Composition and exhalation flux of gases from mud volcanoes in Taiwan

Many mud volcanoes are distributed along the tectonic sutures in southern Taiwan and can be divided into five zones based on their relative positions in different tectonic domains. Most active mud volcanoes are exhaling methane-dominated gases. Nevertheless, some gases show unusual carbon dioxide-dominated and/or nitrogen-excess compositions. This implies that there are multiple sources for the gas compositions of mud volcanoes in Taiwan.For better understanding the total amount of exhalation gases and its flux, the gas flow and compositions were continuously measured in the interval of two minutes at Chung-lun (CL) bubbling mud pool for a few months. The major compositions of gases exhaling from this site were 75~90% of CO₂ and 5~12% of CH₄. The amount of gases exhaling from the mud pool can be estimated to be about 1.4 ton/year for CH₄ and 28 ton/year for CO₂, respectively. The preliminary results of exhaling gas flux from the major vents of representative active mud volcanoes, yielded an estimated total CH₄ output of the mud volcanoes in Taiwan of ca. 29 ton/year during quiescent period.     

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks,Roman Magmatic Province,Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.70926 to 0.70999, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for ²⁰⁶Pb/²⁰⁴Pb, 15.685–15.701 for ²⁰⁷Pb/²⁰⁴Pb, and 39.048–39.076 for ²⁰⁸Pb/²⁰⁴Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. ⁸⁷Sr/⁸⁶Sr ranges from 0.70665 to 0.70745, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51234 to 0.51238, ²⁰⁶Pb/²⁰⁴Pb ranges from 18.924 to 19.153, ²⁰⁷Pb/²⁰⁴Pb ranges from 15.661 to 15.694, and ²⁰⁸Pb/²⁰⁴Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Lichens as bioindicators in volcanic areas: Mt. Etna and Vulcano Island (Italy)

 The aim of this paper is to verify whether lichens have the capacity to accumulate atmospheric contaminators linked to volcanic activity. About 100 lichens were collected in 1994 and 1995 from two active volcanic areas in Italy: Mount Etna and Vulcano Island. Twenty-seven elements were analyzed for each individual lichen using Instrumental Neutronic Activation Analysis and Inductively Coupled Plasma-Mass Spectrometry. Lichen composition reflects the contribution of the volcanic particulate material, and the two areas investigated can be distinguished on the basis of the concentration of some lithophile elements. Moreover, the distribution in lichens of the elements (As, Sb, Br, Pb) – derived from gas emissions (plume, fumaroles) – also shows different geochemical trends on Mt. Etna and Vulcano. Received: 20 April 1998 · Accepted: 4 July 1998     

Stress Field Estimations for Colima Volcano, Mexico, Based on Seismic Data

 For first time, during 1991, seismic activity was recorded during an eruption at Colima volcano. We analyze these data to obtain a stress pattern using a composite focal mechanism technique. From the analysis of regional seismicity, the Tamazula Fault and the Armeria River appear as active features and the dip of the slab east of the Jalisco Block is approximately 12°. Southwest of Colima volcano a vertical alignment of seismic events was observed. We estimate five different composite focal mechanism solutions from our data set, which indicate a change of the stress field at the volcano after the 1991 eruption. These solutions suggest that the stress field in the volcanic edifice was controlled by stresses related to the emplacement of magma superimposed on the regional stress field. No evidence of active local faults in the volcanic edifice was found. We propose a model for the eruptive process that involves tilting of the volcanic edifice. Received: 15 October 1995 / Accepted: 26 October 1998     

吉林龙冈火山群金龙顶子火山喷发物中幔源包体的基本特征及其地质意义

龙冈火山群金龙顶子火山喷发发生在距今1500年前,其火山喷发物中所含的幔源橄榄岩包体是我国在最新火山喷发物中所发现的幔源包体。这些包体以普遍含有韭闪石为特征。包体的结构和位错构造反映这些包体在上地幔条件下经历过一定程度的变形作用。包体的平衡温度大多数集中在800～950℃之间,只有个别样品温度达到1050℃;平衡压力大多数集中在1.0～1.5 GPa之间。由包体平衡温度、压力得到的上地幔地温线稍低于中国东部由新第三纪包体得到的上地幔地温线,但接近第四纪包体得到的地温线。包体在上地幔条件下变形时的差异应力在30～44MPa,应变速率为10-18s-110-15s-1。角闪石的出现反映上地幔流体的渗透及交代作用。金龙顶子火山发的玄武岩浆直接来自35～50km的上地幔顶部。     

美国西部活火山及其灾害考察

美国西部是世界上最不寻常的第四纪火山区之 一,形成多种多样的火山地形和具有不同的岩石成分。本文主要介绍美国西部在过去2 ka内有过爆发活动的11座火山及其灾害,其中包括著名的圣?海伦斯火山。     

