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1

Petrology and geochemistry of augite trachytes and porphyritic trachytes from the Gölcük volcanic region, Isparta, SW Turkey: A case study  

Microsoft Academic Search

The Plio-Quaternary alkaline volcanic rocks at Gölcük, Isparta, SW Turkey, include older porphyritic trachytes, augite trachytes and younger trachyandesitic and tephriphonolitic dikes. In order to better understand their pre-eruptive evolution the geochemical variations of porphyritic trachytes and augite trachytes have been modeled quantitatively using the MELTS algorithm (Ghiorso and Sack, 1995). Their geochemical variations are also compared with those of

Mustafa Kumral; Hakan Coban; Atasever Gedikoglu; Attila Kilinc

2006-01-01

2

Severe leaching of trachytic glass without devitrification, Terceira, Azores  

Microsoft Academic Search

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

James E Mungall; Robert F Martin

1994-01-01

3

Trace-element partitioning in pantellerites and trachytes  

Microsoft Academic Search

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

Gail A Mahood; James A Stimac

1990-01-01

4

Facies characteristics and magma–water interaction of the White Trachytic Tuffs (Roccamonfina Volcano, southern Italy)  

Microsoft Academic Search

The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic,\\u000a pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions\\u000a from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT\\u000a Galluccio. The WTT Galluccio eruption was the largest and

Guido Giordano

1998-01-01

5

Mixing Experiments with Natural Shoshonitic and Trachytic Melts  

NASA Astrophysics Data System (ADS)

Evidence of cyclic replenishment of the shallow magmatic reservoir with deeper alkali basaltic (shoshonitic) magma (Campi Flegrei, in Italy; e.g. Arienzo et al., 2008, Bull. Volc.) motivated this study. Based on previous isotopic data, Agnano-Monte Spina trachyte and Minopoli shoshonite have been chosen as the most suitable end-member melts for simulating magma mixing in this system. Results from different mixing experiments with natural volcanic samples from this region will be presented. For this purpose time series using two different techniques have been performed: 1) a high-temperature centrifuge and 2) a viscometer. For the centrifuge experiments the rotating speed was 1850 revolutions per minute and the acceleration 1000 g. This way, dynamic conditions closer to those calculated for magma chambers (Reynolds Numbers [Re] around 100) could be simulated. For every experiment, a 4 mm thick disk of previously homogenized crystal free shoshonitic glass and an 8 mm thick disk of homogenized crystal free trachytic glass were loaded in a 5mm diameter Pt capsule. The capsule was then sealed on both sides, but for a small opening on the upper end, allowing intersticial degassing during the acceleration. Samples were arranged in a buoyantly unstable geometry, where the denser material is placed at the inner side of the rotating circle (basaltic trachyandesite, ?=2.63 g/cm3 at 1169° C) and the lighter material at the external side (trachyte, ?=2.45 g/cm3 at ~1000°C). Temperature has been kept constant at 1,200° during all experimental runs, with a negligible thermal gradient (<1°C). Forced convection was applied via centrifugal acceleration and density instabilities. Results from three experimental runs with the centrifuge: after 5, 20 and 120 min will be presented and discussed. The second set of experiments consisted of two runs (25- and 168-hours duration) under Taylor-Couette flow, according to De Campos et al. (2008, Chem. Geol.). Higher amounts of the same end-members, in different proportions, have been mixed together using a concentric cylinder viscometer. For the 2nd set of experiments forced convection has been simulated by stirring with a spindle. Experimental conditions were constrained by: 1) constant angular velocity (0.5 rotations per minute) and 2) constant temperature (1,300°C). The experiments terminated by stopping all movement, extracting the spindle from the sample and letting the sample cool to room temperature. Cylinders of the resultant mixed glasses were recovered by drilling and, prepared for microprobe analysis. Microprobe and ICP-MS analyses along longitudinal lines from sections of all the resulting products reveal a complex non-linear mixing process with different mobility for different elements. Chemical data from both experiments (with the centrifuge and the viscometer) will be discussed in a comparative way. Our results highlight the importance of chaotic dynamics for the efficiency of the mixing process in silicate melts.

de Campos, C. P.; Perugini, D.; Kolzenburg, S.; Petrelli, M.; Dorfman, A.; Dingwell, D. B.

2010-12-01

6

Volcanology of trachytic and associated basaltic pyroclastic deposits at Roccamonfina volcano, Roman Region, Italy  

Microsoft Academic Search

The 274 ka “Basalt-Trachytic Tuff of Tuoripunzoli” (TBTT) from Roccamonfina volcano (Roman Region, Italy) consists of a basaltic scoria lapilli fall (Unit A) overlain by a trachytic sequence formed by a surge (Unit B), repetitive pumice lapilli and ash-rich layers both of fallout origin (Unit C) and a pyroclastic flow deposit (Unit D). The TBTT is widespread (40 km2) in

Bernardino Giannetti

1996-01-01

7

Severe leaching of trachytic glass without devitrification, Terceira, Azores  

NASA Astrophysics Data System (ADS)

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 DNa between 1.8 × 10 -9 cm 2 s -1 and 6.1 × 10 -19 cm 2 s -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. We suggest that a simultaneous influx of molecular water and cation exchange between Na + in the glass and H + 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, James E.; Martin, Robert F.

1994-01-01

8

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

9

A basalt-trachyte-phonolite series from Ua Pu, Marquesas Islands, Pacific Ocean  

Microsoft Academic Search

The paper describes a suite of volcanic and intrusive rocks from Ua Pu, one of the Marquesas Islands, situated in the central Pacific Ocean. The rocks comprise alkali olivine basalts, hawaiites, mugearites, trachytes, and phonolites. Their petrographic characters are briefly described and 24 new chemical analyses presented. The rocks fall into a sodic and a potassic series, since some rocks

Arthur Clive Bishop; Alan Robert Woolley

1973-01-01

10

Emplacement of multiple magma sheets and wall rock deformation: Trachyte Mesa intrusion, Henry Mountains, Utah  

Microsoft Academic Search

A detailed structural and rock magnetic study of the Trachyte Mesa intrusion and deformed sedimentary wall rocks, Henry Mountains, Utah, indicates that the intrusion grew vertically and horizontally by the accumulation of multiple horizontal magma sheets. 2–3cm thick shear zones recognized by intensely cataclasized plagioclase phenocrysts define the contact between sheets. Sheets have bulbous and \\/ or steep frontal terminations

Sven Morgan; Amy Stanik; Eric Horsman; Basil Tikoff; Michel de Saint Blanquat; Guillaume Habert

2008-01-01

11

Open System evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya rift  

NASA Astrophysics Data System (ADS)

Suswa is the southernmost volcanic center in the Central Kenya Peralkaline Province (CKPP) and represents the only salic center to have erupted significant volumes of peralkaline silica-undersaturated lavas and tuffs (trachyte, nepheline trachyte and phonolite). The eruptive products of Suswa can be clearly divided into two series, which correspond closely to the volcano's eruptive history. The earlier series (C1) includes lavas and tuffs that built the initial shield volcano (pre-caldera, unit S1) and erupted during the first caldera collapse (syn-caldera, units S2-S5); these rocks are dominated by peralkaline, silica-saturated to mildly under-saturated trachyte. The later series (C2) includes lavas and tuffs that erupted within the caldera structure following the initial collapse (post-caldera, units S6-S7) and during the creation of a second smaller, nested caldera and central "island block" (ring trench group, RTG, unit S8); these rocks are dominated by peralkaline phonolite. In this study, we combine mineralogical evidence with the results of major-element, trace-element, and thermodynamic modelling to propose a complex model for the origin of the Suswa volcano. From these results we conclude that C1 is the result of protracted fractional crystallization of a fairly "dry" alkali basalt (< 1 wt.% H2O) under relatively high pressure (400 MPa) and low oxygen fugacity (FMQ to FMQ-1). Although C1 appears to be primarily the result of closed system processes, a variety of open system processes are responsible for C2. We propose that crystallization of C1 trachyte resulted in the formation of a syenitic residue, which was assimilated (Ma/Mc = 0.1) during a later stage of recharge and differentiation of alkali basalt to produce post-caldera ne-trachyte. Post-caldera (S6-7) phonolites were in turn the result of fractional crystallization of this ne-trachyte. RTG phonolites, however, are the result of feldspar resorption prompted perhaps by magma recharge as evidenced by reverse zoning in alkali feldspar and linear compatible trace element patterns.

White, John Charles; Espejel-García, Vanessa V.; Anthony, Elizabeth Y.; Omenda, Peter

2012-11-01

12

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

13

Dredged trachyte and basalt from kodiak seamount and the adjacent aleutian trench, alaska.  

PubMed

Blocky fragments of aegirine-augite trachyte (with accompanying icerafted gravels.) were recovered from the upper slopes of Kodiak Seamount in several dredge hauls. An alkali basalt pillow segment was also dredged from a moatlike depression, at a depth of 5000 meters, near the west base of the seamount. These retrievals confirm the volcanic origin of Kodiak Seamount and further support the view of Engel, Engel, and Havens that the higher elevations of seamounts are composed of alkali basalts or related variants. PMID:17731907

Forbes, R B; Hoskin, C M

1969-10-24

14

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

NASA Astrophysics Data System (ADS)

Fluororichterite, a rare amphibole species, is observed for the first time in trachyte of the Hoggar volcanic province.The fluorian biotite–fluororichterite association resulted from fluids with high fH2F2/fH2O ratios.Fluid compositions evolved from metaluminous H2O-dominated to peralkaline F-enriched.

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

2013-09-01

15

Phase relations in trachytes: implication for magma storage conditions in the Chaîne des Puys (French Massif Central)  

NASA Astrophysics Data System (ADS)

Trachytes from the Chaîne des Puys, French Massif Central, have been studied by performing phase equilibria in order to (i) constrain the storage conditions of the trachytic magmas that lead to explosive eruptions (either dome destructions as concentrated or diluted pyroclastic density currents or highly explosive events) and (ii) provide phase relationships and compositions for differentiated alkaline magmas. Phase assemblage, proportion, and compositions have been determined on six trachytes (62-69 wt % SiO2 and 10.5-12.0 wt % alkali) mostly coming from the actual domes dated from 9.2 to 15 Ka and aligned along a 10-km distance in the Chaîne des Puys. All samples contain ~23-30 % of phenocrysts, mainly consisting of plagioclase (15-17 % and K-feldspars for the two SiO2-richest samples), biotites (2-6 % except in the SiO2-poorest sample, where it is absent), and Fe-Ti oxides (1-3 %). The three SiO2-poorest samples also contain ~2 % of amphibole and the SiO2-richest one has 1 % of clinopyroxene. All samples have apatite and zircon as minor phases and sphene for the SiO2-richest one. Glasses (melt inclusions and residual glasses) analysed in pumices resulting from highly explosive events, show trachytic to rhyolitic compositions (65-73 wt % SiO2 and 10.5-13.0 wt % alkali). Analyses of melt inclusions (EMP by-difference method) and the biotite+K-feldspar+magnetite hygrobarometer both suggest pre-eruptive H2O contents up to 7-8 wt %, which are so far the highest contents ever reported for alkaline liquids. The melt inclusions also contain ~3400 ppm chlorine, ~700 ppm fluorine, and ~300 ppm sulphur (EMP analyses). Phase equilibria of six representative trachytes have been performed between 200 and 400 MPa, 700-900°C, H2O saturation, and oxygen fugacity from NNO-1 to NNO+1. The comparison between the natural and experimental products suggests magma storage conditions at pressures of 300-350 MPa, temperatures increasing from 700 to 825°C with decreasing bulk SiO2 contents, oxygen fugacity from NNO to NNO+1, and melt H2O contents close to saturation conditions (~8 wt. %). The high H2O contents of the trachytes show that wet conditions prevail during the differentiation of continental alkaline series.

Martel, C.; Champallier, R.; Prouteau, G.; Pichavant, M.; Arbaret, L.; Balcone-Boissard, H.; Boudon, G.; Boivin, P.; Bourdier, J. L.; Scaillet, B.

2012-04-01

16

Cenozoic intra-plate magmatism in the Darfur volcanic province: mantle source, phonolite-trachyte genesis and relation to other volcanic provinces in NE Africa  

NASA Astrophysics Data System (ADS)

Chemical and Sr, Nd and Pb isotopic compositions of Late Cenozoic to Quaternary small-volume phonolite, trachyte and related mafic rocks from the Darfur volcanic province/NW-Sudan have been investigated. Isotope signatures indicate variable but minor crustal contributions. Some phonolitic and trachytic rocks show the same isotopic composition as their primitive mantle-derived parents, and no crustal contributions are visible in the trace element patterns of these samples. The magmatic evolution of the evolved rocks is dominated by crystal fractionation. The Si-undersaturated strongly alkaline phonolite and the Si-saturated mildly alkaline trachyte can be modelled by fractionation of basanite and basalt, respectively. The suite of basanite-basalt-phonolite-trachyte with characteristic isotope signatures from the Darfur volcanic province fits the compositional features of other Cenozoic intra-plate magmatism scattered in North and Central Africa (e.g., Tibesti, Maghreb, Cameroon line), which evolved on a lithosphere that was reworked or formed during the Neoproterozoic.

Lucassen, Friedrich; Pudlo, Dieter; Franz, Gerhard; Romer, Rolf L.; Dulski, Peter

2013-01-01

17

The effect of H 2O on the viscosity of K-trachytic melts at magmatic temperatures  

Microsoft Academic Search

Viscosity of hydrous trachytes from the Agnano Monte Spina eruption (Phlegrean Fields, Italy) has been determined at 1.0 GPa and temperatures between 1200 and 1400 °C using the falling sphere method in a piston cylinder apparatus. The H2O content in the melts ranged from 0.18 to 5.81 wt.%. These high-temperature hydrous viscosities, along with previous ones determined at low-temperature (anhydrous and hydrous) and

Valeria Misiti; Carmela Freda; Jacopo Taddeucci; Claudia Romano; Piergiorgio Scarlato; Antonella Longo; Paolo Papale; Brent T. Poe

2006-01-01

18

Mineralogical constraints on the petrogenesis of trachytic inclusions, Carpenter Ridge Tuff, Central San Juan volcanic field, Colorado  

Microsoft Academic Search

Although bulk-rock normative analyses of the trachytic inclusions from the Carpenter Ridge Tuff yield abundant quartz and minor corundum, a portion of the phenocryst assemblage is indicative of an alkaline parentage. Sanidine and biotite contain up to 8 and 5 wt% BaO respectively. In addition, both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica-undersaturated magma.

Michael J. Dorais I; James A. Whitney; John C. Stormer Jr

1991-01-01

19

Mixing Experiments with Shoshonitic and Trachytic Melts using a High-Temperature Centrifuge and a Viscometer: a comparative study  

NASA Astrophysics Data System (ADS)

Evidence of cyclic replenishment of the shallow magmatic reservoir with deeper alkali basaltic (shoshonitic) magma (Campi Flegrei, in Italy; see Arienzo et al., 2008, Bull. Volcanol.) motivated this study. Results from different mixing experiments using natural volcanic samples from this region will be presented. The end-member melts derive from the Agnano-Monte Spina (trachytic) and Minopoli (shoshonitic) eruptions. Based on previous isotopic data, these are thought to be the most suitable counterparts for simulating the extreme interacting compositions in this system. In order to study the mixing dynamics between these natural magmas, we performed time series of convection-diffusion experiments using two different techniques: 1) a high-temperature centrifuge and 2) a viscometer. For the centrifuge experiments the rotating speed was 1850 revolutions per minute and the acceleration 10 3 g. In this way, dynamic conditions closer to those calculated for magma chambers (Reynolds Numbers [Re] around 102) could be simulated. For every experiment, a 4 mm thick disk of previously homogenized crystal free shoshonitic glass and an 8 mm thick disk of homogenized crystal free trachytic glass were loaded in a 5mm diameter Pt capsule. The capsule was then sealed on both sides, but for a small opening on the upper end, allowing intersticial degassing during the acceleration. Samples were arranged in a buoyantly unstable geometry, where the denser material is placed at the inner side of the rotating circle (basaltic trachyandesite, ?=2.63 g/cm3 at 1169oC) and the lighter material at the external side (trachyte, ?=2.45 g/cm3 at ~1000oC). Temperature has been kept constant at 1,200oC during all experimental runs, with a negligible thermal gradient (< 1°C). Forced convection was applied via centrifugal acceleration and density instabilities. Results from three experimental runs with the centrifuge: after 5, 20 and 120 min will be discussed. The second set of experiments consisted of two experimental runs (25- and 168-hours duration) under Taylor-Couette flow, according to De Campos et al. (2008, Chem. Geol.). Higher amounts of the same end-members, in different proportions, have been mixed together using a concentric cylinder viscometer. For the 2nd set of experiments forced convection has been simulated by stirring with a spindle. Experimental conditions were constrained by: 1) constant angular velocity (0.5 rotations per minute) and 2) constant temperature (1,300° Celsius). The experiments terminated by stopping all movement, extracting the spindle from the sample and letting the sample cool to room temperature. Cylinders of the resultant mixed glasses were recovered by drilling and, prepared for microprobe analysis. Microprobe and ICP-MS analyses along longitudinal lines from sections of all the resulting products reveal a complex non-linear mixing process with different mobility for different elements. Chemical data obtained from both experiments (with the centrifuge and the viscometer) will be discussed in a comparative way.

de Campos, Cristina; Dorfman, Alexander; Perugini, Diego; Kolzenburg, Stephan; Petrelli, Maurizio; Dingwell, Donald B.

2010-05-01

20

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

Microsoft Academic Search

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\\u000a 1,000–5,000 m of basanite surmounted by trachyte and subordinate intermediate rocks, plus phonolite, or pantellerite, or comendite.\\u000a The volumes of felsic sections are large (~30–700 km3), but these rocks probably make up <10% of volcanic

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

21

Geochronology of Mount Morning, Antarctica: two-phase evolution of a long-lived trachyte-basanite-phonolite eruptive center  

NASA Astrophysics Data System (ADS)

Mount Morning is a Cenozoic, alkaline eruptive center in the south-west Ross Sea, Antarctica. New ages on 17 Mount Morning volcanic rocks (combined with 34 existing ages) allows division of Mount Morning volcanism into two phases, erupted between at least 18.7 Ma and 11.4 Ma, and 6.13 and 0.02 Ma. The position of Mount Morning on the active West Antarctic Rift System within the stationary Antarctic plate is a key factor in the eruptive center’s longevity. The earliest, mildly alkaline, Phase I volcanism comprises predominantly trachytic rocks produced by combined assimilation and fractional crystallization processes over 7.3 m.y. Strongly alkaline Phase II volcanism is dominated by a basanite - phonolite lineage, with the youngest (post last glacial maximum) activity dominated by small volume primitive basanite eruptions. The evolution from mildly to strongly alkaline chemistry between phases reflects magma residence time in the crust, the degree of mantle melting, or the degree of magma—country-rock interaction. Phase I magmatism occurred over a comparable area to the present-day, Phase II shield. The 5.2 m.y. volcanic hiatus separating Phase I and II coincides with a cycle of eruption and glacial erosion at the nearby Minna Bluff eruptive center. Mount Morning is the likely source of volcanic detritus in Cape Roberts drill-core (about 24.1 to 18.4 Ma) and in ANDRILL drill-hole 1B (about 13.6 Ma), located 170 km north and 105 km north-east respectively, of Mount Morning. Based upon the timing of eruptions and high heat-flow, Mount Morning should be considered a dormant volcano.

Martin, Adam P.; Cooper, Alan F.; Dunlap, W. James

2010-04-01

22

Time scales of magmatic evolution from a parental alkali basalt to trachytic andesite beneath Rishiri Volcano, Japan  

NASA Astrophysics Data System (ADS)

Timescales of the thermal and chemical evolution of a cooling magma chamber were investigated for alkali basalt lavas (Kutsugata lava) and trachytic andesite lavas (Tanetomi lava) from Rishiri Volcano, northern Japan, by geochronological approach using a 238U-230Th radioactive disequilibrium. Pre-eruption magmatic history of the lavas has been investigated by detailed petrology and geochemistry, and it has been shown that these lavas represent a series of magmas evolved with assimilation and fractional crystallization in the same magma reservoir located at depth corresponding to ~2 kbar pressure. No evidence of magma replenishment is found during the magmatic evolution, except for the replenishment to have triggered the eruption of the Tanetomi lava. They therefore provide an excellent opportunity to investigate a timescale of the magmatic evolution in a cooling magma chamber without any other heat budget. The eruption age of the Kutsugata North lava has been constrained to be >37 ka and <55 ka, by 14C dating and K-Ar dating, respectively, by previous workers. Therefore, the timescales of the magmatic evolution from the basalt to the andesite can be estimated if we know the eruption age of the Tanetomi lava. In this study, the eruption age of the Tanetomi lava was determined utilizing a 238U-230Th radioactive disequilibrium. The age of the Kutsugata lava is also evaluated from a model age, using an assumption that primary magmas have an identical (230Th/232Th) ratio in each volcano, as has been done by previous workers. All the Rishiri samples plot on the left-hand side of the equiline in a (230Th/232Th) - (238U/232Th) diagram. Considering that the depth to the Wadati-Benioff zone is ~300 km at Rishiri Volcano, the 230Th excess signature may reflect that the effect of U addition by fluids from the subducting slab was not so large as to counterbalance the effect of Th enrichment due to melting in the presence of garnet or aluminous clinopyroxene. The (230Th/232Th) and (238U/232Th) ratios of the lava samples show marked variations, and these ratios tend to increase systematically with increasing the whole- rock SiO2 contents, although the variations in the Tanetomi lava are insignificant. This variation is considered to reflect an ageing effect, as well as assimilation and fractional crystallization. The eruption age of the Tanetomi lava was determined from a whole rock isochron, produced by selective fractionation of U during degassing from the solidifying lavas after eruption, and the age of 20.2 ka was obtained. We also obtained the model age of the Kutsugata lava to be 42.8 49.9 ka, using an assumed initial ratio of (230Th/232Th) estimated from the (230Th/232Th) ratio of the most primitive basalt erupted in the latest activity of the volcano. This model age is consistent with the possible eruption age of 37 55 ka estimated by other dating methods. From these results, it is concluded that it took about 20-30 kyr for the parental alkali basalt to have evolved to the andesite by assimilation and fractional crystallization in the magma chamber beneath Rishiri Volcano. The estimated long timescale is consistent with large eruption volume of ~3 km3 of the alkali basalt lavas.

Kuritani, T.; Yokoyama, T.; Nakamura, E.

2006-12-01

23

Tectonic Controls on the Volumes and Petrologic Evolution of Pantellerite-Trachyte-Phonolite Volcanoes in a Continental Rift Setting, Marie Byrd Land, Antarctica  

NASA Astrophysics Data System (ADS)

The 18 alkaline volcanoes in Marie Byrd Land (MBL) are characterized by large volumes of felsic rock and a large range in composition, from trachyte to pantellerite, comendite, and phonolite. These characteristics are controlled largely by mantle plume activity, a stationary plate environment, and lithospheric structure of the West Antarctic rift system in coastal MBL. Felsic rocks occur either as summit sections of basalt volcanoes, or comprising all the rock exposed above ice level. Their exposed volumes range from ~30-780 km3 in individual volcanoes. Seven fall between 200-780 km3. Field observations and one seismic traverse show that felsic sections are underlain by thick (1-5 km) sequences of basalt, dominated by basanite. Thus, in spite of their large volumes, felsic rocks appear to make up only ~10% of all the volcanic rock in the province. In four of these volcanoes, pantellerite, trachyte, and phonolite eruptions alternated with each other, and/or took place coevally from the same edifice, indicating that these magmas were available throughout the ~15 m.y. histories of these volcanoes from isolated, but closely adjacent upper crustal reservoirs. Isotope data record crustal contamination in some felsic rocks, but constrain it to <3%, at most. This, plus the results of major and trace element modeling, imply that pantellerites, trachytes and phonolites all evolved from basanite magma by fractional crystallization (FC). Phonolites could have evolved largely by low-pressure FC of basanite in the upper crust. However, modeling and experimental data suggest that 90-95% of pantellerite evolution took place below the crust, where inclusion of kaersutite among fractionated phases, in a low fO2 environment, were key to developing an FeO-rich and SiO2-rich pantellerite lineage from basanite. The complexity of the felsic suite seems related to the presence of mechanical boundaries at the base of the lithosphere (~50 km) and base of the crust (~25 km), that trapped some basanite magmas within the stability field of kaersutite, but allowed others to rise directly to upper crustal reservoirs, allowing a range of FC schemes to produce diverse felsic rock types. Mantle plume activity has produced syn-volcanic doming and large volumes of basalt magma in MBL over the past ~30 m.y. A stationary plate environment has allowed a continued focus of magma generation, storage, and eruption beneath the same volcanic centers throughout this time. The sum of these tectonic factors has resulted in an environment that was optimal for producing large volumes of felsic rock by FC.

Lemasurier, W. E.

2010-12-01

24

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

25

Anisotropy of Magnetic Susceptibility of a Magma Flow Sheet in the Trachyte Mesa Laccolith of the Henry Mountains in South-Central Utah  

NASA Astrophysics Data System (ADS)

The Trachyte Mesa Laccolith (TML) in the Henry Mountains of south-central Utah is thought to have been formed by the coalescing of numerous horizontal magma sheets that stemmed from the nearby igneous intrusion, Mount Hillers. To understand the flow behavior within a magma sheet, cores (ranging in length from 60 cm to 1.2 m) were cut through the top sheet of the laccolith with a construction grade drill. Individual samples were then cut every 5 cm down the core to obtain a downward gradient of foliation. These samples were cut parallel to lineation and were used for fabric analysis and the collection of Anisotropy of Magnetic Susceptibility (AMS) data. To determine average crystal percentage versus matrix in the sheet, thin sections were made from two samples from the top of the sheet. Each sample was divided into rows and columns and a microphotograph was taken for each section of the grid. The area calculation and crystal analysis of the microphotographs was done using NIH software. The software allowed each plagioclase and oxide crystal to be colored in and deleted from the total area which provided the crystal percentage of each photograph. An average percentage for each row of the thin sections was calculated and determined to be approximately 35 to 40%. In both samples, one row, several cm from the top of the sheet, deviates by 22% less than the average crystal percentage. The row lying directly underneath shows a 22% increase in crystallinity. These changes in crystal content are possibly due to grain dispersive forces that cause crystals to be displaced away from the contact shear zone. This is known as the Bagnold effect and helps to define the plug zone within the sheet where no shearing in occurring. Analysis of the AMS data revealed the bulk magnetic susceptibility (Km) to range from 2.71E-3 to 1.54E-2 SI with the average being 7.69E-3 SI. The Km values appear to be changing at approximately 40 cm down. At this same level the AMS foliation begins to curve from a northwesterly dip to a southeasterly dip in each of the core samples drilled. These data suggest there is a magma sheet contact at this depth. All evidence collected suggests the magma sheet was emplaced as a plug flow. This plug flow behavior is supported by the displacement of the crystal percentages, the curvature in the AMS foliation, and a change in the bulk magnetic susceptibility that corresponds to the change in foliation curvature.

Serwatowski, T. M.; Morgan, S. S.; Tikoff, B.

2008-12-01

26

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

27

Hydrothermal Conversion of Trachytic Glass to Zeolite. 3. Monocationic Model Glasses  

Microsoft Academic Search

Experiments on zeolitization were conducted on four synthetic monocationic glasses (Na, K, Ca, or Mg-rich glass) with Si\\/A1 molar ratios of 2.67, similar in acidity to many volcanic glasses of partially zeolitized Italian tufts. The products of the hydrothermal treatment at 100, 150, and 200~ of single glasses or glass mixtures with deionized H20 or monosaline solutions (NaC1, KC1, CaC12)

Maurizio de' Gennaro; ALESSIO LANGELLA; PIERGIULIO CAPPELLETTI; CARMINE COLELLA

1999-01-01

28

Geochronology of Mount Morning, Antarctica: two-phase evolution of a long-lived trachyte-basanite-phonolite eruptive center  

Microsoft Academic Search

Mount Morning is a Cenozoic, alkaline eruptive center in the south-west Ross Sea, Antarctica. New ages on 17 Mount Morning volcanic rocks (combined with 34 existing ages) allows division of Mount Morning volcanism into two phases, erupted between at least 18.7 Ma and 11.4 Ma, and 6.13 and 0.02 Ma. The position of Mount Morning on the active West Antarctic

Adam P. Martin; Alan F. Cooper; W. James Dunlap

2010-01-01

29

Geochronology of Mount Morning, Antarctica: two-phase evolution of a long-lived trachyte-basanite-phonolite eruptive center  

Microsoft Academic Search

Mount Morning is a Cenozoic, alkaline eruptive center in the south-west Ross Sea, Antarctica. New ages on 17 Mount Morning\\u000a volcanic rocks (combined with 34 existing ages) allows division of Mount Morning volcanism into two phases, erupted between\\u000a at least 18.7 Ma and 11.4 Ma, and 6.13 and 0.02 Ma. The position of Mount Morning on the active West Antarctic Rift System\\u000a within

Adam P. Martin; Alan F. Cooper; W. James Dunlap

2010-01-01

30

Mineralogy and geochemistry of the « Low potassium » series of the roccamonfina volcanic suite (Campania, South Italy)  

Microsoft Academic Search

In the Campania area volcanic rocks belonging both to the trachytic (low K) and tephritic (high K) series occur, the high\\u000a K series being well subordinate to the low K one. Volcanology of the trachytic series shows that: 1) the most widespread products\\u000a are trachytes and alkali-trachytes, 2) in the stratigraphic sequences the most evolved (salic) volcanics are the oldest.

