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Hansulrich Schmincke - One of the best experts on this subject based on the ideXlab platform.
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petrogenesis of rhyolite trachyte basalt composite ignimbrite p1 gran canada canary islands
Journal of Geophysical Research, 1995Co-Authors: Armin Freundt, Hansulrich SchminckeAbstract:The 14 Ma caldera-forming composite ignimbrite P1 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: (1) Crystal-poor to highly phyric rhyolite (∼10 km3), (2) sodic trachyandesite through mafic to evolved trachyte (∼6 km3), (3) Na-poor trachyandesite (<1 km3), and (4) basalt zoned from 5.2 to 4.3 wt % MgO (∼26 km3). P1 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 P1 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” P1 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 P1 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 P1 Magma chamber only shortly before eruption. The lower temperature for trachyte compared to rhyolite and the strong crustal contamination of trachyte and sodic trachyandesite support their residence along the walls of the vertically and laterally zoned P1 Magma chamber. The complex Magmatic evolution of P1 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 P1 characterize processes operating during this important transition, which also occurs on other volcanic ocean islands.
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mixing of rhyolite trachyte and basalt Magma erupted from a vertically and laterally zoned reservoir composite flow p1 gran canaria
Contributions to Mineralogy and Petrology, 1992Co-Authors: Armin Freundt, Hansulrich SchminckeAbstract:The 14.1 Ma composite welded ignimbrite P1 (45 km3 DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component Magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km3) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite Magma group (6 km3); (3) a minor fraction of Na-poor trachyandesite (<1 km3); and (4) nearly aphyric basalt (26 km3) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic Magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite Magma upon massive anorthoclase precipitation and accumulation. (b) All component Magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of Magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the Magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of Magma density changes during crystallization, Magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 Magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of Magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite Magma mantled by a sheath of vertically zoned trachyte-trachyandesite Magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.
Jon D Blundy - One of the best experts on this subject based on the ideXlab platform.
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experimental petrology of monotonous intermediate Magmas
Geological Society London Special Publications, 2015Co-Authors: Luca Caricchi, Jon D BlundyAbstract:Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates', provide a unique opportunity to investigate the evolution of crustal Magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite Magmas as a function of temperature. The experiments (200 MPa, 720–1100C) span the inferred pre-eruptive conditions of FCT Magmas, and are shown to provide the best match to the chemical and physical properties of the erupted Magmas at 790+-10C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of Magma, and the contrasted behaviour of water-saturated v. water-undersaturated Magmas. In both cases, however, there is a broad interval of temperature (200C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system,such that small perturbations to the heat content of the system (e.g. by cooling or new Magmainjections) results in very little change in Magma properties. The crystal content in this intervalis 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich Magmas tend to settle in this ‘petrological trap', changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate Magma to accumulate in the shallow crust until suchtime as the net buoyancy force of these crystal-rich Magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.
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the temporal record of Magmatism at cerro uturuncu bolivian altiplano
Geological Society London Special Publications, 2015Co-Authors: Jon D Blundy, Duncan Muir, Dan N Barfod, Alison C Rust, R S J Sparks, K M ClarkeAbstract:Twenty-six new 40Ar/39Ar plateau ages for 23 lavas and domes from the Uturuncu volcano in the Altiplano of SW Bolivia reveal a protracted eruptive history from 1050±5 to 250±5 ka. Eruptions have been exclusively effusive, producing some 50 km3 of high-K dacites and silicic andesites. Bimodal mineral compositions, complex mineral textures, the presence of andesitic Magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that Magma mixing is an important petrogenetic process at Uturuncu. Post-458 ka, distinct high and low MgO–Cr Magmas are resolved. These Magmas erupt during similar times, suggesting that eruptions are tapping different parts of the Magma system, albeit from the same vent system. Volcanic and petrological features are consistent with the existence of a vertically extensive Magma mush column beneath Uturuncu, and calculated buoyancy forces are sufficient to drive effusive eruptions. Eruptive activity is episodic, with six eruptive periods separated by hiatuses of >50 kyr. Cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka. The episodicity of eruptions is most likely to be related to fluctuations in the Magma supply to the underlying Altiplano–Puno Magma Body.
