Sanidine Crystals Research Articles

Petrogenesis of extension-related alkaline volcanism in Karaburhan (Sivrihisar–Eskisehir), NW Anatolia, Turkey

Alkaline lavas were erupted as phonolites and trachytes around Karaburhan (Sivrihisar–Eskisehir, NW Anatolia) within the Izmir–Ankara–Erzincan suture zone. These volcanic rocks were emplaced as domes, close and parallel to the ophiolite thrust line. According to 40Ar/ 39Ar geochronological analyses of sanidine crystals from the phonolites, the age of the alkaline volcanics is 25 Ma (Late Oligocene–Early Miocene). The flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid crystals. The clinopyroxenes show compositional zoning, with aegirine (Na 0.82–0.96Fe +3 0.68–0.83) rims and aegirine–augite cores (containing calcium, magnesium, and Fe +2). Some aegirine–augites are replaced with sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. The fine-grained trachytes with a trachytic texture consist of feldspar (oligoclase and sanidine) phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites. Although there are some differences in their element patterns, the phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component. In addition, the high 87Sr/ 86Sr (0.706358–0.708052) and low 143Nd/ 144Nd (0.512546–0.512646) isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction-derived fluids. Petrogenetic modeling indicates that the alkaline lavas generated from the subduction-modified lithospheric mantle have undergone assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent through the thickened crust in an extensional setting.

The effects of heterogeneity in magma water concentration on the development of flow banding and spherulites in rhyolitic lava

This study focuses on the origin of flow-banded rhyolites that consist of compositionally similar darker and lighter flow bands of contrasting texture and color. Infrared radiation was used to obtain Fourier transform infrared (FTIR) spectra from which water concentrations were calculated, and to map variations in water concentrations across zones of spherulites and glass from the ~ 23 million year old Sycamore Canyon lava flow of southern Arizona. Lighter-colored, thicker flow bands consist of gray glass, fine-grained quartz, and large (1.0 to 1.5 mm) spherulites. Darker-colored, thinner flow bands consist of orange glass and smaller (0.1 to 0.2 mm) spherulites. The centers of both large and small spherulites are occupied by either (1) a quartz or sanidine crystal, (2) a granophyric intergrowth, or (3) a vesicle. Mapping of water concentration (dominantly OH − in glass and OH − and H 2O in sanidine crystals) illustrates fluctuating water availability during quenching of the host melt. Textures of large spherulites in the lighter (gray) bands in some cases indicate complex quenching histories that suggest that local water concentration controlled the generation of glass versus crystals. Small spherulites in darker (orange) bands have only one generation of radiating crystal growth. Both the glass surrounding spherulites, and the crystals in the spherulites contain more water in the gray flow bands than in the orange flow bands. Flow banding in the Sycamore Canyon lava flow may have originated by the stretching of a magma that contained pre-existing zones (vesicles or proto-vesicles) of contrasting water concentration, as the magma flowed in the conduit and on the surface. Variation in the original water concentration in the alternating layers is interpreted to have resulted in differences in undercooling textures in spherulites in the lighter compared to the darker flow bands.

