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The application of petrography, mineral chemistry, geochemistry, and experimental petrology, including mineral–melt thermodynamic and diffusion modelling, on quenched basanitic magma samples from the recent (2011–2012) submarine eruption of El Hierro (Canary Islands) has permitted the identification of major physico-chemical variations prior to and during magma eruption that correlate in time with monitored geophysical changes. After nearly 3 months of seismic unrest the eruption of El Hierro started on October 10, 2011 and ended by late February 2012. We studied 10 lava balloons and pyroclastic fragments collected floating on the sea surface between October 15 and late January. Based on petrological and geophysical data we distinguish two main eruptive episodes. Magma erupted from the beginning of the eruption until late November 2011 was an evolved basanite (≈5 wt % MgO), changing to more primitive compositions (≈8–9 wt % MgO) with time, thus suggesting extraction from a compositionally zoned magma system. Experimental data and mineral–melt thermodynamic modelling indicate that the erupted magma equilibrated at a pressure of about 400 MPa, which corresponds to a depth of 12–15 km. This depth is consistent with the location of the crust–mantle discontinuity beneath El Hierro and with the hypocentral location of seismicity during the unrest episode. Preliminary modelling of the olivine chemical zoning of crystals erupted in this first episode suggests that the time scale for basanite fractionation and magma replenishment in the shallower reservoir was of the order of a few months. This is within the same time frame as the duration of the unrest episode preceding the eruption. The first eruption episode coincided with intense seismicity mostly located north of the island, first at a depth of 20–25 km and a few days later also at 10–15 km depth, with strong seismic tremor beneath the vent site. An abrupt change in magma composition and crystal content was observed at the end of November 2011. After that, more primitive and less viscous magma erupted contemporaneously with a change in the frequency and intensity of seismic events. During this period, seismicity was mostly north of the island at depths of 10–15 km. At the same time the tremor intensity at the eruption site significantly dropped. This marked the onset of the second eruption episode, which is correlated with an intrusion of fresh, more primitive magma into the shallow magmatic system that raised the temperature of the remaining magma. Experiments reveal that subtle changes in temperature of about 50°C (i.e. 1100–1150°C) were enough to produce large changes in the crystal content (10–60 wt %). This non-linear behaviour between crystal content and temperature had important effects on magma dynamics during transport and cooling. Our results suggest the existence of two interconnected mafic magma reservoirs during the El Hierro eruption, which agrees with the pattern shown by the seismicity. Stress readjustments of the plumbing system, caused by decompression during the eruption, influenced the thermodynamic evolution of the erupting magma and facilitated the intrusion of the deeper magma into the shallow reservoir, thus forcing a change in its rheological characteristics and eruption dynamics. [ABSTRACT FROM AUTHOR]

Phase equilibrium experiments were performed to determine the pre-eruptive conditions of the explosive eruption of Montaña Blanca (2020 bp) that occurred from a satellite vent located on the east flank of Teide volcano (Tenerife). Crystallization experiments used a phonolitic obsidian from the fall-out deposit as the starting material; this contains 5 wt % anorthoclase, diopside and magnetite with minor amounts of biotite and ilmenite, set in a glassy matrix that contains microlites of Ca-rich alkali feldspar. Temperature was varied between 850 and 800°C, and pressure between 200 and 50 MPa. The oxygen fugacity (fO2) was varied between NNO + 0·2 (0·2 log units above the Ni–NiO solid buffer) and NNO − 2, and dissolved water contents varied from 7 to 1·5 wt %. Comparison between natural and experimental phase proportions and compositions indicates that the main body of phonolitic magma was stored at 850 ± 15°C, 50 ± 20 MPa, 2·5 ± 0·5 wt % H2O at an fO2 around NNO − 0·5 prior to eruption, equivalent to depths of between 1 and 2 km below the surface. Some clinopyroxene crystals hosting H2O-rich melt inclusions possibly originate from an intermittent supply of phonolitic magma stored at somewhat deeper levels (100 MPa). The Ca- and Fe-rich composition of alkali feldspar phenocryst rims and microlites attests to the intrusion of a more mafic magma into the reservoir just prior to eruption; this is evidenced by the appearance of banded pumices in the later products of the eruptive sequence. The comparison with other phonolitic magmas from Tenerife and elsewhere (e.g. Vesuvius, Laacher See) shows that differences in the eruption dynamics of phonolitic magmas can be correlated with differences in magma storage depths, along with variations in pre-eruptive volatile contents. [ABSTRACT FROM AUTHOR]

