Eruption characteristics and cycles at Pavlof Volcano, Alaska, and their relation to regional earthquake activity

Pavlof Volcano (55° 25′N, 161° 54′W) exhibits two eruption styles: magmatic eruptions of one-to-two-days duration, and phreatic-phreatomagmatic activity lasting several days to two months. Thirty-four eruptions have occurred in historic times; of these the largest are Volcano Explosivity Index=3. Nine magmatic and 13 phreatomagmatic eruptions occurred between 1973–1983. All the magmatic eruptions occurred in the fall, between Sept. 9–Nov. 20. Four magmatic eruptions occurred during November 11–15, but in four different years. A 3-year-long period of eruptive activity between 1973–1976 bears striking resemblance to a period of activity between 1980–1983. No locatable shallow earthquakes (<50 km) have occurred within 30 km of Pavlof since 1973, which is quite unusual for an active island-arc volcano. Shallow events in the adjacent are segments have focal mechanisms with P-axes perpendicular to the arc (and parallel to plate convergence). Deep earthquakes (> 100 km) are clustered beneath Pavlof and several other volcanoes. Their T-axes show downdip tension within the slab. Deep teleseisms (> 160 km) mostly occurred between 1977–1979 when the volcano was not erupting. Catalogued volcanic activity throughout the Alaska/Aleutian arc shows a weak tendency to increase around the time of great (M > 7.8) earthquakes.     

Calc-alkaline and alkaline miocene and calc-alkaline recent volcanism in the Southernmost Patagonian Cordillera, Chile

In the southernmost Patagonian Cordillera (south of Lat. 54°45′) previously unrecorded calc-alkaline dacites and andesites, and adjacent alkali-basalts, were dated at 21 and 18 Ma (K-Ar), respectively. In the same area post-glacial low-K calc-alkaline andesites, identified at Isla Cook, represent the southernmost Recent volcanism recorded so far in South America, about 400 km south of Monte Burney, previously known as the southernmost Recent volcano. The association of contemporaneous calc-alkaline and alkali volcanics suggests a subduction-related environment for the Miocene volcanism, and the Recent calc-alkaline volcanics indicate that currently there is a small component of subduction of the Antarctic Plate beneath the South American part of the Scotia Plate.     

Mantle flow and melting beneath oceanic ridge–ridge–ridge triple junctions

Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This study investigates how triple junction (TJ) geometry may influence mantle dynamics. An earlier study [Georgen, J.E., Lin, J., 2002. Three-dimensional passive flow and temperature structure beneath oceanic ridge-ridge-ridge triple junctions. Earth Planet. Sci. Lett. 204, 115–132.] suggested that the effects of a ridge–ridge–ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity model that focuses on the slowest-diverging ridge of a triple junction with geometry similar to the Rodrigues TJ. This spreading axis may be considered to be analogous to the Southwest Indian Ridge. Within 100 km of the TJ, temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by ~ 40 °C and 1 km, respectively. We also investigate the effects of differential motion of the TJ with respect to the underlying mantle, by imposing bottom model boundary conditions replicating (a) absolute plate motion and (b) a three-dimensional solution for plate-driven and density-driven asthenospheric flow in the African region. Neither of these basal boundary conditions significantly affects the model solutions, suggesting that the system is dominated by the divergence of the surface places. Finally, we explore how varying spreading rate magnitudes affects TJ geodynamics. When ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to the Azores TJ), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~ 150 °C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the TJ. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of TJ geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.     

Experimental evidence for the role of accessory phases in magma genesis

Recent experimental studies have established petrogenetic models based on melting processes involving major phases. The possible residual character of trace-element-enriched accessory phases is not considered for temperatures well above the solidus in these models. In contrast, geochemists, applying trace element data to independently test the experimentally-based models, have concluded that residual (or fractionating) accessory phases may have an essential role in controlling the trace element (especially REE) distributions in magmas.Some recent experimental work provides data on the stability of potentially significant accessories such as sphene, rutile, apatite, zoisite and mica in basaltic compositions at elevated P and T. Sphene is stable to 1000°C with 60% melting of a hydrous tholeiite at 15 kbar. At higher pressure, rutile is the only Ti-rich accessory phase, and is present to at least 1000°C and high degrees of melting. Published REE data on sphene and rutile suggest that these phases may be important in controlling REE distribution in some magmas. For example, island are high-Mg, low-Ca-Ti tholeiites with low REE abundances and U-shaped patterns (Hickey and Frey, 1979) may reflect the role of sphene. In addition to rutile, similar close-packed Ti-rich accessory phases such as priderite, perovskite, crichtonite and loveringite may occur in mantle-derived magmas. These phases readily accommodate the REE but their possible role needs experimental confirmation.Apatite is recorded in hawaiite (1.16% P₂O_s) with 2% H₂O added at 5–6 kbar and 1050°C within 30°C of the liquidus, but at present no other experimental data are available on its high P, T stability, although thermodynamic calculations indicate that F may increase its stability markedly. Apatite is well known in high-pressure inclusions and as a phenocryst phase in rocks of the alkaline and calc-alkaline series.Ilmenite is known as a near-liquidus phase in some mafic magmas at 5–10 kbar, but its stability decreases to near-solidus at 25–30 kbar. Zoisite occurs in hydrous mafic compositions at mantle pressures, but it is confined to temperatures < 780°C. Finally, mica has a wide temperature range of stability at mantle pressures, especially in potassic magmas, and phlogopitic mica is stable to 1040°C at 20–25 kbar in a hydrous, K-rich “tholeiite” (1.6% K₂O).     

