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1.
Thirty-four ash layers of Pleistocene and Pliocene age from DSDP Site 192, northwestern Pacific Ocean, have been subjected to detailed chemical and optical study to evaluate: (1) the chemical and optical variability in glass shards from deep-sea ash layers, and (2) secondary changes brought about by prolonged exposure to seawater. Glass shards from approximately half of the ash layers studied were found to have uniform compositions which approach the precision of the microprobe chemical analyses, whereas the remainder are compositionally diverse (e.g., SiO2, variations of 5–15% among shards from the same ash layer) and appear to be the eruptive products of compositionally zoned magma chambers. Optical studies of glass shards confirm the absence of devitrification or the formation of pervasive secondary alteration products. By contrast, chemical studies suggest that the glass shards have experienced progressive hydration with possible minor ion exchange of K, Mg, Ca and Si. The hydration occurs rapidly and leads to a rather uniform water content of 4.5–5% after several hundred thousands of years exposure to seawater. Step-wise heating dehydration experiments, optical effects, and published'oxygen isotope studies indicate that the water of hydration is incorporated uniformly within the glass. Systematic chemical differences between electron microprobe analyses of glass shard interiors and corresponding bulk chemical study by atomic absorption lead us to postulate that glass shard margins have undergone a minor chemical exchange with major cations in seawater. They have gained 0.10–0.20 wt. % K20, MgO, and CaO while losing a corresponding amount of Si2O. Although the glass shards from DSDP Site 192 are hydrated and may have experienced subtle, surficial ion exchange, we stress that they are the most chemically representative samples available of magmas that were explosively erupted from volcanic arcs.  相似文献   

2.
Mapping and analysis of marine magnetic anomalies generated during the past 3.78 m.y. at the East Pacific Rise crest near 31°S reveals a history of ongoing small-scale migration of the spreading center. The axis first became curved and then broke when the curvature became too severe, forming a 10-km offset. The offset healed rapidly and the topographic axis of the rise is now continuous and essentially linear. Sea-floor spreading has occurred asymmetrically in this area with east and west flank rates of 86 and 77 mm/yr, respectively, since 2.41 m.y. ago. Total spreading rates show an overall decline from 176 to 145 mm/yr prior to the Jaramillo event, 0.9 m.y. ago. For the last 0.7 m.y. the total spreading rate has been 162 mm/yr.  相似文献   

3.
The Sierra La Primavera volcanic complex consists of late Pleistocene comenditic lava flows and domes. ash-flow tuff, air-fall pumice, and cold caldera-lake sediments. The earliest lavas were erupted about 120,000 years ago, and were followed approximately 95,000 years ago by the eruption of about 20 km3 of magma as ash flows that form the compositionally-zoned Tala Tuff. Collapse of the roof zone of the magma chamber led to the formation of a shallow 11-km-diameter caldera. It soon filled with water, forming a caldera lake in which sediment began to collect. At about the same time, two central domes erupted through the middle of the lake and a “giant pumice horizon”, an important stratigraphic marker, was deposited. Shortly thereafter ring domes erupted along two parallel arcs: one along the northeast portion of the ring fracture, and the other crossing the middle of the lake. All these events occurred during a period of approximately 5,000–10,000 years. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine-grained ashy sediments virtually free from pumice lapilli. Approximately 75,000 years ago, a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only 10–20 m of sediments, as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 20,000–30,000 years ago.The four major fault systems in the Sierra La Primavera are related to caldera collapse or to uplift caused by the insurgence of the southern are magma. Steam vents and larga-discharge 65°C hot springs are associated with the faulting. Calculated equilibrium temperatures of the geothermal fluids are 170°C, but temperatures in excess of 240°C have been encountered in an exploratory drill hole.A seismic survey showed attenuation of both S and P waves within the caldera, P waves attenuated more severely than S waves. The greatest attenuation is associated with an area of steam vents, and the rapid lateral variations in attenuation suggest that they are produced by a shallow geothermal system rather than by underlying magma.  相似文献   

