共查询到20条相似文献,搜索用时 265 毫秒
1.
Richard J. Stevenson † Donald B. Dingwell Nikolai S. Bagdassarov Curtis R. Manley 《Bulletin of Volcanology》2001,63(4):227-237
Hazardous explosive activity may sporadically accompany the extrusion of silicic lava domes. Modelling of the emplacement of silicic domes is therefore an important task for volcanic hazard assessment. Such modelling has been hampered by a lack of a sufficiently accurate rheological database for silicic lavas with crystals and vesicles. In the present study, the parallel-plate viscometry method was applied to determine the shear viscosity of five natural rhyolitic samples from a vertical section through the Ben Lomond lava dome, Taupo Volcanic Centre, New Zealand. Rheological measurements were performed at volcanologically relevant temperatures (780-950°C) and strain rates (10-5-10-7 s-1). Although these samples are in the metastable state, viscosity determinations, melt composition, as well as water and crystal contents of samples were demonstrably stable during experiments. For samples containing up to 5 vol.% microlites, the composition of the melt, rather than the physical effect of suspended crystals, had greater influence on the effective viscosity of the silicic magma. Samples with 10 vol.% microlites and containing a flow banding defined by microlites show no significant orientational effects on apparent viscosity. The rheological measurements were used together with a simple cooling model to construct thermal and viscosity profiles revealing conditions during the emplacement of the Ben Lomond lava dome. 相似文献
2.
To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians
of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures
(650–950 °C), stresses (103–105 Pa) and strain rates (10–5 to 10–7 s–1). The glass transition temperatures (T
g
) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the
equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing
samples from the T
g
data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing
samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of <5 vol.% rod-like
microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with
identical bulk chemistry show a difference of 0.6 log10 units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The
only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.%
branching crystals, and a peralkaline rhyolite containing crystal-rich bands with >45 vol.% crystals. For both of these samples
a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence
the apparent viscosity.
Received: 14 March 1996 / Accepted: 30 May 1996 相似文献
3.
Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā 总被引:1,自引:1,他引:0
Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling
and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150 °C) cooled and crystallized
rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14 °C over the first 2 km of transport.
At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50 °C/h. Cooling rates this high can be explained by
radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel.
Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50%
per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼104/cm3s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation
delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due
to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km
from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137 °C and crystallinity
of ∼15%, and a flow surface temperature of ∼1100 °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins,
where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that
the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield
strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation
in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass
crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite
poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields
are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava
issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization.
Received: 3 September 1998 / Accepted: 12 April 1999 相似文献
4.
Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986–88, 1992–93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts+microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab63An29Or8) surrounded by alkali feldspar rims (Ab53An5Or42), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock.As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (PH2O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992–93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic. 相似文献
5.
Jacob B. Lowenstern Heather Bleick Jorge A. Vazquez Jonathan M. Castro Peter B. Larson 《Bulletin of Volcanology》2012,74(10):2303-2319
We investigated the distribution of Cl, F, Li, and Be in pumices, obsidians, and crystallized dome rocks at Chaitén volcano in 2008?C2009 in order to explore the behavior of these elements during explosive and effusive volcanic activity. Electron and ion microprobe analyses of matrix and inclusion glasses from pumice, obsidian, and microlite-rich dome rock indicate that Cl and other elements were lost primarily during crystallization of the rhyolitic dome after it had approached the surface. Glass in pumice and microlite-free obsidian has 888?±?121?ppm Cl, whereas residual glass in evolved microlite-rich dome rock generally retains less Cl (as low as <100?ppm). Estimated Cl losses were likely >0.7?Mt Cl, with a potential maximum of 1.8?Mt for the entire 0.8-km3 dome. Elemental variations reflect an integrated bulk distribution ratio for Cl?>?1.7 (1.7 times more Cl was degassed or incorporated into crystals than remained in the melt). Because Cl is lost dominantly as the very last H2O is degassed, and Cl is minimally (if at all) partitioned into microlites, the integrated vapor/melt distribution ratio for Cl exceeds 200 (200 times more Cl in the evolved vapor than in the melt). Cl is likely lost as HCl, which is readily partitioned into magmatic vapor at low pressure. Cl loss is accelerated by the change in the composition of the residual melt due to microlite growth. Cl loss also may be affected by open-system gas fluxing. Integrated vapor-melt distribution ratios for Li, F, and Be all exceed 1,000. On degassing, an unknown fraction of these volatiles could be immediately dissolved in rainwater. 相似文献
6.
