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1.
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   

2.
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   

3.
Volcanic glass shards from tephra layers in the Byrd Station ice core were chemically analyzed by electron microprobe. Tephra in seven layers have similar peralkaline trachyte compositions. The tephra are believed to originate from Mt. Takahe, on the basis of their chemical similarity to analyzed rocks from Mt. Takahe and because dated rock samples from the volcano are younger than 250,000 years old. Glass shards from 726 m deep in the Dome C ice core, which is 2400 km from Byrd Station, are composed of peralkaline trachyte and may have also been derived from Mt. Takahe. The tephra could have resulted from eruptions which were triggered by increased ice loading during the late Wisconsin glaciation. Preliminary grain size data suggest the eruptions were only minor and they were unlikely to have instantaneously altered global climate as have explosive eruptions in the tropics. Nevertheless, the effect of this localized volcanic activity upon the Antarctic energy budget warrants further investigation.  相似文献   

4.
Volcanic glass shards from three tephra layers at 788, 1457, 1711 m depth in the 2164-m Byrd Station ice core from the West Antarctic Ice Sheet were analysed by electron microprobe. Glass shards within each tephra layer are homogeneous and have peralkaline trachyte compositions. Mt. Takahe, 450 km north-northwest of the drill site is considered the most likely eruptive source, although Toney Mountain, 460 km to the north is also a possible source. Tephra layers in ice cores from the West Antarctic ice sheet may offer a valuable tool for stratigraphic correlation.  相似文献   

5.
The fission track dating method is applied to glass shards from volcanic ash layers in deep sea sediments which have also been dated by other methods. Measured ages for three samples are in excellent agreement with previously determined K-Ar, paleomagnetic and paleontologic ages. The fission track method of dating of glass shards seems to offer itself as a valuable tool for dating marine sediments.  相似文献   

6.
Tephra layers occur in deep-sea sediments of the northeastern Indian Ocean, adjacent to western Indonesian are. The layers range in age from Recent to Late Miocene. Relative abundance of light and heavy mineral species in all tephra layers have been determined, and pure glass shards from representative samples have been analyzed chemically for major oxides. On the basis of the chemical data, three distinct provinces can be recognized: (1) an extensive province of rhyolitic tephra layers, ranging in age back to Late Miocene, is found adjacent to Sumatra; (2) a more restricted province of dacitic layers, adjacent to Sunda Strait and western Java; and (3) a province of andesitic layers, found adjacent to eastern Java and the Lesser Sunda Islands. Chemical composition of tephra layers in each province remains constant with time. As an example, tephra layers from the rhyolitic province are characterized by a high and restricted range of SiO2 (75–77%) when expressed on an H2O-free basis.Tephra layers recovered from the study area were examined for chemical evidence of secondary alteration. The analyses revealed that H2O is the only major oxide in the glass shards which increases progressively with the age of the tephra layers regardless of the bulk composition. H2O, however, reaches a “saturation point” of 4–5% in the layers 250–400 thousands of years old and remains constant to the oldest recovered tephra layer (7.5 m.y. old).The decrease in silica content in deep-sea tephra layers eastward along the Indonesian volcanic arc coincides with a similar eastward decrease in average silica content in Indonesian lavas. A relatively high silica content in lavas from Sumatra, with associated ignimbrites and their deep-sea ash-fall equivalents is closely linked to thick pre-Cenozoic crust. In the portion of the arc to the east of Sumatra, the crust is Cenozoic and thin. Difference in silica content of both the lavas and deep-sea tephras along the Indonesian arc is considered in regard to the hypothesis of “magma filtering” which is based on the contrasting density gradients of ascending magma and the upper crust.  相似文献   

7.
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   

8.
Visible and non-visible (cryptotephra) volcanic ash layers are increasingly being used to underpin the chronology and high-precision correlation of sequences dating to the last glacial–interglacial transition (LGIT). As the number of sediment records analysed for tephra content rises, and methodological developments permit the detection, extraction and chemical analysis of increasingly scantily represented glass shard concentrations, greater complexity in shard count profiles is revealed. Here we present new evidence from sites in Scotland, and review published evidence from sites elsewhere in NW Europe, that indicate complexity in the eruptive history of Katla volcano during the mid-Younger Dryas and Early Holocene. We propose evidence for a previously-overlooked tephra isochron, here named the Abernethy Tephra, which is consistently found to lie close to the Younger Dryas/Holocene transition. It has a major-element chemical composition indistinguishable from that of the Vedde Ash, which was erupted from the Katla volcano at 12,121 ± 114 cal a BP. The new data suggest that Katla may have erupted again between 11,720–11,230 cal a BP and the subsequent ash fall increases the potential to assess environmental response to Holocene warming across north and west Europe.  相似文献   

