Silicic Lavas Research Articles

Miocene basaltic magmatism in the Goldfield-Superstition volcanic province, central Arizona: Geochemistry, mineralogy, and petrology

Basaltic lavas form part of the Miocene (∼20.5 to 18 Ma) Goldfield-Superstition silicic large igneous province in central Arizona. Most of the basalt erupted early in the development of this southern Basin and Range volcanic province, and only small amounts of basalt co-erupted with the silicic volcanism. We examined 50 samples of basalt from lower, middle, and upper stratigraphic positions of the province to establish basalt petrogeneses, the characteristics of basalt sources, relationships among compositionally different lavas, and the igneous processes that relate various basalt types. We base our study largely on major and trace elements, mineral compositions, and a small data set for Sr, Nd, and Hf isotopes. Lower-section basalts include the Weekes Wash basalts, which are transitional between alkalic and tholeiitic (SiO2 49–51 wt. %) and with MgO 8.1–10.2 wt. %. They are associated with lavas that are andesitic (SiO2 ∼59–62 wt. %; MgO 8–2.5 wt. %) or are seemingly andesitic due to alteration and felsic xenocrysts. Weekes Wash basalts have incompatible-element abundances that correlate positively with MgO. Their87Sr/86Sr ratios are ∼0.705. Olivines, rarely fresh, have Fo86–89 cores, and clinopyroxenes have Mg#s 86–89. Overlying the Weekes Wash are the Cottonwood Spring basalts, which are alkalic (SiO2 ∼45.5–47.5 wt. %), and can be categorized into subgroups defined by lower and higher incompatible-element abundances, or low Ti-P and high Ti-P (e.g., Ba ∼1100 versus 1800 ppm; La ∼75 versus 110 ppm). The Cottonwood Spring basalts are the closest to primary lavas we observed (MgO 10.1–11.8 wt. %), and their87Sr/86Sr ratios are ∼0.705–0.706. Their olivines are Fo86–89, and their clinopyroxenes have Mg#s 86–90. Overlying the Cottonwood Spring basalts is the Apache Gap Fe-Ti-enriched basalt (TiO2 ∼2.6 wt. %), which has the lowest MgO (∼5.6–7.5 wt. %) and incompatible-element abundances observed for any basalts in the province (e.g., Ba ∼400 ppm; La ∼25 ppm). All lower-section basalts have primitive-mantle normalizations showing Nb-Ta negative anomalies. Middle-section basalts erupted among silicic lava and pyroclastic flows from ∼19 to 18.5 Ma, and they compositionally resemble Weekes Wash basalts. Upper-section (post-18.5 Ma) lavas are the Willow Springs hawaiite (∼9.5 wt. % MgO;87Sr/86Sr ∼0.705) and Black Mesa basanite (SiO2 ∼44 wt. %; MgO ∼8 wt. %; CaO 14.7 wt. %;87Sr/86Sr ∼0.706). A Nb-Ta anomaly is clear in the hawaiite but weak in the basanite, as the basanite has the highest Nb and Ta observed (∼60 and ∼3 ppm; lowest Zr/Nb, ∼4 versus all others >7). Relevant interpretations are the following. Absence of ultramafic mantle xenoliths in Goldfield-Superstition basalts suggests that magmas occupied crustal reservoirs. The two subgroups of the Cottonwood Spring basalts attest to small-scale trace element and isotopic heterogeneities in lithospheric mantle sources that, based on Nb-Ta and Ce versus Ce/Yb modeling, had subduction zone characteristics and were garnet bearing. For Weekes Wash basalts, the decreasing incompatible-element abundances with decreasing MgO is consistent with a hybrid origin by the low-Ti-P subgroup of Cottonwood Spring basalts having assimilated lower crust. Trace-element modeling based on assimilation fractional crystallization (AFC) demonstrates that mixing Cottonwood Spring basalt with ∼50 percent (partial) melts of lower-crust pyroxenite in proportions from 80:20 to 50:50 yields Weekes Wash basalt compositions. Apache Gap lava represents AFC processes of high-Ti-P Cottonwood Spring basalts in a crustal reservoir, enabling Fe-Ti enrichment after ∼60 percent crystallization, as estimated by mass balancing, of mainly clinopyroxene and plagioclase. Upper-section hawaiite models as a differentiate after ∼50 percent crystallization of mainly clinopyroxene and plagioclase from a composition resembling low-Ti-P Cottonwood Spring basalt. Black Mesa basanite originated in a garnet-bearing lithospheric mantle source generally similar to those for Cottonwood Spring basalts, but additionally carbonitized (e.g., basanite has SiO2-undersaturation; high CaO) to yield magma relatively enriched in Nb and Ta. Basanite undersaturation is consistent with a smaller percent of source melting at the close of Goldfield-Superstition basaltic magmatism compared to initial source melting. This study of the Goldfield-Superstition volcanic province demonstrates that magmas produced from Miocene lithospheric mantle spanning ∼2.5 million years took various paths, from erupting as nearly primitive lavas from their lithospheric sources, to interacting with lower crust where they assimilated, differentiated, and provided heat to create a silicic large igneous province. Finally, the basalts reflect some changes in source and melting characteristics over the time of their emplacements, but they sustained lithospheric-source and ancient subduction characteristics throughout.

