Effusive natrocarbonatite activity of Oldoinyo Lengai,June 1988

Oldoinyo Lengai in the Tanzanian rift valley is the only active carbonatite volcano in the world and its natrocarbonatitic lavas are unique in composition. The characteristics of effusive natrocarbonatite activity in June 1988 were studied and fresh samples were directly collected from active carbonatitic lava lakes and flows. Analyses of these samples provide the first information on natrocarbonatites since these unusual volcanic rock type was first described from the 1960–1961 eruptions. The analytical results constrain the original chemistry of fresh natrocarbonatite. Temperatures in lava lakes and of carbonatite lava flows range 491–544°C. The natrocarbonatite lava is extremely fluid at these temperatures and reaches incandescence. The most common variety of natrocarbonatite is porphyritic with abundant phenocrysts of nyerereite (Na_0.82K_0.19)₂(Ca, Sr, Ba)_0.975(CO₃)₂ and gregoryite Na_1.74K_0.1(Ca, Sr, Ba)_0.16CO₃, with complex substitution of (CO₃)^2- by (SO₄)^2-, (PO₄)^3-, F^-, and Cl^-. A phenocryst-poor to aphyric natrocarbonatite variety reflects residual liquids separating from the crystal-rich porphyritic flows. Sylvite, fluorite, and Fe-alabandite (Mn_0.7Fe_0.3S) have been identified as additional primary magmatic phases. Rare phases in the matrix are witherite (BaCO₃) and sellaite (MgF₂). Sylvite and gregoryite, and to a lesser extent nyerereite, are water-soluble and are responsible for the immediate decomposition and chemical alteration of natrocarbonatites under atmospheric conditions. A peralkaline combeite-bearing nephelinite lava is closely related to the natrocarbonatite activity, and is isotopically indistinguishable. It is likely that these two magma compositions are related by liquid immiscibility. The unusual hyperalkaline composition of both magma types makes Oldoinyo Lengai an exotic volcano, and its carbonatites have extreme compositions, and are not representative of carbonatites in general.     

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940–1941, and 1966–1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007–2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe–Ti oxides, and a Na–Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite–wollastonite–melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25–30 wt.% SiO₂, with 7–11 wt.% CO₂, high alkalies (Na₂O 15–19%, K₂O 4–5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite–wollastonite nephelinite (CWN) of OL by enrichment of CO₂ and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.     

Vapour transport of rare earth elements (REE) in volcanic gas: Evidence from encrustations at Oldoinyo Lengai

Fumarolic encrustations and natrocarbonatite lava from the active crater of Oldoinyo Lengai volcano, Tanzania, were sampled and analysed. Two types of encrustation were distinguished on the basis of their REE content, enriched (~ 2800–5600 × [REE_chondrite]) and depleted (~ 100–200 × [REE_chondrite]) relative to natrocarbonatite (1700–1900 × [REE_chondrite]. REE-enriched encrustations line the walls of actively degassing fumaroles, whereas REE-depleted encrustations occur mainly along cracks in and as crusts on cooling natrocarbonatite lava flows; one of the low REE encrustation samples was a stalactite from the wall of a possible fumarole. The encrustations are interpreted to have different origins, the former precipitating from volcanic gas and the latter from meteoric/ground water converted to steam by the heat of the overlying lava flow(s). REE-profiles of encrustations and natrocarbonatite are parallel, suggesting that there was no preferential mobilization of specific REE by either volcanic vapour or meteoric water vapour. The elevated REE-content of the first group of encrustations suggests that direct REE-transport from natrocarbonatite to volcanic vapour is possible. The REE trends observed in samples precipitating directly from the volcanic vapour cannot be explained by dry volatility based on the available data as there is no evidence in the encrustation compositions of the greatly enhanced volatility predicted for Yb and Eu. The observed extreme REE-fractionation with steep La/Sm slopes parallel to those of the natrocarbonatite reflects solvation and complexation reactions in the vapour phase that did not discriminate amongst the different REE or similar transport of REE in both the natrocarbonatite magma and its exsolving vapour. The low concentrations of REE in the encrustations produced by meteoric vapour suggest that the temperature was too low or that this vapour did not contain the ligands necessary to permit significant mobilization of the REE.     

Voluminous lava flows at Oldoinyo Lengai in 2006: chronology of events and insights into the shallow magmatic system

The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO₂). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 10⁵ m³. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.     

