Rhyolitic Glass Research Articles

Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico

Los Humeros is the largest caldera volcano in the Mexican volcanic belt. Its second largest caldera-forming eruption, the ca. 0.1 Ma Zaragoza eruption, is recorded by two Plinian pumice-fall layers and a zoned intra-Plinian ignimbrite. Diverse pumice types within the ignimbrite provide insights about the way that different magmas within a single magmatic system interact, and the way in which this can give rise to a major explosive ignimbrite-forming eruption. Normal-and-reverse compositional zoning in the ignimbrite is defined by vertical variations in the relative abundance of rhyodacitic (69–71 wt%% SiO 2 ) and andesitic (54–63 wt% SiO 2 ) pumice lapilli: Lower parts are dominated by rhyodacite and pass gradationally up into a central part with andesitic and rhyodacite pumice, and this passes up into a rhyodacitic uppermost part, with no andesite. Petrographic and microprobe analyses of coexisting glass and phenocrysts provide mixed evidence of equilibrium and disequilibrium conditions in the magmas at the time of eruption. The Fe-Ti oxides record magma temperatures of ∼850 °C (andesite) and 780 °C (rhyodacite). The andesitic pumice contains euhedral labradorite (∼An 60 ), and orthopyroxene and clinopyroxene, in a dacitic glass groundmass, which yield equilibrium Na-Ca K d pl/liq and Fe-Mg K d pl/liq ratios. It also contains highly calcic plagioclase (to An 82 ) that in some cases is highly resorbed and mantled by the more sodic plagioclase, which may record early mixing between andesitic and plagioclase-bearing basaltic magmas, followed by equilibrium crystallization within the hybrid magma. The rhyodacite contains euhedral crystals of more-evolved plagioclase (∼An 30-40 ) and euhedral pyroxenes in a rhyolitic glass groundmass (74–75 wt% SiO 2 ). The pyroxenes yield disequilibrium Fe-Mg K d pl/liq ratios and indicate formation from a liquid that was more mafic than the liquid that formed the glass groundmass of the dacitic pumice. Subordinate pumices with interbanded rhyodacite and a scarcity of intermediate-composition pumices indicate that the magmas remained separate for most of the time, and mingled only immediately prior to, and during eruptive quenching. Rather than a simple density-stratified magma chamber, the Zaragoza eruption may have occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma reservoir, possibly arranged as semiconnected high-melt lenses or zones within a partially consolidated crystal mush. However, contrary to assumptions of simple replenishment, tapping, and fractionation-type systems, the Zaragoza magmas contain no record of previously erupted highly evolved rhyolites that developed when zircon joined the fractionating assemblage. This absence indicates that the highly evolved rhyolites had either been completely tapped or solidified prior to the Zaragoza eruption eruption, or that interaction was prevented by contrasting magma densities and viscosities.

Using dissolved H2O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur (Torfajökull, Iceland)

The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H2O and CO2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to Blahnukur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ∼0.7 wt.% at the base to ∼0.3 wt.% at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption. As Blahnukur rises 350 m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1 wt.%) or water-poor (∼0.2 wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from Blahnukur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on Blahnukur’s eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed.

Petrogenesis of the Sólheimar ignimbrite (Katla, Iceland): Implications for tephrostratigraphy

