Outline volcanic history of the region west of Vatnajökull, Central Iceland

This paper outlines the structure and volcanic geology of a 25 × 50 km region of central Iceland including part of the eastern neovolcanic zone and its western margin. It includes an extinct Brunhes epoch silicic centre, the Hagangas, offset en échelon from a zone of major postglacial basaltic activity forming a northeasterly extension of the Torfajökull centre. Stratigraphic subdivisions restricted to the last 690,000 years comprise, in order of decreasing age, interglacial flood tholeiites, major centres of intraglacial hyaloclastite eruption, and postglacial lavas, which are mostly olivine basalts. The Hagangas centre and interglacial tholeiites lie on crust predominantly of Matuyama age (0.69–2.30 m.y.) but the bulk of the present volcanic activity may be taking place through crust belonging entirely to the present polarity epoch; this latter zone is characterised by normal faulting and extensive hydrothermal alteration. The widespread hydrothermal alteration and voluminous basaltic eruption distinguish this neovolcanic zone from the western zone, and the relationship of the region to growth of the upper crust in Iceland is briefly discussed.     

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.     

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     

Seismic constraints on magma chambers at Hekla and Torfajökull volcanoes,Iceland

Hekla and Torfajökull are active volcanoes at a rift–transform junction in south Iceland. Despite their location next to each other they are physically and geologically very different. Hekla is an elongate stratovolcano, built mainly of basaltic andesite. Torfajökull is a prominent rhyolitic centre with a 12-km-diameter caldera and extensive geothermal activity. The scope of this study is to examine the propagation of body waves of local earthquakes across the Hekla–Torfajökull area and look for volumes of anomalous S-wave attenuation, which can be evidence of magma chambers. So far the magma chamber under Hekla has been modelled with various geophysical means, and its depth has been estimated to be 5–9 km. A data set of 118 local earthquakes, providing 663 seismic rays scanning Hekla and Torfajökull, was used in this study. The major part, 650 seismograms, did not show evidence for S-wave attenuation under these volcanoes. Only six seismograms had clear signs of S-wave attenuation and seven seismograms were uncertain cases. The data set samples Hekla well at depths of 8–14 km, and south part of it also at 4–8 km and 14–16 km. Western Torfajökull is sampled well at depths of 4–14 km, eastern and southern Torfajökull at 6–12 km. Conclusions cannot be drawn regarding the existence of magma beyond these depth ranges. Also, magma volumes of smaller dimensions than about 800 m cannot be detected with this method. If a considerable molten volume exists under Hekla, it must be located either above 4 km or below 14 km. The former possibility seems unlikely, because Hekla lacks geothermal activity and persistent seismicity, usually taken as expressions of a shallow magma chamber. An aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of Torfajökull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber. Our results indicate that this volume cannot be molten to a great extent because S-waves travelling through it are not attenuated. Intense geothermal activity and low-frequency earthquakes are possibly signs of magma in the south part of Torfajökull, but a magma chamber was not detected there in the areas sampled by this study.Editorial responsibility: T. Druitt     

Explosive silicic volcanism in Iceland and the Jan Mayen area during the last 6 Ma: sources and timing of major eruptions

Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ¹⁸O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K₂O/SiO₂>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island.     

Volcanoes as elastic inclusions: Their effects on the propagation of dykes,volcanic fissures,and volcanic zones in Iceland

