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1

The viscosity of hydrous phonolites and trachytes  

Microsoft Academic Search

The 1-atm viscosities of hydrated synthetic iron-free phonolite and trachyte melts are reported between 108.4 and 1013.1 Pa s for water contents between 0 and 5 wt.%. These show a very strong reduction with increasing water content, particularly at low contents. Empirical formulae are derived for the dependence of viscosity on temperature and water content. At magmatic temperatures of about

Alan Whittington; Pascal Richet; Yannick Linard; François Holtz

2001-01-01

2

Water diffusion in potassium-rich phonolitic and trachytic melts Sara Fanara a,  

E-print Network

Water diffusion in potassium-rich phonolitic and trachytic melts Sara Fanara a, , Harald Behrens Available online 27 September 2012 Keywords: Water diffusion Phonolite Trachyte IR spectroscopy Water diffusivity was investigated in phonolitic and trachytic melts containing up to 6 wt.% of dissolved water

Zhang, Youxue

3

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

4

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

5

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

6

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

7

Petrogenesis of basalt-trachyte lavas from Olmoti Crater, Tanzania  

NASA Astrophysics Data System (ADS)

Olmoti Crater is part of the Plio-Pleistocene Ngorongoro Volcanic Highland (NVH) in northern Tanzania to the south of Gregory Rift. The Gregory Rift is part of the eastern branch of the East African Rift System (EARS) that stretches some 4000 km from the Read Sea and Gulf of Aden in the north to the Zambezi River in Mozambique. Here, we (1) characterize the chemistry and mineral compositions of lavas from Olmoti Crater, (2) determine the age and duration of Olmoti volcanic activity through 40Ar/ 39Ar dating of Olmoti Crater wall lavas and (3) determine the genesis of Olmoti lavas and the relationship to other NVH and EARS volcanics and (4) their correlation with volcanics in the Olduvai and Laetoli stratigraphic sequences. Olmoti lavas collected from the lower part of the exposed crater wall section (OLS) range from basalt to trachyandesite whereas the upper part of the section (OUS) is trachytic. Petrography and major and trace element data reflect a very low degree partial melt origin for the Olmoti lavas, presumably of peridotite, followed by extensive fractionation. The 87Sr/ 86Sr data overlap whereas Nd and Pb isotope data are distinct between OLS and OUS samples. Interpretation of the isotope data suggests mixing of enriched mantle (EM I) with high-?-like reservoirs, consistent with the model of Bell and Blenkinsop [Bell, K., Blenkinsop, J., 1987. Nd and Sr isotopic compositions of East African carbonatites: implications for mantle heterogeneity. Geology 5, 99-102] for East African carbonatite lavas. The isotope ratios are within the range of values defined by Oceanic Island Basalt (OIB) globally and moderate normalized Tb/Yb ratios (2.3-1.6) in these lavas suggest melting in the lithospheric mantle consistent with other studies in the region. 40Ar/ 39Ar incremental-heating analyses of matrix and anorthoclase separates from Olmoti OLS and OUS lavas indicate that volcanic activity was short in duration, lasting ˜200 kyr from 2.01 ± 0.03 Ma to 1.80 ± 0.01 Ma. The age of Olmoti activity overlaps with ages reported for Ngorongoro Caldera, implying contemporaneous activity of multiple NVH volcanic centers during part of the eruption interval. Olmoti is considered the source for the bulk of interbedded volcanics and volcaniclastic deposits that comprise much of the upper Bed I section of nearby Olduvai Gorge, and part of the Laetoli sequence, both known for their well preserved fossils and archaeological remains. Age and chemical data reported here are compatible with those derived from tephra and lava interbedded in Bed I at Olduvai Gorge and from the Olpiro Beds at Laetoli.

Mollel, Godwin F.; Swisher, Carl C., III; McHenry, Lindsay J.; Feigenson, Mark D.; Carr, Michael J.

2009-08-01

8

Petrogenesis of rhyolites and trachytes from the Deccan Trap: Sr, Nd and Pb isotope and trace element evidence  

Microsoft Academic Search

Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking

P. C. Lightfoot; C. J. Hawkesworth; S. F. Sethna

1987-01-01

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

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

11

Mineral stability in peralkaline silicic rocks: Information from trachytes of the Menengai volcano, Kenya  

NASA Astrophysics Data System (ADS)

Electron microprobe analyses are presented for phenocrysts and matrix glass in peralkaline, silica-oversaturated trachytes from the Menengai volcano, Kenya Rift Valley. The dominant phenocryst assemblage is alkali feldspar-hedenbergite-fayalite-titanomagnetite-apatite. Aenigmatite, amphibole and quartz occur rarely in more peralkaline rocks. QUILF calculations indicate that the trachytic magmas crystallised at temperatures of 854-870 °C, at relatively low oxidation states (?FMQ - 1.6 to - 1.7) and silica activity (aSiO2 (Qtz) of 0.60-0.66). The new analyses are used, along with published data, to outline the distribution of the main phases in the compositional spectrum of peralkaline quartz trachytes and rhyolites. There is uncertainty about the nature, or even existence, of a low-temperature zone in the alkali feldspar primary phase region, the equivalent of the thermal valley in the haplogranite system. Quartz phenocrysts may appear early or late in the crystallisation sequence, even in rocks of similar bulk composition, its appearance perhaps being a function of the F content of the melts. Whereas hedenbergite and fayalite show fairly systematic compositional trends with increasing host rock peralkalinity, amphibole compositions are variable, for reasons not yet understood. Aenigmatite crystallisation is at least partly controlled by oxygen fugacity and silica activity. With rare exceptions, ilmenite and titanomagnetite are incompatible phases but the factors controlling their relative stabilities are not clear. It appears that peralkaline trachyte-rhyolite sequences evolve along many crystallisation paths, the paths perhaps being strongly influenced by pH2O, pF2, melt F/Cl ratios and perhaps total pressure.

Macdonald, R.; Bagi?ski, B.; Leat, P. T.; White, J. C.; Dzier?anowski, P.

2011-07-01

12

Trace element diffusion and viscous flow in potassium-rich trachytic and phonolitic melts  

Microsoft Academic Search

Trace element diffusion was experimentally investigated in nominally dry (0.03–0.10 wt.% H2O) and hydrous (1.13–1.86 wt.% H2O) melts of trachytic and phonolitic composition at temperatures from 1050 to 1250 °C and a pressure of 500 MPa. Experiments with a large set of trace elements (Rb, Sr, Ba, Cr, Ni, Cu, Zn, Y, La, Nd, Sm, Eu, Gd, Yb, Zr, Nb, Hf, and Sn) were

Harald Behrens; Matthias Hahn

2009-01-01

13

Origin of xenoliths in the trachyte at Puu Waawaa, Hualalai Volcano, Hawaii  

USGS Publications Warehouse

Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe2O3 during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO2 fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO2 was introduced from alkalic basalt magma between the time CO2-inclusion-free xenoliths erupted at 106??6 ka and the time CO2-inclusion-rich xenoliths erupted within the last 15 ka. ?? 1991 Springer-Verlag.

Clague, D.A.; Bohrson, W.A.

1991-01-01

14

Melting and remobilization of felsic protoliths through mafic recharge: evidence from basalt-trachyte mingling in Mumbai, Deccan Traps  

NASA Astrophysics Data System (ADS)

Contrasting, mingled magmas are widespread, and often erupt at volcanoes, or form intrusive bodies. Field evidence, petrography, mineral chemistry, and whole-rock major, trace and Pb isotope chemistry of trachytes and their mafic enclaves from the Manori-Gorai area of Mumbai, in the Deccan Traps flood basalt province, are used here to assess the petrogenetic processes that operated to produce these rocks. Local hybridization has occurred and has produced hybrid trachytes and intermediate rocks such as a quartz monzonite. Feldspar crystals have complex textures and an unusually wide range in chemical composition. Crystals from the trachytes cover the alkali feldspar compositional range and include plagioclase crystals with Ca contents up to An47. Crystals from the mafic enclaves are dominated by plagioclase An72-90, but include orthoclase microphenocrysts and other feldspars covering the entire compositional range sampled in the trachytes. And feldspars from the hybridized quartz monzonite yield mineral chemical clusters of An80-86, An47-54, Ab94-99, Or45-60 and Or96-98, all sampled within individual phenocrysts. We show that these feldspar crystals provide a record of remobilization of felsic protoliths by influx of mafic magmas, followed by magma mixing and hybridization processes that result in bulk rock major and trace element variations which can be modeled by essentially simple bulk mixing. However, heterogeneities in Pb isotopic compositions of trachytes and their microscopic to meter-sized enclaves are observed on the scale of individual outcrops, likely reflecting initial variations in the isotopic compositions of the involved magmas and protoliths, and provide evidence for one or more shallow trachyte magma chambers disturbed by multiple injections of trachytic, porphyritic alkali basaltic, and variably hybridized magmas from depth.

Zellmer, Georg; Sheth, Hetu; Iizuka, Yoshiyuki; Lai, Yi-Jen

2010-05-01

15

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

16

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

Microsoft Academic Search

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

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

2003-01-01

17

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

18

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

19

Partial melting and fractionation in the Mesa Chivato alkali basalt-trachyte series, Mount Taylor Volcanic Field, New Mexico  

NASA Astrophysics Data System (ADS)

Mesa Chivato comprises a series of alkaline cones, flows, and domes within the Mount Taylor Volcanic Field (MTVF) in northwest New Mexico. Compositions range from alkali basalt to trachyte. Intermediate magmas are less well represented than mafic and felsic rocks, but benmoreites and transitional benmoreite-trachytes provide a window into the differentiation processes. Major element, trace element, and isotopic data suggest that petrogenesis of benmoreite proceeded by fractional crystallization of mafic liquids and magma mixing with partially melted mafic rocks. Major element mass balance models permit the derivation of transitional benmoreite/trachyte from the benmoreite by 20-25% crystallization of microphenocryst phases (olivine, plagioclase, Ti-magnetite, and apatite) and further fractionation to trachyte by 10-15% crystallization of olivine, plagioclase and alkali feldspar, Fe-Ti oxide, and apatite. These models are supported by SiO2-Sr and -Ba systematics. However, the hawaiite to benmoreite gap cannot be crossed by fractional crystallization alone. While major element models permit the mafic lavas to yield the benmoreite, they require extensive fractionation of clinopyroxene and plagioclase - this is unsupported by petrography (clinopyroxene phenocrysts are rare in the mafic rocks and lacking in the intermediate rocks) and cannot explain the benmoreite's very high Sr contents (>1800 ppm), which would have been depleted by plagioclase fractionation. From LA-ICPMS analysis of plagioclase: 87Sr/86Sr of early alkali basalt (0.70285-0.70300) and late hawaiite (0.70406-0.70421) bracket the 87Sr/86Sr of the benmoreite (0.70361-0.70406). Thus, either could represent the fractionated liquid parental to the benmoreite and the other the partially melted source.

Schrader, C. M.; Schmidt, M. E.; Crumpler, L. S.; Wolff, J. A.

2012-12-01

20

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

21

Petrogenesis of trachyte and rhyolite magmas on Ponza Island (Italy) and its relationship to the Campanian magmatism  

NASA Astrophysics Data System (ADS)

Magmatism on Ponza island (Italy) has been reviewed, considering both new and literature data, and compared with the Campanian Volcanic District (CVD) rocks. Different origins have been inferred for the two main rock types (trachyte and rhyolite). The trachytes were formed by fractional crystallization (FC) processes coupled with minor upper crustal contamination from magmas similar to the least evolved magma erupted into CVD. The rhyolites formed by partial melting of a lower crust component. The two lithotypes evolved through intra-suite FC, and they are correlated with the volcanic rocks of the CVD by major element, trace element and isotope data, extending the Campanian compositional spectrum. To explain the FC and the melting processes, a tectonic model is developed in which most of the FC for the CVD occurred in the lower-intermediate crust where magma rises from the upper mantle and is stored in a process of magma accumulation and fractionation. These processes have produced enough heat to melt the crust and cause several rhyolite episodes in Ponza Island. A subduction-related setting must be inferred to explain the origin of the Ponza trachytes and rhyolites and the rest of the CVD volcanism.

Angelo, Paone

2013-11-01

22

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

NASA Astrophysics Data System (ADS)

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

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

2009-09-01

23

Magma Recharge and Mixing Processes That Triggered the Eruption of Trachytes and Phonolites at Suswa Volcano, Kenya Rift, East Africa  

NASA Astrophysics Data System (ADS)

Suswa Volcano, one of a series of Holocene central vent volcanoes located in the central part of the Kenya Rift, is divided into four major stages: 1) pre-caldera; 2) syn-caldera; 3) post-caldera I; and 4) post-caldera II. In addition to central vent volcanoes are basalt and basaltic-trachyandesite (BTA) flows (e.g. Tandamara and Elmenteita) that occur in low-lying areas adjacent to the central volcanoes. Both pre- and syn-caldera rocks include trachyte to phonolite. Syn-caldera rocks also include BTA similar to Tandamara. Matrix glass in pre-caldera samples is similar to whole-rock. However, for syn-caldera samples, light and dark mixed glasses are observed petrographically and compositions range from trachyandesite to trachyte. Pre-caldera samples have a phenocryst assemblage of anorthoclase (An0-5Ab50-60Or30-45), clinopyroxene (En27Fs28Wo45) and Fe-Ti oxide. Syn-caldera trachyte-phonolite contain this same assemblage but also plagioclase (An52Ab45Or3), with a composition identical to BTA samples. Clearly, the syn-caldera event represents magma mixing between BTA and trachyte. With the developing East Africa rifting, introduction of mafic magmas into the Suswa trachytic chamber was possible, similar to the rupture events in Ethiopia in 2005 (Wright et al., 2006). Post-caldera rocks have phenocrysts of alkali feldspar, olivine (Fa70), clinopyroxene and Fe-Ti oxides. Post-caldera II alkali feldspars are zoned and contain two different core compositions, one with low anorthite content similar to pre- and syn-caldera samples (An3Ab64Or33) and the other with higher anorthite content (An17Ab69Or14). They exhibit oscillatory zoning, with compositional variation between Ca2O and K2O and have thin rims with composition similar to the matrix feldspars. The thin rims may represent magma recharge that triggered eruption of the phonolite. Matrix glass in post-caldera rocks includes both trachyandesite and phonolite, indicating that hybridization of the contrasting magmas is still ongoing. Processes in addition to mixing contribute to this dynamic volcanic setting. Post-caldera rocks have compositions of essentially all elements that are intermediate between BTA and trachyte, as appropriate to mixing. However, samples show variable Na20 content at constant K20, Fe0, MgO, and CaO. A possible explanation for this variable Na2O content is assimilation of sodalite-bearing syenite roof and sidewall rocks into the evolving magma chamber. References: Wright, T.J., et al., 2006, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature, 442: 291-294.

Espejel-Garcia, V.; Anthony, E. Y.; Ren, M.; MacDonald, R.; White, J. C.

2007-12-01

24

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

25

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

NASA Astrophysics Data System (ADS)

Products of contrasting mingled magmas are widespread in volcanoes and intrusions. Subvolcanic trachyte intrusions hosting mafic enclaves crop out in the Manori-Gorai area of Mumbai in the Deccan Traps. The petrogenetic processes that produced these rocks are investigated here with field data, petrography, mineral chemistry, and whole rock major, trace, and Pb isotope chemistry. Local hybridization has occurred and has produced intermediate rocks such as a trachyandesitic dyke. Feldspar crystals have complex textures and an unusually wide range in chemical composition. Crystals from the trachytes cover the alkali feldspar compositional range and include plagioclase crystals with anorthite contents up to An47. Crystals from the mafic enclaves are dominated by plagioclase An72-90, but contain inclusions of orthoclase and other feldspars covering the entire compositional range sampled in the trachytes. Feldspars from the hybridized trachyandesitic dyke yield mineral compositions of An80-86, An47-54, Ab94-99, Or45-60, and Or96-98, all sampled within individual phenocrysts. We show that these compositional features are consistent with partial melting of granitoid rocks by influx of mafic magmas, followed by magma mixing and hybridization of the partial melts with the mafic melts, which broadly explains the observed bulk rock major and trace element variations. However, heterogeneities in Pb isotopic compositions of trachytes are observed on the scale of individual outcrops, likely reflecting initial variations in the isotopic compositions of the involved source rocks. The combined data point to one or more shallow-level trachytic magma chambers disturbed by multiple injections of trachytic, porphyritic alkali basaltic, and variably hybridized magmas.

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

2012-01-01

26

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

27

Experimental study of Cl solubility in hydrous alkaline melts: constraints on the theoretical maximum amount of Cl in trachytic and phonolitic melts  

Microsoft Academic Search

. Cl solubility in evolved alkaline melts was investigated at 860-930 °C and pressures of 25 to 250 MPa using natural trachytes and a synthetic phonolite equilibrated with subcritical fluids in the H2O-(Na,K)Cl system (i.e. silicate melt coexisted with water-rich aqueous fluid and a saline brine). Fluid phase characteristics were identified by examination of fluid inclusions present in the run

S. Signorelli; M. Carroll

2002-01-01

28

The partitioning behavior of Cl, S, and H 2O in aqueous vapor- ± saline-liquid saturated phonolitic and trachytic melts at 200 MPa  

Microsoft Academic Search

Hydrothermal experiments were conducted with molten Mt. Somma–Vesuvius phonolite and mixed-composition fluids comprised of O–H–Cl–S-cations to determine the solubilities and melt-fluid(s) partitioning behavior of the dominant volatile components at 896–1022 °C, 200 MPa, and fO2 of NNO+0.54 to NNO+1.6. The final melt compositions ranged from phonolitic to trachytic due to component exchange between the melt and fluid(s) and limited crystallization of plagioclase

J. D. Webster; M. F. Sintoni; B. De Vivo

2009-01-01

29

Deformation Structures associated with the emplacement of high level intrusions: A study of Trachyte Mesa Intrusion, Henry Mountains, Utah  

NASA Astrophysics Data System (ADS)

Most studies of sill and laccolith complexes have focused on the internal architecture and thermal effects of these intrusions, while few have looked in detail at host rock deformation structures associated with their emplacement. Various sill and laccolith emplacement mechanisms have been proposed (e.g. radial growth/ bulldozing, and two-stage growth), each with their own distinct deformation style. Compressional structures likely dominate during radial growth (bulldozing) emplacement, while extensional structures are more likely to form during two-stage growth emplacement. In this study we focus on deformation structures (faults, deformation bands and joints) associated with emplacement of Tertiary sills and laccolith intrusions in the Henry Mountains, Utah. Trachyte Mesa, the most distal satellite intrusion to the Mt. Hilliers intrusive centre, is an elongate (NE-SW) laccolith concordant with the Entrada sandstone it intrudes. The intrusion is comprised of multiple, stacked intrusive sheets. Two structural transects across the northwest lateral margin have identified distinct structural domains within the host rock that reflect both temporal and kinematic variations in deformation. Three deformation phases are identified, interpreted to be pre-, syn- and late-emplacement structures. A background set of deformation bands (phase 1), trending oblique to the intrusion margin, is apparent across the entire area. A second set of deformation bands (phase 2) overprint the early phase. These are characterised by conjugate deformation bands that parallel the intrusion margin, and increase in intensity and spacing towards the intrusion. Within this same zone a series of calcite filled normal faults, striking parallel and perpendicular to the intrusion margin, are apparent. Due to their spatial, kinematic and overprinting relationships we interpret these to be linked to the emplacement of the intrusive body. Overprinting all other structures, are two sets of tensile joints (phase 3), often infilled with calcite crystals. These occur over the top surface and lateral margin of the intrusion. Similar to phase 2 faults, these joints strike both parallel and perpendicular to the margin of the intrusion. Phase 2 and 3 structures both indicate extensional strain normal to the intrusion margin, which is consistent with a two-stage growth mechanism for the overall intrusion. Furthermore, the presence of calcite precipitation indicates that these structures have acted as good conduits for fluids. Deformation structures associated with emplacement of sills, laccoliths and dykes have the potential to affect fluid flow through a porous sandstone reservoir, which may have important implications in fields such as hydrocarbon reservoir deliverability and CO2 sequestration. Assuming deformation structures are intrinsically linked to the emplacement mechanism, predictions about fluid flow around intrusive bodies may be possible, i.e. enhancement (two-stage growth) or suppression (radial growth) of fluid flow.

