Experimental solidification of anhydrous latitic and trachytic melts at different cooling rates: The role of nucleation kinetics

Two sets of cooling experiments were run at atmospheric conditions for two anhydrous starting latitic and trachytic melts: 1) five cooling rates (25, 12.5, 3, 0.5, and 0.125 °C/min) between 1300° and 800 °C, and 2) a 0.5 °C/min cooling rate from 1300 °C with quench temperatures at 1200°, 1100°, 1000° and 900 °C. Trachytic run-products are invariably glassy. Nucleation is also suppressed in the latitic run-products at the three highest cooling rates. Conversely, in the 0.5 and 0.125 °C/min runs, latites have a crystal content of  90 vol.%. The phases are: plagioclase, clinopyroxene, glass and iron-bearing oxide (in order of abundance). The variable quench temperatures, investigated by coupling experiments with Pt wire and Pt capsule sample containers in set 2, again did not produce crystallization of trachyte, whereas latitic samples are characterized by  10 vol.% of oxides, pyroxenes and plagioclase (in order of appearance), at temperature < 1000 °C. Effects of (preferential) heterogeneous nucleation on sample holders, of superheating degree, and chemical species loss during cooling are absent for both melt compositions. The difference of solidification paths between these two silicate melts can be ascribed only to their small chemical differences. In comparison with calculated equilibrium conditions all the experimental latitic and trachytic run-products revealed strong kinetic effects, interpretable in the light of the nucleation theory. The glass-forming ability (GFA) of trachyte is higher, whereas their critical cooling rate (Rc) is lower (< 0.125 °C/min), in comparison to latitic melts (Rc > 0.5 °C/min). The experimental results carried out in this study can be applied to lava flows and domes; trachytic lavas are able to flow for longer period with respect to latitic ones in a metastable condition. Glass-rich terrestrial lavas, i.e. obsidians, can be the result of sluggish nucleation kinetics due to the relative high polymerisation of evolved silicate melts.     

Timing of UHP metamorphism in the Hong’an area,western Dabie Mountains,China: evidence from zircon U–Pb age,trace element and Hf isotope composition

The Hong’an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U–Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong’an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U–Pb ages, and trace element and Lu–Hf isotope compositions. One group with an average age of 239 ± 2 Ma show relatively high and variable HREE contents (527 ≥ Lu_N ≥ 14) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.00008–0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 ± 2 Ma, and shows consistent low HREE contents and ¹⁷⁶Lu/¹⁷⁷Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP–HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 ± 86 and 752 ± 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (T _DM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An experimental study of the grain-scale processes of peridotite melting: implications for major and trace element distribution during equilibrium and disequilibrium melting

The grain-scale processes of peridotite melting were examined at 1,340°C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P–T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal-melt interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxene-melt partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements in residual minerals reequilibrate with their surrounding melt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Magma Ascent and Crustal Accretion at Ultraslow-Spreading Ridges: Constraints from Plagioclase Ultraphyric Basalts from the Arctic Mid-Ocean Ridge

Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclasephenocrysts were dredged in the rift valley and adjacent flanksof the ultraslow-spreading Mohns and Knipovich ridges. The PUBsshow large variations in crystal morphologies and zoning. Thelarge variations suggest that single basalt samples containa mixture of plagioclase crystals that aggregated at differentlevels in the magma conduits. Resorbed crystals and repeatedreverse zones suggest that the magma reservoirs were replenishedand heated several times. Thin concentric zones with melt inclusions,and sharp reductions in the anorthite content of 3–7%,are common between the reverse zones. These zones, and skeletalcrystals with distinctly lower anorthite contents than massivecrystals, are interpreted to be the result of rapid crystalliztionduring strong undercooling. The changes between short periodsof cooling and longer periods with reheating are explained bymultiple advances of crystal-rich magma into cool regions followedby longer periods of gradual magma inflow and temperature increase.The porphyritic basalts are characterizd by more depleted andmore fractionated compositions than the aphyric basalts, withlower (La/Sm)_N, K₂O and Mg-numbers. This relationship, and theobservation that PUBs are sampled only close to segment centresalong these ridges, suggests that the PUBs formed by higherdegrees of melting and evolved in more long-lived magma reservoirs.We propose that the zoning patterns of plagioclase crystalsand crystal morphologies of these PUBs reflect the developmentand flow of magma through a stacked sill complex-like conduitsystem, whereas the aphyric equivalents represent later flowof magma through the conduit. The formation of voluminous higher-degreemelts may trigger the development of the magma conduits andexplain the generally depleted compositions of PUB magmas. KEY WORDS: basalt; mineral chemistry; MORB; magma mixing; magma chamber; major element     

Numerical estimation of REV and permeability tensor for fractured rock masses by composite element method

The Monte Carlo method is used to generate parent stochastic discrete fracture network, from which a series of fractured rock samples of different sizes and orientations are extracted. The fracture network combined with a regular grid forms composite element mesh of the fractured rock sample, in which each composite element is composed of sub‐elements incised by fracture segments. The composite element method (CEM) for the seepage is implemented to obtain the nodal hydraulic potential as well as the seepage flow rates through the fractured rock samples. The application of CEM enables a large quantity of stochastic tests for the fractured rock samples because the pre‐process is facilitated greatly. By changing the sizes and orientations of the samples, the analysis of the seepage characteristics is realized to evaluate the variation of the permeability components, the existence of the permeability tensor and the representative element volume. The feasibility and effectiveness are illustrated in a numerical example. Copyright © 2008 John Wiley & Sons, Ltd.     

Finite Element Modeling of Swelling Soil Pressures Using Joint/Interface Elements

There could be some discontinuities in a soil media such as layers, earth structures, cracks, and fissures so that estimation of stresses and deformations in these types of soil masses are somewhat different from continuous masses. The discontinuities in a soil mass could be considered as a special link between two blocks. Transmitted swelling pressures affected by the soil properties of the backfill especially at the interface between the backfill and retaining wall. In order to estimate the transmitted swelling pressure distribution behind a retaining wall, using with interface element, a new finite element model and a code (SWELPRES) have been developed. To evaluate the effect of backfill characteristics and interface element from the transmitted lateral swelling pressures, four cases of thickness of backfills with or without interface elements are investigated.     

贵州修文小山坝铝土矿中镓等伴生元素分布规律研究

通过ICP-MS等分析方法对贵州小山坝铝土矿中伴生元素的研究,发现该矿床TiO2、Ga、Ge、Sc、Nb、Li和REE等有用伴生元素在铝土矿矿体中均有较高富集,其含量大多接近或超过了工业综合利用标准,具有较大的经济价值,特别是矿区富铁铝土矿石中Ga和Sc等含量异常高.贵州铝土矿资源丰富,其伴生元素在铝土矿冶炼过程中所产生的赤泥中更为富集,因此,贵州铝土矿综合利用前景十分巨大,这些元素潜在的经济价值,在某种程度上,甚至会超过主导产品氧化铝的价值.开展铝土矿中镓等伴生有用元素的分布及赋存状态的研究,对于镓等稀散元素在特殊地质地球化学环境中超常富集机制、矿产资源综合价值的重新认识评估和矿产资源的综合利用将十分重要,不仅可为铝工业注入新的活力,增加新的经济增长点,而且还可以解决大量赤泥堆放和利用问题,同时获得较大的社会效益与环境效益.