The Sólheimar ignimbrite was one of the largest eruptions from the Katla caldera (Iceland) and is important for tephra studies in the North Atlantic because of its possible linkage with the Vedde Ash, a compositionally bimodal tephra layer used for correlation of sedimentary records in the North Atlantic and Northern Europe. The composition of the Sólheimar ignimbrite extends from rhyolite to basaltic-icelandite, a trend that defines a coherent magma mixing line. Mixing is evident both in mingled textures seen in hand specimens and thin sections and as binary mixing trends in major and trace element and 87Sr/86Sr isotopes of the volcanic glasses. The Sólheimar rhyolite is slightly more radiogenic than the basaltic-icelandite in terms of Sr isotopes, which is inconsistent with generation of the rhyolite by fractionation of the basaltic-icelandite. Alternatively, the Sólheimar rhyolite may have been produced by partial melting of Icelandic crust. Major and trace element modelling indicates that partial melting of Icelandic tholeiite does not replicate the observed rhyolite composition, in particular K2O is significantly lower in the modelled melt. However, partial melting of Katla alkali basalt does produce a comparable melt. We suggest a two-stage model in which 30–40% melting of basalt generated a dacitic magma which underwent subsequent ∼30% fractionation of the observed phenocryst phases (feldspar, clinopyroxene, spinel and FeTi oxide) form rhyolite. The eruption of the Sólheimar ignimbrite was triggered by the intrusion of basaltic-icelandite magma, which mixed with resident rhyolite magma during eruption.The Sólheimar ignimbrite has been linked to the Vedde Ash (Lacasse et al., 1995), a compositionally bimodal tephra layer used to link sedimentary records in the North Atlantic and Northern Europe. Despite the importance of the Vedde Ash in late Quaternary studies, its provenance remains equivocal. We demonstrate that Vedde rhyolite glasses share the same major and trace element chemistry as the Sólheimar rhyolite, carrying the implication that these deposits may be produced by the same eruption. However, the Sólheimar ignimbrite lacks the basaltic component that is sometimes associated with rhyolitic shards of the Vedde Ash at far distal locations, therefore this correlation cannot be confirmed.

Effect of alkalis on the Fe oxidation state and local environment in peralkaline rhyolitic glasses

Iron oxidation state and coordination geometry have been determined by Fe K-edge X-ray absorption near edge spectroscopy (XANES) for three sets of silicate glasses of peralkaline rhyolitic composition with different peralkalinity values. These compositions were chosen to investigate the effect of alkali content (and oxygen fugacity) on the Fe oxidation state. The samples were produced by means of hydrothermal vessels at 800 °C with oxygen fugacity conditions ranging from NNO-1.61 to NNO+2.96 log units.

Aluminum coordination in rhyolite and andesite glasses and melts: Effect of temperature, pressure, composition and water content

The coordination state of the network formers Si and Al is one of the primary parameters describing the structure of silicate glasses and melts. In this experimental study, we use 27Al MAS NMR spectroscopy to investigate the effect of temperature, pressure and composition on the coordination of Al in dry and hydrous rhyolite and andesite glasses, quenched from pressures of up to 3.5GPa. The spectra were collected at a high magnetic field of 20 Tesla to increase spectral resolution and allow for a direct quantification of the different Al coordinations. The disorder around [4]Al increases with increasing fictive temperature and pressure and with decreasing water content or peralkalinity. Al is predominantly (>98%) in fourfold coordination in all investigated samples. Nevertheless, systematic trends in Al coordination have been observed as a function of pressure, composition and water content. For the investigated pressure range, the proportion of [5]Al increases with increasing pressure, but decreases with increasing water content and proximity to the metaluminous join for rhyolite glasses. Within analytical uncertainty, the amount of [5]Al is independent of fictive temperature. The addition of water to andesite glasses does not decrease the amount of [5]Al, in contrast to the rhyolite glasses. Based on the glass data, we conclude that the majority of Al and Si will be present in fourfold coordination in metaluminous dry and hydrous rhyolite magmas at the temperature and pressure conditions relevant for the crust and the subducting slab (up to 3.5GPa). However, consistent with previous predictions from model systems, more higher-coordinated Al may be present in andesite and peralkaline or peraluminous rhyolite melts, although the exact concentrations cannot be predicted accurately from the available data. Compared to more depolymerized melts, rhyolite melts can accommodate a higher degree of densification without a significant increase in Al coordination.