The Granadilla eruption at 600 ka was one of the largest phonolitic explosive eruptions from the Las Cañadas volcano on Tenerife, producing a classical plinian eruptive sequence of a widespread pumice fall deposit overlain by an ignimbrite. The eruption resulted in a major phase of caldera collapse that probably destroyed the shallow-level magma chamber system. Granadilla pumices contain a diverse phenocryst assemblage of alkali feldspar + biotite + sodian diopside to aegirine–augite + titanomagnetite + ilmenite + nosean/haüyne + titanite + apatite; alkali feldspar is the dominant phenocryst and biotite is the main ferromagnesian phase. Kaersutite and partially resorbed plagioclase (oligoclase to sodic andesine) are present in some eruptive units, particularly in pumice erupted during the early plinian phase, and in the Granadilla ignimbrite at the top of the sequence. Associated with the kaersutite and plagioclase are small clots of microlitic plagioclase and kaersutite interpreted as quenched blebs of tephriphonolitic magma within the phonolite pumice. The Granadilla Member has previously been recognized as an example of reverse-then-normal compositional zonation, where the zonation is primarily expressed in terms of substantial variations in trace element abundances with limited major element variation (cryptic zonation). Evidence for cryptic zonation is also provided by the chemistry of the phenocryst phases, and corresponding changes in intensive parameters (e.g. T, f O2, f  H2O). Geothermometry estimates indicate that the main body of phonolite magma had a temperature gradient from 860 °C to ∼790 °C, with hotter magma (≥900 °C) tapped at the onset and terminal phases of the eruption. The reverse-then-normal chemical and thermal zonation reflects the initial tapping of a partially hybridized magma (mixing of phonolite and tephriphonolite), followed by the more sequential tapping of a zoned and relatively large body of highly evolved phonolite at a new vent and during the main plinian phase. This suggests that the different magma types within the main holding chamber could have been laterally juxtaposed, as well as in a density-stratified arrangement. Correlations between the presence of mixed phenocryst populations (i.e. presence of plagioclase and kaersutite) and coarser pumice fall layers suggest that increased eruption vigour led to the tapping of hybridized and/or less evolved magma probably from greater depths in the chamber. New oxygen isotope data for glass and mineral separates preclude syn-eruptive interaction between the vesiculating magma and hydrothermal fluids as the cause of the Sr isotope disequilibrium identified previously for the deposit. Enrichment in radiogenic Sr in the pumice glass has more likely been due to low-temperature exchange with meteoric water that was enriched in 87Sr by sea spray, which may be a common process affecting porous and glassy pyroclastic deposits on oceanic islands. [ABSTRACT FROM AUTHOR]

Laser ablation microprobe data are presented for olivine, orthopyroxene and clinopyroxene in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro, and Lanzarote, and new whole-rock trace-element data for xenoliths from Hierro and Lanzarote. The xenoliths show evidence of strong major, trace element and Sr isotope depletion (87Sr/86Sr ≤ 0·7027 in clinopyroxene in the most refractory harzburgites) overprinted by metasomatism. The low Sr isotope ratios are not compatible with the former suggestion of a mantle plume in the area during opening of the Atlantic Ocean. Estimates suggest that the composition of the original oceanic lithospheric mantle beneath the Canary Islands corresponds to the residues after 25–30% fractional melting of primordial mantle material; it is thus significantly more refractory than ‘normal’ mid-ocean ridge basalt (MORB) mantle. The trace element compositions and Sr isotopic ratios of the minerals least affected by metasomatization indicate that the upper mantle beneath the Canary Islands originally formed as highly refractory oceanic lithosphere during the opening of the Atlantic Ocean in the area. During the Canarian intraplate event the upper mantle was metasomatized; the metasomatic processes include cryptic metasomatism, resetting of the Sr–Nd isotopic ratios to values within the range of Canary Islands basalts, formation of minor amounts of phlogopite, and melt–wall-rock reactions. The upper mantle beneath Tenerife and La Palma is strongly metasomatized by carbonatitic or carbonaceous melts highly enriched in light rare earth elements (REE) relative to heavy REE, and depleted in Zr–Hf and Ti relative to REE. In the lithospheric mantle beneath Hierro and Lanzarote, metasomatism has been relatively weak, and appears to be caused by high-Si melts producing concave-upwards trace element patterns in clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-rich harzburgites/lherzolites, dunites, wehrlites and clinopyroxenites formed from mildly alkaline basaltic melts (similar to those that dominate the exposed parts of the islands), and appear to be mainly restricted to magma conduits; the alkali basalt melts have caused only local metasomatism in the mantle wall-rocks of such conduits. The various metasomatic fluids formed as the results of immiscible separations, melt–wall-rock reactions and chromatographic fractionation either from a CO2-rich basaltic primary melt, or, alternatively, from a basaltic and a siliceous carbonatite or carbonaceous silicate melt. [ABSTRACT FROM AUTHOR]

