Segregation structures in vapor-differentiated basaltic flows

 Vesicle cylinders represent a spectacular kind of segregation structure involving residual liquids formed in situ during the cooling of lava flows. These vertical pipes, commonly found within basalt flows typically 2–10 m thick, are interpreted as the product of a vapor-driven differentiation process. The olivine phenocrysts and the earliest generation of groundmass olivines found in cylinder-bearing basalts appear to have been generally affected by magmatic oxidation, resulting in high-temperature iddingsite (HTI) alteration. This feature is also observed within cylinder-free basalt flows which exhibit other kinds of vesicular segregation structures, such as vesicle-rich pegmatoid segregation sheets and/or segregation vesicles. Detailed textural, petrological, and geochemical characteristics of two types of cylinders, three types of vesicle sheets, and five types of segregation vesicles are described, based on the study of 12 occurrences of HTI-bearing basalt flows from oceanic shield volcanoes or continental basalt plateaus. We propose a general classification of these segregation structures likely to derive from vapor differentiation. Flow thickness is probably the main factor influencing their morphology. Finally, we suggest that the concomitant occurrence of olivine oxidation and vapor-differentiation effects results from the late persistence of water oversaturation after eruption, perhaps due to a high rate of magma ascent. Received: 27 March 1999 / Accepted: 15 February 2000     

条石鲷消化道发育的组织学观察

对人工养殖的条石鲷（Oplegnathus fasciatus）仔、幼稚鱼消化道及消化腺的发育过程进行了系统的组织学观察。研究表明： 初孵仔鱼 （体长1.66～2.20 mm）的消化道完全处于未分化状态。1 日龄仔鱼全长1.83～2.32 mm, 消化道中部开始形成空腔, 为初始的消化道腔。2 日龄仔鱼全长2.61～2...     

721铀矿可培养嗜酸性微生物多样性分布

嗜酸性微生物是一类生活在极端酸性环境下的微生物,已被广泛用于生物冶金。对721铀矿嗜酸性微生物的群落分布规律进行了初步的研究与探讨。运用双层固体平板培养技术,以琼脂糖为凝固剂,以SJH为底层添加菌,分离样品中的微生物．得到8种嗜酸性微生物,根据不同菌在不同双层固体平板表面的菌落形态特征,并结合16SrRNA基因扩增技术鉴定菌种。占优势的嗜酸性微生物为氧化亚铁硫杆菌（Acidithiobacillus ferrooxidans,简称A．f）.     

Micromorphology of surface crusts in the Knersvlakte, South Africa

Soils in the Knersvlakte are particularly prone to crusting and have lower inherent infiltrability than other soils across western southern Africa. Micromorphological techniques were used to examine the structure and porosity of soil crusts in the Knersvlakte to ascertain why crusting is so intense in this region. Quantile regression using boundary lines was employed to examine the relationships between infiltrability and soil properties for all samples （n = 67）. This analysis showed that infiltrability is potentially maximal at low water- dispersible ＇clay plus silt＇ content and low silt content （r^2 = 0.72 and 0.64; respectively, n = 67） （Figure 2）. The strength of crusts, pH, EC, clay mineralogy, and water-dispersible clay, silt and ＇clay plus silt＇ content were compared, and a pore analysis using optical microscopy was undertaken on images of six soil thin sections （n = 6） （circular and parallel polarizers）. Pore analysis was further undertaken on five horizontal slices of equal dimensions taken through each soil thin section. The porosity samples with low infiltrability （〈 100 mm·hr^-1, n = 4） had greater crust strength, lower porosity （both total and in the least porous slice） and greater water-dispersible ＇day plus silt＇ and silt content than the porosity samples with high infiltrability （〉 100mm·hr^-1, n = 2）. The porosity samples with low infiltrability showed a trend of lower pH and greater water dispersible clay percentage. Porosity varied within the porosity samples due to the presence of dense clay/silt bands （〈 0.5 mm in width） with relatively few air vesicles. The porosity samples with horizontal slices of low porosity （but large numbers of air vesicles） had low infiltrability, while those without slices of low porosity （and relatively few air vesicles） had high infiltrability. We conclude that the intense crusting and resultant low infiltrability of soils in the Knersvlakte appears to be related to the formation of thin, dense clay/silt bands in the pedoderm.     

From alkali basalt to phonolite in hand-size samples: vapor-differentiation effects in the Bouzentès lava flow (Cantal, France)

The Bouzentès lava flow is a 20-m-thick alkali basalt flow emplaced during the last stage of formation of the Cantal stratovolcano at 4.2 Ma. Its upper part has 1- to 20-cm-thick vesicle-rich segregation sheets which recur every 0.1–2 m. These horizontal veins are hawaiitic in composition. They are characterized by hypertrophic development of their minerals (‘pegmatoids’) and by glassy phonolitic segregation vesicles. Internal differentiation within the Bouzentès lava flow was triggered by an unusually high water content, as suggested by pre-emptive iddingsite alteration of olivine phenocrysts. The proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow. Olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles. Most of the features observed in Bouzentès recall the internal differentiation processes usually described within thick Hawaiian lava lakes. The segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process.     