M. R. Ghiara; L. Lirer

1976-01-01

31

Hydrothermal Alteration of Intra-caldera Deposits, Tejeda Caldera, Gran Canaria  

Microsoft Academic Search

The Tejeda caldera, Gran Canaria erupted about 20 rhyolite-trachyte ignimbrites (Mogan Group 14 - 13.25Ma) followed by about 20 phonolitic lava flows and ignimbrites (Fataga Group 12.5 - 7Ma). The Mogan and Fataga ignimbrites (ash-flow tuffs) occur as intra- and extra-caldera deposits. At the caldera margin, late rhyolite-trachyte Mogan tuffs are severely altered but overlain by unaltered Fataga units, implying

V. R. Troll; A. O'Halloran; C. Harris; T. R. Walter

2004-01-01

32

Volatile emission during the eruption of Baitoushan Volcano (China\\/North Korea) ca. 969 AD  

Microsoft Academic Search

3   [magma volume (DRE): 24?±?5?km3]. The main phase (ca. 95?vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and\\u000a proximal ignimbrites. The eruption column is estimated to have reached ca. 25?km and thus entered the stratosphere. A late\\u000a phase (5?vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain ???3,\\u000a and the trachytes

Susanne Horn; Hans-Ulrich Schmincke

2000-01-01

33

Evolution of the Campanian Ignimbrite Magmatic System II: Trace Element and Th Isotopic Evidence for Open-System Processes  

Microsoft Academic Search

The Campanian Ignimbrite, a large volume (~200 km3 DRE) trachytic to phonolitic ignimbrite was deposited at ~39.3 ka and represents the largest of a number of highly explosive volcanic events in the region near Naples, Italy. Thermodynamic modeling of the major element evolution using the MELTS algorithm (see companion contribution by Fowler et al.) provides detailed information about the identity

W. A. Bohrson; F. J. Spera; S. Fowler; H. Belkin; B. de Vivo

2005-01-01

34

Chemical and mineralogic trends within the Timber Mountain-Oasis Valley caldera complex, Nevada: Evidence for Multiple cycles of chemical evolution in a long-lived silicic magma system  

Microsoft Academic Search

Rocks of the Miocene and Pliocene Timber Mountain-Oasis Valley (TM-OV) caldera complex in the southwest Nevada volcanic field are dominantly rhyolites and quartz latites (trachytes). Minor basaltic to dacitic rocks were erupted peripherally to the caldera complex and in the moat of the youngest caldera. We divided the petrologic evolution of the caldera complex into a series of petrochemical cycles

David E. Broxton; Richard G. Warren; Frank M. Byers; Robert B. Scott

1989-01-01

35

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

36

The influence of environmental parameters on CO2 soil diffuse degassing in S. Miguel Island (Azores)  

Microsoft Academic Search

Continuous monitoring of carbon dioxide soil diffuse degassing in S. Miguel Island is being carried out since October 2001. The first station was installed at Furnas volcano caldera in the large degassing area that extends into the Furnas village. Two trachytic eruptions with explosive and effusive phases occurred in this volcano in historical times (1439-1443 and 1630). A second station

F. Viveiros; T. Ferreira; C. Vieira; J. L. Gaspar

2003-01-01

37

Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado  

Microsoft Academic Search

The Guffey volcanic center is the largest within the 2000 km2 mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which represent the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the

Reinhard A. Wobus; David W. Mochel; Elizabeth A. Eide; Miriam T. Rothwarf; Bruce M. Loeffler; David A. Johnson; Gordon N. Keating; Kimberly Sultze; Anne E. Benjamin; Edward A. Venzke; Tammy Filson

1990-01-01

38

Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado  

Microsoft Academic Search

The Guffey volcanic center is the largest within the 2000 km² mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which present the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the

Reinhard A. Wobus; David W. Mochel; Elizabeth A. Eide; Miriam T. Rothwarf; Bruce M. Loeffler; David A. Johnson; Gordon N. Keating; K. Sultz; Anne E. Benjamin; Edward A. Venzke; Tammy Filson

1990-01-01

39

A Suevite in Black and White: SEM Study on the Samples from the El'gygytgyn Drill Core  

NASA Astrophysics Data System (ADS)

The El'gygytgyn structure, N-E Siberia (Russia), is the only impact crater on Earth in rhyolitic-trachytic volcanic rocks; it provides a unique opportunity to improve our knowledge of shock metamorphism at the microscopic scale in such a target.

Pittarello, L.; Koeberl, C.

2012-03-01

40

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

Microsoft Academic Search

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

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

2007-01-01

41

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

42

Chlorine-Bearing Phases in the Campanian Ignimbrite, Italy  

Microsoft Academic Search

The Campanian volcanic zone, located west of Naples, Italy, has been active for more than 300 ky. The Campanian ignimbrite, a trachyte-phonolite pyroclastic deposit, erupted at 39 ka with an estimated volume > 200 km3 and spread predominantly eastward overlapping the nearby western Apennines. Recent studies have distinguished a complex stratigraphy of at least two cooling units with variable degrees

H. E. Belkin; B. de Vivo; A. Lima

2006-01-01

43

Age of Fault Movements in Tanzanian Sector of East African Rift System  

Microsoft Academic Search

THE Neogene volcanic province of northern Tanzania is a southerly extension of the more extensive volcanic areas of Ethiopia and Kenya. It stands astride the Eastern Rift Valley and represents a complex interplay of volcanic activity and widespread Earth movements with associated faulting. A general picture is that an older series of basaltic-trachytic shield volcanoes, together with smaller nephelinitic centres,

R. M. MacIntyre

1974-01-01

44

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

45

Tectonic controls on the genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy  

USGS Publications Warehouse

The Campanian Plain is an 80 x 30 km region of southern Italy, bordered by the Apennine Chain, that has experienced subsidence during the Quaternary. This region, volcanologically active in the last 600 ka, has been identified as the Campanian Volcanic Zone (CVZ). The products of three periods of trachytic ignimbrite volcanism (289-246 ka, 157 ka and 106 ka) have been identified in the Apennine area in the last 300 ka. These deposits probably represent distal ash flow units of ignimbrite eruptions which occurred throughout the CVZ. The resulting deposits are interstratified with marine sediments indicating that periods of repeated volcano-tectonic emergence and subsidence may have occurred in the past. The eruption, defined as the Campanian Ignimbrite (CI), with the largest volume (310 km3), occurred in the CVZ 39 ka ago. The products of the CI eruption consist of two units (unit-1 and unit-2) formed from a single compositionally zoned magma body. Slightly different in composition, three trachytic melts constitute the two units. Unit-1 type A is an acid trachyte, type B is a trachyte and type C of unit-2 is a mafic trachyte. The CI, vented from pre-existing neotectonic faults, formed during the Apennine uplift, Initially the venting of volatile-rich type A magma deposited the products to the N-NE of the CVZ. During the eruption, the Acerra graben already affected by a NE-SW fault system, was transected by E-W faults, forming a cross-graben that extended to the gulf of Naples. E-W faults were then further dislocated by NE-SW transcurrent movements. This additional collapse significantly influenced the deposition of the B-type magma of unit-1, and the C-type magma of unit-2 toward the E-SE and S, in the Bay of Naples. The pumice fall deposit underlying the CI deposits, until now thought to be associated with the CI eruption, is not a strict transition from plinian to CI-forming activity. It is derived instead from an independent source probably located near the Naples area. This initial volcanic activity is assumed to be a precursor to the CI trachytic eruptions, which vented along regional faults.

Rolandi, G.; Bellucci, F.; Heizler, M. T.; Belkin, H. E.; De Vivo, B.

2003-01-01

46

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

47

Procida volcanic history: new insights into the evolution of the Phlegraean Volcanic District (Campania region, Italy)  

Microsoft Academic Search

New stratigraphic, major- and trace-element, and Sr-, Nd- and Pb- isotopic data on volcanic deposits older than 14 ka from the island of Procida, Italy, are presented and compared with published analyses from the rest of the Phlegraean Volcanic District (PVD). Procida rocks range in composition from basalt to shoshonite and trachyte and show 87Sr\\/ 86Sr, 143Nd\\/ 144Nd, 206Pb\\/ 204Pb, 207Pb\\/

Gianfilippo De Astis; Lucia Pappalardo; Monica Piochi

2004-01-01

48

Belmore Volcanic Province, northeastern New South Wales, and some implications for plume variations along Cenozoic migratory trails  

Microsoft Academic Search

The little known Belmore Volcanic Province, near Baryugil, northeast New South Wales, is a half-eroded structure, 21 – 19.5 Ma in age. The remnants occupy 140 km and suggest an original volcanic field ?14 km in volume. The eruptive sequence, fed by dykes, plugs and compound centres ranges up to 350 m thick. Sodic, metaluminous trachytes predominate and include ne and qtz normative types, with Na2O\\/K2O

F. L. Sutherland; I. T. Graham; H. Zwingmann; R. E. Pogson; B. J. Barron

2005-01-01

49

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

50

The roles of fractional crystallization, magma mixing, crystal mush remobilization and volatile-melt interactions in the genesis of a young basalt-peralkaline rhyolite suite, the greater Olkaria volcanic complex, Kenya Rift valley  

USGS Publications Warehouse

The Greater Olkaria Volcanic Complex is a young (???20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro-syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ?? corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt-trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. ??? 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing. ?? The Author 2008. Published by Oxford University Press. All rights reserved.

MacDonald, R.; Belkin, H. E.; Fitton, J. G.; Rogers, N. W.; Nejbert, K.; Tindle, A. G.; Marshall, A. S.

2008-01-01

51

A Sr, Nd, Hf, and Pb isotope perspective on the genesis and long-term evolution of alkaline magmas from Erebus volcano, Antarctica  

Microsoft Academic Search

We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling

Kenneth W. W. Sims; Janne Blichert-Toft; Philip R. Kyle; Sylvain Pichat; Pierre-Jean Gauthier; Jurek Blusztajn; Peter Kelly; Lary Ball; Graham Layne

2008-01-01

52

Isotopic evidence for open system processes within the Campanian Ignimbrite (Campi Flegrei–Italy) magma chamber  

Microsoft Academic Search

New Sr and Nd isotope data for whole rocks, glasses and minerals are combined to reconstruct the nature and origin of mixing\\u000a end-members of the 200 km3 trachytic to phonolitic Campanian Ignimbrite (Campi Flegrei, Italy) magmatic system. The least-evolved magmatic end-member\\u000a shows equilibrium between host glass and the majority of the phenocrysts and is less radiogenic in Sr and Nd than

I. Arienzo; L. Civetta; A. Heumann; G. Wörner; G. Orsi

2009-01-01

53

Geochronology of Gran Canaria, Canary Islands: Age of shield building volcanism and other magmatic phases  

Microsoft Academic Search

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic\\u000a rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km3 was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (?100 km3) of silicic volcanics (trachyte

I. McDougall; H.-U. Schmincke

1976-01-01

54

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

Microsoft Academic Search

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

Stevan Christian Benker

2010-01-01

55

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

56

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian craton, Brazil  

Microsoft Academic Search

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts

Ronaldo Pierosan; Evandro Fernandes de Lima; Lauro Valentin Stoll Nardi; Artur Cesar Bastos Neto; Cristina P. de Campos; Kym Jarvis; José Maximino Tadeu Miras Ferron; Mauricio Prado

2011-01-01

57

Interaction between caldera collapse and eruptive dynamics during the Campanian Ignimbrite eruption, Phlegraean Fields, Italy  

Microsoft Academic Search

The Campanian Ignimbrite (36000 years B.P.) was produced by the explosive eruption of at least 80 km3 DRE of trachytic ash and pumice which covered most of the southern Italian peninsula and the eastern Mediterranean region. The eruption has been related to the 12-x15-km-diameter caldera located in the Phlegraean Fields, west of Naples. Proximal deposits on the periphery of the

M. Rosi; L. Vezzoli; P. Aleotti; M. De Censi

1996-01-01

58

Volcanic eruptions from ghost magma chambers  

Microsoft Academic Search

Recent studies have proposed that magma reservoirs crystallized to a virtually rigid crystal-mush can be partially remelted by diffusion of hot fluids. We show that for a crystal mush with the composition of a K-trachyte from the Campanian Ignimbrite (CI) Eruption, remelting can occur without a significant increase of the magma temperature, but simply by diffusion of H2O by the

Raffaello Trigila; Maurizio Battaglia; Gianluca Sottili; Mauro Brilli

2008-01-01

59

Tephrochronology of core C from Lago Grande di Monticchio  

Microsoft Academic Search

Tephrochronological studies of sediment from Lago Grande di Monticchio have begun with detailed electron microprobe analyses\\u000a of two thickest tephra layers found in Core C. A total of 76 analyses of both layers reveals them to be of trachytic composition,\\u000a of the type erupted from the Campi Flegrei during the last 35,000 years. LGM C41, the lower layer, is a

Anthony J. Newton; Andrew J. Dugmore

60

Lava stones from Neapolitan volcanic districts in the architecture of Campania region, Italy  

Microsoft Academic Search

Results of a research carried out on the lavas from Campi Flegrei and Somma-Vesuvius volcanic districts are reported here.\\u000a The lavas have been widely employed, since Roman age, in several important monumental buildings of the Campania region, mainly\\u000a in the town of Naples and in its province. They are classified as trachytes (Campi Flegrei products), tephri-phonolites and\\u000a phono-tephrites (Somma-Vesuvius complex)

Alessio Langella; Domenico Calcaterra; Piergiulio Cappelletti; Abner Colella; Maria Pia D’Albora; Vincenzo Morra; Maurizio de Gennaro

2009-01-01

61

Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands  

Microsoft Academic Search

The Miocene Tejeda caldera on Gran Canaria erupted ~20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites,

Eleanor Donoghue; Valentin R. Troll; Chris Harris; Aoife O'Halloran; Thomas R. Walter; Francisco J. Pérez Torrado

2008-01-01

62

Eruption Mechanism of the 10th Century Eruption in Baitoushan Volcano, China/North Korea  

NASA Astrophysics Data System (ADS)

Baitoushan volcano, China/North Korea, is one of the most active volcanoes in Northeastern Asia, and the 10th century eruption was the most voluminous eruption in the world in recent 2000 years. The sequence of the eruption reconstructed recently consists mainly of 6 units of deposits (Miyamoto et al., 2004); plinian airfall (unit B), large pyroclastic flow (unit C), plinian airfall with some intra- plinian pyroclastic flows (unit D), sub-plinian airfall (unit E), and large pyroclastic flow (unit F) with base surge (unit G) in ascending order. The magma erupted during steady eruption in earlier phase was comendite (unit B-C; Phase 1), whereas the magma during fluctuating eruptions in later phase is characterized by trachyte to trachyandesite with various amount of comendite (unit D-G; Phase 2). The wide variety in composition and occurrence of banded pumices strongly indicate mixing or mingling of the two magmas just prior to or during the eruption. The initial water contents had been determined for comendite by melt inclusion analyses (ca. 5.2 wt.%; Horn and Schmincke, 2000). Although the initial water content of the trachytic magma has not been correctly determined yet, the reported water contents of trachytic melt inclusions are lower (3-4 wt.%) than those of comenditic melt (Horn and Schmincke, 2000). We investigated juvenile materials of the eruption sequentially in terms of vesicularity, H2O content in matrix glass and textural characteristics. The vesicularity of pumices are generally high (>0.75) for all units. The residual water contents of the comenditic pumices during Phase 1 are relatively uniform (1.6 wt.%), whereas those of the trachytic scoria during Phase 2 and gray pumices during Phase 1 are low (ca. 0.7-1.3 wt.%). These facts may indicate that the difference in the initial water content, rather than the ascent mechanism of magma, controls the steadiness or fluctuation in eruption styles and the mass flux during the eruption.

Shimano, T.; Miyamoto, T.; Nakagawa, M.; Ban, M.; Maeno, F.; Nishimoto, J.; Jien, X.; Taniguchi, H.

2005-12-01

63

Trace element distribution coefficients in alkaline series  

Microsoft Academic Search

Mineral\\/groundmass partition coefficients for U, Th, Zr, Hf, Ta, Rb, REE, Co and Sc have been systematically measured in olivine, clinopyroxene, amphibole, biotite, Ti-magnetites, titanite, zircon and feldspars, in basaltic to trachytic lavas from alkaline series (Velay, Chaîne des Puys: Massif Central, France and Fayal: Azores). Average partition coefficients are denned within the experimental uncertainty for limited compositional ranges (basalt-hawaiite,

Fabienne Lemarchand; Benoît Villemant; Georges Calas

1987-01-01

64

Jebel Marra, a dormant volcano in Darfur Province, Western Sudan  

Microsoft Academic Search

The geology of the Tertiary-Recent Marra mountains is shown on photogeological maps and described using data from short field\\u000a visits and from previous literature. The 200 km long mass is aligned N-S parallel to basement trends; it comprises a stratified\\u000a complex 2000 m thick of olivine basalts, pyroclastics and trachytes. Coarse detrital deposits testify to concomitant erosion.\\u000a The spectacular Deriba

J. R. Vail

1972-01-01

65

The pliocene volcanism of the voras Mts (Central Macedonia, Greece)  

Microsoft Academic Search

Major element chemistry, K\\/Ar ages and trace element data are reported for volcanic rocks from the Voras volcanic complex\\u000a of Central Macedonia (Greece). Petrological data show that the Voras volcanic rocks consist essentially of intermediate members\\u000a of the high-K calc-alkaline and shoshonitic series, the most abundant rock types being high-K andesites and dacites, latites\\u000a and trachytes. K\\/Ar ages determined on

N. Kolios; F. Innocenti; P. Manetti; A. Peccerillo; O. Giuliani

1980-01-01

66

The relationship between calc-alkaline volcanism and within-plate continental rift volcanism: evidence from Scottish Palaeozoic lavas  

Microsoft Academic Search

Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial 87Sr\\/86Sr (0.7029-0.7046), high ?Nd (-0.4 to

Pauline L. Smedley

1986-01-01

67

Quality assessment of replacement stones for the Cologne Cathedral: mineralogical and petrophysical requirements  

Microsoft Academic Search

Owing to its long building history, different types of building stones comprised the construction of the Cologne Cathedral.\\u000a Severe damage is observed on the different stones, e.g., sandstones, carbonate, and volcanic rocks, especially when the different\\u000a stone materials neighbor the medieval “Drachenfels trachyte” from the “Siebengebirge”. The question arises, “Is the insufficient\\u000a compatibility of the implemented building materials causatively related

B. Graue; S. Siegesmund; B. Middendorf

2011-01-01

68

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

69

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

70

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

71

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

72

Mafic Clasts with Unusual REE Patterns in Felsic Volcanic Rocks: Evidence of Subsolidus Alteration of Mafic Rocks from Ascension Island, South Atlantic Ocean  

NASA Astrophysics Data System (ADS)

Incompatible trace element abundances and patterns, particularly those of the rare earth elements (REE), have traditionally been used in igneous petrogenesis as indicators of processes relating to crystal fractionation, partial melting, and characteristics of mantle source regions of ocean island basalt (OIB) suites. However, unusual features of REE patterns, e.g. negative Ce anomalies, have been reported from basalt (island arc and OIB) and even from mantle peridotite. Mafic clasts hosted in trachyte from Ascension Island show unusual distribution of REE. Ascension Island lava flows and pyroclastic deposits are transitional to mildly alkaline and are a continuous fractionation series of basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite. The major element variations from basalt to trachyte conform to those expected from crystal fractionation of the observed phenocryst phases from basalt parent magma. The mafic clasts in trachyte have the same compositional range (~47 to 55 wt.% SiO2) from basalt to hawaiite, mugearite, and benmoreite with the majority of the clasts collected having a mugearitic composition. The mafic lava flows and pyroclastic deposits have been divided into four distinct genetically identifiable groups based on trace element and isotopic characteristics: 1) high Zr/Nb (5.6 to 6.1) basalt; 2) Dark Slope Crater type (Zr/Nb of 4.9 to 5.4) hawaiite; 3) low Zr/Nb (4.1) hawaiite; 4) intermediate Zr/Nb (4.7 to 5.4) basalt-hawaiite-mugearite-benmoreite. The mafic clasts are geochemically heterogeneous with compositions varying from the high Zr/Nb basalt to the intermediate Zr/Nb hawaiite-mugearite-benmoreite. Oblong shape and uniformly smooth nature of the mafic clasts point to mechanical abrasion. The felsic magmatism that brought the clasts to the surface could not have mechanically abraded them into such shapes. Hence the rocks the clasts were derived from must have been abraded by mechanical weathering. Some of the mafic clasts also have extremely high abundance and anomalous distributions of rare-earth elements (REE) and yttrium (Y), whereas other incompatible trace element concentrations and O, Sr, Nd, and Pb isotopic ratios do not differ from those of mafic (basalt to benmoreite) flows. The possibility of inheriting the REE from the host trachyte is not likely as the REE enrichment is different in different clasts. Also, the REE enrichment in the mafic clasts is commonly much greater than in the host trachyte. REE and Y enrichment in the clasts with respect to the mafic flows is a post-magmatic alteration feature. This is supported by negative Ce anomalies in these mafic clasts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotopic ratios between the clasts and the mafic flows preclude the possibility that additional REE and Y are derived from marine sediment or guano, but rather suggest an origin under subsolidus conditions perhaps from interaction of mafic rocks with meteoric water circulating through the volcanic edifice.

Kar, A.; Weaver, B. L.; Davidson, J.; Ghazi, A. M.

2001-12-01

73

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

NASA Astrophysics Data System (ADS)

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.

Calvert, Andrew T.; Moore, Richard B.; McGeehin, John P.; Rodrigues da Silva, Antonio M.

2006-08-01

74

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

75

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

76

Dike injection and magma mixing in Kenya rift volcanoes  

NASA Astrophysics Data System (ADS)

A nexus of volcanoes in the rift graben at approximately the latitude of Nairobi consist of central vent trachyte, phonolite, and peralkaline rhyolite and cinder cone and fissure-fed flows of basalt to benmoreite. The volcanoes are referred to as the Central Kenya Peralkaline Province (CKPP, Macdonald and Scaillet, 2006, Lithos 91, 59-73) and formed by a combination of processes including fractional crystallization, magma mixing, and volatile transport (Ren et al., 2006, Lithos 91, 109-124; Macdonald et al., 2008, JPet 49, 1515-1547). This presentation focuses on magma mixing for trachytes and phonolites for Suswa rocks, which are the southernmost part of the CKPP. We also explore the contribution of magma process studies to the interpretation of recent geodetic data, which indicate inflation/deflation of up to 21 cm for Kenyan volcanoes from 1997 to present (Biggs et al., 2009, Geology, in press). Incontrovertible evidence for magma mixing is found in field evidence, where a basaltic trachyandesite ash horizon is found interbedded with syncaldera trachyte (Skilling, 1993, J. Geol. Society London 150, 885-896), hand-specimen and thin-section petrography, and disequilibrium mineral chemistry. Precaldera lavas contain a homogeneous group of anorthoclase crystals with An content 6% or less. Syncaldera samples contain this same group and two other populations: polysynthetic twinned labradorite and andesine and anorthoclase with An content of 17%. Textures for all three groups indicate disequilibrium. Postcaldera flows contain the high and low An anorthoclase populations but lack the polysynthetic twinned labradorite and andesine. These observations suggest a model of injection of mafic magmas via diking into shallow trachtytic magma systems. Recent geodetic studies of dike injection and subsequent seismic/volcanic activity in both Ethiopia and Lengai point to the ongoing importance of these processes to rift evolution in East Africa.

Anthony, E. Y.; Espejel, V.; Biggs, J.

2009-12-01

77

Major, trace element and Sr isotopic composition of lavas from Vico volcano (Central Italy) and their evolution in an open system  

NASA Astrophysics Data System (ADS)

Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974 0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.

Barbieri, M.; Peccerillo, A.; Poli, G.; Tolomeo, L.

1988-08-01

78

Assessment of a shallow magmatic system: the 1888 90 eruption, Vulcano Island, Italy  

NASA Astrophysics Data System (ADS)

The magmatic system feeding the last eruption of the volcano La Fossa, Vulcano Island, Italy was studied. The petrogenetic mechanisms controlling the differentiation of erupted rocks were investigated through petrography, mineral chemistry, major, trace and rare earth element and Sr, Nd and Pb isotopic geochemistry. In addition, melt inclusion and fluid inclusion data were collected on both juvenile material and xenolithic partially melted metamorphic clasts to quantify the P T conditions of the magma chamber feeding the eruption. A regular and continuous chemical zoning has been highlighted: rhyolites are the first erupted products, followed by trachytes and latites, whereas rhyolitic compositions were also found in the upper part of the sequence. The chemical and isotopic composition of the rhyolites indicates that they originated by fractional crystallization from latitic magmas plus the assimilation of crustal material; the trachytes represent hybrid magmas resulting from the mixing of latites and rhyolites, contaminated in the shallow magmatic system. The erupted products, primarily compositionally zoned from latites to rhyolites, are heterogeneous due to syn-eruptive mingling. The occurrence of magma crust interaction processes, evidenced by isotopic variations (87Sr/86Sr = 0.70474±3 to 0.70511±3;143Nd/144Nd = 0.512550±6 to 0.512614±8;206Pb/204Pb = 19.318 19.489;207Pb/204Pb=15.642 15.782;208Pb/204Pb = 39.175 39.613), is confirmed by the presence of partially melted metamorphic xenoliths, with 87Sr/86Sr = 0.71633±6 to 0.72505±2 and 143Nd/144Nd = 0.51229±7, in rhyolites and trachytes. AFC calculations indicate a few percentage contribution of crustal material to the differentiating magmas. Thermometric measurements on melt inclusions indicate that the crystallization temperatures of the latites and trachytes were in the range of 1050 1100° C, whereas the temperature of the rhyolites appears to have been around 1000° C at the time of the eruption. Compositional data on melt inclusions reveal that the magmas involved in the eruption contained about 1 1.5 wt.% dissolved H2O in pre-eruptive conditions. Secondary fluid inclusions found in metamorphic xenoliths give low equilibration pressure data (30 60 MPa), giving the location of the higher portions of the chamber at around 1500 2000 m of depth.

Clocchiatti, R.; Moro, A. Del; Gioncada, A.; Joron, J. L.; Mosbah, M.; Pinarelli, L.; Sbrana, A.

1994-12-01

79

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

80

Using Glaciovolcanic Processes and Products at the Mount Edziza Volcanic Complex (MEVC), British Columbia, Canada to Constrain Paleo-Ice Conditions: Initial Results  

NASA Astrophysics Data System (ADS)

The Mount Edziza Volcanic Complex (MEVC) of the northern cordillieran volcanic province (NCVP) erupted basaltic, trachytic and rhyolitic effusive and explosive products in a wide variety of subaerial and subaqueous environments from about 8Ma to about 2000yrs BP. Souther (1992) suggested that several eruptive products were emplaced in a sub-ice or ice-contact environment; an interpretation confirmed by our investigations in 2006. Glaciovolcanic rocks at the MEVC are significant because they record evidence of ice presence and thickness in a region for which there is very little data on ice conditions prior to the most recent glaciation Several areas of likely basaltic and trachytic glaciovolcanic products were examined in detail on the western side of MEVC, in order to confirm their glaciovolcanic nature, and to ultimately better constrain paleo-ice presence and thickness. Our initial results presented here are focused on documenting the processes and products in detail. For example, a sequence of basaltic subaqueous lavas erupted from Tennana Cone, displayed spectacular evidence for the role of sub-ice bedrock topography on the geomorphology and structure of such lava flows. Proximal flows were emplaced in a sheet-like form as both pillow and lobate flows with mounding of pillow lavas at the crest of sub-ice cliffs (ie at contact with thicker ice). Pillow lavas were emplaced in sinuous drainage tunnels at the base of the cliffs. Trachytic lava flows and domes form an important part of the MEVC sequence and many also display evidence of ice-contact or sub-ice emplacement. In particular, Triangle Dome on the northwestern side of the MEVC displays a pattern of columnar cooling joints that indicates that it was emplaced as several very steep-sided domes in a sub-ice environment. Geochronology, geochemistry and volatile analysis of basaltic and trachytic glass from these and several other localities are currently ongoing, and with detailed study of the products, will help us constrain the paleo-ice thicknesses during the Pleistocene at the MEVC. We hope that the methods we develop for constraining paleo-ice conditions can be used at other glaciovolcanic centers and perhaps become part of a standard palaeoclimatological toobox for terrestrial environments

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

2006-12-01

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

Origin of Large Felsic Rock Volumes in Marie Byrd Land Volcanoes:Possible Influence of Tectonic Environment and Continental Lithospheric Structure  

NASA Astrophysics Data System (ADS)

The Marie Byrd Land volcanic province, in the West Antarctic rift system, includes several volcanoes with large volumes of peralkaline trachyte, phonolite, and rhyolite. The roughly 780 cu. km. of trachyte at Mt. Takahe volcano, for example, is about twice the volume of Mt. Shasta, the largest Cascade volcano.Geochemical data provide evidence for only minor amounts of crustal contamination, affecting just a small proportion of the felsic rocks, hence crustal assimilation is unlikely to have supplemented the volume of felsic rocks. Most appear to have been derived entirely by fractional crystallization of basaltic magma; but the volumes of felsic rock in Marie Byrd Land volcanoes is much larger than is found in their counterparts in oceanic islands, produced by essentially the same process, suggesting that continental structure and/or tectonic environment may have played a role in felsic rock evolution. The absence of Antarctic plate motion, over at least the past 26 m.y., seems an obvious factor. This has apparently led to repeated replenishment of the same magma chambers, rather than formation of linear chains of volcanoes. In addition, modeling results suggest that a multi-level, polybaric plumbing system within the lithosphere has acted as a filter, delaying the rise of at least some large volume magma batches, thereby prolonging and refining the fractionation process.

Lemasurier, W. E.

2002-12-01

83

Xenopumices from the 2011-2012 submarine eruption of El Hierro (Canary Islands, Spain): Constraints on the plumbing system and magma ascent  

NASA Astrophysics Data System (ADS)

Textures, petrography and geochemical compositions of products emitted during the onset of the 2011-2012 submarine eruption (15 October, 2011) off the coast of El Hierro have been investigated to get information on interaction mechanism between the first rising magma and the crust during the onset of the eruption as well as to get information on magma storage and plumbing systems beneath El Hierro volcano. Studied products consist of 5-50 cm bombs with an outer black to greenish, vesicular crust with bulk basanite composition containing pumiceous xenoliths (xenopumices). Our results show that xenopumices are much more heterogeneous that previously observed, since consist of a macro-scale mingling of a gray trachyte and white rhyolite. We interpreted xenopumices as resulting from the interaction (heating) between the basanitic magma feeding the eruption, a stagnant trachytic magma pocket/s and an associated hydrothermally altered halo with rhyolitic composition. Our findings confirm the importance of the study of the early products of an eruption since they can contain crucial information on the plumbing system geometry and the mechanism of magma ascent.

Meletlidis, S.; Di Roberto, A.; Pompilio, M.; Bertagnini, A.; Iribarren, I.; Felpeto, A.; Torres, P. A.; D'Oriano, C.

2012-09-01

84

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

85

Mineralogy, geochemistry and petrogenesis of the recent magmatic formations from Mbengwi, a continental sector of the Cameroon Volcanic Line (CVL), Central Africa  

NASA Astrophysics Data System (ADS)

The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62 wt.% SiO2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031 < (87Sr/86Sr)initial < 0.7043; 1.03 < ?Ndi < 5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2 Ma, almost similar to 27.40 ± 0.6 Ma K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL.

Mbassa, Benoît Joseph; Njonfang, Emmanuel; Benoit, Mathieu; Kamgang, Pierre; Grégoire, Michel; Duchene, Stephanie; Brunet, Pierre; Ateba, Bekoa; Tchoua, Félix M.

2012-11-01

86

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

87

Petrography, geochemistry and geodynamic environment of potassic alkaline rocks in Eslamy peninsula, northwest of Iran  

NASA Astrophysics Data System (ADS)

Eslamy peninsula, 360 km2 in area, is located in the eastern coast of Urmieh lake in the northwest of Iran. This peninsula is a complex stratovolcano with a collapsed center, which is elevated due to later intrusions of sub-volcanic masses with trachytic to microsyenitic composition. The composite cone consists of a sequence of leucite tephrite, tephrite, leucite basanite, basanite and related pyroclastic rocks. Magmatic activities in the Eslamy peninsula begin with potassic alkaline to ultrapotassic and basic, silica-undersaturated shoshonitic rocks and they are followed by intrusions of lamprophyric dykes and end with acidic magmatism including trachytic, microsyenitic, syenitic and phonolitic domes. The original magma of the Eslamy peninsula rocks has a potassic alkaline nature (Roman type) rich in LREE and LILE and depleted of HREE. These characteristics suggest that the origin of magma can be from deep mantle with a garnet lherzolite composition, a low partial melting rate which has been contaminated by crustal materials in its way up. Fractional crystallization of olivine, diopsidic clinopyroxene and leucite played an important role in the evolution of magmas. Scrutinizing the geodynamic environment of Eslamy peninsula rocks in discrimination diagrams indicates that these rocks must have been formed in a post-collision magmatic arc setting.

Hajalilou, B.; Moayyed, M.; Hosseinzadeh, Gh.

