Abrupt shift in δ18O values at Medicine Lake Volcano (California, USA)

 Oxygen-isotope analyses of lavas from Medicine Lake volcano (MLV), in the southern Cascade Range, indicate a significant change in δ¹⁸O in Holocene time. In the Pleistocene, basaltic lavas with <52% SiO₂ averaged +5.9‰, intermediate lavas averaged +5.7‰, and silicic lavas (≥63.0% SiO₂) averaged +5.6‰. No analyzed Pleistocene rhyolites or dacites have values greater than +6.3‰. In post-glacial time, basalts were similar at +5.7‰ to those erupted in the Pleistocene, but intermediate lavas average +6.8‰ and silicic lavas +7.4‰ with some values as high as +8.5‰. The results indicate a change in the magmatic system supplying the volcano. During the Pleistocene, silicic lavas resulted either from melting of low-¹⁸O crust or from fractionation combined with assimilation of very-low-¹⁸O crustal material such as hydrothermally altered rocks similar to those found in drill holes under the center of the volcano. By contrast, Holocene silicic lavas were produced by assimilation and/or wholesale melting of high-¹⁸O crustal material such as that represented by inclusions of granite in lavas on the upper flanks of MLV. This sudden shift in assimilant indicates a fundamental change in the magmatic system. Magmas are apparently ponding in the crust at a very different level than in Pleistocene time. Received: 6 March 1997 / Accepted: 12 January 1998     

A strontium isotope evolution model for cenozoic magma genesis,Estern Great Basin,U.S.A.

Cenozoic volcanism in the Great Basin is characterized by an outward migration of volcanic centers with time from a centrally located core region, a gradational decrease in the initial Sr⁸⁷/Sr⁸⁶ ratio with decreasing age and increasing distance from the core, and a progressive change from calc-alkalic core rocks to more alkalic basin margin rocks. Generally each volcanic center erupted copious silicic ignimbrites followed by small amounts of basalt and andesite. The Sr⁸²/Sr⁸⁶ ratio for old core rocks is about 0.709 and the ratio for young basin margin rocks is about 0.705. Spatially and temporally related silicic and mafic suites have essentially the same Sr⁸⁷/Sr⁸⁶ ratios. The locus of older volcanism of the core region was the intersection of a north-south trending axis of crustal extension and high heat flow with the northeast trending relic thermal ridge of the Mesozoic metamorphic hinterland of the Sevier Orogenic Belt. Derivation of the Great Basin magmas directly from mantle with modification by crustal contamination seems unlikely. Initial melting of lower crustal rocks probably occurred as a response to decrease in confining pressure related to crustal extension. Volcanism was probably also a consequence of the regional increase in the geothermal gradient that is now responsible for the high heat flow of the Basin and Range Province. High Sr isotopic ratios of the older core volcanic rocks suggests that conditions suitable for the production of silicic magmas by partial fusion of the crust reached higher levels within the crust during initial volcanism than during production of later magmas with lower isotopic ratios and more alkaline chemistry. As the Great Basin became increasingly attenuated, progressively lower portions of the crust along basin margins were exposed to conditions suitable for magma genesis. The core region became exhausted in low temperature melting components, and volcanism ceased in the core before nearby areas had completed the silicic-mafic eruption cycle leading to their own exhaustion of crustal magma sources.     

Sr–Nd isotopic and chemical characteristics of the silicic magma reservoir of the Aira pyroclastic eruption, southern Kyushu, Japan

