Zoned Magma Chamber Research Articles

Apoyo caldera, Nicaragua: A major quaternary silicic eruptive center

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 km 3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km 3 (10.7 km 3 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.

Vent Evolution and Lag Breccia Formation during the Cape Riva Eruption of Santorini, Greece

The 18,500 yr. b.p. Cape Riva (CR) eruption of Santorini vented several km3 or more of magma, generating four eruption units: a basal Plinian fall deposit (CR-A) and three pyroclastic flow deposits (CR-B to CR-D upwards). CR-B and CR-D are welded ignimbrites; CR-C consists predominantly of up to 25 m thick coarse, lithic-rich co-ignimbrite lag breccias resulting from a climactic phase of the eruption. The initial Plinian phase occurred from a localized vent in N Santorini, and subsequent column collapse resulted in emplacement of CR-B. Towards the end of CR-B, new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias (CR-C). CR-D probably records a return to a localized vent as the eruption waned. The eruption sampled a zoned magma chamber containing rhyodacite overlying andesite, and leaks of these magmas were manifested as the Skaros-Therasia lavas preceding the CR eruption. Plinian and initial ignimbrite stages occurred while the magma chamber was ove...

Petrochemistry of quaternary pumiceous pyroclastic products in southern Guadeloupe (F.W.I.)

Pumiceous pyroclastic products are present as flows and falls at several stages of the evolution of the southern Guadeloupe volcanic island. An understanding of this volcanism had to rely on detailed petrochemical data of these products to complement similar data for effusive rocks so as to yield complete stratigraphical coverage. On the other hand most pumiceous rocks are more or less conspicuousily banded suggesting that mixing phenomena occurred to different degrees in their genesis. Three major classes of pumiceous products are found: (1) the Axial Chain deposits (2.0–0.5 My) are characterized by An90, 75-55 + En55 + Wo42En37 + Usp35-37 ± Fo68 ± Hble, SiO2 60%, SiO2/Th 35.6, and La/Th 3.9. Banded samples have components that differ in evolution indices by about 50%; (2) the Bouillante Chaine pyroclastics (0.3–0.1 My) consist of scattered deposits with variable mineralogical and geochemical compositions that seem to have erupted from a number of small eruptive centers. Qz-dacitic pumice is common with An90, 70-45 + En66-56 + Usp32-38 + Ilm94 + Hble ± Wo40-42En40-42, SiO2 62%, SiO2/Th 22.9, and La/Th >4. Mixed pumice samples have highly contrasted evolution indices differing by up to 120%; (3) the Pintade pumice flows and falls correspond to the major pyroclastic event (approx. 10 km3) in the southern Basse Terre area. They are characterized by An85-70 + En66-56 + Usp32-37 ± Wo42En42, SiO2/Th 18.7-22?6, and La/Th 3.0-4.0. Banded pumice lumps are scarce and show slight compositional contrasts; differences in evolution indices do not exceed 38%. Axial and Bouillante chain pyroclastics and Grande Decouverte volcano pumice respectively, form two different families in trace element plots. Minor elements in pyroxenes also are distinctive. These trends are similar to those obtained for effusive rocks and define comagmatic series. Major and trace element data for the separated components of inhomogeneous pumice in each formation always plot in the corresponding series. These chemical discriminants can be used to attribute samples of unknown provenance to a given volcanic ensemble. An inverse relationship between differences in evolution indices in inhomegeneous pumice and the volume of any single eruptive sequence is noted. This is an indication that pumiceous pyroclastic rocks were erupted from a zoned magma chamber. We favor an interpretation where zonation is produced by influx of less envolved magma in superficial differentiated chambers which is a direct cause for eruption.