In situ ion-microprobe determination of trace element partition coefficients for hornblende, plagioclase, orthopyroxene, and apatite in equilibrium with natural rhyolitic glass, Little Glass Mountain Rhyolite, California

Partially crystalline hornblende gabbro inclusions from the Little Glass Mountain Rhyolite contain euhedral plagioclase, orthopyroxene, hornblende, and apatite crystals in contact with interstitial rhyolitic (71-76% SiO 2 ) glass. Textural and mineral compositional data indicate that the gabbros crystallized sufficiently slowly that surface equilibrium was closely approached at the interface between crystals and the liquid. This rare occurrence represents a natural dynamic crystallization experiment with a “run time” that is not realistically achievable in the laboratory. SIMS analysis of mineral rim-glass pairs have permitted the determination of high-quality, equilibrium trace-element partition coefficients for all four minerals. These data augment the limited partition coefficient database for minerals in high-SiO 2 rhyolitic systems. For all minerals, the D values are consistent with those anticipated from crystal-chemical considerations. These data further support a liquid SiO 2 control on the REEs (and presumably other elements) partitioning wherein D values systematically increase with increasing liquid SiO 2 content.

Amorphous Materials: Properties, Structure, and Durability: Compositional dependent compressibility of dissolved water in silicate glasses

The sound velocities and elastic properties of a series of hydrous rhyolite, andesite, and basalt glasses have been determined by Brillouin scattering spectroscopy at ambient conditions to elucidate the effect of glass composition on the compressibility of dissolved water. Both the adiabatic bulk (K S ) and shear modulus (μ) of the dry glasses decrease with increasing silica content (K S,basalt > K S,andesite > K S,rhyolite and μ basalt > μ andesite > μ rhyolite ). For each composition, the shear modulus systematically decreases with increasing water content. Although the addition of up to 14 mol% water decreases the K S of andesite and basalt glasses by up to 6%, there is no discernable effect of water on the K S of the rhyolite glasses. The partial molar KS of dissolved water (K S ) in rhyolite, andesite, and basalt glasses are 37 ± 5, 19 ± 7, and 40 ± 3 GPa, corresponding to partial molar isothermal compressibilities (β̅ T ) of 0.029 ± 0.005, 0.042 ± 0.004, and 0.026 ± 0.002 GPa −1 , respectively. These results indicate that the compressibility of dissolved water strongly depends on the bulk composition of the glass; hence, the partial molar volume of water cannot be independent of the bulk composition at elevated pressure. If the compressibility of dissolved water also depends on composition in the analog melts at high temperature and pressure, these observations will have important consequences for magmatic processes such as magma mixing/unmixing and fractional crystallization.

Unravelling provenance from Eocene–Oligocene sandstones of the Thrace Basin, North‐east Greece

AbstractNew sandstone petrology and petrostratigraphy provide insights on Palaeogene (Middle Eocene to Oligocene) clastics of the Thrace Basin in Greece, which developed synchronously with post‐Cretaceous collision and subsequent Tertiary extension. Sandstone petrofacies are used as a tool to unravel complex geodynamic changes that occurred at the southern continental margin of the European plate, identifying detrital signals of the accretionary processes of the Rhodope orogen, as well as subsequent partitioning related to extension of the Rhodope area, followed by Oligocene to present Aegean extension and wide magmatic activity starting during the Early Oligocene. Sandstone detrital modes include three distinctive petrofacies: quartzolithic, quartzofeldspathic and feldspatholithic. Major contributions are from metamorphic basement units, represented mostly by low to medium‐grade lithic fragments for the quartzolithic petrofacies and high‐grade metamorphic rock fragments for the quartzofeldspathic petrofacies. Volcaniclastic sandstones were derived from different volcanic areas, with a composition varying from dominantly silicic to subordinate intermediate products (mainly rhyolitic glass, spherulites and felsitic lithics). Evolution of detrital modes documents contributions from three key source areas corresponding to the two main crystalline tectonic units: (i) the Variegated Complex (ultramafic complex), in the initial stage of accretion (quartzolithic petrofacies); (ii) the Gneiss–Migmatite Complex (quartzofeldspathic petrofacies); and (iii) the Circum‐Rhodope Belt. The volcaniclastic petrofacies is interbedded with quartzofeldspathic petrofacies, reflecting superposition of active volcanic activity on regional erosion. The three key petrofacies reflect complex provenance from different tectonic settings, from collisional orogenic terranes to local basement uplift and volcanic activity. The composition and stratigraphic relations of sandstones derived from erosion of the Rhodope orogenic belt and superposed magmatism after the extensional phase in northern Greece provide constraints for palaeogeographic and palaeotectonic models of the Eocene to Oligocene western portions of the Thrace Basin. Clastic detritus in the following sedimentary assemblages was derived mainly from provenance terranes of the Palaeozoic section within the strongly deformed Rhodope Massif of northern Greece and south‐east Bulgaria, from the epimetamorphic units of the Circum‐Rhodope Belt and from superposed Late Eocene to Early Oligocene magmatism related to orogenic collapse of the Rhodope orogen. The sedimentary provenance of the Rhodope Palaeogene sandstones documents the changing nature of this orogenic belt through time, and may contribute to a general understanding of similar geodynamic settings.