Tectonics of the Nazca-Antarctic plate boundary

We have constructed a new bathymetric chart of part of the Chile transform system, based mainly on an R/V “Endeavor” survey from 100°W to its intersection with the East Ridge of the Juan Fernandez microplate at 34°30′S, 109°15′W. A generally continuous lineated trend can be followed through the entire region, with the transform valley being relatively narrow and well-defined from 109°W to approximately 104°30′W. The fracture zone then widens to the east, with at least two probable en echelon offsets to the south at 104° and 102°W. Six new strike-slip mechanisms along the Chile Transform and one normal fault mechanism near the northern end of the Chile Rise, inverted together with other plate motion data from the eastern portion of the boundary, produce a new best fit Euler pole for the Nazca-Antarctic plate pair, providing tighter constraints on the relative plate motions.     

Alvin-SeaBeam studies of the Sumisu Rift, Izu-Bonin arc

Bimodal volcanism, normal faulting, rapid sedimentation, and hydrothermal circulation characterize the rifting of the Izu-Bonin arc at 31°N. Analysis of the zigzag pattern, in plan view, of the normal faults that bound Sumisu Rift indicates that the extension direction (080° ± 10°) is orthogonal to the regional trend of the volcanic front. Normal faults divide the rift into an inner rift on the arc side, which is the locus for maximum subsidence and sedimentation, and an outer rift further west. Transfer zones that link opposing master faults and/or rift flank uplifts further subdivide the rift into three segments along strike. Volcanism is concentrated along the ENE-trending transfer zone which separates the northern and central rift segments. The differential motion across the zone is accommodated by interdigitating north-trending normal faults rather than by ENE-trending oblique-slip faults. Volcanism in the outer rift has built 50–700 m high edifices without summit craters whereas in the inner rift it has formed two multi-vent en echelon ridges (the largest is 600 m high and 16 km long). The volcanism is dominantly basaltic, with compositions reflecting mantle sources little influenced by arc components. An elongate rhyolite dome and low-temperature hydrothermal deposits occur at the en echelon step in the larger ridge, which is located at the intersection of the transfer zone with the inner rift. The chimneys, veins, and crusts are composed of silica, barite and iron oxide, and are of similar composition to the ferruginous chert that mantles the Kuroko deposits. A 1.2-km transect of seven alvin heat flow measurements at 30°48.5′N showed that the inner-rift-bounding faults may serve as water recharge zones, but that they are not necessarily areas of focussed hydrothermal outflow, which instead occurs through the thick basin sediments. The rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as Sumisu and South Sumisu volcanoes.     

Chronology of cenozoic igneous and tectonic events in the central Chilean Andes — latitudes 35° 30′ to 36°S