Robert B. Watts Shanaka L. de Silva Guillermina Jimenez de Rios Ian Croudace 《Bulletin of Volcanology》1999,61(4):241-264
The Cerro Chascon-Runtu Jarita Complex is a group of ten Late Pleistocene (∼85 ka) lava domes located in the Andean Central
Volcanic Zone of Bolivia. These domes display considerable macroscopic and microscopic evidence of magma mixing. Two groups
of domes are defined chemically and geographically. A northern group, the Chascon, consists of four lava bodies of dominantly
rhyodacite composition. These bodies contain 43–48% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole in
a microlite-poor, rhyolitic glass. Rare mafic enclaves and selvages are present. Mineral equilibria yield temperatures from
640 to 750 °C and log ƒO2 of –16. Geochemical data indicate that the pre-eruption magma chamber was zoned from a dominant volume of 68% to minor amounts
of 76% SiO2. This zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions.
The mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas. A southern
group, the Runtu Jarita, is a linear chain of six small domes (<1 km3 total volume) that probably is the surface expression of a dike. The five most northerly domes are composites of dacitic
and rhyolitic compositions. The southernmost dome is dominantly rhyolite with rare mafic enclaves. The composite domes have
lower flanks of porphyritic dacite with ∼35 vol.% phenocrysts of plagioclase, orthopyroxene, and hornblende in a microlite-rich,
rhyodacitic glass. Sieve-textured plagioclase, mixed populations of disequilibrium plagioclase compositions, xenocrystic quartz,
and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid. The central cores of the composite
domes are rhyolitic and contain up to 48 vol.% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole. This
is separated from the dacitic flanks by a banded zone of mingled lava. Macroscopic, microscopic, and petrologic evidence suggest
scavenging of phenocrysts from the silicic lava. Mineral equilibria yield temperatures of 625–727 °C and log ƒO2 of –16 for the rhyolite and 926–1000 °C and log ƒO2 of –9.5 for the dacite. The rhyolite is zoned from 73 to 76% SiO2, and fractionation within the rhyolite composition produced this variation. Most of the 63–73% SiO2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite. Eruption
of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile
and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects
of recharge by mafic magmas. For the Runtu Jarita group, the eruption is best explained by intrusion of a dike of dacite into
a chamber of crystal-rich rhyolite close to its solidus. The rhyolite was encapsulated and transported to the surface by the
less-viscous dacite magma, which also acted as a lubricant. Simultaneous effusion of the lavas produced the composite domes,
and their zonation reflects the subsurface zonation. The role of recharge by hotter, more fluid mafic magma appears to be
critical to the eruption of some highly viscous silicic magmas.
Received: 23 August 1998 / Accepted: 10 March 1999 相似文献
7.
The Mururoa and Fangataufa atoll basement consists of superimposed submarine and subaerial lava flows which have been intruded
by late volcanics. The intrusions have developed large hydrothermal alteration haloes throughout the basaltic wall rock. The
cuttings of the Natice-1 and Mitre-1 holes, drilled into the submarine volcanic pile at Fangataufa atoll, show a vertical
zonation of clay minerals ranging from 270 to 850 m depth. The newly formed clay minerals occurring from top to bottom of
the altered pile are: dioctahedral aluminous smectites, saponite, an intimate assemblage of saponite with two random chlorite/saponite
mixed layers and an intimate assemblage of one random chlorite/saponite mixed-layer with one ordered chlorite/saponite mixed
layer and one chlorite below 816 m depth. These clay mineral assemblages indicate a general increase in the chloritic component
with depth. They are associated throughout the pile with secondary carbonates and quartz. The ∂18O and ∂13C of calcite and ∂18O of clay minerals, on the one hand, and the intimate mixtures of trioctahedral species, on the other, suggest a general cooling
with the evolution of a paleogeothermal gradient from approximately 300 °C/km during the crystallization of chlorite to 150 °C/km
for the late calcite precipitation.
Received: 2 October 1995 / Accepted: 14 January 1997 相似文献
8.
