Trace element abundances of high-MgO glasses from Kilauea, Mauna Loa and Haleakala volcanoes, Hawaii

We performed an ion-microprobe study of eleven high-MgO (6.7–14.8 wt%) tholeiite glasses from the Hawaiian volcanoes Kilauea, Mauna Loa and Haleakala. We determined the rare earth (RE), high field strength, and other selected trace element abundances of these glasses, and used the data to establish their relationship to typical Hawaiian shield tholeiite and to infer characteristics of their source. The glasses have trace element abundance characteristics generally similar to those of typical shield tholeiites, e.g. L(light)REE/H(heavy)REE_C1 < 1. The Kilauea and Mauna Loa glasses, however, display trace and major element characteristics that cross geochemical discriminants observed between Kilauea and Mauna Loa shield lavas. The glasses contain a blend of these discriminating chemical characteristics, and are not exactly like the typical shield lavas from either volcano. The production of these hybrid magmas likely requires a complexly zoned source, rather than two unique sources. When corrected for olivine fractionation, the glass data show correlations between CaO concentration and incompatible trace element abundances, indicating that CaO may behave incompatibly during melting of the tholeiite source. Furthermore, the tholeiite source must contain residual garnet and clinopyroxene to account for the variation in trace element abundances of the Kilauea glasses. Inversion modeling indicates that the Kilauea source is flat relative to C1 chondrites, and has a higher bulk distribution coefficient for the HREE than the LREE. Received: 4 February 1997 / Accepted: 27 August 1997     

Sublithospheric flows in the mantle

The estimated rates of upper mantle sublithospheric flows in the Hawaii–Emperor Range and Ethiopia–Arabia–Caucasus systems are reported. In the Hawaii–Emperor Range system, calculation is based on motion of the asthenospheric flow and the plate moved by it over the branch of the Central Pacific plume. The travel rate has been determined based on the position of variably aged volcanoes (up to 76 Ma) with respect to the active Kilauea Volcano. As for the Ethiopia–Arabia–Caucasus system, the age of volcanic eruptions (55–2.8 Ma) has been used to estimate the asthenospheric flow from the Ethiopian–Afar superplume in the northern bearing lines. Both systems are characterized by variations in a rate of the upper mantle flows in different epochs from 4 to 12 cm/yr, about 8 cm/yr on average. Analysis of the global seismic tomographic data has made it possible to reveal rock volumes with higher seismic wave velocities under ancient cratons; rocks reach a depth of more than 2000 km and are interpreted as detached fragments of the thickened continental lithosphere. Such volumes on both sides of the Atlantic Ocean were submerged at an average velocity of 0.9–1.0 cm/yr along with its opening. The estimated rates of the mantle flows clarify the deformation properties of the mantle and regulate the numerical models of mantle convection.     

火山学述评

火山学研究有了长足的进步。笔者总结近些年全球火山学研究各个方面的成果,包括对火山基本概念的新的认识、火山机构、火山的各种分类、火山岩石学和地球化学、火山岩相学、评估火山爆发大小的火山爆发指数、岩石和地球化学分类、各种常量和微量元素区分图、活火山分布与板块构造理论的关系、活火山给人类带来的灾害与利益和活火山的监测、曾经的火山活动与生物毁灭、单成因火山研究等。火山—构造是未来火山学研究的一个方向,通过火山与构造关系的研究以揭示火山的分布和地球的演化。火山喷出的岩浆是其通过地下以岩墙或管道形式为通道运移到地表的结果。中朝边境上的长白山的位置是个特例,应当值得深入的研究。中国分布有许多新生代火山,它们是否为单成因火山、这些火山在成分上是否有演化规律、它们的分布与大地构造的关系等都有待深入和系统的研究。     

Boron isotopic constraints on the source of Hawaiian shield lavas

Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original ¹¹B/¹⁰B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, ¹¹B/¹⁰B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive ¹¹B/¹⁰B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ¹¹B values for the shield lavas are −4.5 to −5.4‰ for Koolau (N = 8), −3.6 to −4.6‰ for Kilauea (N = 11), and −3.0 to −3.8‰ for Mauna Loa (N = 6).Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ¹¹B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ¹¹B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau—decreasing δ¹¹B coupled with decreasing εNd as well as increasing ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb—is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas.     

Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

Multi-scale topographic analysis of Merbabu and Merapi volcanoes using wavelet decomposition

With the increasing availability of data, geo-sciences have experienced deep changes in handling and processing it. One of the presently explored research directions concerns the systematic decomposition and understanding of topographical features, without the subjective interactions of humans. This can eventually lead to fully automated algorithms for topographic analysis and understanding. This paper aims at being a contribution to this broad research area for the specific cases of stratovolcanoes, whose general geometry are very similar to a perfect cone. More specifically, this paper addresses two issues: (1) is it possible to separate erosion features (local variations) from structural features (large variations) on stratovolcanoes, through mathematical expression; (2) can information on volcanic activity??intensity, age, etc.??be retrieved from a topographic analysis? The study has been conducted from two volcanoes in Central Java (Indonesia): the Merapi and the Merbabu. The DEM of these volcanoes has been sampled using concentric circles with a radius ranging from 500 to 5,000?m (horizontal distance) to the summit. The data conversion and sampling was performed in ArcMap^?, while the data analysis was carried out with Matlab^?, using Discrete Meyer wavelet decomposition. Results provide an insight on large-scale topographic variations (long-wave wavelet) that have been separated from rapidly varying topographic features such as lahar channels (short-wave wavelets). Observations proved that flanks where the most recent volcanic activity occur??like at Merapi Volcano on the S-SE flank??present a very low variability of long-wave variations, whereas short-wave variations are important. The author argues that this feature is due to highly erosive lahars that dig the valleys combined with a recent production of material and volcanic growth keeping the overall structure regular. Flanks with lesser activity are characterized at the two volcanoes by important long-wave variations??most certainly due to long-term differential erosion??and different level of short-waves variations. Comparing the two volcanoes, results show that the valleys of Merapi and Merbabu volcanoes are deeply incised, indicating recent periods of high activity, with reworkable material eroded by lahars and other channels deepening processes. The topography of the summit area of Merapi Volcano is smoother than at Merbabu Volcano, where deep erosion features extend up to the summit area. This difference is most certainly due by the material production at the summit of Merapi Volcano. Developing such classification is important for automated mapping and computer recognition of volcanic past activities and their impacts on landscapes. It is the based for the development of decision trees that assist computer assisted and automated computer vision.     

Tungsten in Hawaiian picrites: A compositional model for the sources of Hawaiian lavas

Concentrations of tungsten (W) and uranium (U), which represent two of the most highly incompatible elements during mantle melting, have been measured in a suite of Hawaiian picrites and primitive tholeiites from nine main-stage shield volcanoes. Tungsten abundances in the parental melts are estimated from correlations between sample W abundances and MgO contents, and/or by olivine correction calculations. From these parental melt determinations, along with independent estimates for the degree of partial melting at each volcanic center, we extrapolate the W content of the mantle sources for each shield volcano. The mantle sources of Hualalai, Mauna Loa, Kohala, Kilauea, Mauna Kea, Koolau and Loihi contain 9 ± 2 (2σ), 11 ± 5, 10 ± 4, 12 ± 4, 10 ± 5, 8 ± 7 and 11 ± 5 ng/g, respectively. When combined, the mean Hawaiian source has an average of 10 ± 3 ng/g W, which is three-times as enriched as the Depleted MORB Mantle (DMM; 3.0 ± 2.3 ng/g).The relatively high abundances of W in the mantle sources that contribute to Hawaiian lavas may be explained as a consequence of the recycling of W-rich oceanic crust and sediment into a depleted mantle source, such as the depleted MORB mantle (DMM). However, this scenario requires varying proportions of recycled materials with different mean ages to account for the diversity of radiogenic isotope compositions observed between Kea- and Loa-trend volcanoes. Alternatively, the modeled W enrichments may also reflect a primary source component that is less depleted in incompatible trace elements than the DMM. Such a source would not necessarily require the addition of recycled materials, although the presence of some recycled crust is permitted within our model parameters and likely accounts for some of the isotopic variations between volcanic centers.The physical admixture of ?0.5 wt.% outer core material with the Hawaiian source region would not be resolvable via W source abundances or W/U ratios; however, W isotopes may provide a more sensitive to this mixing process. Recent W isotopic studies show no indication of core-mantle interaction, indicating that either such a process does not occur, or that mechanisms other than physical mixing may operate at the core-mantle boundary.     

