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1.
Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan.Recent experimental studies on calc-alkaline andesite-dacite under H2O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H2O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H2O content and bulk chemical composition of the magma.It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H2O contents decrease, towards the volcanic front in the East Japan volcanic arc.Such lateral variations in the H2O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H2O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H2OMg2SiO4z.sbnd;CaMgSi2O6z.sbnd;SiO2, the decrease of H2O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model.  相似文献   

2.
Calc-alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively: (1) fractional crystallization involving separation of titanomagnetite; (2) selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion; and (3) mixing of magmas on continuous fractionation trends. The association of calc-alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas, but the calc-alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends. Chemical variations of most arc volcanic rocks, including calc-alkalic ones, can accordingly be essentially accounted for by the low-pressure fractional crystallization of phenocrystic phases from primary basaltic magmas.Crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages. The crystallization sequence changes laterally across-arc, suggesting increasing water contents in magmas toward the back-arc side, as is also seen for other incompatible elements such as K and Rb. Systematic differences in the characteristic crystallization sequence are also observed among arcs, roughly correlating with the crustal thickness. Water content in magma, like other incompatible elements, tends to increase with increasing crustal thickness. The variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle, which is broadly controlled by the crustal thickness.The variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite. This strongly constrains the maximum water contents in primary basaltic magma, at most 2.5 wt.%. This value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite. Diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus. Diapirs that begin to rise from the subduction zone may stop at or near the crust-mantle boundary because of the surrounding density change, and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature. Across-arc variation is also explained by the stopped depth of diapirs, but is not controlled by crustal thickness.  相似文献   

3.
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   

4.
The Fe/Mg+Fe) ratios (XFe) of the Quaternary basalts (SiO2 < 53 wt.%) in the Japanese arcs were examined. The XXFe of relatively magnesian basalts decreases from the volcanic front toward the Japan Sea across the arcs. Based on the partition coefficient of Mg-Fe2+ between olivine and liquid, it is suggested that all the basalts near the volcanic front, which are mostly tholeiitic basalts, are significantly fractionated, whereas many basalts near the Japan Sea, which are mostly alkali basalts, are little fractionated. The K2 O content in the primary basalt magmas increases toward the Japan Sea. Combining the XFe and K2 O data, it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front, but they fractionate during their ascent, whereas smaller amounts of alkali basalt magmas are formed near the Japan Sea, but they can ascend with less fractionation. The density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas. It is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front, whereas primary alkali basalt magmas can ascend through the upper crust without fractionation, as far as buoyancy is the principal ascending force. In the Japanese arcs, the stress field may be less compressional near the Japan Sea than near the volcanic front, so that magmas can ascend more rapidly in the latter region than in the former. These two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs. The variation in volume of the Quaternary volcanic rocks across the arcs can be explained by the presence of a melt-rich zone above but nearly parallel to the subducted slab.  相似文献   

5.
Fluorine contents in about 160 representative Quaternary volcanic rocks and 15 hornblende and biotite phenocrysts in a calc-alkali series in Japan have been determined by a selective ion-electrode method. Tholeiites have the lowest contents and the narrowest range (58–145 ppm), while alkali basalts have the highest contentws and the widest range (301–666 ppm), high-alumina basalts have intermediate values (188–292 ppm). F contents in basalts clearly increase from east to west across the Japanese Islands, as do alkalies, P2O5 REE, U, Th and H2O.The volcanic rocks studied are divided into two groups on the basis of F: (1) witt, increasing % SiO2 or advancing fractionation, F contents show either progressive enrichment; or (2) with increasing fractionation, F contents show rather constant values. The former is produced by fractionation of anhydrous phases from basalt to mafic andesite magmas; the tholeiite series of Nasu volcanic zone (outer zone), northeastern, Japan is a typical example. The latter group is derived through separation of amphibole-bearing phases from basaltic magmas at various depths from upper mantle (about 30 km) to upper crust; the alkali series in southwestern Japan and the calc-alkali series of Chokai volcanic zone (inner zone), northeastern Japan, are examples.  相似文献   

