35 research outputs found
The late Pleistocene Sacarosa tephra-fall deposit, Misti Volcano, Arequipa, Peru: its magma, eruption, and implications for past and future activity
Entre 38.5 ka cal BP y 32.4 ka cal BP el volcĂĄn Misti generĂł una erupciĂłn dacĂtica con Indice de Explosividad
VolcĂĄnica 5 que emplazo el depĂłsito de caĂda de tefra âSacarosaâ. La presencia de fenocristales de biotita, el tamaño fino de sus granos, escasos lĂticos y la abundancia de cristales libres caracterizan el depĂłsito en los lugares muestreados. El magma tuvo una temperatura de ~ 800 °C, el cual ascendiĂł rĂĄpidamente de ~ 10 km de profundidad y resultĂł en una erupciĂłn Pliniana que tuvo una tasa de descarga de masa de 7.7 Ă 106â
4.1 Ă 107 kg/s, y deposito alrededor de 3 km3 de tefra dentro de decenas de horas. El depĂłsito tiene dos capas con espesores casi similares, separados por un contacto difuso y con una capa superior que se caracteriza por contener granos un poco mĂĄs gruesos y ser un poco menos sorteado que la capa inferior. La capa superior gruesa indica condiciones culminantes o un menor grado de fragmentaciĂłn durante la Ășltima mitad de la erupciĂłn.
Fuertes vientos distribuyeron el depĂłsito al suroeste del Misti cubriendo al menos 800 km2, incluyendo la actual ciudad de Arequipa donde el depĂłsito de tefra tiene hasta 100 cm de espesor. El depĂłsito âSacarosaâ es el primero entre los depĂłsitos de la etapa Cayma (un grupo distintivo de unidades fĂ©lsicas que contienen biotita) que es descrito detalladamente y con su erupciĂłn caracterizada. Varios depĂłsitos de la etapa Cayma fueron generados por erupciones explosivas voluminosas similares a la erupciĂłn âSacarosaâ, representando un intervalo de ~ 8.9â15.5 ky de poderosas erupciones. Una erupciĂłn tan explosiva hoy amenazarĂa a los mĂĄs de 1,100,000 habitantes de Arequipa, muchos de ellos viven dentro del ĂĄrea de distribuciĂłn del depĂłsito âSacarosaâ
Recommended from our members
Origin of minor and trace element compositional diversity in anorthitic feldspar phenocrysts and melt inclusions from the Juan de Fuca Ridge
Melt inclusions trapped in phenocryst phases are important primarily due to their potential of preserving a significant proportion of the diversity of magma composition prior to modification of the parent magma array during transport through the crust. The goal of this investigation was to evaluate the impact of formational and post-entrapment processes on the composition of melt inclusions hosted in high anorthite plagioclase in MORB. Our observations from three plagioclase ultra-phyric lavas from the Endeavor Segment of the Juan de Fuca Ridge document a narrow range of major elements and a dramatically greater range of minor and trace elements within most host plagioclase crystals. Observed host/inclusion partition coefficients for Ti are consistent with experimental determinations. In addition, observed values of D[subscript]Ti are independent of inclusion size and inclusion TiOâ content of the melt inclusion. These observations preclude significant effects from the re-homogenization process, entrapment of incompatible element boundary layers or dissolution/precipitation. The observed wide range of TiOâ contents in the host feldspar, and between bands of melt inclusions within individual crystals rule out modification of TiOâ contents by diffusion, either pre-eruption or due to re-homogenization. However, we do observe comparatively small ranges for values of KâO and Sr compared to PâOâ
and TiOâ in both inclusions and crystals that can be attributed to diffusive processes that occurred prior to eruption.KEYWORDS: crustal processes, magma transport, plagioclas
Recommended from our members
The petrogenesis of plagioclase-phyric basalts at mid-ocean ridges
Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with
ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin
assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of
plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined,
this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and
densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of
PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a
magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have
never been sampled on axis at fast-spreading centers or from locations with noted contemporaneous axial
magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid-ocean ridge basalt
(MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction
between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by
loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater
than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for
these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB
eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit
system that bypassed an axial chamber.Keywords: Mid-ocean ridges, Plagioclase, Magma transport, PU
Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development
The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBMâs oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs
Semi-groupe de Lie associé à un cÎne symétrique
Volcanic arcs are the surface expression of magmatic systems that result from the subduction of mostly oceanic lithosphere at convergent plate boundaries. Arcs with a submarine component include intraoceanic arcs and island arcs that span almost 22,000 km on Earth\u27s surface, the vast majority of which are located in the Pacific region. Hydrothermal systems hosted by submarine arc volcanoes commonly contain a large component of magmatic fluid. This magmatic-hydrothermal signature, coupled with the shallow water depths of arc volcanoes and their high volatile contents, strongly influences the chemistry of the fluids and resulting mineralization and likely has important consequences for the biota associated with these systems. The high metal contents and very acidic fluids in these hydrothermal systems are thought to be important analogs to numerous porphyry copper and epithermal gold deposits mined today on land
A record of spontaneous subduction initiation in the IzuâBoninâMariana arc
The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and ageâas constrained by the biostratigraphy of the overlying sedimentsâto the 52â48-million-year-old basalts in the adjacent IzuâBoninâMariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of IzuâBoninâMariana subduction in a mode consistent with the spontaneous initiation of subduction
Recommended from our members
Mineral-scale Sr isotope variation in plutonic rocks - a tool for unravelling the evolution of magma systems
Isotope ratios of elements such as Sr, Nd, Pb and Hf can be used as tracers of magmatic sources and processes. Analytical capabilities have evolved so that isotope ratios can now be analysed in situ, and isotopic tracers can therefore be used within single minerals to track the changing magmatic environment in which a given mineral grew. This contribution shows that Sr isotope ratios in feldspars that make up plutonic rocks will typically preserve initial isotopic variations, provided precise and accurate age corrections can be applied. Variations in initial isotope ratio can give a core-to-rim record of magmatic evolution and can be used to diagnose open system events such as contamination and magma recharge and mixing.
New single grain Sr isotope data are presented from the Dais Intrusion, Antarctica, which reflect an open system origin for the crystals. The crystal cargo appears to be aggregated and assembled during transport and emplacement. This model, as opposed to a magma body crystallising post emplacement, may be more applicable to plutonic rocks in general, and is testable using the in situ isotopic determination methods described here
Isotopic Characteristics of Neogene-Quaternary Tephra From IODP Site U1438:A Record of Explosive Volcanic Activity in the Kyushu-Ryukyu Arc
International Ocean Discovery program (IODP) Site U1438 is located within the AmamiâSankaku Basin, ~50 km SW of the KyushuâPalau Ridge and ~500 km SE of the presentâday volcanic front of the KyushuâRyukyu arc. Thirtyâeight rhyolitic tephra layers (1â15 cm thick) deposited over the last 4 Myr were recovered at this site. Representative ash layers dominated by unaltered, colorless glass shards reaching 100â300 ÎŒm in size, in association with rare pyroxene, amphibole, biotite, and zircon, were sampled for isotopic analysis. The isotopic compositions of the tephra have a narrow range in 87Sr/86Sr (0.704 to 0.706), 206Pb/204Pb (18.32 to 18.46), 207Pb/204Pb (15.57 to 15.62), and 208Pb/204Pb (38.46 to 38.75) and are more variable in ΔNd (â3.5 to +4.8) and ΔHf (+2.0 to +13.3). This record indicates the tephra were not sourced from the IzuâBoninâMariana arc or from Central Japan but derived from felsic volcanism from the KyushuâRyukyu arc. The 4.8âkmâdeep AmamiâSankaku Basin was located up to 600â900 km from the KyushuâRyukyu arc over the last 4 Ma, which is significantly more distal than other drill sites from which extensive tephra layers sourced from SW Japan have been recovered. Site U1438 tephra are thus likely related to widely distributed (M > 6) rhyolitic eruptions. We propose the unique tephra record and highâprecision isotope analysis of recovered tephra from Site U1438 can be used as an important marker to identify submerged calderas within the Ryukyu arc and/or constrain the history of activity (>1 Ma) of major calderasâforming eruptions within the KyushuâRyukyu arc.A. M. acknowledges support from the Swiss National Science Foundation
(grant 200020/135511 to Othmar
MĂŒntener and grant P2LAP2_171819 to
A. M.) and Swiss IODP. This work was
also supported by grants to G. M. Y. and
M. B. from NSF (OCEâ1537135) and
IODP
Closed- to open-system differentiation at Arenal volcano (1968-2003)
Arenal volcano, located in northern Costa Rica, has been continuously erupting since 1968. Magmas during the first half of the eruption by volume (Stage 1: 1968â1971) were related by largely closed-system crystal fractionation that had produced a compositionally zoned magma chamber prior to 1968. It erupted downward from the most differentiated magma in 1968 to the most mafic by early 1971. In contrast, the second half of the eruption has been dominated by recharge and compositions have become more evolved with time (Stage 2: 1971âcurrent). We base these conclusions on new major and trace element plus SrâNdâHfâPb isotope analyses of 56 whole rocks from throughout the eruption. Differentiates are enriched in incompatible elements in both stages, but compatible element concentrations drop much more during Stage 1 than 2. Changes during Stage 1 were successfully modeled using least squares and MELTS models despite the mineral complexity of the rocks. About 19% fractional crystallization of phenocryst phases (plagioclase \u3e orthopyroxene \u3e clinopyroxene \u3e magnetite) is required, consistent with crystallization at 4 kb and from 1145 to 1088 °C of a melt initially containing 2.5 wt.% H2O at quite oxidizing conditions (QFM + 2). An implication is that most phenocrysts formed during decompression and degassing. Changes during the second stage were successfully modeled using EC-EâČRAÏFC with the ratio of recharge to crystallization decreasing from 17 to 5 over ⌠30 years. Crystallization rates (dFS / dt) increase from 0.05 to 0.4%/a from closed- to open-system behavior and are even faster than inferred from U-series disequilibria. The recharging magma results from a smaller degree of flux melting of a mostly similar source than for the resident magma prior to the eruption, with the two events separated by ⌠450 years. The most recent compositions have no precedent at Arenal