 94.12

29.03 27.31 29.66 26.90 29.86 30.15 28.35 30.27 31.64 28.19

23.82 28.08 26.03 31.22 25.05 25.73 31.16 27.33 20.30 27.35

XIlm 0.72 0.68 0.72 0.65 0.72 0.72 0.67 0.71 0.78 0.70

and devitrification especially towards the upper zones of the deposit (Wright et al., 2011). Moreover, volatiles released during devitrification could catalyze additional devitrification in the middle and the upper parts of the deposit (Keating, 2005). Indeed, Kızılkaya ignimbrite was affected by significant deuteric alteration, illustrated by the increased amount of devitrification and secondary crystallization of ash matrix towards the top of the flow in all three sampling localities (KK1, KK2 and

KK3). The basal zones of Kızılkaya ignimbrite are devoid of devitrification, secondary crystallization or vapor-phase alteration (Fig. 3), and contain titanomagnetites that lack significant oxyexsolution textures (Fig. 4). Audunsson et al. (1992) interpret a similar zonation in a basaltic flow, where titanomagnetites of the basal layer are least oxidized and oxidation increases with height, possibly due to hydrogen loss by

H.E. Çubukçu / Journal of Volcanology and Geothermal Research 308 (2015) 10–18

17

Fig. 6. Hysteresis loops of titanomagnetite separates from Kızılkaya ignimbrite exhibit significant decrease in saturation magnetization towards the stratigraphically topmost zone.

dissociation of magmatic water, as well as unknown contributions of circulating air and percolating water from above. Similarly, Saito et al. (2004) indicate that the degree of deuteric oxidation is variable across a lava dome, where oxygen-rich, outer zone is represented by exsolved Fe–Ti oxides, while the oxygen-poor inner core contains titanomagnetite and titanohematite. Although petrographical observations on Kızılkaya samples reveal that secondary carbonate crystallization does not exist in two localities, probable impact of lacustrine waters (in which Early-Middle Pliocene Kışladağ limestone precipitated) and the ambient air on oxidation in uppermost zones of the ignimbrite cannot be ruled out. Besides, degassing of the accumulated pyroclasts for prolonged duration might ease and promote the release of volatiles which in turn oxidizes (and devitrifies) upper zones of Kızılkaya ignimbrite. The degassing of volatiles present at deposition in rhyolitic ash flow tuffs occurs within days to a few weeks (Riehle et al., 1995). These timescales are sufficient to produce trellis-type oxyexsolution textures in titanomagnetite since such textures have been observed on synthetic crystals within a few seconds to a few hours (Lattard et al., 2012).

in pyroclastic deposits (Mullick and Majumdar, 2002). However, posteruptive deuteric and hydrothermal mechanisms could operate to enhance or reduce remanent intensity (Piper et al., 2002). Moreover, the occurrence of different ferromagnetic minerals resulting from chemical processes during the cooling of the ignimbrite deposits can inhibit the reliable determination of flow orientation via AMS (Agrò et al., 2015). According to Butler (1998), oxidation induced exsolution of intermediate composition titanomagnetites has two effects on the rock magnetic susceptibility: 1) unmixing of titanomagnetite grains into composite Tirich and Ti-poor grains alters magnetic properties such as Js (saturation magnetization) and Tc (Curie temperature) depending on the chemical composition of the individual phases; 2) exsolution dramatically decreases effective grain size. The uppermost zones of Kızılkaya ignimbrite have titanomagnetites with dominantly C3 type exsolution where the Ti-rich intracrystal lamellae are the thickest and the volume of the homogeneous titanomagnetite hosts are the smallest among the samples. This suggests an effect of the decreased effective grain sizes in the oxidized titanomagnetites on Js, especially for the uppermost zones of Kızılkaya ignimbrite where Fe-rich titanomagnetite hosts are surrounded by Ti-rich exsolution lamellae (Fig. 6). The exact location of the source area of the Kızılkaya ignimbrite is a matter of debate and several areas have been proposed by Pasquare et al. (1988), Le Pennec et al. (1994, 1998), Schumacher and MuesSchumacher (1996), Le Pennec (2000) and Agrò et al. (2015) using various methods. Among these, the studies that rely on magnetic data (Le Pennec et al., 1994, 1998; Le Pennec, 2000; Agrò et al., 2015) proposed the SW of the town of Derinkuyu as the probable source area of the Kızılkaya ignimbrite. Although the AMS technique uses a bulk measurement of the ferromagnetic and paramagnetic componentry of any pyroclastic rock, the observations of this study suggest that the Js values of magnetic titanomagnetites, a basic parameter (or a measured signal) for magnetic susceptibility, can dramatically decrease towards the top of an ignimbrite body due to either deuteric oxidation or vapor-induced crystallization (Fig. 6). The impact of vertical variation in oxidation on magnetic zonation in some localities of

5.2. Variation in saturation magnetization Ferromagnetic minerals such as titanomagnetites exhibit magnetic properties which are useful for the determination of magnetic fabric of volcanic rocks. The anisotropy of magnetic susceptibility (AMS) is the preferred method for the determination of pyroclastic flow directions, and to identify possible source regions. However, the interpretation of AMS results is sometimes difficult even in vertical sections across the same deposit due to the variation of magnetic fabric caused either by syn- or post-depositional processes (c.f. Canon-Tapia and MendozaBorunda, 2014). The AMS of pyroclastic density current deposits can be mainly associated with a combination of ferromagnetic and paramagnetic mineral sources (Canon-Tapia and Mendoza-Borunda, 2014). In the absence of ferromagnetic minerals, the paramagnetic fabric dominates the AMS