Methods for determination of the age of Pleistocene tephra, derived from eruption of Toba, in central India

Tephra, emplaced as a result of Pleistocene eruption of the Indonesian ‘supervolcano’ Toba, occurs at many localities in India. However, the ages of these deposits have hitherto been contentious; some workers have argued that these deposits mark the most recent eruption (eruption A, ca 75 ka), although at some sites they are stratigraphically associated with Acheulian (Lower Palaeolithic) artefacts. Careful examination of the geochemical composition of the tephras, which are composed predominantly of shards of rhyolitic glass, indicates that discrimination between the products of eruption A and eruption D (ca 790 ka) of Toba is difficult. Nonetheless, this comparison favours eruption D as the source of the tephra deposits at some sites in India, supporting the long-held view that the Lower Palaeolithic of India spans the late Early Pleistocene. In principle, these tephra deposits should be dateable using the K–Ar system; however, previous experience indicates contamination by a small proportion of ancient material, resulting in apparent ages that exceed the true ages of the tephras. We have established the optimum size-fraction in which the material from Toba is concentrated, 53–61 μm, and have considered possible origins for the observed contamination. We also demonstrate that Ar–Ar analysis of four out of five of our samples has yielded material with an apparent age similar to that expected for eruption D. These numerical ages, of 809 ± 51, 714 ± 62, 797 ± 45 and 827 ± 39 ka for the tephras at Morgaon, Bori, Gandhigram and Simbhora, provide a weighted mean age for this eruption of 799 ± 24 ka (plus-or-minus two standard deviations). However, these numerical ages are each derived from no more than 10–20% of the argon release in each sample, which is not ideal. Nonetheless, our results demonstrate that it is feasible, in principle, to date this difficult material using the Ar–Ar technique; future follow-up studies will therefore be able to refine our preparation and analysis procedures to better optimize the dating.

Origin of the sequential Shirakawa ignimbrite magmas from the Aizu caldera cluster, northeast Japan: Evidence for renewal of magma system involving a crustal hot zone

