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1.
The 1998 eruption of Volcán Cerro Azul in the Galápagos Islands produced two intra-caldera vents and a flank vent that erupted more than 1.0×108 m3 of lava. Lava compositions changed notably during the 5-week eruption, and contemporaneous eruptions in the caldera and on the flank produced different compositions. Lavas erupted from the flank vent range from 6.3 to 14.1% MgO, nearly the entire range of MgO contents previously reported from the volcano. On-site monitoring of eruptive activity is linked with petrogenetic processes such that geochemical variations are evaluated in a temporal context. Lavas from the 1998 eruption record two petrogenetic stages characterized by progressively more mafic lavas as the eruption proceeded. Crystal compositions, whole rock major and trace element compositions, and isotope ratios indicate that early lavas are the product of mixing between 1998 magma and remnant magma of the 1979 eruption. Intra-caldera lavas and later lavas have no 1979 signature, but were produced by the 1998 magma incorporating olivine and clinopyroxene xenocrysts. Thus, early magma petrogenesis is characterized by mixing with the 1979 magma, followed by the magma progressively entraining wehrlite cumulate mush.Editorial Responsibility: M.R. Carroll  相似文献   

2.
 The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume (>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode 30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized (to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle processes. Received: 8 March 1996 / Accepted: 30 April 1996  相似文献   

3.
The magmatic plumbing system of Kilauea Volcano consists of a broad region of magma generation in the upper mantle, a steeply inclined zone through which magma rises to an intravolcano reservoir located about 2 to 6 km beneath the summit of the volcano, and a network of conduits that carry magma from this reservoir to sites of eruption within the caldera and along east and southwest rift zones. The functioning of most parts of this system was illustrated by activity during 1971 and 1972. When a 29-month-long eruption at Mauna Ulu on the east rift zone began to wane in 1971, the summit region of the volcano began to inflate rapidly; apparently, blockage of the feeder conduit to Mauna Ulu diverted a continuing supply of mantle-derived magma to prolonged storage in the summit reservoir. Rapid inflation of the summit area persisted at a nearly constant rate from June 1971 to February 1972, when a conduit to Mauna Ulu was reopened. The cadence of inflation was twice interrupted briefly, first by a 10-hour eruption in Kilauea Caldera on 14 August, and later by an eruption that began in the caldera and migrated 12 km down the southwest rift zone between 24 and 29 September. The 14 August and 24–29 September eruptions added about 107 m3 and 8 × 106 m3, respectively, of new lava to the surface of Kilauea. These volumes, combined with the volume increase represented by inflation of the volcanic edifice itself, account for an approximately 6 × 106 m3/month rate of growth between June 1971 and January 1972, essentially the same rate at which mantle-derived magma was supplied to Kilauea between 1952 and the end of the Mauna Ulu eruption in 1971.The August and September 1971 lavas are tholeiitic basalts of similar major-element chemical composition. The compositions can be reproduced by mixing various proportions of chemically distinct variants of lava that erupted during the preceding activity at Mauna Ulu. Thus, part of the magma rising from the mantle to feed the Mauna Ulu eruption may have been stored within the summit reservoir from 4 to 20 months before it was erupted in the summit caldera and along the southwest rift zone in August and September.The September 1971 activity was only the fourth eruption on the southwest rift zone during Kilauea's 200 years of recorded history, in contrast to more than 20 eruptions on the east rift zone. Order-of-magnitude differences in topographic and geophysical expression indicate greatly disparate eruption rates for far more than historic time and thus suggest a considerably larger dike swarm within the east rift zone than within the southwest rift zone. Characteristics of the historic eruptions on the southwest rift zone suggest that magma may be fed directly from active lava lakes in Kilauea Caldera or from shallow cupolas at the top of the summit magma reservoir, through fissures that propagate down rift from the caldera itself at the onset of eruption. Moreover, emplacement of this magma into the southwest rift zone may be possible only when compressive stress across the rift is reduced by some unknown critical amount owing either to seaward displacement of the terrane south-southeast of the rift zone or to a deflated condition of Mauna Loa Volcano adjacent to the northwest, or both. The former condition arises when the forceful emplacement of dikes into the east rift zone wedges the south flank of Kilauea seaward. Such controls on the potential for eruption along the southwest rift zone may be related to the topographic and geophysical constrasts between the two rift zones.  相似文献   

