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1

Trace-element partitioning in pantellerites and trachytes  

SciTech Connect

In order to investigate the effect of increasing melt peralkalinity on partitioning, partition coefficients have been determined using neutron activation analyses of coexisting phenocrysts and glass of five samples from Pantelleria spanning the range trachyte to pantellerite. Alkali feldspar partition coefficients for Fe, Rb, Ba, Sr, and Eu vary with melt peralkalinity due to changes in melt polymerization and to the systematic increase in X{sub or} and decrease in X{sub an} of the feldspar. In going from trachyte to pantellerite, Fe partition coefficients increase from 0.04 to 0.10, presumably because Fe{sup +3} increasingly substitutes in the feldspar tetrahedral site as melt activity of Al declines and Fe concentrations increase. Partition coefficients for trivalent light REEs (rare earth elements) decrease and the partitioning pattern becomes flatter, the most evolved samples having some of the lowest published values for feldspar. The hundredfold decline in Eu partition coefficients (2.5 to 0.024) and the decrease in the size of the positive partitioning anomaly are attributed to increasing Eu{sup 3+}/Eu{sup 2+} in the melt as it becomes more peralkaline, as well as to concomitant decrease in the Ca content of feldspar. As a result, the behavior of Eu during fractional crystallization of peralkaline suites is fundamentally different from that in metaluminous suites; absolute abundances rise and the size of the negative Eu anomaly changes little with fractionation beyond pantelleritic trachyte.

Mahood, G.A.; Stimac, J.A. (Stanford Univ., CA (USA))

1990-08-01

2

Severe leaching of trachytic glass without devitrification, Terceira, Azores  

SciTech Connect

Pumice produced during the subplinian phase of eruption of a peralkaline trachyte lava on the island of Terceira, Azores, has undergone extensive leaching at ambient conditions by groundwater, while remaining in a glassy state. Over 25% of F, Na, K, Si, Fe, Ti, and Mn originally present has been removed, whereas U, Al, Nb, Ca, Y, and Rb show smaller but significant losses. Thorium , Zr, Hf, Ta, and Lu have remained immobile. Water content (measured as LOI) has increased up to tenfold, whereas the Sr content has risen by up to 80%. The absence of devitrification requires that leached elements escaped by diffusing through the glass; a coefficient of chemical diffusion D[sub Na] between 1.8 X 10[sup [minus]19] cm[sup 2] s[sup [minus]1] and 6.1 X 10[sup [minus]19] cm[sup 2] s[sup [minus]1] has been calculated, in excellent agreement with previously published experimentally derived values for comparably low temperatures. Water has not replaced the leached cations on a charge-equivalent basis, a finding that requires an explanation other than simple binary interdiffusion. The authors suggest that a simultaneous influx of molecular water and cation exchange between Na[sup +] in the glass and H[sup +] in the groundwater opens the glass structure and frees cations, which can diffuse outward by hydrolysis of bridging oxygen atoms. Reported mobilities have important implications for such diverse fields as igneous petrology, nuclear waste management, and agriculture.

Mungall, J.E.; Martin, R.F. (McGill Univ., Montreal, Quebec (Canada))

1994-01-01

3

Geology and geochemistry of the Ol Doinyo Nyokie trachyte ignimbrite vent complex, south Kenya rift valley  

Microsoft Academic Search

The Ol Doinyo Nyokie complex is of late Pleistocene age and occurs in the floor of the south Kenya rift valley. It consists of a shallow depression 5 km long and 3 km wide occupied by ash-flows, surrounded by a zone of trachyte dykes, and with a dome-shaped ignimbrite vent at its eastern end. The complex began to form approximately

B. H. Baker

1975-01-01

4

Geology and geochemistry of the Ol Doinyo Nyokie trachyte ignimbrite vent complex, south Kenya rift valley  

Microsoft Academic Search

The Ol Doinyo Nyokie complex is of late Pleistocene age and occurs in the floor of the south Kenya rift valley. It consists\\u000a of a shallow depression 5 km long and 3 km wide occupied by ash-flows, surrounded by a zone of trachyte dykes, and with a\\u000a dome-shaped ignimbrite vent at its eastern end. The complex began to form approximately

B. H. Baker

1975-01-01

5

Crystallization kinetics of alkali feldspars in cooling and decompression-induced crystallization experiments in trachytic melt  

NASA Astrophysics Data System (ADS)

Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspar, the effect of the degree of undercooling ( ?T = T liquidus - T experimental) and time on nucleation and crystal growth process. This experimental work gives us new data about crystallization kinetics of trachytic melts, and it that will be useful to better understand the natural system of Campi Flegrei volcanoes. Experiments have been conducted using cold seal pressure vessel apparatus, at pressure between 30 and 200 MPa, temperature between 750 and 855 °C, time between 7,200 and 57,600 s and redox condition close to the NNO +0.8 buffer. These conditions are ideal to reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes, where the "conditions" pertain to the complete range of pressures, temperatures and time at which the experiments were performed. Alkali feldspar is the main phase present in this trachyte, and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the crystallization kinetics. The obtained results show that crystallization kinetics are strictly related to ?T, time, final pressure, superheating (- ?T) and water content in the melt. ?T is the driving force of the crystallization, and it has a strong influence on nucleation and growth processes. In fact, the growth process dominates crystallization at small ?T, whereas the nucleation dominates crystallization at large ?T. Time also is an important variable during crystallization process, because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations. This is an important aspect to understand magma evolution in the magma chamber and in the conduit, which in turn has strong effects on magma rheology.

Arzilli, Fabio; Carroll, Michael R.

2013-10-01

6

Polybaric Evolution of Phonolite, Trachyte, and Rhyolite Volcanoes in Eastern Marie Byrd Land, Antarctica: Controls on Peralkalinity and Silica Saturation  

Microsoft Academic Search

In the Marie Byrd Land volcanic province, peralkaline and metaluminous trachytes, phonolites, and rhyolites occur in 18 large shield volcanoes that are closely associated in time and space. They are arrayed radially across an 800 km wide structural dome, with the oldest at the crest and the youngest around the flanks. Several lines of evidence suggest that these rocks evolved

Wesley E. LeMasurier; Kiyoto Futa; Malcolm Hole; Yosuke Kawachi

2003-01-01

7

Structural study of the Foréké-Dschang trachytic dome (Mount Bambouto, West Cameroon): An anisotropy of magnetic susceptibility (AMS) approach  

NASA Astrophysics Data System (ADS)

The Tertiary trachytic dome of Foréké-Dschang is located on the southwestern slope of Mount Bambouto. This dome, made of porphyritic and microlitic lavas which enclose sanidine, pyroxene and oxides, is dated at 16-8.8 Ma and was emplaced in gneisses belonging to the Pan-African basement. The mean magnetic susceptibility magnitude Km values of the trachytic dome show a ferromagnetic behavior in 79% of the stations, likely due to the presence of more or less maghemitized titanomagnetite based on thermomagnetic and hysteresis data. The Foréké-Dschang dome is elliptical and strikes NE-SW, parallel to a border fracture. The Pan-African host gneisses display N70° striking and steeply dipping foliations crosscut by the dome. Some Tertiary sinistral reworking of these foliations may have induced the opening of a local tension gash in agreement with the regional model suggesting that the N30° Cameroon Volcanic Line opened as a mega-tension gash with respect to the N70° Adamawa fault zone. Two petrographic types are recognized in the trachytic dome, aphyric (domain I) or porphyritic (domain II). The magnetic foliations show an outward-dipping concentric pattern in domain I. A flat lying zone with the highest magnetic anisotropy (18%) is regarded as overlying the vent. The organization of magnetic fabrics suggests that domain II was emplaced before domain I. As a whole, the Foréké-Dschang trachytes represent a multilobate dome emplaced in a pulsatory manner.

Bella Nké, B. E.; Njanko, T.; Kwékam, M.; Njonfang, E.; Naba, S.; Tcheumenak, K. J.; Gountié, M.; Rochette, P.; Nédélec, A.

2014-07-01

8

Geology, Geochemistry, and Petrology of the Alkaline Subvolcanic Trachyte-Hosted Iron Deposit in the Karakuz Area, Northwestern Hekimhan-Malatya, Turkey  

Microsoft Academic Search

The Karakuz iron deposit is hosted by Maastrichtian subvolcanic trachyte-trachyandesite, which has a total alkali content of 9-13 wt%, with Na2O less than 1% of the total alkalies. SiO2, Al2O3, and Fe2O3, in order of relative abundance, are other common oxides, whereas the P2O5 concentration is less than 0.5% in all of the host-rock samples. The trachytic host rock has

Ali Ucurum; Lawrence T. Larson; Durmus Boztug

1996-01-01

9

Clinopyroxene/liquid trace element partitioning in natural trachyte-trachyphonolite systems: insights from Campi Flegrei (southern Italy)  

NASA Astrophysics Data System (ADS)

Trace element partition coefficients between clinopyroxenes and associated glassy matrix (Cpx/L D) have been determined for 13 REE, HFSE4+,5+, U, Th, Sr, Pb, Sc and V from combined LA-ICP-MS/EMP analyses in selected trachytes and trachyphonolites from Campi Flegrei. Composition of clinopyroxene and glass is pretty homogeneous in the trachyphonolites, pointing to an overall attainment of the equilibrium conditions. In trachytes, conversely, phases show some compositional heterogeneity (due to the presence of clinopyroxene xenocrysts) that requested a more careful petrographic and geochemical inspection of the samples to assess the equilibrium clinopyroxene composition. In the trachyte clinopyroxenes, REE are compatible from Nd to Lu (Cpx/L D up to 2.9), like Y, Ti, Sc and V. The Cpx/L D for Eu is lower than those of the adjacent REE, highlighting Eu2+ contribution. High D values are also shown by U, Th, Pb, Zr, Hf, Nb and Ta relatively to basaltic and andesitic systems, whereas the D Sr is roughly similar to that found for less evolved magmas. Trachyphonolites are characterized by an overall decrease of the Cpx/L D for highly-charged cations (with the exception of V), and by a slight increase of D Sr. REE are still compatible from Nd to Lu (Cpx/L D up to 2.1), like Ti, Y, Sc and V. This variation is also predicted for REE and Y by models based on the elastic strain theory, being consistent with the slightly lower polymerization degree estimated for the trachyphonolites. However, the observed Cpx/L D (REE,Y) are matched by the modelled ones only considering very low T (?825°C), which are believed unlikely. This mismatch cannot be attributed to effects induced by the water-rich composition of the trachyte-trachyphonolite suite, since they would lower the observed Cpx/L D (REE,Y). Moreover, the anomalous inflections of measured Cpx/L D for HREE suggests some crystal-chemical control, such as the entrance of these elements in a site distinct from M2. It is concluded that the large Cpx/L D determined for trachytes and trachyphonolites are likely induced by hitherto unconstrained changes of the Z3+ activities related to the composition of melt and/or solid. All these considerations strongly highlight the importance of a direct characterization of trace element partitioning in natural samples from magmatic systems poorly characterized by experimental studies.

Fedele, Lorenzo; Zanetti, Alberto; Morra, Vincenzo; Lustrino, Michele; Melluso, Leone; Vannucci, Riccardo

2009-09-01

10

Volcanic and magmatic evolution of a small trachytic vent complex, north Burro Mesa, Big Bend National Park, Texas  

USGS Publications Warehouse

Volcanic rocks exposed on the northern end of Burro Mesa in Big Bend National Park portray the evolution of an Oligocene central volcanic vent complex that produced two generations of welded block and ash deposits associated with 1) initial dome collapse and 2) subsequent central spine collapse. Peripheral to the vent complex, isolated breccia deposit exposures overlie ignimbrites, tephras, and lavas. These blocks are a few meters to several hundred meters long and 30 m high and consist of monolithic angular and welded trachytic lava clasts in finer-grained matrix. Rheomorphic structures in the breccia deposit show ductile deformation and suggest it formed while above the glass transition temperature.

Morgan, Lisa A.; Shanks, Pat

2009-01-01

11

The Axum-Adwa basalt-trachyte complex: a late magmatic activity at the periphery of the Afar plume  

NASA Astrophysics Data System (ADS)

The Axum-Adwa igneous complex consists of a basalt-trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K-Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19-15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na-Cl-rich deuteric fluids (600-400 °C), as indicated by mineral and bulk rock chemical budgets as well as by ?18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

Natali, C.; Beccaluva, L.; Bianchini, G.; Siena, F.

2013-08-01

12

Occurrence of fluororichterite and fluorian biotite in the In Tifar trachyte neck (Tazrouk district, Hoggar volcanic province, Sahara, Algeria)  

NASA Astrophysics Data System (ADS)

The unusual occurrence in the In Tifar trachyte neck (Tazrouk district, Hoggar volcanic province, Sahara, Algeria) of the fluorian biotite-fluororichterite association is presented. The two mineral species were previously unknown in the Hoggar and their association is uncommon worldwide. Ti-rich biotite has 28-40% OH sites occupied by fluorine, hence the use of the modifier "fluorian". Sodic-calcic fluororichterite has more than 55% OH sites filled by fluorine, hence the use of the prefix "fluoro". Well-defined F-Mg affinities are documented in both cases, while Cl remains very low. Temperatures are estimated roughly at 775-700 °C at low pressures. The fluorian biotite ? fluororichterite sequence of crystallisation implies increasingly high fH2F2/fH2O ratios in metaluminous H2O-dominated evolving to peralkaline F-enriched fluids.

Azzouni-Sekkal, Abla; Bonin, Bernard; Ben El Khaznadji, Riad

2013-09-01

13

Petrology and geochemistry of Late Holocene felsic magmas from Rungwe volcano (Tanzania), with implications for trachytic Rungwe Pumice eruption dynamics  

NASA Astrophysics Data System (ADS)

Rungwe in southern Tanzania is an active volcanic centre in the East African Rift System, characterised by Plinian-style explosive eruptions of metaluminous to slightly peralkaline trachytic silica-undersaturated magmas during its late Holocene history. Variations in whole-rock major and trace element compositions of erupted products have been investigated, in combination with electron microprobe data for melt inclusions and phenocrysts comprising sanidine, biotite, clinopyroxene, titanomagnetite, ilmenite, haüyne, titanite, apatite and traces of plagioclase and amphibole. Compositional variations largely reflect fractional crystallisation, with a limited influence of magma mixing. Subtle variations in whole-rock composition and mineralogical characteristics between and within deposits, suggest the existence of a chemically zoned trachytic magma chamber beneath Rungwe. For the two most important studied deposits, the Isongole and Rungwe Pumice, co-existing Fe-Ti oxides constrain pre-eruptive temperature to 915-950 °C and oxygen fugacity to NNO + 0.25-NNO + 0.45. For the Rungwe Pumice, melt inclusions suggest that the melt was water-undersaturated (maximum inferred H2O concentration 5.5 wt.%). In the range of the defined pre-eruptive temperatures, this corresponds to melt viscosities as low as 103.3 Pa · s, i.e. significantly lower than magmas that typically generate highly explosive eruptions. Because no microlites formed in the conduit during ascent, which would have strongly increased the effective magma viscosity, the highly explosive nature of the eruptions may be attributable to a crucial role of exsolved CO2 and S phases, and very high ascent rates.

Fontijn, Karen; Elburg, Marlina A.; Nikogosian, Igor K.; van Bergen, Manfred J.; Ernst, Gerald G. J.

2013-09-01

14

Remobilization of granitoid rocks through mafic recharge: evidence from basalt-trachyte mingling and hybridization in the Manori–Gorai area, Mumbai, Deccan Traps  

Microsoft Academic Search

Products of contrasting mingled magmas are widespread in volcanoes and intrusions. Subvolcanic trachyte intrusions hosting\\u000a mafic enclaves crop out in the Manori–Gorai area of Mumbai in the Deccan Traps. The petrogenetic processes that produced these\\u000a rocks are investigated here with field data, petrography, mineral chemistry, and whole rock major, trace, and Pb isotope chemistry.\\u000a Local hybridization has occurred and has

Georg F. Zellmer; Hetu C. Sheth; Yoshiyuki Iizuka; Yi-Jen Lai

15

Phenocryst\\/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy)  

Microsoft Academic Search

Summary A set of phenocryst\\/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mt. Etna system, Sicily). Partition coefficients were determined for plagioclase, clinopyroxene, olivine, kaersutite and Ti-magnetite. These phases, along with apatite (not analysed in this work), constitute the common fractionating solid assemblage of alkaline

M. D'Orazio; P. Armienti; S. Cerretini

1998-01-01

16

Geochemistry of a transitional ne-trachybasalt — Q-trachyte lava series from Patmos (Dodecanesos), Greece: further evidence for fractionation, mixing and assimilation  

Microsoft Academic Search

Trace-element and preliminary Sr- and O-isotopic data are reported for a transitional alkaline-sub-alkaline lava series (MVS) from Patmos, Greece. The lava types belonging to this series are ne-trachybasalt, hy-trachybasalt, hy-trachyandesite and Q-trachyte. Rb, Sr and Ba contents, as well as K\\/Rb ratios, of the ne-trachybasalts differ from those of alkali basalts of oceanic islands and those of K-rich alkaline lavas

G. Paul Wyers; Michael Barton

1987-01-01

17

Combined Effect of Crystals and Bubbles on the Rheology of Trachytic Magmas from Monte Nuovo (Campi Flegrei, Italy) (Invited)  

NASA Astrophysics Data System (ADS)

We present results of high-temperature, uniaxial deformation experiments on samples of natural trachytic magma from the Monte Nuovo (1538 AD) eruption. The experiments were performed using a high-temperature uniaxial press at dry atmospheric conditions and under controlled deformation rates. Each experiment involved deforming cores of natural (i.e., crystal- and vesicle-bearing) scoriaceous samples isothermally (600 to 800° C) at constant displacement rates (CDR) corresponding to strain rates between 10-7 and 10-4 s-1. Measured viscosities vary between 1010 and 1013 Pa s. The rheology of these complex natural materials is fully described by a simplified Herschel-Bulkely equation involving two rheometric parameters: consistency K and flow index n. We estimate the combined effects of crystals and pores on the rheology of these multiphase suspensions. Our results demonstrate that the presence of pores has a major impact on the rheological response of magmas and may produce a marked decrease in their apparent viscosity. At the same time, the presence of pores leads to a strong decrease in the strength of the magma inducing local and temporal variation in the deformation regimes (ductile vs. brittle). Brittle failure was in fact observed at T=600°C and strain rates of 10-5 s-1 and at T=800°C for the highest applied strain rate (10-4 s-1), respectively. This study constitutes an important step toward the estimation of multiphase rheological evolution of Monte Nuovo magmas and toward the general understanding of the full complexities governing the dynamics of magma transport in natural systems.

Vona, A.; Romano, C.; Giordano, D.; Russell, K.

2013-12-01

18

Preliminary Studies of the Emplacement of Trachytic Lava Flows and Domes in an Ice- Contact Environment: Mount Edziza, British Columbia, Canada  

NASA Astrophysics Data System (ADS)

Mount Edziza Volcanic Complex (MEVC) lies within the Northern Cordilleran Volcanic Province (NCVP), in northwest British Columbia, Canada. The eruption products include basalt, trachyte, and rhyolite that have been emplaced in a variety of subaerial, sub-ice and subaqueous environments from about 8Ma to less than 2000 y.b.p. The Ice Peak Formation (IPF) is approximately 1Ma (Souther, 1992) and includes basaltic and trachytic effusive and explosive products. Two IPF bluffs on the western side of Edziza, Ornostay Bluff (OB) and Koosick Bluff (KB), and another nearby construct (Triangle Dome, TD) were interpreted by Souther (1992) as sequences of trachytic lavas flows (OB, KB) and a lava dome (TD) formed in an ice-contact environment. This interpretation was confirmed through detailed field examination of joint patterns, visible textures, and other structures. Both OB and KB consist of several lava flows, which vary from several to 75 or more meters in thickness, emplaced on a steep surface. Basal flow breccias (locally hydrothermally-altered), are overlain by massive conchoidally-fractured lava with large-sized (greater than 1m in width) poorly-developed columns, local flow banding, and folded incorporated flow breccia. The tops of the flows display better-developed and smaller- scale columns with well-developed perpendicular planar joints. Textures are comparable to those in rhyolite domes, implying a similar viscosity on emplacement. Column orientations and joint measurements at OB and KB indicate local cooling against a steep surface, which is interpreted as ice. Spectacular fanning columns at TD likely indicate emplacement of several very steep-sided domes within a sub-ice cavity or cavities. Geochronology and volatile analysis of glass is currently ongoing. Detailed study of the products will help constrain paleo-ice thicknesses at the MEVC at the time of lava emplacement. Preliminary interpretation based upon the thicknesses of TD and OB suggests a minimum ice thickness of at least 225 meters at both centers.

Lamoreaux, K. A.; Skilling, I. P.; Endress, C.; Edwards, B.; Lloyd, A.; Hungerford, J.

2006-12-01

19

Polybaric evolution of phonolite, trachyte, and rhyolite volcanoes in Eastern Marie Byrd Land, Antarctica: Controls on peralkalinity and silica saturation  

USGS Publications Warehouse

In the Marie Byrd Land volcanic province, peralkaline and metaluminous trachytes, phonolites, and rhyolites occur in 18 large shield volcanoes that are closely associated in time and space. They are arrayed radially across an 800 km wide structural dome, with the oldest at the crest and the youngest around the flanks. Several lines of evidence suggest that these rocks evolved via open-system, polybaric fractionation. We have used mass balance modeling of major elements together with trace-element data and mineral chemistry to help explain the evolution of this diverse suite of felsic rocks, which appear to have been generated coevally in isolated magma chambers, and erupted close to each other in patterns related to tectonic uplift and extension within the West Antarctic rift system. Isotopic and trace-element data indicate that this occurred with only minimal crustal contamination. We focus on volcanoes of the Executive Committee Range and Mount Murphy, where we find good representation of basalts and felsic rocks within a small area. Our results suggest that the felsic rocks were derived from basaltic magmas that differentiated at multiple levels during their passage to the surface: first to ferrogabbroic compositions near the base of the lithosphere, then to intermediate compositions near the base of the crust, and finally to felsic compositions in mid- to upper crustal reservoirs. The high-pressure history has been largely masked by low-pressure processes. The best indications of a high-pressure history are the mineral phases in cumulate nodules and their correlation with modeling results, with REE anomalies, and with the composition of an unusual gabbroic intrusion. Silica saturation characteristics are believed to have originated in magma chambers near the base of the crust, via fractionation of variable proportions of kaersutite and plagioclase. Development of peralkalinity in felsic rocks took place in upper crustal reservoirs by fractionating a high ratio of plagioclase to clinopyroxene under conditions of low pH2O. With increasing pH2O, the ratio plagioclase/clinopyroxene in the fractionated assemblage decreases and metaluminous liquids resulted. Crustal contamination seems to have had a role in suppressing peralkalinity, and was probably a factor in the origin of high-silica metaluminous rhyolite, but metaluminous rocks are uncommon. The volume and diversity of felsic rocks were probably enhanced by the structure of the lithosphere, the persistence of plume activity, and the immobility of the Antarctic plate. Mechanical boundaries at the base of the lithosphere and crust, and within the crust, appear to have acted as filters, trapping magmas at multiple levels, and prolonging the fractionation process. Final volumes would have been further enhanced by repeated refluxing of the same magma chambers, controlled by plume activity and plate immobility.

LeMasurier, W. E.; Futa, K.; Hole, M.; Kawachi, Y.

2003-01-01

20

Which came first: the pumice or the obsidian? Complex degassing transitions during the 114ka trachytic Pu'u Wa'aWa'a eruption (Hawaii)  

NASA Astrophysics Data System (ADS)

Fragmental obsidian clasts are highly correlated with coeval pumice in eruptions that produce obsidian (e.g., Lipari, Aeolian Islands; Little Glass Mountain, California; Mono-Inyo chain, California; Taupo, New Zealand), implying that at least some magma is able to degas quiescently prior to or during the explosive stage of an eruption. However, gross stratigraphic relationships reveal a consistent pattern of explosive activity transitioning to effusive activity (e.g., obsidian flows), suggesting subsurface stratification of magmatic volatiles. A prevailing conceptual model of obsidian formation reconciles these observations through (1) formation of dense glassy material by collapse of vesicles in bubbly magma, occuring in the shallow conduit or at the surface, (2) subsequent ascent of gas-rich magma and fragmentation/assimilation of the previously-emplaced obsidian clasts, followed by (3) transition to dominantly effusive eruptive activity. The Pu'u Wa'aWa'a trachytic pumice cone is unique feature in the Hawaii island volcanic landscape, otherwise dominated by basaltic lava. Around 114 ka, a pulsating explosive eruption at Hualalai Volcano expelled trachytic pumice, forming a ~150-200 m high cone. This phase was immediately followed by the outpouring of a large trachyte flow (the most voluminous silicic lava flow identified in Hawaii ~5 km3), identical in bulk composition to the pumice. The tephra deposits of the cone contain abundant obsidian clasts, as well as pyroclasts bearing striking gradual textural transitions and discretely banded pumiceous, scoriaceous and aphanitic material. The intricate variations in glass H2O contents (measured by microRaman), microlite and vesicle abundances (textural analysis), along with the chemical traits (EMPA) displayed by glasses from the diverse textural end-members suggest a complex ascent and eruption history. We test three hypotheses: (a) the obsidian clasts formed during ascent, stalling and outgassing of the magma (i.e., similar to the prevailing models mentioned above), (b) the obsidian clasts formed thorough shear-induced degassing-outgassing during the entire ascent stage, and (c) obsidian derives from volatile poor portions of the magma reservoir, which were disrupted and mingled with volatile-rich magma.

Hammer, J. E.; Shea, T.; Hellebrand, E.

2012-12-01

21

Evolution of pantellerite-trachyte-phonolite volcanoes by fractional crystallization of basanite magma in a continental rift setting, Marie Byrd Land, Antarctica  

NASA Astrophysics Data System (ADS)

The Marie Byrd Land province includes 18 large (up to 1,800 km3) central volcanoes distributed across an active volcano-tectonic dome. The typical volcano structure consists of a basal 1,000-5,000 m of basanite surmounted by trachyte and subordinate intermediate rocks, plus phonolite, or pantellerite, or comendite. The volumes of felsic sections are large (~30-700 km3), but these rocks probably make up <10% of volcanic rock in the province. This paper describes pantellerite volcanoes in the Ames and Flood Ranges, which include a large and varied suite of these iron-rich, silica-poor rhyolites. Isotopic and trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic range, and the results of modeling, all exclude significant crustal contamination and point to fractional crystallization as the process that controls magmatic evolution. The most unusual feature of these volcanoes is the apparent need to derive pantellerites from basanite, the long interval of fractionation at the base of the lithosphere and crust, involving kaersutite as the key phase in developing pantellerite, and a plumbing system that permitted coeval eruption of pantellerite and phonolite from the same edifice. Peralkalinity most likely developed in upper crustal reservoirs during the final 4-5% of magmatic history, by fractionating a high proportion of plagioclase under low pH2O. Mantle plume activity appears to drive doming and volcanism. This, a stationary plate, and continental lithospheric structure seem to provide an optimal environment for the evolution of a diverse, large volume suite of felsic rocks by fractional crystallization.

Lemasurier, Wesley E.; Choi, Sung Hi; Kawachi, Y.; Mukasa, Samuel B.; Rogers, N. W.

2011-12-01

22

Multiple explosive rhyolite/trachyte eruptions of alkaline-peralkaline Nemrut and dacite/rhyolite eruptions of neighboring subduction zone-related Süphan volcano over 600 000 years: the East Anatolian tephra province  

NASA Astrophysics Data System (ADS)

The active Nemrut stratovolcano (2918 m asl) (Eastern Anatolia) is topped by a spectacular caldera and dominates the area west of huge Lake Van that covers its lower flanks. The stratovolcano has been active explosively for at least ca. 600 ka based on drilling evidence (ICDP Paleovan project). We have identified, correlated and compositionally characterized some 40 fallout sheets on land - none previously known - the largest ones probably with magma volumes exceeding 30 km3(DRE). The alkaline to peralkaline tephras are dominated by anorthoclase, Fe-rich clinopyroxene and fayalite with quartz and aenigmatite in some. Large-volume comenditic to pantelleritic rhyolite eruptions occurred in intervals of 20 000 - 40 000 years with smaller volume trachytic tephra deposits in between reflecting overall fairly constant magma transfer rates periodically fractionating to highly evolved rhyolite in larger magma reservoirs. Many of the ca. 10 widespread ignimbrite sheets, nearly all newly recognized, commonly followed on the heels of rhyolitic fallout sheets. They are more mafic than the underlying fallout deposits, magma mixing being common. Widespread spectacular agglutinates represent a late phase of the youngest large-volume fallout/ignimbrite eruption at ca. 30 ka. Active Süphan stratovolcano (4158 m asl), some 50 km NE of Nemrut and bordering Lake Van to the north, is dominated in contrast by subduction-related chemistry and mineralogy, smaller-volume eruptions and more advanced crystallization of magmas prior to eruption. Chief phenocrysts comprise complex disequilibrium assemblages of clinopyroxene, hypersthene, olivine, strongly zoned plagioclase, biotite and/or amphibole and common clots of fractionating phases. Many of the highly viscous and crystal-laden Süphan magmas were emplaced as domes and debris avalanches next to fallout sheets and ignimbrites. The dominant NE direction of fan axes of partial isopach maps of ca. 15 major fallout deposits reflecting prevailing wind directions for more than half a million years suggest that well-dated tephra markers of alkaline/peralkaline Nemrut, and sofar less well-dated "calcalkaline" Süphan and Ararat volcanoes represent a major tephrostratigraphic framework that should provide for excellent tephra markers in neighboring countries (e.g. Iran, Armenia, Aserbeidschan) and the Caspian Sea.

Schmincke, H.-U.; Sumita, M.; Paleovan scientific Team

2012-04-01

23

Step-filling and development of a three-layer magma chamber: the Neapolitan Yellow Tuff case history  

Microsoft Academic Search

The Neapolitan Yellow Tuff, the product of the largest known trachytic phreatoplinian eruption, gives a good opportunity to investigate the filling mechanisms and internal dynamics of a trachytic magma chamber. A detailed study of the geochemical, mineralogical and isotopical features of the deposit was carried out to investigate the behaviour of the magma chamber before the eruption. The collected data

G. Orsi; L. Civetta; M. D'Antonio; P. Di Girolamo; M. Piochi

1995-01-01

24

Geochemistry and Petrogenesis of the Late Jurassic-Early Cretaceous Mansouri Ring Complex, Southeastern Desert, Egypt  

Microsoft Academic Search

The Mansouri Ring Complex is an eccentric mass (9 × 6 km) located in the extreme southwestern part of the Eastern Desert of Egypt. The complex exhibits a poorly defined ring structure and a limited variety of rock types: trachytes, alkali feldspar syenites and rhyolitic dykes. The rocks are hypersolvus, porphyritic and less commonly show trachytic textures. The complex has

S. A. El-nisr; G. M. Saleh

2001-01-01

25

Petrology and geochemistry of Easter Island  

Microsoft Academic Search

Easter Island has developed around three volcanoes—Poike, an older (3 m.y.) strato-volcano, Rano Kau, a caldera, and the fissure complex of Terevaka and its associated cones. The lavas show a wide compositional spread from tholeiites and olivine tholeiites to hawaiites, mugearites, benmoreites, trachytes and rhyolites (comendites). Hawaiite is by far the most abundant rock type and trachytes and rhyolites are

P. E. Baker; F. Buckley; J. G. Holland

1974-01-01

26

The petrology and geochemistry of the Azores Islands  

Microsoft Academic Search

Forty lavas from the Azores Islands have been analyzed for 87Sr\\/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define

William M. White; Maria D. M. Tapia; Jean-Guy Schilling

1979-01-01

27

Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition  

Microsoft Academic Search

New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552±5Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543±9Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531±5Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to

Dominique Gasquet; Gilles Levresse; Alain Cheilletz; Moulay Rachid Azizi-Samir; Abdellah Mouttaqi

2005-01-01

28

Geology of ascension Island, South Atlantic Ocean  

Microsoft Academic Search

Ascension Island is the exposed part of a large volcano located about 80 km west of the Mid-Atlantic Ridge axis in the notrhern South Atlantic Ocean. The volcanic rocks of the island form an alkaline suite with a compositional range of basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite. Trachyte and rhyolite compose approximately 14% of surface exposures. Petrochemical studies by other investigators have shown that the

Dennis L. Nielson; Bruce S. Sibbett

1996-01-01

29

The origin of K-feldspar megacrysts hosted in alkaline potassic rocks from central Italy: a track for low-pressure processes in mafic magmas  

NASA Astrophysics Data System (ADS)

In situ Sr-isotope and microchemical studies were used to determine the provenance of K-feldspar megacrysts hosted in mafic alkaline potassic, ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central Italy. At Monte Cimino volcanic complex, mafic leucite-free ultrapotassic megacryst-bearing rocks of olivine latitic composition are associated with evolved latite and trachyte. Here, latites and trachytes straddle the sub-alkaline field. Age-corrected 87Sr/ 86Sr values (Sr i) of the analysed Cimino olivine latites vary from 0.71330 and 0.71578 and strongly increase at constant Mg value. Latite and trachyte have lower Sr i than olivine latites ranging between 0.71331 and 0.71361. Sr i of K-feldspar megacrysts from olivine latites are between 0.71352 and 0.71397, but core and rim 87Sr/ 86Sr ratios within individual megacryst are indistinguishable. In all the mafic rocks, the megacrysts are not in isotopic equilibrium with the hosts. K-feldspar megacrysts from both the latite and trachyte have similar Sr-isotope compositions (Sr i=0.71357-0.71401) to those in the olivine latites. However, Sr i of megacryst in the trachyte vary significantly from core to rim (Sr i from 0.71401 to 0.71383). As with the olivine latites, the K-feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte. At Vico volcano, megacryst-bearing rocks are mafic leucite-free potassic rocks, mafic leucite-bearing ultrapotassic rocks and old trachytic rocks. The mafic leucite-bearing and leucite-free rocks are a tephri-phonolite and an olivine latite, respectively. A megacryst in Vico trachyte is isotopically homogeneous (Sr i core=0.71129, rim=0.71128) and in equilibrium with the host rock (Sr i bulk rock=0.71125). Sr i of megacryst from tephri-phonolite is clearly not in isotopic equilibrium with its host (Sr i bulk rock=0.71158), and it increases from core (Sr i=0.71063) to rim (Sr i=0.71077). A megacryst in Vico olivine latite is isotopically homogeneous (Sr i core=0.71066, rim=0.71065), but not in equilibrium with the host rock (Sr i bulk rock=0.71013). The Sr isotope microdrilling technique reveals that Cimino megacrysts were crystallised in a Cimino trachytic magma and were subsequently incorporated by mixing/mingling processes in the latitic and olivine latitic melts. A model invoking the presence of a mafic sub-alkaline magma, which was mixed with the olivine latite, is proposed to justify the lack of simple geochemical mixing relation between Cimino trachytes and olivine latites. This magmatological model is able to explain the geochemical characteristics of Cimino olivine latites, otherwise ascribed to mantle heterogeneity. The similarity of core Sr i of megacrysts hosted in Vico tephri-phonolite and olivine latite suggests that the K-feldspar megacrysts are co-genetic. Isotopic equilibrium between megacryst and Vico host trachyte indicates that the trachyte is the parent of this megacryst. On the contrary, the megacrysts hosted in tephri-phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim Sr isotope increase of the xenocryst hosted in the tephri-phonolite. The megacrysts hosted in the Vico mafic rocks might derive from a trachytic melt similar in composition to the old Vico trachytes.

Perini, Giulia; Tepley, Frank J.; Davidson, Jon P.; Conticelli, Sandro

2003-02-01

30

Petrogenesis of Cenozoic volcanic rocks in the NW sector of the Gharyan volcanic field, Libya  

NASA Astrophysics Data System (ADS)

The north-western sector of the Gharyan volcanic field (northern Libya) consists of trachytic-phonolitic domes emplaced between ~ 41 and 38 Ma, and small-volume mafic alkaline volcanic centres (basanites, tephrites, alkali basalts, hawaiites and rare benmoreites) of Middle Miocene-Pliocene age (~ 12-2 Ma). Two types of trachytes and phonolites have been recognized on the basis of petrography, mineralogy and geochemistry. Type-1 trachytes and phonolites display a smooth spoon-shaped REE pattern without negative Europium anomalies. Type-2 trachytes and phonolites show a remarkable Eu negative anomaly, higher concentration in HFSE (Nbsbnd Tasbnd Zrsbnd Hf), REE and Ti than Type-1 rocks. The origin of Type-1 trachytes and phonolites is compatible with removal of clinopyroxene, plagioclase, alkali feldspar, amphibole, magnetite and titanite starting from benmoreitic magmas, found in the same outcrops. Type-2 trachytes and phonolites could be the result of extensive fractional crystallization starting from mafic alkaline magma, without removal of titanite. In primitive mantle-normalized diagrams, the mafic rocks (Mg# = 62-68, Cr up to 514 ppm, Ni up to 425 ppm) show peaks at Nb and Ta and troughs at K. These characteristics, coupled with low 87Sr/86Sr(i) (0.7033-0.7038) and positive ?Nd (from + 4.2 to + 5.3) features typical of the mafic anorogenic magmas of the northern African plate and of HIMU-OIB-like magma in general. The origin of the mafic rocks is compatible from a derivation from low degree partial melting (3-9%) shallow mantle sources in the spinel/garnet facies, placed just below the rigid plate in the uppermost low-velocity zone. The origin of the igneous activity is considered linked to passive lithospheric thinning related to the development of continental rifts like those of Sicily Channel (e.g., Pantelleria and Linosa) and Sardinia (e.g., Campidano Graben) in the Central-Western Mediterranean Sea.

Lustrino, Michele; Cucciniello, Ciro; Melluso, Leone; Tassinari, Colombo C. G.; dè Gennaro, Roberto; Serracino, Marcello

2012-12-01

31

Volcanic hazard assessment at the restless Campi Flegrei caldera  

Microsoft Academic Search

Eruption forecasting and hazard assessments at the restless Campi Flegrei caldera, within the Neapolitan volcanic area, have been performed using stratigraphical, volcanological, structural and petrological data. On the basis of the reconstructed variation of eruption magnitude through time, we hypothesize that the most probable maximum expected event is a medium-magnitude explosive eruption, fed by trachytic magma. Such an eruption could

Giovanni Orsi; Mauro Antonio Di Vito; Roberto Isaia

2004-01-01

32

The seasonal variation for the discharge and water quality of a stream in volcanic island, Korea  

Microsoft Academic Search

Assessment of the groundwater resources in a volcanic island is so difficult, because permeable and impermeable layers were formed from lava flows in different times and various lithologies. Jeju island is the largest volcanic island in Korea, and is composed of plateau and shield forming basaltic to trachytic lava flows, numerous tuff rings\\/cones, scoria cones during its long volcanic history

K. Ha; D. Moon

2007-01-01

33

Lithospheric thickness beneath the southern Kenya Rift: implications from basalt geochemistry  

Microsoft Academic Search

Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor.

Anton P. Roex; Andreas Späth; Robert E. Zartman

2001-01-01

34

49. GEOCHEMISTRY AND PETROLOGY OF IGNEOUS ROCKS, DEEP SEA DRILLING PROJECT LEG 621  

Microsoft Academic Search

Igneous rocks were recovered from three sites on Hess Rise during Deep Sea Drilling Project Leg 62: altered basalt at Site 464, at the northern end of Hess Rise; and altered trachyte from Site 465, and rounded basalt pebbles in upper Albian to middle Miocene sediments from Site 466, both at the southern end of Hess Rise. Major-, minor-, and

K. E. Seifert; K. E. Windom; S. R. Morgan

35

Petrology of the bimodal Cenozoic volcanism of the Kapsiki plateau (northernmost Cameroon, Central Africa)  

NASA Astrophysics Data System (ADS)

The Kapsiki Plateau is the northernmost volcanic zone of the Cameroon Line. The volcanism (27-35 Ma) is of alkaline type and has a typical bimodal lava series diversity with basalts and hawaiite as mafic lavas and phonolites, trachytes, and rhyolites as felsic lavas. Mg-rich olivine phenocrysts occur only in basalts. The hawaiite contains andesine, olivine and Ca-rich pyroxene phenocrysts, and sanidine and quartz xenocrysts. The phonolites contain alkali-feldspar and Na-rich clinopyroxene phenocrysts. Two types of trachytes occur: peralkaline trachytes, with Ti-, Na- and F-rich aegirine augite, richterite, arfvedsonite phenocrysts and non-peralkaline trachytes, with an aenigmatite-type undetermined mineral, Ti-rich biotite and zircon phenocrysts. Similarly, two types of rhyolites occur: peralkaline with quartz and arfvedsonite phenocrysts and non-peralkaline with quartz and biotite phenocrysts. Differentiation indices ( Thornton and Tuttle, 1960) of the lavas range from 22 to 97 with a large gap in the range 34-82. Some basalts are primitive (530 ppm Ni, 1100 ppm Cr). In basaltic lavas, phonolites and non-peralkaline trachytes and rhyolites, Zr and Nb covary with approximately constant ratios (3.1trachytes and rhyolites have high concentrations in Zr (up to 2180 ppm) and Nb (up to 780 ppm), with correlative higher Zr/Nb ratios (6.3-8.3). Some rhyolites have abnormal REE patterns (with kinks), depleted in light-REE, probably resulting from stability of Na-REE-F complexes under hydrothermal conditions. Despite a large gap between basaltic and felsic lavas, major- and trace-element distributions indicate co-magmatism for both the basaltic and felsic lavas. The differentiation of the lava series is dominated by crystal fractionation, the role of fluids in rhyolite genesis being of minor effect, as evidenced by constant values of Y/Ho and Zr/Hf throughout the series. The Kapsiki Plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors of the Cameroon Line. The continental crust appears to have no significant role in their genesis. The hawaiite mineralogical and geochemical characteristics are consistent with an origin by mixing of basaltic and felsic (phonolitic) magmas. The Kapsiki Plateau basaltic magmas may have originated from an infra-asthenospheric reservoir similarly to other basaltic magmas generated throughout the Cameroon Line. The Sr-isotope variations observed in trachytes and rhyolites point to some contamination of the magmas by crustal materials, while the Nd isotopic composition is only slightly affected.

Ngounouno, I.; Déruelle, B.; Demaiffe, D.

2000-10-01

36

Volcanic geology and eruption frequency, São Miguel, Azores  

NASA Astrophysics Data System (ADS)

Six volcanic zones comprise São Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoação caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoação caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on São Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150 250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau.

Moore, Richard B.

1990-11-01

37

Volcanic geology and eruption frequency, Sa??o Miguel, Azores  

USGS Publications Warehouse

Six volcanic zones comprise Sa??o Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoac??a??o caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoac??a??o caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on Sa??o Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150-250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau. ?? 1990 Springer-Verlag.

Moore, R. B.

1990-01-01

38

Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley  

NASA Astrophysics Data System (ADS)

The second caldera-forming ash-flow tuff erupted from the Menengai volcano is up to 80 m thick and represents ca. 30 km 3 of peralkaline trachytic magma. Activity started with the emplacement of air-fall tuffs, possibly from a central vent, followed by welded ash-flow tuffs and then by unwelded flow material, the proximal facies of which is lag breccia. At least part of the ash-flow deposits was emplaced through the ring fracture. The tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Na, Nb, Rb, Th, Y, Zr and the REE, and depletion in Al, Mg, Ca, K, Ti, P, and Ba, corresponding to pantelleritic trachyte at the top, grading down through comenditic trachyte to more primitive (Ba-rich) trachyte. The maximum observed enrichment factor is ×4 for Zr. The chamber may also have been zoned isotopically, 87Sr/ 86Sr becoming less radiogenic (from 0.7062 to 0.7048) and Nd more radiogenic ( ? from 0 to -2), upwards. Alkali feldspar phenocrysts generally became more sodic, and olivine and clinopyroxene phenocrysts more Ferich, towards the roof. Variable matrix glass compositions and unusually sodic cores of feldspar phenocrysts in some rocks are thought to result from mixing of liquids from more than one compositional layer during magma withdrawal. The partitioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid was apparently dependent on melt peralkalinity. As the eruption proceeded, a range of compositions was progressively tapped. Compositional zonation in the chamber is inferred to have been continuous. The zonation resulted from up to 75% fractional crystallization of an alkali feldspar-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the least-evolved trachytes, accompanied by minor assimilation of wall rocks, especially in the deeper parts of the chamber.

Macdonald, R.; Navarro, J. M.; Upton, B. G. J.; Davies, G. R.

1994-05-01

39

Step-filling and development of a three-layer magma chamber: the Neapolitan Yellow Tuff case history  

NASA Astrophysics Data System (ADS)

The Neapolitan Yellow Tuff, the product of the largest known trachytic phreatoplinian eruption, gives a good opportunity to investigate the filling mechanisms and internal dynamics of a trachytic magma chamber. A detailed study of the geochemical, mineralogical and isotopical features of the deposit was carried out to investigate the behaviour of the magma chamber before the eruption. The collected data show three distinct compositional groups separated by gaps. Single depositional units contain glass shards formed contemporaneously. Although each of these shards is homogeneous they display the same compositional variations and gaps detected in pumice clasts. This feature is taken as an evidence for interpreting the detected compositional gaps as real gaps in the chamber. Therefore the chamber was filled by three distinct magma bodies separated by compositional gaps. The uppermost magma was alkali-trachyte and highly homogeneous, likely a consequence of vigorous convection. The intermediate magma was trachyte with a slight and continuous compositional variation, likely resulting from less intense convection. The lowermost magma was compositionally zoned from alkali-trachyte to latite downward. This compositional zonation was most likely acquired during uprise from a deeper reservoir. The three magmas entered the chamber sequentially from the uppermost to the lowermost. The latter entered the chamber short before the beginning of the eruption. Its input was interpreted as a possible triggering factor for the eruption. The results of this study strongly support a step-filling mechanism for the Neapolitan Yellow Tuff magma chamber and allow definition of the temporal succession of input of magma batches. Furthermore they also suggest that the magma bodies did not mix although, at least the uppermost two, coexisted inside the chamber for a time long enough to allow internal homogeneization by convection.

Orsi, G.; Civetta, L.; D'Antonio, M.; Di Girolamo, P.; Piochi, M.

1995-09-01

40

Crystal fractionation, magma step ascent, and syn-eruptive mingling: the Averno 2 eruption (Phlegraean Fields, Italy)  

NASA Astrophysics Data System (ADS)

The 3.7 ka year-old Averno 2 eruption is one of the rare eruptions to have occurred in the northwest sector of the Phlegraean Fields caldera (PFc) over the past 5 ka. We focus here on the fallout deposits of the pyroclastic succession emplaced during this eruption. We present major and trace element data on the bulk pumices, along with major and volatile element data on clinopyroxene-hosted melt inclusions, in order to assess the conditions of storage, ascent, and eruption of the feeding trachytic magma. Crystal fractionation accounts for the evolution from trachyte to alkali-trachyte magmas; these were intimately mingled (at the micrometer scale) during the climactic phase of the eruption. The Averno 2 alkali trachyte represents one of the most evolved magmas erupted within the Phlegraean Fields area and belongs to the series of differentiated trachytic magmas erupted at different locations 5 ka ago. Melt inclusions record significant variations in H2O (from 0.4 to 5 wt%), S (from 0.01 to 0.06 wt%), Cl (from 0.75 up to 1 wt%), and F (from 0.20 to >0.50 wt%) during both magma crystallization and degassing. Unlike the eruptions occurring in the central part of the PFc, deep-derived input(s) of gas and/or magma are not required to explain the composition of melt inclusions and the mineralogy of Averno 2 pumices. Compositional data on bulk pumices, glassy matrices, and melt inclusions suggest that the Averno 2 eruption mainly resulted from successive extrusions of independent magma batches probably emplaced at depths of 2-4 km along regional fractures bordering the Neapolitan Yellow Tuff caldera.

Fourmentraux, Céline; Métrich, Nicole; Bertagnini, Antonella; Rosi, Mauro

2012-06-01

41

Preliminary Interpretation of Glacial and Glaciofluvial Deposits Associated with 1 Ma Glaciovolcanism of the Ice Peak Formation, Edziza volcanic complex, British Columbia, Canada  

NASA Astrophysics Data System (ADS)

The Mount Edziza Volcanic Complex (MEVC) comprises ~775 km3 of basalt, trachyte and rhyolite erupted in a variety of subaerial, sub-ice and subaqueous environments from about 8Ma to <2000 y.b.p. It forms a major component of the Stikine peralkaline subprovince of the central part of the northern Cordilleran volcanic province (NCVP), northwestern British Columbia, Canada. The Ice Peak Formation (IPF) is a widespread unit of the MEVC erupted about 1Ma (Souther, 1992) and includes basaltic and trachytic lava flows, trachytic domes, pyroclastic rocks, and a variety of glacial and glaciofluvial volcaniclastic rocks. Souther's (1992) interpretation of some of the trachytic effusive rocks as ice-contact has been confirmed by our recent study of flow morphologies and joint patterns (LaMoreaux et al., 2006). No evidence of mechanical interaction of the trachytic lava flows with the underlying glaciogenic deposits was observed, so they may be significantly older than 1Ma. However, these deposits may provide a critical record of local and/or regional paleoclimate conditions during a period of the Pleistocene that is poorly constrained in terrestrial settings. Massive and bedded, poorly sorted, pebble-cobble volcaniclastic sandstones exposed immediately beneath IPF trachytic lava flows were examined at 5 widely-spaced localities on the western and northern sides of the MEVC plateau. One of the locations, on the north-central edge of the Sezill Creek drainage, was described by Spooner et al. (1992) as meltout till and glaciolacustrine sediments. The other sequences have not previously been described and include three areas in the headwaters of Sezill Creek and one area at the northwestern end of the MEVC plateau. We interpret these sequences as deposits of meltout tills, debris flows, hyperconcentrated flood flows, with minor stream flows and subaqueous suspension in local areas. The widespread distribution of similar, stratigraphically equivalent glaciogenic deposits is perhaps more consistent with an "ice-sheet" environment, than valley glaciers. The presence of abundant grains of fresh volcanic glass in the glaciogenic deposits is consistent with deposition of the sediments penecontemporaneously with the eruption of IPF volcanics. Mineralogical, geochemical, geochronological and componentry studies are in progress to constrain possible relationships between IPF eruptions and the glaciogenic sediments, to document the palaeoenvironments in detail, and to understand the ice sheet conditions at Edziza at this time.

Endress, C.; Edwards, B.; Skilling, I.; Lloyd, A.; Lamoreaux, K.; Hungerford, J.

2006-12-01

42

Volcanic-hosted rare-metals deposit at Brockman, Western Australia  

NASA Astrophysics Data System (ADS)

Rare-metals mineralization at Brockman, Western Australia, is the product of early pyroclastic eruption of trachytic magma enriched in volatiles and incompatible elements such as Zr, Hf, Nb, Ta, Be, Y and REE and Ga. The mineralization is fine-grained (<20 ?m) and is the result of alteration and re-mobilization of comparatively simple magmatic precursor minerals such as columbite and zircon by F-rich deuteric solutions that were retained in an ash-flow tuff (the Niobium Tuff) following eruption. Chondrite normalized REE distributions show strong enrichment in HREE. Gel-zircon is the principal residence of the HREE, disseminated bastnaesite (±parisite and synchisite) carries the LREE and bertrandite, in late-stage calcite veins, is the host for Be. Ga occurs in K-mica in the groundmass. Trachytic flows overlying the Niobium Tuff contain many of the same ore minerals, but in trace amounts.

Ramsden, A. R.; French, D. H.; Chalmers, D. I.

1993-01-01

43

Mid-Tertiary magmatism in western Big Bend National Park, Texas, U.S.A.: Evolution of basaltic source regions and generation of peralkaline rhyolite  

NASA Astrophysics Data System (ADS)

Tertiary magmatism in the Big Bend region of southwestern Texas spanned 47 to 17 Ma and included representatives of all three phases (Early, Main and Late) of the Trans-Pecos magmatic province. Early phase magmatism was manifested in the Alamo Creek Basalt, an alkalic lava series ranging from basalt to benmoreite, and silicic alkalic intrusions of the Christmas Mountains. Main phase magmatism in the late Eocene/early Oligocene produced Bee Mountain Basalt, a lava series ranging from hawaiite and potassic trachybasalt to latite, widespread trachytic lavas of Tule Mountain Trachyte and silicic rocks associated with the Pine Mountain Caldera in the Chisos Mountains. Late main phase magmatism produced trachyte lava and numerous dome complexes of peralkaline Burro Mesa Rhyolite (~ 29 Ma) in western Big Bend National Park. Late stage basaltic magmatism is sparsely represented by a few lavas in the Big Bend Park area, the adjacent Black Gap area and, most notably, in the nearby Bofecillos Mountains, where alkalic basaltic rocks were emplaced as lava and dikes concurrent with active normal faulting. Trace element modeling, Nd isotope ratios and calculated depths of segregation for estimated ancestral basaltic magmas suggest that Alamo Creek basalts (?Ndt ~ 6.15 to 2.33) were derived from depths (~ 120 to 90 km) near the lithosphere/asthenosphere boundary at temperatures of ~ 1600 to1560 °C, whereas primitive Bee Mountain basalts (?Ndt ~ 0.285 to - 1.20) may have been segregated at shallower depths (~ 80 to 50 km) and lower temperatures (~ 1520 to 1430 °C) within the continental lithosphere. Nb/La versus Ba/La plots suggest that all were derived from OIB-modified continental lithosphere. Late stage basaltic rocks from the Bofecillos Mountains may indicate a return to source depths and temperatures similar to those calculated for Alamo Creek Basalt primitive magmas. We suggest that a zone of melting ascended into the continental lithosphere during main-phase activity and then descended as magmatism died out. Variation within Burro Mesa Rhyolite is best explained by fractional crystallization of a mix of alkali feldspar, fayalite and Fe-Ti oxide. Comendite of the Burro Mesa Rhyolite evolved from trachyte as batches in relatively small independent magma systems, as suggested by widespread occurrence of trachytic magma enclaves within Burro Mesa lava and results of fractionation modeling. Trachyte may have been derived by fractional crystallization of intermediate magma similar to that erupted as part of Bee Mountain Basalt. ?Ndt values of trachyte lava (0.745) and two samples of Burro Mesa Rhyolite (- 0.52 and 1.52) are consistent with the above models. In all, ~ 5 wt.% comendite may be produced from 100 parts of parental trachybasalt. Negative Nb anomalies in some Bee Mountain, Tule Mountain Trachyte and Burro Mesa incompatible element plots may have been inherited from lithospheric mantle rather than from a descending plate associated with subduction. Late phase basalts lack such a Nb anomaly, as do all of our Alamo Creek analyses but one. Even if some slab fluids partially metasomatized lithospheric mantle, these igneous rocks are much more typical of continental rifts than continental arcs. We relate Big Bend magmatism to asthenospheric mantle upwelling accompanying foundering of the subducted Farallon slab as the convergence rate between the North American and the Farallon plates decreased beginning about 50 Ma. Upwelling asthenosphere heated the base of the continental lithosphere, producing the Alamo Creek series; magmatism climaxed with main phase magmatism generated within middle continental lithosphere, and then, accompanying regional extension, gradually died out by 18 Ma.

Parker, Don F.; Ren, Minghua; Adams, David T.; Tsai, Heng; Long, Leon E.

2012-07-01

44

Paleomagnetic study of Deception Island, South Shetland Islands, Antarctica  

Microsoft Academic Search

A paleomagnetic study was carried out on recent volcanic rocks exposed on Deception Island (63.0°S, 60.6°W), Antarctica. Sampling comprised all stratigraphic units exposed on the island, which include basaltic, andesitic and trachytic lavas, basaltic dykes and pyroclastic flows. Following stepwise thermal and alternating field demagnetization procedures, consistent characteristic remanence directions were determined at 21 sites, using principal-component analysis. The overall

Andrés Baraldo; Augusto E. Rapalini; Harald Böhnel; Mabel Mena

2003-01-01

45

Petrogenesis and tectonic setting of the peralkaline Pine Canyon caldera, Trans-Pecos Texas, USA  

NASA Astrophysics Data System (ADS)

The Pine Canyon caldera is a small (6-7 km diameter) ash-flow caldera that erupted peralkaline quartz trachyte, rhyolite, and high-silica rhyolite lavas and ash-flow tuffs about 33-32 Ma. The Pine Canyon caldera is located in Big Bend National Park, Texas, USA, in the southern part of the Trans-Pecos Magmatic Province (TPMP). The eruptive products of the Pine Canyon caldera are assigned to the South Rim Formation, which represents the silicic end member of a bimodal suite (with a "Daly Gap" between 57 and 62 wt.% SiO 2); the mafic end member consists primarily of alkali basalt to mugearite lavas of the 34-30 Ma Bee Mountain Basalt. Approximately 60-70% crystallization of plagioclase, clinopyroxene, olivine, magnetite, and apatite from alkali basalt coupled with assimilation of shale wall rock ( Ma/ Mc = 0.3-0.4) produced the quartz trachyte magma. Variation within the quartz trachyte-rhyolite suite was the result of ˜70% fractional crystallization of an assemblage dominated by alkali feldspar with subordinate clinopyroxene, fayalite, ilmenite, and apatite. High-silica rhyolite is not cogenetic with the quartz trachyte-rhyolite suite, and can be best explained as the result of ˜ 5% partial melting of a mafic granulite in the deep crust under the fluxing influence of fluorine. Variation within the high-silica rhyolite is most likely due to fractional crystallization of alkali feldspar, quartz, magnetite, biotite, and monazite. Lavas and tuffs of the South Rim Formation form A-type rhyolite suites, and are broadly similar to rock series described in anorogenic settings both in terms of petrology and petrogenesis. The Pine Canyon caldera is interpreted to have developed in a post-orogenic tectonic setting, or an early stage of continental rifting, and represents the earliest evidence for continental extension in the TPMP.

White, John Charles; Benker, S. Christian; Ren, Minghua; Urbanczyk, Kevin M.; Corrick, Donald W.

2006-10-01

46

Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography  

Microsoft Academic Search

We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness\\u000a of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12?km and average\\u000a 40–60?m (up to >140?m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map

Mary E. MacKay; Scott K. Rowland; Peter J. Mouginis-Mark; Harold Garbeil

1998-01-01

47

First finding of fish in the diet of a water-dwelling extinct frog Palaeobatrachus from the Upper Oligocene Fossil-Lagerstätte Enspel (Westerwald Mountains, Western Germany)  

Microsoft Academic Search

Lake Enspel was a deep lake which developed in a small trachytic caldera or a bims volcano. One of two finds of aquatic palaeobatrachid\\u000a specimens from Enspel contains a fish prey as stomach content, which is described for the first time. At the critical time,\\u000a the lake was meromictic, eutrophic, and without an outlet. The rarity of such finds of

Michael Wuttke; Markus Poschmann

2010-01-01

48

Constraining the recent plumbing system of Vulcano (Aeolian Arc, Italy) by textural, petrological, and fractal analysis: The 1739 A.D. Pietre Cotte lava flow  

NASA Astrophysics Data System (ADS)

The 1739 A.D. Pietre Cotte lava flow is part of a sequence of low-explosive to weak effusion events occurred at La Fossa Cone, the active vent on Vulcano Island (Aeolian Arc, Italy). This lava is rhyolitic, texturally heterogeneous, and contains lati-trachytic enclaves. These compositions are recurrent in the La Fossa volcanic products and are representative of the recent Vulcano plumbing system. The host lava is vesicular, relatively phenocryst-free, and locally contains microlites and millimeter-sized spherulites. The enclaves are up to 10 cm in size, display angular to spherical shapes, and can form the core of spherulites. Enclaves mostly consist of plagioclase and augitic phenocrysts set in a weakly vesicular groundmass characterized by variable abundance of glass and feldspar microlites. Field, textural, and fractal data allow us to constrain the rheological features of the rhyolitic and lati-trachytic magmas. In situ major, trace, and volatile element analyses provide evidence for heterogeneities in the glassy matrix and zoning of phenocrysts. Processes of magma evolution have been quantitatively constrained by using the apparent distribution ratios of trace elements measured between mineral phases and glassy matrices. The collected data in combination with petrological and fluid inclusion data from the literature provides evidence for (1) a genetic relationship between the two magmas through assimilation fractional crystallization process; (2) a mingling mechanism between an uprising rhyolitic magma and a shallower partly crystallized lati-trachytic magma plug; (3) the desegregation (enclaves) at variable scales of the lati-trachyte within the rhyolite; and (4) the possible eruptive scenarios consequent to a future magmatic unrest.

