A. R. L. Nichols

Timescales of spherulite crystallization in obsidian inferred from water concentration profiles

Abstract We determined the kinetics of spherulite growth in obsidians from Krafla volcano, Iceland. We measured water concentration profiles around spherulites in obsidian by synchrotron Fourier transform infrared spectroscopy. The distribution of OH- groups surrounding spherulites decreases exponentially away from the spherulite-glass border, reflecting expulsion of water during crystallization of an anhydrous paragenesis (plagioclase + SiO2 + clinopyroxene + magnetite). This pattern is controlled by a balance between the growth rate of the spherulites and the diffusivity of hydrous solute in the rhyolitic melt. We modeled advective and diffusive transport of the water away from the growing spherulites by numerically solving the diffusion equation with a moving boundary. Numerical models fit the natural data best when a small amount of post-growth diffusion is incorporated in the model. Comparisons between models and data constrain the average spherulite growth rates for different temperatures and highlight size-dependent growth among a small population of spherulites.

The influence of H2O and CO2 on the glass transition temperature: insights into the effects of volatiles on magma viscosity

CO 2 can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H 2 O and initiate degassing. The effect of such CO 2 loss on magma viscosity is not well constrained, especially compared to the established effects of H 2 O loss. In terms of understanding the CO 2 solubility mechanism, recent spectroscopic studies have shown that CO 2 speciation is strongly temperature dependent and that CO 2 speciation preserved in quenched glasses below T g is different from the true CO 2 speciation observed in the melts. However, the effect of CO 2 on the glass transition temperature, and by inference the viscosity, has not been previously established. In this study, calorimetric measurements were conducted on synthetic H 2 O- and CO 2 -bearing phonolite and jadeite glasses in order to investigate the volatile’s effect on the glass transition interval, by defining a single glass transition temperature ( T g onset ). The samples were synthesised in a piston-cylinder apparatus between 1300 and 1550 °C, at 1.0 to 2.5 GPa, and contained up to 2.29 wt.% CO 2 and up to 5.49 wt.% H 2 O. For both compositions, H 2 O has a large effect in reducing T g onset , but CO 2 appears to have little or no effect. For the entire range of H 2 O contents, T g onset decreases exponentially with H 2 O content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the CO 2 content. No measurable effect of CO 2 on T g onset was observed. These results suggest that compared to H 2 O, CO 2 contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of CO 2 speciation from the bulk silicate melt structural relaxation process at T g .

Lead isotope analysis of melt inclusions by LA-MC-ICP-MS

Pb isotope compositions of melt inclusions provide unique information about the composition of primary magmas and their source. In this study, we have developed a method for measuring Pb isotopes in small olivine-hosted melt inclusions (>40 μm) from young and old volcanoes by LA-MC-ICP-MS. We used a new interface cone assemblage consisting of a Jet sample cone and X skimmer cone. A small flow of N2 gas was added to the carrier gas and passed through the assemblage to enhance the signal intensity. In addition the energy and repetition rate of the laser conditions were reduced and the signal integration time was shortened in order to lengthen the laser ablation time and to collect enough data. Mass bias and instrument drift were corrected using a standard–sample–standard bracketing method. The analysis routine employed eight ion counters to receive 238U, 235U, 232Th, 208Pb, 207Pb, 206Pb, 204Pb and 202Hg signals simultaneously, which allowed Hg interference to be corrected on 204Pb, and in old samples U–Th decay to be age-corrected. Using the Jet and X cones, under the same laser ablation conditions, the precisions for almost all the measured standard glasses are improved by at least a factor of two compared to using standard cones. At 208Pb signal intensity >200 000 cps, external precisions of ratios involving 204Pb are better than 1.3% (2RSD) and precisions of 208Pb/206Pb and 207Pb/206Pb are better than 0.23% (2RSD). The results of Pb isotopes in olivine-hosted melt inclusions, using 45 μm laser spots, show that the internal precisions of 208Pb/206Pb and 207Pb/206Pb for most analyzed melt inclusions are better than 0.2% (2RSE) and for ratios involving 204Pb are better than 0.8% (2RSE). We are able to present the first ever Pb isotope data from ∼260 Ma Emeishan flood basalt olivine-hosted melt inclusions. They show the importance to do age correction which results in the reduction of the spread of data in old samples. The mean values of age-corrected 208Pb/206Pb and 207Pb/206Pb have 1.2% and 2.8% deviations from the uncorrected mean values, respectively. The method developed here provides a fast, precise and accurate in situ Pb isotopic composition analysis, applicable not only to melt inclusions from young basalts, but also from old samples that require correction for U–Th decay.