2009-12-01

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

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 and the internal textural evolution of Tertiary sills and laccoliths in the Henry Mountains. We have carried out kinematic and geometrical studies of emplacement-related structures in the host rocks. This work is supplemented by micro-scale textural and geochemical studies of plagioclase feldspar and amphibole phenocryst populations within the intrusions. 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 Entrada Formation sandstone. Preliminary results highlight the importance of shear zones in accommodating the extra volume of magma at depth. Trachyte Mesa is an elongate (NE-SW) laccolith comprised of multiple, stacked intrusive sheets. Semi-brittle shear fabrics (Riedel shear fractures) can be identified on the top surface of the intrusion. Furthermore, sub-horizontal shear zone fabrics can also be observed adjacent to the frontal propagating tip of individual intrusive sheets, e.g. at the northwest lateral margin of Trachyte Mesa. A well-developed shear zone was also identified above the Maiden Creek intrusion. Maiden Creek is a sill with a complex elliptical shape and several finger-like lobes. Detailed outcrop studies across two neighbouring lobes have identified a sub-horizontal shear zone which may be traced from the top of each intrusive lobe. This shear zone separates low/ moderately-deformed sandstones above from highly deformed sandstones below and between the two lobes, hence acting as a detachment zone. Fabrics (stretched plagioclase phenocrysts) within the igneous rock, seen on the top surface of the intrusive lobes directly beneath this shear zone, support the timing of the shear zone being contemporaneous with emplacement of the intrusive lobes. The shear zone appears to have played a critical role in accommodating the volumetric changes associated with magma emplacement.

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

2012-04-01

90

An overview of long-lived, peralkaline glacivolcanism within the northern Cordilleran volcanic province  

NASA Astrophysics Data System (ADS)

The Neogene-to-Recent northern Cordilleran volcanic province (NCVP) comprises >50 separate volcanic occurences whose morphologies and deposits record interaction between volcanism and ice. NCVP glacivolcanism includes a broad range of alkaline rock types, from basanite to phonolite, and several different glacial setting (alpine, local ice sheets, regional ice sheets). Although many of the smaller volcanoes erupted beneath the Cordilleran ice sheet, the larger and longer-lived volcanic complexes are high enough in elevation to be associated with local ice caps in addition to valley glaciers. Of the three largest volcanic centers in the NCVP, Mount Edziza#1 and Hoodoo Mountain show a variety of peralkaline glacivolcanic deposits; predominantly trachytic to phonolitic at Hoodoo, but covering a broader compositional spectrum from trachyte to peralkaline rhyolite at Edziza. The third, Level Mountain#2, is probably older and is more heavily dissected than the other two, but also contains trachytic deposits that appear to be glacivolcanic in origin. Taken together these three complexes form a distinct peralkaline magmatic subprovince within the NCVP, referred to as the ``Stikine subprovince'', whose glacivolcanic deposits appear to be unique in the North American Cordillera. Massive and fragmental glacivolcanic deposits at these centers have a variety of distinctive characteristics. Massive deposits include domical shaped lava bodies covered by pervasive, small-diameter cooling joints, and lava flows up to 200m in thickness that have been interpreted as ice-dammed. Fragmental deposits interpreted as having formed beneath thicker ice tend to be non-vesicular, whereas those interpreted as having formed under thinner ice locally have glassy, pumiceous clasts in highly altered, ash-rich matrices. Other distinctive deposits show lobate lava bodies totally enclosed by volcanic breccias with aphanitic clasts. None of the peralkaline deposits described to date contain pillow-like structures. Given the peralkaline characteristics of the Stikine subprovince, it is ideal for geochronology studies aimed at refining the Pleistocene record of glacivolcanism in the North American Cordillera. Hoodoo preserves a detailed chronology of glacivolcanic interaction associated with the ebb and flow of at least local ice over the last 80 k.y., whereas at Edziza and Level Mountain have the potential documenting local and Cordilleran-wide glacivolcanism. Efforts are currently underway to try and provide a more detailed record of glacivolcanism from Edziza. #1Souther, J.G. 1992. The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia, GSC Memoir 420, 320 p. #2Hamilton, T.S. 1981. Late Cenezoic Alkaline Volcanics of the Level Mountain Range, Northwestern British Columbia: Geology, Petrology, and Paleomagnetism [Ph.D. Dissert.], Edmonton, University of Alberta, p. 490.

Edwards, B. R.

2005-12-01

91

Age of Ko??ko Seamount, Emperor Seamount chain  

USGS Publications Warehouse

KAr ages obtained by the conventional isotope-dilution and the 40Ar/39Ar techniques on two sanidine trachytes, four basalts, and a phonolite dredged from the top of Ko??ko Seamount, 300 km north of the Hawaiian-Emperor bend, show that the seamount is 46.4 ?? 1.1 my old. These data indicate that the volcanoes in the Hawaiian-Emperor chain continue to increase in age to the west and north beyond Midway Atoll, as predicted by the melting-spot hypothesis for the origin of the chain, and that the rate of volcanic migration along the chain was nonlinear between the time of formation of the island of Hawaii and Ko??ko Seamount. ?? 1973.

Clague, D. A.; Brent, Dalrymple, G.

1973-01-01

92

Radiocarbon dates for lava flows and pyroclastic deposits on Sao Miguel, Azores  

USGS Publications Warehouse

We report 63 new radiocarbon analyses of samples from Sao Miguel, the largest island in the Azores archipelago. The samples are mainly carbonized tree roots and other plant material collected from beneath 20 mafic lava flows and spatter deposits and from within and beneath 42 trachytic pyroclastic flow, pyroclastic surge, mudflow, pumice-fall and lacustrine deposits and lava flows. One calcite date is reported. These dates establish ages for 48 previously undated lava flows and pyroclastic deposits, and revise three ages previously reported. These data are critical to deciphering the Holocene and late Pleistocene eruptive history of Sao Miguel and evaluating its potential volcanic hazards. Average dormant intervals during the past 3000 years are about 400 years for Sete Cidades volcano, 145 years for volcanic Zone 2, 1150 years for Agua de Pau volcano and 320 years for Furnas volcano. No known eruptions have occurred in volcanic Zone 4 during the past 3000 years. -from Authors

Moore, R. B.; Rubin, M.

1991-01-01

93

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

94

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

95

The Breccia Museo formation, Campi Flegrei, southern Italy: Geochronology, chemostratigraphy and relationship with the Campanian Ignimbrite eruption  

USGS Publications Warehouse

The Breccia Museo is one of the most debated volcanic formations of the Campi Flegrei volcanic district. The deposit, made up of six distinctive stratigraphic units, has been interpreted by some as the proximal facies of the major caldera-forming Campanian Ignimbrite eruption, and by others as the product of several, more recent, independent and localized events. New geochemical and chemostratigraphical data and Ar - Ar age determinations for several units of the Breccia Museo deposits (???39 ka), correlate well with the Campanian Ignimbrite-forming eruption. The chemical zoning of the Breccia Museo deposits is interpreted here to be a consequence of a three-stage event that tapped a vertically zoned trachytic magma chamber. ?? Springer-Verlag 2008.

Fedele, L.; Scarpati, C.; Lanphere, M.; Melluso, L.; Morra, V.; Perrotta, A.; Ricci, G.

2008-01-01

96

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

97

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

98

Relation of peralkaline magmatism to heterogeneous extension during the Middle Miocene, southeastern Nevada  

USGS Publications Warehouse

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17-14 Ma) as smaller volumes of rhyolite-trachyte-basalt suites were erupted from the relatively small Kane Springs Wash caldera complex in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15-13.5 Ma) with peralkaline magmatism (14.7-14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post-middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. -from Authors

Scott, R. B.

1995-01-01

99

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.

100

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

101

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

102

Chemically diverse, episodic volcanism offshore southern and peninsular California  

SciTech Connect

Volcanic rocks recovered from eight seamounts offshore southern and peninsular California are chemically diverse. Compositions of lavas from seven small to moderately sized seamounts between 30.5[degree] and 34.0 N latitudes include low-K[sub 2]O tholeiitic, transitional, and mildly to moderately alkalic basalt. Volcanic rocks from the upper part of the much larger and morphologically complex edifice of Rocas Alijos, offshore central Baja California at about 25.0 N latitude, are highly differentiated trachyandesite and trachyte. The low-K[sub 2]O basalts are MORB-like with low abundances of incompatible elements and lower [sup 87]Sr/[sup 86]Sr and higher [sup 143]Nd/[sup 144]Nd ratios than MORB from the East Pacific Rise. The alkalic compositions have higher abundances of incompatible elements and isotopic data indicate more variably enriched mantle sources than those of seamounts near the East Pacific Rise, but the compositions of all samples are within the mantle array defined by other ocean-island basalts. [sup 40]Ar/[sup 39]Ar laser fusion ages for the seamounts span a large range. MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust ([approximately]23 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger (16.6--9.1 Ma) than the underlying oceanic crust. The trachytes from Rocas Alijos are less than 300,000 years old, indicating that the last volcanism on this large edifice occurred recently. The region offshore southern and peninsular California is tectonically complex and has many volcanic edifices of varying sizes, shapes, and orientations. The data available for volcanic rocks from this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism occurring sporadically from early Miocene to Recent.

Davis, A.S.; Gunn, S.H. (Geological Survey, Menlo Park, CA (United States)); Bohrson, W.A. (Univ. of California, Los Angeles, CA (United States). Dept. Earth and Space Sciences)

1993-04-01

103

Geochemical investigation of magma evolution in Campi Flegrei volcanic field (Italy) based on melt inclusions  

NASA Astrophysics Data System (ADS)

Melt inclusions are a powerful tool for understanding the evolutionary history (crystallization, volatile evolution, mixing, etc.) of magmas. They are also useful to interpret the conditions of primary melt generation and extraction. In particular the volatile content in magmas (e.g. H2O, CO2) is of critical importance in determining the eruptive style and magma evolution, because degassing is usually one of the major phenomena before and during an eruption. In populated and active volcanic areas such as Campi Flegrei (Italy), understanding the role of volatile content in magmas is a necessary requirement to assess volcanic risks. The goal of our project is to define the geochemical parameters that control the internal dynamics of magmatic systems of this active volcanic area. The eruptive products of Campi Flegrei were selected for study based on age, eruptive characteristic, mineralogy, chemical compositions and structural position of eruptive centers, in order to verify the existence of a possible change in plumbing systems between different eruptions. Here we report preliminary data on latitic-to-trachytic products from Fondo Riccio eruption. Compositions of the inclusions and host phenocryst were obtained by electron microprobe techniques. The compositions of melt inclusions in pyroxenes vary from trachybasalt to trachite, in those Mg-rich diopside are less evolved ( ranging from trachybasalt to shoshonite) than in Fe-rich diopside (from latite to trachyte). The compositions of inclusions in Fo 85-87 olivine are similar to those in Mg-rich pyroxenes. Homogenization temperatures were obtained by microthermometric experiments and range from 950 to 1000° C for feldspars, from 1030 to 1080° C for pyroxenes and from 1070 to 1120° C for olivines. Volatile contents of melt inclusions, based on electron microprobe analyses, are < 2 wt %. The chemical composition of minerals and melt inclusions, as well thermometric data, are consistent with a continuos crystallization process of an evolving magma body.

Cannatelli, C.; Bodnar, R. J.; de Vivo, B.; Fedele, L.; Lima, A.; Piochi, M.

2004-12-01

104

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

105

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

106

Shield volcanoes of Marie Byrd Land, West Antarctic rift: oceanic island similarities, continental signature, and tectonic controls  

NASA Astrophysics Data System (ADS)

The Marie Byrd Land volcanic province is largely defined by 18 large (up to ~1,800 km3) alkaline shield volcanoes, each surmounted by a summit section of varied felsic rocks dominated by trachytic flows. They are distributed over a 500 × 800-km block-faulted dome within the West Antarctic rift. The basement contact of volcanic sections is ~500 masl at one site and 3,000 mbsl at another, 70 km away, which illustrates the scale of block faulting but complicates an understanding of volcanic structure. Furthermore, the continental ice sheet buries 16 volcanoes to progressively greater heights inland. However, five are sufficiently exposed to allow meaningful comparisons with alkaline oceanic island volcanoes; these comparisons are used as a guide to estimate the structure of Marie Byrd Land volcanoes. The type example for this study is Mt. Murphy, the most completely exposed volcano. It consists of a 1,400-m section of alkaline basalt overlain by trachyte and benmoreite flows that make up ~7-13 % of the volcano volume. In gross structure and composition, Mt. Murphy is similar to Gran Canaria volcano, Canary Islands, but the percent of felsic rock may be three times that of Gran Canaria, if the estimate is approximately correct. Departures from the oceanic island example are believed to represent the imprint of the Marie Byrd Land lithosphere and tectonic environment on volcano evolution. These include a lack of order in the sequence of felsic rock types, lack of progression toward more silica undersaturated compositions with time, absence of a highly undersaturated mafic resurgent stage, and perhaps, a relatively large volume of felsic rock.

LeMasurier, Wesley

2013-06-01

107

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

108

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

109

Exceptional Volumes of Rejuvenated Volcanism in Samoa  

NASA Astrophysics Data System (ADS)

The internal structure of within-plate volcanoes is typically compared to the stages of volcanic evolution in Hawaii. In Samoa, these stages show some differences with the Hawaiian model, in terms of the duration, volume and geochemical composition of the stages. Particularly, the rejuvenated stage of volcanism in Samoa is significantly more voluminous, with increasing geographic coverage with age, completely repaving the island of Savai’i. This unusual outpouring of rejuvenated lavas has previously been proposed to be related to the tectonic setting, near the northern terminus of the Tonga Trench. Therefore, Samoan volcanism might be caused by lithospheric fracturing, a mantle plume, or potentially a combination of the two. We collected new samples from a deeply eroded canyon on Savai’i to determine a time evolution of the transition from shield to eventual rejuvenated lavas. The canyon exposes several hundred meters of lavas, and we collected samples about 200m vertically down into the canyon. These samples are dominantly olivine basalts, and their Pb isotope compositions fall within the compositional field of young rejuvenated lavas on Savai’i and Upolu. This canyon section, therefore, represents a minimum thickness for the rejuvenated lavas of 200m. Assuming eruption of rejuvenated lavas only occurred subaerially, with a universal thickness of 200m, the new data suggest more than one percent of the volume of Savai’i consists of rejuvenated lavas. This is an order of magnitude greater than the largest relative volumes in Hawaii (Kauai), and implies a different cause for rejuvenated volcanism in Samoa. Another feature that suggests different processes may be important is the transition between the shield and rejuvenated stage. Although Samoan volcanoes do not seem to erupt exactly the same rock types as characteristic Hawaiian post-shield stage lavas, there is a definite shift to more evolved compositions (including trachytes) during the later stages of Samoan shield volcanism, as revealed on the islands of Upolu and Tutuila. On Savai’i, a previously collected trachyte cobble, combined with several evolved plagioclase-phyric lavas collected as cobbles for this study, suggest that this transitional stage is exposed in the deepest parts of the canyon. Interestingly, a number of these evolved samples define an intermediate trend in Pb isotopes between the rejuvenated lavas and Fagaloa shield lavas on Upolu. This suggests the source composition shifted at the end of shield volcanism when more evolved rocks were erupted. The age of the trachyte cobble suggests this shift occurred around 2 Ma, implying the shield stage lasted 3 Ma, given the age (5 Ma) of recently dredged Savai’i samples. Therefore, the shield stage may have lasted longer than typical Hawaiian shields, the post-shield stage did not erupt all expected rock types, and the rejuvenated lavas erupted an unusually large volume.

Konter, J. G.; Jackson, M.; Storm, L.

2010-12-01

110

The late MIS 5 Mediterranean tephra markers: a reappraisal from peninsular Italy terrestrial records  

NASA Astrophysics Data System (ADS)

We present new tephrostratigraphic records from the late MIS 5 (ca 110-80 ka) terrestrial sediments from southern and central Italy. On the one hand, the central Italy record consists of an outcropping lacustrine sequence from the Sulmona intermountain basin that contains four trachytic-phonolitic tephra layers (POP3, POP2a, POP2b, POP1), all of which show a K-alkaline affinity that is typical for the Roman co-magmatic Province. The POP3 and POP1 layers were dated by 40Ar/39Ar method at 106.2 ± 1.3 ka (2?) and 92.4 ± 4.6 ka (2?), respectively. The sequence in southern Italy, on the other hand, is represented by post-Tyrrhenian coastal deposits of the Cilento area, Campania, which contain two trachytic layers (CIL2, CIL1) that show the same K-alkaline affinity. Based on their chemical compositions and radiometric ages, POP3 and POP1 are firmly correlated with the marine tephra layers X-5 (105 ± 2 ka) and C-22 (ca 90 ka), which, in turn, match tephras TM-25 and TM-23-11, respectively, in the lacustrine sequence of Lago Grande di Monticchio (southern Italy). Of note, the POP1 layer also matches the Adriatic Sea tephra PRAD 2517 that was previously correlated with the older X-5 layer. The tephra couplet POP2a and POP2b (ca 103 and 103.5 ka, extrapolated ages) are compatible with the TM-24b and TM-24-3 tephras in Monticchio, which match both the stratigraphic positions and the chemical compositions. In the Cilento area, as well as the already described X-6 layer (ca 108 ka) (CIL2), we recognise a new stratigraphic superimposed layer (CIL1) that matches the POP3/TM-25/C-27/X-5 Mediterranean marker(s). In summary, the data presented here provide new chemical and 40Ar/39Ar chronological constraints towards a robust late MIS 5 tephrostratigraphy of the central Mediterranean, although at the same time, they also reveal how the tephrostratigraphy itself might be flawed when dealing with tephra markers that are not adequately constrained and characterised.

Giaccio, Biagio; Nomade, Sebastien; Wulf, Sabine; Isaia, Roberto; Sottili, Gianluca; Cavuoto, Giuseppe; Galli, Paolo; Messina, Paolo; Sposato, Andrea; Sulpizio, Roberto; Zanchetta, Giovanni

2012-11-01

111

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

112

Bubble Nucleation in Hydrous Magmas: The Impact of Melt Composition, Water Content, and Temperature  

NASA Astrophysics Data System (ADS)

When magma rises toward the surface, water - the most abundant volatile dissolved in most magmas - exsolves and forms bubbles, because its solubility in magma decreases with lower pressure. The kinetics of bubble nucleation has been linked to the eruptive behavior of magma, and may even control whether eruptions are violently explosive or effusively passive. Assuming bubbles nucleate homogeneously, both the onset and rate of their nucleation are controlled mainly by the degree to which water is supersaturated in the melt (?P) and the surface tension (?) between the bubble and its host melt. In fact, ? is critical, because even small changes in ? lead to large differences in ?P needed to trigger nucleation. Despite the importance of ? in controlling bubble nucleation, and thus possibly in determining the eruptive behavior of volcanoes, we lack predictive models of ?, and even know little about how it varies between magmas. From what data exist, we know that dissolved water content impacts ?, as does temperature and melt composition. To expand the database of measured values of ?, we have carried hydrothermal decompression experiments on hydrous rhyolite, pantellerite, and trachyte. Those melts were first hydrated at high pressure and temperature, typically for 5 days, and then decompressed to lower pressure within a few seconds (hence, instantaneous decompression), and then the melt was quenched rapidly after spending some fixed amount of time at the new lower pressure. Bubble number densities are measured in 3D, and divided by time to estimate nucleation rate. From those rates as a function of decompression, we then estimate the value of ?. Our preliminary results suggest that, at 900° C, pantellerite (highly per-alkaline rhyolite) with 6.2 wt.% dissolved water has a ? of 0.063 N/m and trachyte with 4.7 wt.% water has a ? of ~0.078 N/m. Our results on rhyolite reveal that at constant water content (~5 wt.%) ? decreases about 20% as temperature increases from 775° to 875° C, roughly equal to the impact of isothermally increasing water content by a weight percent. When we combine our results with published surface tensions determined by similar methods we find that ? correlates, albeit weakly, with melt viscosity, suggesting that the structure of hydrous melt may play an important role in determining surface tension. Indeed, we find that ? correlates with the concentration of structural modifiers in the melt, modified for water content and temperature; that same modified parameter correlates strongly with melt viscosity. These preliminary findings suggest that surface tension may be predictable by knowing magma composition and temperature. We plan to expand our database by carrying out further decompression studies using different melt compositions, water contents, and temperatures.

Gardner, J. E.; Lewis, A. E.

2009-12-01

113

Evolution of the Hawaiian Mantle Plume: Shield and Rejuvenescent Magmatism at Middle Bank, the Youngest Sunken Hawaiian Volcano  

NASA Astrophysics Data System (ADS)

Post-shield volcanism provides unique insight into the structure of mantle plumes and the magmatic processes responsible for the evolution of ocean islands. Middle Bank is the closest seamount to the main Hawaiian archipelago, thus providing a perspective into the processes related to the dying phase of a Hawaiian volcano. We conducted a detailed survey of the volcano in 2007 using multibeam sonar coupled with Jason2 ROV imaging and sampling. According to plate tectonic models, Middle Bank volcano should be about 9.6 Ma, if it formed near the present location of Kilauea. Middle Bank is 100 km in diameter and rises nearly 5000 m from base level. Its morphology is dominated by three major rift zones that emanate to the east, west, and south from the beveled summit platform. The rifts are separated by talus fans, and the volcano is surrounded by dozens of satellite cones. Many of the satellite cones are covered by remarkably unsedimented lavas that were erupted in the submarine environment, which we interpret as a rejuvenated stage of volcanism. Most of the sampled rocks are strongly alkaline and range from basanite to hawaiite and trachyte. Samples from two sites are tholeiitic, which is consistent with them forming during the shield stage of volcanism. If so, then most of the late history of volcanism, from shield building to rejuvenated volcanism is preserved at Middle Bank. The alkaline basalts and basanites have La/Sm and La/Yb ratios that are higher than the tholeiites, and all of the rocks are strongly LREE enriched. Major and trace element compositions of hawaiites and trachytes are consistent with large amounts of crystal fractionation, which especially affected magmas erupted on the outer flanks of the volcano. The tholeiites have Sr/Nb and Zr/Nb that suggest that the Middle Bank shield is akin to the modern-day "Kea" trend geochemically. Thus, Middle Bank has preserved the archetypical tholeiitic-shield to alkaline-rejuvenated evolutionary stages that characterize the subaerial Hawaiian volcanoes.

Geist, D.; Garcia, M.; Ito, G.; Harpp, K.; Weis, D.

2008-12-01

114

Topographic Attributes of Three Hawaiian Lava Flows: Implications for Evaluation of Lava Flow Emplacement on Mars  

NASA Astrophysics Data System (ADS)

Differential Global Positioning System surveys were carried out recently across portions of three lava flows on the Big Island of Hawaii. Transects crossed an entire flow in several cases, and in other cases provided detailed information about selected flow margins. The 1907 basalt (a'a) flow from the southwestern rift zone of Mauna Loa has easy access at several points via the Ocean View Estates road system; flow thickness ranges from about 1 m near the middle of the eastern flow lobe to more than 10 m toward the distal end of this flow. Several components of a benmoreite (alkali-rich basaltic andesite) flow complex from Mauna Kea were examined near the small community of Mana (with permission of the Parker Ranch management), on the western flank of the volcano. The flows are more than 14,000 years old and completely covered with soil more than a meter thick, but flow morphology at the decameter scale remains very evident in aerial photographs; some benmoreite flows have up to 30 m of relief along their middle reaches. A trachyte flow more than 100,000 years old extends down slope from Puu Waawaa, on the northern flank of Hualalai; Puu Anahulu represents a very advanced stage of magmatic differentiation that resulted in a flow complex with more than 120 m of relief at its southern margin. Width/thickness represents a good discriminator between these three Hawaiian lava flows. Unfortunately, width is often the most difficult parameter to measure remotely for flows on other planets. Recent imaging data from the Thermal Emission Imaging System on the Mars Odyssey spacecraft reveal important new details of lava flows in the Tharsis region of Mars, some of which can be combined with elevation information from the Mars Orbiter Laser Altimeter. The precise topographic characteristics of diverse Hawaiian lava flows provide a new tool for evaluating the potential emplacement conditions for some Martian lava flows, which appear to be more consistent with the basalt to basaltic andesite lava flows than with the highly evolved trachyte flows. Future work, supported by a grant from the NASA Planetary Geology and Geophysics Program, will obtain additional precise topographic information for several Hawaiian flows to expand the topographic data set for comparison with the Martian flows, as well as lava flows on other planetary bodies.

Zimbelman, J. R.

2004-12-01

115

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

116

The Averno 2 Eruption (campi Flegrei Caldera, Italy): Influence of Structural Setting On Magma Evolution and Eruption History  

NASA Astrophysics Data System (ADS)

The Averno 2 eruption (Av2 - 3.7 ka) is a low-magnitude explosive event which, to- gether with Averno 1 (4.5 ka) and Mt. Nuovo (1538 AD), is one of the only three erup- tions occurred in the NW sector of the Campi Flegrei caldera (CFc). Their vents were located at the intersection of NE-SW and NW-SE fault systems, bordering to the NW the most uplifted block of the resurgent caldera floor. The eruption included magmatic and phreatomagmatic explosions which produced fallout and surge deposits. 6 fallout layers dominate the lower portion of the eruption sequence, while surge beds prevail upsection. These beds are mostly wet in the central, and dry in the uppermost part of the sequence. Magmatic explosions prevailed only during the first phase of the erup- tion generating columns 10 km high, at most. The fifth erupted fallout deposit contains a greater amount of lithic clasts relatively to the earlier beds. Isopachs and isopleths maps, and areal distribution of ballistic clasts, suggest that during this stage of the eruption the vent migrated from SW towards the centre of the present crater, likely along the NE-SW fault system. Arrival of significant amount of water into the sys- tem, favoured by intense fracturing, produced explosions driven by variably efficient water-magma interaction with generation of wet to dry surges. The magma emitted during this eruption was about 0.05 km3 (DRE). The erupted rocks are aphyric to sub- aphyric, with up to 3% vol. of plagioclase, alk-feldspar, biotite and clinopyroxene. The rocks range from first erupted weakly peralkaline alkali-trachyte (DI about 90) to last erupted alkali-trachyte (DI about 85). Trace element and isotopic variations suggest that during the eruption two distinct magma layers were tapped. The most evolved, uppermost magma layer was tapped during the prevailing magmatic, first phase; the least evolved, lowermost magma layer was extruded during the phreatomagmatic last phase, whereas mingled magma fed the phreatomagmatic intermediate phase of the eruption. The slightly peralkaline character of the Av2 rocks as well as those of the other volcanoes active in the W sector of the CFc over the past 5 ka, suggests that only in this sector small batches of magmas can stagnate at shallow depth for a time long enough to drive their chemical character toward peralkalinity.

D'Antonio, M.; di Vito, M. A.; Braia, G.; Carroll, M.; Civetta, L.; Isaia, R.; Orsi, G.; Piermattei, M.

117

Activity/composition relations in the ternary feldspars  

NASA Astrophysics Data System (ADS)

Activity/composition relations are presented for high-structural state feldspars whose bulk compositions lie within the ternary system NaAlSi3O8 CaAl2Si2O8-KAlSi3O8. The expressions are parameterized from the data for coexisting feldspars of Seck (1971a) using an asymmetric regular solution approximation for the excess Gibbs free energy of mixing and an Al-avoidance model for the configurational entropy of solution. The solution properties of the plagioclase and alkali-feldspar binaries have been made to conform to the recent work of Thompson and Hovis (1979) and Newton et al. (1980). Using the proposed model the ternary feldspar solvus is extrapolated in temperature (up to 1,500° C) and pressure (up to 5kbars). A new two-feldspar geothermometer is presented which provides somewhat more reasonable estimates of crystallization temperatures than the equations and graphs of Stornier (1975), Powell and Powell (1977), Brown and Parsons (1981) and Haselton et al. (1983). In conjunction, some criteria are suggested for establishing the existence of “equilibrium” tie-lines between coexisting ternary feldspars in rhyolites and trachytes. Calculated values of the activity of KAlSi3O8 in plagioclase are examined in some detail. These compare favorably with independent estimates obtained from experimentally grown plagioclases precipitating at liquidus temperatures from igneous rocks of widely varying alkali contents.

Ghiorso, Mark S.

1984-09-01

118

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

119

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

120

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

121

The contribution of synchrotron X-ray computed microtomography to understanding volcanic processes.  

PubMed

A series of computed microtomography experiments are reported which were performed by using a third-generation synchrotron radiation source on volcanic rocks from various active hazardous volcanoes in Italy and other volcanic areas in the world. The applied technique allowed the internal structure of the investigated material to be accurately imaged at the micrometre scale and three-dimensional views of the investigated samples to be produced as well as three-dimensional quantitative measurements of textural features. The geometry of the vesicle (gas-filled void) network in volcanic products of both basaltic and trachytic compositions were particularly focused on, as vesicle textures are directly linked to the dynamics of volcano degassing. This investigation provided novel insights into modes of gas exsolution, transport and loss in magmas that were not recognized in previous studies using solely conventional two-dimensional imaging techniques. The results of this study are important to understanding the behaviour of volcanoes and can be combined with other geosciences disciplines to forecast their future activity. PMID:20157274

Polacci, Margherita; Mancini, Lucia; Baker, Don R

2010-02-02

122

Dyella jiangningensis sp. nov., a ?-proteobacterium isolated from the surface of potassium-bearing rock.  

PubMed

A Gram-stain-negative, aerobic, motile with one polar flagellum ?-proteobacterium, designated strain SBZ3-12(T), was isolated from surfaces of weathered potassic trachyte. Phylogenetic analysis of this strain based on 16S rRNA gene sequences showed that it was most closely related to Dyella japonica XD53(T) (97.9?% 16S rRNA gene sequence similarity), Dyella terrae JS14-6(T) (97.7?%), Dyella soli JS12-10(T) (97.5?%) and Dyella koreensis BB4(T) (97.0?%). The DNA G+C content of strain SBZ3-12(T) was 64.0 mol%. In addition, iso-C17?:?1?9c, iso-C15?:?0 and iso-C16?:?0 were the major cellular fatty acids and ubiquinone Q-8 was the predominant respiratory quinone. The low DNA-DNA relatedness values between strain SBZ3-12(T) and recognized species of the genus Dyella and the many phenotypic properties supported the classification of strain SBZ3-12(T) as a representative of a novel species of the genus Dyella, for which the name Dyella jiangningensis sp. nov. is proposed. The type strain is SBZ3-12(T) (?=?CCTCC AB 2012160(T)?=?KACC 16539(T)?=?DSM 26119(T)). PMID:23435246

Zhao, Fei; Guo, Xin-Qi; Wang, Peng; He, Lin-Yan; Huang, Zhi; Sheng, Xia-Fang

2013-02-22

123

Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado  

SciTech Connect

The Guffey volcanic center is the largest within the 2000 km{sup 2} mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which present the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the Guffey center was followed by extrusion of a thick series of basalt, trachybasalt, and shoshonite flows and lahars. Plugs, dikes, and vents ranging from basalt to rhyolite cut the thick mafic deposits, and felsic tuffs breccias chemically identical to the small rhyolitic plutons are locally preserved. Whole-rack major and trace element analyses of 80 samples, ranging almost continuously from 47% to 78% SiO{sub 2}, indicate that the rocks of the Guffey center are among the most highly enriched in K{sub 2}O (up to 6%) and rare earth elements (typically 200-300 ppm) of any volcanic rocks in Colorado. These observations, along with the relatively high concentrations of Ba and Rb and the depletion of Cr and Ni, suggest an appreciable contribution of lower crustal material to the magmas that produced the Thirtynine Mile volcanic rocks.