Evolution of the mechanics of the 2004–2008 Mt. St. Helens lava dome with time and temperature

The 2004–2008 eruption of Mount St. Helens, Washington, USA, formed a typical example of a Pelean spiny lava dome, with solid spines of crystalline silicic lava extruded along shear zones bounded by fault gouge (Cashman et al., 2008; Iverson et al., 2006). Creation of and movement along shear zones in this erupting lava, recorded as shallow micro-earthquakes, were thus key controls on eruption style and rate, in addition to dome stability. As eruption style and lava dome stability often change within individual eruptions, it is important to identify how the strength of dome rocks changes with temperature, time, texture and extrusion rate. However, critical values of the shear strength of dome lava at eruptive temperatures have never been measured. We have found in controlled laboratory experiments simulating eruption conditions that dome lava strength increased during the course of the 2004–2008 eruption of Mount St. Helens, as the textures became more crystalline and less porous. Increasing lava strength during the eruption would lead to deformation becoming increasingly localised to shear zones, explaining why the extrusion style and centre were so consistent. Acoustic emissions and microstructure analysis indicate that fracturing was more localised at higher temperatures, resulting in higher strengths at eruptive temperature compared to ambient temperatures for later erupted samples. The strength of these later erupted samples at eruptive temperatures increased markedly with strain rate. This would damp any increases in effusion rate that may result from changes in magma pressure, explaining the steady effusion rate at Mount St. Helens from 2004 to 2008.

Fluid flow associated with silicic lava domes and faults, Ohaaki hydrothermal field, New Zealand

Fault-controlled fluid flow is a well-recognised feature of hydrothermal systems. Yet, the role of extinct lava domes in governing fluid flow is less established. Here we present fluid-flow models for the spatial distribution and origins of permeability within the two thermal areas (Ohaaki West (OHW) and Ohaaki East (OHE)) of the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand. We develop conceptual fluid-flow models using detailed mapping of surface heat and mass flow gradients and structural and volcanic/stratigraphic data. Heat and mass flow models based on extinct lava domes are of two main types: (i) a ‘volcanic conduit’ model where hydrothermal fluids ascending from an underlying upflow exploit remnant permeability structures associated with dome growth (dyke margins, conduit shear zones and eruptive vents) and discharge at the surface of OHW, and; (ii) a dome and regional fault, which collectively produce a channel-barrier fluid-flow system within surficial calcite-cemented alluvium and ash fall deposits that mantle the dome (OHE). These models show that silicic lava domes localise fluid flow through either remnant eruptive structures, or by providing islands of locally competent and low-porosity rock that favour the development of high-permeability fault and fracture systems. In each model, hydrothermal sealing and cementation of otherwise unconsolidated and low strength materials, mantling the domes, play an important role in permeability generation. From these observations and recent mass and heat flow surveys of other fields, we argue that the Quaternary dome complexes of the TVZ, overlying long lived (≥ 0.15–0.5 Myr) hydrothermal systems, may constitute a major pathway of magmatic heat and mass release to the atmosphere long after eruption and dome formation.