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

2012-04-01

30

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

USGS Publications Warehouse

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

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

2003-01-01

31

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

32

Na2O and Trace Elements Behavior in Trachytes and Phonolites at Suswa Volcano, Kenya: the Result of Combined Magma Mixing and Volatile-rich Na-Trace Element Fluids  

NASA Astrophysics Data System (ADS)

The evolution of Suswa, a Quaternary volcano in the Kenya Rift, was dominated by the eruption of two rock suites, separated by a caldera event. Suswa is part of the Central Kenya Peralkaline Province (CKPP), which includes the Greater Olkaria Volcanic Complex (GOVC) and inter-center mafic fields, e.g. Tandamara and Elmenteita, whose compositions range from basalt to basaltic trachy-andesite (BTA). Both suites at Suswa range from trachyte to phonolite, but are distinguished by the amount of SiO2: pre- and syn-caldera rocks have 60-62%, and post-caldera rocks 57-59%. Trachyte to phonolite trends within each suite result from increasing Na2O, which is accompanied by increases in a number of trace elements (Be, Hf, Nb, Rb, Th, Y, Zn, Zr, and REE, except Eu). Magmatic processes included magma mixing, in which BTA magma similar to those of Tandamara and Elmenteita intruded the pre-caldera Suswa trachytic chamber, and fluid complexing, which was responsible for the enrichment in Na2O and trace elements. The importance of magma mixing in the CKPP has been recently documented at the GOVC by Macdonald et al. (2008, J Pet 49, 1515-1547), for which mafic-intermediate magmatic inclusions within comendites and disequilibrium phenocryst assemblages are part of the evidence. Evidence for mixing at Suswa includes: 1) mixed feldspar assemblages, e.g. syn-caldera ignimbrite samples contain both alkali feldspar (An2Ab62Or36), and xenocrystic plagioclase (An45Ab52Or3), and 2) heterogeneous matrix glass compositions. Glass in pre-caldera rocks is trachytic, similar to whole-rock compositions. Syn-caldera rocks have glass compositions both trachytic and intermediate between trachyte and BTA, while Tandamara BTA rocks contain trachytic glass. Glass in post-caldera rocks is mostly phonolitic. Glass inclusions in plagioclase xenocrysts are basaltic, similar to flows in the area. X-Y elemental plots do not show linear trends, as would be predicted from a mixing process. We attribute this to the short time scale between mixing and eruption. Experimental studies by De Campos et al. (2008, Chem Geol) and Perugini et al. (2008, Chem Geol) show that short time scales of mixing result in insufficient time for diffusion of elements to distribute linearly. The second important process at Suswa is halogen complexing, evidence for which includes: 1) High concentrations of F in matrix glass. For instance, syn-caldera matrix glass F varies from 0.5% for an early phreatomagmatic group to 1 - 2% for a later ring feeder and fissure eruptions, and post-caldera rocks have 0.5%, increasing to 1% from early to late eruptions; 2) Precipitation of LREE-rich fluorapatite and fluorite in the groundmass in syn-caldera rocks, and fluorapatite as a daughter mineral in melt inclusions in post-caldera rocks; 3) A positive correlation between Na2O and F in melt inclusions and matrix glasses. Macdonald et al. (1993, CMP 114, 276-287) documented immiscible carbonatite in syn-caldera trachytes from Suswa. Buhn and Rankin (1999, GCA 63, 3781-3797) showed that interaction with Na -F -REE - trace element-bearing carbonatitic fluids in Namibia resulted in enrichment of magmas with Na and trace elements similar to those observed at Suswa.

Espejel-Garcia, V. V.; Anthony, E. Y.; Ren, M.; MacDonald, R.; Skilling, I. P.; White, J. C.

2008-12-01

33

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

34

Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya  

Microsoft Academic Search

Two suites of felsic eruptives and intrusives are represented in a set of samples from the summit region of the Plio-Pleistocene\\u000a volcano, Mt. Kenya. Most of the samples are moderately or strongly undersaturated and have 87Sr\\/86Sr initial ratios in the range 0.70360–0.70368 (mean=0.70362). Members of this phonolitic suite are phonolites, nepheline\\u000a syenites or kenytes and as a group they show

R. C. Price; R. W. Johnson; C. M. Gray; F. A. Frey

1985-01-01

35

The viscosity of trachytes, and comparison with basalts, phonolites, and rhyolites  

Microsoft Academic Search

The viscosity of natural liquids representative of the glassy portion of pumice collected from the deposits of the Campanian Ignimbrite (IGC) and Monte Nuovo (MNV) eruption of Phlegrean Fields has been measured in the temperature range from 1770 K down to the glass transition, and for a dissolved water content range from dry to nearly 4 wt.%. Measurements were performed

D. Giordano; C. Romano; P. Papale; D. B. Dingwell

2004-01-01

36

Relationships between pre-eruptive conditions and eruptive styles of phonolite-trachyte magmas  

NASA Astrophysics Data System (ADS)

Phonolitic eruptions can erupt either effusively or explosively, and in some cases develop highly energetic events such as caldera-forming eruptions. However, the mechanisms that control the eruptive behaviour of such compositions are not well understood. By combining pre-eruptive data of well studied phonolitic eruptions we show that the explosive-effusive style of the phonolitic magma is controlled by the amount of volatiles, the degree of water-undersaturation and the depth of magma storage, the explosive character generally increasing with pressure depth and water contents. However, external factors, such as ingestion of external water, or latter processes occurring in the conduit, can modify the starting eruptive dynamic acquired at the levels of magma ponding.

Andújar, Joan; Scaillet, Bruno

2012-11-01

37

The distribution of Sr and Ba between the alkali feldspar, plagioclase and groundmass phases of porphyritic trachytes and phonolites  

Microsoft Academic Search

Data for the distribution of Sr and Ba between the feldspar phenocrysts and groundmasses of porphyritic volcanic rocks are given. It is assumed that the composition of the bulk rock represents an original liquid composition and the composition of the groundmass represents that of a residual liquid. Distribution ratios of the type, element in feldspar\\/element in original liquid, have been

C. M. B. Henderson

1969-01-01

38

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

39

TRACHYTE PHASE RELATIONS AND IMPLICATION FOR MAGMA STORAGE CONDITIONS IN THE CHAINE DES PUYS (FRENCH MASSIF CENTRAL)  

E-print Network

or pumice-and-ash flows) and (ii) provide phase relationships and chemical compositions for differentiated has been found in one sample. Pristine glasses (melt inclusions or residual glasses) in pumice from

Paris-Sud XI, Université de

40

Influence of composition and thermal history of volcanic glasses on water content as determined by  

E-print Network

and (Fe3+ )IV in calcalkaline (rhyolite to basaltic andesite) and alkaline (trachyte, phonolite to alkali to an external standard is only faintly composition-dependent for Si-rich alkaline glasses (trachytes­phonolites

Paris-Sud XI, Université de

41

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai`i  

E-print Network

mineral compositions (e.g. albitic feldspar, clinopyroxene Mg- number 67­78). Fine-grained diorites between the xenoliths and the trachyte, suggests that the shift from shield to post-shield magmatism that trachyte was derived by shallow (3­7 km) crystallization of an alkalic parent magma, based on Pb isotope

Hammer, Julia Eve

42

Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 157  

E-print Network

and Mari Sumita2 ABSTRACT Sixty-eight fallout trachytic to phonolitic ash layers recovered from Sites 953 samples changes from trachytic to phonolitic in the time interval between 2 and 0.4 Ma. The concentration composed of lavas and pyroclastics of evolved phonolitic to trachyphonolitic composition. The cone of Pico

43

Landslides density map of S. Miguel Island, Azores archipelago  

Microsoft Academic Search

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

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

2002-01-01

44

Eur. J. Mineral. 2007, 19, 849857  

E-print Network

for the emplacement of earlier, volumetrically dominant phonolites and nepheline syenites (phase I). This age is 13 such as phonolites and trachytes are also common, either directly associated with basalts, or as isolated occurrences

45

Tchabal Nganha volcano in Adamawa (Cameroon): petrology of a continental alkaline lava series  

Microsoft Academic Search

The Mio-Pliocene Tchabal Nganha volcano in the Adamawa Plateau (Cameroon) contains a broad range of rock types from ankaramites to phonolites and trachytes. Tchabal Nganha is located on large N70°E strike-slip faults that have been active since the Cretaceous and delimit the Adamawa Plateau horst. Geological mapping revealed a succession of basaltic-hawaiitic lava flow units, mugearite-benmoreite breccias, phonolite and trachyte

A. Nono; B DERUELLE; D DEMAIFFE; R KAMBOU

1994-01-01

46

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

47

Eruption and emplacement of a basaltic welded ignimbrite during caldera formation on Gran Canaria  

Microsoft Academic Search

The 14.1?Ma old composite ignimbrite cooling unit P1 (45?km 3) on Gran Canaria comprises a lower mixed rhyolite–trachyte tuff, a central rhyolite–basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3–5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly

Armin Freundt; Hans-Ulrich Schmincke

1995-01-01

48

First data on REE-bearing silicates from volcanic rocks of Kamchatka  

NASA Astrophysics Data System (ADS)

The first finds of the REE-bearing silicates in the Kamchatka island-arc system were reported from K-Na alkaline trachytes from a large volcanic body of Bolshoi volcano in the back-arc zone of the Sredinny Ridge volcanic belt (56°24'10? N and 157°56'45? E) of Kamchatka. Chevkinite microlites are found in a trachytic groundmass in association with microlites of ferropigeonite, ferroaugite, anorthoclase, Na-sanidine, richterite, apatite, Nb-bearing ilmenite, zirkelite, baddeleyite, and zircon. The chevkinites from the Bolshoi volcano trachytes have higher Nb and Zr and similar REE contents compared to the chevkinite-group minerals from igneous rocks formed in different geodynamic settings. The crystallization conditions for chevkinites from Bolshoi volcano were determined by comparing the compositions of trachytes and their mineral assemblages with those of trachytes from Belogolovskii volcano of Sredinny Ridge, Kamchatka. The results show that chevkinites were formed during prolonged crystallization of a trachytic magma under reducing conditions at ranging from -1.36 to -1.55 NNO, relatively low temperatures of 794-741°C, and moderate HF concentrations of magmatic fluids (0.003-0.0029 mol/dm3). These results, combined with the other data, are of crucial interest for studies of the REE budget during the evolution of high-alkaline magmas and can be used as indicators for determining the nature and role of different magma sources in the active continental margin setting of Kamchatka.

Shcherbakov, Yu. D.; Perepelov, A. B.; Karmanov, N. S.; Puzankov, M. Yu.; Tsypukova, S. S.

2014-12-01

49

Oligocene volcanism and multiple caldera formation in the Chinati Mountains, Presidio County, Texas  

SciTech Connect

The Chinati Mountains caldera, which lies in Trans-Pecos Texas in the southern Basin and Range Province, was formed by eruption of the Mitchell Mesa Rhyolite. Volcanism in the Chinati Mountains area began several million years before formation of the Chinati Mountains caldera. Rocks of the Morita Ranch Formation, Infiernito caldera, and Shely Group ring the caldera on the south, east, and north. After its collapse, the caldera was filled by rhyolitic to trachytic lava flows and an ash-flow tuff of the Chinati Mountains Group. These include, from oldest to youngest, the lower trachyte, middle trachyte, lower rhyolite, upper trachyte, and upper rhyolite (ash-flow tuff). The Chinati Mountains Group was then intruded by the West Chinati Stock, the resurgent dome of the caldera. Three cycles of rhyolitic to trachytic magmatism, all derived from a zoned magma chamber, are represented by (1) Mitchell Mesa Rhyolite to lower and middle trachytes, (2) lower rhyolite to upper trachyte, and (3) upper rhyolite to West Chinati Stock. Dominant caldera collapse followed eruption of the Mitchell Mesa Rhyolite, but collapse is also associated with rhyolitic eruptions in the second and third cycles. The entire sequence erupted between 32 and 33 mya. The Chinati Mountains area is the site of one major, inactive silver mine and numerous prospects for silver, lead, zinc, copper, molybdenum, uranium, and fluorite. The Shafter silver district produced 31 million ounces of silver from Permian dolomitic limestones just south of the southern boundary of the caldera. Major prospects are associated with a quartz-monzonite porphyry intrusion (copper-molybdenum) just west of Shafter and with the West Chinati Stock (silver, lead, zinc, copper, and fluorite). All mineralization is probably genetically related to the caldera. 74 references, 15 figures, 3 tables.

Cepeda, J.C.; Henry, C.D.

1983-01-01

50

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

51

A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)  

NASA Astrophysics Data System (ADS)

The island of Boa Vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world (50% of the total outcropping igneous rocks of Boa Vista). The felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years.

Ancochea, Eumenio; Huertas, María José; Hernán, Francisco; Brändle, José Luis

2014-03-01

52

Evolution of a mafic volcanic field in the central Great Basin, south central Nevada  

E-print Network

Nd = +4.2, but episode 1 samples vary to high 87Si/86Sr (up to 0.7060) over a narrow range of ?Nd (+0.8 to +4.5). Trachytic rocks (MgO ? 0.5%) are isotopically akin to the episode 1 basalts. Geochemical variation requires the addition of a crustal...

Yogodzinski, G. M.; Naumann, T. R.; Smith, E. I.; Bradshaw, T. K.; Walker, J. Douglas

1996-08-10

53

Thermoluminescence dating of Hawaiian alkalic basalts  

Microsoft Academic Search

The thermoluminescence (TL) properties of plagioclase separates from 10 independently dated alkalic basalts from the Hawaiian Islands were studied for the purpose of developing a TL dating method for young volcanic rocks. The samples range in composition from alkalic olivine basalt to trachyte and in age from 4500 years to 3.3 m.y. The TL process in basaltic plagioclase appears to

Rodd J. May

1977-01-01

54

Application of in situ zircon geochronology and accessory phase chemistry to constraining basin development  

E-print Network

000 Vandoeuvre-les-Nancy, France Accepted 7 August 2003 Abstract A series of five volcanic ash layers formation and sedimentation. Weighted mean 206 Pb/238 U ages for the five studied tuffs of a mantle component in the source of the magmas and a trachytic affiliation. The 295­300 Ma volcanic episode

Demouchy, Sylvie

55

Integrated tephrochronology of the West Antarctic region-Implications for a potential tephra record in the West Antarctic Ice Sheet (WAIS) Divide Ice Core  

E-print Network

record in the West Antarctic Ice Sheet (WAIS) Divide Ice Core N.W. Dunbar,1 W.C. McIntosh,1 A.V. Kurbatov and Mt. Takahe A number of volcanoes protrude through the West Antarctic Ice Sheet (Le the last 500,000 years . Mt. Takahe, a flat-topped stratovolcano, has produced a number of recent trachytic

Dunbar, Nelia W.

56

A232 Goldschmidt Conference Abstracts 2008 Integrated tephrochronology of the  

E-print Network

- Implications for a potential tephra record in the West Antarctic Ice Sheet (WAIS) divide ice core N.W. DUNBAR1 the last 500,000 years. Mt. Takahe has produced a number of recent trachytic eruptions, including events. Berlin, and is therefore likely to contain a number of known tephra layers from the two volcanoes

Dunbar, Nelia W.

57

Abstract Hoodoo Mountain volcano (HMV) is a Qua-ternary phonolitic volcano situated on the north side of  

E-print Network

Abstract Hoodoo Mountain volcano (HMV) is a Qua- ternary phonolitic volcano situated on the north- out the 100,000-year history of the HMV. All samples are phonolite or trachyte with (micro, petrological data sets, and ther- modynamic modeling support derivation of the phonolite magmas at HMV from

Russell, Kelly

58

Origin of oceanic phonolites by crystal fractionation and the problem of the Daly gap: an example from Rarotonga  

Microsoft Academic Search

Felsic alkalic rocks are a minor component of many ocean island volcanic suites, and include trachyte and phonolite as well as various types of alkaline and peralkaline rhyolite. However, there is considerable debate on the nature of their formation; for example, are they formed by partial melting of anomalous mantle or the final products of fractional crystallization of mafic magmas.

G. Thompson; I. Smith; J. Malpas

2001-01-01

59

Sheet-like emplacement of satellite laccoliths, sills, and bysmaliths of the Henry Mountains, Southern Utah  

Microsoft Academic Search

Small intrusions (<3 km2) on the margins of the Henry Mountains intrusive com- plex of southern Utah are exceptionally well exposed in three dimensions and have a variety of shapes. Our examination of the geometry, structures, and fabric of the Maiden Creek sill, Trachyte Mesa laccolith, and the Black Mesa bysmalith (cylindri- cal intrusion bounded by vertical faults) suggests that

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

60

Leg 192 Preliminary Report52 -7 -6 -5 -4 -3 -2 -1 0  

E-print Network

(wt%) Site 807 (A) Site 807 (C­G) Site 803 Site 289 Picro- basalt Basalt Basaltic andesite Andesite Dacite Rhyolite Trachy- basalt Basaltic trachy- andesite Trachy- andesite Trachyte Trachydacite Basanite Tephrite 1185 (Lower) Site 1186 Site 1187 Picro- basalt Basalt Basaltic andesite Dacite Rhyolite Trachy- basalt

61

Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 157  

E-print Network

-GRADE IGNIMBRITE P1 IN THE SEA AROUND GRAN CANARIA1 Armin Freundt2 and Hans-Ulrich Schmincke2 ABSTRACT Drilling change from basaltic to felsic volcanism marked by the rhyolite-trachyte-basalt mixed ignimbrite cooling to the compositional zonation of the P1 ignimbrite on land. The large volume of foreign material in the P1 layers

62

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

Microsoft Academic Search

Two distinctive ash-flow tuffs occur around the base of Volcan Las Navajas, a Pleistocene trachyte - peralkaline rhyolite center located in the northwestern segment of the Mexican Volcanic belt. The lower ash-flow unit is locally up to 65 m thick, is lithic rich and contains pumice blocks of comenditic rhyolite. The unit is not extensively exposed, and thus its areal

S. A. Nelson; J. A. Hebre

1985-01-01

63

Contrib Mineral Petrol (1995) 119:43-55 ~_?Springer-Verlag 1995 J.E. Mungall. R.F. Martin  

E-print Network

for the origin of ocean-island rhyolites Received: 29 June 1994 / Accepted: 20 October 1994 Abstract Petrogenetic mafic and felsic modes. Rhyolite and trachyte form extensive flows, pumice fall deposits and even oceanic islands presents a petroge- netic puzzle: rhyolite flows account for more than 50% of the total

Wells, Mathew G. - Department of Physical and Environmental Sciences, University of Toronto

64

Triassic alkaline magmatism of the Hawasina Nappes: Post-breakup melting of the Oman lithospheric mantle modified by the Permian Neotethyan Plume  

E-print Network

and development stages of the southern Neotethyan margin. For instance, Middle Permian flood basalts Mountains. They are predominantly alkali basalts and trachybasalts, associated with minor sub-alkaline basalts, trachyandesites, trachytes and rhyolites. Their major, trace elements and Nd­Pb isotopic

Demouchy, Sylvie

65

The rainbow range, British Columbia: A miocene peralkaline shield volcano  

NASA Astrophysics Data System (ADS)

The Rainbow Range is a Late Miocene shield volcano (30 km diameter, 370 km 3) whose stratiform flanks surround a complex central zone. Over a 2-m.y. interval, extrusion of low viscosity, silicic peralkaline lavas and minor basaltic lavas built up the gently sloping (5-8°) flanks, forming a shield volcano rather than a composite cone. Comenditic trachytes are the lowest flows exposed on the north flank of the volcano. Thin mugearite flows rest unconformably on the comenditic trachytes. Comendites unconformably overlie the mugearites and account for at least 75% of the volume of flows within the flank zone. These lavas are distinguished from the comenditic trachytes by lower Al 2O 3 (13%), higher total iron as Fe 2O 3, (7%), and extremely depleted Sr (1-10 ppm) and Ba (10-100 ppm). Strontium isotopic studies combined with petrologic data suggest that Rainbow Range lavas originated from alkali basalt magma trapped in an intracrustal magma chamber and tapped at several intervals after it underwent crystal fractionation. A best-fit mathematical model for the origin of the suite involves step-wise derivation of the lavas in the order hawaiite ? mugeartite ? comenditic trachyte ? comendite, with the major phases precipitating in the order: olivine, clinopyroxene, plagioclase, iron-titanium oxides, and alkali feldspar.

Bevier, Mary Lou

1981-12-01

66

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

67

Volcanic geology and eruption frequency, São Miguel, Azores  

NASA Astrophysics Data System (ADS)

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

Moore, Richard B.