The Bandas del Sur Formation preserves a Quaternary extra-caldera record of central phonolitic explosive volcanism of the Las Can˜adas volcano at Tenerife. Volcanic rocks are bimodal in composition, being predominantly phonolitic pyroclastic deposits, several eruptions of which resulted in summit caldera collapse, alkali basaltic lavas erupted from many fissures around the flanks. For the pyroclastic deposits, there is a broad range of pumice glass compositions from phonotephrite to phonolite. The phonolite pyroclastic deposits are also characterized by a diverse, 7–8-phase phenocryst assemblage (alkali feldspar + biotite + sodian diopside + titanomagnetite + ilmenite + nosean–hau¨yne + titanite + apatite) with alkali feldspar dominant, in contrast to interbedded phonolite lavas that typically have lower phenocryst contents and lack hydrous phases. Petrological and geochemical data are consistent with fractional crystallization (involving the observed phenocryst assemblages) as the dominant process in the development of phonolite magmas. New stratigraphically constrained data indicate that petrological and geochemical differences exist between pyroclastic deposits of the last two explosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57 Ma) pyroclastic fall deposits commonly show a cryptic compositional zonation indicating that several eruptions tapped chemically, and probably thermally stratified magma systems. Evidence for magma mixing is most widespread in the pyroclastic deposits of Cycle 3 (0·37–0·17 Ma), which includes the presence of reversely and normally zoned phenocrysts, quenched mafic glass blebs in pumice, banded pumice, and bimodal to polymodal phenocryst compositional populations. Syn-eruptive mixing events involved mostly phonolite and tephriphonolite magmas, whereas a pre-eruptive mixing event involving basaltic magma is recorded in several banded pumice-bearing ignimbrites of Cycle 3. The periodic addition and mixing of basaltic magma ultimately may have triggered several eruptions. Recharge and underplating by basaltic magma is interpreted to have elevated sulphur contents (occurring as an exsolved gas phase) in the capping phonolitic magma reservoir. This promoted nosean–hau¨yne crystallization over nepheline, elevated SO3 contents in apatite, and possibly resulted in large, climatologically important SO2 emissions. [ABSTRACT FROM PUBLISHER]

Mantle xenoliths from Tenerife show evidence of metasomatism and recrystallization overprinting the effects of extensive partial melting. The evidence includes: recrystallization of exsolved orthopyroxene porphyroclasts highly depleted in incompatible trace elements into incompatible-trace-element-enriched, poikilitic orthopyroxene with no visible exsolution lamellae; formation of olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depleted clinopyroxene at the expense of orthopyroxene; the presence of phlogopite; whole-rock CaO/Al2O3 ≫ 1 (Ca metasomatism) in recrystallized rocks; and enrichment in incompatible elements in recrystallized rocks, relative to rocks showing little evidence of recrystallization. The ‘higher-than-normal’ degree of partial melting that preceded the metasomatism probably results from plume activity during the opening of the Central Atlantic Ocean. Sr–Nd isotopic compositions are closely similar to those of Tenerife basalts, indicating resetting from the expected original mid-ocean ridge basalt composition by the metasomatizing fluids. Metasomatism was caused by silicic carbonatite melts, and involved open-system processes, such as trapping of elements compatible with newly formed acceptor minerals, leaving residual fluids moving to shallower levels. The compositions of the metasomatizing fluids changed with time, probably as a result of changing compositions of the melts produced in the Canary Islands plume. Spinel dunites and wehrlites represent rocks where all, or most, orthopyroxene has been consumed through the metasomatic reactions. [ABSTRACT FROM PUBLISHER]