Sr and Nd isotope and geochemical investigations were performed on a remarkably homogeneous, high-silica rhyolite magma reservoir of the Aira pyroclastic eruption (22,000 years ago), southern Kyushu, Japan. The Aira caldera was formed by this eruption with four flow units (Osumi pumice fall, Tsumaya pryoclastic flow, Kamewarizaka breccia and Ito pyroclastic flow). Quite narrow chemical compositions (e.g., 74.0–76.5 wt% of SiO₂) and Sr and Nd isotopic values (⁸⁷Sr/⁸⁶Sr=0.70584–0.70599 and _Nd=−5.62 to −4.10) were detected for silicic pumices from the four units, with the exception of minor amounts of dark pumices in the units. The high Sr isotope ratios (0.7065–0.7076) for the dark pumices clearly suggest a different origin from the silicic pumices. Andesite to basalt lavas in pre-caldera (0.37–0.93 Ma) and post-caldera (historical) eruptions show lower ⁸⁷Sr/⁸⁶Sr (0.70465–0.70540) and higher _Nd (−1.03 to +0.96) values than those of the Aira silicic and dark pumices. Both andesites of pre- and post-caldera stages are very similar in major- and trace-element characteristics and isotope ratios, suggesting that the both andesites had a same source and experienced the same process of magma generation (magma mixing between basaltic and dacitic magmas). Elemental and isotopic signatures deny direct genetic relationships between the Aira pumices and pre- and post-caldera lavas. Relatively upper levels of crust (middle–upper crust) are assumed to have been involved for magma generation for the Aira silicic and dark pumices. The Aira silicic magma was derived by partial melting of a separate crust which had homogeneous chemistry and limited isotope compositions, while the magma for the Aira dark pumice was generated by AFC mixing process between the basement sedimentary rocks and basaltic parental magma, or by partial melting of crustal materials which underlay the basement sediments. The silicic magma did not occupy an upper part of a large magma body with strong compositional zonation, but formed an independent magma body within the crust. The input and mixing of the magma for dark pumices to the base of the Aira silicic magma reservoir might trigger the eruptions in the upper part of the magma body and could produce a slight Sr isotope gradient in the reservoir. An extremely high thermal structure within the crust, which was caused by the uprise and accumulation of the basaltic magma, is presumed to have formed the large volume of silicic magma of the Aira stage.     

Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan

We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO₂ 57 wt%) with a small amount of more silicic pumice (SiO₂ 62–63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (pre-eruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.     

Volcanic stratigraphy of large-volume silicic pyroclastic eruptions during Oligocene Afro-Arabian flood volcanism in Yemen

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km³. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×10⁷ km² and estimated maximum total volumes of up to 5,000 km³, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km³) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.Electronic Supplementary Material Supplementary material is available for this article at     

Trans-Atlantic correlation of eruptive sequences and individual silicic volcanic units within the Paraná-Etendeka igneous province

The Mesozoic volcanic rocks of the Serra Geral Formation in the Paraná Basin, South America, and of the Etendeka Group in northwestern Namibia were erupted shortly before the opening of the South Atlantic. The major widespread silicic volcanic units in the Etendeka Group are interpreted as rheoignimbrites (Milner et al., 1992) and are interbedded with tholeiitic basalts and basaltic andesites.The southern portion of the Etendeka Group is subdivided into a basal Awahab Formation which is overlain disconformably by the Tafelberg Formation. Both formations contain silicic and mafic units. Bulk composition, initial ⁸⁷Sr/⁸⁶Sr ratios, phenocryst assemblages and mineral compositions are used to correlate silicic units of the Awahab Formation with the basal units of the Palmas silicic volcanic rocks in the southern Paraná Basin. Silicic units of the Tafelberg Formation can similarly be correlated with silicic units in the upper portion of the Palmas succession, which are also disconformable on the units below them. Not all silicic units in these successions are present in both the Etendeka and Paraná areas, but where correlation of individual units is possible, then this is found to be consistent with the overall stratigraphic sequence.Silicic units in the Awahab Formation were erupted from the Messum Igneous Complex in Namibia and their correlation into Brazil indicates that individual eruptive units must have travelled over 340 km from their source. Serial changes in the composition of silicic units in the Awahab Formation and their correlatives indicates that they were erupted from a single magma system from which a total of ˜ 8600 km³ of material was erupted.     