Michael McCurry Karl P. Hayden Lee H. Morse Stan Mertzman 《Bulletin of Volcanology》2008,70(3):361-383
Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated
volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River
Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical
signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine
± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and
Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite
lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s
of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the
volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions
correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas
are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves.
Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and
rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or
previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme
depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares
mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain
rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites
and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing
and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic
rhyolites. 相似文献
9.
A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system 总被引:5,自引:0,他引:5
A. Gioncada R. Clocchiatti A. Sbrana P. Bottazzi D. Massare L. Ottolini 《Bulletin of Volcanology》1998,60(4):286-306
This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the
shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary
series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements,
with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20 °C)
and similar volatile abundances. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages
and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic
mantle. The younger primitive melt is characterized with respect to the older one by higher K2O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree
of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies
in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination.
Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary
paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions
during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive
mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions
suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively
homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical
configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs.
Received: 11 March 1998 / Accepted: 14 July 1998 相似文献
10.
A first-order leveling survey across the northeast part of the Yellowstone caldera in September 1998 showed that the central
caldera floor near Le Hardy Rapids rose 24±5 mm relative to the caldera rim at Lake Butte since the previous survey in September
1995. Annual surveys along the same traverse from 1985 to 1995 tracked progressive subsidence near Le Hardy Rapids at an average
rate of –19±1 mm/year. Earlier, less frequent surveys measured net uplift in the same area during 1923–1976 (14±1 mm/year)
and 1976–1984 (22±1 mm/year). The resumption of uplift following a decade of subsidence was first detected by satellite synthetic
aperture radar interferometry, which revealed approximately 15 mm of uplift in the vicinity of Le Hardy Rapids from July 1995
to June 1997. Radar interferograms show that the center of subsidence shifted from the Sour Creek resurgent dome in the northeast
part of the caldera during August 1992 to June 1993 to the Mallard Lake resurgent dome in the southwest part during June 1993
to August 1995. Uplift began at the Sour Creek dome during August 1995 to September 1996 and spread to the Mallard Lake dome
by June 1997. The rapidity of these changes and the spatial pattern of surface deformation suggest that ground movements are
caused at least in part by accumulation and migration of fluids in two sill-like bodies at 5–10 km depth, near the interface
between Yellowstone's magmatic and deep hydrothermal systems.
Received: 30 November 1998 / Accepted: 16 April 1999 相似文献
11.
Following a period of net uplift at an average rate of 15±1 mm/year from 1923 to 1984, the east-central floor of Yellowstone Caldera stopped rising during 1984–1985 and then subsided 25±7 mm during 1985–1986 and an additional 35±7 mm during 1986–1987. The average horizontal strain rates in the northeast part of the caldera for the period from 1984 to 1987 were:
1 = 0.10 ± 0.09 strain/year oriented N33° E±9° and
2 = 0.20 ± 0.09 strain/year oriented N57° W±9° (extension reckoned positive). A best-fit elastic model of the 1985–1987 vertical and horizontal displacements in the eastern part of the caldera suggests deflation of a horizontal tabular body located 10±5 km beneath Le Hardys Rapids, i.e., within a deep hydrothermal system or within an underlying body of partly molten rhyolite. Two end-member models each explain most aspects of historical unrest at Yellowstone, including the recent reversal from uplift to subsidence. Both involve crystallization of an amount of rhyolitic magma that is compatible with the thermal energy requirements of Yellowstone's vigorous hydrothermal system. In the first model, injection of basalt near the base of the rhyolitic system is the primary cause of uplift. Higher in the magmatic system, rhyolite crystallizes and releases all of its magmatic volatiles into the shallow hydrothermal system. Uplift stops and subsidence starts whenever the supply rate of basalt is less than the subsidence rate produced by crystallization of rhyolite and associated fluid loss. In the second model, uplift is caused primarily by pressurization of the deep hydrothermal system by magmatic gas and brine that are released during crystallization of rhyolite and them trapped at lithostatic pressure beneath an impermeable self-sealed zone. Subsidence occurs during episodic hydrofracturing and injection of pore fluid from the deep lithostatic-pressure zone into a shallow hydrostatic-pressure zone. Heat input from basaltic intrusions is required to maintain Yellowstone's silicic magmatic system and shallow hydrothermal system over time scales longer than about 105 years, but for the historical time period crystallization of rhyolite can account for most aspects of unrest at Yellowstone, including seismicity, uplift, subsidence, and hydrothermal activity. 相似文献
12.