Petrogenesis of the Zoned Laacher See Tephra

The late Quaternary Laacher See phonolitic tephra deposit (East Eifel, W. Germany) is mineral-ogically and chemically zoned from highly evolved, volatile-rich and crystal-poor at its base towards a mafic, crystal-rich phonolite at the top (Wörner & Schmincke, 1984). This zonation is interpreted as the result of a continuous eruption from a zoned magma column. Major and trace element evidence shows that the last erupted mafic ULST (Upper Laacher See Tephra) phonolite can be derived from a basanite parent magma via fractional crystallization of 30 per cent clinopyroxene, 24 per cent amphibole, 4 per cent phlogopite, 3.8 per cent magnetite, 2.5–3.0 per cent olivine and 1 per cent apatite, leaving a derivative of 30 per cent evolved magma. Starting from the mafic (ULST) phonolite as a parent, the zoned sequence is postulated to have been formed by progressive fractional crystallization of the observed phenocryst phases. This model was tested by a series of 7 step-by-step mass balance fractionation calculations. Abundance, modal composition and relative variations of calculated fractionated phases agree well with the observed phenocryst abundances: sanidine followed by plagioclase and minor amounts of mafic phases are to be fractionated to give the observed zoned sequence. The most evolved phonolite, however, cannot be generated by subtraction of phenocrysts from the underlying phonolite. Processes such as liquid-state differentiation may therefore have chemically modified the upper part (cupola) of the Laacher See magma column subsequent to crystal fractionation. The erupted phonolite magma (5.3 km3) was calculated to have started with a volume of 56 km3 of parental basanite magma which fractionated to form 16.6 km3 of mafic phonolite. This magma further differentiated to give a 5.3 km3 zoned (erupted) phonolite column. The non-erupted volume of 50 km3 is postulated to form a cooling cumulate body below the present day Laacher See volcano. The Laacher See magma system represents a complex end-member type of a highly evolved small volume composition ally zoned magma chamber with steep major and trace element gradients, the uppermost volatile rich magma layer resembling the stable roof part of rhyolitic chambers.

Geology, geochronology and chemical evolution of the island of Pantelleria

AbstractPotassium–argon dating, field relations, geochemical and strontium-isotope compositions are reported for the island of Pantelleria (Strait of Sicily, Italy). These data support the following model for the genesis and evolution through time of the volcanic system: the peralkaline rocks originated from mantle-derived parental magmas; the trachytic magma differentiated in a low pressure magma chamber by crystal–liquid fractionation. This process led to a chemically zoned magma chamber tapped at different levels by successive eruptions. During low-pressure differentiation the 87Sr/86Sr ratios of some of the most evolved Sr-poor rhyolitic magmas increased from 0.703 up to 0.708 by contamination with crustal material.The chemical variation displayed by the products of each of the defined eruptive cycles in the last 50000 years suggests an open system behaviour of the magma chamber which is episodically refilled by more mafic parent magma, differentiated at high rate and episodically erupted.

Rhyolitic volcanism and the geochemical evolution of an Archaean central ring complex: the Blake River Group volcanics of the southern Abitibi belt, Superior province

The stratigraphic and geochemical relations of the Blake River Group volcanics of the southern Abitibi metavolcanic belt are explained within the context of an Archaean ring complex. Tholeiitic eruptions were limited to the periphery of the complex, whilst the central complex was characterized by voluminous eruptions of calc-alkaline andesites and rhyolites. Relative to the tholeiitic lavas, these andesites are enriched in LIL-elements and are interpreted to represent contaminated tholeiitic magmas. The rhyolites associated with the central complex are high—SiO 2 (75–80% SiO 2) and show variable trace element enrichments. The development of such SiO 2-rich rhyolite liquids is interpreted as taking place within a high-level, zoned magma chamber. The presence of the ore bodies in the Blake River Group is directly related to the rhyolitic volcanics and it is suggested that these ore bodies may, in part, represent magmatic hydrothermal fluids.

