Lava Compositions Research Articles

40Ar/39Ar Geochronology of Subaerial Ascension Island and a Re-evaluation of the Temporal Progression of Basaltic to Rhyolitic Volcanism

Ar/Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight Ar/Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602^1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/ Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. Ar/Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (4931ka) Ascension evolved magmas, whereas zircon^titanite fractionation was predominant in younger felsic magmas.The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of highSiO2 trachyte and rhyolite.

Sr, Nd and Pb isotopic and chemical compositions of central Deccan Traps lavas and relation to southwestern Deccan stratigraphy

Sr, Nd and Pb isotopic compositions, and major and trace element abundances of lavas in five central Deccan Traps sections are generally very similar to those of lavas in the southwestern Deccan (Western Ghats escarpment). The combination of strong isotopic and chemical similarities between lavas of the two regions indicates that they shared a closely similar petrogenesis. Our results indicate that, unlike many lavas in the northeastern Deccan, most of the lavas studied in the central Deccan sections can be grouped into different stratigraphic members and chemical types (CTs) belonging to four formations (Thakurvadi, Bhimashankar, Khandala, and Poladpur) present in the type sections of the Western Ghats. Bhimashankar- and Thakurvadi-type lavas are found in a section near Outram, whereas only Khandala-type lavas make up the Mhaishmal, Ellora and Ajanta sections to the east, and only Poladpur-type lavas are present in the easternmost section at Lonar. This west-east sequence is the same as that seen going upward in the Western Ghats stratigraphy, except that Bushe-type lavas, which are located stratigraphically between the Khandala and Poladpur formations in the Western Ghats, have not been found in this study. Overall, our results indicate that the Khandala and Poladpur formations extend over west-east distances of more than 300km, and some individual members (e.g., the Dhak Dongar) may extend over as much as 500km. The regional dip from Igatpuri in the southwest to Lonar in the east-central Deccan is less than 0.5° to the east, which is similar to the southward regional dip in the southwestern Deccan. Near-horizontal flows in the central and southwestern Deccan areas suggest that post-Deccan tectonic activity has had limited effects on the studied area. Large volumes of basaltic lavas derived from eruptive centers in the western Deccan may have covered both the central and southwestern Deccan areas.

Melt–peridotite reactions in upwelling eclogite bodies: Constraints from EM1-type alkaline basalts in Payenia, Argentina

The processes of magma generation in upwelling eclogite bodies of recycled lithospheric material are not fully understood but are important for our understanding and modelling of major and trace element variations in many ocean island basalts (OIB). The primitive alkaline intraplate basalts from the Payenia volcanic province (34–38 °S) in Argentina, for which Sr, Nd and double-spike Pb isotope ratios are presented, and from other north Patagonian volcanic fields may provide details of the eclogite melt–peridotite reactions taking place in the melting column of an upwelling OIB-type mantle. The isotopic composition of the uncontaminated lavas is highly restricted but the Payenia basalts fall in two distinct trace element groups termed the high and low Nb/U groups, which both have EM1-type trace element patterns but with subtle differences that cannot be explained by contamination, fractionation or simple variations in degrees of mantle melting. The difference is also clear in major elements where the low Nb/U basalts have markedly higher alkali contents but lower FeO and Ni than the high Nb/U basalts. Four melt components have been identified based on olivine fractionation corrected compositions: a low Nb/U pyroxenite melt component which is interpreted as an eclogite melt that experienced low degrees of melt–peridotite interaction, a high Nb/U pyroxenite melt component which is interpreted as an eclogite melt that experienced high degrees of melt–peridotite interaction, a low pressure peridotite melt component which contributes to most high Nb/U, Payún Matrú, Matancilla and Auca Mahuida basalts and finally, a melt component with similarities to peridotite melts contributing to many low Nb/U lavas. The high Nb/U type basalts are interpreted to have been formed from mantle with higher temperature than ambient mantle while the low Nb/U basalts may have been formed from normal temperature mantle.

