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11.
Julia E. Hammer Malcolm J. Rutherford Wes Hildreth 《Contributions to Mineralogy and Petrology》2002,144(2):144-162
New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H2O+CO2 fluid saturation were determined for an andesite (58.7 wt% SiO2) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H2O-saturated conditions (i.e., PH2O=PTOT) in a negatively sloping region in T-P space from 930 °C/100 MPa to 960 °C/75 MPa with fO2~NNO+1. The H2O-saturated equilibration conditions of the dacite are constrained to a T-P region from 850 °C/50 MPa to 880 °C/25 MPa. If H2O-saturated, these magmas equilibrated at (and above) the level where co-erupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H2O + CO2) fluid for the same range in PH2O. Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H2O-saturated rhyolite is consistent with experimental findings only if the intermediates have XH2Ofl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days. 相似文献
12.
Hildreth Wes Fierstein Judy Siems David F. Budahn James R. Ruíz Joaquin 《Contributions to Mineralogy and Petrology》2004,147(3):243-275
Physical and compositional data and K-Ar ages are reported for 14 rear-arc volcanoes that lie 11–22 km behind the narrowly linear volcanic front defined by the Mount Katmai-to-Devils Desk chain on the Alaska Peninsula. One is a 30-km3 stratocone (Mount Griggs; 51–63% SiO2) active intermittently from 292 ka to Holocene. The others are monogenetic cones, domes, lava flows, plugs, and maars, of which 12 were previously unnamed and unstudied; they include seven basalts (48–52% SiO2), four mafic andesites (53–55% SiO2), and three andesite-dacite units. Six erupted in the interval 500–88 ka, one historically in 1977, and five in the interval 3–2 Ma. No migration of the volcanic front is discernible since the late Miocene, so even the older units erupted well behind the front. Discussion explores the significance of the volcanic front and the processes that influence compositional overlaps and differences among mafic products of the rear-arc volcanoes and of the several arc-front edifices nearby. The latter have together erupted a magma volume of about 200 km3, at least four times that of all rear-arc products combined. Correlation of Sr-isotope ratios with indices of fractionation indicates crustal contributions in volcanic-front magmas (0.7033–0.7038), but lack of such trends among the rear-arc units (0.70298–0.70356) suggests weaker and less systematic crustal influence. Slab contributions and mantle partial-melt fractions both appear to decline behind the front, but neither trend is crisp and unambiguous. No intraplate mantle contribution is recognized nor is any systematic across-arc difference in intrinsic mantle-wedge source fertility discerned. Both rear-arc and arc-front basalts apparently issued from fluxing of typically fertile NMORB-source mantle beneath the Peninsular terrane, which docked here in the Mesozoic.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: T.L. Grove 相似文献
13.
Wes Hildreth 《Bulletin of Volcanology》1987,49(5):680-693
New data extend our understanding of the 1912 eruption, its backfilled vent complex at Novarupta, and magma-storage systems beneath adjacent stratovolcanoes. Initial Plinian rhyolite fallout is confined to a narrow downwind sector, and its maximum thickness may occur as far as 13 km from source. In contrast, the partly contemporaneous rhyolite-rich ash flows underwent relatively low-energy emplacement, their generation evidently being decoupled from the high column. Flow veneers 1–13 m thick on near-vent ridge crests exhibit a general rhyolite-to-andesite sequence like that of the much thicker valley-confined ignimbrite into which they merge downslope. Lithics in both the initial Plinian and the ignimbrite are predominantly fragments of the Jurassic Naknek Formation, which extends from the surface to a depth of ca. 1500 m. Absence of lithics from the underlying sedimentary section limits to < 1.5 km the fragmentation level and the structural depth of the vent, which is thought to be funnel-shaped, flaring shallowly to a surface diameter of 2 km. Overlying the ignimbrite are layers of Plinian dacite fallout, > 100 m thick near source and 10 m thick 3 km away, which dip back into an inner vent <0.5 km wide, nested inside the earlier vent funnel of the ignimbrite. The dacite fallout is poor in Naknek lithics but contains abundant fragments of vitrophyre, most of which was vent-filling, densely welded tuff reejected during later phases of the 3-day eruption. Adjacent to the inner vent, a 225-m-high asymmetrical accumulation of coarse near-vent ejecta is stratigraphically continuous with the regional dacite fallout. Distensional faulting of its crest may reflect spreading related to compaction and welding. Nearby andesite-dacite stratovolcanoes, i.e., Martin, Mageik, Trident, and Katmai, display at least 12 vents that define a linear volcanic front trending N65°E. The 1912 vent and adjacent dacite domes are disposed parallel to the front and ca. 4 km behind it. Mount Griggs, 10 km behind the front, is more potassic than other centers, taps isotopically more depleted source materials, and reflects a wholly independent magmatic plumbing system. Geochemical differences among the stratovolcanoes, characteristically small eruptive volumes ( < 0.1 to 0.4 km3), and the dominance of andesite and low-SiO2 dacite suggest complex crustal reservoirs, not large integrated magma chambers. Linear fractures just outside the 1912 vent strike nearly normal to the volcanic front and may reflect dike transport of magma previously stored beneath Trident 3–5 km away. Caldera collapse at Mount Katmai may have taken place in response to hydraulic transfer of Katmai magma toward Novarupta via reservoir components beneath Trident. The voluminous 1912 eruption (12–15 km3 DRE) was also unusual in producing high-silica rhyolite (6–9 km3 DRE), a composition rare in this arc and on volcanic fronts in general. Isotopic data indicate that rhyolite genesis involved little assimilation of sedimentary rocks, pre-Tertiary plutonic rocks, or hydrothermally altered rocks of any age. Trace-element data suggest nonetheless that the rhyolite contains a nontrivial crustal contribution, most likely partial melts of Late Cenozoic arc-intrusive rocks. Because the three compositions (77%, 66–64.5%, and 61.5–58.5% SiO2) that intermingled in 1912 vented both concurrently and repeatedly (after eruptive pauses hours in duration), the compositional gaps between them must have been intrinsic to the reservoir, not merely effects of withdrawal dynamics. 相似文献