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small volume andesite Magmas and melt mush interactions at ruapehu new zealand evidence from melt inclusions
Contributions to Mineralogy and Petrology, 2013Co-Authors: Jon D Blundy, Katharine V Cashman, Geoffrey N Kilgour, Heidy M MaderAbstract:Historical eruptions from Mt. Ruapehu (New Zealand) have been small (<0.001 km3 of juvenile Magma) and have often occurred without significant warning. Developing better modelling tools requires an improved understanding of the Magma storage and transport system beneath the volcano. Towards that end, we have analysed the volatile content and major element chemistry of groundmass glass and phenocryst-hosted melt inclusions in erupted samples from 1945 to 1996. We find that during this time period, Magma has been stored at depths of ~2–9 km, consistent with inferences from geophysical data. Our data also show that Ruapehu Magmas are relatively H2O-poor (<2 wt%) and CO2-rich (≤1,000 ppm) compared to typical arc andesites. Surprisingly, we find that melt inclusions are often more evolved than their transporting melt (as inferred from groundmass glass compositions). Furthermore, even eruptions that are separated by less than 2 years exhibit distinct major element chemistry, which suggests that each eruption involved Magma with a unique ascent history. From these data, we infer that individual melt batches rise through, and interact with, crystal mush zones formed by antecedent Magmas. From this perspective, we envision the Magmatic system at Ruapehu as frequently recharged by small Magma inputs that, in turn, cool and crystallise to varying degrees. Melts that are able to erupt through this network of crystal mush entrain (to a greater or lesser extent) exotic crystals. In the extreme case (such as the 1996 eruption), the resulting scoria contain melt inclusion-bearing crystals that are exotic to the transporting Magma. Finally, we suggest that complex interactions between recharge and antecedent Magmas are probably common, but that the small volumes and short time scales of recharge at Ruapehu provide a unique window into these processes.
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Magma heating by decompression driven crystallization beneath andesite volcanoes
Nature, 2006Co-Authors: Jon D Blundy, Kathy Cashman, Madeleine C S HumphreysAbstract:Explosive volcanic eruptions are driven by exsolution of H2O-rich vapour from silicic Magma1. Eruption dynamics involve a complex interplay between nucleation and growth of vapour bubbles and crystallization, generating highly nonlinear variation in the physical properties of Magma as it ascends beneath a volcano2. This makes explosive volcanism difficult to model and, ultimately, to predict. A key unknown is the temperature variation in Magma rising through the sub-volcanic system, as it loses gas and crystallizes en route3. Thermodynamic modelling of Magma that degasses, but does not crystallize, indicates that both cooling and heating are possible4. Hitherto it has not been possible to evaluate such alternatives because of the difficulty of tracking temperature variations in moving Magma several kilometres below the surface. Here we extend recent work on glassy melt inclusions trapped in plagioclase crystals5 to develop a method for tracking pressure–temperature–crystallinity paths in Magma beneath two active andesite volcanoes. We use dissolved H2O in melt inclusions to constrain the pressure of H2O at the time an inclusion became sealed, incompatible trace element concentrations to calculate the corresponding Magma crystallinity and plagioclase–melt geothermometry to determine the temperature. These data are allied to ilmenite–magnetite geothermometry to show that the temperature of ascending Magma increases by up to 100 °C, owing to the release of latent heat of crystallization. This heating can account for several common textural features of andesitic Magmas, which might otherwise be erroneously attributed to pre-eruptive Magma mixing.
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rapid decompression driven crystallization recorded by melt inclusions from mount st helens volcano
Geology, 2005Co-Authors: Jon D Blundy, Kathy CashmanAbstract:Crystals in hydrous Magmas can form in response to falling temperature (Magma cooling) or degassing (Magma decompression). It remains unclear which process dominates beneath explosive silicic volcanoes. Because decompression and cooling operate on very different time scales, resolving the driving force behind crystallization is of fundamental importance for determining Magma dynamics and eruption hazard. Here we use ion-microprobe measurements of dissolved H 2 O in phenocryst-hosted melt inclusions from pumices erupted between May and October 1980 at Mount St. Helens volcano to show that all microlites and a significant proportion of phenocrysts were formed by near isothermal decompression. Magmas erupted after 18 May show evidence for subsequent crystallization of both phenocrysts and microlites, indicating that the time scales of crystal nucleation and growth are on the order of months or less.
Ian A Nairn - One of the best experts on this subject based on the ideXlab platform.