STRONTIUM IN FELDSPARS OF HIGH-K PROTEROZOIC IGNEOUS ROCKS OF THE ROBITAILLE SUITE, BUCKINGHAM, QUEBEC

Near Buckingham, Quebec, Sr-, K-rich rocks exposed in small, Mesoproterozoic, volcano-plutonic bodies of the Robitaille suite overlie or intrude the Grenville metasedimentary sequence and are cut by Neoproterozoic diabase dykes. They may represent near-surface equivalents of the Mont Laurier – Marmora alkaline plutons. The rocks includes latite and alkali-feldspar trachyte, lamprophyres (kersantite and minette), monzodiorite and alkali-feldspar syenite. Ultrapotassic compositions are common. The complexes, up to 2.7 km long and 1.0 km wide, are partly fringed with aureoles of Sr-rich, dolomitized calcitic marble. In 20 samples from three igneous complexes, Sr (up to 3960 ppm) is present in celestine (very rare), barite (rare), fluorapatite and calcite, but feldspars are the main hosts. K-feldspar without plagioclase occurs in samples with K 2 O/Na 2 O > 15 (weight ratio), and K-feldspar + plagioclase in samples with a lower ratio. Individual grains of plagioclase in the groundmass of lamprophyre are unzoned, but distinct ranges of An content are found in grains from restricted areas within a single polished thin section. The amount of Sr increases linearly with An content, from near zero at end-member albite to 1.0–2.6 wt% SrO at An 40 . In one sample of latite, Sr peaked at An 30 . Variation of Sr within a single sample is due to both differentiation (mainly by crystallization of sanidine) and equilibrium factors (such as temperature and An content). Clustering of point compositions on An – wt% diagrams for individual samples may reflect periods of volcanicity, separated by quiescent intervals, as differentiation evolved continuously in the parent magma-chamber. Barium is low, but can be correlated with Sr in one kersantite sample. The distribution of Sr between contiguous K-feldspar and plagioclase grains is governed by disequilibrium factors at low An content. For feldspar pairs, Sr preferentially enters more calcic plagioclase (An > ca. 10), but K-feldspar where plagioclase is more sodic (An ca. 10).

Magma chamber recharge at Vesuvius in the century prior to the eruption of A.D. 79

Evaluating volcanic hazards requires knowledge of the processes that trigger eruptions and the nature and timing of geophysical signals related to these processes. One approach to addressing this need is to link geophysically observable signals (e.g., those related to seismic, aeromagnetic, inflationary, or degassing processes) to pre-eruptive magmatic events deduced (in hindsight) from studies of erupted magmas. Here we present data on sanidine crystals from the A.D. 79 eruption of Vesuvius that show abrupt changes in Ba concentration caused by magma chamber recharge events prior to eruption. These changes have been degraded by diffusion during the time interval between recharge and eruption, and we have determined the length of this time interval by modeling the measured Ba concentration gradients. The results identify three distinct recharge events in the century before the eruption, the most recent occurring ∼20 yr beforehand.

Ar40-Ar39 dating of biotite and sanidine in tuffaceous sediments and related intrusive rocks: Implications for the Early Miocene evolution of the Gördes and Selendi basins, W Turkey

The precise ages of the sedimentary successions within two prominent NE-SW-trending basins, the Gördes Basin and the Selendi Basin, are critical to an understanding of the Neogene evolutionary setting of western Turkey and associated calc-alkaline magmatism. Early radiometric dating was not always sufficiently precise to resolve alternative interpretations. During this study, high-precision Ar40-Ar39 radiometric ages were determined on single crystals of biotite and sanidine from silicic tuffs and associated intrusive rocks. Finegrained tuffaceous sediments near the top of the sedimentary succession in the Selendi Basin gave ages of 18.89 ± 0.58 Ma to 16.42 ± 0.09 Ma. Coarser-grained tuffaceous sediments near the top of the equivalent sedimentary succession in the Gördes Basin to the NW yielded ages of 18.78 ± 0.3 Ma to 17.04 ± 0.35 Ma. Associated intrusive rocks were dated at 20.86 ± 0.08 Ma to 17.62 ± 0.07 Ma. An andesitic body on the northern margin of the Gediz (Alaşehir) Graben further south gave ages of 16.08 ± 10.91 to 14.65 ± 0.06 Ma. Combined with published radiometric age data, these new results confirm an Early Miocene age for the clastic sedimentary fills of the Gördes and Selendi basins. The results from the Gediz Graben are consistent with its formation in Early Miocene time, earlier than the Late Miocene or Plio-Quaternary ages suggested in some interpretations.