1990-11-01

68

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

69

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

70

Late Quaternary primary tephras in Sacred Lake sediments, northeast Mount Kenya, Kenya  

NASA Astrophysics Data System (ADS)

In this paper, a sequence of five Late Quaternary tephras occurring as discrete, well-preserved horizons in lake sediments on the northeastern flank of Mount Kenya are characterised and their ages determined by a combination of high-resolution indirect radiocarbon dating and direct {U}/{Th} dating. The grain size characteristics suggest that the tephras are of fairly local origin. These Na?K-rich alkali pyroclasts with a trachytic chemical composition have a highly correlated chemistry and mineralogy, suggesting that they were probably derived from the same genetic series and possibly erupted from a single source vent. Morphological differences are attributed to the peculiar characteristics of each eruption episode. The magma source was probably a small, highly differentiated magma chamber following the olivine basalt-trachyandesite-trachyte-phonolite series, which broadly reflects the Quaternary rock suite of Mount Kenya.

Olago, D. O.; Street-Perrott, F. A.; Perrott, R. A.; Ivanovich, M.; Harkness, D. D.

2000-05-01

71

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

72

Petrogenesis of extension-related alkaline volcanism in Karaburhan (Sivrihisar-Eskisehir), NW Anatolia, Turkey  

NASA Astrophysics Data System (ADS)

Alkaline lavas were erupted as phonolites and trachytes around Karaburhan (Sivrihisar-Eskisehir, NW Anatolia) within the Izmir-Ankara-Erzincan suture zone. These volcanic rocks were emplaced as domes, close and parallel to the ophiolite thrust line. According to 40Ar/ 39Ar geochronological analyses of sanidine crystals from the phonolites, the age of the alkaline volcanics is 25 Ma (Late Oligocene-Early Miocene). The flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid crystals. The clinopyroxenes show compositional zoning, with aegirine (Na 0.82-0.96Fe +30.68-0.83) rims and aegirine-augite cores (containing calcium, magnesium, and Fe +2). Some aegirine-augites are replaced with sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. The fine-grained trachytes with a trachytic texture consist of feldspar (oligoclase and sanidine) phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites. Although there are some differences in their element patterns, the phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component. In addition, the high 87Sr/ 86Sr (0.706358-0.708052) and low 143Nd/ 144Nd (0.512546-0.512646) isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction-derived fluids. Petrogenetic modeling indicates that the alkaline lavas generated from the subduction-modified lithospheric mantle have undergone assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent through the thickened crust in an extensional setting.

Sar?fak?o?lu, Ender; Özen, Hayrettin; Hall, Chris

2009-08-01

73

Phenocrysts Crystallisation Pressures and Temperatures and Melts Evolution at La Fossa Volcano (Vulcano Island, Italy)  

NASA Astrophysics Data System (ADS)

La Fossa Volcano (Vulcano Isl., Aeolian Arch., Italy) erupted last, explosively, in 1888-1890. Its eruptive history includes at least four eruptive cycles of mixed eruptions with strombolian and hydromagmatic phases followed generally by small lava flows ranging in composition from latites to trachytes and rhyolites. Crystallisation temperatures and pressures of phenocrysts and melts chemical evolution, have been modeled via thermochemical calculations and HP-HT laboratory experiments. The crystallisation temperatures and pressures of olivine and clinopyroxene phenocrysts of latitic and trachitic lavas (Punte Nere, Grotte dei Palizzi) were obtained via the empirical olivine-clinopyroxene-liquid thermobarometer of Sugawara (2000) and the olivine geothermometer of Ariskin et al. (1993) which gave consistent values of 1120°C - 60 MPa and 1080°C - 50 MPa. For the trachytic lava of Grotte dei Palizzi and the rhyolitic blocks of 1888-90 eruption, T - P of 1030°C - 50 MPa and 1000°C - 40 MPa were obtained using the two feldspars thermochemical equilibrium model of Green and Usdansky (1986). The %(H2O)m of trachytic and rhyolitic melts, in the range of 1.7 - 2.7% and 2.4 - 2.7% respectively, was obtained after the experimental calibration at P = 100 MPa and T = 1000-1020°C of X(An)plg against the %(H2O)m. The phase relations and melts composition under the above indicated conditions were finally investigated by the MELTS code (Ghiorso and Sack, 1995), allowing us to show how the more primitive latitic melt can evolve toward the more evolved trachytic and rhyolitic compositions.

Masotta, M.; Trigila, R.

2008-12-01

74

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

75

Stratigraphy and evolution of the trachy-rhyolitic volcanism of the Senafe area (Eastern Eritrean Plateau)  

NASA Astrophysics Data System (ADS)

The Senafe area reveals a pile of stratoid volcanic rocks ("Senafe" ignimbrite), of considerable extent and thickness, which are the products of the first volcanic event which took place in this sector, close to the upper margin of the Afar escarpment. The Senafe ignimbrite is composed prevalently of trachyte with differing degrees of alkalinity: trachy-dacite of transitional series, and trachyte s. s. of mildly alkaline series. K/Ar radiometric measurements carried out on three samples give ages ranging between 21 and 23 Ma (Lower Miocene) and show that the Senafe ignimbrite with transitional character is an extension of the Serae rhyolite of the Central Eritrean Plateau, and may also be correlated with the Miocene Alaji rhyolite of the Central Ethiopian Plateau. In contrast, the more alkaline ignimbrite shows good correlations with the trachyte emitted by the Miocene Termaber alkaline central volcanoes of Ethiopia. It is noted that, in the course of the Miocene volcanism in Eritrea, the volumetric ratio between associated basalt and ignimbrite diminishes from west to east, i.e., approaching the Afar escarpment. The stratoid volcanic rocks are injected by thick trachytic and rhyolitic dykes. As radiometric measurements on them could not be performed, their age is unknown, but it is probably more recent than that of the injected ignimbrite, according to Merla and Minucci [Merla, G., Minucci, E., 1938. Missione geologica nel Tigrai. In: La serie dei terreni, vol. 1. Regia Accademia d'Italia, Centro Studi per l'Africa Orientale Italiana, Rome, Italy, pp. 1-362] for similar dykes and domes occurring in the Adwa-Axum area (Tigrai, Ethiopia), not far from Senafe. A section is devoted to the dyke feeders of the Eritrean and Adwa-Axum volcanism.

Zanettin, B.; Bellieni, G.; Visentin, E. Justin

2006-08-01

76

A numerically calibrated reference level (MP28) for the terrestrial mammal-based biozonation of the European Upper Oligocene  

Microsoft Academic Search

The fauna of the Enspel (Westerwald) and the neighbouring Kärlich (Neuwied basin) fossil deposits correspond to the Upper\\u000a Oligocene Mammal Paleogene (MP) reference level 28 and 28–30, respectively. Basaltic flows and a trachyte tuff terminating\\u000a and predating the fossil deposit sedimentation allow to numerically calibrate the MP reference levels by radioisotope dating.\\u000a Laser fusion 40Ar\\/39Ar step heating on volcanic feldspars

Dieter F. Mertz; Paul R. Renne; Michael Wuttke; Clemens Mödden

2007-01-01

77

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

Microsoft Academic Search

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

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

1997-01-01

78

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

79

Geology of the volcanic-hosted Brockman rare-metals deposit, Halls Creek Mobile Zone, northwest Australia. l. Volcanic environment, geochronology and petrography of the Brockman volcanics  

Microsoft Academic Search

Summary Rare-metals mineralization of the Brockman deposit (Halls Creek Mobile Zone, NW Australia) is hosted in a fluorite-bearing, rhyolitic volcaniclastic unit informally termed the “Niobium Tuff”. The Tuff, more correctly described as a tuffaceous volcaniclastic deposit, is the lowermost unit of a sequence of trachyte-to-rhyolite lavas, trachyandesite subvolcanic rocks, and volcaniclastic units of the Brockman volcanics located within the Halls

W. R. Taylor; R. W. Page; G. Esslemont; N. M. S. Rock; D. I. Chalmers

1995-01-01

80

Geochemical and isotopic insights into the assembly, evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)  

NASA Astrophysics Data System (ADS)

New geochemical and isotopic data on volcanic rocks spanning the period ~75-50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5-10 km3 of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4-6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr-Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8-12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4-6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted.

Brown, R. J.; Civetta, L.; Arienzo, I.; D'Antonio, M.; Moretti, R.; Orsi, G.; Tomlinson, E. L.; Albert, P. G.; Menzies, M. A.

2014-09-01

81

Wide dispersal and deposition of distal tephra during the Pleistocene ‘Campanian Ignimbrite\\/Y5’ eruption, Italy  

Microsoft Academic Search

A trachytic volcanic ash layer is widely distributed across south-western Russia, where it is found both in well-characterised archaeological contexts close to the Don River (the Paleolithic sites of Kostenki-Borschevo (51.4°N, 39.0°E), and in undisturbed geological contexts. This ash layer has all of the characteristics of a distal tephra fall deposit: it is fine grained and unimodal with a grain

David M. Pyle; Graham D. Ricketts; Vasiliki Margari; Tjeerd H. van Andel; Andrei A. Sinitsyn; Nicolai D. Praslov; Sergei Lisitsyn

2006-01-01

82

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

Microsoft Academic Search

Summary ¶The Campanian Plain is an 80?×?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

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

2003-01-01

83

Geology and Petrology of the Woods Mountains Volcanic Center, southeastern California: Implications for the Genesis of Peralkaline Rhyolite Ash Flow Tuffs  

Microsoft Academic Search

The Woods Mountains Volcanic Center is a middle Miocene silicic caldera complex located at the transition from the northern to the southern Basin and Range provinces of the western United States. It consists of a trachyte-trachydacite-rhyolite-peralkaline rhyolite association of lava flows, domes, plugs, pyroclastic rocks, and epiclastic breccia. Volcanism began at about 16.4 Ma, near the end of a local

Michael McCurry

1988-01-01

84

20. PETROLOGY AND KAr AGE OF VOLCANIC TUFF AND ASH FROM THE WALVIS SEAMOUNT PROVINCE, DSDP SITE 359, LEG 39  

Microsoft Academic Search

An ash-flow tuff of 3.3 meters thickness cored from a seamount at the southwest extremity of the Walvis Ridge (~ 1000 km east of the Mid-Atlantic Ridge) has trachytic bulk composition (SiCh 62.5 wt %; total alkalis 12 wt %) and contains abundant alkali feldspar (avg. AmAbsyOns>), clinopyroxene (FsisEn37Wo*j), biotite, and Fe-Ti oxides in a dusty, partly devitrified matrix. Moderately

R. V. Fodor; K. Keil; J. W. Husler; E. H. McKee

85

High 18 O igneous rocks from the Tuscan Magmatic Province, Italy  

Microsoft Academic Search

The 18O\\/16O ratios were measured for 60 rocks and coexisting minerals from the Plio-Pleistocene, calc-alkaline, Tuscan Magmatic Province, Italy. The d18O values of these magmas were as follows: Elba, Giglio, and Montecristo granodiorites (11.4 to 12.1); M. Cimini rhyolites and trachytes (11.2 to 11.7); Roccastrada, S. Vincenzo, and M. Amiata rhyolites (12.3 to 13.4); and the Tolfa rhyolites and quartz

Hugh P. Taylor; Bruno Turi

1976-01-01

86

Eruptive history and magmatic stability of Erebus volcano, Antarctica: Insights from englacial tephra  

NASA Astrophysics Data System (ADS)

tephrostratigraphy of the active Antarctic Erebus volcano was determined from englacial tephra on the ice-covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine-grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the nonmagmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40Ar/39Ar ages, shard morphology, and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA-ICP-MS. Trachybasalt, trachyte, and phonolite tephra were identified. All phonolitic tephra are Erebus-derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40 ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long-term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated and could provide an important time-stratigraphic marker in Antarctic ice cores.

Iverson, Nels A.; Kyle, Philip R.; Dunbar, Nelia W.; McIntosh, William C.; Pearce, Nicholas J. G.

2014-11-01

87

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

88

A basic radial dike swarm of Boa Vista (Cape Verde Archipelago); its significance in the evolution of the island  

NASA Astrophysics Data System (ADS)

A basic radial dike swarm unrelated to other basic units of Boa Vista (Cape Verde Archipelago) has been localized and characterized in the central sector of the island. According to new radiometric data three main stages in the evolution of Boa Vista are distinguished: the earlier (the Old Volcanic Complex: 17-16 Ma) is equivalent to the shield building stage of Hawaii and the later (the Recent Volcanics (8-4 Ma) is in some aspects comparable to the post-erosional stage. An important intermediate essentially felsic stage (the Trachytic-Phonolitic Complex: 14.3-12.8 Ma) followed the basaltic shield stage. This felsic stage has equivalents in some other oceanic islands as the Canary Islands, specially Gran Canaria, Tenerife, and La Gomera. The central sector of Boa Vista is also occupied by the Felsic Subvolcanic Complex, a unit consisting of phonolitic breccias, syenites and monzonites that represent the Trachytic-Phonolitic Complex hypabyssal roots. The felsic rocks as a whole constitute half of the total amount of igneous rocks on the island making up Boa Vista, the island with the highest percentage of felsic rocks in the Central Atlantic Ocean. More than 200 dikes of the basic radial swarm intruding the Felsic Subvolcanic Complex have been measured. The intensity of the multiple dike injection is sometimes rather high, roughly a dike every 5 m. The individual dikes have an observable mean length of about 300 m. The composition of these dikes is always foiditic (nephelinites, melilitites, and limburgites), slightly different in composition (more alkaline and richer in incompatible elements) to the other basic units of the island (the Old Volcanic Complex and the Recent Volcanics). The radial dikes converge in an area located NW of the geometrical center of Boa Vista, a zone where the hypothetical center of the Old Volcanic Complex and the Trachytic-Phonolitic Complex edifices must also have been situated. The ages obtained from the dikes (between 14.8 and 11.5 Ma) indicate that the radial injections are contemporary with the phonolites and the trachytes of the Trachytic-Phonolitic Complex and yet are part of the intermediate evolutionary stage of Boa Vista.

Ancochea, Eumenio; Hernán, Francisco; Huertas, María José; Brändle, José Luis

2012-10-01

89

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

NASA Astrophysics Data System (ADS)

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

Sumita, Mari; Schmincke, Hans-Ulrich

2013-05-01

90

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

91

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

92

Glass and mineral analyses from first deposits of Peach Spring Supereruption (SW USA) illuminate initial tapping of a zoned magma chamber  

NASA Astrophysics Data System (ADS)

The Peach Spring supereruption (18.78±0.02 Ma) was sourced from Silver Creek caldera in the southern Black Mountains, Arizona (Ferguson et al. in press). The resulting ignimbrite, the Peach Spring Tuff (PST), blanketed >32,000 km2 of Arizona, California, and Nevada (Buesch, 1993). Underlying the ignimbrite is a thin (? 1m thick) basal layered deposit that consists of texturally distinct layers 1a-e (Valentine et al. 1989) and is present up to ~100 km from the source caldera. Basal layered deposits contain the first material erupted during the PST supereruption, preceding the main eruption event. Petrography and geochemistry of minerals and pumice clasts from basal layered deposits collected ~15-100 km from the caldera, combined with a survey of glass and crystal compositions from both outflow and basal deposits, permit (1) comparisons with the overlying ignimbrite, and (2) insights into the initial stages of the supereruption and extraction of magma from the chamber. Pumice clasts from a pumice-rich layer (1a2) of the basal deposit were characterized by LA-ICPMS and SEM. Unaltered glass has a uniform high-Si rhyolite composition (76.7% SiO2, 13.0% Al2O3, 3.6% Na2O, 5.3% K2O, 0.6% FeO, <0.1% MgO, 0.6% CaO, 0.1% TiO2). Mildly altered glass is similar but has lower Na2O and higher K2O. Pumice clasts are relatively crystal poor (<10% phenocrysts) with an assemblage dominated by sanidine (~Or55Ab43An2), with lesser plagioclase (~Ab73An19Or8), minor hornblende and biotite, and accessory magnetite, sphene, zircon, chevkinite, and apatite; no quartz was identified. Initial LA-ICPMS results for glass reveal REE patterns with large negative Gd (0.21: i.e. U-shaped REE pattern) and Eu (0.31) anomalies, very low Ba and Sr (?10 ppm), and high Rb (~250 ppm). These compositions are essentially identical to those of the most common pumice from distal outflow ignimbrite, but very different from crystal-rich (>30%) trachyte pumice that dominates the intracaldera fill and is present at the tops of proximal outflow sections (Pamukcu et al. in press). A broader survey by EMP of minerals and glass shards from basal layered deposits and outflow ignimbrite (Buesch, 1993 and unpublished data, 1992) support the general conclusion that distal outflow is dominated by high-Si rhyolite, but demonstrate that trachyte is present throughout ignimbrite and basal deposits. Although SiO2 concentration is dominantly ~76-78 wt%, a small but persistent population ranges from ~66-72 wt%. Likewise, 6% (in layer 1a2) to 40% of all plagioclase in basal and outflow deposit samples is relatively calcic (An>27), indicating trachyte origin. The above data (1) confirm the existence of two compositions of erupted magma (phenocryst-poor, high-Si rhyolite, crystal-rich trachyte); (2) demonstrate that the initial eruption (basal layer) tapped the same magmas as the ignimbrite; and (3) reveal that, although rhyolite dominates outside the caldera, trachyte like that in intracaldera deposits was also tapped throughout the eruption. These relations are consistent with eruption from a vertically stratified magma chamber with trachyte (cumulate?) beneath rhyolite, but indicate that either chamber stratification was imperfectly developed or some magma from deeper levels was entrained throughout the eruption.

Mccracken, R. G.; Miller, C. F.; Buesch, D.; Gualda, G. A.; Covey, A.

2012-12-01

93

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

94

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

95

Primary origin of some trachytoid magmas: Inferences from naturally quenched glasses in hydrothermally metasomatized gabbroic xenoliths (Hyblean area, Sicily)  

NASA Astrophysics Data System (ADS)

Hydrothermally-modified gabbroic xenoliths from the Hyblean tuff-breccia deposits (Sicily) consist of albitized plagioclase, Fe-Mg-rich clays, aegirine-augite, ± zeolites, titanite, apatite, magnetite, and hydrothermal zircon. Pockets of silicate glass with perlitic cracking occur in some samples forming 15-20% (by volume) of the rock modal assemblage. Electron microprobe analyses show the trachytic composition of the glass, with generally peralkaline sodic affinity [molar Al 2O 3/(Na 2O + K 2O) ~ 0.8 (average value); molar Al 2O 3/(Na 2O + K 2O + CaO) ~ 0.7 (average value); Na 2O/K 2O (wt.%) = 1.7-2.3]. The glass trace element abundances, obtained by secondary ion mass spectrometry (SIMS) analyses are consistent with those of world-wide trachytes (e.g. Zr/Ti = 0.15-018; Nb/Y = 0.73-1). Relatively high abundances of Cl (700-1600 ppm) and F (> 500 ppm) were also detected in the glass. Careful macroscopic and microscopic observations exclude the possibility that external silicate melt infiltrated the xenolith. The occurrence of glass pockets between the mafic clay assemblages and the feldspar grains, along with comparisons between chemical compositions of the glass and the surrounding minerals, suggest that the glass is due to the melting of a eutectoid system consisting of Na-rich alkali feldspar, Fe-Mg-rich clays and aegirine-augite. Halogens had probably played an important role in the partial melting process by decreasing the melting temperature of modal minerals, especially feldspar. The occurrence of these trachytic glasses lends support to petrologic models suggesting that partial melting of a hydrothermally altered, brine-rich oceanic crust induced by shallow-seated basic intrusions can produce primary trachytoid melts. This may explain the "Daly-gap" characterizing some oceanic within-plate volcanoes.

Viccaro, Marco; Scribano, Vittorio; Cristofolini, Renato; Ottolini, Luisa; Manuella, Fabio C.