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silicic recharge of multiple rhyolite Magmas by basaltic intrusion during the 22 6 ka okareka eruption episode new zealand
Lithos, 2008Co-Authors: Phil Shane, Ian A Nairn, Victoria C Smith, Miles Darragh, Kate Beggs, J. W. ColeAbstract:Abstract Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite Magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption (∼ 0.01 km3 Magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite Magma (T1) with a volume of ∼ 0.3 km3. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite Magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite Magma (T2) (∼ 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite Magma (T3) (∼ 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 Magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite Magmas. The eruption sequence and the restricted direct basaltic intrusion into only one Magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary Magma storage depths of
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multiple rhyolite Magmas and basalt injection in the 17 7 ka rerewhakaaitu eruption episode from tarawera volcanic complex new zealand
Journal of Volcanology and Geothermal Research, 2007Co-Authors: Phil Shane, Victoria C Smith, Miles Darragh, Kate Beggs, J. W. Cole, S B Martin, Ian A NairnAbstract:Abstract The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km3 DRE rhyolite Magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous rhyolite lava extrusions. Two different rhyolite Magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 Magma was a crystal-poor orthopyroxene-hornblende rhyolite that is highly evolved (whole rock SiO2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende rhyolite that is less evolved (SiO2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two Magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third rhyolite Magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO2 = 74 wt.%) and hottest (∼ 820 °C) of the three Magmas. Saturation pressures calculated using dissolved H2O and CO2 contents of melt inclusions in quartz crystals indicate that T2 Magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 Magma grew during ascent through ∼ 8 km depths. Some T1 and T2 rhyolite clasts contain vesicular brown blebs with widely variable (andesite to rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic Magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera rhyolite eruption episodes. However, the rhyolite Magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite Magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 rhyolite Magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 Magma, and lubricated its transport to the surface.
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geochemistry and Magmatic properties of eruption episodes from haroharo linear vent zone okataina volcanic centre new zealand during the last 10 kyr
Bulletin of Volcanology, 2006Co-Authors: Ian A Nairn, Victoria C Smith, Phil Shane, Catherine M WilliamsAbstract:Post-10 ka rhyolitic eruptions from the Haroharo linear vent zone, Okataina Volcanic Centre, have occurred from several simultaneously active vents spread over 12 km. Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous Magma batches. Three Magmas totalling ~8 km3 were erupted during the 9.5 ka Rotoma episode. The most evolved Rotoma Magma (SiO2=76.5–77.9 wt%, Sr=96–112 ppm) erupted from a southeastern vent, and is characterised by a cummingtonite-dominant mineralogy, a temperature of 739±14°C, and fO2 of NNO+0.52±0.11. The least evolved (SiO2=75.0–76.4 wt%, Sr=128–138 ppm, orthopyroxene+ hornblende-dominant) Rotoma Magma erupted from several vents, and was hotter (764±18°C) and more reduced (NNO+0.40±0.13). The ~11 km3 Whakatane episode occurred at 5.6 ka and also erupted three Magmas, each from a separate vent. The most evolved (SiO2=73.3–76.2 wt%, Sr=88–100 ppm) Whakatane Magma erupted from the southwestern (Makatiti) vent and is cummingtonite-dominant, cool (745±11°C), and reduced (NNO+0.34±0.08). The least evolved (SiO2=72.8–74.1 wt%, Sr=132–134 ppm) Magma was erupted from the northeastern (Pararoa) vent and is characterised by an orthopyroxene+ hornblende-dominant mineralogy, temperature of 764±18°C, and fO2 of NNO+0.40±0.13. Compositionally intermediate Magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids. A single ~13 km3 Magma erupted during the intervening 8.1 ka Mamaku episode was relatively homogeneous in composition (SiO2=76.1–76.8 wt%, Sr=104–112 ppm), temperature (736±18°C), and oxygen fugacity (NNO+0.19±0.12). Some of the vents tapped a single Magma while others tapped several. Deposit stratigraphy suggests that the eruptions alternated between Magmas, which were often simultaneously erupted from separate vents. Both effusive and explosive activity alternated, but was predominantly effusive (>75% erupted as lava domes and flows). The plumbing systems which fed the vents are inferred to be complex, with Magma experiencing different conditions in the conduits. As the eruption of several Magmas was essentially concurrent, the episodes were likely triggered by a common event such as Magmatic intrusion or seismic disturbance.
Victoria C Smith - One of the best experts on this subject based on the ideXlab platform.