4.
 Lava drainback has been observed during many eruptions at Kilauea Volcano: magma erupts, degasses in lava fountains, collects in surface ponds, and then drains back beneath the surface. Time series data for melt inclusions from the 1959 Kilauea Iki picrite provide important evidence concerning the effects of drainback on the H2O contents of basaltic magmas at Kilauea. Melt inclusions in olivine from the first eruptive episode, before any drainback occurred, have an average H2O content of 0.7±0.2 wt.%. In contrast, many inclusions from the later episodes, erupted after substantial amounts of surface degassed lava had drained back down the vent, have H2O contents that are much lower (≥0.24 wt.% H2O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39–0.51 wt.% H2O in tephra from high fountains to 0.10–0.28 wt.% H2O in spatter from low fountains. The low H2O contents of many melt inclusions from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal that prior to crystallization of the enclosing olivine host, the melts must have exsolved H2O at pressures substantially less than those in Kilauea's summit magma reservoir. Such low-pressure H2O exsolution probably occurred as surface degassed magma was recycled by drainback and mixing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H2O for differentiated tholeiitic magma in Kilauea's summit magma storage reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995) and melt inclusions from Kilauea Iki demonstrate that primary Kilauean tholeiitic magma has an H2O/K2O mass ratio of ∼1.3. At transition zone and upper mantle depths in the Hawaiian plume source, H2O probably resides partly in a small amount of hydrous silicate melt. Received: 31 March 1997 / Accepted: 17 November 1997  相似文献   

5.
The lavas of the 1955 east rift eruption of Kilauea Volcano have been the object of considerable petrologic interest for two reasons. First, the early 1955 lavas are among the most differentiated ever erupted at Kilauea, and second, the petrographic character and chemical composition of the lava being erupted changed significantly during the eruption. This shift, from more differentiated (MgO=5.0–5.7%) to more magnesian (MgO=6.2–6.8%) lava, has been variously interpreted, as either due to systematic excavation of a zoned, differentiated magma body, or to invasion of the differentiated magma by more primitive magma, followed by rapid mixing and eruption of the resulting hybrid magmas. Petrologic examination of several nearvent spatter samples of the late 1955 lavas shows abundant evidence for magma mixing, including resorbed and/or reversely zoned crystals of olivine, augite and plagioclase. In addition, the compositional ranges of olivine, plagioclase and groundmass sulfide are very large, implying that the assemblages are hybrid. Core compositions of olivine phenocrysts range from Fo85 to Fo77. The most magnesian olivines in these samples must have originally crystallized from a melt containing 8.0–8.5% MgO, which is distinctly more magnesian than the bulk composition of the late 1955 lavas. The majorelement and trace-element data are either permissive or supportive of a hybrid origin for the late 1955 lavas. In particular, the compositional trends of the 1955 lavas on plots of CaO vs MgO, and the virtual invariance of Al2O3 and Sr in these plagioclase-phyric lavas are more easily explained by magma mixing than by fractionation. The pattern of internal disequilibrium/re-equilibration in the late 1955 spatter samples is consistent with reintrusion and mixing having occurred at least twice, during the latter part of the 1955 eruption. Plagioclase zonation preserves possible evidence for additional, earlier reintrusion events. Least-squares modelling the mixing of early 1955 bulk compositions with various summit lavas±olivine pick the 1952 summit lava as most like the primitive component. The results also indicate the primitive component had MgO=7.5–8.0%, corresponding to liquidus temperatures of 1165–1175°C. The absence of Fe-Ti oxide phenocrysts in the late 1955 lavas implies that the cooler component of the hybrid had T>1110°C. Thus the thermal contrast between the two components may have been as much as 55–65°C, sufficient to produce the conspicuous disequilibrium effects visible in the spatter samples.  相似文献   