Geochemistry of gas emanations: A case study of the Réunion Hot Spot,Indian Ocean

The results of analysis of natural emanations in Réunion Island show a clear magmatic origin for CO₂ and He, while N₂ and Ar are predominantly derived from the atmosphere. The distribution of magmatic gases in the Piton des Neiges massif fits the local volcanotectonic context well and suggests that the areas concerned are still subject to volcanic activity at depth. A simple method is proposed for correcting gas concentration and isotope composition for water degassing. In doing so, the isotope and elemental (C, He) composition of gases is homogeneous for the two volcanoes. The isotope ratio of He (12.5 ± 0.5R/Ra) in the present discharges is in agreement with the results of previous studies on rocks of various ages from the two volcanoes. The isotope ratio of C(δ¹³C= −5 ‰ to −4 ‰ vs PDB) and the C/³ He ratio (∼4 × 10⁹) are similar to those found in other Hot Spot volcanic systems such as Kilauea (Hawaii) and Hengill (Iceland). These similarities suggest comparable volatile history for the respective mantle sources, the main differences being in the relative proportions of radiogenic ⁴He. In detail, Hot Spots appear enriched in C having a light composition with respect to MORB, possibly due to the addition of a C-rich (e.g. subducted) component, in addition to a relatively undegassed, ³He-rich, component.     

A fast GIS-based risk assessment for tephra fallout: the example of Cotopaxi volcano, Ecuador-Part II: vulnerability and risk assessment

In order to develop efficient strategies for risk mitigation and emergency management, planners require the assessment of both the expected hazard (frequency and magnitude) and the vulnerability of exposed elements. This paper presents a GIS-based methodology to produce qualitative to semi-qualitative thematic risk assessments for tephra fallout around explosive volcanoes, designed to operate with datasets of variable precision and resolution depending on data availability. Due to the constant increase in population density around volcanoes and to the wide dispersal of tephra from volcanic plumes, a large range of threats, such as roof collapses, damage to crops, blockage of vital lifelines and health problems, concern even remote communities. To address these issues, we have assessed the vulnerability and the risk levels for five themes relevant to tephra fallout: (1) social, (2) economic, (3) environmental, (4) physical and (5) territorial. Risk and vulnerability indices for each theme are averaged to the fourth level of administrative unit (parroquia, parish). In a companion paper, Biass and Bonadonna (this volume) present a probabilistic hazard assessment for tephra fallout at Cotopaxi volcano (Ecuador) using the advection-diffusion model TEPHRA2, which is based on field investigations and a global eruption database (Global Volcanism Program, GVP). The scope of this paper is to present a new approach to risk assessment specifically designed for tephra fallout, based on a comprehensive hazard assessment of Cotopaxi volcano. Our results show that an eruption of moderate magnitude (i.e. VEI 4) would result in the possible collapse of ??9,000 houses in the two parishes located close to the volcano. Our study also reveals a high risk on agriculture, closely linked to the economic sector, and a possible accessibility problem in case of an eruption of any size, as tephra is likely to affect the only major road running from Quito to Latacunga (Panamerican Highway). As a result, this method fits into the ongoing effort to better characterize and evaluate volcanic risk, and more specifically the risk associated with tephra fallout. Although this methodology relies on some assumptions, it can serve as a rapid and efficient starting point for further investigations of the risk level around explosive volcanoes.     