Geochemical evolution of the Menengai Caldera Volcano, Kenya

The Menengai volcano is composed almost entirely of strongly peralkaline, Si‐oversaturated trachytes. No mafic or intermediate products have yet been identified. The volcano has had a complex geochemical evolution, resulting from the interplay of magma mixing, crystal fractionation and liquid state differentiation. The controlling mechanism at any time was related to the growth stage of the complex, the presence of volatile gradients in the chamber, and the distribution of magma densities in the chamber. Prior to a major ash flow eruption, the magma reservoir was growing by the addition and mixing of two or more trachytic melts, only slightly different in composition. A volatile‐rich cap eventually separated from the lava‐forming zone and became compositionalIy zoned by liquid state processes. In late pfecaldera times, trachyte magma was able to penetrate into the cap zone, resulting in the eruption of mixed magma. The first Menengai ash flow tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Cs, Hf, Nb, Ni, Pb, Rb, Ta, Th, U, Y, Zn, Zr, and the REE (including Eu) and probably also Na, Cl, and F, and roofward depletion in Al, Mg, Ca, K, Ti, P, Ba, and Sc. Observed enrichment factors of up to 5 make this one of the most strongly zoned ash flow units yet recorded. Zonation was achieved by liquid state differentiation, probably involving volatile transfer and thermodiffusion, and minor crystal fractionation. After a period of rehomogenization of the magma remaining in the upper parts of the chamber, the second Menengai ash flow tuff was erupted, with formation of the present caldera. This unit is also compositionalIy zoned, although with lower observed enrichment factors than the first sheet. Caldera collapse was followed by convective overturn within the magma chamber and the rise to the roof zone of a Ba‐rich magma from a level not tapped by the as flow. Enrichment of volatiles in this zone resulted in the establishment of a stable density interface between an upper, tuff‐producing zone and the lower, lava‐forming zone. In the latter, 25% crystallization of syenite took place against the side walls. Crystallization in the tuff‐forming zone was more extensive (75%) and produced a series of chemically evolved tuffs. At some critical stage, liquid state processes became dominant in both zones and compositional variations comparable to those in the ash flow sheets were established. Some mixing of the upper and lower zone magmas may have occurred relatively recently. The Menengai volcano provides evidence of the development by liquid state mechanisms of extensive compositional zonations through thicknesses in excess of 102 m in times of 102–103 years. It also gives unique information on the senses and amounts of elemental enrichments and depletions during liquid state differentiation of trachytic magma.

Large‐volume rhyolite ash flow eruptions and submarine caldera collapse in the Lower Mesozoic Sierra Nevada, California

The Mineral King roof pendant in the southern Sierra Nevada, California, provides a view of a Triassic and early Jurassic submarine caldera complex which was tilted to vertical and intruded by middle Cretaceous granitoids. An estimated 90 km3 of rhyolitic and 10 km3 of andesitic volcanic rocks are interstratified with marine sandstones, siltstones, limestones, and conglomerates. Periods of volcanotectonic activity alternated with periods of marine sedimentation, Volcanic eruptions at Mineral King resulted in (1) deposition of widespread, very thick sheets of rhyolite ash flow tuff and (2) construction of small (1–8 km3) andesitic stratocones. Growth faults, fault talus aprons, slide blocks, slumps, and debris flows found in association with these volcanic deposits indicate that the eruptive phases represent periods of great seismic activity. Interuptive phases were periods of volcanic and tectonic quiescence during which sequences of shallow to deep marine sediments were deposited. Four different times at Mineral King, eruption of large volumes of rhyolite ash flow tuff resulted in catastrophic caldera collapse and ponding of single ash flow tuffs to thicknesses greater than 0.5 km. Only minimum volumes can be estimated for three of these intracaldera tuffs because they are truncated by younger granites. Volumes are estimated to be at least 25 km3 each, placing these tuffs in the lower size range of epicontinental ring structures. The fourth caldera, the Vandever Mountain caldera, is entirely preserved in the roof pendant. The vertically dipping strata of the Vandever Mountain caldera provide an unusual opportunity to examine a submarine caldera in cross section. Evidence for caldera subsidence during the eruption of the Vandever Mountain rhyolite ash flow tuff includes the following: (1) the great thickness of the ash flow tuff and the rapid accumulation of the tuff during progressive draw‐down of a compositionally zoned magma chamber, (2) high‐angle faults which pond the ash flow tuff and offset underlying but not overlying stratigraphy, and (3) megabreccia and slide blocks up to 0.5 km long that were derived from the caldera wall and floor and incorporated into the ash flow tuff as it filled the caldera. Disruption of the caldera floor was intense near the caldera margin but negligible along most of its length, owing to pistonlike subsidence of an intact cylinder of crust. Small‐volume ash flow eruptions occurred along an incipient, “leaky” ring fracture zone prior to the caldera‐forming eruption of the Vandever Mountain ash flow tuff. The Vandever Mountain caldera was not affected by resurgent doming or late stage rhyolite magmatism in the cross‐sectional view provided by the Mineral King roof pendant.