6.
The Spurr volcanic complex (SVC) is a calc-alkaline, medium-K, sequence of andesites erupted over the last 250000 years by the eastern-most currently active volcanic center in the Aleutian arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58%–60% SiO2), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) formed in the caldera. Both the ash flows and dome are made of acid andesite more silicic (60%–63% SiO2) than any analyzed lavas from the ancestral Mt. Spurr, yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53%–57% SiO2) erupted during and after dome emplacement from a separate vent only 3 km away. Hybrid block-and-ash flows and lavas were also produced. The vents for the silicic and mafic lavas are in the center and in the breach of the 5-by-6-km horseshoe-shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic. SVC lavas are plag ±ol+cpx±opx+mt bearing. All postcaldera units contain small amounts of high-Al2O3, high-alkali amphibole, and proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately preexisting magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. Elevated Sr, Pb, and O isotope ratios and trace-element systematics incompatible with fractional crystallization suggest that a significant amount of continental crust from the upper plate has been assimilated by SVC magmas during their evolution.  相似文献   

7.
The Adak volcanic center is located in the central part of the Aleutian arc and consists of three main volcanic vents. Andrew Bay Volcano, the oldest center, has been mostly removed by erosion. The next youngest vent, Mount Adagdak, was built in three major volcanic stages whereas Mount Moffett, the largest volcanic edifice, consists of a main cone and a parasitic cone each with several magmatic phases. Adak is unique compared to other modern Aleutian volcanic centers in that it contains two xenolith suites (Conrad and Kay, 1984; Debari et al., 1987). One suite consisting predominantly of mafic xenoliths occurs on Mount Moffett whereas an assemblage of ultramafic and mafic xenoliths is found on Mount Adagdak. Lavas erupted at Adak span the compositional range from 48.4 to 65.0 wt.% SiO2 and are characterized by significant variations in Al2O3, MgO, Sr, Ni and Cr. On Harker diagrams, this variability produces compositional trends with significant scatter. The Adak suite has total REE contents that vary from 32 to 154 ppm but do not correlate systematically with silica. ( )n ratios range from 2.41 to 21.72 with the majority of lavas between 2.41 and 6.06. On process identification diagrams, the Adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other Aleutian centers. Measured isotopic ranges are large and nearly equal to those for the entire arc. Although they span similar silica ranges, subtle geochemical and isotopic differences distinguish the different volcanic vents of Adak. On Mount Moffett, a geochemically and isotopically distinct group of andesites (55.5–57.9 SiO2), the mafic andesites, occur on its NE flank. These lavas have elevated MgO, Ni and Cr but are depleted in Al2O3 relative to other Mount Moffett andesites with similar silica. They also have more heterogeneous REE abundances and isotopic ratios than most of the other andesites. Significant compositional differences exist between Adak and the other volcanic centers of the central Aleutian arc. Although these differences are characteristic of all geochemical systems, they are greatest for major and rare-earth elements and isotopic ratios. The lack of coherent relationships on major- and trace-element Harker diagrams, the isotopic variability, as well as the steeply sloping trends on REE process identification diagrams suggest that the Adak volcanic suite was not formed predominantly by closed-system crystal fractionation, but must be the product of a complex open-system process(es). The significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite. REE data are also suggestive of a role for magma mixing. Such a complex petrologic evolution is consistent with an immature lithospheric plumbing system. Based on REE systematics, the xenolith suites of Adak cannot, as previously proposed, be related to the host lavas or the rest of the Adak suite through crystal fractionation schemes. Rather they are probably accidental fragments derived from various depths along lithospheric conduits. In light of their relation to xenolith-bearing units, the mafic andesites of Adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock. They are, therefore, characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas. Because of the close proximity of the vents and the subtle compositional differences between their lavas, the Adak volcanic center was probably supplied by a single, deep lithospheric plumbing system that fed separate crustal magma chambers. The absence of historic volcanic activity on Adak suggests this plumbing system was abandoned before complete conduit development. This decline in magmatism may reflect a re-adjustment of volcano spacing within this part of the Aleutian arc.  相似文献   