The Shirakawa ignimbrites were generated by caldera-forming eruptions in the Aizu caldera cluster, northeast Japan, during the Early Pleistocene. They have a total volume of > 300 km 3 dense rock equivalent and consist of the Kumado ignimbrite (1.4 Ma) from Ono caldera, the Ashino ignimbrite (1.3 Ma) and the Nagurasawa ignimbrite (1.2 Ma) from Tonohetsuri caldera, the Nishigo ignimbrite (1.1 Ma) from Narioka caldera and the Ten-ei ignimbrite (1.0 Ma) from an unknown, concealed caldera. These calderas overlap each other within a 20 × 20 km area in the eastern part of the cluster. The Shirakawa ignimbrites are relatively small in the eruption rate and have sufficient interval times that are longer than the residence times for their magma volume. The individual ignimbrites are composed of 20–40% phenocrysts set in high-silica rhyolite glass and they lack vertical chemical zoning. The SiO 2 content of the Kumado, Ashino, Nagurasawa, Nishigo and Ten-ei ignimbrites are 68–70 wt.%, 67–70 wt.%, 68–69 wt.%, 72–74 wt.% and 69–70 wt.%, respectively. They plot within the medium-K field, but show distinct trends for most major elements at the same SiO 2 ranges. Although they all have similar arc-related trace-elements patterns with Nb, Ta and Ti negative anomalies, high LREEs and flat HREEs, they differ in their Zr/Nb, Ba/Th and K/La ratios. Also, isotopic ratios of these pumices from each unit are distinct; 87Sr/ 86Sr varies from 0.7044 to 0.7047 and 143Nd/ 144Nd from 0.51271 to 0.51278. These results imply that the ignimbrite sources were renewed with every caldera-forming eruption. Seismic tomography and pre-eruption uplifting of the caldera cluster suggest that a substantial amount of mantle-derived magma was stored within the lower crust and contributed to the evolution of a crustal hot zone. Observed geochemical features also suggest that silicic magmas were generated by the melting of variably assimilated mafic intrusions. To explain the renewal of the Shirakawa magma system, melted materials must have been completely evacuated from a deep-seated source zone by each caldera-forming eruption and different mafic rocks subsequently subsided into the hot zone as raw materials for the next eruption.

U–Th–Pb geochronology of zircon and monazite from syenite and pincinite xenoliths in Pliocene alkali basalts of the intra-Carpathian back-arc basin

Phreatomagmatic eruptions in the Lučenec Basin in Slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite – pincinite. Two kinds of syenites with different Fe 3+ contents composed of anorthoclase and interstitial metaluminous trachyte-to-alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15–18 km. Pincinite composed of plagioclase, quartz, orthopyroxene, ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17–20 km by high-temperature dehydration melting of biotite-bearing gneissic protolith. Zircon from pincinite and syenite, and monazite from syenite were dated using excimer laser ablation ICP-MS. The obtained ages cluster at 5.8 ± 0.1 and 5.4 ± 0.1 Ma, thus corroborating the genetic link between the two rock-types and constraining the residence time of at least 300 ± 100 ka of the parental basalt in the deep crustal magma storage zones. The inferred ages overlap the Upper Miocene–Pliocene boundary, and are considerably younger than spatially associated basaltic lava flows dated using the K/Ar method. Consequently, the fluviolacustrine Poltár Formation overlying the maar structure must have sedimented during the Pliocene and not during the Upper Miocene (Pontian) as previously assumed.

Multi‐Elemental Analysis of ATHO‐G Rhyolitic Glass (MPI‐DING Reference Material) by Femtosecond and Nanosecond LA‐ICP‐MS: Evidence for Significant Heterogeneity of B, V, Zn, Mo, Sn, Sb, Cs, W, Pt and Pb at the Millimetre Scale

This paper reports on the application of variants of LA‐ICP‐MS – including infrared femtosecond laser ablation (fs‐LA) inductively coupled plasma‐quadrupole mass spectrometry (ICP‐QMS) and nanosecond laser ablation (ns‐LA) coupled with single‐collector sector‐field (SF‐) ICP‐MS – to the in situ determination of trace elements in different splits of the reference material (RM) ATHO‐G (MPI‐DING). Analyses of the materials performed by fs‐ and ns‐LA‐ICP‐MS demonstrated the efficiency of the techniques with typical accuracy at a level of ≤ ± 20%. One ‘anomalous’ split, however, displayed a significant discrepancy from the reference concentrations for B, V, Zn, Mo, Sn, Sb, Cs, W and Pb. Three‐ to six‐fold enrichment of V, Mo, Cs and Pt relative to the reference contents in this split is likely to have been due to direct contact of the silicate melt with Pt crucible walls and ceramics. Boron, Zn, Sn, Sb, W and Pb depletion relative to the reference concentrations is probably due to siderophile element adsorption by the Pt walls and/or related to the formation of volatile‐depleted compositional cords during the preparation process. Our results imply that additional precautions should be taken against volatile/siderophile element heterogeneity in marginal/surface layers (≤ 10 mm) during the preparation of RMs by the fusion technique.