Alder Creek sanidine (ACs-2): A Quaternary 40Ar/ 39Ar dating standard tied to the Cobb Mountain geomagnetic event

Accurate 40Ar/ 39Ar dating of Quaternary and Holocene samples requires a standard (neutron fluence monitor) of appropriate age. This standard should be isotopically homogenous at the scale of analysis, and should be well intercalibrated with other standards. Several such standards have been proposed, including sanidine from the ∼1.19 Ma Alder Creek Rhyolite (ACs; Turrin et al., 1994 [Turrin, B.D., Donnely-Nolan, J.M., Hearn, B.C. Jr., 1994. 40Ar/ 39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited. Geology 22, 251–254]). This sanidine is satisfactorily homogeneous, well intercalibrated with Fish Canyon sanidine (FCs; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/ 39Ar dating. Chem. Geol. 145, 117–152]), and is linked to the astronomically calibrated magnetic polarity time scale, since the Alder Creek Rhyolite lava is the type occurrence of the Cobb Mountain event (CM). We collected 408 kg of this rock, from which we anticipate being able to extract more than 8 kg of >425 μm sanidine crystals. Analyses of sanidine phenocryts from the ACs-2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1.190±0.004 to 1.194±0.003 Ma 1 1 All uncertainties in this paper are given at the 95% confidence level. with a weighted mean age of 1.193±0.001 Ma (MSWD=0.74, n=225; based on FCs=28.02 Ma; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117–152]). The value of the intercalibration factor ( Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/ 39Ar dating. Chem. Geol. 145, 117–152]) is R FCs ACs-2=0.04229±0.00012 ( n=8), identical with that of the original ACs measurements (0.04229±0.00012; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/ 39Ar dating. Chem. Geol. 145, 117–152]). The age of ACs-2 is in agreement with the recent astronomical calibration of the CM event (1.215 to 1.190 Ma; [ Channell, J.E.T., Mazaud, A., Sullivan, P., Turner, S., Raymo, M.E., 2002. Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin). J. Geophys. Res. 107 (6), 10.1029/2001JB000491.]). The apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements. Aliquots from various grain sizes will be available upon request. Zircon is also potentially available, which may be of interest to (U–Th)/He researchers.

Impact melt rocks from the Ries structure, Germany: an origin as impact melt flows?

The production of impact melt rocks and glasses is a characteristic feature of hypervelocity impact events on Earth and other planetary bodies. This investigation represents the first detailed study of an unusual series of coherent impact melt rocks intermittently exposed around the periphery of the ~24-km diameter, ~14.5 Ma Ries impact structure, Germany. Optical and analytical scanning electron microscopy (SEM) reveals that the groundmass comprises sanidine, plagioclase, quartz and ilmenite (decreasing order of abundance) with the interstices filled by either fresh or devitrified glassy mesostasis. Primary crystallites display skeletal, dendritic and/or spherulitic textures indicating rapid crystallization from a melt. The mesostasis is characterized by extreme chemical heterogeneity (e.g., FeO and Al 2O 3 contents from ~1 to ~62–80 wt.%). This is likely due to a combination of crystal–liquid fractionation during rapid cooling and crystallization of an originally incompletely homogenized melt. Vapor phase crystallization of sanidine and cristobalite occurred in miarolitic cavities during late-stage cooling of the impact melts. The most likely protolith for the impact melt rocks are granitic rocks present in the crystalline basement target. The high volatile content of the mesostasis suggests that a large volatile component was retained from this protolith. Field observations together with analytical data and micro-textures indicate that the Ries impact melt rocks were molten at the time of, and after, deposition. Field relations with other impactites also suggest that these rocks were emplaced subsequent to the excavation stage of crater formation and that they are not, therefore, ballistic ejecta. Thus, it is proposed that the Ries impact melt rocks were emplaced as ground-hugging impact melt flows that emanated from different regions of the evolving transient cavity during the modification stage of crater formation. This is consistent with, and in fact predicted, by field relations, which indicate that these rocks overlie unequivocal ejecta. This represents the first recognition of impact melt flows near the rim of a terrestrial impact structure and is consistent with observations of such features on the Moon and Venus.