Piochi, Monica; de Astis, Gianfilippo; Petrelli, Maurizio; Ventura, Guido; Sulpizio, Roberto; Zanetti, Alberto

2009-01-01

49

Geochemical zoning, mingling, eruptive dynamics and depositional processes — the Campanian Ignimbrite, Campi Flegrei caldera, Italy  

Microsoft Academic Search

The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km2 including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.l. The pumice fragments of the CI range in composition

L. Civetta; G. Orsi; L. Pappalardo; R. V. Fisher; G. Heiken; M. Ort

1997-01-01

50

New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)  

Microsoft Academic Search

Summary   The ??150?km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the\\u000a Mediterranean region in the last 200?ky. Despite centuries of investigation, the age and eruptive history of the Campanian\\u000a Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last

B. De Vivo; G. Rolandi; P. B. Gans; A. Calvert; W. A. Bohrson; F. J. Spera; H. E. Belkin

2001-01-01

51

Geochemical characteristics of volcanic rocks from ascension island, South Atlantic Ocean  

Microsoft Academic Search

The volcanic rocks of Ascension Island are a transitional to mildly alkaline basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite suite. Although the overall major element variations in the suite are consistent with derivation of the more evolved compositions by crystal fractionation from parental basalt magma, trace element variations among basalt and hawaiite compositions define four distinct magma types. Three of these types are discriminated by variations

Barry Weaver; Aditya Kar; Jon Davidson; Mike Colucci

1996-01-01

52

18. GEOCHEMISTRY AND PETROLOGY OF PLEISTOCENE ASH LAYERS ERUPTED AT LAS CAÑADAS EDIFICE (TENERIFE)1  

Microsoft Academic Search

Sixty-eight fallout trachytic to phonolitic ash layers recovered from Sites 953, 954, and 956 during Leg 157 are interpreted as a result of explosive eruptions of the Las Cañadas edifice on Tenerife. Their biostratigraphic ages range from 0.3 to 3.8 Ma . Tephra layers consist dominantly of highly vesicular pumice and 1-10 vol% phenocrysts. Alkali feldspar occurs in almost all

Uta Rodehorst; Hans-Ulrich Schmincke; Mari Sumita

53

Lithostratigraphy, magnetostratigraphy, and radiometric dating of the Stanislaus Group, CA, and age of the Little Walker Caldera  

Microsoft Academic Search

The Miocene Stanislaus Group (Stanislaus, Calaveras, Tuolomne, Alpine and Mono counties, CA), composed of intercalated latite and quartz-latite (trachyandesite and trachyte\\/trachydacite) lavas and ignimbrites, provides an important marker for reconstructing the elevation history and tectonic development of the Sierra Nevada and Walker Lane. We present new Ar\\/Ar geochronology and magnetostratigraphy indicating that the Stanislaus Group was emplaced in two pulses:

Christopher J. Pluhar; Alan L. Deino; Nathan M. King; Cathy Busby; Brian P. Hausback; Tracy Wright; Collin Fischer

2009-01-01

54

Granitic ring complexes and Precambrian hot-spot activity in the St. Francois terrane, Midcontinent region, United States  

Microsoft Academic Search

Surface and subsurface geologic data have led to the recognition of granitic ring complexes in the 1.5-b.y.-old St. Francois terrane of southeastern Missouri. The distinctive rock association of the ring complexes comprises alkaline silicic rocks with minor quartz monzonite, trachyte, and basalt. Three periods of granite emplacement are recognized: sub-volcanic granite massifs were emplaced below comagmatic rhyolites and ash-flow tuffs,

Eva B. Kisvarsanyi

1980-01-01

55

Deccan volcanism in Rajasthan: 40Ar-39Ar geochronology and geochemistry of the Tavidar volcanic suite  

NASA Astrophysics Data System (ADS)

The Tavidar volcanic suite in western Rajasthan, India, comprises a group of lava flows (and subordinate pyroclastic deposits) of highly diverse compositions ranging from basalt through trachyte to rhyolite. We have dated five samples of the Tavidar volcanic rocks by the 40Ar-39Ar incremental heating technique. One trachyte and two rhyolite samples yield very good plateau, isochron and inverse isochron ages of 67-65 Ma, typical of the Deccan Traps large igneous province. A subalkalic basalt and a basaltic trachyandesite yield saddle-shaped argon release spectra and show evidence for excess argon. Importantly, all five samples have very similar Nd-Sr isotopic ratios, and constitute a broadly cogenetic magmatic suite with the rhyolites possibly derived by closed-system fractional crystallization of trachytic magmas. The Tavidar rocks' isotopic data overlap, or are very close to, those of the Mahabaleshwar and Panhala Formation basalts in the Western Ghats type section 700-800 km to the southeast. We therefore infer that the Tavidar rocks, having initial Nd values of +3.2 to +0.7, have incorporated only small amounts of lower continental crust. The Tavidar volcanic suite attests to the great areal extent of the Deccan Traps, and reaffirms the great compositional diversity evident in the northwestern Deccan Traps.

Sen, Archisman; Pande, Kanchan; Hegner, Ernst; Sharma, Kamal Kant; Dayal, A. M.; Sheth, Hetu C.; Mistry, Harish

2012-10-01

56

The volcano-pluton interface; The Longonot (Kenya) and Kûngnât (Greenland) peralkaline complexes  

NASA Astrophysics Data System (ADS)

An important step in studying volcanic processes is to consider the coeval processes in the subjacent magma reservoir(s). The trachytic caldera volcano Longonot (Kenya Rift Valley) and the Kûngnât syenite complex (Gardar province, Greenland) are taken to represent complementary magmatic systems, providing evidence of the volcanic and plutonic stages of evolution, respectively. The systems show many features in common; they have similar sizes, experienced two periods of caldera collapse, and were dominated by trachytic magmas, with smaller volumes of basic magma. Magmatic differentiation was dominantly by fractional crystallization of basaltic parents, with minor episodes of magma mixing and, at Kûngnât, some crustal contamination of parental basalts. A model is presented of a single, hypothetical trachytic centre, showing how evidence from one complex can be used to infer processes at the other. For example, an active convective system, with formation of wall and floor syenitic cumulates, can be inferred to exist in the Longonot magma chamber. At Kûngnât, the intermittent development of compositionally zoned caps to the magma chamber is postulated and the nature of syn-caldera eruptive activity is outlined.

Macdonald, R.; Bagi?ski, B.; Upton, B. G. J.

2014-05-01

57

Provenance of the Gueydan Formation, south Texas: Implications for the late Oligocene—early Miocene tectonic evolution of the Trans-Pecos volcanic field  

NASA Astrophysics Data System (ADS)

Coarse fluvial deposits of the Soledad Member of the Gueydan Formation were derived from the Trans-Pecos volcanic field (TPVF), but the proportions of clast types in the deposits do not match present outcrop abundances in the TPVF. Basalt and mafic trachyte are most abundant in Soledad conglomerate and sandstone, durable trachyte forming the largest clasts. In contrast, the surviving remnant of the TPVF is dominated by silicic lava and tuff. The basaltic and trachytic clasts most closely resemble rocks within some of the younger (˜32-27 Ma) units in the Davis, Chinati, and Bofecillos mountains of the TPVF. Our data suggest that these and similar units were much more widespread at the time of Gueydan deposition and were largely stripped from the TPVF during erosion (1-2 km). Gueydan deposition may have been associated with regional uplift of the TPVF accompanying the onset of Basin and Range extension. This uplift would have provided the steep gradient necessary to transport coarse detritus from the TPVF to the Gulf Coast. Basin and Range extension eventually disrupted the course of the ancestral Rio Grande-Rio Conchos, thereby cutting off the supply of volcanic detritus and ending Gueydan deposition.

Parker, Don F.; Krystinik, Jon G.; McKee, Bryce J.

1988-12-01

58

Geology and petrology of the Woods Mountains Volcanic Center, southeastern California: Implications for the genesis of peralkaline rhyolite ash flow tuffs  

NASA Astrophysics Data System (ADS)

The Woods Mountains Volcanic Center is a middle Miocene silicic caldera complex located at the transition from the northern to the southern Basin and Range provinces of the western United States. It consists of a trachyte-trachydacite-rhyolite-peralkaline rhyolite association of lava flows, domes, plugs, pyroclastic rocks, and epiclastic breccia. Volcanism began at about 16.4 Ma, near the end of a local resurgence of felsic to intermediate magmatism and associated crustal extension. Numerous metaluminous high-K trachyte, trachydacite, and rhyolite lava flows, domes, and pyroclastic deposits accumulated from vents scattered over an area of 200 km2 forming a broad volcanic field with an initial volume of about 10 km3. At 15.8 Ma, about 80 km3 of metaluminous to mildly peralkaline high-K rhyolite ash flows were erupted from vents in the western part of fhe field in three closely spaced pulses, resulting in the formation of a trap door caldera 10 km in diameter. The ash flows formed the Wild Horse Mesa Tuff, a compositionally zoned ash flow sheet that originally covered an area of about 600 km2 to a maximum thickness of at least 320 m. High-K trachyte pumice lapilli, some of which are intimately banded with rhyolite, were produced late in the two later eruptions, Intracaldera volcanism from widely distributed vents rapidly filled the caldera with about 10 km3 of high-K, mildly peralkaline, high-silica rhyolite lava flows and pyroclastic deposits. These are interlayered with breccia derived from the caldera scarp. They are intruded by numerous compositionally similar plugs, some of which structurally uplifted and fractured the center of the caldera. The center evolved above a high-K trachyte magma chamber about 10 km in diameter that had developed and differentiated within the upper crust at about 15.8 Ma. Petrological, geochemical, and geophysical data are consistent with the idea that a cap of peralkaline rhyolite magma formed within the trachyte chamber as a result of fractional crystallization within chemical boundary layers.

McCurry, Michael

1988-12-01

59

Impact of volcanism on the evolution of Lake Van I: evolution of explosive volcanism of Nemrut Volcano (eastern Anatolia) during the past >400,000 years  

NASA Astrophysics Data System (ADS)

The historically active Nemrut Volcano (2,948 m asl) (Eastern Anatolia), rising close to the western shore of huge alkaline Lake Van, has been the source of intense Plinian eruptions for >530,000 years (drilled lake sediments). About 40 widespread, newly recognized trachytic and less common rhyolitic fallout tephras and ca. 12 interbedded ignimbrites, sourced in Nemrut Volcano, are documented in stratigraphic traverses throughout an area of >6,000 km2 mostly west of Lake Van. Phenocrysts in the moderately peralkaline trachytes and rarer large-volume comenditic rhyolites comprise anorthoclase, hedenbergite-augite, fayalite and, especially in trachytic units, augite, minor aenigmatite, apatite and quartz, and rare chevkinite and zircon. Dacitic to rhyolitic tephras from nearby calcalkalic Süphan Volcano (4,058 m asl), locally interbedded with Nemrut tephras, are characterized by disequilibrium phenocryst assemblages (biotite, augitic clinopyroxene and hypersthene, minor olivine, common crystal clots and/or, in some deposits, amphibole). The magma volume (DRE) of the largest Nemrut tephra sheet (AP-1) described in detail may exceed 30 km3. Extreme facies and systematic compositional changes are documented in the ca. 30 ka Nemrut Formation (NF) deposits formed from one large and complex eruption (thick rhyolitic fallout overlain by ignimbrite, welded agglutinate, overbank surge deposits, and final more mafic fallout deposits). Common evidence of magma mixing in Nemrut ignimbrites reflects eruption from compositionally zoned magma reservoirs. Several young Çekmece Formation trachytes overlying ca. 30 ka old NF deposits and the late trachytes of the NF deposits show compositional affinities to tephra from Süphan Volcano possibly due to temporary influx of Süphan magmas into the Nemrut system following the evacuation of >10 km3 magma (DRE) during the caldera-forming NF eruption. Axes of large fallout fans are dominantly SW-NE but W-E in the younger sheets resembling the direction of the present dominant wind field. Growth of Nemrut volcanic edifice and its peripheral domes since before 0.5 Ma in the hinge area between the Van and Mu? tectonic basins is likely to have been the major factor in isolating Lake Van basin thus initiating the origin and subsequent alkaline evolution of the lake. This alkalinity was later significantly controlled by climate forcing. Internal forcing mechanisms (volcanic and geodynamic) may also have contributed to major lake level changes in addition to climate forcing.

Sumita, Mari; Schmincke, Hans-Ulrich

2013-05-01

60

Ages of zircons from pre-, syn- and post-caldera eruption products of the Changbaishan Volcano, indicating rapid magmatic development  

NASA Astrophysics Data System (ADS)

The Millennium Eruption of Changbaishan Volcano with its eponymous 1000 CE eruption age represents one of the two largest volcanic eruptions on Earth in the past 2000 years. In addition to this major caldera-producing eruption of comendite magma, the Changbaishan Volcano also erupted smaller-scale comendite at ~5 ka and trachyte at ~0.3 ka. Here we report U-Th disequilibrium ages of zircons from pumice and lava of the pre-caldera (~5 ka), syn-caldera (the 1 ka Millennium eruption), and post-caldera (~0.3 ka) events. The zircon isochron ages are 12.4+/-1.5 ka (2?, n=29, MSWD=0.87) for the 5 ka eruption, and 12.2 +/- 1.7 ka (n=16, MSWD=1.0) for the 1 ka eruption which is in agreement within uncertainty with a previously reported isochron age of 10.6+/-1.6 ka (n=11, MSWD=0.61) for a different 1 ka sample (Zou et al., 2010, Lithos). The zircon storage times in the pre-eruptive magma body are thus 7 ka for the pre-caldera eruption and 11 ka for the syn-caldera eruption. Furthermore, identical zircon ages in pre-caldera and syn-caldera rocks suggest that both comenditic eruptions tapped the same magma body. In contrast to the uniform zircon ages for the 5 ka and 1 ka comenditic eruptions, zircon ages for the post-caldera 0.3 ka trachytic eruption define multimodal age populations. The youngest peak for the 0.3 ka eruption is 2.6+/-1.8 ka (n=11, MSWD=0.90), an older peak is 130+/-10 ka (n=13, MSWD=1.7), and the oldest population is ?230 ka (near U-Th equilibrium). The youngest mode represents zircon microphenocrysts (autocrysts) that crystallized in a trachytic magma chamber in the built-up to the 0.3 ka eruption, whereas the 130 ka and ?230 ka zircons are interpreted as antecrysts derived from earlier episodes of magmatism. If this interpretation is correct, the zircon storage time in the eruptible magma body for the 0.3 ka eruption is extremely short at 2.3+/-1.8 ka. The distinct multimodal zircon age distributions for the 0.3 ka eruption of trachytic magma suggests this post-caldera eruption tapped a separate magma chamber, and argues against any significant mixing between comendite and trachyte magma bodies at Changbaishan Volcano.

Zou, H.; Fan, Q.; Zhang, H.; Schmitt, A. K.

2013-12-01

61

Geochemistry and petrogenesis of the late Cretaceous potassic-alkaline volcanic rocks from the Amasya Region (northern Turkey)  

NASA Astrophysics Data System (ADS)

The Cretaceous Lokman Formation (Alp, 1972) , is a volcano-sedimantary unit that comprises high- to ultra high-K alkaline volcanic rocks in Amasya Region (40°N, 35°E). The volcanic rocks expose as small outcrops and interfingered with pyroclastic and epiclastic rocks, and are classified as leucitite, tephriphonolite (LT), lamprophyres, trachytes and rarely andesites. LT and lamprophyres occur as dikes cutting each other, and rare lava flows. Trachytes are observed as small domes in the field and lots of pebbles and blocks within the clastic deposits derived from the domes. Samples of LT comprise lct+cpx (diopsite)+plg+mag+ap and classified as leucite-basanite mineralogically and tephri-phonolite geochemically. Ar-Ar age dating from leucites show that the leucite-bearing volcanic activity formed 75.6±3.7 Ma. The mineralogic composition of melanokratic lamprophyre dikes are represented by Kfs+cpx+mica+ap+mag. They defined geochemically as phono-tephrite and phonolite. The Ar-Ar plateau ages from the phlogopites from two different outcrops are 76.78 and 77.48 Ma. The main minerals of trachytic rocks are amp + bt + pl + Kfs + spn + ap +opq. They are classified as alkaline trachyandesite, geochemically. Radiometric age data from Kfs minerals reveal that the trachytic volcanism occurred 75.83±0.09 Ma. Except one andesitic sample, lamprophyres and trachytes of the Lokman Formation are the high- and ultra high-K and alkaline rocks. LT and lamprophyres are characterized by relatively high MgO (3.25-7.04 wt.%), K2O (4.34-6.54 wt.%), Na2O (3.42-5.74 wt.%). Total analcimization of leucite minerals let to decreasing its K2O, and increasing the Na2O contents. Therefore, K2O/Na2O values for LT and the lamprophyres (0.92-2.27) are relatively low. Trachytic suite is also high-K and alkaline in nature. On MORB normalized plots, all of the volcanic rocks from Lokman Formation display enrichment of LIL elements significantly relative to HFSE, and depletions of Nb-Ta and Ti elements. Mg# (44.78 - 62.24), FeO (4.74-7.80 wt.%), Nb (6.3-14.4 ppm) and Ni (20-81 ppm) contents of these rocks imply that these rocks were not originated directly from the primitive melts. The geochemical findings suggest a source that is similar with subduction-related magmas. The evaluation of the geological data and combined with the geochemical findings suggest that the high- to ultrahigh-K alkaline volcanic rocks of the Lokman formation were generated by the partial melting processes of a heterogeneous magma source that was modified by the subduction of the Neo-Tethys ocean during the late Cretaceous period.

Gülmez, Fatma; Genç, Can; Tüysüz, Okan; Karac?k, Zekiye; Roden, Mike; Billor, Zeki; Hames, Willis

2013-04-01

62

The `Daly Gap' and implications for magma differentiation in composite shield volcanoes: A case study from Akaroa Volcano, New Zealand  

NASA Astrophysics Data System (ADS)

The origin of compositional gaps in volcanic deposits that are found worldwide, and in a range of different tectonic settings, has challenged petrologists since Daly’s first observations at mid-ocean ridges. In the shield-forming Akaroa Volcano (9.6 - 8.6 Ma) of Banks Peninsula, New Zealand, a dramatic compositional gap exists in both eruptive and co-genetic intrusive products between basalt and trachyte, and between gabbro and syenite respectively. Rock compositions display mildly alkaline affinities ranging from picritic basalt, olivine alkali basalt and hawaiite to trachyte. Intermediate mugearite and benmoreite (50 - 60 wt. % SiO2) are not exposed or absent. Equivalent plutonic diorite, monzodiorite and monzonite (45 - 65 wt. % SiO2) are also absent. Previously, the formation of the more evolved trachyte (and syenite) has been ascribed to crustal melting. However, our analysis of new major and trace element data from minerals and bulk-rocks of this hy-normative intraplate alkalic suite provide evidence for an alternative model based on crystal fractionation and punctuated melt extraction. Observed major and trace element trends in bulk-rocks can be reproduced by Rayleigh fractional crystallization from dry melts (< 0.5 wt. % H2O) at oxygen fugacities one unit below the quartz-fayalite-magnetite-buffer (QFM -1). The results of our MELTS models are in agreement with experimental studies, and indicate a fractionation-generated compositional gap, where trachytic liquid (62 - 64 wt. % SiO2) has been extracted after the melt has reached a crystallinity of 65 - 70 %. The fractionated assemblage of clinopyroxene, olivine, plagioclase, magnetite and apatite is left in a mafic cumulate residue (44 - 46 wt. % SiO2). Calculated values of specific trace and minor elements (Sr, Cr, P) from a theoretical cumulate are consistent with measured concentrations from cumulate xenoliths. Compositional trends from individual mineral analysis are also supportive of fractional crystallization, but illustrate a disrupted liquid-line-of-decent for each mineral phase. Olivine compositions progressively decrease in Mg concentration (Fo83-42) in basaltic melts and show high Fe concentration in trachytic melts (Fo5-10). Clinopyroxene analyses also display higher Fe/Mg ratios in more evolved rocks. Ternary feldspar compositions shift from plagioclase (An84-56) in basalt to alkali feldspar (Or8-65Ab53-33An39-2) in trachyte, but also lack the intermediate compositions. On the other hand, analysis of mafic cumulate xenoliths reflect more evolved mineral compositions towards the rim than volcanic equivalents and complete observed fractionation trends. In summary, our results indicate that these compositional gaps formed from punctuated melt extraction within an optimal crystal fraction window (60 - 70 % crystallinity).

Hartung, E.; Kennedy, B.; Deering, C. D.; Trent, A.; Gane, J.; Turnbull, R. E.; Brown, S.

2010-12-01

63

The evolution of the Peach Spring Tuff magmatic system as revealed by accessory mineral textures and compositions  

NASA Astrophysics Data System (ADS)

The Peach Spring Tuff (PST), a large Miocene ignimbrite located in the southwestern USA, is distinctive in its abundance of U, Th, and REE concentrating accessory minerals (zircon, sphene, allanite, chevkinite). We have examined textures and compositions of these accessory minerals and magnetite, as well as glasses, in pumice clasts and fiamme from the PST outflow and intracaldera by a variety of methods. Textures of crystal populations were assessed qualitatively in thin sections and crystal separates, and quantitatively by differential absorption x-ray tomography (DAT) to obtain quantitative textural information (i.e. crystal size distributions, CSDs). We have also analyzed REE compositions of glasses by LA-ICPMS and zircon and sphene by SHRIMP-RG. Pumice clasts and fiamme from the outflow sheet and intracaldera deposits range in composition and crystal content, from relatively crystal-poor rhyolites to crystal-rich trachytes, with intracaldera fiamme on the less silicic end of this spectrum. REE trends in zircon and sphene grains reveal a simple fractionation history in rhyolites, but MREE enrichment in sphene edges in trachytes suggest final growth from a less evolved melt. Ti-in-zircon and Zr-in-sphene thermometry reveals lower temperature growth at edges of grains from rhyolites (down to ~730 °C), while edges from trachytes record warmer temperatures (up to ~980 °C). Trace element variations and estimated temperatures also suggest that zircon has a more protracted history of growth than other accessory phases. Textures are consistent with the geochemical results. Phenocrysts in rhyolites tend to be euhedral, while those from intracaldera trachytes display resorption features. Zircon and allanite+chevkinite size distributions in outflow pumice clasts and intracaldera fiamme generally display exponential CSDs, consistent with a simple growth and nucleation history. Sphene and magnetite size distributions in outflow samples are generally kinked, with large numbers of small (<100 µm) crystals, suggestive of enhanced nucleation due to rapid decompression. In intracaldera fiamme, the abundance of large sphene crystals (>200 µm) is considerably lower than what is found for the other accessory phases, and sphene CSDs have concave-down shapes for crystals <100 µm. These features are consistent with resorption. The lack of a concave-down section in zircon and allanite+chevkinite CSDs is likely due to comparatively slower growth rates of these phases. These results suggest that the PST was a zoned system affected by a late-stage heating event, which may have triggered eruption, followed by eruptive decompression. Timescales of crystallization calculated from magnetite size distributions suggest this decompression event occurred at most months to a year before eruption. One outflow pumice clast of trachyte composition follows compositional trends of intracaldera trachytes and textural trends of outflow rhyolites, suggesting that these events affected different regions of the chamber to different extents.

Pamukcu, A. S.; Gualda, G. A.; Miller, C. F.; Wooden, J. L.

2010-12-01

64

Volcaniclastic facies architecture of a long-lived, nested silicic tuff ring: the Los Loros volcano, Mendoza, Argentina  

NASA Astrophysics Data System (ADS)

Los Loros is a small, well-preserved volcanic depression. New K/Ar age dating revealed that the volcano is least 1 million years old. The circular morphology of the ~50 m deep and ~ 1 km broad crater of Los Loros was initially inferred to be of phreatomagmatic origin. Recent work, however, concluded that Los Loros is a far more complex volcano than originally thought; with multiple eruptive phases produced by magmatic and minor phreatomagmatic explosive fragmentation-dominated eruptive processes that consequently formed a low aspect ratio volcano. Previous work also suggested that volcanic rocks of Los Loros were entirely basaltic in composition (Puente Formation) and their age was mid-upper Pleistocene. Newly obtained geochemical data, alongside a new age determination, underlies the fact that the volcano is far older than had been expected from its morphology, and its composition shows no signs of basalt, instead it is trachytic. The volcanic succession forms a ~100 m thick pile sitting directly on Cretaceous continental red beds. The base of the volcanic succession is a polymict volcaniclastic conglomerate with variable bed thickness and occasional cross stratification, indicating a channel-filling nature and an origin from a braided river system. The diversity of the clasts suggests that they likely have been transported from a nearby Miocene back-arc volcanic complex, the Sierra Cachahuén (~40 km), which is a lava dome dominated multiple volcano with thick silicic pyroclastic successions. The volcaniclastic conglomerate is covered by a trachytic pumiceous unit thickening toward the SSE. They are composed of loosely packed rounded pumice lapilli. These beds have a well-sorted texture with no characteristic internal stratification, indicating that they are fall in origin and the bed thickness variation is inferred to reflect the paleo-wind direction. A thick pile of pumiceous tuff (up to 20 m) overlies the basal pumice fall unit. It is stratified, cross-bedded and having erosional contacts to the underlying pumice fall beds suggest deposition from high particle concentration pyroclastic density currents. This succession is inferred to represent an original pumice ring formation in a braided river network, where external surface and shallow sub-surface water were available to influence the eruption, causing slight phreatomagmatic affinity. This initial volcaniclastic succession is covered by immature, but thick (dm-to-m) pelitic palosoils and/or channel-filling volcanic conglomerates, suggesting a significant time break (tens of thousands of years), erosion and landscape resetting by fluvial networks after the pumice ring was formed. The rejuvenation of the volcanic vent is represented by a thick pyroclastic and lava capping unit. At least three units of trachytic pyroclastic breccias can be separated on the basis of their welding textures and pumice-to-lithic ratios. The gradual transition from stratified trachytic pumiceous beds to welded units indicates that these units are formed from laterally moving pumiceous pyroclastic density currents (e.g. small-volume ignimbrites). The topmost unit of Los Loros is a trachytic lava flow, which is well-preserved in the East. Monomict volcanic conglomerate covers the eastern sector of the lower slopes of Los Loros, suggesting long-lasting alluvial deposition since the volcanism. The eruptive sequence preserved at Los Loros indicates an initial pumice ring formation on an active alluvial plain. The significant time gap between the basal and capping volcanic units suggests a long-lasting inter-eruptive period prior to resumption of volcanic activity, forming small-volume, low aspect ratio trachytic ignimbrites and capping lava flows. Los Loros is a unique volcano in the sense that it "mimics" a tuff ring in its morphology and geometrical parameters; however, its eruptive sequence is more typical to those eruptions associated with large-volume silicic composition volcanoes with significant inter-eruptive periods.

Németh, Károly; Risso, Corina; Nullo, Francisco

2010-05-01

65

The Astroni volcano: the only example of closely spaced eruptions in the same vent area during the recent history of the Campi Flegrei caldera (Italy)  

NASA Astrophysics Data System (ADS)

The Astroni volcano formed during the third and most recent epoch of activity (4.8-3.8 ka) of the Campi Flegrei caldera (CFc). The activity of the volcano was dominated by explosive, mostly phreatomagmatic eruptions, with only subordinate lava effusions. We have grouped the sequence of deposits into seven distinct units, separated by erosional unconformities or very thin paleosols. The units include mostly surge beds, with subordinate strombolian deposits and lavas, and one plinian fallout layer. The total volume of erupted magma is 0.45 km 3 (DRE), while the total mass is 1.12×10 12 kg. The magma feeding the first five eruptions was alkali-trachytic and slightly zoned, while the last two eruptions tapped a magma batch resulting from mixing of the previously extruded alkali-trachytic and a less evolved trachytic magma. The volcano grew at the northwestern edge of the polygonal volcano-tectonic collapse, northwest-southeast elongated, which accompanied the Agnano-Monte Spina eruption (4.1 ka), the largest of the third epoch. Available radiometric dates and stratigraphical data constrain the age of the volcano in the final part of the 4.1-3.8 ka time span. This implies that the seven eruptions followed each other at very short time intervals. This conclusion is also supported by constancy in archaeological facies of findings within the paleosols between variable Astroni units, in the plain north of the caldera. The sequence of close eruptions in the same area, although with a slight migration of the vent from northwest to southeast, makes the Astroni volcano peculiar in the recent history of the CFc. Therefore, the definition of its history is very important in order to understand one of the past phenomenologies of the caldera, relevant elements to forecast its behavior.

Isaia, Roberto; D'Antonio, Massimo; Dell'Erba, Francesco; Di Vito, Mauro; Orsi, Giovanni

2004-05-01

66

Volatiles in pantellerite magmas: A case study of the Green Tuff Plinian eruption (Island of Pantelleria, Italy)  

NASA Astrophysics Data System (ADS)

The Green Tuff (GT) Plinian eruption, the largest in magnitude at Pantelleria, erupted 3 to 7 km3 DRE of pantellerite magma and a small volume of trachyte. Fifty-nine anorthoclase-hosted melt inclusions from the two basal pumice members were analyzed by FT-IR spectroscopy in order to assess the pre-eruptive H2O content in the pantellerite melt. Microanalytical methods were used to determine major element, Cl, F and S contents. Melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition (peralkaline index = 1.9-2.2) and variable water contents ranging from 1.4 to as high as 4.2 wt %, i.e. much higher than the 1.4 wt % of earlier published studies. The chlorine content is constant at about 1 wt %. Combined Cl and H2O data were used to estimate a confining pressure of about 50 MPa (depth around 2-3 km) for the GT magma chamber. The chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma. Soon after the GT eruption, intra-caldera volcanism was dominated by the eruption of voluminous trachyte lava flows, while pantellerite melt production resumed after about 20 ka with numerous low-volume, mildly explosive (Strombolian) to effusive eruptions. Comparison with data from the literature reveals that, despite the different explosivity, the post-caldera Strombolian eruptions and the GT Plinian eruption were fed by pantelleritic magmas with similar water contents. Chlorine and CO2 contents suggest that the young magma reservoirs feeding the Strombolian to effusive activity were deeper (h ? 4.5 km) than the much larger (based on erupted volumes) magma chamber which fed the GT eruption.

Lanzo, Giovanni; Landi, Patrizia; Rotolo, Silvio G.

2013-07-01

67

Volcanic history and 40Ar/39Ar and 14C geochronology of Terceira Island, Azores, Portugal  

USGS Publications Warehouse

Seven new 40Ar/39Ar and 23 new radiocarbon ages of eruptive units, in support of new geologic mapping, improve the known chronology of Middle to Late Pleistocene and Holocene volcanic activity on the island of Terceira, Azores and define an east-to-west progression in stratovolcano growth. The argon ages indicate that Cinco Picos Volcano, the oldest on Terceira, completed its main subaerial cone building activity by about 370-380??ka. Collapse of the upper part of the stratovolcanic edifice to form a 7 ?? 9??km caldera occurred some time after 370??ka. Postcaldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of Cinco Picos have refilled much of the caldera. The southern portion of Guilherme Moniz Volcano, in the central part of the island, began erupting prior to 270??ka and produced trachyte domes, flows, and minor pyroclastic deposits until at least 111??ka. The northern part of Guilherme Moniz Caldera is less well exposed than the southern part, but reflects a similar age range. The northwest portion of the caldera was formed sometime after 44??ka. Several well-studied ignimbrites that blanket much of the island likely erupted from Guilherme Moniz Volcano. The Pico Alto Volcanic Center, a tightly spaced cluster of trachyte domes and short flows, is a younger part of Guilherme Moniz Volcano. Stratigraphic studies and our new radiocarbon ages suggest that most of the Pico Alto eruptions occurred during the period from about 9000 to 1000??years BP. Santa Barbara Volcano is the youngest stratovolcano on Terceira, began erupting prior to 29??ka, and has been active historically. ?? 2006.

Calvert, A. T.; Moore, R. B.; McGeehin, J. P.; Rodrigues, da, Silva, A. M.

2006-01-01

68

Petrology of two continental alkaline intraplate series at Emi Koussi volcano, Tibesti, Chad  

NASA Astrophysics Data System (ADS)

Tibesti volcanoes (Chad) are poorly documented, as much for volcanological evolution as for mineralogy and geochemistry. Emi Koussi volcano is a wide ignimbritic shield-like volcano that results from the succession of three eruptive sequences, each one related to a caldera collapse. Activity occurred from 2.4 Ma to the recent Quaternary. The continental alkaline volcanics of Emi Koussi exhibit two bimodal lava series, silica-saturated (up to 11% normative quartz) and silica-undersaturated (up to 29% normative nepheline). Basanite and tephrite, trachyte and phonolite are dominant while trachyandesite is very scarce. The basanites and tephrites contain olivine, diopside, plagioclase, nepheline, kaersutite and titanomagnetite. The trachytes contain alkali feldspar, plagioclase, Na-Ca amphibole (kataphorite, richterite), mica, augite, aegirine augite, titanomagnetite and ilmenite. In phonolites, alkali feldspar, andesine, nepheline or sodalite, hedenbergite, aegirine augite, amphibole (ferropargasite and kaersutite), titanite and titanomagnetite occur. Trachyandesite contains olivine, augite, edenite, phlogopite, andesine, alkali feldspar, titanomagnetite and ilmenite. The two suites exhibit a wide compositional gap of about 10% SiO 2. Trachytes are peralkaline and comenditic. Major and trace element data are compatible with differentiation dominated by fractional crystallisation. The fractional crystallisation has been modelled using major-element least-squares mass balance models. Crystal fractionation involving olivine, clinopyroxene, amphibole and plagioclase is the best way to explain the petrogenesis of the undersaturated suite. The silica-saturated suite recorded removal of olivine, pyroxene, amphibole, mica, plagioclase, alkali feldspar, magnetite and apatite. Trace element data, including rare earth elements, calculated using the Rayleigh equation, confirm the models for the undersaturated suite, because there is a close agreement between the observed and calculated data. But the petrogenesis of the silica-saturated suite is probably related to other processes, such as crustal contamination.

Gourgaud, A.; Vincent, P. M.

2004-01-01

69

Geochemistry and Petrogenesis of the Late Jurassic-Early Cretaceous Mansouri Ring Complex, Southeastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

The Mansouri Ring Complex is an eccentric mass (9 × 6 km) located in the extreme southwestern part of the Eastern Desert of Egypt. The complex exhibits a poorly defined ring structure and a limited variety of rock types: trachytes, alkali feldspar syenites and rhyolitic dykes. The rocks are hypersolvus, porphyritic and less commonly show trachytic textures. The complex has a high fracture density consisting of two dominant fault systems trending west-northwest and east-west. The rocks are silica-oversaturated; trachytes and syenites are mildly metaluminous to peralkaline, while rhyolites are mainly peraluminous due to the relative loss of a peralkaline fluid phase. The rocks are enriched in high field strength elements (Nb, Zr, Y) reflecting a within-plate tectonic setting. The curvilinear trends defined by some major and trace element variations, together with the decrease in the K/Rb ratio and concomitant increase in Rb/Ba and Rb/Sr ratios, indicate the important role of feldspar fractionation during the evolution of the Mansouri Complex. In some Mansouri rocks, radioactive anomalies are mainly associated with shear zones aligned along the east-west direction and characterised by abnormal accumulations of U- and Th-bearing accessory minerals like monazite, allanite, apatite and titanite. The Mansouri Ring has carbonatite rocks of calcitic, calcitic-dolomitic and dolomitic varieties; the latter is widely abundant. Apatite, monazite, pyrochlore, sphalerite, pyrrohotite and magnetite are the common accessory minerals associated with the Mansouri carbonatites. The U and Th contents of the carbonatites range from 10-25 ppm and 40-250 ppm, respectively. The U and Th contents of the alkali feldspar syenites range from 1-9 ppm and 3-10 ppm, respectively. The low Y/Nb ratios (< 0.6), together with the relatively low initial 87Sr/ 86Sr ratio (0.705) of the Mansouri rocks, substantiates the dominant role of mantle-derived magma in their genesis.

El-nisr, S. A.; Saleh, G. M.

2001-01-01

70

The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands  

Microsoft Academic Search

Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up\\u000a to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine)\\u000a and trachytes (plagioclase–amphibole–magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates\\u000a relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides

D. J. Smith; M. G. Petterson; A. D. Saunders; I. L. Millar; G. R. T. Jenkin; T. Toba; J. Naden; J. M. Cook

2009-01-01

71

Generation of compositionally atypical hydrocarbons in CO2-rich geologic environments  

NASA Astrophysics Data System (ADS)

Bitumen seepages from a trachyte flow on the Suswa volcano (East African Rift) are anomalously enriched in O and N heteroatomic organic compounds and depleted in aliphatics. Compositional and geologic data suggest that the biogenically derived bitumens were extracted and/or transported by liquid or supercritical CO2, rather than H2O, possibly from caldera-lake sediments. Compositionally atypical hydrocarbons and reduced carbon associated with other alkaline-peralkaline igneous complexes, as well as Hg and Au deposits, may also reflect CO2, fluids. Hydrocarbon extraction and migration in CO2-rich fluids are considered realistic mechanisms in certain geologic environments.

Gize, Andrew; MacDonald, Ray

1993-02-01

72

Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites: variation with crystal fractionation and interaction with sea water  

Microsoft Academic Search

Lavas and pyroclastics on Ascension Island contain plutonic blocks that include fluid-inclusion-bearing peralkaline-granite. 18O\\/16O ratios, F and Cl have been analysed on whole rocks and\\/or minerals for lavas and granites, and D\\/H ratios and H2O+ for comenditic obsidians and granites. Whole rock 18O\\/16O ratios of fresh alkali-basalt, hawaiite, trachyandesite, trachyte and comendite range from 6.0 to 6.9‰ with 18O tending

Simon M. F. Sheppard; Chris Harris

1985-01-01

73

Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya  

SciTech Connect

K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.

Tatsumi, Yoshiyuki (Univ. of Tasmania (Australia) Kyoto Univ. (Japan)); Kimura, Nobukazu (Kyoto Univ. (Japan)); Itaya, Tetsumaru (Okayama Univ. of Science (Japan)); Koyaguchi, Takehiro (Kumamoto Univ. (Japan)); Suwa, Kanenori (Nagoya Univ. (Japan))

1991-06-01

74

Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy  

NASA Astrophysics Data System (ADS)

Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma 39Ar/40Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-39Ar/40Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-39Ar/40Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-39Ar/40Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km3) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km3) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km3) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria.

Parker, Don F.; White, John C.

2008-01-01

75

Petrology of Hualalai volcano, Hawaii: Implication for mantle composition  

USGS Publications Warehouse

Hualalai is one of five volcanoes whose eruptions built the island of Hawaii. The historic 1800-1801 flows and the analyzed prehistoric flows exposed at the surface are alkalic basalts except for a trachyte cone and flow at Puu Waawaa and a trachyte maar deposit near Waha Pele. The 1800-1801 eruption produced two flows: the upper Kaupulehu flow and the lower Huehue flow. The analyzed lavas of the two 1800-1801 flows are geochemically identical with the exception of a few samples from the toe of the Huehue flow that appear to be derived from a separate magmatic batch. The analyzed prehistoric basalts are nearly identical to the 1800-1801 flows but include some lavas that have undergone considerable shallow crystal fractionation. The least fractionated alkalic basalts from Hualalai are in equilibrium with mantle olivine (Fo87) indicating that the Hawaiian mantle source region is not unusually iron-rich. The 1800-1801 and analyzed prehistoric basalts can be generated by about 5-10% partial fusion of a garnet-bearing source relatively enriched in the light-rare-earths. The mantle underlying the Hawaiian Islands is chemically and mineralogically heterogeneous before and after extraction of the magmas that make up the volcanoes. ?? 1980 Intern. Association of Volcanology and Chemistry of the Earth's Interior.

Clague, D. A.; Jackson, E. D.; Wright, T. L.

1980-01-01

76

Petrologic evolution of divergent peralkaline magmas from the Silent Canyon caldera complex, southwestern Nevada volcanic field  

USGS Publications Warehouse

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60-100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. Weakly peralkaline silicic comendites (PI 1.0-1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low-silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center. -from Authors

Sawyer, D. A.; Sargent, K. A.

1989-01-01

77

Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas  

NASA Astrophysics Data System (ADS)

We present new thermometers and barometers based on clinopyroxene-liquid equilibria specific to alkaline differentiated magmas. The new models were calibrated through the regression analyses of experimental datasets obtained by merging phase equilibria experiments from the literature with new experiments performed by using trachytic and phonolitic starting compositions. The regression strategy was twofold: (1) we have tested previous thermometric and barometric equations and recalibrated these models using the new datasets; (2) we have calibrated a new thermometer and a new barometer including only regression parameters that closely describe the compositional variability of the datasets. The new models yield more precise estimates than previous thermometers and barometers when used to predict temperatures and pressures of alkaline differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene-liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this validates the use of the proposed models for precise estimates of crystallization temperatures and pressures in differentiated alkaline magmas. Because these magmas feed some of the most voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of the environmental conditions of their reservoirs is mandatory and this is now possible with the new models provided here.

Masotta, M.; Mollo, S.; Freda, C.; Gaeta, M.; Moore, G.

2013-12-01

78

The VORISA Project: An Integrated Approach to Assessing Volcanic Hazard and Risk in the Kingdom of Saudi Arabia  

NASA Astrophysics Data System (ADS)

The Kingdom of Saudi Arabia has numerous large monogenetic volcanic fields, known locally as 'Harrat'. The largest of these, Harrat Rahat, produced a basaltic fissure eruption in 1256 AD with lava flows travelling within 20 km of the Islamic holy city Al-Madinah. With over 900 visible basaltic and trachytic vents and periodic seismic swarms indicating stalled eruptions, an understanding of the risk of future eruptions in this volcanic field is vital. To systematically address this need we developed the Volcanic Risk in Saudi Arabia (VORISA) Project, a 3-year, multi-disciplinary international research collaboration that integrates geological, geophysical, hazard and risk studies. Detailed mapping and geochemical studies are being combined with new and existing age determinations to determine the style and sequence of events during past basaltic and trachytic eruptions. Data from gravity and magnetotelluric surveys are being integrated with microearthquake data from an 8-station borehole seismic research array to geophysically characterise the structure and nature of the crust, and thus constrain possible physical controls on magma propagation. All available data are being synthesised in hazard models to determine patterns in eruption frequency, magnitude, and style of past activity, as well as the probable location and style of a future event. Combined with geospatial vulnerability data, these hazard models, which include a reconstruction of the 1256 AD eruption, enable us to calculate and communicate volcanic risk to the city of Al-Madinah.

Lindsay, J. M.; Moufti, R.

2013-12-01

79

Age and petrology of the Tertiary As Sarat volcanic field, southwestern Saudi Arabia  

USGS Publications Warehouse

Harrat As Sarat forms the second smallest and southernmost of the basalt fields of western Saudi Arabia and is part of a voluminous Red Sea rift-related continental alkali basalt province. The rocks of the As Sarat were emplaced during the first stage of Red Sea rifting and represent the northernmost extension of the Tertiary Trap Series volcanics that occur mainly in the Yemen Arab Republic and Ethiopia. The field consists of up to 580 m of basalt flows, that are intruded by basaltic plugs, necks, minor dikes, and highly evolved peralkaline trachyte intrusions. K-Ar ages indicate that the As Sarat field formed between 31 and 22 Ma and contains an eruption hiatus of one million years that began about 25 Ma ago. Pre-hiatus flows are primarily hypersthene normative intersertal subalkaline basalt, whereas the majority of post-hiatus flows are nepheline normative alkali basalt and hawaiite with trachytic textures. Normative compositions of the basalts are consistent with their genesis by partial melting at varying depths. Trace element abundances in the basalt indicate that varying degrees of partial melting and fractional crystallization (or crystal accumulation) had major and minor roles, respectively, in development of compositional variation in these rocks. Modeling indicates that the pre-hiatus subalkaline basalts represent 8-10 percent mantle melting at depths of about 70 km and the post-hiatus alkali basalts represent 4-9 percent mantle melting at depths greater than 70 km. ?? 1991.

du, Bray, E. A.; Stoeser, D. B.; McKee, E. H.

1991-01-01

80

Evolution of anorthoclase phonolite, Mt. Erebus, Antarctica  

SciTech Connect

Over the last 1 m.y. Mt. Erebus (3794 m) has erupted mainly anorthoclase phonolite (AP) and lesser trachyte, kaersutite phonolite and intermediate differentiates. An active convecting AP lava lake, identical in composition to the older lavas, existed from 1972 until late 1984. Most of the rocks define a strongly undersaturated continuous sodic differentiation series, composed of basanite, Ne-hawaiite, Ne-mugearite, Ne-benmoreite and AP. The main phenocryst phases and their ranges are: olivine (Fo 81-43), clinopyroxene (Wo 50-44, En 42-24, Fs 11-30), opaque oxides (Usp 52-79) and feldspar. Major, trace and REE analyses exhibit smooth trends on variation diagrams. REE are strongly LREE enriched and increase from La/sub N/=220 in the basanites to 400 in AP. There are no significant Eu anomalies. Published isotopic data show derivation of the basanite parental magmas from a depleted (/sup 87/Sr//sup 86/Sr=0.703) heterogeneous mantle source similar to that for oceanic island basalts. Crustal contamination is insignificant except in the trachytes. Evolution of the Erebus lineage by fractional crystallization of the above phases plus apatite is supported by mass balance models. Differentiation probably occurred in larger, hotter and lower P/sub H20/ magma chambers compared to the basanite to kaersutite phonolite DVDP lineage (Kyle, 1981) of the neighboring Hut Point Peninsula. Mt. Erebus may mark the site of a major mantle upwelling.

Moore, J.A.; Kyle, P.R.

1985-01-01

81

Landslides density map of S. Miguel Island, Azores archipelago  

NASA Astrophysics Data System (ADS)

The Azores archipelago is located in the Atlantic Ocean and is composed of nine volcanic islands. S. Miguel, the largest one, is formed by three active, E-W trending, trachytic central volcanoes with caldera (Sete Cidades, Fogo and Furnas). Chains of basaltic cinder cones link those major volcanic structures. An inactive trachytic central volcano (Povoação) and an old basaltic volcanic complex (Nordeste) comprise the easternmost part of the island. Since the settlement of the island early in the 15th century, several destructive landslides triggered by catastrophic rainfall episodes, earthquakes and volcanic eruptions occurred in different areas of S. Miguel. One unique event killed thousands of people in 1522. Houses and bridges were destroyed, roads were cut, communications, water and energy supply systems became frequently disrupted and areas of fertile land were often buried by mud. Based on (1) historical documents, (2) aerial photographs and (3) field observations, landslide sites were plotted on a topographic map, in order to establish a landslide density map for the island. Data obtained showed that landslide hazard is higher on (1) the main central volcanoes where the thickness of unconsolidated pyroclastic deposits is considerable high and (2) the old basaltic volcanic complex, marked by deep gullies developed on thick sequences of lava flows. In these areas, caldera walls, fault scarps, steep valley margins and sea cliffs are potentially hazardous.

Valadão, P.; Gaspar, J. L.; Queiroz, G.; Ferreira, T.

82

Sm-Nd and Rb-Sr isotopic systematics of the Pea Ridge Fe-P deposit and related rocks, southeast Missouri  

SciTech Connect

Pea ridge is a discordant Middle Proterozoic Fe-P deposit hosted in rhyolite tuffs and flows of the 1.4--1.5 Ga St. Francois terrane. Host rocks and the deposit are cut by basalt and aplite/pegmatite dikes. The deposit overlies a blind pluton which is partially surrounded by a trachytic ring complex. In the deposit, which is mined for Fe, early Qtz+Amph+Mag+Ap rock is cut by Mag+Ap+Qtz rock. Subsequently, portions of the deposit and host rocks were brecciated, oxidized and silicified to produce a complex suite of rocks enriched in Hem+Qtz+Ksp+Mu. Late breccia pipes/dikes cut the complex and were mineralized with Bar+Ksp+Flu+Chl+Cc+REE-phosphates. Sm/Nd and Rb/Sr isotopic systematics have been studied to: (1) constrain source(s) of igneous rocks and deposit components, (2) refine ages of magmatism, mineralization, and later hydrothermal activity, (3) begin regional comparison of isotopic systematics in SE Missouri Fe deposits, and (4) complement ongoing Missouri DGLS/USGS studies. Fourteen combined Sm-Nd and Rb-Sr analyses were done on materials including two host rhyolites, two nearby trachytes, two gneiss samples representing plausible basement, two intramineral dikes, and six samples of mineralization.

Marikos, M.A.; Barton, M.D. (Univ. of Arizona, Tucson, AZ (United States). Dept. of Geosciences)

1993-03-01

83

Petrology of the Guenfalabo ring-complex: An example of a complete series along the Cameroon Volcanic Line (CVL), Cameroon  

NASA Astrophysics Data System (ADS)

In the Guenfalabo ring-complex (GRC), two non-comagmatic rock suites have been identified as a result of two volcanic episodes: Suite 1 (68.8 ± 1.7 Ma by K/Ar on trachyte) of peralkaline trachytes and pantellerites cogenetic with alkaline syenites, granites and rhyolitic flows and tuffs; Suite 2 (62 ± 2 Ma by K/Ar on basalt), a bimodal and complete series of alkali olivine basalts and associated microgabbro dykes, diorites, syenites and granites, cross-cutting the former. Kaersutite in the trachytes of Suite 1 has mantle-derived signatures: TiO2 > 4%, MgO < 15%, FeO > 8%, Ti = 0.63 c.p.f.u. and Al = 2, characteristic of kaersutites of HP and HT origin: 13-23 kbar, 1100-1220 °C. The trachytes are probably products of FC of a basaltic parent that did not attain higher crustal levels. The Suite 1 rocks are enriched in Rb, K, Zr, Nb, LREE, alkalis, and (Ce/Yb)N = 7-15 probably due to some effect of metasomatism during the magma ascension. Fe-Ti enrichment is corroborated by the presence of ferropseudobrookite-ilmenite-ulvospinel in the syenites and ilmenite in the pantellerite. The Rb/Ba > 1 in the trachytes (2.44, 26.7), pantellerite (6.33), alkaline granites (0.63-1.8) and the 87Sr/86Sr in the alkaline granites (=0.74060) depict the role of AFC. The ankaramites of the Suite 2 rocks are olivine-phyric (25%), Fo85-88, have 50-52% clinopyroxene (salite), 5% plagioclase (An55-36) and 7% Fe-Ti oxides. Trace element modeling indicates an origin from a basaltic magma of about 25% PM of spinel lherzolite mixed with a magma from <1% PM of garnet lherzolite (3-4% garnet) in a proportion of 1:4. The cogenetic alkali basalts and the microgabbro-diorite-syenite-granite that constitute the Suite 2 rocks, with a Daly gap of 54% > SiO2 < 58%, result from this Early Cenozoic magmatic event. The basalts have: Zr = 225-253, Nb = 98-111, Y = 33-56, typical of FOZO, a HIMU-type OIB related magmas (Sr/Sri = 0.70202-0.7034; Nd/Nd = 0.51282-0.512545; 206Pb/204Pb = 19.13, 207Pb/204Pb = 15.59 and 208Pb/204Pb = 38.64. The (La/Yb)N ratios (9-20) indicate a high REE fractionation and garnet-lherzolite source. Elemental ratios as La/Nb = 0.49-0.81, Nb/Y = 1.00-2.04 suggest crustal contamination of minor importance. The (La/Sm)N ratios (2-3.5) and LREE enrichment corroborate the contribution of a subcontinental lithospheric mantle (SCLM). REE spectra and minor element spidergrams are concordant for each rock suite.

Donald Ngonge, E.; Hollanda, Maria Helena B. M.; Nsifa, E. Nkonguin; Tchoua, Felix M.

2014-08-01

84

Plio-Pleistocenic igneous activity in Sardinia (Italy)  

NASA Astrophysics Data System (ADS)

New geochemical and isotopic data are presented for five Plio-Pleistocene volcanic districts of Sardinia (Italy). From north to south they are: Logudoro (3.18-0.11 Ma), Montiferro (3.8-2.3 Ma), Capo Frasca (1.6 Ma), Mt. Arci (3.7-2.7 Ma) and Capo Ferrato (5.9-5.0 Ma). Most of the Logudoro rocks are mugearite with fewer hawaiite, benmoreite and phonotephrite; subordinate tholeiitic rocks (basaltic andesite) also occur. The Montiferro volcanics range from hawaiite to mugearite and trachyte; rarer tholeiitic rocks (basaltic andesites) occur as well. Capo Frasca samples are basaltic andesites with tholeiitic affinity. Mt. Arci is made up of tholeiitic (basaltic andesite, andesite, dacite and rhyolite), mildly alkaline (hawaiite, mugearite and trachyte) with rarer transitional types. Capo Ferrato is entirely made up of mildly alkaline mugearite and trachyte. Except few primitive samples from Logudoro and Montiferro (Mg#=65, Ni=210 ppm and Cr up to 360 ppm), most of the rocks are evolved, testifying fractional crystallization processes in all the volcanic districts. Primitive mantle-normalized diagrams show patterns with positive peak at Ba coupled with negative peaks at U and smooth or no peaks at Ta-Nb, a feature already observed for other Plio-Pleistocene volcanic rocks of Sardinia. LREE are over 150 x chondrite whereas HREE are mostly over 8 x chondrite suggesting a spinel-bearing source, with alkaline rocks showing REE patterns subparallel to tholeiitic products but with higher (La/Yb)N ratios (13.2-29.9 and 8.4-10.1 for alkaline and tholeiitic rocks, respectively). Although REE analyses point out a possible common source between tholeiitic and alkaline products, the different slopes (for both series of each volcanic district) observed in trends of some incompatible element ratios plots seem to suggest the existence of distinct parental magmas for the volcanics of each district, with the exception of Logudoro and Montiferro. Isotopic data show that the less evolved samples have 87Sr/86Sr around 0.70428 whilst the more differentiated reach 0.70913; eNd varies from -8.3 to +2.1. Geochemical and isotopic data share some features akin to an EM-I source, being shifted toward the left of the mantle array, with high and variable Ba/Nb ratios (from 18 to 42 for both tholeiitic and alkaline rocks; only Mt. Arci show values between 8 to 10, more similar to the other anorogenic Cenozoic European Volcanic Province products (Ba/Nb<20). An enriched lithospheric source, possibly modified by previous subduction events, could explain the geochemical features of the Sardinian rocks.

Petteruti Lieberknecht, A. M.; Fedele, L.; D Amelio, F.; Lustrino, M.; Melluso, L.; Morra, V.

2003-04-01

85

Petrologic evolution of divergent peralkaline magmas from the Silent Canyon Caldera Complex, Southwestern Nevada Volcanic Field  

NASA Astrophysics Data System (ADS)

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60-100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. The most mafic rocks are precollapse and postcollapse trachytes (65-69% SiO2). Low-silica comendites (69-73% SiO2) were erupted as the mafic upper part of the chemically zoned Grouse Canyon Member and as postcollapse lavas. The lower part of the Grouse Canyon Member and the underlying rhyolite of Split Ridge are moderately peralkaline comendite (PI is molar ratio Na + K/Al is 1.17-1.26). These comendites have major element characteristics and trace element enrichments approaching those of pantellerites. The Tub Spring Member, by contrast, is a weakly peralkaline chemically unzoned silicic comendite (75-76% SiO2) ash flow tuff. Weakly peralkaline silicic comendites (PI 1.0-1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low-silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. These contrasting differentiation paths are shown in the behavior of Fe and other transition metals, Al, Na, K; the trace elements Ba, Zr, Nb; and probably F and Cl. Weakly peralkaline silicic comendites show a LREE/heavy REE crossover in early erupted/late erupted rocks; moderately peralkaline comendites are enriched in all REE. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center.

Sawyer, David A.; Sargent, K. A.

1989-05-01

86

U-series zircon age constraints on the plumbing system and magma residence times of the Changbai volcano, China/North Korea border  

NASA Astrophysics Data System (ADS)

The Changbai (or Baitoushan, Paektu) volcano on the China/North Korea border is best known for its climactic caldera-forming eruption of 100 km3 of comendite materials 1000 years (1 ka) ago. The polygenetic Changbai volcano also erupted moderate-volume pre-caldera comendite lava at ~ 4 ka and small-volume post-caldera trachyte ignimbrite at ~ 0.3 ka. Here we report 238U-230Th disequilibrium ages of zircons from lavas and ignimbrites of the pre-caldera (~ 4 ka), syn-caldera (1 ka), and post-caldera (~ 0.3 ka) events. The zircon isochron ages are 12.2 ± 1.1 ka (2?) for the 4-ka comendite lava and 12.2 ± 1.7 ka for the 1-ka comendite ignimbrite. Zircons from the 0.3-ka trachyte ignimbrite exhibit 3 respective peaks at 2.6 ± 1.8 ka, 130 ± 10 ka and > 230 ka. The indistinguishable zircon ages for the 4-ka pre-caldera eruption of comendite lava and the 1-ka caldera-forming eruption of comendite pumice and ignimbrite suggest that the 4-ka lava provides an early sampling of a much larger magma body at depth and thus serves as a kind of petrologic early-warning signal. In addition, the 4-ka lava may represent the lowest-temperature magma in the roof zone of a thermally zoned magma chamber that usually escapes first. The distinctive multi-modal zircon age distributions of the 0.3-ka trachytic eruption, however, reveal that this post-caldera eruption tapped a different magma body and indicate that Changbai's magmatic plumbing system had changed after the 1-ka caldera-forming climactic eruption. Our results suggest very short zircon and magma residence times for the Changbai volcano (8 kyr for the 4-ka eruption, 11-12 kyr for the 1-ka eruption and 2.3 kyr for the 0.3-ka eruption).

Zou, Haibo; Fan, Qicheng; Zhang, Hongfu; Schmitt, Axel K.