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
13.
Cynthia A. Gardner Katharine V. Cashman Christina A. Neal 《Bulletin of Volcanology》1998,59(8):537-555
The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume
and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of
similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly
stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts
have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have
average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average
maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event
(and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in
the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma
reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive
magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater.
Received: 5 September 1997 / Accepted: 1 February 1998 相似文献
14.
B. F. Houghton C. J. N. Wilson M. D. Rosenberg I. E. M. Smith R. J. Parker 《Bulletin of Volcanology》1996,58(1):59-66
A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill
tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous
wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid
lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile
clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating
magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic
pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred.
Received: July 11, 1995 / Accepted: February 13, 1996 相似文献
15.
M. Nakagawa K. Wada T. Thordarson C. P. Wood J. A. Gamble 《Bulletin of Volcanology》1999,61(1-2):15-31
Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic
scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions.
The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger
events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of
magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic
magma by mixing between relatively high (1000–1200 °C)- and low (∼1000 °C)- temperature (T) end members. Oscillatory zoning
in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although
the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst
clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the
four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature
(high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived
from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation
from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system
comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in
the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected
into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions
sequentially tapped these magma pockets.
Received: 1 April 1998 / Accepted: 22 December 1998 相似文献
16.
Molly C. McCanta Malcolm J. Rutherford Julia E. Hammer 《Journal of Volcanology and Geothermal Research》2007,160(3-4):263-284
A series of experiments and petrographic analyses have been run to determine the pre-eruption phase equilibria and ascent dynamics of dacitic lavas composing Black Butte, a dome complex on the flank of Mount Shasta, California. Major and trace element analyses indicate that the Black Butte magma shared a common parent with contemporaneously erupted magmas at the Shasta summit. The Black Butte lava phenocryst phase assemblage (20 v.%) consists of amphibole, plagioclase (core An77.5), and Fe–Ti oxides in a fine-grained (< 0.5 mm) groundmass of plagioclase, pyroxene, Fe–Ti oxides, amphibole, and cristobalite. The phenocryst assemblage and crystal compositions are reproduced experimentally between 890 °C and 910 °C, ≥ 300 MPa, XH2O = 1, and oxygen fugacity = NNO + 1. This study has quantified the extent of three crystallization processes occurring in the Black Butte dacite that can be used to discern ascent processes. Magma ascent rate was quantified using the widths of amphibole breakdown rims in natural samples, using an experimental calibration of rim development in a similar magma at relevant conditions. The majority of rims are 34 ± 10 μm thick, suggesting a time-integrated magma ascent rate of 0.004–0.006 m/s among all four dome lobes. This is comparable to values for effusive samples from the 1980 Mount St. Helens eruption and slightly faster than those estimated at Montserrat. A gap between the compositions of plagioclase phenocryst cores and microlites suggests that while phenocryst growth was continuous throughout ascent, microlite formation did not occur until significantly into ascent. The duration of crystallization is estimated using the magma reservoir depth and ascent rate, as determined from phase equilibria and amphibole rim widths, respectively. Textural analysis of the natural plagioclase crystals yields maximum growth rates of plagioclase phenocryst rims and groundmass microlites of 8.7 × 10− 8 and 2.5 × 10− 8 mm/s, respectively. These rates are comparable to values determined from time-sequenced samples of dacite erupted effusively from Mount St. Helens during 1980 and indicate that syneruptive crystallization processes were important during the Black Butte eruptive cycle. 相似文献
17.