Wobus, R.A.; Mochel, D.W. (Williams College, Williamstown, MA (USA)); Mertzman, S.A.; Eide, E.A.; Rothwarf, M.T. (Franklin Marshall College, Lancaster, PA (USA)); Loeffler, B.M.; Johnson, D.A. (Colorado College, Colorado Springs (USA)); Keating, G.N.; Sultz, K. (Carleton College, Northfield, MN (USA)); Benjamin, A.E. (Smith College, Northampton, MA (USA)); Venzke, E.A. (Beloit College, WI (USA)); Filson, T. (Whitman College, Walla Walla, WA (USA))

1990-07-01

124

Active synchronous counterclockwise rotation and northwards translation of Africa toward Eurasia during the Late Cretaceous: A paleomagnetic study on the Alkaline volcanic field of Wadi Natash (ca. 100-86Ma), South Eastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

In order to shed light on the paleo-tectonic movement of Africa during the Late Cretaceous, the two end members of the alkaline volcanic field of Wadi Natash (ca. 100-86Ma) in the South Eastern Desert of Egypt were studied paleomagnetically. The Wadi Natash volcanic field (24.5°N-34.25°E) is made up of a succession of differentiated flows grading from alkali olivine basalt [AOB] to trachyte-phonolite [Tr/Ph]. The oldest flows of the AOB (104±7 Ma) and the youngest Tr/Ph plugs and ring dykes (86Ma) as well as the interflows sandstones [ previously know as Nubian sandstone were sampled allover the field > 400km2. The isothermal remanent magnetization [IRM] study revealed that the remanence in Wadi Natash volcanics reside mainly in magnetite with some subsidiary goethite/hematite sites. On the other hand, goethite/hematite are the sole remanence carriers in the Nubian-type interflow sandstone. After the progressive stepwise thermal demagnetization of all samples, the visual isolation and subsequent calculation of the best-fit line of the characteristic remanence [ChRM] direction of each sample, followed by the calculation of the site and rock-unit means revealed that: 1- In the tilt-corrected coordinates, the mean ChRM of the oldest AOB flows [N=12 sites

Lotfy, H.

2009-04-01

125

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

NASA Astrophysics Data System (ADS)

We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12km and average 40-60m (up to >140m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1km3 for an individual flow, and 1.8km3 for all the identified flows) and yield strength of several flows (23-124kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2×1012 to 5.5×1012Pas for surface viscosity, and 7.5×1010 to 5.2×1011Pas for interior viscosity, for a strain interval of 24h). We use shaded-relief images of the DEM to map basic flow structures such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.

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

126

40Ar/(39)Ar geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya.  

PubMed

(40)Ar/(39)Ar single-crystal laser-fusion dating, K-Ar dating, and paleomagnetic reversal stratigraphy have been used to determine the chronostratigraphy of the Kabarnet Trachyte, Lukeino Formation, Kaparaina Basalt Formation, and Chemeron Formation at the sites of Kapcheberek (BPRP#77) and Tabarin (BPRP#77) in the Tugen Hills, Kenya. The succession ranges in age from 6.56-3.8 Ma. The upper Lukeino Formation at Kapcherberek, including the fauna from the site BPRP#76, was deposited during chron C3r and can be constrained to the interval 5.88-5.72 Ma. The Chemeron Formation at Tabarin includes at the base an ignimbrite and associated basal air-fall tuff with a combined age of 5.31+/-0.03 Ma. Sedimentary and volcaniclastic rocks of the Chemeron Formation which unconformably overlie the ignimbrite record chrons C3n.2n through C2Ar. The combined(40)Ar/(39)Ar and paleomagnetic data constrain the age of this sequence to 4.63-3.837 Ma. The age of the Tabarin mandible fragment (KNM-TH 13150) and associated fauna at site BPRP#77 in the Chemeron Formation is 4.48-4.41 Ma, marginally older than similar early hominids from Aramis, Ethiopia. Basin subsidence appears to be defining an overall accumulation rate of about 17 cm/ka over the 2.7 Ma represented at Tabarin and Kapcheberek, despite episodes of rapid accumulation and hiatuses. PMID:11795971

Deino, Alan L; Tauxe, Lisa; Monaghan, Marc; Hill, Andrew

127

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

128

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; Métour, Joël Le; Sergeev, Sergey; Tegyey, Monique

2013-02-01

129

Spectral analysis of the Namarunu volcanic complex in the Northern Kenya Rift  

NASA Astrophysics Data System (ADS)

The Namarunu volcanic complex, situated on the western side of Suguta Valley in the northern part of the Kenya Rift, is dominated by trachytic and basaltic volcanics with a Pliocene to Holocene age range. The analysis of ASTER satellite imagery with special focus on the VNIR and SWIR bands covering a wavelength from 0.5µm to 2.4µm provides the possibility to distinguish these different volcanic rock types by means of spectral characteristics. The visualisation of calculated ratio bands also shows a distinct gradient within alluvial fans and scree surrounding Namarunu, indicating varying source areas. Based on this satellite information, samples both from in-place volcanics and from the enclosing fans were taken for additional spectral analysis. With hyperspectral lab measurements, high resolution spectra of the rock samples were acquired. These spectral signals allow establishing a basic provenance analysis of the fans. Due to limitations of the spectral characteristics of volcanics, the spectral rock classification additionally depends on alteration patterns. As Namarunu itself is active since at least 0.87Ma, its volcanic rocks comprise the latest history of the Rift Valley, including rift tectonics and the influence of lacustrine environment, thus the superimposed climate fluctuations; the analysis of the fans delivers an further insight into the volcanic evolution in Suguta Valley. Presented are the possibilities and limitations of the technique that uses the connection of remote sensing data and field samples, as well as the approach to comprehend the volcanic history of Namarunu with the help of spectral analysis.

Riedl, S.; Trauth, M. H.

2009-04-01

130

Italian zeolitized rocks of technological interest  

NASA Astrophysics Data System (ADS)

Large areas of Italian territory are covered by thick and widespread deposits of zeolite-bearing volcaniclastic products. The main zeolites are phillipsite and chabazite spread over the whole peninsula, and clinoptilolite recorded only in Sardinia. A trachytic to phonolitic glassy precursor accounts for the formation of the former zeolites characterized by low Si/Al ratios (?3.00), while clinoptilolite is related to more acidic volcanism. The genesis of most of these zeolitized deposits is linked to pyroclastic flow emplacement mechanisms characterized by quite high temperatures and by the presence of abundant fluids. The main utilization of these materials has been and still is as dimension stones in the building industry. Currently, limited amounts are also employed in animal farming (dietary supplement, pet litter and manure deodorizer) and in agriculture as soil improvement and slow-release fertilizers. New fields of application have been proposed for these products on account of their easy availability, very low cost, their high-grade zeolites (50 70%), and good technological features such as high cation exchange capacities and adsorption properties.

de'Gennaro, M.; Langella, A.

1996-09-01

131

Magnetic fabric on recent lava flows of Deception Island, South Shetland islands, Antarctica  

NASA Astrophysics Data System (ADS)

We report Anisotropy of the Magnetic Susceptibility results (AMS) from Plio-Quaternary lava flows exposed on Deception Island. It is located in the Bransfield Strait marginal basin that separates the South Shetland Islands from the Antarctic Peninsula and was generated by the collapse of the central part of a stratovolcano. The magnetic fabric of two pre-caldera and two post-caldera units, including basaltic, andesitic and trachytic terms, has been compared. Pre-caldera samples correspond to the older units (Fumarole Bay and Basaltic Shield Formations). The two other studied lava flows, included within the Baily Head and Pendulum Cove Formations, postdate the principal caldera collapse stage. The bulk susceptibility (Km) and the anisotropy degree (Pj) data suggest that the total AMS is related to both paramagnetic and ferromagnetic minerals. The magnetic fabric of all samples defines dominant foliated ellipsoids consistent with the primary magma flow surface. The directional analysis reveals that, excluding sites corresponding to the Pendulum Cove Formation (late post-caldera stage), a conspicuous NE-SW magnetic lineation has been preserved in both the pre- and post-caldera samples around the island. This orientation is parallel with one of the main fracture systems controlling the morphology of the island and to the faulting in the Bransfield Strait. Further low-field and high-field AMS measurements will be necessary to understand the coincidence between the magnetic and structural lineations.

Gil-Imaz, Andrés.; Gil-Peña, Inmaculada; Soto, Ruth; Galindo-Zaldívar, Jesús; Rey, Jorge; Maestro, Adolfo; López-Martínez, Jerónimo; Oliva, Belén.; Rull, Fernando

2010-05-01

132

Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado  

NASA Astrophysics Data System (ADS)

The Guffey volcanic center is the largest within the 2000 km2 mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which represent the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the Guffey center was followed by extrusion of a thick series of basalt, trachybasalt, and shoshonite flows and lahars. Plugs, dikes, and vents ranging from basalt to rhyolite cut the thick mafic deposits, and felsic tuffs and tuff breccias chemically identical to the small rhyolitic plutons are locally preserved. Whole-rock major and trace element analyses of 80 samples, ranging almost continuously from 47% to 78%SiO2, indicate that the rocks of the Guffey center are among the most highly enriched in K2O (up to 6%) and rare earth elements (typically 200-300 ppm) of any volcanic rocks in Colorado. These observations, along with the relatively high concentrations of Ba and Rb and the depletion of Cr and Ni, suggest an appreciable contribution of lower crustal material to the magmas that produced the Thirtynine Mile volcanic rocks.

Wobus, Reinhard A.; Mochel, David W.; Mertzman, Stanley A.; Eide, Elizabeth A.; Rothwarf, Miriam T.; Loeffler, Bruce M.; Johnson, David A.; Keating, Gordon N.; Sultze, Kimberly; Benjamin, Anne E.; Venzke, Edward A.; Filson, Tammy

1990-07-01

133

Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions  

NASA Astrophysics Data System (ADS)

One of the largest explosive eruptions in the past several thousand years occurred at Tianchi volcano, also known as Changbaishan, on the China-North Korea border. This historically active polygenetic central volcano consists of three parts: a lower basaltic shield, an upper trachytic composite cone, and young comendite ash flows. The Millennium Eruption occurred between 938 and 946 ad, and was preceded by two smaller and chemically different rhyolitic pumice deposits. There has been at least one additional, small eruption in the last three centuries. From 2002 to 2005, seismicity, deformation, and the helium and hydrogen gas contents of spring waters all increased markedly, causing regional concern. We attribute this event to magma recharge or volatile exhalation or both at depth, followed by two episodes of addition of magmatic fluids into the overlying aquifer without a phreatic eruption. The estimated present magma accumulation rate is too low by itself to account for the 2002-2005 unrest. The most serious volcanic hazards are ash eruption and flows, and lahars. The available geological information and volcano monitoring data provide a baseline for comprehensive assessment of future episodes of unrest and possible eruptive activity.

Wei, Haiquan; Liu, Guoming; Gill, James

2013-04-01

134

Gels composed of sodium-aluminum silicate, Lake Magadi, Kenya  

USGS Publications Warehouse

Sodium-aluminum silicate gels are found in surftcial deposits as thick as 5 centimeters in the Magadi area of Kenya. Chemical data indicate they are formed by the interaction of hot alkaline springwaters (67?? to 82??C; pH, about 9) with alkali trachyte flows and their detritus, rather than by direct precipitation. In the process, Na2O is added from and silica is released to the saline waters of the springs. Algal mats protect the gels from erosion and act as thermal insulators. The gels are probably yearly accumulates that are washed into the lakes during floods. Crystallization of these gels in the laboratory yields analcite; this fact suggests that some analcite beds in lacustrine deposits may have formed from gels. Textural evidence indicates that cherts of rocks of the Pleistocene chert series in the Magadi area may have formed from soft sodium silicate gels. Similar gels may have acted as substrates for the accumulation and preservation of prebiological organic matter during the Precambrian.

Eugster, H. P.; Jones, B. F.

1968-01-01

135

?7Li variation along a continental volcanic Suite: evidences for the Li characteristic of the involved reservoir  

NASA Astrophysics Data System (ADS)

The intraplate volcanic suite of the Chaîne des Puys (French Massif Central) shows a complete petrological suite, from alkali basalts with HIMU affinities to trachytes. The significant variations of trace elements, radiogenic isotopes and Li isotopes within the series strongly suggest the effect of lower crust assimilation associated with fractional crystallization (AFC). The Li isotopic compositions of the lavas range from high ?7Li (> +7‰) in basalts to lighter values in more evolved lavas (down to ?7Li ? 0‰). The mantle component, as expressed in the least evolved lavas, has a heavy Li isotopic signature, in good agreement with previous ?7Li measurements in HIMU-type OIB. Along with our observations, these results suggest that heavy Li isotope compositions of altered oceanic crust can be reintroduced and partially preserved in the deeper mantle. Mixing relationships throughout the AFC process gives constraints on the Li signature of the lower crustal end-member and also an indirect method to assess the in situ value of ?7Li in the lower crust. Although the behavior of Li isotopes during assimilation processes is currently poorly constrained, our preliminary calculations suggest that at least a portion of the lower crust beneath the Chaîne des Puys is characterized by a light Li isotopic composition (?7Li < -5‰).

Hamelin, C.; Barrat, J.; Seitz, H.; Dosso, L.; Chaussidon, M.

2009-12-01

136

40Ar/39Ar Dating of the Pleistocene Peninj Group, Lake Natron, Tanzania  

NASA Astrophysics Data System (ADS)

40Ar/39Ar incremental-heating experiments on basaltic lavas and single-crystal total-fusion analyses of trachytic tuffs provide for the first time accurate resolution of the chronostratigraphy of the Pleistocene Peninj Group west of Lake Natron, northern Tanzania. These new data force a major revision of the chronology of the entire sequence: the base of the group is now ~1.75 Ma, the Wa Mbugu basalt within the Main Tuff is 1.19 ± 0.03 Ma (Cobb Mountain paleomagnetic event), and the top of the Peninj Group is 1.01 ± 0.03 Ma. Thus the Achulean and Olduwan artifact assemblages found in the Upper Sands and Clays of the upper Humbu Formation above the Wa Mbugu basalt are about ~1.2 to 1.1 Ma, half a million years younger than previously believed. The revised chronology also clarifies the age of the major lake expansion recorded within the upper part of the Peninj Group. This lacustrine phase is now constrained to ~1.1 to 1.0 Ma, corresponding to a previously identified episode of lake expansion in East Africa between 1.1 and 0.9 Ma (Trauth et al., 2005).

Deino, A. L.; Dominguez-Rodrigo, M.; Luque, L.

2006-12-01

137

Geochemistry of tephra from Bed I, Olduvai Gorge, Tanzania: Stratigraphic correlations and implications for magmatic evolution  

NASA Astrophysics Data System (ADS)

At least 10 predominantly trachytic and rhyolitic tuffs are preserved interbedded in volcaniclastic sediments of Plio-Pleistocene Bed I, Olduvai Gorge, Tanzania. Physical correlation of the tuffs is complicated by faulting and variation in preservation and lithofacies. Differences in the degree and type of tephra alteration (clay, zeolitic, none) and preservation of glass shards within the various depositional environments (saline-alkaline lake, lake margin, wetlands, alluvial fan) make correlation by conventional glass chemistry methods impossible. However, variations in overall mineralogy and chemical compositions of co-magmatic phenocrysts (feldspar, augite, titanomagnetite, amphibole) have proven useful to uniquely characterize the tuffs for correlation purposes. Samples of 10 major tuffs in the Olduvai Bed I sequence were collected from various depostional and preservational environments situated up to 15 km apart. Thin sections and mineral separates (10-60 grains of each type of phenocryst/ sample, 2-3 samples/ tuff) were analyzed by electron microprobe for major and minor elements. The lower Bed I tuffs are rhyolitic and easily distinguished from the upper tuffs by the presence of quartz and high-Fe augite. Feldspar composition has been previously found to separate all of the upper tuffs (1B-1F) except the two trachyandesitic tuffs (1D and the "unnamed" tuff between 1E and 1F). Mn and Ti concentrations in the titanomagnetites separate the upper tuffs (MnO%: 1B=1.5-2, 1C=1.3-1.6, 1D=1.1-1.4, 1E=1.5-1.7, unnamed= 0.9-1.2, 1F=1.6-2; TiO2%: 1B, 1E=23-26, 1C=18-22, 1D=25-27, unnamed=20-21, 1F= 12-20). Tuffs 1B, unnamed, and 1F contain abundant amphibole, 1D contains none. Mn and Fe concentrations in the augites also separate the tuffs (MnO%: 1B=1.2-1.5, 1C=0.9-1.2, 1D=0.6-0.9, 1E=0.9-1.1, unnamed=0.5-0.7, 1F=variable; FeO%: 1B=19-21, 1C=15-19, 1D=12-16, 1E=13-16, unnamed=11-14, 1F=variable). Results of these findings provide new widespread markers in the Olduvai sequence for detailed lateral correlations of depositional and paleoecological environments as well as various archaeological and fossil faunal localities. Highlights include the identification of tuffs1C, IE and a new unnamed tuff as widespread airfall units in the western part of Olduvai Gorge, where they had not been previously identified, as well as the confirmation of the presence of tuffs 1B, 1D, 1E, unnamed, and 1F in the saline-alkaline lake sequence. This sequence of tuffs documents the magmatic changes in the Ngorongoro crater highlands from the final alkaline rhyolitic eruptions of Ngorongoro volcano (~2 My) to the development of Olmoti with its trachyandesitic/trachytic tuff compositions (1.8-1.7 My), to the beginning of the silica-undersaturated volcanism which persists to the present day.

McHenry, L.

2003-04-01

138

Understanding the Peach Spring supereruption through its basal layer deposits (Southwestern USA)  

NASA Astrophysics Data System (ADS)

The Peach Spring Tuff (PST) supereruption occurred 18.8 Mya, depositing a thick ignimbrite over 32,000 km^2 of the NW Arizona, SE California, and S Nevada. At the base of the ignimbrite is a thin (?1 m thick) layered unit that has been identified as a surge deposit (Valentine et al 1989, 1990; Wilson & Self 1989 offer a contrary interpretation) that extends 100 km east and 65 km west from its source, the Silver Creek caldera in the southern Black Mountains, AZ (Ferguson 2008). These deposits record the first material ejected from the PST magma body immediately before the main ignimbrite-forming event, and provide (1) samples of the first magma to be tapped and (2) evidence for the initial eruptive process and potentially eruption triggers. We are investigating textures and petrology of pumice clasts taken from an internally structureless layer within the basal unit. We have determined roundness of pumice clasts from six exposures located 22 to 96 km from the source caldera, using the approach of Manga et al (2011) (the first application of their metric to a putative surge deposit). Roundness (R) is defined as 4?A/P^2, where A is the cross-sectional area of a clast and P is its perimeter. Mean R values range from 0.76 to 0.80, showing no systematic change with distance from the caldera. Generally, the R values for samples show negatively skewed distributions, with the majority of the pumice clasts having values between 0.75 and 0.85 but with values as low as 0.60 and as high as 0.90; standard deviations also vary little from sample to sample. This relative uniformity suggests that pumice clasts may have reached a saturation roundness (beyond which they would round little) in the high-energy environment near the vent and early in the transport processes of the surge. The pumice clasts are relatively crystal-poor, with a phenocryst assemblage comprising abundant sanidine, lesser plagioclase, minor hornblende and biotite, and accessory magnetite, sphene, zircon, chevkinite, apatite, and fluorite; quartz is minimal or absent. Phenocrysts in the pumice are mostly fragments, suggesting syn-eruption fragmentation within magma chamber and/or conduit. Preliminary SEM analysis of the glass indicates dominantly high-silica rhyolite compositions (?77% wt SiO_2) consistent with previous electron microprobe values. The siliceous glass composition and low phenocryst abundance are similar to typical pumice clasts in the outflow sheet. These pumice clasts are much more evolved than crystal-rich, trachytic pumice clasts that form a minor component in the basal layer unit, are more common in the overlying ignimbrite, and dominate the intracaldera fill. Trachytic magma is interpreted as a mush from the base of the chamber that was heated and remobilized by a recharging episode prior to the climactic eruption. Basal layer pumice compositions suggest the initial magma was drawn primarily from the chamber's top with minor contribution from near the bottom and that extent of entrainment of deep-level magma increased over the course of the eruption.

McCracken, R. G.; Miller, C. F.; Dufek, J.; Gualda, G. A.; Buesch, D.; Brooks, C. E.

2011-12-01

139

A new look at the collision-related volcanism in Eastern Anatolia, Turkey: Volcanic history of the Northern-Van neovolcanic province  

NASA Astrophysics Data System (ADS)

The region including the Eastern Anatolian - Northern Iranian High Plateau and Greater and Lesser Caucasus mountain ranges is one of the best examples of an active continental collision zone in the world, which is thought to have been formed by the closure of the northern branch of the Neotethyan Ocean. It comprises one of the high plateaus of the Alpine-Himalaya mountain belt (i.e. the Eastern Anatolia High Plateau) with an average elevation of ~2 km above the sea level. The volcanic activity initiated immediately after the block uplift of the region (at around 15 Ma as our new isotope-geochronological database indicates) and produced great volumes of volcanic material in a number of countries including Turkey, Russia, Georgia, Azerbaijan, Armenia and Iran. At present, the volcanic province extends from Eastern Anatolia (Turkey) into Caucasus of Southern Russia, spanning a distance of some 1000 km. Perhaps the most striking aspect of the Eastern Anatolian - N Iranian High Plateau and Caucasus is the volume and compositional variability of collision-related volcanic products erupted in a time interval from Neogene to Quaternary. Only in E Turkey, the collision-related volcanic units cover over half of the region (i.e.˜43,000 km2). In order to better understand the spatial and temporal compositional variations in volcanic rocks and their implications on magma genesis and geodynamic evolution of the region, we conducted joint research on this spectacular volcanic province. One of the largest Cenozoic volcanic areas on the EAHP is located in the north of Lake Van as we named "the Northern-Van neovolcanic province". It covers an area of about 6000 km2 starting from the northern cost of Lake Van. It is composed of a series of volcanic edifices (e.g. Girekol, Meydandag and Etrusk volcanoes). Remarkably, these volcanoes sit almost on the culmination of a regional domal structure called "Lake Van dome" in the vertex of the eastern Turkish high plateau. We intentionally started working from the southern part of the Turkish side because little is known about the initiation dates of volcanism there, as good dates on these rocks are quite limited. Collision-related volcanism in the Northern-Van neovolcanic province lasted around 15 My and followed four stages of intense activity, each lasted 1-2 My but divided by long pose periods. (1) During the Middle Miocene (15.0-13.5 Ma) period, andesitic lavas and pyroclastics with a distinct subduction signature erupted along a zone extending from S of the Tendurek volcano to Zilan Valley and Deliçay in the N and NE of the town of Ercis, basically around Mt Aladag. These are the oldest lavas in the E Anatolian volcanic province. (2) During Late Miocene (10-9 Ma) volcanism restarted along the same belt, producing lavas ranging in composition from basalts, trachybasalts to dacites. These lavas overly the volcanic units of the Middle Miocene period. (3) After a 3.2 My time break, volcanism restarted in the region during Pliocene (5.8-3.9 Ma) with the eruption of basalts, trachydacites and trachytes in the NW, N and NE of the town of Ercis. Early-Pliocene basaltic flows formed a vast plateau in the north from Etrusk volcano. The final phase of the Pliocene magmatic activity was marked by the eruptions of trachytic, trachyandesitic, rtrachydaitic and rhyolitic lavas from the Etrusk volcano (4.3 to 3.9 Ma), whose final stage was marked by a caldera collapse at around 3.7 Ma. (4) Volcanism restarted in Quaternary (1.0-0.4 Ma) with the eruption of basalts and trachybasalts. The diverse character of the volcanism in the region can be explained by variations in magma genesis, magma chamber processes and geodynamic reasons, e.g. detachment by means of slab breakoff and/or delamination.

Keskin, Mehmet; Lebedev, Vladimir; Sharkov, Evgenii; Oyan, Vural; Ünal, Esin

2010-05-01

140

Reviving Moribund Intrusive Complexes: Mafic Thermal Input, the Accessory Mineral Record, and the Pluton-Volcano Connection  

NASA Astrophysics Data System (ADS)

Intrusive and extrusive silicic systems spanning subduction zone, oceanic, and intracontinental settings and ranging from small, persistent eruptive centers to supereruptions show evidence for periodic thermal input. We present examples demonstrating that this input is responsible for remobilizing stagnant intrusions and apparently triggering eruptions. Mafic recharge is the evident heat source in some and implicated indirectly in others. Field relations, textures, accessory mineral zoning, and U-Pb and U-Th geochronology provide evidence for the inferred thermal reinvigoration of these systems: --Mount St. Helens, WA (active subduction zone volcano): Combined U-Th dating and elemental analysis demonstrates that zircon grew from residual silicic melt in relatively cool storage zones,10's to 100's of ky prior to eruption, and that its growth was episodic over the history of the system. Zircon crystals are thus samples of stagnant intrusive ("plutonic") parts of the St. Helens system, entrained in hotter ascending magmas during replenishment and local rejuvenation. --Highland Range volcanics & Searchlight pluton, NV (Miocene volcanic-plutonic system, incipient crustal extension): Evidence for late-stage mingling between intermediate and highly evolved, crystal-rich magma is preserved in the pluton, large dikes, a near-surface plug, and the final erupted products of the system, which were accompanied and followed by andesite lavas. Resorption and reaction rimming of sphene and large feldspar and quartz crystals in the dikes, plug, and volcanic rocks attest to heating. We infer that voluminous andesite invaded nearly-solid leucogranite, remobilized it, and triggered final eruptions. --Peach Spring Tuff, AZ, CA, NV (Miocene supereruption, incipient crustal extension): In contrast to phenocryst-poor rhyolitic outflow, intracaldera tuff is crystal-rich trachyte. Feldspar and sphene phenocrysts are extensively resorbed and rims of zircon crystals record a >100 degree C spike in T. We conclude that the trachyte tuff is cumulate from the base of the pre-eruption chamber that was remobilized by an intense heating event. Based on Rhyolite-MELTS modelling, we estimate that this cumulate was initially uneruptible "rigid sponge" with 10-20% melt. Sparse andesitic enclaves may reflect the source of thermal rejuvenation of the mush. --Austurhorn intrusive complex (Late Miocene) & historic silicic eruptions, Iceland (hyperactive mid-ocean rift adjacent to hot spot): Austurhorn records extensive mingling between basaltic and silicic magma. Zircon U-Pb data reveal a major episode of mafic intrusion at 6.0 Ma. This intrusion partially remobilized felsic material and induced a second episode of zircon growth in the dominantly 6.5 Ma complex. Historic silicic eruptions are also usually accompanied by mafic lavas and/or enclaves, and zircon records T significantly lower than erupting magma and ages of 10's of ka. Our experience suggests that thermal rejuvenation plays a vital role in the dynamic behavior of many and perhaps most silicic systems. It is further consistent with the hypothesis that such systems are long-lived but that their normal state is as melt-poor, immobile, uneruptible bodies that only sporadically spring to life.

Miller, C. F.; Gualda, G. A.; Padilla, A. J.; Pamukcu, A. S.; Claiborne, L. L.; Carley, T. L.; Flanagan, D. M.

2011-12-01

141

U-Pb geochronology of the Kap Washington Volcanic Province, North Greenland: Constraints on the timing of continental rifting and implications for the development of the Arctic Basin  

NASA Astrophysics Data System (ADS)

The Kap Washington volcanic sequence at the north coast of Greenland is bimodal with alkaline basalts, trachytic to rhyolitic lavas, tuffs and ignimbrites predominating. In terms of geochemistry and distribution of rock types, the sequence bears resemblance to presently active continental rift systems, e.g. the Main Ethiopian Rift. Associated with the volcanics is a swarm of coast-normal alkaline basaltic dykes which intensifies towards the outer coast. The volcanics are believed to be linked to rifting in the Arctic Basin and have featured prominently in geotectonic reconstructions of the Arctic region (e.g. Batten et al. 1981). Here we report the first U-Pb zircon ages from silicic lavas and intrusions of the Kap Washington sequence. A total of ten samples have been dated and the duration of magmatism is constrained at present to ca. 10 million years - from 71 to 61 Ma (based on 206Pb/238U ages of concordant analyses). Three age ‘groups’ have been identified: 71-69 Ma (n = 6); 68-65 Ma (n = 2); and 64-61 Ma (n = 2). The oldest group comprises trachytic and rhyolitic lava flows from Kap Kane and a rhyolitic sill from the Kap Washington peninsula. These ages agree well with new 40Ar/39Ar ages obtained on amphiboles from benmoreitic tuffs exposed on Kap Kane (Holm et al., this session) and suggest that most of the ~1.5 km thick Kap Kane sequence was extruded within a period of 1-2 million years. The two younger groups comprise silicic lavas exposed on Lockwood Island. The exposed sequence on Lockwood Island is estimated to be 3-4 km thick and was previously thought to be the oldest part of the succession (Brown et al. 1987). The large scatter in ages on Lockwood Island indicates that magmatism was episodic rather than continuous. The new age data from the Kap Washington volcanics together with 40Ar/39Ar ages for the associated dyke swarm (Kontak et al. 2001) suggest that continental extension and magmatism occurred in the area between ca. 82 and 61 Ma. This age bracket seems to preclude any relation to initial spreading on the Nansen-Gakkel Ridge (52 Ma-present) and an association with spreading in the Labrador Sea-Baffin Bay-Makarov Basin system seems the most probable. We propose that the Kap Washington magmatism ceased with the onset of seafloor spreading in the Northeast Atlantic-Eurasia Basin system, which shifted the tectonic regime in North Greenland from extensional to compressional, culminating with the Eurekan deformation in early Eocene (Holm et al., this session). References: Batten et al. (1981): Nature 294: 150-152. Brown et al. (1987): J Geol. Soc. Lond. 144: 707-715. Kontak et al. (2001): Can. Mineral. 39: 997-1020

Thorarinsson, S. B.; Holm, P. M.; Tappe, S.; Heaman, L.; Tegner, C.