Silicic magmatism associated with Late Cretaceous rifting in the Arctic Basin—petrogenesis of the Kap Kane sequence, the Kap Washington Group volcanics, North Greenland

The bimodal, Late Cretaceous–Palaeocene (71–61Ma) Kap Washington Group volcanic sequence on the north coast of Greenland was erupted in a continental rift setting during the opening of the Arctic Ocean. On Kap Kane ca. 70Ma silicic lavas and ignimbrites dominate over mildly alkaline basalts. Intermediate rocks are scarce (SiO2=57–62wt.%) and cognate mafic inclusions, resorbed crystals, and highly variable clinopyroxene compositions indicate that they formed by magma mixing. Silicic lavas (66–74wt.% SiO2) on Kap Kane show geochemical features typical of A-type granitoids and form two chemically and mineralogically distinct suites: (i) one-feldspar, peralkaline trachytes and rhyolites with elevated contents of high field strength elements (HFSE); and (ii) two-feldspar, weakly peraluminous, HFSE-poor trachytes and rhyolites. The peralkaline lavas have Sr–Nd–Pb isotopic compositions which overlap with the compositions of mildly alkaline Kap Kane basalts. The peralkaline lavas are inferred to have originated from an evolved basaltic parent by ca. 90% fractional crystallisation of an assemblage consisting of plagioclase, alkali feldspar, clinopyroxene, Fe-Ti oxide, olivine, kaersutite and apatite. The peraluminous lavas have mixed mantle–crust Sr–Nd–Pb isotopic signatures and it is proposed that they were formed by partial melting of hybridised mafic crust with >70% contribution from juvenile, mantle-derived material in the form of lower crustal gabbros. Feldspar phenocrysts have near pure end-member compositions and many crystals display a patch antiperthite texture characterised by patches of Or98-99 in a host of Ab99, reflecting low-temperature dissolution–reprecipitation reactions during hydrothermal alteration. Leaching experiments, moreover, indicate that the Sr isotopic system of Sr-poor peralkaline lavas has been disturbed by interaction with a fluid with relatively low Rb/Sr and 87Sr/86Sr. The Nd and Pb isotopic systems do not appear significantly affected by this process.

Mechanisms of oxidation and degassing in the Takanoobane rhyolite lava of Aso Volcano, Japan

Vertical variation of the oxidation state as recorded by Fe–Ti oxides of Takanoobane rhyolite lava (TR lava) is revealed on the basis of petrographic and rock magnetic studies. Petrographic observations of the interior of TR lava showed that Fe–Ti oxides are preferentially oxidized along the flow structure that is characterized by the light-colored band composed of aggregates of deformed minute cavities, whereas both the top and bottom parts of the lava are not oxidized. Rock magnetic studies measuring thermal demagnetization of a three-component isothermal remanent magnetization (IRM) and magnetic susceptibility revealed that hematite of a highly oxidized state is formed preferentially along the light-colored band, consistent with the petrographic observations. The observed preferential oxidation of the light-colored band is adequately explained by post-eruptive degassing of hydrogen from the interior of the lava, rather than migration of atmospheric oxygen into the hot interior of the cooling lava against the back-streaming volcanic gas. The magmatic volatiles emitted from the interior can be transported effectively through the light-colored band and microcracks which ubiquitously develop nearly vertical to the flow direction. The degassing mechanism should be limited to highly viscous silicic lavas such as rhyolite because the mechanism requires development of the flow structure and the microcracks as the pathways for gases. These structures are achieved only in highly viscous silicic lavas.

The opaline silica-rich sedimentary rocks of Milos Island, Greece and their behaviour as pozzolanas in the manufacture of cement

In the Pliocene–Pleistocene succession of Milos Island, Greece, siliceous rocks of biogenic origin (diatomites) occur interbedded with volcanic tuffs. Locally lava sills penetrate the diatomaceous rocks causing their diagenesis. In order to identify the nature of lava and related tuffites and the influence that lava intrusion had on the diatomaceous rocks, X-ray diffraction, scanning electrom microscopy, X-ray fluorescence and light microscopy analyses were performed on representative samples. The original diatomaceous rocks are mainly composed of opal-A in the form of broken diatom frustules and well preserved silicoflagellate skeletons. The affected diatomite beds are characterised as porcelanites containing neoformed opal-CT lepispheres, in expense of opal-A. To utilise the diatomites and porcelanite as pozzolanas, measurements on the Blaine, water demand and compressive strength of cements were carried out. Slightly lower strength was measured in porcelanite cement, due to the higher crystallinity of opal-CT compared to opal-A. However, both rocks improve the pozzolanic cement properties compared to that currently produced, using glassy tuffs.

