Pumice Clasts Research Articles

Rock types characteristics of Prambanan and Sambisari Temples, Yogyakarta Province, Indonesia

Many Hindu and Buddhist temples built in the 8th to 10th centuries are found in the Yogyakarta Province, Indonesia, including Prambanan and Sambisari temples. Based on petrographic and geochemical observations, both temples are built using basaltic to andesitic lavas and pumice breccias. The rock types are relatively similar to those the basaltic to andesitic lavas around the Quaternary age Merapi volcano, and the pumice breccia of the Tertiary age Semilir Formation in the Piyungan area situated at the south of the Prambanan temple. Selected rocks samples were analized using petrographic and X-ray fluorescence methods to identify minerals, textures and geochemical characteristics. Petrographic observations show that the rocks used to build Prambanan temple are hornblende-pyroxene andesite and pyroxene basalts, while the rocks of Sambisari temple consist of hornblende-pyroxene andesites and pumice breccias. Hornblende-pyroxene andesite is generally gray, showing porphyritic texture, 1-3 mm crystal size; phenocrysts are composed by plagioclase, hornblende, pyroxene, and opaque minerals set in groundmass of plagioclase microcrysts and volcanic glass. Pumice breccia, showing a light gray colour, moderate sorting, fragments, consist of andesite and pumice clasts in size range 2-6 cm, embedded in sandy to clay matrix. Most of the rocks are generally weathered or oxidized, which is reflected by the presence of secondary iron oxide. This is probably most of the basaltic to andesitic lavas used had a high vesicular texture, and that Yogyakarta has a tropical climate.

Pumice clasts in cross stratified basalt-dominated sandstones and conglomerates. Characteristics and depositional significance: Huarenchenque Fm (Neuquén, Argentina)

The Huarenchenque Fm is a Pleistocene clastic sedimentary unit that crops out in the SW of the Neuquen Basin, Argentina. This unit is made up of cross stratified basalt-dominated sandstones and conglomerates with pumice clasts in a fluvial context. The roundness of these pumice clasts, the numerous fragments of shards and their association with polymodal basalt-rich clasts in cross-bedded conglomerates strongly suggest that these clasts were transported together during hydraulic flow. Pumice clasts are usually buoyant in cold water whenever their density is lower than 1 g/cm3. However, since their density exceeds that of water during sediment transport, they form part of the sedimentary record. A flume was employed to yield fresh insight into the behavior of these clasts mixed with sands during transport and accumulation. The hydraulic equivalence of both materials was calculated by evaluating the density of the pumice clasts and that of the quartz sands. The pumice clasts sank as a result of the interaction of pumice-rich pyroclastic flows with a moving mass of shallow water. However, they retained their buoyancy after recovery from the sedimentary record. The sinking process is therefore rapid and reversible.

Event stratigraphy and emplacement mechanisms of the last major caldera eruption on Sete Cidades Volcano (São Miguel, Azores): the 16 ka Santa Bárbara Formation

Sete Cidades is an active yet dormant central volcano located in the westernmost part of Sao Miguel Island (Azores archipelago), characterized by a 5-km-wide multi-stage caldera formed in the last 36 ka. In this work, we present new stratigraphic, grain size, petrographic, and geochemical data of the 16 ka Santa Barbara Formation, related to the last stage of caldera formation. We define the lithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position, lithofacies, and distribution of the deposits. The preserved deposits outcrop in a narrow area (over < 5 km between the caldera rim and the coastal cliffs) and include ash beds with dispersed lapilli (Lower Member), coarse-grained pumice and lithic-rich lapilli tuffs (Middle Member), and pumice fall deposits (Upper Member). The minimum bulk volume preserved on land is about 0.03 km3; however, the majority of the pyroclastic products were deposited in the ocean. The presence of magmatic mafic enclaves (48 wt% SiO2) within the juvenile pumice clasts (63 wt% SiO2) is testimony to the injection of trachybasaltic magma into the trachytic magma reservoir shortly prior to eruption. The mingling of these two magmas acted as the eruption trigger. The eruption history can be divided into three main phases each characterized by different eruption styles and deposit emplacement mechanisms, accompanied by enlargement of the caldera. The eruption started with a sequence of short-lived explosions generating unstable columns that produced ash fallout and generated dilute pyroclastic density currents, mostly deposited in the SW sector of the volcano. The climax of the eruption was marked by successive low fountaining events that generated high particle concentration pyroclastic density currents emplaced on the SW and NE flanks. The final phase of the eruption was characterized by establishment of a sub-Plinian eruption column that produced pumice fall deposits on the NE flank of the volcano. Our study allows delineation of eruptive scenarios for future major explosive eruptions at Sete Cidades, a volcano which is located on the most populated island of the Azores (Sao Miguel Island), and can provide a framework for the assessment of volcanic hazards on oceanic islands elsewhere.

Age and area predict patterns of species richness in pumice rafts contingent on oceanic climatic zone encountered.

