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71.
Yuri Taran Tobias P. Fischer Boris Pokrovsky Yuji Sano Maria Aurora Armienta Jose Luis Macias 《Bulletin of Volcanology》1998,59(6):436-449
The 1982 eruption of El Chichón volcano ejected more than 1 km3 of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 300-m-deep crater and uncovered the
upper 500 m of an active volcano-hydrothermal system. Instead of the weak boiling-point temperature fumaroles of the former
lava dome, a vigorously boiling crater spring now discharges / 20 kg/s of Cl-rich (∼15 000 mg/kg) and sulphur-poor ( / 200 mg/kg
of SO4), almost neutral (pH up to 6.7) water with an isotopic composition close to that of subduction-type magmatic water (δD=–15‰,
δ18O=+6.5‰). This spring, as well as numerous Cl-free boiling springs discharging a mixture of meteoric water with fumarolic
condensates, feed the crater lake, which, compared with values in 1983, is now much more diluted (∼3000 mg/kg of Cl vs 24 030 mg/kg),
less acidic (pH=2.6 vs 0.56) and contains much lower amounts of S ( / 200 mg/kg of SO4, vs 3550 mg/kg) with δ34S=0.5–4.2‰ (+17‰ in 1983). Agua Caliente thermal waters, on the southeast slope of the volcano, have an outflow rate of approximately
100 kg/s of 71 °C Na–Ca–Cl water and are five times more concentrated than before the eruption (B. R. Molina, unpublished
data). Relative N2, Ar and He gas concentrations suggest extensional tectonics for the El Chichón volcanic centre. The 3He/4He and 4He/20Ne ratios in gases from the crater fumaroles (7.3Ra, 2560) and Agua Caliente hot springs (5.3Ra, 44) indicate a strong magmatic contribution. However, relative concentrations of reactive species are typical of equilibrium
in a two-phase boiling aquifer. Sulphur and C isotopic data indicate highly reducing conditions within the system, probably
associated with the presence of buried vegetation resulting from the 1982 eruption. All Cl-rich waters at El Chichón have
a common source. This water has the appearence of a "partially matured" magmatic fluid: condensed magmatic vapour neutralized
by interaction with fresh volcaniclastic deposits and depleted in S due to anhydrite precipitation. Shallow ground waters
emerging around the volcano from the thick cover of fresh pumice deposits (Red waters) are Ca–SO4–rich and have a negative oxygen isotopic shift, probably due to ongoing formation of clay at low temperatures.
Received: 21 July 1997 / Accepted: 4 December 1997 相似文献
72.
甘肃公婆泉铜矿田中—晚志留世浅海相火山喷发旋回及火山作用演化 总被引:9,自引:2,他引:7
根据甘肃公婆泉铜矿田内4条典型地层剖面,确立了甘肃公婆泉群中-上志留统浅海相火山喷发沉积岩系的标准层和层序,并将其划分为14个韵律和5个旋回。火山喷发旋回由早→晚,火山岩岩性由安山质凝灰岩、安山岩→英安岩、钠流纹岩→石英粗面岩→安山岩,火山岩中铁族元素的丰度由高→低→最低(在石英粗面岩中)→高,形成一个较完整的演化周期。中晚期火山岩所表现出的岩性(出现碱性岩-石英粗面岩)和铁族元素丰度(出现最低值)的突变与区域北西走向断裂的深切活动有关,为铜、银等金属成矿提供主要的地球化学条件。铜、银等成矿元素主要富集在火山喷发旋回中期的英安岩中,表明火山活动中期是主要的成矿期,英安岩可作为重要的找矿标志。此外,早中期火山旋回主要发育于矿田的中西部,而晚期旋回则主要分布在矿田的东部,并且火山喷发的强度和频率增高,表明晚期火山活动中心已明显向东部迁移,因此矿田的东部仍有较大的找矿潜力 相似文献
73.