9.
Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash.Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.  相似文献   

10.
PeterD.  Clift & Jongman  Lee 《Island Arc》1998,7(3):496-512
The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3–4 Ma, ∼ 3–4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front.  相似文献   

11.
Records of Toba eruptions in the South China Sea   总被引：1，自引：0，他引：1  
Three layers of volcanic tephra, sampled from ODP 1143 Site in the South China Sea, were observed at the mcd depth of 5.55 m, 42.66 m, and 48.25 m, and named, in this paper, layers of A, B, and C, respectively. All of these tephra layers have an average thickness of ca. 2 cm. They were constrained in age of ca. 0.070 Ma, ca. 0.80 Ma, and ca. 1.00 Ma, respectively, by the microbiostratigraphy data. These tephra layers were predominated by volcanic glass shards with a median grain size of 70–75 μm in diameter. Major chemical compositions analyzed by EMPA and comparison with the previous data from other scatter areas suggest that these three layers of tephra can correspond to the three layers of Toba tephra, YTT, OTT, and HDT, respectively, erupting during the Quaternary. The occurrence of these tephra layers in the South China Sea implies that the Toba eruptions often occurred in the summer monsoon seasons of the South China Sea during the Quaternary, and that the strength of eruptions was probably stronger than that previously estimated.  相似文献   

12.
The results of magnetic and microprobe studies of the rock sequences in the Mezmaiskaya cave at the unique multilayer Paleolithic site are discussed. The magnetic properties of rocks are analyzed for 17 layers (upwards from layer 4 to layer 1\1) dated from over 73 ka ago to recent times. The rocks of layer 1C (Early Upper Paleolithic, ∼38 ka) are found to have the highest magnetic susceptibility (K) (up to 2500 × 10−6 SI), which is related to the intensive activity of [Homo sapiens]. The minimum K corresponds to the rocks of layer 2, which is overlain by layer 1D. The sizes of magnetic grains vary throughout the rock section. The largest grains are found in the middle part of the section in the Middle Paleolithic layers 2B3, 2B2, 2B1, 2A, and 2. The superparamagnetic fraction is identified in all layers. This fact supports the view that the cave was open as early as the formation of layer 4. According to the thermomagnetic data on the saturation magnetization and the temperature curves of magnetic susceptibility, magnetite is the main carrier of the rock magnetization; some samples contain iron hydroxides. Samples with iron sulfides (pyrite) are abundant. The study of the hysteresis parameters of rocks showed that the question on whether sulfide-bearing rocks are suitable for reliable paleomagnetic determinations requires further laboratory research into the origin of magnetite in the rocks. The chemical composition of rocks composing layer 2B3 and layers 1D (∼39 ka) and 2B1 (∼45 ka), in which the presence of volcanic ash has been previously established according to the presence of volcanic glass, was determined by detailed microprobe analysis. A wide variety of chemical elements (up to 18 items) was recognized in layers 1D and 2B1. The iron, titan, chrome, manganese content, and concentrations of other components vary from grain to grain. The microprobe analysis of samples from layers 1D and 2B1 revealed a set of magnetic particles with compositions characteristic of volcanic rocks, which supports the ash origin of these layers. Layer 2B3 is established not to be volcanic ash. The results on the volcanic glass in the rocks of layers 1D and 2B1 were published by Golovanova and her colleagues in Current Anthropology in October 2010 [Golovanova et al., 2010].  相似文献   

13.
The plinian eruptions of 1912 at Novarupta,Katmai National Park,Alaska   总被引：1，自引：0，他引：1  
The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to 15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23–26 km for A, 22–25 km for CD, and 17–23 km for FG; and peak mass eruption rates of 0.7–1x108, 0.6–2x108, and 0.2–0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of co-plinian rather than co-ignimbrite origin.  相似文献   

14.
Three layers of volcanic tephra, sampled from ODP 1143 Site in the South China Sea,were observed at the mcd depth of 5.55 m, 42.66 m, and 48.25 m, and named, in this paper, lay ers of A, B, and C, respectively. All of these tephra layers have an average thickness of ca. 2 cm.They were constrained in age of ca. 0.070 Ma, ca. 0.80 Ma, and ca. 1.00 Ma, respectively, by the microbiostratigraphy data. These tephra layers were predominated by volcanic glass shards with a median grain size of 70-75 μm in diameter. Major chemical compositions analyzed by EMPA and comparison with the previous data from other scatter areas suggest that these three layers of tephra can correspond to the three layers of Toba tephra, YTT, OTT, and HDT, respectively, erupt ing during the Quaternary. The occurrence of these tephra layers in the South China Sea implies that the Toba eruptions often occurred in the summer monsoon seasons of the South China Sea during the Quaternary, and that the strength of eruptions was probably stronger than that previously estimated.  相似文献   