A 250 k.y. record of Cascade arc pyroclastic volcanism from late Pleistocene lacustrine sediments near Summer Lake, Oregon, USA

Distal tephra beds provide important records of pyroclastic volcanism that enhance our overall understanding of eruptive frequencies, magnitudes, compositions, and hazards. Some beds also serve as widespread chronostratigraphic markers. Lacustrine sediments near Summer Lake, Oregon (United States), record numerous eruptions of Cascade arc sources over a period exceeding 2.5 m.y. Late Pleistocene sediments exposed in outcrop have yielded 88 visible tephra beds, including many beds not previously documented. Of these beds, 44 are characterized by rhyolitic glass, 40 contain predominantly basaltic or intermediate glass, and 4 are strongly heterogeneous in composition. Only 23 have been correlated to deposits outside of the Summer Lake basin. The remaining 65 beds provide a record of Cascade arc volcanism that is as yet unique to Summer Lake. Age-depth relations are well constrained for the upper 6 m of section, but are less certain in the lower 12.4 m. Tephra correlations and an overall age model suggest the following: bed B1 originates from an eruption of Mount Mazama (Crater Lake) ca. 20 ka. Beds I and W likely originate from eruptions of Mount St. Helens ca. 80 and 190 ka. A 7-cm-thick tephra bed correlated to Shevlin Park Tuff probably dates to ca. 198 ka. Tephra correlated to the Antelope Well tuff from Medicine Lake volcano dates to ca. 215 ka. Bed NN, at the base of the section, has an estimated age of at least 240–250 ka and probably originated from Newberry Volcano. Overall, this record significantly refines the Pleistocene tephrostratigraphic framework for western North America.

Bacterial Diversity of Weathered Terrestrial Icelandic Volcanic Glasses

The diversity of microbial communities inhabiting two terrestrial volcanic glasses of contrasting mineralogy and age was characterised. Basaltic glass from a <0.8 Ma hyaloclastite deposit (Valafell) harboured a more diverse Bacteria community than the younger rhyolitic glass from ∼150-300 AD (Dόmadalshraun lava flow). Actinobacteria dominated 16S rRNA gene clone libraries from both sites, however, Proteobacteria, Acidobacteria and Cyanobacteria were also numerically abundant in each. A significant proportion (15-34%) of the sequenced clones displayed <85% sequence similarities with current database sequences, thus suggesting the presence of novel microbial diversity in each volcanic glass. The majority of clone sequences shared the greatest similarity to uncultured organisms, mainly from soil environments, among these clones from Antarctic environments and Hawaiian and Andean volcanic deposits. Additionally, a large number of clones within the Cyanobacteria and Proteobacteria were more similar to sequences from other lithic environments, included among these Icelandic clones from crystalline basalt and rhyolite, however, no similarities to sequences reported from marine volcanic glasses were observed. PhyloChip analysis detected substantially greater numbers of phylotypes at both sites than the corresponding clone libraries, but nonetheless also identified the basaltic glass community as the richer, containing approximately 29% unique phylotypes compared to rhyolitic glass.

