Petrologic constraints on the decompression history of magma prior to Vulcanian explosions at the Soufrière Hills volcano,Montserrat

A series of 88 Vulcanian explosions occurred at the Soufrière Hills volcano, Montserrat, between August and October, 1997. Conduit conditions conducive to creating these and other Vulcanian explosions were explored via analysis of eruptive products and one-dimensional numerical modeling of magma ascent through a cylindrical conduit. The number densities and textures of plagioclase microlites were documented for twenty-three samples from the events. The natural samples all show very high number densities of microlites, and > 50% by number of microlites have areas < 20 μm². Pre-explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions. Our comparisons suggest quench pressures < 30 MPa (origin depths < 2 km) and multiple rapid decompressions of > 13.75 MPa each during ascent from chamber to surface. Values are consistent with field studies of the same events and statistical analysis of explosion time-series data. The microlite volume number density trend with depth reveals an apparent transition from growth-dominated crystallization to nucleation-dominated crystallization at pressures of ∼ 7 MPa and lower. A concurrent sharp increase in bulk density marks the onset of significant open-system degassing, apparently due to a large increase in system permeability above ∼ 70% vesicularity. This open-system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to Vulcanian explosions. The corresponding inferred depth of overpressure at 250–700 m, near the base of the dense plug, is consistent with depth to center of pressure estimated from deformation measurements. Here we also illustrate that one-dimensional models representing ascent of a degassing, crystal-rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis. The comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open-system degassing through conduit walls.     

Quench textures in pillow basalt from the Andaman-Nicobar Islands,Bay of Bengal,India

Tholeiitic pillow basalt from the South Andaman island, an integral part of the outer sedimentary arc of the Sunda-Burmese double chain arc system in the Bay of Bengal, is characterized by the occurrence of several morphologies of quenched crystals of plagioclase and pyroxene. Plagioclase shows a swallow tail, belt-buckle, rosette and closely spaced fan-spherulites pattern while pyroxene has elongate parallel chain, dendritic, spherulitic and finely ornamented feathery spherulitic habit. Most of these textures are identical to those reported from submarine basalts, lunar basalts, spinifex textured rocks and experimentally produced textures. The occurrence of these quench textures in the Andaman basalt suggests that they were formed by rapid cooling at 30–70°C/h in a submarine environment.     

Quantulua-1220型低本底液闪仪的性能测定

Quantulua-1220型低本底高灵敏度液闪仪是14C测年系统中的关键设备,其质量直接影响14C测年数据的准确性,文中使用中国糖碳、合成本底和成对样品来评估新购置的Quantulua-1220型低本底液闪仪的本底、探测效率及仪器的长期稳定性等重要性能指标是否满足14C测年要求。测试结果表明,在一般工作环境下,液闪仪的本底低于0.5cpm,最大可测14C年龄估计值为48ka;由于猝灭因素引起的年龄偏差不超过100a,新液闪仪经过10个月的运行后,所测定的14C年龄具有可对比性     

牙 板 渗 碳 淬 火 工 艺 的 改 进

针对钻井用吊钳牙板使用时易崩碎、不耐磨的特点，分析了原因，改进了牙板渗碳淬火的热处理工艺。实践证明改进后的工艺切实可行。     

Study of Distal Pyroclastic-flow Stratum from Tianchi Volcano in 1215 (±15) Eruption: Pyroclastic-flow Over Water

In this paper,we describe three strata at the distal part of the pyroclastic-flow from the Tianchi volcano in 1215(±15) eruption.One of the strata with crosslayers that are different from typical pyroclastic-flow strata may come from a ground-surge.The grain-size and scanning electron microscopy(SEM) analysis was performed to study the origin of the pyroclastic-flow.Characteristics of grain-size distribution show that it is similar with the ash cloud.Through the SEM analyses,we found some quench structures with less damage on the surfaces of the vitric pumices.These phenomena indicate that there has been hydration in the transportation processes at the distal of pyroclastic-flow.It has partly changed the transportation mechanism of pyroclastic-flow,which transitions form dense flow to diluted flow.This paper develops a new distal pyroclastic-flow model in the Tianchi volcano that can be divided into three stages,i.e.the quench stage,expanding stage and depositing stage.     

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW-ESE to ENE-WSW or NE-SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite-pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near-surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite. Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline-type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km. The bulk chemical composition of the pseudotachylite veins is characterized by low SiO₂ and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high-Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two-pyroxene geothermometer. Using published data relating water solubilities in high-temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites. Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.     

Widespread strombolian eruptions of mid-ocean ridge basalt

Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele's hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent.     

一类具有非线性迁移项的拟线性抛物方程解的熄灭

在Rn有界域上讨论了具有非线性迁移项的平均曲率型方程ut-div[σ(| u|2) u]+b(u)· u=0的初边值问题。若初值u0∈Lq,q≥n,问题的解将在有限时刻熄灭,并且给出了解的L∞估计。