15.
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   

16.
Il-Soo  Kim  Myong-Ho  Park  Byong-Jae  Ryu Kang-Min  Yu 《Island Arc》2006,15(1):178-186
Abstract   Data on the late Quaternary tephra layers, tephrostratigraphy, geochemistry and environment were determined in two sediment cores from the southwestern part of Ulleung Basin (East Sea/Sea of Japan), representing marine-oxygen isotope stages 1–3. The cores consist mainly of muddy sediments that are partly interbedded with silty sands, lapilli tephra and ash layers. The lapilli tephra layers (Ulleung-Oki tephra, 9.3 ka) originating from Ulleung Island consist mainly of massive-type glass shards, whereas the ash layers (Aira-Tanzawa ash, 22.0–24.7 ka) derived from southern Kyushu Island are mainly composed of typical plane-type and bubble-wall glasses that are higher in SiO2 and lower in Na2O + K2O than the lapilli tephra layers. Except for the tephra layers, fine-grained sediments throughout the core sections are mostly of marine origin based on geochemical data (C/N ratios, hydrogen index, S2 peak) and Tmax. In particular, organic carbon contents increased during Termination I, probably as a result of an influx of the deglacial Tsushima Current through the Korea Strait.  相似文献   

17.
The active crater of Nakadake at Aso Volcano, southwestern Japan, has been occupied by a lake during its dormant periods. Multiple ash emissions were observed from the crater lake between July 2003 and August 2005. The largest events occurred on 10 July 2003 and 14 January 2004. On 10 July 2003, ash (41 tons) was dispersed throughout an area extending 14 km east-northeast of the vent, and on 14 January 2004 ash (32 tons) extended 8 km to the east-southeast. Thereafter, small ash emissions were frequent at the crater lake, in which the water level fell considerably from April to August 2005. During this period major ash emitting events, producing mostly white aggregated ash, occurred on June 10–12, June 21 (4.2 tons) and July 25 (1.2 tons). Ash emissions at the Nakadake crater in 2003–2005 were classified into three types: gentle release of white aggregated ash from fumaroles inside the crater lake (e.g. daytime of 14 April, 10–12 June and 25 July 2005); emission of black ash from an almost dried-up vent (21 June 2005); and short-time (ca. 20 s) small-explosion-triggered gas-and-ash emission through the crater lake (10 July 2003, 14 January 2004 and 14 April 20h41m 2005). All products from these ash emissions consisted of fine-grained (< 1 mm) glass shards, crystals and lithic fragments, and contained neither lapilli nor blocks. Although the glass shards show varying degrees of crystallization and alteration, clear glass shards, which appear fresh, are probably juvenile materials. These observations suggest that the 2003–2005 ash emissions from the crater lake of Nakadake Volcano are related to newly ascending magma.  相似文献   

18.
New field observations with the submersible ALVIN and photographic evidence from a study of the summits of seamounts near the East Pacific Rise show that hyaloclastite deposits occur commonly. Hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m. These new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies. Most of the hyaloclastite samples are of hydrovolcanic eruptive origin, but a few show evidence of a predominantly sedimentary origin. Primarily from morphology, we identify several vent areas from which hyaloclastite presumably erupted. The surface appearance of the hyaloclastite deposits varies with distance to these vents, leading us to propose a facies model for deep-sea hyaloclastites on seamount summits. Hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding-parallel alignment of platy shards. They consist of blocky, sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment. The shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous. We propose that the shards form mainly by cooling-contraction granulation, but cannot rule out the possibility of limited steam explosivity. Hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate, promoting clastic-dominated versus flow-dominated eruptive behavior, is the dominant control on hyaloclastite formation. We propose that shard formation occurs during submarine lava fountaining. Gravitational instability of the resulting slurry of shards, sea water and possibly steam causes gravity flow that carries the shards outward from the vent. Further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism, eruption dynamics and deposition of deep-sea hyaloclastites.  相似文献   

19.
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20.
Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An25 plagioclase, Fs42 orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An45 plagioclase and Fs35, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.  相似文献   

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