Sixty-six K---Ar dates from igneous rocks in the central Chilean Andes between 33° and 38°S are reported in this study. From these results and observed field relations, major Cenozoic volcanic and intrusive rock units are divided into chronologic groups representing igneous events.Volcanic units of Oligocene (33.3–27.9 m.y.) and Early Miocene (20.2 m.y.) age have been dated west of the present range at 33°S but neither the magnitude nor extent of these volcanic events has yet been established. Extensive Middle to Late Miocene volcanism (15.3–6.4 m.y.) followed by regional folding is recognized in the map area between 35° 20′ and 36°S. Partly contemporaneous Middle Miocene volcanism (18.4–13.7 m.y.) also followed by regional folding is recorded in the Andes between 37° 30′ and 38°S. General volcanic quiescence from 6.4 to 2.5 m.y. is observed in the map area but whether this volcanic hiatus is of regional significance is not known.The majority of the K---Ar dates document a history of nearly continuous volcanism throughout the last 2.5 m.y. in the map area. The abundant and diverse sequences of volcanic strata formed during this time, have been divided into four successive age groups which as map units show the evolution and distribution of latest volcanic activity.Landforms preserved by this volcanic series show that topographic relief similar to the present has prevailed during this time. Deep incision of rivers into young volcanic terrain, estimated to be on the order of 1–2 m/1000 years, has produced a complex volcanic and morphologic record.Four plutons dated in this study give ages of 62.0, 41.3, 19.5, and 7.0 m.y. No spatial pattern of emplacement is observed in the map area where three of these plutons are represented.Similarities in structural style, orientation and degree of deformation of Miocene and Mesozoic strata suggest that Late Miocene regional folding may have accounted for a significant part of the observed deformation in older basement strata previously ascribed to earlier orogenies.A regional comparison of ages of recognized igneous and tectonic event at different latitudes in the central and southern Andes shows the gross chronology of Cenozoic events which can be correlated with sea-floor spreading and subduction events.     

Deglacial pattern of circulation and marine productivity in the upwelling region off central-south Chile

A high-resolution sea surface temperature and paleoproductivity reconstruction on a sedimentary record collected at 36°S off central-south Chile (GeoB 7165-1, 36°33′S, 73°40′W, 797 m water depth, core length 750 cm) indicates that paleoceanographic conditions changed abruptly between 18 and 17 ka. Comparative analysis of several cores along the Chilean continental margin (30°–41°S) suggests that the onset and the pattern of deglacial warming was not uniform off central-south Chile due to the progressive southward migration of the Southern Westerlies and local variations in upwelling. Marine productivity augmented rather abruptly at 13–14 ka, well after the oceanographic changes. We suggest that the late deglacial increase in paleoproductivity off central-south Chile reflects the onset of an active upwelling system bringing nutrient-rich, oxygen-poor Equatorial Subsurface Water to the euphotic zone, and a relatively higher nutrient load of the Antarctic Circumpolar Current. During the Last Glacial Maximum, when the Southern Westerlies were located further north, productivity off central-south Chile, in contrast to off northern Chile, was reduced due to direct onshore-blowing winds that prevented coastal upwelling and export production.     

Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific)

The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets.During the January 1987 Rapanui expedition on the N/O “Jean Charcot” we conducted a Sea Beam/magnetics/ gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern boundary of the microplate is expressed as a series of parallel NW-SE trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift).The northern end of the Pito Rift merges with an E-W to 083° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates.The northern triple junction between the Easter (Rapanui), Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately E-W direction.The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E-W zone where shear must be predominant.The southern triple junction is a RRF junction with an overlapping ridge system.The structural data acquired during the survey provide strong constraints for kinematic models of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.     

Groundmass pargasite in the 1991–1995 dacite of Unzen volcano: phase stability experiments and volcanological implications

Pargasite commonly occurs in the dacitic groundmass of the 1991–1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991–1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe–Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880±30°C, suggesting that the groundmass pargasite crystallized at >70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991–1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.     

Seasonal variations in pCO2 and its controlling factors in surface seawater of the northern East China Sea

Surface partial pressure of CO₂ (pCO₂), temperature, salinity, nutrients, and chlorophyll a were measured in the East China Sea (ECS; 31°30′–34°00′N to 124°00′–127°30′E) in August 2003 (summer), May 2004 (spring), October 2004 (early fall), and November 2005 (fall). The warm and saline Tsushima Warm Current was observed in the eastern part of the survey area during four cruises, and relatively low salinity waters due to outflow from the Changjiang (Yangtze River) were observed over the western part of the survey area. Surface pCO₂ ranged from 236 to 445 μatm in spring and summer, and from 326 to 517 μatm in fall. Large pCO₂ (values >400 μatm) occurred in the western part of the study area in spring and fall, and in the eastern part in summer. A positive linear correlation existed between surface pCO₂ and temperature in the eastern part of the study area, where the Tsushima Warm Current dominates; this correlation suggests that temperature is the major factor controlling surface pCO₂ distribution in that area. In the western part of the study area, however, the main controlling factor is different and seasonally complex. There is large transport in this region of Changjiang Diluted Water in summer, causing low salinity and low pCO₂ values. The relationship between surface pCO₂ and water stability suggests that the amount of mixing and/or upwelling of CO₂-rich water might be the important process controlling surface pCO₂ levels during spring and fall in this shallow region. Sea–air CO₂ flux, based on the application of a Wanninkhof [1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97, 7373–7382] formula for gas transfer velocity and a set of monthly averaged satellite wind data, were −5.04±1.59, −2.52±1.81, 1.71±2.87, and 0.39±0.18 mmol m⁻² d⁻¹ in spring, summer, early fall, and fall, respectively, in the northern ECS. The ocean in this study area is therefore a carbon sink in spring and summer, but a weak source or in equilibrium with the atmosphere in fall. If the winter flux value is assumed to have been the mean of autumnal and vernal values, then the northern ECS absorbs about 0.013 Pg C annually. That result suggests that the northern ECS is a net sink for atmospheric CO₂, a result consistent with previous studies.     