8.
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   

9.
Calc-alkaline intermediate rocks are spatially and temporally associated with high-Mg andesites (HMAs, Mg#>60) in Middle Miocene Setouchi volcanic belt. The calc-alkaline rocks are characterized by higher Mg# (strongly calc-alkaline trend) than ordinary calc-alkaline rocks at equivalent silica contents. Phenocrysts in the intermediate rocks have petrographical features such as: (1) coexisting reversely and normally zoned orthopyroxene phenocrysts in single rock; (2) sieve type plagioclase in which cores are mantled by higher An%, melt inclusion-rich zone; and (3) reversely zoned amphibole phenocrysts with opacite cores. In addition, mingling textures and magmatic inclusions were observed in some rocks. These petrographic features and the mineral chemistry indicate that magma mixing was the most important process in producing the strongly calc-alkaline rocks. The core composition of normally zoned orthopyroxene phenocrysts and the mantle composition of reversely zoned orthopyroxene phenocrysts have relatively high Mg# (85–90) in maximum. Although basaltic and high-Mg andesitic magmas are candidate as possible mafic end-member magmas, basaltic magma is excluded in terms of phenocryst assemblage and bulk composition. HMA magmas are suitable mafic end-member magmas that precipitated high Mg# (90) orthopyroxene, whereas andesitic to dacitic magma are suitable felsic end-members. In contrast, it is difficult to produce the strongly calc-alkaline trend through fractional crystallization from a HMA magma, because it would require removal of plagioclase together with mafic minerals from the early stage of crystallization, whereas the precipitation of plagiolase is suppressed due to the high water content of HMA magmas. These results imply that Archean Mg#-rich TTGs (>45–55), which are an analog of the strongly calc-alkaline rocks in terms of chemistry and magma genesis, can be derived from magma mixing in which a HMA magma is the mafic end-member magma, rather than by fractional crystallization from a HMA magma.  相似文献   

10.
Quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic and more siliceous type (tholeiite) on the oceanic side to more alkalic and less siliceous type (alkali olivine basalt) on the continental side. In the northeastern part of the Japanese Islands and in Kamchatka, zones of tholeiite, high-alumina basalt, and alkali olivine basalt are arranged parallel to the Pacific coast in the order just named, whereas in the southwestern part of the Japanese Islands, the Aleutian Islands, northwestern United States, New Zealand, and Indonesia, zones of high-alumina basalt and alkali olivine basalt are arranged parallel to the coast. In the Izu-Mariana, Kurile, South Sandwich and Tonga Islands, where deep oceans are present on both sides of the island arcs, only a zone of tholeiite is represented. Thus the lateral variation of magma type is characteristic of the transitional zone between the oceanic and continental structures. Because the variation is continuous, the physico-chemical process attending basalt magma production should also change continuously from the oceanic to continental mantle. Suggested explanations for the lateral variation assuming a homogeneous mantle are: 1) Close correspondence between the variations of depth of earthquake foci in the mantle and of basalt magma type in the Japanese Islands indicates that different magmas are produced at different depths where the earthquakes are generated by stress release: tholeiite at depths around 100 km, high-alumina basalt at depths around 200 km, and alkali olivine basalt at depths greater than 250 km. 2) Primary olivine tholeiite magma is produced at a uniform level of the mantle (100–150 km), and on the oceanic side of the continental margin, it leaves the source region immediately after its production and forms magma reservoirs at shallow depths, perhaps in the crust, where it undergoes fractionation to produce SiO2-oversaturated tholeiite magma, whereas on the continental side, the primary magma forms reservoirs near the source region and stays there long enough to be fractionated to produce alkali olivine basalt magma, and in the intermediate zone, the primary magma forms reservoirs at intermediate depths where it is fractionated to produce high-alumina basalt magma.  相似文献   

11.
Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite.  相似文献   

12.
REE abundances in gabbros and peridotites from Site 334 of DSDP Leg 37 show that these rocks are cumulates produced by fractional crystallization of a primitive oceanic tholeiite magma. They may be part of a layered oceanic complex. The REE distributions in the residual liquids left after such a fractionation are similar to those of incompatible element-depleted oceanic tholeiites. The REE data indicate that the basalts which overlie the gabbro-peridotite complex, are not genetically related to plutonic rocks.  相似文献   

13.
The 2730-Ma-old Hunter Mine Group (HMG), a dominantly felsic subaqueous volcanic sequence, was formed during early arc construction in the Abitibi greenstone belt (Quebec, Canada). The western part of the HMG contains a felsic dyke swarm up to 1.5 km wide and traceable up-section for 2.5 km. Five distinct generations were identified: (1) aphanitic to feldspar-phyric dykes; (2) quartz-feldspar-phyric dykes with < 5% quartz phenocrysts; (3) quartz-feldspar-phyric dykes with 10–25% quartz phenocrysts; (4) dacitic feldspar-phyric dykes; and (5) mafic dykes. The felsic dykes collectively constitute more than 90% of the dyke swarm. Geochemically, they resemble modern calc-alkaline dacites and rhyolites. Their mantle-normalized incompatible trace-element patterns display a moderate enrichment of Th and light REE relative to HFSE and heavy REE as well as negative Nb, Ta, Eu and Ti anomalies. Most of the major- and trace-element abundance variations in these rocks can be explained by crystallization of feldspars. Geochemical data including depleted mantle-like Nd values suggest that an older sialic substrate was not involved in their genesis. We infer that the felsic rocks were generated by melting of mafic oceanic crust. The swarm was emplaced during nascent oceanic island-arc development and was related to rifting of the arc. The conformably overlying MORB-like basalts and basaltic komatiites of the Stoughton-Roquemaure Group used the same conduits and further indicate splitting of the arc. HMG and associated parts of the Abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western Pacific.  相似文献   