2009-12-01

142

Hydrothermal Alteration of Intra-caldera Deposits, Tejeda Caldera, Gran Canaria  

NASA Astrophysics Data System (ADS)

The Tejeda caldera, Gran Canaria erupted about 20 rhyolite-trachyte ignimbrites (Mogan Group 14 - 13.25Ma) followed by about 20 phonolitic lava flows and ignimbrites (Fataga Group 12.5 - 7Ma). The Mogan and Fataga ignimbrites (ash-flow tuffs) occur as intra- and extra-caldera deposits. At the caldera margin, late rhyolite-trachyte Mogan tuffs are severely altered but overlain by unaltered Fataga units, implying that alteration of the tuffs occurred between 13 and 12.5Ma and was associated with the emplacement of the high-level Fataga magma system (Schminke 1998, Geol. Field Guide Gran Canaria). Unaltered extra-caldera tuffs are beige to brown in colour and contain K-feldspar, plagioclase and to a lesser degree pyroxene and amphibole and groundmass quartz. The intra-caldera tuffs are vividly coloured and still contain relics of primary K-feldspar, plagioclase, pyroxenes, amphiboles and groundmass quartz. In addition, however, they contain various clays, zeolites and alteration feldspars (adularia) characteristic for secondary fluid alteration of the rock, not found in the unaltered extra-caldera samples. Water concentrations up to 4wt% are found in the intra-caldera tuffs compared to less than 0.5wt% in unaltered samples. Moreover, there is a positive correlation between presence of alteration minerals and d18O values. The d18O values of the intra-caldera tuffs (13-18 permil, n=8) are considerably higher than the igneous values of unaltered extra-caldera tuffs (6-8 permil, Hansteen and Troll 2003, Chem.Geol., 193, 181-192), indicating a low-T alteration environment. The dD values of the altered tuffs lie between -52 and -78 permil. Ambient meteoric water at the alteration site is estimated at approximately -30 permil, assuming an average recharge altitude of about 500m, (calculated from an average present day Gran Canaria value of -20 (Javoy et al, 1986, CMP, 92, 225-235). This estimated altitude of recharge is lower than the present day altitude of about 1000m as it takes into account an increase in elevation caused by post-caldera cone sheet emplacement (Schirnick et al., 1998, Geology, 27, 207-210), Holocene igneous activity, and a fall in sea-level of more than 100m (Haq et al, 1987, Science, 235, 1156-1166). Interaction of this meteoric water (dD = -30%) with the rocks at low-T would have resulted in at least partial equilibration between the water and the minerals. The H-isotope fractionation between most clays and water is typically around -30% (Sheppard and Gilg 1996, Clay Minerals, 31, 1-24.), accounting for the observed rock dD values down to -60%. Three samples have dD values substantially lower than -60%, which indicate interaction with meteoric water having lower dD values. At 100 degree C, H2O liquid - vapor = -28 permil (Horita and Wesolowski 1994, GCA, 58, 3425-3437). Condensation of steam fumaroles might, therefore, be a mechanism to produce such isotopically negative water. This type of low-T system may be seen in modern analogue in New Zealand and Indonesia, where low-T steam fumaroles (80 - 150 degree C) are a common phenomenon and are known to fluctuate in intensity due to environmental factors. The Gran Canaria results are consistent with the shallow epithermal part of a larger, fault controlled, hydrothermal cell associated with a high-level Fataga chamber system where fluid temperatures did not exceed 200 - 250 degree C in the near surface environment.

Troll, V. R.; O'Halloran, A.; Harris, C.; Walter, T. R.

2004-05-01

143

Origin of Volcanic Seamounts Offshore California Related to Interaction of Abandoned Spreading Centers with the Continental Margin  

NASA Astrophysics Data System (ADS)

The numerous NE-SW trending volcanic seamounts at the continental margin offshore central to Southern California owe their existence to the complex tectonics that resulted when small spreading ridge segments intersected and partly subducted beneath the continental margin during the Miocene plate reorganization. A limited number of dredged samples had indicated multiple episodes of coeval, alkalic volcanism at geographically widely separated sites (Davis et al., 2002, GSA Bull. 114, 316-333). 450 new samples were collected from 8 seamounts from 37. 5°N to 32.3°N with MBARI's ROV Tiburon. Ar-Ar ages for 50 of these samples extend the ages of volcanism from 18 Ma to 2.8 Ma. The dominant whole rock compositions are differentiated alkalic basalt, hawaiite, and mugearite, but include minor benmoreite, trachyte, and rare tholeiitic basalt. This entire range of compositions is also present in glassy margins or in volcaniclastic breccias, except for the trachyte, which had no glassy margins. Trace element abundances and ratios (e.g. REE, Zr, Nb, Ta, Th, Ba, etc.) are typical for ocean island basalt, whether the seamount is located on the Pacific plate (e.g. Pioneer, Gumdrop, Guide, Davidson, San Juan, San Marcos) or on the continental slope (Rodriguez) or within the Southern Continental Borderland (Northeast Bank). Nine samples, predominantly from Rodriguez Seamount, show a calc-alkaline trend with lower Nb, Ta, and higher Th. These samples may be erratics (Paduan et al., 2007, Marine Geology, in press). Sr, Nd, and Pb isotopic compositions plot within the Pacific N-MORB field for the northern seamounts (Pioneer, Gumdrop, Guide) but suggest progressively more radiogenic sources southward. There is considerable scatter at each site, especially with regard to 87Sr/86Sr, despite severe acid-leaching of the samples. Isotopic and trace element compositions indicate sources that are heterogeneous at a small scale. Chondrite-normalized Ce/Yb suggest smaller degree of melting and more alkalic compositions with decreasing age, although there is again considerable scatter. Chondrite-normalized La/Sm versus Zr/Nb form a continuum from the seamount lavas to depleted N-MORB and E-MORB suggesting a common origin by decompression melting of a mantle source with randomly distributed enriched heterogeneities, which are incorporated to a greater degree with decreasing degree of melting. Based on symmetric magnetic anomalies, only Davidson Seamount has been identified as straddling a fossil spreading center (Lonsdale, 1991, AAPG Mem. 47, 87-125). However, the other seamounts along the continental margin with the same NE-SW orientation and similar geochemical characteristics probably originated in a similar setting, erupting lavas along zones of weakness in the ocean floor fabric related to past seafloor spreading. Small volumes of magma can apparently rise long after spreading ceases if there is enough enriched source component to facilitate melting combined with zones of weakness in the underlying ocean crust fabric and/or extensional tectonics.

Davis, A. S.; Clague, D. A.; Paduan, J. B.; Cousens, B. L.; Huard, J.

2007-12-01

144

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

145

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

2013-03-27

146

Behavior of halogens during the degassing of felsic magmas  

NASA Astrophysics Data System (ADS)

Residual concentrations of halogens (F, Cl, Br, I) and H2O in glass (matrix glass and melt inclusions) have been determined in a series of volcanic clasts (pumice and lava-dome fragments) of plinian, vulcanian and lava dome-forming eruptions. Felsic magmas from calc-alkaline, trachytic and phonolitic systems have been investigated: Montagne Pelée and Soufrière Hills of Montserrat (Lesser Antilles), Santa Maria-Santiaguito (Guatemala), Fogo (Azores) and Vesuvius (Italy). The behavior of halogens during shallow H2O degassing primarily depends on their incompatible character and their partitioning between melt and exsolved H2O vapor. However, variations in pre-eruptive conditions, degassing kinetics, and syn-eruptive melt crystallization induce large variations in the efficiency of halogen extraction. In all systems studied, Cl, Br and I are not fractionated from each other by differentiation or by degassing processes. Cl/Br/I ratios in melt remain almost constant from the magma reservoir to the surface. The ratios measured in erupted clasts are thus characteristic of pre-eruptive magma compositions and may be used to trace deep magmatic processes. F behaves as an incompatible element and, unlike the other halogens, is never significantly extracted by degassing. Cl, Br and I are efficiently extracted from melts at high pressure by H2O-rich fluids exsolved from magmas or during slow effusive magma degassing, but not during rapid explosive degassing. Because H2O and halogen mobility depends on their speciation, which strongly varies with pressure in both silicate melts and exsolved fluids, we suggest that the rapid pressure decrease during highly explosive eruptions prevents complete equilibrium between the diverse species of the volatiles and consequently limits their degassing. Conversely, degassing in effusive eruptions is an equilibrium process and leads to significant halogen output in volcanic plumes.

Balcone-Boissard, H.; Villemant, B.; Boudon, G.

2010-09-01

147

Middle Miocene nepheline-bearing mafic and evolved alkaline igneous rocks at House Mountain, Arizona Transition Zone, north-central Arizona  

SciTech Connect

The Middle Miocene House Mountain shield volcano is located on the northern margin of the Arizona Transition Zone, about 7 km SW of Sedona, AZ. Deep erosion has exposed internal structural and stratigraphic relationships of the volcano. Mapping documents two igneous suites: (1) alkali basalt to trachyte and alkali-feldspar syenite, and (2) olivine melanephelinite, nepheline monzodiorite, nepheline monzosyenite and nepheline syenite. The rocks of the first suite occur as dikes and flows, which, with a thick pyroclastic section, are the principal units of the volcano. The melanephelinite is nonvesicular and intruded as a large irregular dike and several smaller dikes. The nepheline-bearing syenitic rocks, which are phaneritic with nepheline and clinopyroxene crystals up to 1 cm in diameter, occur as pods and sheets within the melanephelinite. Also within the melanephelinite are wispy leucocratic segregations, syenitic fracture-fillings, and ocelli. The largest phaneritic sheet is [approx]18 m thick; it displays crude subhorizontal compositional banding and vuggy surfaces. The latter indicate that the magmas were fluid-rich. Compositions intermediate between the melanephelinite and syenitic rocks have not been found. Although the syenitic rocks are coarse-grained, mapping indicates the they are near the summit of the volcano and were probably emplaced at a depth of less than 1 km, possibly of only a few hundred meters. The field relationships of the phaneritic rocks can be explained by ascent and coalescence of immiscible syenitic liquids within the melanephelinite dike. Calculated density contrasts between melanephelinite and syenitic liquids exceed 0.2 g/cm[sup 3].

Wittke, J.; Holm, R.F.; Ranney, W.D.R. (Northern Arizona Univ., Flagstaff, AZ (United States). Dept. of Geology)

1993-04-01

148

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 (Fe2O3 - 1.45-3.84%, Av. 3.13, Al2O3 - 39.31-57.24, Av. 45.80) and laterites (Fe2O3 - 9.84-32.21, Av. 25.13%, Al2O3 - 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

149

Cutting Costs by Locating High Production Wells: A Test of the Volcano seismic Approach to Finding ''Blind'' Resources  

SciTech Connect

In the summer of 2000, Duke University and the Kenyan power generation company, KenGen, conducted a microearthquake monitoring experiment at Longonot volcano in Kenya. Longonot is one of several major late Quaternary trachyte volcanoes in the Kenya Rift. They study was aimed at developing seismic methods for locating buried hydrothermal areas in the Rift on the basis of their microearthquake activity and wave propagation effects. A comparison of microearthquake records from 4.5 Hz, 2 Hz, and broadband seismometers revealed strong high-frequency site and wave-propagation effects. The lower frequency seismometers were needed to detect and record individual phases. Two-dozen 3-component 2- Hz L22 seismographs and PASSCAL loggers were then distributed around Longonot. Recordings from this network located one seismically active area on Longonot's southwest flank. The events from this area were emergent, shallow (<3 km), small (M<1), and spatially restricted. Evidently, the hydrothermal system in this area is not currently very extensive or active. To establish the nature of the site effects, the data were analyzed using three spectral techniques that reduce source effects. The data were also compared to a simple forward model. The results show that, in certain frequency ranges, the technique of dividing the horizontal motion by the vertical motion (H/V) to remove the source fails because of non-uniform vertical amplification. Outside these frequencies, the three methods resolve the same, dominant, harmonic frequencies at a given site. In a few cases, the spectra can be fit with forward models containing low velocity surface layers. The analysis suggests that the emergent, low frequency character of the microearthquake signals is due to attenuation and scattering in the near surface ash deposits.

Eylon Shalev; Peter E. Malin; Wendy McCausland

2002-06-06

150

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

USGS Publications Warehouse

The ???150 km3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200-300 ky. New 40Ar/39Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ?? 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component >315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region.

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

2001-01-01

151

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

152

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

153

K Ar and 40Ar/39Ar ages of dikes emplaced in the onshore basement of the Santos Basin, Resende area, SE Brazil: implications for the south Atlantic opening and Tertiary reactivation  

NASA Astrophysics Data System (ADS)

New K Ar and 40Ar/39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb)N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb)N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64 59 Ma for felsic dikes. Because Jurassic Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

Guedes, Eliane; Heilbron, Monica; Vasconcelos, Paulo M.; de Morisson Valeriano, Cláudio; César Horta de Almeida, Júlio; Teixeira, Wilson; Thomaz Filho, Antonio

2005-03-01

154

Structure, stratigraphy, tectonic evolution, and petroleum source potential of the Hope basin, southern Chukchi Sea, Alaska  

SciTech Connect

Multichannel seismic reflection data from the Hope basin of the southern Chukchi Sea, Alaska, reveal a fill of as much as 5.8 km of probable Eocene and younger sedimentary rock. The basin evolved in three main stages. First, isolated half-grabens formed during the early Paleogene in response to transtensional movement along the Kobuk fault zone. The main fault zone trends northeast-southwest across central Kotzebue Sound and displays predominantly normal displacement. However, structural styles observed in seismic data and radically different thermal maturities of juxtaposed Upper Cretaceous rocks along the Kobuk River valley suggest that some component of strike-slip movement occurred. The half-grabens filled with trachytic and basaltic volcanic flows and nonmarine and possibly marine or lacustrine sedimentary rocks. Second, the basin thermally subsided in the early Miocene, and nonmarine sediment was deposited over the entire basin. Third, an extensional phase characterized by reactivation of the Eocene faults and initiation of numerous normal faults commenced during the middle to late Miocene. Extension was coeval with deposition of nonmarine and marine sediment in basin lows not necessarily coincident with the Eocene half-grabens. At the close of this extensional phase, the Kotzebue arch, a basement high trending east-west across the basin, was uplifted. Hydrocarbon potential of the Hope basin remains low primarily because of the abundance in the Kotzebue Sound area of thermally immature sedimentary deposits that are nonmarine and contain predominantly gas-prone organic matter. However, seismic stratigraphic evidence indicates that more oil-prone marine or lacustrine sedimentary rocks may be present in some of the deeper, more thermally mature, undrilled parts of the basin.

Tolson, R.B.

1987-01-01

155

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

2012-06-15

156

Immiscible separation of metalliferous Fe/ Ti-oxide melts from fractionating alkali basalt: P-T-fO2 conditions and two-liquid elemental partitioning  

NASA Astrophysics Data System (ADS)

Globules of iron-dominated (59-69 wt% FeOtot) and titanium-dominated (43.5 wt% TiO2) oxide melts have been detected in igneous xenoliths from Pliocene-to-Pleistocene alkali basalts of the Western Carpathians. Fluid inclusion and mineral composition data indicate immiscible separation of the high-iron-oxide melt (HIM) at magmatic temperatures. The HIM separation occurred during clinopyroxene (augite) accumulation in an alkali trachybasalt and continued during crystallization of amphibole (kaersutite) and K-feldspar (anorthoclase), the latter coexisting with trachyte and alkalic rhyolite residual melts. Some HIM was also expelled from sub-alkalic rhyolite (70-77% SiO2), coexisting with An27-45 plagioclase and quartz in granitic (tonalite-trondhjemite) xenoliths. Oxygen fugacities during HIM separation range from -1.4to +0.6log units around the QFM buffer. A close genetic relationship between HIM-hosted xenoliths and mantle-derived basaltic magma is documented by mineral 18O values ranging from 4.9 to 5.9‰ V-SMOW. ?D values of gabbroic kaersutite between -61 and -86‰ V-SMOW are in agreement with a presumed primary magmatic water source. Most trace elements, except Li, Rb and Cs, have preferentially partitioned into the HIM. The HIM/Si-melt partition coefficients for transition elements (Sc, V, Cr, Co, Ni) and base metals (Zn, Cu, Mo) are between 2-160, resulting in extreme enrichment in the HIM. La and Ce also concentrate in the silicic melt, whereas Tb-Tm in the HIM. Hence, the immiscible separation causes REE fractionation and produces residual silicic melt enriched in LREE and depleted in HREE. The weak fractionation among Tb-Tm and Yb, Lu can be attributed to recurrent extraction of the HIM from the magmatic system, while flat HREE chondrite-normalized patterns are interpreted to indicate no or little loss of the HIM.

Hurai, Vratislav; Simon, Klaus; Wiechert, Uwe; Hoefs, Jochen; Kone?ný, Patrik; Huraiová, Monika; Pironon, Jacques; Lipka, Jozef

157

Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania  

NASA Astrophysics Data System (ADS)

Sadiman volcano is located in the Crater Highlands area of northern Tanzania, which lies next to the western escarpment of the Gregory rift—a part of the eastern branch of the East African Rift system. It consists of interlayered phonolitic tuffs, tuff breccias (with blocks of nephelinites) and nephelinitic lava flows. Rare xenoliths of phonolite lava and ijolite were observed within the nephelinite lavas with ijolite blocks occurring in phonolitic tuffs. No evidence for the presence of melilite-bearing and/or carbonatitic rocks was found during this study. On the basis of petrography, mineralogy and geochemistry the nephelinites are divided into highly porphyritic nephelinite, wollastonite nephelinite and phonolitic nephelinite, the latter of which is the dominant variety at Sadiman. Nepheline + clinopyroxene + titanite ± perovskite ± andradite-schorlomite ± wollastonite ± sanidine ± sodalite are the principle pheno- and microphenocryst phases. The nephelinites are highly evolved (Mg# = 0.17-0.26) alkaline to peralkaline (AI = 0.88-1.21) rocks enriched in incompatible elements such as Rb, Ba, Th, U, Nb, Pb, Ta, Sr and light REEs, and strongly depleted in P and Ti. This suggests derivation from an enriched mantle source and fractionation of apatite and Ti-rich mineral(s). Primary melt inclusions in nepheline phenocrysts (Thomogenization = 860-1100 °C) indicate enrichment of volatile components in the melts, particularly of fluorine (up to 1.8 wt.% in silicate glass) resulting in the formation of daughter fluorite in partly and complete crystallized inclusions. The Sadiman nephelinites crystallized under relatively oxidizing conditions (above the FMQ buffer), which differ from the reducing conditions reported for trachytic and pantelleritic rocks from other parts of the Gregory rift. Similar rock types and relatively oxidizing conditions are known from Oldoinyo Lengai and other localities, all of which are closely associated with carbonatites. By analogy, we conclude that andradite-schorlomite-rich nephelinites may indicate a pre-stage on the evolutionary path towards carbonatitic magmatism.

Zaitsev, A. N.; Marks, M. A. W.; Wenzel, T.; Spratt, J.; Sharygin, V. V.; Strekopytov, S.; Markl, G.

2012-11-01

158

Non-Arrhenian Multicomponent Melts Viscosity: Extension of the Model.  

NASA Astrophysics Data System (ADS)

Viscosity exerts a strong control on many magmatic and petrological processes on micro- and macroscopic scales. Furthermore, knowledge of such a property gives important constraints for structural theories of silicate melts. Giordano and Dingwell (GD)[1] recently described the viscosity of a large number of silicate melts coping a wide compositional range, on the basis of an empirical parameter (SM), sum on a molar basis of the network modifying cations. That model does not consider the contribution of the iron oxidation state to influencing the viscosity. Here, this is rectified. In this experimental study we have doubled (over 700 data) the number of viscosity determinations input into the model provided by [1] extending the compositional basis to further rhyolitic, trachytic, moldavitic, andesitic, latitic, pantelleritic, basaltic and basanitic melt compositions. The effect of Fe2+/Fe3+ on viscosity has been expressly examined. The temperature-dependence of the new liquid compositions have been investigated at high temperature (1050-1600° C) and low temperature (616-860° C) by using a concentric cylinder apparatus and the micropenetration technique, respectively. Fe2+/Fe3+ ratio has been determined by combining a wet chemistry technique (potassium dichromate titration) with microprobe analysis for each sample, before and after the high and low-T viscometry. Viscosity parameterization along the lines of the GD model, and incorporating the constraint that the high-T viscosities converge to a common value [2], reveal very good agreement with those calculated by [1]. Explicit redox influences appear to be small. [1] D.Giordano, D.B. Dingwell, 2003. Earth Planet. Sci. Lett. 208, 337-349; [2] J.K. Russell, D. Giordano, D.B. Dingwell, 2003, Am. Mineral. 88, 1390-1394.

Mangiacapra, A.; Giordano, D.; Potuzak, M.; Dingwell, D. B.; Russell, J. K.

2003-12-01

159

Geology and mineral deposits of the Hekimhan-Hasancelebi iron district, Turkey  

USGS Publications Warehouse

An area of 210 sq km was investigated in the Hekimhan-Hasancelebi district. of central Turkey as part of the Maden Tetkik ve Arama Institusu(MTA)-U. S. Geological Survey(USGS) mineral exploration and training project to explore for iron deposits and to provide on-.the-job training for MTA geologists. The rocks of the area are Cretaceous and Tertiary sedimentary and volcanic rocks intruded by syenite and a serpentinized mafic and ultramafic complex and overlain unconformably by late .Tertiary basalt. The base of the section is a thick mafic volcanic-sedimentary sequence with diverse rocks that include conglomerate, sandstone, shale, tuff, limestone, and basalt. The upper part of the sequence is metasomatized near syenite contacts. The sequence is conformably overlain by trachyte and unconformably overlain by massive limestone. Overlying the limestone is a Tertiary sedimentary sequence which is dominantly conglomerate and sandstone with local limestone and volcanic rocks. This series is in turn overlain by olivine basalt. Mineral deposits are associated with the two types of intrusive rocks. Hematite-magnetite in the Karakuz mine area and in the Bahcedami-Hasancelebi area is related to the syenite, and siderite in the Deveci mine area is possibly related to the mafic-ultramafic rocks. Significant iron resources are found, only in the Karakuz and Deveci areas. In the Karakuz area disseminations, veins, and replacements consisting of hematite and magnetite are present. Most of the material is low grade. In the Deveci mine area a large deposit of siderite apparently is a replacement of carbonate beds adjacent to serpentinized igneous rock. The upper part of the siderite deposit is weathered and enriched to a mixture of iron and manganese oxides of direct shipping ore grade. Additional investigation of both the Karakuz and .Deveci mine areas is recommended including: 1. A detailed gravity and magnetic survey of part of the Karakuz area. 2. Diamond drilling at both the Karakuz and Deveci areas.

Jacobson, Herbert S.; Kendiro'glu, Zeki; Ozdemir; Celil, Bogaz; Resat; Onder, Osman; Gurel, Nafis.

1972-01-01

160

Uranium in igneous rocks of central Davis Mountains, west Texas  

SciTech Connect

The central Davis Mountains are remnants of a Tertiary eruptive center composed of layered extrusives and a few shallow intrusions. Rock types range from basalts to trachyandesites, latites, trachytes, syenites, and rhyolites. Uranium abundance increases in general with SiO/sub 2/ and K/sub 2/O content, and ranges from a mean of 1 ppM in basalts and andesites to 7 ppM in rhyolites. Uranium is most abundant in welded tuffs, in contrast to lava flows and shallow intrusions. Hydrothermal alteration redistributes uranium; unaltered rocks have significantly more uranium than altered rocks of all types. Glassy rocks contain up to one-third more uranium than their crystalline counterparts. In a vertical section through three rhyolitic-welded tuff units, uranium increases progressively from the oldest unit to the youngest; within each of two of these units, uranium also increases upsection. Further, rhyolitic-welded tuffs from the southwest part of the area contain 50% more uranium than those from the east; the eruptive source, however, has not been located. Within a given rock type, uranium occurs preferentially in accessory minerals, in areas surrounding hydrothermally leached zones, and in vein fillings. Coarse-grained rocks have more localized concentrations of uranium than aphanitic or glassy rocks. Ground-water leaching of uranium from igneous rocks of the central Davis Mountains is not considered an effective mechanism for uranium redistribution and enrichment because of the low permeability of the rocks and the nature of occurrence of uranium. Therefore, the probability of occurrence of large secondary uranium deposits in the area is not high.

Schaftenaar, W.E.; Tieh, T.T.

1983-03-01

161

Origin of volcanic seamounts at the continental margin of California related to changes in plate margins  

NASA Astrophysics Data System (ADS)

Volcanic samples collected with the Monterey Bay Aquarium Research Institute's ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB-like to relatively enriched OIB. High-precision 40Ar/39Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction-dominated to a transform margin. They differ in composition, age, and origin from other more widespread near-ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling-Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.

Davis, A. S.; Clague, D. A.; Paduan, J. B.; Cousens, B. L.; Huard, J.

2010-05-01

162

Petrogenesis of Plio-Quaternary basalts in Mahabad, NW Iran  

NASA Astrophysics Data System (ADS)

The Mahabad1:100000 sheet is located mostly in the Kurdestan district on southern part of west Azerbijan province between east Longitude 45? 30'- 46 ?, and northern Latitude 36? 30'- 37?. Geographic position, geological and structural setting as well as general geological characters of this zone is very similar to Sanandaj-Sirjan zone. Topography is dominated by mountainous terrain with an average elevation around 1800 meters. The oldest rocks belong to intrusive rocks, Mahabad Rhyolite. The younger ones include Plio-Quaternary basalt to alkali basalt, andesite, trachyte and alluvium terraces and salt marsh. The young quaternary volcanoes occur in the southern range east and east of mahabad map sheet. The Plio-Quaternary volcanic lava are seen in the Borhan village? It is built almost entirely of fluid lava flows?. The volcanic rocks are basic in composition (basalt, tephrit basanite). The petrographic and geochemical evidences, related diagrams show fractionation. By studying the major and trace elements variation diagrams, a trend of normal crystallization can be seen crustal contamination in extensional environments. It seems that the original magma has an ultrabasic composition. Some of the phenocrysts of olivine, pyroxene & plagioclase are seen in thin sections. These rocks have microlitic porphyritic? hyallo microlithic porphyritic textures in thin sections. On the basis of chemical analysis? magma that has formed the rocks had alkaline nature. The ratio of nephelin norm is around 5.3 in this rocks. A primitive mantle- normalized incompatible trace element diagram shows close similarity to typical OIB pattern. All of documents denote that magma originated from an enriched asthenospheric mantle and low degree of partial melting in source. Key words: alkali basalt, Quaternary, Volcanic, Compression. asthenospher

Shojaei, Masoomeh; Kheirkhah, Monireh; Hashem Emami, Mohamad; Maleki, Glavig

2010-05-01

163

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

164

238U- and 232Th-decay series constraints on the timescales of crystal fractionation to produce the phonolite erupted in 2004 near Tristan da Cunha, South Atlantic Ocean  

NASA Astrophysics Data System (ADS)

Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29-30, 2004 was analyzed for 238U- and 232Th-series nuclides to determine initial 230Th, 226Ra, 210Pb, 210Po, 228Ra, and 228Th activities. The initial ( 210Po/ 210Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its 210Po upon eruption. The ( 230Th/ 232Th) and ( 238U/ 232Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of 210Pb, 226Ra, and 230Th in the phonolite contrast with those of the trachyandesites, in that 210Pb and 230Th are both strongly enriched with respect to 226Ra. In addition, the phonolite had a small deficit in 228Ra with respect to 232Th. The Ra deficits likely resulted from partitioning into feldspars and hornblende in a time frame that extended over several decades to a century. These disequilibria can be explained by crystal fractionation at a decreasing rate through time at an average of 3-5 × 10 -3 year -1. The calculated crystallization rate is about an order of magnitude faster than has been calculated for most other phonolites and trachytes, and about half that calculated for crystallization of the Makaopuhi lava lake. These data imply that the 2004 magma was not the differentiated cap of a much larger body that remained at depth. Instead, it was likely the residue of a relatively small body of more mafic magma that was injected into the crust southeast Tristan and underwent extensive and rapid crystal fractionation before it erupted.

Reagan, Mark K.; Turner, Simon; Legg, Matthew; Sims, Kenneth W. W.; Hards, Victoria L.

2008-09-01

165

Felsic lavas of Terceira Island, Azores: distribution, morphology and mode of emplacement  

NASA Astrophysics Data System (ADS)

Terceira Island in the Azores archipelago is a remarkable example of effusive felsic volcanism. It is located in a geodynamic setting dominated by the WNW-ESE slow-spreading Terceira Rift that separates the Eurasian and Nubia plates, east of the Mid-Atlantic Ridge. Terceira differs from the other islands of the archipelago for the abundance of peralkaline felsic domes and coulees, which are the product with the largest volumetric expression (4.1 km3 DRE) in the recent eruptive history of the island (< 20 ka). These lavas fill and overflow the calderas of Pico Alto and Santa Bárbara volcanoes, but also occur along the flanks of the two volcanoes. Morphological, morphometric and geological analysis provided the means to constraint the emplacement modes of these peralkaline felsic lavas. From the spatial distribution of the eruptive centres it was possible to determine the presence of extensive WNW-ESE, NW-SE and ENE-WSW alignments, suggesting that these lavas were fed from depth by dykes strongly influenced by regional stress fields, although sometimes locally subjugated by magmatic stress. Lavas from both volcanoes are peralkaline trachytes and comendites very uniform in appearance with black, scoriaceous, rubbly surfaces, ranging from almost aphyric to porphyritic. They show surface morphologies typical of viscous magmas such as ogive-like rigdes, convex in the direction of flow, high levees, lava channels and spines. The lava domes are 14-183 m in height, with radius of 50-372 m, ranging in volume from 7x104 to 4x107 m3. Coulees can reach lengths in excess of 2800 m, with widths ranging from 110 to 900 m and thicknesses of 15-70 m. The calculated volumes range from about 3x105 to 108 m3. The morphometric analysis indicate that domes follow a geometrical growth pattern of low domes (H = 0,36R), dominated essentially by an endogenous regime, although exogenous growth involving extrusions of small lobes is also present. This suggests a low magma viscosity at time of extrusion, compatible with the yield strengths (3x104 - 7x105 Pa) and plastic viscosities (3x107 - 7x1010 Pa.s) estimated and the peralkaline nature of these magmas. The low correlations observed between the morphometric parameters of the coulees suggest a more complex emplacement. The transition from dome to coulee may result from the increase of the effusion rate or the failure of one side of the dome allowing lava to flow downhill. As the lava supply becomes exhausted levees develop in the coulee by drain-out of the central part.

Pimentel, Adriano

2010-05-01

166

An historic subplinian/phreatomagmatic eruption: the 1630 AD eruption of Furnas volcano, Sa˜o Miguel, Azores  

NASA Astrophysics Data System (ADS)

The 1630 AD eruption on the island of Sa˜o Miguel in the Azores took place from a vent in the southern part of the 7 × 5 km caldera of Furnas volcano. Precursory seismic activity occurred at least 8 hours before the eruption began and was felt over 30 km away. This seismic activity caused extensive damage destroying almost all buildings within a 10 km radius and probably triggered landslides on the southern coast.The explosive activity lasted ~ 3 days and ashfall occurred as far as 550 km away. Published models yield a volume of 0.65 km3 (DRE) for the explosive products. Throughout the course of the eruption more than six discrete airfall lapilli layers, each of subplinian magnitude, were generated by magmatic explosive activity. Dispersal directions initially to the west and finally northeast of the vent indicate a change in wind direction during the eruption. Isopleth maps suggest column heights of up to 14 km and wind speeds varying between < 5 and 30 m/s when compared to published plume models. On steep southern slopes (> 20°) at least one lapilli layer (L2) shows pinch and swell thickness variations, and rounded pumice clasts suggesting instant remobilisation as grain flows.Ash-rich layers with abundant accretionary lapilli and vesicular textures are interbedded with the lapilli layers and represent the deposits formed by phreatomagmatic phases that punctuated the purely magmatic activity. The ash-rich layers show lateral thickness variations, as well as cross-bedding and sand-wave structures suggesting that low-concentration, turbulent flows (surges) deposited material on topographic highs. These pyroclastic surges were probably responsible for the 80 people reported burned to death 4 km southwest of the vent. High-particle-concentration, non-turbulent pyroclastic flows were channelled down steep valleys to the southern coast contemporaneously with the low-concentration surges. The massive flow deposits (~ 2 m thick) pass laterally into thin, stratified, accretionary lapilli-rich ashes (~ 20 cm thick) over 100 m horizontally. Lateral transition between thick massive and thin stratified facies occurs on a flat surface unconfined by topography indicating that the flows had an effective yield strength.Effusive activity followed the explosive activity building a trachytic lava dome with a volume of ~20 × 106 m3 (0.02 km3 DRE) within the confines of the tuff/pumice cone formed during the explosive phase. Historic records suggest that dome building occurred over a period of at least two months. Calculated durations for eruptive phases and the fluctuation in eruptive style suggest that the eruption was pulsatory which may have been controlled by variable magma supply to the surface.