Structure and Dynamics of the Laacher See Magma Chamber (Eifel, Germany) from Major and Trace Element Zoning in Sanidine: a Cathodoluminescence and Electron Microprobe Study

Microtextures and zoning patterns in sanidine phenocrysts from the phonolitic Laacher See Tephra (Germany) were investigated in order to constrain processes occurring in the magma chamber before eruption. The Laacher See Tephra unit is chemically zoned and has been inferred to represent the product of eruption of an inverted, layered magma chamber. Samples from various levels in the deposit were investigated. We used a combination of textural studies (optical microscopy and back-scattered electron (BSE) imaging), electron microprobe (EMP) analysis of major, minor and trace elements (Ca, Na, K, Al, Si, Ba, Sr, Fe and Ti) and element mapping. The samples studied contain two feldspar phases and the ternary composition of the sanidine thus constrains the temperature of crystallization, whereas its trace element content reflects the melt composition. The large diversity in textures found in the sanidine crystals can be classified into three types: composite (C-type), pseudooscillatory (PO-type) and resorbed/patchy (R-type). Trace elementpoor, lamellar composite alkali feldspars (C-type) are found in samples inferred to represent the top and the middle part of the magma chamber. They grew as composite crystals from the melt at temperatures as low as 650 � C in a highly differentiated and volatilerich boundary layer at the magma chamber roof or wall. Pseudooscillatory zoning with resorption surfaces (PO-type) is found in sanidinesfromsamplesinferredtorepresentthemiddlepartandthebase of the magma chamber. Repeated, large An variations (1–4 mol %) reflect temperature variations of 100–300 � Ca ssociated with changes in water content. Variations of minor and trace elements (Ba, Sr and Ti) in sanidine, decoupled from the pseudo-oscillations of the major elements, reflect chemical changes in the melt, and are damped by chemicaldiffusioninthemelt.Bothmajorandminorelementvariations inPO-typecrystalsareinterpretedastheprogressiveinfluenceofamore mafic, hotter melt. This may be partly explained by the settling of crystals through the thermal and chemical gradient existing in the magma chamber; however, the additional role of magma recharge may be required to explain the large temperature variations. Resorbed/patchy-zoned crystals (R-type), found mainly in samples corresponding to the middle part of the magma chamber, reflect early growth in a differentiated boundary layer, followed by resorption and overgrowthinthemainmagmabody.ManyoftheLaacherSeesanidine crystals did not crystallize for the most part in the melt in which they were erupted. In the presence of preserved major and trace element zonation in the magma, this observation indicates crystal dispersion within a layered magma chamber without large-scale mixing and overturn.

Chronostratigraphy of the key Upper Miocene (Lower Turolian) sequence of la Montagne d’Andance (Ardèche, France). Implications of new 40Ar/39Ar laser fusion and unspiked K-Ar dating of trachytic tephra and basalts

Abstract A trachytic tephra layer deposited in the lower part of the Upper Miocene (Turolian) diatomite sequence at la Montagne d’Andance (Coiron basaltic province, France) contains glass shards (73 % SiO2), plus crystals of sanidine, ilmenite, biotite, amphibole, clinopyroxene, zircon, sphene and apatite. This composition and other volcanological data suggest a correlation to the Cantal stratovolcano, 150 km distant, better than to the nearer (50 km) Mézenc area. The 7.54 ± 0.06 Ma (± 2 σ) isochron defined by eight 40Ar/39Ar laser fusion analyses of sanidine gives a precise age for the beginning of the fossiliferous sequence, known for its rich flora and fauna. Moreover, unspiked K-Ar ages determined from three basaltic lava flows that pre- and post-date the lacustrine sedimentation are 7.37 ± 0.06, 7.17 ± 0.06 and 7.13 ± 0.06 Ma (± 2σ), thereby bracketing the sequence to a 400 kyr period between ca. 7.5 and 7.1 Ma. These are the first radioisotopic constraints for the lower part of the Turolian continental stage. They imply an age about 20 % younger for the fauna than proposed by previous biostratigraphic correlation. This new chronology is consistent with correlation of the lower limit of the Turolian to the Tortonian-Messinian boundary (7.30 ± 0.15 Ma) of the global marine stages.