The theory of island biogeography predicts that area and age explain species richness patterns (or alpha diversity) in insular habitats. Using a unique natural phenomenon, pumice rafting, we measured the influence of area, age, and oceanic climate on patterns of species richness. Pumice rafts are formed simultaneously when submarine volcanoes erupt, the pumice clasts breakup irregularly, forming irregularly shaped pumice stones which while floating through the ocean are colonized by marine biota. We analyze two eruption events and more than 5,000 pumice clasts collected from 29 sites and three climatic zones. Overall, the older and larger pumice clasts held more species. Pumice clasts arriving in tropical and subtropical climates showed this same trend, where in temperate locations species richness (alpha diversity) increased with area but decreased with age. Beta diversity analysis of the communities forming on pumice clasts that arrived in different climatic zones showed that tropical and subtropical clasts transported similar communities, while species composition on temperate clasts differed significantly from both tropical and subtropical arrivals. Using these thousands of insular habitats, we find strong evidence that area and age but also climatic conditions predict the fundamental dynamics of species richness colonizing pumice clasts.

Geological evolution of Paniri volcano, Central Andes, northern Chile

Paniri volcano, in northern Chile, belongs to a volcanic chain trending across the main orientation of the Central Andean volcanic province. Field work mapping, stratigraphic sequences, and one new 40Ar/39Ar and eleven previous published 40Ar/39Ar, and K/Ar ages, indicate that the evolution of Paniri involved eruption of seven volcanic units (Malku, Los Gordos, Las Lenguas, Las Negras, Viscacha, Laguna, and Llareta) during four main stages occurring over more than 1 Myr: Plateau Shield (>800 ka); Main Edifice (800–400 ka); Old Cone (400–250 ka); and New Cone (250–100 ka). Considering glacial and fluvial action, an estimated 85.3 km3 of volcanic material were erupted during the eruptive history of Paniri volcano, giving a bulk eruption rate of 0.061 km3/ka, with major activity in the last 150 kyr (eruption rate of 0.101 km3/ka).Lava flows from Paniri show abundant plagioclase together with subordinate ortho-, and clino-pyroxene, and amphibole as main phenocrysts. Moreover, although true basalts are scarce in the Central Andes, olivine-bearing lavas were erupted at Paniri at ∼400 ka. Also, scarce phenocrysts of biotite, quartz, rutile, and opaque minerals (Fe-Ti oxides) were identified. The groundmass of these flows is composed mainly of glass along with pyroxene and plagioclase microlites. Consolidated and unconsolidated pyroclastic deposits of dacitic composition are also present. The consolidated deposits correspond to vitreous tuffs, whilst unconsolidated deposits are composed of pumice clasts up to 5 cm in diameter. Both pyroclastic deposits are composed of glassy groundmass (up to 80% vol.), and subordinated plagioclase, hornblende, and biotite phenocrysts up to 1 cm in length. Results of twenty-four new, coupled with previous published compositional analyses show that volcanic products of Paniri vary from 57% (basaltic-andesite) to 71% (rhyolite) vol. SiO2, with significant linear correlations between major element-oxide and trace-element concentrations. 87Sr/86Sr isotope ratios range from 0.7070 to 0.7075, indicating that Paniri, similar to other volcanoes of the San Pedro – Linzor volcanic chain, have undergone significant crustal contamination of its parental magmas. However, the almost constant Sr-isotope compositions of the different volcanic units defined for Paniri volcano, suggested later fractional crystallization of magmas at upper crustal levels.

Causes of fragmented crystals in ignimbrites: a case study of the Cardones ignimbrite, Northern Chile

Broken crystals have been documented in many large-volume caldera-forming ignimbrites and can help to understand the role of crystal fragmentation in both eruption and compaction processes, the latter generally overlooked in the literature. This study investigates the origin of fragmented crystals in the > 1260 km3, crystal-rich Cardones ignimbrites located in the Central Andes. Observations of fragmented crystals in non-welded pumice clasts indicate that primary fragmentation includes extensive crystal breakage and an associated ca. 5 vol% expansion of individual crystals while preserving their original shapes. These observations are consistent with the hypothesis that crystals fragment in a brittle response to rapid decompression associated with the eruption. Additionally, we observe that the extent of crystal fragmentation increases with increasing stratigraphic depth in the ignimbrite, recording secondary crystal fragmentation during welding and compaction. Secondary crystal fragmentation aids welding and compaction in two ways. First, enhanced crystal fragmentation at crystal-crystal contacts accommodates compaction along the principal axis of stress. Second, rotation and displacement of individual crystal fragments enhances lateral flow in the direction(s) of least principal stress. This process increases crystal aspect ratios and forms textures that resemble mantled porphyroclasts in shear zones, indicating lateral flow adds to processes of compaction and welding alongside bubble collapse. In the Cardones ignimbrite, secondary fragmentation commences at depths of 175–250 m (lithostatic pressures 4–6 MPa), and is modulated by both the overlying crystal load and the time spent above the glass transition temperature. Under these conditions, the existence of force-chains can produce stresses at crystal-crystal contacts of a few times the lithostatic pressure. We suggest that documenting crystal textures, in addition to conventional welding parameters, can provide useful information about welding processes in thick crystal-rich ignimbrites.

Geochemical characterization of mid-distal Nisyros tephra on Datça peninsula (southwestern Anatolia)

We present new distal records of tephra deposits that overly the Kos ignimbrite in seven locations of Datça peninsula. Tephra in one of these locations were previously associated with Nisyros Kyra sub-unit based only on the field characteristics. We use different proxies such as field observations, petrography, mineral, glass, and whole-rock chemistry in order to characterize and correlate the previously and recently identified pumice fall deposits on Datça. The total thickness of the fall deposit reaches to 3.5m. The size of the pumice clasts is generally within the range of lapilli, and they have vitrophyric texture consisting mainly of plagioclase (andesine to labradorite) with scarce clinopyroxene (diopside to augite), olivine (Fo48–50), amphibole (magnesio-hastingsite), and biotite crystals. Amphibole is a ubiquitous phenocryst in all Datça tephra units and used as a criterion for the correlation. Glass major element analyses by EMPA reveal two different groups with andesitic and dacitic compositions. Difference in silica content (up to ca. 4wt%) detected in the same specimen also designates the heterogeneity in pumice glass. This heterogeneity in glass composition is also supported by the frequent occurrence of banded pumice clasts in Datça tephra. Whole-rock composition of the pumice is mainly andesitic with calc-alkaline affinity. Multi-element patterns on primitive-mantle normalized diagram display typical arc-magmatism signature (i.e. depletion in Nb, Ta, Ti, and P).In order to check and eliminate the potential alternatives, we compared the distal deposits on Datça not only with Kyra, but also with other Nisyros tephra units. Yet, Kyra is the only unit that has comparable depositional characteristics, calcic amphibole crystals, andesitic-dacitic glass and whole-rock chemistry, and distal tephra deposits on neighboring islands (Tilos and Chalki). Therefore, we associate Datça tephra deposits with some proximal Kyra subunits of intermediate composition. However, if further geochemical (especially glass and mineral chemistry) data are provided for the Lakkí and Melisserí tephra units, a more thorough tephra correlation will be possible. Finally, our results may have implications on Nisyros tephra dispersal and geochemical characterization.

Pómez Bosque de Tlalpan, producto de una erupción de gran magnitud en el margen suroeste de la cuenca de México

In this contribution we describe a white pumice fall deposit, informally named pómez Bosque de Tlalpan (PBT), found in several outcrops in the southern Mexico basin. The most representative sequence occurs at the Bosque de Tlalpan park, Delegación Tlalpan, in Mexico City. In this site the deposit is 3 m thick, massive, and contains 80–90 vol. % of pumice clasts, predominantly of coarse lapilli-sized and some block-sized fragments. The PBT has a dacitic composition (64.9–66 wt. % SiO2, on anhydrous basis) of calc-alkaline affinity, and a mineral assemblage represented by plagioclase &gt; amphibole &gt; orthopyroxene &gt; biotite, ±Fe-Ti oxides, with quartz in lesser proportions, and zircon as accesory mineral, in a vesicular and glassy matrix. According to data collected from seven outcrops, thickness and clast size of the deposit decrease towards the NE and therefore the vent source should be located to the SW of Bosque de Tlalpan. Possible sources are the volcanoes Ajusco and San Miguel, which are part of the Sierra de Las Cruces volcanic range, and are located in this direction at ~11 km from Bosque de Tlalpan. The age of the PBT deposit was determined by radiocarbon dating of underlying paleosoils that yielded 25,730 ± 130 to 37,450 ± 330 yrs B.P. (Late Pleistocene). Additionally, thermoluminscence dating of one sample yielded an age of 30,300 ± 5,000 years, similar to the radiocarbon ages. Hence, the PBT represents the youngest reported deposit from the Sierra de Las Cruces volcanic range. Although only seven outcrops were described in this work, we interpret the PBT deposit as produced by a plinian-type eruption, based on its massive structure, its ca. 3 m thickness, and the presence of pumice clasts in coarse lapilli to block sizes, as well as 3.5 cm sized lithics, at 11 km from the possible vent source, characteristic of this kind of deposits. We discard the monogenetic volcanoes from the Chichinautzin volcanic field as the vent source of the PBT deposit, because this kind of volcanoes is relatively mafic in composition and do not produce plinian-type deposits.

Mineralogical and petrological features of the Cemilköy ignimbrite, Cappadocia, Turkey

The Cemilköy ignimbrite is one of the voluminous ignimbrite deposits in Cappadocia. The Cemilköy ignimbrite contains pumice and lithic clasts of volcanic and ophiolitic origin in an ash matrix. The unwelded Cemilköy ignimbrite is distinguished from other deposits in Cappadocia by flattened pumices, elongate vesicles and a slaty fabric. The mineral assemblage of the Cemilköy ignimbrite is plagioclase, biotite, quartz and oxides (magnetite and Ti-magnetite) and the matrix is glassy. Eutaxitic texture is dominant and all pumice clasts have a vitrophyric-porphyritic texture. The same textural properties were observed throughout the spatial distribution of Cemilköy ignimbrite. Microprobe studies reveal that plagioclase compositions range from albite through oligoclase-andesine. Estimated plagioclase-liquid temperatures (T) and pressures (P) are varying between 806 and 847 °C and 4.2–7.1 (kbar), and the H2O content of the melt is estimated to have been 5 wt.% from the pumice clasts. Based on geochemical data, the Cemilköy ignimbrite is rhyolitic and calc-alkaline in character, and all pumice clasts are enriched in LIL and LRE elements relative to HFS elements. Negative Nb, Ta and Ti anomalies, ratios of Ba/Nb >28 (56–77), Ba/Ta ˃450 (590–700) and Th/Yb vs. Ta/Yb are consistent with a subduction-related origin. According to the geochemical and mineralogical-petrographical data, the Cemilköy ignimbrite originated from partial melting of a mantle source which was enriched during previous subduction processes with variable degrees of assimilation fractional crystallization (AFC) through time and Cemilköy ignimbrite erupted from a crustal magma chamber at shallow to intermediate depth.

Experimental and petrological constraints on long-term magma dynamics and post-climactic eruptions at the Cerro Galán caldera system, NW Argentina

Cerro Galán in NW Argentina records >3.5Myr of magmatic evolution of a major resurgent caldera complex. Beginning at 5.72Ma, nine rhyodacitic ignimbrites (68–71wt% SiO2) with a combined minimum volume of >1200km3 (Dense Rock Equivalent; DRE) have been erupted. The youngest of those ignimbrites is the eponymous, geochemically homogenous, caldera-forming 2.08±0.02Ma Cerro Galán Ignimbrite (CGI; >630km3 DRE). Following this climactic supereruption, structural and magmatic resurgence led to the formation of a resurgent dome and post-climactic lava domes and their associated pyroclastic deposits. A clear transition from amphibole to sanidine-bearing magmas occurred during the evolution of Cerro Galán and is inferred to represent a shallowing of the magma system. We test this hypothesis here using experimental phase equilibria.We conducted a series of phase equilibria experiments on the post-climactic dome lithologies under H2O-saturated conditions using cold seal Waspaloy pressure vessels with an intrinsic log fO2 of NNO+1±0.5 across a temperature-pressure range of 750–900°C and 50–200MPa (PH2O=Ptotal), respectively. Petrologic and geochemical analysis of the post-climactic lithologies shows that the natural phase assemblage (plagioclase+quartz+biotite+sanidine+Fe-Ti oxides±apatite±zircon) is stable at <50MPa (PH2O) and 805–815°C. Applying experimental results to the CGI pumice, which has the same phenocryst phase assemblage and modal abundance, whole rock and phenocryst chemistry, and overlapping temperature and fO2 as the post-climactic deposits, suggests that these pre-eruptive conditions (PH2O<50MPa) are relevant for the magmas that sourced the climactic CGI supereruption as well.Amphibole in the early Cerro Galán ignimbrites (Toconquis Group; 5.72–4.51Ma, and the Cueva Negra Ignimbrite, 3.77±0.08Ma) records crystallization across a range of pressures (500 to 200±60MPa). In the interval between the eruption of the Cueva Negra ignimbrite and the CGI (2.08±0.02Ma) the complex magma system shallowed and stalled at low pressures (<100MPa), resulting in a more simple magma reservoir configuration represented by a large-volume, geochemically homogenous magma body. The shallowing of the Cerro Galán magma system during this time explains the marked transition from amphibole to sanidine-bearing magmas and seems to characterize many large silicic caldera-forming magma systems that erupt over million year timescales to generate long-lived volcanic complexes.The post-climactic history of Cerro Galán is informed through a detailed investigation of the textural differences among the post-climactic dome lithologies, and a comparison of those textures with previously published decompression experiments. These suggest that the highly vesiculated, pumiceous clasts with rare microlites represent magma stored within the core of the lava dome that decompressed relatively rapidly (0.003–0.0003MPas−1) and evolved via closed system degassing. Resulting over-pressure of the dome may have triggered superficial explosion. In contrast, dense clasts with abundant crystalline silica precipitates represent more typical dome-forming magmas that decompressed more slowly (<0.00005MPas−1), evolved via open system degassing, and form the outer carapace of a lava dome. Integrating decompression histories with results from new phase equilibria experiments suggests that during post-climactic volcanic activity at Cerro Galán, remnant CGI dome-forming magmas ascended from the shallow magma reservoir (<4km) to motivate resurgent uplift and erupt as lava domes either explosively as vesiculated clasts or effusively as dense clasts that make up the outer structure of lava domes.

Syn-eruptive breakdown of pyrrhotite: a record of magma fragmentation, air entrainment, and oxidation

Air entrainment in fragmented magmas controls the dynamics of volcanic eruptions. Pyroclast oxidation kinetics may be applied to quantify the degree of magma–air interaction. Pyrrhotite (Po) in volcanic rocks is often oxidized to form magnetite (Mt) and hematite (Hm), and its reaction mechanisms are well constrained. To test utilizing Po oxidation as a marker for magma–air interactions, we compared the occurrence of Po oxidation products from three different eruption styles during the Sakurajima 1914–1915 eruption. Pumices from the Plinian eruption include columnar-type Fe oxides (Mt with subordinate width of Hm) often accompanied by relict Po. This columnar type is also found in clastogenic lava, where it is almost completely oxidized to Hm. The effusive lava contains framboidal aggregates of subhedral to anhedral Mt crystals without Hm. The formation mechanisms of columnar and framboidal Fe oxides were estimated. The columnar type Fe oxides were formed syn-eruptively through gaseous reactions, as opposed to the melt in a magma chamber, as demonstrated by the Ti-free nature of the columnar Mt and its synchronous oxidation to Hm. By contrast, the framboidal type was formed in a melt with decreasing fS2. The calculation of Hm growth in a conductively cooling pumice clast constrains the surface temperature of pumice in the eruption column. The paragenesis and oxidation degree of Po and Fe oxides are consistent with the eruption processes in terms of magma fragmentation, air entrainment, and welding, and can, therefore, be a responsive marker for the magma–air interaction.

40Ar/39Ar geochronology and revised stratigraphy of the late Eocene Taxco volcanic field, southern Mexico

The late Eocene Taxco volcanic succession constitutes an important record of explosive silicic volcanism in the north-central Sierra Madre del Sur of southern Mexico. Detailed stratigraphic studies with age dating of individual units in this area were lacking in spite of it being an important mining district. Re-examination of the stratigraphy together with geochronologic studies were used to define the eruptive style and constrain the age of the main episode of silicic activity. Major element data show a rhyolitic composition for all the units analyzed. The volcanic succession records silicic explosive eruptions during which ash-flows, surge and ash-fall deposits, lava flows and domes were emplaced. Earliest activity was contemporary with the end of sediment accumulation of the continental Balsas Group. The first main episode of volcanic activity produced massive, moderately welded, crystal-poor lithic ignimbrites and ash-fall deposits, which evolved into crystal-rich, densely welded ignimbrites with flattened pumice clasts (San Gregorio ignimbrite) thought to represent erupted mush related to caldera collapse during emptying of the magma chamber. A second episode of non-welded, vapor-phase, indurated ignimbrites and surge deposits (Sombrerito ignimbrite), which are rich in pumice but crystal-poor, suggest volatile-rich magmatism and replenishment of the magma chamber by evolved magmas. Rhyolite emplaced in volcanic units of both episodes point to volatile-poor magmas during which rhyolite flows and domes formed.40Ar/39Ar dating of the units, from the crystal-rich San Gregorio ignimbrite near the base of the succession to rhyolites at the top, has constrained the age of the Taxco volcanic field to a relatively short period between ∼36 and 34.5 Ma. Most of the sanidine ages obtained are indistinguishable within error: the densely welded San Gregorio ignimbrite (34.75 ± 0.26, 35.22 ± 0.26 Ma) and the overlying Peral ignimbrite (35.29 ± 0.41, 35.68 ± 0.20). Plagioclase of the Tenería Formation rhyolites gave the same age within error (34.45 ± 1.10, 34.94 ± 0.34 Ma), documenting the end of the main volcanic episodes. A younger plagioclase age of 30.89 ± 1.45 Ma of a dome near Tetipac gives evidence of later silicic magmatism in the area.The source of the ignimbrites of the Taxco region is inferred to be located in the Tenería area, where abundant rhyolite dome structures seem to obliterate the main caldera structure. Differences in age, mineralogy and geochemistry between the Taxco succession and nearby volcanic zones where caldera structures have been identified preclude sources in these areas.

The emplacement dynamics of pumice lobes ascertained from morphology and granulometry: Examples from the 1993 deposits at Lascar Volcano, Chile

The work presented here focuses on lobe shapes and clast populations within lobate termini of the 1993 pumice flow deposits at Lascar Volcano, Chile. A new method to analyze a coarse-tail grain size population with field photographs is presented. Using this method, >33,000 (>0.5cm) clasts from the pumice lobes of the 1993 pumice flow deposits were measured at 36 sites, and the resultant grain size distributions were then related to lobe morphology. Lobe margins (i.e., levees, clefts, and snouts) were found to contain significantly larger pumice clasts and be more poorly sorted than lobe central channels (i.e., locations away from the margins). Previous laboratory experiments suggest lobe margins form by the floatation and deflection of larger clasts to the margins of an advancing flow lobe. Results here indicate that the same sorting process efficiently segregates clasts into two flow regimes: 1) a mobile central channel depleted in coarse clasts, and 2) friction-dominated margins enriched in clasts ≥15cm. The lobe margins, 60% enriched in larger particles with matrix <20%, slow and frictionally freeze from the base up and before the material in the central channel stops flowing. The advancing pumice lobes finally stop when the margins reach ~12 clasts thick and the central channel has insufficient mass flux or momentum to break through or over-top the static margins. These processes form a unique lobe and channel morphology deposit that is diagnostic of granular flow and typical of small to intermediate volume pumice flow emplacement.

Magma mush chemistry at subduction zones, revealed by new melt major element inversion from calcic amphiboles

We have used multiple-regression methods to calibrate new, pressure-independent empirical chemometric equations to calculate the major element composition of basanitic to rhyolitic melts in equilibrium with calcic amphibole. The equations are based on amphibole stoichiometric formula components ± temperature from published experimental P-T-X data and avoid some problems of previous studies associated with uncertainties in pressure determination. Compared with the pressure-dependent equations of Ridolfi and Renzulli (2012), tests run using an independent data set indicate that the new equations yield improved precision and accuracy, in particular for SiO2, TiO2, CaO, and K2O. The results are only marginally more precise when temperature is used as a dependent variable, demonstrating that temperature has a relatively minor role in controlling amphibole crystal chemistry compared with melt composition. This allows us to accept a small decrease in precision in excluding temperature from the analysis, which is very convenient for application of the equations to natural amphiboles where temperature is typically unknown. Using the new chemometric equations, reconstructed melt compositions in equilibrium with the rims of amphiboles in pumice clasts of the Ongatiti ignimbrite are in good agreement with coexisting matrix glass compositions, lending support for our analysis. The compositionally variable cores of the amphiboles give predicted melt compositions with large compositional variations from andesitic (63 wt% SiO2) to high-Si rhyolite. These compositional variations in the predicted melt compositions suggest that there may be a range of heterogeneous melts undergoing progressive differentiation within a major crustal magma storage region underneath the volcano. The results support the existence of genuine intermediate composition melts within the storage region. Interaction between these stored melts, disaggregating mush fragments and replenishing magmas gives rise to the chemical complexity observed in erupted magmas. We also used our multiple regression model to predict the compositions of melts that were in equilibrium with amphiboles in plutonic nodules from Grenada lavas. The predicted melts cover a wide range of compositions, perhaps as a result of in situ fractionation, but are consistent with melt inclusions hosted in those cumulates, as reported by Stamper et al. (2014). Overall, our new pressure- and temperature-independent equations resolve issues associated with previous pressure-dependent studies and represent a useful tool for further investigation of crustal processes at subduction zones.

The ~ 23,500 y 14C BP White Pumice Plinian eruption and associated debris avalanche and Tochimilco lava flow of Popocatépetl volcano, México

The White Pumice (WP) is one of the thickest and most voluminous Plinian fallouts produced by Popocatépetl volcano in central Mexico during the Late Pleistocene-Holocene. Its eruption ~23,500 14C y BP (27,800cal BP) was triggered by the catastrophic failure of the SW flank of the volcano. The resulting debris avalanche was highly mobile reaching 72km from the cone with an apparent coefficient of friction (L/H) of 0.06. The deposit covers an area of ~1200km2, and has a volume of 10.4km3. This gigantic landslide, characterized by exceptionally large proximal hummocks (>400m) provoked the sudden decompression of the hydrothermal and magmatic systems, which produced an initial blast followed by the rise of a Plinian column that reached an altitude of ~33km. The isopach map allows the recognition of a dispersal axis pointing toward the south, where an area of ~2490km2 was covered by >10cm of pumice and ash. The total volume of the pumice fallout was estimated at ~1.9km3 DRE (Dense Rock Equivalent). Pumice clasts are dacitic (62–66wt.% SiO2, anhydrous basis), highly vesicular (55–88vol.%) and display a seriate texture with phenocrysts of plagioclase+hornblende+augite+hypersthene+oxides (Ti-magnetite and ilmenite)+apatite.As the eruption advanced, discharge rates became more intermittent and the height of the column fluctuated and finally collapsed, generating pumice-and-ash flows that were emplaced around the volcano. This short but intense activity was followed during subsequent years by rain-induced lahars that reached great distances from the volcano. At the same time, more degassed andesitic-dacitic (61–65wt.% SiO2) magma was erupted effusively (4.4km3, DRE) in the new horseshoe-shaped ~5km-wide crater from which the Tochimilco lava flow descended toward the SSE, where it inundated an area of ~68km2 and reached as far as ~22km from its source. Since then, multiple eruptions have reconstructed the summit cone, almost completely obliterating the horseshoe-shaped crater.During the course of this catastrophic eruption (VEI=5) a total volume of ~6.3km3 (DRE) of juvenile magma (pumice and lava) were emitted and at least an additional ~10km3 of pre-existing rocks (debris avalanche) mobilized. It surpasses in magnitude most other known Plinian eruptions from Popocatépetl and can be envisaged as an example of a worst-case scenario for hazard evaluation purposes. It dramatically changed the morphology of the volcano and had profound and far-reaching effects beyond its immediate vicinity along rivers draining surrounding plains as well as on the lacustrine basins (e.g. Chalco) to the NE. A repeat of such an eruption in this densely populated area would certainly cause a major calamity of unprecedented dimensions.

テフラ編年と&lt;sup&gt;14&lt;/sup&gt;C年代に基づく鹿児島湾奥，新島（燃島）の海成堆積物の編年とその意義

Shinjima (Moeshima) Island in Kagoshima Bay, southern Kyushu, Japan is noteworthy in volcanology, paleontology, and palaeo-environmental studies, because the island emerged from the bay bottom during volcanic activity at Sakurajima in 1780 AD. As a result, Holocene and late-Pleistocene deposits of the bay including muddy deposits, a thick pyroclastic flow deposit, and prominent molluscan shell beds occur on this island. Several tephras included in the bay deposits are critical for deriving their precise chronology. The chronology of those tephras was constructed on the basis of their identification using both refractive indices and major element compositions of constituent glass shards, as well as stratigraphic features in the field. Tephras identified in sediments from younger to older are Sakurajima-Taisho (Sz-Ts)-/P1, Sakurajima-Sueyoshi(Sz-Sy)-/P11, Yonemaru, Sakurajima-Uwaba(Sz-Ub)-/P12, Sakurajima-Takatoge3 (Sz-Tk3)-/P13, Sakurajima-Satsuma (Sz-S)-/P14, and Shinjima pyroclastic flow deposit. The Shinjima pyroclastic flow deposit, which differs stratigraphically between northern and southern areas of Shinjima Island, is the same tephra in the two areas, and is estimated to be c. 13,000 cal BP in age. The pumice clasts of Sz-Sy/P11 provided a suitable environment for the habitat of a prominent shell bed (Moeshima Shell Bed) composed mainly of Neopycnodonte musashiana. Sz-Ub/P12, Sz-Tk3/P13, and associate secondary deposits of Sz-S/P14 indicate that the deposit was formed in the last 13,000 cal BP. The chronology of the deposits of Shinjima Island is based on the findings of stratigraphic positions and ages of those tephras, and 14C ages obtained in this study, and will play an important role when examining the palaeo-environmental history of Kagoshima Bay since the last deglaciation.

Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)

Pyroclastic density currents have been observed to both enter the sea, and to travel over water for tens of kilometers. Here, we identified a 1.2-m-thick, stratified pumice lapilli-ash cored at Site U1396 offshore Montserrat (Integrated Ocean Drilling Program [IODP] Expedition 340) as being the first deposit to provide evidence that it was formed by submarine deposition from pumice-rich pyroclastic density currents that traveled above the water surface. The age of the submarine deposit is ca. 4 Ma, and its magma source is similar to those for much younger Soufriere Hills deposits, indicating that the island experienced large-magnitude, subaerial caldera-forming explosive eruptions much earlier than recorded in land deposits. The deposit’s combined sedimentological characteristics are incompatible with deposition from a submarine eruption, pyroclastic fall over water, or a submarine seafloor-hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline. The stratified pumice lapilli-ash unit can be subdivided into at least three depositional units, with the lowermost one being clast supported. The unit contains grains in five separate size modes and has a >12 phi range. Particles are chiefly subrounded pumice clasts, lithic clasts, crystal fragments, and glass shards. Pumice clasts are very poorly segregated from other particle types, and lithic clasts occur throughout the deposit; fine particles are weakly density graded. We interpret the unit to record multiple closely spaced (<2 d) hot pyroclastic density currents that flowed over the ocean, releasing pyroclasts onto the water surface, and settling of the various pyroclasts into the water column. Our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate. The overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column, attesting to complex sedimentation. Slow-settling particles were deposited on the seafloor together with faster-descending particles that were delivered at the water surface by subsequent pyroclastic flows. The final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies. This study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water.

Pleistocene volcaniclastic units from North-Eastern Sicily (Italy): new evidence for calc-alkaline explosive volcanism in the Southern Tyrrhenian Sea

AbstractA well-preserved volcaniclastic sequence crops out in Pleistocene marine sediments along the Tyrrhenian coastline of the Calabrian-Peloritani arc (Sicily, Italy), testifying the occurrence of Lower-Middle Pleistocene volcanic activity in Southern Tyrrhenian Sea. The presence of dominant highly vesicular and minor blocky glassy particles indicates that the volcanic clasts were originated by explosive events related to the ascent and violent emission of volatile-rich magmas accompanied by and/or alternated with hydromagmatic fragmentation due to magma-sea water interaction. Field investigations and sedimentological features of the studied volcaniclastic units suggest a deposition from sediment-water density flows. The chemical classification of the pumice clasts indicates prevalent rhyolitic and dacitic compositions with calc-alkaline to high-K calc-alkaline affinity. The geochemical features of immobile trace elements together with the presence of orthopyroxene are indicative of a provenance from an arc-type environment. The age (from 980-910 to 589 ka), the chemical composition and the evidence of subaerial explosive volcanic activity constrain the origin nature and temporal evolution of the arc-type volcanism in the Southern Tyrrhenian domain. Finally, the new information here provided contribute to a better understanding of the temporal geodynamic evolution of this sector of the Mediterranean domain.

The timing of compositionally-zoned magma reservoirs and mafic ‘priming’ weeks before the 1912 Novarupta-Katmai rhyolite eruption

The June, 6, 1912 eruption of more than 13 km3 of dense rock equivalent (DRE) magma at Novarupta vent, Alaska was the largest of the 20th century. It ejected >7 km3 of rhyolite, ∼1.3 km3 of andesite and ∼4.6 km3 of dacite. Early ideas about the origin of pyroclastic flows and magmatic differentiation (e.g., compositional zonation of reservoirs) were shaped by this eruption. Despite being well studied, the timing of events that led to the chemically and mineralogically zoned magma reservoir remain poorly known. Here we provide new insights using the textures and chemical compositions of plagioclase and orthopyroxene crystals and by reevaluating previous U–Th isotope data. Compositional zoning of the magma reservoir likely developed a few thousand years before the eruption by several additions of mafic magma below an extant silicic reservoir. Melt compositions calculated from Sr contents in plagioclase fill the compositional gap between 68 and 76% SiO2 in whole pumice clasts, consistent with uninterrupted crystal growth from a continuum of liquids. Thus, our findings support a general model in which large volumes of crystal-poor rhyolite are related to intermediate magmas through gradual separation of melt from crystal-rich mush. The rhyolite is incubated by, but not mixed with, episodic recharge pulses of mafic magma that interact thermochemically with the mush and intermediate magmas. Hot, Mg-, Ca-, and Al-rich mafic magma intruded into, and mixed with, deeper parts of the reservoir (andesite and dacite) multiple times. Modeling the relaxation of the Fe–Mg concentrations in orthopyroxene and Mg in plagioclase rims indicates that the final recharge event occurred just weeks prior to the eruption. Rapid addition of mass, volatiles, and heat from the recharge magma, perhaps aided by partial melting of cumulate mush below the andesite and dacite, pressurized the reservoir and likely propelled a ∼10 km lateral dike that allowed the overlying rhyolite to reach the surface.

Crystal-melt elemental partitioning in silicic magmatic systems: An example from the Peach Spring Tuff high-silica rhyolite, Southwest USA

Partition coefficients (Kd) are critical for the quantitative modeling of the evolution of magmatic systems. High-silica rhyolites (HSR) are characterized by saturation in numerous phases, such that a comprehensive Kd database is necessary. While there exists a large body of published Kd values, various methods have been used by different authors over time, focusing on different compositions and conditions, and most investigations are limited to small suites of elements (e.g. rare earth elements - REE), typically in few (2–3) mineral phases. In this study, we present Kd values for a suite of 45 elements in 8 mineral phases from a HSR pumice clast from the Peach Spring Tuff (PST): sanidine, plagioclase, biotite, amphibole, titanite, apatite, zircon, and chevkinite.We determine Kd by measuring crystal rim compositions and unaltered glass from a single volcanic sample, which reflect equilibration between crystals and melt in the system at or near the time of eruption. We measure compositions using a combination of SEM-EDS and LA-ICPMS analysis.We show that titanite strongly partitions REE from the melt, with a significant preference for middle (MREE) over light (LREE) and heavy-REE (HREE). Apatite and amphibole share a similar MREE-enriched pattern, though with absolute Kd values more than an order of magnitude lower than titanite; while apatite displays a slight preference for LREE over HREE, amphibole shows a higher affinity for HREE than LREE. Zircon strongly partitions HREE, Hf, and U, with little affinity for LREE. Chevkinite concentrates LREE and Th significantly more than any other phase we analyzed. Biotite Kd are generally low (<10), with the highest values for transition metals and Ba. Sanidine and plagioclase display a strong preference for Sr, Ba, and Eu.Our dataset is largely consistent with published studies that use similar (in-situ) techniques in samples of similar composition (HSR). We observe a large variation in other studies that we attribute in part to contamination from inclusions in crystals, particularly when measuring compositions by whole-crystal methods.Our dataset suggests that accessory minerals play a dominant role in the partitioning of trace elements. They collectively control the distribution of REE and high field strength elements (HFSE) in the system. On the other hand, large ion lithophile elements (LILE) are most dominantly influenced by feldspars, but, if abundant, phases such as biotite and apatite may also influence the partitioning of LILE from the melt.We analyze our Kd values in light of the theoretical crystal lattice strain model (LSM), by fitting our data to curves that have the expected form in ln(Kd) versus ionic radius space (Onuma diagrams). The fact that we are successful in finding good fits for our Kd values, especially the REE, reinforces the idea that mineral-melt equilibrium was attained and analytical data are of appropriate quality. Additionally, we use the fitted curves to estimate Ce3+/Ce and Eu2+/Eu ratios in the melt, which are controlled by the oxidation state of the magma. We estimate that >97% of all Ce in the PST melt existed as Ce3+. The proportion of Eu2+ in the melt is not as well constrained; we estimate Eu2+ ranges mainly from ~10 to ~50% of the total Eu.