2009-12-01

96

Paleolatitudes of the Permo-Triassic Ukrainian Shield with Implications for Pangaea A and B  

NASA Astrophysics Data System (ADS)

Three research teams jointly collected and independently analyzed 311 Permian and Triassic dikes and minor intrusions from the Ukrainian Shield, the southwest portion of the East European Craton. This paper presents well-documented paleomagnetic results of the shield from samples that are not affected by inclination shallowing, are from a tectonically stable region, and have good age dates. The results are: andesites, Dec/Inc = 240.1 degrees / -64.4 degrees, k = 96, alpha-95 = 4.5, N = 12; trachytes, Dec/Inc = 202.6 degrees / -27.1 degrees, k = 32, alpha-95 = 6.4, N = 17. Argon-argon dating places the andesites in the upper Late Triassic at 202.6-216.9 Ma and the trachytes in the mid-Early Permian at 282.6 ± 2.6 Ma. These are the first paleomagnetic results from Ukraine that are based on a fully demagnetized large collection from reliably dated igneous rocks. It indicates the paleolatitude of andesite emplacement is 46.2 degrees N. Notably, the paleolatitude of the trachytes is 14.4 degrees N. With Gondwana in the paleoposition used by Muttoni et al. (2003) for about 280 Ma, this paleolatitude of Baltica neither allows Pangaea A, nor disproves Pangaea B. However, if the Gondwana paleoposition is changed to a paleopole at 30 degrees S, 54.9 degrees E (from Torsvik and Van der Voo, 2002 in GJI for 280 Ma), then Pangaea A-type reconstructions become possible. A more reliable Early Permian paleopole set for Gondwana would provide a more definitive conclusion.

Yuan, K.; van der Voo, R.; Bazhenov, M. L.; Bakhmutov, V.; Alekhin, V.; Hendriks, B.

2009-12-01

97

Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line  

NASA Astrophysics Data System (ADS)

The concentrations of major and trace elements and the isotopic compositions were measured in felsic lavas of the Bamenda Mountains, an extinct volcanic center of the West Cameroon Highlands (WCH), in order to assess the relative roles of mantle versus crustal magma sources in the evolution of silicic magmatic systems. These silicic volcanics are composed of benmoreites, trachytes and rhyolites associated with alkaline basaltic lavas and have characteristics of A-type magmas. Chemical diagrams show two magmatic lineages based on the ratios of HFSE/LILE (e.g. Hf/Rb: 0.19-0.34 and 0.11-0.23), the degree of fractionation (La/Sm: 5.2-6.4 and 6.5-7.5) and the contents of REE; they are respectively referred to as less-differentiated felsic lavas (LDFL) and high-differentiated felsic lavas (HDFL). Their evolution is mainly governed by alkali feldspar-dominated fractional crystallization. The K/Ar ages on trachytes range from 27.40 ± 0.50 to 12.74 ± 0.25 Ma with the HDFL being mostly younger. Their ?Nd range mostly between -0.88 and +2.43 with one value at -3.43 precluding their derivation from melting of a continental crust. Besides the most negative value (-3, 43) obtained in trachyte BA63 remains high compared to -20.28 calculated for a Bamemda Pan-African granitoid suggesting very low crustal contamination. However, the fact that this lower ?Nd in BA63 is not followed by the corresponding low Ce/Pb and high La/Nb rather indicates a selective role of crustal contamination. This feature seems characteristic of the felsic lavas of the whole Cameroon Volcanic Line. The isotopic and trace element variations observed for the Bamenda felsic lavas are most consistent with a source in the continental lithospheric mantle.

Kamgang, Pierre; Njonfang, Emmanuel; Nono, Alexandre; Dedzo, Merlin Gountie; Tchoua, Félix M.

2010-09-01

98

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

99

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

100

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

101

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

102

Yield strengths of flows on the earth, Mars, and moon. [application of Bingham plastic model to lava flows  

NASA Technical Reports Server (NTRS)

Dimensions of flows on the earth, Mars, and moon and their topographic gradients obtained from remote measurements are used to calculate yield strengths with a view to explore the validity of the Bingham plastic model and determine whether there is a relation between yield strengths and silica contents. Other factors are considered such as the vagaries of natural phenomena that might contribute to erroneous interpretations and measurements. Comparison of yield strengths of Martian and lunar flows with terrestrial flows suggests that the Martian and lunar flows are more akin to terrestrial basalts than they are to terrestrial andesites, trachytes, and rhyolites.

Moore, H. J.; Arthur, D. W. G.; Schaber, G. G.

1978-01-01

103

Stable isotope analyses of the peralkaline volcanics Gregory Rift Valley, Kenya  

NASA Technical Reports Server (NTRS)

Delta O-18 analyses of the Naivasha rhyolites, basalts, Menengai trachytes and the Yatta phonolite are presented together with D/H analyses of the Naivasha rhyolites. Delta O-18 results vary from 5.7 to 8.9 per mill which is within the reported range of delta O-18 analyses for continental volcanics. Closure temperatures calulated from the basalts and rhyolites show equilibration to be at magmatic temperatures. D/H values range from -40 to -148 per mill indicating that the rhyolites have undergone large scale degasssing.

Black, S.; Macdonald, R.; Fallick, A. E.; Kelly, M.

1993-01-01

104

Formation of the Late Permian Panzhihua plutonic-hypabyssal-volcanic igneous complex: Implications for the genesis of Fe-Ti oxide deposits and A-type granites of SW China  

NASA Astrophysics Data System (ADS)

The Late Permian (260 Ma) Emeishan large igneous province of SW China contains numerous magmatic Fe-Ti oxide deposits. The Fe-Ti oxide deposits occur in the lower parts of evolved layered gabbroic intrusions which are spatially and temporally associated with A-type granitic rocks. The 260 Ma Panzhihua layered gabbroic intrusion hosts one of the largest magmatic Fe-Ti oxide deposits in China and is coeval with a peralkaline A-type granitic pluton. The granite has intruded the overlying Emeishan flood basalts and fed at least one dyke which erupted onto the surface producing columnar jointed trachytes. The presence of syenodiorite between the layered gabbro and granite is evidence for compositional evolution from mafic to intermediate to felsic rocks. The syenodiorites have intermediate to felsic composition with SiO 2 = 61 to 65 wt.%, MgO = 0.27 to 0.6 wt.% and CaO = 1.0 to 2.5 wt.% as compared to the granite SiO 2 = 65 to 72 wt.%, MgO = 0.1 to 0.4 wt.%, CaO = < 1.0 wt.%. Primitive-mantle-normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks. The chondrite-normalized REE patterns show Eu anomalies changing from > 1(Eu/Eu* = 1.1 to 2.6) in the gabbroic intrusion, to < 1 in the syenodiorite (Eu/Eu* = 0.75 to 0.83), granites and trachytes (Eu/Eu* = 0.55-0.87). Previously published ?Nd (T) values from clinopyroxenes ( ?Nd (T) = + 1.1 to + 3.2) of the gabbroic intrusion match the whole-rock values of the syenodiorite ( ?Nd (T) = + 2.1 to + 2.5), granite and trachyte ( ?Nd (T) = + 2.2 to + 2.9), suggesting that all rock types originated from the same mantle source. MELTS and trace element modeling confirm that all rock types can be generated by fractional crystallization of high-Ti Emeishan basalt. The jump in SiO 2 from the gabbro to the syenodiorite is attributed to the en masse crystallization of the Fe-Ti oxides. The geological and geochemical data indicate that fractional crystallization of a common parental magma produced the layered gabbroic intrusion and Fe-Ti oxide deposit, the syenodiorite, granites and trachyte of the Panzhihua region, which thus form a genetically related plutonic-hypabyssal-volcanic complex. Other granite-gabbro complexes in the region likely formed in a similar manner.

Shellnutt, J. G.; Jahn, B.-M.

2010-01-01

105

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

106

Ignimbrite sequence on Gran Canaria  

Microsoft Academic Search

The Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units:\\u000a \\u000a \\u000a a) \\u000a \\u000a A lower unit of trachytic rhyolites (lavas, composite flows, ignimbrites) characterized by a phenocryst assemblage of anorthoclase\\u000a (Or15–20, wt%), clinopyroxene, hypersthene (amphibole substituted for both in ignimbrites), and Fe\\/Ti-oxides. The commonest groundmass\\u000a minerals are anorthoclase and alkali-amphibole, with

H. U. Schmincke

1969-01-01

107

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

108

Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

109

Th-230 - U-238 series disequilibrium of the Olkaria basalts Gregory Rift Valley, Kenya: Petrogenesis  

NASA Technical Reports Server (NTRS)

Strong mixing trends on a (Th-230/Th-232) versus Th diagram show that the basalts are mixed magmas which have undergone interaction with the crust. Instantaneous Th/U ratios are less than time integrated ones but these exceed the Th/U ratios in the MORB and OIB sources. This indicates that the mantle may have undergone some metasomatic fluxing, crustal contamination of the basalts will also enhance these ratios. Early activity on the Akira plain is represented by early basalts and hawaiites. The early basalt samples are known to predate the earliest comendites. The most recent phase of activity is represented by another cinder cone 40-50 m high being feldspar and clinopyroxene phyric. Inclusions which occur in the comendites vary in size and distribution. The largest and most porphyritic are the trachytes (up to 40 cm) with alkali feldspar phases up to 6 mm and small pyroxenes in the ground mass. The second set of inclusions are smaller (up to 10 cm) and are largely aphyric. The distribution of the inclusions are not uniform, the Broad Acres (C5) lavas contain 2-5 percent. The size of the inclusions decrease from south to north, as does the abundance of the trachytic inclusions. The major element variations in the Naivasha basalts, hawaiites and magmatic inclusions are discussed.

Black, S.; Macdonald, R.; Kelly, M.

1993-01-01

110

The composition and sources of magmas of Changbaishan Tianchi volcano (China-North Korea)  

NASA Astrophysics Data System (ADS)

The Changbaishan Tianchi volcano is the greatest stratovolcano within the bounds of the Late Cenozoic intraplate volcanic province of East Asia. Changbaishan Tianchi volcanic cone consists mostly of trachytes and pantellerites. It was found that the lavas composing the shield platform of Changbaishan Tianchi volcano are weakly differentiated basic rocks whose geochemical characteristics are generally similar. All the alkaline salic rocks composing the cone of the volcano are characterized by conformable normalized trace element patterns. The concentrations of rare-earth elements in these rocks are high and amount up to 1000 ppm. The character of the distribution of trace elements in the basic rocks of Changbaishan Tianchi volcano is close to that in the OIB-type basalts. Within the series from basalts to pantellerites, the rocks are enriched in REE and zirconium, but depleted in barium, strontium, and europium. According to the obtained geochemical data, it was shown that the rock series of Changbaishan Tianchi volcano, varying from basalts to trachytes and pantellerites comprises compositions geochemically interrelated by the processes of crystal fractionation. The parental magma for the rocks of the volcano was derived from plume sources of the same type as those of OIB and sources of the Late Cenozoic intraplate province of East Asia.

Andreeva, O. A.; Yarmolyuk, V. V.; Andreeva, I. A.; Ji, J. Q.; Li, W. R.

2014-05-01

111

Communities of uropodine mites (Acari: Mesostigmata) in selected oak-hornbeam forests of the Wielkopolska region (Poland).  

PubMed

Pristine oak-hornbeam forests are among the richest flora and fauna environments in Poland. The agricultural development of the Wielkopolska region has led to the replacement of forest area with farmland. Consequently, the oak-hornbeam forests became fragmented, resulting in the isolation of local arthropod populations. The aim of this study was to compare the communities of uropodine mites in selected parts of a forest, differing in stand age and composition, physical soil condition and degree of anthropogenic pressure. Species composition of mite communities in a forest near Duszniki (West Poland), transformed by humans, was compared with the mite species composition observed in three nature reserves in its close vicinity. The analyses showed that Trachytes aegrota and Olodiscus minima constitute more than 50% of all communities in each type of tree stand. Diversity in Uropodina communities was higher in older tree stands, as well as in protected areas. Some species, such as Uroobovella pulchella, Uroobovella pyriformis and Dinychus woelkei, are related to specific microhabitats (e.g., they inhabit only dead wood) but there are also ubiquitous species, occurring in all types of environment, e.g., Oodinychus ovalis. Species like Oodinychus karawaiewi and Dinychura cordieri indicate a high degree of forest disturbance. Presence of such species as Trachytes lamda, Cilliba rafalskii, Cilliba cassideasimilis and Trematurella elegans points at high naturalness of soil in oak-hornbeam forests. These species have been found in old (>100 years old) tree stands, where Uropodina communities were also the richest. PMID:19326248

Napiera?a, Agnieszka; B?oszyk, Jerzy; Bruin, Jan

2009-12-01

112

Silicic magmas from the Emeishan large igneous province, Southwest China: Petrogenesis and their link with the end-Guadalupian biological crisis  

NASA Astrophysics Data System (ADS)

The Emeishan large igneous province in SW China comprises a bimodal mafic-silicic suite with the silicic rocks occurring at the uppermost part of the thick lava sequence. The silicic rocks have an age of 257-263 Ma, and are thus roughly coeval with the Guadalupian-Loping (G-L) boundary event. Most silicic rocks (trachyte and rhyolite) from the Emeishan province have rather uniform ?Nd values (+ 1 to + 2.9) that are comparable with the uncontaminated high-Ti basalts. This fact and the remarkably narrow ranges of incompatible element ratios (e.g., Zr/Nb) of the basalt-silicic suite indicate a genetic relationship between basalt and silicic members. The significant difference between the Emeishan rocks and the experimental melts of hydrated basaltic crust, suggests the fractional crystallization of basaltic magma, rather than crustal melting, as the major petrogenetic process for the formation of silicic rocks. Indeed, their major and trace element trends can be modeled by fractionation of the observed mineral phases (feldspar, clinopyroxene, Fe-Ti oxide and apatite). In contrast to the virtually closed system differentiation processes associated with trachytes, the rhyolites may have experienced interaction with upper crustal material during ascent. The geochemical characteristics and recent assessment of timing of the Emeishan volcanism suggest the Emeishan rhyolites as the potential source of the widespread clay bed at the G-L boundary in south China. This enhanced the causal link between the Emeishan eruption and the end-Guadalupian biological crisis.

Xu, Yi-Gang; Chung, Sun-Lin; Shao, Hui; He, Bin

2010-09-01

113

The discovery of late Quaternary basalt on Mount Bambouto: Implications for recent widespread volcanic activity in the southern Cameroon Line  

NASA Astrophysics Data System (ADS)

At the north-eastern flank of Mount Bambouto, a lateral cone, the Totap volcano, is dated at 0.480 ± 0.014 Ma, which corresponds to the most recent activity of this area. The lava is a basanite similar to the older basanites of Mount Bambouto. Two new datations of the lavas of the substratum are 11.75 ± 0.25 Ma, and 21.12 ± 0.45 Ma. A synthetic revision of the volcanic story of Mount Bambouto is proposed as follows. The first stage, ca. 21 Ma, corresponds to the building of the initial basaltic shield volcano. The second stage, from 18.5 to 15.3 Ma, is marked by the collapse of the caldera linked to the pouring out of ignimbritic rhyolites and trachytes. The third stage, from 15 to 4.5 Ma, renews with basaltic effusive activity, together with post-caldera extrusions of trachytes and phonolites. The 0.5 Ma Totap activity could be a fourth stage. In the recent Quaternary, a number of basaltic activities, similar to that of the Totap volcano, are encountered elsewhere in the Cameroon Line, from Mount Oku to Mount Cameroon. The very long-live activity at Mount Bambouto and the volcanic time-space distribution in the southern Cameroon Line are linked to the working of a hotline.

Kagou Dongmo, Armand; Nkouathio, David; Pouclet, André; Bardintzeff, Jacques-Marie; Wandji, Pierre; Nono, Alexandre; Guillou, Hervé

2010-04-01

114

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

SciTech Connect

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

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

1993-03-01

115

New radiometric age of volcanic rocks in the central Eritrean plateau (from Asmara to Adi Quala): Considerations on stratigraphy and correlations  

NASA Astrophysics Data System (ADS)

New radiometric data have recently been acquired on basalt and rhyolite sampled at various levels of the volcanic sequence occurring in the central Eritrean plateau, confirming the stratigraphic reconstruction suggested in a previous paper [Zanettin, B., Bellieni, G., Justin Visentin, E., Haile, T., 1999. The volcanic rocks of the Eritrean plateau: stratigraphy and evolution. Acta Volcanologica 11(1), 183-193]. New considerations indicate the tholeiitic, not alkaline, nature of the Asmara basalt. Doubts about the relative age of the Aiba/Alaji and Asmara basalts have now been clarified: they are, at least partly, coeval (about 30 Ma old). The Serae rhyolite intercalated in the Adi Ugri basalt turns out to be about 24 Ma old, like the more abundant ignimbrite outcropping in the Senafe area, of which it is the westernmost extension. Its age confirms that it does not correspond to the trachyte intercalated in the Oligocene stratoid basalt of the Adwa-Axum area (where the Adi Ugri basalt probably also occurs, intercalated with the Serae trachyte and rhyolite). The upper part of the Adi Ugri basalt is 22 Ma old (an age consistent with the finding of a Deinotherium tooth). The radiometric age of these rocks also confirms already indicated correlations between Eritrean and Ethiopian volcanic formations.

Zanettin, B.; Bellieni, G.; Visentin, E. Justin

2006-06-01

116

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

NASA Astrophysics Data System (ADS)

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

Lindsay, J. M.; Moufti, R.

2013-12-01

117

The Buem volcanic and associated sedimentary rocks, Ghana: a field and geochemical investigation  

NASA Astrophysics Data System (ADS)

The Buem volcanic rocks of Ghana form part of the Dahomeyide Chain on the southeastern flank of the West African Craton. They have been regarded as an ophiolite complex indicating a continental collision origin for the Dahomeyides. Mapping and geochemical analysis of the volcanic outcrop has shown that the volcanics and associated sediments form a conformable eastward dipping succession with minor folding. Pillow lavas and agglomerates were followed by hawaiites, mugearites, trachytes and phonolitic trachytes. These lavas make up a consanguineous alkaline suite. Gravity data suggest that the outcrop is a section across a rift zone of a large volcano whose eroded core now lies under the Voltaian Basin. The sediments enclosing the volcanics are red shales, feldspathic to quartz arenite sands, conglomerates, mixtites of possibly glacial origin, jasper and minor limestone. They were deposited in a shallow water to subaerial environment northwest of a mountain front. Regional correlation suggests that the volcanics were erupted at about 650-600 M.a. but they gave K/Ar ages of about 500 M.a. The latter age conforms with the later stages of the Pan-African orogeny and may date the folding and metasomatism of the Buem. The evidence does not support, but does not necessarily rule out a continental collision origin for the Dahomeyide Belt in Ghana.

Jones, W. B.

118

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

119

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.

120

Occurrence of an unknown Atlantic eruption in the Chaîne des Puys volcanic field (Massif Central, France)  

NASA Astrophysics Data System (ADS)

A volcanic ash layer, called MF1, was recently identified in Holocene sediments from the Gourgon and Molhiac peat bogs (Monts du Forez, French Massif Central). This ash layer consists of colorless shards with a heterogeneous trachytic to rhyolitic composition. The trace elements analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) attest to a local origin. Radiocarbon dating of peat samples taken within and below the ash layer indicates the best age at 6339 ± 61 cal yr BP, i.e. an age contemporaneous with the volcanic activity of Montchal, Montcineyre and Pavin volcanoes from the Chaîne des Puys volcanic field. These volcanoes are characterized by basaltic and trachytic products, thus the rhyolitic composition of MF1 tephra suggests that it is likely originated from an unknown eruption. These results again confirm the interest of studying the distal volcanic ash fallouts in order to establish or specify records of past eruptions of volcanic fields. Identification of this new tephra layer also provides an additional tephrochronological marker for Eastern French Massif Central.

Jouannic, G.; Walter-Simonnet, A. V.; Bossuet, G.; Cubizolle, H.; Boivin, P.; Devidal, J. L.; Oberlin, C.

2014-08-01

121

Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes  

NASA Astrophysics Data System (ADS)

The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.

Lim, Chungwan; Kim, Seonyoung; Lee, Changyeol

2014-12-01

122

Topographic stress perturbations in southern Davis Mountains, west Texas 2. Hydrogeologic implications  

USGS Publications Warehouse

As part of a regional groundwater investigation, geophysical logs were obtained in two municipal water wells located near the west Texas city of Alpine. These boreholes are 252 and 285 m deep and penetrate extrusive rocks of Tertiary age. The deeper well was drilled in the central valley and the other along the northern flank of an east-west trending valley-ridge setting. Analysis and interpretation of the logs reveal that the two wells are subjected to significantly different stress environments because of topographic effects and exhibit significantly different hydrogeologic properties. Water production is associated with two specific types of features common to both wells: (1) the upper and lower contacts of a dense trachyte unit located in the shallow part of the wells and (2) deeper zones of highly fractured rocks within the interior of a basalt formation. The transmissivity of the trachyte boundaries is twice as large in the central valley well as it is in the ridge flank well, whereas the transmissivity of the deeper basalts is an order of magnitude greater in the flank well than it is in the central well. This discrepancy is examined from the perspective of rock failure, fracture opening, and flow enhancement by computing values for a Drucker-Prager stability factor that is based on the magnitudes of the normal and deviatoric stress invariants as a function of depth. Thus the field measurements and subsequent stress analysis offer evidence of a coupled tectonic-hydrologic interaction at this site.