Geochemistry of the Chagai–Raskoh arc, Pakistan: Complex arc dynamics spanning the Cretaceous to the Quaternary

The Chagai–Raskoh arc is located in western Pakistan and extends into Iran and Afghanistan. The arc forms an elongate body trending EW and is roughly 500 km long by 150 km wide. Activity along the arc began in the Late Cretaceous and continued through into the Quaternary. The oldest volcanic rocks in the arc belong to the Sinjrani and Kuchakki Formations. These rocks are primarily basalts and andesites which form both pillow sequences and massive flows. Geochemically these units are very similar. They are tholeiitic lavas with typical island arc characteristics and an N-MORB source. For example when normalized to N-MORB they are large ion lithophile element (LILE) enriched, high field strength element (HFSE) depleted, and have negative Nb and Ti anomalies. Also within the Sinjrani Formation are a sequence of Senonian basalt–dacite lavas that were generated from an N-MORB type mantle source, and although they are initially tholeiitic the more evolved lavas are calc-alkaline. Trace element data indicates these lavas have a very minor continental affiliation. The generation of these lavas may mark the increasing proximity of the Afghan continental block. The youngest units in this study belong to the Quaternary Koh-e-Sultan and the Miocene Koh-e-Dalil. These units have a calc-alkaline fractionation trend and contain more silicic lavas, including dacites, than the older lavas. Chemically these units are very similar; they both contain continental arc signatures and were generated from low degrees of partial melting of an enriched source. Current theories to explain the multiple phases of volcanism in the Chagai–Raskoh arc propose that these lavas are the result of intra-oceanic convergence in the Neo-Tethys. Our data supports this model in that the initial phases of volcanism are entirely formed in an oceanic arc. However the increasing proximity of the Afghan Block, ca. 65 Ma, is evidenced by increasing continental signatures in the lavas, followed by much younger continental arc volcanism. We suggest the north–south distribution of oceanic versus continental signatures indicates that the location of the Afghan Block–Makran accretionary complex boundary is beneath the Chagai–Raskoh arc.

Episodic growth and homogenization of plutonic roots in arc volcanoes from combined U–Th and (U–Th)/He zircon dating

Tracing the fate of unerupted magma is challenging because plutonic roots of young volcanoes are largely inaccessible. Here we develop the use of zircon age spectra to determine crystal provenance and source rocks for volcanic products, in analogy to detrital crystals in sediments. U–Th zircon crystallization ages for the Soufrière Volcanic Complex, Saint Lucia (Lesser Antilles) frequently predate their eruption as determined from combined U–Th and (U–Th)/He zircon dating. The oldest dated eruptions are 273±15ka and 264±8ka (1σ uncertainty) for Morne Bonin dacite and Bellevue pumice deposit, respectively. The most recent eruptions formed morphologically pristine domes in the center of the Qualibou depression (Belfond: 13.6±0.4ka; Terre Blanche: 15.3±0.4ka). U–Th (U–Pb) zircon crystallization ages determined for crystal rims and interiors range between near-eruption ages to ∼600ka. Older xenocrysts are absent. Zircon crystallization age distributions are complex, yet systematic: crystal rim ages in the most recently erupted volcanic rocks match those of co-erupted plutonic inclusions, whereas crystal interiors are equivalent to the cumulative distribution of zircon ages from older eruptions. This is evidence that silicic lava domes and pyroclastic flows share a common source that is located underneath the Qualibou depression, where the intrusive roots of this long-lived arc volcanic system became homogenized through thermal and mechanical reprocessing of individual batches of unerupted magma from earlier volcanic episodes within timescales of <100ka.