6.
Kilauea's 1955 eruption was the first major eruption (longer than 2 days) on its east rift zone in 115 years. It lasted 88 days during which 108 × 106 m3 of lava was erupted along a discontinuous, 15-km-long system of fissures. A wide compositional range of lavas was erupted including the most differentiated lavas (5.0 wt% MgO) from a historic Kilauea eruption. Lavas from the first half of the eruption are strongly differentiated (5.0–5.7 wt% MgO); later lavas are weakly to moderately differentiated (6.2–6.7 wt% MgO). Previous studies using only major-element compositions invoked either crystal fractionation (Macdonald and Eaton 1964) or magma mixing (Wright and Fiske 1971) as models to explain the wide compositional variation in the lavas. To further evaluate these models detailed petrographic, mineralogical, and whole-rock, major, and trace element XRF analyses were made of the 1955 lavas. Plagioclase and clinopyroxene in the early and late lavas show no petrographic evidence for magma mixing. Olivines from both the early and late lavas show minor resorption, which is typical of tholeiitic lavas with low MgO contents. Core-to-rim microprobe analyses across olivine, augite, and plagioclase mineral grains give no evidence of disequilibrium features related to mixing. Instead, plots of An/Ab vs distance from the core (D) and %Fo vs (D)4.5 generated essentially linear trends indicative of simple crystal fractionation. Least-squares, mass-balance calculations for major- and trace-element data using observed mineral compositions yield excellent results for crystal fractionation (sum of residuals squared <0.01 for major elements, and <5% for trace elements); magma mixing produced less satisfactory results especially for Cr. Furthermore, trace-element plots of Zr vs Sr, Cr, and A12O3 generate curved trends indicative of crystal fractionation processes. There is no evidence that mixing occurred in the 1955 lavas. Instead, the data are best explained by crystal fractionation involving a reservoir that extends at least 15 km along Kilauea's east rift zone. A dike was intruded into the rift zone from the summit reservoir eight days after the eruption started. Instead of causing magma mixing, the dike probably acted as a hydraulic plunger forcing more of the stored magma to be erupted.  相似文献   

7.
Caldera morphology on the six historically active shield volcanoes that comprise Isabela and Fernandina islands, the two westernmost islands in the Galapagos archipelago, is linked to the dynamics of magma supply to, and withdrawal from, the magma chamber beneath each volcano. Caldera size (e.g., volumes 2–9 times that of the caldera of Kilauea, Hawai'i), the absence of well-developed rift zones and the inability to sustain prolonged low-volumetric-flow-rate flank eruptions suggest that magma storage occurs predominantly within centrally located chambers (at the expense of storage within the flanks). The calderas play an important role in the formation of distinctive arcuate fissures in the central part of the volcano: repeated inward collapse of the caldera walls along with floor subsidence provide mechanisms for sustaining radially oriented least-compressive stresses that favor the formation of arcuate fissures within 1–2 km outboard of the caldera rim. Variations in caldera shape, depth-to-diameter ratio, intra-caldera bench location and the extent of talus slope development provide insight into the most recent events of caldera modification, which may be modulated by the episodic supply of magma to each volcano. A lack of correlation between the volume of the single historical collapse event and its associated volume of erupted lava precludes a model of caldera formation linked directly to magma withdrawal. Rather, caldera collapse is probably the result of accumulated loss from the central storage system without sufficient recharge and (as has been suggested for Kilauea) may be aided by the downward drag of dense cumulates and intrusives.  相似文献   

8.
 Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. 210Po–210Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify that they crystallized at moderate depths (∼8–9 km) within the volcano, which is approximately 1 km below the hypocenters for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current magma chamber is considerably deeper than the shallow magma chamber (∼3–4 km) in the adjoining active shield volcanoes. Received: 21 August 1997 / Accepted: 15 February 1998  相似文献   