The eruptive history of the Tequila volcanic field, western Mexico: ages, volumes, and relative proportions of lava types

The eruptive history of the Tequila volcanic field (1600 km²) in the western Trans-Mexican Volcanic Belt is based on ⁴⁰Ar/³⁹Ar chronology and volume estimates for eruptive units younger than 1 Ma. Ages are reported for 49 volcanic units, including Volcán Tequila (an andesitic stratovolcano) and peripheral domes, flows, and scoria cones. Volumes of volcanic units 1 Ma were obtained with the aid of field mapping, ortho aerial photographs, digital elevation models (DEMs), and ArcGIS software. Between 1120 and 200 kyrs ago, a bimodal distribution of rhyolite (~35 km³) and high-Ti basalt (~39 km³) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km³ of andesite and dacite erupted from more than 15 isolated vents; these lavas are crystal-poor and show little evidence of storage in an upper crustal chamber. Approximately 200 kyr ago, ~31 km³ of andesite erupted to form the stratocone of Volcán Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at ~2–3 km depth. After a hiatus of ~110 kyrs, ~15 km³ of andesite erupted along the W and SE flanks of Volcán Tequila at ~90 ka, most likely from a second, discrete magma chamber located at ~5–6 km depth. The youngest volcanic feature (~60 ka) is the small andesitic volcano Cerro Tomasillo (~2 km³). Over the last 1 Myr, a total of 128±22 km³ of lava erupted in the Tequila volcanic field, leading to an average eruption rate of ~0.13 km³/kyr. This volume erupted over ~1600 km², leading to an average lava accumulation rate of ~8 cm/kyr. The relative proportions of lava types are ~22–43% basalt, ~0.4–1% basaltic andesite, ~29–54% andesite, ~2–3% dacite, and ~18–40% rhyolite. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and chemical composition, the lavas do not reflect the differentiation of a single (or only a few) parental liquids in a long-lived magma chamber. The rhyolites are geochemically diverse and were likely formed by episodic partial melting of upper crustal rocks in response to emplacement of basalts. There are no examples of mingled rhyolitic and basaltic magmas. Whatever mechanism is invoked to explain the generation of andesite at the Tequila volcanic field, it must be consistent with a dominantly bimodal distribution of high-Ti basalt and rhyolite for an 800 kyr interval beginning ~1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsility: J. Donnelly-NolanThis revised version was published online in January 2005 with corrections to Tables 1 and 3.An erratum to this article can be found at     

Late cenozoic bimodal magmatism in the northern basin and range province of southeastern Oregon

Compositional features of 93 samples of primitive Pliocene to recent basalts erupted along the Brothers Fault Zone in the northernmost Basin and Range indicate that they were derived from a shallow mantle source and underwent only minor shallow-level fractionation. Simple mass-balance modelling can derive these basaltic bulk compositions by removal of small amounts of observed crystalline phases from glass compositions produced in peridotite melting experiments. Additional support comes from phase equilibria data on other magnesian basalts having similar bulk compositions. The eruption of these lavas without substantial subcrustal fractionation was probably promoted by progressive extension along the Brothers Fault Zone. This origin is in sharp contrast to that generally proposed for mid-Miocene Columbia River and Steens Mountain basalts, which show clear evidence in their evolved compositions (e.g. Mg # ~ 40) of having stagnated at shallow depth where they differentiated to nearly basaltic andesite compositions. Bulk compositions of northern Basin and Range silicic rocks, together with physical and thermal considerations, suggest that they, like their counterparts in the Snake River Plain, were products of crustal anatexis driven by the injection of mafic magmas, but with meta-volcaniclastic protoliths rather than Archaean basement rocks, as in the case of the Snake River Plain rhyolites. These petrologic features suggest that the arrival of the mantle plume presently beneath Yellowstone produced or strongly influenced most late Cenozoic magmatism in the Oregon northern Basin and Range. This model accounts for many features of the northern Basin and Range in Oregon: (1) the change in basaltic character about 10 to 8 Ma ago from voluminous, evolved Columbia River/Steens lavas to smaller-volume primitive lavas and the lack of younger lavas atop the Columbia River Basalts; (2) the lack of an obvious track of the Yellowstone hot spot west of the Oregon-Idaho-Nevada tri-state area; (3) the “mirror-image” age relationship of silicic rocks in the northern Basin and Range and Snake River Plain; (4) the formation of silicic rocks by crustal anatexis and the general decrease in their volumes with time in Oregon but not along the Snake River Plain; (5) the high elevation of the region; and (6) the high surface heat flow in the Oregon northern Basin and Range. The proposed model obviates the controversy surrounding the pre-Miocene history of the Yellowstone plume by proposing that the plume initiated about 18 Ma ago.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Characteristics of crustal structures in the Yamato Basin,sea of Japan,deduced from seismic explorations