2014-07-01

87

Geochemical and petrographic characteristics of potassium-rich pyroclastic and lava samples from Vulsini volcano, Roman magmatic region, Italy  

NASA Astrophysics Data System (ADS)

Pyroclastic deposits are the dominant eruptive product at Vulsini and at other potassic volcanoes in the Roman magmatic region, central Italy. Geochemical data from 139 K-rich undersaturated Vulsinian samples are used in the first systematic study of both pyroclastic and lava eruptions from a large volcano in this region. The pyroclastic samples span the complete range of rock chemistries; however, compared to the lavas they are concentrated in the more evolved products and thus significantly enlarge the erupted volume of phonolitic and trachytic magma. The pumice and lava major- and trace-element trends generally form a continuum consistent with a fractionated sequence of K-rich magmas as follows: leucite basanite/Mg-rich leucitite?leucitite/tephritic leucitite?leucite phonolite/leucite trachyte/saturated trachyte. Olivine is conspicuous in the primitive basanitic pumice and lava samples and is also present in the less evolved tephritic leucitites. Clinopyroxene crystallizes at all stages of magma evolution along with ±leucite, ± plagioclase, ± mica, ± opaque oxides and ±sanidine. Leucite phenocrysts are prominant in the intermediate to more evolved compositions, particularly in the leucite phonolites where they form large macrophenocrysts. Sanidine is a significant phenocryst phase in the more evolved samples, particularly in the evolved pumices where it dominates the modal assemblage: in these samples increasing Rb and rapidly decreasing Sr and Ba, as well as reducing K 2O/Na 2O ratios, are linked to sanidine fractionation. High Ni contents (˜ 250 ppm), high Mg numbers (˜ 78) and forsteritic olivine (Fo 92) suggest that the basanitic pumice and lava magmas had a mantle origin through partial melting. Similarly, Mg-rich leucitites are also inferred to be mantle partial melts. On the basis of high K, CaO, Rb and F concentrations and high K/Na, CaO/MgO, low K/Rb and high di/ol ratios found in the primary basanites and leucitites, the mantle source is inferred to be rich in phlogopite and clinopyroxene. By comparison with various purported primary alkaline magmas the Vulsinian primary magmas show considerable enrichment of K, Rb and Th, while other elements such as Nb, Zr, Sr, P and Ti are significantly depleted, thus suggesting a decoupling between these groups of incompatible elements in the mantle beneath Vulsini. Variable Nb/Th ratios are a feature of Vulsinian magmas and such variations along with the relative decoupling of incompatible elements are linked to metasomatic processes and the development of a phlogopite- and clinopyroxene-enriched mantle.

Parker, R. J.

1989-12-01

88

Eruptive and Transportation Processes During Caldera-Forming Eruptions of Sete Cidades Volcano, São Miguel, Azores  

NASA Astrophysics Data System (ADS)

Sete Cidades volcano forms the Western part of the island of São Miguel, Azores, which is hosting three active trachytic central volcanoes (Sete Cidades, Fogo, Furnas). Volcanic activity in the archipelago exhibits a strong tectonic control and on São Miguel, the NW-SE trending basaltic Terceira Rift is intersecting the central volcanoes. All three have erupted since the settlement of the island in the 15{th} century. The Eastern part of the island is considered extinct. The oldest dated subaerial rocks of Sete Cidades exhibit an age of 210 ka. Morphology of the present summit caldera (5 km diameter, up to 350 m deep), stratigraphy, and distribution of the deposits suggest a multiple-stage evolution and at least three caldera-forming eruptions (CFE) are assumed to have occurred. 14C-dating revealed ages of 36, 29, and 16 ka, respectively, for the most recent ones. Today, the average slope angle is 12° and the maximum distance of the coastline from the caldera rim approx. 5 km. Assuming a comparable situation at the time of the CFE, a large portion of the eruptive products has probably not been deposited on land. After a pause of several thousand years, eruptive activity resumed approx. 5 ka ago and started filling the caldera. As deposits of minor thickness and distribution can be found between the deposits of the CFE, it is unclear whether the caldera formation is completely finished. Climatic factors (e.g. precipitation, air humidity) have affected the deposits by erosion, weathering, and possibly significant reworking and caused dense vegetation on all flanks of the volcano. Still, it was possible to establish distribution and thickness of the deposits of the CFE and constrain differences in eruptive behaviour and transport/emplacement mechanisms. They are composed of air-fall deposits and pyroclastic density currents but show significant differences amongst them: (1) Degree of pre- and syn-eruptive magma-magma interaction and syn-eruptive magma-water interaction. (2) Ratio of juvenile/lithic content and basaltic/trachytic magma. (3) Degree of vesiculation and crystal content of the juvenile material. (4) Percentage of air-fall deposits within the deposits of a single CFE and the timing of their deposition. (5) Distribution of air-fall deposits. (6) Degree of welding. The results highlight the bandwidth of possible eruptive scenarios at this trachytic central volcano cut by an active rift. Based on the study of these eruptions, volcanic hazard maps can be produced that are essential for adequate risk assessment.

Kueppers, U.; Queiroz, M. G.; Pacheco, J. M.

2007-12-01

89

The effect of air pollution on the stone decay of the Cologne Cathedral  

NASA Astrophysics Data System (ADS)

Different building stones of the Cologne Cathedral show a large variation of weathering phenomena. The Drachenfels trachyte, which was the construction material for the medieval part of the cathedral, shows significant surface deterioration, back-weathering coexisting with flaking, crumbling or the massive formation of gypsum crusts. Wolff (1992) first mentioned the negative interferences between the Schlaitdorfer sandstone and the Londorfer basalt lava or the Drachenfels trachyte and the Krensheimer muschelkalk. Crust formation on limestone, sandstone, and volcanic rock from the Cologne Cathedral as well as from the Xanten and Altenberg Cathedral are investigated. These three buildings are located in different areas and exposed to varying industrial, urban, and rural environmental situations. The material investigated range from dark grey to black framboidal crusts. This 3 to 10 mm thick cauliflower-like form of gypsum crust incorporates particles from the pollution fluxes. It covers the stone surface and mainly occurs at sites protected from wind and direct rain. Secondly, thin laminar black crusts trace the stone surface and may cover complete sections of the building's structure not necessarily preferring protected sites. This kind of crust seems to have very strong bonds between the thin black crust and the stone surface. Major and trace element distribution show an enrichment of sulfur, indicating the presence of gypsum, lead and other typical pollutants (arsenic, antimony, bismuth, tin etc.), which generally can be linked to traffic and industry. This indicates that even though the SO2 emission has decreased due to i.e. stronger regulations of waste incineration plants and the ban of leaded petrol, the pollutants are still present in the crusts on the building stones. From systematic SEM observations it becomes evident that the total amount of pollution is less pronounced in the Altenberg and Xanten Cathedrals as compared with the Cologne Cathedral. The formation of gypsum occurs at lower amounts in Altenberg, which correlates well with the measured SO2 content. On the other hand, the increasing H2O content in the trachyte and the crusts correlates well with an increasing phyllosilicate formation. Through the combination of different analytical techniques it was possible to clearly distinguish samples from the industrial or rural environment. If the data is compared to actual pollutant emissions, the analyzed samples imply present but also past pollution fluxes. Thus, the soiled zones of the built environment can function as environmental indicators.

Graue, B.; Siegesmund, S.; Licha, T.; Simon, K.; Oyhantcabal, P.; Middendorf, B.

2012-04-01

90

The relationship between potassic and calc-alkaline post-orogenic magmatism at Vico volcano, central Italy  

NASA Astrophysics Data System (ADS)

Post-orogenic magmatism in Italy took place during the Plio-Pleistocene. Alkaline ultrapotassic magmas and sub-alkaline magmas were erupted from closely-spaced volcanoes throughout the central part of Italy. Therefore, central Italy is one of the best site to study the relationship between calc-alkaline and alkaline magmatism in complex tectonic regimes. Activity at Vico volcano developed in three periods between 0.419 Ma and 0.095 Ma. Vico products overlie leucite-free volcanic rocks of Monte Cimino volcano that have a variable affinity from high-potassium calc-alkaline to ultrapotassic. Most of the Vico rocks are leucite-bearing Roman-type (ultrapotassic). However, during the first period, leucite-free silicic rocks are interbedded with leucite-bearing products. The early-erupted Vico magmas are isotopically distinct from the youngest magmas. Age-corrected Sr (Sr i=0.71110-0.71149) and Nd (Nd i=0.51210-0.51211) isotope ratios fall within the gap between Roman-type (including younger Vico) and Monte Cimino rocks. Some leucite-free Vico rocks contain orthopyroxene crystals and Al contents in clinopyroxenes are lower than those in clinopyroxenes from Roman-type rocks. These characteristics are, however, also found in Monte Cimino rocks. At the beginning of the first period, leucite-free latites to trachytes and rhyolites with high Sr i (0.71127-0.71149) were erupted. Trachytes and leucite-bearing latites with lower Sr i (0.71110-0.71138) were erupted later. The younger trachytes have higher Rb, and LRE element contents than older leucite-free rocks with similar SiO 2 contents. Furthermore, the onset of leucite crystallization is marked by a change in 87Sr/ 86Sr and SiO 2. Sr-isotope ratios and their time-related variations, as well as the presence of two different trends in some trace element variation diagrams, define two distinct modes of magma differentiation. Magmas evolved along a stratigraphically "lower trend" at the beginning of the activity whereas they evolved differently along a stratigraphically "upper trend" later in the volcanic history. Modelling of geochemical and Sr-isotope variations indicates that, along the lower trend, fractional crystallisation was the main evolutionary process, whereas along the upper trend mixing between a leucite-free magma and a leucite-bearing magma occurred during crystallisation. The Sr-isotope compositions of rocks from the early stage of activity at Vico suggest mixing between magmas having different petrological affinities: a leucite-bearing Roman-type magma and a high-potassium calc-alkaline magma.

Perini, Giulia; Conticelli, Sandro; Francalanci, Lorella; Davidson, Jon P.

2000-01-01

91

Évolution volcanologique du mont Manengouba (Ligne du Cameroun) ; nouvelles données pétrographiques, géochimiques et géochronologiquesVolcanological evolution of the mount Manengouba (Cameroon line), new petrographical, geochemical, and geochronological data  

NASA Astrophysics Data System (ADS)

The mount Manengouba is a polygenic volcanic complex of the Cameroon line, which was built in the Quaternary, between 1.55 and 0 Ma, during three stages. The first stage, from 1.55 to 0.7 Ma, corresponds to the building of the Elengoum volcano. The second, between 0.7 and 0.56 Ma, points to the collapse of the Elengoum summit. The third, from 0.56 to 0 Ma, fits with the erection of the Eboga volcano and its caldera sinking, and, from 0.48 Ma, with the adventive fissural activity. The products define an alkaline sodic series, from nepheline-bearing basalts to trachytes. All the lavas evolved by fractional crystallization in a periodically replenished magmatic chamber.

Kagou Dongmo, Armand; Wandji, Pierre; Pouclet, André; Vicat, Jean-Paul; Cheilletz, Alain; Nkouathio, David Guimolaire; Alexandrov, Pavel; Tchoua, Félix M.

2001-08-01

92

Reconnaissance geology of the Jabal Khatam Quadrangle, sheet 26/39 D, Kingdom of Saudi Arabia  

USGS Publications Warehouse

The Jabal Khatam quadrangle encompasses an area of about 2,725 km 2 on the edge of the Precambrian shield in northwestern Saudi Arabia, between lat 26?00' and 26?30' N. and long 39?30' and 40?00' E. The southern one-third and northeastern corner of the area are covered by Tertiary and Quaternary basalt flows of Harrat Khaybar and Harrat Ithnayn. In one small area on its northern edge, Harrat Khaybar is composed of Tertiary rhyolitic tuff and trachyte. Most of the northern one-third and eastern part of the quadrangle are covered by Paleozoic Siq Sandstone. The remainder of the quadrangle is composed of trachyandesite flows, agglomerate, graywacke, and sparse marble of the Hulayfah group and intrusive rocks that range in composition from alkali-feldspar granite to diabase. No deposits with economic potential were found. However, the geothermal potential of the quadrangle warrants further investigation.

Fairer, G. M.

1983-01-01

93

The Petrochemistry of Jake_M: A Martian Mugearite  

NASA Astrophysics Data System (ADS)

“Jake_M,” the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes).

Stolper, E. M.; Baker, M. B.; Newcombe, M. E.; Schmidt, M. E.; Treiman, A. H.; Cousin, A.; Dyar, M. D.; Fisk, M. R.; Gellert, R.; King, P. L.; Leshin, L.; Maurice, S.; McLennan, S. M.; Minitti, M. E.; Perrett, G.; Rowland, S.; Sautter, V.; Wiens, R. C.; Kemppinen, Osku; Bridges, Nathan; Johnson, Jeffrey R.; Cremers, David; Bell, James F.; Edgar, Lauren; Farmer, Jack; Godber, Austin; Wadhwa, Meenakshi; Wellington, Danika; McEwan, Ian; Newman, Claire; Richardson, Mark; Charpentier, Antoine; Peret, Laurent; Blank, Jennifer; Weigle, Gerald; Li, Shuai; Milliken, Ralph; Robertson, Kevin; Sun, Vivian; Edwards, Christopher; Ehlmann, Bethany; Farley, Kenneth; Griffes, Jennifer; Grotzinger, John; Miller, Hayden; Pilorget, Cedric; Rice, Melissa; Siebach, Kirsten; Stack, Katie; Brunet, Claude; Hipkin, Victoria; Léveillé, Richard; Marchand, Geneviève; Sánchez, Pablo Sobrón; Favot, Laurent; Cody, George; Steele, Andrew; Flückiger, Lorenzo; Lees, David; Nefian, Ara; Martin, Mildred; Gailhanou, Marc; Westall, Frances; Israël, Guy; Agard, Christophe; Baroukh, Julien; Donny, Christophe; Gaboriaud, Alain; Guillemot, Philippe; Lafaille, Vivian; Lorigny, Eric; Paillet, Alexis; Pérez, René; Saccoccio, Muriel; Yana, Charles; Armiens-Aparicio, Carlos; Rodríguez, Javier Caride; Blázquez, Isaías Carrasco; Gómez, Felipe Gómez; Gómez-Elvira, Javier; Hettrich, Sebastian; Malvitte, Alain Lepinette; Jiménez, Mercedes Marín; Martínez-Frías, Jesús; Martín-Soler, Javier; Martín-Torres, F. Javier; Jurado, Antonio Molina; Mora-Sotomayor, Luis; Caro, Guillermo Muñoz; López, Sara Navarro; Peinado-González, Verónica; Pla-García, Jorge; Manfredi, José Antonio Rodriguez; Romeral-Planelló, Julio José; Fuentes, Sara Alejandra Sans; Martinez, Eduardo Sebastian; Redondo, Josefina Torres; Urqui-O'Callaghan, Roser; Mier, María-Paz Zorzano; Chipera, Steve; Lacour, Jean-Luc; Mauchien, Patrick; Sirven, Jean-Baptiste; Manning, Heidi; Fairén, Alberto; Hayes, Alexander; Joseph, Jonathan; Squyres, Steven; Sullivan, Robert; Thomas, Peter; Dupont, Audrey; Lundberg, Angela; Melikechi, Noureddine; Mezzacappa, Alissa; DeMarines, Julia; Grinspoon, David; Reitz, Günther; Prats, Benito; Atlaskin, Evgeny; Genzer, Maria; Harri, Ari-Matti; Haukka, Harri; Kahanpää, Henrik; Kauhanen, Janne; Kemppinen, Osku; Paton, Mark; Polkko, Jouni; Schmidt, Walter; Siili, Tero; Fabre, Cécile; Wray, James; Wilhelm, Mary Beth; Poitrasson, Franck; Patel, Kiran; Gorevan, Stephen; Indyk, Stephen; Paulsen, Gale; Gupta, Sanjeev; Bish, David; Schieber, Juergen; Gondet, Brigitte; Langevin, Yves; Geffroy, Claude; Baratoux, David; Berger, Gilles; Cros, Alain; d'Uston, Claude; Forni, Olivier; Gasnault, Olivier; Lasue, Jérémie; Lee, Qiu-Mei; Meslin, Pierre-Yves; Pallier, Etienne; Parot, Yann; Pinet, Patrick; Schröder, Susanne; Toplis, Mike; Lewin, Éric; Brunner, Will; Heydari, Ezat; Achilles, Cherie; Oehler, Dorothy; Sutter, Brad; Cabane, Michel; Coscia, David; Israël, Guy; Szopa, Cyril; Teinturier, Samuel; Dromart, Gilles; Robert, François; Le Mouélic, Stéphane; Mangold, Nicolas; Nachon, Marion; Buch, Arnaud; Stalport, Fabien; Coll, Patrice; François, Pascaline; Raulin, François; Cameron, James; Clegg, Sam; DeLapp, Dorothea; Dingler, Robert; Jackson, Ryan Steele; Johnstone, Stephen; Lanza, Nina; Little, Cynthia; Nelson, Tony; Williams, Richard B.; Kirkland, Laurel; Baker, Burt; Cantor, Bruce; Caplinger, Michael; Davis, Scott; Duston, Brian; Edgett, Kenneth; Fay, Donald; Hardgrove, Craig; Harker, David; Herrera, Paul; Jensen, Elsa; Kennedy, Megan R.; Krezoski, Gillian; Krysak, Daniel; Lipkaman, Leslie; Malin, Michael; McCartney, Elaina; McNair, Sean; Nixon, Brian; Posiolova, Liliya; Ravine, Michael; Salamon, Andrew; Saper, Lee; Stoiber, Kevin; Supulver, Kimberley; Van Beek, Jason; Van Beek, Tessa; Zimdar, Robert; French, Katherine Louise; Iagnemma, Karl; Miller, Kristen; Summons, Roger; Goesmann, Fred; Goetz, Walter; Hviid, Stubbe; Johnson, Micah; Lefavor, Matthew; Lyness, Eric; Breves, Elly; Fassett, Caleb; Blake, David F.; Bristow, Thomas; DesMarais, David; Edwards, Laurence; Haberle, Robert; Hoehler, Tori; Hollingsworth, Jeff; Kahre, Melinda; Keely, Leslie; McKay, Christopher; Wilhelm, Mary Beth; Bleacher, Lora; Brinckerhoff, William; Choi, David; Conrad, Pamela; Dworkin, Jason P.; Eigenbrode, Jennifer; Floyd, Melissa; Freissinet, Caroline; Garvin, James; Glavin, Daniel; Harpold, Daniel; Mahaffy, Paul; Martin, David K.; McAdam, Amy; Pavlov, Alexander; Raaen, Eric; Smith, Michael D.; Stern, Jennifer; Tan, Florence; Trainer, Melissa; Meyer, Michael; Posner, Arik; Voytek, Mary

2013-09-01

94

Isotopic composition of lead and strontium from Ascension and Gough Islands  

USGS Publications Warehouse

Isotopic composition of lead and strontium has been determined in a series of rock samples from two islands on the Mid-Atlantic Ridge. Both inter-and intra-island variations exist in the abundance of radiogenic isotopes of both elements. Lead from basalt of Ascension Island has a Pb206-Pb 204 ratio of 19.5, while the corresponding ratio at Gough Island is only 18.4. The Pb208-Pb204 ratios from the two islands do not differ. Conversely, strontium from basalt of Ascension Island is less radiogenic than that from Gough Island basalts. The trachytes of both islands have lead and strontium that is more radiogenic than that found in the basalts. The inter-island differences indicate the existence of regional variations in the uranium-lead and rubidium-strontium ratios of the upper mantle source of these rocks and show that isotope compositions are a means for investigating chemical heterogeneities in the mantle.

Gast, P. W.; Tilton, G. R.; Hedge, C.

1964-01-01

95

The petrochemistry of Jake_M: a martian mugearite.  

PubMed

"Jake_M," the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes). PMID:24072927

Stolper, E M; Baker, M B; Newcombe, M E; Schmidt, M E; Treiman, A H; Cousin, A; Dyar, M D; Fisk, M R; Gellert, R; King, P L; Leshin, L; Maurice, S; McLennan, S M; Minitti, M E; Perrett, G; Rowland, S; Sautter, V; Wiens, R C

2013-09-27

96

Isotopic Composition of Lead and Strontium from Ascension and Gough Islands.  

PubMed

Isotopic composition of lead and strontium has been determined in a series of rock samples from two islands on the Mid-Atlantic Ridge. Both interand intra-island variations exist in the abundance of radiogenic isotopes of both elements. Lead from basalt of Ascension Island has a Pb(206)-Pb(204) ratio of 19.5, while the corresponding ratio at Gough Island is only 18.4. The Pb(208)-Pb(204) ratios from the two islands do not differ. Conversely, strontium from basalt of Ascension Island is less radiogenic than that from Gough Island basalts. The trachytes of both islands have lead and strontium that is more radiogenic than that found in the basalts. The inter-island differences indicate the existence of regional variations in the uranium-lead and rubidium-strontium ratios of the upper mantle source of these rocks and show that isotope compositions are a means for investigating chemical heterogeneities in the mantle. PMID:17743662

Gast, P W; Tilton, G R; Hedge, C

1964-09-11

97

East Sakhalin island arc paleosystem of the Sea of Okhotsk region  

NASA Astrophysics Data System (ADS)

It has been established that volcanic rocks of the Schmidt, Rymnik, and Terpeniya terranes are fragments of the compound Early to Late Cretaceous-Paleogene East Sakhalin island arc system of the Sea of Okhotsk region. This island arc paleosystem was composed of back-arc volcano-plutonic belt, frontal volcanic island arc, fore-arc, inter-arc, and back-arc basins, and the Sakhalin marginal paleobasin. The continental volcanic rocks dominate in the back-arc volcano-plutonic belt and frontal volcanic island arc. The petrochemical composition of basalts, basaltic andesites, andesites, and trachytes from the frontal island arc formed in submarine conditions are typical of oceanic island arc or marginal sea rocks (IAB). The petrochemical composition of volcanic rocks from the island arc structures indicates its formation on the heterogeneous basement including the continental and oceanic blocks.

Grannik, V. M.

2012-08-01

98

Tephrochronological study in the quaternary Val d'Agri intermontane basin (Southern Apennines, Italy)  

NASA Astrophysics Data System (ADS)

In the south-eastern depocentre of the Val d'Agri basin (Southern Apennines), a volcanic ash layer crops out interbedded within poorly structured alluvial fan deposits of Late Pleistocene age. Textural, depositional and pedological features of this weathered layer suggest a primary deposition from a pyroclastic fall-out of volcanic ash. Chemical analyses of feldspars show an alkali trachytic composition and accessory minerals association allow to correlate this tephra layer with the regionally dispersed Y-7 marine tephra layer (Tufo Verde Epomeo eruption, Ischia volcano), dated at 56 ± 4 ka. The Val d'Agri tephra here described for the first time was deposited during MIS Stage 3. Its recovery and characterization permit to contribute to regional correlation of the Mediterranean climatic and volcanic events from marine to continental successions and to describe landscape evolution of the Southern Apennines during glacial-interglacial cycles.

Zembo, Irene; Vignola, Pietro; Andò, Sergio; Bersezio, Riccardo; Vezzoli, Luigina

2011-01-01

99

The Scythian Platform north of Dobrogea (Romania, Moldova, and Ukraine)  

NASA Astrophysics Data System (ADS)

The Scythian Platform, along the southern margin of the East European Craton (EEC) from the East Carpathians foreland through the Crimean plains to the Fore-Caucasus, represents a highly controversial issue for the geology of the northern Black Sea area. Its westernmost part, known as the Predobrogea Depression (PDD), is a Mesozoic feature superimposed on a pre-Triassic basement, extending north of the North Dobrogea Orogen from the Vrancea zone to the Odessa shelf. Geological and geophysical data were integrated in order to investigate issues related to the age of basement consolidation and evolution of the PDD. A thick pile of Vendian, Devonian-Permian and, locally, Triassic sediments overlies cratonic basement. Buried beneath a flat-lying Jurassic to Tertiary cover, the main structure of the PDD is a Permo-Triassic rift. This structure, derived from interpretation of borehole data, is supported by 2D gravity and magnetic modelling, carried out along a trans-Scythian cross-section. Accompanied by bimodal volcanism, rifting has disrupted the Vendian to Carboniferous pre-rift successions accumulated on the craton margin in various tectonic settings. Syn-rift sediments, located in narrow, EW elongated half-grabens, are represented by continental terrigenous and volcano-sedimentary sequences, overlain in places by shallow marine carbonates. Rifting was accommodated by a system of EW faults that parallel the southern margin of the East European Craton. Syn-rift alkali volcanism of the bimodal basalt-trachyte association resulted in volcano-sedimentary successions interbedded with continental red-beds and evaporites; several syenite bodies and dykes of basalts and trachytes intruding the pre-rift sediments are likely to represent the feeder channels of this Permian intraplate volcanism. The evolution of the PDD rift ended in the Late Triassic when compressional deformation took place along its southern border, as a consequence of basement inversion in North Dobrogea. Meanwhile a compressional regime continued in the neighbouring North Dobrogea Orogen throughout most of the Jurassic.

Seghedi, A.; Stephenson, R. A.; Neaga, V.; Dimitriu, R.; Ioane, D.; Stovba, S.

2003-04-01

100

New 230Th- 238U and 14C age determinations from Piton des Neiges volcano, Reunion — A revised chronology for the Differentiated Series  

NASA Astrophysics Data System (ADS)

230Th- 238U age measurements were carried out on 18 samples of lava from Piton des Neiges. These lavas range from basalts to trachytes. The ages extend from 270,000 to 12,000 a B.P. and cover most of the period in which Differentiated Series magmas were erupted. New 14C ages on charcoal sampled in young pyroclastic deposits are also presented. The late-stage evolution of the volcano is discussed in the light of these new ages and a chronology of the explosive activity is given. The oldest pyroclastic flow (Ste. Suzanne) is more than 220,000 a; other pyroclastic units were emitted between 220,000 and 110,000 a. There were at least two episodes of caldera formation: the first episode is inferred to have occurred about 190,000 a ago; the last occurred between 190,000 and 110,000 a. Although the most recent lava activity was mainly concentrated on the eastern flank of the volcano and around the summit area, it also occurred, about 70,000 a ago, in the southwestern part of the massif. The youngest explosive activity (about 50,000 to 20,000 a) on the eastern flank of the volcano has now been dated by three different methods ( 14C and Th-U ages on block- and ash-flow tuffs, K-Ar ages on associated domes). Youthful explosive activity (about 35,000 to 12,000 a) on the western flank of the volcano is also demonstrated by 14C dating on charcoal and by a Th-U age of 12,000 a on a trachytic nuée ardente deposit. This last age indicates that activity may have persisted later than previously supposed and implies a high rate of erosion for the Cirque de Mafate.

Deniel, C.; Kieffer, G.; Lecointre, J.

1992-07-01

101

Classification and Geochemical Characterization of Igneous Rocks: Southern Part of Chihuahua City, Mexico  

NASA Astrophysics Data System (ADS)

Chihuahua City is the capital of the state with the same name, located in northern Mexico. The city was established near the Chuviscar River, but in the last decades it has been extended to the nearby areas (mountains), with volcanic (rhyolitic tuffs), and sedimentary rocks (limestone). The study area includes areas in the south part of Chihuahua City, where we can still find unbuilt lands and it is possible to appreciate outcrops of igneous rocks. This project includes 5 study spots, which are located about 9 km. far from the south extreme of the city. This research is developed in order to complement the geological information in this area, as there is no is detailed record of it. In the geological map H13-10 (SGM, 1997), it is said that the urban area is covered by Quaternary conglomerates, while exploring the region we have located several igneous rocks outcrops. In three of the sampling points, dark colored intrusive igneous rocks with large crystals appear in blocks without noticeable fractures. While in the other two sampling points, highly fractured blocks of pink aphanitic igneous rocks, showing traces of pyrolusite were observed. The petrographic study shows the two different textures that classify these rocks as extrusive (aphanitic) or intrusive (phaneritic), both with quartz and feldspars being the dominant minerals. Geochemical analyses confirm the felsic composition of the rocks, varying form trachytes to rhyolites. The trace element results show high contents of Sr, Ba, V, Rb, and Zr in trachytic compositions, while there are high concentrations of Mn, W, Rb and Co for rhyolitic compositions.

Fontes, I. D.; Espejel-Garcia, V. V.; Villalobos-Aragon, A.

2013-05-01

102

Tectonic significance of Neoproterozoic magmatism of Nakora area, Malani igneous suite, Western Rajasthan, India  

NASA Astrophysics Data System (ADS)

Three magmatic phases are distinguished in the Neoproterozoic Nakora Ring Complex (NRC) of Malani Igneous Suite (MIS), namely (a) Extrusive (b) Intrusive and (c) Dyke phase. Magmatism at NRC initiated with minor amount of (basic) basalt flows and followed by the extensive/voluminous acid (rhyolites-trachytes) flows. The ripple marks are observed at the Dadawari area of NRC in tuffaceous rhyolite flow which suggests the aqueous condition of flows deposition. The emplacement of the magma appears to have been controlled by a well defined NE-SW tectonic lineament and cut by radial pattern of dykes. These NE-SW tectonic lineaments are the linear zones of crustal weakness and high heat flow. The spheroidal and rapakivi structures in the Nakora acid volcanics indicate the relationship between genetic link and magma mixing. Basalt-trachyte-rhyolite association suggests that the large amount of heat is supplied to the crust from the magma chamber before the eruption. The field (elliptical/ring structures), mineralogical and geochemical characteristics of Nakora granites attest an alkaline character in their evolution and consistent with within plate tectonic setting. The emplacement of these granites and associated volcanics is controlled by ring structures, a manifestation of plume activity and cauldron subsidence, an evidence of extensional tectonic environment. NRC granites are the product of partial melting of rocks similar to banded gneiss from Kolar Schist Belt of India. The present investigations suggest that the magmatic suites of NRC rocks are derived from a crustal source and the required heat supplied from a mantle plume.

Kumar, Naresh; Vallinayagam, G.

2014-05-01

103

Petrochemical features of Miocene volcanism around the Çubukluda? graben and Karaburun peninsula, western Turkey: Implications for crustal melting related silicic volcanism  

NASA Astrophysics Data System (ADS)

Widespread Neogene volcanism, mainly intermediate and rarely mafic and felsic in composition, was controlled by the extensional tectonic regime in western Turkey. The Karaburun and Cumaovas? volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic volcanism. The Karaburun volcanics (KV) are mainly oriented north to south in the Karaburun peninsula and span a wide spectrum from basalt (20 Ma) to rhyolite (16 Ma), and younger trachyte and trachydacites (13 Ma). The products of the subaerial silicic volcanism (the Cumaovas? volcanics, CV; 17 Ma) which are represented by cluster of rhyolite domes, related pyroclastics occur within the NE–SW trending Çubukluda? graben, and intermediate and mafic volcanic rocks are lack in this area. The lavas of the Cumaovas? volcanics are high silica rhyolites and rare dacites which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, extremely Eu depletions and very low LaN/YbN ratios are typical for the rhyolites of CV, similar to the topaz rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. 87Sr/86Sr ratios of the KV and dacitic samples of CV are close to each other and range from 0.708 to 0.709; while Sr isotopic ratios of the rhyolites are significantly high and variable (0.724–0.786). 143Nd/144Nd ratios of the CV and KV, except for the alkaline samples, are similar for both sequences vary from 0.51230 to 0.51242.

Karac?k, Z.; Genç, ?. C.; Gülmez, F.

2013-09-01

104

Volcanic unrest in Kenya: geological history from a satellite perspective  

NASA Astrophysics Data System (ADS)

The East African Rift (EAR) system is a 5,000 km long series of fault bounded depressions that run from Djibouti to Mozambique. In the Kenyan Rift, fourteen Quaternary volcanoes lie along the central rift axis. These volcanoes are principally composed of trachyte pyroclastics and trachyte and basaltic lavas forming low-angle multi-vent edifices. Between 1997 and 2008, geodetic activity has been observed at five Kenyan volcanoes, all of which have undergone periods of caldera collapse and explosive activity. We present a remote-sensing study to investigate the temporal and spatial development of volcanic activity at Longonot volcano. High-resolution mapping using ArcGIS and an immersive 3D visualisation suite (GeovisionaryTM) has been used with imagery derived from ASTER, SPOT5 and GDEM data to identify boundaries of eruptive units and establish relative age in order to add further detail to Longonot's recent eruptive history. Mapping of the deposits at Longonot is key to understand the recent geological history and forms the basis for future volcanic hazard research to inform risk assessments and mitigation programs in Kenya. Calderas at Kenyan volcanoes are elliptical in plan view and we use high-resolution imagery to investigate the regional stresses and structural control leading to the formation of these elliptical calderas. We find that volcanoes in the central and northern segments of the Kenyan rift are elongated nearly parallel to the direction of least horizontal compressive stress, likely as a reflection of the direction of the plate motion vector at the time of caldera collapse. The southern volcanoes however are elongated at an acute angle to the plate motion vector, most likely as a result of oblique opening of the Kenyan rift in this region.

Robertson, E.; Biggs, J.; Edmonds, M.; Vye-Brown, C.

2013-12-01

105

Comenditic and pantelleritic ash-flow tuffs from Volcan Las Navajas, Nayarit, Mexico  

SciTech Connect

Two distinctive ash-flow tuffs occur around the base of Volcan Las Navajas, a Pleistocene trachyte - peralkaline rhyolite center located in the northwestern segment of the Mexican Volcanic belt. The lower ash-flow unit is locally up to 65 m thick, is lithic rich and contains pumice blocks of comenditic rhyolite. The unit is not extensively exposed, and thus its areal extent and volume cannot be determined. Its chemical characteristics and stratigraphic relationship to other products erupted from Las Navajas suggest that it is related to the formation of the older of the two calderas which occur on Las Navajas. Unconformably overlying this unwelded ash-flow is a pantelleritic airfall pumice unit which is locally welded. This airfall unit is conformably overlain by a welded as-flow tuff that contains fiamme of pantelleritic composition (72 %SiO/sub 2/, 8% FeO*, 900 ppm Zr, agpaitic index of 1.7) as well as pumice blocks that show evidence of various degrees of mixing between pantellerite and trachyte. This suggests eruption from a chemically zoned magma chamber. This unit is locally up to 20 m thick, although its top has been removed by erosion. It is found on all sides of Las Navajas except on the south where it may be covered by Volcan Sanganguey, a Pleistocene to Recent calc-alkaline volcano. The welded ash-flow has been dated by K - Ar at 0.2+/-0.1 m.y. Stratigraphically and chemically this ash-flow appears to be related to the formation of younger of the two calderas.

Nelson, S.A.; Hebre, J.A.

1985-01-01

106

Acoustic emissions generated during uniaxial compressive strength tests on Lyttelton volcano rocks, Christchurch, New Zealand  

NASA Astrophysics Data System (ADS)

Earthquakes comprising the ongoing Canterbury sequence, South Island, New Zealand, have exhibited disproportionately large energy magnitude (Me) to moment magnitude (Mw) ratios (Fry and Gerstenberger, 2011). The 22 February 2011 Mw 6.3 event, for example, had an energy magnitude of 6.7 (USGS). The 22 February event may have ruptured immature faults with high apparent stress formed during the emplacement of Banks Peninsula volcanic rocks (12 Ma-6 Ma); these faults may have been further strengthened by cross-cutting intrusive rocks of the Lyttelton volcano (Fry and Gerstenberger, 2011). We measured P-wave velocity (Vp), S-wave velocity (Vs), density, elastic moduli, and unconfined compressive strength of Lyttelton volcano basalt, trachyte and rhyolite. Unconfined compressive strength (UCS) tests were conducted on specimens fitted with axial and radial strain gauges using a stress-controlled unconfined compression apparatus following ASTM standard method. UCS values range between 165 and 232 MPa for the trachyte and basalt samples; rhyolite UCS values range between 122 and 126 MPa. During UCS testing, acoustic emissions were recorded using 2 broadband AE sensors (PAC WS? 20kHz-1MHz) mounted in the end platens. AE event waveforms, magnitude-frequency relationships, and spectrograms were analyzed. Deformation of each rock type involved brittle-failure-generated AE events with broadband waveforms; numbers of AE events increased exponentially at failure. The magnitude-frequency plots of AE events display a sharp decrease in relative energy emitted at frequencies greater than 600 kHz. Quantifying absolute energy emitted at high frequencies remains challenging; we present preliminary results from experiments designed to characterize broadband frequency attenuation. Our experiments quantify the unconfined compressive strengths, elastic moduli, and characteristic AE waveforms emitted during failure of intraplate volcanic rocks comprising the Lyttelton volcano. Reference: Fry, B., and M. Gerstenberger (2011). Large apparent stresses from the Canterbury earthquakes of 2010 and 2011. Seismological Research Letters 82, 833-838.

Boulton, C.; Villeneuve, M.; Goodin, C.

2012-04-01

107

Petrological and geochemical comparition between the upper and lower rhyolite of the Binchuan basaltic profile,Emeishan LIP succession  

NASA Astrophysics Data System (ADS)

Emeishan basalt is well known worldwide,and it has been well tested from the geochemistry and petrology.However,the eruptional rock sequences is rare reported. Some former work reported that on the top of Emeishan basalt,there is always sit with rhyolite(or felsic-composition,mainly are rhyolite and trachyte ).This work is focusing on newly found rhyolite and trachyte succession at the bottom of Binchuan basalt pofile,and the comparition between the bottom and top felsic-composition of the Binchuan pofile basaltic related succession from petrological and geochemistry points. The bottom rhyolite is lack of phenocryst,and the filling of blowhole is observed quartz only. Otherwise,the phenocryst of the top rhyolite is mainly alkaline-feldspar. The upper layered rhyolite is less sillical rich than the lower part with the content of SiO2 65-70,68-74 respectively. Additionally,the content of TiO2 (0.82-0.87,0.57-0.70),Total Fe2O3(5.15-5.87,2.89-4.88),MgO(2.13-2.64,0.19-0.48),CaO(1.18-1.49,0.13-0.42), P2O5(0.18-0.25,0.02-0.25) of the upper layered rhyolite is more abundant than the lower layer. However,the amount of the Na2O,K2O of both upper and lower rhyolite can not be distinguished clearly as weathering effect suspectively. This may note that the two kinds of rhyolite are formed from different geological process. The upper may due to the crystallization differentiation of the mafic magmas ,while the lower is formed in the result of crust remelting.

Huixin, H.; Yu, W.

2013-12-01

108

Preliminary AMS Study in Cretaceous Igneous Rocks of Valle Chico Complex, Uruguay: Statistical Determination of Magnetic Susceptibility  

NASA Astrophysics Data System (ADS)

The Valle Chico Complex, at southeast Uruguay, is related Paraná-Etendeka Province. The study involved basaltic lavas, quarz-syenites, and rhyolitic and trachytic dikes. Samples were taken from 18 sites and the AMS of 250 specimens was analyzed. The AMS is modeled by a second order tensor K and it graphical representation is a symmetric ellipsoid. The axes relations determine parameters which describe different properties like shape, lineation, and foliation, degree of anisotropy and bulk magnetic susceptibility. Under this perspective, one lava, dike, or igneous body can be considered a mosaic of magnetic susceptibility domains (MSD). The DSM is an area with specific degree of homogeneity in the distribution of parameters values and cinematic conditions. An average tensor would weigh only one MSD, but if the site is a mosaic, subsets of specimens with similar parameters can be created. Hypothesis tests can be used to establish parameter similarities. It would be suitable considered as a MSD the subsets with statistically significant differences in at least one of its means parameters, and therefore, be treated independently. Once defined the MSDs the tensor analysis continues. The basalt-andesitic lavas present MSD with an NNW magnetic foliation, dipping 10. The K1 are sub-horizontal, oriented E-W and reprsent the magmatic flow direction. The quartz-syenites show a variable magnetic fabric or prolate ellipsoids mayor axes dispose parallel to the flow direction (10 to the SSE). Deformed syenites show N300/11 magnetic foliation, consistent with the trend of fractures. The K1 is subvertical. The MSD defined in rhyolitic dikes have magnetic foliations consistent with the structural trend. The trachytic dikes show an important indetermination in the magnetic response. However, a 62/N90 magnetic lineation was defined. The MSDs obtained are consistent with the geological structures and contribute to the knowledge of the tectonic, magmatic and kinematic events.

Barcelona, H.; Mena, M.; Sanchez-Bettucci, L.

2009-05-01

109

Volcanic ash particles as carriers of remanent magnetization in deep-sea sediments from the Kerguelen Plateau  

NASA Astrophysics Data System (ADS)

Carbonate sediments from the Kerguelen Plateau (ODP Leg 120) of Eocene to Pliocene age were investigated with rock magnetic, petrographic and geochemical methods to determine the carriers of remanent magnetization. Magnetic methods showed that the major magnetic minerals were titanomagnetites slightly larger than single domain particles. Submicrometre to micrometre-size grains of titanomagnetite were identified as inclusions in volcanic glass particles or as crystals in lithic clasts. Volcanic fallout ash particles formed the major fraction of the magnetic extract from each sediment sample. Three groups of volcanic ashes were identified: trachytic ashes, basaltic ashes with sideromelane and tachylite shards, and palagonitic ashes. These three groups could be equally well defined based on their magnetic hysteresis properties and alternating field demagnetization curves. The highest coercivities of all samples were found for the tachylite, due to the submicrometre-size titanomagnetite inclusions in the matrix. Trachytic ashes had intermediate magnetic properties between the single-domain-type tachylites and the palagonitic (altered) basaltic ashes with low coercivities. Samples which contained mixtures of these different volcanic ashes could be distinguished from the three types of ashes based on their magnetic characteristics. There was neither evidence of biogenic magnetofossils in the transmission electron micrographs nor did we find magnetic particles derived from continental Antarctica. The presence of dispersed volcanic fallout ashes between visible ash layers suggests continuous explosive volcanic activity on the Kerguelen Plateau in the South Indian Ocean since the early Eocene. The continuous fallout of volcanic ash from explosive volcanism on the Kerguelen Archipelago is the source of the magnetic particles and thus responsible for the magnetostratigraphy of the nannofossil oozes drilled during Leg 120.

Heider, Franz; Ko¨rner, Ulrike; Bitschene, Peter

1993-07-01

110

Crystallization paths of leucite-bearing lavas: Examples from Italy  

NASA Astrophysics Data System (ADS)

The salic phases found in leucite-basanites, -trachytes, and -phonolites may be used to portray crystallization in the system NaAlSiO4-KAlSiO4-CaAl2Si2O8-SiO2, the phonolite pentahedron. Only two lavas have been found that contain the assemblage leucite-nepheline-plagioclase-sanidine and liquid, a natural pseudo-invariant assemblage (at 900° C±100) equivalent to the isobaric invariant point of the four component system. The diversity of phases in this group of lavas illustrates the role of halogens in controlling their crystallization paths. Thus the presence of F in the leucite-basanites has stabilized magnesian biotite and suppressed sanidine, as has been found in other basanitic lavas (Brown and Carmichael 1969). The presence of Cl in these same lavas has induced the crystallization of sodalite, which takes the place of nepheline in the groundmass. However in the leucite-trachytes, biotite has suppressed olivine and coexists with sanidine and leucite. The presence of S may produce haüyne at the expense of nepheline, and in general sulphate minerals, which include apatite, have the role in lavas of low silica activity that pyrrhotite plays in liquids of high silica activity. Both pyroxenes and titaniferous magnetites in this suite of lavas are very aluminous. Groundmass crystals of pyroxene may have one-fifth of Si replaced by Al. Other phases which occur occasionally are melanite garnet and a potassium-rich hastingsite, but neither ilmenite nor a sulphide mineral has been found. Phenocryst equilibration temperatures, derived from olivine and Sr-rich plagioclase, are generally in the range from 1,050° C to 1,150° C. The high content of incompatible elements (e.g., K, Ba, Rb, F, Sr, P) in these lavas suggests that they represent a small liquid fraction from a mantle source which possibly contains phlogopite.

Baldridge, W. Scott; Carmichael, I. S. E.; Albee, A. L.

1981-05-01

111

Maximum Historical Seismic Intensity Map of S. Miguel Island (azores)  

NASA Astrophysics Data System (ADS)

The Azores archipelago is situated in the Atlantic Ocean where the American, African and Eurasian lithospheric plates meet. The so-called Azores Triple Junction located in the area where the Terceira Rift, a NW-SE to WNW-ESE fault system with a dextral component, intersects the Mid-Atlantic Ridge, with an approximate N-S direction, dominates its geological setting. S. Miguel Island is located in the eastern segment of the Terceira Rift, showing a high diversity of volcanic and tectonic structures. It is the largest Azorean island and includes three active trachytic central volcanoes with caldera (Sete Cidades, Fogo and Furnas) placed in the intersection of the NW-SE Ter- ceira Rift regional faults with an E-W deep fault system thought to be a relic of a Mid-Atlantic Ridge transform fault. N-S and NE-SW faults also occur in this con- text. Basaltic cinder cones emplaced along NW-SE fractures link that major volcanic structures. The easternmost part of the island comprises an inactive trachytic central volcano (Povoação) and an old basaltic volcanic complex (Nordeste). Since the settle- ment of the island, early in the XV century, several destructive earthquakes occurred in the Azores region. At least 11 events hit S. Miguel Island with high intensity, some of which caused several deaths and significant damages. The analysis of historical documents allowed reconstructing the history and the impact of all those earthquakes and new intensity maps using the 1998 European Macrosseismic Scale were produced for each event. The data was then integrated in order to obtain the maximum historical seismic intensity map of S. Miguel. This tool is regarded as an important document for hazard assessment and risk mitigation taking in account that indicates the location of dangerous seismogenic zones and provides a comprehensive set of data to be applied in land-use planning, emergency planning and building construction.

Silveira, D.; Gaspar, J. L.; Ferreira, T.; Queiroz, G.

112

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

NASA Astrophysics Data System (ADS)

The bimodal, Late Cretaceous-Palaeocene (71-61 Ma) 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. 70 Ma silicic lavas and ignimbrites dominate over mildly alkaline basalts. Intermediate rocks are scarce (SiO 2 = 57-62 wt.%) and cognate mafic inclusions, resorbed crystals, and highly variable clinopyroxene compositions indicate that they formed by magma mixing. Silicic lavas (66-74 wt.% SiO 2) 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 Or 98-99 in a host of Ab 99, 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.

Thorarinsson, Sigurjon B.; Holm, Paul Martin; Duprat, Helene; Tegner, Christian

2011-07-01

113

High to ultrahigh potassic alkaline volcanic belt along the Ankara-Erzincan suture (northern Turkey): new geochemical and Ar-Ar data constraining petrogenesis with implications for the late Cretaceous subduction of the Neotethys Ocean  

NASA Astrophysics Data System (ADS)

Remnants of some high- to ultrahigh-K alkaline volcanic rocks crop out as isolated small and discontinuous bodies along the Ankara-Erzincan suture belt in northern Turkey. These rocks are represented by leucite-bearing lavas (LB), basaltic andesites, trachytes, monzonite/syenites) and lamprophyres. Leucite-bearing rocks are small stocks, dikes and lava flows. Pebbles and blocks of the LB are found in the coeval volcanic debris avalanche deposits and volcanoclastic breccias. Leucite-bearing rocks are mainly phonotephrite, tephriphonolite, trachyandesite and basaltic trachyandesites (shoshonite) and have mineral assemblages of lct + cpx + ol + pl + Kfs + mag+ ap. Leucites were almost totally analcimized. Trachytes and monzonite/syenites, which are seen as small stocks and dikes, are characterized by amp + bt + pl + Kfs + spn + ap + opq paragenesis. Lamprophyres are mica-rich melanocratic dikes, and include cpx + mica (phlogopitic) + Kfs + ap + opq. Rarely leucite, olivine and plagioclase are also present. Ar-Ar data reveal that this volcanic activity occurred between 73.6±0.18 and 76.78±0.19 Ma, corresponding to latest Cretaceous. All the samples from the high- and ultrahigh-K volcanic belt are alkaline in nature. Leucite-bearing lavas are characterized by their MgO (2.70-5.81, av. 4.58 wt.%), K2O (0.79-4.81, av. 2.35 wt.%), Na2O (4.86-7.48, av. 3.58 wt.%) and K2O/Na2O (0.13-0.92, av. 0.42 wt.%). The low K2O and K2O/Na2O contents of these rocks are due to extensive analcimization of the leucites. Major oxide contents in lamprophyric rocks are 3.25-7.48 (MgO), 1.35-7.76 (K2O), 1.77-4.00 (Na2O) and 0.31-2.69 (K2O/Na2O). The silica content of these rocks are variable and range from 47.18-50.26 (wt.%) (LB) to 39.14-53.28 (lamprophyres). Based on their major element contents, these rocks are classified as plagioleucitites or ultrapotassic rocks of the active orogenic zones (Foley, 1992). Leucite-bearing rocks, lamprophyres and the trachytes (with their hypabyssal equivalents) display similar geochemical behavior. Their typical features are high LILE relative to HFSE, and Nb-Ta and Ti depletions on the P-MORB normalized plots. In the lamprophyre group, Hf and Zr depletions are also evident. Mg# for all the rock series are highly variable (34 - 60), implying that these rocks formed from evolved melts. Considering the Ar-Ar ages, Nb-Ta depletions, LILE enrichments, some inter-elemental ratios and the geological evolution of the Ankara-Erzincan suture belt, we conclude that the late Cretaceous alkaline high- to ultrahigh-K magmatic products are common in space, time and origin in this region. All the rock groups were derived from partial melting of lithospheric mantle modified by subduction-related fluids, and from the melting of the subducted crustal material. Additionally, lamprophyric melts were possibly generated from the melting of veined SCLM. Veins were probably rich in phlogopitic mica and clinopyroxene. We propose that this alkaline high- to ultrahigh-K volcanic belt is the product of subduction-related magmatism, and resulted from northerly subduction of the Neotethyan Ocean crust during the latest Cretaceous period.

Genc, S. Can; Gulmez, Fatma; Tuysuz, Okan; Karacik, Zekiye; Roden, Michael F.; Zeki Billor, M.; Hames, Willis E.

2013-04-01

114

Peralkaline magma evolution and the tephra record in the Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75-76 wt% SiO2) in comparison with the pumice (64-75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50-60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50-60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal-proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.

Rooney, Tyrone O.; Hart, William K.; Hall, Chris M.; Ayalew, Dereje; Ghiorso, Mark S.; Hidalgo, Paulo; Yirgu, Gezahegn

2012-09-01

115

Hazards associated with alkaline glaciovolcanism at Hoodoo Mountain and Mt. Edziza, western Canada: comparisons to the 2010 Eyjafjallajokull eruption  

NASA Astrophysics Data System (ADS)

The hazards associated with 2010 eruption from Eyjafjallajokull were well documented, and included flooding, pyroclastic activity, and local/regional ash and aerosol dispersal (e.g., Gudmundsson et al, 2010, Session V27). At least two ice-capped, alkaline volcanoes in northwestern British Columbia could produce similar styles of eruptive activity with associated local and regional hazards: Hoodoo Mountain and the Mt. Edziza volcanic complex. Similar to Eyjafjallajokull, both of the Canada volcanoes have likely had multiple Holocene eruptions of lava flows from beneath snow/ice cover, both eruption mildly alkaline basaltic to trachytic lavas, and both also have a history of explosive eruptions. Hoodoo Mountain volcano, which is approximately 17 cubic kilometers in volume, erupts dominantly trachyte-phonolite composition lavas, although it also has closely associated basaltic centers. Most of its history has been dominated by effusive eruptions, but at least one thick sequence of eutaxitic pyroclastic materials has been erupted in the past 50 ka (Edwards et al, 2002). It is presently covered by snow and a 3 km diameter ice cap that feeds a few small, flank alpine glaciers. The Mt. Edziza volcanic complex is much larger, with an estimated total eruptive volume of approximately 650 cubic km; it has had an extended eruptive history during the Plio-Pleistocene (Souther et al, 1984) including basaltic and trachytic eruptions. It presently hosts an ice-filled summit caldera approximately 2.8 by 2 km, which feeds several alpine glaciers radiating outwards in all directions. Edziza has several striking morphological similarities to Eyjafjallajokull, including the similar-sized summit ice cap dominated by silicic eruption products, an overall elongate morphology, and flanking fields of basaltic lava flows. Although Hoodoo and Edziza volcanoes are located in relatively remote parts of British Columbia, eruptions from either would likely partly melt existing snow and ice cover, generating locally important lahars and flooding along major BC water courses (Iskut and Stikine rivers). More importantly, fine silicic ash produced by phreatomagmatic activity could be a significant hazard for North American airspace, just as ash from Alaskan eruptions (e.g. Redoubt 1989/1990) has caused occasional air traffic problems. This possibility has recently been re-enforced by studies of lacustrine-deposited ash across western Canada (Lakeman et al 2008). *Edwards et al (2002) Subglacial, phonolitic volcanism at Hoodoo Mountain volcano, Canadian Cordillera, Bull Volc. DOI: 10.1007/s00445-002-0202-9 *Lakeman et al (2008) Holocene tephras in lake cores from northern British Columbia: Can. J. Earth Sci. 45, 935-947. *Souther et al (1984) Chronology of the peralkaline late Cenozoic Mount Edziza volcanic complex, northern British Columbia, Canada. Geol. Soc. Am. Bull. 95, 337-349.

Edwards, B. R.

2010-12-01

116

The last 5000 years of activity at Sete Cidades volcano (São Miguel Island, Azores): Implications for hazard assessment  

NASA Astrophysics Data System (ADS)

Sete Cidades is a central volcano with a summit caldera at the western end of São Miguel Island, Azores. Its stratigraphy comprises two main geological groups: the Inferior Group, the units of which date from more than 200 000 years ago through to 36 000 years before present, consisting of thick lava flows and subaerial volcaniclastic deposits that built the base of the central volcano; and the Superior Group which comprises all the activity from the last 36 000 years, including pumice and scoria fallout and PDC deposits with minor lava flows. The volcanostratigraphy is divided into six main formations — Risco, Ajuda, Bretanha, Lombas, Santa Bárbara and Lagoas, each defined by different activity phases in the volcano's evolution. The present caldera developed in three phases associated with massive paroxysmal eruptions which occurred approximately 36 000, 29 000 and 16 000 years before present. Since the last caldera forming event, eruptive activity has been predominantly explosive and a variety of deposits have been produced by different eruptive styles, associated with three specific settings: (1) pumice fall deposits, pyroclastic density currents and hydromagmatic events from eruptions inside the caldera; (2) lava domes, cinder cones and basaltic lava flows from eruptions on the volcano flanks; (3) tuff ring and tuff deposits from surtseyan eruptions offshore. The more recent history of the volcano is marked by a change in the intracaldera activity from dominantly magmatic behaviour to a hydromagmatic character, which happened about 5000 years ago. Since then, at least 15 basaltic subaerial and submarine eruptions have occurred on the volcano flanks and 17 trachytic (s.l.) explosive eruptions have occurred within the caldera, the most recent of which took place about 700 years ago. This eruptive frequency makes Sete Cidades probably the most active volcanic centre in the Azores. In this paper we present the stratigraphy and a description of the deposits which originate from the last 5000 years, in order to delineate Sete Cidades' recent eruptive history for the purpose of estimating the hazard associated with this volcano. An event tree has been designed to systematize possible future eruptive scenarios and to aid the assessment of relative probabilities of occurrence of different potential eruptive styles. The probabilities for all nodes on the event tree, together with their associated uncertainties, were obtained by expert elicitation, providing results that accord with the geologic record. These probability estimates indicate that (1) basaltic strombolian and submarine eruptions are the most likely to occur, (2) trachytic activity from the central volcano magmatic system is more probable in the intracaldera environment than on the flanks, and (3) explosive activity is more likely than effusive for the trachytic system. Worst-case scenario and maximum expected events are identified according to the eruptive scenarios and their probabilities of occurrence.

Queiroz, G.; Pacheco, J. M.; Gaspar, J. L.; Aspinall, W. P.; Guest, J. E.; Ferreira, T.

2008-12-01

117

Volcanisms and Earthquakes Related to the Pacific Plate Subduction in Northeast Asia  

NASA Astrophysics Data System (ADS)

It is very known that an integrated plate system displays in Northeast Asia from the Pacific Plate subduction zone via arc islands and back-arc basin to the continental margin with rifting system. Based on this geological background many huge earthquakes and volcanic eruptions occurred in this area from the Mesozoic to the present such as Fujiyama Volcano in Japan, Cheju Volcano in South Korea, Changbaishan Volcano in China and M 7.3 deep focus earthquake in Wangqing, Northeast China of June 28,2002, M9 earthquake in Northeast Japan of March 11,2011 and so on. Now it is tectonic active phase in the Northeast Asia, even in the globe. The Changbaishan Volcano is huge volcanic group with some 12-103 km2 area and hundreds volcanic cones crossed the boundary between China and Korea covered 41° -42.5° latitude north and 127° -129° longitude east. It is among largest active and dangerous volcanoes on the Globe and composed of three main volcanoes (eruptive centers): Tianchi(2755 m a.s.l.), Wangtian'e (2438m a.s.l.) and South Paotaishan (2434m a.s.l.), which distribution assumes as tripod. These three eruptive centers have similar magma system and different ages. They were built from the Early Miocene to the Recent by basaltic flow as lava plateau, trachyte composing of volcanic cones and pyroclastic deposits covering the tops of the mountains and other places. Tianchi volcano is younger than others. According to historic documents the largest eruption of Tianchi volcano occurred in 1014-1019 AD., after that there were still several eruptions until 1903 AD. The frequencies of Changbaishan volcanic eruptions corresponded to those of the Pacific, especially Japan. There is systematic magma evolution from basic basalt, intermediate trachyte to acid pantellerite with 87Sr/86Sr 0.704771-0.710096, 143Nd/144Nd 0.512487-0.512602, which indicated that the magma derived from rich mantle. Geophysical data reveal a buried magmatic reservoir is lying below the volcanoes. Recently, the west Pacific fire ring is very active accompanied with frequent volcanic eruptions and earthquakes; the earthquake frequency and intensity and geochemical anomaly also obviously strengthen in the surrounding of Changbaishan volcanoes. It reveals that volcanic activity and possibility of re-eruption is going to strengthen. Therefore we must put attention to volcanic action.

Liu, J.; Chen, X.

2012-04-01

118

Spatio-temporal evolution of a dispersed magmatic system and its implications for volcano growth, Jeju Island Volcanic Field, Korea  

NASA Astrophysics Data System (ADS)

Jeju Island is the emergent portion of a basaltic volcanic field developed over the last c. 1.8 Ma on continental crust. Initial volcanism comprised dispersed, small-volume (< 0.01 km3) alkali basaltic eruptions that incrementally constructed a tuff pile. Lavas and scoria from continuing small-scaled monogenetic volcanism capped this foundation. From c. 0.4 Ma large-volume (> 1 km3) eruptions began, with lavas building a composite shield. Three magma suites can be recognized: Early Pleistocene high-Al alkali (HAA), and Late Pleistocene to Holocene low-Al alkali (LAA) and subalkali (SA). The chemical similarity between small-volume and primitive large-volume eruptions suggests analogous parent magmas and fractionation histories that are independent of erupted volumes. The large-volume magmas evolved to trachyte, which erupted in two distinct episodes: the HAA Sanbangsan suite at c. 750 ka and the LAA Hallasan suite at c. 25 ka. Sr and Nd isotopes indicate that the early trachytes were contaminated by upper crustal material, whereas the later magmas were not. Both suites bear a Nd isotope signature indicative of lower crustal interaction. Sub-suites transitional between HAA and LAA, and between LAA and SA, indicate that melting occurred in discrete, but adjacent, mantle domains. Throughout the evolution of this volcano, each magma batch erupted separately, and a centralized plumbing system was never created. The Island's central peak (Mt. Halla 1950 m a.s.l.) is therefore not a sensu stricto stratovolcano, but marks the point of peak magma output in a distributed magmatic system. Jeju's shape and topography thus represent the spatial variation of fertility of the mantle below it. An increase in melt production in the Late Pleistocene was related to a deepening of the melting zone due to regional tectonic rearrangements. Temporal coincidences between magmatic pulses on Jeju and large-scale caldera eruptive events along the nearest subduction system in Kyushu, Japan, suggest that tectonic extension and changing strain rates may drive volcanism on a regional basis, influencing the intraplate volcanism of Jeju Island.