Mechanically, many volcanoes may be regarded as elastic inclusions, either softer (with a lower Young's modulus) or stiffer (with a higher Young's modulus) than the host-rock matrix. For example, many central volcanoes (stratovolcanoes, composite volcanoes) are composed of rocks that are softer than the crustal segments that host them. This is particularly clear in Iceland where central volcanoes are mostly made of soft rocks such as rhyolite, pyroclastics, hyaloclastites, and sediments whereas the host rock is primarily stiff basaltic lava flows. Most active central volcanoes also contain fluid magma chambers, and many have collapse calderas. Fluid magma chambers are best modelled as cavities (in three dimensions) or holes (in two dimensions), entire calderas as holes, and the ring faults themselves, which commonly include soft materials such as breccias, as soft inclusions. Many hyaloclastite (basaltic breccias) mountains partly buried in the basaltic lava pile also function as soft inclusions. Modelling volcanoes as soft inclusions or holes, we present three main numerical results. The first, using the hole model, shows the mechanical interaction between all the active central volcanoes in Iceland and, in particular, those forming the two main clusters at the north and south end of the East Volcanic Zone (EVZ). The strong indication of mechanical interaction through shared dykes and faults in the northern cluster of the EVZ is supported by observations. The second model, using a soft inclusion, shows that the Torfajökull central volcano, which contains the largest active caldera in Iceland, suppresses the spreading-generated tensile stress in its surroundings. We propose that this partly explains why the proper rift zone northeast of Torfajökull has not managed to propagate through the volcano. Apparently, Torfajökull tends to slow down the rate of southwest propagation of the rift-zone part of the EVZ. The third model, again using a soft inclusion, indicates how the lateral propagation of a segment of the 1783 Laki fissure became arrested in the slopes of the hyaloclastite mountain Laki.     

New multibeam mapping and geochemistry of the 30°–35° S sector,and overview,of southern Kermadec arc volcanism

New multibeam mapping and whole-rock geochemistry establish the first order definition of the modern submarine Kermadec arc between 30° and 35° S. Twenty-two volcanoes with basal diameters > 5 km are newly discovered or fully-mapped for the first time; Giggenbach, Macauley, Havre, Haungaroa, Kuiwai, Ngatoroirangi, Sonne, Kibblewhite and Yokosuka. For each large volcano, edifice morphology and structure, surficial deposits, lava fields, distribution of sector collapses, and lava compositions are determined. Macauley and Havre are large silicic intra-oceanic caldera complexes. For both, concentric ridges on the outer flanks are interpreted as recording mega-bedforms associated with pyroclastic density flows and edifice foundering. Other stratovolcanoes reveal complex histories, with repeated cycles of tectonically controlled construction and sector collapse, extensive basaltic flow fields, and the development of summit craters and/or small nested calderas.Combined with existing data for the southernmost arc segment, we provide an overview of the spatial distribution and magmatic heterogeneity along ∼780 km of the Kermadec arc at 30°–36°30′ S. Coincident changes in arc elevation and lava composition define three volcano–tectonic segments. A central deeper segment at 32°20′–34°10′ S has basement elevations of > 3200 m water-depth, and relatively simple stratovolcanoes dominated by low-K series, basalt–basaltic andesite. In contrast, the adjoining arc segments have higher basement elevations (typically < 2500 m water-depth), multi-vent volcanic centres including caldera complexes, and erupt sub-equal proportions of dacite and basalt–basaltic andesite. The association of silicic magmas with higher basement elevations (and hence thicker crust), coupled with significant inter- and intra-volcano heterogeneity of the silicic lavas, but not the mafic lavas, is interpreted as evidence for dehydration melting of the sub-arc crust. Conversely, the crust beneath the deeper arc segments is thinner, initially cooler, and has not yet reached the thermal requirements for anatexis. Silicic calderas with diameters > 3 km coincide with the shallower arc segments. The dominant mode of large caldera formation is interpreted as mass-discharge pyroclastic eruption with syn-eruptive collapse. Hence, the shallower arc segments are characterized by both the generation of volatile-enriched magmas from crustal melting and a reduced hydrostatic load, allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions. Proposed initiation parameters for submarine pyroclastic eruptions are water-depths < 1000 m, magmas with 5–6 wt.% water and > 70 wt.% SiO₂, and a high discharge rate.     

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.     

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.     