Experiments and models on H2O retrograde solubility in volcanic systems

Abstract We present a suite of 36 high-temperature (900-1100 °C) experiments performed on 10 × 10 mm unjacketed cores of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland, under atmospheric pressure. The obsidian is bubble- and crystal-free with an H2O content of 0.11(4) wt%. The obsidian cores were heated above the glass transition temperature (Tg), held for 0.25-24 h, then quenched. During each experiment the volume of the samples increased as H2O vapor-filled bubbles nucleated and expanded. Uniquely, the bubbles did not nucleate on the surface of the core, nor escape, conserving mass during all experiments. Within each isothermal experimental suite, the cores increased in volume with time until they reached a maximum, after which continued heating caused no change in volume (measured by He-pycnometry). We interpret these T-t conditions as representing thermochemical equilibrium between the melt and exsolved vapor. These experiments are modeled to recover the 1-atm, temperature-dependent solubility of water in the rhyolite melt. Our results define the magnitude of retrograde solubility (-7.1 × 10-3 wt% H2O per 100 °C) and provide estimates of the enthalpy and entropy of the H2O exsolution reaction [ΔH° = 17.8 kJ/mol, ΔS° = 107 J/(K·mol)]. We conclude by modeling the implications of retrograde solubility for the glass transition temperatures (Tg) of cooling volcanic systems at pressures relevant to volcanic conduits and the Earth’s surface. All volcanic systems cool; the effects of retrograde solubility are to allow melts to rehydrate by H2O dissolution as they cool isobarically, thereby depressing Tg and expanding the melt window. Ultimately, the melt is quenched at higher H2O contents and lower temperatures where the isobaric retrograde solubility curve “catches” the evolving Tg.

Volcanoes of the diamante cross-chain: Evidence for a mid-crustal felsic magma body beneath the southern izu-bonin-mariana arc

Abstract Three submarine Diamante cross-chain volcanoes in the southern Mariana arc mark a magma-healed zone of along-arc (north–south) extension that allows either mafic mantle-derived basalts or felsic magmas from the middle of thickened arc crust to erupt. The largest volcano is East Diamante, with a well-developed (5×10 km) caldera that formed via violent felsic submarine eruptions beginning nearly 0.5 Ma. One or more of these eruptions also formed a giant submarine dune field extending 30 km to the NW of the volcano. Felsic igneous activity continues at least as recently as c. 20 000 years ago, with emplacement of resurgent dacite domes, some hot enough to power the only black smoker hydrothermal system known in the Mariana arc. In contrast, felsic eruptions do not occur on the two volcanoes to the west, implying that the mid-crustal felsic zone does not underlie the thinner crust of the Mariana Trough back-arc basin. Diamante cross-chain lavas define a medium K suite; mafic lava phenocryst assemblages show arc-like associations of anorthite-rich plagioclase with Fe-rich olivine. Magmatic temperatures for a basaltic andesite and three dacites are c. 1100 °C and c. 800 °C, respectively, typical for cool, wet, subduction-related felsic magmas. Felsic magmas formed under low-P crustal conditions. The Diamante cross-chain is the southernmost of at least seven and perhaps eight Mariana arc volcanoes in a c. 115 km long arc segment characterized by felsic eruptions. This is the ‘Anatahan Felsic Province’, which may have formed above a mid-crustal tonalite body that formed by fractionation or was re-melted when heated by c. 1200 °C mafic, mantle-derived magmas. Across- and along-arc variations suggest that felsic eruptions and dome emplacement occurred when midcrustal tonalite was remobilized by intrusions of mafic magma, while north–south extension facilitated the development of conduits to the surface. Supplementary material: Detailed Hyperdolphin ROV dive tracks, Cook 7 dredge locations, 40Ar/39Ar analytical data, analytical methods, major and selected trace element analyses of whole rock samples, and compositional data for minerals are available at http://www.geolsoc.org.uk/SUP18611