4.
The tempo of Cenozoic volcanism on opposite sides of the Pacific Ocean has been examined by compiling the numbers of radiometric dates reported for terrestrial volcanic sequences and the numbers of volcanic ash (glass) horizons recorded in Neogene deep-sea (DSDP) sedimentary sections. Within certain limits these data are believed to provide a reliable record of extrusive and explosive volcanism. Although terrestrial and marine records for individual regions reveal important differences in the episodicity of volcanism, a correlation is found between activity in the Southwestern Pacific, Central America and the Cascade Range of western North America. Two important pulses of Neogene volcanism (the Cascadian and Columbian episodes) occurred during the Quaternary (t = 2 m.y. to present) and within the Middle Miocene (t = 16 to 14 m.y. ago), with less important episodes in the latest Miocene to Early Pliocene (t = 6 to 3 m.y. ago) and Late Miocene (11 to 8 m.y. ago). The names Fijian and Andean are proposed to these episodes. Dating of terrestrial sequences indicates that these episodes of intense volcanism took place in relatively short intervals of time, separated by longer more quiescent periods.It has been suggested that synchronous episodic volcanism is related to changes in rates of sea-floor spreading and subduction. If so, volcanism must amplify these changes, because the variations in tempo of volcanism are much too great for proportional rate changes. An apparent correlation of volcanism in orogenic zones of the circum-Pacific region with world-wide changes of sea level and changes of activity in the Hawaiian-Emperor chain suggests that volcanism records fundamental tectonic changes throughout the entire Pacific region.  相似文献   

5.
Glass separates from 115 ash layers derived from the Kamchatkan (DSDP Site 192; 34 layers), the eastern Aleutian (DSDP Site 183; 56 layers) and the Alaska Peninsula (DSDP Site 178; 25 layers) volcanic arcs have been analyzed for up to 28 elements. In addition, the abundance and diversity of associated mafic phenocrysts have been evaluated. The resulting data set has made possible an evaluation of the late Miocene to Recent changes in composition of ashes derived from North Pacific volcanic arcs and of the factors controlling the evolution of highly siliceous magmas.We find no evidence for a general transition from arc tholeiite to calc-alkalic magma parentage of ashes derived from the volcanic arcs during the last 10 m.y., but instead find 0.1- to 0.5-m.y. intervals during which particular types of volcanism are prevalent. Most convincing is the transition from arc tholeiite to calc-alkalic for ashes derived from Kamchatka during the last 0.8 m.y., a change believed to be associated with a landward shift in the site of magma generation. Considered together, ashes derived from North Pacific volcanic arcs have been becoming more siliceous during the last 1.5 m.y. and may be associated with accelerated subduction during the same time interval.Hydrous phenocrysts (e.g., biotite) are typically associated with low-silica deep-sea ashes, but not with terrestrial volcanic rocks of comparable silica contents, suggesting the important role of water in the evolution of siliceous magma. REE patterns and relative abundances of mafic phenocrysts demonstrate the importance of fractional crystallization in controlling the evolution of highly siliceous arc magmas. REE increase with increasing silica, but become less concentrated in ashes with SiO2 > 64%. Eu anomalies increase throughout the SiO2 range. Initial fractionation is dominated by clinopyroxene and plagioclase with amphibole strongly influencing fractionation above 64% SiO2.  相似文献   

6.
Seven piston cores, 7–16 m long, taken between the Kuril Islands and Emperor Seamounts, have been dated using radiolarian and diatom extinction levels and correlated using volcanic ash layers. The average rate of deposition in the cores decreases from 6 cm/1000 years near the Kuril Trench to about 3.5 cm/1000 years near the seamounts. Dispersed volcanic ash is the main constituent of the cores and it comprises up to 80% of the sediments. The percentage of the ash in the sediments decreases eastward from the Kuril Islands as the rates of deposition decrease.The total thickness of the sediments in the same latitudinal belt also decreases eastward. The thickness of the sediment inferred from seismic data near the Kuril trench is about 600 m and rates of deposition are approximately 6 cm/1000 years in the Pleistocene cores. Sediment thickness near the seamounts is about 300 m, and rates of deposition are approximately 3 cm/1000 years in the Pleistocene cores. Extrapolated rates of deposition in these cores suggest that the age of the base of the sediment to the east of the Kurils is only about 10 m.y.The anomalously young age for the base of the sediments obtained by extrapolation of an assumed constant rate of deposition can be explained by Deep Sea Drilling Project data from the northwest Pacific. The sediment thickness at DSDP site 192 east of Kamchatka includes sediments from all the Cenozoic epochs except the Paleocene. Rates of deposition of sediment younger than Middle Miocene are an order of magnitude higher than those prior to this time. At DSDP sites east of Japan, either Late Miocene sediments lie directly on the basement, or sediments older than Late Miocene are very thin. Post-Middle Miocene sediments are composed primarily of glass shards. Thus, about 90% of the total thickness of sediments in the northwest Pacific is composed of sediments younger than Middle Miocene with volcanic ash as the main constituent. The volcanic ash results from the present phase of explosive volcanic activity which began in the Late Miocene in the northwest Pacific volcanic arcs.  相似文献   