Crustal structures around the Yamato Basin in the southeastern Sea of Japan, inferred from recent ocean bottom seismography (OBS) and active-source seismological studies, are reviewed to elucidate various stages of crustal modification involved from rifting in the crust of the surrounding continental arc to the production of oceanic crust in the Yamato Basin of the back-arc basin. The northern, central, and southern areas of the Yamato Basin have crustal thicknesses of approximately 12–16 km, and lowermost crusts with P-wave velocities greater than 7.2 km/s. Very few units have P-wave velocities in the range 5.4–6.0 km/s, which corresponds to the continental upper crust. These findings, combined with previous geochemical analysis of basalt samples, are interpreted to indicate that a thick oceanic crust has been formed in these areas of the basin, and that this oceanic crust has been underplated by mantle-derived magma. In the central Yamato Basin, the original continental crust has been fully breached and oceanic crust has been formed. Conversely, the presence of a unit corresponding to the continental upper crust and the absence of a high-velocity part in the lower crust implies that the southwestern edge of the Yamato Basin has a rifted crust without significant intrusion. The Oki Trough has a crust that is 17–19 km thick with a high-velocity lower crust and a unit corresponding to the continental upper crust. The formation of the Oki Trough resulted from rifting with magmatic intrusion and/or underplating. We interpret these variations in the crustal characteristics of the Yamato Basin area as reflecting various instances of crustal modification by thinning and magmatic intrusion due to back-arc extension, resulting in the production of a thick oceanic crust in the basin.     

Temporal variations in magma composition at Merapi Volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity

Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from 1900 ¹⁴C yr BP to the present. Radiocarbon dating of pyroclastic deposits indicates an almost continuous activity with periods of high eruption rates alternating with shorter time spans of distinctly reduced eruptive frequency since the first appearance of high-K volcanic rocks. Geochemical data of 28 well-dated, prehistoric pyroclastic flows of the Merapi high-K series indicate systematic cyclic variations. These medium-term compositional variations result from a complex interplay of several magmatic processes, which ultimately control the periodicity and frequency of eruptions at Merapi. Low eruption rates and the absence of new influxes of primitive magma from depth allow the generation of basaltic andesite magma (56–57 wt% SiO₂) in a small-volume magma reservoir through fractional crystallisation from parental mafic magma (52–53 wt% SiO₂) in periods of low eruptive frequency. Magmas of intermediate composition erupted during these stages provide evidence for periodic withdrawal of magma from a steadily fractionating magma chamber. Subsequent periods are characterised by high eruption rates that coincide with shifts of whole-rock compositions from basaltic andesite to basalt. This compositional variation is interpreted to originate from influxes of primitive magma into a continuously active magma chamber, triggering the eruption of evolved magma after periods of low eruptive frequency. Batches of primitive magma eventually mix with residual magma in the magmatic reservoir to decrease whole-rock SiO₂ contents. Supply of primitive magma at Merapi appears to be sufficiently frequent that andesites or more differentiated rock types were not generated during the past 2000 years of activity. Cyclic variations also occurred during the recent eruptive period since AD 1883. The most recent eruptive episode of Merapi is characterised by essentially uniform magma compositions that may imply the existence of a continuously active magma reservoir, maintained in a quasi-steady state by magma recharge. The whole-rock compositions at the upper limit of the total SiO₂ range of the Merapi suite could also indicate the beginning of another period of high eruption rates and shifts towards more mafic compositions.     