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silicic recharge of multiple rhyolite Magmas by basaltic intrusion during the 22 6 ka okareka eruption episode new zealand
Lithos, 2008Co-Authors: Phil Shane, Ian A Nairn, Victoria C Smith, Miles Darragh, Kate Beggs, J. W. ColeAbstract:Abstract Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite Magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption (∼ 0.01 km3 Magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite Magma (T1) with a volume of ∼ 0.3 km3. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite Magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite Magma (T2) (∼ 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite Magma (T3) (∼ 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 Magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite Magmas. The eruption sequence and the restricted direct basaltic intrusion into only one Magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary Magma storage depths of
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multiple rhyolite Magmas and basalt injection in the 17 7 ka rerewhakaaitu eruption episode from tarawera volcanic complex new zealand
Journal of Volcanology and Geothermal Research, 2007Co-Authors: Phil Shane, Victoria C Smith, Miles Darragh, Kate Beggs, J. W. Cole, S B Martin, Ian A NairnAbstract:Abstract The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km3 DRE rhyolite Magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous rhyolite lava extrusions. Two different rhyolite Magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 Magma was a crystal-poor orthopyroxene-hornblende rhyolite that is highly evolved (whole rock SiO2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende rhyolite that is less evolved (SiO2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two Magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third rhyolite Magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO2 = 74 wt.%) and hottest (∼ 820 °C) of the three Magmas. Saturation pressures calculated using dissolved H2O and CO2 contents of melt inclusions in quartz crystals indicate that T2 Magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 Magma grew during ascent through ∼ 8 km depths. Some T1 and T2 rhyolite clasts contain vesicular brown blebs with widely variable (andesite to rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic Magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera rhyolite eruption episodes. However, the rhyolite Magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite Magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 rhyolite Magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 Magma, and lubricated its transport to the surface.
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geochemistry and Magmatic properties of eruption episodes from haroharo linear vent zone okataina volcanic centre new zealand during the last 10 kyr
Bulletin of Volcanology, 2006Co-Authors: Ian A Nairn, Victoria C Smith, Phil Shane, Catherine M WilliamsAbstract:Post-10 ka rhyolitic eruptions from the Haroharo linear vent zone, Okataina Volcanic Centre, have occurred from several simultaneously active vents spread over 12 km. Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous Magma batches. Three Magmas totalling ~8 km3 were erupted during the 9.5 ka Rotoma episode. The most evolved Rotoma Magma (SiO2=76.5–77.9 wt%, Sr=96–112 ppm) erupted from a southeastern vent, and is characterised by a cummingtonite-dominant mineralogy, a temperature of 739±14°C, and fO2 of NNO+0.52±0.11. The least evolved (SiO2=75.0–76.4 wt%, Sr=128–138 ppm, orthopyroxene+ hornblende-dominant) Rotoma Magma erupted from several vents, and was hotter (764±18°C) and more reduced (NNO+0.40±0.13). The ~11 km3 Whakatane episode occurred at 5.6 ka and also erupted three Magmas, each from a separate vent. The most evolved (SiO2=73.3–76.2 wt%, Sr=88–100 ppm) Whakatane Magma erupted from the southwestern (Makatiti) vent and is cummingtonite-dominant, cool (745±11°C), and reduced (NNO+0.34±0.08). The least evolved (SiO2=72.8–74.1 wt%, Sr=132–134 ppm) Magma was erupted from the northeastern (Pararoa) vent and is characterised by an orthopyroxene+ hornblende-dominant mineralogy, temperature of 764±18°C, and fO2 of NNO+0.40±0.13. Compositionally intermediate Magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids. A single ~13 km3 Magma erupted during the intervening 8.1 ka Mamaku episode was relatively homogeneous in composition (SiO2=76.1–76.8 wt%, Sr=104–112 ppm), temperature (736±18°C), and oxygen fugacity (NNO+0.19±0.12). Some of the vents tapped a single Magma while others tapped several. Deposit stratigraphy suggests that the eruptions alternated between Magmas, which were often simultaneously erupted from separate vents. Both effusive and explosive activity alternated, but was predominantly effusive (>75% erupted as lava domes and flows). The plumbing systems which fed the vents are inferred to be complex, with Magma experiencing different conditions in the conduits. As the eruption of several Magmas was essentially concurrent, the episodes were likely triggered by a common event such as Magmatic intrusion or seismic disturbance.
Nicholas T. Arndt - One of the best experts on this subject based on the ideXlab platform.
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differentiation crustal contamination and emplacement of Magmas in the formation of the nantianwan mafic intrusion of the 260 ma emeishan large igneous province sw china
Contributions to Mineralogy and Petrology, 2012Co-Authors: Christina Yan Wang, Yali Sun, Nicholas T. ArndtAbstract:The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved Magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic Magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of crustal contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved Magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as Magma differentiation, crustal contamination and sulfide saturation at different stages in Magma chambers may have intervened during formation of the intrusion. Parental Magmas were derived from picritic Magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging Magma chamber, where the parental Magmas crystallized olivine, assimilated minor crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved Magmas in the upper part of the chamber. The evolved Magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow Magma chamber. The Magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow Magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive Magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.