7.
Abstract Arc volcanic activity on opposite sides of the Pacific Ocean (Japan and Central America) has been investigated by examining the number of volcanic ash layers recorded in Neogene and Quaternary deep-sea sediments. The data suggest that ash layers counted in deep-sea sediments may provide a reliable record of arc volcanism. The study is based on a quantitative analysis of arc volcanic activity using cores collected on DSDP (Deep-Sea Drilling Project) and ODP (Ocean Drilling Program) legs. Five distinct parameters which might affect ash distribution in marine sediments were reviewed: nature of the eruption, wind influence, settling conditions, diagenesis, and plate motion. Of these five, past atmospheric circulation was the most significant. The main constraint on the analysis is that temporal scattering of ash is not directly related to wind pattern variations. Results of this analysis are correlated with dating of terrestrial volcanic sequences. Although marine tephra records for individual regions reveal minor differences in the episodes of volcanic activity, a general correlation exists between activity of arc volcanism in Japan and in Central America. Two important pulses of arc volcanism occurred during Middle Miocene times (18–13 Ma) and Plio-Quaternary times (5–0 Ma). These episodes of intense volcanism are separated by a well recorded quiescent period during Late Miocene times. These correlating episodes of the volcanic record indicate a direct link between arc volcanism and the global tectonic evolution of the Pacific ocean margins.  相似文献   

8.
The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced > 4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5–11.5 km. Curiously < 400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (> 600 °C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting > 28 m.y., and induced by the steep continent–ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.  相似文献   

9.
Potassium-argon dating of volcanic and plutonic rocks in the Andean region of central Chile has revealed previously unrecognized episodes of igneous activity during Cretaceous and Cenozoic time. These results indicate the need to re-evaluate the classic stratigraphic subdivisions that have evolved on lithologic rather than time-stratigraphic criteria.Four radiometric age groups have been identified in the coast range volcanic belt:
1. (1) Las Chilcas Formation — Early Cretaceous continental volcanic strata (120-110 m.y.).
2. (2) Lo Valle Formation — Late Cretaceous continental volcanic strata (78-65 m.y.).
3. (3) Late Oligocene extrusive volcanics (31-28 m.y.).
4. (4) Early Miocene intrusive volcanics (20.6–19.5 m.y.).
Two radiometric age groups have also been identified in the adjacent Andean Cordillera:
1. (1) Farellones Formation — continental volcanic strata (18.5–17.3 m.y.).
2. (2) Early Pliocene extrusive volcanics (5-4 m.y.).
An older group of continental volcanic strata in the Andes represented by the Abanico Formation remains undated but is intruded by plutons dated at 19.5 and 24 m.y.Available chronologic evidence indicates that volcanic activity moved eastward from the coast range volcanic belt to the Andean Cordillera between 20 and 18 m.y. ago and remained there to the present time.  相似文献   

10.
In southern British Columbia the terrestrial heat flow is low (44 mW m–2) to the west of the Coast Plutonic Complex (CPC), average in CPC (50–60 mW m–2),and high to the east(80–90 mW m–2). The average heat flow in CPC and the low heat generation (less than 1 W m–3) indicate that a relatively large amount of heat flows upwards into the crust which is generally quite cool. Until two million years ago the Explorer plate underthrust this part of the American plate, carrying crustal material into the mantle. Melted crustal rocks have produced the inland Pemberton and Garibaldi volcanic belts in the CPC.Meager Mountain, a volcanic complex in the CPC 150 km north of Vancouver, is a possible geothermal energy resource. It is the product of intermittent activity over a period of 4 My, the most recent eruption being the Bridge River Ash 2440 y B.P. The original explosive eruption produced extensive fracturing in the granitic basement, and a basal explosion breccia from the surface of a cold brittle crust. This breccia may be a geothermal reservoir. Other volcanic complexes in the CPC have a similar potential for geothermal energy.Earth Physics Contribution No. 704.  相似文献   