The Hasan Dagi stratovolcano (Central Anatolia, Turkey): evolution from calc-alkaline to alkaline magmatism in a collision zone

The Hasan Dagi volcano is one of the two large Plio-Quaternary volcanoes in Cappadocia (Central Anatolia, Turkey). Three stages of edifice construction have been identified for this volcano: Paleovolcano, Mesovolcano and Neovolcano. Most samples from Hasan Dagi volcano are calc-alkaline and define an almost complete trend from basaltic andesite to rhyolite. However, the more recent (Neovolcano) mafic samples are alkaline basalts. The mineralogical and geochemical characteristics of the oldest lavas (Keçikalesi (13 Ma) and Paleo-Hasan Dagi (7 Ma)) are significantly different from those of the younger lavas (Meso- and Neo-Hasan Dagi (<1 Ma)). Calcic plagioclase and pigeonite are typically observed in these older lavas. The Paleovolcano basalts are depleted in alkalis and display a tholeiitic tendency whereas the differentiated lavas are depleted in Na₂O but enriched in K₂O compared to younger lavas. There is an evolution through time towards higher TiO₂, Fe₂O₃^*, MgO, Na₂O and K₂O and lower Al₂O₃ and SiO₂ which is reflected in the basalt compositions. All the basalts display multi-element patterns typical of continental margin magmas with a significant enrichment in LILE (K, Rb, Ba and Th) and LREE and strong (Paleovolcano) to moderate (Meso- and Neovolcano) negative Nb, Zr and Ti anomalies. However, the younger basalts are the most enriched in incompatible elements, in agreement with their alkaline affinities and do not systematically display negative HFSE anomalies. REE data suggest an hydrous amphibole-bearing crystallization history for both Meso- and Neovolcano lavas. The distinction between the older and younger lavas is also apparent in trace element ratios such as Nb/Y, Ti/Y and Th/Y. These ratios indicate the role of a subducted component±crustal contamination in the genesis of the Hasan Dagi lavas, particularly for the oldest lavas (Keçikalesi and Paleo-Hasan Dagi). The decreasing influence of this component through time, over the last 6–7 m.y., has been accompanied by an increasing contribution of melt-enriched lithosphere. Although the range of variation of Sr, Nd and Pb isotopic ratios is small (0.70457–0.70515; 0.51262–0.51273; 18.80–18.94; 15.64–15.69; 38.87–39.10), it also reflects the evolution of the magma sources through time. Indeed, the youngest (Neovolcano) and most primitive basalts display significantly lower ⁸⁷Sr/⁸⁶Sr than the Paleo- and Mesovolcano basalts, whereas the Mesovolcano basalts display more radiogenic Pb than Paleovolcano samples. Magma mixing processes between initially heterogeneous and/or variably contaminated magmas may account for the genesis of the less differentiated and intermediate lavas (48–57% SiO₂). Meso- and Neovolcano differentiated lavas (60–68% SiO₂) are either derived from the analyzed basalts or from more primitive and more depleted magmas by fractional crystallization±some crustal contamination (AFC). Furthermore, the highly differentiated samples (72–75% SiO₂) are not strongly contaminated. The strong calc-alkaline character of Hasan Dagi lavas, in the absence of contemporaneous subduction, must reflect the heritage of the early subduction of the Afro–Arabian plate under the Eurasian plate. The evolution towards alkaline compositions through time is clearly related to the development of extensional tectonics in Central Anatolia in the Late Miocene.     

Subglacial to emergent basaltic volcanism at Hlöðufell, south-west Iceland: A history of ice-confinement