Morin, R.H.; Savage, W.Z.

2002-01-01

123

The Povoação Ignimbrite, Furnas Volcano, São Miguel, Azores  

NASA Astrophysics Data System (ADS)

The Povoação Ignimbrite Formation (PIF) was emplaced by one of the larger explosive trachytic eruptions of Furnas Volcano, São Miguel, Azores. Trachytic ignimbrites are common in the products of Furnas Volcano and examples of welding occur in at least three ignimbrites of which the Povoação Ignimbrite is the most extensive. The PIF may correlate with the formation of the main caldera of Furnas. In the Povoação Ignimbrite, the welded horizons thicken, without evidence of rheomorphism, into palaeovalleys and can be seen to thin and in some places become completely attenuated over old ridges. The welded horizons are intimately associated with non-welded ignimbrites and in some places there is an alternation between welded and non-welded horizons. On interfluves, the ignimbrite is stratified and some of the welded horizons show pinch and swell and occasional cross-bedding. The welding is interpreted as a primary depositional feature with the clasts sintering on emplacement. It is argued that this ignimbrite was emplaced from a turbulent pulsatory pyroclastic flow. Some pulses were hotter which enabled more extensive development of welding. The flows became more concentrated and denser down valleys favouring the emplacement of thicker welded units.

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

1999-09-01

124

Petrological and geochemical evolution of the basaltic growth stages of La Gomera, Canary Islands  

NASA Astrophysics Data System (ADS)

La Gomera is the only island of the canaries without Quaternary volcanism, and compared to the others it is the geologically least known, specially regarding its subaerial volcanic growth stages. In order to reconstruct its volcanic evolution we report a comprehensive data set including new field observations, mineral chemistry, whole rock major and trace element composition for lavas from the two subaerial volcanic edifices, the Old Edifice and the Young Edifice. The Old Edifice was built up in two main stages, starting with the Lower Old Edifice (LOE; 10.5- 8.7 Ma) represented by a large basaltic shield and followed by the growth stage of the Upper Old Edifice (UOE; 8.6- 6.2 Ma), composed of a thick succession of basalt and trachybasalt lava flows and pyroclastics, with two important trachytic to phonolitic episodes associated. The Young Edifice was built up in two main stages too: the Young Edifice I (YE-I: 5.9-4.9 Ma) and the Young Edifice II (YE-II: 4.7-2.3 Ma). The first stage is represented by lava flows, flowing essentially south and south-eastwards and ranging between basalts and trachyandesites (Young Basalts I). The second stage is mainly made up by a thick sequence of horizontal picrobasalt-trachyandesite lava flows covering the central area and parts of the NE and SW sectors (the Young Basalts II), but also by felsic domes, and the Trachytic Unit, composed of basaltic trachyandesites, trachyandesites and trachytes lava flows. The subaerial volcanic activity of La Gomera is characterised by alkaline rocks ranging from picrites to trachytes as a whole. However, each of the main growth stages shows specific compositional characteristics. As for the petrographic and geochemical characteristics, the LOE is the less variable suite (MgO=15.4-4.5 %). The other units display a greater variation range: 18.6-2.9% MgO in the UOE mafic suites; 13.2-1.2% MgO in the Young Basalts I; 14.4-1.5% MgO in the Young Basalts II and 3.6-0.6% MgO in the Trachytic Unit. The mafic rocks are alkaline OIB basalts coming from a very similar mantle source, that is enriched in incompatibles elements (Ta, Nb) at least two or three times the primordial mantle concentration, and less in Th, U and LREE, being the basalts of the LOE the most enriched and the UOE ones the least enriched. Most of the incompatible elements ratios are typical of HIMU mantle reservoir, as the ratios of Ba/Th vs Th/Nb. We detected differences in the rate of partial melting and in the fractioned crystallization processes. The rate of partial melting decreases from LOE to UOE basalts, whereas it increases from YE I to YE-II basalts, with the maximum degree during the LOE eruption. In addition, the graphic and numerical modelling of the fractioned crystallization processes allows us to detect the differences between the Old basalts and the Young basalts. The data presented support the important differences between the basaltic growth stages of La Gomera, including the both Young Edifice ones, and point out that its volcanic evolution is most complex that the until now accepted.

Huertas, M.; Herrera, R.; Ancochea, E.

2011-12-01

125

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

126

Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands  

NASA Astrophysics Data System (ADS)

Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77-79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe-Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970-890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes.

Sigmarsson, Olgeir; Laporte, Didier; Carpentier, Marion; Devouard, Bertrand; Devidal, Jean-Luc; Marti, Joan

2013-03-01

127

The Mount Manengouba, a complex volcano of the Cameroon Line: Volcanic history, petrological and geochemical features  

NASA Astrophysics Data System (ADS)

The volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94 Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89 Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70 Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11 Ma. The volume of the volcano is calculated at 320 km3 ± 5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39 km in the upper mantle for the Group 1 lavas, and between 42 and 44 km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3-6% of partial melting of a moderately enriched source containing 3-7% of garnet. Melting took place in the spinel to garnet transition zone located at 70-90 km and around 25 kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component linked to the contaminated lithosphere of the Neoproterozoic mobile belt, and an enriched component or the lithosphere possibly related to pre-rifting magmatic processes.

Pouclet, André; Kagou Dongmo, Armand; Bardintzeff, Jacques-Marie; Wandji, Pierre; Chakam Tagheu, Pulchérie; Nkouathio, David; Bellon, Hervé; Ruffet, Gilles

2014-09-01

128

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

129

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

130

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

131

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

132

Mantle sources and magma crust interactions in volcanic rocks from the northern Kenya rift: geochemical evidence  

NASA Astrophysics Data System (ADS)

Miocene to Quaternary mafic (SiO 2<54 wt.%) and evolved (SiO 2>54 wt.%) lavas from Samburu Hills, northern Kenya rift, exhibit significant variations in major and trace element compositions, and Sr-Nd isotopic ratios. Bimodal lavas were erupted in the early and middle Miocene (20-15 Ma and 15-10 Ma) and Pliocene (4.1-3.6 Ma), whereas only mafic lavas were produced in the upper Miocene (7.5-5.3 Ma) and Quaternary (2.0-0.1 Ma). Incompatible element concentrations (Zr, Nb, Y, Rb, and K) vary widely in the Miocene mafic lavas, but are depleted in Pliocene and Quaternary equivalents. In the Miocene and Pliocene mafic lavas, initial 87Sr/ 86Sr ratios vary from 0.70315 to 0.703808, and initial 143Nd/ 144Nd ratios vary from 0.51254 to 0.51288. 143Nd/ 144Nd ratios of the Quaternary lavas are similar (0.512724-0.512873), but 87Sr/ 86Sr ratios are higher (0.70432-0.70468). On a Nd- vs. Sr-isotopic diagram, the Quaternary lavas plot near bulk earth, whereas all other mafic lavas lie within the depleted mantle quadrant and at the left end of the East African Carbonatite Line (EACL). The geochemical variations of the mafic lavas originated from heterogeneous mantle sources produced by interaction of plume and lithospheric mantle, and subsequently modified by variable degrees of partial melting. The evolved lavas are divisible into evolved high-alkali (Na 2O+K 2O>9 wt.%) early Miocene latites, trachyphonolites, and middle Miocene and Pliocene trachytes; less evolved low-alkali (Na 2O+K 2O<9 wt.%) middle Miocene mugearite, and Pliocene benmoreite and trachyte suites. The high-alkali trachyphonolites and trachytes are enriched in Nb, Zr, Rb, and K, and depleted in Ba, Sr, P, and Ti relative to latites and evolved low-alkali lavas. The evolved high-alkali lavas have higher initial Sr-isotopic (0.70372-0.705939) ratios than their mafic equivalents, but their initial Nd-isotopic ratios (0.512656-0.512777) lie within the same range. The low-alkali suites, however, have lower initial Nd-isotopic ratios (0.51235-0.51250), and slightly higher Sr-isotopic ratios (0.70347-0.70407) than their associated mafic lavas. Assimilation and fractional crystallization (AFC) of mafic magma is considered to be the dominant process for the generation of evolved lavas. The high-alkali and low-alkali evolved lavas require low and high degrees of crustal assimilation, respectively.

Kabeto, Kurkura; Sawada, Yoshihiro; Iizumi, Shigeru; Wakatsuki, Toshiyuki

2001-03-01

133

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

134

Geochemistry and geochronology of mafic rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon Volcanic Line  

NASA Astrophysics Data System (ADS)

Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model.

Kamgang, Pierre; Chazot, Gilles; Njonfang, Emmanuel; Tchoua, Félix

2008-12-01

135

The Petrochemistry of Jake_M: A Martian Mugearite  

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

136

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

137

Geomechanical parameters of intact rocks and rock masses from the Canary Islands: Implications on their flank stability  

NASA Astrophysics Data System (ADS)

New data on the geomechanical properties of the highly cohesive volcanic rocks of the Canary Archipelago and their role in the flank stability on oceanic islands are provided in this work. On the basis of the textural and petrological features, a preliminary classification of rocks, grouped into lithotypes, was carried out. This classification includes vesicular and non vesicular basalts, trachybasalts, trachytes, phonolites, welded and non welded ignimbrites. Strength and strain-related features are summarized here for each distinctive lithotype. Taking into account the results of the uniaxial and triaxial compressive tests, the geological strength index of rock masses and their textural-structural features, an estimate of the rock mass parameters and Mohr-Coulomb fit has been carried out. A final discussion on the impact of those geomechanical parameters as factors governing the stability of steep slopes in volcanic islands is then made here as a contribution in volcanic risk.

Rodríguez-Losada, J. A.; Hernández-Gutiérrez, L. E.; Olalla, C.; Perucho, A.; Serrano, A.; Eff-Darwich, A.

2009-05-01

138

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

PubMed

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

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

1964-09-11

139

Non-omnia moriantur-toxicity of mancozeb on dead wood microarthropod fauna.  

PubMed

The effect of Dithane M-45 (dithiocarbamate fungicide; active substance: mancozeb) was studied on microarthropod fauna inhabiting dead wood. Although the exposure was almost never 100% lethal for the majority of observed taxa, almost all (Mesostigmata, Oribatida, some Uropodina, Actinedida, Collembola and Diplopoda) showed very high correlation between concentration of the fungicide and mortality (r > 0.86). Only Stigmaeidae showed low correlation (r = 0.293). For the majority of taxa LC(50 )values were close to the concentrations used during agrochemical activities in woods. Only Trachytes aegrota showed full susceptibility to the fungicide within the range of recommended field concentrations used in forestry (characterised by the low LC(95 )value). Tolerance of mesostigmatid and oribatid mites was found to differ between juveniles and adults, but not consistently. Related Uropodina species varied in susceptibility to the fungicide. PMID:17522956

Adamski, Zbigniew; Bloszyk, Jerzy; Bruin, Jan; Ziemnicki, Kazimierz

2007-01-01

140

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

141

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

142

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

143

Geological investigations of volcanic rocks at Mount Discovery, Mount Morning, and Mason Spur, McMurdo Sound  

SciTech Connect

This work includes mapping of volcanic geology, description of geologic sections, and collection of samples for geochemical analysis and potassium-argon dating. Reconnaissance mapping of Mount Discovery shows that this 2681-meter-high composite volcano comprises a core of plagioclase-phyric nepheline-benmoreite flows, lahars, and volcanoclastic fluviatile sediments, which are thought to be the unit dated at 5.44 million years. Reconnaissance mapping of Mount Morning shows that this is a young volcano principally composed of kaersutite-bearing phonolite flows erupted both from the summit crater and from small parasitic domes on the upper northern slopes of the mountain. A sample from one of these parasitic domes has been dated at 1.15 million years. Description of five geologic sections at Mason Spur has provided a more detailed understanding of the older trachytic volcanic complex, which is now dated at 11.5 to 12.8 years and is divided into seven mapped units.

Wright, A.C.; Kyle, P.R.; More, J.A.; Meeker, K.

1986-01-01

144

Mantle sources and magma-continental crust interactions during early Red Sea-Gulf of Aden rifting in southern Yemen - Elemental and Sr, Nd, Pb isotope evidence  

NASA Astrophysics Data System (ADS)

Major and trace elements and Nd, Sr, and Pb isotopes from the southern Yemen area are studied. Oligo-Miocene traps, dykes, and plutons are associated with the early Red Sea-Gulf of Aden rifting in the area. They display a bimodal suite ranging from basalts and ferrobasalts to trachytes-rhyolites, and comendites-pantellerites. Major and trace element distributions and mass balance calculations indicate that the evolution from basalts to felsic rocks is partly governed by fractional crystallization. The genesis of the felsic rocks requires assimilation of the crustal basement involving an old continental component. The isotopic diversity of the felsic rocks is best explained by bulk mixing in the case of the traps and by fractional crystallization processes in the case of the dykes. Nd, Sr, and Pb isotope relationships within the basalts strongly suggest mixing processes mainly involving a depleted asthenospheric source and enriched lithospheric mantle reservoirs.

Chazot, Gilles; Bertrand, Herve

1993-02-01

145

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

146

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

147

The use of leaves and roots of Laurus novocanariensis as an indicator for soil and rock chemical composition in the environment of a subtropical cloud forest (Tenerife, Canary Islands, Spain).  

NASA Astrophysics Data System (ADS)

On the Canary Island of Tenerife exists a sensitive and endangered ecosystem called "laurel forest". Laurel forest is an endemic type of a humid subtropical forest and it still covers a terrain of roughly 60 km2 on Tenerife (nearly 7% of the territory) (FERNANDEZ et al. 2001). The existing Laurel forest soils have been developed on different rocks. Corresponding to different moisture regimes, Vertisols, Alfisols, Ultisols, and Inceptisols are developed on basaltic lava flows. Inceptisols, allophanic Andisols, and vitric Andisols are present on pyroclastic rocks (ARNALDS et al., 2007). Three volcanic rock types of the basanite-phonolite assemblage are recognised (Rothe, 2008): Basic (basanites, ankaramites), intermediate (trachybasanites, plagioclase phonolites), and salic (trachyte, trachyphonolite, phonolite). Trachytes (sensu stricto) are comparatively rare. The present study aims to understand the element cycle and feed back mechanism between volcanic rocks, soils, roots, and leaves. Laurus novocanariensis stands as a key example how leaves and roots in a subtropical cloud forest, such as on Canary Islands, can be used as an indicator for soil and rock geochemistry. To obtain a wide spectrum of inorganic elements, we chose for our samples a combination of ICP-OES and ICP-MS. Our results show clearly that certain elements are enriched or depleted in leaves and roots. Other elements mirror the chemical composition of the soils and the volcanic rocks in great detail. This study indicates that Laurus novocanariensis can be used to trace the element distribution of certain elements from volcanic rocks thru soils to roots and leaves without a large disturbance of a sensitive ecosystem.

Heidak, M.; Glasmacher, U. A.; Schöler, H. F.; Hernández-Moreno, J. M.

2012-04-01

148

Halogen and trace-element chemistry in the Gardar Province, South Greenland: Subduction-related mantle metasomatism and fluid exsolution from alkalic melts  

NASA Astrophysics Data System (ADS)

XRF analyses of 152 magmatic dyke samples from a broad area (150 × 60 km) of the Gardar Province in Southern Greenland span the time 1280 to 1163 Ma and represent a wide compositional range (transitional olivine basalts to trachytes, alkalinity index of 0.3 to 1.5). Among those, 16 dyke samples were additionally analysed for Cl and Br. Generally, the dykes represent a continuous fractionation trend from relatively unfractionated basalts to more highly fractionated trachytes. Dykes from different areas exhibit a diverse geochemistry suggesting a heterogeneous and metasomatised mantle source. Enrichment in LILE, LREE and Sr and depletion in HFSE, Nb and Ti suggest that some of the metasomatism may have been associated with subduction processes pre-dating Gardar activity by some 600 Ma (Ketilidian orogeny). The dykes are characterised by high F contents up to 1.2 wt.%, particularly in the vicinity of the Ivigtut fluoride deposit. F was probably derived from partial melting of lithospheric mantle enriched in F-apatite and F-phlogopite. High Cl/Br (>500) and low Cl/F (<1) ratios of the dykes point to a fluid degassing/separation process which is supported by mineral/rock and fluid inclusion data from the Ilímaussaq and Ivigtut intrusions. There, the analysed rocks and minerals generally show high Cl/Br (>300) and low Cl/F (<1) weight ratios whereas the fluid inclusions have complementary low ratios (Cl/Br ±100; Cl/F >10). Our investigations are in accordance with experimental data which show that F is preferentially enriched in the melt whereas Cl and especially Br are lost with the fluid phase. Accordingly, the halogens show an increase in incompatible behaviour in the magma in the order of F < Cl < Br.

Köhler, Jasmin; Schönenberger, Johannes; Upton, Brian; Markl, Gregor

2009-12-01

149

Fluid-rock interaction in the Miocene Tejeda intrusive complex, Gran Canaria, Canary Islands  

NASA Astrophysics Data System (ADS)

The intra-caldera volcaniclastic deposits of the Miocene Tejeda caldera on Gran Canaria host a ~12 km diameter intrusive complex, comprising ~500 peralkaline trachytic to phonolitic cone sheets, surrounding a central core of hypabyssal syenite stocks. Both intrusive rock types display textural and mineralogical features indicative of secondary fluid-rock interaction, including (i) deuteric mineral phases (e.g. aegirine, riebeckite, analcime), (ii) exsolved alkali feldspar and Fe-Ti oxide phenocrysts, (iii) pervasive phyllosilicate and zeolite replacement, and (iv) dendritic Mn-oxide coatings, suggesting the intrusive complex sustained an active hydrothermal system during emplacement, cooling, and after solidification. Altered cone sheets have ?18O values ranging from 0.1 to 10.0 per mil (n = 22), and ?D values between -62 to -149 per mil (n = 28). Altered syenites have ?18O values of 0.9, 1.5, and 2.5 per mil, and corresponding ?D values of -91, -99, and -121 per mil. The majority of samples are depleted in 18O relative to the typical ?18O-range for unaltered trachytes and syenites (?18O = 6-8 per mil), indicative of interaction with low-?18O fluids at high temperature. A positive correlation between ?D and ?18O, which parallels the global meteoric water line, suggests local meteoric water was the dominant hydrothermal fluid source. No systematic variation in ?18O or ?D was detected across the cone sheet swarm, reflecting overprinting of isotopic compositions during successive intrusive events. However, the highest ?18O values (8.2-10.0 per mil) occur in clay- and zeolite-rich cone sheets from the central part of the intrusive complex, suggesting overall inward-cooling or 'shrinking' of the hydrothermal system. By combining mineralogical and isotopic data, two phases of alteration can be distinguished in the Tejeda Intrusive Complex: (i) high-temperature (~300-500C) deuteric alteration by late-magmatic, alkali-rich fluids, and (ii) lower temperature (

Donoghue, E.; Troll, V. R.; Harris, C.

2009-04-01

150

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

NASA Astrophysics Data System (ADS)

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

Kumar, Naresh; Vallinayagam, G.

2014-05-01

151

A volcanological and geochemical investigation of Boa Vista, Cape Verde Islands; 40Ar/ 39Ar geochronology and field constraints  

NASA Astrophysics Data System (ADS)

Boa Vista, the easternmost island in the Cape Verde archipelago, consists of volcanic products, minor intrusions and a thin partial sedimentary cover. The first 15 age results from 40Ar/ 39Ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from Boa Vista are presented. The combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages: (1) > 16 Ma, (2) 15-12.5 Ma and (3) 9.5-4.5 Ma. The first stage, restricted to the north eastern part of the island, is dominated by ankaramitic lavas. The second stage, consisting of evolved lavas of phonolitic-trachytic compositions and nepheline syenites, makes up large central parts of the island. The large volume of evolved rocks and the extended eruption period of several Ma make stage 2 in Boa Vista unique to Cape Verde. Mainly basanites and nephelinites were erupted during the third stage, initially dominated by eruption of subaerial mafic lavas around 9 Ma. Pillow lavas are erupted around 7 Ma whereupon dominantly subaerial mafic lavas were erupted. Stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time. The volcanic evolution of Boa Vista constrains the initiation of volcanic activity in the Cape Verde archipelago to the eastern islands. Major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on Boa Vista is presented, revealing an extremely well developed Daly Gap. Only a little was modified from the mafic magmas that rose in small batches from the mantle. Compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts. The highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas. Two of these may have been derived by crystal fractionation of primitive Boa Vista melts, whereas the third was not.