Origin and emplacement of the andesite of Burroughs Mountain,a zoned,large-volume lava flow at Mount Rainier,Washington, USA

Burroughs Mountain, situated at the northeast foot of Mount Rainier, WA, exposes a large-volume (3.4 km³) andesitic lava flow, up to 350 m thick and extending 11 km in length. Two sampling traverses from flow base to eroded top, over vertical sections of 245 and 300 m, show that the flow consists of a felsic lower unit (100 m thick) overlain sharply by a more mafic upper unit. The mafic upper unit is chemically zoned, becoming slightly more evolved upward; the lower unit is heterogeneous and unzoned. The lower unit is also more phenocryst-rich and locally contains inclusions of quenched basaltic andesite magma that are absent from the upper unit. Widespread, vuggy, gabbronorite-to-diorite inclusions may be fragments of shallow cumulates, exhumed from the Mount Rainier magmatic system. Chemically heterogeneous block-and-ash-flow deposits that conformably underlie the lava flow were the earliest products of the eruptive episode. The felsic–mafic–felsic progression in lava composition resulted from partial evacuation of a vertically-zoned magma reservoir, in which either (1) average depth of withdrawal increased, then decreased, during eruption, perhaps due to variations in effusion rate, or (2) magmatic recharge stimulated ascent of a plume that brought less evolved magma to shallow levels at an intermediate stage of the eruption. Pre-eruptive zonation resulted from combined crystallization–differentiation and intrusion(s) of less evolved magma into the partly crystallized resident magma body. The zoned lava flow at Burroughs Mountain shows that, at times, Mount Rainier’s magmatic system has developed relatively large, shallow reservoirs that, despite complex recharge events, were capable of developing a felsic-upward compositional zonation similar to that inferred from large ash-flow sheets and other zoned lava flows.     

Replenishment of a mafic magma in a zoned felsic magma chamber beneath Rishiri Volcano, Japan

The trachytic Tanetomi lava from Rishiri Volcano, northern Japan, provides useful information concerning how a replenished mafic magma mixes with a compositionally zoned felsic magma in a magma chamber. The Tanetomi lava was erupted in the order of Lower lava 1 (LL1, 59.2-59.8 wt.% in SiO₂), Lower lava 2 (LL2, 58.4-59.1 wt.%), and Upper lava (UL, 59.9-65.1 wt.%). Evidence for mixing with a mafic magma is observed only in the LL2, in which a greater amount of crystals derived from the mafic magma occurs in rocks with higher SiO₂ content. The whole-rock compositional trend of the Tanetomi lavas is fairly smooth except for the LL2 lava composition, which scatter along the main composition trend. There is no reasonable composition of basaltic magma on the extrapolation of the LL2 composition trend, and the trend cannot be explained by a simple two-component magma mixing. Before the replenishment, the felsic magma was zoned in composition (58-65 wt.% in SiO₂) and temperature (1030-920°C) in the magma chamber located at the pressure of ~2 kbar. The compositional variation of the main felsic magma was produced by extraction of a fractionated interstitial melt from mush zones along the chamber walls and its subsequent mixing with the main magma (boundary layer fractionation). The LL1 magma tapped the magma chamber soon after the replenishment, before the mafic magma mixed with the overall felsic magma. Then the basalt magma mixed heterogeneously with the upper part of the felsic magma by forced convection as a fountain during injection. The mixing of the basalt magma with compositionally zoned felsic magma resulted in the characteristic composition trend of the LL2. The fraction of basaltic magma in the LL2 magma is estimated to be at most 10%. Despite such a small proportion, the basalt magma was mixed completely with the felsic magma, probably because the crystallinity of undercooled basalt magma was low enough to behave as a liquid.     

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.     

Petrologic constraints on rift-zone processes

The Puu Oo eruption in the middle of Kilauea volcano's east rift zone provides an excellent opportunity to utilize petrologic constraints to interpret rift-zone processes. Emplacement of a dike began 24 hours before the start of the eruption on 3 January 1983. Seismic and geodetic evidence indicates that the dike collided with a magma body in the rift zone. Most of the lava produced during the initial episode of the Puu Oo eruption is of hybrid composition, with petrographic and geochemical evidence of mixing magmas of highly evllved and more mafic compositions. Some olivine and plagioclase grains in the hybrid lavas show reverse zoning. Whole-rock compositional variations are linear even for normally compatible elements like Ni and Cr. Leastsquares mixing calculations yield good residuals for major and trace element analyses for magma mixing. Crystal fractionation calculations yield unsatisfactory residuals. The highly evolved magma is similar in composition to the lava from the 1977 eruption and, at one point, vents for these two eruptions are only 200 m apart. Possibly both the 1977 lava and the highly evolved component of the episode 1 Puu Oo lava were derived from a common body of rift-zone-stored magma. The more mafic mixing component may be represented by the most mafic lava from the January 1983 eruption; it shows no evidence of magma mixing. The dike that was intruded just prior to the start of the Puu Oo eruption may have acted as a hydraulic plunger causing mixing of the two rift-zone-stored magmas.     