Cluster linkages between luminescence emission spectra and continuous trap distribution in a volcanic sanidine

Volcanic sanidine shows two important characteristics involved in the luminescence emission: (i) a near-infrared emission centred around 700 nm , attributed to Fe 3+ substituted for Al 3+, and (ii) a Si–Al highly disordered crystallographic lattice of natural, recently pre-annealed feldspars. Thermoluminescence emission spectra have been obtained from well-characterised transparent crystals of sanidine using a high sensitivity spectrometer. Tests of thermal stability at different temperatures show a continuous trap distribution with progressive changes in the glow curve shape, intensity and temperature position of the peak maximum. The characteristic broad blue emission band displays cluster linkages between blue and red emissions which can be explained by thermal alkali self-diffusion through both bulk and interfaces producing redox changes of the iron ion valence. These complex phenomena could also explain the large fading and tunnelling recombination of sanidine.

Paleomagnetism of the early Triassic Semeitau igneous series, eastern Kazakstan

Paleomagnetic results obtained from the early Triassic Semeitau igneous series (SIS) yield a high‐quality pole for eastern Kazakstan that is concordant with reference poles for Siberia and Europe, strongly supporting the hypothesis that Kazakstan was already joined with Eurasia at this time. Of the 25 sites that were measured, data were straightforward for 20 sites, 2 of which recorded normal polarity and 18 of which recorded reverse; 5 sites were discarded because of severe overprinting, unstable demagnetizations, or extremely scattered within‐site directions. Combining sites judged to be the same cooling unit and taking the average of 15 cooling units of equal weight, we calculated a pole located at 56°N, 139°E (N = 15, K = 24.6, α95 = 7.9) for the SIS. We consider this pole to be reliable. The sampling of temporally spaced extrusives and more slowly cooled intrusives and the presence of both polarities strongly suggest that this pole averages secular variation. The 40Ar/39Ar determinations on sanidine crystals from two extrusive units yielded highly consistent ages of 248.2 ± 0.5 and 248.8 ± 0.5 Ma. Comparison of our pole for the SIS with the Siberian reference pole shows relative translation (−8° ± 10°) and rotation (1°±16°) that are indistinguishable from zero at 95% confidence. This result fails to confirm hypothesized post‐Permian rotation of the Siberian craton relative to its margins and provides no evidence for left‐lateral motion on the east‐west shear zones in which the Semeitau is located. The similarity of ages between the SIS and the Siberian Traps supports speculation of a genetic relationship between the two.

The Santa Rosa Event: 40Ar/ 39Ar and paleomagnetic results from the Valles rhyolite near Jaramillo Creek, Jemez Mountains, New Mexico

The Jaramillo Event was originally defined by Doell and Dalrymple in 1966 on the basis of K–Ar ages from sanidine in the normally, transitionally and reversely magnetized rhyolite domes named Cerro del Abrigo, Cerro Santa Rosa I and Cerro Santa Rosa II, respectively, that erupted following collapse of the archetypal Valles Caldera, New Mexico. We have collected new paleomagnetic data from the three domes and new 40Ar/ 39Ar laser fusion and furnace incremental heating experiments on sanidine crystals from the Cerro Santa Rosa I rhyolite. Step-wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of Doell and Dalrymple [Science 152 (1966) 1060–1061] are valid. Cerro del Abrigo is normally magnetized, whereas the Santa Rosa I dome is transitional with an inclination of −63° toward the east and the Santa Rosa II dome is of reversed polarity. Twenty-five laser fusion experiments on sanidine crystals from the the Cerro Santa Rosa I dome, together with the saddle-shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption. Our preferred age of 936±8 ka (±2σ) for the Santa Rosa I rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages. This age is significantly older than was inferred on the basis of earlier laser fusion 40Ar/ 39Ar results that suggested a trapped component characterized by a 40Ar/ 36Ar ratio lower than the atmospheric value of 295.5. Our new age distinguishes the Santa Rosa I dome by 65 kyr from the termination of the Jaramillo normal subchron 1001±10 ka and by 37 kyr from basaltic lavas at Haleakala and Tahiti, which record the Kamikatsura Event 899±6 ka. Moreover, this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period. The Cerro Santa Rosa I rhyolite dome, once intimately linked with the end of the Jaramillo Event and the acceptance of plate tectonic theory, now defines a highly resolved feature, the Santa Rosa Event, in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments.