Dyhr, Charlotte T.; Holm, Paul M.

2010-01-01

152

Late Oligocene OIB-like lavas in northern Idaho  

NASA Astrophysics Data System (ADS)

The 26.3 to 25.3 Ma Potlatch volcanics in northern Idaho (Kuffman et al., 2006) consist of a suite of basalts, hawaiites, mugearites, benmoreites, trachytes and nepheline trachytes. The volcanic field was erupted on North American cratonic basement well to the northeast of the regional crustal suture with Phanerozoic terranes accreted during the Mesozoic, and predates Columbia River flood basalt activity in the area by 9 million years. The most primitive Potlatch lavas are porphyritic olivine basalts with 6 percent MgO and strongly OIB-like chemical affinities (La/Nb = 0.69 - 0.76, Th/Ta = 0.92 to 1.08, Pb/Ce = 0.029 to 0.033, 87Sr/86Sr = 0.70367 to 0.70476, 206Pb/204Pb = 19.254 to 19.504). Similarly, intermediate and felsic lavas and pyroclastics closely resemble differentiated members of typical sodic ocean island suites, but have additionally been affected by AFC involving small amounts of regional continental crust, which has acted to increase 87Sr/86Sr up to 0.70516. The Potlatch volcanics are geochemically unlike other regional Cenozoic volcanic suites including Eocene Challis rocks, basalts and rhyolites of the John Day Formation and other volcanic fields around the Blue Mountains to the south and southwest, and the later Columbia River basalts. Their occurrence represents a modification to the southward retreat pattern of early to mid-Cenozoic magmatism in northwestern North America. Kauffman, Bush, and Lewis (2006) ID Geol. Surv. Tech. Rep. 06-7, 11 pp.

Stadnik, S.; Wolff, J. A.; Hart, G. L.

2008-12-01

153

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

NASA Astrophysics Data System (ADS)

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

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

2011-07-01

154

Characterizing weathering intensity and trends of geological materials in the Gilgel Gibe catchment, southwestern Ethiopia  

NASA Astrophysics Data System (ADS)

Detailed geological and geochemical characterization is crucial to support soil studies in such geologically and topographically complex systems as the Gilgel Gibe catchment in southwestern Ethiopia. Field studies, as well as mineralogical, petrological and geochemical analyses on selected rock samples and their weathering products revealed that the catchment is dominantly underlain by rhyolites and trachytes, which occur as both lava flows and pyroclastic associations. Most of the lavas have a trachytic texture, while few others are massive or show spherulitic or perlitic texture. The rocks have a SiO2-content ranging from about 62 to 73 wt% (intermediate to felsic composition, on an anhydrous base) and a relatively high Na2O + K2O content ranging from about 9 to 12 wt% (anhydrous base). The dominant phenocrysts present in the rocks are plagioclase, sanidine and Fe-Ti oxide minerals. Alkali-rich amphiboles and quartz occur in most of them, while hornblende, titanite and clinopyroxene are rare. The amount of phenocrysts varies from less than 1 vol.% to about 30 vol.%. The pyroclastic associations are discontinuously scattered within the study area. They all have a glassy matrix (vitrophyric texture) and are composed of a mixture of lithics, crystals and glass. In comparison with the lava samples, the pyroclastic samples exhibit a more variable chemistry. In contrast, the X-ray diffractograms of the pyroclastic deposits and the lavas show little difference. The Chemical Index of Alteration values for the studied samples vary from 53 to 99 indicating moderate to high intensity of weathering. Samples from lava flows have shown less degree of weathering than samples of the pyroclastic associations.

Regassa, Alemayehu; Van Daele, K.; De Paepe, P.; Dumon, M.; Deckers, J.; Asrat, Asfawossen; Van Ranst, E.

2014-11-01

155

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

156

Petrogenesis of the Miocene felsic volcanism from the south of Izmir (Western Turkey) and its regional tectono-magmatic implications  

NASA Astrophysics Data System (ADS)

Miocene volcanism, mainly intermediate and rarely mafic and felsic in composition, was produced under the effects of the extensional tectonic regime in western Turkey. The Karaburun and Cumaovasi volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic bimodal volcanism. The Karaburun volcanics (KV) are mainly distributed north to south direction within the Karaburun peninsula and span a wide compositional spectrum from basalt (20 Ma) to rhyolite (16 Ma), and younger trachyte and trachydacites (13 Ma). The products of the subaerial felsic volcanism (the Cumaovasi volcanics, CV; 17 Ma) which are represented by cluster of topaz bearing rhyolite domes, related pyroclastic rocks occur within the NE-SW trending Cubukludag graben. The intermediate and mafic volcanic rocks are lack in the volcanic succession of CV. The lavas of the Cumaovasi volcanics are silicic which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, Eu depletions and very low LaN/YbN ratios are typical for rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. The petrological data revealed that the KV and CV were formed in extensional tectonic setting, but evolved by different petrological processes in different magma chambers. The Cumaovasi lavas have a unique chemical composition, and closely similar to the extension related topaz bearing rhyolites formed from small magma bodies. Our data reveal that extension related mafic injections caused crustal anatectic melting and produced felsic melts that rapidly ascended into the upper crust.

Karacik, Z.; Genç, C.

2013-12-01

157

Electrical conductivity and streaming potential coefficient in a moderately alkaline lava series  

NASA Astrophysics Data System (ADS)

Coupled hydraulic and electrical problems are being investigated with renewed interest in geophysical applications. In this study, electrical conductivity and streaming potential coefficient (SPC) have been measured in the laboratory for crushed oceanite, hawaite, and trachyte samples from an almost complete moderately alkaline lava series from the Mururoa atoll (French Polynesia). For pH varying from 7 to 9.5 and ionic strength varying from 0.1 to 100 mmol/l, despite the closely related mineralogy of the rocks, contrasted results are obtained for the surface conductivity as defined by Revil and Glover: 0.10±0.07 mS/m, 8.3±0.2 mS/m and 1.05±0.16 mS/m for oceanite, hawaite, and trachyte, respectively, and for the inferred ? potential: -6.6±0.3 mV, -26.3±0.5 mV, and -14.6±0.3 mV, respectively. The higher values obtained with hawaite suggest that electrical properties are controlled by secondary and accessory minerals such as clay minerals and zeolites resulting from low-temperature seawater alteration, rather than by the magmatic differentiation. Furthermore, a synthesis of results obtained with 20 rock samples indicates that signatures of the alteration process may emerge from the study of the ? potential as a function of surface conductivity. The results with the lava series can be used to derive estimates of the scaling of basalt resistivity and associated SPC as a function of permeability. For permeability values smaller than 10 -15 m 2, the value of the SPC can be dramatically affected by the value of the surface conductivity. At least in the brittle crust, a better knowledge of the contributions of clay minerals and zeolites to surface conduction is therefore needed, both for the interpretation of electrical conductivity profiles and for the estimation of electrical variations associated with groundwater flow, as could be produced for example during volcanic or tectonic cycles.

Perrier, Frédéric; Froidefond, Thierry

2003-05-01

158

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

SciTech Connect

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

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

1985-01-01

159

The Chaîne des Puys: how complicated can monogentic get?  

NASA Astrophysics Data System (ADS)

The Chaîne des Puys (Massif Centrale of France) is part of a prospective volcano-tectonic UNESCO World Heritage site including the Limagne Rift fault (http://www.chainedespuys-failledelimagne.com/). The strategy is to present the monogentic field that is as representative of other such fields, but which itself is uniquely special. Effectively, the Chaîne des Puys would become a sort of ambassador for monogentic volcanism, raising the profile of all other sites. Here, I want to go through some recent work on the chain, looking at morphology, morphometry, structure, lithology and petrology and show some of the intriguing complexities of this classic highly variable monogentic alignment. Also, I want to build on the historical development of ideas that can be traced back to characters such as Montlosier, Humphrey Davey, Faraday, Lyell, and Von Humbolt... and many more. I focus, first, on the central the Puy de Dôme. This classic trachyte dome has been known for some time to be the product of two eruptions, and recently we have found that it is related to a number of cryptodome intrusions that have created flanking bulges, fed at least two major explosive eruptions, and extensively modified the topography of the field. Strangely, Von Humbolt's concept of 'craters of elevation' rises up in a reanalysis of these structures. Secondly, I visit Lemptégy (www.auvergne-volcan.com/), that in 1857, Scrope called 'an insignificant cone grazed by sheep', but now quarried out and showing the internal structure expected for the Puy de Dôme bulges, as well as illustrating that a seemingly simple scoria cone plumbing can be infernally complicated. Thirdly, I consider the Beaunit, a bucolic village in a maar, where the process of crustal ingestion suggests an intimate relationship between eruptive dynamics and assimilation. With these three examples I show some of the complications and interactions of monogentic basaltic to trachytic volcanism typified by the Chaîne des Puys.

Van Wyk de Vries, B.; Grosse, P.; Marquez, A.; Petronis, M. S.; Kervyn, M.; Delcamp, A.; Mossoux, S.; Troll, V. R.

2012-12-01

160

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.

161

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

162

The potassic series of Karisimbi volcano (Virunga range, Rwanda): Volcanological and petrological aspects  

NASA Astrophysics Data System (ADS)

The Karisimbi volcano (Rwanda) is one of the principal eruptive centers of the Virunga volcanic range. It is located at the southern tip of a NE-SW volcano-tectonic axis on the western branch of the East African Rift. The different morphological units (caldera "Branca", pit crater "Muntango", domes and flow domes) built up on the main edifice make Karisimbi the most complex central volcano of the Virunga range. The products of Karisimbi's activity are essentially fluid lavas of basic or intermediate chemical composition and viscous differentiated lavas. Ash falls and pumice flows are evidence of pyroclastic episodes which occurred during the last stages of volcanic activity around 30,000 years ago. The upper Pleistocene potassic series of Karisimbi is composed of a diverse range of lavas, closely associated in space and time. These lavas show many common features with rocks from rift provinces, such as high K 2O, TiO 2, Rb and Ba contents. The general evolution of the series from potassic basanites to trachytes is qualitatively compatible with the successive separations of the mineral phases observed in the lavas (olivine, clinopyroxene, leucite, ferro-titanic oxides, plagioclase, biotite, alkali feldspar). Coexisting Fe-Ti oxides indicate equilibration temperatures lying between 1100° and 750°C and fO 2 between N-NO and QFM buffers. Felsic magmas are considered to have crystallized at a relatively shallow level in the crust, under near water-saturated conditions, which is consistent with the presence of pyroclastics with trachytic composition and the presence of hydrous mineral phases.

Marcelot, G.; Rançon, J. Ph.; Demange, J.

1985-10-01

163

Volcanic unrest in Kenya: geological history from a satellite perspective  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

164

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

165

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

166

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

167

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

168

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

NASA Astrophysics Data System (ADS)

The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km 2 including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.l. The pumice fragments of the CI range in composition from trachyte to phonolitic-trachyte (DI = 75-90). They do not show any systematic compositional variation with stratigraphic height, but the analyzed sections can be divided into three groups on the basis of chemical composition of pumices. Least-evolved pumices (DI = 75-83) occur in the ignimbrite in the central sector of the Campanian Plain up to 30 km from the vent, while the most-differentiated pumices (DI = 88-90) characterize the cogenetic fallout deposit and the ignimbrite in the western sector of the Campanian Plain, on the Tyrrhenian scarp of the Apennines between Caserta and Mt. Maggiore, on Roccamonfina volcano, and on the Sorrento Peninsula, up to 50 km from the source. Pumice fragments of intermediate composition (DI = 84-87) occur in the ignimbrite on the Apennine Mountains and Roccamonfina volcano, up to 65 km from the vent. In one exposure at Mondragone, at the base of a calcareous ridge, an ignimbrite with pumices of most-evolved composition is overlain by an ignimbrite with pumices of intermediate composition. The observed compositional variation between most- and least-evolved ignimbrite was generated in part by crystal-liquid fractionation, although other magmatic processes such as syn-eruptive mingling between most- and least-evolved magmas accounts for the mineralogical disequilibria and for the bimodality of the glass compositions in the intermediate-composition rocks. Pumice Sr-isotope ratios are positively correlated with degree of differentiation. Feldspar crystals separated from pumices of different compositions have a homogeneous Sr-isotope composition similar to that of the least-evolved pumices. Interaction between fluids and strongly fractionated Sr-poor less-dense magma can account for these isotopic features. Geochemical, mineralogic, stratigraphic and volcanologic data, together with the stratigraphic relations between most-, intermediate- and least-evolved ignimbrite as detected at Mondragone and from bore-hole drillings suggest that: (1) the CI magmatic system was composed of two distinct magma layers — the upper layer was more differentiated and homogeneous in composition, while the deeper was less evolved and slightly zoned; and (2) the CI was mostly emplaced in three main pulses of pyroclastic flows that tapped the chamber at variable levels and with distinct withdrawal dynamics. The eruption began with emission of the most differentiated magma, which gave rise to the fallout deposit. It continued with generation of expanded, turbulent pyroclastic flows that reached the Sorrento Peninsula in the southeast and Roccamonfma volcano in the northwest. These flows, whose thickness was greater than the overtopped relief, were able to travel over the water of the bay of Naples. Subsequently an intermediate-composition magma resulting from mingling of different portions of the magma chamber generated similar flows that spread radially and traveled not less than 65 km from the vent. During the last pulse the least-evolved magma was tapped and generated flows that spread within the Campanian Plain. Variation in eruptive dynamics and composition of magma during the course of the eruption likely reflected variations of both geometry of vent and plumbing system, and efficiency of water/magma interaction, which in turns affected the dynamics of the magma chamber and the withdrawal mechanism, and resulted from the dynamics of the caldera collapse.

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

1997-02-01

169

Incision of the Colorado River in southern Utah - insights from channel profiles, local incision rates, and modeling of lithologic controls  

NASA Astrophysics Data System (ADS)

The Colorado River and its tributaries in southern Utah and northern Arizona provide an opportunity to study the propagation of bedrock incision through a large heterogeneous fluvial network, as the system is continuing to adjust to the baselevel fall responsible for the Grand Canyon. Although the carving of the Grand Canyon was largely complete by ~1 Ma, the canyon ends at Lee's Ferry and the incision history of Colorado River system upstream of the associated large knickpoint has been the subject of debate. In conjunction with existing incision rate estimates based on the dating of strath terraces, we use longitudinal profiles of the Colorado and tributaries between Marble Canyon and Cataract Canyon to investigate the incision history of the Colorado in this region. We find that all but two of the tributaries in this region steepen as they enter the Colorado River. The consistent presence of oversteepened reaches with similar elevation drops in the lower section of these channels, and their coincidence within a corridor of high local relief along the Colorado, suggest that the tributaries are steepening in response to an episode of increased incision rate on the Colorado River. This analysis is supported by available incision rate data, as the spatial distribution of incision rates predicted by the channel profiles is consistent with existing rate estimates. The two analyzed tributaries that show no evidence for this incision pulse, Trachyte Creek and Bullfrog Creek, have smoothly concave profiles and do not contain knickpoints. In order to evaluate the significance of these anomalous channel profiles, we measure in situ 10Be concentrations on four gravel-covered strath surfaces elevated from 1 m to 110 m above Trachyte Creek. The surfaces yield exposure ages that range from approximately 2.5 ka to 267 ka and suggest incision rates that vary between 350 and 600 m/my. These incision rates are similar to other rates determined within the high-relief corridor, and suggest that despite the lack of knickpoints in their long profiles, Trachyte and Bullfrog Creeks are also responding to the increase in incision rate on the Colorado. Sustained high incision rates combined with a smoothly concave profile suggest that these channels are responding in a continuous, transport-limited manner, perhaps driven by the combination of extremely durable diorite sediment and weak, easily abraded bedrock found in the channels of the Henry Mountains. Finally, we use a simple numerical model of detachment-limited bedrock incision to investigate the relationship between the large convexity in the Colorado River at Lee's Ferry and the incision we observe farther upstream. Model results suggest that the pulse of incision we observe may be related to the interaction between the propagation of headward incision through the Grand Canyon and the presence of an upstream-dipping lithologic boundary at Lee's Ferry. This suggests that the large knickpoint at Lee's Ferry is neither the upstream extent of Grand Canyon incision nor solely related to lithology, but instead results from a combination of lithologic and transient effects.

Cook, K.; Whipple, K.; Heimsath, A.

2009-04-01

170

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

171

Petrology and geochemistry of Marion and Prince Edward Islands, Southern Ocean: Magma chamber processes and source region characteristics  

NASA Astrophysics Data System (ADS)

New bulk rock geochemical data on an extensive suite of samples from the Marion and Prince Edward Islands, located in the Southern Ocean at ~ 37°45'E, 46°55'S, show highly coherent major and trace element variations. Rock types are dominated by alkali basalt, with lesser hawaiite and minor mugearite and benmoreite. Prince Edward Island has in addition rare trachytes exposed on the plateau, and xenoliths of comenditic rhyolite pumice were found in a basalt flow of the 1980 eruption on Marion Island. Lavas from Prince Edward Island differ from those erupted on Marion Island by having subtly distinct incompatible trace element ratios (e.g. Zr/Nb = 9.1 vs 6.7; Ba/Nb = 7.8 vs 6.7; Ce/Pb = 31 vs 36), and lower 87Sr/86Sr (0.70302) and higher 143Nd/144Nd (0.51300) isotope ratios than found on Marion Island (0.70336-0.70349; 0.51292-0.51295). The two islands are indistinguishable in terms of Pb isotope ratios (206Pb/204Pb = 18.61-18.75, 207Pb/204Pb = 15.54-15.56). Quantitative modelling suggests that the range in rock types found on each island can be ascribed to extensive low pressure crystal fractionation of phenocryst phases. For example, hawaiite can be related to the more primitive alkali basalts by some 24% fractional crystallisation of olivine, clinopyroxene, plagioclase and minor FeTi-oxides, whereas the more evolved mugearites and benmoreites require, respectively a further 36% and 24% fractional crystallisation to account for their felsic compositions. The trachytes of Prince Edward Island appear to represent the residual 28% magma following extensive crustal fractionation of dominantly clinopyroxene, plagioclase and FeTi-oxide with minor apatite. None of the sampled lavas have compositions characteristic of true primary magmas, but the least evolved, with Mg# ~ 63, appear to have experienced as much as 18% olivine fractionation from a primary magma with ~ 15 wt.% MgO. The rhyolite pumice is distinct in composition from Bouvet rhyolite or South Sandwich Island pumice and may belong to an earlier, submarine eruption at or near Marion Island. The enriched incompatible element ratios (Zr/Nb = 6-9; La/Ybn = 10.6; Y/Nb = 0.73) and radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd isotope ratios show that the lavas erupted on Marion and Prince Edward Islands resulted from melting an enriched mantle source, consistent with derivation from an upwelling deep mantle plume. Pb isotope ratios (?8/4Pb ~ 33) confirm the absence of any DUPAL signature in the source region of these magmas.

le Roex, A. P.; Chevallier, L.; Verwoerd, W. J.; Barends, R.

2012-04-01

172

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

NASA Astrophysics Data System (ADS)

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

Seghedi, Ioan; Helvac?, Cahit

2014-05-01

173

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

NASA Astrophysics Data System (ADS)

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

Gioncada, A.; Landi, P.