14.
Batur is an active stratovolcano on the island of Bali, Indonesia, with a large, well-formed caldera whose formation is correlated with the eruption about 23,700 years ago of a thick ignimbrite sheet. Our study of the volcanic stratigraphy and geochemistry of Batur shows the formation of the caldera was signalled by a change in the composition of the erupting material from basaltic and andesitic to dacitic. The dacitic rocks are glassy, possess equilibrium phenocryst assemblages, and display compositional characteristics consistent with an origin by crystal-liquid fractionation from more mafic parent magmas in a shallow chamber, possibly at 1.5 km depth and 1000–1070°C.However, although separated by a gap of 6 wt.% SiO2, the dacitic rocks are clearly related in their minor- and trace-element geochemistry to those basalts and basaltic andesites erupted after the caldera was formed rather than to the andesites erupted immediately before the dacites first appeared. We infer from this and published experimental modelling of the possible crystallization behaviour of basaltic magma chambers that a magmatic cycle involving caldera formation began independently of the previous activity of Batur by formation of a new, closed-system magma chamber beneath the volcano. Fractional crystallization, possibly at the walls of the chamber, led to the early production of derivative siliceous magmas and, consequently, to caldera formation, while most of the magma retained its original composition. The postcaldera Batur basalts represent the largely undifferentiated core liquids of this chamber.This model contrasts with the traditional evolutionary model for stratovolcano calderas but may be applicable to the origins of calderas similar to that of Batur, particularly those in volcanic island arcs.  相似文献   

15.
The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to 2000 ppm sulfur, but concentrations decrease rapidly with increasing SiO2 down to matrix glass values (50–100 ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (200 ppm S). Chlorine concentrations increase from 750 to 2200 ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2 Mt is not in accordance with a petrologic estimate of 0.09 Mt. Using a published value of 550 Mt of erupted material about 0.34 km3 fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26 km3), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3 Mt chlorine, which agrees well with an output of 0.47 Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.  相似文献   

16.
Compositional heterogeneity (56–64 wt% SiO2 whole-rock) in samples of tephra and lava from the 1986 eruption of Augustine Volcano, Alaska, raises questions about the physical nature of magma storage and interaction beneath this young and frequently active volcano. To determine conditions of magma storage and evolutionary histories of compositionally distinct magmas, we investigate physical and chemical characteristics of andesitic and dacitic magmas feeding the 1986 eruption. We calculate equilibrium temperatures and oxygen fugacities from Fe-Ti oxide compositions and find a continuous range in temperature from 877 to 947°C and high oxygen fugacities (ΔNNO=1–2) for all magmas. Melt inclusions in pyroxene phenocrysts analyzed by Fourier-transform infrared spectroscopy and electron probe microanalysis are dacitic to rhyolitic and have water contents ranging from <1 to ∼7 wt%. Matrix glass compositions are rhyolitic and remarkably similar (∼75.9–76.6 wt% SiO2) in all samples. All samples have ∼25% phenocrysts, but lower-silica samples have much higher microlite contents than higher-silica samples. Continuous ranges in temperature and whole-rock composition, as well as linear trends in Harker diagrams and disequilibrium mineral textures, indicate that the 1986 magmas are the product of mixing between dacitic magma and a hotter, more mafic magma. The dacitic endmember is probably residual magma from the previous (1976) eruption of Augustine, and we interpret the mafic endmember to have been intruded from depth. Mixing appears to have continued as magmas ascended towards the vent. We suggest that the physical structure of the magma storage system beneath Augustine contributed to the sustained compositional heterogeneity of this eruption, which is best explained by magma storage and interaction in a vertically extensive system of interconnected dikes rather than a single coherent magma chamber and/or conduit. The typically short repose period (∼10 years) between Augustine's recent eruptive pulses may also inhibit homogenization, as short repose periods and chemically heterogeneous magmas are observed at several volcanoes in the Cook Inlet region of Alaska.  相似文献   