Incipient melt segregation as preserved in subaqueous pyroclasts

Melt segregation is the extraction of residual melt from the rigid but permeable network of growing crystals with which it co-evolved. It is recognized as an effective mechanism of igneous differentiation that acts over many geologic time and length scales. Here we present evidence for rapid melt segregation in subaqueous basaltic pyroclasts. Segregation produced intravesicular extrusions, i.e., partly hollow balloons of glass penetrating the walls of earlier formed vesicles set in a microcrystalline groundmass. The segregation process can be described as a form of gas filter pressing, where microlite crystallization produced local gradients in volatile supersaturation and vapor pressure that drove melt extrusion into adjacent vesicles. Unlike in previously described segregation features, the structures presented here are shown to have formed after the extruded melt became enriched in fast-diffusing H2O, but before it became measurably enriched or depleted in slower diffusing major elements. We show that melt segregation of this embryonic type must occur in seconds, rather than in the days to centuries required for similar processes to occur in lava flows or magma chambers, and that it occurs within the short time scales that characterize explosive fragmentation of basaltic melt.

The missing half of the subduction factory: shipboard results from the Izu rear arc, IODP Expedition 350

ABSTRACT IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.

Advent of Continents: A New Hypothesis

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10–20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs.

Accounting for the species-dependence of the 3500 cm−1 H2Ot infrared molar absorptivity coefficient: Implications for hydrated volcanic glasses

Abstract Fourier transform infrared (FTIR) spectroscopy can be used to determine the concentration and speciation of dissolved water in silicate glasses if the molar absorptivity coefficients (ε) are known. Samples that are thin and/or water-poor typically require the use of the mid-IR 3500 cm−1 total water (H2Ot) and 1630 cm−1 molecular water (H2Om) absorbance bands, from which hydroxyl water (OH) must be determined by difference; however, accurate determination of H2Ot and OH is complicated because ε3500 varies with water speciation, which is not usually known a priori. We derive an equation that uses end-member ε3500 values to find accurate H2Ot and OH concentrations from the 3500 cm−1 absorbance for samples where only the H2Om concentration need be known (e.g., from the 1630 cm−1 band). We validate this new species-dependent ε3500 method against published data sets and new analyses of glass standards. We use published data to calculate new end-member ε3500 values of ε3500OH = 79 ± 11 and ε3500H2Om = 49 ± 6 L/mol·cm for Fe-bearing andesite and ε3500OH = 76 ± 12 and ε3500H2Om = 62 ± 7 L/mol·cm for Fe-free andesite. These supplement existing end-member values for rhyolite and albite compositions. We demonstrate that accounting for the species-dependence of ε3500 is especially important for hydrated samples, which contain excess H2Om, and that accurate measurement of OH concentration, in conjunction with published speciation models, enables reconstruction of original pre-hydration water contents. Although previous studies of hydrous silicate glasses have suggested that values of ε decrease with decreasing tetrahedral cation fraction of the glass, this trend is not seen in the four sets of end-member ε3500 values presented here. We expect that future FTIR studies that derive end-member ε3500 values for additional compositions will therefore not only enable the species-dependent ε3500 method to be applied more widely, but will also offer additional insights into the relationship between values of ε and glass composition.

Erratum: Limited latitudinal mantle plume motion for the Louisville hotspot (Nature Geoscience (2012) 5 (911-917))

Nature Geoscience 5, 911–917 (2012); published online 25 November 2012. In the print version of this Article originally published, the present address for Toshitsugu Yamazaki was erroneously omitted. It is as follows: Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.