The vesiculation of a peralkaline rhyolite melt (initially containing ∼0.14 wt.% H2O) has been investigated at temperatures above the rheological glass transition (T
g≈530 °C) by (a) in situ optical observation of individual bubble growth or dissolution and (b) dilatometric measurements
of the volume expansion due to vesiculation. The activation energy of the timescale for bubble growth equals the activation
energy of viscous flow at relatively low temperatures (650–790 °C), but decreases and tends towards the value for water diffusion
at high temperatures (790–925 °C). The time dependence of volume expansion follows the Avrami equation
ΔV (t)∼{1–exp [–(t/τav)
n
]}
with the exponent n=2–2.5. The induction time of nucleation and the characteristic timescale (τav) in the Avrami equation have the same activation energy, again equal to the activation energy of viscous flow, which means
that in viscous melts (Peclet number <1) the vesiculation (volume expansion), the bubble growth process, and, possibly, the nucleation of vesicles, are
controlled by the relaxation of viscous stresses. One of the potential volcanological consequences of such behavior is the
existence of a significant time lag between the attainment of a super-saturated state in volatile-bearing rhyolitic magmas
and the onset of their expansion.
Received: March 20, 1995 / Accepted: October 24, 1995 相似文献
18.
Crystallization history of Obsidian Dome,Inyo Domes,California 总被引:4,自引:0,他引:4
Samuel E. Swanson Michael T. Naney H. R. Westrich J. C. Eichelberger 《Bulletin of Volcanology》1989,51(3):161-176
Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (T) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low T. A drastic increase in T, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H2O) magma crystallized spherulites and fine-grained rhyolite at high T. A granophyric texture, representing crystallization at a moderate T, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H2O) rhyolitic magma at shallow depths. 相似文献
19.
The purpose of this work was to study jointly the volcanic-hydrothermal system of the high-risk volcano La Soufrière, in
the southern part of Basse-Terre, and the geothermal area of Bouillante, on its western coast, to derive an all-embracing
and coherent conceptual geochemical model that provides the necessary basis for adequate volcanic surveillance and further
geothermal exploration. The active andesitic dome of La Soufrière has erupted eight times since 1660, most recently in 1976–1977.
All these historic eruptions have been phreatic. High-salinity, Na–Cl geothermal liquids circulate in the Bouillante geothermal
reservoir, at temperatures close to 250 °C. These Na–Cl solutions rise toward the surface, undergo boiling and mixing with
groundwater and/or seawater, and feed most Na–Cl thermal springs in the central Bouillante area. The Na–Cl thermal springs
are surrounded by Na–HCO3 thermal springs and by the Na–Cl thermal spring of Anse à la Barque (a groundwater slightly mixed with seawater), which are
all heated through conductive transfer. The two main fumarolic fields of La Soufrière area discharge vapors formed through
boiling of hydrothermal aqueous solutions at temperatures of 190–215 °C below the "Ty" fault area and close to 260 °C below
the dome summit. The boiling liquid producing the vapors of the Ty fault area has δD and δ18O values relatively similar to those of the Na–Cl liquids of the Bouillante geothermal reservoir, whereas the liquid originating
the vapors of the summit fumaroles is strongly enriched in 18O, due to input of magmatic fluids from below. This process is also responsible for the paucity of CH4 in the fumaroles. The thermal features around La Soufrière dome include: (a) Ca–SO4 springs, produced through absorption of hydrothermal vapors in shallow groundwaters; (b) conductively heated, Ca–Na–HCO3 springs; and (c) two Ca–Na–Cl springs produced through mixing of shallow Ca–SO4 waters and deep Na–Cl hydrothermal liquids. The geographical distribution of the different thermal features of La Soufrière
area indicates the presence of: (a) a central zone dominated by the ascent of steam, which either discharges at the surface
in the fumarolic fields or is absorbed in shallow groundwaters; and (b) an outer zone, where the shallow groundwaters are
heated through conduction or addition of Na–Cl liquids coming from hydrothermal aquifer(s).
Received: 9 November 1998 / Accepted: 15 July 1999 相似文献
20.
M. Alidibirov D. B. Dingwell R. J. Stevenson K.-U. Hess S. L. Webb J. Zinke 《Bulletin of Volcanology》1997,59(2):103-111
Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory.
The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log10
η (Pa s) (T=900–982 °C), and shrinkage effects were dilatometrically observed at T>900 °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log10
η (Pa s) (T=736–802 °C) and viscosities measured by rotational viscometry are 3.22–1.66 log10
η (Pa s) (T=1298–1594 °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding
water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a
combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis
for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of
the 18 May 1980 Mount St. Helens eruption was residing at 900 °C with a phenocryst content of 30 vol.%, a vesicularity of
36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 1010.8 Pa s.
Received: 25 August 1996 / Accepted: 19 July 1997 相似文献