Hlöðufell is a familiar 1186 m high landmark, located about 80 km northeast of Reykjavík, and 9 km south of the Langkjökull ice-cap in south-west Iceland. This is the first detailed study of this well-exposed and easily accessible subglacial to emergent basaltic volcano. Eight coherent and eleven volcaniclastic lithofacies are described and interpreted, and its evolution subdivided into four growth stages (I–IV) on the basis of facies architecture. Vents for stages I, II, and IV lie along the same fissure zone, which trends parallel to the dominant NNE–SSW volcano-tectonic axis of the Western Volcanic Zone in this part of Iceland, but the stage III vent lies to the north, and is probably responsible for the present N–S elongation of the volcano. The basal stage (I) is dominated by subglacially erupted lava mounds and ridges, which are of 240 m maximum thickness, were fed from short fissures and locally display lava tubes. Some of the stage I lavas preserve laterally extensive flat to bulbous, steep, glassy surfaces that are interpreted to have formed by direct contact with surrounding ice, and are termed ice-contact lava confinement surfaces. These surfaces preserve several distinctive structures, such as lava shelves, pillows that have one flat surface and mini-pillow (< 10 cm across) breakouts, which are interpreted to have formed by the interplay of lava chilling and confinement against ice, ice melting and ice fracture. The ice-contact lava confinement surfaces are also associated with zones of distinctive open cavities in the lavas that range from about 1 m to several metres across. The cavities are interpreted as having arisen by lava engulfing blocks of ice, that had become trapped in a narrow zone of meltwater between the lava and the surrounding ice, and are termed ice-block meltout cavities. The same areas of the lavas also display included and sometimes clearly rotated blocks of massive to planar to cross-stratified hyaloclastite lapilli tuffs and tuff–breccias, termed hyaloclastite inclusions, which are interpreted as engulfed blocks of hyaloclastite/pillow breccia carapace and talus, or their equivalents reworked by meltwater. Some of the stage I lavas are mantled at the southern end of the mountain by up to 35 m thickness of well-bedded vitric lapilli tuffs (stage II), of phreatomagmatic origin, which were erupted from a now dissected cone, preserved in this area. The tephra was deposited dominantly by subaqueous sediment gravity flows (density currents) in an ice-bound lake (or less likely a sub-ice water vault), and was also transported to the south by sub-ice meltwater traction currents. This cone is onlapped by a subaerial pahoehoe lava-fed delta sequence, formed during stage III, and which was most likely fed from a now buried vent(s), located somewhere in the north-central part of the mountain. A 150 m rise in lake level submerged the capping lavas, and was associated with progradation of a new pahoehoe lava-fed delta sequence, produced during stage IV, and which was fed from the present summit cone vent. The water level rise and onset of stage IV eruptions were not associated with any obviously exposed phreatomagmatic deposits, but they are most likely buried beneath stage IV delta deposits. Stage IV lava-fed deltas display steep benches, which do not appear to be due to syn- or post-depositional mass wasting, but were probably generated during later erosion by ice. The possibility that they are due to shorter progradation distances than the underlying stage III deltas, due to ice-confinement or lower volumes of supplied lava is also considered.     

Temporal variation in the lavas of Mayon volcano, Philippines

Chemical and petrographic analyses of 51 sequential lava flows from the central vent of Mayon volcano show cyclical variation. In the two most recent cycles, from 1800 to 1876 and from 1881 to the present, one to three basaltic flows are followed by six to ten andesitic flows. Modal and whole-rock chemical parameters show the most regular cyclical variation; calculated groundmass chemical parameters vary less regularly. There is also a long-term trend, over approximately 1700 years of exposed section, toward more basic compositions.The cyclical variation in modes and the chemical composition of the lavas apparently results from periodic influxes of basaltic magma from depth into a shallow magma system. Fractional crystallization of olivine, augite, hypersthene, calcic plagioclase, magnetite and pargasitic hornblende produces successively more andesitic lavas until the next influx of basaltic magma. Differentiation in a deep zone of magma generation is not excluded by the data, but is more likely responsible for the overall change toward more basic compositions than for the cyclical variation.Three points in a cycle — the beginning of basaltic lavas, the beginning of andesitic lavas and a leveling-off of SiO₂, K₂ O and K₂O/Na₂O values — correspond roughly to the beginning of frequent effusive eruptions (with or without an early Plinian eruption), frequent weak to moderately explosive (Strombolian) eruptions, and less frequent explosive (Vulcanian) eruptions, respectively. Recognition of the current stage in a cycle can give a qualitative indication of the nature of forthcoming eruptions. Changes in several specific parameters may precede basaltic lavas and allow early detection of basaltic influxes. These include minima in the glass inclusion/plagioclase phenocryst and phenocryst/groundmass ratios, vesicularity and groundmass TiO₂, a decrease in hypersthene phenocrysts, and constant values for the whole-rock K₂O/Na₂O ratio. The Mayon area is densely populated, making prediction of eruption type important for safety and land-use planning.     