Phenocryst crystallization during ascent of alkali basalt magma at Rishiri Volcano, northern Japan

Phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers. A fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions, on the basis of their large sizes. However, this assumption is not always true as demonstrated here for a porphyritic alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan. All phenocryst phases in the Kutsugata lava, plagioclase, olivine, and augite, have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals. Rarely, a core region with distinct composition is present in all phenocryst phases. Phenocrysts, excluding this core, are occasionally in direct contact with each other, forming crystal aggregates. The equilibrium liquidus temperature of plagioclase, the dominant phase (35 vol%) in the Kutsugata lava, can never exceed the estimated magmatic temperature, unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent. This suggests that most phenocrysts in the Kutsugata lava were formed by decompression of the magma during ascent in a conduit, rather than by cooling during residence in a magma reservoir. In the magma chamber before eruption, probably located at depth of more than 7 km, only cores of the phenocrysts were present and the magma was nearly aphyric (<5 vol% crystals), though the observed rock is highly porphyritic with up to 40 vol% crystals. The Kutsugata magma is inferred to have been rich in dissolved H₂O (>4 wt.%) in the magma chamber, and liquidus temperatures of phenocryst phases were significantly suppressed. Large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution. Low ascent rate of the Kutsugata magma, which is suggested by pahoehoe lava morphology and no association of pyroclastics, gave sufficient time for crystallization. Furthermore, the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size, which characterizes the porphyritic nature of the Kutsugata lava. Alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems.     

Effects of eruption and lava drainback on the H2O contents of basaltic magmas at Kilauea Volcano

 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     

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ～ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ～ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions.The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H₂O, several hundred ppm of CO₂, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ～ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H₂O, 8700 ppm Cl, 6000 ppm F, and CO₂ below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ～ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H₂O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.     

Gas analyses from the Pu'u O'o eruption in 1985, Kilauea volcano,Hawaii

Volcanic gas samples were collected from July to November 1985 from a lava pond in the main eruptive conduit of Pu'u O'o from a 2-week-long fissure eruption and from a minor flank eruption of Pu'u O'o. The molecular composition of these gases is consistent with thermodynamic equilibrium at a temperature slightly less than measured lava temperatures. Comparison of these samples with previous gas samples shows that the composition of volatiles in the magma has remained constant over the 3-year course of this episodic east rift eruption of Kilauea volcano. The uniformly carbon depleted nature of these gases is consistent with previous suggestions that all east rift eruptive magmas degas during prior storage in the shallow summit reservoir of Kilauea. Minor compositional variations within these gas collections are attributed to the kinetics of the magma degassing process.     

Genesis of the Nyiragongo lavas

Mt. Nyiragongo is one of the eight major volcanoes of the large Virunga volcanic field in the Lake Kivu area in the Eastern Congo. The lavas of Nyiragongo are rather unique. Starting from the top of the mountain, the rocks are nephelinites with some leucite and melilite. The molten material of the present-day lava lake belongs to this type of lava. Under the nephelinites, there is a thin series of leucite-rich lava beds. The main part of the volcano consists of bergalitic melitite lavas alternating with pyroclastics of similar composition. The nephelinitic material is considered to constitute the main portion of the pre-volcanic magma under the future volcano. It is pointed out that the Nyiragongo represents just the type of volcano with which the African volcanic carbonatites are associated. It is concluded that the Nyiragongo nephelinite must be interpreted in a way accepted for the Central African volcanic nephelinites in general. The bergalitic melilitite material is interpreted as a product of carbonation of the nephelinitic magma.     

The November 2002 eruption of Piton de la Fournaise,Réunion: tracking the pre-eruptive thermal evolution of magma using melt inclusions

The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo₈₁) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo_83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo_83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe²⁺ loss by diffusion during cooling. Melt inclusions in Fo₈₁ olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo₈₁ olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo_83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe²⁺ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.     

Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore,Massif Central,France)

Three magmatic units (Grande Cascade pyroclastic deposits, Grande Cascade lava flow, Durbise nuée ardente deposits) from the Quaternary volcano Sancy (Mont-Dore area, Auvergne, France) show textural evidences of magma mixing between a silica undersaturated basic magma (alkali basalt and hawaiite) and an acid magma (quartz-bearing trachyte). Three kinds of mixed rock types are described: basic inclusions within an acid host, « emulsified rocks » showing infracentimetric basic globules disseminated within an acid groundmass, and « banded rocks » in which elongated acid and basic zones alternate. The chemical compositions of mixed rocks plot systematically onto linear trends in petrographic diagrams. Microprobe analyses of the groundmass show similar linear variations between basic and acid end-members. The mineralogical associations of these mixed rocks are highly complex and present many disequilibrium features. Olivine is stable in the basic component and becomes rimmed by orthopyroxene in the acid one. Zoning patterns of feldspars are complex. Clinopyroxene, kaersutite and phlogopite phenocrysts have increasing component Mg contents from core to rim both in the basic and the acid. Titanomagnetite and hemoilmenite phenocrysts were equilibrated at 900-800° C under high oxygen fugacities.Mixing results primarily from a mechanical disintegration of partly liquid basic inclusions within their acid host, and also from a mechanical transfer of phenocrysts from one component to the other, in which they often remain surrounded by a coating of their original groundmass. Chemical data on the groundmass indicates that some « true » hybridization between coexisting acid and basic liquids may also have occurred. The extent of mixing is controlled by the type of emplacement. For pyroclastic deposits a chemical gap exists between basic inclusions and their acid hosts; in contrast, mechanical mixing was enhanced during the emplacement of the viscous Grande Cascade lava flow, and complete transitions occur between basic and acid components. The two end-members are genetically associated, the latter deriving from the former by crystal fractionation. Mixing appears as a late-stage phenomenon in the petrogenetic history of the Mont-Dore series; in the case of the Grande Cascade lava flow, its extent is primarily dependent on emplacement modalities.     

Lava lake oscillations and the magma reservoir beneath a volcano

Stagnation of magma beneath a volcano very likely produces a considerable body of magma, the so called magma reservoir. Assuming an active lava lake being connected with an underneath magma reservoir through a vertical conduit, the height of the surface of the lava lake may be expected to show tidal fluctuations which are caused by squeezing out and draining back of magma from a magma reservoir due to earth tides. Examples are shown in the case of Halemaumau lava lake, Kilauea, in 1919. A similar behaviour also appeared in 1968 which showed semidiurnal tilt of the summit area. It is interesting to notice that the semidiurnal oscillation of the surface of the lava lake appeared only at the heighest level of the lava lake activity. This evidence implies that during the early stage of the activity, a part of the lava filled feeding dikes and open cracks and consequently tidal oscillations of the lava lake were masked and could not be observed.     

The magma budget of Volcan Arenal, Costa Rica from 1968 to 1980

The volume of magma emitted by Volcan Arenal from July 1968 to March 1980 has been calculated to be 304 × 10⁶ m³ (dense rock equivalent). Most of this magma has been emplaced as block lava flows on the western flanks of the volcano following the initial explosive eruptions in 1968. From 1968 to 1973 the volumetric discharge rate of magma decreased from about 3-2 m³ s⁻¹ to about 1 m³ s⁻¹. During a break in activity in late 1973 the site of effusion moved from Crater A to Crater C about 400 m higher. Subsequent effusion was at a lower rate (0.3 m³ s⁻¹) which remained constant for the next six years. Comparison of dry-tilt measurements during this latter period of steady-state effusion with numerical finite-element models of Arenal's elastic response to the evacuation of magma from an underlying reservoir favor a very shallow reservoir (< 2 km depth) to explain the data. However, the constraints imposed by the measured volumes of magma are not compatible with such a reservoir. Instead, it is argued that the steady downward tilting of the volcano's summit was caused by the loading of the western side of the volcano by about 19 × 10⁶ m³ of lava. Surface loading by lava flows may be an important deformational effect at other volcanoes. A system of magma supply involving open conduits (pipes) for the uppermost one kilometer and transitory conduits (cracks) to a crustal reservoir is proposed. This crustal reservoir initially contained a compositionally graded magma which was evacuated from 1968 to 1973. The subsequent abrupt decrease in effusion rate is compatible with the increased magmatic head required to reach Crater C. The constancy of magma composition and effusion rate from 1974 to 1980 implies a homogeneous magma reservoir.     