Agronomic impact of tephra fallout from the 1995 and 1996 Ruapehu Volcano eruptions, New Zealand 总被引:2,自引:0,他引:2
Eruptions from Ruapehu Volcano on 11 and 14 October 1995 and 17 June 1996 distributed at least 36×106 m3 of sulphur(S)-rich tephra over the central and eastern North Island of New Zealand. The tephras added between 30–1500 kg
ha–1 S to at least 25 000 km2 of land in primary production. Smaller but beneficial amounts of selenium (Se) and in some areas potassium and magnesium
were also supplied. Addition of S to the soils in the form of sulphate and elemental S resulted in a drop in soil pH and an
increase in pasture S contents within seven weeks of the eruptions. The soils affected by the tephra are naturally low in
S and Se, but following the eruptions S was not required in fertilizer applications in many areas. The strongest and longest
lasting effects of S and Se deposition were in high anion-retention soils particularly Hapludands (moist, moderately weathered
soils, derived from volcanic ash). Soluble fluorine concentrations within the tephras were low compared to historic Icelandic
and Chilean examples. However, pastoral livestock deaths were apparently caused by fluorosis in addition to starvation when
tephra covered feed. The Ruapehu tephra contained very low concentrations of other soluble toxic elements.
Received: 17 January 1997 · Accepted: 31 March 1997 相似文献
74.
长白山天池火山研究进展 总被引:25,自引:0,他引:25
长白山天池火山距今4105年以来至少发生过两次大规模喷发。其最近的一次大喷发发生于公元850~1040年间。估计了这次喷发的喷发物体积和C1,F,S等的释放量。数值模拟表明,该次大喷发对全球气候变化产生过重要影响,最近的地球物理探测结果显示,在该火山下面存在地壳和上地幔双层岩浆房。认为该火山现在仍是一座具潜在灾害性大喷发危险的现代活动火山。 相似文献
75.
The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main
deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced
varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these
units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and
scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial
age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller
Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together
cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as
well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated
with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial
lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding
lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y.
Received: 24 June 1996 / Accepted: 6 December 1996 相似文献
76.
Stephen R. Hammond 《Bulletin of Volcanology》1997,58(8):617-627
Forty-three volcanoes located along the flanks of the Juan de Fuca Ridge were selected to study relationships between their
morphologies and off-axis magmatic processes. The volcanoes occur both in chains consisting of up to seven distinct cones
and isolated edifices. Nearly all of the volcanoes are circular, truncated cones with steep flanks and large, relatively flat
summit plateaus. In addition, most of these volcanoes also have prominent and distinctly offset calderas or craters. The most
striking characteristic of the volcanoes' morphology is that nearly all of their collapse structures are located on the sides
of the volcanoes which face the Juan de Fuca Ridge and many are breached with openings toward the ridge. A simple model based
on these observations accounts for these ridge-facing features. As plate motion transports a volcano away from its magma source
beneath the lithosphere, the volcano's magma supply conduits tend to lag behind. Eventually these conduits are abandoned and
ridgeward collapse structures are formed. It can be inferred from the model that, on average, individual volcanoes were active
for approximately 50 000 years and that most eruptions took place early in this interval. If most of the cone-building eruptions
occurred during the first thousand years or so, associated hydrothermal activity may have temporarily rivaled the present-day
yearly time-averaged hydrothermal output along the entire Juan de Fuca ridge axis.
Received: 1 September 1996 / Accepted: 13 January 1997 相似文献
77.
A new airborne radar technique can generate digital topographic data for volcanoes at a scale of 10 m spatial and 1–5 m vertical, with a swath width of 6.4 km. Called TOPSAR, the intrument is an interferometric radar flown on the NASA DC-8 aircraft. TOPSAR data permit the quantification of volcano slopes, volumes, and heights, and as such will be valuable for the analysis of lava flows, domes, and lahar channels. This instrument will be flown over several volcanoes in the near future, providing volcanologists with valuable data sets for the analysis of high-resolution topography. We briefly illustrate the potential use of TOPSAR data through examples from Mt Somma and Vesuvius, Italy. 相似文献
78.