Rhyolite volcanism in the Krafla central volcano,north-east Iceland

At the Krafla central volcano in north-east Iceland, two main phases of rhyolite volcanism are identified. The earlier phase (last interglacial) is related to the formation of a caldera, whereas the second phase (last glacial) is related to the emplacement of a ring dike. Subsequently, only minor amounts of rhyolite have been erupted. The volcanic products of Krafla are volumetrically bimodal. Geochemically, there is a series of basaltic to basalto-andesitic rocks and a cluster of rhyolitic rocks. Rocks of intermediate to silicic composition (icelandites and dacites) show clear signs of mixing. The rhyolites are Fe-rich (tholeiitic), and aphyric to slightly porphyritic (plagioclase, augite, pigeonite, fayalitic olivine and magnetite). They are minimum melts on the quartz-plagioclase cotectic plane in the granite system (Qz-Or-Ab-An). The rhyolites at Krafla were produced by near-solidus, rather than nearliquidus fractionation. They are interpreted as silicic minimum melts of hydrothermally altered crust, mainly of basaltic composition. They were primarily generated on the peripheries of an active basaltic magma chamber or intrusive domain, where sufficient volumes of crust were subjected to temperatures favorable for rhyolite genesis (850–950° C). The silicic melts were extracted crystal-free from their source in response to crustal deformation.     

Eruptive history and tectonic setting of Medicine Lake Volcano,a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km² of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km³, giving an overall effusion rate of ∼ 1.2 km³ per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H₂O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.     

Strontium,lead and oxygen isotopic investigation of the Skaergard intrusion,East Greenland

New Pb, Sr and O isotopic analyses of rocks from the Skaergard intrusion indicate the following: (1) initial⁸⁷Sr/⁸⁶Sr of the gabbroic magma was less than or equal to 0.7041; (2) limited contamination of magma with crustal Sr and Pb may have occurred in a deep reservoir below the presently exposed parts of the intrusion; (3) marked crustal contamination occurred at high level in marginal border group rocks, but these rocks effectively shielded the main magma body from further interaction with country rock gneisses; (4) subsolidus interaction between Skaergard gabbros and hydrothermal fluids modified δ¹⁸O values but had little effect on Sr and perhaps Pb isotopic ratios; (5) late-stage melanogranophyres may be comagmatic with the Skaergard magma, but silicic granophyres are not; (6) silicic granophyres contain large and varied proportions of crustal Sr and Pb; some may be largely anatectic melts derived from the deep crust whereas others may represent mixing of such anatectic melts with late-stage differentiated liquids of the Skaergard intrusion (e.g. Sydtoppen sill).     

Ar–Ar ages and geochemistry of the basaltic shield stage of Tenerife, Canary Islands, Spain