The mineralogy and petrology of compositionally zoned ash flow tuffs, and related silicic volcanic rocks, from the McDermitt Caldera Complex, Nevada‐Oregon

The McDermitt caldera complex, Nevada‐Oregon, is a composite collapse structure formed following eruption of three ash flow tuffs from a single compositionalIy zoned magma chamber. The major and early erupted portions of the tuffs are mildly peralkaline high‐silica comendite. Later erupted portions of two ash flow sheets are metaluminous low‐silica rhyolite. Associated with the caldera complex are precaldera dacite to rhyolite lava flows and postcaldera rhyolite domes and intrusives. Systematic variations in mineral compositions and whole‐rock chemistry throughout the entire silicic volcanic suite record the petrogenesis of highly silicic comendite through fractional crystallization of dacitic or rhyodacitic parental magma. Fractionation from daciterhyodacite to metaluminous rhyolite is characterized by initially decreasing light and middle rare earth elements; little change in Zr and Hf; and increasing U, Th, K, Rb, and Ba; controlled by removal of plagioclase, low and high‐Ca pyroxenes and significant (5%) amounts of apatite, magnetite, and ilmenite. Fractionation from rhyodacite‐low‐silica rhyolite to high‐silica comendite is characterized by increasing rare earth elements, U, Th, K, Rb, Zr, and Hf, with drastically decreasing Eu and Ba, controlled by removal of ternary‐alkali feldspar, quartz, ferroaugite‐ferrohedenbergite, fayalite and lesser ilmenite, magnetite, and apatite. Heavy rare earth element enrichment (HREE) from dacite to rhyolite is pronounced in ash flow tuffs and postcaldera volcanic rocks but not in precaldera lavas; this is possibly due to the variable presence of F as a HREE complexing agent during crystal fractionation in the caldera magma chamber. Fe‐Ti oxide compositions and phase equilibria indicate progressively decreasing magmatic temperature and oxygen fugacity: 865–940°C in dacite‐low‐silica rhyolite, with fo2 below the fayalite‐magnetite‐quartz buffer, and approximately 800°C in high‐silica comendite, with fo2 slightly above the wustite‐magnetite buffer. In the ash flow tuffs, observed patterns of vertical compositional zonation and phenocryst distributions reveal preeruptive processes important for the genesis of mildly peralkaline silicic magmas. Influx of heat is required to maintain crystallization of relatively mafic, high‐temperature mineral assemblages, notably with ternary feldspar, and prohibits further crystallization of derivative melts. Heat transfer occurred throughout a density‐stratified magma chamber via convection within compositionally discreet zones of low‐ and high‐silica rhyolite.

Origin of some mixed-magma and net-veined ring intrusions

Most of the ring complexes in the British Tertiary Igneous Province include at least one net-veined or mixed-magma ring-dyke. Net-veined ring-dykes consist of a granophyre host rock containing mafic pillows, with areas of granophyric net-veined mafic rock. Mixed-magma ring-dykes contain mafic micropillows dispersed in a silicic host rock. The ‘mafic’ component covers a wide compositional range with basaltic-andesite to andesite abundant. These magmas represent both fractionates of basaltic magma and hybrids of silicic, intermediate and mafic magma. Analogous compositional diversity in extrusive rocks of Hekla and Askja, Iceland, indicate that the Hebridean composite ring-dykes originate from compositionally zoned magma chambers. Density relationships suggest that zoning in chambers of tholeiitic systems can only result for magmas more evolved than ferrobasalt. Zonation can result from crystallization on the sidewalls and roof of the chamber (producing a boundary layer flow of less dense fluid), wall melting and magma replenishment. Hybrid mafic pillows originate by mixing within the chamber, especially when lower-density olivine tholeiite is emplaced beneath ferrobasalt magma. Subsidence of a crustal block into an underlying compositionally and thermally zoned chamber results in commingling of silicic, intermediate and mafic magmas within both the reservoir and the ring-dyke, producing pillows and net-veining. Mixing is incomplete due to the contrasted viscosities and solidus temperatures of the magmas. Mixing and net-veining is also enhanced by volatile exsolution and explosive emplacement of magma at sub-volcanic levels, with mixed magma eruptions sometimes occurring at the surface.