Miocene shoshonite volcanism in Sardinia: Implications for magma sources and geodynamic evolution of the central-western Mediterranean

In this paper we document the existence of a Miocene shoshonite (SHO) volcanism in Northern Sardinia (Anglona). This occurrence completes the spectrum of orogenic magmas related to the subduction process which developed from the Eocene along the Palaeo-European continental margin, in concert with the opening of the Ligurian–Balearic back-arc basin and southeastward drift/rotation of the Sardinia–Corsica continental block. K–Ar ages show that the oldest volcanics of the area are calcalkaline (CA) basalts and andesites (~21Ma), overlain by 19.7–18.4Ma-old more potassic products such as high-potassium calcalkaline (HK-CA) and SHO lavas. CA, HK-CA and SHO suites include basalts and differentiated lavas of andesite and latite composition, respectively, that (according to the PELE software modelling) represent ~40–45% residual liquid fraction after shallow fractional crystallization. Application of the “Arc Magma Simulator” software suggests that the generation of primary melts of the distinct suites may occur at similar degrees of partial melting (5–8%) and melting pressures (2–2.2GPa, ~60–70km depth) in the mantle wedge. By contrast, the potassic character of parental melts of CA, HK-CA and SHO suites is controlled by 1) the amount of subducted continental components (possibly terrigenous sediments) and 2) the pressure (depth) at which these metasomatic agents are released from the slab. Results suggest that the slab depth beneath the volcanic district increased from ~80–100 to 100–120km for CA and SHO magmas, respectively. Accordingly, the evolution from CA to SHO magmatism in the same plumbing system could be related to slab deepening and increase of the subduction angle of ~5–10° in the time span of 2–3Ma. This tectono-magmatic scenario conforms to the major anticlockwise rotation (~30°) event of the Sardinia block (between 20.5 and 18Ma). This geodynamic evolution preludes the development of the volcanism in the Apennine–Tyrrhenian domains, where the final collisional/post-collisional stages of subduction were characterized by accentuated slab retreat and roll back, inter-arc extension and eruption of highly potassic magmas in the frontal arc (Roman and Aeolian Provinces).

Renewed melting at the abandoned Húnafloí Rift, northern Iceland, caused by plume pulsing

The abandoned Húnafloí Rift (HR), near Skagi in northern Iceland, provides an opportunity to investigate how relocation of an oceanic spreading ridge influences melt generation. Melting recommenced beneath HR several million years after spreading ceased at 7–4 Ma. Both the older (pre-HR abandonment) and younger (post-abandonment) lavas are tholeiitic basalts with high Fe–Ti and enriched incompatible trace element compositions. The HR lavas are compositionally similar to modern lavas from central Iceland (above the Iceland Plume conduit) and the Eastern Volcanic Zone (a propagating rift). Although older and younger HR lavas are compositionally similar, geological and geophysical observations indicate different mantle melting conditions. For the older lavas, a plume migration reconstruction suggests that the combination of incompatible trace element enrichment and crustal thickness of ∼25 km can be explained by plume-driven upwelling, a scenario similar to central Iceland at present. The smaller volume of the younger HR lavas (<1 km melt thickness) and their greater distance from the plume centre argues against plume-driven upwelling to explain their enrichment. Rare earth element inversion modelling indicates that the younger lavas come from reactivation of the deepest part of the melting region beneath HR. We suggest that the base of the HR melting region was reactivated when a pulse of unusually hot asthenosphere, known to be producing unusually thick crust ∼700 km away from the plume centre today, passed laterally beneath the HR at ∼3 Ma. A thermal and kinematic ridge–plume interaction model indicates that this scenario can explain both the volume and composition of the younger Skagi lavas, provided that a small amount of decompression occurs. The decompression may result either from extension or from buoyancy-driven upwelling of hot mantle beneath the relatively thin lithosphere of the abandoned HR. The hypothesis that a hot plume pulse caused the younger HR lavas is significant not only as an example of interaction between mid-ocean ridge melting and time-dependent mantle convection, but also because this same hot pulse may have subsequently helped promote intensification of the Northern Hemisphere Glaciation when it uplifted the oceanic gateways either side of Iceland.

A Palaeocene intracanyon-style lava emplaced during the early shield-building stage of the Cuillin Volcano, Isle of Skye, NW Scotland