Geochemical Constraints on Possible Subduction Components in Lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines 总被引:5,自引:0,他引:5
Mayon is the most active volcano along the east margin of southernLuzon, Philippines. Petrographic and major element data indicatethat Mayon has produced a basaltic to andesitic lava seriesby fractional crystallization and magma mixing. Trace elementdata indicate that the parental basalts came from a heterogeneousmantle source. The unmodified composition of the mantle wedgeis similar to that beneath the Indian Ocean. To this mantlewas added a subduction component consisting of melt from subductedpelagic sediment and aqueous fluid dehydrated from the subductedbasaltic crust. Lavas from the highly active Taal Volcano onthe west margin of southern Luzon are compositionally more variablethan Mayon lavas. Taal lavas also originated from a mantle wedgemetasomatized by aqueous fluid dehydrated from the subductedbasaltic crust and melt plus fluid derived from the subductedterrigenous sediment. More sediment is involved in the generationof Taal lavas. Lead isotopes argue against crustal contamination.Some heterogeneity of the unmodified mantle wedge and differencesin whether the sediment signature is transferred into the lavasource through an aqueous fluid or melt phase are needed toexplain the regional compositional variation of Philippine arclavas. KEY WORDS: Mayon Volcano; Philippines; sediment melt; subduction component; Taal Volcano 相似文献
79.
80.
Geology and geochemistry of basaltic lava flows and dikes from the Trans-Koolau tunnel, Oahu, Hawaii
A 200-m section of Koolau basalt was sampled in the 1.6-km Trans-Koolau (T–K) tunnel. The section includes 126 aa and pahoehoe
lava flows, five dikes and ten thin ash units. This volcanic section and the physical characteristics of the lava flows indicate
derivation from the nearby northwest rift zone of the Koolau shield. The top of the section is inferred to be 500–600 m below
the pre-erosional surface of the Koolau shield. Therefore, compared with previously studied Koolau lavas, this section provides
a deeper, presumably older, sampling of the shield. Shield lavas from Koolau Volcano define a geochemical end-member for Hawaiian
shields. Most of the tunnel lavas have the distinctive major and trace element abundance features (e.g. relatively high SiO2 content and Zr/Nb abundance ratio) that characterize Koolau lavas. In addition, relative to the recent shield lavas erupted
at Kilauea and Mauna Loa volcanoes, most Koolau lavas have lower abundances of Sc, Y and Yb at a given MgO content; this result
is consistent with a more important role for residual garnet during the partial melting processes that created Koolau shield
lavas. Koolau lavas with the strongest residual garnet signature have relatively high 87Sr/86Sr, 187Os/188Os, 18O/16O, and low 143Nd/144Nd. These isotopic characteristics have been previously interpreted to reflect a source component of recycled oceanic crust
that was recrystallized to garnet pyroxenite. This component also has high La/Nb and relatively low 206Pb/204Pb, geochemical characteristics which are attributed to ancient pelagic sediment in the recycled crust. Although most Koolau
lavas define a geochemical endmember for Hawaiian shield lavas, there is considerable intrashield geochemical variability
that is inferred to reflect source characteristics. The oldest T–K tunnel lava flow is an example. It has the lowest 87Sr/86Sr, Zr/Nb and La/Nb, and the highest 143Nd/144Nd ratio found in Koolau lavas. In most respects it is similar to lavas from Kilauea Volcano. Therefore, the geochemical characteristics
of the Koolau shield, which define an end member for Hawaiian shields, reflect an important role for recycled oceanic crust,
but the proportion of this crust in the source varied during growth of the Koolau shield.
Received: 1 June 1998 / Accepted: 30 August 1998 相似文献