Brenna, Marco; Cronin, Shane J.; Smith, Ian E. M.; Sohn, Young Kwan; Maas, Roland

2012-09-01

119

Boron, Thorium and Oxygen Isotopes in Icelandic Tephra  

NASA Astrophysics Data System (ADS)

Important Th-isotope variations in silicic rocks from Iceland have been interpreted in terms of partial melting of a metabasic crust. This hypothesis was tested by boron systematics, since both the B-isotope ratios and their concentration are significantly different in the altered oceanic crust and the underlying mantle. Here we present new boron isotope compositions and concentrations measured by ion microprobe (3f, Nancy, France) on Holocene tephra samples from 6 Icelandic volcanoes. The B concentrations vary by more than a factor 20, from 1.48±0.03 to 36.38±4.46 ppm in basaltic tholeiite and trachytes, respectively. A large range is also observed for B isotope compositions, between - 9.8±2.0 in a trachyte to +7.6±2.6‰ in a subalkaline rhyolite from Askja volcano. Furthermore, the ?^{11}B correlates positively with Th/B (0.26 < Th/B < 2.90) and negatively with 230Th/232Th (0.940 < (230Th/232Th) < 1.067) and with ?18O (-0.2 to +5.3‰). The variations of B isotope compositions in the Icelandic tephra and the correlations with both ?18O and (230Th/232Th) strongly support the crustal melting model for most Icelandic rhyolites. In fact, the co-variation is interpreted as a mixing between mantle derived basalts and crustal melts derived from the anatexis of hydrothermally altered basaltic crust. The highest ?^{11}B (+7.6±2.6 and +7.1±2.1‰) measured in rhyolites from the rift-related Askja and Krafla volcanoes represent the B-isotope composition for the altered Icelandic crust. These values are indeed well within the range of values proposed for altered oceanic crust (between +0.1 and +9.2‰, Spivack and Edmond, 1987 ; +3.4±1.1‰, Smith et al., 1995). In contrast, the lowest ?^{11}B values (-9.8 to -5.7‰) are observed in samples with the highest ?18O (+4.95 to +5.30‰) and (230Th/232Th; 0.990 to 1.067) corresponding to a "normal-mantle" signature (?^{11}B =- 9.9±1.3‰; Chaussidon and Marty, 1995).

Rose-Koga, E. F.; Sigmarsson, O.

2006-12-01

120

Monogenetic volcanic fields and their geoheritage values of western Saudi Arabia and their implication to holistic geoeducation projects locally and globally (Invited)  

NASA Astrophysics Data System (ADS)

Monogeneitc volcanic fields are the most common manifestation of volcanism on Earth and other planets. They composed of small volume and short lived volcanoes each of them with a relatively simple eruption history. In spite of recent researches demonstrated complex, repeated and geochemically distinct eruption histories commonly associated with te formation of small-volume volcanoes, they are still considerred as volcanoes that are in human-scale and therefore ideal to use them as educational tools or part of volcanic geoheritage projects including geopark developments. In the western margin of the Kingdom of Saudi Arabia there are at least 9 intracontinental volcanic fields subparalell with the Red Sea Rift ranging from alkaline basaltic to basalt-trachyte bimodal dispersed volcanic systems. Among these volcanic fields the geoheritage value of three fields were recently evaluated and proposed that they are suitable for further development to establish the first volcanic geoparks in the Arabian Peninsula in the area of 1) Al Madinah (AMVF) 2) Kishb (KVF) and 3) Hutaymah Volcanic Fields (HVF). The AMVF offers a natural concept based on specific volcanic precinct ordering of its volcanic geoheritages from the most accessable and most common volcanism that is historically significant (eg. scoria and lava spatter cones with extensive lava fields) toward a more adventure geotourism style approach in remote, less common but more destructive type of volcanism (eg. trachytic explosion craters). In the contrary, the KVF is a perfect site where phreatomagmatic volcanism and their consequences were identified as a major driving force for further geopark developments. The HVF with its rich archaeological and cultural sites and superbly exposed variously eroded tuff rings and maars offer a good location to develop geoeducation programs to highlight short- and long-term climatic and hydrologic changes in an area a volcanic field evolved. The three Saudi projects also demonstrate the need to arrange and coordinate geoeducational projects locally and globally around common geological assets such as monogenetic volcanic fields. We also provide a conceptual model to link various sites of monogenetic volcanic fields along a volcanologically valid holistic geoconservation and geoeducation programs that are scientifically well-established. Well-preserved maar craters such as Harrat Hutaymah (A) and erosionally enlarged maars such as Tabah (B) are internationally significant geotopes of volcanic geoheritage sites of Saudi Arabia

Nemeth, K.; Moufti, R.

2013-12-01

121

Viscosity of hydrous Etna basalt: implications for Plinian-style basaltic eruptions  

NASA Astrophysics Data System (ADS)

Water dissolved in a silicate melt can strongly influence its physical properties and thus magma behavior during crystallization, degassing, foaming and fragmentation. Etna is a basaltic volcano whose activity is dominated by effusive eruptions which have long represented a threat to the densely populated, surrounding area. Recently, recognition of the products of a Plinian eruption (122 B.C.) has raised further issues for hazard assessment at Etna and other basaltic volcanoes. Constraining the behavior of Etna magma under conditions relevant to both effusive and explosive hazards requires viscosity data under conditions near the glass transition. Here we have investigated the viscosity of hydrous Etna lava in order to better understand eruptive processes which characterize this volcano. The experimental methods which have been used include piston cylinder synthesis of the hydrated melts, micropenetration viscometry for low-temperature viscosity measurements, and near-infrared spectroscopy for the evaluation of sample homogeneity and measurements of water content. Additionally, scanning calorimetric determinations were performed to check whether incipient crystallization had occurred. Sample compositions were determined using electron microprobe analysis and 57Fe Mössbauer spectroscopy. Results from this study are compared with previous reports of trachytic, phonolitic and model calc-alkaline rhyolite (HPG8) compositions. The viscosity of the basaltic melt (dry and wet) has been parameterized in terms of temperature and water content via the non-Arrhenian equation: log10?=-4.643+(5,812.44-427.04×H2O)/(T(K)-499.31+28.74×ln(H2O)) where ? is the viscosity in Pa s, H2O is the water content in wt%, and T is the temperature in Kelvin. We observe that the viscosity of alkali basalt (at more than 0.5 wt% H2O) is similar to that of an alkaline trachyte (Agnano-Monte Spina eruption, Phlegrean Fields) and much higher than that of a peralkaline phonolite (Teide, Tenerife) at similar silica contents and NBO/T. For water contents above 1.5 wt%, the viscosity of the basalt is similar to that of rhyolitic melts with similar water contents. At temperatures ranging from 1,050 to 1,150 °C and with water contents between 0.5 and 2.3 wt% (eruptive conditions), the viscosities calculated using the equation defined in this study are (1) in reasonable agreement with those calculated using Shaw's model, and (2) much lower than those experimentally determined in a previous study. However, outside these temperature and water content ranges, the agreement with Shaw's model (1972) breaks down.

Giordano, D.; Dingwell, D. B.

2002-07-01

122

Viscosity of hydrous Etna basalt: implications for Plinian-style basaltic eruptions  

NASA Astrophysics Data System (ADS)

Water dissolved in a silicate melt can strongly influence its physical properties and thus magma behavior during crystallization, degassing, foaming and fragmentation. Etna is a basaltic volcano whose activity is dominated by effusive eruptions which have long represented a threat to the densely populated, surrounding area. Recently, recognition of the products of a Plinian eruption (122 B.C.) has raised further issues for hazard assessment at Etna and other basaltic volcanoes. Constraining the behavior of Etna magma under conditions relevant to both effusive and explosive hazards requires viscosity data under conditions near the glass transition. Here we have investigated the viscosity of hydrous Etna lava in order to better understand eruptive processes which characterize this volcano. The experimental methods which have been used include piston cylinder synthesis of the hydrated melts, micropenetration viscometry for low-temperature viscosity measurements, and near-infrared spectroscopy for the evaluation of sample homogeneity and measurements of water content. Additionally, scanning calorimetric determinations were performed to check whether incipient crystallization had occurred. Sample compositions were determined using electron microprobe analysis and 57Fe Mössbauer spectroscopy. Results from this study are compared with previous reports of trachytic, phonolitic and model calc-alkaline rhyolite (HPG8) compositions. The viscosity of the basaltic melt (dry and wet) has been parameterized in terms of temperature and water content via the non-Arrhenian equation: log10?=-4.643+(5,812.44- 427.04×H2O)/(T(K)- 499.31+28.74×ln(H2O)) where ? is the viscosity in Pa s, H2O is the water content in wt%, and T is the temperature in Kelvin. We observe that the viscosity of alkali basalt (at more than 0.5 wt% H2O) is similar to that of an alkaline trachyte (Agnano-Monte Spina eruption, Phlegrean Fields) and much higher than that of a peralkaline phonolite (Teide, Tenerife) at similar silica contents and NBO/T. For water contents above 1.5 wt%, the viscosity of the basalt is similar to that of rhyolitic melts with similar water contents. At temperatures ranging from 1,050 to 1,150 °C and with water contents between 0.5 and 2.3 wt% (eruptive conditions), the viscosities calculated using the equation defined in this study are (1) in reasonable agreement with those calculated using Shaw's model, and (2) much lower than those experimentally determined in a previous study. However, outside these temperature and water content ranges, the agreement with Shaw's model (1972) breaks down.

Giordano, D.; Dingwell, D. B.

123

Petrogenesis of the Sabongari alkaline complex, cameroon line (central Africa): Preliminary petrological and geochemical constraints  

NASA Astrophysics Data System (ADS)

The petrography, mineral chemistry and geochemical features of the Sabongari alkaline complex are presented and discussed in this paper with the aim of constraining its petrogenesis and comparing it with other alkaline complexes of the Cameroon Line. The complex is mainly made up of felsic rocks: (i) granites predominate and include pyroxene-amphibole (the most abundant), amphibole-biotite, biotite and pyroxene types; (ii) syenites are subordinate and comprise amphibole-pyroxene and amphibole-biotite quartz syenites; (iii) pyroxene-amphibole-biotite trachyte and (iv) relatively abundant rhyolite. The minor basic and intermediate terms associated with felsic rocks consist of basanites, microdiorite and monzodioites. Two groups of pyroxene bearing rocks are distinguished: a basanite-trachyte-granite (Group 1) bimodal series (SiO2 gap: 44 and 63 wt.%) and a basanite-microdiorite-monzodiorite-syenite-granite (Group 2) less pronounced bimodal series (reduced SiO2 gap: 56-67 wt.%). Both are metaluminous to peralkaline whereas felsic rocks bare of pyroxene (Group 3) are metaluminous to peraluminous. The Group 1 basanite is SiO2-undersaturated (modal analcite in the groundmass and 11.04 wt.% normative nepheline); its Ni (240 ppm) and Cr (450 ppm) contents, near mantle values, indicate its most primitive character. The Group 2 basanite is rather slightly SiO2-saturated (1.56 wt.% normative hypersthene), a marker of its high crustal contamination (low Nb/Y-high Rb/Y). The La/Yb and Gd/Yb values of both basanites (1: 19.47 and 2.92; 2: 9.09 and 2.23) suggest their common parental magma composition, and their crystallization through two episodes of partial melting (2% and 3% respectively) of a lherzolite mantle source with <4% residual garnet. The effects of crustal contamination were selectively felt in the values of HFSE/LREE, LREE/LILE and LREE/HFSE ratios, known as indicators. Similar features have been recently obtained in the felsic lavas of the Cameroon Volcanic Line.

Njonfang, Emmanuel; Tchoneng, Gilbert Tchuenté; Cozzupoli, Domenico; Lucci, Federico

2013-07-01

124

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas  

NASA Astrophysics Data System (ADS)

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO3 (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce2O3. Sulfur is incorporated in apatite via several independent exchange reactions involving (P5+, Ca2+) vs. (S6+, Si4+, Na+, and Ce3+). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO3 (sulfate-saturated systems). The chlorine partition coefficients (D{Cl/apatite/glass}) range from 0.4 to 1.3 (average D{Cl/apatite/glass} = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D{F/apatite/glass} decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to D{F/apatite/glass} determined experimentally in felsic rock. D{S/apatite/glass} decreases from 9.1 to 2.9 with increasing SO3 in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO3). D{S/apatite/glass} is only slightly dependent on the melt composition and can be expressed as: SO3 apatite (wt%) = 0.157 * ln SO3 glass (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.

Parat, Fleurice; Holtz, François; Klügel, Andreas

2011-09-01

125

Early Miocene Kirka-Phrigian caldera, western Anatolia - an example of large volume silicic magma generation in extensional setting  

NASA Astrophysics Data System (ADS)

Large rhyolitic ignimbrite occurrences are close connected to the Early Miocene initiation of extensional processes in the central-west Anatolia along Ta?vanl?-Afyon zones. Field correlations, petrographical, geochemical and geochronological data lead to a substantial reinterpretation of the ignimbrite surrounding K?rka area, known from its world-class borate deposits, as representing the climatic event of a caldera collapse, unknown up to now and newly named "K?rka-Phrigian caldera". The caldera, which is roughly oval (24 km x 15km) in shape, one of the largest in Turkey, is supposed to have been formed in a single stage collapse event, at ~19 Ma that generated huge volume extracaldera outflow ignimbrites. Transtensive/distensive tectonic stresses since 25 Ma ago resulted in the NNW-SSE elongation of the magma chamber and influenced the roughly elliptical shape of the subsided block (caldera floor) belonging to the apex of Eski?ehir-Afyon-Isparta volcanic area. Intracaldera post-collapse sedimentation and volcanism (at ~ 18 Ma) was controlled through subsidence-related faults with generation of a series of volcanic structures (mainly domes) showing a large compositional range from saturated silicic rhyolites and crystal-rich trachytes to undersaturated lamproites. Such volcanic rock association is typical for lithospheric extension. In this scenario, enriched mantle components within the subcontinental lithospheric mantle will begin to melt via decompression melting during the initiation of extension. Interaction of these melts with crustal rocks, fractionation processes and crustal anatexis driven by the heat contained in the ascending mantle melts produced the silicic compositions in a large crustal reservoir. Such silicic melts generated the initial eruptions of K?rka-Phrigian caldera ignimbrites. The rock volume and geochemical evidence suggests that silicic volcanic rocks come from a long-lived magma chamber that evolved episodically; after caldera generation there is a shift to small volume episodic rhyolitic, trachytic and lamproitic volcanism, the last ones indicating a more primitive magma input with evident origin in an enriched mantle lithosphere. The volcanic rock succession provides a direct picture of the state of the magmatic system at the time of eruptions that generated caldera and post-caldera structures and offer an excellent example for silicic magma generation and associated potassic and ultrapotassic intermediate-mafic rocks in post-collisional extensional setting.

Seghedi, Ioan; Helvac?, Cahit

2014-05-01

126

The campi flegrei (Italy) geothermal system: A fluid inclusion study of the mofete and San Vito fields  

USGS Publications Warehouse

A fluid inclusion study of core from the Mofete 1, Mofete 2, Mofete 5, San Vito 1, and San Vito 3 geothermal wells (Campi Flegrei, Campania, Italy) indicates that the hydrothermal minerals were precipitated from aqueous fluids (??CO2) that were moderately saline (3-4 wt.% NaCl equiv.) to hypersaline (> 26 wt.% NaCl equiv.) and at least in part, boiling. Three types of primary fluid inclusions were found in authigenic K-feldspar, quartz, calcite, and epidote: (A) two-phase [liquid (L) + vapor (V)], liquid-rich inclusions with a range of salinity; (B) two-phase (L + V), vaporrich inclusions with low salinity; and (C) three-phase [L + V + crystals (NaCL)], liquid-rich inclusions with hypersalinity. Results of microthermometric and crushing studies are reported for twenty drill core samples taken from the lower portions of the five vertical wells. Data presented for selected core samples reveal a general decrease in porosity and increase in bulk density with increasing depth and temperature. Hydrothermal minerals commonly fill fractures and pore-spaces and define a zonation pattern, similar in all five wells studied, in response to increasing depth (pressure) and temperature. A greenschist facies assemblage, defined by albite + actinolite, gives way to an amphibolite facies, defined by plagioclase (andesine) + hornblende, in the San Vito 1 well at about 380??C. The fluid inclusion salinity values mimic the saline and hypersaline fluids found by drilling. Fluid inclusion V/L homogenization temperatures increase with depth and generally correspond to the extrapolated down-hole temperatures. However, fluid inclusion data for Mofete 5 and mineral assemblage data for San Vito 3, indicate fossil, higher-temperature regimes. A limited 87Sr/86Sr study of leachate (carbonate) and the leached cores shows that for most samples (except San Vito 3) the carbonate deposition has been from slightly 87Sr-enriched fluids and that Sr isotopic exchange has been incomplete. However, San Vito 3 cores show an approach to fluid/rock Sr equilibrium with a fluid similar to modern ocean water in 87Sr/86Sr ratio. The Campi Flegrei volcanic system has evolved undersaturated products, mostly trachyte, and defines a large (??? 12 km) caldera. The hydrothermal system developed in this location can be used as an analog for fossil systems in similar trachytic environments. The potential for ore mineralization is expressed by the recognition, from fluid inclusion and drilling data, of ore-forming environments such as boiling and brine stratification. ?? 1989.

de, Vivo, B.; Belkin, H. E.; Barbieri, M.; Chelini, W.; Lattanzi, P.; Lima, A.; Tolomeo, L.

1989-01-01

127

Major-element geochemistry of the Silent Canyon-Black Mountain peralkaline volcanic centers, northwestern Nevada Test Site: applications to an assessment of renewed volcanism  

USGS Publications Warehouse

The Silent Canyon and Black Mountain volcanic centers are located in the northern part of the Nevada Test Site. The Silent Canyon volcanic center is a buried cauldron complex of Miocene age (13-15 m.y.). Black Mountain volcanic center is an elliptical-shaped cauldron complex of late Miocene age. The lavas and tuffs of the two centers comprise a subalkaline-peralkaline association. Rock types range from quartz normative subalkaline trachyte and rhyolite to peralkaline comendite. The Gold Flat Member of the Thirsty Canyon Tuff (Black Mountain) is a pantellerite. The major-element geochemistry of the Black Mountain-Silent Canyon volcanic centers differs in the total range and distribution of Si02, contents, the degree of peralkalinity (molecular Na2O+K2O>Al2O3) and in the values of total iron and alumina through the range of rock types. These differences indicate that the suites were unrelated and evolved from differing magma bodies. The Black Mountain volcanic cycle represents a renewed phase of volcanism following cessation of the Timber Mountain-Silent Canyon volcanic cycles. Consequently, there is a small but numerically incalculable probability of recurrence of Black Mountain-type volcanism within the Nevada Test Site region. This represents a potential risk with respect to deep geologic storage of high-level radioactive waste at the Nevada Test Site.

Crowe, Bruce M.; Sargent, Kenneth A.

1979-01-01

128

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallaj?kull, Iceland  

PubMed Central

The 39-day long eruption at the summit of Eyjafjallajökull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011?kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05?km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27?km3) transported by 3–10?km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010?kg (70?Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010?kg (2%) are considered to have been transported longer than 600–700?km with <108?kg (<0.02%) reaching mainland Europe.

Gudmundsson, Magnus T.; Thordarson, Thorvaldur; Hoskuldsson, Armann; Larsen, Gudrun; Bjornsson, Halldor; Prata, Fred J.; Oddsson, Bjorn; Magnusson, Eyjolfur; Hognadottir, Thordis; Petersen, Gu?run Nina; Hayward, Chris L.; Stevenson, John A.; Jonsdottir, Ingibjorg

2012-01-01

129

Interaction of a rheomorphic peralkaline ash-flow tuff and underlying deposits, Menengai volcano, Kenya  

NASA Astrophysics Data System (ADS)

In the caldera wall of the volcano Menengai, Kenya, a late Quaternary peralkaline trachyte ash-flow tuff is densely welded and shows evidence (e.g. a well-defined lineation) or rheomorphism. The ash-flow tuff overlies a variety of volcanic and volcaniclastic deposits, with some of which it has intimate and complex relationships interpreted to have formed during post-emplacement processes. Where emplaced over a low-density pumice-fall deposit, the dense rheomorphic ash-flow tuff invaded the pumice deposit as tongues, and locally the two deposits mixed intimately. Blocks of welded, foliated ash-flow tuff were incorporated, when cooled and rigid, into the underlying pumice deposit, indicating mobility of the pumice deposit. Locally, the pumice-fall deposit was modified, during processes associated with the emplacement of the ash-flow tuff, to a pumice-bearing silt. The modifying process apparently involved smashing of fragile pumice lumps during physical agitation of the pumice deposit. Where emplaced over dense deposits, shear at the base of the rheomorphic ash contributed to the development of a gently dipping, nearly planar erosional surface. A microbreccia (buff silt) deposit formed along this shear plane consists of material from the overlying ash-flow and underlying deposits and was probably rich in gas and mobile during its formation.

Leat, P. T.

1985-10-01

130

The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands  

NASA Astrophysics Data System (ADS)

Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase-clinopyroxene-magnetite ± amphibole ± olivine) and trachytes (plagioclase-amphibole-magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates; it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic agent.

Smith, D. J.; Petterson, M. G.; Saunders, A. D.; Millar, I. L.; Jenkin, G. R. T.; Toba, T.; Naden, J.; Cook, J. M.

2009-12-01

131

Vent distribution in the Quaternary Payún Matrú Volcanic Field, western Argentina: Its relation to tectonics and crustal structures  

NASA Astrophysics Data System (ADS)

The Payún Matrú Volcanic Field consists of two polygenetic and mostly trachytic volcanoes (Payún Matrú with a summit caldera and Payún Liso) along with around 220 scoria cones and basaltic lava flows. This volcanic field belongs to the Payenia Basaltic Province (33° 30?-38° S), a Quaternary Andean back-arc basaltic province of the Southern Volcanic Zone, in western Argentina. The vent density distribution of the Payún Matrú Volcanic Field is different from the other volcanic fields within Payenia. The Payún Matrú volcano and the scoria cones are distributed in an E-W oriented fringe about 15 km wide and 70 km long, with the Payún Matrú caldera in the middle of this fringe. The structural framework in which the volcanic field is located allows to infer that this vent density distribution is strongly conditioned by pre-existing crustal anisotropies. The volcanic field is located in a transfer zone related to Jurassic extensional structures of the Neuquén Basin, which were inverted also as a transfer zone during the Miocene compressive deformation that formed the Malargüe fold and thrust belt, and, in addition, it is located in the southern margin of a Neogene syn-orogenic basin. The analysis of vent center location and vent morphology is helpful to determine basaltic vent alignments within the Payún Matrú Volcanic Field and to infer the syn-eruptive stress field. This analysis shows that vent alignments are compatible with the present-day maximum horizontal stress, as measured by break-out of oil wells.

Hernando, I. R.; Franzese, J. R.; Llambías, E. J.; Petrinovic, I. A.

2014-05-01

132

Geochemical evolution of Kohala Volcano, Hawaii  

USGS Publications Warehouse

Kohala Volcano, the oldest of five shield volcanoes comprising the island of Hawaii, consists of a basalt shield dominated by tholeiitic basalt, Pololu Volcanics, overlain by alkalic lavas, Hawi Volcanics. In the upper Pololu Volcanics the lavas become more enriched in incompatible elements, and there is a transition from tholeiitic to alkalic basalt. In contrast, the Hawi volcanics consist of hawaiites, mugearites, and trachytes. 87Sr/86Sr ratios of 14 Pololu basalts and 5 Hawi lavas range from 0.70366 to 0.70392 and 0.70350 to 0.70355, respectively. This small but distinct difference in Sr isotopic composition of different lava types, especially the lower 87Sr/86Sr in the younger lavas with higher Rb/Sr, has been found at other Hawaiian volcanoes. Our data do not confirm previous data indicating Sr isotopic homogeneity among lavas from Kohala Volcano. Also some abundance trends, such as MgO-P2O5, are not consistent with a simple genetic relationship between Pololu and Hawi lavas. We conclude that all Kohala lavas were not produced by equilibrium partial melting of a compositionally homogeneous source. ?? 1987 Springer-Verlag.

Lanphere, M. A.; Frey, F. A.

1987-01-01

133

Silurian magmatism in eastern Senegal and its significance for the Paleozoic evolution of NW-Gondwana  

NASA Astrophysics Data System (ADS)

Submarine basalt and trachyte of the Nandoumba group occur in eastern Senegal within the Bassarides branch of the Mauritanides orogen. The unit forms part of the parautochthonous domain which is stacked between underlying Neoproterozoic to Paleozoic foreland and overlying Variscan nappes. The crystallisation age of the volcanic to subvolcanic rocks has been determined by U-Pb single zircon SHRIMP method at 428 ± 5.2 Ma whereas zircon xenocryst ages vary from 500 to 2200 Ma. The shape of the xenocryst grains document proximal Neo- and Paleoproterozoic and distal Mesoproterozoic provenance areas for assimilated sediments. This is compatible with the Paleoproterozoic Birimian basement and Neoproterozoic cover rocks nearby whereas an origin from the Amazonian craton could be assumed for distal Mesoproterozoic zircons. Geochemical and Sm-Nd isotope whole rock analysis show that basalts of the Nandoumba group are similar to modern transitional to alkaline volcanic lavas in intraplate settings. Those basalts have a deep mantle source with a great contribution of a recycled mantle component such as EM1 and/or EM2. The basalts resemble in their composition those from the Meguma terrane of Nova Scotia which are of similar age suggesting a common source and therefore connection of Meguma with Gondwana during this period. Review of circum-Atlantic Silurian magmatism indicates ongoing fragmentation of NW-Gondwana that started in Cambro/Ordovician times.

Fullgraf, Thomas; Ndiaye, Papa Moussa; Blein, Olivier; Buscail, François; Lahondère, Didier; Le Métour, Joël; Sergeev, Sergey; Tegyey, Monique

2013-02-01

134

Geochemical trends through time and lateral variability of diatom floras in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley  

NASA Astrophysics Data System (ADS)

The Olorgesailie Formation (1.2-0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin ( Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8-9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200-16,000 ?S cm - 1 and pH of 7.5-9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution.

Owen, R. B.; Renaut, R. W.; Potts, R.; Behrensmeyer, A. K.

135

The `Cameroon Hot Line' (CHL): A unique example of active alkaline intraplate structure in both oceanic and continental lithospheres  

NASA Astrophysics Data System (ADS)

The Cameroon Hot Line (CHL) is an active N30°E tectono-magmatic alignment, extending from Pagalu Island to Lake Chad. Its oceanic sector is comprised of six major volcanoes, and the continental sector of seven major volcanoes, numerous monogenic cones and more than sixty plutonic anorogenic ring complexes. The lavas and plutonic rocks have the typical mineralogy of the alkaline series. Nephelinites and alkaline lamprophyres are also present. The basalts from the oceanic and continental sectors have similar trace elements and Sr and Nd isotope compositions, attesting that the source is sublithospheric and that the continental crust plays no significant role in the magma genesis. Uncommon intermediate lavas originate through mixing between basaltic and felsic magmas. The evolution from basaltic to phonolitic magmas is explained through crystal fractionation. Compositions of most continental trachytes and of rhyolites require a crustal component. Metasomatism of carbonatitic affinity affected the lithospheric mantle during the magma ascent. DM and FOZO mantle components were involved in the genesis of the CHL parental magmas.

Déruelle, Bernard; Ngounouno, Ismaïla; Demaiffe, Daniel

2007-08-01

136

Calibration of the Mars Science Laboratory Alpha Particle X-ray Spectrometer  

NASA Astrophysics Data System (ADS)

The alpha-particle X-ray spectrometer (APXS) for the Mars Science Laboratory (MSL) mission was calibrated for routine analysis of: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Br, Rb, Sr, and Y. The following elements were also calibrated, but may be too low to be measured (10s-100s ppm) for their usual abundance on Mars: V, Cu, Ga, As, Se and W. An extensive suite of geological reference materials, supplemented by pure chemical elements and compounds was used. Special attention was paid to include phyllosilicates, sulfates and a broad selection of basalts as these are predicted minerals and rocks at the Gale Crater landing site. The calibration approach is from first principles, using fundamental physics parameters and an assumed homogeneous sample matrix to calculate expected elemental signals for a given instrument setup and sample composition. Resulting concentrations for most elements accord with expected values. Deviations in elements of lower atomic number (Na, Mg, Al) indicate significant influences of mineral phases, especially in basalts, ultramafic rocks and trachytes. The systematics of these deviations help us to derive empirical, iterative corrections for different rock groups, based on a preliminary APXS analysis which assumes a homogeneous sample. These corrections have the potential to significantly improve the accuracy of APXS analyses, especially when other MSL instrument results, such as the X-ray diffraction data from CheMin, are included in the overall analysis process.

Campbell, John L.; Perrett, Glynis M.; Gellert, Ralf; Andrushenko, Stefan M.; Boyd, Nicholas I.; Maxwell, John A.; King, Penelope L.; Schofield, Céleste D. M.

2012-09-01

137

Calibration of the Mars Science Laboratory Alpha Particle X-Ray Spectrometer  

NASA Astrophysics Data System (ADS)

We have used a suite of over 60 geochemical reference standards for the calibration of the Mars Science Laboratory (MSL) Alpha Particle X-ray Spectrometer (APXS). For the elements P, S, Cl and Br we have supplemented this suite by adding various amounts of relevant chemical compounds to a powdered basalt standard. Special attention has been paid to include phyllosilicates, sulphates and a broad selection of igneous basalts as these are predicted key deposits at the MSL landing site, Gale Crater. The calibration is performed from first principles using x-ray excitation cross sections for the alpha particle and x-ray radiation source and an assumed homogeneous sample matrix. Remaining deviations indicate significant influences of mineral phases especially for light elements in basalts, ultra-mafic rocks and trachytes. Supporting x-ray diffraction work has helped to derive empirical, iterative corrections for distinct rock types, based on the first APXS analysis, assuming a homogeneous sample. These corrections have the potential to significantly improve the accuracy of APXS analyses, especially when other MSL instrument results, such as x-ray diffraction data from ChemMin, are included in the overall analysis process.

Perrett, G. M.; Campbell, J. L.; Gellert, R.; King, P. L.; Maxwell, J. A.; Andrushenko, S. M.

2011-12-01

138

Age discrimination among eruptives of Menengai Caldera, Kenya, using vegetation parameters from satellite imagery  

NASA Technical Reports Server (NTRS)

Results are presented of an investigation to determine the degree to which digitally processed Landsat TM imagery can be used to discriminate among vegetated lava flows of different ages in the Menengai Caldera, Kenya. A selective series of five images, consisting of a color-coded Landsat 5 classification and four color composites, are compared with geologic maps. The most recent of more than 70 postcaldera flows within the caldera are trachytes, which are variably covered by shrubs and subsidiary grasses. Soil development evolves as a function of time, and as such supports a changing plant community. Progressively older flows exhibit the increasing dominance of grasses over bushes. The Landsat images correlated well with geologic maps, but the two mapped age classes could be further subdivided on the basis of different vegetation communities. It is concluded that field maps can be modified, and in some cases corrected by use of such imagery, and that digitally enhanced Landsat imagery can be a useful aid to field mapping in similar terrains.

Blodget, Herbert W.; Heirtzler, James R.

1993-01-01

139

Inner structure of the Puy de Dôme volcano: cross-comparison of geophysical models (ERT, gravimetry, muon imaging)  

NASA Astrophysics Data System (ADS)

Muon imaging of volcanoes and of geological structures in general is actively being developed by several groups in the world. It has the potential to provide 3-D density distributions with an accuracy of a few percent. At this stage of development, comparisons with established geophysical methods are useful to validate the method. An experiment has been carried out in 2011 and 2012 on a large trachytic dome, the Puy de Dôme volcano, to perform such a comparison of muon imaging with gravimetric tomography and 2-D electrical resistivity tomography. Here, we present the preliminary results for the last two methods. North-south and east-west resistivity profiles allow us to model the resistivity distribution down to the base of the dome. The modelling of the Bouguer anomaly provides models for the density distribution within the dome that are directly comparable with the results from the muon imaging. Our ultimate goal is to derive a model of the dome using the joint interpretation of all sets of data.

Portal, A.; Labazuy, P.; Lénat, J.-F.; Béné, S.; Boivin, P.; Busato, E.; Cârloganu, C.; Combaret, C.; Dupieux, P.; Fehr, F.; Gay, P.; Laktineh, I.; Miallier, D.; Mirabito, L.; Niess, V.; Vulpescu, B.

2013-01-01

140

Spherulites and thundereggs from pitchstones of the Deccan Traps: geology, petrochemistry, and emplacement environments  

NASA Astrophysics Data System (ADS)

Spherulites and thundereggs are rounded, typically spherical, polycrystalline objects found in glassy silicic rocks. Spherulites are dominantly made up of radiating microscopic fibers of alkali feldspar and a silica mineral (commonly quartz). They form due to heterogeneous nucleation in highly supercooled rhyolitic melts or by devitrification of glass. Associated features are lithophysae ("stone bubbles"), which have an exterior (sometimes concentric shells) of fine quartz and feldspar, and internal cavities left by escaping gas; when filled by secondary silica, these are termed thundereggs. Here, we describe four distinct occurrences of spherulites and thundereggs, in pitchstones (mostly rhyolitic, some trachytic) of the Deccan Traps, India. The thundereggs at one locality were previously misidentified as rhyolitic lava bombs and products of pyroclastic extrusive activity. We have characterized the thundereggs petrographically and geochemically and have determined low contents of magmatic water (0.21-0.38 wt.%) in them using Fourier transform infrared spectroscopy. We consider that the spherulite-bearing outcrops at one of the localities are of lava flows, but the other three represent subvolcanic intrusions. Based on the structural disposition of the Deccan sheet intrusions studied here and considerations of regional geology, we suggest that they are cone sheets emplaced from a plutonic center now submerged beneath the Arabian Sea.

Kshirsagar, Pooja V.; Sheth, Hetu C.; Seaman, Sheila J.; Shaikh, Badrealam; Mohite, Poonam; Gurav, Trupti; Chandrasekharam, D.

2012-03-01

141

Effects of crystallization and bubble nucleation on the elastic properties of magmas  

NASA Astrophysics Data System (ADS)

Seismic tomography of potentially hazardous volcanoes is a prime tool to assess the physical state of magma reservoirs. Processes occurring in the conduit or in the chamber, such as crystallization and bubble exsolution, control the magma rheology, hence the style of volcanic eruption. Elastic parameters of vapor-saturated, partially molten systems are thus providing fundamental information for the identification of such reservoirs under active and seemingly dormant volcanoes. This knowledge will potentially serve to assess their risk. We present preliminary data on compression and shear wave propagation velocities of a chemically simplified melt analogous to andesite and trachyte, in the system CaO-Na2O-Al2O3-SiO2-H2O-CO2. These ultrasonic velocities are measured simultaneously in a Paterson-type internally-heated gas pressure apparatus at confining pressures up to 300 MPa and temperatures up to 1000°C. Using the pulse transmission technique, the experiments are performed at frequencies ranging from 0.1 to 3 MHz. Variations in the elastic parameters induced by the presence of bubbles or dissolved water in glassy samples are discussed for various pressures and temperatures. As the investigated melt undergoes plagioclase crystallization, a thermal plateau is maintained over specific time duration in order to measure the changes in seismic properties of in-situ crystallizing magmas. This maintained temperature varies between 800° and 1000°C depending on the amount of dissolved water in the system.

Tripoli, B. A.; Ulmer, P.; Eric, R.; Cordonnier, B.; Burg, J.

2012-12-01

142

Assessment of the atmospheric impact of volcanic eruptions  

NASA Technical Reports Server (NTRS)

The dominant global impact of volcanic activity is likely to be related to the effects of volcanic gases on the Earth's atmosphere. Volcanic gas emissions from individual volcanic arc eruptions are likely to cause increases in the stratospheric optical depth that result in surface landmass temperature decline of 2 to 3 K for less than a decade. Trachytic and intermediate magmas are much more effective in this regard than high-silica magmas, and may also lead to extensive ozone depletion due to effect of halogens and magmatic water. Given the assumed relationship between arc volcanism and subduction rate, and the relatively small variation in global spreading rates in the geologic record, it is unlikely that the rates of arc volcanism have varied greatly during the Cenozoic. Hotspot related basaltic fissure eruptions in the subaerial environment have a higher mass yield of sulfur, but lofting of the valcanic aerosol to levels above the tropopause is required for a climate impact. High-latitude events, such as the Laki 1783 eruption can easily penetrate the tropopause and enter the stratosphere, but formation of a stratospheric volcanic aerosol form low-latitude effusive basaltic eruptions is problematical, due to the elevated low-latitude tropopause. Due to the high sulfur content of hotspot-derived basaltic magmas, their very high mass eruption rates and the episodic behavior, hotspots must be regarded as potentially major modifiers of Earth's climate through the action of their volcanic volatiles on the chemistry and physics of the atmosphere.

Sigurdsson, H.

1988-01-01

143

Distribution, geochemistry and age of the Millennium eruptives of Changbaishan volcano, Northeast China — A review  

NASA Astrophysics Data System (ADS)

Large explosive volcanic eruptions generate extensive regional tephra deposits that provide favorable conditions for identifying the source of volcanoes, comparing the sedimentary strata of a region and determining their ages. The tephra layer, referred to as B-Tm, generated by the Millennium eruption of Changbaishan volcano, is widely distributed in Northeast China, Japan, D.P.R. Korea, and the nearby coastal area of Russia. It forms part of the widespread northeast Asian strata and is significant for establishing an isochronal stratigraphic framework. However, research on the temporal characterization and stratigraphic correlation of associated strata using this tephra layer is mainly concentrated in and near Japan. In northeastern China, this tephra layer is seldom seen and its application in stratigraphic correlations is even rarer. More importantly, the determination of accurate ages for both distal and proximal tephras has been debated, leading to controversy in discussions of its environmental impacts. Stratigraphic records from both distal and proximal Changbaishan ash show that this eruption generally occurred between 1,012 and 1,004 cal yr BP. Geochemical comparison between Changbaishan ash and the Quaternary widespread ash around Japan illustrates that Changbaishan ash is a continuous composition from rhyolitic to trachytic and its ratio of FeOT to CaO is usually greater than 4, which can be used as a distinguishing identifier among worldwide contemporary eruptions.

Sun, Chunqing; You, Haitao; Liu, Jiaqi; Li, Xin; Gao, Jinliang; Chen, Shuangshuang

2014-06-01

144

The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution  

NASA Technical Reports Server (NTRS)

A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

Mohr, P. A.; Potter, E. C.

1976-01-01

145

Use of single-grain geochemistry of cryptic tuffs and volcaniclastic sandstones improves the tephrostratigraphic framework of Olduvai Gorge, Tanzania  

NASA Astrophysics Data System (ADS)

Single-grain geochemical composition of volcaniclastic sandstones can be a potential tool to improve correlations of mixed pyroclastic/epiclastic deposits. To test this, trachytic tuffs of the paleoanthropologically important FLK, FLK N, and FLK NN sites of Pleistocene Olduvai Gorge Bed I (Tanzania) are used as an established tephrostratigraphic framework against which to test volcaniclastic sandstone correlations. Fluvio-lacustrine sandstones and tuff samples were collected from eight archeological trenches between Tuffs IB and ID across a 500-m transect, including Leakey's famous Zinjanthropus (FLK) and OH 7/OH 8 (FLK NN) sites. A previously unknown, thin, fine, mineralogically unique, black trachyandesitic fallout ash was discovered below Tuff IC. Compositions of individual augite, feldspar and titanomagnetite grains from sandstones between Tuffs IB and IC reveal some IB-equivalent material, and a new compositional assemblage distinct from the sandwiching marker tuffs. Mineral compositions of the "tripartite" volcaniclastic sandstone between Tuffs IC and ID are similar to ID. Volcaniclastic sandstone grain fingerprints further refine correlations between fluvio-lacustrine sections within the area, providing support for proposed high-resolution stratigraphic reconstruction of the Zinjanthropus and OH 7/OH 8 land surfaces. This method might be applied to other sections where pyroclastic particles are admixed but distinct tuffs are not preserved.

McHenry, Lindsay J.; Stollhofen, Harald; Stanistreet, Ian G.

2013-09-01

146

Distribution, geochemistry and age of the Millennium eruptives of Changbaishan volcano, Northeast China -- A review  

NASA Astrophysics Data System (ADS)

Large explosive volcanic eruptions generate extensive regional tephra deposits that provide favorable conditions for identifying the source of volcanoes, comparing the sedimentary strata of a region and determining their ages. The tephra layer, referred to as BTm, generated by the Millennium eruption of Changbaishan volcano, is widely distributed in Northeast China, Japan, D.P.R. Korea, and the nearby coastal area of Russia. It forms part of the widespread northeast Asian strata and is significant for establishing an isochronal stratigraphic framework. However, research on the temporal characterization and stratigraphic correlation of associated strata using this tephra layer is mainly concentrated in and near Japan. In northeastern China, this tephra layer is seldom seen and its application in stratigraphic correlations is even rarer. More importantly, the determination of accurate ages for both distal and proximal tephras has been debated, leading to controversy in discussions of its environmental impacts. Stratigraphic records from both distal and proximal Changbaishan ash show that this eruption generally occurred between 1,012 and 1,004 cal yr BP. Geochemical comparison between Changbaishan ash and the Quaternary widespread ash around Japan illustrates that Changbaishan ash is a continuous composition from rhyolitic to trachytic and its ratio of FeOT to CaO is usually greater than 4, which can be used as a distinguishing identifier among worldwide contemporary eruptions.

Sun, Chunqing; You, Haitao; Liu, Jiaqi; Li, Xin; Gao, Jinliang; Chen, Shuangshuang

2014-04-01

147

Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies  

USGS Publications Warehouse

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16-9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial e{open}Nd values (???1 e{open}Nd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar e{open}Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher e{open}Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low e{open}Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20-40% (by mass) wall-rock into magmas that were injected into the upper crust. The low e{open}Nd magmas most likely formed via the incorporation of low ??18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ??18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13-14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low e{open}Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70-80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0-10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing "basification" of a lower crustal magma source by repeated injection of mantle-derived mafic magmas. ?? 1991 Springer-Verlag.

Farmer, G. L.; Broxton, D. E.; Warren, R. G.; Pickthorn, W.

1991-01-01

148

Impact of volcanism on the evolution of Lake Van II: Temporal evolution of explosive volcanism of Nemrut Volcano (eastern Anatolia) during the past ca. 0.4 Ma  

NASA Astrophysics Data System (ADS)

Thirty-two new single crystal ages document 400 000 years of widespread explosive volcanism of historically active Nemrut Volcano towering over huge alkaline Lake Van (Eastern Anatolia). The dated deposits were selected to monitor the volcanic and compositional evolution of Nemrut Volcano through time and thus to provide a rigorous temporal framework for the tephra record of the PaleoVan Drilling Project. Tephra samples were taken from large-volume deposits or those that occur in medial to distal localities, well-exposed stratigraphic sections or from the initial phase of an eruptive sequence. Mainly fallout deposits were chosen because most ignimbrites show more complex and corroded feldspar populations owing to compositional zoning and magma mixing. Moreover, fallout deposits held the promise to be more clearly identifiable with—and correlatable to—> 300 tephra layers in the PaleoVan drill cores, even though commonly in amounts marginal or insufficient in thickness to allow well-supported single crystal dating. The crystals dated are dominantly anorthoclase, the main phenocryst phase in the trachytic to rhyolitic, slightly to strongly peralkaline Nemrut magmas. Ages obtained so far range from ca. 400 ka to ca. 30 ka for Nemrut Volcano. The causes of significant changes in the frequency, volume and composition of tephra layers per unit time are discussed in terms of external (erosion, climate changes, geodynamic factors) and internal forcing (changes in magma supply and composition and incubation periods preceding large volume rhyolitic eruptions). For example, the low frequency of tephra layers deposited prior to ca. 200 ka may be due to low explosive activity, severe erosion between MIS 9 and MIS 11, or both. Nevertheless, the overall frequency of explosive eruptions appears to have increased during the past ca. 200 ka. We also recognize a slight peak in explosive eruptions during warm periods (e.g. MIS 5 and MIS 7) and speculate on lithospheric unloading triggering increased partial melting or magma reservoir unloading following massive glacier melting. The ages of 5 dated ignimbrites span ca. 250 000 years suggesting that Nemrut Volcano went through a polycyclic evolution with multiple caldera collapses and major pyroclastic flow eruptions, the oldest dated so far as 265 ka. The widely held view of the impressive Nemrut Caldera now dated to have formed at ca. 30 ka, as the main paroxysmal event during the evolution of the volcano is no longer tenable. Distinct and coherent compositional characteristics, especially in trace element concentrations, characterize several groups of trachytic tephras. We speculate that the growth of Nemrut Volcano caused the isolation of the Lake Van basin. On account of their mineralogical (anorthoclase, hedenbergite, fayalite, aenigmatite) and alkalic chemical compositions and large volume, dated Nemrut fallout tephras are likely to represent excellent markers in lakes and other sites of paleoclimatological or archeological interest in neighboring countries to the northeast of Lake Van as far as the Caspian Sea in what may be called the East Anatolian Tephra Province.

Sumita, Mari; Schmincke, Hans-Ulrich

2013-03-01

149

Some New Constraints On The Stratigraphic And Structural Setting Of The Soda Lake Geothermal Field, Churchill County, Nevada - McLACHLAN, Holly S. and FAULDS, James E., Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557  

NASA Astrophysics Data System (ADS)

Our research group is currently conducting a regional survey to identify favorable structural settings of producing and prospective geothermal fields in the Great Basin. The Soda Lake geothermal field - one of the oldest consistently producing fields in this study region - is located in west-central Nevada near the heart of the Carson Sink. Producing and prospective geothermal fields in the surrounding highlands are hosted in 1) fault termination zones (Desert Queen), 2) accommodation zones (Brady's Hot Springs) and 3) fault step-overs (Desert Peak). However, the structural setting is challenging to identify at the Soda Lake field, because it lies in the central part of a large basin with no nearby bedrock exposures. The well field at Soda Lake is centered ~3.5 km NNE of the Holocene Soda Lake maar, from which it takes its name. The geothermal field was identified serendipitously during the drilling of an irrigation survey well in the early 20th century. Modern exploratory drilling at the field began in the mid-1970s and has continued sporadically to the present. There are currently more than 28 500+ m wells at and near the production site. The exceptional drilling density at Soda Lake allows for comparatively reliable correlation of stratigraphy in the subsurface below the feature-poor Carson Sink. Stratigraphy in the Soda Lake geothermal area is relatively "layer cake" at the scale of the well field. Unconsolidated sediments extend more than 1000 m below surface. The upper few hundred meters are composed of fluvial and lacustrine sediments derived from Sierran batholith source rocks. The deeper basin fill derives from more proximal mafic to felsic Miocene volcanic rocks along the basin margins. At ~450-650 m depth, basin sediments are interrupted by a 5.11 Ma trachytic basalt of restricted lateral extent and variable thickness. Most wells intercept ~50-250 m of fine lacustrine sediments below this basalt body before intercepting the basin floor. Basin floor rocks consist of a thick (>1500 m) package of fine-grained altered basalts and interbedded sedimentary rocks. Within this package, in the central portion of the well field, a ~300-500 m thick marker of laminated siltstones + coarse-grained, porphyritic plagioclase basalt has been identified in cuttings. Variations in thickness within the marker suggest older faults with significant throw were primarily northwest striking. Large local variations in the thickness of the 5.11 Ma trachytic basalt body support this interpretation and indicate NW-striking faulting likely continued through ~5 Ma B.P. However, all evidence indicates near-surface (<1000 m depth) faults at the Soda Lake geothermal field strike NNE, perpendicular to the contemporary extension direction. Structural interpretation is in progress for the Soda Lake geothermal field. In conjunction with recently obtained 3D seismic and microgravity surveys, stratigraphic information obtained from cuttings broadly constrains the structural setting. These data may permit determination of the specific structural host environment and should allow for assessment of how the prevailing faults at the site correlate with regional scale trends.

McLachlan, H. S.

2012-12-01

150

Eocene potassic and ultrapotassic volcanism in south Tibet: New constraints on mantle source characteristics and geodynamic processes  

NASA Astrophysics Data System (ADS)

In the Yangbajing area, southern Tibet, several monogenic volcanoes were conformably superimposed on the Linzizong calc-alkaline volcanic successions. According to their petrologic and geochemical characteristics, these monogenic volcanoes are composed of three rock varieties: tephritic phonolitic plugs and shoshonitic and trachytic lavas. Their geochemical systematics reveals that low-pressure evolutionary processes in the large voluminous Linzizong calc-alkaline magmas were not responsible for the generation of these potassic-ultrapotassic rocks, but the significant change in petrologic and geochemical characteristics from the Linzizong calc-alkaline to potassic-ultrapotassic magma is likely accounted for the change of metasomatic agents in the southern Tibetan lithospheric mantle source during the Paleocene to Eocene. The tephritic phonolites containing both leucite and plagioclase show primary ultrapotassic character similar to that of Mediterranean plagioleucititic magmas. Radiogenic Sr increases with SiO 2 in the xenolith-bearing trachytes strongly suggesting significant crustal assimilation in the shoshonitic magmas. The Yangbajing ultrapotassic rocks have high K 2O and Al 2O 3, and show depletion of high field strength elements (HFSEs) with respect to large ion lithophile elements. In primitive mantle-normalized element diagrams, all samples are characterized by positive spikes at Th (U) and Pb with negative anomalies at Ba, Nb-Ta and Ti, reflecting the orogenic nature of the ultrapotassic rocks. They are characterized by highly radiogenic 87Sr/ 86Sr (i) ratios (0.7061-0.7063) and unradiogenic 143Nd/ 144Nd (i) (0.5125), and Pb isotopic compositions ( 206Pb/ 204Pb = 18.688-18.733, 207Pb/ 204Pb = 15.613-15.637, and 208Pb/ 204Pb = 38.861-38.930) similar to the global subducting sediment. Strong enrichment of incompatible trace elements and high Th fractionation from the other HFSEs (such as Nb and U) clearly indicate that the Th-enriched sedimentary component in a network veined mantle source was mainly introduced by sediment-derived melts. In addition, the ultrapotassic rocks have significant Ce (Ce/Ce* = 0.77-0.84) and Eu (Eu/Eu* = 0.72-0.75) anomalies, suggesting a subduction sediment input into the southern Tibetan lithospheric mantle source. In contrast, high U/Th (> 0.20) and Ba/Th (> 32) and low Th/La (< 0.3) in the shoshonites indicate that the Eocene potassic magma originated from partial melting of the surrounding peridotite mantle pervasively affected by slab-related fluid addition from the dehydration of either the subducting oceanic crust or the sediment. Thus, at least two different subduction-related metasomatic agents re-fertilized the upper mantle. According to the radiometric ages and spatial distribution, the Gangdese magmatic association shows a temporal succession from the Linzizong calc-alkaline to ultrapotassic magmas. This indicates a late arrival of recycled sediments within the Tibetan lithospheric mantle wedge. The most diagnostic signatures for the involvement of continent-derived materials are the super-chondritic Zr/Hf (45.5-49.2) and elevated Hf/Sm values (0.81-0.91) in the ultrapotassic rocks. Therefore, the occurrence of orogenic magmatism in the Gangdese belt likely represents the volcanic expression of the onset of the India-Asia collision, preceding the 10 Ma Neo-Tethyan slab break-off process at 42-40 Ma. The absence of residual garnet in the mantle source for the ultrapotassic volcanism seems to imply that the southern Tibetan lithosphere was not been remarkably thickened until the Eocene (˜ 50 Ma).

Gao, Yongfeng; Yang, Zhusen; Hou, Zengqian; Wei, Ruihua; Meng, Xiangjin; Tian, Shihong

2010-06-01

151

Edaphics, active tectonics and animal movements in the Kenyan Rift - implications for early human evolution and dispersal  

NASA Astrophysics Data System (ADS)

The quality of soils (edaphics) and the associated vegetation strongly controls the health of grazing animals. Until now, this has hardly been appreciated by paleo-anthropologists who only take into account the availability of water and vegetation in landscape reconstruction attempts. A lack of understanding the importance of the edaphics of a region greatly limits interpretations of the relation between our ancestors and animals over the last few million years. If a region lacks vital trace elements then wild grazing and browsing animals will avoid it and go to considerable length and take major risks to seek out better pasture. As a consequence animals must move around the landscape at different times of the year. In complex landscapes, such as tectonically active rifts, hominins can use advanced group behaviour to gain strategic advantage for hunting. Our study in the southern Kenya rift in the Lake Magadi region shows that the edaphics and active rift structures play a key role in present day animal movements as well as the for the location of an early hominin site at Mt. Olorgesailie. We carried out field analysis based on studying the relationship between the geology and soil development as well as the tectonic geomorphology to identify 'good' and 'bad' regions both in terms of edaphics and accessibility for grazing animals. We further sampled different soils that developed on the volcanic bedrock and sediment sources of the region and interviewed the local Maasai shepherds to learn about present-day good and bad grazing sites. At the Olorgesailie site the rift valley floor is covered with flood trachytes; basalts only occur at Mt. Olorgesailie and farther east up the rift flank. The hominin site is located in lacustrine sediments at the southern edge of a playa that extends north and northwest of Mt. Olorgesailie. The lakebeds are now tilted and eroded by motion on two north-south striking faults. The lake was trapped by basalt flows from Mt. Olorgesailie and was released by the fault motion leading to deep river incision and exposure of the site. To the west and the north steep fault scarps bound the playa forming a natural barrier for animals. Field observations and information from local shepherds suggest that the trachytes at the valley floor produce rather poor soils whereas the soils developed on lacustrine and alluvial sediments close to the hominin site are much more attractive grazing sites for present-day animals. This is supported by first results from soil analysis. With a lake in the past the Olorgesailie site represents an key example of how early hominins may have used strategic advance of the landscape. While steep fault scarps blocked the northern pathway, the southern lakeshore represented one of the few accessible places for animals to be suffiently provided with nutrients and thus, was an excellent location for hominins to stalemate and hunt down prey. Future studies will include additional sites in the central and northern Kenya rift.

Kübler, Simon; Owenga, Peter; Rucina, Stephen; King, Geoffrey C. P.

2014-05-01

152

Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies  

NASA Astrophysics Data System (ADS)

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16 9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial ? Nd values (˜1 ? Nd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar ? Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher ? Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low ? Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20 40% (by mass) wall-rock into magmas that were injected into the upper crust. The low ? Nd magmas most likely formed via the incorporation of low ? 18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ? 18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13 14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low ? Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70 80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0 10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing “basification” of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.

Farmer, G. Lang; Broxton, David E.; Warren, Richard G.; Pickthorn, William

1991-03-01

153

Genesis and evolution of mafic and felsic magmas at Quaternary volcanoes within the Main Ethiopian Rift: Insights from Gedemsa and Fanta 'Ale complexes  

NASA Astrophysics Data System (ADS)

This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta 'Ale complexes, located in the Main Ethiopian Rift, the site of an intense magmatism since Eocene-Oligocene. The earlier NW-SE direction of extension of the Rift, which generated NE-SW trending faults, rotated around E-W in Quaternary times, and produced the still active N to N-NE Wonji Fault System. The Gedemsa volcano is located in the central part of the Ethiopian Rift, about 100 km SE of Addis Ababa. It is characterized by a wide central caldera, about 8 km in diameter. The general stratigraphic sequence in the area includes, from base upwards, rift-floor ignimbrites, pantelleritic and subordinate trachytic pyroclastic deposits and lava flows and domes, and widespread basaltic deposits. The Fanta 'Ale volcanic complex is located in the northern part of the Main Ethiopian Rift, where the Afar depression begins. It is characterized by a summit caldera of which the diameter is about 4 km. This volcano erupted trachytic and rhyolitic lavas, whereas the most diffuse unit is an ignimbrite related to the caldera collapse. Explosive activity has occurred inside and outside the caldera, forming tuff cones and thick pumice-fallout deposits. The only mafic unit is represented by a basaltic eruption that occurred in 1870 AD. Historical eruptions and intense fumarolic activity are evidence for the persistence activity of the Fanta 'Ale in this part of the Main Ethiopian Rift. New geochemical and Sr-Nd-Pb isotope data on representative samples from Gedemsa and Fanta 'Ale volcanoes are presented and discussed in order to shed light on the genesis of mafic and felsic magmas, the genetic link between them, and their possible interaction with the local crust. Volcanic rocks show a typical mafic-felsic bi-modal distribution with few intermediate terms (Daly Gap), as observed at regional scale along the Main Ethiopian Rift as well as on the plateau. Geochemical data and modeling suggest that magmas evolved mainly through fractional crystallization processes, accounting for the entire mafic-felsic compositional variation. However, Sr-Nd-Pb isotope data reveal also open-system evolution processes. The most differentiated, Sr-poor rhyolites suffered important low temperature contamination by shallow fluids of hydrothermal and/or meteoric origin. This affected mostly the Sr isotopic composition of whole-rocks, and much less that of separated feldspars that provide more reliable 87Sr/86Sr values. Mafic rocks, as well as the least contaminated felsic rocks, provide evidence for two components involved in the genesis and evolution of mafic magmas: a mantle component, carrying the isotopic composition of the Afar plume, and a crustal component, likely Pan-African sialic lower crust, that might have been added in small amounts, about 2%, to mafic magmas. The origin of the primary magmas is inferred to have occurred by 7% partial melting of a mixed source region including both depleted and enriched mantle components.

Giordano, F.; D'Antonio, M.; Civetta, L.; Tonarini, S.; Orsi, G.; Ayalew, D.; Yirgu, G.; Dell'Erba, F.; Di Vito, M. A.; Isaia, R.

2014-02-01

154

Tephro- and chemo-stratigraphy of the Vulcanello Peninsula (Vulcano, Aeolian Islands)  

NASA Astrophysics Data System (ADS)

New stratigraphic studies of the Vulcanello Peninsula have been used to better define the small-scale evolution of this young (1000 AD and 325±100 BP) volcanic center and to re-investigate the last 1000 years of volcanic history for the Island of Vulcano (Aeolian Islands, Southern Italy). Vulcanello Peninsula is the northern-most part of the Island of Vulcano. It comprises a shoshonitic lava platform and a volcanic edifice made up of three overlying cones, which are shoshonitic to trachytic in composition. Volcanic activity in this area was coeval with the recent eruptions of the La Fossa Cone, the present-day active center of the island. Our goal is to constrain the recent volcanic development of this mafic volcano and to focus on the historic eruptive activity of the two other recent or active centres in the southern Aeolian Islands, Mt. Pilato (Island of Lipari) and La Fossa Cone. In order to do so, we reconstructed the stratigraphical setting of the proximal deposits of the three Vulcanello cones, through the investigation of 25 outcrops. We analyzed the stratigraphy of the tephra blankets deposited on the lava platform, studying 10 trenches. Our intention is to integrate morphological, textural and chemical data in order to correlate these deposits with the Vulcanello, La Fossa Cone or Mt Pilato. LA-MC-ICPMS (RHUL) analysis of juvenile clasts is underway in order to investigate the evolution of the Vulcanello juvenile clasts. In addition 14C dating is planned on selected organic matter from the volcanostratigraphic sections. Our preliminary data for the Vulcanello proximal deposits suggest that each of the three cones experienced several eruptions, with a wide spectrum of eruptive styles and a diversity of chemistry. The oldest cone (Vulcanello I) is characterised by four different eruptions separated by minor unconformities or reworking material indicative of little or not time breaks in the eruptive cycle. The eruptions shift from Violent Strombolian to Hawaiian in style, testifying to a reduction in fragmentation and dispersal. The second cone (Vulcanello II), contains volcanic deposits from Strombolian eruptions only. The third cone (Vulcanello III), displays a complex evolution with an initial effusive episode of a trachytic lava flow, followed by phreatic explosions, evident as altered fine ash layers. These deposits are interbedded with scoriaceous fall deposits, attesting the occurrence of some mild explosive activity during this eruptive phase. This detailed study of the effusive and explosive products from Vulcanello reveals rapid evolution of Vulcanello during the initial phases (1000 AD to 1200 AD) with voluminous mafic eruptions, both effusive and explosive. A progressive reduction in emitted volume is apparent. The presence of abundant explosive deposits related to phreatic explosions during the Vulcanello III phase, is related to the presence of water, a reduction in magma volume and the presence of intense hydrothermal activity in the latter stage of the evolution of Vulcanello evolution until 1878. This may indicate the presence of a stable shallow thermal anomaly.