The petrology of basalts from Surtla (Surtsey), Iceland

Surtla is the entirely submarine volcanic pile of a satellite vent which was active shortly after initiation of the main Surtsey eruption. Blocks of lava and glassy elastic deposits collected from Surtla are of mildly alkaline olivine basalt and are compositionally the most evolved material determined from the Surtsey volcano. Petrographical and chemical analyses confirm the previously postulated continuum between the evolved (derivative) and more primitive (parental) compositions widely occurring in the Vestmannaeyjar volcanic system but not hitherto found in a single eruption. A compositionally zoned magma chamber is invoked.

Zoned Magma Chamber of the Osuzuyama Acid Rocks, Southwest Japan

Zoned Magma Chamber of the Osuzuyama Acid Rocks, Southwest Japan

Relationships between mineralization and silicic volcanism in the Central Andes

Studies of late Tertiary silicic volcanic centres in the Western and Eastern Cordilleras of the Central Andes show that three volcanic environments are appropriate sites for mineralization: (1) ring-fracture extrusions post-dating large calderas; (2) similar extrusions within ignimbrite shields; and (3) isolated, small silicic volcanoes. Subvolcanic tin mineralization in the Eastern Cordillera is located in silicic stocks and associated breccias of Miocene age. The Cerro Rico stock, Potosi, Bolivia, contains tin and silver mineralization and has an intrusion age apparently millions of years younger than that of the associated Kari Kari caldera. Similar age relationships between mineralization and caldera formation have been described from the San Juan province, Colorado. The vein deposits of Chocaya, southern Bolivia, were emplaced in the lower part of an ignimbrite shield, a type of volcanic edifice as yet unrecognized in comparable areas of silicic volcanism. The El Salvador porphyry copper deposit, Chile, is related to silicic stocks which may have been intruded along a caldera ring fracture. Cerro Bonete, Chile, provides a modern example of the volcanic superstructure which may have overlain isolated mineralized stocks and breccia pipes such as that of Salvadora at Llallagua, Bolivia. Existing models for the genesis of porphyry copper deposits suggest that they formed in granodioritic stocks located in the infrastructure of andesitic stratovolcanoes. Sites of porphyry-type subvolcanic tin mineralization in the Eastern Cordillera of Bolivia are distinguished by the absence of such andesitic structures. The surface expression of a typical subvolcanic porphyry tin deposit was probably an extrusive dome of quartz latite porphyry, sometimes related to a larger caldera structure. Evidence from the El Salvador porphyry copper deposit in the Eocene magmatic belt in Chile suggests that it too may be more closely related to a silicic volcanic structure than to an andesitic stratovolcano. The dome of La Soufriere, Guadeloupe is proposed as a modern analog for the surface expression of subvolcanic mineralization processes, the phreatic eruptions there suggesting the formation of hydrothermal breccia bodies in depth. Occurrence of mineralized porphyries, millions of years after caldera formation, does not necessarily indicate that intrusions and mineralization are not genetically related to the sub-caldera pluton, but may be a consequence of the long thermal histories (1–10 million years) of the lowermost parts of large plutons. Caldera formation can only inhibit mineralization by dispersal of ore metals when these are of magmatic origin, and ignimbrites should not be taken as being unlikely to be associated with porphyry mineralization. Whether ore metals are of wall rock or magmatic origin, the key to understanding the relationships between silicic volcanism and mineralization lies in the fractionation of trace elements within large zoned magma chambers during their igneous history, and their subsequent hydrothermal migration. Small, highly mineralized intrusions formed late in a caldera cycle (such as the Cerro Rico) may be due to the introduction of fresh supplies of mafic magma into the lower parts of the main pluton.

Petrologic Monitoring of 1981 and 1982 Eruptive Products from Mount St. Helens

New material from the dacite lava dome of Mount St. Helens, collected soon after the start of each successive extrusion, is subjected to rapid chemical and petrologic analysis. The crystallinity of the dacite lava produced in 1981 and 1982 is 38 to 42 percent, about 10 percent higher than for products of the explosive 1980 eruptions. This increase in crystallinity accompanies a decrease in the ratio of hornblende to hornblende plus orthopyroxene, which suggests that the volatile-rich, crystal-poor material explosively erupted in 1980 came from the top of a zoned magma chamber and that a lower, volatile-poor and crystal-rich region is now being tapped. The major-element chemistry of the dacite lava has remained essentially constant (62 to 63 percent silica) since August 1980, ending a trend of decreasing silica seen in the products of the explosive eruptions of May through August 1980.