利用接收函数方法研究中国东北东南部地区不同构造体的地壳特征

基于我们布设的探测深俯冲的中国东北地震台阵NECsaids台阵和固定地震台长时段的观测记录及NECESSArray流动台阵共计152个台站数据提取得到的33752条P波接收函数,采用H-κ叠加分析和共转换点（CCP）叠加成像等方法进行统一分析处理,并汇集他人接收函数研究结果得到中国东北东南部地区迄今为止最高分辨率的地壳厚度和平均波速比分布图像.对中国东北东南部地区不同构造体的地壳特征综合分析研究表明：研究区不同陆块的地壳属性存在明显差别,张广才岭地块中南部的地壳厚度和波速比与华北克拉通东北缘相当,地壳厚度同地表地形之间显示有明显的正相关关系;松辽地块东南缘地壳最薄、波速比最高,地壳厚度同壳内波速比之间显示出明显的负相关关系;兴凯地块西部地区的地壳结构表现为稍厚的地壳厚度和研究区内最低的壳内波速比,其地壳厚度同壳内波速比之间亦显示出明显的负相关关系;佳木斯地块西南缘在具有"正常"的壳内波速比同时地壳最厚.研究区内的郯庐断裂带北延段在切穿其下Moho面的同时表现出南北分段的特征：北段（44.4°N—47°N）两分支之下的Moho面整体下凹,而南段（41.5°N—43.3°N）两分支之下的Moho面则整体上隆.长白山天池火山下方表现为Moho面下凹沉落及高壳内波速比特征,推测其壳内岩浆囊很可能存在于火山口东北隅至少10 km的范围内.     

长白山天池火山天文峰期黄色浮岩成因研究

长白山天池火口北侧天文峰之上,一套醒目的黄色浮岩引起广泛的关注,其颜色成因问题更是讨论的热点。本文通过野外地质调查、显微形貌和地球化学分析等方法,探索了黄色浮岩的颜色成因问题,并对此次喷发活动(天文峰期喷发)有了更进一步的认识。黄色浮岩与其下部灰白色浮岩应为同一期喷发所形成,两者成分一致且特征相似。黄色浮岩初始颜色为灰白色,后期受所处环境(降水丰富)与本身气孔特征的影响,浮岩内发生了元素析出和元素沉淀的过程。首先,浮岩内Si与H2O结合形成弱硅酸(H2Si O3),而大气中CO2与H2O结合形成弱碳酸(H2CO3),在弱酸环境下火山玻璃逐渐析出Si、K、Al、Ca和Fe等阳离子,而析出的元素易溶于水的部分被流水带走,难溶于水的Fe与Al富集并粘附在火山玻璃壁上,同时由于Fe可与H2O络合形成黄色的Fe的水合物(Fe2O3·n H2O),而Al与H2O络合形成凝胶状白色水合物(Al2O3·n H2O),两者混合形成了黄色胶状物粘附在火山玻璃壁上,改变了浮岩原本的灰白色,形成了黄色浮岩。因此,天文峰期浮岩的黄色是由于后期风化淋滤作用所造成,属于次生色。本研究提高了对火山喷发堆积物风化淋滤作用过程的认识,也为其他地区相似颜色变化问题的讨论提供了借鉴。     

强震观测仪器接地与防雷的思考

就四川省地震局实施的“十五”强震建设项目中的强震仪器Etna的接地和防雷措施进行了探讨.强震仪Etna的接地要独立于其它设备或建筑物接地,而且相互间的距离要大于15米,接地电阻要小于4 欧姆.当相互间的距离小于15米时要增加防雷器,以防止反击雷对于仪器的破坏.在场地无法达到所要求的距离时,则要采用联合接地的办法,以防反击雷造成设备损坏.而且在接地极时不能简单地采用铜板接地,必须严格按照有关标准采用接地网才能真正达到防雷要求。     

petrological and geochemical characteristics of quarternary volcanic rocks in haixing area,eastern north china

Field investigation and lab analysis on samples were carried out for Quaternary volcanoes, including Xiaoshan volcano, Dashan volcano and Bianzhuang hidden volcano, in Haixing area, east of North China. Results show that Xiaoshan volcano with the eruptive material of volcanic scoria, crystal fragments and volcanic ash is a maar volcano, the eruptive pattern is pheatomagmatic eruption, and the influence scope is near the crater. Dashan volcano exploded in the early stage, and then the magma intruded, forming the volcanic neck. The eruption strength and scale are limited, and the eruptive materials are scoria, volcanic agglomerate and dense lava neck. The volcanic rocks in Bianzhuang are porosity and dense volcanic rocks and volcanic breccia, reflecting the pattern of weak explosive eruption and lava flow, and the K-Ar age dating on volcanic rocks indicates that the eruption happened in early Pleistocene. Xiaoshan volcanic scoria and Bianzhuang hidden volcanic rocks are mainly basaltic, Dashan volcanic rocks with lower SiO₂ content are nephelinite in composition. Their oxide contents have no linear relationship, indicating that there is no magma evolution relationship between these magmas from the three places. Three volcanic rocks all have enrichment of light rare earth. The Bianzhuang volcanic rocks are rich in large ion lithophile elements, and have no high field strength elements Zr and Hf, Ti losses. The volcanic materials from Xiaoshan and Dashan are intensively rich in Th, U, Nb and Ta, and significantly poor in K and Ti. Although the magmas from these three places in Haixing area may all come from asthenosphere, the volcanic materials have different petrological and geochemical features, and relatively independent volcanic structures, therefore, they experienced different magma processes.