Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores (original) (raw)
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Petrogenesis of the Peralkaline Ignimbrites of Terceira, Azores
Journal of Petrology, 2017
The recent (< 100 ka) volcanic stratigraphy of Terceira, Azores, includes at least seven peralkaline trachytic ignimbrite formations, attesting to a history of explosive eruptions. In this study, the petrogenesis and pre-eruptive storage conditions of the ignimbrite-forming magmas are investigated via whole-rock major and trace element geochemistry, melt inclusion and groundmass glass major element and volatile compositions, mineral chemistry, thermobarometric models, and petrogenetic modelling. Our primary aims are to develop a model for the magmatic plumbing system from which the ignimbrite-forming trachytes of Terceira were produced by evaluating various petrogenetic processes and constraining pre-eruptive magma storage conditions. We also place the ignimbriteforming magmas into the context of the Terceira suite and discuss the potential implications of preeruptive magma conditions for eruptive behaviour. Results indicate that ignimbrite-forming, comenditic trachytes are generated predominantly by extended fractional crystallization of basaltic parental magmas at redox conditions around 1 log unit below the fayalite-magnetite-quartz buffer. This is achieved via a polybaric fractionation pathway, in which mantle-derived basalts stall and fractionate to hawaiitic compositions at lower crustal depths ($15 km), before ascending to a shallow crustal magma storage zone ($2-4 km) and fractionating towards comenditic trachytic compositions. The most evolved pantelleritic magmas of Terceira (not represented by the ignimbrites) are plausibly generated by continued fractionation from the comenditic trachytes. Syenite autoliths represent portions of peralkaline trachytic melt that crystallized in situ at the margins of a silicic reservoir. Trachytic enclaves hosted within syenitic autoliths provide direct evidence for a two-stage mingling process, in which ascending hawaiites are mixed with trachytic magmas in the shallow crustal magma storage zone. The resulting hybridized trachytes then ascend further and mix with the more evolved peralkaline trachytes in the uppermost eruptible cap of the system, passing first through a syenitic crystal mush. The reduced viscosities of the peralkaline silicic magmas of this study in comparison with their metaluminous counterparts facilitate rapid crystal-melt segregation via crystal settling, generating compositionally zoned magma bodies and, in some instances, relatively crystalpoor erupted magmas. Reduced viscosity may also inhibit highly explosive activity (e.g. formation of a sustained eruption column), and limit the majority of explosive eruptions to low pyroclastic fountaining or 'boil-over' eruption styles. The formation of intermediate composition magmas within the system is considered to be limited to episodic mixing between mafic and silicic magmas.
Journal of Volcanology and Geothermal Research, 2005
Large 226 Ra excesses in a number of mafic arc magmas, and geophysical observations of earthquake hypocenters locations, indicate that mafic melts can get transferred from source to surface within days to~1 kyr. Decreasing 226 Ra-230 Th disequilibria with increasing SiO 2 in co-magmatic suites of individual arc volcanoes suggest that magma differentiation often occurs on timescales of a few thousand years in closed systems, or less in open systems. However, the rapid decrease of global 238 U-230 Th disequilibria with increasing SiO 2 within basaltic andesite compositions suggests that rapid closed system differentiation does not typically produce more evolved compositions from mafic parental magmas with large 238 U excesses. In the case of rapid magmatic evolution, open system processes are frequently involved in the production of andesites and dacites, and the data imply that the open system component is close to 238 U-230 Th secular equilibrium. MELTS modeling of magma evolution at Santorini in the Aegean arc provides corroborating evidence from an individual well-studied arc volcano, that closed system fractional crystallization fails to produce the more evolved compositions, and increasing 87 Sr/ 86 Sr ratios with increasing SiO 2 from andesites to rhyolites suggests assimilation of old crust is involved. These observations can be conceptualized through thermal modeling of basaltic sill injections into the lower and upper crust. Melt production in deep crustal hot zones provides an explanation for low 238 U-230 Th disequilibria of the evolved magmatic component, as significant accumulation of evolved melts through partial re-melting of previously intruded basalts requires N100 kyr incubation time at typical magmatic flux rates. Thermal modeling also provides a framework for understanding assimilation of old upper crustal rocks in more mature magmatic systems. Further insights into magmatic evolution can be gained from dating of minerals from mafic, intermediate and felsic arc magmas. (a) Many U-Th mineral isochrons in mafic arc magmas yield ages significantly older than those obtained from Ra-Th mineral isochrons from the same samples. A crystal size distribution case study from Soufrière, St.
Geological Magazine, 2013
Magmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized b...