Geochemistry of basalts from the Dumisseau Formation, southern Haiti: implications for the origin of the Caribbean Sea crust

Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO₂ contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm.La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO₂ (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO₂ (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts.⁸⁷Sr⁸⁶Sr ratios vary from 0.70280 to 0.70316,¹⁴³Nd¹⁴⁴Nd from 0.512929 to 0.513121, and²⁰⁶Pb²⁰⁴Pb from 19.00 to 19.27. LREE-depleted lavas have high¹⁴³Nd¹⁴⁴Nd (0.51309–0.51310) typical of MORBs, whereas¹⁴³Nd¹⁴⁴Nd in the LREE-enriched lavas varies widely (0.512929–0.513121).Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.     

Intracrustal origin of Arenal basaltic andesite in the light of solid–melt interactions and related compositional buffering

The origin of Arenal basaltic andesite can be explained in terms of fractional crystallization of a parental high-alumina basalt (HAB), which assimilates crustal rocks during its storage, ascent and evolution. Contamination of this melt by Tertiary calc-alkalic intrusives (quartz–diorite and granite, with ⁸⁷Sr/⁸⁶Sr ratios ranging 0.70381–0.70397, nearly identical with those of the Arenal lavas) occurs at upper crustal levels, following the interaction of ascending basaltic magma masses with gabbroic–anorthositic layers. Fragments of these layers are found as inclusions within Arenal lavas and tephra and may show reaction rims (1–5 mm thick, consisting of augite, hypersthene, bytownitic–anorthitic plagioclase, and granular titanomagnetite) at the gabbro–lava interface. These reaction rims indicate that complete `assimilation' was prevented since the temperature of the host basaltic magma was not high enough to melt the gabbroic materials (whose mineral phases are nearly identical to the early formed liquidus phases in the differentiating HAB). Olivine gabbros crystallized at pressure of about 5–6 kbar and equilibrated with the parental HAB at pressures of 3–6 kbar (both under anhydrous and hydrous conditions), and temperatures ranging 1000–1100°C. In particular, `deeper' interactions between the mafic inclusions and the hydrous basaltic melt (i.e., with about 3.5 wt.% H₂O) are likely to occur at 5.4 (±0.4) kbar and temperatures approaching 1100°C. The olivine gabbros are thus interpreted as cumulates which represent crystallized portions of earlier Arenal-type basalts. Some of the gabbros have been `mildly' tectonized and recrystallized to give mafic granulites that may exhibit a distinct foliation. Below Arenal volcano a zoned magma chamber evolved prior the last eruptive cycle: three distinct andesitic magma layers were produced by simple AFC of a high-alumina basalt (HAB) with assimilation of Tertiary quartz–dioritic and granitic rocks. Early erupted 1968 tephra and 1969 lavas (which represent the first two layers of the upper part of a zoned magma chamber) were produced by simple AFC, with fractionation of plagioclase, pyroxene and magnetite and concomitant assimilation of quartz–dioritic rocks. Assimilation rates were constant (r<sub>1</sub>=0.33) for a relative mass of magma remaining of 0.77–0.80, respectively. Lavas erupted around 1974 are less differentiated and represent the `primitive andesitic magma type' residing within the middle–lower part of the chamber. These lavas were also produced by simple AFC: assimilation rates and the relative mass of magma remaining increased of about 10%, respectively (r₁=0.36, and F=0.89). Ba enrichment of the above lavas is related to selective assimilation of Ba from Tertiary granitic rocks. Lava eruption occurred as a dynamic response to the intrusion of a new magma into the old reservoir. This process caused the instability of the zoned magma column inducing syneruptive mixing between portions of two contiguous magma layers (both within the column itself and at lower levels where the new basalt was intruded into the reservoir). Syneruptive mixing (mingling) within the middle–upper part of the chamber involved fractions of earlier gabbroic cumulitic materials (lavas erupted around 1970). On the contrary, within the lower part of the chamber, mixing between the intruded HAB and the residing andesitic melt was followed by simple fractional crystallization (FC) of the hybrid magma layer (lavas erupted in 1978–1980). By that time the original magma chamber was completely evacuated. Lavas erupted in 1982/1984 were thus modelled by means of `open system' AFCRE (i.e., AFC with continuous recharge of a fractionating magma batch during eruption): in this case assimilation rates were r₁=0.33 and F=0.86. Recharge rates are slightly higher than extrusion rates and may reflect differences in density (between extruded and injected magmas), together with dynamic fluctuations of these parameters during eruption. Ba and LREE (La, Ce) enrichments of these lavas can be related to selective assimilation of Tertiary granitic and quartz–dioritic rocks. Calculated contents for Zr, Y and other REE are in acceptable agreement with the observed values. It is concluded that simple AFC occurs between two distinct eruption cycles and is typical of a period of repose or mild and decreasing volcanic activity. On the contrary, magma mixing, eventually followed by fractional crystallization (FC) of the hybrid magma layer, occurs during an ongoing eruption. Open-system AFCRE is only operative when the original magma chamber has been totally replenished by the new basaltic magma, and seems a prelude to the progressive ceasing of a major eruptive cycle.     