Growth of megaspherulites in a rhyolitic vitrophyre

Megaspherulites occur in a rhyolitic vitrophyre, near the base of a thick rhyolitic vitrophyre that occupies a late Eocene to early Oligocene volcanic-tectonic basin near Silver Cliff, Colorado. Diameters of the megaspherulites range from 0.3 to over 3.66 m, including an alteration layer. The megaspherulites are compound spherulites. They are holocrystalline, very fine- to fine-grained, composed of sanidine and quartz, and surrounded by a thin sanidine-quartz rind, and an alteration layer containing mordenite and 15 Å montmorillonite. Megaspherulite crystallization began soon after the vitrophyre was emplaced as the result of sparse heterogeneous nucleation, under highly nonequilibrium conditions. Spherulite growth proceeded in a diffusion-controlled manner under lowered viscosity as a result of a high water content (5 to 7 wt%), which also contributed to sparse nucleation. When nucleation did occur, it was at a large ΔT (245-350 °C) on near critical-sized nuclei. Once the spherulites began to grow, continued nucleation was on existing growth cones and new, independent spherulites did not form. Sanidine crystals grew with a fibrous habit, which is favored by a large ΔT, restricting fibril lengths and diameters between 10-30 and 3-7 μm, respectively. During crystallization, these growth cones impinged upon each other, resulting in fibril cone-free areas. These cone-free (interconal) areas consist of coarser, fine-grained phases, dominated by quartz, which crystallized from the melt as it accumulated between the crystallizing sanidine fibrils of the cones. The anhydrous nature of the sanidine and quartz suggests that their crystallization concentrated a water-rich residual fluid, enriched in Ca, Mg, and Fe, and depleted in Si, K, and Na, at the megaspherulite-vitrophyre interface. This fluid phase enclosed the megaspherulites resulting in the hydrothermal alteration of the vitrophyre, forming a hydrothermal alteration layer, as indicated by the presence of the minerals mordenite and montmorillonite.

40Ar/39Ar dating of latest Pleistocene (41 ka) marine tephra in the Mediterranean Sea: implications for global climate records