ABSTRACTThe twin summits of Preshal More and Preshal Beg, near Talisker, Isle of Skye, comprise the erosional remnants of a thick (at least 120 m) compound olivine tholeiite lava, or flow field, that ponded in palaeo-valleys within the Palaeocene lava field of west-central Skye. This unique flow field constitutes the Talisker Formation and is the youngest preserved extrusive unit of the Skye Lava Field. The lava inundated a complex of palaeo-valleys incised into the higher stratigraphical levels of the existing lava field, and remnants of the original sedimentary fill of these valleys still exist, the Preshal Beg Conglomerate Formation. The lava displays spectacularly well-developed two-tier (colonnade-entablature) columnar joint sets that formed as a consequence of slow, uninterrupted cooling through its base and sidewalls, aided by groundwater circulation and water ingress (from displaced drainage) directed into the lava's interior by master-joint systems. Intrusive phenomena developed at both the base and the top of the lava and there is evidence for the existence of subsurface feeder tubes. The tholeiitic composition of the Talisker Formation lava contrasts with the transitional, mildly alkaline characteristics of the remainder of the (older) lavas of Skye Lava Field. In broad terms, the Talisker Formation lava is compositionally very similar to the suite of cone-sheets emplaced into the oldest of the four intrusive centres that comprise the Skye Central Complex – the Cuillin Intrusive Centre – together with a high proportion of the Skye regional dyke swarm. The stratigraphical position, field relationships and compositional characteristics of the lava indicate that it was erupted and emplaced as an intracanyon-style flow field during the early shield-building stage in the growth of the (tholeiitic) Cuillin Volcano, which post-dates the main Skye ‘plateau’ Lava Field. Although the remnant outcrops are detached from their likely source area through erosion, this tholeiitic lava provides the first direct evidence linking the central complexes of the British Palaeogene Igneous Province and their eruptive products.

Temperature and variability of Pillan, Wayland Patera, and Loki Patera on Io from Cassini ISS data

Cassini spacecraft images of Io obtained during its flyby of Jupiter in late 2000 and early 2001 were used to determine the lava composition and eruption style of three faint hot spots, Pillan, Wayland Patera, and Loki Patera. We found a maximum color temperature of 1130±289K for Pillan and maximum color temperatures of 1297±289K and 1387±287K for Wayland Patera and Loki Patera, respectively. These temperatures are suggestive of basaltic lava but an ultramafic composition cannot be ruled out. The temperatures with the best signal-to-noise ratios also suggested basaltic lava and were found to be 780±189K, 1116±250K, and 1017±177K for Pillan, Wayland Patera, and Loki Patera, respectively. Pillan showed constant thermal output within error over three days of observations. The data also suggest Pillan may be surrounded by topography that blocked emission in the middle of the observation and caused a more dramatic decrease in emission. Wayland Patera’s intensity decreased over the three eclipse observations, consistent with a cooling lava flow or decreasing effusion rate. Intensities at Loki Patera over the course of the observations varied, consistent with previous determinations that Loki Patera is an often quiescent lava lake with periods of overturning, fountaining, and crustal foundering.

On the relationships between the Bushveld Complex and its felsic roof rocks, part 1: petrogenesis of Rooiberg and related felsites

A major question concerning the Bushveld Complex is the relationship between the layered mafic rocks and the overlying Rooiberg Group felsites and related granophyres. Here, we assemble bulk-rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks. The data indicate that the Rooiberg Group consists of distinctive magnesian and ferroan lavas. The former dominates the basalts to rhyolites of the basal Dullstroom Formation, while nearly all the dacites to rhyolites of the overlying Damwal, Kwaggasnek, and Schrikkloof Formations are ferroan. The ferroan rocks also include the Stavoren Granophyre, which exists regionally as a several-hundred-meter-thick concordant sheet between the Bushveld Complex and Rooiberg lavas. The compositions of the magnesian lavas are similar to calc-alkaline granitoids found in convergent margins, suggesting that the lavas could have originated in a mantle affected by previous Archean subduction events that are recorded by xenoliths and inclusions in diamonds from most Kaapvaal kimberlites. In contrast, the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings. The hypothesis that these rocks are fractional crystallization products of Bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations. Based on compositional characteristics and geologic relations, the Stavoren Granophyre is the most likely candidate for the residual liquid that escaped from the top of the Bushveld Complex. Whether the bulk of the Bushveld Province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper, hidden crustal magma reservoir remains unclear.

Redox Heterogeneity in Mid-Ocean Ridge Basalts as a Function of Mantle Source

The oxidation state of Earth's upper mantle both influences and records mantle evolution, but systematic fine-scale variations in upper mantle oxidation state have not previously been recognized in mantle-derived lavas from mid-ocean ridges. Through a global survey of mid-ocean ridge basalt glasses, we show that mantle oxidation state varies systematically as a function of mantle source composition. Negative correlations between Fe(3+)/ΣFe ratios and indices of mantle enrichment--such as (87)Sr/(86)Sr, (208)Pb/(204)Pb, Ba/La, and Nb/Zr ratios--reveal that enriched mantle is more reduced than depleted mantle. Because carbon may act to simultaneously reduce iron and generate melts that share geochemical traits with our reduced samples, we propose that carbon creates magmas at ridges that are reduced and enriched.