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

1995-12-01

167

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

168

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

169

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

170

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

171

The Rungwe Pumice (Rungwe, Tanzania): a wind-still Plinian eruption  

NASA Astrophysics Data System (ADS)

The Late Holocene eruptive history of Rungwe Volcano in South-West Tanzania indicates a dominance of sustained explosive eruptions ranging from violent Strombolian to Plinian scale, generating extensive pumice fall deposits. The most significant deposit is that of the Rungwe Pumice, a ca. 4 ka old Plinian pumice fall deposit of trachytic composition. The comprehensive documentation in the field of the preserved part of the deposit enables us to estimate a set of crucial eruptive parameters. A maximum eruption column height HT of 30.5 - 35 km, associated with a peak mass discharge rate of 2.8-4.8 × 108 kg/s, is suggested by empirical models. These values are consistent with estimations from TEPHRA2 inversion on grain size data, which suggest a maximum HT of 33±4 km, corresponding to mass discharge rates of 2.3-6.0 × 108 kg/s. The total grain size distribution of the preserved part of the deposit shows Mdf of -1.5f to -2.4f. The deposit thinning trend can be extrapolated far beyond on-land exposures using lake core data from Lake Malawi. Volume estimates from thickness data range between 3.2 and 5.8 km3, corresponding to an erupted mass of 1.1 - 2.0 × 1012 kg, which is consistent with TEPHRA2 inversion (1.1 × 1012 kg). The deposit dispersal is consistent with an eruption happening during nearly no-wind conditions. The plume corner is estimated at 11 - 12 km from the vent. The Rungwe Pumice eruption clearly classifies as Plinian (VEI 5, Magnitude 5 - 5.3). The eruption intensity gradually increased during the opening phase, after which a high discharge rate was maintained throughout the eruption. A lack of pyroclastic density current deposits, including in proximal-to-medial locations, suggests that there was no fountain collapse, which may be caused by the extremely low pumice density of order of 400 - 450 kg/m3.

Fontijn, Karen; Ernst, Gerald; Bonadonna, Costanza; Elburg, Marlina; Mbede, Evelyne; Jacobs, Patric

2013-04-01

172

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

173

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

174

Silicate-carbonate-salt liquid immiscibility and origin of the sodalite-haüyne rocks: study of melt inclusions in olivine foidite from Vulture volcano, S. Italy  

NASA Astrophysics Data System (ADS)

Melt inclusions in clinopyroxenes of olivine foidite bombs from Serra di Constantinopoli pyroclastic flows of the Vulture volcano (Southern Italy) were studied in detail. The rocks contain abundant zoned phenocrysts and xenocrysts of clinopyroxene, scarce grains of olivine, leucite, haüyne, glass with microlites of plagioclase and K-feldspar. The composition of clinopyroxene in xenocrysts (Cpx I), cores (Cpx II), and in rims (Cpx III) of phenocrysts differs in the content of Mg, Fe, Ti, and Al. All clinopyroxenes contain two types of primary inclusion-pure silicate and of silicate-carbonate-salt composition. This fact suggests that the phenomena of silicate-carbonate immiscibility took place prior to crystallization of clinopyroxene. Homogenization of pure silicate inclusions proceeded at 1 225 - 1 190°C. The composition of conserved melts corresponded to that of olivine foidite in Cpx I, to tephrite-phonolite in Cpx II, and phonolite-nepheline trachyte in Cpx III. The amount of water in them was no more than 0.9 wt.%. Silicate-carbonate inclusions decrepitated on heating. Salt globules contained salts of alkali-sulphate, alkali-carbonate, and Ca-carbonate composition somewhat enriched in Ba and Sr. This composition is typical of carbonatite melts when decomposed into immiscible fractions. The formation of sodalite-haüyne rocks from Vulture is related to the presence of carbonate-salt melts in magma chamber. The melts conserved in clinopyroxenes were enriched in incompatible elements, especially in Cpx III. High ratios of La, Nb, and Ta in melts on crystallization of Cpx I and Cpx II suggest the influence of a carbonatite melt as carbonatites have extremely high La/Nb and Nb/Ta and this is confirmed by the appearance of carbonatite melts in magma chamber. Some anomalies in the concentrations and relatives values of Eu and especially Ga seems typical of Italian carbonatite related melts. The mantle source for initial melts was, most likely, rather uniform, undepleted and was characterized by a low degree of melting and probable presence of garnet in restite.

Panina, Liya I.; Stoppa, Francesco

2009-12-01

175

Magmatism and Eurekan deformation in the High Arctic Large Igneous Province: 40Ar- 39Ar age of Kap Washington Group volcanics, North Greenland  

NASA Astrophysics Data System (ADS)

The High Arctic Large Igneous Province is unusual on two counts: first, magmatism was prolonged and has been suggested to include an initial tholeiitic phase (130-80 Ma) and a second alkaline phase (85-60 Ma); second, it was subsequently deformed during the Eurekan Orogeny. New 40Ar- 39Ar dating of alkaline volcanics from Kap Kane, part of the Kap Washington Group volcanics at the northern tip of Greenland, provides an emplacement age of 71.2 ± 0.5 Ma obtained from amphibole in lapilli tuffs, and a thermal resetting age of 49-47 Ma obtained in feldspar and whole-rocks from trachyte flows. Patch perthite feldspars and coeval resetting of Rb-Sr isotopes by hydrothermal fluids provide further support for thermal overprinting. This thermal event is interpreted as a result of compressional tectonism of the Kap Cannon Thrust Zone in which older Palaeozoic metasediments were thrusted northwards over the Kap Washington Group volcanics. The formation of the tholeiitic suite (130-80 Ma) is linked to the opening of the Canada Basin and may involve mantle plume action. Formation of the alkaline suite (85-60 Ma) is attributed to continental rifting in the Lincoln Sea area linked to seafloor spreading in the Labrador Sea and the Baffin Bay, and to eastwards displacement of Greenland relative to North America. The alkaline suite, therefore, may be unrelated to the main tholeiitic phase of the High Arctic Large Igneous Province. The subsequent initiation of continental rifting and ensuing seafloor spreading in the Northeast Atlantic resulted in spreading and volcanism (61-25 Ma) on both sides of Greenland, pushing Greenland northwards relative to North America. The tectonic setting in the High Arctic thus changed from extensional to compressional and volcanic activity was terminated. 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, C.; Storey, M.; Holm, P. M.; Thorarinsson, S. B.; Zhao, X.; Lo, C.-H.; Knudsen, M. F.

2011-03-01

176

The role of magmatic and hydrothermal processes in the chemical evolution of the Strange Lake plutonic complex, Québec-Labrador  

NASA Astrophysics Data System (ADS)

The Strange Lake plutonic complex consists of three annular Mid-Proterozoic arfvedsonite-aegirine-bearing alkali granites emplaced in the Rae province of the Canadian Shield. The mineralogy, chemistry and structural setting of the complex are very similar to that of many peralkaline central salic complexes associated with the development of the Gardar rift in southern Greeland. The Strange Lake granites are highly fractionated (Rb/Sr=5 to 160 and K/Rb=27 to 120) and carry unusually high abundances of HFSE and REE-bearing exotic minerals (e.g. pyrochlore, gittinsite, elpidite, gadolinite and kainosite) which are reflected in the elevated HFSE (e.g.Zr=307 to 16800 ppm) and REE (e.g. La=84 to 1337 ppm) contents of the granites. HFSE and REE increase from the oldest intrusive unit, which is hypersolvus and unaltered, to the youngest, which is subsolvus and metasomatized. The unaltered granites display a restricted range of ?18O values (+8.2 to+9.6‰) and low ?18O signatures for fresh arfvedsonite/aegirine (+4.8 to+5.2‰). Anomalously high CaO (0.7 to 3.2 wt%) and MgO (0.1 to 0.6wt%) concentrations characterize the altered subsolvus granites. These rocks also have elevated whole rock ?18O values (+9.6 to +11.9‰), negative ??18Oquartz-alk.feld. (-0.1 to-1.6), and high ?18O values of altered arfvedsonite (i.e.+6.5 to 13.75‰) that correlates positively with whole rock ?18O values. The chemical and isotopic data are consistent with a model in which the least evolved alkali granites are formed through differentiation from trachytic (syenitic) parents. Extreme HFSE and REE-enrichment may have been accomplished by differentiation through fractional crystallization and heterogenous distribution of F-rich silicic residual melts in which the REE and HFSE are transported as fluorocomplexes. The O-isotopic values are consistent with the circulation of low temperature (lt;200° C) hydrothermal fluids in the youngest subsolvus intrusive unit which caused extensive Ca (Mg and Sr) metasomatism and fluorine leaching, widespread hematization, and remobilization of the HFSE and REE.

Boily, M.; Williams-Jones, A. E.

1994-03-01

177

Origin of chemical variation of the magmas formed Aso caldera  

NASA Astrophysics Data System (ADS)

Aso caldera has been considered to form by four large scale pyroclastic flow eruptions, Aso 1-4, small scale Plinian eruption, Aso 2/1-4/3, and lavas. The volcanic rocks consist basalt, trachyte, dacite and rhyolite. These volcanic rocks are characterized by high alkaline contents compared with volcanic rocks occur on the volcanic front elsewhere. There are many evidences that geochemical characteristics of Aso caldera can not be able to explain cooling process in single magma chamber (Yasuhara and Yokose, 1998, 1999): (1) LILE-HFSE have a positive correlations, (2) incompatible elements (K, Y, Zr, Nb and REE) / SiO2 ratios decrease through eruption cycles, (3) incompatible trace element abundances pattern on spider diagram does not similar to island arc signature, (4) chemical variations cannot indicate as a single trend, (5) volcanic glass in essential fragments does not change through eruption cycles and have a similar composition systematically, (6) there are geochemical differences between pumice from pyroclastic flow deposits and fall deposits. These chemical variations in the magmas of Aso caldera can not be explained by AFC or magma mixing models. In order to understand the contribution of fluids on magma genesis, we have analyzed on chlorine and sulfur contents in the pumices. Chlorine contents indicates positive correlation with SiO2 and LILE, and negative correlation with Sr. This positive correlations imply that increasing of volatile elements in a melt correspond to increasing with degree of melting. The negative correlation may be linked with stability of plagioclase. The ratios of HFSE / Cl have wide variation and become small to the order of lava, scoria, pyroclastic fall deposit and pyroclastic flow deposits. As eruption cycle becomes new, the ratio decrease gradually. It suggests that volatile components represented by Cl may control the degree of melting and abundance of incompatible elements. Based on the above observations, geochemical variation in volcanic rocks from Aso caldera is not created in the magma chamber, but in the source region by progressive dehydration melting. It is conceivable that there were few volatile components in the partial melting at the early stage of eruption cycles, and the volatile component increased in the partial melting at the late stage of eruption cycles. Concentrations of the volatile elements play an important role in the reaction.

YASUHARA, M.; YOKOSE, H.

2001-12-01

178

Hydrogeochemical characterization of groundwater using multivariate statistical analysis and tritium in the mountainous area of Jeju Island, South Korea  

NASA Astrophysics Data System (ADS)

The baseline groundwater quality of Jeju volcanic island was investigated for the mountainous area where natural area is dominant and basaltic rocks and hydrovolcanic tuffs are distributed. Principal component analysis (PCA) results showed that principal component (PC) 1 represented natural mineralization by CO2- charged water and PC 2 corresponded to contamination from nitrate sources among four PCs considered. Hierarchical cluster analysis (HCA) was performed to PC scores generated from PCA to reduce clustering biases due to mutually-dependant variables, which resulted in 5 sample groups; high-altitude springs, low- mineral water, nitrate-contaminated water, intermediate-mineral water and high-mineral water which are distinguished by total dissolved solids (TDS), nitrate concentration and water type. High-mineral water has higher PCO2 and calcite saturation states than other groups. The high-mineral water also has 3H values lower than 1.5 TU indicating contribution of old groundwater is significant while low-mineral water has 3H values of about 3 TU which is close to those in rainwater. Geographically, the high-mineral water is concentrated in the southwestern area accompanying with intermediate-mineral water. This feature is likely to be related to (1) the presence of extensive trachytic rocks which could form locally isolating aquifers to enable prolonged water-rock interactions under high PCO2 and (2) higher location of low-permeable hydrovolcanic tuffs in subsurface in the southern area which increases chances to tap the aquifers below the hydrovolcanic tuffs. Cumulative probability of TDS showed two break points of 50 and 150 mg/L which distinguished high-altitude springs, low-mineral water and high-mineral water. Cumulative probability of nitrate provided possible threshold values for anthropogenic contamination of 2.5 mg/L and 5.5 mg/L. 32% of samples have nitrate concentration higher than 2.5 mg/L indicating high vulnerability to surface contamination sources in the mountainous area. These threshold values were consistent with the result of PCA and HCA. PCA and HCA are more effective than other threshold calculating statistics because PCA and HCA have implication of geochemical processes and anthropogenic contamination. It is suggested that PCA and HCA could be used as complementary method for determining threshold values.

Koh, D.; Chae, G.; Kang, B.; Koh, G.; Yoon, Y.; Ko, K.; Park, K.

2008-12-01

179

The Influence of Volatiles on the Glass Transition  

NASA Astrophysics Data System (ADS)

The glass transition is the temperature interval across which melt relaxation times change appreciably, from instantaneous at high temperatures (super-cooled liquid) to infinite at low temperatures (glass). It has several important implications, being where the speciation of water is locked into the melt structure, viscous flow and welding stop and elastic response to mechanical stress commences. It depends on composition, thermal history and the timescale of the experimental method being used. In this study we use the variation of heat capacity with temperature measured by differential scanning calorimetry to define glass transition temperatures (Tg) for a series of synthetic and natural samples with a range of volatile contents. Two methods have been used to define Tg: a) the temperature of the onset of the glass transition, and b) the temperature of the peak in heat capacity across the glass transition. For each sample the thermal history (cooling and heating at 10 K/min) is the same, allowing direct comparison of Tg. The samples include natural glasses with a wide range of compositions (trachyte, dacite, phonolite, basalt) that have been doped with varying amounts of water, basaltic glasses from the Hawaiian drill core that contain naturally occurring amounts of water and jadeite glasses synthesised with varying amounts of water and carbon dioxide. We will show that for each natural composition water affects Tg with the same overall pattern. Increasing water contents cause Tg to decrease (2 wt.% water causes drops of 190 to 280° C), with the largest effect occurring with the addition of the first 1 wt.% of water (drops of 110 to 200° C). The drop varies depending on the glass composition. The effect of carbon dioxide on the Tg of jadeite glass is less clear, no systematic variation with concentration occurs. Water affects the Tg of jadeite glass in exactly the same way as in the natural compositions. We will also examine how the width of the glass transition (in terms of temperature) and the height of the heat capacity peak are affected by composition and volatile content.

Nichols, A. R.; Giordano, D.; Morizet, Y.; Dingwell, D. B.; Kohn, S. C.; Brooker, R. A.

2004-12-01

180

Adakites along oceanic transforms ?  

NASA Astrophysics Data System (ADS)

Quaternary dacites and trachytes from the Aird Hills and Lusancay Islands in Papua New Guinea show some of the clearest slab melt geochemical signatures (Mg# 73-93, Sr = 1520-2650 ppm, Sr/Y = 140-445, La/Yb = 135-238), yet there is no slab currently subducting beneath Papua New Guinea. Alternatively, they may be melts from orogenic mafic crustal underplate, yet they do not occur above an arc crustal keel, nor are they part of an active convergent tectonic setting. Instead, they occur at the tip of a propagating rift-tectonic system within rift-related mafic to silicic alkaline magmatic suites. Although some adakites lie up to 100 km off the present rift-front, they connect to a curved line after their relative positions are adjusted for 16° of late Cenozoic rotation that accounts for active oceanic spreading in the Woodlark Rift. The timing of rift-propagation is consistent with the Quaternary age of Papua New Guinea adakites. The geochemical signature of these rocks is similar to other modern adakites (Western Aleutians, Cerro Pampa, Cook Island). Their Mg#, Sr contents, Sr/Y and La/Yb ratios are significantly higher than those of adakitic melts from orogenic mafic underplate. Trace element modeling indicates that their high Sr/Y and La/Yb ratios requires both small partial melting degrees (<5 vol.%) with residual garnet-amphibolite and/or eclogite (30-42 vol.% garnet), and subsequent interaction with metasomatised peridotite. We present a model of rift-propagation over previously subduction-modified mantle. Papua New Guinea was influenced by rapid creation and destruction of oceanic microplates since Mesozoic times. In a new rift-tectonic regime, rift-related magmas encountered and partially melted some remnant eclogite and/or garnet-amphibolite slab fragments, and subsequently interacted geochemically with hydrated and veined peridotite, resulting in adakites in a non-convergent tectonic setting. Other rift melts traveled through this paleo-mantle wedge without encountering any slab fragments, and formed MORB and OIB-type magmatic suites. Support for our model comes from geophysical tomographic imaging which indicates a number of flat-lying, anomalously fast regions interpreted as former subduction zone remnants. Earlier ophiolite studies show (adakite-like) trondhjemitic rocks along oceanic paleo-transform faults. Papua New Guinea is a key area for testing the adakite = slab melts story, because it not only simulates the geochemical, but also the geodynamic context which presumably led to widespread continental crustal growth in the Archean. This challenges existing adakite and Archean crustal growth models which suggest that the generation of adakitic melts are restricted to convergent plate margins.

Haschke, M.; Ben-Avraham, Z.

2003-04-01

181

Geochemistry of Miocene glass in sediments from the AND-2A Core, McMurdo Sound, Antarctica: magma petrogenesis and response to glacial unloading  

NASA Astrophysics Data System (ADS)

Volcanic glass recovered from the AND-2A core between ~354 and 765 mbsf (~15.9-18.4 Ma) occurs as accumulations (2-50 vol.%) within sediments. Glass-rich sediments consist of muddy to fine sandstone and stratified diamictite. Glass varies in color, size, vesicularity, crystal content, angularity, and from fresh to moderately altered. The common occurrence of delicate cuspate forms and fresh glass suggest that they were introduced directly into the water as ashfall with minimal reworking. Therefore, key samples currently being dated using the 40Ar/39Ar method will likely provide important time horizons for the core. Pristine basaltic glass (MgO 3-8 wt.%) is ne-normative (5-30 wt.%) and has a restricted average SiO2 content (ca. 45.2 ± 0.8 wt.%). Variations in CaO and TiO2, positively correlated with MgO content, indicate differentiation controlled by fractionation of plagioclase, olivine, clinopyroxene and magnetite. Trace element concentrations, determined on glasses by LA-ICP-MS, are typical for alkaline volcanism of the Erebus Volcanic Province (EVP). Specifically, our data extends the known compositional range of Mt. Morning, the only known Early-Middle Miocene source in the EVP, to more mafic compositions, revealing the roots of both high alkaline (basanite to phonolite) and lower alkaline (alkali basalt to trachyte) differentiation series. Glass-rich sediments in glacimarine sequences from the AND-2A core record numerous cycles of glacial advance and retreat [1, 2]. Increases in rate, explosivity and volume of volcanism have been related to crustal loading and unloading by ice-sheets in Iceland and elsewhere. AND-2A sediments with high glass contents correspond with ice minimums and, geochemically, Ba/La and Ba/Hf ratios are correlated to intervals of ice expansion (decreasing values) and contraction (increasing values) at multiple depths. Within a single cycle, glass angularity, vesicularity, and composition also vary systematically. Results provide new information from which to evaluate the potential impact of glacial cyclicity on EVP volcanism during the Early-Middle Miocene. [1] Fielding et al. (2011) PALEO3, 305, 337-351. [2] Passchier et al. (2011) GSA Bull.

Nyland, R.; Rocchi, S.; Di Vincenzo, G.; Del Carlo, P.; Tiepolo, M.; Field, B.

2011-12-01

182

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

NASA Astrophysics Data System (ADS)

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 (about 1.8 Ma). Most of streams in Jeju island are dry in normal times. Owing to high permeable geologic features such as clinkers, stream run-off occurs when precipitation is over 40 mm/day. To understand runoff phenomena in Jeju island, some streams are monitored automatically about stream stage, and water quality. Oedocheon (cheon means stream) is monitored during the normal and runoff period. Oedocheon is a permanent stream in Jeju island, and its water quantity and quality is originated by a spring water from lava flow boundaries. The surface limit and watershed parameters for the Oedocheon watershed were created and calculated by WMS software. Stream stages respond very quick during the runoff time, but the duration of the runoff is so short about a few hours. Parameters such as landuse, soil condition, preconditioned rainfall, and vegetables influence runoff phenomena. Periodic stream discharge measurements and chemical analyses of the water were preformed in order to analyze the seasonal variation of the stream water quantity and quality in normal times. Considering water intake quantities, ordinary stream discharge is 2,569~50,415 m3/day, average 21,215m3/day. Water qualities are dependent on each measurement season. Electrical conductivity is 101.7-202.0 ¥ìS/cm, pH is 7.38-8.38, and water temperature is 10.8-23.3¡É. Major ion concentrations also varies seasonally. Mg is 2.39-7.45 mg/l, Ca is 4.11-11.54 mg/l, Na is 4.80-13.24 mg/l, K is 1.64-3.47 mg/l, SO4 is 2.78-8.25 mg/l, HCO3 is 17.78-36.61 mg/l, and Cl is 6.43-22.77 mg/l. The stream discharge and water quality are so correlated with rainfall. The analyses of the responses to the rainfall in each season can be used to estimate the size of the underground groundwater watershed.

Ha, K.; Moon, D.; Park, K.

2007-12-01

183

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

184

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

185

40Ar/39Ar AGE, petrology, and tectonic significance of some seamounts in the Gulf of Alaska  

NASA Astrophysics Data System (ADS)

New petrographic, geochemical, and 40Ar/39Ar age data on Welker Guyot, Patton Seamount, Murray Guyot, Miller Seamount, and Pathfinder Seamount, when considered with previously published data, show that the majority of seamounts in the Gulf of Alaska formed by mid-plate, rather than ridge associated, volcanic processes. Lavas recovered by dredging from Welker, Patton, Murray, and Pathfinder include alkalic basalt, hawaiite, mugearite, benmoreite, and trachyte, which are typical of mid-plate volcanoes. Lavas from Miller Seamount are tholeiitic basalt and basalt transitional between tholeiitic and alkalic basalt, which are typical of volcanoes associated with accretionary processes at spreading ridges. These genetic associations are confirmed by the age data, which show that Miller was constructed on younger crust than the other four seamounts. 40Ar/39Ar age spectra show the following ages for the five seamounts: Welker 14.9±0.3 m.y., Patton 29.7±0.3 m.y., Murray 27.6±0.2 m.y., Miller 25.8±2.1 m.y., and Pathfinder 23.1±0.2 m.y. Welker Guyot is too old to be part of the hot spot trace proposed by Turner et al. [1980] for other dated volcanoes in the Pratt Welker chain unless the velocity of volcanic propagation changed just before and after Welker formed. It seems more likely that the volcanoes in this chain were formed by at least two episodes of mid-plate volcanism as well as volcanism associated with spreading ridges. The 40Ar/39Ar ages of Patton and Murray are much older than the 2-8 m.y. age suggested by their shallow depth and basement swell height; they apparently sit on crust that has been recently thermally rejuvenated. The volcanic "chains" in the Gulf of Alaska were formed by multiple episodes of intermittent mid-plate volcanism and by volcanism associated with spreading ridges rather than by the persistent volcanic activity typically associated with hot-spot chains elsewhere on the Pacific Plate. Their apparent alignment is in part fortuitous.

Dalrymple, G. Brent; Clague, David A.; Vallier, Tracy L.; Menard, H. William

186

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

187

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs  

USGS Publications Warehouse

Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb Sr and Nd Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb 204Pb vs 206Pb 204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb 204Pb vs 206Pb 204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components-a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction. ?? 1991.

Basu, A. R.; Wang, Junwen; Huang, Wankang; Xie, Guanghong; Tatsumoto, M.

1991-01-01

188

Volcanic activity and its link to glaciation cycles: Single-grain age and geochemistry of Early to Middle Miocene volcanic glass from ANDRILL AND-2A core, Antarctica  

NASA Astrophysics Data System (ADS)

In the frame of the ANtarctic DRILLing Program, volcanic glass fragments were collected from the AND-2A core between ~ 354 and 765 m below sea floor (mbsf) as accumulations (5-70 vol.%) within sediments. Here, we present the physical characteristics, age and geochemistry of the glass, which enable us to reconstruct Early to Middle Miocene volcanic activity in southern McMurdo Sound and, for the first time, document the response of volcanism to climate change in Antarctica.Glass-rich sediments include muddy-to-fine sandstone and stratified diamictite. Glass varies in color, size, vesicularity, crystal content, angularity, and degree of alteration. The mostly fresh glass exhibits delicate cuspate forms indicating deposition as primary ash fall. 40Ar-39Ar age determinations on individual glass grains are in good agreement with the depositional age model of the sediments (ca. 15.6 to 18.6 Ma), supporting for most of them a primary origin, however, some samples do contain older fragments that indicate glass recycling during times of enhanced glacial erosion.Most glasses are mafic (MgO = 3 to 9 wt.%) and vary from hypersthene to nepheline normative with a restricted range in SiO2 (45.2 ± 0.8 wt.%, 1?) and trace element concentrations typical of the rift-related alkaline rocks in the Erebus Volcanic Province. The glass extends known composition of early phase Mount Morning activity (ca. 11-19 Ma), the only known Early to Middle Miocene source, to a more mafic end, revealing a previously unknown explosive, strongly alkaline, basaltic phase and the most primitive forms of both strongly alkaline (basanite to phonolite) and moderately alkaline (alkali basalt to trachyte) magma associations.The glass-rich sediments occur in glacimarine sequences that record 56 cycles of glacial advance and retreat. Volcanic response to glacial cyclicity is observed both physically and geochemically in AND-2A glass. Higher glass volumes in sediments correlate with ice minimum conditions between 300 and 800 mbsf. Ratios of Ba to Hf, Nb, La and Zr in mafic glasses (? 5 wt.% MgO) show a systematic increase in mean values during intervals of ice retreat and decreasing values with ice expansion, suggesting tapping of magmas with variable incompatible to compatible trace element ratios. This may be related to changes in the stress state of the crust in response to rapid ice volume fluctuations over the volcano, which may influence magma chemistry by varying the duration and depth of magma storage.

Nyland, R. E.; Panter, K. S.; Rocchi, S.; Di Vincenzo, G.; Del Carlo, P.; Tiepolo, M.; Field, B.; Gorsevski, P.

2013-01-01

189

Chlorine-Bearing Phases in the Campanian Ignimbrite, Italy  

NASA Astrophysics Data System (ADS)

The Campanian volcanic zone, located west of Naples, Italy, has been active for more than 300 ky. The Campanian ignimbrite, a trachyte-phonolite pyroclastic deposit, erupted at 39 ka with an estimated volume > 200 km3 and spread predominantly eastward overlapping the nearby western Apennines. Recent studies have distinguished a complex stratigraphy of at least two cooling units with variable degrees of welding. We have examined more than 80 samples of scoria, pumice, and massive gray and yellow facies ignimbrite with scanning electron microscopy and electron microprobe techniques in conjunction with detailed analytical chemistry of major, minor, and trace elements. Marialitic scapolite, Cl-rich cancrinite, sodalite, halite, and sylvite have been identified in massive yellow and gray facies ignimbrite and in piperno. Marialitic scapolite contains 4.1 wt.% Cl; the cancrinite, a probable davyne-type, contains 8 wt.% Cl (SO3= 4 wt.%); and sodalite contains 7.2 wt.% Cl. Halite and sylvite appear to be relatively pure from SEM-EDS analyses. All phases occur as well-formed crystals, with many scapolites doubly-terminated. Scapolite is more common than cancrinite in the studied samples. All the Cl-bearing phases occur in open spaces on a thin-section scale. Scapolite contains numerous fluid inclusions; all appear to be 'empty', contain low-density vapor, and are coeval with various solid inclusions such as titanite, Mn-bearing pyroxene, Mn-bearing titanomagnetite, and apatite. Based on the details of the textures, contained fluid and solid inclusions, and crystal habit, we have concluded that all these minerals formed from supercritical to subcritical aqueous fluids post-emplacement. Cooling, fluid migration, and perhaps post-emplacement fluid exsolution mobilized sufficient material to form a variety of volatile-rich phases, alter glass, and to form overgrowths on some feldspars. Some of these Cl- bearing phases are observed to be partially altered. We suggest that given enough time, these Cl-rich phases will alter, releasing Cl to the hydrosphere. Hence, the chemistry of the bulk ignimbrite will, in time, become Cl depleted. Studies of older deposits comparing Cl concentrations in early pristine melt inclusions to the chemistry of the erupted products may assume a greater atmospheric Cl emission than had actually happened.

Belkin, H. E.; de Vivo, B.; Lima, A.

2006-05-01

190

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

191

Rheological and Thermodynamic Properties of Volatile-bearing Magmas from Pantelleria, Etna and Phlegrean Fields Magmas  

NASA Astrophysics Data System (ADS)

The rheological and thermodynamic properties of silicate melts control fluid-dynamics of transport, eruption style and rates of physico-chemical processes (degassing and crystallization) in natural magmas. In this study we investigated the effect of H2O and CO2 on the liquid viscosity and heat capacity on several multicomponent systems. Measurements were conducted on four series of melts, obtained by remelting and homogenization of natural pantelleritic (Khaggiar dome, Pantelleria), trachytic (Agnano Monte Spina eruption, Phlegrean Fields), latitic (Fondo Riccio eruption, Phlegrean Fields) and trachybasaltic (Etna 1992 eruption) magmas. CO2 or H2O synthesis experiments were conducted in piston cylinder apparatus. The volatile -bearing samples were measured with a differential scanning calorimeter (DSC) and a vertical dilatometer (micropenetration technique). Water and CO2 content were measured by Karl Fisher Titration and FTIR spectroscopy. Compositions were analyzed over a range of water contents up to 5.3 wt% and CO2 content up to 4000 ppm. Viscosity ranged from 108 to 1013 Pa s with decreasing temperature from 630 to 1100 K. Viscosity is strongly affected by H2O and CO2. The effect of CO2 on viscosity appears to be a function of speciation and chemical composition. The heat capacity of glasses and liquids and the glass transition interval were also investigated. Base chemical composition have a strong influences on Tg: high alkali contents can lower Tg of low NBO/T liquids. This behavior is demonstrated by Pantellerite samples. Glass transition temperatures are also strongly affected by H2O and CO2. The CO2 effect, such as water, is to decrease Tg and it appears to be a function of chemical composition. We present data for partial molar CpH2Omol and CpOH- and derive a simple expression to evaluate the relative contributions of different H-bearing species to the total heat capacity of hydrous melts. Experimental viscosity and calorimetric data were fitted according to the Adam and Gibbs theory in which configurational entropy (Sconf) is the main factor controlling the viscosity of melts. From calorimetric measurements, and assuming that the vibrational contribution to the liquid Cp remains constant above Tg, we determined the configurational contribution to Cpliq and thus calculated the variation of the Sconf as a function of T, H2O and CO2 content in the liquid state. Combining viscosity measurements with the configurational entropies for our liquids, we parameterized the variation of viscosity as a function of temperature and volatiles content within the framework of the Adam and Gibbs theory of structural relaxation.