Evidence of magma mixing: Petrological study of Shirouma-Oike calc-alkaline andesite volcano, Japan

Shirouma-Oike volcano, a Quaternary composite volcano in central Japan, consists mostly of calc-alkaline andesitic lavas and pyroclastic rocks. Products of the earlier stage of the volcano (older group) are augite-hypersthene andesite. Hornblende crystallized during the later stage of this older group, whereas biotite and quartz crystallized in the younger group.Assemblages of phenocrysts in disequilibrium, such as magnesian olivine(Fo₃₀)/quartz, iron-rich hypersthene(En₅₅)/iron-poor augite(Wo_43.5, En_42.5, Fs_14.0), and two different types of zoning on the rim of clinopyroxene are found in a number of rocks. Detailed microprobe analyses of coexisting minerals reveal that phenocrysts belong to two distinctly different groups; one group includes magnesian olivine + augite which crystallized from a relatively high-temperature (above 1000°C) basaltic magma; the second group, which crystallized from relatively low temperature (about 800°C) dacitic to andesitic magma, includes hypersthene + hornblende + biotite + quartz + plagioclase + titanomagnetite ± ilmenite (in the younger group) and hypersthene + augite + plagioclase + titanomagnetite ± hornblende (in the older group). The temperature difference between the two magmas is clarified by Mg/Fe partition between clinopyroxene and olivine, and Fe-Ti oxides geothermometer. The compositional zoning of minerals, such as normal zoning of olivine and magnesian clinopyroxene, and reverse zoning of orthopyroxene, indicate that the basaltic and dacitic-andesitic magmas were probably mixed in a magma reservoir immediately before eruption. It is suggested that the basaltic magma was supplied intermittently from a deeper part to the shallower magma reservoir, in in which dacitic-andesitic magma had been fractionating.     