Atmospheric control of the cooling rate of impact melts and cryolavas on Titan’s surface

As on Earth, Titan’s atmosphere plays a major role in the cooling of heated surfaces. We have assessed the mechanisms by which Titan’s atmosphere, dominantly N 2 at a surface pressure of 1.5 × 10 5 Pa, cools a warm or heated surface. These heated areas can be caused by impacts generating melt sheets and (possibly) by endogenic processes emplacing cryolavas (a low-temperature liquid that freezes on the surface). We find that for a cooling cryolava flow, lava lake, or impact melt body, heat loss is mainly driven by atmospheric convection. Radiative heat loss, a dominant heat loss mechanism with terrestrial silicate lava flows, plays only a minor role on Titan. Long-term cooling and solidification are dependent on melt sheet or flow thickness, and also local climate, because persistent winds will speed cooling. Relatively rapid cooling caused by winds reduces the detectability of these thermal events by instruments measuring surface thermal emission. Because surface temperature drops by ≈50% within ≈1 day of emplacement, fresh flows or impact melt may be difficult to detect via thermal emission unless an active eruption is directly observed. Cooling of flow or impact melt surfaces are orders of magnitude faster on Titan than on airless moons (e.g., Enceladus or Europa). Although upper surfaces cool fast, the internal cooling and solidification process is relatively slow. Cryolava flow lengths are, therefore, more likely to be volume (effusion) limited, rather than cooling-limited. More detailed modeling awaits constraints on the thermophysical properties of the likely cryomagmas and surface materials.

Late Shield-Stage Silicic Magmatism at Wai'anae Volcano: Evidence for Hydrous Crustal Melting in Hawaiian Volcanoes

Late shield-stage silicic (icelandite and rhyodacite) lavas and dikes are exposed within the caldera region of Wai‘anae Volcano, O’ahu, and comprise the most continuous silicic, subalkaline and transitional suites in Hawai‘i. The silicic rocks can be divided into two groups. Group I consists of transitional icelandite dikes and icelandite to basaltic icelandite lava flows that follow a tholeiitic liquid line of descent. Group II includes the Mauna Kūwale Rhyodacite and several intermediate-silica lavas that follow a K-rich, calc-alkaline evolutionary trend. Group II lavas are highly porphyritic, with biotite and hornblende in the rhyodacites, and are selectively enriched in Rb, U and Pb. Pb, Sr and Nd isotope ratios of a Group I dike are similar to those of other Wai‘anae basaltic samples, whereas Group II lavas have lower 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and e Nd , and high ratios of radiogenic 208 Pb*/ 206 Pb*, similar to some lavas from Ko’olau, Lāna‘i, and Kaho’olawe volcanoes. Most Group I dikes define a trend consistent with fractional crystallization at ∼0·5 kbar with low initial water (0·3 wt %) contents, whereas the chemical trends for Group II samples cannot be reproduced by simple crystallization models. These results, together with trace element modeling and isotopic characteristics, suggest derivation of the Group II silicic flows by hydrous melting of lower Hawaiian crust that had been altered to amphibolite. Crustal melting toward the end of the shield stage may be related to an increase in lithostatic pressure following rapid accumulation of lavas in the shield stage and ponding of basaltic magmas at depths below the level of crack closure (∼9–10 km). Contemporaneously, Group I silicic magmas formed by shallow-level crystallization as a consequence of decreased magma supply to the upper crust as a result of waning activity at the end of the shield stage. Geochemical evidence indicates significant mixing between the different types of silicic magma and between basaltic and silicic magmas at this stage of volcano evolution. Although melting of recycled crustal material entrained in the upwelling Hawaiian plume has previously been proposed, this is the first documentation of a role for melting of lower Hawaiian crust in the genesis of Hawaiian volcanoes.