11.
Deep-sea sediments in the Panama Basin-Carnegie Ridge area contain biogenic material, detrital carbonate, mineral clay and volcanic ash layers. Ash layer “L” of Bowles et al. (1973) is correlated mineralogically and by the physical property of glass shards through sixteen cores. Isopachs and grain-size analyses of the ash layer indicate that it originated in Colombia or Ecuador, and was carried by easterly winds. The distribution of the ash and of mica percentage in the ash form a W-shaped pattern opening towards the west. This suggests that two branches of the Cromwell current, one moving along the equator and one along 3°S, had a significant influence on the distribution of the ash.CaCO3 content has been measured down thirteen cores and oxygen isotope content of benthonic Foraminifera obtained in two. Two cores penetrate theStylatractus universus extinction datum. Ash “L” fell 230,000 years ago during the cold substage 7b of isotope stage 7. Deposition rates vary between 2.5 and 6 cm/1000 yr and show no relationship with bottom topography or proximity to land.  相似文献   

12.
Sixty-six K---Ar dates from igneous rocks in the central Chilean Andes between 33° and 38°S are reported in this study. From these results and observed field relations, major Cenozoic volcanic and intrusive rock units are divided into chronologic groups representing igneous events.Volcanic units of Oligocene (33.3–27.9 m.y.) and Early Miocene (20.2 m.y.) age have been dated west of the present range at 33°S but neither the magnitude nor extent of these volcanic events has yet been established. Extensive Middle to Late Miocene volcanism (15.3–6.4 m.y.) followed by regional folding is recognized in the map area between 35° 20′ and 36°S. Partly contemporaneous Middle Miocene volcanism (18.4–13.7 m.y.) also followed by regional folding is recorded in the Andes between 37° 30′ and 38°S. General volcanic quiescence from 6.4 to 2.5 m.y. is observed in the map area but whether this volcanic hiatus is of regional significance is not known.The majority of the K---Ar dates document a history of nearly continuous volcanism throughout the last 2.5 m.y. in the map area. The abundant and diverse sequences of volcanic strata formed during this time, have been divided into four successive age groups which as map units show the evolution and distribution of latest volcanic activity.Landforms preserved by this volcanic series show that topographic relief similar to the present has prevailed during this time. Deep incision of rivers into young volcanic terrain, estimated to be on the order of 1–2 m/1000 years, has produced a complex volcanic and morphologic record.Four plutons dated in this study give ages of 62.0, 41.3, 19.5, and 7.0 m.y. No spatial pattern of emplacement is observed in the map area where three of these plutons are represented.Similarities in structural style, orientation and degree of deformation of Miocene and Mesozoic strata suggest that Late Miocene regional folding may have accounted for a significant part of the observed deformation in older basement strata previously ascribed to earlier orogenies.A regional comparison of ages of recognized igneous and tectonic event at different latitudes in the central and southern Andes shows the gross chronology of Cenozoic events which can be correlated with sea-floor spreading and subduction events.  相似文献   

13.
The ages of reversals of the Earth's magnetic field have been dated accurately back to 3.4 m.y. ago. Between this time and the age of the Cretaceous-Tertiary boundary, dates for reversals have been calculated assuming a constant rate of sea-floor spreading in the South Atlantic Ocean. The presence of thick piles of lava flows in Iceland allows us to produce independent evidence for the ages of reversals back to 13.0 m.y. B.P. Because of the extreme regularity of extrusion of these lava flows, the measurement of their magnetic polarity allows us to correlate the lava flows which were extruded during the polarity intervals associated with sea-floor spreading anomalies. The measurement of many K-Ar ages on these lava flows also allows us to compare the ages of reversals assumed by the linear interpolation between the ages of 3.4 m.y. and the Cretaceous-Tertiary boundary at 66.5 m.y., with those suggested by the radiometric dates. We find that in general the assumption of constant spreading has been a good one, but suggest a small change in the ages of reversals, amounting to an increase of about 0.27 m.y. in ages of reversals between 8.5 and 13.0 m.y. ago.  相似文献   