Quantifying the effect of rheology on lava-flow margins using fractal geometry

This study aims at quantifying the effect of rheology on plan-view shapes of lava flows using fractal geometry. Plan-view shapes of lava flows are important because they reflect the processes governing flow emplacement and may provide insight into lava-flow rheology and dynamics. In our earlier investigation (Bruno et al. 1992), we reported that flow margins of basalts are fractal, having a characteristic shape regardless of scale. We also found we could use fractal dimension (D, a parameter which quantifies flow-margin convolution) to distinguish between the two endmember types of basalts: a a (D: 1.05–1.09) and pahoehoe (D: 1.13–1.23). In this work, we confirm those earlier results for basalts based on a larger database and over a wider range of scale (0.125 m–2.4 km). Additionally, we analyze ten silicic flows (SiO₂: 52–74%) over a similar scale range (10 m–4.5 km). We note that silicic flows tend to exhibit scale-dependent, or non-fractal, behavior. We attribute this breakdown of fractal behavior at increased silica contents to the suppression of small-scale features in the flow margin, due to the higher viscosities and yield strengths of silicic flows. These results suggest we can use the fractal properties of flow margins as a remote-sensing tool to distinguish flow types. Our evaluation of the nonlinear aspects of flow dynamics indicates a tendency toward fractal behavior for basaltic lavas whose flow is controlled by internal fluid dynamic processes. For silicic flows, or basaltic flows whose flow is controlled by steep slopes, our evaluation indicates non-fractal behavior, consistent with our observations.     

Silicic volcanism in Iceland: Composition and distribution within the active volcanic zones

Silicic volcanic rocks within the active volcanic zones of Iceland are mainly confined to central volcanoes. The volcanic zones of Iceland can be divided into rift zones and flank zones. Each of these zones contains several central volcanoes, most of which have produced minor amounts of silicic rocks. The silicic rocks occur as lavas and domes or as tephra layers, welded tuffs and ignimbrites, formed both in effusive and explosive eruptions. They tend to be glassy or very fine-grained, containing small amounts of phenocrysts. Plagioclase (andesine–oligoclase), anorthoclase or occasionally sanidine coexist with minerals such as augite, fayalite, pigeonite, orthopyroxene and magnetite. Quartz phenocrysts are exceedingly rare. Zoning of phenocrysts is limited and the pattern is variable. A set of 90 samples representing all active central volcanoes that have erupted silicic rocks was analysed for major- and trace-elements. The silicic rocks can be classified as dacites, trachytes, low-alkali rhyolites and alkalic rhyolites. Some of the trachytes and alkalic rhyolites are peralkaline (mostly comenditic). Trachytes and alkalic rhyolites are only found within the flank zones, while dacites and low-alkali rhyolites are mostly confined to the rift zones. The Icelandic rhyolites plot close to the thermal minimum in the “granite” system, while dacites and trachytes plot within the plagioclase field and towards the alkali feldspar temperature minimum. The silicic rocks are relatively Fe-rich and Ca-poor indicating low water pressure in the source. Trace element concentrations follow similar patterns in most central volcanoes. Exceptions are Torfajökull where silicic rocks display a negative correlation of Ba to Th and unusually high Th-contents, and the western flank zone where Ba-concentrations are highly variable. The ratios of different high field-strength elements are generally similar within each central volcano or region, which probably reflects different ratios in the source materials. Isotope systematics indicate that the silicic rocks are derived from older basaltic rocks similar to those from the same volcano, and that meteoric water has played a role in the genesis of the silicic rocks. Traditionally, the petrogenesis of silicic rocks in Iceland has been explained by various models of fractional crystallization or partial melting. The available data seems to be better explained by near-solidus differentiation than by near-liquidus differentiation. The silicic minimum melts can be extracted from the rigid framework of the near-solidus source by the process of solidification front instability or by deformation-assisted melt segregation. The source of the silicic rocks is within the intrusive complex beneath a central volcano rather than in a large, long-lived magma chamber.     