2010-01-01

174

Geochemical and B-Sr-Nd isotopic evidence for mingling and mixing processes in the magmatic system that fed the Astroni volcano (4.1-3.8 ka) within the Campi Flegrei caldera (southern Italy)  

NASA Astrophysics Data System (ADS)

The Astroni volcano was built through seven eruptions that generated pyroclastic deposits and lava domes within the Campi Flegrei caldera (southern Italy) 4.1-3.8 ka BP. Whole-rock geochemical and B-Sr-Nd isotopic investigations were carried out on representative samples of all seven eruptions. The products vary from tephriphonolites to phonolites, and from latites to trachytes. They show textural, mineralogical and isotopic evidence of disequilibrium, including distinct clinopyroxene populations, rounded and/or resorbed plagioclase and alkali-feldspar, and reverse-zoned phenocrysts of all these mineral phases. The Sr, Nd and B isotopic composition of whole rocks is variable and correlated with the degree of chemical evolution, suggesting open-system processes in addition to fractional crystallisation. Moreover, significant Sr-isotopic disequilibrium between the phenocrysts and glass has been documented for one sample. The chemostratigraphy of the products indicates that Astroni eruptions 1 through 5 were fed by magmas of trachytic to phonolitic composition that were less enriched in radiogenic Sr and 11B up-section. This variability has been interpreted as the result of mingling between at least two distinct magmatic end-members, one more evolved and the other less evolved. Another heterogeneous batch of magma, resulting from almost complete mixing between the same two end-members, was drained during eruptions 6 and 7. The more evolved end-member, characterised by 87Sr/ 86Sr ? 0.7075, 143Nd/ 144Nd ? 0.51247 and ?11B ? - 8‰, was very similar to the magma that fed the final phases of the Agnano-Monte Spina eruption, which occurred a few centuries earlier in the Astroni vent area. The less evolved end-member had 87Sr/ 86Sr ? 0.70726, 143Nd/ 144Nd ? 0.51251 and ?11B ? 10‰, and was likely derived by fractional crystallisation of a mantle-derived magma. An abrupt decrease in both the Sr isotope ratio and the Th content, detected at the transition between Unit 4 and 5, suggests that another magma with a 87Sr/ 86Sr ratio intermediate between those of the two identified end-members may have been involved in Astroni activity. The more evolved end-member is interpreted as a residue of the Agnano-Monte Spina eruption that was invaded by either the intermediate or the less evolved magmatic end-member, promoting mingling and triggering Astroni activity. This study of Astroni provides insights for both short- and long-term volcanic hazard assessment, as the Astroni volcano is the best example of a very close sequence of eruptions from the same vent area in the Campi Flegrei caldera.

Tonarini, Sonia; D'Antonio, Massimo; Di Vito, Mauro Antonio; Orsi, Giovanni; Carandente, Antonio

2009-02-01

175

A Gradual Compositional Change from Samoan Shield to Rejuvenated Lavas?  

NASA Astrophysics Data System (ADS)

The geochemical evolution of intraplate volcanoes is often compared to the archetypal model derived from Hawaiian volcanoes that involves a mantle plume source with multiple components. In Samoa, a range in rock types, ages, and isotopic compositions have been obtained across a number of volcanic seamounts and islands. However, due to dense vegetation on the islands, stratigraphic relations are not well known, and therefore a sequence for the construction of Samoan volcanoes is not as well-defined as in Hawaii. On the three largest islands, a shield and a rejuvenated stage have been defined, whereas the existence of a post-shield stage has been suggested and questioned (Natland and Turner, 1985). Moreover, the existing shield isotopic compositions (particularly Sr) are distinct, spanning a larger range than in Hawaii. Tutuila only hosts a small amount of rejuvenated lavas, but they are similar to those of Upolu, and there is an isotopic and compositional overlap between Tutuila shield volcanics and the Upolu lavas. On the island of Upolu, roughly half the surface area is covered in rejuvenated lavas, while the other half consists of shield stage lavas. The shield lavas around Fagaloa Bay are compositionally similar to some of the Tutuila lavas. On the island of Savaii, rejuvenated volcanism covers nearly the entire island, showing similar compositions to Upolu rejuvenated volcanism. We here present new data for samples from a deep canyon in the interior of Savaii, which form the missing link in understanding the geochemical sequence of Samoan volcanic construction. In this canyon, an isotopically distinct composition is found that resembles the composition of Fagaloa lavas on Upolu, instead of the extremely radiogenic Sr isotope compositions dredged from the submarine base of Savaii that represent the early shield stage. In Fagaloa Bay, a slope break on the mountainside has been suggested to form the outline of a crater along which late-stage evolved lavas such as trachytes erupted. A similar slope break is found in the canyon sampled on Savaii, and trachytes have previously been reported as cobbles in the draining river. Therefore, we infer that early shield volcanism in Savaii erupted extremely radiogenic Sr isotope compositions, and that by the end of shield building, compositions had changed to a composition similar to Upolu's Fagaloa. Some of the samples have Pb-Sr isotope compositions between Upolu and rejuvenated lavas, while their trace element compositions correspond to those of rejuvenated lavas. Thus, it appears that during the final stages of shield building, a shift to rejuvenated composition takes place. This contrasts with the definition of rejuvenated volcanism in Samoa, based on erosional contacts, and suggests rejuvenated source material may be sampled before the volcanic rejuvenation really occurs.

Konter, J. G.; Jackson, M. G.; Koppers, A. A.

2013-12-01

176

Megacrystic Clinopyroxene Basalts Sample A Deep Crustal Underplate To The Mount Taylor Volcanic Field, New Mexico  

NASA Astrophysics Data System (ADS)

The alkaline and compositionally diverse (basanite to high-Si rhyolite) Mount Taylor Volcanic Field (MTVF), New Mexico comprises 4 regions that cover ~75 x 40 km2: (1) Mount Taylor, a large composite volcano and a surrounding field of basaltic vents; (2) Grants Ridge, constructed of topaz rhyolitic ignimbrite and coulees; (3) Mesa Chivato, a plateau of alkali basalts and mugearitic to trachytic domes; and (4) the Rio Puero basaltic necks. Distributed throughout its history (~3.6 to 1.26 Ma; Crumpler and Goff, 2012) and area (excepting Rio Puerco Necks) is a texturally distinct family of differentiated basalts (Mg# 43.2-53.4). These basalts contain resorbed and moth-eaten megacrysts (up to 2 cm) of plagioclase, clinopyroxene, and olivine ±Ti-magnetite ±ilmenite ±rare orthopyroxene. Some megacrystic lava flows have gabbroic cumulate inclusions with mineral compositions similar to the megacrysts, suggesting a common origin. For instance, gabbroic and megacrystic clinopyroxenes form linear positive arrays in TiO2 (0.2-2.3 wt%) with respect to Al2O3 (0.7-9.3 wt%). The lowest Al clinopyroxenes are found in a gabbroic inclusion and are associated with partially melted intercumulus orthopyroxene. Megacrystic and gabbroic plagioclase (An 41-80) in 4 representative thin sections were analyzed for 87Sr/86Sr by Laser Ablation ICP-MS. 87Sr/86Sr values for the suite range from 0.7036 to 0.7047. The low 87Sr/86Sr plagioclases (0.7036 to 0.7037) are associated with high Ti-Al clinopyroxenes. Likewise, the higher 87Sr/86Sr plagioclases (0.7043 to 0.7047) are associated with the low-Al clinopyroxenes. Taken together, these megacrysts track the differentiation of an intrusive body (or related bodies) from alkaline to Si-saturated conditions by fractional crystallization and crustal assimilation. The intrusive body likely underplates portions of the MTVF that have generated silicic magmas (Mount Taylor, Grants Ridge, Mesa Chivato). Although disequilibrium is implied by resorbed grain boundaries, clinopyroxene-liquid geobarometry calculations (Putirka, 2008) suggest crystallization occurred in the lower crust at ~1200°C and 12.4 kbar. This intrusive body may represent a common deep crustal heat source to link the diverse silicic magmas (mugerites, trachytes, trachy-andesites and dacites, high-Si rhyolites, and topaz rhyolites) of the MTVF.

Schmidt, M. E.; Schrader, C. M.; Crumpler, L. S.; Wolff, J. A.

2012-12-01

177

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

178

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

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

179

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

NASA Astrophysics Data System (ADS)

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

Nemeth, K.; Moufti, R.

2013-12-01

180

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

NASA Astrophysics Data System (ADS)

Widespread Neogene volcanism, mainly intermediate and rarely mafic and felsic in composition, was controlled by the extensional tectonic regime in western Turkey. The Karaburun and Cumaovas? volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic volcanism. The Karaburun volcanics (KV) are mainly oriented north to south in the Karaburun peninsula and span a wide spectrum from basalt (20 Ma) to rhyolite (16 Ma), and younger trachyte and trachydacites (13 Ma). The products of the subaerial silicic volcanism (the Cumaovas? volcanics, CV; 17 Ma) which are represented by cluster of rhyolite domes, related pyroclastics occur within the NE-SW trending Çubukluda? graben, and intermediate and mafic volcanic rocks are lack in this area. The lavas of the Cumaovas? volcanics are high silica rhyolites and rare dacites which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, extremely Eu depletions and very low LaN/YbN ratios are typical for the rhyolites of CV, similar to the topaz rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. 87Sr/86Sr ratios of the KV and dacitic samples of CV are close to each other and range from 0.708 to 0.709; while Sr isotopic ratios of the rhyolites are significantly high and variable (0.724-0.786). 143Nd/144Nd ratios of the CV and KV, except for the alkaline samples, are similar for both sequences vary from 0.51230 to 0.51242. Geological, geochemical, isotopic and radiochronologic data reveal that the KV and CV were formed in extensional tectonic setting, but evolved by different petrological processes in different magma chambers. During the Neogene, underplated mafic magma was injected into the crust and hybridized by mantle and crustal derived materials. Geochemical features and trace element modeling for the mafic members of the KV indicate that they were derived from enriched lithospheric mantle and modified by fractional crystallization from basalt to rhyolite (Helvac? et al., 2009). Unexpectedly, the felsic lavas from Cumaovas? region have a unique chemical composition, and similar to the extension related rhyolites formed from small magma bodies. Our data reveal that extension related mafic inputs caused crustal anatectic melting and formed felsic melts that rapidly ascended into the upper crust. The Cumaovas? felsic rocks were differentiated into the highly evolved silica-rich melts within the magma chambers trapped near the surface.

Karac?k, Z.; Genç, ?. C.; Gülmez, F.

2013-09-01

181

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

182

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

183

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

184

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

185

40Ar/(39)Ar dating of the Kapthurin Formation, Baringo, Kenya.  

PubMed

The(40)Ar/(39)Ar radiometric dating technique has been applied to tuffs and lavas of the Kapthurin Formation in the Tugen Hills, Kenya Rift Valley. Two variants of the(40)Ar/(39)Ar technique, single-crystal total fusion (SCTF) and laser incremental heating (LIH) have been employed to date five marker horizons within the formation: near the base, the Kasurein Basalt at 0.61+/-0.04 Ma; the Pumice Tuff at 0.543+/-0.004 Ma; the Upper Kasurein Basalt at 0.552+/-0.015 Ma; the Grey Tuff at 0.509+/-0.009 Ma; and within the upper part of the formation, the Bedded Tuff at 0.284+/-0.012 Ma. The new, precise radiometric age determination for the Pumice Tuff also provides an age for the widespread Lake Baringo Trachyte, since the Pumice Tuff is the early pyroclastic phase of this voluminous trachyte eruption. These results establish the age of fossil hominids KNM-BK 63-67 and KNM-BK 8518 at approximately 0.510-0.512 Ma, a significant finding given that few Middle Pleistocene hominids are radiometrically dated. The Kapthurin hominids are thus the near contemporaries of those from Bodo, Ethiopia and Tanzania. A flake and core industry from lacustrine sediments in the lower part of the formation is constrained by new dates of 0.55-0.52 Ma, a period during which the Acheulian industry, characterized by handaxes, is known throughout East Africa. Points, typical of the Middle Stone Age (MSA), are found in Kapthurin Formation sediments now shown to date to between 0.509+/-0.009 Ma and 0.284+/-0.012 Ma. This date exceeds previous estimates for the age of the MSA elsewhere in East Africa by 49 ka, and establishes the age of Acheulian to MSA transition for the region. Evidence of the use of the Levallois technique for the manufacture of both small flakes and biface preforms, the systematic production of blades, and the use and processing of red ochre also occurs in this interval. The presence of blades and red ochre at this depth is important as blades signify a high degree of technical competence and red ochre suggests symbolic behavior. PMID:11795974

Deino, Alan L; McBrearty, Sally

2002-01-01

186

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

NASA Astrophysics Data System (ADS)

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 ( ±CO 2) 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.

de Vivo, Benedetto; Belkin, Harvey E.; Barbieri, Mario; Chelini, Walter; Lattanzi, Pierfranco; Lima, Annamaria; Tolomeo, Luigia

1989-02-01

187

Carbonatites and primary carbonates in the Rio Apa and Amambay regions, NE Paraguay  

NASA Astrophysics Data System (ADS)

In the Rio Apa and Amambay regions, north-eastern Paraguay (NPAA), potassic, alkaline-carbonatitic rocks (138-139 Ma) predate the eruption of tholeiitic flood basalts (133 ± 1 Ma). These rocks, mainly outcropping as dykes or ring-like complexes, intrude a Cambro-Ordovician carbonate platform and Precambrian metamorphic rocks along with their Silurian and Permo-Carboniferous sediments. The main rock-types range from basanite to trachyte and trachyphonolite (and intrusive equivalents) to carbonatite, in addition to glimmeritic and pyroxenitic veins. Geological and geophysical evidence indicate that the NPAA magmatism is related to extensional tectonics, like the Early Cretaceous alkaline-carbonatitic complexes from central-eastern Paraguay (127 ± 1 Ma) and the eastern magmatic occurrences of the Paraná Basin. Oxygen and carbon isotope compositions (whole rocks and carbonates) vary from values close to the field of continental lithospheric mantle, or that of primary carbonatites, up to values typical of a hydrothermal environment. An isotope exchange model implies that the main isotope variations, ranging from a magmatic (e.g., 1200 °C) to a low temperature (< 400 °C) environment, involved fluids with CO2/H2O ratios between 0.8 and 1.0. In particular, the Osbnd C isotopic variations, in combination with the La vs La/Yb ratios, suggest an increasingly higher level of carbonate in the silicate liquids, with increasing evolution, i.e., basanite ? phonotephrite ? trachyphonolite-phonolite (trachyte) ? carbonatite. Srsbnd Nd isotopes show that the carbonatites are mantle derived without significant crustal contamination and that they can be related to isotopically enriched sources where newly formed veins (enriched component) and peridotite matrix (depleted component) underwent differing isotopic evolution. TDM model ages for NPAA range from 1.2 to 2.3 Ga. Considering that in the whole Paraná Basin isotopically distinct K-alkaline and tholeiitic magmas were generated following the enrichment of the subcontinental mantle mainly between 1.0 and 2.3 Ga (Paleo-Mesoproterozoic events), the mantle sources preserved the isotopic heterogeneities over long time periods, suggesting a non-convective, i.e., lithospheric, mantle source beneath different cratonic or intercratonic areas.

Comin-Chiaramonti, Piero; De Min, Angelo; Girardi, Vicente A. V.; Gomes, Celso B.

2014-02-01

188

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland  

PubMed Central

The 39-day long eruption at the summit of Eyjafjallajökull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011?kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05?km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27?km3) transported by 3–10?km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010?kg (70?Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010?kg (2%) are considered to have been transported longer than 600–700?km with <108?kg (<0.02%) reaching mainland Europe. PMID:22893851

Gudmundsson, Magnús T.; Thordarson, Thorvaldur; Höskuldsson, Ármann; Larsen, Gudrún; Björnsson, Halldór; Prata, Fred J.; Oddsson, Björn; Magnússon, Eyjólfur; Högnadóttir, Thórdís; Petersen, Guðrún Nína; Hayward, Chris L.; Stevenson, John A.; Jónsdóttir, Ingibjörg

2012-01-01

189

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

190

The volcanic-subvolcanic rocks of the fernando de noronha archipelago, southern atlantic ocean: Mineral chemistry  

NASA Astrophysics Data System (ADS)

Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16-19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.

Lopes, Rosana Peporine; Ulbrich, Mabel N. Costas; Ulbrich, Horstpeter

2014-12-01

191

Triassic alkaline magmatism of the Hawasina Nappes: Post-breakup melting of the Oman lithospheric mantle modified by the Permian Neotethyan Plume  

NASA Astrophysics Data System (ADS)

Middle to Late Triassic lavas were sampled within three tectonostratigraphic groups of the Hawasina Nappes in the Oman Mountains. They are predominantly alkali basalts and trachybasalts, associated with minor sub-alkaline basalts, trachyandesites, trachytes and rhyolites. Their major, trace elements and Nd-Pb isotopic compositions are very similar to those of the Permian plume-related high-Ti basalts which also occur in the Hawasina Nappes. The Triassic lavas derive from low-degree melting of an enriched OIB-type mantle source, characterized by ?Ndi = 0.3-5.3 and (206Pb/204Pb)i = 16.96-19.31 (for t = 230 My). With time, melting depths decreased from the garnet + spinel to the spinel lherzolite facies and the degree of melting increased. The oldest are distinguished from the others by unradiogenic Nd and Pb signatures, with ?Ndi = - 4.5 to - 1.2 and (206Pb/204Pb)i = 16.35-17.08, which we attribute to their contamination by Arabo-Nubian lower crust. The lavas likely derived from the Oman lithospheric mantle, the original DMM-HIMU signature of which was overprinted during its pervasive metasomatism by the Permian plume-related melts. We suggest that these lavas were emplaced during post-breakup decompression-triggered melting in the Middle Triassic during global kinematic reorganization of the Tethyan realm.

Chauvet, François; Lapierre, Henriette; Maury, René C.; Bosch, Delphine; Basile, Christophe; Cotten, Joseph; Brunet, Pierre; Campillo, Sylvain

2011-02-01

192

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

193

?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

194

Paleozoic and early mesozoic magmatism manifestations in the early Precambrian structure of the South Yenisei Ridge  

NASA Astrophysics Data System (ADS)

This study characterizes some issues of the Paleozoic and Mesozoic tectonomagmatic evolution of Precambrian structures from the southwestern margin of the Siberian craton. The relationship between the Devonian and Triassic magmatic events is demonstrated from the example of the Severnaya rift-related structure, South Yenisei Ridge. U-Pb SHRIMP dating yielded ages of 387 ± 5 Ma for leucogranites and 240 ± 3 Ma for the overlying alkaline trachytes. These ages show good agreement with Ar-Ar geochronological data (392-387 Ma) obtained for micas from paragneisses and leucogranite dykes of the Yenisei suture zone, the extension of which is superimposed by the studied rift-related structure. The previous geological evidence and the Devonian age estimate first obtained for magmatic rocks of the Yenisei Ridge allow us to interpret the studied leucogranites as products of Devonian continental rifting, similar to volcanic and intrusive rocks of the North Minusa depression and Agul graben. Like other localities within the western margin of Siberian craton, the formation of Triassic alkaline rocks may be related to the Siberian superplume activity.

Vernikovsky, V. A.; Vernikovskaya, A. E.; Matushkin, N. Yu.; Romanova, I. V.; Berezhnaya, N. G.; Larionov, A. N.; Travin, A. V.

2010-05-01

195

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

NASA Technical Reports Server (NTRS)

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

Blodget, Herbert W.; Heirtzler, James R.

1993-01-01

196

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

197

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

NASA Astrophysics Data System (ADS)

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

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

198

Volcanology, geochemistry and age of the Lausitz Volcanic Field  

NASA Astrophysics Data System (ADS)

The Lausitz (Lusatia) Volcanic Field is part of the Central European Volcanic Province, and its magmas represent an alkaline trend from olivine nephelinites and basanites to trachytes and phonolites, typical for intraplate settings. Neighbouring volcanic fields are the ?eské St?edoho?í Mountains to the south-west and the Fore-Sudetic Basin in Lower Silesia to the east. More than 1000 volcanic structures associated with approximately 500 vents have been located within this volcanic field. Residuals of scoria cones, lava lakes, lava flows and maar-diatreme in filling occur in situ near the level of the original syn-volcanic terrain. In more deeply eroded structures, volcanic relicts outcrop as plugs or feeders. Evolved rocks occur as monogenetic domes or intrusions in diatremes, while their volcaniclastic equivalents are rare. Twenty-three localities were dated using the 40Ar/39Ar method. The ages range from 35 to 27 Ma, with a focus around 32-29 Ma, indicating Late Eocene and mainly Oligocene volcanism for the LVF. Differentiated rocks appear to be slightly younger than less differentiated. No geographical age clusters are apparent.

Büchner, J.; Tietz, O.; Viereck, L.; Suhr, P.; Abratis, M.