14.
Airborne and ground-based (correlation spectrometer, cascade impactor, and photoelectric counter together with intake filter probes) measurements are described for the volcanic emissions from Popocatépetl volcano (Mexico) from December 23, 1994 to January 28, 1995. Measurements of SO2 restarted 48 h after the eruption onset of December 21, 1994. Maximum sulfur dioxide (4560 t d−1) plus 3.8×104 t d−1 of particulate matter were ejected on December 24, 1994. The maximum rate of ejection occurred coincidentally with the maximum amplitude of harmonic tremor and the maximum number of seismic type B events. Sulfur dioxide emission rates ranged from 1790 to 2070 t d−1 (December 23–24, 1994). Afterwards, sulfur dioxide emission rates clearly indicated a consistent decline. However, frequent gas and ash emission puffs exhibited SO2 fluxes reaching values as high as 3060 t d−1. The emission SO2 baseline for the period of study (February 1994–January 1995) was about 1000 t d−1. Ejection velocity of particulate matter was approximately 270 m s−1 reaching a height of about 2.5 km over the summit. The immediate aerosol dispersion area was estimated at 6.0×104 km2 maximum. The microscopic structure of particles (aerosol and tephra) showed a fragile material, probably coming from weathered crustal layers. X-ray fluorescence and neutron-activation analysis from the impactor samples found the following elements: Si, Al, Ca, S, P, Cl, K, Ni, Fe, Ti, Sc, Cu, Zn, Mn, Sr, Cr, Co, Y, Br, Se, Ga, Rb, Hg and Pb. Morphological analysis shows that ash samples might be from pulverized basaltic rock indicating that the Popocatépetl eruption of December 21, 1994 was at low temperature. The microscopic structure of puff material showed substance aggregates consisted of fragile rock, water and adsorbed SO2. These aggregates were observed within water droplets of approximately 1 mm and even larger. Sulfur transformations in the droplets occurred intensively. Volcanic ash contained 5–6% of sulfur during the first expulsion hours. Elemental relative concentrations with respect to Al show that both Si and S have relative concentrations >1, i.e., 13.73 and 2.17, respectively in agreement with the photoelectric counter and COSPEC measurements.  相似文献   

15.
Sea-floor spreading data from the Southwest Pacific have recently been used to predict the Cainozoic geological history along the Indo-Australian/Pacific plate boundary. Geologic and sedimentologic data pertaining to this plate boundary where it crosses southern New Zealand, as the Alpine Fault, are summarised and discussed. It is concluded that there is a close accord between the plate-tectonic predictions and South Island Cainozoic geological history. In particular, (1) no Cainozoic plate boundary traversed the New Zealand region prior to 38 m.y. B.P. (late Eocene); (2) transcurrent movement on the Alpine Fault took place largely between ca. 30 m.y. B.P. (middle Oligocene) and ca. 10 m.y. B.P. (late Miocene); and (3) the period 10 m.y. B.P. to present corresponds to a phase of oblique compression, continental collision, and mountain building along the Alpine Fault sector of the plate boundary. There is a close correlation between the sites and histories of Cainozoic sedimentation and this tectonic timetable.  相似文献   

16.
Geostationary Operational Environmental Satellite (GOES) Imager and Sounder data were evaluated to determine the potential effects of volcanic ash detection without the use of a 12 μm infrared (IR) band, on GOES-M (12) through Q (a period of at least 10 years). Principal component analysis (PCA) images with and without 12 μm IR data were compared subjectively for six weak to moderate eruptions using pattern recognition techniques, and objectively by determining a false detection rate parameter. GOES Sounder data were also evaluated in a few instances to assess any potential contributions from the new 13.3 μm Imager band.Results indicated that, during periods of daylight, there was little apparent difference in the quality of IR detection without the 12 μm IR, likely due to a maximum in solar reflectance of silicate ash in a shortwave IR (SWIR) band centered near 3.9 μm. At night when SWIR reflectance diminished, the ash detection capability appeared to be significantly worse, evidenced by increased ambiguity between volcanic ash and meteorological clouds or surface features. The possible effects of this degradation on aviation operations are discussed. The new 13.3 μm IR band on GOES has the capability to help distinguish ash from cirrus clouds, but not from low level clouds consisting of water droplets.Multi-spectral data from higher resolution polar orbiting satellites may also be used to supplement analyses from lower resolution GOES for long-lived ash cloud events. The Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) instruments appear to be the best options in accomplishing this, with additional satellite missions becoming available later in the decade. In summary, it will still be possible to observe and track significant volcanic ash clouds in the GOES-M through Q era (2003–2012) without the benefit of 12 μm IR data, but with some degradation that will be most significant at night.  相似文献   