We report the first ³⁹Ar–⁴⁰Ar ages from the three early basic shield-like massifs of Tenerife, Canary islands, and couple these with detailed major and trace element chemistry to constrain the nature and timing of the mantle melting processes. The massifs have chemically different sources, and independent evolutionary histories. The Teno and Roque del Conde massifs appear chemically to represent the products of single mantle melting cycles, with progressive decrease in mean melt fraction and increase in mean melting depth in younger rocks. The Teno massif (NW) was erupted in a short time period around 6.0–6.4 Ma, while at least the lower half of the Roque del Conde massif (SW) is older than 11 Ma. In contrast, the Anaga massif (NE) is polygenetic, with ³⁹Ar–⁴⁰Ar ages ranging from 8.0–4.2 Ma, and no simple stratigraphic chemical progression. These ages run counter to published suggestions of progressive younging of Canary shield stages to the southwest. Basic rocks in all three massifs are the result of much deeper melting and smaller melt fractions than equivalent units in Gran Canaria, but nevertheless the melting column must have extended significantly into the spinel facies, requiring substantial disruption of the local lithosphere. The age and melting relationships broadly support the mantle blob model for Canary magmatism proposed by Hoernle and Schmincke (Hoernle, K., Schminke, H.-U., 1993. The role of partial melting in the 15-Ma geochemical evolution of Gran Canaria: a blob model for the Canary hotspot. J. Petrol. 34, 599–626). In all three massifs, extensive fractional crystallisation has taken place at crustal levels so that mean MgO contents are only some 6–7%. The fractionation sequence is olivine–clinopyroxene–magnetite in basaltic compositions, with the involvement of plagioclase, amphibole and apatite only to generate the infrequent more evolved hawaiites to benmoreites. Despite the abundance of basanitic magmas in the Tenerife older massifs, these follow a differentiation trend towards weakly undersaturated benmoreite rather than to phonolite. This probably reflects early crystallisation of magnetite, perhaps resulting from somewhat high oxygen fugacity. The chemical evidence for replenished magma chambers in Tenerife described by Neumann et al. (Neumann, E.R., Wulff-Oedersen, E., Simonsen, S.L., Pearson, N.J., Martí, J., Mitjavila, J., 1999. Evidence for fractional crystallisation of periodically refilled magma chambers in Tenerife, Canary Islands. J. Petrol. 40, 1089–1123) is a consequence of treating as a single cogenetic suite the products of several magmatic systems that differ in parental melt fraction.     

Volcanic history of Mount Sidley,a major alkaline volcano in Marie Byrd Land,Antarctica

Mount Sidley is a complex, polygenetic stratovolcano composed primarily of phonolitic and trachytic lavas and subordinate pyroclastic lithologies at the southern extremity of the Executive Committee Range, a linear chain of volcanoes in central Marie Byrd Land, Antarctica. Detailed field investigation coupled with 14 high precision ⁴⁰Ar/³⁹Ar age determinations reveal a 1.5 million year life span between 5.7 and 4.2 Ma in which three major phonolitic central vent edifices (Byrd, Weiss and Sidley volcanoes) and their calderas were developed (5.7–4.8 Ma). This was followed (4.6–4.5 Ma) by the eruption of trachytic magmas from multiple vent localities further south, and then by small volume benmoreite-mugearite lavas and tephras around 4.4–4.3 Ma at the southern end of Mount Sidley. The final phase of activity was the eruption of basanite cones at approximately 4.2 Ma. The southward migration of volcanic activity was accompanied by distinct changes in magma composition and is best explained by the sequential release of magmas stored within an intricate system of conduits and chambers in the crust by tectonically driven (magma assisted?) fracture propagation. The style of volcanic migration at Mount Sidley is emulated on a larger scale by other volcanoes in the Executive Committee Range, in which progressive southward displacement of volcanic activity corresponds with significant petrological variations between major centers.     

Petrochemistry of the Gjálp-1996 subglacial eruption, Vatnajökull, SE Iceland

In October 1996 a subglacial fissure to the north of the Grimsvötn caldera in W-Vatnajökull produced about 0.4 km³ of Fe-rich basaltic andesite–icelandite—in an area characterized mostly by tholeiitic basalt. In this paper the chemical composition of volcanic systems in the region is discussed with the help of six new analyses and others from the literature, and a tentative model for their evolution is proposed, in which magma produced by the partial melting of a two-component mantle mixes with hydrous, silicic melt in the crust. The Vatnajökull 1996 magma belongs to a separate volcanic system, intermediate between Bardarbunga and Grimsvötn.     

Effusive eruptions from a large silicic magma chamber: the Bearhead Rhyolite,Jemez volcanic field,NM