Quaternary tephra marker beds and their potential for palaeoenvironmental reconstruction on Chatham Island, east of New Zealand, southwest Pacific Ocean

AbstractTephras provide one of the most reliable methods of time control and synchronisation within Quaternary sequences. We report on the identification of two widespread rhyolitic tephras – the Kawakawa and Rangitawa tephras – preserved in extensive peat deposits on Chatham Island ∼900 km east of New Zealand. The tephras, both products of supereruptions from the Taupo Volcanic Zone, occur as pale, fine‐ash dominated layers typically 10–150 mm thick. Mineralogically they are dominated by rhyolitic glass, together with subordinate amounts of quartz, feldspar, hypersthene, hornblende, Fe–Ti oxides and zircon. Phlogopite/biotite was identified additionally in Rangitawa Tephra. Ages for each tephra were obtained via mineralogical and major element glass composition‐based correlation with well‐dated equivalent deposits on mainland New Zealand, and we also obtained a new zircon fission‐track age for Rangitawa Tephra (350 ± 50 ka) on Chatham Island. Both tephras were erupted at critical times for palaeoenvironmental reconstructions in the New Zealand region: the Kawakawa at ca. 27 cal. ka, near the beginning of the ‘extended’ LGM early in marine isotope stage (MIS) 2; and the Rangitawa at ca. 350 ka near the end of MIS 10. The time constraints provided by the tephras demonstrate that Chatham Island peats contain long‐distance pollen derived from mainland New Zealand, which provides a reliable proxy for identifying glacial–interglacial climate conditions, in this case during the MIS 11–10 and MIS 2–1 cycles. The two tephras thus provide important chronostratigraphic tie‐points that facilitate correlation and synchronisation not only across the Quaternary deposits of the Chatham Islands group but also with climatically significant terrestrial and marine records in the wider New Zealand region. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Ar, CO 2 and H 2O diffusion in silica glasses at 2 kbar pressure

The diffusion of volatiles into silica glasses was studied by sorption experiments in argon gas pressure vessels at 2 kbar and temperatures from 521 to 1097 °C. In the experiments either Ar and H 2O (AH series, samples in open containers), or CO 2 and H 2O (CH series, samples in closed noble metal capsules with silver oxalate added as source for CO 2) could exchange between the fluid and the glasses. Both sample assemblies were run simultaneously to enable direct comparison of diffusion data. Profiles were recorded on sections perpendicular to the initial surfaces by electron microprobe (Ar) and FTIR spectroscopy (CO 2 and H 2O). Ar diffusion is systematically slower by 0.17 log units in average in water-rich silica glass (∼ 0.05 wt.% H 2O, Suprasil ®) than in water-poor silica glasses (< 0.01 wt.% H 2O, Infrasil ® and Herasil ®) while the activation energy for argon diffusion agrees within error. This effect is attributed to partial closure of doorways for argon in the silicate network by the formation of hydroxyl groups. CO 2 is dissolved only as molecular species in silica glasses and its diffusivity is about the same as that of argon, similar as found for rhyolitic glasses. Dissolved H 2O was detected in the run products exclusively in form of silanol groups. The H 2O profiles show systematic deviations from error function shape. The shape of the profiles could originate from either a positive correlation of water diffusivity to water content or a continuous decrease in water fugacity in the fluid. H 2O was only a minor component in the fluids and the condition of an infinite reservoir was not necessarily always ensured during the whole run. Assuming negligible decrease of water fugacity during experiment, diffusion coefficients of H 2O ( D H2O) were determined by a Boltzmann–Matano analysis. The data are consistent with D H2O being proportional to the concentration of dissolved water.

Complex relationships among coexisting pyroxenes: the Palaeogene Eskdalemuir dyke, Scotland

AbstractThe composite Eskdalemuir dyke was formed by the intrusion of rhyolitic magma into partly crystallized basaltic magma, the proportion of the silicic component increasing towards the dyke centre. The less silicic parts were effectively supercooled and crystallization was halted when the residual melt quenched to rhyolitic glass which constitutes up to 50% of the dyke's centre. Pyroxene compositions in the mixed magma rocks include augite, pigeonite and orthopyroxene of very variable composition. Individual crystals in the matrix as small as 200 μm can contain compositions occupying a considerable part of the Di-Hd-En-Fs quadrilateral. Pyroxene crystallization took place under extreme disequilibrium conditions, as shown by highly variable zonation patterns; apparently opposite zoning trends can occur in adjacent grains. Small-scale variations in melt composition were probably the dominant control over the compositional and textural complexity in the pyroxenes.