Compositional heterogeneity of tephras from the 1980 eruptions of Mount St. Helens

Samples collected at hourly intervals on May 18–19, 1980, at three sites 200 km downwind from Mount St. Helens, have made possible a detailed reconstruction of the conditions that contribute to the compositional heterogeneity of mineral and glass components observed in distal tephra layers. The air fall tephra deposited at the sites during the first 7 hours of the May 18 eruption is mostly coarse grained, microlite‐rich, nonjuvenile glass and feldspar. Grain‐size maxima in this initial tephra can be related to the cataclysmic blast at 0832 and a subsequent pulse of the eruption at 1200. Juvenile, microlite‐free glass increases in relative abundance at the sampling sites beginning at about 1900. Such a change between nonjuvenile and juvenile tephra can be related to a 5‐km increase in column height associated with the last major pulse of the eruption which occurred at 1700 at the volcano. Electron microprobe study of both microlite‐rich and microlite‐free pumice in the time series samples reveals significant compositional differences. Interstitial glass in nonjuvenile pumice deposited during the first few hours at the sampling sites is enriched in SiO2 and K2O and depleted in TiO2, FeO*, and MgO relative to juvenile glass. By comparison, major element composition of the least evolved juvenile glass sampled during the last several hours of the eruption displays a slight trend toward less evolved composition. Least squares calculations suggest that the more evolved character of the nonjuvenile glass can be explained by greater fractional crystallization brought about by enhanced cooling in a cryptodome prior to eruption, whereas the temporal changes observed in juvenile glass composition during the last several hours of the eruption suggest the presence of a small, slightly zoned magma chamber at depth. Electron microprobe study of glass‐coated ilmenites, magnetites, and plagioclases provides the following estimates of the physical conditions in this reservoir: 865°±50°C, PH2O = 2.2 kbar and ‐log ƒO2 = 11.7. Analyses of bulk pumice, glass and selected mineral phases from May 25, June 12, July 22, and October 16–18 pumices erupted from Mount St. Helens indicate that the bulk pumice (magma) compositions have become slightly more andesitic with time, while mineral and co‐existing glass compositions have changed significantly in post‐May 18 eruptions with both being more highly evolved than those associated with the May 18 eruption. An application of the magnetite‐ilmenite geothermometer to June 12 and July 22 samples indicates temperatures of 919°±30°C and 930°±50°C, respectively. Least squares calculations suggest that such evolved post‐May 18 glass and mineral phases can be derived by fractional crystallization of a magma composition like bulk May 18 pumice into approximately 50% crystals and 50% residual liquid. Such partitioning between crystals and residual liquid appears to have occurred on the scale of centimeters and is interpreted as a consequence of accelerated crystallization under reduced water pressure.

Co-existing calcic amphiboles in calc-alkaline andesites: Possible evidence of a zoned magma chamber

Hornblende-biotite andesites erupted from Mount Price and Clinker Peak volcanoes, southwestern British Columbia, contain two texturally and compositionally distinct calcic amphiboles: pargasitic hornblende xenocrysts and magnesio-hornblende microphenocrysts. Disequilibrium relationships exhibited by these amphiboles and associated minerals suggest that the magnesio-hornblendes precipitated under chemical and thermal conditions that were intermediate between those under which pargasitic hornblende and biotite, respectively, crystallized. Experimental studies of crystallization in double-diffusive systems (Chen and Turner, 1980; Turner, 1980; McBirney, 1980) suggest that these varied magmatic environments can be explained as a consequence of progressive crystallization within a zoned magma chamber. Although gravitational settling may have played a role, the observed mineral assemblages probably developed by convective mixing of crystals precipitated at the cooling margins with those crystallized in the interior of the compositionally stratified magma column.