Geochemistry of Volcanic Rocks from Faial Island (Azores)
2008
Whole rock geochemistry of nine rocks from Faial Island (Azores) was used to characterize Faial Island volcanism. Studied rocks are lavas and were classified based on chemical data as basalts, hawaiites and trachyte. These rocks represent five stratigraphic units of island, from the base to the top: Ribeirinha Volcanic Complex, Cedros Volcanic Complex, Almoxarife and Capelo formations. The rocks belong to the sodic alkaline series, with the exception of the trachytic rock, that exhibits potassic affinity. Faial Island volcanism is characterized by low SiO2 contents (45 to 49 %), high TiO 2 (2 to 3 %) and P 2 O 5 (0.29 to 0.74 %). MgO shows values varying from medium to high (2 to 15 %). The trachyte has 62 % of SiO 2 , low MgO (0.42 %), TiO 2 (0.53 %) and P 2 O 5 (0.13 %) contents. Al 2 O 3 varies from low to high (11 to 18 %) and reflects the plagioclase abundance variation. Na 2 O+K 2 O values increase with fractionation and K 2 O/Na 2 O ratio is less than 0.54, with the exception of the trachyte (0.73). Ni content decreases with magmatic evolution, from 356 to 5 ppm. The presence of the cumulate phases in basalts of Almoxarife and Capelo formations explains the MgO and Ni high values. Almost all the samples show Rb, Ba, Ta, Nb, Zr enrichment and Th, Sm impoverishment. The trachyte shows Ba enrichment and K depletion. Zr positive anomaly and U, Sr, P, Ti negative anomalies are observed in the trachyte. The REE patterns are typical of oceanic island lavas, showing LREE enrichment relative to MREE and HREE. The REE patterns suggest a continuous fractionation from a common melt generated by low partial melting rates. The rocks of Faial Island are the result of fractional crystallization and derived from a magma, which has mixed characteristics, mainly reflecting PREMA and EM II reservoirs, with minor contribution of HIMU.
Petrogenesis of rhyolite-trachyte-basalt composite ignimbrite P1, Gran Canada, Canary Islands
Journal of Geophysical Research, 1995
The 14 Ma caldera-forming composite ignimbrite PI on Gran Canaria (Canary Islands) represents the first voluminous eruption of highly differentiated magmas on top of the basaltic Miocene shield volcano. Compositional zonation of the ignimbrite is the result of vertically changing proportions of four component magmas, which were intensely mixed during eruption: (I) Crystal-poor to highly phyric rhyolite (-10 km 3), (2) sodic trachyandesite through mafic to evolved trachyte (-6 km 3), (3) Nafoor trachyandesite (<I km 3), and (4) basalt zoned from 5.2 to 4.3 wt% MgO (-26 km). PI basalt is composed of two compositionally zoned magma batches, B2 basalt and B3 basalt. B3 basalt is derived from a mantle source depleted in incompatible trace elements compared to the shield basalt source. Basaltic magmas were stored in a reservoir probably underplating the crust, in which zoned B2 basaltic magma formed by mixing of "enriched" (shield) and "depleted" (B3) mafic melts and subsequent crystal fractionation. Evolved magmas formed in a shallow crustal chamber, whereas intermediate magmas formed at both levels. Abundant pyroxenitic to gabbroid cumulates in PI support crystal fractionation as the major differentiation process. On the basis of major and trace element modeling, we infer two contemporaneous fractional crystallization series: series I from "enriched" shield basalt through Na-poor trachyandesite to rhyolite, and series II from "depleted" PI basalt through sodic trachyandesite to trachyte. Series II rocks were significantly modified by selective contamination involving feldspar (Na, K, Ba, Eu, Sr), zircon (Zr) and apatite (P, Y, rare earth elements) components; apatite contamination also affected series I Na-poor trachyandesite. Substantial sodium introduction into sodic trachyandesite is the main reason for the different major element evolution of the two series, whereas their different parentage is mainly reflected in the high field strength trace elements. Selective element contamination involved not only rapidly but also slowly diffusing elements as well as different saturation conditions. Contamination processes thus variably involved differential diffusion, partial dissolution of minerals, partial melt migration, and trace mineral incorporation. Magma mixing between trachyte and rhyolite during their simultaneous crystallization in the PI magma chamber is documented by mutual mineral inclusions but had little effect on the compositional evolution of both magmas. Fe-Ti oxide thermometry yields magmatic temperatures of around 850°C for crystal-poor through crystal-rich rhyolite,-815°C for trachyte and-850°-900°C for the trachyandesitic magmas. High 