9.
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   

10.
The 1960 Kapoho lavas of Kilauea’s east rift zone contain 1–10 cm xenoliths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Textures are poikilitic (ol+sp+cpx in pl) and intergranular (cpx+pl±ol±opx). Poikilitic xenoliths, which we interpret as cumulates, have the most primitive mineral compositions, Fo82.5, cpx Mg# 86.5, and An80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, and overall ‘open’ textures to suggest that they represent ‘mush’ and ‘crust’ of a magma crystallization environment. Their phase compositions are more evolved (Fo80–70, cpx Mg# 82–75, and An73–63) than those of the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO2 3.7–5.8 wt%). The gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various Kilauea lavas to one another. FeO/MgO crystal–liquid partitioning is consistent with the poikilitic ol-gabbro assemblage forming as a crystallization product from Kilauea summit magma with ∼8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to early 1955 rift-zone lavas (∼5 wt% MgO) through ∼28–34% crystallization of the ol+sp+cpx+pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivine gabbro-norite assemblages crystallized from evolved liquids, such as represented by the early 1955 and late 1955 lavas (∼6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, then, scoured the rift-zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift-zone magmas. Received: January 7, 1993/Accepted: November 23, 1993  相似文献   

11.
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   

12.
Apoyo caldera, near Granada, Nicaragua, was formed by two phases of collapse following explosive eruptions of dacite pumice about 23,000 yr B.P. The caldera sits atop an older volcanic center consisting of lava flows, domes, and ignimbrite (ash-flow tuff). The earliest lavas erupted were compositionally homogeneous basalt flows, which were later intruded by small andesite and dacite flows along a well defined set of N—S-trending regional faults. Collapse of the roof of the magma chamber occurred along near-vertical ring faults during two widely separated eruptions. Field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast, followed by eruption column collapse, which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse. A period of quiescence was marked by the eruption of scoria-bearing tuff from the nearby Masaya caldera and the development of a soil horizon. Violent plinian eruptions then resumed from a vent located within the caldera. A second phase of caldera collapse followed, accompanied by the effusion of late-stage andesitic lavas, indicating the presence of an underlying zoned magma chamber. Detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume. Mapping of the Apoyo airfall and ignimbrite deposits gives a volume of 17.2 km3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km3 (10.7 km3 Dense Rock Equivalent), approximately the volume necessary to fill the caldera. A vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions. This vent area is controlled by N—S-trending precaldera faults related to left-lateral motion along the adjacent volcanic segment break. Fractional crystallization of calc-alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at Apoyo. Prior to caldera collapse, highly atypical tholeiitic magmas resembling low-K, high-Ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber. The injection of tholeiitic magmas may have contributed to the paroxysmal caldera-forming eruptions.  相似文献   

13.
The main break-in-slope on the northern submarine flank of Molokai at −1500 to −1250 m is a shoreline feature that has been only modestly modified by the Wailau landslide. Submarine canyons above the break-in-slope, including one meandering stream, were subaerially carved. Where such canyons cross the break-in-slope, plunge pools may form by erosion from bedload sediment carried down the canyons. West Molokai Volcano continued infrequent volcanic activity that formed a series of small coastal sea cliffs, now submerged, as the island subsided. Lavas exposed at the break-in-slope are subaerially erupted and emplaced tholeiitic shield lavas. Submarine rejuvenated-stage volcanic cones formed after the landslide took place and following at least 400–500 m of subsidence after the main break-in-slope had formed. The sea cliff on east Molokai is not the headwall of the landslide, nor did it form entirely by erosion. It may mark the location of a listric fault similar to the Hilina faults on present-day Kilauea Volcano. The Wailau landslide occurred about 1.5 Ma and the Kalaupapa Peninsula most likely formed 330±5 ka. Molokai is presently stable relative to sea level and has subsided no more than 30 m in the last 330 ka. At their peak, West and East Molokai stood 1.6 and 3 km above sea level. High rainfall causes high surface runoff and formation of canyons, and increases groundwater pressure that during dike intrusions may lead to flank failure. Active shield or postshield volcanism (with dikes injected along rift zones) and high rainfall appear to be two components needed to trigger the deep-seated giant Hawaiian landslides.  相似文献   