Rosi, M.; Fusillo, R.; di Traglia, F.; Pistolesi, M.; Todman, A.; Menzies, M. A.

2009-12-01

155

Peralkaline syenite autoliths from Kilombe volcano, Kenya Rift Valley: Evidence for subvolcanic interaction with carbonatitic fluids  

NASA Astrophysics Data System (ADS)

Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the "residuum system" (ne = 0-1.03; qz = 0-0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg-hedenbergite by aegirine-augite, Ca-aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti-magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids. By contrast, the Fe-Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite-magnetite equilibria indicate fO 2 between - 19.5 and - 23.1 log units ( T 679-578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids. Irrespective of silica saturation, the syenites can be divided into (1) "normal" syenites, characterized by Ce/Ce * ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce * 0.77-0.24). "Normal" silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE. The silica-saturated syenites contain REE-fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce * 0.4-0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Ce mean* = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce * is less than 0.2. It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization. Chevkinite-(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite-(Ce) and thorite. These latter represent new mineral occurrences in Kenya.

Ridolfi, Filippo; Renzulli, Alberto; Macdonald, Ray; Upton, Brian G. J.

2006-10-01

156

Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons  

USGS Publications Warehouse

The Solitario is a large, combination laccolith and caldera (herein termed "laccocaldera"), with a 16-km-diameter dome over which developed a 6 x 2 km caldera. This laccocaldera underwent a complex sequence of predoming sill, laccolith, and dike intrusion and concurrent volcanism; doming with emplacement of a main laccolith; ash-flow eruption and caldera collapse; intracaldera sedimentation and volcanism; and late intrusion. Detailed geologic mapping and 40Ar/39Ar dating reveal that the Solitario evolved over an interval of approximately 1 m.y. in three distinct pulses at 36.0, 35.4, and 35.0 Ma. The size, duration, and episodicity of Solitario magmatism are more typical of large ash-flow calderas than of most previously described laccoliths. Small volumes of magma intruded as abundant rhyolitic to trachytic sills and small laccoliths and extruded as lavas and tuffs during the first pulse at 36.0 Ma. Emplacement of the main laccolith, doming, ash-flow eruption, and caldera collapse occurred at 35.4 Ma during the most voluminous pulse. A complex sequence of debris-flow and debris-avalanche deposits, megabreccia, trachyte lava, and minor ash-flow tuff subsequently filled the caldera. The final magmatic pulse at 35.0 Ma consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture. Solitario rocks appear to be part of a broadly cogenetic, metaluminous suite. Peralkaline rhyolite lava domes were emplaced north and west of the Solitario at approximately 35.4 Ma, contemporaneous with laccolith emplacement and the main pulse in the Solitario. The spatial and temporal relation along with sparse geochemical data suggest that the peralkaline rhyolites are crustal melts related to the magmatic-thermal flux represented by the main pulse of Solitario magmatism. Current models of laccolith emplacement and evolution suggest a continuum from initial sill emplacement through growth of the main laccolith. Although the Solitario laccocaldera followed this sequence of events, our field and 40Ar/39Ar data demonstrate that it developed through repeated, episodic magma injections, separated by 0.4 to 0.6 m.y. intervals of little or no activity. This evolution requires a deep, long-lived magma source, well below the main laccolith. Laccoliths are commonly thought to be small, shallow features that are not representative of major, silicic magmatic systems such as calderas and batholiths. In contrast, we suggest that magma chambers beneath many ashflow calderas are tabular, floored intrusions, including laccoliths. Evidence for this conclusion includes the following: (1) many large plutons are recognized to be laccoliths or at least tabular, (2) the Solitario and several larger calderas are known to have developed over laccoliths, and (3) magma chambers beneath calderas, which are as much as 80 km in diameter, cannot be as deep as they are wide or some would extend into the upper mantle. The Solitario formed during a tectonically neutral period following Laramide deformation and preceding Basin and Range extension. Therefore, space for the main laccolith was made by uplift of its roof and possibly subsidence of the floor, not by concurrent faulting. Laccolith-type injection is probably a common way that space is made for magma bodies of appreciable areal extent in the upper crust.

Henry, C. D.; Kunk, M. J.; Muehlberger, W. R.; McIntosh, W. C.

1997-01-01

157

REE variation in alkaline mafic lavas across the North Tanzanian Divergence zone, a possible indicator of varying lithospheric thickness  

NASA Astrophysics Data System (ADS)

The magmatic evolution of the North Tanzanian Divergence zone (NTD), the southern termination of the eastern branch of the East African Rift (EAR), is of interest as a currently active magmatic-rich continental rift. In fact, the role of magmatism in continental rift initiation and evolution is of much debate and contrasts are currently drawn between amagmatic and magmatic-rich rift systems. The NTD possesses a wide array of pre-rift Miocene volcanoes to currently active volcanoes broadly distributed across the valley floor to the adjacent rift margins and characterized by having very heterogeneous chemical compositions. A highly diverse array of magmas from basalt to rhyolite, trachyte, phonolite and carbonatite occur at various volcanic centers, some of which have erupted more then one magma type. We analyzed 11 samples from Ketumbeine volcano for whole rock major and trace element abundances, Sr-Nd-Pb isotopic signatures and Ar/Ar ages. Two alkaline rock series are distinguished: a basanite to tephra-phonolite magma series and an alkali basalt to basaltic trachy-andesite magma series. Interestingly, the two magma series represent distinct ages of volcanism: the alkali basalt series is older with ages ranging between 2.2 and 1.9 Ma, while the basanite series erupted at about 1.2 Ma. The temporal separation of the two magma series occurs on a regional scale as well with the basanite to phonolite lavas erupted during the entire period of the NTD volcanism (5.8 Ma to Present), whereas the basalt to trachyte magma series is restricted to two brief intervals: the first between 2.4 and 1.6 Ma, immediately before the major rifting event dated by MacIntyre et al. (1974) at 1.2 Ma, and the second at about 0.5 Ma at Kibo and Mawenzi (Kilimanjaro). We model the source of the NTD as being derived from a metasomatized lithospheric mantle including variable amounts of garnet, amphibole and phlogopite. The most mafic and presumably less contaminated NTD volcanics have systematic REE variations (Sm/Yb versus La/Sm) that are consistent with different degrees of partial melting (La/Sm), and varying amounts of garnet and amphibole in the source (Sm/Yb). Many of the more evolved samples have greater radiogenic isotopic ratios and evolve via assimilation and fractional crystallization to lower Sm/Yb and higher La/Sm. Different degree of partial melting may reflect heat flow variations, while heterogeneity in the source is consistent with pressure conditions that imply changes in the depth of melting. In fact, REE abundances of key NTD volcanics (e.g. Essimingor) indicate melting in the garnet and phlogopite stability zone indicating the presence of a relatively thick lithosphere, while the absence of garnet indicate areas characterized by thinning of the lithosphere. MacIntyre, R.M., Mitchell, J.G., Dawson, J.B., 1974. Age of fault movements in Tanzanian sector of East African Rift System. Nature 247, 354-356.

Mana, S.; Carr, M. J.; Feigenson, M.; Furman, T.; Swisher, C. C.

2012-12-01

158

Calibration of cosmogenic 36Cl production rates from Ca and K spallation in lava flows from Mt. Etna (38°N, Italy) and Payun Matru (36°S, Argentina)  

NASA Astrophysics Data System (ADS)

Published cosmogenic 36Cl production rates from Ca and K spallation differ by almost a factor of 2. In this paper we determine production rates of 36Cl from Ca and K in samples of known age containing little Cl. Ca-rich plagioclases and K-feldspars were separated from a total of 13 samples collected on the surfaces of four basaltic lava flows at Mt. Etna (38°N, Italy) and from a trachyte lava flow at Payun Matru volcano (36°S, Argentina). Eruption ages, determined by independent methods, range between 0.4 and 32 ka. Sample site elevations range between 500 and 2500 m. Corresponding scaling factors were calculated using five different published scaling models, four of which consider paleo-geomagnetic field variations integrated over the exposure durations. The resulting five data sets were then analyzed using a Bayesian statistical model that incorporates the major inherent uncertainties in a consistent way. Spallation production rates from Ca and K, considering all major uncertainties, are 42.2 ± 4.8 atoms 36Cl (g Ca) -1 a -1 and 124.9 ± 8.1 atoms 36Cl (g K) -1 a -1 normalized to sea level and high latitude using the scaling method of Stone (2000). Scaling models that account for paleo-geomagnetic intensity changes yield very similar mean values (at most +4%). If the uncertainties in the independent ages are neglected in the Bayesian model, the calculated element specific production rates would be about 12% higher. Our results are in agreement with previous production rate estimations both for Ca and K if only low Cl (i.e. ?20 ppm) samples are considered.

Schimmelpfennig, Irene; Benedetti, Lucilla; Garreta, Vincent; Pik, Raphaël; Blard, Pierre-Henri; Burnard, Pete; Bourlès, Didier; Finkel, Robert; Ammon, Katja; Dunai, Tibor

2011-05-01

159

Kizilcaören ore-bearing complex with carbonatites (northwestern Anatolia, Turkey): Formation time and mineralogy of rocks  

NASA Astrophysics Data System (ADS)

The results of isotope-geochronological and mineralogical studies of the rocks making up the Kizilcaören fluorite-barite-REE deposit, northwestern Anatolia, Turkey are discussed in the paper. The ore is a constituent of the subvolcanic complex localized in a large fault zone. The complex combines (from earlier to later rocks): (1) phonolite and trachyte stocks, (2) carbonatite and carbonate-silicate dikelike bodies; and (3) fluorite-barite-bastnaesite ore in the form of thick homogeneous veins and cement in breccia. The K-Ar dating of silicate igneous rocks and carbonatites shows that they were formed in the Chattian Age of the Oligocene 25-24 Ma ago. Mineralogical observations show that the ore is the youngest constituent in the rock complex. Supergene alteration deeply transformed ore-bearing rocks, in particular, resulting in leaching of primary minerals, presumably Ca-Mn-Fe carbonates, and in cementation of the residual bastnaesitefluorite framework by Fe and Mn hydroxides. Most of the studied rocks contain pyrochlore, LREE fluorocarbonates, Nb-bearing rutile, Fe-Mg micas, and K-feldspar. The genetic features of the deposit have been considered. In general, the ore-bearing rock complex is compared in the set of rocks and their mineralogy and geochemistry with deposits of the Gallinas Mountains in the United States, the Arshan and Khalyuta deposits in the western Transbaikalia region, and Mushugai-Khuduk deposit in Mongolia. The Kizilcaören deposit represents a variant of postmagmatic mineralization closely related to carbonatite magmatism associated with alkaline and subalkaline intermediate rocks.

Nikiforov, A. V.; Öztürk, H.; Altuncu, S.; Lebedev, V. A.

2014-02-01

160

The Ngorongoro Volcanic Highland and its relationships to volcanic deposits at Olduvai Gorge and East African Rift volcanism.  

PubMed

The Ngorongoro Volcanic Highland (NVH), situated adjacent and to the east of Olduvai Gorge in northern Tanzania, is the source of the immense quantities of lava, ignimbrite, air fall ash, and volcaniclastic debris that occur interbedded in the Plio-Pleistocene sedimentary deposits in the Laetoli and Olduvai areas. These volcanics have proven crucial to unraveling stratigraphic correlations, the age of these successions, the archaeological and paleontological remains, as well as the source materials from which the bulk of the stone tools were manufactured. The NVH towers some 2,000 m above the Olduvai and Laetoli landscapes, affecting local climate, run-off, and providing varying elevation - climate controlled ecosystem, habitats, and riparian corridors extending into the Olduvai and Laetoli lowlands. The NVH also plays a crucial role in addressing the genesis and history of East African Rift (EAR) magmatism in northern Tanzania. In this contribution, we provide age and petrochemical compositions of the major NVH centers: Lemagurut, basalt to benmorite, 2.4-2.2 Ma; Satiman, tephrite to phonolite, 4.6-3.5 Ma; Oldeani, basalt to trachyandesite, 1.6-1.5 Ma; Ngorongoro, basalt to rhyolite, 2.3-2.0 Ma; Olmoti, basalt to trachyte, 2.0-1.8 Ma; Embagai, nephelinite to phonolite, 1.2-0.6 Ma; and Engelosin, phonolite, 3-2.7 Ma. We then discuss how these correlate in time and composition with volcanics preserved at Olduvai Gorge. Finally, we place this into context with our current understanding as to the eruptive history of the NVH and relationship to East African Rift volcanism. PMID:22404967

Mollel, Godwin F; Swisher, Carl C

2012-08-01

161

Preliminary Interpretation on the Role of Sub-Ice Topography on the Emplacement of Basaltic Pillow Lavas at Tennana Cone, Mount Edziza Volcanic Complex, British Columbia, Canada  

NASA Astrophysics Data System (ADS)

The Mount Edziza Volcanic Complex (MEVC) is a northeast trending, 75km long volcanic edifice in Northern British Columbia. Erupting cyclically over the last 7.5 ma, volcanic centers along the complex have erupted alkalic basalts to rhyolites and a range of intermediate alkalic lavas. Several episodes of sub-ice or ice- contact volcanism are recorded since 2Ma (Souther, 1992), including a basaltic volcaniclastic ridge and associated lava flows at Tennena Cone on the western side of the MEVC. Tennena Cone (TC) is a 200m high sequence, dominated by pillow-fragment breccias, that overlies trachytic lava flows of the approximately 1Ma-old Ice Peak Formation (IPF). The IPF flows vary from 3 to 7 meters in thickness and form a cliff-forming stepped topography. Underlying the IPF lavas are glaciogenic sediments, of dominantly glaciofluvial origin. A widespread area of pillow and subaqueous lobate lava flows that originated from TC were emplaced on its western margin. These lavas overlie both the steep topography of the IPF, and distally, the glaciogenic sediments. The geomorphology and structures of the TC lava flows suggest that were initially emplaced as sheet-like pillow lavas with local ponding on a dissected plateau of underlying IPF lavas, but as they approached the steep sub-ice cliffs of IPF lavas (and thicker ice), the growth of pillow mounds at the flow termini occurred. However, at one locality the pillow lavas spilled over a gap in the sub-ice cliffs, and were then focused downstream within sub-ice sinuous drainage tunnels. We intend to conduct volatile analyses on samples of glass rinds of pillow lavas in both pillow mounds and sheets to constrain the thickness of overlying ice. Knowing ice thickness and reconstructing the paleo- topography, we can begin to interpret how ice flow dynamics may control the emplacement of sub-ice basaltic lavas.

Hungerford, J. D.; Skilling, I.; Lloyd, A.; Edwards, B.

2006-12-01

162

Floating sandstones off El Hierro (Canary Islands, Spain): the peculiar case of the October 2011 eruption  

NASA Astrophysics Data System (ADS)

The eruption that started off the south coast of El Hierro, Canary Islands, in October 2011 has emitted intriguing eruption products found floating in the sea. These specimens appeared as floating volcanic "bombs" that have in the meantime been termed "restingolites" (after the close-by village of La Restinga) and exhibit cores of white and porous pumice-like material. Currently the nature and origin of these "floating stones" is vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have collected and analysed the structure and composition of samples and compared the results to previous work on similar rocks found in the archipelago. Based on their high silica content, the lack of igneous trace element signatures, and the presence of remnant quartz crystals, jasper fragments and carbonate relicts, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary rocks that were picked up and heated by the ascending magma causing them to partially melt and vesiculate. They hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies in the Canary Islands as well as in similar Atlantic islands that rest on sediment/covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of these "restingolites" does therefore not indicate the presence of an explosive high-silica magma that is involved in the ongoing eruption.

Troll, V. R.; Klügel, A.; Longpré, M.-A.; Burchardt, S.; Deegan, F. M.; Carracedo, J. C.; Wiesmaier, S.; Kueppers, U.; Dahren, B.; Blythe, L. S.; Hansteen, T.; Freda, C.; Budd, D. A.; Jolis, E. M.; Jonsson, E.; Meade, F.; Berg, S.; Mancini, L.; Polacci, M.

2011-12-01

163

Post 19 ka B.P. eruptive history of Ulleung Island, Korea, inferred from an intra-caldera pyroclastic sequence  

NASA Astrophysics Data System (ADS)

Ulleung Island is a Quaternary volcanic island located in the mid-western part of the East Sea (Sea of Japan) back-arc basin, which has erupted from the Pliocene until the late Holocene. This study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent, trachytic/phonolitic pyroclastic sequence, named the Nari Tephra Formation. This formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the Nari Caldera. The embayment acted as a topographic trap for proximal pyroclastic deposits, and contains a complete record of the past 19,000 years of eruption history. The formation includes evidence for five separate eruptive episodes (Member N-1 to N-5), with intervening weathered and/or soil horizons indicating hundreds to thousands of years of repose between each eruption. Eruption styles and depositional mechanisms varied between and during individual episodes, reflecting changing dynamics of the magma plumbing system, magmatic gas coupling, and a variable role of external water. Extra-caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents (PDCs) large enough to overtop the caldera wall. Thus tephra sequences outside the caldera provide an underestimate of eruption frequency, and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records. In addition, topographic effects of caldera structures should be considered for the assessment of PDC-related hazards in such moderately sized pyroclastic eruptions.

Kim, G. B.; Cronin, S. J.; Yoon, W. S.; Sohn, Y. K.

2014-04-01

164

Influence of alteration on physical properties of volcanic rocks  

NASA Astrophysics Data System (ADS)

Physical properties of some weathered/altered volcanic rocks and their variation with the degree of alteration are described in detail. A series of tests was performed to identify and quantify the progressive degradation of the properties: 1) petrographycal and chemical studies; 2) effective (?e) and total porosity (?t) measurements and 3D pore reconstruction 3) ultrasonic pulse velocity and spatial attenuation (?s) measurements. Four different volcanic lithologies have been tested: i) trachytic lava with abundant crystals; ii) pyroclastic deposits, with lava clasts and pumice elements; iii) green tuff, made prevalently of pumice clasts; and iv) non-welded ignimbrite deposits. Chemical indices of weathering (CIW) reveal large differences (42.73 < Chemical Index of Alteration [CIA] < 69.24) not only between lithotypes, but also between samples. These differences are reflected by physical properties, in particular ?t (6.0 < ?t from X-ray tomography images < 49.8%), ?e (11.0 < ?e from mercury porosimeter < 65.0%) and shear wave velocity values (0.50 < Vs < 2.90 km/s). Pore network evolution with alteration for each lithology is well documented by fractal dimension (D) and ?s. Mean values of porosity are strictly related to P and S wave velocity (Vp and Vs) and the degree of alteration. Values of CIA are well correlated with the degradation trend exhibited by measured physical properties. The combination of techniques to measure the ?t and ?e provides a good estimate of grain size and pore size distribution and rock structure. Defects and particular characteristics in the rock sample (e.g. micro-fractures, voids, cavities and orientation and sizes of certain minerals and clasts) are revealed by ?s values: the smaller the ?s the more homogeneous and less altered is the sample.

Pola, Antonio; Crosta, Giovanni; Fusi, Nicoletta; Barberini, Valentina; Norini, Gianluca

2012-09-01

165

Magmatic (silicates/saline/sulfur-rich/CO2) immiscibility and zirconium and rare-earth element enrichment from alkaline magma chamber margins : Evidence from Ponza Island, Pontine Archipelago, Italy  

USGS Publications Warehouse

Fluid inclusions were measured from a feldspathoid-bearing syenite xenolith entrained in trachyte from Ponza, one of the islands of the Pontine Archipelago, located in the Gulf of Gaeta, Italy. The feldspathoid-bearing syenite consists mainly of potassium feldspar, clinopyroxene, amphibole, biotite, titanite, manganoan magnetite, apatite with minor nosean, Na-rich feldspar, pyrrhotite, and rare cheralite. Baddeleyite and zirkelite occur associated with manganoan magnetite. Detailed electron-microprobe analysis reveals enrichments in REE, Y, Nb, U, Th as well as Cl and F in appropriate phases. Fluid inclusions observed in potassium feldspar are either silicate-melt or aqueous inclusions. The aqueous inclusions can be further classified as. (1) one-phase vapor, (2) two-phase (V + L) inclusions, vapor-rich inclusions with a small amount of CO2 in most cases; homogenization of the inclusions always occurred in the vapor phase between 359 and 424??C, salinities vary from 2.9 to 8.5 wt. % NaCl equivalent; and. (3) three-phase and multiphase inclusions (hypersaline/sulfur-rich aqueous inclusions sometimes with up to 8 or more solid phases). Daughter minerals dissolve on heating before vapor/liquid homogenization. Standardless quantitative scanning electron microscope X-ray fluorescence analysis has tentatively identified the following chloride and sulfate daughter crystals; halite, sylvite, glauberite. arcanite, anhydrite, and thenardite. Melting of the daughter crystals occurs between 459 and 536??C (54 to 65 wt. % NaCI equivalent) whereas total homogenization is between 640 and 755??C. The occurrence of silicate-melt inclusions and high-temperature, solute-rich aqueous inclusions suggests that the druse or miarolitic texture of the xenolith is late-stage magmatic. The xenolith from Ponza represents a portion of the peripheral magma chamber wall that has recorded the magmatic/hydrothermal transition and the passage of high solute fluids enriched in chlorides, sulfur, and incompatible elements.

Belkin, H. E.; De Vivo, B.; Lima, A.; Torok, K.

1996-01-01

166

The double solid reactant method: II. An application to the shallow groundwaters of the Porto Plain, Vulcano Island (Italy)  

NASA Astrophysics Data System (ADS)

This paper presents an example of application of the double solid reactant method (DSRM) of Accornero and Marini (Environmental Geology, 2007a), an effective way for modeling the fate of several dissolved trace elements during water-rock interaction. The EQ3/6 software package was used for simulating the irreversible water-rock mass transfer accompanying the generation of the groundwaters of the Porto Plain shallow aquifer, starting from a degassed diluted crateric steam condensate. Reaction path modeling was performed in reaction progress mode and under closed-system conditions. The simulations assumed: (1) bulk dissolution (i.e., without any constraint on the kinetics of dissolution/precipitation reactions) of a single solid phase, a leucite-latitic glass, and (2) precipitation of amorphous silica, barite, alunite, jarosite, anhydrite, kaolinite, a solid mixture of smectites, fluorite, a solid mixture of hydroxides, illite-K, a solid mixture of saponites, a solid mixture of trigonal carbonates and a solid mixture of orthorhombic carbonates. Analytical concentrations of major chemical elements and several trace elements (Cr, Mn, Fe, Ni, Cu, Zn, As, Sr and Ba) in groundwaters were satisfactorily reproduced. In addition to these simulations, similar runs for a rhyolite, a latite and a trachyte permitted to calculate major oxide contents for the authigenic paragenesis which are comparable, to a first approximation, with the corresponding data measured for local altered rocks belonging to the silicic, advanced argillic and intermediate argillic alteration facies. The important role played by both the solid mixture of trigonal carbonates as sequestrator of Mn, Zn, Cu and Ni and the solid mixture of orthorhombic carbonates as scavenger of Sr and Ba is emphasized.

Lelli, Matteo; Cioni, Roberto; Marini, Luigi

2008-11-01

167

Two mantle sources, two plumbing systems: Tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province  

USGS Publications Warehouse

Rocks of two distinctly different magma series are found in a ???4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and ??Nd values of 0 to + 2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO2 from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al2O3/TiO2 ??? 1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of ??Nd values, from -2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. ?? Springer-Verlag 1998.

Arndt, N.; Chauvel, C.; Czamanske, G.; Fedorenko, V.

1998-01-01

168

Middle Pleistocene activity of the Hekla volcano  

NASA Astrophysics Data System (ADS)

Hekla volcano is one of the most active volcanoes in SE Iceland. Hekla is a ridge-shape stratovolcano, located near the apex Icelandicelandic hot spot. It is located on the SVZ, initiated with the last rift jump, c.3 Ma and the polarity of the basement lavas yields an age younger than 700 ka. Even if Holocene and late glacial eruptions are well constrained, little is known about the effective age of this volcano. Hekla old lavas are mostly hyaloclastites and are difficult to date, while dykes are deeply weathered by late hydrothermal activity. Field data in the Rangavellir (ou mettre Ytri-Rangá valley) provide evidence for eruptions around the last Interglacial within a large coastal sedimentary prism, the Rangá Formation (130-80 ka) that is buried by the Búdi terminal moraines and Hekla Holocene lava flows. Emplaced after a highly erosive glaciation, this Rangá Formation contains a reworked trachytic tephra in form of pumice pellets that display a vesiculation similar to Hekla pumices. element'sd trace this tephra ts composition of these tephra is very similar to the Holocene pumice from Hekla volcano and confirm that they are coming from an eruption produced by this volcano. 40Ar-39Ar dating of these pumices yielded c.410ka. This age is very similar to those of other acidic volcanoes around the Hofsjökull and the Vatnajökull (Kerlingarfjöll, Torfajökull, Laufafell, Nyðry Hagánga, Snæfell, Kverkfjöll) and also from the Snæfelness peninsula. This confirms that very large glaciations such as MIS 12 and 10 are followed by intense felsic volcanic activity at the onset of the deglaciations

Chazot, Gilles; Guillou, Hervé; Schneider, Jean-luc; Van Vliet-Lanoe, Brigitte

2013-04-01

169

Contact metasomatic and hydrothermal minerals in the SH2 deep well, Sabatini Volcanic District, Latium, Italy  

SciTech Connect

Metasomatic and hydrothermal minerals were logged throughout the SH2 geothermal well, which reached a depth of 2498 m in the Sabatini volcanic district. Below 460 m of volcanics, where the newly formed minerals were mainly chlorite, calcite and zeolites (mostly phillipsite), drilling entered the Allochthonous Flysch Complex. Evidence of the ''Cicerchina facies'' was found down to 1600 m depth. Starting from 1070 m, down to hole bottom, a contact metasomatic complex was defined by the appearance of garnet. Garnet together with K-fledspar, vesuvianite, wilkeite, cuspidine, harkerite, wollastonite and apatite prevail in the top part of the contact metasomatic complex. Vesuvianite and phlogopite characterize the middle part. Phlogopite, pyroxene, spinel and cancrinite predominate in the bottom part. The 1500 m thick metasomatic complex indicates the presence at depth of the intrusion of a trachytic magma which released hot fluids involved in metasomatic mineral-forming reactions. Minerals such as harkerite, wilkeite, cuspidine, cancrinite, vesuvianite and phlogopite indicate the intrusive melt had a high volatile content which is in agreement with the very high explosivity index of this volcanic district. The system is at present sealed by abundant calcite and anhydrite. It is proposed that most, if not all, of the sulphates formed after reaction of SO/sub 2/ with aqueous calcium species rather than from sulphates being remobilized from evaporitic (Triassic) rocks as previously inferred. The hypothesis of a CO/sub 2/-rich deep-derived fluid ascending through major fracture systems and contrasting cooling in the hottest areas of Latium is presented.

Cavarretta, G.; Tecce, F.

1987-01-01

170

Fluoride content in drinking water supply in São Miguel volcanic island (Azores, Portugal).  

PubMed

High fluoride contents in the water supply of the city of Ponta Delgada, located in the volcanic island of São Miguel (Azores, Portugal) have been reported. Dental fluorosis in São Miguel has been identified and described in several medical surveys. The water supply in Ponta Delgada consists entirely of groundwater. A study was carried out in order to characterize the natural F-pollution of a group of springs (30) and wells (3), that are associated to active central volcanoes of a trachytic nature. Two springs known for their high content in fluoride were sampled, both located in the central volcano of Furnas. The sampled waters are cold, ranging from slightly acidic to slightly alkaline (pH range 6.53-7.60), exhibiting a low electrical conductivity (springs range 87-502 ?S/cm; wells range 237-1761 ?S/cm), and are mainly from the Na-HCO(3), Na-HCO(3)-Cl and Na-Cl-HCO(3) water types. Results suggest two main trends of geochemical evolution: silicate weathering, enhanced by CO(2) dilution, and seawater spraying. Fluoride contents range between 0.17 mg/L and 2 mg/L, and no seasonal variations were detected. Results in the sources of the water supply system are lower than those of the Furnas volcano, which reach 5.09 mgF/L, demonstrating the effect of F-rich gaseous emanations in this area. Instead, the higher fluoride contents in the water supply are mainly due to silicate weathering in aquifers made of more evolved volcanic rocks. PMID:22705903

Cordeiro, S; Coutinho, R; Cruz, J V

2012-08-15

171

Fluvial geochemistry in São Miguel Island (Azores, Portugal): source and fluxes of inorganic solutes in an active volcanic environment.  

PubMed

River water chemistry in two of the major rivers in São Miguel Island (Azores archipelago, Portugal) has been characterized monthly between June 2010 and October 2011 in order to estimate the main geochemical processes that explain water composition and to estimate solute fluxes and the thermal water input to rivers. Both rivers (Ribeira Grande - RRG, and Ribeira Quente - RRQ) drain active trachytic central volcanoes. The number of sampling stations is seven in RRG and six in RRQ. Rivers are mainly slightly acid to basic in nature (pH in the range 5.41-8.70 in RRG and 5.90-8.10 in RRQ) and from the Na-HCO3 and Na-HCO3-Cl water types. In both cases water temperature increases towards the river mouth and median values are slightly higher in Ribeira Quente (15.5-22.4°C) comparing to Ribeira Grande river (15.2-19.7°C). Electrical conductivity suggests poorly mineralized waters (139-456 ?S/cm in RRG and 209-402 ?S/cm in RRQ, and values increase downstream. Higher solute fluxes are associated to HCO3 and Na, and the total CO2 flux is equal to 3647 t/yr in RRQ and 7546 t/yr in RRG. Mixture with thermal water discharges also influences river water chemistry and in RRQ the contribution to the annual average discharge rate was estimated in 2.96×10(6)m(3)/yr (12.8% of overall discharge rate in the watershed). In RRG thermal water discharges were estimated in 2.4×10(6)m(3)/yr (14.9% of the discharge rate). The minimum total CO2-consumption associated with low-temperature weathering is equal to 0.58×10(6)mol/km(2)/yr in Ribeira Quente river and equal to 0.78×10(6)mol/km(2)/yr in Ribeira Grande river. PMID:23542489

Freire, P; Andrade, C; Coutinho, R; Cruz, J V

2013-06-01

172

Magmatic processes inferred from chemical composition, texture and crystal size distribution of the Heikongshan lavas in the Tengchong volcanic field, SW China  

NASA Astrophysics Data System (ADS)

The products of volcanic eruption contain abundant information about the magmatic processes in a magma chamber. In this paper, we study the magmatic processes associated with the Heikongshan volcano in the Tengchong volcanic field (TVF), Yunnan Province of southwestern China, through chemical composition, texture and crystal size distribution (CSD) analysis of volcanic lavas. The Heikongshan lavas comprise five flow units associated with three different episodes of volcanic eruption since the middle Pleistocene. Bulk rock chemical analyses indicate that unit I is a basaltic trachyandesite, units II-IV are of trachyandesite, and unit V is composed of trachyte. The textures of these lavas can be further classified into three main groups accordingly, each represented by different phenocryst and groundmass assemblages, reflecting the changes of physical environment in the magma chamber. CSD analysis of plagioclase phenocrysts yields a mixture of CSD curves, some of which are nearly straight lines, while others are weakly concave-up. These concave-up CSD curves are consistent with magma mixing between host magma and early residual dacitic magma. In specifics, the CSD curve of large plagioclase phenocrysts (>1.6 mm) in unit I reflects the characteristic of early dacitic magma. Whereas, the CSD curves of small plagioclase phenocrysts (<1.6 mm) in units II-IV yield straight lines, indicating a relatively steady open system in which they formed. In contrast, influenced by the plagioclase crystals in early dacitic magma, the CSD curves of large plagioclase phenocrysts (>1.6 mm) in units II-IV deviated from their original trend and formed in concave-up shape. Finally, in the last episode when unit V formed, the magma temperature decreased, resulting in the formation of relatively large amount of small-sized plagioclase crystals. Such process is reflected in the CSD diagram by a relatively higher value of intercept and the steeper slope.

Yu, Hongmei; Xu, Jiandong; Lin, Chuanyong; Shi, Lanbin; Chen, Xiaode

2012-09-01

173

Phonolites and peralkaline rhyolites from a single magma source in the mantle : A new look at some Black Hills rocks  

SciTech Connect

A re-evaluation of existing data from the Deer Mountain-Terry Peak-Sugarloaf Mountain area of the Black Hills, plus some new data, suggests the real possibility that both silica-undersaturated and silica-oversaturated alkaline-peralkaline rocks evolved from the same mantle-derived parent magma. Mineralogically, aegirine rhyolites, phonolites, a minette and the mantle are linked by an association of Mg-rich olivine-phlogopite structures, zenocrystic phlogopite and diopside-cored pyroxene phenocrysts. Trends of silica vs. major elements, trace elements (V,Sc,Ni) and MgO/FeOt are continuous and preclude being fortuitous. Peralkalinity also increases with silica in a well-defined trend. Increasing ferric oxide to total iron oxide indicates increasing oxygen fugacity with silica saturation. A mantle origin for the phonolites is supported by Sr-isotope data of Beintema (1986) and Beintema and Montgomery (1986). Higher Sr-isotope ratios for the aegirine rhyolites, suggesting a lower crustal origin, actually may result from magmatic processes, as shown by others for ocean island basalt-phonolite-comendite associations. Early fractionation of mafic phases drives trends away from the Ne-minimum on the residua diagram, indicating that magma evolution took place above residua temperatures, thus avoiding the thermal divide. Later fractionation of alkali feldspars accounts for variation in the aegirine rhyolites. Rising alkalies and oxygen explain variations in peralkalinity and ferric iron content. Pressure-dependent immiscibility possibly may be the cause of a silica gap in rock types, as rocks with low quartz contents are not found, except as phaneritic inclusions. A model is suggested in which either a fractionating minette or trachyte magma could yield the series of rocks under study.

Kirchner, J.G. (Illinois State Univ., Normal, IL (United States). Dept. of Geography-Geology)

1993-03-01

174

An Initial Report of Research Into the Identification of Lava Flows at the Broken Top and North Crater Cinder Cones in the Craters of the Moon Lava Field by Their Chemical and Petrographic Composition (the Great Rift of Idaho, Snake River Plain)  

NASA Astrophysics Data System (ADS)

Craters of the Moon lava field was formed during the Great Rift of Idaho volcanic activity for more than 15 Ka. There are still unsolved questions about chemical and petrographic compositions of lava flows inside the Craters of the Moon lava field, their relative and absolute ages, and depths of their magma generation chambers. The research undertaken by the author is based on results of field work, petrographic and microprobe analysis of lava samples, and published materials on the Great Rift and adjacent territories. The chemical and petrographic composition of North Crater and Broken Top cinder cones and lava flows, and the South Highway and Blue Dragon lava flows was analyzed. The North Crater lava flow and cinder cone mainly consist of trachybasalts and basaltic trachyandesite. The South Highway lava flow can be divided into three groups of flow and cinder, which are 1) dacite-trachydacite-trachyte; 2) basalt-trachybasalt, and 3) andesite-trachyandesite. The main lava flow of Broken Top is composed of trachybasalt and basaltic trachyandesite. The cinder cone of Broken Top consists of basaltic andesite and basaltic trachyandesite. It is shown that the chemical composition of glass, olivine and the spinel group minerals is unique in each lava flow or cinder cone, which serves as a tool to identify each lava flow. Depths of magma generation were estimated for North Crater, South Highway, Broken Top and Blue Dragon lava flows. It was determined that during the evolution of volcanic activity of the Great Rift the depth of magma generation has decreased. This is explained by the decompression which took place as the Great Rift stretched, allowing the magma chamber to rise closer to the surface. This can be observed in the eruptive and non-eruptive fissures that run parallel to the rift.

Lendyel, P.; Koronovsky, N.

2013-12-01

175

The Deccan tholeiite lavas and dykes of Ghatkopar–Powai area, Mumbai, Panvel flexure zone: Geochemistry, stratigraphic status, and tectonic significance  

NASA Astrophysics Data System (ADS)

Mumbai City, situated on the western Indian coast, is well known for exposures of late-stage Deccan pillow basalts and spilites, pyroclastic rocks, rhyolite lavas, and trachyte intrusions. These rock units, and a little-studied sequence of tholeiitic flows and dykes in the eastern part of Mumbai City, constitute the west-dipping limb of a regional tectonic structure called the Panvel flexure. Here we present field, petrographic, major and trace element and Sr–Nd isotopic data on these tholeiitic flows and dykes, best exposed in the Ghatkopar–Powai area. The flows closely resemble the Mahabaleshwar Formation of the thick Western Ghats sequence to the east, in Sr–Nd isotopic ratios and multielement patterns, but have other geochemical characteristics (e.g., incompatible trace element ratios) unlike the Mahabaleshwar or any other Formation. The flows may have originated from a nearby eruptive center, possibly offshore of Mumbai. Two dykes resemble the Ambenali Formation of the Western Ghats in all geochemical characteristics, though they may not represent feeders of the Ambenali Formation lavas. Most dykes are distinct from any of the Western Ghats stratigraphic units. Some show partial (e.g., Sr–Nd isotopic) similarities to the Mahabaleshwar Formation, and these include several dykes with unusual, concave-downward REE patterns suggesting residual amphibole and thus a lithospheric source. The flows and dykes are inferred to have undergone little or no contamination, by lower continental crust. Most dykes are almost vertical, suggesting emplacement after the formation of the Panvel flexure, and indicate considerable east–west lithospheric extension during this late but magmatically vigorous stage of Deccan volcanism.

Sheth, Hetu C.; Zellmer, Georg F.; Demonterova, Elena I.; Ivanov, Alexei V.; Kumar, Rohit; Patel, Rakesh Kumar

2014-04-01

176

Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism  

NASA Astrophysics Data System (ADS)

We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000-1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (?=1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050-1075 °C, while phlogopite-breakdown occurs at 1150-1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite-lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ?3.9 wt.% in lherzolite to ?6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5-52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field.

Condamine, Pierre; Médard, Etienne

2014-07-01

177

Igneous rocks of the East Pacific Rise  

USGS Publications Warehouse

The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte). In contrast, the more submerged parts of the Rise are largely composed of a tholeiitic basalt which has low concentrations of K, P, U, Th, Pb, and Ti. This tholeiitic basalt is either the predominant or the only magma generated in the earth's mantle under oceanic ridges and rises. It is at least 1000-fold more abundant than the alkali suite, which is probably derived from tholeiitic basalt by magmatic differentiation in and immediately below the larger volcanoes. Distinction of oceanic tholeiites from almost all continental tholeiites is possible on the simple basis of total potassium content, with the discontinuity at 0.3 to 0.5 percent K2O by weight. Oceanic tholeiites also are readily distinguished from some 19 out of 20 basalts of oceanic islands and seamount cappings by having less than 0.3 percent K2O by weight and more than 48 percent SiO2. Deep drilling into oceanic volcanoes should, however, core basalts transitional between the oceanic tholeiites and the presumed derivative alkali basalts.The composition of the oceanic tholeiites suggests that the mantle under the East Pacific Rise contains less than 0.10 percent potassium oxide by weight; 0.1 part per million of uranium and 0.4 part of thorium; a potassium:rubidium ratio of about 1200 and a potassium: uranium ratio of about 104.

Engel, A. E. J.; Engel, C. G.

1964-01-01

178

Geochemical study of laterites of the Jamnagar district, Gujarat, India: Implications on parent rock, mineralogy and tectonics  

NASA Astrophysics Data System (ADS)

The laterite deposits occur in a linear stretch along the northern Arabian Sea coast in the Jamnagar and Porbandar districts of, Gujarat state, India. These deposits are characterised by presence of gibbsite, kaoline, calcite, quartz, anatase, natroalunite, goethite and hematite, and relicts of mafic minerals and plagioclase. On the basis of petro-mineralogy and geochemistry, these deposits are grouped as aluminous laterites (Fe 2O 3 - 1.45-3.84%, Av. 3.13, Al 2O 3 - 39.31-57.24, Av. 45.80) and laterites (Fe 2O 3 - 9.84-32.21, Av. 25.13%, Al 2O 3 - 34.74-49.59, Av. 41.27). The major, trace and REE characteristics of laterites indicate that these were formed in situ by the alteration of parent rocks of trachytic/andesitic composition, and the process of bauxitisation followed the path of destruction of kaolinite and deferruginisation. The correlation patterns of several trace and rare earth elements and their preferential enrichment have indicated that there is an influence of precursor rock on the distribution of trace elements. The Jamnagar laterite deposits occur as capping over the Deccan Trap basaltic lava flows and pyroclasic deposits. Lateritisation prevailed during Palaeocene age when India was separated from the Seychelles and passing over the equator. During this time climate, morphology and drainage conditions were favourable for lateritisation that result in the formation of Jamnagar and other laterite deposits within the Deccan Province. Flood basaltic provinces of Deccan, Columbia, North Australia and Hawaii appear good location for hosting laterite deposits due to their wide areal extent, small geological time span and uniform chemical composition. However, comparison of the major flood basaltic provinces of the world has indicated that their palaeopositions along with palaeoclimate, morphology and drainage are equally important factors for facilitating lateritisation.

Meshram, R. R.; Randive, K. R.

2011-11-01

179

Volcanic geology of Furnas Volcano, São Miguel, Azores  

NASA Astrophysics Data System (ADS)

Furnas is the easternmost of the three active central volcanoes on the island of São Miguel in the Azores. Unlike the other two central volcanoes, Sete Cidades and Fogo, Furnas does not have a well-developed edifice, but consists of a steep-sided caldera complex 8×5 km across. It is built on the outer flanks of the Povoação/Nordeste lava complex that forms the eastern end of São Miguel. Constructive flanks to the volcano exist on the southern side where they form the coastal cliffs, and to the west. The caldera margins tend to reflect the regional/local tectonic pattern which has also controlled the distribution of vents within the caldera and areas of thermal springs. Activity at Furnas has been essentially explosive, erupting materials of trachytic composition. Products associated with the volcano include plinian and sub-plinian pumice deposits, ignimbrites and surge deposits, phreatomagmatic ashes, block and ash deposits and dome materials. Most of the activity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera. The eruptive history consists of at least two major caldera collapses, followed by caldera infilling. Based on 14C dates, it appears that the youngest major collapse occurred about 12,000-10,000 years BP. New 14C dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption occurred at about 30,000 years BP. Recent eruptions (<5000 years old) were mainly characterised by alternating episodes of magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude, forming deposits of interbedded ash and lapilli. An historical eruption is documented in 1630 AD; new evidence suggests that another occurred during the early occupation of the area at about 1440 AD.

Guest, J. E.; Gaspar, J. L.; Cole, P. D.; Queiroz, G.; Duncan, A. M.; Wallenstein, N.; Ferreira, T.; Pacheco, J.-M.

1999-09-01

180

The Campanian Ignimbrite and its precursor eruptions - implications for tephrochronology  

NASA Astrophysics Data System (ADS)

The Campanian Ignimbrite (CI, Campi Flegrei, Naples, Italy) has been the focus of research for many decades and is well constrained in terms of eruptive volume (>200 km3), chemistry (phonolitic-trachytic to tephriphonolite-trachyandesite), 40Ar/39Ar age (39 ka) and distribution (plinian fall deposits cover >1500 km2.; proximal deposits are correlated with Y5, a widespread ash horizon found in cores from the eastern Mediterranean). Given its fall footprint across Europe it is a pivotal marker horizon in marine and lacustrine cores and archaeological sites as far afield as Russia (Pyle et al., 2006). However, its use in tephrochonology is only possible if “diagnostic chemistry” can be determined in proximal units where the age is well-constrained. Defining a diagnostic chemistry for the CI is complicated by the compositional variability of it’s erupted products, and by the existence of several proximal fall units erupted during the period 59 to 39 ka. Juvenile clasts (glass, phenocrysts, & melt inclusions) from fall deposits in the Campanian and several of the largest pre-Campanian units (TLa, TLc, TLf from Trefola Quarry, Naples) were analysed by LA-ICP-MS in order to: (a) define the proximal variation of juvenile clasts; (b) investigate the processes that lead to compositional variation among the eruptive products from Campi Flegrei, in order to determine which signatures provide the most reliable geochemical fingerprint; and (c) to decide upon the CI fingerprint for use in tephrochronology in distal archaeological sites and marine/lacustrine cores. The pre-CI juvenile products are highly evolved, undersaturated magmas (dominantly phonolites) and are less chemically heterogeneous than the CI. Geochemical plots support fractionation of magnetite and feldspar, Consistent ratios of elements with similar degrees of incompatibility indicate that the pre-CI and the first erupted CI products may share a common source at depth before source and/or crustal contamination, as also evidences by Sr and Nd isotope compositions (Arienzo et al., 2009).

Tomlinson, E.; Smith, V.; Mueller, W.; Arienzo, I.; Carandente, A.; Civetta, L.; Orsi, G.; Rosi, M.; Menzies, M. A.

2009-12-01

181

Duration of gas accumulation before the 2010 Eyjafjallajökull eruption constrained by 210Po-210Pb-226Ra disequilibria  

NASA Astrophysics Data System (ADS)

Excess gas phase in magmas erupting explosively is well known world-wide. However, the origin of this gas phase, in excess of what can be dissolved in the erupting magma at depth, and the duration of gas accumulation, is less well defined. The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreiitic and trachytic composition whereas alkali basalt (MgO > 8 %) was emitted during the preceding flank eruption. The silicic tephra of the first explosive phase is composed of three glass types, alkaline rhyolite, mixed benmoreiite, and evolved basalt (MgO < 5 %). The rhyolitic glass is indistinguishable from tephra glass composition emitted during the penultimate eruption of Eyjafjallajökull in 1821-23 AD (Sigmarsson et al., 2011). Tephra from the first explosive phase, emitted on 15 and 17 April, had large 210Po in excess of 210Pb ((210Po/210Pb)0 as high as 2!) and a small, but significant, 210Pb excess over its parent 226Ra ((210Pb/226Ra)0= 1.05 and 1.04, respectively). These excesses suggest rapid accumulation of Po and Rn together with the major gas species in the residual rhyolitic magma from the 1821-23 eruption. The gas most likely originates from the basalt recharge that eventually provoked the eruption. Basalts emitted a month earlier during the flank eruption at Fimmvörðuháls lost all their Po upon eruption and had (210Po/210Pb)0 equal to 0). From a simple model of radon and polonium degassing and accumulation, the mass of basalt magma degassing over the mass of silicic magma accumulating the excess gas can be calculated. Moreover, the duration of gas accumulation can be shown to be close to 300 days. This duration suggests that gas was liberated from the basaltic magma since June 2009, a month that corresponds to the initial seismic swarm beneath Eyjafjallajökull preceding the explosive eruption of 14 April 2010.

Sigmarsson, Olgeir; Gauthier, Pierre-Jean; Condomines, Michel

2014-05-01

182

Digital mapping of accommodating structures and deformation associated with the emplacement of high level magmatic intrusions, Henry Mountains, Utah  

NASA Astrophysics Data System (ADS)

High-level sill and laccolith complexes form an important part of volcanic plumbing systems in which magma is emplaced as a series of sub-horizontal tabular sheet-like intrusions. Few studies of these intrusion types have looked in detail at the host rock, emplacement-related deformation structures, and how the additional volume of rock is accommodated within the crust, i.e. the 'space problem'. The aim of this study is to develop an understanding of the stages of emplacement (style of emplacement versus style of host rock deformation) and the internal textural evolution of Tertiary sills and laccoliths in the Henry Mountains, whilst also attempting to resolve the 'space problem'. Conventional field mapping, outcrop studies and detailed data collection of deformation structures has been combined with digital mapping using FieldMove™ and terrestrial laser scanning (TLS) in order to enable 3D modelling of the intrusive bodies and emplacement-related host rock deformation. Kinematic and geometrical studies of emplacement-related structures in the host rocks are supplemented by micro-scale textural and geochemical studies of deformed host rocks, plagioclase feldspar and amphibole phenocryst populations within the intrusions, and the intrusion-host rock contact zone. Fabric studies recognise micro-structural fabrics (associated with accommodating structures) from magmatic fabrics (associated with magma flow). Crystal size distribution (CSD) studies help constrain the crystal:molten rock ratio and mechanical properties of the intruding magma, in addition to helping identify individual magma pulses. Fieldwork to date has focused on two satellite intrusions to Mt. Hilliers: Trachyte Mesa (the most distal intrusion; simple geometries); and Maiden Creek (closer to Mt. Hilliers; more complex geometries) both of which are emplaced into the Jurassic Entrada Formation sandstone. Preliminary results highlight the importance of faults, fractures, deformation bands and newly identified shear zones in accommodating the extra volume of magma at depth.

Wilson, P. I.; McCaffrey, K. J.; Holdsworth, R. E.; Davidson, J. P.; Murphy, P. J.; Jarvis, I.

2012-12-01

183

Volcanic time-markers for Marine Isotopic Stages 6 and 5 in Southern Ocean sediments and Antarctic ice cores: implications for tephra correlations between palaeoclimatic records  

NASA Astrophysics Data System (ADS)

Three megascopic and disseminated tephra layers (which we refer to as layers A, B, and C) occur in late Quaternary glaciomarine sediments deposited on the West Antarctic continental margin. The stratigraphical positions of the distal tephra layers in 28 of the 32 studied sediment cores suggest their deposition during latest Marine Isotopic Stage (MIS) 6 and MIS 5. One prominent tephra layer (layer B), which was deposited subsequent to the penultimate deglaciation (Termination II), is present in almost all of the cores. Geochemical analyses carried out on the glass shards of the layers reveal a uniform trachytic composition and indicate Marie Byrd Land (MBL), West Antarctica, as the common volcanic source. The geochemical composition of the marine tephra is compared to that of ash layers of similar age described from Mount Moulton and Mount Takahe in MBL and from ice cores drilled at Dome Fuji, Vostok and EPICA Dome C in East Antarctica. The three tephra layers in the marine sediments are chemically indistinguishable. Also five englacial ash layers from Mt. Moulton, which originated from highly explosive Plinian eruptions of the Mt. Berlin volcano in MBL between 142 and 92 ka ago, are chemically very similar, as are two tephra layers erupted from Mt. Takahe at ca 102 ka and ca 93 ka. Statistical analysis of the chemical composition of the glass shards indicates that the youngest tephra (layer A) in the marine cores matches the ash layer that erupted from Mt. Berlin at 92 ka, which was previously correlated with tephra layers in the EPICA Dome C and the Dome Fuji ice cores. A tephra erupted from Mt. Berlin at 136 ka seems to correspond to a tephra layer deposited at 1733 m in the EPICA Dome C ice core. Additionally, the oldest tephra (layer C) in the marine sediments resembles an ash layer deposited at Vostok around 142 ka, but statistical evidence for the validity of this correlation is inconclusive. Although our results underscore the potential of tephrostratigraphy for correlating terrestrial and marine palaeoclimate archives, our study also reveals limitations of this technique, which may result in the miscorrelation of tephra. Such pitfalls comprise failure to recognise the occurrence of various tephra layers in marine sediment cores, 'swamping' of records with chemically indistinguishable tephra from a single volcanic source, and exclusive use of 'geochemical fingerprinting' for correlating ash layers.

Hillenbrand, C.-D.; Moreton, S. G.; Caburlotto, A.; Pudsey, C. J.; Lucchi, R. G.; Smellie, J. L.; Benetti, S.; Grobe, H.; Hunt, J. B.; Larter, R. D.

2008-03-01

184

Geochemical and temporal patterns of felsic volcanism in Ethiopia  

SciTech Connect

At least three major geochemical groups characterize late Cenozoic felsic volcanism exposed in the central Ethiopian Rift graben or along its uplifted margins. Each group is distinguished by age and/or position with distinctive compositional traits that are probably tectonically controlled. They include: (1) large-volume Pliocene tholeiitic to calcalkaline rhyolitic ignimbrites that form >500m thick exposures along the rift margins. These have moderate FeO/sub T/ (1.5-3.0%), low CaO (<0.5%) and moderately steep REE slopes (Ce/Yb = 21-24) with large negative Eu anomalies (Eu/Eu* = 0.2-0.4). K/Ar ages range from 4.5 to 3.0 Ma. These ignimbrites must have been associated with major caldera-forming events, but no calderas of this age or chemistry have yet been found; (2) Plio-Pleistocene trachytic volcanoes occur on the Ethiopian Highland, parallel to the rift axis. These have low SiO/sub 2/ (60-70%), high FeO/sub T/ (4-8%) and steep REE patterns (Ce/Yb = 25-37) with very small negative Eu anomalies (0.8-0.9). The volcanoes of Chilalo, Kaka, Hunkulu and the Bada Range are of this group. K/Ar ages range from 3.5 to 1.0 Ma; (3) Pleistocene peralkaline rhyolitic volcanoes of the Ethiopian Rift graben have high FeO/sub T/ (5-8%), high Na/sub 2/O (6-8%) and shallow REE profiles (Ce/Yb = 14-21) with small Eu anomalies (0.5 to 0.6). K/Ar ages range from 0.5 to <0.05 Ma. The volcanoes of Aluto and Dofen are of this group. Eruption of Group 1 signaled the modern rapid development of the present rift; Group 2 overlapped in time and mainly was confined to the Plateau margins; and Group 3 represents the present mature stage of rift floor volcanism.

Walter, R.C.; Westgate, J.A.; Giday, W.G.; Aronson, J.L.; Hart, W.K.

1985-01-01

185

Genesis of the pliocene to recent bimodal mafic-felsic volcanism in the Debre Zeyt area, central Ethiopia: volcanological and geochemical constraints  

NASA Astrophysics Data System (ADS)

The Debre Zeyt volcanic district is located about 40 km southeast of Addis Ababa, on the western shoulder of the Ethiopian Rift. Two main phases of volcanic activity are shown to have occurred in the area, both younger than the main episodes of rifting. Eruption of acid lavas and pyroclastics, between 4 and 1 Ma, brought to the formation of the central volcanoes of Yerer, Bede Gebabe and Zikwala. Younger activity generated basaltic cinder cones, maars and lava flows, all aligned along the main rift direction. A few intermediate and acid volcanics are associated with the younger basalts. Petrological and geochemical investigations have shown that the rocks from central volcanoes consist of peralkaline rhyolites and trachytes with a few trachyandesitic lava flows. All these rocks have high concentrations of Rb, Zr, LREE and other incompatible elements, and variable Ba and Sr contents. Measured Nd isotopic ratios are close to the bulk earth value, whereas Sr isotopic ratios are very variable, due to the combined effects of the high to extreme Rb/Sr values, variable ages and, possibly, different initial isotopic signatures. Basalts range from transitional to weakly alkaline in composition and display relatively homogeneous incompatible element contents and Nd and Sr isotopic ratios. Incompatible element ratios such as Ba/Rb are very variable in the basalts. The younger acid rocks have a large range of incompatible elements concentration with some rhyolites displaying very low values of some hygromagmaphyle trace elements (HYGE), such as Zr, Nb and LREE. The younger intermediate rocks have comparable HYGE contents as the basalts and define linear trends between basalts and low-HYGE rhyolites on several interelement variation diagrams. Geochemical modelling indicates that the major and trace element composition of the acid rocks from central volcanoes can be satisfactorily explained by a derivation from basaltic parents by fractional crystallization. Nd-isotopic ratios which, for the largest part, fall within the range of younger basalts suggest that, except for the Yerer rhyolites, the assimilation of the upper continental crust did not play a major role during magma evolution. The relatively constant HYGE contents of basaltic and intermediate younger volcanics exclude an evolution by fractional crystallization for this suite. The linear trends on inter-element diagrams suggest that mixing processes between basaltic magmas and an acid rock or liquid may be responsible for the generation of the intermediate rocks. This hypothesis is supported by Ba/Rb vs. Rb relationships revealing hyperbolic mixing trends between acid and basaltic end-members. This process also generated important geochemical variations within the basalts.

Gasparon, M.; Innocenti, F.; Manetti, P.; Peccerillo, A.; Tsegaye, A.

1993-08-01

186

Subduction-related High- to Ultrahigh-Potassic Rocks of the Ankara-Erzincan Suture Belt of Turkey: a geochemical and isotopic approach to source and petrogenesis  

NASA Astrophysics Data System (ADS)

A Late Cretaceous Volcano-sedimantary Succession (LCVS) trends parallel to Neo-Tethyan Suture in North Central Anatolia. Volcanic members of the LCVS consist mainy of coeval leucite phonolite/tephrites, trachytes, lamprophyres and andesitic rocks. Obtained Ar-Ar ages reveal that the volcanic activity occurred between 73.6±0.18 and 76.78±0.19 Ma, contemporaneous with the subduction of the Neo-Tethyan ocean beneath the Pontides. The volcanic rocks of LCVS are classified as alkaline, High- to ultrahigh-K, and silica-saturated and silica-unsaturated, geochemically. Rare calc-alkaline andesitic lavas are also occur within the volcanic sucession. Except the calc-alkaline samples, magmatic members of LCVS have similar major and trace element concentrations similar to the plagioleucitites or ultrapotassic rocks of the active orogenic zones (i.e. the Roman Province ultrapotassic series, Peccerillo, 2005). The multi element patterns on N-MORB- and Chondrite-normalized spider diagrams are characterized by significant LILE and LREE enrichments relative to HFSE and HREE, and display apparent Nb and Ta depletions, implying the subduction-related magmas. 87Sr/86Sr(i) (0.704493-0.706090) and 143Nd/144Nd(i) (0.512523-0.512680) isotope ratios are close to the mantle array, and are also in between the Aeolian Islands CA-Potassic rocks (Peccerillo, 2005), BSE and the circum-Mediterranean anorogenic Cenozoic igneous province (CiMACI, Lustrino&Wilson, 2007). Variable Mg# (33-60) evidences that these rocks are the products of evolved melts. The lead isotope values display a trend between EMI and DM, suggesting that the crustal involvement is more effective process during the generation of some lamprophyres relative to the other ultrapotassic rocks of LCVS. The results of clinopyroxene thermobarometry calculations reveal significant differences in depth of crystallization for the rock suites. Some trace element abundances and inter elemental ratios together with their co-variations show that the magmatic rock suites of LCVS had different crystallization history. REE melting modelings suggest a common mantle source, which contains variable amount of spinel lerzolite and garnet lerzolitic proportions. Considering the presence of some phlogopite-bearing clinopyroxenite xenolits in the lamprophyre and the leucite-bearing rocks, we propose that the LCVS ultrapotassic rocks were possibly derived from the low degree partial melting of veined SCLM. Keywords: High- to Ultrahigh-K, Isotope, Ar-Ar, Cretaceous, Turkey, Veined SCLM Lustrino, M., Wilson, M., 2007. ESR 81, 1-65. Peccerillo, A., 1995. Springer-Verlag. 365pp.

Genc, S. Can; Gulmez, Fatma; Karacik, Zekiye; Tuysuz, Okan; Prelevic, Dejan; Roden, Michael F.; Hames, Willis E.; Zeki Billor, M.