Di Genova, D.; Romano, C.; Alletti, M.; Behrens, H.; Scaillet, B.

2011-12-01

192

Degassing of CO 2 and H 2O in submarine lavas from the Society hotspot  

NASA Astrophysics Data System (ADS)

We analyzed CO 2- ?13C-H 2O- ?D in vesicles of a suite of 23 vesicular submarine volcanic glasses (4 alkalic basalts and 19 trachytes) from Teahitia, Volcano 4 and Rocard seamounts and Mehetia Island in the active area of the Society hotspot (French Polynesia, South Pacific). Vesicle C and H 2O concentrations ([C vesicle] and [H 2O vesicle]) vary in the range 1-1487 ppm C and ˜0-5431 ppm H 2O, respectively. The ?13C of CO 2 in vesicles ( ?13C vesicle) vary from - 3.8‰ to - 9.7‰ and the ?D of H 2O in vesicles ( ?D vesicle) vary from - 32‰ to - 71‰. The ?13C vesicles values are correlated with the vesicularity, Log([C vesicle]) and the ?D vesicle values. The Log([H 2O vesicle]) is also correlated to the ?D vesicle values and Log([C vesicle]). These observations show that late-stage open-system degassing, following a first stage of closed-system degassing, is the chief process controlling the concentration and isotopic ratio variations observed in the vesicles. The highest measured ?13C vesicle values fall in a typical mantle range (- 4.0 ± 0.5‰). Taking into account the effect of vesicle bursting and crystal fractionation, the reconstructed pre-eruptive carbon and water concentrations in the alkalic basalt parental magma are ˜3810 ppm C and 14,360 ppm H 2O (1.43 wt.%). These concentrations correspond to carbon and water fluxes for the Society hotspot during the last 900 kyr of 2.57 × 10 4 tons C yr - 1 and 97 × 10 3 tons H 2O yr - 1 and volatile concentrations of the source of 190 ppm C and 720 ppm H 2O. These values are similar in magnitude to previous estimates for other hotspots. Our study emphasizes that H 2O degassing must be taken into account before discussing H 2O content and ?D value variations in the lavas in terms of source variation in the case of the Society hotspot.

Aubaud, Cyril; Pineau, Françoise; Hékinian, Roger; Javoy, Marc

2005-07-01

193

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

194

Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia  

NASA Astrophysics Data System (ADS)

In order to constrain better the distribution, age, geochemistry and origin of widespread Cenozoic intraplate volcanism on Zealandia, the New Zealand micro-continent, we report new 40Ar/ 39Ar and geochemical (major and trace element and Sr-Nd-Hf-Pb isotope) data from offshore (Chatham Rise, Campbell and Challenger Plateaus) and onland (North, South, Auckland, Campbell, Chatham and Antipodes Islands of New Zealand) volcanism on Zealandia. The samples include nephelinite, basanite through phonolite, alkali basalt through trachyte/rhyolite, and minor tholeiite and basaltic andesite, all of which have ocean island basalt (OIB)-type trace element signatures and which range in age from 64.8 to 0.17 Ma. Isotope ratios show a wide range in composition ( 87Sr/ 86Sr = 0.7027-0.7050, 143Nd/ 144Nd = 0.5128-0.5131, 177Hf/ 176Hf = 0.2829-0.2831, 206Pb/ 204Pb = 18.62-20.67, 207Pb/ 204Pb = 15.54-15.72 and 208Pb/ 204Pb = 38.27-40.34) with samples plotting between mid-ocean-ridge basalts (MORB) and Cretaceous New Zealand intraplate volcanic rocks. Major characteristics of Zealandia's Cenozoic volcanism include longevity, irregular distribution and lack of age progressions in the direction of plate motion, or indeed any systematic temporal or spatial geochemical variations. We believe that these characteristics can be best explained in the context of lithospheric detachment, which causes upwelling and melting of the upper asthenospheric mantle and portions of the removed lithosphere. We propose that a large-scale seismic low-velocity anomaly, that stretches from beneath West Antarctica to Zealandia at a depth of > 600 km may represent a geochemical reservoir that has been in existence since the Cretaceous, and has been supplying the upper mantle beneath Zealandia with HIMU-type plume material throughout the Cenozoic. In addition, the sources of the Cenozoic intraplate volcanism may be at least partially derived through melting of locally detached Zealandia lower lithosphere.

Timm, Christian; Hoernle, Kaj; Werner, Reinhard; Hauff, Folkmar; den Bogaard, Paul van; White, James; Mortimer, Nick; Garbe-Schönberg, Dieter

2010-01-01

195

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe  

NASA Astrophysics Data System (ADS)

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different continental intra-plate volcanic fields The in the Rhön Mts. and the Heldburg dike swarm tapped mantle source is characterized by an enriched Pb-isotope geology. The highest HIMU component has been measured in the lherzolite-bearing Veste Heldburg phonolite. This higher enriched Pb isotope signature compared to the mafic magmas cannot be explained by crustal contamination. Assimilation fractionation crystallization (AFC) modeling of the Heldburg phonolite allows us to petrogenetically link this melt with HIMU rich shallow mantle amphibole-bearing xenoliths. These new observations suggest that melting started in more depleted mantle segments. And that these melts interacted with more enriched metasomatic overprinted lithospheric mantle domains.

Meyer, R.; Song, X.; Elkins-Tanton, L. T.

2009-12-01

196

Increasing Interaction of Alkaline Magmas with Lower Crustal Gabbroic Cumulates over the Evolution of Mt. Taylor Volcanic Field, New Mexico  

NASA Astrophysics Data System (ADS)

The Mount Taylor Volcanic Field at the southeastern edge of the Colorado Plateau, New Mexico erupted diverse alkaline magmas from ~3.8 to 1.5 Ma (Crumpler, 1980; Perry et al., 1990). The earliest eruptions include high silica topaz rhyolites of Grants Ridge (plagioclase, quartz, biotite) and Si-under saturated basanites and trachytes at Mt Taylor stratovolcano. Mt. Taylor was later constructed of stacks of thick, trachyandesitic to rhyolitic lava flows that were subsequently eroded into a ~4-km across amphitheatre opening toward the southeast. Early Mt. Taylor rhyolitic lavas exposed within the amphitheatre contain quartz, plagioclase, hornblende, and biotite (± sanidine) phenocrysts. Later cone-building trachydacite to trachyandesite lavas are crystal-rich with plagioclase and augite megacrysts (± hornblende, ± quartz) and record an overall trend of decreasing SiO2 with time. The last eruptions ~1.5 Ma from the stratovolcano (Perry et al. 1990) produced thick (>70 m), viscous lava flows that contain up to 50% zoned plagioclase phenocrysts. While SiO2 decreased among the silicic magmas, the degree of silica saturation increased among peripheral basaltic magmas from basanite to ne-normative hawaiite to hy-normative basalts. Evidence of increasing crustal contamination within the basalts includes zoned plagioclase megacrysts, augite and plagioclase cumulate texture xenoliths with accompanying xenocrysts. These textures within the basalts combined with abundant, complex plagioclase among the cone-building silicic magmas imply interaction and mixing with gabbroic cumulate mush in the lower crust beneath Mt. Taylor Volcano. Contemporaneous basanitic to trachytitc volcanism in the northern part of the volcanic field at Mesa Chivato (Crumpler, 1980) was more widely distributed, smaller volume, and produced mainly aphyric magmas. The lower crustal gabbroic cumulates either do not extend northward beneath Mesa Chivato, or they were not accessed by lower magma flux rate in that part of the volcanic field. Future work will include quantifying the magma volumes combined with new 40Ar/39Ar dates to understand heat and mass flux rates for the entire volcanic field. In addition, we hope to investigate if early F-rich fluids of Grants Ridge topaz rhyolite are sourced in the mantle, and whether F is an important volatile component over the history of the magma system.

Schmidt, M. E.; Crumpler, L. S.; Schrader, C.

2010-12-01

197

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe  

NASA Astrophysics Data System (ADS)

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different continental intra-plate volcanic fields The in the Rhön Mts. and the Heldburg dike swarm tapped mantle source is characterized by an enriched Pb-isotope geology. The highest HIMU component has been measured in the lherzolite-bearing Veste Heldburg phonolite. This higher enriched Pb isotope signature compared to the mafic magmas cannot be explained by crustal contamination. Assimilation fractionation crystallization (AFC) modeling of the Heldburg phonolite allows us to petrogenetically link this melt with HIMU rich shallow mantle amphibole-bearing xenoliths. These new observations suggest that melting started in more depleted mantle segments. And that these melts interacted with more enriched metasomatic overprinted lithospheric mantle domains.

Meyer, Romain; Elkins-Tanton, Linda T.

2010-05-01

198

The mantle and basalt-crust interaction below the Mount Taylor Volcanic Field, New Mexico  

NASA Astrophysics Data System (ADS)

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 ~3.8-1.5 Ma (K-Ar, Perry et al., 1990). 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 (Perry et al., 1990). Mantle xenolith-bearing alkali basalts and basanites occur on Mesa Chivato (Crumpler, 1980) 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 cone-building. 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 (Perry et al., 1990), 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, C. M.; Crumpler, L. S.; Schmidt, M. E.

2010-12-01

199

Heterogeneous refertilization of the upper mantle beneath the Azorean volcanoes. Evidence from mantle xenoliths  

NASA Astrophysics Data System (ADS)

Several mantle xenoliths from the island of Sao Miguel (Azores, Portugal) have been studied to investigate the nature of the mantle beneath the Azorean archipelago. Ultramafic xenoliths are porphyroclastic spinel harzburgite and subordinate clinopyroxene-poor lherzolite, range between 3 and 10 cm in size and show clear signs of plastic deformation. In harzburgites, olivine porphyroclasts have Fo89-91, while in neoblasts it ranges between 80 and 87. Mg# in orthopyroxene and clinopyroxene spans between 85-91 and 84-92 respectively, while spinel is characterized by Cr# between 64 and 78. Two harzburgites show phlogopite. In lherzolite, olivine porphyroclasts show Fo89-91, orthopyroxenes and clinopyroxenes have Mg# 91-92 and 90-92 respectively, and spinel has Cr# 76-84. Abundant silica- and alkali-rich glass is present as intergranular micro-veins, and as primary melt inclusions in both porphyroclasts and neoblasts. Orthopyroxene porphyroclasts (1 - 6 mm) have exsolution-free rims, strong ondulatory extinction, and clinopyroxene exsolution at the core as well as abundant primary Silicate Melt Inclusions (SMIs). SMIs are quartz-hyperstene normative trachydacitic silicate glass (SiO2 = 62.3-67.9 wt%), containing CO2 bubbles. Trachydacitic glass is particularly rich in alkali, LILE (e.g., Rb, Ba, and Sr), LREE-enriched, and strongly depleted in HFSE, MREE and HREE, compatible with a melt containing a large component of deep eclogitic source (e.g. residual garnet and rutile). Olivine, clinopyroxene and rare orthopyroxene polygonal neoblasts (< 0.5 mm) also contain primary SMIs. However, these last SMIs are trachyte-phonolite, mostly nepheline normative and are REE- and HFSE-enriched, have low LREE/MREE ratios and selective LILE enrichment, consistent with small fractions of metasomatic melts generated from an upper mantle source. Some xenoliths contain melt inclusions of both kinds, but some record just one of the two metasomatic events that have refertilized the refractory mantle. We interpret the high local mantle fertility beneath the Azores to be derived by the recycling of buoyant oceanic crust material that refertilizes the refractory mantle by percolation of eclogite partial melts, such as those hosted in orthopyroxene porphyroclasts. Our data provide evidence for storage of mafic oceanic slab enclaves in a heterogeneous upper mantle in a region of ocean island basalt volcanism.

Cannatelli, C.; Frezzotti, M.; Zanon, V.; Petrelli, M.; Neumann, E.; Peccerillo, A.

2011-12-01

200

Magmatic Processes in Monogenetic Eruptions, Procida Island, Campi Flegrei, Italy: Geochemical Evidence From Melt Inclusions  

NASA Astrophysics Data System (ADS)

Campi Flegrei is an active volcanic complex located in the greater Naples area, which has produced more than 50 eruptions over the past 60,000 years. These have ranged from small eruptions such as Monte Nuovo eruption of 1538 CE to extremely large eruptions such as the Campanian Ignimbrite (150-200 DRE; Barbieri et al., 1978). The volcanic field includes the mainland area located to the west of Naples and also the two islands of Ischia and Procida. The volcanic products range from basalts to shoshonitic phonolites and trachytes, with the more evolved magmas being more abundant. Three eruptive units from Procida Island have been studied to observe geochemical trends over time within a small area and to better understand magmatic processes between monogenetic eruptions. Juvenile samples from Pozzo Vecchio, Breccia Museo, and Solchiara were collected to examine the geochemistry of the mineral phases present and melt inclusions (MIs) found within the phenocrysts. Solchiara contained phenocrysts of olivine and clinopyroxene, whereas Breccia Museo and Pozzo Vecchio samples contained clinopyroxene and sanidine as the dominant phenocryst phases. Melt inclusions from Solchiara have narrow compositional ranges in major and trace elements (i.e., CaO, TiO2, Zr, Dy, La) over a large range in SiO2 contents (47 to 55 wt%) while MI from the Breccia Museo have a limited range of SiO2 contents (57 to 61 wt%) with a wider range for major and trace elements (i.e., FeO, Al2O3, CaO, La, Th, Rb). Pozzo Vecchio MI from clinopyroxene and sanidine define different chemical compositions, but petrographic evidence does not suggest a xenocrystic origin for either mineral phase. This suggests that Pozzo Vecchio is the result of magma mixing. Modeling of fractional crystallization of olivine, clinopyroxene, and sanidine are capable of producing most of the trends in major and trace elements between the most primitive samples to the most evolved samples. Volatile concentrations between the Breccia Museo and Solchiara MI are not significantly different for CO2, and S, but Cl, H2O, and F are slightly higher in Breccia Museo MIs compared to those from Solchiara. Water contents vary significantly, with many displaying statistically zero water up to approximately 2 wt% H2O. The wide range in water contents is due to reequilibration of the MIs during laboratory heating that was conducted to homogenize devitrified inclusions.

Severs, M. J.; Fedele, L.; Esposito, R.; Bodnar, R.; Petrosino, P.; Lima, A.; de Vivo, B.; Shimizu, N.

2008-12-01

201

A new chronostratigraphical and evolutionary model for La Gomera: Implications for the overall evolution of the Canarian Archipelago  

NASA Astrophysics Data System (ADS)

A review of the general volcano-stratigraphy and geochronology of La Gomera, one of the lesser known Canary Islands, has led to the establishment of a new evolutionary model. The oldest edifice corresponds to the submarine stage built up between 20 and 15 Ma. The construction of the Submarine Edifice was followed by an important break in the activity (about 4 Ma) and deep erosion of the edifice. About 10.5 Ma ago, the main present-day edifice (the Old Edifice 10.5 6.4 Ma) emerged, which was also submarine in its initial phases. Two different main stages are distinguishable. The first stage was represented by a large, some 22 km wide basaltic shield volcano (the Lower Old Edifice). Several lateral collapse events (Tazo and San Marcos avalanches) occurred during this time and were responsible for the removal of an important part of its northern flank. In the second growth stage (the Upper Old Edifice), the activity migrated southwards. A 25-km wide composite volcano arose covering part of the remaining earlier shield volcano. The felsic (trachytic to phonolitic) activity occurring in two separate episodes formed a significant component of this composite volcano. Finally, one more recent large edifice (the Young Edifice) built up from 5.7 to 4 Ma. The lava flows of this younger edifice covered completely the centre and the south of the island and filled deep ravines in the north. More evolved magmas, including significant felsic magmas (the third and last felsic episode), occurred in this phase of activity. The growth of La Gomera was long-lasting, separated by an important gap in the activity in the Middle Miocene, with no Quaternary activity at all. At the same time on Tenerife (the nearest island east of La Gomera), three large edifices grew separately: Roque del Conde, Anaga and Teno (initially three separated islands). From the available data, it is inferred that the subaerial activity started earlier in the Roque del Conde Edifice, then on La Gomera and later in Teno in the NW and Anaga in NE of Tenerife, which is the youngest of all these edifices. These facts, together with the irregular general progress of the volcanic activity, support more complex views of the genesis for the Canary Islands than the simple hotspot model.

Ancochea, E.; Hernán, F.; Huertas, M. J.; Brändle, J. L.; Herrera, R.

2006-10-01

202

Geologic Mapping in Nogal Peak Quadrangle: Geochemistry, Intrusive Relations and Mineralization in the Sierra Blanca Igneous Complex, New Mexico  

NASA Astrophysics Data System (ADS)

Nogal Peak quadrangle is located in the northern Sierra Blanca Igneous Complex (SBIC) and contains most of the White Mountain Wilderness (geologic map is available at http://geoinfo.nmt.edu/publications/maps/geologic/ofgm/details.cfml?Volume=134). The geology of the quad consists of a late Eocene to Oligocene volcanic pile (Sierra Blanca Volcanics, mostly alkali basalt to trachyte) intruded by a multitude of dikes, plugs and three stocks: Rialto, 31.4 Ma (mostly syenite), Three Rivers, ca. 29 to 27 Ma (quartz syenite intruded by subordinate alkali granite), and Bonito Lake, 26.6 Ma (mostly monzonite). Three Rivers stock is partially surrounded by alkali rhyolites that geochemically resemble the alkali granites. The circular shape of the stock and surrounding rhyolites suggests they form the root of a probable caldera. SBIC rocks have compositions typical of those found within the Rocky Mountain alkaline belt and those associated with continental rift zone magmatism. Because the volcanic host rocks are deeply eroded, intrusive relations with the stocks are well exposed. Most contacts at stock margins are near vertical. Roof pendants are common near some contacts and stoped blocks up to 700 m long are found within the Three Rivers stock. Contacts, pendants and stoped blocks generally display some combination of hornfelsing, brecciation, fracturing, faulting and mineralization. Sierra Blanca Volcanics display hydrothermal alteration increasing from argillic in the NW sector of the quad to high-temperature porpylitic near stock margins. Retrograde phyllic alteration occurs within breccia pipes and portions of the stocks. Mineral deposits consist of four types: Placer Au, fissure veins (mostly Ag-Pb-Zn±Au), breccia pipes (Au-Mo-Cu), and porphyry Mo-Cu. A singular pipe on the SW margin of Bonito Lake stock contains sapphire-lazulite-alunite. Although Au has been intermittently mined in the quad since 1865, best production of Au originated around the turn of the last century from the Parsons Mine, a breccia pipe in the southern Rialto stock. The Great Western Mine deposit, located within three breccia pipes on the north margin of Three Rivers stock, apparently contains 150,000 troy ounces of low-grade, disseminated Au. Three Rivers syenites and alkali granites are slightly enriched in REE compared to typical intrusive rock standards but are not high enough to be exploitable. One alkali granite sample contains 2850 ppm Zr, about 5 to 10 times the values of typical rock standards.

Goff, F.; Kelley, S. A.; Lawrence, J. R.; Cikowski, C. T.; Krier, D. J.; Goff, C. J.; McLemore, V. T.

2011-12-01

203

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

204

Trace element and oxygen isotope composition of Hawaiian hotspot zircon  

NASA Astrophysics Data System (ADS)

The trace element and isotopic compositions of zircon are increasingly used to delimit the provenance of detrital minerals in sedimentary rocks, as well as the dynamics and timescales of magmagenesis and metamorphism. Numerous studies document the characteristics of zircon hosted by continental and MORB-related rocks, but the characteristics of zircon generated in magmas from oceanic hotspots are relatively unknown. We present the trace element and O-isotope compositions of Hawaiian zircon from gabbroic and dioritic xenoliths sampled by Holocene basalts at Hualalai volcano. The plutonic zircon yield U-Pb and 238U-230Th ages of ca. 250 ka and 40 ka and grew from intrusions of highly fractionated alkalic magma lodged at >10 km depths (Shamberger & Hammer, 2006; Vazquez et al., 2007). Individual Hualalai zircon are euhedral to anhedral with inclusions of feldspars, pyroxenes, and trachytic glass. Zoning apparent in cathodoluminescence images is generally indistinct, but a significant minority of crystals has rims with relatively high luminescence. Ion microprobe analyses of individual zircon crystals reveal trace element zoning that generally correlates with luminescence. Up to 20-fold variation in HREE concentrations, with overall positive correlation between Eu/Eu* and Ti and inverse correlation between Hf and Ti, characterize Hualalai zircon. These relations are similar to those reported for zircon from some felsic suites related by cooling-induced fractionation. Luminescent rims have relatively low REE, Hf, and Y, but have Ti concentrations like their corresponding interiors. Ti-in-zircon thermometry yields temperatures between 800-1000°C after adjustments for sub-unity TiO2 and SiO2 activities using silicate-oxide equilibria. These temperatures are generally consistent with temperatures derived from two-feldspar and glass-pyroxene pairs included by single zircon crystals. Despite compositional differences, zircon interiors and luminescent rims yield 238U-230Th model ages that are within analytical error of each other, suggesting a relatively brief (?104 yr) period of zircon growth and changing magma composition. Bulk zircon separates from two xenoliths yield ?18O values of 5.5-5.7 ‰, indicating that oceanic hotspot zircon has an O-isotope composition similar to zircon derived from fractionation of mantle-derived basalts such as MORB. On recently developed discrimination diagrams for zircon trace element composition and provenance (Grimes et al., 2007), Hualalai zircon plot as transitional between continental and oceanic fields, which is consistent with the transitional trace element composition of Hawaiian magmas relative to those from continental and mid-ocean ridge settings. Shamberger, PJ, and Hammer, JE, 2006, J. Pet., 47: 1785; Vazquez, JA, Shamberger, PJ, and Hammer, JE, 2007, Geology, 35: 695; Grimes et al., 2007, Geology, 35: 643

Vazquez, J. A.; Bindeman, I. N.; Shamberger, P. J.; Hammer, J. E.

2010-12-01

205

Anisotropy of magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite, Italy  

NASA Astrophysics Data System (ADS)

The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions.

Ort, Michael; Orsi, Giovanni; Pappalardo, Lucia; Fisher, Richard

2002-11-01

206

Anisotropy of magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite, Italy  

NASA Astrophysics Data System (ADS)

The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions.

Ort, Michael; Orsi, Giovanni; Pappalardo, Lucia; Fisher, Richard

207

Evolution of the Campanian Ignimbrite Magmatic System II: Trace Element and Th Isotopic Evidence for Open-System Processes  

NASA Astrophysics Data System (ADS)

The Campanian Ignimbrite, a large volume (~200 km3 DRE) trachytic to phonolitic ignimbrite was deposited at ~39.3 ka and represents the largest of a number of highly explosive volcanic events in the region near Naples, Italy. Thermodynamic modeling of the major element evolution using the MELTS algorithm (see companion contribution by Fowler et al.) provides detailed information about the identity of and changes in proportions of solids along the liquid line of descent during isobaric fractional crystallization. We have derived trace element mass balance equations that explicitly accommodate changing mineral-melt bulk distribution coefficients during crystallization and also simultaneously satisfy energy and major element mass conservation. Although major element patterns are reasonably modeled assuming closed system fractional crystallization, modeling of trace elements that represent a range of behaviors (e.g. Zr, Nb, Th, U, Rb, Sm, Sr) yields trends for closed system fractionation that are distinct from those observed. These results suggest open-system processes were also important in the evolution of the Campanian magmatic system. 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 processes can profoundly change isotopic characteristics of a magma body, these results illustrate that it is critical to understand the contribution that open-system processes make to silicic magma bodies prior to assigning relevance to age or timescale information derived from isotope systematics. 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 suggest large-scale fluid-melt interaction at liquidus temperatures is unlikely. In the case of the magma body associated with the Campanian Ignimbrite, the most likely source of open-system signatures is assimilation of partial melts of compositionally heterogeneous basement composed of older cumulates and intrusive equivalents of volcanic activity within the Campanian region. Additional trace element modeling, explicitly evaluating the mass and energy balance effects that fluid, solids, and melt have on trace element evolution, will further elucidate the contributions of open vs. closed system processes within the Campanian magma body.

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

2005-12-01

208

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

209

Temporal Variation and Bifurcating Differentiation Trends from Olivine Tholeiite Among Mafic Volcanic Rocks in the Bimodal High Lava Plains, Oregon  

NASA Astrophysics Data System (ADS)

The High Lava Plains are part of a large basaltic footprint that has dominated Neogene volcanism in the Pacific Northwest. Regional basalts, aka the HAOT, are distinctive for high alumina (> 16 wt%) and low K and low incompatible trace-element concentrations (K a few tenths % and Rb a few ppm). The HAOT are associated with high silica rhyolite tuffs and domes, making a strongly bimodal suite; we here focus on the diversity within the mafic compositions of the suite in the central High Lava Plains. The mafic suite ranges from about 8.5 to < 1 Ma and includes: 1) HAOT, that are compositionally and isotopically in keeping with regional trends; 2) calcalkaline basalts to basaltic andesites; and 3) a distinct set of Fe-rich basalts to trachyandesites. The latter crops out around the inferred source region of the ~300 cubic km (DRE) Rattlesnake Tuff. Primitive members of the HAOT suite last equilibrated with the depleted subcontinental asthenosphere near the Moho at about 35 km depth, based on comparison to phase equilibria and on seismic work (Eagar et al, 2010). From these primitive beginnings, the HAOT have a distinct differentiation trend from ~10 to 7.5 wt% MgO with an increase in FeO* from about 9 to12 wt% and a 10 to 20 fold enrichment in incompatible trace elements with a twofold decrease in Ni. Strong enrichments in La/Yb (1-4.5) preclude the variation being controlled mostly by variable melting, even of a garnet bearing source. An increasing tilt in rare earth element patterns with decreasing MgO is best modeled by minor assimilation of a rhyolite. At 7-8 wt% MgO, the HAOT trend bifurcates to give rise to Fe-poor (FeO* 10 to 6) basalts to basaltic andesites that have a larger amount of crustal contamination as indicated by somewhat elevated incompatible elements (La 10-30; La/Yb 4-12) coupled with crystal fractionation; These are the only basalts to have radiogenic isotope ratios slightly displaced toward crustal values. The ferrobasaltic to trachytic mafic rocks diverge from the HAOT trend in also at 7-8 wt% MgO and have increasing FeO (to 14 wt %) associated with the greatest incompatible element concentration increase (La 30 to 70; La/Yb ~4), but without Sr- or Nd-isotope shift relative to HAOT. These samples are also enriched in P (to > 2 wt%). These magmas appear to have an extensive recharge history as a well as having selectively assimilated apatite from wall rocks. The general stratigraphic context indicates that the HAOT were widespread regionally around 8 Ma and came through the crust fairly uncontaminated. In the western Harney Basin heralded, the oldest HAOT heralded the Rattlesnake Tuff (RST). Calcalkaline mafic compositions joined HAOT after the RST, during a possible smaller regional pulse of basalt, where crust was largely assimilated by basalt. The centers that erupted ferro-trachy compositions cluster and are generally younger and reflect passage through a more mafic and more refractory crust where fractionation, recharge and scavenging from mafic precursors impart a distinctive geochemical signature.

Grunder, A.; Streck, M. J.

2011-12-01

210

Paleomagnetism of Eocene Intrusive Rocks, Black Hills of South Dakota and Wyoming  

NASA Astrophysics Data System (ADS)

The Black Hills of South Dakota and Wyoming are a large Precambrian-cored Laramide uplift. Intruding the Black Hills are a diverse suite of igneous rocks, which include phonolites, trachytes, latites, garnet-bearing rhyolites, and pyroxenites. These intrusive bodies range in size from several meter outcrop-scale bodies, to several 10s of km wide intrusive complexes. New geochronology (40Ar-39Ar) data indicate many of these intrusive rocks are between 58 and 45 Ma in age (Duke at al, 2002). As part of a larger paleomagnetic study aimed at Jurassic strata surrounding the Black Hills, a collection of 20 sites and 145 samples of the Eocene intrusive rocks was made. A combination of alternating field, thermal, and liquid nitrogen step-wise demagnetization revealed that, with a few exceptions, these rocks have two well-defined magnetization components. The first-removed component is interpreted to be a present (dipole) field magnetization, and is removed by 10 to 30 mT a.f., or 200 C thermal demagnetization steps. The second-removed components have either positive or negative inclinations, and are defined by demagnetization steps between 30 and 200 mT a.f., or 300 to 630 C thermal demagnetization steps. These components are interpreted to be ancient, presumably Eocene, magnetizations. A preliminary mean of the normal-polarity sites is D=352, I=59.3, k=26.7, a95=18.2, N=4, and of the reverse-polarity sites is D=154.9, I=-61.3, k=23.1, a95=18.2, N=4. The combined mean direction is D=344.9, I=60.3, k=28.8, a95=10.5, N=8. Two sites of rhyolites at Mt. Theodore Roosevelt have well-defined magnetization components, but either mixed polarity (Site 99Trr1), or reverse-polarity with what might be a transitional-field direction (D=27.7, I=-37.4, k=18.0, a95=18.6, n=5), and are not included in the calculation of means. The magnetizations recorded by these Eocene rocks are essentially identical to the expected direction for the Black Hills calculated from the Diehl et al., 1983 Eocene reference pole for North America. This result indicates that the Black Hills have experienced no rotation or large-scale tilting since the Eocene, that these intrusive rocks are suitable for additional study of geomagnetic field behavior. In addition, the mean direction reported here is similar to the Jurassic Morrison Formation from the Black Hills (D=349.7, I=61.8, k=87.4, a95=4.5, N=13), supporting an assertion that the Jurassic rocks had been remagnetized during the Eocene.

Housen, B. A.; Fawcett, T. C.; Gregiore, P.

2003-12-01

211

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

212

An enriched mantle source for potassic basanites: evidence from Karisimbi volcano, Virunga volcanic province, Rwanda  

NASA Astrophysics Data System (ADS)

Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K2O/Na2O?1, and enriched in incompatible elements, with chondrite normalised (La/Yb)n>18 and Nb/Zr>0.25. The 87Sr/86Sr and 143Nd/144Nd ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances >4% there is no systematic variation of isotope ratios with fractionation. At >4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a “primitive” K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both endmembers are mantle-derived and isotopically distinct, with the PKB being characterised by 87Sr/86Sr up to 0.707 and 143Nd/144Nd as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties 206Pb/204Pb remains roughly constant at ˜19.2 while 207Pb/204Pb and 208Pb/204Pb vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at ˜1 Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated 208Pb/204Pb and 208Pb*/206Pb* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated 207Pb/204Pb ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (˜27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at ˜1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was ?5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high 208Pb/204Pb and 208Pb*/206Pb* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.

Rogers, N. W.; de Mulder, M.; Hawkesworth, C. J.