17.
For some time, sea-floor spreading has been hypothesized for the Mid-Cayman Rise based on inferences from seismicity, heat flow, topography and plate geometry. Here we present magnetic anomaly inversions from which a reasonable record of sea-floor spreading emerges. We obtain total opening rates of 20 ± 2 mm/yr for 0–2.4 m.y. B.P. and 40 ± 2 mm/yr for 2.4–6.0 m.y. B.P. Data on the west flank extend the half-opening rate of 20 mm/yr back to 8.3 m.y. B.P. Spreading has been very nearly symmetric. These new observations place important constraints on plate tectonic reconstructions by defining the relative motion between the North American and Caribbean plates. They also shed some light on sea-floor spreading processes in which the spreading center is a secondary feature in the sense that it is over an order of magnitude shorter than the adjoining transform faults.  相似文献   

18.
Glass and mineral fragments from discrete volcanic ash layers were sampled from DSDP/IPOD Site 450 in the Parece Vela Basin, Philippine Sea and analyzed by electron microprobe. The ashes are interpreted as eruptive products of the adjacent West Mariana arc system between 25 and 14 Ma B.P., and have compositions between basaltic andesite and rhyolite, and rarely, boninite. ‘Continuous’ chemical trends appear to reflect mixing of mafic and silicic magmas. ‘Discontinuous’ trends between these end-members are relatively few, and are consistent with ‘liquid lines’ produced by fractional crystallization. Andesitic tephra become progressively richer in MgO and CaO through the middle Miocene, while boninite appears towards the end of the sequence, between 14 and 15 Ma B.P. Coeval rhyolitic glasses become richer in K2O and Na2O, with maximum concentrations at about 15 Ma B.P. Chronologic changes in fractionation type and composition of parent magmas are interpreted to reflect the subaerial volcanic evolution of the West Mariana arc. The appearance of boninite is believed to signal early stages of arc sundering, and corresponds temporally with regional uplift of the sea floor above the carbonate compensation depth, precursor to a new pulse of back-arc spreading.  相似文献   

19.
Melt generation and extraction along the Hawaiian volcanic chain should be largely controlled by the thermal structure of the Hawaiian swell and the heat source underneath it. We simulate numerically the time- and space-dependent evolution of Hawaiian volcanism in the framework of thermal evolution of the Hawaiian swell, constrained by residual topography, geoid anomalies, and anomalous heat flow along the Hawaiian volcanic chain. The transient heat transfer problem with melting relationships and variable boundary conditions is solved in cylindrical coordinates using a finite difference method. The model requires the lithosphere to be thinned mechanically by mantle plume flow. Melting starts quickly near the base of the plate when the hotspot is encountered. Thermal perturbation and partial melting are largely concentrated in the region where the original lithosphere is thinned and replaced by the mantle flow. The pre-shield Loihi alkalic and tholeiitic basalts are from similar sources, which are a mixture of at least three mantle components: the mantle plume, asthenosphere, and the lower lithosphere. The degree of partial melting averages 10–20%, with a peak value of 30% near the plume center. As a result of continuous compaction, melts are extracted from an active partial melting zone of about 10–20 km thickness, which moves upwards and laterally as the heating and compaction proceed. The rate of melt extraction from the swell increases rapidly to a maximum value of 1 × 105 km3/m.y. over the center of the heat source, corresponding to eruption of large amounts of tholeiitic lavas during the shield-building stage. This volume rate is adequate to account for the observed thickness of the Hawaiian volcanic ridge. Melts from direct partial melting of the mantle plume at depth may be important or even dominant at this stage, although the amount is uncertain. At the waning stage, mixing of melts from the mantle flow pattern with those from low-degree partial melting of the lithosphere may produce postshield alkalic basalts. After the plate moves off the heat source, continuous conductive heating can cause very low degree partial melting (less than 1%) of the lithosphere at shallow depths for about one million years. This process may be responsible for producing post-erosional alkalic basalts. The extraction time for removing such small amount of melts is about 0.4–2 m.y., similar to the time gap between the eruption of post-erosional alkalic lavas and the shield-building stage. Our results show that multi-stage Hawaiian volcanism and the general geochemical characteristics of Hawaiian basalts can be explained by a model of plume-plate interaction.  相似文献   

20.
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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