Abstract Fifty-one 40 Ar/ 39 Ar laser heating analyses of sanidine crystals in the C-13 tephra layer, recovered out of a piston core raised from 3000 m depth in the Tyrrhenian Sea, yielded apparent ages between 25 and 112 ka. Twenty-four of these measurements define an age probability maximum and generate an isochron of 41.1±2.1 ka (2σ) with a 40 Ar/ 36 Ar intercept of 288±21 that is indistinguishable from the modern atmosphere. However, the entire population is skewed toward older apparent ages with smaller peaks at 48 ka, 55 ka, 67 ka, and 110 ka. The composition of glass shards in the C-13 tephra overlaps that of the SiO 2 -rich phase of the 150 km 3 Campanian Ignimbrite which erupted from the Campi Flegrei caldera 130 km northwest of the core site. Although the 41.1±2.1 ka isochron is ∼15% older than K–Ar and 14 C ages for the Campanian Ignimbrite, it is indistinguishable from recent 40 Ar/ 39 Ar age determinations. The Campanian Ignimbrite eruptions were also chemically similar to, and correlated with, the Y-5 ash layer – the most voluminous and widely dispersed late Pleistocene tephra in the Mediterranean region. The age of the Y-5 layer was constrained originally at 38.0±2.5 ka on the basis of astronomical dating of O-isotope stages. The 40 Ar/ 39 Ar ages, astronomical dating, plus chemical similarities permit correlation of the C-13 tephra layer with both the Campanian Ignimbrite and the Y-5 tephra layer. The older ages preserved in the C-13 tephra layer may reflect xenocrysts incorporated during the explosive eruption accompanying collapse of the Campi Flegrei caldera 41 ka. The C-13 tephra layer is correlated with a 10 Be peak in an adjacent marine core, O-isotope stage 3.12, and immediately preceded a cooling episode corresponding to Heinrich event 4, thus the age of these features is constrained by radioisotope dating. The 41 ka peak in 10 Be deposition is a global phenomenon whose age matches that of the Laschamp geomagnetic excursion. Thus, our results provide a new tie point among several polar ice core and marine proxy records of paleoclimate during the past 50 kyr. Precise age information, independent of radiocarbon dating, can be recovered from latest Pleistocene marine tephra using the 40 Ar/ 39 Ar laser heating method, despite the presence of multiple components that may have been incorporated during or after its deposition.

New 40Ar/39Ar age of the Bishop Tuff from multiple sites and sediment rate calibration for the Matuyama‐Brunhes boundary

Precise dating of sanidine from proximal ash flow Bishop Tuff and air fall Bishop pumice and ash, California, can be used to derive an absolute age of the Matuyama Reversed‐Brunhes Normal (M‐B) paleomagnetic transition, identified stratigraphically close beneath the Bishop Tuff and ash at many sites in the western United States. An average age of 758.9±1.8 ka, standard error of the mean (SEM), was obtained for individual sanidine crystals or groups of several crystals, determined from ∼70 individual analyses of sanidine separates from 11 sample groups obtained at five localities. The basal air fall pumice (757.7±1.8 ka) and overlying ash flow tuff (762.2±4.7 ka) from near the source yield essentially the same dates within errors of analysis, suggesting that the two units were emplaced close in time. A date on distal Bishop air fall ash bed at Priant, California, ∼100 km to the west of the source area, is younger, 750.1±4.3 ka, but not significantly different within analytical error (±1 standard deviation). Previous dates of the Bishop Tuff, obtained by others using conventional K‐Ar and the fission track method on zircons, ranged from ∼650 ka to ∼1.0 Ma. The most recent, generally accepted date by the K‐Ar method on sanidine was 738±3 ka. We infer, as others before, that many K‐Ar dates on sanidine feldspar are too young owing to incomplete degassing of radiogenic Ar during fusion in the K‐Ar technique and that many older K‐Ar dates are too old owing to detrital or xenocrystic contamination in the larger samples that are necessary for the technique. The new dates are similar to recent 40Ar/39Ar ages of the Bishop Tuff determined on individual samples by others but are derived from a larger proximal sample population and from multiple analysis of each sample. The results provide a definitive and precise age calibration of this widespread chronostratigraphic marker in the western United States and northeastern Pacific Ocean. We calculated the age of the M‐B transition at five sites, assuming constant sedimentation rates, the age of the Bishop ash bed and one or more well‐dated chronostratigraphic horizons above and below the Bishop Tuff ash bed and M‐B transition, and stratigraphic separations between these datum levels. The age of the M‐B transition is 774.2±2.8 ka, based on the average of eight such calculations, close to other recent determinations, and similar to that determined from the astronomically tuned polarity timescale. Our approach provides an alternative and surprisingly precise method for determining the age of the M‐B and other chronostratigraphic levels. The above dates, calculated using U.S. Geological Survey values of 27.92 Ma for the Taylor Creek (TC) sanidine can be recalculated to other widely used values for these monitors. For example, using recently published values of 28.34 Ma (TC) and 523.1 Ma (McLure Mountain hornblende, MMhb‐1), the resulting ages are ∼774 ka for the Bishop Tuff and ash bed and ∼789 ka for the M‐B transition.