プロト伊豆‐マリアナ島弧の衝突付加テクトニクス―レビュー―

This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny. Since the first pioneering work by Sugimura (1972), which placed the plate boundary on-land Japan north of the Izu peninsula collision-accretion tectonics of the Proto-Izu-Mariana arc against the Honshu arc started to be extensively investigated through multi-disciplinary methods. Aoike (1999) proposed a comprehensive scenario of five successive accretions of Proto-Izu to the present, with all bounded by top- and bottom-faults since 15 Ma. Oroclinal bending of the Honshu arc began simultaneously with the collision and indentation of the Proto-Izu arc, as is well documented by the Neogene paleomagnetic declination of sedimentary rocks in the Kanto district (Niitsuma et al., see summary, 1989). The entire Izu-Mariana arc is subducting without any accretion now under Honshu, as is well documented by seismic tomography imaging by Hasegawa et al. (see Hasegawa et al., 2010). Tanzawa Tonalite–Trondhjemite–Granodiolite (TTG) pluton intruded into the already-accreted Tanzawa Group at 4 Ma (Tani et al., 2010), resulting in a contact metamorphic aureole ranging from amphibolite through greenschist to zeolite facies at a depth of ca. 10 km, and tectonically denudated along the Kan-nawa fault to be exposed on the surface by 1.0 Ma. The Proto-Izu-Mariana arc was built on the Pre-Paleogene, presumably Cretaceous MORB crust on which boninitic and fractionated silicic lava flows accumulated since 48 Ma, which is older than the bending of the Hawaii–Emperor seamount chain at 43 Ma. Hence, we need another plate off Proto-Ogasawara arc, called North New Guinea plate (Seno, 1985), and ridge subduction underneath Ogasawara arc to initiate subduction magmatism. Both calk-alkali rock series (CA) and coeval blueschist–eclogite rocks are exposed along the inner wall of the Izu–Mariana trench. This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation (Kawate and Arima, 1998) and delaminated mafic lower crust (Nakajima and Arima, 1998). In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue. Instead, extensive tectonic erosion that transports not only the upper crust but also the lower mafic crust together with parts of the descending slab could be more plausible than these two models in order to present a thinner mafic lower crust.

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ∼ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ∼ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions. The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H 2O, several hundred ppm of CO 2, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ∼ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H 2O, 8700 ppm Cl, 6000 ppm F, and CO 2 below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ∼ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H 2O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.

New 40Ar/39Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley caldera, California

We undertook a 40 Ar/ 39 Ar study of young mafic and silicic lavas at Mammoth Mountain and the Long Valley caldera (east-central California) to better understand the frequency of these eruptions and the magmatic plumbing system that drives them. Our results show that most of Mammoth Mountain, a lava-dome complex straddling the southwestern topographic rim of the caldera, consists of trachydacite lavas erupted at ca. 68 ka. These ages and new 29- and 41 ka ages for trachydacite lavas in the northwest quadrant of the caldera indicate that these silicic lavas are considerably younger than previously thought. Mafic lavas vented widely in the western third of the caldera in the past 190,000 years, suggesting that this area has not been underlain by large bodies of silicic magma during this interval, as such magma would have prevented the rise of the denser basaltic magma. We identify four eruptive sequences over the past 190,000 years: the western moat sequence (∼190–160 ka), the Mammoth sequence (∼120–58 ka), the northwest caldera sequence (∼41–29 ka), and the Inyo chain sequence (∼9 ka–present). In each eruptive sequence mafic and silicic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of alkali basalt into the shallow crust led to the silicic eruptions. If the seismic unrest and deformation of the past three decades is a result of basalt injected beneath Mammoth Mountain and perhaps the western third of the caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards. Our data bear on Pleistocene glaciation in the region. Ages of 162 ± 2 ka and 99 ± 1 ka for bracketing mafic lava flows better constrain the age of the Casa Diablo till. Our results provide equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California (Glazner et al., 1999).

Clastic and core lava components of a silicic lava dome

The formation of a lava dome involves fractionation of the lava into core and clastic components. We show that for three separate, successive andesitic lava domes that grew at Soufriere Hills volcano, Montserrat, between 1999 and 2007, the volumetric proportion of the lava converted to talus or pyroclastic flow deposits was 50%–90% of the lava extruded. Currently, only 8% of the total magma extruded during the 1995–2007 eruption remains as core lava. The equivalent representation in the geological record will probably be even lower. Most of the lava extruded at the surface flowed no further than 150–300 m from the vent before disaggregation, resulting in a lava core whose shape tends to a cylinder. Moderate to high extrusion rates at the Soufriere Hills domes may have contributed to the large clastic fraction observed. Creating talus dissipates much of the energy that would otherwise be stored in the core lava of domes. The extreme hazards from large pyroclastic flows and blasts posed by wholesale collapse of a lava dome depend largely on the size of the lava core, and hence on the aggregate history of the partitioning process, not on the size of the dome.