17.
This paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO2; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.  相似文献   

18.
Origin of andesite and its bearing on the Island arc structure   总被引：1，自引：0，他引：1  
The hypothesis that andesite magmas originate from basalt magmas through fractionation is supported for the following reasons: 1) A close association of andesite and dacite with basalt in many volcanoes and a complete gradation in chemistry and mineralogy throughout this suite. 2) Formation of andesite magmas from basalt magmas by differentiation in situ of some intrusive and extrusive bodies. 3) Agreement between the calculated compositions of solid materials to be subtracted from basalt magmas to yield andesite magmas and the observed mineralogy of phenocrysts in these rocks. 4) Higher alkali contents in andesite and dacite associated with high-alumina basalt than in those associated with tholeiite. 5) A complete gradation from the high iron concentration trend of basalt magma fractionation (Skaergaard) to the low or noniron concentration trend (the calc-alkali series) which can be ascribed to the difference of the stage of magnetite crystallization. 6) Similarity between the orogenic rock suite and plateau basalts in the preferential eruption of magmas of middle fractionation stage, givin rise to the great volume of andesite in the orogenic belts and iron-rich basalt in the plateau lavas. Petrological and seismic refraction studies suggest that a great volume of gabbroic materials are present in the lower crust underneath the volcanic belts as a complementary material for the andesite lavas. The island arc structure would develop by repeated eruption of andesite on the surface and by thickening of the oceanic crust underneath the arc due to the addition of gabbroic materials. The suitable portion of the lower crust may be subjected to partial melting to produce granitic magma in the later stage of development of the arc, successively changing it to a part of the adjacent continent.  相似文献   

19.
Spinel-lherzolite xenoliths have been found in olivine tholeiite near Andover in the Tasmanian Tertiary volcanic province. They show a high-pressure mineralogy of predominant olivine (Mg90), with aluminous enstatite (Mg90) and lesser aluminous diopside and chrome-bearing spinel, and resemble lherzolite xenoliths commonly found in undersaturated lavas. Such xenoliths are unusual in tholeiitic basalts and the occurrence directly attests to a mantle origin for at least some tholeiitic magmas.The lherzolites are accompanied by doleritic and pyroxenitic xenoliths and by olivine, orthopyroxene, clinopyroxene and plagioclase xenocrysts. If near-liquidus phases are represented amongst the xenocrysts, then the magnesian number of the host basalt and its xenocryst assemblage provisionally suggest a magma derived by more than 15–20% partial melting of mantle peridotite, before commencing xenocryst crystallisation at pressures between 8–13 kbar.With this new record, lherzolite-bearing lavas in Tasmania now cover an extremely wide compositional range, extending from highly undersaturated olivine melilitite to olivine tholeiite. They also include a considerable number of fractionated alkaline rocks that are only sparsely reported in the literature as lherzolite hosts. This latter group contains representatives of a previously suggested but unestablished alkaline fractionation series based on olivine nephelinite, viz. calcic olivine nephelinite → sodic olivine nephelinite → potassi-sodic olivine nephelinite → mafic nepheline benmoreite → mafic phonolite.Lherzolite and megacryst-bearing lavas are relatively more abundant in peripheral parts to the main basalt sequences in Tasmania. This suggests that they developed in fringing zones of less intense mantle melting which enhanced stagnation and fractionation of magmas within the mantle before eruption. Calculated crustal thicknesses under these areas suggest that the magmas were generated at pressures exceeding 6–11 kbar, with the Andover tholeiitic magma exceeding 9 kbar.  相似文献   

20.
Refractory megacrysts of olivine, plagioclase, chromian diopside and Cr-Al spinel, which were not in equilibrium with the host oceanic tholeiite on eruption, are present in samples from several dredge sites and DSDP drill sites in the Atlantic and Pacific Oceans. They have multiple origins: (1) cognate or accidental mantle fragments; (2) relict fragments from fractional crystallization of parental liquids considerably more primitive than oceanic tholeiite; and most commonly (3) the fractional crystallization products of such liquids mixed with oceanic tholeiite magma. Melt inclusions in chrome-spinel phenocrysts provide evidence for this postulated Mg- and Ca-rich magma which has counterparts in the Scottish Tertiary Province and in west Greenland.  相似文献   

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