头足类眼睛的研究综述

头足类是现存最古老的海洋生物之一,为视觉性动物,视觉在它们的捕食、避敌、求偶、交配等活动中发挥了重要作用。在长期的进化过程中,头足类的眼睛为它们带来了视觉上的强大优势。目前国内关于头足类眼睛的研究较少,主要集中于头足类眼睛视网膜的研究,国外的研究则主要集中于头足类眼睛的结构特点和一些视觉特性,本文对头足类眼睛的形成、形态、结构、视敏度变化、偏振光敏感性、趋光性及眼外光感受器——感光囊进行了论述。综合分析认为,除鹦鹉螺外,头足类的眼睛发达,虽然它们的眼睛保留了无脊椎动物无法识别颜色这一特性,但已经具备了形成物像,进行屈光和光调节的完善而精细的系统结构,分辨率高,甚至还具有一些高等脊椎动物所不具备的功能,同时还有眼外光感受器——感光囊进行辅助作用。因此,和它们的分类地位一样,头足类的眼睛一定程度上也处于海洋动物由低等进化到高等的一个过渡型的地位。此外,本文也对目前头足类眼睛研究进展的问题和未来研究方向进行了总结和展望。希望可以引起国内外同行的重视,并为相关研究提供借鉴和参考。     

Changing eruptive styles in basaltic explosive volcanism: Examples from Croscat complex scoria cone, Garrotxa Volcanic Field (NE Iberian Peninsula)

The Croscat pyroclastic succession has been analysed to investigate the transition between different eruptive styles in basaltic monogenetic volcanoes, with particular emphasis on the role of phreatomagmatism in triggering Violent Strombolian eruptions. Croscat volcano, an 11 ka basaltic complex scoria cone in the Quaternary Garrotxa Volcanic Field (GVF) shows pyroclastic deposits related both to magmatic and phreatomagmatic explosions.Lithofacies analysis, grain size distribution, chemical composition, glass shard morphologies, vesicularity, bubble-number density and crystallinity of the Croscat pyroclastic succession have been used to characterize the different eruptive styles. Eruptions at Croscat began with fissural Hawaiian-type fountaining that rapidly changed to eruption types transitional between Hawaiian and Strombolian from a central vent. A first phreatomagmatic phase occurred by the interaction between magma and water from a shallow aquifer system at the waning of the Hawaiian- and Strombolian-types stage. A Violent Strombolian explosion then occurred, producing a widespread (8 km²), voluminous tephra blanket. The related deposits are characterized by the presence of wood-shaped, highly vesicular scoriae. Glass-bearing xenoliths (buchites) are also present within the deposit. At the waning of the Violent Strombolian phase a second phreatomagmatic phase occurred, producing a second voluminous deposit dispersed over 8.4 km². The eruption ended with a lava flow emission and consequent breaching of the western-side of the volcano. Our data suggest that the Croscat Violent Strombolian phase was related to the ascent of deeper, crystal-poor, highly vesicular magma under fast decompression rate. Particles and vesicles elongation and brittle failure observed in the wood-shaped clasts indicate that fragmentation during Violent Strombolian phase was enhanced by high strain-rate of the magma within the conduit.     

Hawaiian magma-reservoir processes as inferred from the petrology of gabbro xenoliths in basalt,Kahoolawe Island

Gabbro xenoliths in a tholeiitic lava of Kahoolawe Island, Hawaii, a 1.3–1.4 Ma shield volcano, are 1–3 cm in size and comprised of plagioclase, clinopyroxene, and orthopyroxene. Gabbro textures — while intergranular and in part subophitic-are open due to 28–48 vol.% of vesicular basalt occupying xenolith space. Vesicles in and around the xenoliths are lined or filled with rhyolitic glass (segregation vesicles). The host is evolved tholeiite (MgO 6.1 wt%) with phenocrysts, microphenocrysts, and glomerocrysts of olivine, clinopyroxene, orthopyroxene, and plagioclase, and megacrysts (1 cm) of plagioclase. The Sr-isotope ratio of one xenolith is 0.70489; the host basalt ratio is 0.70460. Xenolith isotope composition, grain resorption, and clinopyroxene (Fs_12.5–15Wo_38–35.5), orthopyroxene (Fs_19.5–24Wo_4.1), and plagioclase (An_68–65Or_0.8–1.2) compositions suggest that these gabbros crystallized from Kahoolawe tholeiitic magma of essentially the same composition as the host basalt, but pre-dating the magma represented by the host. Based on the absence of intergranular Fe–Ti oxide phases from the pl+cpx+opx assemblages, and the open, vuggy textures, we envision crystallization on a reservoir roof at temperatures >1100°C. Entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush/suspension zone occurred during convection associated with replenishment of the magma reservoir. These open-textured gabbro xenoliths are therefore not fragments of preexisting coarse-grained bodies such as sills or segregation veins. Rhyolitic glass in vesicles represents a gas-effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths.Sirrine Environmental Consultants, Fremont, CA 94538     

Scaling vesicle distributions and volcanic eruptions

Models of coalescence-decompressive expansion of the later stages of bubble growth predict that for diverse types of volcanic products the vesicle number densities (n(V)) are of the scaling form where V is the volume of the vesicles and B₃ the 3-dimensional scaling (power law) exponent. We analyze cross sections of 9 pumice samples showing that over the range of bubble sizes from 10 m to 3 cm, they are well fit with B₃0.85. We show that to within experimental error, this exponent is the same as that reported in the literature for basaltic lavas, and other volcanic products. The importance of the scaling of vesicle distributions is highlighted by the observation that they are particularly effective at packing bubbles allowing very high vesicularities to be reached before the critical percolation threshold, a process which—for highly stressed magmas—would trigger explosion. In this way the scaling of the bubble distributions allows them to be key actors in determining the rheological properties and in eruption dynamics.Editorial responsibility: D. Dingwell