Table 2 Fe-Ti two oxide geothermometry calculations based on Fe-Mg exchange (Ghiorso and Evans, 2008) on coexisting titanomagnetite and ilmenite pairs observed in basal zones of Kızılkaya ignimbrite. Stratigraphical Position

Sampling Locality

Sample

Mineral

SiO2

TiO2

Al2O3

FeOtotal

MnO

MgO

1

KK1

KK1-01

1

KK2

KK2-01

2

KK3

KK3-02

2

KK2

KK2-02

2

KK1

KK1-02

titanomagnetite ilmenite titanomagnetite ilmenite titanomagnetite ilmenite titanomagnetite ilmenite titanomagnetite ilmenite

0.23 0.38 0.18 0.27 0.01 0.19 0.16 0.11 0.25 0.23

4.74 38.56 5.15 36.56 4.93 35.04 4.83 35.57 4.60 37.45

1.36 0.49 1.18 0.41 1.11 0.26 1.21 0.21 1.32 0.33

82.84 50.46 82.57 52.58 82.95 54.99 82.92 54.86 82.65 53.30

0.44 1.04 0.43 0.93 0.61 1.18 0.58 0.94 0.74 0.77

1.01 2.84 1.04 2.78 0.83 2.05 0.96 1.84 1.13 1.95

T°C

log10fO2 (Δ NNO)

746

1.36

775

1.39

763

1.44

752

1.47

735

1.49

18

H.E. Çubukçu / Journal of Volcanology and Geothermal Research 308 (2015) 10–18

the Kızılkaya ignimbrite has been documented by Agrò et al., 2015, where they have observed systematic decrease of the magnetic susceptibility values from the base to the top of the section. Since titanomagnetite is the main magnetic mineral (in the Kızılkaya ignimbrite), determination of its oxidation state would play a vital role for the evaluation of the magnetic data. 6. Conclusions This study demonstrates that titanomagnetites in an ignimbrite deposit exhibit vertical variation of oxyexsolution features, which can be considered as the direct result of varying degrees of the oxidation states. The basal zones in Kızılkaya ignimbrite bear unoxidized, chemically (and optically) homogeneous titanomagnetites. However, the middle and the upper zones of the deposit comprise increasingly oxidized titanomagnetites where the oxyexsolved Ti-rich lamellae get thicker and more populated towards the top. Oxidation results in the decrease of magnetically effective grain size of ferromagnetic titanomagnetites, which are surrounded by paramagnetic Ti-rich oxyexsolution lamellae consequently reducing the Js (saturation magnetization). The vertical variation in oxidation states of the Kızılkaya ignimbrite can be explained by a) the relative differences in cooling rates of basal, middle and upper zones and b) increased degrees of degassing which might be enhanced by increased devitrification towards the top of the deposit. Vertical variation in titanomagnetite oxyexsolution textures requires careful selection of the sampling position in an ignimbrite deposit if magnetic properties are to be determined. Acknowledgements I would like to thank Tamas Mikes and Andreas Mulch for the field assistance during a short expedition in Cappadocia in 2012. Burak Kaynar is thanked for the room temperature magnetization measurements. Lütfiye Akın Karakülah and Efe Akkaş were particularly helpful during sample preparation. An earlier draft of the manuscript was significantly benefited from the careful, constructive and inspiring discussions with Axel K. Schmitt. I gratefully acknowledge detailed comments and suggestions from M. Marcaida and an anonymous reviewer. Margaret T. Mangan is thanked for editorial handling. References Agrò, A., Zanella, E., Le Pennec, J.-L., Temel, A., 2015. Magnetic fabric of ignimbrites: a case study from the Central Anatolian Volcanic Province. Geol. Soc. Lond. Spec. Publ. 396 (1), 159–175. Aragon, R., McCallister, R.H., Harrison, H.R., 1984. Cation diffusion in titanomagnetite. Contrib. Mineral. Petrol. 85, 174–185. http://dx.doi.org/10.1007/BF00371707. Audunsson, H., Levi, S., Hodges, F., 1992. Magnetic property zonation in a thick lava flow. J. Geophys. Res. 97, 4349–4360. Aydar, E., Schmitt, A.K., Cubukcu, H.E., Akin, L., Ersoy, O., Sen, E., Duncan, R.A., Atici, G., 2012. Correlation of ignimbrites in the central Anatolian volcanic province using zircon and plagioclase ages and zircon compositions. J. Volcanol. Geotherm. Res. 213, 83–97. Bacon, C.R., Hirschmann, M.M., 1988. Mg/Mn partitioning as a test for equilibrium between coexisting Fe–Ti oxides. Am. Mineral. 73 (1-2), 57–61. Blackburn, T.J., Stockli, D.F., Walker, J.D., 2007. Magnetite (U-Th)/He dating and its application to the geochronology of intermediate to mafic volcanic rocks. Earth Planet. Sci. Lett. 259 (3-4), 360–371. Branney, M.J., Kokelaar, B.P., 2002. Pyroclastic density currents and the sedimentation of ignimbrites. Geol. Soc. Lond. Mem. 27, 152. Buddington, A.F., Lindsley, D.H., 1964. Iron-titanium oxide minerals and synthetic equivalents. J. Petrol. 5 (2), 310–357. Butler, R.F., 1998. Paleomagnetism: Magnetic Domains to Geologic Terranes. Blackwell Scientific Publications, Oxford (319 pp.). Canon-Tapia, E., Mendoza-Borunda, R., 2014. Magnetic petrofabric of igneous rocks: Lessons from pyroclastic density current deposits and obsidians. J. Volcanol. Geotherm. Res. 289, 151–169.

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