Evolution of the late Pleistocene Mojanda–Fuya Fuya volcanic complex (Ecuador), by progressive adakitic involvement in mantle magma sources

The Mojanda–Fuya Fuya Volcanic Complex consists of two nearby volcanoes, Mojanda and Fuya Fuya. The older one, Mojanda volcano (0.6 to 0.2 Ma), was first constructed by andesites and high-silica andesites forming a large stratovolcano (Lower Mojanda). This edifice was capped by a basaltic andesite and andesitic cone (Upper Mojanda), which collapsed later to form a 3-km-wide summit caldera, after large phreatomagmatic eruptions. The Lower Fuya Fuya edifice was constructed by the extrusion of viscous Si-rich andesitic lavas and dacitic domes, and the emission of a thick sequence of pyroclastic-flow and fallout deposits which include two voluminous rhyolitic layers. An intermediate construction phase at Fuya Fuya is represented by a mainly effusive cone, andesitic in composition (San Bartolo edifice), the construction of which was interrupted by a major sector collapse in the Late Pleistocene. Finally, a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre, forming the Upper Fuya Fuya volcanic centre. This paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source. Although all the rocks of the complex are included in the medium-K field of continental arcs, the Fuya Fuya suite (61–75 wt.% SiO2) shows depletion in Y and HREE and high Sr/Y and La/Yb values, compared to the less silicic Mojanda suite (55–66.5 wt.% SiO2). The Mojanda calc-alkaline suite was generated by partial melting of an adakite-metasomatised mantle source that left a residue with 2% garnet, followed by fractional crystallization of dominant plagioclase + pyroxene + olivine at shallow, intra-crustal depths. For Fuya Fuya, geochemical and mineralogical data suggest either (1) partial melting of a similar metasomatised mantle with more garnet in the residue (4%), followed by fractional crystallization involving plagioclase, amphibole and pyroxene, or (2) mixing of mafic mantle-derived magma from the Mojanda suite and slab melts, followed by the same fractional crystallization process.

Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km 2 of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km 3, giving an overall effusion rate of ∼ 1.2 km 3 per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H 2O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.

Galileo observations of volcanic plumes on Io

Io's volcanic plumes erupt in a dazzling variety of sizes, shapes, colors and opacities. In general, the plumes fall into two classes, representing distinct source gas temperatures. Most of the Galileo imaging observations were of the smaller, more numerous Prometheus-type plumes that are produced when hot flows of silicate lava impinge on volatile surface ices of SO 2. Few detections were made of the giant, Pele-type plumes that vent high temperature, sulfur-rich gases from the interior of Io; this was partly because of the insensitivity of Galileo's camera to ultraviolet wavelengths. Both gas and dust spout from plumes of each class. Favorably located gas plumes were detected during eclipse, when Io was in Jupiter's shadow. Dense dust columns were imaged in daylight above several Prometheus-type eruptions, reaching heights typically less than 100 km. Comparisons between eclipse observations, sunlit images, and the record of surface changes show that these optically thick dust columns are much smaller in stature than the corresponding gas plumes but are adequate to produce the observed surface deposits. Mie scattering calculations suggest that these conspicuous dust plumes are made up of coarse grained “ash” particles with radii on the order of 100 nm, and total masses on the order of 10 6 kg per plume. Long exposure images of Thor in sunlight show a faint outer envelope apparently populated by particles small enough to be carried along with the gas flow, perhaps formed by condensation of sulfurous “snowflakes” as suggested by the plasma instrumentation aboard Galileo as it flew through Thor's plume [Frank, L.A., Paterson, W.R., 2002. J. Geophys. Res. (Space Phys.) 107, doi:10.1029/2002JA009240. 31-1]. If so, the total mass of these fine, nearly invisible particles may be comparable to the mass of the gas, and could account for much of Io's rapid resurfacing.