1160°C for the basalt magma suggest its intrusion into the PI magma chamber only shortly before eruption. The lower temperature for trachyte compared to rhyolite and the strong crustal contamination of trachyte and sodic trachyandcsite support their residence along the walls of the vertically and laterally zoned PI magma chamber. The complex magmatic evolution of PI reflects the transient state of Gran Canaria's mantle source composition and magma plumbing system during the change from basaltic to silicic volcanism. Our results for PI characterize processes operating during this important transition, which also occurs on other volcanic ocean islands. Introduction and Geologic Setting During the major, Miocene magmatic cycle [Schmincke, t976, 1982, t990] on the volcanic ocean island of Gran Can aria (28'00'N, l5°35'W) in the Canarian archipelago (east central Atlantic), a major change from basaltic to silicic
Chemical Geology, 2007
High precision measurements of 226 Ra-230 Th-238 U disequilibria and Ba concentrations are reported for samples from two chemically zoned trachyte deposits from Fogo volcano, São Miguel, Azores. High-precision U-series disequilibria measurements by plasma ionization multicollector mass spectrometry were performed on pumice lapilli and volcanic glass separates from Fogo 1563 A.D. (∼ 0.14 km 3 ) and the ∼4.7 ka Fogo A (∼ 0.7 km 3 ) deposits in order to quantify the time scales of magmatic processes. Observed ( 226 Ra)/Ba-( 230 Th)/Ba relationships in Fogo 1563 are compatible with a conventional instantaneous fractional crystallization model and a pre-eruptive magma residence time of ∼ 50 y. However, the Fogo A data cannot be explained by instantaneous fractional crystallization, and require a prolonged crystallization history. Continuous differentiation models better explain the observed 226 Ra-230 Th variations within the Fogo deposits and may be more realistic in general. Such models suggest magma residence times prior to eruption of ∼ 50-80 years for Fogo 1563, and ∼ 4.7 ka for the larger volume Fogo A eruption. These time scales represent liquid residence ages rather than the crystallization ages documented in most previous magmatic time scale studies, and allow constraints to be placed on the time scales necessary for the development of chemical zonation within the Fogo magma chamber. Our results indicate that calculated time scales are relatively insensitive to the precise nature of the continuous differentiation models, and further indicate that meaningful magma differentiation time scales can be obtained despite open system behavior, because the Ra-Th disequilibria are overwhelmingly controlled by feldspar fractionation. Calculated time scales are, however, extremely sensitive to D Ra /D Ba ratios. We therefore emphasize the crucial importance to better constrain the relative partitioning of Ra and Ba when employing Ra-Th disequilibrium data to constrain the rates and time scales of igneous processes.
Petrological investigations of active volcanoes are often supported by mass balance, thermodynamic calculations and/or experiments performed at key conditions. Conversely, the compositions of mineral phases found in natural products are generally used as input data for predictive models calibrated to derive the intensive variables of the magmatic system. In order to evaluate the extent to which mineral chemistry records crystallization conditions, we have compared the compositions of olivine, clinopyroxene, plagioclase and titanomagnetite in 2001-2012 trachybasaltic lavas at Mt. Etna with those obtained through thermodynamic simulations and experiments conducted under anhydrous, water-undersaturated and water-saturated conditions. This systematic comparison allows us to track recent differentiation processes beneath Mt. Etna, as well as the P-T-fO 2 -H 2 O variables controlling the solidification path of magma. Two compositionally distinct populations of olivine and clinopyroxene phenocrysts are found in these lavas: Mg-rich and Mg-poor minerals formed at 600-1100 MPa and 1100-1250°C, and 0.1-500 MPa and 1050-1175°C, respectively. The oxygen fugacity varies by 1-2 log units suggesting water exsolution during magma ascent in the conduit and magma emplacement near the surface. The nucleation and growth of normally zoned plagioclases occur at P b100 MPa, when the amount of H 2 O dissolved in the melt abruptly decreases from about 3.0 to 0.2 wt.% due to magma decompression and degassing. This leads to the conclusion that Etnean magmas fractionate throughout the entire length of the vertically developed plumbing system where magma mixing, volatile exsolution and degassing are the most important processes driving eruptions.