14.
The Hilina Formation comprises the oldest sequence of lava flows and tuffs exposed on Kilauea Volcano. These rocks are only exposed in kipukas in younger Puna Formation lavas along cliffs on the south flank of Kilauea Volcano. Locally, tuffs and flows of the Pahala Formation separate the underlying Hilina Formation rocks rom the overlying Puna Formation rocks. Charcoal collected from the base of the Pahala Formation yielded a C14 age of 22.800±340 years B.P. which defines a minimum age for the Hilina Formation. Hilina Formation lavas crop out over a wide region and probably originated from the summit area and from both rift zones. The Hilina Formation contains both olivine-controlled and differentiated lavas (using the terminology ofWright, 1971). The olivine-controlled lavas of the Hilina Formation are distinguishable mineralogically and geochemically from younger olivine-controlled Kilauea lavas. The younger lavas generally contain discrete low-calcium pyroxene grains. greater glass contents, higher K2O/P2O5 ratios and lower total iron contents. Similar geochemical trends prevail for Manuna Loa lavas, and may typify the early lavas of Hawaiian shield volcanoes. Despite these similarities, the Hilina Formation (and all Kilauea) lavas have higher TiO2 and CaO, and lower SiO2 and Al2O3 contents than Mauna Loa Lavas. These differences have existed for over 30,000 years. Therefore, it is unlikely that the older lavas of Kilauea are compositionally similar to recent Mauna Loa lavas as was previously suggested. K2O, TiO2, Na2 and Zr contents of lavas from a stratigraphic sequence of Hilina Formation lavas are variable. These variations may be utilized to subdivide the sequence into geochemical groups. These groups are not magma batches. Rather, they represent lavas from batches whose compositions may have been modified by crystal fractionation and magma mixing.  相似文献   

15.
Three major phases are distinguished during the growth of Nyiragongo, an active volcano at the western limit of the Virunga Range, Zaire. Lavas erupted during phase 1 are strongly undersaturated melilitites characterized by the presence of kalsilite phenocrysts, perovskite, and the abundance of calcite in the matrix. Such lavas crop out mainly on the inner crater wall and progressively evolve toward more aphyric melilite nephelinites well represented on the flanks of the volcano. Adventive vents lying at the base of the cone developed along radial fracture systems and erupted olivine and/or clinopyroxene – rich melilitites or nephelinites. Stage 2 lavas are melilite-free nephelinites. Clinopyroxene is the main phenocryst and feldspathoids are abundant in the lavas exposed on the crater wall. These flows result from periodic overflowing of a magma column from an open crater. Extensive fissure flows which erupted from the base of the cone at the end of this stage are related to widespread draining out of magma which in turn induces the formation of the summit pit crater. Magmas erupted during stage 3 are relatively aphyric melilite nephelinites and the main volcanological characteristic is the permanent lava lake observed into the pit crater until the 1977 eruption. Fluctuations of the level of the lava lake was responsible for the development of the inner terraces. Periodic overflowing of the lava lake from the central pit formed the nepheline aggregate lava flows. Petrography and major element geochemistry allow the determination of the principal petrogenetic processes. Melilitites and nephelinites erupted from the summit crater are lavas derived, via clinopyroxene fractionation, from a more primitive melt. The abundance of feldspathoids in these lavas is in keeping with nepheline flotation. Aphyric melilite nephelinites covering the flanks and the extensive fissure flows have a homogeneous chemical composition; rocks from the historical lava lake are slightly more evolved. All these lavas differentiated in a shallow reservoir. Lavas erupted from the parasitic vents are mainly olivine and/or clinopyroxene-phyric rocks. Rushayite and picrites from Muja cone are peculiar high-magnesium lavas resulting from the addition of olivine xenocrysts to melilitic or nephelinitic melts. Fluid and melt inclusions in olivine and clinopyroxene phenocrysts indicate a crystallization depth of 10–14 km. A model involving two reservoirs located at different depths and periodically connected is proposed to explain the petrography of the lavas; this hypothesis is in accordance with geophysical data. Received: July 8, 1993/Accepted: September 10, 1993  相似文献   

16.
Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan   总被引：1，自引：0，他引：1  
During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO2), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO2). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt  相似文献   

17.
A summit eruption of Kartala commenced on September 8th, 1972 and finished on October 5th, 1972. In the course of this eruption, approximately 5×106 m3 of alkali olivine basalt was erupted from a N-S fissure system within and adjacent to the caldera. Aa flows were partly ponded within the caldera, almost filling the 1918 Choungou Chagnoumeni crater pit, and partly spilled NW down the flanks of the volcano. The lavas are of uniform composition, almost identical to those erupted in 1965 and closely resembling the majority of flows erupted during the last 115 years. One-atmosphere melting experiments support petrographic and chemical evidence that the lavas are coctetic, with coprecipitation of olivine, augite and plagioclase. The lavas were crupted at, or close to, their liquidus temperature, determined at approximately 1170°C. Whereas eruptions of Kartala in the nineteenth century were distributed widely along a fissure system approximately 45 km long by 7 km wide, the eruptions since 1918 have been confined to the vicinity of the summit caldera.  相似文献   