1992-09-01

213

Mafic intrusion remobilising silicic magma under El Hierro, Canary Islands  

NASA Astrophysics Data System (ADS)

The 2011 submarine eruption at El Hierro, Canary Islands, has produced volcanic bombs that degas at sea surface, boil seawater and sink when cooled and degassed. At the beginning of the eruption white coloured pumices enveloped in darker coloured spatters floated on land. These composite pumices show evidence of magma mingling with folds and undulations of the darker coloured magma within the white pumice suggesting magma mingling in a viscous regime. The white pumice is highly vesicular and resembles foam. Most of the vesicular structure is made of tightly packed, polygonal bubbles of uniform size (˜ 100 ?m), suggesting a single event of homogeneous bubble nucleation. An earlier event of heterogeneous bubble nucleation is indicated by the presence of a few large bubbles developed around tiny quartz crystals. Both the darker and lighter coloured pumices are almost aphyric. A few olivine crystals with perfect euhedral morphology occur within the darker part. Rare olivines of same composition are also found in the white pumice glass but then display somewhat rounded outlines and hopper-type structure. Melt inclusions in olivines of the darker pumice are of the same composition as the enveloping mafic glass, whereas olivines in the mixing boundary layer have melt inclusions of less mafic composition. The whole-rock composition and slightly more evolved glass composition are of basanitc and alkali rhyolitic composition (at the limit of the trachyte field) according to the TAS classification. Such rhyolitic compositions are rare in the Canaries. Analyses of residual volatile concentration in the glasses show that the silicic glass is highly degassed (F: 511 ±222; Cl: 202 ±58; S: below detection limit; values in ppm,1SD, n=10), whereas the basanitic glass still has very high halogene concentrations (F: 1354 ±151; Cl: 1026 ±47; S: 362 ±29; 1SD, n=10). In-situ analysis of trace element compositions of the dark glasses reveal typical basanitic compositions with elevated incompatible element concentrations and primitive mantle normalised spectra characteristic for the Canary Island basanites (e.g. La is of 100 times higher concentration than primitive mantle with important LREE enrichments). In contrast, the trace element composition of the alkali rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (such as Rb, Ba, K and Th). All other measured incompatible LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart. This differences increase with the atomic number of the REE reaching maximum for the MREE and thus forming an intriguing U-shaped rhyolite spectra. Furthermore, unusual U-depletion is observed in the rhyolite. Other negative spikes, such as those for Sr and P, are readily accounted for by the removal of plagioclase and apatite during magma evolution from a basanite to a more evolved melt. The results obtained so far suggest an intrusion of gas-rich basanitic melt at the base of an evolved intrusion remobilising a stagnant phonolitic melt present as late differentiate in the crust. Interaction with old oceanic crust and the volcanic edifice can be quantified and shown to have modified the phonolite melt composition and produced the alkali rhyolitic composition of the white floating pumice. Extensive gas exsolution shortly before the melt-glass transition explains the foam texture and the low volatile concentrations in the quenched alkali rhyolite.

Sigmarsson, O.; Laporte, D.; Marti, J.; Devouard, B.; Cluzel, N.

2012-04-01

214

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

215

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

216

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

217

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

218

Evolution of the Jan Mayen Ridge - new geochemical and geophysical data from the Jan Mayen Fracture Zone  

NASA Astrophysics Data System (ADS)

Geochronologic and geochemical data derived from sea-floor samples dredged from the Jan Mayen Fracture Zone together with seismic data provide new insight into the tectonomagmatic evolution of the Jan Mayen Ridge. Based on the seismic data, the Jan Mayen Ridge is believed to represent an off-rifted fragment of East Greenland continental lithosphere that since early Miocene has drifted 400 km into the North Atlantic as a result of sea-floor spreading along the Kolbeinsey Ridge. At present the Jan Mayen Ridge is uniquely located at the Mid-Atlantic Ridge north of Iceland. During the recent G.O.SARS research cruises a suite of volcanic rocks, as well as sandstones and conglomerates that are predominantly made up of volcaniclastic material were recovered from the southern escarpment of Jan Mayen Fracture Zone east of Jan Mayen. The conglomerates contain carbonate shell fragments that yielded 87Sr/86Sr age of ca. 32 Ma, which probably reflects the time of deposition of these volcano-sedimentary rocks. U-Pb ages of detrital zircon from the samples show age distribution consistent with an East Greenland source region characterized by a wide age pattern with significant Archaean and Early Proterozoic component. A population of angular zircons provides the youngest ages around 30 Ma, which are consistent with the Sr-age data from the shell fragment. These young zircons are most likely derived from the local volcanic material and do accordingly date the volcanic activity. Chemical analyses of individual volcanic clasts in the conglomerates show that they belong to the trachytic suite, and correspond mainly to hawaiites and trachyandesites. They are geochemically very similar to the recent volcanic rocks of the Jan Mayen Island. The maximum age of some of the volcanic clasts obtained by Ar-Ar whole-rock dating is consistent with the age of the youngest detrital zircons and with the Sr-age of the shell fragment. The new data suggest that the alkaline volcanism in the Jan Mayen area may be traced 30 My back in time. It is yet unknown however, whether or not the volcanic activity has been continuous since that time. The lack of a significant crustal contamination of the volcanic rocks of the Jan Mayen Ridge and in the Jan Mayen Fracture Zone is consistent with the results of seismic survey that suggests an existence of continental lithosphere beneath the northern part of the Jan Mayen Ridge farther south of the Jan Mayen Fracture Zone, i.e. farther from the volcanic center. Geophysical data suggest that spreading along the Kolbeinsey Ridge started ca. 25 My ago. The ca. 30 Ma magmatic event recorded in the dredged samples from the Jan Mayen Fracture Zone seems to reflect an episode of alkaline break-up magmatism associated with the off-rifting of the Jan Mayen micro-continent.

Slama, J.; Pedersen, R. B.; Kosler, J.; Kandilarov, A.; Hendriks, B. W. H.

2009-04-01

219

Evolution of elastic properties and acoustic emission, during uniaxial loading of rocks, from the Fogo Volcano in the island of Sao Miguel, Azores; Preliminary results.  

NASA Astrophysics Data System (ADS)

A Computerized Uniaxial Press working up to 250 kN was installed in the middle 2011 in the Laboratory of Microseismic Monitoring of ISEL. The system is able to record continuous time, pressure and axial strain (1 µm resolution) at 1s sampling rate. The loading platens were designed to integrate acoustic emission (AE) transducers. Signals are acquired and processed through an 8-channel ESG Hyperion Ultrasonic Monitoring System (10 MSPS, 14/16-bit ADC). The first experiments, presented here, were applied to a set of rock samples from the Fogo, an active central volcano in the island of Sao Miguel. Two different volcanic rock types were studied: a fine grained alkali basaltic rock with a porphyritic texture, a porosity of 4.5% and bulk density of 2700 kg m-3 (sample #3); and a benmoreitic rock with a trachytic texture, a porosity of 8.1 %, and bulk density of 2400 kg m-3 (sample #4). Cores from sample #3 were subjected to continuous increasing pressure, until failure. They show a uniaxial compressive strength (UCS) spanning from 60 to 85 MPa and a stress-strain curve with two phases: a first one with relative low Young's Module (YM) followed by a second phase were the YM increases roughly 3 times. The stress transition value occurs broadly in a stress level 50% of the UCS. The AE produced in the process is almost negligible until the YM transition stress level and increases after that. Important pulses of high AE rate occur, (> 100 s-1), associated with the occurrence and propagation of fractures, which are always parallel to the principal stress, showing an evident pattern of tensile fractures. About 20s before the failure, very important deformation rate is observed, the YM strongly decrease, and continuous AE events, with low rate, usually <50 s-1. The failure is accompanied with a sudden rise of AE events with rate > 200 s-1. Cycling stress experiences were also performed showing reversible stress-strain relation for axial pressure below the YM transition level, and important hysteresis for axial pressure above that level. The associated AE events show a characteristic Kaiser effect pattern. Cores from sample #4 undergo the same continuous increasing stress process, but failure is attained at a considerable lower pressure of 20-25 MPa. The stress-strain curves show an almost linear relation, but approaching the stress level of failure, the YM decreases. The AE events are constant but with a reduced rate until the decrease of the YM, when a significant rise in the AE occurs, achieving emission rates greater that 200 s-1. The fracture shows a characteristic shear pattern. Differences in stress-strain behavior, fracture mode and AE rates are associated with the very different structure of the rocks, once the basaltic sample is very fine grained with some very scattered and almost spherical vesicles or voids, while the benmoreitic core shows high values of porosity in a structure with vesicles and voids with very irregular shapes. Work supported by FCT, Portugal, projet FreeRock, PTDC/CTE-GIX/100687/2008

Moreira, M.; Wallenstein, N.

2012-04-01

220

Interpretation of 1.5-m resolution AUV bathymetry using ROV observations and samples at Davidson and Rodriguez Seamounts  

NASA Astrophysics Data System (ADS)

The summits and upper flanks of Rodriguez and Davidson Seamounts off California were mapped at 1.5-m resolution by the MBARI Mapping AUV. The seamounts were built by episodic eruptions on abandoned spreading ridges 10-12 and 10-15 Ma, respectively. They consist of ridges and elongate cones that parallel the old spreading axes, yet have strikingly different summit morphologies. Video observations and samples from prior ROV Tiburon dives are used to interpret the textures revealed in the AUV data, and are extrapolated to make geologic maps of the seamounts. The summit of Davidson is rugged and studded with cones of three general classes: completely smooth cones with nearly circular bases, mounds elongated into subparallel ridges, and disorganized mounds of rounded shapes. The elongated mound ridge-lines are roughly rectangular in cross-section, and smooth apron-like slopes descend below. They and the smooth cones occupy the highest points on the seamount but also occur deeper, whereas the disorganized mounds occur only deeper. Smooth, flat pockets lie between the cones. The disorganized mounds were identified as pillow lavas during ROV dives. The mounds that form ridges are blocky ’a’a-like flows, probably oriented over eruptive fissures. Lava samples vary from basalt to trachyte, and there is no correlation between the presumed fluidity of the lavas and occurrence of pillows. The smooth aprons below the blocky flows, and presumably the smooth cones, are glass-rich, volcaniclastic debris produced by explosive activity above. The debris has bedding parallel to the steep slopes, and has lithified into pavement. Pelagic sediment has accumulated between the cones. An inflated flow drained at its distal end in a valley between two ridges; collapses in the flow have drainback veneers like bath-tub rings on the inner surfaces. The summit of Rodriguez has no cones, but they dot the flanks. They are smooth with nearly circular bases and mounds elongated into ridges with smooth aprons. Their shapes and sizes are similar to cones at Davidson. The summit is a broadly domed, smooth platform, with subtle contour-parallel steps on the margins. It is punctuated by small, irregular-shaped outcrops, one of which has a leveed channel emanating from it. Faults cross the platform and large and small landslide scars indent the rim. The smooth summit was identified during ROV dives to be horizontally bedded beach deposits of volcanic sand, and the summit outcrops to be degassed blocky lava flows eroded by wave action. Volcaniclastic debris is abundant on the cones and flanks downslope. Pillow flows do not occur in the AUV survey, but were observed with the ROV much deeper on the seamount. ROV samples and observations are consistent with Rodriguez having been subaerial while active, whereas Davidson may have erupted in shallow water but was not an island. The AUV maps clearly show the contrast between the morphologies of the summits, and the types of cones produced by different eruption styles. The summit shapes, eruption styles, and subsequent erosion were profoundly influenced by proximity to the sea surface millions of years ago.

Paduan, J. B.; Clague, D. A.; Caress, D. W.; Thomas, H.; Conlin, D.; Thompson, D.

2010-12-01

221

Volcanic history and petrography of the Pliocene Etrüsk Stratovolcano, E Turkey  

NASA Astrophysics Data System (ADS)

The Pliocene Etrusk volcano, with its 3100 m elevation and ~500 km2 area, is one of the major centers of the collision-related volcanism in E Anatolia. It is located in the northeast of Lake Van, sitting almost on the culmination of the "Lake Van dome" structure forming the vertex of the eastern Turkish high plateau (Sengor et al., 2008). A ~5-km-wide horseshoe-shaped caldera, open to the south, is located in the center of the volcano. Apart from two trace element analyses and two K/Ar dates, there are virtually no data available in the literature on this major eruption center. Our study intends to fill this gap with a detailed petrographical, geochemical and geochronological study. Our new K/Ar age determinations indicate that the main volcanic edifice of the Etrusk volcano was formed in period between 4.3 and 3.9 Ma, with the eruption of several intermediate to acid lavas from a central vent. This phase ended up with the formation of a small collapse caldera that produced pyroclastic material emplaced on the earlier lavas. The final impulse of the volcano activity from the Etrusk volcanic center was the eruption of a post-caldera rhyolitic lava flow from the southern flank of the volcano (~3.8-3.7 Ma). After about 2.7 Myr of magmatic quiescence, during the Quaternary time between ~1 and 0.43 Ma, basalts erupted from the SW flank of the Etrusk volcano. They were generated predominantly from a ~N-S extending fissure, as well as from a scoria cone (Karniyarik hill) and a maar-shaped volcanic center (i.e. Düzgeyikçukuru). Edifice-forming products of the Etrüsk stratovolcano are represented by sanidine-plagioclase-biotite-clinopyroxene-phyric trachytes and plagioclase-clinopyroxene-orthopyroxene-phyric trachyandesites containing sporadic olivine phenocrysts. K-feldspar is the most abundant mineral phase in trachitic lavas of the Etrüsk volcanic system. Post caldera lavas, on the other hand, have relatively more evolved compositions ranging from trachydacite to rhyolite. All these units and also caldera walls are cut by a set of radial dykes. Both trachydacitic/rhyolitic lavas and radial dykes are made up of plagioclase, biotite and quartz phenocrysts. Some textures in the intermediate and felsic lavas (e.g., glass inclusions and sieve texture in plagioclase phenocrysts etc.) suggest that magma mixing might have been an important process in the magma chamber beneath Mt. Etrusk although such textures can also be related to the variations of volatile content of the magma. The Quaternary eruptions on the W flank of the Etrusk volcano are represented by plagioclase-olivine-clinopyroxene-phyric basalts. Our geochemical database indicates that the edifice-forming trachitic lavas are alkaline in character whereas overlying thracyandesites and post caldera thrachydacites/rhyolites plot on the subalkaline-alkaline divide on TAS diagram, displaying transitional characteristics. Quaternary fissure eruptions in the SW of the volcano classify as subalkaline basalts. All these lavas display enrichment in LIL and LREE elements relative to HFS and HREE respectively. These characteristics may be a reflection of the composition of mantle source region, although the effects of magma chamber processes (e.g. AFC and mixing) on magma composition cannot be ruled out. REFERENCE ?engör, A.M.C., Özeren, M.S., Keskin, M., Sak?nç, M., Özbak?r, A.D. and Kayan, I. (2008). Eastern Turkish high plateau as a small Turkic-type orogen: implications for post-collisional crust-forming processes in Turkic-type orogens, Earth Science Reviews, 90(1-2), 1-48.

Oyan, Vural; Keskin, Mehmet; Lebedev, Vladimir; Sharkov, Evgenii; Lustrino, Michele; Mattioli, Michele

2010-05-01

222

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

223

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

224

Mineralogical, Chemical, and Isotopic Characterization of Fracture-Coating Minerals in Borehole Samples from Western Pahute Mesa and Oasis Valley, Nevada  

SciTech Connect

This report summarizes the results of a mineralogical and geochemical investigation of fracture-coating phases obtained from archived borehole core and cuttings samples from the western Pahute Mesa-Oasis Valley region. The objective is to provide data needed to validate UGTA flow and transport models for this region. Fracture-lining minerals were characterized using micrographic techniques (SEM-EDS), and selected calcite samples were analyzed for their stable isotope ({sup 13}C/{sup 12}C and {sup 18}O/{sup 16}O) and rare earth element (REE) abundances. The main conclusions are as follows: (1) The distribution of fracture-lining mineral phases is a function of primary rock type, the style and degree of syn-depositional alteration, effects of post-depositional hydrothermal alteration, and fracture location relative to recharge waters (in the unsaturated zone) or through going groundwater (in the saturated zone). (2) Fracture-lining minerals within the welded tuff aquifers (principally the Timber Mountain and Paintbrush Tuffs) are characterized by the assemblage calcite + chalcedony + Fe- and Mn-oxyhydroxides + mixed illite/smectite (in approximate decreasing order of abundance). The predominant mode of host rock alteration is quartzofeldspathic. (3) Interbedded rhyolitic lava flow aquifers are characterized by the fracture-lining assemblage chalcedony + mixed illite/smectite + Fe- and Mn-oxyhydroxides {+-} calcite {+-} quartz {+-} K-feldspar (in approximate decreasing order of abundance). These include lava flow aquifers from the Thirsty Canyon, Beatty Wash, Paintbrush, and Quartz Mountain groups. The predominant mode of host rock alteration is quartzofeldspathic. (4) Fracture-lining zeolite minerals are abundant only within one of the basaltic lava flow aquifers (Trachyte of Ribbon Cliff) where they occur with chalcedony + calcite + clay minerals. (5) Stable isotope analyses ({sup 13}C/{sup 12}C and {sup 18}O/{sup 16}O) of secondary calcite samples were used to deduce the origin and temperature of formation of the calcite. These data are also useful for constraining the geochemical evolution of dissolved inorganic carbon in groundwater flowpath models. Two general types of secondary calcite are recognized on the basis of temperature conditions. (6) Low- to moderate-temperature calcite (<45 C) was deposited from local recharge or from ambient regional groundwater flow. It contains carbon from either pedogenic (soil zone) or carbonate bedrock sources, and occurs at depths up to {approx}2200 ft. It commonly forms transparent, euhedral crystals that may reflect recent precipitation from groundwater, especially in ER-EC-4 and -7. (7) High-temperature calcite ({approx}50 to 116 C) was deposited from groundwater that previously equilibrated with carbonate bedrock, and is observed at depths greater than {approx}1900 ft. It typically occurs as dense, opaque crystalline veins. It is most prevalent in wells located within the moat of the Timber Mountain caldera (particularly ER-EC-1, -5 and -6), and was probably deposited under hydrothermal conditions following earlier periods of volcanic activity. (8) Variations in REE patterns for secondary calcite reflect the influence of groundwater chemistry, host rock chemistry, the carbonate source, and the physiochemical conditions of deposition. The partitioning of LREE relative to HREE is influenced by water chemistry (e.g. complexing with HCO{sub 3}{sup -} and SO{sub 4}{sup 2-} ions) and by the preferential substitution of LREE for Ca{sup 2+} in the calcite crystal lattice. (9) Changes in the oxidation state of Ce and Eu are reflected as anomalies in chondrite- and shale-normalized REE plots, and are useful indicators of redox conditions during calcite formation. Strong negative Ce and Eu anomalies are most pronounced in samples from relatively shallow depths, but are not observed in all ''shallow'' samples. In general, comparative studies of REE patterns in secondary calcite with REE patterns in groundwater are needed to determine the applicability of these data for constraining react

Benedict, F C; Rose, T P; Zhou, X

2000-09-01

225

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

226

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr-Nd-Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy  

NASA Astrophysics Data System (ADS)

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr-Nd-Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of 87Sr/86Sr range from 0.70926 to 0.70999, 143Nd/144Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788-18.851 for 206Pb/204Pb, 15.685-15.701 for 207Pb/204Pb, and 39.048-39.076 for 208Pb/204Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. 87Sr/86Sr ranges from 0.70665 to 0.70745, 143Nd/144Nd ranges from 0.51234 to 0.51238, 206Pb/204Pb ranges from 18.924 to 19.153, 207Pb/204Pb ranges from 15.661 to 15.694, and 208Pb/204Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10-20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved.

Conticelli, Sandro; Marchionni, Sara; Rosa, Davide; Giordano, Guido; Boari, Elena; Avanzinelli, Riccardo

2009-01-01

227

Chemical and Physical Characteristics of Groundwater in the Western Coastal Area in Jeju Volcanic Island, Korea  

NASA Astrophysics Data System (ADS)

Residents in Jeju volcanic island use most part of water resources from groundwater. Actually, in the island, there exist no perennial streams or rivers due to extremely high infiltration rate of water into surface soils and rocks (basalt and trachyte). In the western part of Jeju Island, high pumping rate of wells caused great drawdown especially during drought period. By this current trend, great decline of groundwater level as well as seawater intrusion is predictable. According to drill data from 13 wells for monitoring seawater intrusion installed in the western part of the island by the authority of Jeju Special Governed Island, the geology of the western area is composed of five units: lava sequence (hyaloclastic breccia, acicular feldspar basalt, olivine basalt, aphanitic feldspar basalt, augite feldspar basalt, and porphyritic feldspar basalt), sedimentary layer (containing gravel and sand) intercalated in lava sequences, Seoguipo Formation (gravels, unconsolidated sands, shell fossils, and sandy mudstone), trachyandesite and tuff occurring in Seoguipo Formation, and U Formation. Geophysical well logging on the five monitoring wells (Panpo (PP), Kosan (KS), Shindo (SD), Ilgwa (IG), and Hamo (HM)), resulted in approximately 20~40 cps (counts per second) of natural gamma intensity in lava sequence. High gamma intensity of approximately 60 cps is noticeble in the sedimentary layer intercalated in lava sequence, and in Seoguipo Formation, especially clay minerals. Electric conductivity (EC) on PP, KS and IG wells showed 100~400 ?S/cm with fresh water range. However, EC on SD and HM wells increased up to around 20,000~10,000 ?S/cm with depth, which indicates variation from freshwater to salt water. Pumping tests were performed on nine monitoring wells in the range of 900~2,300m3/d and with an average discharge rate of 1,371m3/d. Among them, data from only five monitoring wells were used for pumping test analysis, since the other four wells were highly affected by tide. Transmissivity was estimated using transmissivity (T) ~ specific capacity (Q/s) relationsip: T = 0.99(Q/s)0.89/ proposed by Hamm et al. (2005). T estimates ranged from 21.9 to 2664.3m2/d, and Q/s estimates ranged from 32.4 to 7,143m2/d. The average drawdown is 12.9 m, between 0.1 and 40 m, presenting a wide variation of drawdown on different monitoring wells. From drill data, geophysical logs, and pumping tests, it is concluded that main aquifers develops in jointed parts in lava sequence, especially hyaloclastic breccia, and gravels and unconsolidated sands in Seoguipo Formation. Keywords: transmissivity, specific capacity, geophygical log, pumping test, Jeju volcainc Island Acknowledgement This work was financially supported by of the 21st Century Frontier R&D Program (project no. 3-4-3 of the Sustainable Water Resources Research Center) and by the 2nd stage of the BK21 Project, Ministry of Education, Republic of Korea.

Lee, S.; Hamm, S.; Lee, J.; Koh, G.; Hwang, S.

2008-12-01

228

Mingling processes at Panarea Volcano (Aeolian Islands, Italy): results from M73/2 cruise drilled cores  

NASA Astrophysics Data System (ADS)

The last Meteor 73/2 cruise drilled several lava cores in the southern Tyrrhenian Sea, close to Panarea Island and surrounding islets (Aeolian archipelago, Italy), at depths comprised between 50 and 70 m bsl. These rocks - unconformably covered by unconsolidated lapilli tuffs - revealed different lithologies and mineralogical assemblages corresponding to different compositions (hereafter A & B), as then evidenced by ICP-MS analyses (major and trace elements) performed on selected rock-samples. The cores also displayed several, cm-sized, rounded enclaves of the A-type dispersed in the B-type. The petrographic study on textures and microprobe analyses on glass shards and mineral phases finally concurred in identifying two magmas with different history and quite complex interaction. Rock A is a holocrystalline shoshonite (SHO) - showing plagioclase (pl - An%=62-74) and clinopyroxene (cpx) as main phases, plus subordinate amphibole and biotite phenocrysts, rare and small olivines (Fo?89%) - which represents the first magma, usually in form of enclaves. Notably, the SHO shows intersertal vesicularity and scarce glass. Rock B is a porphyritic rhyodacite (RD) characterized by pl (An%=32-52), and biotite phenocrysts, with minor cpx phenocrysts and microphenocrysts. Pl and cpx show both alternate and normal zoning, and the former have frequent K-rich reaction rims. Similar mineral phases and frequent sanidine microlites characterize the alkali-trachyte glassy groundmass of rock B. This rock hosts the SHO and represent the most voluminous magma. Overall, these features indicate a quite complex history of magma interaction(s) as well as a polybaric crystallization, which lead the volatiles abundance and behaviour. From the study of the highly irregular edges observed along their contacts, we argue intrusive and visco-plastic relationships between A and B. Moreover, the presence of irregular vesicles and vugs bounded by pl microlites suggest an emplacement at shallow level where cooling favoured both slow degassing and pervasive crystallization. Textural and compositional data concur in indicating that the two magmas mingled at depth. Noteworthy, enclaves of a third rock type - very limited in volume - is present along some of the collected cores. It is a reddish low-porphyritic lava similar to the RD lava in terms of mineralogical composition, but showing a higher amount of microlites with smaller size if compared to the main RD host-rock. This could indicate that at some extent also mixing occurred. The multiple similarities of our rocks with lavas of the Panarea islets or other acid volcanics containing mafic-intermediate enclaves and outcropping on other Aeolian Islands, suggest that mafic magma uprising "within" resident magma with subsequent mingling is a recurrent process in these volcanic systems and may be the trigger for the eruption of acid melts.

De Benedetti, A. A.; De Astis, G.; Raffaele, V.; Esposito, A.; Giordano, G.; Petersen, S.; Monecke, T.

2012-04-01

229

The Magma Chamber Simulator: A Comprehensive Tool for Modeling the Evolution of Magmatic Systems  

NASA Astrophysics Data System (ADS)

The Magma Chamber Simulator (MCS) is a computational tool for modeling the chemical and physical evolution of magmatic systems. Closed-system fractional or equilibrium crystallization as well as open-system processes such as assimilation of wallrock partial melt, equilibration of stoped country rock, magma recharge, and eruption are incorporated self-consistently by combining results from the thermodynamic model of Ghiorso &Sack (1995, i.e. MELTS) with trace element and isotope conservation equations (e.g., Spera & Bohrson, 2001, 2002; Bohrson &Spera 2001, 2003). Input includes initial compositions and masses of magma, wallrock, and recharge magma and mineral-melt-fluid partition coefficients for trace elements. Compositions (major, trace element, isotopes) and abundances of solids and melt, and thermodynamic and physical properties of the system (e.g., viscosity, density, volume fraction fluid) are output. A critical aspect of the MCS is the direct coupling of phase information generated by minimization of appropriate thermodynamic potentials with partition coefficients governing trace element distribution among solid, melt, and fluid. Each MCS simulation results in a thermodynamically-based description of the chemical and energetic state of a magma body as it evolves along a complex P-T-X path. The MCS is structured so that improvements in thermodynamic and trace element partition coefficient databases can be efficiently incorporated into the code. The MCS provides for interactive visual output so that the geochemist can compare observed geochemical data with model results. Although completion of the MCS is several years away, results are presented here on a prototype version used to study the petrologic-geochemical evolution of the Campanian Ignimbrite (39.3 ka trachytic-phonolitic ignimbrite located near Naples, Italy). We show that magma evolution was dominated by crystal fractionation in a fluid-saturated environment with minor assimilation of upper crust. A trachybasaltic parent (Tliq = 1235°C) initially crystallized olivine, clinopyroxene, spinel and apatite. At 883°C, identified as a pseudo-invariant temperature, melt simultaneously saturated in alkali feldspar, plagioclase and biotite, causing a dramatic decrease in fraction of melt from ~0.5 to 0.1. Crystallization also led to a striking decrease in melt viscosity (1700 Pa s to 200 Pa s), a marked change in volume fraction of water in magma (~0.1 to 0.8), and a significant decrease in melt and magma density; these changes acted as a destabilizing eruption trigger (see Fowler et al.). Trace element modeling in the prototype MCS couples mineral and fluid abundances along the liquid line of descent with fluid-present fractional crystallization conservation equations (see Spera et al.) and estimates of mineral-melt-fluid partition coefficients. Preliminary results indicate that trace elements can be divided into 5 groups: (1) compatible elements (Ba, Sr, Cr) which are influenced by crystallizing phases; (2) REE, which show evidence of being incompatible, although behavior of Sm and Nd may reflect crystallization of apatite; (3) HFSE, which are dominated by incompatible behavior but require input from wallrock; (4) elements (e.g., As, Sb) which may owe their behavior to interaction between magma and hydrothermally altered wallrock; (5) elements such as Rb and Pb, which may be scavenged from magma by supercritical fluid.

Bohrson, W. A.; Spera, F. J.; Ghiorso, M. S.; Fowler, S. J.

2006-12-01

230

Phase Equilibria Impetus For Large-Volume Explosive Volcanic Eruptions  

NASA Astrophysics Data System (ADS)

We have investigated the phase equilibria and associated variations in melt and magma thermodynamic and transport properties of seven large-volume silicic explosive volcanic systems through application of the MELTS (Ghiorso &Sack, 1995) algorithm. Each calculation is based on fractional crystallization along an oxygen buffer at low-pressure (0.1 - 0.3 GPa), starting from a mafic parental liquid. Site-specific geological constraints provide starting conditions for each system. We have performed calculations for seven tuffs; the Otowi (~400 km3) and Tshirege (~200 km3) members of the Bandelier Tuff, the ~600 km3 Bishop Tuff, and the 2500, 300, and 1000 km3 Yellowstone high-silica rhyolite tuffs. These represent the six largest eruptions within North America over the past ~2 million years. The seventh tuff, the 39.3 ka Campanian Ignimbrite, a 200 km3 trachytic to phonolitic ignimbrite located near Naples, Italy, is the largest explosive eruption in the Mediterranean area in the last 200 kyr. In all cases, MELTS faithfully tracks the liquid line of descent as well as the identity and composition of phenocrysts. The largest discrepancy between predicted and observed melt compositions is for CaO in all calculations. A key characteristic for each system is a pseudoinvariant temperature, Tinv, where abrupt shifts in crystallinity (1-fm, where fm is the fraction of melt), volume fraction of supercritical fluid (?), magma compressibility, melt and magma density, and viscosity occur over a small temperature interval of order 1 - 10 K. In particular, the volume fraction of vapor increases from ? ~0.1 just below Tinv to ? >0.7 just above Tinv for each case. The rheological transition between melt-dominated (high viscosity) and bubble-dominated (low viscosity) magma occurs at ? ~0.6. We emphasize that this effect is observed under isobaric conditions and is distinct from the oft-studied phenomenon of volatile exsolution accompanying magma decompression and subsequent explosive eruption. Cast in terms of the variation of melt fraction (fm) and volume fraction bubbles (?) versus the nondimensional (liquidus to solidus) enthalpy (H) change, the conditions dfm/dH >2 and d?/dH <-3 appear to be required in order to generate large-volume explosive eruption. The isobaric condition implies that country rock surrounding the evolving and expanding magma body is weak and able to accommodate the increase in system volume that occurs at Tinv. We have also explored isochoric (constant volume) fractional crystallization, noting a dramatic pressure increase at Tinv due to copious exsolution of H2O and crystallization of phases of lower density than melt. In the case of the 1875 0.8 km3 oceanic rhyolitic eruption of Askja, Iceland, MELTS results show that whereas closed-system fractionation carried out either isobarically or isochorically does not generate explosive eruption, addition of water via assimilation yields values of dfm/dH and d?/dH consistent with explosive eruption despite initially rather dry parental MORB (~<1 wt. % H2O). In general, pseudoinvariant behavior is suppressed at higher pressure (e.g., P >0.3 GPa), thus suggesting that low pressure is essential to this mechanism. Recently, we have found that the 4 km3 79 AD Pompei eruption also satisfies the thermodynamic criteria developed in this study.

Fowler, S. J.; Spera, F. J.; Bohrson, W. A.; Ghiorso, M. S.

2006-12-01

231

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