长白山天池火山全新世3期浮岩长石斑晶中熔体包裹体高温热台实验研究

长白山天池火山在全新世曾多次喷发,以往研究结果基本上认定至少有3期喷发物:距今约5,000a前淡黄色碱流质浮岩、距今约1,000a前灰白色碱流质浮岩和碎屑流以及距今约300a的黑色粗面质浮岩和熔结凝灰岩。喷发物斑晶矿物中含有众多熔体包裹体,形貌特征各有不同。经Leitz1350高温热台对天池火山全新世3期喷发物长石中熔体包裹体的均一法测温实验,结果表明第1,3喷发期的岩浆温度相差不大,而千年大喷发期较为复杂,揭示当时有2个不同温度段的熔体存在,为大喷发可能由2种岩浆注入和混合而触发的论点提供了线索。实验还证实,个体小的熔体包裹体易于均一,而个体较大的、尤其是含有大量子晶的熔体包裹体则很难均一;均一温度与熔体包裹体大小、何时从较快升温速率变为较慢升温速率、是否为初次升温时测量得出有关。同一颗熔体包裹体多次数升温至均一,每次得到的均一温度不同,而且每升温一次得到的均一温度均比前一次高,证实了均一过程中的确有H扩散的现象     

Factors influencing the height of Hawaiian lava fountains: implications for the use of fountain height as an indicator of magma gas content

The heights of lava fountains formed in Hawaiian-style eruptions are controlled by magma gas content, volume flux and the amounts of lava re-entrainment and gas bubble coalescence. Theoretical models of lava fountaining are used to analyse data on lava fountain height variations collected during the 1983–1986 Pu'u 'O'o vent of Kilauea volcano, Hawaii. The results show that the variable fountain heights can be largely explained by the impact of variations in volume flux and amount of lava re-entrainment on erupting magmas with a constant gas content of 0.32 wt.% H₂O. However, the gas content of the magma apparently declined by 0.05 wt.% during the last 10 episodes of the eruption series and this decline is attributed to more extensive pre-eruption degassing due to a shallowing of the sub-vent feeder dike. It is concluded that variations in lava fountain height cannot be simply interpreted as variations in gas content, as has previously been suggested, but that fountain height can still be a useful guide to minimum gas contents. Where sufficient data are available on eruptive volume fluxes and extent of lava entrainment, greatly improved estimates can be made of magma gas content from lava fountain height.     

Ultramafic inclusions from San Carlos,Arizona: Petrologic and geochemical data bearing on their petrogenesis

Ultramafic inclusions from San Carlos, Arizona, are classified into two groups. Group I inclusions are dominated by magnesian (Mg/Mg + ΣFe= 0.86 – 0.91), olivine-rich peridotites containing Cr-rich clinopyroxene and spinel. The less abundant Group I pyroxenites (containing Mg- and Cr-rich pyroxenes) occur as discrete inclusions and as portions of composite inclusions where they have a sharp, planar interface with lherzolite. Group II inclusions are dominated by clinopyroxene-rich peridotites containing Al- and Ti-rich augite and commonly abundant, Al-rich spinel. Compared to Group I inclusions, they are more Fe-rich (Mg/Mg + ΣFe= 0.62 – 0.78) and more hetereogeneous in composition and modal proportions. Similar groups occur at many ultramafic inclusion localities.Our petrographic and geochemical results lead to the following conclusions. Olivine-rich Group I inclusions are not genetically related to the host basanite, and they are formed from two components. Component A is a partial melting residue; it comprises the major portion of these inclusions and determines the modal mineralogy and major and compatible trace element composition. Component B results from a small degree (<5%) of garnet peridotite melting (probably, within the low-velocity zone). This highly LIL-element-enriched melt has migrated upwards into the overlying component A where it crystallized primarily as clinopyroxene and amphibole, and thus, introduced LIL elements into the residual component A. Subsequent cooling and subsolidus recrystallization have removed textural evidence of this mixing. This model has also been proposed for olivine-rich Group I inclusions from Victoria, Australia. At Victoria and San Carlos some relatively clinopyroxene-rich Group I lherzolites are not contaminated by component B, and they represent the best estimates of upper mantle composition prior to melting. Group I orthopyroxenites may be fragments of tectonic layers formed in lherzolite, but they could also be early cumulates (now metamorphosed) from the melt in equilibrium with component A. Group I clinopyroxenites have geochemical features of clinopyroxene in equilibrium with a magma. Thus, they could also represent early cumulates (now metamorphosed) from a magma unrelated to the host basanite. Alternatively, their geochemical characteristics could result from more complex models such as residues from partial remelting of pyroxenite dikes and veins or intradike segregation processes such as filter pressing. All Group II inclusions studied appear to be cumulates derived from a SiO₂-undersaturated magma, possibly an early magma in the same volcanic episode which culminated with eruption of the host basanite. The poikilitic texture of amphibole-rich (kaersutite) inclusions is consistent with a cumulate origin. The bulk compositions of Group II inclusions are not equivalent to typical basaltic compositions.