Geologic mapping of the Zal region of Io

We produced a regional geologic map of the Zal region of Io's antijovian hemisphere using Galileo mission data. We discuss the geologic features, summarize the map units and structures that are present, discuss the nature of volcanic activity, and present an analysis of the volcanic, tectonic, and gradational processes that affect the region. The Zal region consists of five primary types of geologic materials: plains, mountains, paterae floors, flows, and diffuse deposits. The flows and patera floors are similar, but are subdivided based on uncertainties regarding emplacement environments and mechanisms. The Zal region includes two hotspots detected by Galileo : one along the western scarp of the Zal Patera volcano and one at the Rustam Patera volcano (name submitted to IAU). A third hotspot at the nearby At'am Patera volcano (name submitted to IAU) is the source of diffuse and pyroclastic materials that blanket north Zal Mons. The western bounding scarp of Zal Patera is the location of a fissure vent that is the source of multiple silicate lava flows. The floor of Zal Patera has been partially resurfaced by dark lava flows, although portions of the patera floor appear bright and unchanged during the Galileo mission. This suggests that the floor did not undergo complete resurfacing as a flooding lava lake but does contain a compound flow field. Mountain materials exhibit stages of degradation; lineated material degrades into mottled material. We have explored the possibility that north and south Zal Mons were originally one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units, opened a fissure which serves as a vent for lava flow, and created a depression which, by further extension during the rifting event, became Zal Patera. With comparison to other regional maps of Io, this work provides insight into the general geologic evolution of Io.

Evidence for seismogenic fracture of silicic magma

It has long been assumed that seismogenic faulting is confined to cool, brittle rocks, with a temperature upper limit of approximately 600 degrees C (ref. 1). This thinking underpins our understanding of volcanic earthquakes, which are assumed to occur in cold rocks surrounding moving magma. However, the recent discovery of abundant brittle-ductile fault textures in silicic lavas has led to the counter-intuitive hypothesis that seismic events may be triggered by fracture and faulting within the erupting magma itself. This hypothesis is supported by recent observations of growing lava domes, where microearthquake swarms have coincided with the emplacement of gouge-covered lava spines, leading to models of seismogenic stick-slip along shallow shear zones in the magma. But can fracturing or faulting in high-temperature, eruptible magma really generate measurable seismic events? Here we deform high-temperature silica-rich magmas under simulated volcanic conditions in order to test the hypothesis that high-temperature magma fracture is seismogenic. The acoustic emissions recorded during experiments show that seismogenic rupture may occur in both crystal-rich and crystal-free silicic magmas at eruptive temperatures, extending the range of known conditions for seismogenic faulting.

Butalón igneous rocks, Neuquén, Argentina: Age, stratigraphic relationships and geochemical features

Three main groups of igneous rocks of different age are recognized in the Butalón area, including volcanic rocks of the Choiyoi Group (Permian–Triassic), stocks and dikes of the Granodiorita Varvarcó (Upper Cretaceous–Paleocene) and stocks and dikes of the Tonalita Butalón (Paleocene–Eocene?). The Choiyoi Group consists of ignimbrites, breccias, tuffs and silicic lavas. Most are subalkaline and four groups can be distinguished on the basis of their trace element patterns. Most samples are enriched in Ba, but depleted in Nb, Sr, P, Ti and Eu. The subvolcanic rocks of the Granodiorita Varvarcó and Tonalita Butalón are mainly calc-alkaline, metaluminous to peraluminous, with low to medium potassium contents. They have similar subparallel REE and multielement patterns with low La/Yb and Eu/Eu ∗ ratios. These patterns mimic those observed in Pleistocene–Holocene volcanic rocks, of similar SiO 2 content, from the CSVZ of the Andes. They are depleted in Nb and Ti, have normal to low P, and are enriched in Sr and Zr. Hornfels-skarn type hydrothermal alteration is produced by Tonalita Butalón in the adjacent igneous rocks. Magmatic-hydrothermal breccias are developed on top of some of the stocks. While some of the breccias exhibit Mo anomalies, Au and Ag anomalies are present in the Choiyoi Group.