Major and trace element data on basanitic to phonolitic lavas of different ages and from different parts of Tenerife (Canary Islands), and their mafic silicates have been used to obtain more detailed information about processes taking place in crustal magma chambers associated with ocean island magmatism in Tenerife. Clinopyroxene phenocrysts in basanitic to phonolitic lavas consist of diopside-salite (referred to as Al-salite) with alternating normal and reverse zoning, and commonly contain a rounded or corroded core of homogeneous Na-rich diopside-salite (referred to as Na-salite). In general Al-salite contains lower amounts of rare earth elements (REE) and Y, and more Mg, Al, Ti, Cr, Sc and Ni than Na-salite. Variations in trace element concentrations and ratios are only weakly related to variations in mg-number. Petrographic and compositional relations among the lavas and mafic silicates are interpreted as the results of fractional crystallization in periodically refilled magma chambers (FCM processes). The FCM processes took place at temperatures of 1040-1260°C and pressures of about 0·2-0·5 GPa; that is, partly within the old oceanic crust and partly within the overlying sequence of Canary Islands lavas. FCM processes may lead to significant fractionation between incompatible trace elements whose ratios in mafic magmas are used to characterize their mantle source(s). Melts subjected to FCM processes will, furthermore, produce significantly larger masses of cumulates than melts of similar mg-number that have only been subjected to simple fractional crystallization. [ABSTRACT FROM PUBLISHER]

Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr-Nd-Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. 187Os/188Os ratios in the basalts range from 0·129 to 0·183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a `radiogenic' group with high 187Os/188Os from 0·152 to 0·183; (2) an `unradiogenic' group with low 187Os/188Os from 0·129 to 0·138; (3) an `intermediate' group with 187Os/188Os between 0·139 and 0·151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of <= 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher 87Sr/86Sr and 208Pb/204Pb but lower 143Nd/144Nd than samples with similar 206Pb/204Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower 206Pb/204Pb. The radiogenic Os and moderate 206Pb/204Pb signature of the Canary plume is consistent with a plume which contains 25-35% of relatively young (~1·2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in 206Pb/204Pb isotopic composition. [ABSTRACT FROM PUBLISHER]

Migmatization of gabbroic rocks at 2–3 kbar has occurred in the metamorphic contact aureole of a mafic pluton in the Fuerteventura Basal Complex (Canary Islands). Migmatites are characterized by a dense network of closely spaced millimetre-wide leucocratic veins with perfectly preserved igneous textures. They are all relatively enriched in Al, Na, P, Sr, Ba, Nb, Y and the rare earth elements compared with the unaffected country rock beyond the aureole. Migmatization under such low-pressure conditions was possible because of the unusual tectonic and magmatic context in which it occurred. Multiple basic intrusions associated with extrusive volcanic activity created high heat flow in a small area. Alkaline and metasomatized rocks present in the country rock of the intruding pluton were leached by high-temperature fluids during contact metamorphism. These enriched fluids then favoured partial melting of the host gabbroic rocks, and contaminated both the leucosomes and melanosomes. A transpressive tectonic setting at the time of intrusion created shearing along the contact between the intrusion and its host rock. This shearing enhanced circulation of the fluids and allowed segregation of the neo-formed melts from their restite by opening tension veins into which the melts migrated. Depending on the relative timing of melt segregation and recrystallization, leucosomes range in composition from a 40–60% mixture of clinopyroxene (± amphibole) and plagioclase to almost pure feldspathic veins. Comparable occurrences of gabbros migmatized at low pressure are expected only at a small scale in localized areas of high heat flow in the presence of fluids, such as in mid-ocean ridges or ocean-islands. [ABSTRACT FROM AUTHOR]

The petrology, geochemistry and petrogenesis of the active Teide–Pico Viejo volcanic complex provide information about the evolution and internal structure of the Tenerife central magma system during its most recent volcanic cycle. Two petrologically distinct basaniten–phonolite lineages are identified, which correlate essentially with the products of the Pico Teide and Pico Viejo stratovolcanoes. Geochemical modelling supports the evolution of both series from a common evolved basanite parent, by crystal fractionation under different physico-chemical conditions. Pico Viejo series intermediate magmas fractionated mainly kaersutite and low-Or plagioclase at high PH2O, whereas Teide series examples fractionated mainly high–Or plagioclase and clinopyroxene at low PH2O, resulting in lower Ba, Sr, Eu/Eu* and MREE/HREE, and less peralkaline phonolitic residua. The Pico Viejo series shows smooth modal, mineralogical and whole-rock chemical variations, whereas the Teide series shows the additional effects of mineral accumulation, magma mixing, assimilation of hydrothermally altered crust and contamination by felsic magmas. Pyroxene barometry suggests that parental basanites and Pico Viejo series intermediate magmas evolved in the lower crust and uppermost mantle at 6–12 kbar Ptotal. Teide series intermediate magmas experienced polybaric fractionation, also differentiating extensively within a shallow chamber beneath Teide, where low PH2O conditions resulted from low Ptotal and, on the basis of F and Cl systematics, from open–system degassing. Hygrometry and melt-inclusion data from phonolites suggest a shallow Teide chamber at ∼1.5 kbar Ptotal, whereas Pico Viejo series phonolites evolved in a separate shallow chamber at ∼1 kbar Ptotal. [ABSTRACT FROM AUTHOR]