2014-05-01

187

Multiple Magmatic Events Over 40 Ma in the Fish Creek Mountains, North-central Great Basin, Nevada, USA  

NASA Astrophysics Data System (ADS)

The Fish Creek Mountains, located in north-central Nevada south of Battle Mountain, is a site of multiple igneous events ranging from ca. 35 Ma to 1 Ma, covering most of the igneous history of the Great Basin of the western United States. Such extended volcanic activity allows for documentation of mantle sources and petrogenetic processes over time. Beginning approximately 50 Ma, the Great Basin experienced a magmatic front that began migrating southwestward across southern Idaho, central Oregon and into northern Nevada and Utah. Intermediate, "arc-like" andesite and dacite dominated volcanic activity in northeastern Nevada between about 45 and 36 Ma. By 34 Ma, a northwest-trending belt of rhyolitic ash-flow calderas began to develop through central Nevada, the "ignimbrite flare-up". Volcanism then migrated westwards towards the Sierra Nevada. In north-central Nevada, the oldest lavas are ca. 35 Ma basaltic andesites through rhyolites that are exposed in the western Shoshone Range, the eastern Tobin Range, and the northern and eastern Fish Creek Mountains. Plagioclase-rich andesites, dacite intrusions, and volcanic breccias occur in a belt along the western side of the Fish Creek Mountains. The bulk of the Fish Creek Mountains is composed of the 24.7 Ma Fish Creek Mountains rhyolitic tuff that is largely confined to an undeformed caldera structure. The caldera and tuff are anomalously young compared to nearby felsic centers such as the Caetano caldera (33.8Ma) and Shoshone Range (39-35 Ma) and relative to the southwest to west magmatic migration. The basal tuff is unwelded, with abundant pumice and lithic (primarily volcanic) fragments but only rare crystals. Sanidine and smoky quartz phenocrysts become more abundant upsection and glassy fiamme (hydrated to devitrified) are common, but the abundance of lithic fragments diminishes. 16-15 Ma volcanic rocks of the Northern Nevada Rift are exposed in the Battle Mountain area, ranging in composition from subalkaine basalt to rhyolite and rare trachyte. These rocks are linked to the Columbia River flood basalt event. Along the northwestern margin of the Fish Creek Mountains and in the center of the caldera complex are exposed late Pliocene to Quaternary lava flows and cinder cones of the Buffalo Valley volcanic field. The Buffalo Valley volcanic rocks are alkalic basalts that are locally vesicular, with rare plagioclase and olivine phenocrysts as well as plagioclase megacrysts up to several centimeters in size. Trace element and isotopic characteristics are similar to those of the Pliocene-Pleistocene Lunar Craters volcanic field in central Nevada. Ongoing geochemical analyses will outline variations in mantle sources and post-melting processes in the multiple volcanic systems of north-central Nevada.

Cousens, B.; Henry, C. D.; Stevens, C.; Varve, S.

2011-12-01

188

Groundwater and urbanisation, risks and mitigation: The case for the city of Windhoek, Namibia  

NASA Astrophysics Data System (ADS)

The City of Windhoek is underlain by the Kuiseb Schist, locally known as the “Windhoek Schist” and amphibolites. In the low-lying parts of the Windhoek valley, gravels and sands are present. The Windhoek schist has several lithologies, dominated by garnet-muscovite-chlorite-biotite schist, with distinctive cleavage. This pervasive cleavage renders the underlying lithology permeable to fluids percolating from the surface into the aquifer. Other minor lithologies are trachytes, metarhyolites and quartzites found to the east of the city. The amphibolite is part of the Matchless belt, and traverses the city in a NE-SW fashion. When weathered, it forms a perfect aquiclude. North-south and northeast-southwest trending faults with a few splays cut across the Kuiseb Schist. The faults play a significant role in increasing the fracture density of the fissile schist. The faults are the major links that form channels between the surface and the aquifer below. The city of Windhoek uses the aquifer both as a source of fresh water and as a storage facility. The recharge areas of the aquifer lie to the east and south- to southeast of the city in the vicinity of the suburb of Kleine Kuppe. The soil horizon over the Windhoek schist is very shallow and most buildings are built directly on bedrock. The thin soil horizon makes the aquifer prone to pollution, caused either by accidents such as spills or by carelessness due to unsupervised dumping. The fissility and fracture density of the schist imply that leakage of surface waters, phenols, septic-tank spills, sewer-bursts, chemical and industrial contaminants and other such materials can reach the aquifer in unusually high rainfall years. The effects of fuels and oils are much more adverse, as they may remain in soils for long periods. The rapid urbanization and building of informal settlements without sewage reticulation has increased the risk of pollution to the Windhoek aquifer. The close monitoring of sewage pipes, filling stations and dump sites (including cemeteries), preferably using a GIS-based model, is the best way to prevent future pollution.

Mapani, B. S.

189

Geologic map of the Tetilla Peak Quadrangle, Santa Fe and Sandoval counties, New Mexico  

USGS Publications Warehouse

This digital geologic map summarizes all available geologic information for the Tetilla Peak quadrangle located immediately southwest of Santa Fe, New Mexico. The geologic map consists of new polygon (geologic map units) and line (contact, fault, fold axis, dike, flow contact, hachure) data, as well as point data (locations for structural measurements, geochemical and geochronologic data, geophysical soundings, and water wells). The map database has been generated at 1:24,000 scale, and provides significant new geologic information for an area of the southern Cerros del Rio volcanic field, which sits astride the boundary of the Espanola and Santo Domingo basins of the Rio Grande rift. The quadrangle includes the west part of the village of La Cienega along its eastern border and includes the southeasternmost part of the Cochiti Pueblo reservation along its northwest side. The central part of the quadrangle consists of Santa Fe National Forest and Bureau of Land Management lands, and parts of several Spanish-era land grants. Interstate 25 cuts through the southern half of the quadrangle between Santa Fe and Santo Domingo Pueblo. Canada de Santa Fe, a major river tributary to the Rio Grande, cuts through the quadrangle, but there is no dirt or paved road along the canyon bottom. A small abandoned uranium mine (the La Bajada mine) is found in the bottom of the Canada de Santa Fe about 3 km east of the La Bajada fault zone; it has been partially reclaimed. The surface geology of the Tetilla Peak quadrangle consists predominantly of a thin (1-2 m generally, locally as thick as 10? m) layer of windblown surficial deposits that has been reworked colluvially. Locally, landslide, fluvial, and pediment deposits are also important. These colluvial deposits mantle the principal bedrocks units, which are (from most to least common): (1) basalts, basanites, andesite, and trachyte of the Pliocene (2.7-2.2 Ma) Cerros del Rio volcanic field; (2) unconsolidated deposits of the Santa Fe Group, mainly along the western border, in the hanging wall of the La Bajada fault zone, but locally extending 2-3 km east under the Cerros del Rio volcanic field; (3) older Tertiary volcanic and sedimentary rocks (Abiquiu?, Espinaso, and Galisteo Formations); (4) intrusive rocks of the Cerrillos intrusive center that are roughly coeval with the Espinaso volcanic rocks; and (5) Mesozoic sedimentary rocks ranging in age from the Upper Triassic Chinle Formation to the Upper Cretaceous Mancos Shale.

Sawyer, D. A.; Shroba, R. R.; Minor, S. A.; Thompson, R. A.

2002-01-01

190

Petrological and geochemical data of volcanic rocks from the southern Afar Depression, Ethiopia.  

NASA Astrophysics Data System (ADS)

The geological and petrological investigations (FWF Project P15196) in the southern Afar Depression of Ethiopia support an international palaeoanthropological research-team (PAR) under the leadership of Horst Seidler. Mount Galila is the conspicuous centre of the research area [N 9° 44.101', E 40° 27.368'], situated about 20 km E of the NNE-SSW striking, recently active Hertale Graben, which represents a northernmost segment of the Main Ethiopian Rift (MER). Stratigraphically, the fossiliferous lacustrine and fluvial deposits, as well as the intercalated volcanic layers of the Galila area, belong to the "Upper Stratoid Series" (5-1.4 Ma) and will be named the Mount Galila Formation. They are similar to the Awash Group, from which very famous early hominid fossils have been described. In the Mount Galila Fm., 7 main volcanic horizons serve as marker beds comprising basalts, ignimbrites, tuffs and tuffaceous sands. The basalt horizons in the research area represent basaltic lava flows each consisting of one single flow unit c. 5 meters thick with maximum 5 cooling units. A first set of geochemical data from XRF spectrometry comprising main and trace element analysis shows characteristics for the volcanic marker beds as following: The basalts are clearly tholeiitic in the main elements (FeO/MgO/Alk) and show typical trace element distributions (e.g. Zr/Y-Zr; Ti/100-Yx3-Zr) as Within Plate Tholeiit Basalts. All basalt samples contain access 40Ar which can be explained by specific erruption mechanisms that leads to analytical problems for 40Ar/39Ar dating. In the TAS diagram after LeMaitre 1984 the ignimbrites vary at high alkali levels (7-9%) from trachytic to dacitic and rhyolitic composition, whereas at low alkali contents (<7%) they plot into the andesitic field. Compared to the basalts, the geochemistry of the ignimbrites is much more inhomogenous. Tuffs and tuffaceous sands are relevant as marker beds especially for the palaeoanthropological excavations in the Galila area because some great fossil findings, e.g. a tooth of an Australopithecus cf. afarensis and some primate remains, are stratigraphically related to them. Petrologically their compositions vary from basaltic andesitic to andesitic and dacitic dependent on their clastic input.

Urbanek, Ch.; Faupl, P.; Richter, W.; Seidler, H.

2003-04-01

191

Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption  

NASA Astrophysics Data System (ADS)

A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga), appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of "restingolites" indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro.

Troll, V. R.; Klügel, A.; Longpré, M.-A.; Burchardt, S.; Deegan, F. M.; Carracedo, J. C.; Wiesmaier, S.; Kueppers, U.; Dahren, B.; Blythe, L. S.; Hansteen, T. H.; Freda, C.; Budd, D. A.; Jolis, E. M.; Jonsson, E.; Meade, F. C.; Harris, C.; Berg, S. E.; Mancini, L.; Polacci, M.; Pedroza, K.

2012-03-01

192

Petrogenesis and 40Ar/ 39Ar geochronology of the volcanic rocks of the U?ak-Güre basin, western Türkiye  

NASA Astrophysics Data System (ADS)

In spite of much research over the past 30 years, the dynamic evolution, origin of the volcanism and geometrical-stratigraphical relations of the NE-SW-trending basins in western Anatolia are poorly understood. The U?ak-Güre basin is one of the prominent NE-SW-trending basins developed on the northern part of the Menderes Massif core complex. Three distinct volcanic successions are found in the U?ak-Güre basin: (1) the Beyda?? volcanic unit composed of shoshonite, latites and rhyolitic lavas followed by dacitic and andesitic pyroclastic deposits; (2) the Payamtepe volcanic unit composed of potassic intermediate composition lavas (latites and trachytes); and (3) the Karaa?aç dikes composed of andesite and latite. The Beyda?? volcanic unit occurs in three different NE-SW-trending volcanic centers—Beyda??, ?tecektepe and Elmada? calderas from southwest to northeast, respectively. The oldest radiometric ages for the Beyda?? volcanic unit are from the Elmada? volcanic center in the north and range from 17 to 16 Ma. The data indicate that volcanism was active during the latest early Miocene. The youngest radiometric age for the Beyda?? volcanic unit is obtained from the Beyda?? caldera located (12 Ma) in the south. The data indicate that Beyda?? volcanism was active in the late middle Miocene and migrated from north to south with time. 40Ar/ 39Ar ages of the Payamtepe volcanic unit are restricted to a short period between 16.0 and 15.9 Ma. Volcanic rocks of the U?ak-Güre basin are characterized by strong enrichment in LILE and LREE and depletions of Nb-Ta and Ti on MORB-normalized multi-element diagrams. Geochemical features of the volcanic rocks suggest that they experienced mixing processes between mafic and felsic end-members and also fractional crystallization of dominantly plagioclase and pyroxenes from mixed magma compositions. Crustal contributions to the magma sources may also have occurred during magmatic evolution. These processes have resulted in scattered major and trace element variations with respect to increasing silica contents. Geochemical features of the most mafic samples agree with the results of previous studies from other volcanic areas in western Anatolia, suggesting that the volcanic rocks in the region were derived from a mainly lithospheric mantle source that had been heterogeneously metasomatized by previous subduction events during convergence between the African and Eurasia plates. The volcanic activity in the region, which developed synchronously with the formation of the Menderes Massif core complex, is best explained by delamination of lithospheric mantle slices that were heterogeneously enriched by previous subduction-related processes.

Karao?lu, Özgür; Helvac?, Cahit; Ersoy, Yalç?n

2010-10-01

193

The Mantle and Basalt-Crust Interaction Below the Mount Taylor Volcanic Field, New Mexico  

NASA Technical Reports Server (NTRS)

The Mount Taylor Volcanic Field (MTVF) lies on the Jemez Lineament on the southeastern margin of the Colorado Plateau. The field is centered on the Mt. Taylor composite volcano and includes Mesa Chivato to the NE and Grants Ridge to the WSW. MTVF magmatism spans approximately 3.8-1.5 Ma (K-Ar). Magmas are dominantly alkaline with mafic compositions ranging from basanite to hy-basalt and felsic compositions ranging from ne-trachyte to rhyolite. We are investigating the state of the mantle and the spatial and temporal variation in basalt-crustal interaction below the MTVF by examining mantle xenoliths and basalts in the context of new mapping and future Ar-Ar dating. The earliest dated magmatism in the field is a basanite flow south of Mt. Taylor. Mantle xenolith-bearing alkali basalts and basanites occur on Mesa Chivato and in the region of Mt. Taylor, though most basalts are peripheral to the main cone. Xenolith-bearing magmatism persists at least into the early stages of conebuilding. Preliminary examination of the mantle xenolith suite suggests it is dominantly lherzolitic but contains likely examples of both melt-depleted (harzburgitic) and melt-enriched (clinopyroxenitic) mantle. There are aphyric and crystal-poor hawaiites, some of which are hy-normative, on and near Mt. Taylor, but many of the more evolved MTVF basalts show evidence of complex histories. Mt. Taylor basalts higher in the cone-building sequence contain >40% zoned plagioclase pheno- and megacrysts. Other basalts peripheral to Mt. Taylor and at Grants Ridge contain clinopyroxene and plagioclase megacrysts and cumulate-textured xenoliths, suggesting they interacted with lower crustal cumulates. Among the questions we are addressing: What was the chemical and thermal state of the mantle recorded by the basaltic suites and xenoliths and how did it change with time? Are multiple parental basalts (Si-saturated vs. undersaturated) represented and, if so, what changes in the mantle or in the tectonic regime allowed their coexistence or caused the transition?

Schrader, Christian M.; Crumpler, Larry S.; Schmidt, Marick E.

2010-01-01

194

Volcanology and Petrology of Clasts and Tephra in the AND-2A core, ANDRILL Southern McMurdo Sound Project, Antarctica  

NASA Astrophysics Data System (ADS)

Volcanic material is persistent throughout the AND-2A core and include, in order of relative abundance, volcanic sediment, pyroclasts and lava. Volcanic clasts range from <1-13 cm and consist of lava, breccia, and rounded scoria and pumice granules and pebbles. Clasts of lava vary in texture from glassy to fine- grained pilotaxitic to coarsely porphyritic, and in composition from mafic (clinopyroxene (Mg-rich), olivine, plagioclase) to intermediate (plagioclase, clinopyroxene (Fe-rich), amphibole) to felsic (K-feldspar, clinopyroxene (aegirine), amphibole). The relationships between silica and alkali contents indicate an overall alkaline affinity with significant compositional differences. Volcanic clasts from the top of the core belong to a strongly alkaline lineage (basanite to phonolite) while samples deeper in the core belong to a moderately alkaline linage (alkali basalt to trachyte). The moderately alkaline lineage does not exist in the McMurdo Ice Shelf core (AND-1B), which was drilled ~50 km to the east. The distinction may signify an important spatial and temporal change in volcanism within the region. In the AND-2A core, scoria and pumice are considered to be primary or minimally reworked; windblown and/or transported atop or within ice. Several primary to gently reworked tephra layers are found within sandstone and siltstone in the early to mid-Miocene interval (1093 to 640 mbsf). A primary six cm thick layer of lapilli tuff occurs at 640 mbsf and several clast-supported accumulations of pumice, up to 3.5 cm thick, occur in ripple cross-laminated sands at 709 mbsf. Pumice consists of highly vesiculated colourless glass showing tubular vesicles with delicate bubble walls, which in most cases, are filled by authigenic minerals or completely replaced. The pumice contain rare phenocrysts of anorthoclase that are also found as separate dispersed crystals in the tephra horizons. The pumice glass is highly altered, having very low alkali, magnesium and iron contents. Shards of brown glass are relatively fresh and range from basanite to mugearite. Apart from the in-situ volcanic deposits recovered at the top of the core, which are considered to be from a previously unknown proximal vent, the specific point sources for the volcanics are not known. The potential sources are numerous and include many large volcanoes that surround the SMS drill site and range in age from ~19 Ma to currently active.

Del Carlo, P.; Panter, K. S.; Rocchi, S.; Bracciali, L.; Field, B.; Bassett, K.

2008-12-01

195

The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)  

NASA Astrophysics Data System (ADS)

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings.

Busquets, P.; Méndez-Bedia, I.; Gallastegui, G.; Colombo, F.; Cardó, R.; Limarino, O.; Heredia, N.; Césari, S. N.

2013-07-01

196

Explosive volcanism from the Galapgos Hotspot: Evidence from Miocene marine tephras on top of the Cocos Ridge (IODP Exp. 334)  

NASA Astrophysics Data System (ADS)

The Central American Volcanic Arc (CAVA) has generated numerous Plinian eruptions along its 1200 km extension. The best-preserved archive of this volcanism is found as tephra layers in marine sediments downwind from the eruption centers on the floor of the Pacific. Up to now no such deposits are known to exist from the Galapagos Hot Spot. During IODP expedition 334 "CRISP" offshore Costa Rica a total of 31 tephra layers have been recovered at Site U1381 on top of the Cocos Ridge near the Costa Rican Trench. In the first 45 mbsf five of these tephra layers are intercalated with hemiplegic Pleistocene sediments associated with a near trench depositional environment. In contrast, the 26 tephra layers recovered from the deeper parts of Site U1381 (>45 mbsf) are embedded in a succession of Miocene silicic and calcareous ooze dominated sediments. Plate reconstruction shows that these sediments have been deposited in proximity to the Galapagos Islands. Individual tephra layers range in thickness from 1 to 33 cm. Macroscopically the tephra layers are either pinkish/white to gray/brown (20 layers) or pinkish/greenish black (11 layers). Dark tephra beds account for ~36% of the total tephra-bed assemblage at Site U1381. The felsic tephra layers consist of fresh, transparent glass shards, rare plagioclase and traces of pyroxene. Grain size ranges from medium to coarse ash (up to mm size). Mafic ash layers consist predominantly of very coarse, dark to light brown sideromelane glass shards, rare tachylitic particles, and minor plagioclase and trace pyroxene. Most of the sideromelane glass shards have blocky shapes some show a tubular appearance and are medium to poorly vesicular. Glass shards are well preserved without obvious alteration features. Generally, tephras from the older Miocene sequence have a lower crystal content than the tephras of the younger one. First results of electron microprobe analysis show that the tephras have a highly variable composition, ranging from relatively primitive basalts to trachytes and rhyolites. A first provenance analysis conducted based on major elements suggests that some of the tephra layers within the Pleistocene unit of Site U1381 might be derived from the two largest Central American eruptions in the last 200ka, the 191 ka old L-Fall and the 84ka old Los Chocoyos eruption. Within the Miocene unit we can identify both, an evolved tephra with low Ti/K ratios (<2) comparable to Central American Arc tephras as well as a series of evolved and primitive tephras with high Ti/K ratios (>2.5) probably associated with the Galapagos hot spot volcanism. In future, trace element analyses will better constrain the possible source areas and the preliminary correlations shown above.

Strehlow, K.; Kutterolf, S.; Stroncik, N.; Vannucchi, P.; Ujiie, K.; IODP expedition 334 science party

2011-12-01

197

Petrology and source of lavas from seamounts in the Adare Basin, Western Ross Sea: Implications for the origin of Cenozoic magmatism in Antarctica  

NASA Astrophysics Data System (ADS)

Hundreds of volcanic seamounts are randomly distributed in the Adare Basin, northern Ross Sea, and on the adjacent continental shelf of north Victoria Land. The cluster of volcanic seamounts directly east of Cape Adare, which we designate as southern Adare Basin Seamounts (ABS), were intruded through a thick (~2000 m) pile of sediments that have been accumulating in the Adare Basin since its opening ~43 Ma and have youthful morphology suggesting that they were formed contemporaneously with the other Cenozoic volcanoes in West Antarctica. Lavas dredged from the southern ABS on the continental shelf (~600 to 400 mbsl) and within the ocean basin (~2000 to 1400 mbsl) range from basanite and phonotephrite to trachyte and rhyolite (MgO >10 to <1 wt.%; Ni>300 to <10 ppm), are alkaline (K2O >1 to <5 wt.%; Ne-norm 2-18 wt.%, Ba >600 to <100 ppm), are light rare earth element enriched (La/YbN 15-27), and seven samples have low 87Sr/86Sr (<0.7029) and high 143Nd/144Nd (>0.51295) ratios. Lavas are fine-grained to glassy, porphyritic and vesicular with phenocrysts of olivine, pyroxene, feldspars, magnetite and rare amphibole. The incompatible trace element ratios of the most differentiated ABS lava are similar to those of more mafic ABS lavas, but have higher 87Sr/86Sr (0.7077). This is reminiscent of the compositional signature of the differentiated alkaline lavas produced by melting of previously erupted mafic lavas in a few oceanic islands. Overall, results show that volcanism within the Adare Basin is coeval and petrogenetically akin to continental volcanism in West Antarctica and thus expand the known extent of Cenozoic alkaline magmatism associated with the West Antarctic rift. The similarities between ABS and West Antarctic volcanism, as well as with other continental intraplate lavas from the southwest Pacific offer compelling support for an inherent connection between their mantle sources. Furthermore, the coupled extensional history of the oceanic and continental sectors in the northern Ross Sea, along with the broadly coincident age of the volcanic activity, strongly suggests that both continental and oceanic volcanism were triggered by the same mechanism.

Panter, K. S.; Castillo, P.

2008-12-01

198

Geochronology, geochemistry and isotope tracing of the Oligocene magmatism of the Buchim-Damjan-Borov Dol ore district: Implications for timing, duration and source of the magmatism  

NASA Astrophysics Data System (ADS)

Timing, source and magmatic evolution of the intrusions in the Buchim-Damjan-Borov Dol ore district of the Former Yugoslav Republic of Macedonia (F.Y.R.O.M.) have been studied. They intrude the Circum Rhodope Unit close to the contact with the Vardar Zone and are a part of the Late Eocene-Oligocene Macedonian Rhodope-North Aegean belt. The magmatism at Buchim-Damjan-Borov Dol occurred between 24.04 ± 0.77 and 24.51 ± 0.89 Ma, as indicated by chemical-annealing (CA)-LA ICP-MS zircon dating. Major element, trace and rare earth element analyses have been performed on the various intrusive rocks. All ore bearing magmas were classified as trachyandesitic, except the youngest intrusion which is not associated with mineralization; the Black Hill locality (24.04 ± 0.77 Ma) shows a trachytic composition. The distribution of the trace elements, enrichment of large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicates subduction-related magmatism; most of the magmas follow a calc-alkaline fractionation trend with shoshonitic affinities; additionally, Sr/Y (10 to 90) and La/Yb values show some similarities to adakite-like magmas. Sr and Nd isotope ratios (Sri = 0.70658 to 0.70740 and Ndi = 0.512425-0.512497) show that the magmatic products were slightly contaminated by continental crust material, e.g., the Variscan/Cadomian basement. In the Late Eocene-Oligocene belt the magmatism between 29 and 35 Ma is dominated by crustal melting with an increase in the mantle contribution between 20 and 27 Ma. We suggest the following scenario for the magmatic history of the Buchim-Damjan-Borov Dol ore district: a slab rollback of an oceanic slab located further to the SW which led to extensional and compressional features in upper levels of the continental crust. In the middle to upper crust three consecutive crystallization stages occurred at variable depths as indicated by amphibole zonation. Mixing of newly formed crust with mantle-like affinities and continental crust material in variable degrees during the ascent of the magma can explain all geochemical characteristics. The magma crystallized as dykes or stocks near the Earth's surface (> 1 km) after its final emplacement and contemporaneous hydrothermal activity led to different mineralization styles depending on the lithology of the host rocks.

Lehmann, St.; Barcikowski, J.; von Quadt, A.; Gallhofer, D.; Peytcheva, I.; Heinrich, C. A.; Serafimovski, T.

2013-11-01

199

Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore, Massif Central, France)  

NASA Astrophysics Data System (ADS)

Three magmatic units (Grande Cascade pyroclastic deposits, Grande Cascade lava flow, Durbise nuée ardente deposits) from the Quaternary volcano Sancy (Mont-Dore area, Auvergne, France) show textural evidences of magma mixing between a silica undersaturated basic magma (alkali basalt and hawaiite) and an acid magma (quartz-bearing trachyte). Three kinds of mixed rock types are described: basic inclusions within an acid host, « emulsified rocks » showing infracentimetric basic globules disseminated within an acid groundmass, and « banded rocks » in which elongated acid and basic zones alternate. The chemical compositions of mixed rocks plot systematically onto linear trends in petrographic diagrams. Microprobe analyses of the groundmass show similar linear variations between basic and acid end-members. The mineralogical associations of these mixed rocks are highly complex and present many disequilibrium features. Olivine is stable in the basic component and becomes rimmed by orthopyroxene in the acid one. Zoning patterns of feldspars are complex. Clinopyroxene, kaersutite and phlogopite phenocrysts have increasing component Mg contents from core to rim both in the basic and the acid. Titanomagnetite and hemoilmenite phenocrysts were equilibrated at 900-800° C under high oxygen fugacities. Mixing results primarily from a mechanical disintegration of partly liquid basic inclusions within their acid host, and also from a mechanical transfer of phenocrysts from one component to the other, in which they often remain surrounded by a coating of their original groundmass. Chemical data on the groundmass indicates that some « true » hybridization between coexisting acid and basic liquids may also have occurred. The extent of mixing is controlled by the type of emplacement. For pyroclastic deposits a chemical gap exists between basic inclusions and their acid hosts; in contrast, mechanical mixing was enhanced during the emplacement of the viscous Grande Cascade lava flow, and complete transitions occur between basic and acid components. The two end-members are genetically associated, the latter deriving from the former by crystal fractionation. Mixing appears as a late-stage phenomenon in the petrogenetic history of the Mont-Dore series; in the case of the Grande Cascade lava flow, its extent is primarily dependent on emplacement modalities.

Gourgaud, A.; Maury, R. C.

1984-12-01

200

A Devonian to Carboniferous intra-oceanic subduction system in Western Junggar, NW China  

NASA Astrophysics Data System (ADS)

The Devonian and Carboniferous rock assemblages in Western Junggar, southern Altaids, include ophiolitic mélanges and coherent units. At Baijiantan, Yeyagou and Dagun Devonian gabbros and one group of volcanic rocks (type I lava) generally crop out as blocks in a matrix of ultramafic rocks and/or mudstone. They have a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Another group of volcanic rocks (type II lava) from Baijiantan, Yeyagou, Western Karamay and Maliya includes alkaline basalt, basanite, andesite, dacite and trachyte that contain microphenocrysts of clinopyroxene-plagioclase, amygdales filled with chalcedony and calcite, and accessory pyrite and ilmenite. The type II lavas are further subdivided into five sub-types: (1) Type II 1 lava is strongly enriched in LREE with incompatible trace element ratios similar to oceanic island basalt (OIB), suggesting derivation from enriched mantle. (2) Type II 2 lava and tuff are slightly LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment indicating that they were generated in a supra-subduction zone (SSZ). (3) Type II 3 lava is also moderately enriched in LREE suggesting derivation from a more-enriched MORB mantle. (4) Type II 4 lava is enriched in LREE and has high Nb/Yb values close to those of OIB. It is extensively depleted in Nb suggesting a source containing a mixture of enriched and SSZ mantle. (5) Type II 5 lava has MORB-like REE patterns, and Nb/Yb and Th/Yb ratios. Based on field structural data, and the contemporaneity of the Baogutu adakite, a high-Mg diorite dike and the Miaoergou charnockite, these enriched and depleted magmatic features suggest generation in an intra-oceanic subduction zone influenced by ridge-trench interaction. The Altaids in Western Junggar are characterized by multiple intra-oceanic subduction systems, which may have continued to operate to the late Carboniferous.

Zhang, Ji'en; Xiao, Wenjiao; Han, Chunming; Mao, Qigui; Ao, Songjian; Guo, Qianqian; Ma, Chong

201

Baseline geochemical characteristics of groundwater in the mountainous area of Jeju Island, South Korea: Implications for degree of mineralization and nitrate contamination  

NASA Astrophysics Data System (ADS)

SummaryHydrogeochemical characteristics of groundwater were investigated to elucidate baseline quality for basaltic aquifers of a mountainous area dominated by natural land cover in the volcanic island of Jeju, South Korea. Principal component analysis (PCA) resulted in four principal components (PC) of which PC1, PC3 and PC4 represented natural mineralization by water-rock interaction, while PC2 corresponded to anthropogenic contamination from nitrate sources. Hierarchical cluster analysis (HCA) was performed to assess the effect of the geochemical and hydrologic processes on each sample. Six sample groups, distinguished by total dissolved solids (TDS), nitrate concentration, and water type, resulted from the HCA: high-altitude springs, low-mineral water, nitrate-contaminated water, intermediate-mineral water, and two groups of high-mineral water. Water types were transformed from Na(Mg, Ca)-HCO 3 for low-mineral water to Na-HCO 3 for high-mineral water. Nitrate-contaminated water occurred near the boundary of natural and agricultural land uses of the western area. High-mineral water had higher calcite saturation states than other groups. The high-mineral water also had tritium ( 3H) values lower than 1.5 TU, indicating a significant contribution of old groundwater. In contrast, low-mineral water had tritium values of about 3 TU, which is close to those of rainwater. Geographically, the high-mineral water was concentrated in the southwestern area, where intermediate-mineral water was also found. This distribution was likely related to the presence of extensive trachytic rocks, which could have formed locally isolated aquifers, enabling prolonged water-rock interactions; the higher location of low-permeability hydrovolcanic tuffs in the subsurface of the southern area would also increase chances of tapping aquifers below the hydrovolcanic tuffs. The cumulative probability of TDS showed two breakpoints of 50 and 150 mg/L that distinguished high-altitude springs, low-mineral water, and high-mineral water. The cumulative probability of nitrate provided possible threshold values for anthropogenic contamination of 2.5 and 5.5 mg/L. That 32% of samples had nitrate concentrations higher than 2.5 mg/L indicated high vulnerability to surface contamination sources in this mountainous area. HCA seems to be more effective for interpreting the baseline chemistry of groundwater than other threshold-calculating statistics in that HCA revealed geochemical processes including natural mineralization and anthropogenic contamination. Our study suggests that the cumulative probability and HCA are useful as complementary methods for determining threshold values.

Koh, Dong-Chan; Chae, Gi-Tak; Yoon, Yoon-Yeol; Kang, Bong-Rae; Koh, Gi-Won; Park, Ki-Hwa

2009-09-01

202

A model for Ischia hydrothermal system: Evidences from the chemistry of thermal groundwaters  

NASA Astrophysics Data System (ADS)

Ischia volcano, in Central Italy, has long been known for its copious surface hydrothermal manifestations, signs of a pervasive circulation of hot fluids in the subsurface. Because of the significant chemical heterogeneity of fumarolic gas discharges and hot spring discharges, evidences of a complex hydrothermal setting, a definite model of fluid circulation at depth is currently unavailable, in spite of the several previous efforts. Here, we report on the chemical and isotopic composition of 120 groundwater samples, collected during several sampling surveys from 2002 to 2007. The acquired data suggest that the composition of surface manifestations reflect contributions from meteoric water, sea water, and thermal fluids rising from two distinct hydrothermal reservoir, with equilibrium temperatures of respectively ~ 150 °C and ~ 270 °C, and depths of 150-300 m and > 300 m (but possibly > 1000 m). We also make use of an isotopic characterization of the dissolved gas phase in thermal waters to demonstrate that the Ischia hydrothermal system is currently supplied by a deep-rising gas component (DGC), characterized by CO 2 ~ 97.7 ± 1.2 vol.% (on a water-free basis), ?13C CO2 = - 3.51 ± 0.9‰, and helium isotopic ratio of about 3.5 Ra ( 3He/ 4He ratio normalized to the air ratio, Ra), likely magmatic in origin. An assessment of the thermal budget for Ischia hydrothermal system is also presented, in the attempt to derive a first estimate of the size and rate of degassing of the magmatic reservoir feeding the gas emissions. We calculate that a heat flow of about 153-222 MW presently drives hydrothermal circulation on the island, which we suggest is supplied in convective form (e.g., by the ascent of a high- T magmatic vapour phase) by complete degassing of 2.2-3.3 · 10 7 m 3 yr - 1 of trachytic magma (with ~ 2.1 wt.% dissolved H 2O content). If extrapolated to entire period of quiescence lasting since the Arso eruption in 1302 A.D., this volume corresponds to 1.6-2.3·10 10 m 3 of magma degassed in about 700 years of quiescent activity.

Di Napoli, R.; Aiuppa, A.; Bellomo, S.; Brusca, L.; D'Alessandro, W.; Candela, E. Gagliano; Longo, M.; Pecoraino, G.; Valenza, M.

2009-10-01

203

A LA-ICP-MS study of minerals in the Rocche Rosse magmatic enclaves: Evidence of a mafic input triggering the latest silicic eruption of Lipari Island (Aeolian Arc, Italy)  

NASA Astrophysics Data System (ADS)

The volcanic products of Lipari Island (Aeolian Arc, Italy) younger than 10 ka are mostly aphyric rhyolitic pumices and obsidians emitted during unusual strombolian-type eruptions, which ended with the emplacement of lava flows. The last volcanic activity on the island dates back to 1230 ± 40 AD, with the extrusion of Rocche Rosse (RR) obsidian lava flow. Recently, mafic enclaves of latitic to trachytic composition have been identified and an evolution process between these enclaves and the rhyolitic magma has been documented in detail [Davì, M., 2007. The Rocche Rosse rhyolitic lava flow (Lipari, Aeolian Islands): magmatological and volcanological aspects. Plinius, supplement to the European Journal of Mineralogy 33, 1-8]. In this work textural and trace-element investigation of mineral phases of the RR enclaves, such as crystals of clinopyroxene, olivine, plagioclase, alkali-feldspar and biotite, was carried out to delineate the most recent feeding system of the island, since such a reconstruction could be significant in terms of hazard forecasting. The results indicate that most of the mineral phases are reversely or oscillatory zoned with respect to both major and trace elements, suggesting an early crystallization under low fO 2 conditions from melts of intermediate composition, followed by a later growth from a more mafic (presumably shoshonitic-basaltic) magma than that from which their cores crystallized. Crystals of magnesium-rich pyroxene and forsteritic-rich olivine are indicative of the presence of this shoshonitic basaltic magma. Based on microanalytical data, it is suggested here that the feeding system of recent Lipari volcanic activity was characterized by a shoshonitic-basaltic magma originating from a deep reservoir, which may have evolved and stopped in the crust, generating zoned magma chambers at different depths, in which latitic and rhyolitic magmas reside. The sudden arrival of a new input of mafic melt may have interacted with these resident fractionated magmas and triggered the eruption. A plumbing system of this type has been shown to be active in the southern sector of the Aeolian Islands since the last 42 ka [Gioncada, A., Mazzuoli, R., Milton, A., 2005. Magma mixing at Lipari (Aeolian Islands, Italy): Insights from textural and compositional features of phenocrysts. J. Volcanol. Geotherm. Res. 145, 97-118; Peccerillo, A., Frezzotti, M.L., De Astis, G., Ventura, G., 2006. Modeling the magma plumbing system of Vulcano (Aeolian Islands, Italy) by integrated fluid-inclusion geobarometry, petrology and geophysics. Geology 34, 17-20]. It is concluded that the hazard assessment of Lipari Island should take into account the arrival of deep and never erupted mafic melts as eruption triggers of more evolved shallower magma bodies.

Davì, Marcella; De Rosa, Rosanna; Barca, Donatella

2009-05-01

204

Hermann Wilhelm Abich im Kaukasus: Zum zweihundertsten Geburtstag  

NASA Astrophysics Data System (ADS)

Hermann Abich was born in 1806 in Berlin and died in 1886 in Graz. He grew up in a wealthy family which had friendly relations with famous scientists like Alexander von Humboldt, Leopold von Buch or Carl Ritter. After his studies in Heidelberg and Berlin he turned to extended fieldwork at the volcanoes of Italy. In 1833 1834 he published excellent petrological/chemical results and got soon a good scientific reputation. Thus he was nominated as Professor for Geology and Mineralogy of the prestigious Russian University in Dorpat (now Tartu, Esthonia) in 1842. In 1844 he was sent to Armenia by the Russian authorities. For the next three decades his fieldwork with about 190 publications was concentrated on the Great and Lesser Caucasus. This was a period of Russian expansion to the South with long-lasting regional fights. But he enjoyed the support of powerful governors. He was an indefatigable and enthusiastic explorer and a precise observer and designer. His interests covered many fields: morphology, glaciology, structural geology, volcanology with Thermal Springs, mineral resources from hydrocarbons, coal, salt to ores, stratigraphy and paleontology as a base for geological maps. But he also gave advice for practical problems, and he was active in meteorology, botany and archaeology. Alltogether he became “the Father of Caucasus Geology”. The following sketch stresses only on three aspects of his activities. He was one of the first pioneers in hydrocarbon exploration, especially around the anticlines with the mud volcanoes near Baku. In many respects, however, his fundamental ideas were erronous. He explained the structure of the Great Caucasus by the traditional theories of Leopold von Buch and Elie de Beaumont. The Caucasus anticline “was elevated by forces acting from beneath”. Following them he tried to discover regularities in the strike of mountain chains. Similarily he treated volcanism like Alexander von Humboldt and Leopold von Buch with their two groups of phenomena: voluminous, mostly basaltic “elevation craters” versus isolated, mostly trachytic and relatively small cones of “true volcanoes”. In spite of the isolation of the Caucasus region he had cultivated continuously contacts with leading geologists in Europe and was honoured by many institutions. He left Russia in 1876 for Vienna planning to write there the final monograph volumes about his investigations but he died before he could complete them.

Seibold, Ilse; Seibold, Eugen

2006-11-01

205

Chapter 13 Petrogenesis of the Campanian Ignimbrite: implications for crystal-melt separation and open-system processes from major and trace elements and Th isotopic data  

USGS Publications Warehouse

The Campanian Ignimbrite is a large-volume trachytic to phonolitic ignimbrite that was deposited at ???39.3 ka and represents one of a number of highly explosive volcanic events that have occurred in the region near Naples, Italy. Thermodynamic modeling using the MELTS algorithm reveals that major element variations are dominated by crystal-liquid separation at 0.15 GPa. Initial dissolved H2O content in the parental melt is ???3 wt.% and the magmatic system fugacity of oxygen was buffered along QFM+1. Significantly, MELTS results also indicate that the liquid line of descent is marked by a large change in the proportion of melt (from 0.46 to 0.09) at ???884??C, which leads to a discontinuity in melt composition (i.e., a compositional gap) and different thermodynamic and transport properties of melt and magma across the gap. Crystallization of alkali feldspar and plagioclase dominates the phase assemblage at this pseudo-invariant point temperature of ???884??C. Evaluation of the variations in the trace elements Zr, Nb, Th, U, Rb, Sm, and Sr using a mass balance equation that accounts for changing bulk mineral-melt partition coefficients as crystallization occurs indicates that crystal-liquid separation and open-system processes were important. Th isotope data yield an apparent isochron that is ???20 kyr younger than the age of the deposit, and age-corrected Th isotope data indicate that the magma body was an open system at the time of eruption. Because open-system behavior can profoundly change isotopic and elemental characteristics of a magma body, these Th results illustrate that it is critical to understand the contribution that open-system processes make to magmatic systems prior to assigning relevance to age or timescale information derived from such systems. Fluid-magma interaction has been proposed as a mechanism to change isotopic and elemental characteristics of magma bodies, but an evaluation of the mass and thermal constraints on such a process suggests large-scale interaction is unlikely. In the case of the magma body associated with the Campanian Ignimbrite, the most likely source of the open-system signatures is assimilation of partial melts of compositionally heterogeneous basement composed of cumulates and intrusive equivalents of volcanic activity that has characterized the Campanian region for over 300 kyr. ?? 2006 Elsevier B.V. All rights reserved.

Bohrson, W. A.; Spera, F. J.; Fowler, S. J.; Belkin, H. E.; De Vivo, B.; Rolandi, G.

2006-01-01

206

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field, Saudi Arabia: Implications for duration and migration of volcanic activity  

NASA Astrophysics Data System (ADS)

New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma-Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (˜10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma-Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma-Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.

Moufti, M. R.; Moghazi, A. M.; Ali, K. A.

2013-01-01

207

Age of Magmatism and Eurekan Deformation in North Greenland  

NASA Astrophysics Data System (ADS)

The alpine mountains of Northernmost Greenland are composed of Phanerozoic sediments and volcanic rocks that make up a broadly East-West striking orogenic belt. The major components include: 1) Cambrian-Devonian sediments deposited in the Franklinian Basin; 2) Ellesmerian (365-345 Ma) deformation of these sediments into a fold belt; 3) renewed extension and deposition of Carboniferous-Cretaceous sediments and Cretaceous-Paleogene volcanic rocks of the Kap Washington Group; and 4) Eurekan deformation of sediments and volcanic rocks. We present results of 40Ar-39Ar, U-Pb and Rb-Sr dating of volcanic rocks of the Kap Washington Group. This volcanic succesion is part of the High Arctic Large Igneous Province, exceeds 5 km in thickness, and is composed of bimodal alkaline flows, agglomerates and ignimbrites including peralkaline compositions typical of continental rifts such as the East African Rift. Based on zircon U-Pb and amphibole 40Ar-39Ar ages most volcanics were emplaced at 71-68 Ma, but activity continued down to 61 Ma. A thermal resetting age of 49-47 Ma is also identified in 40Ar-39Ar whole-rock data for trachyte flows. Patch perthite feldspars and coeval resetting of Rb-Sr isotopes by hydrothermal fluids provide further support for thermal overprinting, interpreted as a result of Eurekan compressional tectonism. It is striking that North Greenland volcanism terminated at about the same time (c. 61 Ma) as magmatism in the North Atlantic Large Igneous Province began. We suggest that this was a corollary of a change from extensional to compressional tectonism in the High Arctic. In the period when Greenland moved together with Eurasia (>60 Ma), the separation from North America resulted in rift-related alkaline magmatism in the High Arctic. When Greenland subsequently moved as a separate plate (60-35 Ma), overlapping spreading on both sides pushed it northwards and volcanism in the High Arctic stopped due to compression. Evaluation of plate kinematic models shows that the relative northwards movement of Greenland culminated in the Eocene, coinciding with thermal resetting. We conclude that compression in North Greenland peaked at 49-47 Ma and coincided with the Eurekan Orogeny in a belt across the Canadian Arctic Islands and western Svalbard.

Tegner, Christian; Storey, Michael; Holm, Paul M.; Thorarinsson, Sigurjon; Knudsen, Mads F.

2014-05-01

208

The tectonic evolution of Cenozoic extensional basins, northeast Brazil: Geochronological constraints from continental basalt 40Ar/39Ar ages  

NASA Astrophysics Data System (ADS)

The Boa Vista and Cubati Basins, Paraíba, Brazil, are NW-SE extension-related intracratonic basins that resulted from tectonic stresses after the opening of the South Atlantic. These basins contain lacustrine fossiliferous sediments, bentonite beds, and basalt flows that preserve Cenozoic continental records. 40Ar/39Ar ages for six whole-rocks from two distinct basaltic flows underlying the sediments in the Boa Vista basin are 27.3 ± 0.8 and 25.4 ± 1.3 Ma, while three grains from a basaltic flow overlying the sediments yield 22.0 ± 0.2 Ma. The sediments at the nearby Cubati Basin are overlain by a basalt flow with ages of ˜25.4 Ma. Three whole-rocks from an NE-SW-trending trachytic dyke cross cutting the sediments at the Boa Vista Basin yield 40Ar/39Ar ages of ˜12.45 ± 0.06, 12.59 ± 0.07, and 12.58 ± 0.07 Ma. Three whole-rocks from a nearby volcanic plug (Chupador) yield an age of 23.4 ± 0.1 Ma. The geochronological results combined with stratigraphic correlations between the two basins allow bracketing the age of the main sedimentary and bentonic units within the Boa Vista and Cubati Basins between 25.5 ± 1.3 and 24.9 ± 0.1 Ma. The ages, combined with field observations reveal that the formation of the Boa Vista and Cubati basins is associated with mantle-derived magmas channelled through reactivated Precambrian shear zones. Our geochronological results suggest that a temporal link with the Fernando de Noronha and Saint Helena hot spots can be excluded as possible sources of the Boa Vista and Cubati magmas. Rather, the extensional tectonics in the 30-20 Ma interval, long after Gondwana break-up, may be associated with the re-activation of continental-scale shear zones that channelled small batches of mantle-derived magmas.

de Souza, Zorano Sérgio; Vasconcelos, Paulo Marcos; Knesel, Kurt Michael; da Silveira Dias, Luiz Gustavo; Roesner, Eduardo Henrique; Cordeiro de Farias, Paulo Roberto; de Morais Neto, João Marinho

2013-12-01

209

Felsic volcanism in a basic shield (El Hierro, Canary Islands). Implications in terms of volcanic hazards.  

NASA Astrophysics Data System (ADS)

El Hierro, the southwesternmost and smallest island of the Canary Archipelago, is a complex basaltic shield volcano characterized by mainly effusive volcanism with both Strombolian and Hawaiian activity. Explosive felsic volcanism is not a common feature of the archipelago and, so far, it has only been reported on the central islands of Tenerife and Gran Canaria, where it has been responsible for the formation of large central volcanic complexes. The presence of felsic rocks on the other islands of the archipelago and specifically on El Hierro is mostly restricted to subvolcanic intrusions and a few lava flows, generally associated with the oldest parts of the islands. We hereby report the presence of a trachytic pumice deposit on the island of El Hierro, referred to here as the Malpaso Member. A detailed stratigraphic, lithological, and sedimentological study was carried out on the deposits of this explosive episode of felsic composition, which is the only one found on the Canary Islands apart from those of Gran Canaria and Tenerife. Four different subunits were identified on the basis of their lithological and granulometrical characteristics. The products of the eruption correspond to a single eruptive event and cover an area of about 13 km2. This deposit originated from a base-surge-type explosive eruption with a subsequent radial emplacement of dilute PDC currents, was emplaced from the vent that would have been located in a similar position to the volcano of Tanganasoga. The low vesicularity of juvenile fragments and the morphological characteristics of the fine particles, as well as the high proportion of lithic fragments and the ash-rich nature of the deposit, suggest that magma/water interaction controlled the dynamics of the eruption. This study demonstrates that magmas from El Hierro could have the potential for producing an explosive eruption, in an environment in which the majority of the eruptions are basaltic and effusive in nature. Bearing in mind the style and the spatial extent of the studied eruption, a future event with similar characteristics would have a serious impact on the population, infrastructures, and economy of the island of El Hierro. For this reason it is clearly of great importance to assess the potential volcanic hazard on the island. This research was partially funded by the MINECO grant CGL2011-16144-E and the European Commission (FT7 Theme: ENV.2011.1.3.3-1; Grant 282759: "VUELCO").

Pedrazzi, Dario; Becerril Carretero, Laura; Martí Molist, Joan; Meletlidis, Stavros; Galindo Jiménez, Inés

2014-05-01

210

Tephra record from the Sea of Marmara for the last 70 ka and its paleoceanographic implications  

NASA Astrophysics Data System (ADS)

Sea of Marmara (SoM) is a gateway between the Mediterraean and Black seas, and a tectonically active basin located on a transform plate boundary. Tephra record in the SoM is therefore very important for dating palaeoceanographic, paleoclimatic and tectonic events. We report three tephra units in cores from the SoM extending back to ca 70 ka BP and including an upper marine and a lower lacustrine units separated by a 12 ka (uncalib.) boundary. The uppermost tephra unit is up to 8 mm thick layer in the marine unit. It is heterogenous phonolitic with high total alkali content of 12.4-15.7 wt % and K2O/Na2O of 0.9 to 1.2. The middle and lower tephra layers occur in the lacustrine unit in ca 29 m-long Core MD-01-2430. The middle tephra (MT-1) is a 70 mm-thick homogeneously rhyolitic layer. The lower tephra (MT-2) is 140 mm thick and has a phonolitic-trachytic composition with CaO content of 1.7-1.9 wt % and bimodal K2O/Na2O of 1.0-1.4. Using their geochemical composition and stratigraphic analysis, we assign the tephra units, from top to bottom, to Vesuvius AP2 Pumice, Santorini Cape Riva and Campanian Ignimbrite, which have been previously dated at 3.5 ka BP, 21.95 ka BP, and 39.3 ka BP (all calender ka). The continuous sedimentary record in the Core MD-01-2430 covering the last ca 70 ka indicates that the SoM was lacustrine, disconnected from the Mediterraean Sea during MIS4, MIS3 and most of MIS2. This implies that the sill depth of the Çanakkale Strait (Dardanelles) was shallower than the present-day -65 m sill depth during MIS3 and MIS4. Figure 1: Morphotectonic map of the Sea of Marmara showing location of the studied cores (red stars). Figure 2: Geochemical biplots of tephra glass composition. a) Total alkali silica diagram b) FeO versus total alkalies for allocating cryptotephras from core MNTKS34 and ML01 to the AP2 tephra from Vesuvius. c) FeO versus CaO for correlating tephra MT1 with the Y-2 tephra from Santorini. d) SiO2 versus CaO for discriminating the MT2 tephra (Campanian Ignimbrite).

Cagatay, M.; Wulf, S.; Guichard, F.; Ozmaral, A.; Sancar; Akçer-Ön, S.; Henry, P.; Gasperini, L.

2013-12-01

211

Phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite  

USGS Publications Warehouse

The Campanian Ignimbrite is a > 200 km3 trachyte-phonolite pyroclastic deposit that erupted at 39.3 ?? 0.1 ka within the Campi Flegrei west of Naples, Italy. Here we test the hypothesis that Campanian Ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts (5-10 wt%) of crustal rock (skarns and foid-syenites) during crystallization. Comparison of observed crystal and magma compositions with results of phase equilibria assimilation-fractionation simulations (MELTS) is generally very good. Oxygen fugacity was approximately buffered along QFM+1 (where QFM is the quartz-fayalite-magnetite buffer) during isobaric fractionation at 0.15 GPa (???6 km depth). The parental melt, reconstructed from melt inclusion and host clinopyroxene compositions, is found to be basaltic trachyandesite liquid (51.1 wt% SiO2, 9.3 wt% MgO, 3 wt% H2O). A significant feature of phase equilibria simulations is the existence of a pseudo-invariant temperature, ???883??C, at which the fraction of melt remaining in the system decreases abruptly from ???0.5 to < 0.1. Crystallization at the pseudo-invariant point leads to abrupt changes in the composition, properties (density, dissolved water content), and physical state (viscosity, volume fraction fluid) of melt and magma. A dramatic decrease in melt viscosity (from 1700 Pa s to ???200 Pa s), coupled with a change in the volume fraction of water in magma (from ??? 0.1 to 0.8) and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger. Thermal models suggest a timescale of ??? 200 kyr from the beginning of fractionation until eruption, leading to an apparent rate of evolved magma generation of about 10-3 km3/year. In situ crystallization and crystal settling in density-stratified regions, as well as in convectively mixed, less evolved subjacent magma, operate rapidly enough to match this apparent volumetric rate of evolved magma production. ?? Copyright 2007 Oxford University Press.

Fowler, S. J.; Spera, F. J.; Bohrson, W. A.; Belkin, H. E.; De Vivo, B.

2007-01-01

212

The effects of acid leaching on 40Ar/39Ar age dating results using samples from the Walvis Ridge hotspot trail  

NASA Astrophysics Data System (ADS)

In this study we systematically explore how acid leaching can be used to reduce the negative effects of seawater alteration on the 40Ar/39Ar age dating of submarine basalts. Koppers et al (2000) showed that acid leaching of groundmass samples generated more consistent ages as well as ages more concordant with phenocrystic mineral phases, compared to samples that were left untreated. By studying the effects of progressively increasing the strength and length of acid treatment, we will show how acid leaching of groundmass separates reduces alteration while leaving the initial eruption signature intact. Samples were chosen from the Walvis ridge hotspot trail in the southeast Atlantic. Three samples were selected based on degree and style of alteration. Two samples (basalt and basaltic andesite) appear highly altered in thin section. The basalt contains diffuse iddingsite alteration that is pervasive throughout the groundmass. The basaltic andesite displays focused secondary mineral phases within and around abundant vesicles. The third sample, a trachyte, shows relatively minor degrees of alteration in thin section. These groundmass separates were divided into four splits and treated with a progressively stronger acid and for longer duration. One split from each rock was left untreated to act as a baseline. Of the other three splits from each sample, one was treated with a mild leach (1N HCl and 1N HNO3), one a strong leach (1N HCl, 1N HNO3, 6N HCl, and 3N HNO3), and lastly the strong leach performed twice. The samples were then handpicked to remove any remaining visible alteration. The untreated samples were picked as well, removing the most distinctly altered grains. All splits were analyzed by electron microprobe, x-ray fluorescence (XRF) and the incremental heating 40Ar/39Ar dating method. We will report on the results of an image analysis of microprobe backscatter images and elemental maps taken of individual groundmass grains. This analysis will show the location of alteration within the groundmass and how it is changing as acid leaching strength and duration progresses. We will discuss bulk composition comparisons as well as how depletions and enrichments vary amongst major and trace elements. Lastly, we will report 40Ar/39Ar step heating results, in particular K/Ca ratios and age spectra, to better understand the effect of leaching strength on the effective removal of groundmass alteration during 40Ar/39Ar age dating.

Klath, J. F.; Koppers, A. A.; Heaton, D. E.; Schnur, S.

2013-12-01

213

The Ischia Debris Avalanche: The Result of A Catastrophic Collapse of The Island Southern Flank  

NASA Astrophysics Data System (ADS)

The first occurrence of a volcano-related debris avalanche in a non-oceanic setting, arises from the Ischia Island southern flank, where an EC-funded survey using CNR R/V Urania was ruled out using TOBI deep-tow, long-range side scan sonar in 1998. Above the span of coast running from Capo Grosso headland to S.Angelo, a large amphiteatre is present (Serrara Fontana basin) smaller in size but resembling in shape to the similar features overtopping debris avalanche deposits in Canary and Reunion Islands. At sea the continental shelf is missing and a very steep (up to 45) slope scoured by canyons and gullies develops, from 20/35m down to a depth of about 800m. Below this depth the slope is more gentle and the seafloor is completely cov- ered by a chaotic melange of blocks ranging in size from a few hundreds to a several thousands of cubic meters (a maximum dimension of 100x150 m for a single block has been measured). The blocky facies extends down to more than 1000m, form- ing a tongue of debris long at least 40 km (and this is just the downslope limit of the larger blocks). The debris avalanche deposit seems to completely fill-up the 250 m deep Magnaghi canyon. Seafloor sampling collected a quite heterogeneous spec- trum of lithotypes ranging from coarse debris reworked deposits to finer compacted tuffites to more juvenile rocks (trachytic lava, scoriae and pumice) On gravity cores an hemipelagic interval as thick as several decimetres, overlyies poorly sorted volcan- oclastic debris. Such data fit with the absence of blocks smaller than about 1 m in seafloor sonographs, witnessing a certain amount of burial of the deposits, i.e. a cer- tain time span since its emplacement. Dating of the material is in progress an will give age constrain on the occurrence of the failure event that, on the basis of geomorpho- logic/volcanologic considerations would have occurred within the last eustatic cycle (i.e. last 20-30 ky) as a single event, or an event whose products covered all the pre- vious ones. Blocks alignment within the tongue define flow paths, that are off course gravity driven; similarly to other literature example, the high-energy flow seems to have been able to move in uphill direction in certain conditions.

Chiocci, F. L.; de Alteriis, G.; Bosman, A.; Budillon, F.; Martorelli, E.; Violante, C.

214

Cosmogenic He-3 exposure ages of basalts from Ascension Island - implications for evolution of ocean islands  

NASA Astrophysics Data System (ADS)

Ascension Island is the emergent top of a volcano situated at 7°56'S and 14°22'W in the South Atlantic Ocean, approximately 90 km west of the Mid-Atlantic Ridge and 50 km south of the Ascension Fracture Zone [1, 2]. It rises about 860 m above sea level and the base of the volcano covers about 2000 km2 approximately 3200 m beneath sea level [3]. Volcanic activity is associated with the Mid-Atlantic Ridge, and commenced at 6-7 Ma [2, 4, 5]. The volcanic rocks of Ascension Island are transitional to mildly-alkaline basalt to rhyolite volcanic suite and are distinguished by trace element ratios (e.g. Zr/Nb) [3]. Whereas the age of the trachyte intrusions are well constrained (0.6 - 1.0 Ma) [e.g. 3] the younger basalt suites have not been dated reliably and the low K concentration make Ar/Ar dating difficult. In order to reconstruct the volcanic history of Ascension Island we have used cosmogenic He-3 in olivine and pyroxene phenocrysts from uneroded basalt flows to date the three basalt lava suites that appear to span the range of volcanism. Ages of co-genetic pyroxene and olivine agree within analytical uncertainties. Implanted radiogenic He-4 tends to lower He-3 derived exposure ages therefore some olivine samples were etched with HF/HNO3 to remove about 30% of the sample mass. The so etched olivine is systematically older (by less than 10%) than the unetched samples. The three basalt suites have exposure ages of 300 to 190 ka. The high and intermediate Zr/Nb basalts seem to have been erupted contemporaneously or at least in overlapping events about 300 ka ago. These suites were previously thought to be separated in time on the basis of K-Ar chronology and stratigraphy [3]. The third suite which seams to be a more local vent is erupted between 260 and 190 ka. Our new data indicate the co-existence of different magma chambers with different geochemical signatures. [1] E. Bourdon, C. Hemond, Mineralogy and Petrology 71(2001) 127-138. [2] D.L. Nielson, B.S. Sibbett, Geothermics 25(1996) 427-448. [3] B. Weaver et al., Geothermics 25(1996) 449-470. [4] C. Harris, J.D. Bell, F.B. Atkins, Earth and Planetary Science Letters 60(1982) 79-85. [5] F. Klingelhoefer, et al., Earth and Planetary Science Letters 190(2001) 41-56.

Ammon, K.; Dunai, T. J.; Stuart, F. M.; Meriaux, A.-S.; Gayer, E.

2009-04-01

215

Improving the sensitivity of an interferometric fiber optic sensor for acoustic detection in rockfalls  

NASA Astrophysics Data System (ADS)

Being intrinsically EMI free and offering superior hostile environment operation, fiber optic sensor technology represents a valuable alternative to standard sensors technology in landslides monitoring. Here an improved design for a fiber optic sensor to be used for ultrasonic acoustic detection in rockfall monitoring is proposed. Basically, the original sensor consists of a fiber coil tightly wound on an aluminum flanged hollow mandrel that acts as the sensing arm of a Mach-Zehnder interferometer [1]. To further improve sensor sensitivity, the use of a special fiber, with polyimide coating and very large numerical aperture, has been proposed and tested. The polyimide coating, harder and thinner than standard coating, makes the fiber more sensitive to acoustic waves and increase the coupling efficiency between fiber and mandrel. At the same time, a fiber with very large numerical aperture allows for a much smaller bending radius and thus enables the design of a sensor with reduced size, or with the same external size but housing a longer fiber. Part of the research activity has been then focused toward the optimization of the shape and dimensions of the mandrel: to this aim, a large set of numerical simulations has been performed and they are here presented and discussed. The performance assessment gained with new sensors has been carried in a controlled scenario by using a block of trachyte in which the sensors have been screwed in internally threaded chemical anchors housed in holes drilled on one face of the block. Ultrasonic signals have been generated in a repeatable way by dropping a 5-mm-diameter steel ball along a steep slide. Experimental tests, carried out by firstly comparing the performance of a sensor made with special fiber with respect to the original one, have shown an increased sensitivity of almost 35 % in the detected acoustic energy. Further tests, carried out on a sensor with optimized dimensions and made with special fiber, have shown an increased sensitivity of an impressive 400% with respect to the sensor with special fiber, but original dimensions. These results further confirm the viability of fiber optic acoustic sensors for acoustic detection in rockfall monitoring. Moreover, it is shown how an optimized design can be allowed only by choosing the adequate fiber. References: [1] L. Schenato, L. Palmieri, G. Gruca, D. Iannuzzi, G. Marcato, A. Pasuto, A. Galtarossa, "Fiber optic sensors for precursory acoustic signals detection in rockfall events", J. Eur. Opt. Soc, Rapid Publ. 7, 2012.