Age, petrogenesis and tectonic setting of the Thessalon volcanic rocks, Huronian Supergroup, Canada

The ∼2.5Ga Huronian Supergroup volcanic rocks in northern Ontario represent one of the better preserved Paleoproterozoic continental flood basalt sequences on Earth. The youngest age of Huronian Supergroup volcanism is refined to 2452.5±6.2Ma based on new U–Pb zircon analyses of the Copper Cliff rhyolite. Recent mapping and geochemical analyses of Thessalon Formation volcanic rocks define a well-developed chemostratigraphy; a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt–andesite volcanic series. These two mafic volcanic series are separated by felsic volcanic rocks (rhyolite and dacite). The mafic lavas are low-Ti tholeiites, dominated by basalt and andesite, and are characterized by enriched light rare earth element profiles and high Th contents (up to 12ppm). Most basalt–andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and, in some cases, limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle. All magma batches display prominent negative Nb–Ta and Ti anomalies on primitive mantle normalized multi-element diagrams, interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism. At least three distinct mantle source compositions are required to explain the range in the observed lava compositions. The most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high Th/Yb (∼10) and low Zr/Nb (∼11). This reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward Neoarchean subduction along the southern Superior craton margin.

Louisville seamount subduction and its implication on mantle flow beneath the central Tonga–Kermadec arc

Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga-Kermadec arc, west of the contemporary Louisville-Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated (206)Pb/(204)Pb, (208)Pb/(204)Pb, (86)Sr/(87)Sr in lavas from the central Tonga-Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga-Kermadec arc system.

The nature and formation of cristobalite at the Soufrière Hills volcano, Montserrat: implications for the petrology and stability of silicic lava domes

Cristobalite is commonly found in the dome lava of silicic volcanoes but is not a primary magmatic phase; its presence indicates that the composition and micro-structure of dome lavas evolve during, and after, emplacement. Nine temporally and mineralogically diverse dome samples from the Soufrière Hills volcano (SHV), Montserrat, are analysed to provide the first detailed assessment of the nature and mode of cristobalite formation in a volcanic dome. The dome rocks contain up to 11 wt.% cristobalite, as defined by X-ray diffraction. Prismatic and platy forms of cristobalite, identified by scanning electron microscopy (SEM), are commonly found in pores and fractures, suggesting that they have precipitated from a vapour phase. Feathery crystallites and micro-crystals of cristobalite and quartz associated with volcanic glass, identified using SEM-Raman, are interpreted to have formed by varying amounts of devitrification. We discuss mechanisms of silica transport and cristobalite formation, and their implications for petrological interpretations and dome stability. We conclude: (1) that silica may be transported in the vapour phase locally, or from one part of the magmatic system to another; (2) that the potential for transport of silica into the dome should not be neglected in petrological and geochemical studies because the addition of non-magmatic phases may affect whole rock composition; and (3) that the extent of cristobalite mineralisation in the dome at SHV is sufficient to reduce porosity—hence, permeability—and may impact on the mechanical strength of the dome rock, thereby potentially affecting dome stability.

Lead Isotope composition and origin of the quaternary lavas of Elbrus Volcano, the Greater Caucasus: High-precision MC-ICP-MS data

A systematic study of Pb isotope composition was carried out for Elbrus Volcano, one of the Europe’s largest volcanoes, using high-precision method of multi-collector inductively coupled plasma mass spectrometry. The measurement error of Pb isotope ratios was estimated from the results of replicate analyses of international BCR-1 and AGV-1 standards as ±0.03% (±2SD). The study of a representative collection showed that dacites of all three phases of the Elbrus eruptive activity are characterized by relatively small-scale variations of Pb isotope composition: 206Pb/204Pb 18.621–18.670, 207Pb/204Pb 15.636–15.659, and 208Pb/204Pb 38.762–38.845. New Pb isotope geochemical characteristics in combination with existing Sr-Nd data indicate that the parental magmas of Elbrus are of mixed mantle-crust origin. They were formed by interaction of mantle-derived melts with continental crust of the Greater Caucasus during continental collision between the Eurasian, Arabian, Turkish, and Iranian plates.