Magma–conduit interaction at Piton de la Fournaise volcano (Réunion Island): a melt and fluid inclusion study

Major-element, Cl, S, F analyses have been performed on a wide selection of melt inclusions trapped in olivine (Fo_81–87) from scoria and crystal-rich lapilli samples of Piton de la Fournaise volcano. As a whole, they display a transitional basaltic composition. The melt inclusions (8–9 wt.% MgO, 0.62–0.73 wt.% K₂O) are in equilibrium with olivines (Fo_81–85) in the samples from the Central Feeding Zone and the South-East Feeding Zone and show a slight alkaline affinity. The melt inclusions in olivines (Fo_85–87) from the North-West Rift (NWR) contain 9.3–9.7 wt.% MgO and 0.54–0.58 wt.% K₂O, with a more tholeiitic tendency. In oceanitic lavas and crystal-rich lapilli, the olivine xenocrysts are recognisable by the presence of one or more secondary shear plane fracture(s) filled up with CO₂ and alkali-rich basaltic melt inclusions. In dunite nodules, olivines present also contain several secondary shear plane fracture(s) filled up with CO₂ and high-SiO₂ melt inclusions. Secondary CO₂-rich fluid inclusions in olivine (Fo_85–87) from the NWR samples indicate PCO₂ up to 500 MPa whereas, PCO₂ ranges from 95 MPa to few tenths of bars in the other samples. Both the primary melt inclusions and the secondary fluid inclusions strongly suggest that the olivine crystallises and accumulates over a wide depth range (15 km). It is envisioned that cumulative pockets with low residual porosity are repeatedly percolated with a CO₂-rich fluid phase, possibly associated with basaltic to SiO₂-rich melts, and are finally disrupted and entrained to the surface when vigorous magma transfer occurs. The SiO₂-rich residual melts in early-formed dunitic or gabbroic bodies may have acted as contaminant agents for the more alkali character of magmas vented through the central feeding system, where a well-developed cumulative system is thought to exist. Finally, the existence of secondary fluid and melt inclusions in olivines implies that the dunitic bodies are weakened on the micrometric scale.     

Geology of the Heimaey volcanic centre, south Iceland: early evolution of a central volcano in a propagating rift?

Heimaey is the southernmost and also the youngest of nine volcanic centres in the southward-propagating Eastern Volcanic Zone, Iceland. The island of Heimaey belongs to the Vestmannaeyjar volcanic system (850 km²) and is situated 10 km off the south coast of Iceland. Although Heimaey probably started to form during the Upper Pleistocene all the exposed subaerial volcanics (10 monogenetic vents covering an area of 13.4 km²) are of Holocene age. Heimaey is composed of roughly equal amounts of tuff/tuff-breccias and lavas as most eruptions involve both a phreatomagmatic and an effusive phase. The compositions of the extrusives are predominantly alkali basalts belonging to the sodic series. Repeated eruptions on Heimaey, and the occurrence of slightly more evolved rocks (i.e. hawaiite approaching mugearite), might indicate that the island is in an early stage of forming a central volcano in the Vestmannaeyjar system. This is further substantiated by the development of a magma chamber at 10–20 km depth during the most recent eruption in 1973 and by the fact that the average volume of material produced in a single eruption on Heimaey is 0.32 km³ (dense rock equivalent), which is twice the value reported for the Vestmannaeyjar system as a whole. We find no support for the previously postulated episodic behaviour of the volcanism in the Vestmannaeyjar system. However, the oldest units exposed above sea level, i.e. the Norðurklettar ridge, probably formed over a 500-year interval during the deglaciation of southern Iceland. The absence of equilibrium phenocryst assemblages in the Heimaey lavas suggests that magma rose quickly from depth, without long-time ponding in shallow-seated crustal magma chambers. Eruptions on Heimaey have occurred along two main lineaments (N45°E and N65°E), which indicate that it is seismic events associated with the southward propagation of the Eastern Volcanic Zone that open pathways for the magma to reach the surface. Continuing southward propagation of the Eastern Volcanic Zone suggests that the frequency of volcanic eruptions in the Vestmannaeyjar system might increase with time, and that Heimaey may develop into a central volcano like the mature volcanic centres situated on the Icelandic mainland.