Schenato, L.; Palmieri, L.; Autizi, E.; Galtarossa, A.; Pasuto, A.

2013-12-01

216

Case study of an extensive silicic lava: the Bracks Rhyolite, Trans-Pecos Texas  

NASA Astrophysics Data System (ADS)

Field, petrographic, and chemical data indicate that the Bracks Rhyolite of western Trans-Pecos Texas is a single lava flow that traveled as much as 35 km from a source in the north-central part of its outcrop. With a minimum original extent of 1000 km 2 and volume of 75 km 3, the Bracks is far more extensive and voluminous than typical silicic lavas. The Bracks crops out in a 55 × 16 km north-trending belt. It thins radially from a maximum thickness of about 120 m. However, except at flow margins where it thins abruptly, it is everywhere at least 25 m thick. Clusters of vitrophyric domes that intrude and are otherwise identical to the dominantly crystalline lava may represent diapiric rise of hotter, less dense, lower parts of the flow. Some domes may overlie fissure vents that fed the flow. The source area is in the north-central part of the flow, on the basis of thickness and flow patterns and distribution of vitrophyric domes. The Bracks is a slightly peralkaline low-silica rhyolite or trachydacite (68-69% SiO 2). Whole-rock analyses for major oxides and rare earth elements and microprobe analyses of alkali feldspar, Fe-rich clinopyroxene, fayalite, and magnetite phenocrysts show that the Bracks, including vitrophyric domes, is chemically and mineralogically homogeneous, both laterally and vertically. Evidence that the Bracks was emplaced as a lava flow includes autobreccia at the base and top, steep flow fronts, abundant flow bands and folds, elongate vesicles, trachytic texture, and groundmass textures that indicate crystallization from a liquid. Basal breccia that occurs throughout the extent of the flow, is uniformly coarse, and varies in thickness laterally can only form from a lava that flowed over its entire extent. Evidence of a pyroclastic origin, such as shards, pumice, lithics, or welding zonation, is absent. The low aspect ratio of the Bracks (approximately 1 : 500), although in a range typical of many ash-flow tuffs, is considerably higher than that of unequivocal tuffs in Texas, which have comparable outcrop areas but are much thinner. The great extent and sheet-like geometry of the Bracks probably reflects high eruption temperature (? 900°C), low volatile content, moderately low viscosity, rapid eruption, and slow cooling. Unusually low viscosity, on the order of basaltic lavas, was not a factor because, despite peralkalinity and high temperature, the Bracks probably had a low volatile content.

Henry, Christopher D.; Price, Jonathan G.; Rubin, Jeffrey N.; Laubach, Stephen E.

1990-10-01

217

Stratigraphy, Paleomagnetism, and Anisotropy of Magnetic Susceptibility of the Miocene Stanislaus Group, Central Sierra Nevada and Sweetwater Mountains, California and Nevada  

NASA Astrophysics Data System (ADS)

Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic flow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group is a useful stratigraphic marker in regard to the post-9 Ma uplift of the Sierra Nevada and transtensional tectonics within the central Walker Lane. In ascending order, the Stanislaus Group consists of the Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation with alkali-silica compositions ranging from basaltic trachyandesite to trachyte. The stratigraphy of the Eureka Valley Tuff is refined by detailed geologic mapping of the reference section located at Tollhouse Flat, California. We measured directions of remanent magnetization at 32 sites within the Stanislaus Group. The Table Mountain Latite has a distinctively shallow reversed-polarity direction of I=-26.0° and D=162.8° at sampling sites in the Sierran foothills, although the unit mainly consists of normal-polarity flows near the Sierran crest. The Tollhouse Flat Member of the Eureka Valley Tuff has a reversed-polarity magnetization (I=-62.8°, D=159.9°). The By-Day Member (I=52.4°, D=8.6°) and Upper Member (I=27.9°, D=358.0°) of the Eureka Valley Tuff overlie a polarity transition within the Eureka Valley Tuff. The Dardanelles Formation also has normal polarity. We measured vertical-axis rotations of sites in the Walker Lane and Sweetwater Mountains by establishing a virtual geomagnetic reference pole for the Tollhouse Flat Member of the Eureka Valley Tuff in the relatively stable Sierran block. We infer clockwise vertical-axis rotations of approximately 8.5° to 30° in the central Walker Lane north of Mono Lake, California, and to the east in the Anchorite Hills, Nevada. No significant vertical axis rotations relative to the central Sierra Nevada were indicated within the Sweetwater Mountains, California. The AMS results from 19 sites show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, and the flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.

King, N. M.; Hillhouse, J. W.; Gromme, S.; Hausback, B. P.

2006-12-01

218

Development and validation of muon imaging techniques to investigate the internal structure of volcanoes by integrating geophysical and muon tomography methods  

NASA Astrophysics Data System (ADS)

Muon imagery techniques, that use cosmic-ray muons generated in the upper atmosphere, are currently intensively being developed by several international groups to probe the internal structures of volcanic edifices to depths up to several kilometers. The method is based on the measurement, with a muon telescope, of the attenuation of the flux of these high energy atmospheric particles due to their interaction with the constitutive rocks of the volcanic edifice. Muon tomography can thereby be used to construct precise 3D models of rock density distribution, and, even more, its variation with time, within volcanoes. The development and validation of this innovative imaging method are currently being pursued by the multidisciplinary TOMUVOL collaboration that involves both particle and astroparticle physicists, volcanologists and geophysicists. Indeed, a main critical and acknowledged issue among the community is to validate the results of the muon imagery using other geophysical techniques. In particular, the robust reconstruction methodology would require the combined integration of topographic, geological and geophysical information with muon flux interpretation, in order to obtain a reliable internal structure of volcanoes. To achieve this goal, the TOMUVOL project is carrying out this cross-validation approach on an experimental site, the Puy de Dôme volcano. It is a 11000-years old trachytic lava-dome located in the near vicinity of Clermont-Ferrand. A complete interpretation of the internal structure of the dome will be obtained using in situ measurements that started in 2011 and will continue until mid-2012. The field experiment dataset includes two muon radiography surveys acquired from different points of view, two 2km long perpendicular resistivity profiles and a high resolution gravity survey. In addition, a high resolution topographic model has been calculated from a dedicated airborne LIDAR survey, in order to improve the accuracy of geometric reconstructions and to better constrain inversion models. When validated on the Puy de Dôme experimental site, the structure study which integrates muon and traditional geophysical techniques will be used for the study and monitoring of active volcanoes.

Labazuy, P.

2012-04-01

219

Seismic Tomography of Central Sao Miguel, Azores Islands (Portugal)  

NASA Astrophysics Data System (ADS)

The Azores Archipelago consists of nine volcanic islands, located at about 38° N and 28°W, in the triple junction of the American, Eurasian and African plates. The largest island is Sao Miguel. It has rift zones mainly trending NW-SE and E-W; calderas, as Fogo and Furnas, at the intersection of these tectonic lineaments; and thermal springs and fumaroles distributed along these fault systems. Furnas, Sete Cidades and Fogo are the most active volcanic complexes: central volcanoes with a dominantly trachytic production. Furnas is the youngest and consists of a steep-sided caldera structure formed during several collapses. The most important thermal features lie on an E-W lineament which cuts the Furnas caldera complex. The Fogo volcanic edifice is built over an older submarine lava basement and composed by lava flows, domes and pyroclastic flows deposits, with the summit truncated by a caldera.Thermal manifestations are associated with a NW-SE fault system and consist mainly of fumarolic activity. São Miguel was selected as a site for a seismic experiment in a European Union-sponsored project with the aim of quantifying the seismicity of various quiescent volcanoes in inhabited areas.The 3D distribution of P- and S-wave velocities is derived for central São Miguel, by traveltime tomography. We use P- and S-wave arrival times of 289 local earthquakes by a network of 23 seismometers. The model has good resolution in the shallowest 5 km. There are several Vp anomalies, referred to a composite picture of geologic deposits, volcanic structures and tectonic features. Furnas caldera has a shallow, low Vp value probably marking volcaniclastic sediments. A negative Vp anomaly is associated with the geothermal field of Ribeira Grande. Another low Vp area is related to the highly fractured NW-SE tectonic lineament connecting two geothermal areas in central São Miguel. Conversely, high velocity zones mark a central seismogenetic zone at 4-5 km of depth and the Altiprado region. The Sao Bras high Vp is instead interpreted in terms of high-density deposits. These interpretations are supported by distribution of Vp/Vs in the area.

Almendros, J.; Zandomeneghi, D.; Saccorotti, G.; Barclay, A.; Ibáñez, J. M.

2005-12-01

220

The effect of lithology and grainsize on the ablation of glaciers  

NASA Astrophysics Data System (ADS)

Most glaciers are retreating, i.e. the loss of snow and ice (ablation) during the summer is larger than the amount of snow fall during winter. The rate at which ablation occurs generally depends on two independent factors, 1) the atmospheric conditions, such as air moisture, solar radiation, temperature, and 2) the reflectivity which greatly depends on the characteristics of debris coverage (e.g., thickness, grain-size and colour). The atmospheric conditions may vary dramatically on any temporal time scale. In order to decipher the influence of the debris coverage on glacier melting, we prepared eleven different test fields (one without coverage) at the elevation of 3000 m on the Vernagtferner glacier in the Austrian Alps and tested the effects of 1) type of lithology (albedo), 2) grain size, and 3) debris cover thickness. The test area was inclined by less than five degrees to North. Each test field was covered by a certain lithology with controlled grain size and coverage thickness. We used three different lithologies: 1) mica schist (MS), which typifies the local lithology, 2) black, basaltic tephra (EB) of Etna volcano (Italy), and 3) grey, trachytic pumice (SC) of Sete Cidades volcano (Azores, Portugal). We used four different grain size ranges, all in the sand and gravel fraction: 1) between 1 and 2 mm (MS, EB, SC), 2) between 5.6 and 8 mm (MS, EB, SC), 3) between 2 and 3 cm (MS), and 4) larger than 5 cm (MS). The test fields were placed in very close proximity and were up to 1 m2 large. The debris cover thickness usually ranged between 3 and 5 cm, for the coarse gravel, we prepared test fields with up to 18 cm debris coverage. We placed a level in the middle of each test field and measured the individual melting rates over one month (end of June to end of July) on twelve different days, sometimes twice daily (morning and afternoon). Over the 4 weeks period, the uncovered glacier lost 1.55 m of ice thickness whereas the minimum ice loss in one of the test fields was 78 cm (average at all test fields of 96 cm). We will present data on the influence of granulometry and the physical properties (porosity, colour) of the used debris on the melting of glacier ice under identical meteorological conditions.

Lambrecht, A.; Mayer, C.; Kueppers, U.; Juen, M.; Blumenthaler, U.; Seybold, L.; Wirbel, A.

2010-12-01

221

Geochronology and origin of the Pratt-Welker Seamount Chain, Gulf of Alaska: A new pole of rotation for the Pacific Plate  

NASA Astrophysics Data System (ADS)

40K-40Ar and fission-track dating of four seamounts near the southeast end of the Pratt-Welker seamount chain in the Gulf of Alaska, in conjunction with previously published K-Ar and fission-track ages near the northwest end of the chain, documents the complex origin of this seamount chain. Transitional basalts from the adjacent guyots Hodgkins, Davidson, and Denson are dated as 14.3 to 18.2 m.y. These ages, only slightly younger than the ages of the underlying crust, indicate formation of these three seamounts at or very near a spreading center. In contrast, alkalic series lavas (alkali olivine basalts and trachytes) from Kodiak, Giacomini, Dickins, and Hodgkins fit a systematic linear age progression: 23.9±0.6 m.y., 20.9±0.4 m.y., 4.0±0.2 m.y., and 2.8±0.2 m.y., respectively. Hodgkins has apparently experienced two generically different episodes of volcanism, separated by about 12 m.y. The age progression among dated alkali basalts is consistent with the hot spot hypothesis and suggests that for the last 24 m.y. the Pacific plate has moved northwest at 4.4±0.4 cm/yr with respect to the Pratt-Welker hot spot. This volcanic propagation rate, together with the rates from other parallel Neogene Pacific chains, allows an improved estimate of the pole and rate of rotation of the Pacific plate relative to hot spots: 70°N, 95°W, and 0.88°±0.10°/m.y. We conclude that no significant motion of the Pratt-Welker hot spot with respect to other Pacific hot spots has yet been detected. However, the Pratt-Welker age data may alternatively be explained by either the longitudinal roll or propagating crack hypothesis. New K-Ar ages from Horton guyot, in the Cobb seamount chain, indicate alkalic volcanism 20.7± .0 m.y. ago, consistent with a predicted age of 20 m.y. based on the hot spot hypothesis. Guyot depths from Horton and the dated Pratt-Welker seamounts are consistent with the K-Ar ages and normal subsidence of oceanic crust.

Turner, D. L.; Jarrard, R. D.; Forbes, R. B.

1980-11-01

222

Anatomy of a lava dome using muon radiography and electrical resistivity tomography  

NASA Astrophysics Data System (ADS)

For the TOMUVOL Collaboration Previous works (e.g. Tanaka et al., 2008) have demonstrated the capacity of muon radiography techniques to image the internal structure of volcanoes. The method is based on the attenuation of the flux of high energy atmospheric muons through a volcanic edifice, which is measured by a muon telescope installed at some distance from the volcano. The telescope is composed of three parallel matrices of detectors in order to record the angle of incidence of the muons. The aperture of the telescope and its resolution are determined by the distance between the matrices, their surface and their segmentation. TOMUVOL is a project, involving astroparticle and particle physicists and volcanologists, aimed at developing muon tomography of volcanoes. The ultimate goal is to construct autonomous, portable, remote controlled muon telescopes to study and monitor active volcanoes. A first experiment has been carried out on a large, 11000-year-old, trachytic dome, the Puy de Dôme, located in the French Central Massif. The telescope system is derived from particle physics experiments. The sensors are glass resistive plate chambers. The telescope has two 1 m2 and one 1/6 m2 planes. It is located 2 km away from the summit of Puy de Dôme (elevation 1465 m), at 868 m in elevation, Signals have been accumulated during several months. A high resolution LiDAR digital terrain model has been used in computing a density model of the dome, averaged along the path of the muons through the dome. In parallel, an electrical resistivity section of the dome has been obtained using a long (2.2 km) line of electrodes. The internal structure of the dome is thus described with two physical parameters (density and resistivity). This allows us to analyse jointly the results of the two types of measurements. At the time of writing, a new muon radiography campaign is being carried out from a different viewpoint. This is the first step towards a tomographic image of the volcano's internal structure. Reference: Tanaka, H. K. M., T. Nakano, S. Takahashi, J. Yoshida, M. Takeo, J. Oikawa, T. Ohminato, Y. Aoki, E. Koyama, H. Tsuji, H. Ohshima, T. Maekawa, H. Watanabe, and K. Niwa, Radiographic imaging below a volcanic crater floor with cosmic-ray muons, Am. J. Sci., 308, 843-850, 2008.

Lenat, J.

2011-12-01

223

Seismic properties of magmatic processes at laboratory scale: Effects of crystallization and bubble nucleation  

NASA Astrophysics Data System (ADS)

Seismic tomography of potentially hazardous volcanoes is a prime tool to assess the dimensions of magmatic reservoirs and possible magmatic ascent. Magma rheology and volcanic eruptive style are to a first order controlled by processes occurring in the conduit or in the chamber, such as crystallization and bubble exsolution. Seismic velocities are strongly affected by these processes (Carrichi et al, 2009) but the only few constrained measurements don't allow yet to establish a link between seismic tomography and the textural state of the volcanic system. Elastic parameters of vapor-saturated, partially molten systems are thus providing fundamental information for the identification of such reservoirs under active and seemingly dormant volcanoes. We investigated a chemically simplified melt analogous to andesite and trachyte, in the system CaO-Na2O-Al2O3-SiO2-H2O-CO2 (Picard et al, 2011), which undergoes plagioclase crystallization and bubble exsolution. Using a Paterson-type internally-heated gas pressure apparatus, we measured the ultrasonic velocities at a constant pressure of 250 MPa and at a frequency of 0.1 MHz. Samples have been first heated at 850 °C for 30 minutes. Subsequently, the temperature has been decreased to 650 °C at a rate of 0.5 or 0.1 °C/min and velocities were recorded every 45 minutes. In order to characterize the microstructure evolution, series of cold-seal experiments at identical pressure conditions but with rapid-quenching at each of the recorded temperatures have been undertaken. We will present new experimental results that clarify the dependence of the seismic velocities on the evolution of microstructures (bubble and crystal-size distribution) as well as the evolution of composition (melt and crystals). REFERENCES Caricchi, L., Burlini, L., and Ulmer, P. (2009) Propagation of P and S-waves in magmas with different crystal contents: insights into the crystallinity of magmatic reservoirs. Journal of Volcanology and Geothermal Research, 178, 740-750. Picard D., Arbaret L., Pichavant M., Champallier R. and Launeau P. (2011). Rheology and microstructure of experimentally deformed plagioclase suspensions, Geology, 39, 747-750.

Tripoli, Barbara; Cordonnier, Benoit; Ulmer, Peter

2014-05-01

224

Geologic, volcanologic, and tectonic setting of the Vico-Cimino area, Italy  

NASA Astrophysics Data System (ADS)

Although the Cimino and Vico volcanic complexes are located close to each other in a structural low, they belong to two different provinces: Cimino is connected to the Tuscan-Roman anatectic magmatic province and Vico to the Roman-Campanian potassic alkaline province (Barberi et al., 1971; Marinelli, 1975). The two volcanic centers lie on a basement formed by clays and sands of a Miocene-Pliocene aged sedimentary cycle. The Mount Cimino complex forms a central volcano composed of a series of rhyodacitic domes. It was emplaced along a NW-SE trending fracture zone which can be followed for about 16 km. Cimino volcano was active from 1.4 m.y. to 0.95 m.y. ago. Eruption of rhyodacitic ignimbrites were preceded and followed by the emplacement of domes of the same composition. Fluid lavas of latitic and olivine-latitic composition were emitted from the central volcano during the final phase of activity. Vico is a central volcano, mainly composed of lava, with a composite summit caldera. It was emplaced along a NE-SW trending fracture system. The Vico stratovolcano is formed by lavas of various compositions including phonolitic tephrites, tephritic phonolites and slightly undersaturated trachytes. The construction of the stratovolcano, the most ancient exposed products of which have an age of 0.4 m.y., was followed by the eruption of four pyroclastic flows totalling several cubic kilometers, which are dated at 0.3-0.15 m.y. The collapse of the caldera is most likely connected to the eruption of these pyroclastic flows. The ring fractures of the caldera are associated with the emplacement of a pyroclastic unit, most of which erupted along the inner northeast and north rim of the caldera. Post-caldera activity is characterized by the emission of tephritic-phonolitic and tephritic products that form the intra-caldera stratovolcano named Mount Venere. The Vico-Cimino area (northern Latium), located between the Apennine chain and the Tyrrhenian sea, lies along the circum-Tyrrhenian belt (that extends from northern Tuscany to Campania). This zone has experienced post-orogenic tensional movements. Since the Middle Miocene, the teconic evolution of the Tyrrhenian basin is closely associated with acid anatectic and potassic alkaline volcanism along the Tyrrhenian margin of the Apennines. The age, composition, and geographic position of the volcanoes are controlled by different magnitudes of tensional tectonics and crustal thinning processes of the western margin of the Apennine chain.

Sollevanti, Federico

1983-09-01

225

Magmatic Evolution of the Western Azores Islands (Corvo and Flores)  

NASA Astrophysics Data System (ADS)

Corvo and Flores islands belong to the western group of the Azores archipelago, to the west of the Mid-Atlantic Ridge. Several studies have proposed a common magmatic evolution for both islands. However, most of these studies focus on other Azorean islands. In order to investigate the processes that control the evolution of Corvo and Flores we have studied representative samples of the whole volcanostratigraphical sequence in both islands, including lava flows and dikes. Similarly to other oceanic islands, Corvo and Flores are made up of an alternation of porphyritic rocks and microlitic rocks. The former are picrobasalts and basalts with 5 to 60 volume fraction of large (2-15 mm), primitive antecrysts of olivine, clinopyroxene and plagioclase. The latter are Mg-poor hawaites to trachytes. The Mg-rich composition of the porphyritic rocks is due to the accumulation of primitive antecrysts within a more evolved groundmass. In contrast, the composition of the microlitic rocks provides information on the differentiation processes that controlled the evolution of both islands. The microlitic rocks present holocrystalline to hypocrystalline textures with a mineral assemblage mainly composed of microcrysts of plagioclase, olivine, clinopyroxene opaque minerals and accessory amphibole and apatite. Their major element whole rock composition can be best modeled by a polybaric fractional crystallization process (MELTS software) starting at 500 MPa with cooling steps of 5 degrees Celsius and a water content of 1 %, starting from the most primitive analyzed microlitic rock (MgO: 9.04%; Cr: 630 ppm; Ni: 200 ppm). Hence, we confirm that both islands derived from a common primary magma. The crystallization of the antecrysts included in the porphyritic rocks was probably related to the initial stages of the differentiation process. On the other hand, the microlitic rocks and the groundmass of the porphyritic rocks are related to the residual melts of the polybaric fractional crystallization process, probably at shallower magmatic chambers (< 15 km). In conclusion, the processes of fractional crystallization and accumulation of antecrysts control the composition of the products of Corvo and Flores volcanic islands.

Larrea, P.; Galé, C.; Ubide, T.; Widom, E.; Lago, M.; França, Z.; Tierz, P.

2012-12-01

226

The importance of fractional crystallization and magma mixing in controlling chemical differentiation at Süphan stratovolcano, eastern Anatolia, Turkey  

NASA Astrophysics Data System (ADS)

Süphan is a 4,050 m high Pleistocene-age stratovolcano in eastern Anatolia, Turkey, with eruptive products consisting of transitional calc-alkaline to mildly alkaline basalts through trachyandesites and trachytes to rhyolites. We investigate the relative contributions of fractional crystallization and magma mixing to compositional diversity at Süphan using a combination of petrology, geothermometry, and melt inclusion analysis. Although major element chemistry shows near-continuous variation from basalt to rhyolite, mineral chemistry and textures indicate that magma mixing played an important role. Intermediate magmas show a wide range of pyroxene, olivine, and plagioclase compositions that are intermediate between those of basalts and rhyolites. Mineral thermometry of the same rocks yields a range of temperatures bracketed by rhyolite (~750°C) and basalt (~1,100°C). The linear chemical trends shown for most major and trace elements are attributed to mixing processes, rather than to liquid lines of descent from a basaltic parent. In contrast, glassy melt inclusions, hosted by a wide range of phenocryst types, display curved trends for most major elements, suggestive of fractional crystallization. Comparison of these trends to experimental data from basalts and trachyandesites of similar composition to those at Süphan indicates that melt inclusions approximate true liquid lines of descent from a common hydrous parent at pressures of ~500 MPa. Thus, the erupted magmas are cogenetic, but were generated at depths below the shallow, pre-eruptive magma storage region. We infer that chemical differentiation of a mantle-derived basalt occurred in the mid- to lower crust beneath Süphan. A variety of more and less evolved melts with ?55 wt% SiO2 then ascended to shallow level where they interacted. The presence of glomerocrysts in many lavas suggests that cogenetic plutonic rocks were implicated in the interaction process. Blending of diverse, but cogenetic, minerals, and melts served to obscure the true liquid lines of descent in bulk rocks. The fact that chemical variation in melt inclusions preserves deep-seated chemical differentiation indicates that inclusions were trapped in phenocrysts prior to shallow-level blending. Groundmass glasses evolved after mixing and display trends that are distinct from those of melt inclusions.

Özdemir, Yavuz; Blundy, Jon; Güleç, Nilgün

2011-09-01

227

Offshore Oligo-Miocene volcanic fields within the Corsica-Liguria Basin: Magmatic diversity and slab evolution in the western Mediterranean Sea  

NASA Astrophysics Data System (ADS)

The European and Corsica-Sardinia margins of the Ligurian Sea (western Mediterranean) have been affected by a geochemically diverse igneous activity, offshore and onshore, since the Eocene. This magmatism occurred in a global subduction-related framework. On the European side, the oldest Tertiary magmatism dated at ca. 35 Ma was mainly calc-alkaline. It included the emplacement of plutonic bodies of adakitic affinity, such as the quartz microdiorite laccolith locally referred to as "esterellite". Younger magmatic events on-land within the whole Ligurian domain were mostly medium-K or K-rich calc-alkaline. Miocene volcanic activity was important in Sardinia, where andesites and ignimbrites were erupted during several magmatic cycles. In Corsica, it was minor although it emplaced lamprophyres near Sisco at 15 Ma. Dredging and diving cruises conducted in the Ligurian Sea during the last thirty years allowed us to collect a number of submarine samples. We discuss here their geochemistry (major and trace elements) and their whole-rock K-Ar ages and mineral 40Ar-39Ar plateau ages. Around 15 Ma, minor amounts of adakitic lavas were emplaced off southwestern Corsica, in the deepest part of the Liguria-Corsica Basin. They rested over the thinnest southwestern Corsica Hercynian continental crust. Closer to the coast, contemporaneous calc-alkaline rocks erupted on a less thinned crust. The adakitic events could be indicative of either the final stages of active subduction, or alternatively of a slab tearing linked to the southeastern retreat and steepening of the slab. The latter event could be connected with the end of the Corsica-Sardinia block drifting and its correlative eastern collision. Younger volcanic effusions, dated at 14-6 Ma, occurred mostly northwest and north of Corsica. K-rich calc-alkaline basalts, shoshonites and K-rich trachytes were emplaced during this period, and alkali basalts erupted as early as 12 Ma in Sardinia. In the Toulon area, alkali basalts dated at 7-6 Ma represent the last onshore activity just before the Messinian crisis, and the Pliocene alkali basaltic outpouring in Sardinia. We propose to link these latter volcanic events to the development of a slab window in a post-collisional tectonic framework.

Réhault, J.-P.; Honthaas, C.; Guennoc, P.; Bellon, H.; Ruffet, G.; Cotten, J.; Sosson, M.; Maury, R. C.

2012-07-01

228

Pliocene to Quaternary Central American tephrostratigraphy based on marine Tephras from ODP and DSDP sites - first comprehensive study  

NASA Astrophysics Data System (ADS)

The Central American Volcanic Arc (CAVA) is, and has been, one of the most active volcanic regions and generated numerous Plinian eruptions along his 1200 km extension. The best preserved archive of this volcanism can be found as ash layers in the marine sediments downwind from the volcanic sources on the Pacific floor. Numerous ash layers up to 8 Mio old, which occur in ODP and DSDP cores of Legs 66, 67, and 202, originated in Central America and southern Mexico. The cores lie across the ash distribution areas expected from dominant wind directions as identified by mapped fallout deposits. We have chosen 145 ash layers of all three Legs for first detailed analysis of these sites to built up a data base for upcoming IODP cruise 334: Costa Rica Seismogenesis Project. The ash layers commonly have sharp contacts at the bottom and diffuse transitions to terrigenous and pelagic sediments at the top. Ash layer thickness ranges from 0.5 to 60 cm with typical grain sizes from medium silt to coarse sand. The mineral assemblages are typical for arc volcanism (plagioclase, pyroxene, hornblende, and olivine). The most evolved tephras also contain biotite. Electron microprobe analyses of 1300 glass shards yield compositions ranging from basaltic andesite to rhyolite and trachyte. Felsic ashes can be divided into seven compositional groups by means of silica and potassium contents. Correlations between marine ashes and on-land tephras are constrained by petrographical and stratigraphical criteria, major element geochemistry of glasses and minerals, and trace element data from LA-ICP-MS analyses. Due to limited exposure on land, such correlations with individual tephras are only possible for deposits of late Pleistocene to Holocene age. Older ash layers, however, can be correlated with regional arc segments making use of systematic along-arc variations of trace-element characteristics (Zr/Nb, Ba/La, Ce/Yb, La/Yb and Ba/Zr) of the arc rocks. Results show that source areas of the ash layers are distributed along the entire CAVA, as well as at the Southern Mexican Arc. The marine tephra record provides important data for ongoing studies of CAVA volcanism: (a) dating of undated land tephra by correlation with marine ashes and the ages derived by sedimentation rates; (b) stratigraphic correlations along the entire arc can be traced much more completely in the marine sediment cores than by limited onshore outcrops alone; (c) long-term changes in magmatic evolution of volcanic complexes can be reconstructed by using the marine archive of ash layers.

Strehlow, K.; Kutterolf, S.; Freundt, A.; Kwasnitschka, T.

2010-12-01

229

Pb, Nd, and Sr isotopic evidence for a multicomponent source for rocks of Cook-Austral Islands and heterogeneities of mantle plumes  

USGS Publications Warehouse

Sr, Nd, and Pb isotopic compositions were measured in alkaline volcanic rocks (alkali basalt, ankaramite, nephelinite, phonolite, and trachyte) from the South Cook Islands (Aitutaki, Mauke, Rarotonga, Atiu, and Mangaia) and the Austral Islands (Rimatara and Rurutu). The results show that the Cook-Austral rocks have an extremely wide range in isotopic compositions of Pb: 206Pb 204Pb from 18.25 to 21.76, 207pb 204pb from 15.48 to 15.83, and sol208pb 204Pb from 38.37 to 40.62, whereas isotopic compositions of Sr and Nd are less variable. Isotopically, Mangaia, Rimatara, and Rurutu form one group (Mangaia group), which shows extremely radiogenic Pb isotopic compositions but near-MORB (mid-oceanic ridge basalts) values for Sr and Nd isotopic ratios. In contrast, samples from Aitutaki, Rarotonga, Mauke, and Atiu (Aitutaki group) have high 207Pb 204Pb and 208Pb 204Pb and moderately high 87Sr 86Sr (Dupal anomaly). The Aitutaki group could have been derived from heterogeneous mantle plumes, which rose from the enriched deep mantle (the almost primitive lower mantle or recycled continental and oceanic slabs). On the other hand, the Mangaia component could have been derived from the depleted upper mantle which may have been metasomatized with a CO2-rich fluid, as indicated by the near-MORB values of Sr and Nd isotopes. Although Pb isotopic data of the two groups cannot be distinguished from each other statistically, the end components of the Pb-Pb system do not match with those of the Nd-Sr system. Thus, the data must be explained by a multi-, at least three, component mixing model: the mantle plumes (Dupal component and a recycled oceanic slab), metasomatized upper mantle, and lithosphere. The K-Ar ages and isotopic characteristics of the Cook-Austral rocks indicate that if one mantle plume rises from the deep mantle in this region, it has separated into at least two segments on the way to the surface. ?? 1988.

Nakamura, Y.; Tatsumoto, M.

1988-01-01

230

Ascension Island, South Atlantic: Deep Plume or Shallow Melting Anomaly?  

NASA Astrophysics Data System (ADS)

Ascension Island (7° 56' S, 14° 22' W) is the sub-aerial manifestation of a broad region of anomalous volcanism on, and adjacent to, the South Atlantic MAR between the Ascension Fracture Zone (AFZ; approx. 7° S) and the Bode Verde Fracture Zone (BVFC; approx. 11.5° S). Zero age MORB from the MAR in this region have Pb isotope compositions more radiogenic than N-MORB and which plot significantly above (positive ? 8/4) the Northern Hemisphere Reference Line (NHRL), and have LaN/SmN higher and Zr/Nb lower than N-MORB. The enriched character of the MORB volcanism has been suggested to be the product of a plume located near Circe Seamount, at approx. 9° S, 11.7° W, or of a plume located beneath two large on-axis seamounts at 9° 50' S on the MAR. The volcanic rocks of Ascension Island comprise a diverse basalt - hawaiite - mugearite - benmoreite - trachyte suite. There is significant chemical heterogeneity in the mafic lavas, with high Zr/Nb, intermediate Zr/Nb, low Zr/Nb, and Dark Slope Crater lava types reflecting significant source heterogeneity, as indicated by trace element and Sr-Nd-Pb isotope systematics. Data for samples from shallow boreholes and one deep borehole suggest that high Zr/Nb lavas dominate in the subsurface and that there has been a temporal trend toward eruption of increasingly enriched, and more diverse, lava compositions with growth of the volcanic edifice. Ascension Island volcanic rocks have Pb isotope compositions which plot significantly below (negative ? 8/4) the NHRL and trend toward St. Helena HIMU compositions. In addition to Ascension Island, there are numerous seamounts (Circe, Grattan, Stvor, etc.) both on and off the MAR axis between the AFZ and the BVFZ. The locations of the seamounts are closely associated with fracture zones and do not reflect the directions of absolute motion of the South American and African plates (for example, the two large seamounts to the west of Ascension Island are on a flow line parallel to the AFZ). The nature of the distribution of Ascension Island and seamounts between the AFZ and BVFZ does not conform to the classic deep plume model. The geochemistry of Ascension Island and MORB lavas is best explained as the result of melting of shallow chemically heterogeneous mantle, with bursts of excess magmatism when "pods" of enriched (HIMU-type component) mantle pass into the sub-axial MAR melting zone, and with focussing of magma supply being controlled by fracture zone distribution.

Weaver, B.

2004-12-01

231

The Effect of Fe on the Viscosity of Silicate Melts  

NASA Astrophysics Data System (ADS)

Iron is the most abundant transition metal in the Earth's interior, even excluding the iron-rich core. In magmatic systems, Fe can be found in a variety of coordination environments ranging from tetrahedral, to pentahedral to octahedral and in both its bivalent and trivalent oxidation states. Naturally, both coordination number and oxidation state of iron can strongly influence the physical properties of magmatic liquids, even at low concentrations. For example, recent determinations of the viscosity of iron-bearing silicate liquids have revealed strong variations in viscosity as a function of composition (Potuzak et al., 2004), highlighting the importance of understanding the structural role of iron in magmas and how it is linked to their rheological properties. Here we present new viscosity data for a wide range of natural rhyolitic, trachytic, moldavitic, andesitic, latitic, pantelleritic, basaltic and basanitic compositions as well as wet chemistry and synchrotron analysis. Dry Newtonian shear viscosities were investigated at high temperature (1050-1600°C) and low temperature (616-860°C) using the concentric cylinder apparatus and the micropenetration technique, respectively. The glasses obtained by fast quenching the melted sample, during the high temperature viscometry, were used for determining, via potassium dichromate (PD) titration, the Fe2+/Fe3+ ratio. Wet chemistry analyses were found consistent with high-T prediction of Kress and Carmichael (1991) and Ottonello et al. (2001) empirical models. The structural role of iron species was also investigated by Fe K-edge XANES spectroscopy performed at ESRF (Grenoble). High resolution XANES spectra were collected at the BM-8 of the ESRF storage ring operating at 6 GeV and with the ring current ranging from 150 to 200 mA. The energy position of the pre-edge peaks was found intermediate between those of Fe2+ and Fe3+ model compounds, indicating the presence of both Fe oxidation states in the samples examined. The position of the spectral features of the pre-edge peaks has been found compatible with oxidation states intermediate between those of Fe2+ and Fe3+ in both 4 and 5 coordination states. Fe K-edge XANES spectra and wet chemistry analyses allowed to interpret the rheological measurements in terms of structural played by iron species. Kress and Carmichael 1991, Contrib. Min. Petrol. 108, 82 Ottonello et al. 2001, Chem. Geol. 174, 157 Potuzak et al. 2004, EGU 2004 Nizza France

Mangiacapra, A.; Giordano, D.; Potuzak, M.; Romano, C.; Cibin, G.; Poe, B. T.; Dingwell, D. B.

2005-12-01

232

Distal record of multi-sourced tephra in Onepoto Basin, Auckland, New Zealand: implications for volcanic chronology, frequency and hazards  

NASA Astrophysics Data System (ADS)

We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5-9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K2O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K2O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes.

Shane, Phil; Hoverd, Joy

2002-04-01

233

Magnetic Anomaly Modeling of Volcanic Structure and Stratigraphy - Socorro Island, Eastern Pacific Ocean  

NASA Astrophysics Data System (ADS)

Results of a magnetic survey of the volcanic structure of Socorro Island in the Revillagigedo Archipielago are presented. Socorro is part of a group of seamounts and oceanic islands built by volcanic activity at the northern end of the Mathematician ridge and intersection with the Clarion and Rivera fracture zones. Subaerial volcanic activity is characterized by alkaline and peralkaline compositions, marked by pre-, syn- and post-caldera phases of the Evermann volcano, and the Holocene mafic activity of the Lomas Coloradas. The magnetic survey conducted in the central-southern sector of the island permits to investigate the volcanic structure and subsurface stratigraphy. Regional fields for second- and third-degree polynomials show a magnetic low over the caldera, positive anomalies above the pre-caldera deposits and intermediate amplitude anomalies over Lomas Coloradas. Residual fields delineate the structural rim of the caldera, anomaly trends for the pre- and post-caldera deposits and a broad anomaly over Lomas Coloradas. Regional-residual anomalies, first vertical derivative, analytical upward and downward continuations, and forward four-layer modeling are used to construct the geophysical models. Rock magnetic properties were analyzed on samples collected at 24 different sites. Magnetic susceptibility showed wide range of variation from ~10 to ~500 10-3 SI, corresponding to the different lithologies from trachytes and glass-rich tuffs to alkali basalts. Data have been divided into groups with low, intermediate and high values. Rock magnetic analyses indicate that magnetite and titanomagnetites are the main magnetization carriers. Magnetic hysteresis loops indicate low coercivity minerals, with high saturation and remanent magnetizations and PSD domain states. Magnetic susceptibility versus temperature curves show irreversible behavior with Curie temperatures around 560-575 C, suggesting magnetite and Ti-poor titanomagnetites. Paleomagnetic directions determined on samples from one site in the pre-caldera flows and three sites in the post-caldera and Lomas Coloradas units, indicate normal polarity directions with mean declination of 350 and inclination of 37, close to the dipolar direction. Additional data on remanent magnetizations reported in Sbarbori et al. (2009) support dominant normal polarities for pre- and post-caldera units, with mean directions close to the dipolar and the present-day field directions. Implications for the magnetization contrasts used in modeling are to increase the intensities assigned for model units. The effective magnetizations assumed for the model units have dipolar inclinations and northward declinations. The magnetic anomaly shows a broad minimum over the caldera zone, a maximum over the caldera rim and a second maximum closely spaced, followed by a larger wavelength anomaly over the volcano slope and the pre-caldera deposits. The maximum is associated with the caldera rim and the minimum on the outer rim edge is associated with a fracture zone or a deep pre-caldera feature. Preferred models incorporate a topographic relief for the basaltic pre-caldera unit and post-caldera deposits. Top of the pre-caldera basaltic unit lies at depths of about 300 m and up to 600 and 800 m below sea level. The Lomas Coloradas Formation is modeled with thickness of about 200-350 m. Models allow evaluation of stratigraphic distribution and thickness of pre-, syn and post-caldera units and the Lomas Coloradas Formation. Preferred models for the southern flank incorporate a pre-caldera basaltic unit with abrupt relief and syn- and post-caldera silicic deposits with Lomas Coloradas alkaline basalts covering the volcano flanks. Relief for pre-caldera basaltic unit may be associated with the volcanic conduit system for Lomas Coloradas. The structure shown at the southern end of the profile is present in the reduction to the pole, residual field and analytical continuation fields. Models for Evermann volcano show structural features associated caldera collapse, the caldera rim and the pre-caldera morphology

Urrutia-Fucugauchi, Jaime; Escorza-Reyes, Marisol; Pavon-Moreno, Julio; Perez-Cruz, Ligia; Sanchez-Zamora, Osvaldo

2013-04-01

234

Evolution Of An Upper Crustal Plutonic-Volcanic Plumbing System:Insights From High Precision U-Pb Zircon Geochronology Of Intracaldera Tuff And Intrusions In Silver Creek Caldera, Arizona, USA  

NASA Astrophysics Data System (ADS)

Study of both plutonic and volcanic regimes in one single magmatic system is a powerful approach towards obtaining a more complete view of the long-term evolution of magma systems. The recently discovered Silver Creek caldera is the source of the voluminous Peach Spring Tuff (PST) (Ferguson, 2008) and presents a unique opportunity to study a field laboratory of a linked plutonic-volcanic system. This relict west-facing half caldera is predominantly filled with trachytic intracaldera tuff with the caldera margin intruded by several petrologically distinct hypabyssal intrusions. These include porphyritic granite with granophyric texture, felsic leucogranite, porphyritic monzonite exposed on NE side of the caldera that is zoned from more felsic to more mafic, and quartz-phyric dikes that intrude the caldera fill. We present preliminary single zircon ages from 4 samples that have been analyzed using the CA-TIMS method after thermal annealing and chemical leaching (Mattinson 2005), including 1 sample from intracaldera tuff and 3 samples from caldera-related intrusions. 3-D total U/Pb isochron ages from all four samples fall within a range of 18.32-18.90 Ma with uncertainties between 0.09 and 0.39 Ma, although some of them lack precision and are compromised by elevated common Pb. For example, zircon from the dated porphyritic monzonite yields an age of 18.32±0.42 Ma (MSWD=2.7) where the excess scatter may result from real age dispersion and/or different compositions of the common Pb contribution. The PST had been dated to ~18.5 Ma by 40Ar/39Ar techniques (Nielson et al., 1990). In order to be compared to U/Pb ages the 40Ar/39Ar age must be adjusted for a revised age for the then used flux monitor (MMbh-1) and corrected for the now quantified systematic bias between 40Ar/39Ar and U/Pb ages (Renne et al., 2010), which results in a corrected age of 18.8 Ma. Thus, the ages for our samples match that of the PST within error. Based on current results, the age difference between the different phases of the intrusion is very small and the ages of the intrusion match within errors the age of the PST. This tight time range indicates that the super-eruption and the subsequent reactivation of the caldera by hypabyssal intrusions happened on a much shorter timescale than the evolution of large magma systems that have been described with durations of up to 10 m.y. Additional geochronology in combination with geochemical and AMS analyses are aimed at a more detailed reconstruction of the emplacement and eruption history of this plutonic-volcanic system.

Zhang, T.; Mundil, R.; Miller, C. F.; Miller, J. S.; Paterson, S. R.

2010-12-01

235

New geologic mapping combined with geochemical, paleomagnetic, and high-precision 40Ar/39Ar analyses reveal multiple overlapping calderas formed 16.4-15.7 Ma at High Rock caldera complex, northwestern Nevada  

NASA Astrophysics Data System (ADS)

We present new evidence from 1:100,000- and 1:24,000-scale geologic mapping for the presence of at least four overlapping calderas, 24 to 40 km in diameter, that formed in an interval of only 0.7 m.y. during the mid-Miocene at High Rock caldera complex in northwest Nevada and southern Oregon. In total, an estimated minimum volume of ~725 km3 of rhyolitic magma erupted from the complex between 16.5 and 15.5 Ma, covering an area of ~9,000 km2. Rapid eruption of numerous units at volumetric rates as high as 3,000-4,000 km3/m.y., strong welding of lithic-poor ignimbrites, extensive vapor-phase alteration of lavas and ignimbrites alike, a limited range of phenocryst content and assemblage, silicification along faults, and a lack of well-exposed stratigraphic sections has hindered previous reconnaissance-scale mapping and identification of caldera centers. Calderas are located based on truncation of precaldera rhyolitic lavas by caldera topographic walls, by arcuate patterns of rhyolite lavas that erupted along buried caldera ring faults, and by the presence of pumiceous caldera lake sediments. We attribute formation of the Virgin Valley, Badger Mountain, Hanging Rock, and Cottonwood Creek Calderas to collapse on eruption, respectively, of the ca. 16.37 Ma Idaho Canyon Tuff, the 16.34 Ma Summit Lake Tuff, the 16.0 Ma Soldier Meadows Tuff, and the 15.7 Ma Tuff of Yellow Rock Canyon. Additional smaller-volume pyroclastic units erupted during emplacement of geochemically similar rhyolitic lavas. More than 60 new 40Ar/39Ar ages were obtained on ignimbrites, fall deposits, and rhyolitic, trachytic and basaltic lavas. Many of the eruptive units in the HRCC differ in age by less than 100 k.y., which, at ca. 16 Ma, requires precision at the 1-2‰ (2? standard error) level to distinguish units using 40Ar/39Ar geochronology. The high-precision of the analyses of sanidine in the rhyolites, coupled with geochemical and paleomagnetic measurements, allowed us to correlate far-flung exposures of ignimbrite and to assign lavas to particular caldera centers. The same data also demonstrated that some previously defined and mapped ignimbrites consist of similar-looking units that erupted hundreds of thousands of years apart. For example, the densely welded pantelleritic Soldier Meadows Tuff at the type locality is 16.0 Ma, but weakly welded ignimbrite previously mapped as Soldier Meadows Tuff erupted between 15.7 and 15.85 Ma in association with emplacement of geochemically similar lava domes during resurgence at Badger Mountain caldera, and at 15.7 Ma as the less alkali Tuff of Yellow Rock Canyon. Our new data also indicate that the Tuff of Oregon Canyon and Tuff of Idaho Canyon are indeed different units, the more peralkaline Tuff of Oregon Canyon erupting at 16.54 Ma, whereas the Tuff of Idaho Canyon erupted at 16.37 Ma. The integration of geologic mapping with detailed age information, paleomagnetic measurements, and trace element analyses has resulted in a better understanding of the complex eruptive history of the High Rock caldera complex, and allows it to be placed in the regional stratigraphy of other mid-Miocene rhyolites erupted in association with the Steens and Columbia River basalts.

Coble, M. A.; Mahood, G. A.

2012-12-01

236

Hydrogeology study of Faial Island, the Azores  

NASA Astrophysics Data System (ADS)

Azores Archipelago is a Portuguese territory formed by nine islands divided into three groups (eastern, central and western) located in the North Atlantic Ocean. The islands stretch along a NW-SE direction near the so called Azores triple junction, where the North American Plate, the African Plate and the Eurasian Plate meet. Faial Island is the westernmost island of the central group, located between 38°30'56'' to 38°38'40'' N latitude and 28°35'55'' to 28°50'06'' W longitude. Faial is affected by important tectonic features with a WNW-ESE general trend. These features combined with faults with NNW-SSE to NW-SE and NE-SW directions may have conditioned the emplacement of the central volcano with caldera on the central part of the island. In what concerns the geomorphology, besides the central volcano with caldera, one should refer to the graben on the eastern sector, to the scoria cones alignment on the W and to the flattened sector SE of the central volcano. The drainage network is markedly controlled by tectonics and the drainage density is higher on the northern and southern flanks of the central volcano. The origin of the island started more than 800000 years ago with the emplacement of a composite volcano on the NE of the island (Ribeirinha Complex) consisting of a series of lava flows of basaltic to benmoreitic composition and undifferentiated pyroclasts. The complex (Cedros Complex) which followed is about 580000 years old and corresponds to the central volcano formed by suites of basaltic to trachitic lava flows, pyroclasts and domes. Approximately 50000 ago an important fissural activity took place on the eastern part of the island and originated Almoxarife Formation consisting of basaltic to benmoreitic lava flows, scoria cones and tuff cones. The Caldeira Formation (~16000 years) comprises benmoreitic to trachytic materials emitted from the central volcano, whose explosive phases generated ten members formed mainly by pumice fall deposits and pyroclastic flows. Capelo Complex, which is partially contemporary to the previous formation, comprises the western fissural volcanism and the historical eruptions of Cabeço do Fogo and Capelinhos. Groundwater occurs in two main aquifer systems: (1) the basal aquifer that corresponds to the freshwater lens floating over underlying salt water and (2) perched water bodies. 15 drilled wells and 10 hand dug wells extract water from the basal aquifer and about 80 springs drain the volcanic edifices at different altitudes. In what concerns hydrodynamic characterisation, an estimate of the recession constant of the springs yielded values from 2x10-3 to 14.2x10-3. Calculated transmissivity values for the basal aquifer and a perched aquifer are within the range of 9.5x10-3 to 3x10-2 m2/s. Hydraulic diffusivity estimated from tidal effect measurements has a value of 2634 m2/d. Physical and chemical water analysis shows that they are cold, except for two of them. Four water samples show high CO2 contents. Water from springs and from wells not contaminated by salt water intrusion are bicarbonated and sodium rich in composition while water from contaminated wells has higher chloride and magnesium contents. Geochemical modelling indicates that the major mineralizing processes are silicate dissolution and salt water intrusion.

Coutinho, R. M.; Cruz, J. V.

2011-12-01

237

Geological and petrological aspects of the ongoing submarine eruption at El Hierro Island (Canary Islands, Spain)  

NASA Astrophysics Data System (ADS)

The Canarian Archipelago comprises seven major and three minor islands, all of them of volcanic origin. The distribution of the islands forms an east-west volcanic chain, starting about 90 km west of the northwest African continental margin. The canary volcanism is unique among ocean islands (long lifetime, multiple periods of volcanic activity, extensive range of magma compositions) and various theories were developed in order to explain that specific volcanism, with such a variety of volcanic phases and chemical diversity. El Hierro, located at the SW end of this island group, is the youngest island with the oldest subaerial rocks dated at 1.12 Ma and is still in juvenile stage of shield growth. The island is the emergent summit of a 280 km2 volcanic shield which rises from a 3800-4000m depth and grows up to 1500 m above sea level. Although the whole island has been constructed by the volcanic material of two major volcanic edifices, Tiñor in the NE (0.8 -1.2 Ma) and El Golfo edifice in the NW (550 ka-130 ka), rift volcanism (134 ka - AD1793) has been very active after the second major tectonic event (gravitational collapse of El Golfo edifice), specially along the South ridge. Till July 2011 the most recent eruption was the Volcán de Lomo Negro (AD1793) located at the western part of the island. The products of the Tiñor and El Golfo edifice, massive lava flows, are typical mafic basalts with phenocrystals of olivine and only in El Golfo sequence evolved lava flows (trachytes with phenocrystals of plagioclase feldspars) could be observed. However, the recent rift lavas present varied compositional and textural features. During the eruption of 2011-2012 a variety of volcanic material has been observed and sampled. On 15 October, bicoloured lava fragments were observed floating on the sea with a bomb-like shape and sizes between 10 and 40 cm. The outer part, black, vesiculated and no more than 1 cm thick, had a basaltic composition, while the inner part was white, highly vesiculated and rich in silica (>60%). This type of fragments was observed only during the first days of the eruption. On 27 November (and later) new lava fragments were observed while floating and degassing on the sea surface. Many of them were "lava balloons", with a huge cavity in the centre or fragments of pillow lavas, with sizes between 30 and 200 cm; all of them have a highly vesiculated outer crust. The composition is basaltic-basanitic and sideromelane could be observed most of the times. In this work, we describe the petrological evolution observed since the beginning of the eruption through the fragments emitted and the geological characteristics of the submarine edifice.

Meletlidis, S.; Di Roberto, A.; Iribarren, I.; Pompilio, M.; Bertagnini, A.; Torres, P. A.; Felpeto, A.; Lopez, C.; Blanco, M. J.

2012-04-01

238

Volcanic facies and mineral chemistry of Tertiary volcanics in the northern part of the Eastern Pontides, northeast Turkey: implications for pre-eruptive crystallization conditions and magma chamber processes  

NASA Astrophysics Data System (ADS)

Tertiary volcanics in the northern zone of the Eastern Pontides are characterized by subaerial and shallow-subaqueous facieses, and are divided into three volcanic suites: Eocene aged (1) basalt-trachybasalt-basaltic trachyandesite (BTB) and (2) trachyte-trachyandesite (TT), and Miocene aged (3) basanite-tephrite (BT) suites. Clinopyroxene is a common phase in all three volcanic suites, and has different compositions with Mg# varying from 0.57 to 0.91 in BTB suite and 0.57-0.84 in TT suite to 0.65-0.90 in BT suite. Feldspars in all suites generally exhibit wide range of compositions from sanidine to albite or anorthite and have weak normal and reverse compositional zoning. Olivines in BTB and BT suites have Fo60-92. Hornblendes in BTB, TT and BT suites are commonly magnesio-hastingsite and rare pargasite in composition (Mg#: 0.67-0.90). Brown mica is mainly phlogopite with Mg# ranging from 0.56 to 0.92 in the BTB suite, 0.59-0.84 in the TT suite, and 0.75-0.93 in the BT suite. Analcime is present only in the BT suite rocks. Fe-Ti oxides in all suites are mainly composed of magnetite and titanomagnetite. Textural petrographic and mineral chemical data suggest that magmas had undergone hydrous and anhydrous crystallizations in deep-, mid-, and shallow-crustal magma chambers. Clinopyroxene thermobarometric calculations show that Eocene magma chambers were characterized by temperature ranging from 1,100 to 1,244 °C and pressure ranging from 1.84 to 5.39 kbar. Similarly, the Miocene magma chambers were characterized by temperature ranging from 1,119 to 1,146 °C and pressure ranging from 4.23 to 4.93 kbar. Hornblende thermobarometry, oxygen fugacity, and hygrometer reveal that the crystallization temperature of Eocene volcanics range from 956 to 959 °C at pressure ranging from 6.49 to 6.52 kbar. Eocene volcanics were characterized by water content ranging from 7.83 to 8.57 wt.% and oxygen fugacity of 10-9.36 to 10-9.46 (?NNO+2). Miocene volcanics had crystallization temperature ranging from 970 to 978 °C at pressure ranging from 8.70 to 9.00 kbar with water content ranging from 8.04 to 8.64 wt.% and oxygen fugacity ranging from 10-8.75 to 10-8.87 (?NNO+2). Brown mica thermobarometric data show that Eocene volcanics were characterized by relatively high oxygen fugacity varying from 10-10.32 to 10-12.37 (HM) at temperature ranging from 858 to 953 °C and pressure ranging from 1.08 to 1.41 kbar. Miocene volcanics were crystallized at highly oxidized conditions, which are characterized by high oxygen fugacity of 10-12.0 (HM) at temperature of 875 °C and pressure of 2.09 kbar. The wide range of obtained temperatures for clinopyroxenes of the suites denotes that the equilibration of clinopyroxene crystals initiates from depth until close to the surface before magma eruption. The compositional variations, resorbed core and reverse zoning patterns in clinopyroxene phenocrysts, as well as variable pressures of crystallization, further indicate that the magmas that formed the suites were polybaric in origins and were composite products of more than one petrogenetic stage. The observed range of phenocryst assemblage and different compositional trends possibly originated from fractionation of magmas with different initial water contents under variable pressures of crystallization. The repeated occurrence of magmas from different suites during a single period of activity suggests that the magmatic system consists of several conduit systems and that magma reservoirs are dispersed at different levels of crustal magma chambers.

Yücel, Cem; Arslan, Mehmet; Temizel, ?rfan; Abdio?lu, Emel

2014-06-01

239

Petrologic Aspects of Seamount and Guyot Volcanism on the Ancestral Mesozoic Pacific Plate: a Review  

NASA Astrophysics Data System (ADS)

Hundreds of large seamounts and guyots are widely scattered almost in a "shotgun-blast" arrangement in an area about the size of the United States west of the Mississippi River on the Mesozoic Pacific plate between the Mariana Trench and the Gilbert Islands. Most of these formed between ~160-100 Ma while the Pacific plate was surrounded by spreading ridges and growing outward in all directions. There is little to no indication that the seamounts and guyots formed along linear seamount chains; existing radiometric-age data show no age progressions. The volcanoes appear to have formed in response to a uniform stress configuration across the plate, which was either not moving or moving very slowly at the time (1, 2), much like the modern Antarctic plate. When the growing plate started to encounter subduction systems in the western Pacific at ~90 Ma, consistent stress patterns began to develop, and the broad linear Gilbert and Line volcanic ridge systems began to form. Even then, however, considerable overlapping of volcanism occurred, and only the most general age progressions are evident in existing data. Petrologic data from samples obtained from dozens of volcanic summits by dredging and beneath several carbonate platforms by drilling reveal considerable diversity in development of differentiated alkalic magmatic lineages rooted in diverse parental basaltic rocks. These include transitional, alkalic and basanitic compositions, with differentiates of hawaiite, mugearite, trachyte and one phonolite. Many of the basaltic rocks are partly to significantly transformed by alteration under oxidative conditions (dredged rocks) and both oxidative and non-oxidative conditions (drilled rocks). This can make estimations of mantle geochemical provenance difficult. Nevertheless, the province has been linked by backtracking techniques to the modern SOPITA region of the South Pacific (3), and its rocks show enrichments in trace elements and isotopic characteristics similar to lavas from the Cook-Austral, Marquesas, Society, and Samoan linear volcanic chains. Significantly, Hawaiian- type tholeiite has not been sampled in the region, and the diversity of basaltic rocks and differentiates has always been high. Even unusual potassic nephelinites (K2O > Na2O) with phenocrysts of kaersutitic amphibole or phlogopite occur in the Wake and Line Seamounts. These resemble lavas of portions of the East African Rift, but also have counterparts in the Samoan and Society chains, and resemble very young basalts obtained on the outer trench swell of the Pacific plate near Japan. I suggest that variably and often strongly enriched material was originally supplied to the shallow upper mantle beneath a broad region of the Pacific plate during the Mesozoic; that partial melts of this material were subsequently tapped along major fracture systems that developed to form linear island chains as stress configurations changed on the Pacific plate; and that narrow plume conduits of ascending mantle have never figured in the emplacement of the broadly distributed enriched SOPITA volcanoes. 1) Natland, J. H., and Winterer, E.L., 2005, GSA Spec. Paper 388: 687-710. 2) Larson, R.L., et al., 1992, Proc. ODP, Sci Results, 129: p. 615-631; 3) Staudigel, H., et al., 1991, EPSL, 102: 24-44.

Natland, J. H.