Synvolcanic crustal extension during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental, Mexico: Evidence from the Guazapares Mining District region, western Chihuahua

The timing and spatial extent of mid-Cenozoic ignimbrite flare-up volcanism of the Sierra Madre Occidental silicic large igneous province of Mexico in relation to crustal extension is relatively unknown. Extension in the Sierra Madre Occidental has been variably interpreted to have preceded, postdated, or begun during Early Oligocene flare-up volcanism of the silicic large igneous province. New geologic mapping, zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry dating, modal analysis, and geochemical data from the Guazapares Mining District region along the western edge of the northern Sierra Madre Occidental silicic large igneous province have identified three informal synextensional formations. The ca. 27.5 Ma Parajes formation is an ∼1-km-thick succession composed primarily of welded to nonwelded silicic outflow ignimbrite sheets erupted from distant sources. The 27–24.5 Ma Temoris formation is interpreted as an andesitic volcanic center composed of locally erupted mafic to intermediate composition lavas and associated intrusions, with interbedded andesite-clast fluvial and debris flow deposits, and an upper section of thin distal silicic outflow ignimbrites. The 24.5–23 Ma Sierra Guazapares formation is composed of silicic vent facies ignimbrites to proximal ignimbrites, lavas, plugs, dome-collapse deposits, and fluvially or debris flow–reworked equivalents. These three formations record (1) the accumulation of outflow ignimbrite sheets, presumably erupted from calderas mapped ∼50–100 km east of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) development of an andesitic volcanic field in the study area, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental toward the end of and following the Early Oligocene ignimbrite pulse; and (3) the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up. The main geologic structures identified in the Guazapares Mining District region are NNW–trending normal faults, with an estimated minimum of 20% total horizontal extension. Normal faults were active during deposition of all three formations (Parajes, Temoris, and Sierra Guazapares), and bound half-graben basins that show evidence of synvolcanic extension (e.g., growth strata) during deposition. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse to the east and before magmatism began in the study area. In addition, preexisting normal faults localized andesitic volcanic vents of the Temoris formation and silicic vents of the Sierra Guazapares formation, and some faults were reactivated during, as well as after, deposition of these formations. We interpret extensional faulting and magmatism in the Guazapares Mining District region to be part of a regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental. We show that extension accompanied silicic volcanism in the Guazapares region, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is a relationship between lithospheric extension and silicic large igneous province magmatism.

The eruption of Eyjafjallajökull (Iceland) in Spring 2010 and its possible consequences

March 20, 2010, in southern Iceland, the Eyjafjallajokull volcano began to erupt after almost 200 years of silence. Three weeks later, on April 14, a 500-m long fissure opened in the ice cap covering the volcano, ejecting molten lava and huge amounts of steam, volcanic ash, and toxic gases into the atmosphere. This produced a giant ash cloud, which disrupted air traffic across Europe for several days. This paper presents a description of the eruption and gives a brief review of the composition of volcanic lava and gases, as well as examples of the adverse effects of volcanic ash on humans and livestock in many Icelandic farms.

Chemical heterogeneity in the Hawaiian mantle plume from the alteration and dehydration of recycled oceanic crust

Inter-shield differences in the composition of lavas from Hawaiian volcanoes are generally thought to result from the melting of a heterogeneous mantle source containing variable amounts or types of oceanic crust (sediment, basalt, and/or gabbro) that was recycled into the mantle at an ancient subduction zone. Here we investigate the origin of chemical heterogeneity in the Hawaiian mantle plume by comparing the incompatible trace element abundances of tholeiitic basalts from (1) the three active Hawaiian volcanoes (Kilauea, Mauna Loa, and Loihi) and (2) the extinct Koolau shield (a compositional end member for Hawaiian volcanoes). New model calculations suggest that the mantle sources of Hawaiian volcanoes contain a significant amount of recycled oceanic crust with a factor of ∼2 increase from ∼8–16% at Loihi and Kilauea to ∼15–21% at Mauna Loa and Koolau. We propose that the Hawaiian plume contains a package of recycled oceanic crust (basalt and gabbro, with little or no marine sediment) that was altered by interaction with seawater or hydrothermal fluids prior to being variably dehydrated during subduction. The recycled oceanic crust in the mantle source of Loihi and Kilauea lavas is dominated by the uppermost portion of the residual slab (gabbro-free and strongly dehydrated), whereas the recycled oceanic crust in the mantle source of Mauna Loa and Koolau lavas is dominated by the lowermost portion of the residual slab (gabbro-rich and weakly dehydrated). The present-day distribution of compositional heterogeneities in the Hawaiian plume cannot be described by either a large-scale bilateral asymmetry or radial zonation. Instead, the mantle source of the active Hawaiian volcanoes is probably heterogeneous on a small scale with a NW–SE oriented spatial gradient in the amount, type (i.e., basalt vs. gabbro), and extent of dehydration of the ancient recycled oceanic crust.