Large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events. Less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera-forming events produce dominantly effusive eruptions of small-volume rhyolite domes and flows. The Bearhead Rhyolite and associated Peralta Tuff Member in the Jemez volcanic field, New Mexico, represent small-volume eruptions from a large silicic magma system in which no caldera-forming event occurred, and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long-term evolution of continental silicic magma systems.⁴⁰Ar/³⁹Ar dating reveals that most units mapped as Bearhead Rhyolite and Peralta Tuff (the Main Group) were erupted during an ∼540 ka interval between 7.06 and 6.52 Ma. These rocks define a chemically coherent group of high-silica rhyolites that can be related by simple fractional crystallization models. Preceding the Main Group, minor amounts of unrelated trachydacite and low silica rhyolite were erupted at ∼11–9 and ∼8 Ma, respectively, whereas subsequent to the Main Group minor amounts of unrelated rhyolites were erupted at ∼6.1 and ∼1.5 Ma.The chemical coherency, apparent fractional crystallization-derived geochemical trends, large areal distribution of rhyolite domes (∼200 km²), and presence of a major hydrothermal system support the hypothesis that Main Group magmas were derived from a single, large, shallow magma chamber. The ∼540 ka eruptive interval demands input of heat into the system by replenishment with silicic melts, or basaltic underplating to maintain the Bearhead Rhyolite magma chamber.Although the volatile content of Main Group magmas was within the range of rhyolites from major caldera-forming eruptions such as the Bandelier and Bishop Tuffs, eruptions were smaller volume and dominantly effusive. Bearhead Rhyolite domes occur at the intersection of faults, and are cut by faults, suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera-forming eruption.     

The Avellino plinian eruption of Somma-Vesuvius (3760 y.B.P.): the progressive evolution from magmatic to hydromagmatic style

A detailed stratigraphic analysis of the Avellino plinian deposit of the Somma-Vesuvius volcano shows a complicated eruptive sequence controlled by a combination of magmatic and hydromagmatic processes. The role of external water on the eruptive dynamics was most relevant in the very early phase of the eruption when the groundwater explosively interacted with a rising, gas-exolving magma body creating the first conduit. This phase generated pyroclastic surge and phreatoplinian deposits followed by a rapidly increasing discharge of a gas-rich, pure magmatic phase which erupted as the most violent plinian episode. This continuing plinian phase tapped the magma chamber, generating about 2.9 km³ of reverse-graded fallout pumice, more differentiated at the base and more primitive at the top (white and gray pumice). A giant, plinian column, rapidly grew up reaching a maximum height of 36 km.The progressive magma evacuation at a maximum discharge rate of 10⁸ kg/s that accompanied a decrease of magmatic volatile content in the lower primitive magma allowed external water to enter the magma chamber, resulting in a drastic change in the eruptive style and deposit type. Early wet hydromagmatic events were followed by dry ones and only a few, subordinated magmatic phases. A thick, impressive sequence of pyroclastic surge bedsets of over 430 km² in area with a total volume of about 1 km³ is the visible result of this hydromagmatic phase.     

Rates of magma emplacement and volcanic output

This study includes a compilation of about one hundred estimates of volumetric rates of magma emplacement and volcanic output that are average rates for durations of igneous activity greater than 300 yrs. These data indicate that the rate of volcanic output is about 10⁻¹ km³ yr⁻¹ in regions that are the most active magmatically. Factors that correlate with rates of magma emplacement and volcanic output are: magma composition, crustal thickness, tectonic setting, and regional stress. Of the ninety rates of magma emplacement and volcanic output that were studied, the highest for basaltic magmas are greater than the highest for silicic magmas, regardless of the volumes erupted or areal extent of magmatism. Rates of volcanic output for oceanic areas tend to be greater than rates in continental areas, perhaps because of thinner crust, a predominance of basaltic magma, and higher rates of magma generation. Ratios of intrusive to extrusive volumes are typically about 5 to 1 for oceanic localities and 10 to 1 for continental localities. This difference apparently reflects dissimilar rates of magma ascent related to different crustal thicknesses and magma compositions. The total rate of magma emplacement and volcanic output for the Earth, averaged over the last 180 m.y., is between about 26 and 34 km³ yr⁻¹. About 75% of this total is contributed by ocean-ridge magmatism. Oceanic intraplate magmatism contributes about 5%. Igneous activity in subduction zones, about half of which is continental, adds about 20%. Intracontinental magmatism, more than 95% of which is flood and plains basalts, provides less than 5% of the total global rate of magma emplacement and volcanic output.