Trace elements and Sr–Nd–Pb isotopes have been analyzed on sedimentary and igneous (metabasalt, metadiorite and metagabbro) samples from the Jurassic oceanic crust beneath Gran Canaria (Canary Islands). The igneous crust exhibits extreme heterogeneity in 87Sr/86Sr (0.7029–0.7052), 206Pb/204Pb (18.2–20.8) and 208Pb/204Pb (38.1–41.3). Leaching experiments indicate that seawater alteration has elevated the 87Sr/86Sr ratio but has not appreciably affected 143Nd/144Nd (0.51295–0.51306). An Sm–Nd isochron gives an age of 178 ± 17 Ma, which agrees with the age predicted from paleomagnetic data. Hydrothermal alteration near the ridge axis has increased 207Pb/204Pb (15.59−15.73), 208Pb/204Pb (as well as Δ7/4Pb and Δ8/4Pb), 238U/204Pb (μ) and Ce/Pb but has not appreciably changed 206Pb/204Pb. The large range in 206Pb/204Pb and 208Pb/204Pb reflects radiogenic ingrowth with μ being as high as 107. Portions of the Jurassic ocean crust have trace element and isotope characteristics within the range found at St Helena, the Atlantic type locality for the HIMU (high μ) mantle end-member. Evaluation of the published isotopic data for Gran Canaria volcanic rocks indicates that the isotopic composition of these melts primarily represents the composition of their mantle sources rather than crustal assimilation. [ABSTRACT FROM AUTHOR]

Spinel harzburgite, lherzolite, dunite and wehrlite mantle xenoliths from the Canary Islands (La Palma, Hierro, Tenerife and Lanzarote) contain a spectrum of silicate glasses as inclusions in minerals, along grain boundaries, and in interstitial glass pockets. These glasses show a range in composition form basaltic (∼44 wt % SiO2), to highly silicic, TiO2–FeO–MgO–CaO–P2O5-poor types (up to 71 wt % SiO2). Glasses in spinel harzburgites and lherzolites are generally silica oversaturated, whereas those in spinel dunites and wehrlites have somewhat lower SiO2 contents and are generally silica undersaturated. Glasses in xenoliths from La Palma and Tenerife are rich in K2O compared with those from Hierrro and Lanzarote. Daughter minerals coexisting with highly silicic glass in polyphase inclusions are similar in composition to the main phases in the host xenoliths (Fo>90, Cr-diopside, chromite), whereas those in less silicic glasses are richer in Al2O3, TiO2 and FeO, and poorer in MgO. The systematic relations found to exist between glass composition, mineralogy of the host xenolith and locality (island) cannot reflect random variations in the geochemistry of ‘exotic’ melts infiltrating the mantle lithosphere, but instead suggest a cogenetic relationship between the melts and their mantle host xenoliths. The silicic glasses are interpreted as the products of reactions at 8–12 kbar between infiltrating alkali basaltic magmas and peridotitic wall-rocks which, in orthopyroxene-bearing rock-types, involves formation of silicic melt+olivine at the expense of orthopyroxene. In xenoliths from La Palma and Tenerife, where interstitial phlogopite is commonly present, phlogopite has been partly or totally consumed by the reactions between relatively mafic melts and peridotite, giving rise to silicic glasses with high K2O contents and K2O/Na2O ratios. The low K2O concentrations and K2O/Na2O ratios in glasses in anhydrous xenoliths suites from Hierro and Lanzarote are believed to result for reactions between infiltrating melts and anhydrous and/or amphibole-bearing mantle wall-rocks. The silicic melts appear to have been mobile over distances exceeding the diameter of a xenolith, that is, at least 20–30 cm. [ABSTRACT FROM AUTHOR]