Migration paths of magma and fluids and lava compositions in Kamchatka

Geophysical and geochemical data have been analyzed jointly in order to gain better understanding of subduction-related active volcanism in Kamchatka. The velocity structure of lithosphere beneath volcanic arcs has been imaged on three scales. Regional tomography to distances of thousands of kilometers has allowed constraints on slab geometry, which changes markedly in dip angle and thickness beneath the Kuriles-Kamchatka arc, possibly, because of a change in the interplay of the subduction driving forces. Intermediate-scale regional tomography (hundreds of kilometers) has been applied to the cases of Toba caldera in Sumatra, Mount Merapi in Java, and volcanoes in the Central Andes and provided evidence of magma conduits marked by low-velocity zones that link the suprasubduction volcanic arcs with clusters of earthquake hypocenters on the slab top. Local tomography resolves the shallow structure immediately under volcanoes and the geometry of respective melting zones. An example time-lapse (4D) seismic model of the crust beneath the Klyuchevskoy group of volcanoes has imaged a decade-long history of anomalous velocity zones and their relation with the activity cycles of Bezymyanny and Klyuchevskoy volcanoes. As modeling shows, andesitic Bezymyanny and basaltic Klyuchevskoy volcanoes have different feeding patterns during their eruption cycles: the former feeds directly from the mantle while the material coming to the latter passes through a complicated system of intermediate chambers.The local tomography model has been applied as reference to interpret the available major- and trace-element data from the Klyuchevskoy and Bezymyanny volcanoes. The lava compositions of the two volcanoes have becoming ever more proximal since 1945 in many major and trace elements while some parameters remain different. Paroxysmal eruptions of Bezymyanny for several recent decades correlate with the time when Klyuchevskoy erupted lavas with high percentages of high-Mg basalts. The difference in the evolution trends of the Kamchatka volcanic rocks may be due either to fractional crystallization or to the presence of concentrator minerals in the source, titanomagnetite, orthopyroxene, rutile, garnet, and plagioclase being especially active as to uptake of some elements. The natural compositions of rocks have been compared in this context with published experimental data.According to the seismic velocity structure and lava compositions analyzed jointly, there are five levels of crystallization beneath the studied volcanoes, while the number and spatial patterns of magma sources are different for two types of andesitic volcanoes.

Reconciling geochemical and geophysical observations of magma supply and melt distribution at the 9°N overlapping spreading center, East Pacific Rise

Early studies of mid‐ocean ridge discontinuities, such as transform faults and overlapping spreading centers, suggested a lower magma supply compared to ridge segment centers. This is reflected in bathymetrically deeper ridge axes, decreased hydrothermal activity, and the eruption of more evolved lava compositions. While many signatures of lower magma supply are observed at the 9°N overlapping spreading center on the East Pacific Rise, geophysical studies indicate extensive sub‐surface melt in the region, suggesting that the present magmatic system is not diminished. Here major and trace element concentrations of erupted lavas are used to better understand magma supply at a large second‐order ridge discontinuity. We show that the wide range of lava compositions erupted at the 9°N overlapping spreading center is generally consistent with early petrologic models of ridge propagation and require variable degrees of fractional crystallization, extensive magma mixing, and in some instances crustal assimilation. Moderately evolved ferrobasalts and FeTi basalts erupted at the OSC indicate that crustal residence times are long enough for significant crystallization of all magmas within the region, but the presence of dacitic lavas reflects periods of even lower magma supply, where melt replenishment is subordinate to cooling and crystallization. The geophysical observations of extensive melt within the shallow crust are reconciled with the geochemistry of the lavas, if melts are supplied intermittently to the propagating ridge over relatively short timescales.