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Complex proximal deposition during the Plinian eruptions of 1912 at Novarupta,Alaska 总被引：2，自引：1，他引：1
Bruce?F.?HoughtonEmail author C.?J.?N.?Wilson J.?Fierstein W.?Hildreth 《Bulletin of Volcanology》2004,66(2):95-133
Proximal (<3 km) deposits from episodes II and III of the 60-h-long Novarupta 1912 eruption exhibit a very complex stratigraphy, the result of at least four transport regimes and diverse depositional mechanisms. They contrast with the relatively simple stratigraphy (and inferred emplacement mechanisms) for the previously documented, better known, medial–distal fall deposits and the Valley of Ten Thousand Smokes ignimbrite. The proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta, and numerous thin complex deposits of pyroclastic density currents (PDCs) with no regional analogs. The locally dispersed component of the fall deposits forms sector-confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100–300 m (cf. several kilometers for the medial–distal fall deposits). This locally dispersed fall material (and many of the associated PDC deposits) is rich in andesitic and banded pumices and richer in shallow-derived wall-rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition. There are no marked contrasts in grain size in the near-vent deposits, however, between locally and widely dispersed beds, and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial–distal samples. Associated PDC deposits form a spectrum of facies from fines-poor, avalanched beds through thin-bedded, landscape-mantling beds to channelized lobes of pumice-block-rich ignimbrite. The origins of the Novarupta near-vent deposits are considered within a spectrum of four transport regimes: (1) sustained buoyant plume, (2) fountaining with co-current flow, (3) fountaining with counter-current flow, and (4) direct lateral ejection. The Novarupta deposits suggest a model where buoyant, stable, regime-1 plumes characterized most of episodes II and III, but were accompanied by transient and variable partitioning of clasts into the other three regimes. Only one short period of vent blockage and cessation of the Plinian plume occurred, separating episodes II and III, which was followed by a single PDC interpreted as an overpressured "blast" involving direct lateral ejection. In contrast, regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume, which were being strongly affected by short-lived instabilities. These instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent. Of the parameters that control explosive eruptive behavior, only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products.Editorial responsibility: R. Cioni 相似文献
16.
Brian R. Jicha Clark M. Johnson Wes Hildreth Brian L. Beard Garret L. Hart Steven B. Shirey Brad S. Singer 《Earth and Planetary Science Letters》2009,277(1-2):38-49
A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (230Th/238U) > 1 and span a large range in (230Th/232Th) ratios, and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization. 相似文献
17.
Brian R. Jicha Garret L. Hart Clark M. Johnson Wes Hildreth Brian L. Beard Steven B. Shirey John W. Valley 《Contributions to Mineralogy and Petrology》2009,157(2):189-207
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and
its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources,
a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns
for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade
arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators
of slab-fluids such as (Sr/P)
n
and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle
source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification
is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most
Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and
isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass
of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization
in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher
than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ
18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding
involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope
compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile
arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. 相似文献
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19.
Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 μm vesicle diameter and cumulative number densities ranging from 107–109 cm–3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h. 相似文献
20.
A.N. Halliday A.E. Fallick J. Hutchinson W. Hildreth 《Earth and Planetary Science Letters》1984,68(3):379-391
The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial87Sr/86Sr of 0.7060–0.7092 andδ18O of 5.9–10.3‰. Six constituent sanidines yield smaller ranges of initial87Sr/86Sr of 0.7061–0.7069 andδ18O of 6.7–7.9. In contrast143Nd/144Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of0.51258 ± 1. These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had?Nd = ?0.9, ?Sr = +23 andδ18O = 7‰ and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith. 相似文献