A Jurassic-Cretaceous island arc in the Ladakh-Himalayas

The Ladakh Mesozoic ophiolite belt (western Himalaya) contains a pile of volcanic thrust sheets (Dras unit) which differ significantly in structure and composition from the ophiolitic mélange zones. The Dras unit is composed of pillow lavas, doleritic sills, very irregular basaltic (?basaltic andesites) and dacitic flows intercalated with pyroclastics, volcanoclastic sediments and radiolarian cherts. According to fossil evidence, this volcanism must have been active between Upper Jurassic and Upper Cretaceous.The presence of relict primary minerals, such as magnesiochromite, clinopyroxene, hastingsitic hornblende and Ti-magnetite as well as distinctive bulk chemistries, suggests that the volcanics belong to island arc tholeiite and to calc-alkaline rock series, typical of present island arcs in the Caribbean and Pacific.Model calculations incorporating probed phenocryst phases indicate that in addition to olivine, clinopyroxene and plagioclase, amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt. The latter process must have taken place at about 1000°C and at moderate depth of 5–15 km within or underneath the island arc. Today, hornblende-bearing mafic cumulates appear in the vicinity of Kargil within and close to the Dras volcanics.In a Sr-evolution diagram, the Dras volcanics have yielded a “pseudo-isochron” with a low initial ratio of 0.7035 ± 0.0003, which is in the same range as the mean of modern island arc volcanics. However, a geologically unrealistic age of 263 m.y., is obtained from the slope of this isochron.The upper mantle is regarded as the source material for the island arc tholeiitic magmas. Enrichment in K, Ba, Sr and LREE supports the involvement of components derived from dehydration or incipient melting of subducted Tethyan oceanic crust in the mantle.     

SO2 from episode 48A eruption,Hawaii: Sulfur dioxide emissions from the episode 48A East Rift Zone eruption of Kilauea volcano,Hawaii

An SO₂ flux of 1170±400 (1) tonnes per day was measured with a correlation spectrometer (COSPEC) in October and November 1986 from the continuous, nonfountaining, basaltic East Rift Zone eruption (episode 48A) of Kilauea volcano. This flux is 5–27 times less than those of highfountaining episodes, 3–5 times greater than those of contemporaneous summit emissions or interphase Pu'u O'o emissions, and 1.3–2 times the emissions from Pu'u O'o alone during 48A. Calculations based on the SO₂ emission rate resulted in a magma supply rate of 0.44 million m³ per day and a 0.042 wt% sulfur loss from the magma upon eruption. Both of these calculated parameters agree with determinations made previously by other methods.     

Thermal areas on Kilauea and Mauna Loa Volcanoes, Hawaii

Active thermal areas are concentrated in three areas on Mauna Loa and three areas on Kilauea. High-temperature fumaroles (115–362° C) on Mauna Loa are restricted to the summit caldera, whereas high-temperature fumaroles on Kilauea are found in the upper East Rift Zone (Mauna Ulu summit fumaroles, 562° C), middle East Rift Zone (1977 eruptive fissure fumaroles), and in the summit caldera. Solfataric activity that has continued for several decades occurs along border faults of Kilauea caldera and at Sulphur Cone on the southwest rift zone of Mauna Loa. Solfataras that are only a few years old occur along recently active eruptive fissures in the summit caldera and along the rift zones of Kilauea. Steam vents and hot-air cracks also occur at the edges of cooling lava ponds, on the summits of lava shields, along faults and graben fractures, and in diffuse patches that may reflect shallow magmatic intrusions.     

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km² area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.     

Generation of Icelandic rhyolites: silicic lavas from the Torfajökull central volcano

The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (225 km³). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO₂) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An_10-40), Na-rich anorthoclase (<Or₂₃), clinopyroxene (Fs_37-20), FeTi oxides (Usp_32-60; Ilm_93-88), hornblende (edenitic–ferroedenitic) and olivine (Fo_22-37), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The values of the silicic lavas are in the range 3.6–4.4, and these are lower than the values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations, depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low- , hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust.     

Geochemical approach to magmatic evolution of Mt. Erciyes stratovolcano Central Anatolia, Turkey

Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO₂). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low ⁸⁷Sr/⁸⁶Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.     