Geochemical similarities between the Vesuvius, Phlegraean Fields and Stromboli Volcanoes: petrogenetic, geodynamic and volcanological implications

Summary Vesuvius and Stromboli are two active and extensively studied volcanoes that traditionally have been considered as having different styles of eruption, rock composition and tectonic setting. Data reveal close compositional affinities between these two volcanoes. The abundant 13–15 Ka old Stromboli leucite-tephritic rocks have radiogenic isotope signatures, and abundances and ratios of incompatible elements with the exception of Rb and K, which are identical to those of Vesuvius. The Phlegraean Fields also show close affinities to these volcanoes. The similarity between Stromboli leucite-tephrites and Vesuvius rocks cannot be the result of low pressure processes, given the differences between the two volcanoes in terms of structural features, eruptive behaviour and type of basement rocks. Instead, the observed geochemical signatures are likely to represent a primary magma composition and reveal a common homogeneous source for the two suites. The higher K and Rb contents in the Vesuvius rocks suggest either selective enrichment during magma ascent or a role for phlogopite melting during mantle anatexis. The most primitive rocks from Vesuvius, Phlegraean Fields and Stromboli reveal intermediate compositions between arc and intraplate volcanics. It is suggested that the mantle sources beneath these volcanoes consist of a mixture of intraplate- and slab-derived components. Intraplate material was probably provided by inflow of asthenosheric mantle into the wedge above the subducting Ionian Sea plate, either from the Apulian plate and/or from the Tyrrhenian Sea region. Fluids or melts released from the sinking slab and associated sediments generated metasomatic modification of the intraplate material, whose melting gave rise to the Stromboli, Vesuvius and Phlegraean Fields magmas. The present study demonstrates how comparative investigations of various volcanic centres from southern Italy allow clarification of a number of problems involving magma genesis and evolution, composition of mantle sources and geodynamic significance, which have been long debated and are difficult to solve if individual volcanoes are considered in isolation. Received July 20, 2000; revised version accepted March 19, 2001     

Comparison of seismic activity for Llaima and Villarrica volcanoes prior to and after the Maule 2010 earthquake

Llaima and Villarrica are two of the most active volcanoes in the Chilean Southern Volcanic Zone and presently show contrasting types of activity. Llaima is a closed vent edifice with fumarolic activity, while Villarrica has an open vent with a lava lake, continuous degassing and tremor activity. This study is focused on characterizing the relationships between volcanic and seismic activity in the months before and after the 2010 M8.8 Maule earthquake, which was located in NNW direction from the volcanoes. Time series for tremors, long-period and volcano-tectonic events were obtained from the catalogue of the Volcanic Observatory of the Southern Andes (OVDAS) and from the SFB 574 temporary volcanic network. An increase in the amount of tremor activity, long-period events and degassing rates was observed at Villarrica weeks before the mainshock and continued at a high level also after it. This increase in activity is interpreted to be caused by enhanced magma influx at depth and may be unrelated to the Maule event. In Llaima, an increase in the volcano-tectonic activity was observed directly after the earthquake. The simultaneous post-earthquake activity at both volcanoes is consistent with a structural adjustment response. Since this enhanced activity lasted for more than a year, we suggest that it is related to a medium-term change in the static stress. Thus, the Maule earthquake may have affected both volcanoes, but did not trigger eruptions, from which we assume that none of the volcanoes were in a critical state.     

Ages,rare-earth element enrichment,and petrogenesis of tholeiitic and alkalic basalts from Kahoolawe Island,Hawaii

Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K–Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO₂ than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.     

Gravitational spreading and formation of new rift zones on overlapping volcanoes

Large volcanic edifices are often shaped by the coalescence of adjacent volcanoes as well as intrusive rift zones and gravitational spreading. To better understand the structure of such volcanoes we designed analogue experiments simulating gravitational spreading of an edifice made by overlapping cones of different age, and examined the formation of rift zones. The results allow distinction of two main rift geometries. (i) Spreading edifices of similar age that partly overlap, tend to develop a rift zone approximately perpendicular to the boundary of both volcanoes. Such a rift zone causes two volcanoes to grow together and form an elongated topographic ridge. (ii) Partly overlapping volcanoes of different age are spreading at different rates and thus form a rift zone parallel to the boundary of both volcanoes. Such a rift zone causes two volcanoes to structurally separate. The results are widely applicable for large volcanoes subject to rifting and flank spreading, which we demonstrate for Réunion Island and for southern Hawaii.     