Cognate clinopyroxene from Paleogene mantle xenolith-bearing basanite lavas (East Serbia, SE Europe): the role of dissolution of mantle orthopyroxene

The study focuses on clinopyroxene from mantle xenolith-bearing East Serbian basanites and suggests that dissolution of mantle orthopyroxene played an important role in at least some stages of the crystallization of these alkaline magmas. Five compositional types of clinopyroxene are distinguished, some of them having different textural forms: megacrysts (Type-A), green/colourless-cored phenocrysts (Type-B), overgrowths and sieve-textured cores (Type-C), rims and matrix clinopyroxene (Type-D), and clinopyroxene from the reaction rims around orthopyroxene xenocrysts (Type-E). Type-A is high-Al diopside that probably crystallized at near-liquidus conditions either directly from the host basanite or from compositionally similar magmas in previous magmatic episodes. Type-B cores show high VIAl/IVAl≥1 and low Mg# of mostly <75 and are interpreted as typical xenocrysts. Type-C, D and E are interpreted as typical cognate clinopyroxene. Type-D has Mg#<78, Al2O3 = 6–13 wt.%, TiO2 = 1.5–4.5 wt.%, and Na2O = 0.4–0.8 wt.% and compositionally similar clinopyroxene is calculated by MELTS as a phase in equilibrium with the last 30 % of melt starting from the average host lava composition. Type-C has Mg# = 72–89, Al2O3 = 4.5–9.5 wt.%, TiO2 = 1–2.5 wt.%, Na2O = 0.35–1 wt.% and Cr2O3 = 0.1–1.5 wt.%. This clinopyroxene has some compositional similarities to Type-E occurring exclusively around mantle orthopyroxene. Cr/Al vs Al/Ti and Cr/Al vs Na/Ti plots revealed that Type-C clinopyroxene can crystallize from a mixture of the host basanite magma and 2–20 wt.% mantle orthopyroxene. Sieve-textured Type-C crystals show characteristics of experimentally produced skeletal clinopyroxene formed by orthopyroxene dissolution suggesting that crystallization of Type-C was both texturally and compositionally controlled by orthopyroxene breakdown. According to FeO/MgOcpx/melt modelling the first clinopyroxene precipitating from the host basanite was Type-A (T ~ 1250 °C, p ~ 1.5 GPa). Dissolution of orthopyroxene produced decreasing FeO/MgOmelt and crystallization of Type-E and sieve-textured Type-C clinopyroxene (0.3–0.8 GPa and 1200–1050 °C). The melt composition gradually shifted towards higher FeO/MgOmelt ratios precipitating more evolved Type-C and Type-D approaching near-solidus conditions (<0.3 GPa; ~950 °C).

Heat capacities of hydrous silicate glasses and liquids

Differential scanning calorimetry measurements made at atmospheric pressure from 300K to about 100K above the glass transition temperature (Tg) are reported for two new series of hydrated aluminosilicate glasses and supercooled liquids for water contents between 0 and 7.3mol%. The compositions are synthetic iron-free analogs of tephrite and foidite lava compositions, and are depolymerized (NBO/T=0.8 and 1.5, respectively). In all cases, water exerts a marked depressing effect on Tg, in close agreement with the results of viscosity experiments on the same samples. For glasses, the partial molar heat capacity of water previously published for polymerized compositions (Bouhifd et al., 2006) reproduces the present data within the combined uncertainties of the experimental and model values. For the hydrous silicate glasses, the partial molar heat capacity of water can be thus considered as independent of composition, and insensitive to structural factors above room temperature. However, for hydrous liquids the existing data suggest two different coefficients for polymerized and depolymerized melts. In depolymerized liquids, the partial molar heat capacity of water C¯pH2O (is about 237±40J/molK, which is nearly three times higher than that previously determined for polymerized hydrous liquids C¯pH2O (≈85J/molK for three hydrous melts with 0<NBO/T<0.2; Bouhifd et al., 2006), and more than three times that of pure liquid water at ambient conditions. From speciation data, we found that the partial molar heat capacity for hydroxyl groups in depolymerized melts, C¯pOH−, is about 281±47J/molK, and that of molecular water, C¯pH2Omol, is about 30±15J/molK. The present study suggests that C¯pH2Omol is independent of composition, but, C¯pOH− for the depolymerized melts is close to double the value of 153±18J/molK previously obtained for the more polymerized melts. The present result suggests different structural roles of OH− in polymerized and depolymerized melts, and also the predominance of OH− species in depolymerized melts. This striking feature for water is discussed with reference to potential additional species in depolymerized melts, whose formation may have strong effects on hydrous melt properties.