2007-12-01

240

Experimental Study of Dehydration and Partial Melting of Biotite-Amphibole Gneiss Under Influence of the H2O-CO2-(K, Na)cl Fluids at 5.5 Kbar and 750 and 800 C  

NASA Astrophysics Data System (ADS)

Chloride-rich brines coexisting with CO2-rich fluids are an important agent of high-grade metamorphism and metasomatism in the lower to middle crust. Thermodynamic and transport properties of the chloride-rich fluids are well constrained both theoretically and experimentally. Nevertheless, their effects on complex natural assemblages are poorly understood and demand systematic experimental study. We report results of the experiments on interaction of the biotite-amphibole gneiss from the Sand River formation (Limpopo Complex, South Africa) with the H2O-CO2, H2O-CO2-KCl, H2O-CO2-NaCl, and H2O-CO2-(K, Na)Cl fluids at 5.5 kbar, 750 and 800 C, the chloride/(H2O+CO2) ratio varying from 0 to 0.1, and molar CO2/(CO2+H2O) = 0.5. Experiments were carried out using internally-heated gas pressure vessel. Major purposes of the experiments were to trace changes in phase assemblages in dependence on temperature, salt concentration in a fluid, KCl/NaCl ratio and to show a possibility for partial melting. Heating of the gneiss both at 750 and 800 C without any fluid produced no visible changes in its phase assemblage. Interaction of the gneiss with the H2O-CO2 fluid at 750 C did not significantly influence on its phase assemblage, as well. Addition of KCl in the H2O-CO2 fluid at 750 C resulted in the formation of reaction textures consisting of clinopyroxene and K-feldspar around biotite and amphibole at the contacts with plagioclase. No evidence for partial melting was detected in the samples interacted with the H2O-CO2 and H2O-CO2-KCl fluids at 750 C. Addition of NaCl provokes melting with formation of trachytic and trachyandesitic melts along with the new assemblages Cpx+Kfs+Pl+Ti-Mt and Cpx+Amp+Pl+Ti-Mt. Characteristic of feature of amphiboles, forming in the NaCl-rich fluids is their elevated Na2O content. Products of the runs at 750 C do not contain orthopyroxene. This phase appears at 800 C as a result of biotite breakdown in presence of the H2O-CO2 fluid. It is accompanied by dacitic melt and Ti-magnetite. Newly formed assemblages regularly change with an increase of the KCl content in the fluid: Opx+Ti-Mt+melt, Opx+Amp+Ilm+Ti-Mt + melt, Opx+Cpx+Ilm+melt, Cpx+ Ilm+melt. In this sequence, the melt evolves toward potassic rhyolitic composition. Orthopyroxene appears in assemblage with sodic amphibole in equilibrium with NaCl-rich fluids, as well. Present experiments allow conclusion that the interaction of the biotite-amphibole gneiss with the H2O-CO2-(K, Na)Cl fluids can be accompanied by a partial melting at 750-800 C. Melting progress is vitally depends on the NaCl/KCl ratio in a fluid. NaCl intensifies melting because of higher solubility of Cl and CO2 in the sodic melts in comparison to the potassic varieties. Charnockitic and mangeritic assemblages (Opx+Cpx+Kfs+Pl) form in presence of the partial melts at temperature 800 C and are stable in presence of KCl-bearing fluids with relatively low salt concentrations (H2O/(KCl+H2O) < 0.015). Fluids of higher salinity result in formation syenitic and monzonitic assemblages (Cpx+Amp+Kfs+Pl). Applicability of the experimental data is demonstrated by mineral assemblages of local-scale dehydration zones in the high-grade terrains.

Safonov, O.; Kosova, S.

2012-12-01

241

Pleistocene Volcanic Fields in the Anahim Volcanic Belt: What, why, how? - Chilcotin Highland, West-Central British Columbia, Canada  

NASA Astrophysics Data System (ADS)

A large number of volcanic features, including stratovolcanoes, cinder cones, domes, flows and erosional remnants of these exist in the eastern part of the Anahim Volcanic Belt (AVB) around Satah Mountain and Baldface Mountain. These two fields (abbreviated SMVF and BMVF below) are located south and east of the Itcha Ranges in the Chilcotin Highland of west-central British Columbia. Petrographic, geochemical and geochronological studies are hoped to clarify the volcano-tectonic association of these fields and if/how they relate to the nearby AVB. The studies might also provide corroboration of the hot-spot that has been proposed as the source of magmatism in the area, ranging from mid-Miocene ages in the western part of the AVB to Holocene ages at its eastern end at Nazko Cone. During two field campaigns, 20 centres in the SMVF aligned on a NNW-SSE trending topographic high and seven centres in the BMVF were studied with a focus on geochemistry and ages of the erupted lavas. With the exception of Satah Mountain, the most prominent and best-preserved edifice, individual centres are generally small in elevation (200-300 m) and volume. At almost all edifices, there is evidence for glacial modification, which likely removed most of the once-existing (?) pyroclastic material; water-magma interaction could be observed at one centre as well. Extensive coverage by glacial till limits outcrops to cliffs on the edifices' flanks or to local "windows" in the Quaternary deposits. This makes stratigraphic relationships, both within the fields and the surrounding volcanic rocks of the AVB and Chilcotin Flood Basalts (CFB), unclear. Preliminary XRF results indicate a high variability of the lavas, even between centres close to each other. Erupted lavas range from undersaturated basanites (44 wt% SiO2), trachybasalts and trachytes to high-alkali phonolites (14 wt% Na2O+K2O). In general, larger structures in the SMVF appear to have erupted more evolved rocks whereas smaller centres, often just remnants of plugs and necks, and centres in the more easterly BMVF, erupted more primitive rocks. In addition, all new whole-rock ages were determined using the Ar-Ar method for eleven SMVF centres and seven in the BMVF, with clusters around 1.85 Ma for the former and 2.22 Ma for the latter. These ages coincide with existing K-Ar ages for the nearby Itcha Ranges (3.5-1.1 Ma) and would fit well with the hot-spot hypothesis for the AVB. The prevalence of evolved rocks in the SMVF and BMVF might further indicate a relationship to the high-alkaline rocks of the AVB. Further studies will focus (1) on the geochemistry and ages of additional centres, including the yet-unstudied southern part of the Rainbow Range shield volcano in the AVB; (2) the isotopic composition of the lavas to identify possible source regions of the erupted magmas; (3) potential mechanisms and tectonic controls leading to magma ascent along the apparent W-E trend of the AVB, which heretofore has been explained by a mantle plume.

Kuehn, Christian; Guest, Bernard

2013-04-01

242

The Satah Mountain and Baldface Mountain Volcanic Fields, Chilcotin Highland, West-Central British Columbia  

NASA Astrophysics Data System (ADS)

A large number of volcanic features, including stratovolcanoes, cinder cones, domes, flows and erosional remnants of these exist in the Satah Mountain Volcanic Field (SMVF) and Baldface Mountain Volcanic Fields (BMVF), located near the Itcha Ranges in the Chilcotin Highland of west-central British Columbia. Petrographical, geochemical and geochronological studies are hoped to clarify the volcano-tectonic association of these fields and their relation with the nearby Anahim Volcanic Belt (AVB) and possibly provide a confirmation for the hot-spot that has been proposed as the source of magmatism in the area from the mid-Miocene to the Holocene. During field work, 20 centres in the SMVF aligned on a NNW-SSE trending topographic high and seven centres in the BMVF were studied with a focus on geochemistry and ages of the lavas erupted. With the exception of Satah Mountain, the most prominent and best-preserved edifice, individual centres are generally small in height (200-300 m) and volume. There is clear evidence for glacial modification of edifices, which likely removed most of the once-existing pyroclastic material, and water-magma interaction could be observed at one centre as well. Extensive coverage by glacial till limits outcrops to cliffs on the edifices' flanks or to local "windows" in the Quaternary deposits. This makes stratigraphic relationships, both within the fields and the surrounding volcanic rocks of the Anahim Volcanic Belt (AVB) and Chilcotin Flood Basalts (CFB), unclear. Preliminary XRF results indicate a high variability of the lavas, even between centres close to each other. Erupted lavas range from undersaturated basanites (44 wt% SiO2), trachybasalts and trachytes to high-alkali phonolites (14 wt% Na2O+K2O). In general, larger structures in the SMVF appear to have erupted more evolved rocks whereas smaller centres, often just remnants of plugs and necks, and centres in the BMVF erupted more primitive rocks. In addition, whole-rock ages were determined using the Ar-Ar method for eight SMVF centres and four in the BMVF, with clusters around 1.79 Ma for the former and 2.36 Ma for the latter. These ages coincide with existing K-Ar ages for the nearby Itcha Ranges and fit well with the hot-spot hypothesis for the AVB. The prevalence of evolved rocks in the SMVF and BMVF might also indicate a relationship to the high-alkaline rocks of the AVB. Further studies will focus (1) on the geochemistry and ages of additional centres, including the yet-unstudied southern part of the Rainbow Range shield volcano in the AVB, and (2) the isotopic composition of the lavas to identify possible source regions of the erupted magmas.

Kuehn, C.; Guest, B.

2012-12-01

243

Volcanic facies and mineral chemistry of Tertiary volcanics in the northern part of the Eastern Pontides, northeast Turkey: implications for pre-eruptive crystallization conditions and magma chamber processes  

NASA Astrophysics Data System (ADS)

Tertiary volcanics in the northern zone of the Eastern Pontides are characterized by subaerial and shallow-subaqueous facieses, and are divided into three volcanic suites: Eocene aged (1) basalt-trachybasalt-basaltic trachyandesite (BTB) and (2) trachyte-trachyandesite (TT), and Miocene aged (3) basanite-tephrite (BT) suites. Clinopyroxene is a common phase in all three volcanic suites, and has different compositions with Mg# varying from 0.57 to 0.91 in BTB suite and 0.57-0.84 in TT suite to 0.65-0.90 in BT suite. Feldspars in all suites generally exhibit wide range of compositions from sanidine to albite or anorthite and have weak normal and reverse compositional zoning. Olivines in BTB and BT suites have Fo60-92. Hornblendes in BTB, TT and BT suites are commonly magnesio-hastingsite and rare pargasite in composition (Mg#: 0.67-0.90). Brown mica is mainly phlogopite with Mg# ranging from 0.56 to 0.92 in the BTB suite, 0.59-0.84 in the TT suite, and 0.75-0.93 in the BT suite. Analcime is present only in the BT suite rocks. Fe-Ti oxides in all suites are mainly composed of magnetite and titanomagnetite. Textural petrographic and mineral chemical data suggest that magmas had undergone hydrous and anhydrous crystallizations in deep-, mid-, and shallow-crustal magma chambers. Clinopyroxene thermobarometric calculations show that Eocene magma chambers were characterized by temperature ranging from 1,100 to 1,244 °C and pressure ranging from 1.84 to 5.39 kbar. Similarly, the Miocene magma chambers were characterized by temperature ranging from 1,119 to 1,146 °C and pressure ranging from 4.23 to 4.93 kbar. Hornblende thermobarometry, oxygen fugacity, and hygrometer reveal that the crystallization temperature of Eocene volcanics range from 956 to 959 °C at pressure ranging from 6.49 to 6.52 kbar. Eocene volcanics were characterized by water content ranging from 7.83 to 8.57 wt.% and oxygen fugacity of 10-9.36 to 10-9.46 (?NNO+2). Miocene volcanics had crystallization temperature ranging from 970 to 978 °C at pressure ranging from 8.70 to 9.00 kbar with water content ranging from 8.04 to 8.64 wt.% and oxygen fugacity ranging from 10-8.75 to 10-8.87 (?NNO+2). Brown mica thermobarometric data show that Eocene volcanics were characterized by relatively high oxygen fugacity varying from 10-10.32 to 10-12.37 (HM) at temperature ranging from 858 to 953 °C and pressure ranging from 1.08 to 1.41 kbar. Miocene volcanics were crystallized at highly oxidized conditions, which are characterized by high oxygen fugacity of 10-12.0 (HM) at temperature of 875 °C and pressure of 2.09 kbar. The wide range of obtained temperatures for clinopyroxenes of the suites denotes that the equilibration of clinopyroxene crystals initiates from depth until close to the surface before magma eruption. The compositional variations, resorbed core and reverse zoning patterns in clinopyroxene phenocrysts, as well as variable pressures of crystallization, further indicate that the magmas that formed the suites were polybaric in origins and were composite products of more than one petrogenetic stage. The observed range of phenocryst assemblage and different compositional trends possibly originated from fractionation of magmas with different initial water contents under variable pressures of crystallization. The repeated occurrence of magmas from different suites during a single period of activity suggests that the magmatic system consists of several conduit systems and that magma reservoirs are dispersed at different levels of crustal magma chambers.

Yücel, Cem; Arslan, Mehmet; Temizel, ?rfan; Abdio?lu, Emel

2013-09-01

244

Western Tibet relief evolution, insight from sedimentary record and thermochronology  

NASA Astrophysics Data System (ADS)

The Tibetan plateau is defined as a low relief high elevation zone, resulting from India-Asia convergence. However, its morphology is relatively heterogeneous. Especially the western Tibetan plateau is characterized by a strong relief, numerous peaks higher than 6000 m.a.s.l. and large (up to 10 km), deep (1-2 km) valleys. We investigate the origin of this particular morphology, coupling geomorphologic studies with sedimentary records and (U-Th)/He thermochronometry. The western Tibet Tertiary sedimentation is mostly characterized by conglomerates, red sandstone and siltstones related with alluvial fan deposits. Zircon U-Pb dating of interbedded trachyte flows implies that deposition started before 25 Ma and was still ongoing at 20 Ma. These continental, detrital deposits are filling wide open valleys during probable arid climatic conditions. Such valleys are thus interpreted as inherited basins, paleovalleys, formed before detrital sedimentation i.e. at ~25 Ma. Moreover, rare marine sediments were observed below the detrital deposits. Foraminifera suggest an Oligocene age, which implies that the paleovalleys already existed during the Oligocene, and that the emersion of the Western Tibetan Plateau occurred between the Oligocene and 25 Ma. This emersion thus occurred much later than the India-Asia collision (~50-45Ma) but is compatible with the onset of the main thickening phase of the Indian plate. The orientation of the inherited valley axis appears to be that of active strike slip faults that induced eastward extrusion of Western Tibet. This suggests that such extrusion was already active at the time of sedimentation (both marine and continental). Thus extrusion was also active during the plateau emersion at Oligocene time. The morphology of the valleys, and their sedimentary infilling, suggest that a significant relief, similar to present-day one (about 1000-2000m between valleys floor and surrounding peaks) already existed at the time of sedimentation. This implies that very little erosion and only uplift took place following plateau emersion. Assuming an Oligocene emersion and considering the present-day elevation of the valley floor (~ 4300-4500 m.a.s.l.) it yields a mean uplift rate of about 120-170m/Ma. In order to test the slow erosion hypothesis we performed apatite (U-Th)/He dating. Samples have been collected from a vertical profile in a Cretaceous granodiorite. Ages range from 13 to 22Ma. Based on the age variation with sampling elevation an apparent elevation rate of about 0.7 mm/yr is obtained. Modelling of age-elevation relationships indicates that the measured cooling ages are compatible with a slow, continuous exhumation/erosion rate from 25 Ma to the present time. This is similar to previous calculations obtained elsewhere on the Tibetan plateau. Today, the western Tibet is an internally drained area and local erosion induces filling of the paleovalleys. This implies that earlier erosion products were evacuated out of the plateau through the river network. Displacement along the Karakorum fault might be responsible from isolating Western Tibet from the nearby Indus drainage. In conclusion, this study suggests that western Tibet morphology is inherited from the ante-emersion relief (Oligocene?) that was preserved by combination of slow erosion and evacuation of the detrital sediments toward the Indus river network.

Mahéo, Gweltaz; Gourbet, Loraine; Hervé Leloup, Philippe; Sorrel, Philippe; Shuster, David L.; Paquette, Jean-Louis; Quillévéré, Frédéric

2014-05-01

245

Is there a geochemical link between volcanic and plutonic rocks in the Organ Mountains caldera?  

NASA Astrophysics Data System (ADS)

Results from separate volcanic and plutonic studies have led to inconsistent conclusions regarding the origins and thus links between volcanic and plutonic systems in continental arcs and the magmatic processes and time scales responsible for their compositional variations. Some have suggested that there is a geochemical and geochronological disconnect between volcanic and plutonic rocks and hence have questioned the existence of magma mush columns beneath active volcanoes. Investigating contemporary volcanic and plutonic rocks that are spatially connected is thus critical in exploring these issues. The ca. 36 Ma Organ Mountains caldera in New Mexico, USA, represents such a system exposing contemporaneous volcanic and plutonic rocks juxtaposed at the surface due to tilting during extensional tectonics along the Rio Grande Rift. Detailed geologic and structural mapping [1] and 40Ar/39Ar ages of both volcanics and plutons [2] demonstrate the spatial and temporal connection of both rock types with active magmatism over >2.5 myr. Three caldera-forming ignimbrites erupted within 600 kyr [2] from this system with a total erupted volume of 500-1,000 km3 as well as less voluminous pre- and post-caldera trachyte and andesite lavas. The ignimbrite sequence ranges from a crystal-poor, high-SiO2 rhyolite at the base to a more crystal-rich, low-SiO2 rhyolite at the top. Compositional zoning with quartz-monzonite at the base grading to syenite and alaskite at the top is also found in the Organ Needle pluton, the main intrusion, which is interpreted to be the source for the ignimbrites [1]. Other contemporaneous and slightly younger plutons have dioritic to leucogranitic compositions. We examined both volcanic and plutonic rocks with petrography and their textural variations with color cathodoluminescence, and used whole rock element and Sr, Nd and Pb isotope geochemistry to constrain magma compositions and origins. Electron microprobe analyses on feldspars have been completed to determine within-crystal geochemical variations. Our current conclusions and working hypotheses are: 1) All igneous rocks from the Organ Mountains are crustal-mantle melt mixtures indicating two component mixing; 2) the caldera-forming ignimbrites are likely derived from a fractionating Organ Needle pluton; 3) pre- and post-caldera lavas are isotopically similar to the post-caldera Sugarloaf Peak quartz-monzonite; 4) K-feldspar cumulate textures in the structurally top 0.5-1 km of the Organ Needle pluton indicate that interstitial melt was lost from the magma mush, which likely fed the ignimbrite eruptions. 5) Plutonic feldspar textures are complex compared to rather simple zoned volcanic feldspars including K-feldspar rimmed plagioclase, plagioclase rimmed K-feldspar and unrimmed feldspars occurring over a range of grain sizes at thin section scale. Some volcanic feldspar phenocrysts have any previous zonation erased due to late stage albitization. Although the single mineral studies are still work in progress and details need resolving, our data so far suggest a geochemical link between volcanic and plutonic rocks of the Organ Mountains caldera, albeit a complex one; and greater complexity in plutonic versus volcanic minerals. [1] Seager (1980), NM Bureau of Mines and Min. Res. Memoir 36, 97 p. [2] Zimmerer & McIntosh (2013) Journal of Geophysical Research, v. 93, p. 4421-4433

Memeti, V.; Davidson, J.

2013-12-01

246

Geodynamic control on melt production in the central Azores : new insights from major and trace elements, Sr, Nd, Pb, Hf isotopic data and K/Ar ages on the islands of Terceira, Sao Jorge and Faial  

NASA Astrophysics Data System (ADS)

A combined geochronological and geochemical study has been carried out on the volcanic islands of Terceira, São Jorge, and Faial (central Azores) to examine the relationships between mantle dynamics, melt production and regional deformation close to the triple junction between the American, the Eurasian and the Nubian lithospheric plates. The lavas analyzed span the last 1.3 Myr, and have been erupted during two main periods prior to 800 ka and after 750 ka, respectively. They range in composition from alkaline basalts/basanites to trachytes, and overall exhibit a strong enrichment in highly incompatible elements. The whole range of isotopic compositions here reported (87Sr/86Sr: 0.703508-0.703913; 143Nd/144Nd: 0.512882-0.513010; 206Pb/204Pb: 19.0840- 20.0932; 207Pb/204Pb: 15.5388-15.6409; 208Pb/204Pb: 38.7416-39.3921; 176Hf/177Hf: 0.282956-0.283111) suggests the involvement of three components: (1) a weakly radiogenic component reflecting the source of regional MORBs, (2) a main HIMU-type component represented in the three islands, and (3) an additional component in Faial recent lavas, which appears similar to the EM type end-member previously recognized on other Azores eruptive complexes. The geographical distribution of the enriched components and the synchronous construction of various islands at the regional scale rules out a single narrow active plume. They suggest in turn the presence of dispersed residual enriched mantle blobs, interpreted as remnants from a large heterogeneous plume probably responsible for edification of the Azores plateau several Myr ago. The lavas erupted in São Jorge and Faial prior to 800 ka have similar and homogeneous isotopic ratios, which partly overlap the compositional field of MORBs from the adjacent portion of the Mid-Atlantic Ridge (MAR). Their genesis can be explained by the regional development of N150 transtensive tectonic structures, which promoted significant decompression melting of the upper mantle, with correlative dilute expression of the enriched components. In contrast, the youngest lavas (< 750 ka) erupted along the N110 main structural direction on the three islands are significantly enriched in LILE and LREE, and generally have variable but more radiogenic isotopic compositions. Such characteristics suggest low-degree partial melting and greater incorporation of fertile residual mantle anomalies during passive tectonic reactivation of pre-existing transform faults promoted by recent ridge-push at the MAR. We propose that sub-aerial volcanism over the last 1.3 Myr in the central Azores recorded a sudden change in the conditions of melt generation which most probably reveals a major reconfiguration of regional deformation accompanying the recent geodynamic reorganization of the Eurasia-Nubia plate boundary.

Hildenbrand, A.; Weis, D. A.; Madureira, P.; Marques, F. O.

2012-12-01

247

An integrated geophysical study of the northern Kenya rift crustal structure: Implications for geothermal energy prospecting for Menengai area  

NASA Astrophysics Data System (ADS)

In this study, seismic refraction data gathered and interpreted by the Kenya Rift International Seismic Project (KRISP) group has been used as a constraint for the construction of gravity models for the crustal structure of the northern sector of the Kenya rift valley. The gravity data were obtained from the University of Texas at El Paso's (UTEP) database. Additional data were also obtained for the southern Ethiopia and Turkana areas. The analysis and interpretation presented, therefore, takes advantage of this new compilation which has not been incorporated in earlier studies. The other new data set analyzed was an aeromagnetic survey flown in 1987 for the National Oil Corporation of Kenya (NOCK). Maps prepared from these data have been used to corroborate the gravity interpretation for a qualitative assessment of the shapes and trends of the anomalies, in conjunction with the geologic map of Kenya. Therefore, use of integrated methods incorporating well data, KRISP and published industry seismic lines make the presented models better constrained than previous studies. In this study, variations in crustal thickness and upper-mantle structure have been modeled along with evidence for major magmatic modification of the upper crust along the axis of the northern sector of the rift. Results show the following: (1) a decrease in the crustal thickness from about 35 km in the south to 20 km in the north, due to a northward increase in extension, as noted in earlier studies; (2) the gravity highs observed along the axis have been modeled and interpreted as resulting from main magmatic centers underlain by discrete mafic bodies; (3) the axis of the rift is marked by a series of high amplitude magnetic anomalies whose wavelengths are less than 2.5 km, with the positive anomalies coinciding closely with known Quaternary volcanoes; (4) the character of the magnetic field in the southern section of the Kenya rift is significantly different from that of the northern section as shown by a transition zone located just south of Baringo-Bogoria basin. This zone is about 20 km wide and is bounded by pronounced east-west trending faults. In addition to regional analysis of rift structure, a detailed study of the Menengai volcano area was conducted using over 100 DC resistivity soundings. Resistivity anomalies have been highlighted by contouring the apparent resistivity values from the soundings at various depths and investigating how they vary with depth and how they compare with those from the Olkaria geothermal field. The resistivity and gravity signatures suggest the presence of a heat source and a geothermal reservoir hosted within the fractured/faulted brittle trachytic lavas of the rift floor to the north and northeast of Menengai caldera. It is hoped that this analysis will be used in future exploration efforts. (Abstract shortened by UMI.)

Mariita, Nicholas O.

2003-07-01

248

The effect of water on the physical and thermodynamic properties of calc-alkaline basalt and basaltic andesite  

NASA Astrophysics Data System (ADS)

We present corresponding viscosity and heat capacity data obtained on a series of hydrated glasses and supercooled melts for Fe-free calc-alkaline basaltic (sb, NBO/T = 0.67) and basaltic andesite analogues (sba, NBO/T = 0.40). Water contents range from nominally anhydrous to 3.76 wt.% H2O. Density measurements on relaxed (1 atm) hydrous glasses (post-viscometry) yield a partial molar volume of H2O of 12.9 and of 11.4 cm3mol-1 for sba and sb, respectively. Viscosity data were obtained at temperatures in the neighbourhood of the glass transition using the parallel-plate method, and at superliquidus temperatures using the concentric-cylinder and falling-sphere methods. The effect of water on viscosity is most dramatic at low temperatures, with the addition of ~2 wt.% H2O resulting in a reduction of the temperature at which the viscosity of the melt is 1012 Pa s (T12) of 170-180°C. The viscosity of a calc-alkaline basaltic andesite magma with 2 wt.% H2O at depth would increase by a factor of ~100 upon complete degassing during ascent. Calorimetric measurements were made with a Perkin-Elmer 8500 Differential Scanning Calorimeter up to 750°C, the limit of the instrument, depending on the sample water content. The increase in heat capacity associated with the transition from a glass to a supercooled liquid is on the order of ~25-30% and is higher in magnitude the more depolymerized the liquid. Our preliminary results suggest that the heat capacity of the hydrous sba and sb liquids decreases with increasing temperature immediately above the glass transition, similar to borosilicate and titanosilicate melts. In the anhydrous titano- and boro-silicate melts, this anomalous behaviour was linked to T-dependent mixing of B or Ti with Si. In sb and sba, the observed decrease in configurational heat capacity with increasing temperature may be related to the interplay of 'polymerizing' (viscosity-increasing) vs. 'depolymerizing' (viscosity-decreasing) solution mechanisms of water in multi-component depolymerized melts. Measurements to higher temperatures are underway to obtain the heat capacity of the nominally anhydrous sb and sba liquids and rule out other possibilities. The large changes in heat capacity measured at the glass transition for depolymerized melts compared to polymerized ones (25-30 vs. 10-15%) suggest that glasses and liquids in depolymerized systems are structurally significantly different. Large ?CP values correspond to higher fragility, i.e., sharp changes in viscosity at the glass transition. A plot of the fragility of sba shows decreasing fragility with increasing water content, relative to the anhydrous melt, up to 3.76 wt.% H2O, which appears to be slightly more fragile than the dry melt. The magnitude of ?CP(Tg) for sba shows the exact same relationship, with ?CP(Tg) decreasing until 3.76 wt.% H2O. The pattern for sb is less clear. Water in sb and sba behaves calorimetrically like in depolymerized foidite and tephrite, and differently from more polymerized trachyte or phonolite.

Robert, G.; Whittington, A. G.; Stechern, A.; Behrens, H.

2013-12-01

249

Volcanic Unrest of Fogo Volcano in 2011-2012, S.Miguel Island, Azores, Observed by Continuous and Campaign GPS Analysis  

NASA Astrophysics Data System (ADS)

Volcanic eruptions can occur after long time of dormancy as has been seen from the recent examples: Mount St. Helens 1980, Pinatubo 1991, Unzen 1991, Soufrière Hills volcano 1995, Chaitén 2008, and Eyjafjallajökull 2010. By utilizing space geodesy techniques, namely GNSS and InSAR, it has been reported that the inflation-deflation processes exist at several dormant volcanoes in the world, but the mechanism responsible for this phenomena is still controversial. Fundamental questions such as magma vs. hydrothermal fluids and volcanic vs. tectonic process remain unanswered in many cases. In this study, we analyze both continuous and campaign GPS data from Fogo volcano, S. Miguel Island, Azores. Although no geochemical and hydrothermal evidences for a magmatic intrusion were reported during the past seismic swarm episodes (1989, 2003-2006, and 2011-2012), geophysical data, both seismic and ground deformation, indicate possible volcanic sources. GPS time series spanned 2008-2013 period characterize tectonic plate divergence between Eurasian and Nubian, and reveal two different types of ground deformation associated with the 2011-2012 volcanic unrest of Fogo. One is the permanent edifice-scale inflation centered at NE summit which corresponds to the increase of volcano-tectonic events. Another is the subsequent minor-scale inflation-deflation reversals between Congro, a trachyte maar, east of Fogo and Furnas volcano. Calculated strain rates and GPS campaign results indicate that the 2011-2012 deformation is one order smaller than the previous unrest episode. A strong similarity exists to Matsushiro earthquake swarm (1965-1966) and Campi Flegrei volcanic unrests (1969-1972 and 1982-1984), which is the coexistence of an edifice-scale main inflation associated with intense volcano-tectonic earthquakes with inflation to deflation reversal that coincided with a sharp drop of seismicity. High recovery rate of inflation-deflation may be an indicator for the existence of hydrothermal fluids in the highly heterogeneous/porous media. We propose the following hypothesis for the 2011-2012 volcanic unrest of Fogo - (1) a minor magmatic intrusion beneath Fogo which acts as a heat source encourages lateral diffusion of fluids, (2) the fluids are transported through the existing cracks/fissures which are sustained by regional extension due to plate divergence, (3) influx of fluids increases pressure in cracks/fissures and generates lower-frequency earthquakes, and (4) discharge of fluids causes sudden pressure decrease and dilatancy recovery which leads seismic quiescence. Fogo volcano represents 450 years of dormancy since 1563-1564 when the last eruptions took place at the summit caldera. However, we show that the volcano has been experiencing intermittent magma ascents (i.e. repeating "failed eruptions") even its dormant period. Further researches are needed in order to understand the eruption triggering conditions. Nevertheless, we have a continuous GPS network that can detect small changes in the volcano roots and provide important contribution to evaluate future unrest episodes at Azores.

Okada, Jun; Sigmundsson, Freysteinn; Ofeigsson, Benedikt; Ferreira, Teresa; Gaspar, Joao; Lorenzo, Maria; Araujo, Joao; Rodriques, Rita

2014-05-01

250

Timescales of Magma Residence  

NASA Astrophysics Data System (ADS)

The wide variety of approaches by which magma systems can now be imaged provide insights into the duration and dynamics of magma residence in the crust. Many steps may contribute to magma residence, including ascent and emplacement into regions of storage, and episodes of outright magma storage. It is still difficult to distinguish between the various contributions to magma residence and to compare the plumbing systems associated with different magmatic centers because constraints obtained by complementary approaches on single volcanoes are generally lacking. Another limitation of work to date is that most estimates for magma residence times are for volcanic rocks and may therefore preferentially image the most-fluid portions of a magma reservoir. That the various geochemical and geophysical approaches have fidelity for different aspects of a magmatic system is well demonstrated by the range of residence estimates obtained for magma associated with Kilauea. Nonetheless, estimates for magma residence times in mafic systems are on the order of a few thousand years at most and generally years to hundreds of years. These relatively short periods of basaltic residence could reflect the absence of shallow magma reservoirs or, where present, its small volume and/or high turnover rate. Andesitic to dacitic magmas associated with arc stratovolcanoes are complicated by magma recharge, crustal assimilation, and recycling and ascent-related resorption of crystals. Radiometric ages obtained for crystals can predate eruption considerably more than permitted by apparent crystal growth rates which, considered together with the 226Ra excesses of many arc magmas and their crystals, could require either frequent mixing and mingling between recharge melts and reservoir magmas and/or entrainment of crystals from stagnant, mush-dominated magma regimes. Moreover, to reconcile the trachytes from their mafic precursors may be considerably longer (up to hundreds of k.y.) but it is difficult to partition the evolution from mafic to evolved compositions between differentiation, storage in a magma reservoir, and remelting of the underplated mafic magmas. Residence times of silicic magmas can be relatively long, from thousands to a few hundreds of thousands of years. It is possible, however, that once differentiated, silicic magmas solidify and are stored in the crust, only to be later remobilized by reheating. As with mafic magma centers, the same reservoir may be tapped repeatedly but silicic magma residence times seem to be dictated by when storage is interrupted by eruption rather than by the balance between supply and eruption. The several year to few thousand year residence times for the majority of magmas show that individual magma batches are very transient features of the ~0.5 to several m.y. life-spans of many individual volcanic and plutonic systems. Even the timeframes inferred for silicic magmas are short-lived with respect to repose intervals between the more voluminous silicic eruptions and therefore the duration of silicic activity at any particular volcanic system. The liquid (and mush) portions of reservoirs responsible for these magma batches may be equally ephemeral features or may be more longstanding because they are repeatedly renewed/replenished.

Reid, M. R.

2004-05-01

251

Understanding how active volcanoes work: a contribution from synchrotron X-ray computed microtomography  

NASA Astrophysics Data System (ADS)

Volcanoes are complex systems that require the integration of many different geoscience disciplines to understand their behaviour and to monitor and forecast their activity. In the last two decades an increasing amount of information on volcanic processes has been obtained by studying the textures and compositions of volcanic rocks. Five years ago we started a continuing collaboration with the SYRMEP beamline of Elettra Sincrotrone, a third generation synchrotron light source near Trieste, Italy, with the goal of performing high-resolution, phase-contrast X-ray tomographic scans and reconstructing 3-D digital volumes of volcanic specimens. These volumes have been then used for the visualization of the internal structure of rocks and for the quantification of rock textures (i.e., vesicle and crystal volume fraction, individual vesicle volumes and shapes, vesicle connectivity, vesicle volume distributions, permeability simulations etc.). We performed tomographic experiments on volcanic products erupted from different hazardous volcanic systems in Italy and around the world: Campi Flegrei, Stromboli, Etna (Southern Italy), Villarrica (Chile), Yasur and Ambrym (Vanuatu Islands). As an example, we used the results of these studies to constrain the dynamics of vesiculation and degassing in basaltic (Polacci et al., 2006; Burton et al., 2007; Colò et al., 2007; Andronico et al., 2008; Polacci et al., 2008a) and trachytic (Piochi et al., 2008) magmas. A better knowledge of how gas is transported and lost from magmas has led us in turn to draw new implications on the eruptive style of these active, hazardous volcanoes (Polacci et al., 2008b). Work in progress consists of optimizing our procedure by establishing a precise protocol that will enable us to quantitatively study the 3-D texture and composition of rocks in a statistically representative way. Future work will concentrate on the study of the spatial relations between phases (crystals, vesicles and glass) in rocks and their implications on the rheological properties of magmas and on the intensity of explosive activity at volcanoes. Andronico, D., R. A. Corsaro, A. Cristaldi, and M. Polacci (2008), Characterizing high energy explosive eruptions at Stromboli volcano using multidisciplinary data: An example from the 9 January 2005 explosion, J. Volcanol. Geotherm. Res., 176, 541-550. Burton, M. R., H. M. Mader, and M. Polacci (2007), The role of gas percolation in quiescent degassing of persistently active volcanoes, E. Planet. Sci. Lett., 264, 46-60. Colò, L., D. R. Baker, M. Polacci, and M. Ripepe (2007), Magma vesiculation and infrasonic activity in open conduit volcanoes, abstract presented at the AGU 2007 Fall meeting, 10-14 December, San Francisco, California, USA. Piochi, M., M. Polacci, G. De Astis, R. Zanetti, A. Mangiacapra, R. Vannucci, and D. Giordano (2008), Texture and composition of pumices and scoriae from the Campi Flegrei caldera (Italy): implications on the dynamics of explosive eruptions, G-cubed, doi:10.1029/2007GC001746. Polacci, M., D. R. Baker, L. Mancini, G. Tromba, F. Zanini (2006), Three-dimensional investigation of volcanic textures by X-ray microtomography and implications for conduit processes, Geophys. Res. Lett., 33, L13312, doi:10.1029/2006GL026241. Polacci, M., D. R. Baker, L. Bai, and L. Mancini (2008a), Large vesicles record pathways of degassing at basaltic volcanoes, Bull. Volcanol., 70, 1023-1029, doi:10.1007/s00445-007-0184-8. Polacci, M., D. R. Baker, L. Mancini, S. Favretto, and R. Hill (2008b), Vesiculation in magmas from Stromboli (Aeolian Archipelago, Italy) and implications for normal Strombolian activity and paroxysmal explosions in basaltic systems, J. Geophys. Res., doi:10.1029/2008JB005802

Polacci, M.; Baker, D. R.; Mancini, L.

2009-04-01

252

Origin of seamount volcanism in northeast Indian Ocean with emphasis on Christmas Island  

NASA Astrophysics Data System (ADS)

The Northeast Indian Ocean has been a central point of research in the recent past due to its intraplate geophysical and geochemical characteristics. It is dominated by sub-aerial volcanic islands and submerged guyots and two islands, namely, Cocos (Keeling) Island and Christmas Island. Christmas Island, the focus of this study, consists of limestone and mafic intraplate volcanics. The origin of most of the features in northeast Indian Ocean is not fully understood. Christmas Island has experienced multiple stages of intraplate volcanic activity as previously established by 40Ar/39Ar radioisotopic analyses of basalts from the island (Hoernl et al., 2011). Here, we present new 40Ar/39Ar ages where the rock samples from Waterfall Spring (WS), Ethel Beach (EB) & Dolly Beach (DB) on the east coast of the island yielded plateau and mini-plateau ages of 37.75±0.77 Ma, 37.10±0.66 Ma and 43.37±0.45 Ma respectively, whereas a sample from Flying Fish Cove (FFC) in the north of the island yielded a minimum age of 38.6±0.5 Ma. All these units are part of the Lower Volcanics Series. The samples from the west coast (Winifred Beach, WB) are younger with an age of 4.32 ± 0.17 Ma, and are part of the Upper Volcanic Series. This confirms two stages of volcanism at the island with a gap of around 38 Ma. The 40Ar/39Ar radioisotopic ages were overlayed on Gplates and seismic tomography models to determine its paleo motion. The present position of the island is 10.5°S, 105.5°E. During Eocene its reconstructed position was 30°S latitude. Seismic tomography models have highlighted a low velocity zone beneath the island during Eocene. Geochemically, the two volcanic suites (Upper & Lower) are mostly similar in their major and trace element composition. The majority of localities (WS, EB, and WB) are basanites; where as that from Dolly Beach is basaltic. The Dale's (west coast), are trachyte and appear evolved with high SiO2. They also have low Ba and Sr ~25ppm, whereas those from east coast have 550 - 900 ppm Sr. Despite an age difference of more than 38 Ma, there is no significant difference between Hf, Nd and Pb isotopic signatures of the Upper Volcanic and Lower Volcanic series which show an enriched component, interpreted by Hoernle et.al (2011) to be due to contamination by continental material. This signifies a common homogenous source for a period of more than 40 Ma in contrast to many OIBs. Dale's do have a distinct isotopic character from the Lower Volcanic Series and this signature is still under study. These geochemical analyses from the island broadly lie within those sampled by Hoernle et al. (2011) from their regional study. Here, we are looking at the island in detail. We present geochronological, geophysical and geochemical data from the island which addresses this model using Christmas Island as an example. Ref: Hoernle et al., 2011, Origin of Indian Ocean Seamount Province by shallow recycling of continental lithosphere, Nature Geoscience 4: 883-887

Taneja, R.; O'Neill, C.; Rushmer, T. A.; Jourdan, F.; Blichert-Toft, J.; Turner, S.; Lackie, M. A.

2012-12-01

253

The evolution of bimodal volcanism in NW Anatolia (Turkey): Petrologic and geodynamic implications for the origin of compositional gaps in calc-alkaline and shoshonitic lavas.  

NASA Astrophysics Data System (ADS)

The Aegean province is a site of nearly continuous magmatism since the Early Eocene although the tectonic settings and melt sources of this widespread magmatism appear to have varied through time. NW Anatolia (Turkey) is part of the Aegean extensional province, which is one of the most seismically active and rapidly deforming domains of the Alpine-Himalayan mountain belt. The geological record of the Cenozoic magmatic events in the Aegean province is almost complete in western Anatolia, where both the modern landscape and the surface rocks are predominantly volcanic.Neogene young magmatism in NW Anatolia is associated commonly with NNE-SSW-trending lines of vents and/or fault systems that were also bounding local lacustrine depocenters. Magmatism evolved from all association of medium to high-K calc-alkaline, to shoshonitic to mildly alkaline and alkaline series. The early magmatic pulse in the region is represented by the Oligo-Miocene granitoid plutons and volcanic units . Volcanic rocks of this stage is characterized by medium to high-K calc-alkaline andesite, dacite to rhyolite that are overlain by ignimbrite flows, pumiceous air-fall and ash fall deposits that are intercalated with Lower to Middle Miocene lacustrine rocks and coal seams in NW Anatolia. Following this stage of volcanism, compositionally bimodal volcanism occurred by fissure eruptions and formed small cones in the wide area. The change from large-volume outpourings of intermediate magma to small-volume bimodal volcanism started in the the Early Miocene in the north and Middle Miocene in the south. Basic parental magmas of Early Miocene volcanism were produced from sources related to EM1-type mantle previously modified by subduction, whereas silicic rocks were probably produced through fractional crystallization implying the compositional gap between CA basalt and rhyolite has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Significant crustal component was recognized only in some slightly peraluminous granites and rhyolites with low contents of HFS elements in the south. The younger (Early-Middle Miocene) bimodal volcanism belongs to shoshonitic-mildly alkaline series is represented by transitional basalts, basaltic trachy-andesites and trachytes-phonolites-rhyolites. The ensuing Middle Miocene volcanism produced mildly alkaline lavas that are spatially associated with NNE-trending transtensional fault systems. The Early and Middle Miocene bimodal basic-acid volcanism presents a transitional chemical affinity from calc-alkaline collision related affinity to within plate alkaline series. Sr-Nd isotope data suggest that coexisting mafic and felsic magmas derived from lithospheric mantle source yielding depleted but LILE-enriched compositions, with subsequent contamination. The inferred crustal contamination appears to have been diminished by the Middle Miocene, while the asthenospheric mantle source became more dominant. These findings, combined with the bimodal character of the post-collisional volcanism in the study area, suggest that the geochemical changes in the nature of volcanism from calcalkaline to alkaline through time may have been caused by lithospheric delamination and/or partial convective removal of the subcontinental lithospheric mantle beneath western Anatolia. The geochemical and temporal evolution of Cenozoic magmatism in Western Anatolia clearly shows how the plate tectonic events and the mantle dynamics can be closely in tune with each other during the evolution of orogenic belts. The mantle responds to delamination, and lithospheric tearing swiftly within geological time slices, resulting in whole-scale extension and accompanying magmatism and thereby in the collapse of tectonically and magmatically weakened orogenic crust. The change from large-volume outpourings of intemediate magma to small-volume bimodal volcanism is similar to volcanism occurred in East central Nevada (USA), Carpathian region where magmatism and extension associated in space and time.

Altunkaynak, S.

2009-04-01

254

Comparing pre- and post-chemical abrasion ages for Miocene Peach Springs Tuff zircon from ID-TIMS and SIMS analyses  

NASA Astrophysics Data System (ADS)

The Miocene Peach Spring Tuff (PST) is a voluminous (>600 km3), zoned ignimbrite (trachyte to high-SiO2 rhyolite) that is exposed widely in eastern California, western Arizona, and southernmost Nevada, which was erupted from the Silver Creek caldera in the southwestern Black Mountains, AZ. PST serves as a regionally widespread marker unit and its eruption age has been determined to 18.8 to 18.9 Ma by 40Ar/39Ar methods, when corrected for systematic bias and normalized to the U-Pb system (Renne et al., 2010,). We performed ion-microprobe (SIMS) U-Pb dating of zircon from individual pumice clasts from PST to evaluate the growth history of zircon in the PST magma system. Sectioned, polished zircon from conventional epoxy mounts allows dating of internal growth domains (e.g. cores, interiors, and near-rim), whereas mounting unpolished zircon in indium and analyzing unpolished crystal faces provides a means to selectively sample the final increments of crystal growth (Reid and Coath, 2000). Combining U-Pb ages of unpolished zircon rims with near-rim interior analyses on sectioned grains yields a mean age of ca. 18.3 Ma, whereas ages of cores of sectioned crystals yield a mean of ca. 18.9 Ma. Several zircons have rim and/or core ages that are several hundred thousand years older or younger than these means (up to 1 m.y. total spread), although the uncertainties for individual SIMS ages are 2 to 5% (2 sigma uncertainty). Therefore, the distribution of ages is challenging to resolve. A modest number of the older grains are plausibly recycled antecrysts, but we suspect that the youngest zircons may have experienced Pb-loss. Failure to account for the possibility of inheritance and Pb-loss may lead to erroneous interpretations about crystallization in the PST system. In order to evaluate and mitigate the effects of Pb-loss, we employed the chemical abrasion (CA) technique of Mattinson (2005), which effectively eliminates domains in zircon that have suffered Pb-loss, and removes micro-inclusions that typically contain common Pb. Thermal annealing followed by CA techniques were used for ID-TIMS dating of a sub-set of zircon crystals previously analyzed by SIMS. Prior to TIMS analyses, zircon crystals were imaged by scanning electron microscopy (SEM) to evaluate the effects of CA on crystal domains sampled by SIMS. SEM images reveal that whole portions of crystals were removed by the CA technique, and a heterogeneous pattern of etching that was not confined to specific compositional zones visible in cathodoluminescence. Most of the SIMS sputter pits that yield spurious ages, are associated with etching and/or preferential annealing by the combined annealing and CA technique, suggesting that the young ages relative to the 40Ar/39Ar age may be due to Pb loss. ID-TIMS yields a coherent U-Pb age population of 18.8 Ma, with several older and younger crystals that might reflect xenocrysts, Pb-loss, and/or younger crystallization. In order to maintain spatial resolution and further evaluate the effects of Pb-loss in PST zircon, the annealing and CA-technique will be applied to zircon prior to SIMS dating. References: Reid and Coath, 2000, Geology 28: 443 Renne et al., 2010, GCA 78: 5349

Lidzbarski, M. I.; Mundil, R.; Miller, J. S.; Vazquez, J. A.

2012-12-01

255

Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds  

NASA Astrophysics Data System (ADS)

Occurrence and ClassificationFragments of the Earth's mantle are frequently transported to the surface via volcanic rocks that are dominantly alkaline in nature. These fragments range up to sizes in excess of 1 m across. The term "mantle xenoliths" or "mantle nodules" is applied to all rock and mineral inclusions of presumed mantle derivation that are found within host rocks of volcanic origin. The purpose of this contribution is to review the geochemistry of mantle xenoliths. For detailed petrological descriptions of individual locations and suites, together with their geological setting, the reader is referred to the major reference work by Nixon (1987).Despite peridotite xenoliths in basalts being recognized for several centuries and comparisons being made to lherzolite massifs (Lacroix, 1893), it was not until work on garnet peridotites and diamonds in kimberlites by Fermor (1913) and Wagner (1914) that such xenoliths were conceptually associated with a peridotite zone in the Earth beneath the crust, i.e., the zone that we now identify as the mantle. Mantle xenoliths provide snapshots of the lithospheric mantle beneath particular regions at the time of their eruption and hence are crucial direct evidence of the nature of the mantle beneath regions where no samples have been exposed by tectonic activity. As such, xenoliths are an essential compliment to tectonically exposed bodies of mantle (orogenic peridotites and ophiolites) that occur at plate boundaries (see Chapter 2.04). One obvious contrast between the mantle samples provided by xenoliths and those provided by peridotite massifs is the lack of field relationships available for xenoliths. Other drawbacks include the small size of many xenoliths. This makes accurate estimation of bulk compositions difficult and accentuates modal heterogeneities. The frequent infiltration of the host magma also complicates their chemical signature. Despite these drawbacks, xenoliths are of immense value, being the only samples of mantle available beneath many areas. Because they are erupted rapidly, they freeze in the mineralogical and chemical signatures of their depth of origin, in contrast to massifs which tend to re-equilibrate extensively during emplacement into the crust. In addition, many xenolith suites, particularly those erupted by kimberlites, provide samples from a considerably greater depth range than massifs. Over 3,500 mantle xenolith localities are currently known. The location and nature of many of these occurrences are summarized by Nixon (1987). A historical perspective on their study is given by Nixon (1987) and Menzies (1990a). Mantle xenoliths from any tectonic setting are most commonly described from three main igneous/pyroclastic magma types (where no genetic relationships are implied):(i) Alkalic basalts sensu-lato (commonly comprising alkali basalt-basanites and more evolved derivatives), nephelinites and melilitites.(ii) Lamprophyres and related magmas (e.g., minettes, monchiquites, and alnoites) and lamproites.(iii) The kimberlite series (Group I and Group II or orangeites; Mitchell, 1995).Although mantle xenoliths most commonly occur in primitive members of the above alkaline rocks, rare occurrences have been noted in more evolved magmas such as phonolites and trachytes (e.g., Irving and Price, 1981).To simplify matters and to circumvent the petrographic complexities of alkaline volcanic rocks in general, we will use the term "alkalic and potassic mafic magmas" to include alkalic basalts, nephelinites, melilitites, and lamprophyres. Occurrence of xenoliths in such magmas can be compared to those occurring in kimberlites and related rocks. As a general rule, the spectrum of mantle xenoliths at a given location varies with host rock type. In particular, alkalic and potassic mafic magmas tend to erupt peridotites belonging predominantly to the spinel-facies, whereas kimberlites erupt both spinel and garnet-facies peridotites (Nixon, 1987; Harte and Hawkesworth, 1989).Even within either "group" of volcanic rocks the

Pearson, D. G.; Canil, D.; Shirey, S. B.

2003-12-01

256

Experimental quantification of P-T conditions of mantle refertilisation at shallow depth under spreading ridges and formation of plagioclase + spinel lherzolite  

NASA Astrophysics Data System (ADS)

We studied the first-order melting process of differentiation in the Earth, and the major process of rejuvenation of the upper mantle after melting related to plate spreading (Chalot-Prat et al, 2010; 2013). We conducted experiments at High Pressure (0.75 and 0.5 GPa) and High Temperature (1260-1100°C) to obtain magma compositions in equilibrium with the mineral assemblages of a plagioclase + spinel lherzolite. These PT conditions prevail at 17-30km below axial oceanic spreading ridges. We used a "trial and error" approach in a system involving nine elements (Cr-Na-Fe-Ca-Mg-Al-Si-Ti-Ni). This approaches as closely as possible a natural mantle composition, Cr being a key element in the system. Our objectives were : • to determine experimentally the compositions of melts in equilibrium with plagioclase + spinel lherzolite, with emphasis on the role of plagioclase composition in controlling melt compositions; • to test the hypothesis that MORB are produced at shallow depth (17-30kms) • to quantify liquid- and mantle residue compositional paths at decreasing T and low P to understand magma differentiation by "percolation-reaction" at shallow depth in the mantle; • to compare experimental mantle mineral compositions to those of re-fertilised oceanic mantle lithosphere outcropping at the axis of oceanic spreading ridges, enabling quantification of the pressure (i.e. depth) and temperature of the re-fertilisation process that leads to formation of plagioclase and indicates the minimum thickness of the lithosphere at ridge axes. In the normative basalt tetrahedron, liquids plot on two parallel cotectic lines from silica-oversaturated (basaltic andesite at 0.75 GPa or andesite at 0.5 GPa) at the calcic end to silica-undersaturated compositions (trachyte) at the sodic end. The lower the pressure, the greater the silica oversaturation. Besides the plagioclase solid solution has a dominant role in determining the solidus temperature of plagioclase + spinel lherzolites, at a given pressure at shallow depth. The cotectic lines can be interpreted either as defining partial melting compositions or evolution trends for melts invading and metasomatizing refractory peridotite by reactive porous flow, at 17-30 km. The mismatch between the compositions of the most primitive MORB glasses and the experimental liquids from plagioclase+spinel lherzolites confirms that melt equilibration and segregation of parental MORB occurs deeper than 30km. Mantle mineral compositions are mostly pressure dependent, excluding the co-variance of Na2OCpx and AnPlag, which is pressure independent and enables estimation of AnPlag (if the plagioclase is saussuritised), knowing Na2OCpx of the natural mantle Cpx. Our experimental data are thus tools for estimating mantle re-fertilisation depth in natural plagioclase+spinel lherzolites. The Lanzo plagioclase ± spinel re-fertilised lherzolites (Piccardo et al., 2007) clearly match our 0.75 GPa data, suggesting that re-fertilisation of the oceanic lithosphere occurred at depths of 25 - 30 km below the mid-ocean ridge axis. The most exciting result of this HT-HP experimental work, which imparts even more confidence and significance to our data, is the fit of the five phases of an experimental plagioclase + spinel lherzolite with those of natural plagioclase ± spinel lherzolites. References: Chalot-Prat, Falloon, Green & Hibberson, J. Pet., 51, 11, 2349-2376, 2010; Chalot-Prat, Falloon, Green & Hibberson, Lithos, 172-173, 61-80, 2013. Piccardo, G.B., Zanetti, A., Müntener, O., 2007. Lithos 94, 181-209.

Chalot-Prat, Françoise; Falloon, Trevor J.; Green, David H.

2014-05-01

257

Results of new petrologic and remote sensing studies in the Big Bend region  

NASA Astrophysics Data System (ADS)

The initial section of this manuscript involves the South Rim Formation, a series of 32.2-32 Ma comenditic quartz trachytic-rhyolitic volcanics and associated intrusives, erupted and was emplaced in Big Bend National Park, Texas. Magmatic parameters have only been interpreted for one of the two diverse petrogenetic suites comprising this formation. Here, new mineralogic data for the South Rim Formation rocks are presented. Magmatic parameters interpreted from these data assist in deciphering lithospheric characteristics during the mid-Tertiary. Results indicate low temperatures (< 750 °C), reduced conditions (generally below the FMQ buffer), and low pressures (? 100 MPa) associated with South Rim Formation magmatism with slight conditional differences between the two suites. Newly discovered fayalite microphenocrysts allowed determination of oxygen fugacity values (between -0.14 and -0.25 DeltaFMQ over temperature ranges of 680-700 °C), via mineral equilibria based QUILF95 calculations, for Emory Peak Suite. Petrologic information is correlated with structural evidence from Trans-Pecos Texas and adjacent regions to evaluate debated timing of tectonic transition (Laramide compression to Basin and Range extension) and onset of the southern Rio Grande Rift during the mid-Tertiary. The A-type and peralkaline characteristics of the South Rim Formation and other pre-31 Ma magmatism in Trans-Pecos Texas, in addition to evidence implying earlier Rio Grande Rift onset in Colorado and New Mexico, promotes a near-neutral to transtensional setting in Trans-Pecos Texas by 32 Ma. This idea sharply contrasts with interpretations of tectonic compression and arc-related magmatism until 31 Ma as suggested by some authors. However, evidence discussed cannot preclude a pre-36 Ma proposed by other authors. The later section of this manuscript involves research in the Big Bend area using Google Earth. At present there is high interest in using Google Earth in a variety of scientific investigations. However, program developers have disclosed limited information concerning the program and its accuracy. While some authors have attempted to independently constrain the accuracy of Google Earth, their results have potentially lost validity through time due to technological advances and updates to imagery archives. For this reason we attempt to constrain more current horizontal and vertical position accuracies for the Big Bend region of West Texas. In Google Earth a series of 268 data points were virtually traced along various early Tertiary unconformities in Big Bend National Park and Big Bend Ranch State Park. These data points were compared with high precision GPS measurements collected in field and yielded a horizontal position accuracy of 2.64 meters RMSE. Complications arose in determining vertical position accuracy for Google Earth because default keyhole markup language (.kml) files currently do not export elevation data. This drawback forces users to hand record and manually input elevation values listed on screen. This is a significant handicap rendering Google Earth data useless with larger datasets. However, in a workaround solution exempted elevation values can be replaced from other data sources based on Google Earth horizontal positioning. We used Fledermaus 3D three-dimensional visualization software to drape Google Earth horizontal positions over a National Elevation Dataset (NED) digital elevation map (DEM) in order to adopt a large set of elevation data. A vertical position accuracy of 1.63 meters RMSE was determined between 268 Google Earth data points and the NED. Since determined accuracies were considerably lower than those reported in previous investigations, we devoted a later portion of this investigation to testing Google Earth-NED data in paleo-surface modeling of the Big Bend region. An 18 x 30 kilometer area in easternmost Big Ranch State Park was selected to create a post-Laramide paleo-surface model via interpolation of approximately 2900 Google Earth-NED data points representing sections of an early Tertiary

Benker, Stevan Christian

258

Chemistry of amphiboles and clinopyroxenes from Euganean (NE Italy) cumulitic enclaves: implications for the genesis of melts in an extensional setting  

NASA Astrophysics Data System (ADS)

The magmatism of the Euganean Volcanic District (Veneto Volcanic Province, VVP) developed in the last phases of the Alpine orogenesis; the geochemical and geophysical data are consistent with an extensional geodynamic context (Milani et al., 1999). Cumulitic gabbroic enclaves occur within the Euganean trachytes, and Bartoli et al. (2008) pointed to their cogenetic origin with the Euganean host lavas. Sr isotopic data suggest that these cumulates derived from uncontaminated mantle-derived liquids. We analysed both cumulus and intercumulus amphiboles and clinopyroxenes by electron microprobe and LA-ICP-MS. The cumulus-intercumulus Cpx are diopsides and augites. The Mg#Cpx varies in a wide range (Mg#cumulus-Cpx= 0.74-0.84 and Mg#intercumulus-Cpx= 0.67-0.68). They show a MREE enrichment relative to LREE and HREE (LaN/SmN= 0.46-0.68 and TbN/YbN= 2.18-4.77). No significant Eu anomaly (Eu/Eu* = 0.78-1.23) was observed. On a chondrite-normalized spiderdiagram Cpx exhibits significant Pb and Co negative anomalies, and less evident negative anomalies for Sr and Zr. La, Sm and HREE increase, whereas Ba, Ti, Li and V decrease from core to rim. These Cpx exhibit high Cr contents (701-2958 ppm). Moreover, they display trace element differences when compared to Cpx from MORB gabbros. We analyzed also amphiboles: pargasites, edenites and kaersutites. In the cumulus Amph Mg# varies in the range 0.60-0.69, whereas in the intercumulus assemblage from 0.57 to 0.63. The high K2O and TiO2 contents are distinct from that of amphiboles in MORB gabbros. LREE are enriched relative to HREE (LaN/YbN = 5.07-7.56). Moreover, TbN/YbN = 2.50-4.02 indicates a HREE depletion relative to MREE. REE patterns lack a significant Eu anomaly (Eu/Eu* = 1.06-1.19). From core to rim Th and U decrease in cumulus crystals, but they increase in the intercumulus Amph. Ba (258-282 ppm) is enriched relative to other LILE and Nb-Ta are enriched relative to LREE. Cr varies in the range 423-594 ppm. The similar REE and HFSE content of intercumulus and cumulus Amph may suggest the existence of some post-cumulus processes. We calculated the chemistry of the liquids which should have been in equilibrium with cumulus phases, employing a set of Ds?l. In the liquid in equilibrium with Cpx LREE and MREE are enriched up to 40 and 11 times respectively relative to HREE, which are at about N-MORB concentrations (LaN/YbN = 42.5 and SmN/YbN = 11). Some LILE (i.e., Rb and Ba), Th and U are enriched relative to HFSE and REE. The theoretical composition of the liquid in equilibrium with Amph differs from Cpx-liquid in the marked enrichment of U and Th over LILE and HFSE. A LREE and MREE enrichment is observed (LaN/YbN = 35.4 and SmN/YbN = 3.6). The discrepancies of calculated liquid compositions cannot be ascribed only to the uncertainty in the choice of Ds?l. This may indicate trace element modifications in response to post-cumulus processes involving the amphiboles. The concentrations of HFSE in the calculated liquids (Zr/Hf = 60.2-72.7, Zr/Nb = 1.7-6.5 and Th/Hf = 3.8-6.9) and the incompatible element ratios, (e.g., La/Nb = 0.5-0.7, Pb/Ce = 0.01-0.05, La/Y = 2.3-2.8 and Ce/Nd = 1.9-2.8), are not comparable to those of N-MORB but to those of HIMU-OIB suggesting that typical MORB-type mantle couldn't be the source of these liquids. Nb and Ta are variable, possibly due to an heterogeneity in the lithospheric mantle. The existence of some peculiar trace element signatures of the recalculated liquids (LILE enrichment, high LREE/HREE ratio and abrupt enrichment in U and Th) has been attributed to slab-derived melts/fluids with an abundant sedimentary component. Our estimates are in agreement with the geodynamic scenario proposed by Macera et al. (2007), who explained the occurrence of both HIMU-OIB-type magmatism and subduction-related metasomatism in the VVP mantle lithosphere. According to their model, a mantle plume with HIMU-OIB geochemical signature rose from the deep mantle twice with subsequent partial melting episodes of the plume material: the first time during Paleocene,

Bartoli, O.; Meli, S.; Sassi, R.; Magaraci, D.

2009-04-01