Under the volcano: A new dimension in Ar‐Ar dating of volcanic ash

A significant part of the history of the Greek island of Kos has been decoded from a handful of volcanic ash by argon‐argon laser probe dating of single crystals of the minerals sanidine, plagioclase and quartz. On a 3‐D argon isotope plot, the data define six straight lines, corresponding to four distinct volcanic events from 161 to 1728 ka. All four events produced plagioclase crystals, whose ages and Ca/K ratios are consistent with the known evolution of the rest of the Aegean arc. Sanidine appeared only in the final eruption at 161 ka. The joint occurrence of quartz and plagioclase at a 925 ka event implies that quartz did not reside for millennia in the magma, as has been suggested for quartz from the Bishop Tuff, California. The step‐heated cooling age of a schist fragment from the ash provides the first radiometric evidence for ∼250 Ma crust under the volcano.

Features of sound propagation in the vicinity of “symmetry-dictated” isostructural phase transitions in ferroelastics

Using a proper ferroelastic phase transition of the tension-compression type as an example, it is shown that, if the order parameter characterizing a structural phase transition allows the existence of a third-power invariant in the Landau potential, then there must be “symmetry-dictated” isostructural phase transition lines in the vicinity of the line of that structural phase transition. These isostructural transitions may manifest themselves both directly and as supercritical anomalies in the behavior of elastic moduli and lattice parameters. These effects are discovered and investigated without invoking the perturbation theory in terms of which the second-order phase transitions are commonly described. A hypothesis is made on the basis of the results obtained that the sound velocity anomalies observed in orthoclase and sanidine crystals are due to the super-critical behavior of the lattice parameters in the vicinity of a symmetry-dictated isostructural phase transition in the prototype phase of these crystals.

Chemical zoning and crystallization mechanisms in the magma chamber of the Pomici di Base plinian eruption of Somma-Vesuvius (Italy)

Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO2 = 62.5 wt%) to latite (SiO2 ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO2 ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1).

Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events

The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a 40 Ar/ 39 Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. Analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define 40 Ar/ 39 Ar ages which agree with stratigraphic constraints. Plagioclase analyses are distinctly more scattered and do not typically define reasonable ages. The 40 Ar/ 39 Ar ages for the two members of the Bandelier Tuff yield an interval of 380 ± 20 k.y. between these caldera-forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studied, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. 40 Ar/ 39 Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at >1.59, 1.54, 1.48, 1.37 and 1.22 Ma. 40 Ar/ 39 Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38–1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48, and 1.38–1.34 Ma. The interval from 1.34 to 1.38 Ma was particularly active; 7 of 18 units dated are these ages.

Precise 40 Ar/ 39 Ar age for the initiation of Palaeogene volcanism in the Inner Hebrides and its regional significance

Sanidine crystals from two tuff layers at the base of the Palaeocene Eigg Lava Formation on the Isle of Muck, Inner Hebrides, yield step-wise Ar release plateau ages of 62.8 ± 0.6 (2 σ ) and 62.4 ±0.6 Ma using the laser 40 Ar/ 39 Ar dating technique. These determinations, which are in accord with, but significantly more precise than earlier results, provide a definitive age for the inception of igneous activity in the Small Isles. From stratigraphical inferences, the Muck ages provide precise, maximum constraints on the possible age of Skye magmatism and possibly most of the British Tertiary Volcanic Province. The new ages significantly predate the oldest ages determined so far for Tertiary magmatism in NE Greenland but are similar to ages obtained from W Greenland and offshore SE Greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots. The Muck sanidine age appears to precisely date the first manifestation of the impingement of the Iceland mantle plume on the lithosphere beneath NW Scotland.