18.
For any given volcanic field the compositions of primary melts provide important constraints on models of magmatic processes and volcanic eruptions. In this paper, based on petrography, olivine and bulk rock compositions, two tholeiitic picrites (samples C122 and C123) from Haleakala Volcano, east Maui are evaluated as possible primary melts. Sample C122 (bulk rock MgO = 16.6%) has a high apparent Mg-Fe exchange coefficient, KD, between olivine phenocrysts and bulk rock (0.6). However, major-elements and Ni mass-balance calculations show that the olivines in C122 are in equilibrium with the residual melt (matrix) after closed-system equilibrium fractionation of 25 wt.% olivine. Therefore, the Mg/Fe ratio, Ca content, and Ni content of C122 are consistent with the hypothesis that the bulk composition of C122 is close to a primary melt formed by partial melting of a mantle containing olivine with composition around Fo89 to Fo91. The uniform composition and small size (mostly 0.2–0.3 mm) of the olivine, and the glass patches in the matrix suggest fast ascent, and rapid cooling at shallow depth for C122. On the contrary, sample C123, which has an apparent KD (between the most mafic olivine megacrysts and the bulk rock) close to the equilibrium value (0.27), the multiple planar subgrain boundaries in most of the olivine crystals indicate that it may not be a primary melt unless the deformed olivines are generated at magmatic condition as phenocrysts. If the deformed subgrain boundary texture in olivine could indeed be generated at magmatic condition, then the wide compositional range of olivine crystals in C123 (Fo74 to Fo91) suggests multi-stage crystallization over a wide range of cooling temperatures.The compositions of the two picrites, and a differentiated basalt which does not contain xenocrysts suggest that the Haleakala tholeiites are derived from primary melts with at least 16–17 wt.% MgO. Lavas with such high MgO content are rare in Haleakala and other Hawaiian volcanoes; therefore, most Hawaiian tholeiites must have undergone extensive fractionation histories.  相似文献   

19.
Volcán Las Navajas, a Pliocene-Pleistocene volcano located in the northwestern portion of the Mexican volcanic belt, erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite, and is the only volcano in mainland Mexico known to have erupted pantellerites. Las Navajas is located near the northwestern end of the Tepic-Zacoalco rift and covers a 200-m-thick pile of alkaline basaltic lavas, one of which has been dated at 4.3 Ma. The eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation. During the first stage a broad shield volcano made up of alkali basalts, mugearites, benmoreites, trachytes, and peralkaline rhyolites was constructed. Eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera. The second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls. This stage ended about 200 000 years ago with the eruption of pumice falls and ash flows, which led to the collapse of the southern portion of the volcano to form the second caldera. During the third stage, two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano. Finally, the calc-alkaline volcano Sanganguey was built on the southern flank of Las Lavajas. Alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along NW-trending fractures through the area. Although other mildly peralkaline rhyolites are found in the rift zones of western Mexico, only Las Navajas produced pantellerites. Greater volumes of basic alkaline magma have erupted in the Las Navajas region than in the other areas of peralkaline volcanism in Mexico, a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions.  相似文献   

20.
Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60°C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to < 0.02 wt.% in the subaerially erupted lava at the top, and is a measure of the depth of water (or ice) above the eruptive vent. The extensive subsurface crystallization that generates the more evolved, lower-temperature melts during the growth of the tuyas, apparently results from cooling and degassing of magma contained in shallow magma chambers and feeders beneath the volcanoes. Cooling may result from percolation of meltwater down cracks, vaporization, and cycling in a hydrothermal circulation. Degassing occurs when progressively lower pressure eruption (as the volcanic vent grows above the ice/water surface) lowers the volatile vapour pressure of subsurface melt, thus elevating the temperature of the liquidus and hastening liquid-crystal differentiation.  相似文献   

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