Mantle source compositions of the Paraná Magmatic Province (southern Brazil): evidence from trace element and Sr–Nd–Pb isotope geochemistry

An integrated study based on incompatible trace elements and Sr–Nd–Pb isotopes is presented in order to assess the mantle sources involved in the genesis of the Paraná Magmatic Province (PMP) tholeiites. Particular emphasis is given to 33 new Pb isotope and concentration data obtained in representative samples of low-TiO₂ (LTiB) and high-TiO₂ (HTiB) flood basalts that occur in the province. Results show important differences with respect to type and location of these rocks. The LTiB and HTiB from northern PMP exhibit very similar initial Pb isotope ratios (average LTiB vs HTiB: ²⁰⁶Pb/²⁰⁴Pb_i=17.78±0.03 vs 17.65±0.02; ²⁰⁷Pb/²⁰⁴Pb_i=15.53±0.01 vs 15.52±0.01; ²⁰⁸Pb/²⁰⁴Pb_i=38.12±0.03 vs 38.05±0.04). The LTiB from southern PMP, with initial ⁸⁷Sr/⁸⁶Sr_i≤0.7060, show small variation in initial Pb isotope compositions (average ²⁰⁶Pb/²⁰⁴Pb_i=18.20±0.07; ²⁰⁷Pb/²⁰⁴Pb_i=15.61±0.01; ²⁰⁸Pb/²⁰⁴Pb_i=38.32±0.10), which are highly enriched in radiogenic Pb in comparison to the northern PMP analogues. The HTiB from southern PMP have initial Pb isotope ratios (average ²⁰⁶Pb/²⁰⁴Pb_i=17.45±0.09; ²⁰⁷Pb/²⁰⁴Pb_i=15.50±0.01; ²⁰⁸Pb/²⁰⁴Pb_i=37.89±0.03) slightly less radiogenic compared with the HTiB from northern PMP. The data cover a large range of isotope compositions, which are accompanied by systematic changes in incompatible trace element ratios and Sr–Nd isotopes, indicating contributions from different mantle sources. The remarkable chemical and isotope differences between PMP basalts, N-MORB and Tristan da Cunha least evolved volcanics indicate that these asthenospheric sources did not play a significant role in the basalt genesis, suggesting generation from the melting of heterogeneous lithospheric mantle sources. The close similarity between the radiogenic isotopes of the Cretaceous carbonatites that surround the PMP and those of the HTiB rock-types and the LTiB from the northern Paraná suggests the involvement of the same mantle components in their genesis: a dominant EMI end member and a radiogenic isotope enriched component of EMII-type, as some phlogopite-peridotite mantle xenoliths (Japecanga) from the Alto Paranaíba Igneous Province. The latter component seems also to have an important role in the origin of the LTiB from the southern Paraná, where the other end member is highly depleted in radiogenic lead similar to DMM.     

Evaluation of volcanic gas analysis from surtsey volcano, iceland 1964–1967

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H₂O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H₂O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H₂S, S₂, and H₂. The estimated gas compositions are relatively CO₂-rich, low in total sulfur and reduced. They contain approximately 35–50% CO₂ 45–55% H₂O, 1–2% SO₂, 1–2% H₂., 2–3% CO, 1.5–2.5% H₂S, 0.5% S₂ and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f_O2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f_O2 buffered by the rock, results in gas compositions very rich in CH₄ (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH₄-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.     

Possible contribution of the asthenosphere, below the subducted oceanic lithosphere, to the genesis of arc magmas: Geochemical evidence from the andean southern volcanic zone (33–46°S)

The application of the Sr/Ca-Ba/Ca systematics to volcanic rocks of the Andean Southern Volcanic Zone (33°S–46°S) has revealed a good correlation between the estimated degree of partial melting required to generate primary magmas and the projected extensions of the oceanic Nazca plate fracture zones under the continental South American plate. Magmas erupted at volcanic centers situated above these projections are thought to have been derived from primary magmas generated by relatively high degrees of melting, whereas those erupted at other centers are thought to have evolved from magmas produced by comparatively low degree of fusion. We interpret this relationship to reflect the facilitation of heat and mass transfer from the asthenosphere below the subducted oceanic lithosphere to the subarc mantle by the fracture zones. This contribution enhances the degree of melting of the subarc mantle source as well as the fraction of material derived from the subducted oceanic crust. This model predicts the predominance of basalts depleted in incompatible trace elements in centers located above the Nazca plate fracture zone extensions and of basalts enriched in incompatible trace elements in centers situated between boundaries of fracture extensions.