Rhyolites in the Gillies Hill–Woodtick Hill area, Beaver County, Utah

The rhyolite of Gillies Hill forms a cluster of rounded hills between Beaver basin and Cove Fort basin in southwest-central Utah. These rocks were erupted as a series of viscous lava flows, volcanic domes, and minor pyroclastic rocks from centers localized along and near the main fault separating the volcanic rocks in the Marysvale volcanic field to the east from batholithic rocks in the Mineral Mountains to the west. Faulting began in middle Miocene time before eruption of the rhyolite of Gillies Hill and continued episodically into Pleistocene time. Potassium-argon dating indicates that the rhyolite of Gillies Hill formed from a rapid sequence of eruptions about 9.1 m.y. ago. The rhyolite of Gillies Hill is made up of an older high-silica suite (SiO 2 >75%) consisting of numerous short stubby flows and of a younger low-silica suite (SiO 2 = 70%) represented by a single large volcanic dome. The high-silica suite is believed to have formed by eruptions from the top of a compositionally zoned magma chamber. The younger low-silica suite came either from a separate magma chamber or from a significantly different level within the same magma chamber. The proximity of sources for both suites suggests eruptions from different levels within a single source chamber, perhaps from different vertically stacked convection cells.

Formation of the Green Tuff, Pantelleria

The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km2. Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km3 D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism.

A summary of the geology and petrology of the Sierra La Primavera, Jalisco, Mexico

The Sierra La Primavera, near Guadalajara, Mexico, is a Late Pleistocene rhyolitic center consisting of lava flows and domes, ash flow tuff, air fall pumice, and caldera lake sediments. All eruptive units are high‐silica rhyolites, but systematic compositional differences correlate with age and eruptive mode. The earliest lavas erupted approximately 145,000 years ago and were followed approximately 95,000 years ago by the eruption of about 20 km3 of magma as ash flows that form the Tala Tuff. The Tala Tuff is zoned from a mildly peralkaline first‐erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Nb, Sb, HREE, Hf, Ta, Pb, Th, and U to a metaluminous last‐erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. Collapse of the roof zone of the magma chamber led to the formation of a shallow 11‐km‐diameter caldera in which lake sediments began to collect. The earliest postcaldera lava, the south‐central dome, is nearly identical to the last‐erupted portion of the Tala Tuff, whereas the slightly younger north‐central dome is chemically transitional from the south‐central dome to later, more mafic, ring domes. This sequence of ash flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine‐grained ashy sediments. Approximately 75,000 years ago a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only 10–20 m of sediments as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 30,000 years ago. The lavas that erupted 75,000, 60,000, and 30,000 years ago became decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U with time. They represent successive eruption of the uppermost magma in the postcaldera magma chamber. Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≥ quartz ≫ ferrohedenbergite > fayalite > ilmenite ± titanomagnetite. Major element compositions of sanidine, clinopyroxene, and fayalite phenocrysts vary only slightly between eruptive groups, but the concentrations of many trace elements change by factors of 5–10. This is reflected in phenocryst/glass partition coefficients that differ by factors of up to 20 between successively erupted units. Because the major element compositions of the phenocrysts and the pressure, temperature, and ƒO2 of the magmas were essentially constant, the large variations in partitioning behavior are thought to result from small changes in bulk composition of the melt. Crystal settling and incremental partial melting are by themselves incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post‐95,000‐year lavas. Rather, diffusional processes in the silicate liquid are thought to have been the dominant differentiation mechanisms. The zonation in the Tala Tuff is attributed to transport of trace metals as volatile complexes within a thermal and gravitational gradient in a volatile‐rich but water‐undersaturated magma. The evolution of the postcaldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced octahedral site availability in the silicate melt.

Eruptions from zoned magma chambers

SUMMARY: An appreciation of the positions within a magma chamber from which magma will be erupted can be gained from an understanding of the pattern of magma removal. Upwardly converging streamlines followed by a fluid escaping from a flat-topped reservoir into a cylindrical conduit cause magma from widely separated horizontal and vertical locations in a pre-eruption chamber to be simultaneously erupted. The volume erupted ( V ) is related to the maximum depth tapped ( R ) by the equation V = (2π/9) R 3 . Large ash-flow sheets (100 km 3 ) may therefore sample magma up to 5 km below the chamber roof. Analysis of the flow field suggests that about one half of a given sample of erupted magma represents material from the deepest part of the chamber being tapped at the time of eruption. Many petrographic features of chemically zoned ignimbrites can best be explained by the operation of such a flow-field during eruption, where a series of sub-spherical sampling shells intersect the horizontal layers in the magma chamber to produce heterogeneous ejecta. Mixed magmas erupted late in some zoned ash-flow sequences can originate by tapping across the boundary between silicic and underplating basaltic magmas which coexist in the source chamber rather than by convective mixing prior to eruption. Flow banding in rhyolites can result from the same fluid dynamic processes.