河床底部沙火山特征、演化及其研究意义

沉积后尚未固结的砂体在外部动力的干扰下容易发生液化,形成沙火山、液化砂岩脉等沉积构造,地震是触发液化的最常见动力.在河床现代沉积中发现大量的沙火山构造,其产生过程虽受人为活动影响,形成条件却在盆地特殊构造部位可以满足,分析其特征、演化及动力可以为震积岩及其他成因软沉积变形研究提供参考.研究结果显示,不同规模及对应特征的...     

Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes

The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.     

斑点火山的形成机制和岩石圈-软流圈边界(LAB)的性质

近二十年来,在俯冲带外缘发现的斑点火山群代表了一种全新的海底岩浆活动类型.这种火山规模很小,成簇出现,年龄异常年轻,岩样以EM1型碱性玄武岩为主,孔隙度高且富含挥发性组分.有关斑点火山的岩浆起源以及与岩浆上涌相关的动力学过程目前仍然存在广泛的争论.本文系统性地介绍了斑点火山的特征,总结了前人针对其形成机制和岩浆源区研究...     

The Petrology of Moroto Mountain, Eastern Uganda, and the Origin of Nephelinites

Moroto Mountain is one of the Tertiary alkaline volcanoes ofeastern Uganda. Two differentiated series of alkaline undersaturatedrocks occur in the volcano: a nephelinite series which is commonto all the volcanoes of the province, and an alkaline olivinebasalt series which occurs only at Moroto. It is proposed that the nephelinite series derives from theincongruent melting of pargasite. Since pargasite occurs inhydrated ultrabasic rocks, it is also suggested that the twoseries are formed from the partial melting of hydrated peridotite:the nephelinites from the homblendic portions and the alkalineolivine basalts from the less volatile-rich portions. The two series of the Moroto volcano typify the two major associationsof East African alkaline vulcanicity and, spatially, the volcanolies between regions each characterized by one of these twomajor associations. It may be that the volcano lies above azone of transition in the upper mantle between a relativelyanhydrous, volatile-poor portion and a hydrated, volatile-richportion where hornblendites are developed.     

关联规则候选项支持频度的研究

总结了支持频度的性质,提出了独立支持频度、固定项集频度守恒性与差值支持频度等相关定义与性质,这些性质真实再现了关联规则父子频度项集间的关系。在差值支持频度的引用,充分利用差值支持频度在关联规则挖掘中的重要性,使得算法能够更加精确的预测候选项集的支持频度,从而实现再次减少候选项集的数目的目的。     

基于地下电性结构探讨中国东北活动火山形成机制

东北地区是我国现代火山活动最强烈的地区之一，也是许多学者十分关注的地区。本文回顾了前人提出的关于该地区火山成因的研究成果；通过分析在东北活动火山区大地电磁观测研究的地壳上地幔结构和采用大地电磁网观测研究的地幔1000km以上的电性结构成果，发现长白山天池火山区存在地壳岩浆囊，其它活动火山没有发现地壳岩浆囊，但都存在通往地幔的岩浆通道；东北地区在80～120km左右和200～250km可能存在与地幔岩浆囊相关的地幔高温流体。基于电性结构的研究成果，作者提出了一种东北地区可能的活动火山成因假说。认为东北火山的成因可能与西太平洋板块俯冲到中国东北地区的地幔过渡带后产生脱水有密切关系。这种水以矿物组分或流体方式向上运移，在地幔200-250km和80～120km左右聚集，80～120km的聚集区可能是火山喷发的物质来源。     

Simple mixing as the major control of the evolution of volcanic suites in the Ecuadorian Andes

Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations. The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust.