Post-caldera activity in the Alban Hills volcanic district (Italy): 40Ar/39Ar geochronology and insights into magma evolution

We present 24 ⁴⁰Ar/³⁹Ar ages for the youngest volcanic products from the Alban Hills volcanic district (Rome). Combined with petrological data on these products, we have attempted to define the chronology of the most recent phase of activity and to investigate the magma evolution of this volcanic district. The early, mainly explosive activity of the Alban Hills spanned the interval from 561±1 to 351±3 ka. After approximately 50-kyr of dormancy, a mainly effusive phase of activity took place, accompanied by the strombolian activity of a small central edifice (Monte delle Faete). This second phase of activity spanned the interval 308±2 to 250±1 ka. After another dormancy period of approximately 50-kyr, a new, hydromagmatic phase of activity started at 200 ka at several centers located to the southwest of the Monte delle Faete edifice. After an initial recurrence period of approximately 50-kyr, which also characterized this new phase of activity, the longest dormancy period (approximately 80-kyr) in the history of the volcanic district preceded the start of the activity of the Albano and Giuturna centers at 70±1 ka. Results of our study suggest a quasi-continuous magmatic activity feeding hydromagmatic centers with a new acme of volcanism since around 70 ka. Based on data presented in this paper, we argue that the Alban Hills should not be considered an extinct volcanic district and a detailed re-assessment of the volcanic hazard for the area of Rome is in order. Published online: 4 April 2003 Editorial responsibility: J. Donnelly-Nolan     

Ar/Ar and C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District,Central Italy

New ⁴⁰Ar/³⁹Ar and ¹⁴C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The ⁴⁰Ar/³⁹Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined ⁴⁰Ar/³⁹Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence.     

Crustal structure, evolution, and volcanic unrest of the Alban Hills, Central Italy

 The Alban Hills, a Quaternary volcanic center lying west of the central Apennines, 15–25 km southeast of Rome, last erupted 19 ka and has produced approximately 290 km³ of eruptive deposits since the inception of volcanism at 580 ka. Earthquakes of moderate intensity have been generated there at least since the Roman age. Modern observations show that intermittent periods of swarm activity originate primarily beneath the youngest features, the phreatomagmatic craters on the west side of the volcano. Results from seismic tomography allow identification of a low-velocity region, perhaps still hot or partially molten, more than 6 km beneath the youngest craters and a high-velocity region, probably a solidified magma body, beneath the older central volcanic construct. Thirty centimeters of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted from accumulation of magma beneath these craters. The volume of magma accumulation and the amount of maximum uplift was probably at least 40×10⁶ m³ and 40 cm, respectively. Comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of uplift. The magma chamber beneath the western Alban Hills is probably responsible for much of the past 200 ka of eruptive activity, is still receiving intermittent batches of magma, and is, therefore, continuing to generate modest levels of volcanic unrest. Bending of overburden is the most likely cause of the persistent earthquakes, which generally have hypocenters above the 6-km-deep top of the magma reservoir. In this view, the most recent uplift and seismicity are probably characteristic and not precursors of more intense activity. Received: 15 April 1997 / Accepted: 9 August 1997     

Geochemistry and magmatic properties of eruption episodes from Haroharo linear vent zone, Okataina Volcanic Centre, New Zealand during the last 10 kyr

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.     

40Ar/39Ar Ages and stratigraphy of the Latera caldera,Italy

The Latera caldera is a well-exposed volcano where more than 8 km³ of mafic silica-undersaturated potassic lavas, scoria and felsic ignimbrites were emplaced between 380 and 150 ka. Isotopic ages obtained by ⁴⁰Ar/³⁹Ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera. The initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite. They were closely followed by eruption of evolved phonolitic magma. After roughly 25 ky, several phonolitic ignimbrites were deposited, and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash-flow units at 191 ka. Compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs, tephriphonolitic spatter, and scoria-bearing ignimbrites. After 40 ky of mafic surge deposit and scoria cone development around the caldera rim, a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the Latera caldera. This activity marks the end of large-scale explosive eruptions from the Latera volcano at 156 ka.     

A large K-foiditic hydromagmatic eruption from the early activity of the Alban Hills Volcanic District, Italy

In this paper we discuss the uncommon case of an energetic, pyroclastic-flow-forming eruption with a SiO₂-poor (42-45 wt.%), K-foiditic magma composition. The Trigoria-Tor de' Cenci Tuff (TTC; 561 ka) is the product of the first large-scale explosive event (of the order of 1-10 km³ of erupted products) in the Alban Hills Volcanic District, near the city of Rome, Italy. After an initial Plinian phase that produced a scoria fall horizon, pyroclastic current activity emplaced ash deposits with leucite-bearing juvenile scoria lapilli. The abundance of accretionary lapilli, the most distinctive feature of these deposits, together with the high degree of fragmentation, the abundance of minute lithic inclusions and the morphology of ash particles, indicates a hydromagmatic character for the most part of the eruption. The absence of vent-derived carbonate lithic clasts from the deep regional aquifer and the abundance of cognate lithic fragments suggest that the interaction with external water involved a surficial aquifer in the older Alban Hills volcanic terrains. Perhaps the most striking aspect of the TTC is the K-foiditic composition of the pre-eruptive melt, which, to our knowledge, is unique among explosive events of comparable size elsewhere in the world. The pre-eruptive magma system feeding the TTC was controlled mainly by leucite+clinopyroxene fractionation under a_H2O<1 conditions. The low SiO₂ activity prevented plagioclase and K-feldspar crystallization. The depth of the magma chamber can be estimated at 3-6 km within the carbonate substrate. In contrast to the other major pyroclastic-flow-forming eruptions of the Alban Hills, the juvenile volatile exsolution due to magma crystallization is not seen as the main mechanism driving the TTC eruption. We suggest that the explosive behaviour of the TTC magma in the early magmatic phase resulted from a rapid decompression due to a regional seismic event and from magma-water interaction in the succeeding phase.     

Eruptive history of Fisher Caldera,Alaska, USA

New field, compositional, and geochronologic data from Fisher Caldera, the largest of 12 Holocene calderas in Alaska, provide insights into the eruptive history and formation of this volcanic system. Prior to the caldera-forming eruption (CFE) 9400 years ago, the volcanic system consisted of a cluster of several small (∼3 km³) stratocones, which were independently active between 66±144 and 9.4±0.2 ka. Fisher Caldera formed through a single eruption, which produced a thick dacitic fall deposit and two pyroclastic-flow deposits, a small dacitic flow and a compositionally mixed basaltic-dacitic flow. Thickness and grain-size data indicate that the fall deposit was dispersed primarily to the northeast, whereas the two flows were oppositely directed to the south and north. After the cataclysmic eruption, a lake filled much of the caldera during what may have been a significant quiescent period. Volcanic activity from intracaldera vents gradually resumed, producing thick successions of scoria fall interbedded with lake sediments. Several Holocene stratocones have developed; one of which has had a major collapse event. The caldera lake catastrophically drained when a phreatomagmatic eruption generated a large wave that overtopped and incised the southwestern caldera wall. Multiple accretionary-lapilli-bearing deposits inside and outside the caldera suggest significant Holocene phreatomagmatic activity. The most recent eruptive activity from the Fisher volcanic system was a small explosive eruption in 1826, and current activity is hydrothermal. Late Pleistocene to Holocene magma eruption rates range from 0.03 to 0.09 km³ ky⁻¹ km⁻¹, respectively. The Fisher volcanic system is chemically diverse, ∼48–72 wt.% SiO₂, with at least seven dacitic eruptions over the last 82±14 ka that may have become more frequent over time. Least squares calculations suggest that prior to the CFE, Fisher Volcano products were not derived from a single, large magma reservoir, and were likely erupted from multiple, compositionally independent magma reservoirs. After the CFE, the majority of products appear to have derived from a single reservoir in which magma mixing has occurred.     

The geological evolution of Merapi volcano, Central Java, Indonesia

Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuées ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of ⁴⁰K–⁴⁰Ar and ⁴⁰Ar–³⁹Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170?ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109?±?60?ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138?±?3?ka; 135?±?3?ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30?ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8?±?1.5?ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792?±?90 ¹⁴C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 ¹⁴C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuées ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872?AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of the eruptive history of Mount Erebus,Antarctica: summit flows,tephra, and caldera collapse

Eruptive activity has occurred in the summit region of Mount Erebus over the last 95 ky, and has included numerous lava flows and small explosive eruptions, at least one plinian eruption, and at least one and probably two caldera-forming events. Furnace and laser step-heating ⁴⁰Ar/³⁹Ar ages have been determined for 16 summit lava flows and three englacial tephra layers erupted from Mount Erebus. The summit region is composed of at least one or possibly two superimposed calderas that have been filled by post-caldera lava flows ranging in age from 17 ± 8 to 1 ± 5 ka. Dated pre-caldera summit flows display two age populations at 95 ± 9 to 76 ± 4 ka and 27 ± 3 to 21 ± 4 ka of samples with tephriphonolite and phonolite compositions, respectively. A caldera-collapse event occurred between 25 and 11 ka. An older caldera-collapse event is likely to have occurred between 80 and 24 ka. Two englacial tephra layers from the flanks of Mount Erebus have been dated at 71 ± 5 and 15 ± 4 ka. These layers stratigraphically bracket 14 undated tephra layers, and predate 19 undated tephra layers, indicating that small-scale explosive activity has occurred throughout the late Pleistocene and Holocene eruptive history of Mount Erebus. A distal, englacial plinian-fall tephra sample has an age of 39 ± 6 ka and may have been associated with the older of the two caldera-collapse events. A shift in magma composition from tephriphonolite to phonolite occurred at around 36 ka.Editorial responsibility: Julie Donnelly-Nolan     

Large mafic eruptions at Alban Hills Volcanic District (Central Italy): Chronostratigraphy,petrography and eruptive behavior

Despite its ultra-potassic, basic geochemistry (40 ≤ SiO₂ ≤ 50 wt.%), the Alban Hills Volcanic District was characterized by a highly explosive phase of activity, the Tuscolano–Artemisio phase, which emplaced very large volumes (several tens of km³ each cycle) of pyroclastic-flow deposits, mafic in composition (SiO₂ ≤ 45 wt.%) in the time span 600–350 ka. In contrast to the abundance of pyroclastic-flow deposits, very scarce basal Plinian deposits and, more in general, fallout deposits are associated to these products. While some of the pyroclastic-flow deposits have been described in previous literature, no specific work on the Tuscolano–Artemisio phase of activity has been published so far. In particular, very little is known on the products of the early stages, as well as of the final, post-caldera activity of each eruptive cycle. Here we present a comprehensive stratigraphic and geochronologic study of the Tuscolano–Artemisio phase of activity, along with new textural and petrographic data. We describe the detailed stratigraphy and petrography of five reference sections, where the most complete suites of products of the eruptive cycles, comprising the initial through the final stages, are exposed. We assess the geochronology of these sections by means of 18 new ⁴⁰Ar/³⁹Ar age determinations, integrating them with 16 previously performed, aimed to describe the eruptive behavior of the Alban Hills Volcanic District during this phase of activity, and to assess the recurrence time and the duration of the dormancies.The overall explosive activity appears to be strictly clustered in five eruptive cycles, fairly regularly spaced in time and separated by very long dormancies, in the order of several ten of kyr, during which no volumetrically appreciable eruption occurred, as the lack of deposits dated to this time-interval testify. We propose a volcano-tectonic model that explains this peculiar eruptive behavior, unparalleled in the other coeval volcanic districts of the Tyrrhenian margin of Italy, as related to the local transpressive tectonic regime.     

Testing less-conventional methods to date a late-pleistocene to Holocene eruption: Radiocarbon dating of paleosols and 36Cl exposure ages at Pelado volcano,Sierra Chichinautzin,Central Mexico

Despite their significance for estimating hazards and forecasting future activity, dating young volcanic deposits and landforms (<50,000 yrs old) remains a challenge due to the limitations inherent to the different isotopic chronometers used. The Trans-Mexican Volcanic Belt is one of the most active and populated continental arcs worldwide, yet its temporal pattern of activity is poorly constrained. Such deficiency is particularly problematic for the Sierra Chichinautzin Volcanic Field (SCVF) that is located at the doorstep of Mexico City and Cuernavaca and is hence a major source of risk for these cities. Existing ages for this area derive mostly from either radiocarbon on charcoal, which is rare and may be contaminated, or ⁴⁰Ar/³⁹Ar on rock matrix, which is poorly precise for this time period and rock type. Here, we focus on the Pelado monogenetic volcano, which is located in the central part of the SCVF and erupted both explosively and effusively, producing a large lava shield and a widespread tephra blanket. This unique eruptive event was previously dated at ∼12 calibrated (cal) kyrs BP, using radiocarbon dating on charcoal from deposits related to the eruption. To test alternative dating approaches and confirm the age of this significant eruption, we applied two less conventional techniques, radiocarbon dating of bulk paleosol samples collected below the complete tephra sequence at nine sites around the shield, and in-situ ³⁶Cl exposure dating of two samples of an aphyric lava from the base of the shield. Radiocarbon paleosol ages span a continuous time interval from 13.2 to 20.2 cal kyrs BP (2σ), except for one anomalously young sample. This wide age spread, along with the low organic contents of the paleosols, may be due to erosive conditions, related to the sloping topography of the sampling sites and the cool and relatively dry climate of the Younger Dryas (11.7–12.9 ka), during which the Pelado eruption probably occurred. The two ³⁶Cl-dated lava samples have consistent ages at 1σ analytical errors of 15.5 ± 1.4 ka and 13.2 ± 1.2 ka, respectively, yielding an average age of 14.3 ± 1.6 ka for this lava flow. The high full uncertainty in ³⁶Cl ages (24%) is due to high rock Cl content. We conclude that paleosol radiocarbon dating is useful if numerous samples are analyzed and climatic and relief conditions at the time of the eruption and at the sites of tephra deposition are considered. The ³⁶Cl dating technique is an alternative method to date volcanic eruptions, as it gave consistent results, but in the specific case of Pelado volcano, the high Cl content in the analyzed rocks increases the age uncertainties.     

Temporal dissection of the Huckleberry Ridge Tuff using the 40Ar/39Ar dating technique

New high-precision single crystal sanidine ⁴⁰Ar/³⁹Ar ages for the Huckleberry Ridge Tuff (HRT), Yellowstone volcanic field, show that the three HRT members (A, B, and C) represent at least two different eruptions. The new ⁴⁰Ar/³⁹Ar ages (all ages calculated relative to the optimisation model of Renne et al., 2011) are: 2.135 ± 0.006 Ma, 2.131 ± 0.008 Ma, and 2.113 ± 0.004 Ma (2σ, full uncertainty propagation), for members A, B and C, respectively. Members A and B are within uncertainty of one another and both are more precise than, but in agreement with, previously published ages. Member C was erupted later than members A and B. HRT members A and B were deposited during the Reunion Normal Polarity Subchron (C2r.1n). Member C was deposited during Subchron C2r.1r. Previously published radiogenic and stable isotope data show that member C was sourced from an isotopically discrete magma with a higher fraction of crustal material than members A and B. The volume of the first HRT eruption is reduced by c. 12% from previous estimates and explosive eruptions from the Yellowstone volcanic field occurred more frequently, producing more homogeneous magma than was previously believed. High-precision ⁴⁰Ar/³⁹Ar dating is key for resolving the eruptive history of Yellowstone, temporal dissection of voluminous ignimbrites, and rigorous investigation of what constitutes a ‘super-eruption’.     

Mélanges magmatiques à la Montagne Pelée (Martinique) Origine des eruptions de type Saint-Vincent

During the last 40,000 years B.P. the eruptive activity of Mont Pelée (Martinique) has been exclusively pyroclastic, including mainly pumice flow deposits, Pelean-type and Merapi-type nuée ardente deposits, characterized by an andesitic to dacitic magma composition. In addition, a few Saint-Vincent-type nuée ardente deposits are present. Their products are compositionally more basic (basalt, basaltic andesite) and show some characteristic magma mixing features. Two well defined Saint-Vincent type eruptions, named SV1 and SV2 have been studied here. They have been dated by the C¹⁴ method respectively at 25,700±1,200 and 22,300±1,200 years B.P. Both follow a similar eruptive pattern, evolving from an andesitic to a more basic magma composition, through an intermediate stage of magma mixing. The volume of ejected products is extensive (1 km³ or more), compared with other deposits such as the Pelean-type nuée ardente. The moderate and progressive variations of magma composition (3 to 6 % SiO₂), mineralogy and crystallization pressure-temperature conditions (T: 920°–930° to 950°970°C, using Fe-Ti oxides geothermometer) demonstrate the cogenetic nature of these various magmas. These results, as well as the study of the recent activity of Mont Pelée suggest that during a former period (about 40,000 to 20,000 years B.P.), two magmatic chambers existed rather close to one another. The triggering of these Saint-Vincent type nuée ardente eruptions might involve injections of less-differentiated magma from a lower to a shallower reservoir, followed by the emptying of both reservoirs. During the recent period (less than 13,500 years B.P.), the cyclic eruptive activity of Mont Pelée Volcano has been controlled mainly by a relatively shallow and permanent magmatic chamber. The triggering of eruptions has depended on two processes: volatile overpressure and periodic replenishment of this superficial reservoir by deeper and less-differentiated magma injections. This change in eruptive character results perhaps from succession of SV1 and SV2 Saint-Vincent type eruptions; the volcano deep-structure might have changed, as a consequence of the extensive volume of ejected products.     

Magmatic evolution of the Puyehue–Cordón Caulle Volcanic Complex (40° S), Southern Andean Volcanic Zone: From shield to unusual rhyolitic fissure volcanism

Magmas erupted from Quaternary volcanoes of Southern Andes between 37° and 46° S latitude are mainly basaltic to andesitic. However, PCCVC (40° S) shows a singular magmatic evolution due to the abnormal evacuation of rhyolites, especially in the last 100 ka. In addition, PCCVC is the result of juxtaposing products from the NW-trending alignment of Cordillera Nevada caldera, Cordón Caulle fissure volcano and the Puyehue stratocone. Using ⁴⁰Ar/³⁹Ar and ¹⁴C geochronology it can be established that they evolved since ca. 500 ka as coeval but separated vents with a first stage of shield volcanism, followed by repeated collapses that formed an internal NW-elongated graben. From ca. 100 ka, volcanic activity occurred in both a fissure system (Cordón Caulle) and a central volcano (Puyehue). Holocene explosive eruptions, mainly in the Puyehue crater, accompanied the dome growing along a NW-trending fissure system. Last historical eruptions were in 1921 and 1960 when NW fissures of Cordón Caulle fed rhyodacitic lava flows. In 1960, the fissure eruption was triggered by a remote Mw: 9.5 thrust earthquake.Cordillera Nevada caldera presents a reduced compositional range (52–63% SiO₂) and geochemical features of low-pressure magma mixing and assimilation. Instead, Cordón Caulle and Puyehue volcanoes have a wide silica range (48–71% SiO₂) and an outstanding affinity, which can be modelled with initial high-pressure fractional crystallization, moderate magma mixing and subsequent low-pressure fractional crystallization from a common parental source.The exceptional magmatic evolution and eruptive style of PCCVC in Southern Andes could be related with the physics of the plumbing system, which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime.     

Rhyolites at Kerlingarfjöll, Iceland: the evolution and lifespan of silicic central volcanoes

Kerlingarfjöll central volcano is Iceland’s second largest outcrop of Quaternary rhyolite and is part of the Icelandic Western Rift Zone. Geochemical and Ar/Ar age data show that at least 21 different rhyolite eruptions have taken place at Kerlingarfjöll over the last 350 ka. Ar/Ar dating was carried out on samples of obsidian which showed variable reproducibility, illustrating the difficulty in dating young Icelandic volcanics. Nevertheless, reasonable estimates of eruption age have been derived for a number of eruptive units that are consistent with observed stratigraphy, enabling an understanding of the temporal evolution of Kerlingarfjöll. Two rhyolite magma types are present. The first is an older, low-Nb rhyolite that was erupted episodically along a cryptic curved fracture system, to form a discontinuous ring of rhyolite mountains, between 350 and 250 ka. This discontinuous ring is similar to structures observed at other volcanoes in Iceland, suggesting that the development of a curved fracture that acts as a pathway for episodic silicic eruptions is a feature of central volcano development. The second magma is a younger, high-Nb rhyolite that was erupted episodically between 250 and 68 ka in the northern part of Kerlingarfjöll, forming two clusters, both of which have areas of intense hydrothermal activity. Repose periods for rhyolite volcanism are thought to be on the order of tens of thousands of years, and it is possible that Kerlingarfjöll will erupt rhyolite again in the future.     

Limestone assimilation and the origin of CO2 emissions at the Alban Hills (Central Italy): Constraints from experimental petrology

The Alban Hills volcanic region (20 km south of Rome, in the Roman Province) emitted a large volume of potassic magmas (> 280 km³) during the Quaternary. Chemical interactions between ascending magmas and the ∼ 7000–8000-m-thick sedimentary carbonate basement are documented by abundant high temperature skarn xenoliths in the eruptive products and have been frequently corroborated by geochemical surveys. In this paper we characterize the effect of carbonate assimilation on phase relationships at 200 MPa and 1150–1050 °C by experimental petrology. Calcite and dolomite addition promotes the crystallization of Ca-rich pyroxene and Mg-rich olivine respectively, and addition of both carbonates results in the desilication of the melt. Furthermore, carbonate assimilation liberates a large quantity of CO₂-rich fluid. A comparison of experimental versus natural mineral, glass and bulk rock compositions suggests large variations in the degree of carbonate assimilation for the different Alban Hills eruptions. A maximum of 15 wt.% assimilation is suggested by some melt inclusion and clinopyroxene compositions; however, most of the natural data indicate assimilation of between 3 and 12 wt.% carbonate. Current high CO₂ emissions in this area most likely indicate that such an assimilation process still occurs at depth. We calculate that a magma intruding into the carbonate basement with a rate of ∼ 1 – 2 · 10⁶ m³/year, estimated by geophysical studies, and assimilating 3–12 wt.% of host rocks would release an amount of CO₂ matching the current yearly emissions at the Alban Hills. Our results strongly suggest that current CO₂ emissions in this region are the shallow manifestation of hot mafic magma intrusion in the carbonate-hosted reservoir at 5–6 km depth, with important consequences for the present-day volcanic hazard evaluation in this densely populated and historical area.     

40Ar/39Ar constraints on the temporal evolution of Graciosa Island, Azores (Portugal)

Lava flows spanning the eruptive record of Graciosa Island (Azores archipelago) and a gabbro xenolith were dated by ⁴⁰Ar/³⁹Ar in order to constrain the Pleistocene and Holocene volcanic evolution of the island. The results range from 1.05 Ma to 3.9 ka, whereas prior published K–Ar and ¹⁴C ages range from 620 to 2 ka. The formation of the Serra das Fontes shield volcano started at minimum 1.05 Ma, and the magmatic system was active for ca. 600 ky, as suggested by the formation of the gabbro xenolith by magmatic differentiation. Evolved magmas making up the Serra das Fontes–Serra Branca composite volcano were generated at ca. 450 ka. After a period of ca. 110 ky of volcanic inactivity and erosion of volcanic edifices, volcanism was reactivated with the formation of the Vitória Unit NW platform. Later, the development of the Vulcão Central Unit started with the formation of monogenetic cones located to the south of the Serra das Fontes–Serra Branca–Vitória Unit. This volcanism became progressively more evolved and was concentrated in a main eruptive center, forming the Vulcão Central stratovolcano with an age older than 50 ka. The caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera, resulting in the formation of a lava lake, which ultimately spilled over the caldera rim at ca. 11 ka. The most recent eruptions on Graciosa formed two small pyroclastic cones within the caldera and the Pico do Timão cone within the Vitória Unit at ca 3.9 ka.     

Effusive eruptions from a large silicic magma chamber: the Bearhead Rhyolite,Jemez volcanic field,NM

Large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events. Less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera-forming events produce dominantly effusive eruptions of small-volume rhyolite domes and flows. The Bearhead Rhyolite and associated Peralta Tuff Member in the Jemez volcanic field, New Mexico, represent small-volume eruptions from a large silicic magma system in which no caldera-forming event occurred, and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long-term evolution of continental silicic magma systems.⁴⁰Ar/³⁹Ar dating reveals that most units mapped as Bearhead Rhyolite and Peralta Tuff (the Main Group) were erupted during an ∼540 ka interval between 7.06 and 6.52 Ma. These rocks define a chemically coherent group of high-silica rhyolites that can be related by simple fractional crystallization models. Preceding the Main Group, minor amounts of unrelated trachydacite and low silica rhyolite were erupted at ∼11–9 and ∼8 Ma, respectively, whereas subsequent to the Main Group minor amounts of unrelated rhyolites were erupted at ∼6.1 and ∼1.5 Ma.The chemical coherency, apparent fractional crystallization-derived geochemical trends, large areal distribution of rhyolite domes (∼200 km²), and presence of a major hydrothermal system support the hypothesis that Main Group magmas were derived from a single, large, shallow magma chamber. The ∼540 ka eruptive interval demands input of heat into the system by replenishment with silicic melts, or basaltic underplating to maintain the Bearhead Rhyolite magma chamber.Although the volatile content of Main Group magmas was within the range of rhyolites from major caldera-forming eruptions such as the Bandelier and Bishop Tuffs, eruptions were smaller volume and dominantly effusive. Bearhead Rhyolite domes occur at the intersection of faults, and are cut by faults, suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera-forming eruption.     

Time-dependent CO<Subscript>2</Subscript> variations in Lake Albano associated with seismic activity

Lake Albano (Alban Hills volcanic complex, Central Italy) is located in a densely populated area near Rome. The deep lake waters have significant dissolved CO₂ concentrations, probably related to sub-lacustrine fluid discharges fed by a pressurized CO₂-rich reservoir. The analytical results of geochemical surveys carried out in 1989–2010 highlight the episodes of CO₂ removal from the lake. The total mass of dissolved CO₂ decreased from ∼5.8 × 10⁷ kg in 1989 to ∼0.5 × 10⁷ kg in 2010, following an exponential decreasing trend. Calculated values of both dissolved inorganic carbon and CO₂ concentrations along the vertical profile of the lake indicate that this decrease is caused by CO₂ release from the epilimnion, at depth <9 m, combined with (1) water circulation at depth <95 m and (2) CO₂ diffusion from the deeper lake layers. According to this model, Lake Albano was affected by a large CO₂ input that coincided with the last important seismic swarm at Alban Hills in 1989, suggesting an intimate relationship between the addition of deep-originated CO₂ to the lake and seismic activity. In the case of a CO₂ degassing event of an order of magnitude larger than the one that occurred in 1989, the deepest part of Lake Albano would become CO₂-saturated, resulting in conditions compatible with the occurrence of a gas outburst. These results reinforce the idea that a sudden CO₂ input into the lake may cause the release of a dense gas cloud, presently representing the major volcanic threat for this densely populated area.     

An eruptive history of Maderas volcano using new 40Ar/39Ar ages and geochemical analyses

Maderas volcano is a small, andesitic stratovolcano located on the island of Ometepe in Lake Nicaragua, Nicaragua, with no record of historic activity. Twenty-one samples were collected in 2010 from lava flows of Maderas. The selected samples were analyzed for whole-rock geochemistry using ICP-AES and/or were dated using the ⁴⁰Ar/³⁹Ar method. The results of these analyses were combined with previously collected data from Maderas as well as field observations to determine the eruptive history of the volcano and create a geologic map. The results of the geochemical analyses indicate that Maderas has higher concentrations of alkalies than most Nicaraguan and Costa Rican volcanoes including its nearest neighbor, Concepción volcano. It is also different from Concepción in that it displays higher incompatible elements. Determined age dates range from 179.2?±?16.4?ka to 70.5?±?6.1?ka. Based on these ages and the geomorphology of the volcano which is characterized by a bisecting graben, it is proposed that Maderas experienced two generations of development: initial build-up of the older cone including pre-graben lava flows, followed by post-graben lava flows. The ages also indicate that Maderas is markedly older than Concepción which is historically active. Volcanic hazards were also assessed. The ⁴⁰Ar/³⁹Ar ages indicate that Maderas has likely been inactive for tens of thousands of years and future volcanic eruptions are not considered an immediate hazard. However, earthquake and lahar hazards exist for the communities around the volcano. The steep slopes of the eroded older cone are the most likely sources of lahar hazards.