Geochemistry of reversible hydratable tephra from the Trans Mexican Volcanic Belt

Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72 °C, composed of sodic and non-sodic glasses of surface activity caused by IV Al substituting in Q 4 (1A1) and Q 4 (2A1) positions, minor v A1, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H 2 O m in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H 2 O m molecules. Sodic glasses contain 71.80-77.77 wt% SiO 2 , are partially devitrified to crystallites (∼5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (∼10 μm size). Sodium-free glasses are more siliceous, with 74.84-83.88 wt% SiO 2 , show partial devitrification to crystallites (∼5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (∼10 μm size).

Characterization of the states and diffusivity of intergranular water in a chalcedonic quartz by high-temperature in situ infrared spectroscopy

AbstractInfrared spectroscopy was performed on a thin section of a chalcedonic quartz at high temperature in order to investigate the states and diffusivity of intergranular water. The sample contained 0.3 wt.% of silanol (Si—OH) and 0.3 wt.% of molecular H2O, located mainly at intergranular regions but also as fluid inclusions. We monitored the diffusion of molecular H2O associated with dehydration by in situ analyses at 350—500°C and determined the bulk-diffusion coefficients as expressed by an Arrhenius relationship: Dbulk (m2 sec—1) = 10—4.5 exp(—107±17/RT), where R is the gas coefficient and T the temperature. The activation energy for our sample is similar to those previously reported for diffusion in quartz aggregates with incompletely-connected grain boundaries. This result and our previous measurements of electrical conductivity imply that diffusion of molecular H2O at incompletely- connected intergranular regions is the main mechanism for the dehydration. The diffusion coefficients in chalcedony are larger than those previously reported for rhyolitic glass and other granular aggregates. Intergranular regions, inherent larger pores and cracks created during heating can act as efficient diffusion paths within the chalcedony.

Feldspar dissolution rates in the Topopah Spring Tuff, Yucca Mountain, Nevada

Abstract Two different field-based methods are used here to calculate feldspar dissolution rates in the Topopah Spring Tuff, the host rock for the proposed nuclear waste repository at Yucca Mountain, Nevada. The center of the tuff is a high silica rhyolite, consisting largely of alkali feldspar (∼60 wt%) and quartz polymorphs (∼35 wt%) that formed by devitrification of rhyolitic glass as the tuff cooled. First, the abundance of secondary aluminosilicates is used to estimate the cumulative amount of feldspar dissolution over the history of the tuff, and an ambient dissolution rate is calculated by using the estimated thermal history. Second, the feldspar dissolution rate is calculated by using measured Sr isotope compositions for the pore water and rock. Pore waters display systematic changes in Sr isotopic composition with depth that are caused by feldspar dissolution. The range in dissolution rates determined from secondary mineral abundances varies from 10 −16 to 10 −17 mol s −1 kg tuff −1 with the largest uncertainty being the effect of the early thermal history of the tuff. Dissolution rates based on pore water Sr isotopic data were calculated by treating percolation flux parametrically, and vary from 10 −15 to 10 −16 mol s −1 kg tuff −1 for percolation fluxes of 15 mm a −1 and 1 mm a −1 , respectively. Reconciling the rates from the two methods requires that percolation fluxes at the sampled locations be a few mm a −1 or less. The calculated feldspar dissolution rates are low relative to other measured field-based feldspar dissolution rates, possibly due to the age (12.8 Ma) of the unsaturated system at Yucca Mountain; because oxidizing and organic-poor conditions limit biological activity; and/or because elevated silica concentrations in the pore waters (∼50 mg L −1 ) may inhibit feldspar dissolution.