Lithos, 2011
This work addresses the present-day (b100 ka) mantle heterogeneity in the Azores region through the study of two active volcanic systems from Terceira Island. Our study shows that mantle heterogeneities are detectable even when "coeval" volcanic systems (Santa Bárbara and Fissural) erupted less than 10 km away. These volcanic systems, respectively, reflect the influence of the Terceira and D. João de Castro Bank end-members defined by Beier et al. (2008) for the Terceira Rift. Santa Bárbara magmas are interpreted to be the result of mixing between a HIMU-type component, carried to the upper mantle by the Azores plume, and the regional depleted MORB magmas/source. Fissural lavas are characterized by higher Ba/Nb and Nb/U ratios and less radiogenic 206 Pb/ 204 Pb, 143 Nd/ 144 Nd and 176 Hf/ 177 Hf, requiring the small contribution of delaminated sub-continental lithospheric mantle residing in the upper mantle. Published noble gas data on lavas from both volcanic systems also indicate the presence of a relatively undegassed component, which is interpreted as inherited from a lower mantle reservoir sampled by the ascending Azores plume. As inferred from trace and major elements, melting began in the garnet stability field, while magma extraction occurred within the spinel zone. The intra-volcanic system's chemical heterogeneity is mainly explained by variable proportions of the above-mentioned local end-members and by crystal fractionation processes.
Volcanic geology of Furnas Volcano, São Miguel, Azores
Journal of Volcanology and Geothermal Research, 1999
Furnas is the easternmost of the three active central volcanoes on the island of Sao Miguel in the Azores. Unlike the other two central volcanoes, Sete Cidades and Fogo, Furnas does not have a well-developed edifice, but consists of a steep-sided caldera complex 8 = 5 km across. It is built on the outer flanks of the PovoaçaorNordeste lava complex that forms thẽ eastern end of Sao Miguel. Constructive flanks to the volcano exist on the southern side where they form the coastal cliffs, and to the west. The caldera margins tend to reflect the regionalrlocal tectonic pattern which has also controlled the distribution of vents within the caldera and areas of thermal springs. Activity at Furnas has been essentially explosive, erupting materials of trachytic composition. Products associated with the volcano include plinian and sub-plinian pumice deposits, ignimbrites and surge deposits, phreatomagmatic ashes, block and ash deposits and dome materials. Most of the activity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera. The eruptive history consists of at least two major caldera collapses, followed by caldera infilling. Based on 14 C dates, it appears that the youngest major collapse occurred about 12,000-10,000 years BP. New 14 C dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption Ž . occurred at about 30,000 years BP. Recent eruptions -5000 years old were mainly characterised by alternating episodes of magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude, forming deposits of interbedded ash and lapilli. An historical eruption is documented in 1630 AD; new evidence suggests that another occurred during the early occupation of the area at about 1440 AD. q of the most active and dangerous volcanoes in the Azores Archipelago. It is a trachytic centre, and the majority of its activity has involved explosive volcanism. However, throughout its history, it has exhibited almost all known eruptive styles ranging from mild effusive activity to caldera-forming explosive events. From a hazard perspective, it poses 0377-0273r99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.
Journal of Petrology, 2015
Deciphering the evolution of the internal dynamics of magmatic plumbing systems and identifying the key parameters that drive such dynamics are major goals of modern volcanology. Here we present a novel petrological approach that combines kinetic modelling of the diffusive relaxation of chemical zoning patterns in olivine crystals with thermodynamic modelling (MELTS) to constrain the nature and evolution of the plumbing system of Mt. Etna and the processes governing its internal dynamics. We investigated the compositional and temporal record preserved in 180 olivine crystals that were erupted between 1991 and 2008. Detailed systemization of the information stored in the sequential zoning record of the olivines reveals the existence of at least five compositionally different magmatic environments (MEs), characterized by different olivine compositions: M 0 (Fo79-83), M 1 (Fo75-78), M 2 (Fo70-72), M 3 (Fo65-69) and mm 1 (Fo73-75). Several routes of magma transfer connect these environments. We identified three prominent magma passageways between the environments M 0 :M 1 , M 2 :mm 1 and M 1 :M 2 that were active during the entire period of observation between 1991 and 2008. Modelling the diffusive relaxation of the olivine zoning patterns reveals that the transfer of magma along such routes can occur over fairly heterogeneous timescales ranging from days to 2 years. Although some of the passageways have been sporadically active in the months and sometimes years before an eruption, the magma migration activity increases clearly in the weeks and days prior to an eruptive event. In this context, major transfer routes such as M 2 :mm 1 might represent temporary passageways that are activated only shortly before eruptive events. A forward modelling approach was developed using thermodynamic calculations with the MELTS software to identify the key intensive variables associated with the different magmatic environments. In this approach the observed populations of mineral compositions (e.g. Fo79-83), rather than single compositions, are associated with thermodynamic parameters [pressure, temperature, water content, oxygen fugacity (fO 2) and bulk composition of the melt] to identify the most plausible set corresponding to each ME. We found that temperature, water content and possibly oxidation state are the main distinguishing features of the different magmatic environments. The most primitive olivine population M 0 (Fo79-83) and some of its associated clinopyroxenes