2015-03-01

199

Magma mixing in a zoned alkalic intrusion  

SciTech Connect

The Marble Canyon stock is unique among the alkalic intrusions of the Trans-Pecos magmatic province in being zoned from a critically silica-undersaturated rim of alkali gabbro (AG) to a silica-oversaturated core of quartz syenite (QS). Hybrid rocks of intermediate chemical and mineralogical compositions occur between the rim and core. Nepheline-syenite dikes occur only within the AG. Silica-rich dikes of quartz trachyte, pegmatite, and aplite cut the AG, QS, and hybrid rocks. Thermodynamic calculations of silica activity in the magmas illustrate the presence of two trends with decreasing temperature: a silica-poor trend from AG to nepheline syenite and a silica-rich trend from hybrid rocks to QS. Least-square modeling of rock and mineral compositions suggests 1) the nepheline syenites were derived by crystal-liquid fractionation from nearly solidified AG at the rim of the stock, 2) AG magma farther from the rim mixed with a small proportion of granitic magma, and 3) the mixture then differentiated to produce the hybrid rocks and QS. Zirconium dioxide inclusions in plagioclase crystals of the hybrid rocks and QS indicate that the AG magma contained some crystals before it mixed with the granitic magma. Two origins for the granitic magma are possible: 1) a late-stage differentiate of a mantle-derived hypersthene-normative magma and 2) melting of crustal material by the AG magma. Recognition of magma mixing might not have been possible if the AG had been hypersthene-normative.

Price, J.G.; Henry, C.D.; Barker, D.S.; Rubin, J.N.

1985-01-01

200

Trace element evaluation of a suite of rocks from Reunion Island, Indian Ocean  

USGS Publications Warehouse

Reunion Island consists of an olivine-basalt shield capped by a series of flows and intrusives ranging from hawaiite through trachyte. Eleven rocks representing the total compositional sequence have been analyzed for U, Th and REE. Eight of the rocks (group 1) have positive-slope, parallel, chondrite-normalized REE fractionation patterns. Using a computer model, the major element compositions of group 1 whole rocks and observed phenocrysts were used to predict the crystallization histories of increasingly residual liquids, and allowed semi-quantitative verification of origin by fractional crystallization of the olivine-basalt parent magma. Results were combined with mineral-liquid distribution coefficient data to predict trace element abundances, and existing data on Cr, Ni, Sr and Ba were also successfully incorporated in the model. The remaining three rocks (group 2) have nonuniform positive-slope REE fractionation patterns not parallel to group 1 patterns. Rare earth fractionation in a syenite is explained by partial melting of a source rich in clinopyroxene and/or hornblende. The other two rocks of group 2 are explained as hybrids resulting from mixing of syenite and magmas of group 1. ?? 1975.

Zielinski, R.A.

1975-01-01

201

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

202

Eruptive viscosity and volcano morphology  

NASA Technical Reports Server (NTRS)

Terrestrial central volcanoes formed predominantly from lava flows were classified as shields, stratovolcanoes, and domes. Shield volcanoes tend to be large in areal extent, have convex slopes, and are characterized by their resemblance to inverted hellenic war shields. Stratovolcanoes have concave slopes, whereas domes are smaller and have gentle convex slopes near the vent that increase near the perimeter. In addition to these differences in morphology, several other variations were observed. The most important is composition: shield volcanoes tend to be basaltic, stratovolcanoes tend to be andesitic, and domes tend to be dacitic. However, important exceptions include Fuji, Pico, Mayon, Izalco, and Fuego which have stratovolcano morphologies but are composed of basaltic lavas. Similarly, Ribkwo is a Kenyan shield volcano composed of trachyte and Suswa and Kilombe are shields composed of phonolite. These exceptions indicate that eruptive conditions, rather than composition, may be the primary factors that determine volcano morphology. The objective of this study is to determine the relationships, if any, between eruptive conditions (viscosity, erupted volume, and effusion rate) and effusive volcano morphology. Moreover, it is the goal of this study to incorporate these relationships into a model to predict the eruptive conditions of extraterrestrial (Martian) volcanoes based on their morphology.

Posin, Seth B.; Greeley, Ronald

1988-01-01

203

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

204

Promise and pitfalls for characterizing and correlating the zeolitically altered tephra of the Pleistocene Peninj Group, Tanzania  

NASA Astrophysics Data System (ADS)

The Pleistocene Humbu and Moinik formations of the Peninj Group in northern Tanzania preserve an important archaeological and paleontological record, in addition to a record of local volcanism in the form of tephra and lavas. Samples of the major Humbu and Moinik formations' basaltic and trachytic tephra were collected and characterized using phenocryst composition and both primary and authigenic mineral assemblage, since the volcanic glass was completely altered to zeolite. Some tephra are distinguishable solely using phenocrysts, but some are too similar in mineral composition or too poor in phenocrysts to definitively "fingerprint" without glass. Titanomagnetite phenocrysts were mostly altered; characterization was thus limited to feldspar, augite, and hornblende compositions for most tephra. Phenocryst compositions were compared to Olduvai tephra compositions to see if any regional tephra could be identified that could help correlate the sites. Augite or hornblende composition rules out potential correlations of Olduvai Bed I Tuff IF and the Bed II Bird Print Tuff or Tuff IID to otherwise similar Peninj Group tephra. Despite their overlap in age and locations at less than ~ 80 km from the Ngorongoro Volcanic Highlands, Peninj and Olduvai have different tephra records, which limits the possibilities for establishing a regional tephrostratigraphic framework.

McHenry, Lindsay J.; Luque, Luis; Gómez, José Ángel; Diez-Martín, Fernando

2011-05-01

205

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

206

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

207

Experimentally observed iron redox kinetics in silicic liquids: Implications for Fe/sup 3 +//Fe/sup 2 +/ variations in rhyolite lava  

SciTech Connect

Iron redox kinetics in silicate liquids were investigated by melting 100 mg pellets of compacted rhyolite, pantellerite, pantelleritic trachyte, and andesite rock powders at 1243 and 1343/degree/C in a moderately reducing furnace atmosphere (log fO/sub 2/ = /minus/7.83) for periods of 1 to 4320 minutes. The redox state of glasses produced by quenching these liquids was determined by colorimetric analysis of the ferrous iron and total iron content. Redox equilibrium, indicated by the attainment of a constant FeO/FeO/sub tot/ ratio, was observed for all temperature-composition conditions studied, except for 1243/degree/C experiments with USGS rhyolite standard RGM-1. This is consistent with the low diffusivity of reacting components in high viscosity rhyolite liquids. In the 1243/degree/C experiments with RGM-1, no change in the FeO/FeO/sub tot/ ratio was observed after 4320 minutes. This implies that redox equilibrium is not maintained in natural rhyolite lavas which erupt as significantly lower temperatures (720--850/degree/C). We conclude that sluggish redox kinetics precludes major changes in the oxidation state of a rhyolite magma during the eruption process. If this is true, then the quenched magma, represented by glassy rhyolites, preserves the pre-eruption redox signature of the magma. 2 refs.

Naney, M.T.; Swanson, S.E.

1989-01-01

208

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

2002-01-01

209

Rheology and textures of experimental 3-phase magma mixing  

NASA Astrophysics Data System (ADS)

Hybrid rocks, enclaves, crystal dispersion and zonation (…) are common natural features caused by magma mixing. Such a process has been suggested to trigger volcanic eruptions, during or soon after the replenishment of a differentiated magma chamber by a hotter mafic magma. However, so far little experimental work on magma mixing has been performed under realistic strain rate and P-T conditions. In particular, because of the technical difficulties associated to the implementation of deformation experiments at high pressures, most work has been performed at atmospheric pressures. We have thus performed high pressure torsion experiments with a Paterson Press using two contrasted magma compositions: a synthetic haplotonalitic composition (SiO2?69, close to trachyte-rhyolite compositions), and a natural basalt from Santorini Volcano (Greece). The first series of experiments was done between 900-1200°C at 300 MPa, with viscosity contrasts between both magmas varying from 0 to ~ 4 Log units. Results reveal that the transition from unmixed to mixed magmas occurs over a short temperature interval (<10°C) and that mixing and mingling both occur at a very low viscosity contrast only. SEM images show natural-like mixing textures whereas microprobe analyses reveal concomitant basalt melting and mixing producing andesitic melts. Further experiments including hydrous magmas are in progress; to consider water is essential because water has a profound influence on magma properties. These experiments will be a clue in the understanding of such a common process which possibly leads to eruptions.

Laumonier, M.; Arbaret, L.; Scaillet, B.

2012-04-01

210

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

NASA Technical Reports Server (NTRS)

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

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

1976-01-01

211

Mapping of the major structures of the African rift system  

NASA Technical Reports Server (NTRS)

The author has identified the following significant results. ERTS-1 imagery of the African rift system has already proved of great value in structural geological studies. One of the interesting megastructures expressed on the imagery occurs some 40 km east of the eastern margin of the main Ethiopian rift, in Arussi province, and extending between latitude 71/2 and 81/4 deg N. The Badda-Encuolo ridge proves to have been a line of major Tertiary volcanism and probably supplied the thick Trap Series flood basalt sequence exposed farther east in the canyons of the Webi Shebeli drainage system. The ridge itself was built up by the waning activity of the Sagatu line of volcanism. Serendipitious has been the discovery on Mt. Badda of several deeply glaciated valleys, many of which show clearly on the ERTS-1 imagery. It seems that Mt. Badda was one of the most important glacial centers in eastern Africa during the Pleistocene. Three major late-Tertiary trachytic centers lie between the Badda-Encuolo ridge and the rift valley. The relationships of these three volcanoes to each other and to the rift faulting is revealed for the first time by the ERTS-1 imagery, as is the form of the cladera of Baltata and the crater of Chilalo.

Mohr, P. A. (principal investigator)

1973-01-01

212

Early Cretaceous alkaline volcanism of the Sierra Chica de Córdoba (Argentina): Mineralogy, geochemistry and petrogenesis  

NASA Astrophysics Data System (ADS)

Distinct groups of volcanic rocks, mainly of potassic character, were recognized from the different localities of the Sierra Chica de Córdoba (SCC): (1) alkali basalt to trachyte suite, (2) transitional basalt to latibasalt suite and (3) basanites. Magma evolution would have taken place at crustal level(s) from distinct parental magmas, mainly through fractional crystallization in an open system magma chamber. Source composition supports garnet and phlogopite as residual phases in the mantle. Melting degrees (5-7%) from an enriched mantle source are in accordance with the peripheral position related to the great thermal anomaly that triggered the voluminous Paraná flood tholeiites. SCC volcanism is of high-Ti, with multi-elemental patterns similar to those from Alto Paranaíba (Brazil) and clearly distinct from the ones of low-Ti provinces, as Eastern Paraguay. Though associated with a mobile belt of the Pampean Orogeny, volcanism of SCC does not reveal compositional features that can be related to slab-derived mantle metasomatism, so that the small-scale heterogeneity of the SCC lithospheric mantle source must be related to enrichment events involving volatile-rich small-volume melts from the asthenosphere.

Lagorio, Silvia Leonor

2008-09-01

213

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland.  

PubMed

The 39-day long eruption at the summit of Eyjafjallajökull volcano in April-May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·10¹¹?kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05?km³). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27?km³) transported by 3-10?km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·10¹??kg (70?Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km² in Europe and the North Atlantic. Of order 10¹??kg (2%) are considered to have been transported longer than 600-700?km with <10??kg (<0.02%) reaching mainland Europe. PMID:22893851

Gudmundsson, Magnús T; Thordarson, Thorvaldur; Höskuldsson, Armann; Larsen, Gudrún; Björnsson, Halldór; Prata, Fred J; Oddsson, Björn; Magnússon, Eyjólfur; Högnadóttir, Thórdís; Petersen, Guðrún Nína; Hayward, Chris L; Stevenson, John A; Jónsdóttir, Ingibjörg

2012-01-01

214

Assessment of the atmospheric impact of volcanic eruptions  

NASA Technical Reports Server (NTRS)

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

Sigurdsson, H.

1988-01-01

215

Upper mantle magma storage and transport under a Canarian shield-volcano, Teno, Tenerife (Spain)  

NASA Astrophysics Data System (ADS)

We use clinopyroxene-liquid thermobarometry, aided by petrography and mineral major element chemistry, to reconstruct the magma plumbing system of the late Miocene, largely mafic Teno shield-volcano on the island of Tenerife. Outer rims of clinopyroxene and olivine phenocrysts show patterns best explained by decompression-induced crystallization upon rapid ascent of magmas from depth. The last equilibrium crystallization of clinopyroxene occurred in the uppermost mantle, from ˜20 to 45 km depth. We propose that flexural stresses or, alternatively, thermomechanical contrasts create a magma trap that largely confines magma storage to an interval roughly coinciding with the Moho at ˜15 km and the base of the long-term elastic lithosphere at ˜40 km below sea level. Evidence for shallow magma storage is restricted to the occurrence of a thick vitric tuff of trachytic composition emplaced before the Teno shield-volcano suffered large-scale flank collapses. The scenario developed in this study may help shed light on some unresolved issues of magma supply to intraplate oceanic volcanoes characterized by relatively low magma fluxes, such as those of the Canary, Madeira and Cape Verde archipelagoes, as well as Hawaiian volcanoes in their postshield stage. The data presented also support the importance of progressive magmatic underplating in the Canary Islands.

Longpré, Marc-Antoine; Troll, Valentin R.; Hansteen, Thor H.

2008-08-01

216

The influence of antecrysts on the whole-rock composition of lava flows and dykes from Corvo Island (Azores)  

NASA Astrophysics Data System (ADS)

The Corvo volcanic island is one of the Azorean islands located to the west of the Mid-Atlantic Ridge. The volcanostratigraphy of Corvo has been established based on the caldera-formation event, defining three main units: Pre-caldera, Syn-caldera and Post-caldera Units. We have deeply studied the petrology and geochemistry of representative samples of the whole volcanostratigraphical sequence, including lava flows and dykes. They range in composition from picrobasalts to trachytes. Regardless of their age, they can be divided in two groups according to their texture and composition: microlitic rocks and porphyritic rocks. The former are more evolved in composition. The microlitic rocks are holocrystalline to hypocrystalline, with compositions that range from hawaites to trachytes. They are mainly composed of Pl, Ol, Cpx and Opq phenocrysts and microcrysts and occasional glass. The porphyritic rocks are holocrystalline alkaline picrobasalts to alkaline basalts. They are characterized by the presence of variable proportions of large crystals. These are subhedral to anhedral Ol, Cpx, Pl and scarce Opq crystals; they are up to centimetric in size and some of them are corroded and present overgrowth rims. These mineral phases have very primitive compositions (Fo90-81, An88-70 and Cpx with 0.91-0.76 mg*). They have been described as antecryst, as they crystallized from progenitor magmas and have been reincorporated to their host lava during eruption. The porphyritic rocks have a volume fraction of antecryst from 10 to 80%. The major element compositional trend defined by these porphyritic rocks does not agree with a single fractionation process. However, their behavior appears to be directly related to the amount and composition of antecrysts. The composition of the rocks with smaller amounts of antecrysts is more evolved and appears to be controlled by the composition of the groundmass. To elucidate the influence of the antecrysts on the whole-rock compositions, we have added the major element composition of each antecrystic mineral (Ol, Cpx and Pl) to that of the most primitive antecryst-free sample (MgO: 5.55 wt. %). Ol antecrysts produce the greatest changes in all major element contents (adding 20 % of Ol modifies the whole-rock composition more than adding 100 % of any other antecrystic phase). On the other hand, the accumulation of Pl antecrysts produces the smallest effect. The compositional trend defined by the porphyritic samples can be best approached as a combination of the effects of Ol and Cpx antecrysts, with a minor influence of the Pl crystals. This means that these rocks mainly accumulate Ol and Cpx antecrysts, coherently with the petrographical observations. In consequence, the compositional trend defined by the porphyritic samples is due to the presence of varying proportions of antecrysts in the samples. Therefore, the composition of antecryst-bearing volcanic rocks must not be considered representative of natural melt compositions and only antecryst-free samples must be considered as original melts. This is especially important when using whole-rock compositions to draw information on magmatic processes, where the most primitive compositions are commonly selected.

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

2012-04-01

217

Geochemistry and petrology of the Early Miocene lamproites and related volcanic rocks in the Thrace Basin, NW Anatolia  

NASA Astrophysics Data System (ADS)

The extensional Thrace basin (NW Anatolia) contains an association of early Miocene diopside-leucite-phlogopite (Do?anca) and diopside-phlogopite (Korucuköy) lamproites with Oligocene medium-K calc-alkaline andesites (Ke?an volcanics), early Miocene shoshonitic rocks (Alt?nyaz? trachyte) and middle Miocene Na-alkaline basalts (Be?endik basalts). The Do?anca lamproite (K2O = 5.1-5.5 wt.%; K/Na = 2.78-2.89; MgO = 11.4-11.8 wt.%) consists of olivine (Fo71-86), diopside (Al2O3 = 1.0-5.0, Na2O = 0.2-0.6), phlogopite (TiO2 = 1.1-9.4, Al2O3 = 11.1-13.9), spinel (Mg# = 22.9-32.6; Cr# = 64-83.4), leucite, apatite, zircon, Fe-Ti-oxides and magnetite in a poikilitic sanidine matrix. The potassic volcanic units (lamproites and trachytes) in the region have similarly high Sr and low Nd isotopic compositions (87Sr/86Sr(i) = 0.70835-0.70873 and 143Nd/144Nd(i) = 0.51227-0.51232). The major and trace element compositions and Sr-Nd-Pb isotopic ratios of the shoshonitic, ultrapotassic and lamproitic units closely resemble those of other Mediterranean ultrapotassic lamproites (i.e., orogenic lamproites) from Italia, Serbia, Macedonia and western Anatolia. The Be?endik basalts show intraplate geochemical signatures with an Na-alkaline composition, an absence of Nb negative anomalies on primitive mantle-normalized multi-element diagrams, as well as low Sr (~ 0.70416) and high Nd (0.51293) isotopic ratios; and include olivine (Fo72-84), diopside, spinel, Fe-Ti-oxides and magnetite. The Oligocene Ke?an volcanics were emplaced in the earlier stages of extension in Thrace, and represent the typical volcanic products of post-collisional volcanism. The continental crust-like trace element abundances and isotopic compositions of the most primitive early Miocene ultrapotassic rocks (Mg# up to 74) indicate that their mantle sources were intensely contaminated by the continental material. By considering the geodynamic evolution of the region, including oceanic subduction, crustal accretion, crustal subduction and post-collisional extension, it is suggested that the mantle sources of the potassic volcanic units were most likely metasomatized by direct subduction of continental blocks during accretion and assemblage of various Alpine tectono-stratigraphic units. Overall, the magma production occurred in an extensional tectonic setting that controlled the core-complex formation and related basin development, with the middle Miocene Be?endik basalts being derived from asthenospheric sources during the late stages of extension.

Ersoy, Yalç?n E.; Palmer, Martin R.; Uysal, ?brahim; Gündo?an, ?brahim

2014-08-01

218

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

219

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

220

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

NASA Astrophysics Data System (ADS)

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

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

2010-06-01

221

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

NASA Astrophysics Data System (ADS)

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

McLachlan, H. S.

2012-12-01

222

Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada  

SciTech Connect

The natural near-field environment in and around the emplacement drifts of the proposed nuclear waste repository at Yucca Mountain, Nevada, includes the host rock, dust, seepage water, and pore water. The chemical compositions of these components have been analyzed to provide a basis for assessing possible chemical and mineralogical reactions that may occur in and around the emplacement drifts during the heating and cooling cycle. The crystal-poor rhyolite of the Topopah Spring Tuff of Miocene age with an average silica (SiO{sub 2}) content of 76 percent will host the proposed repository. Samples of the rhyolite are relatively uniform in chemical composition as shown by an average coefficient of variation (CV) of 8.6 percent for major elements. The major component of underground dust is comminuted tuff generated during construction of the tunnel. Average CVs for major elements of dust samples collected from the main tunnel (Exploratory Studies Facility, ESF) and a cross drift (Enhanced Characterization of the Repository Block, ECRB) are 25 and 28 percent, respectively. This increased variability is due to a variable amount of dust derived from trachyte with SiO{sub 2} contents as low as 66 percent (from overlying crystal-rich members) and from surface dust with an even lower average SiO{sub 2} content of 60 percent (from the abundance of trachyte in outcrop and carbonate dust derived from nearby ranges). The composition of the water-soluble fraction of dust is of interest with regard to possible salt deliquescence on waste canisters. The nitrate-to-chloride (NO{sub 3}{sup -}/Cl{sup -}) ratio (weight) is used to assess the potential corrosive nature of the salts because an excess of NO{sub 3}{sup -} over Cl{sup -} may inhibit the formation of the more corrosive calcium chloride brines in deliquescing salts. The soluble fractions of dust samples typically have NO{sub 3}{sup -}/Cl{sup -} ratios between 1 and 10. About 30 samples of seepage into the south ramp of the ECRB have an average NO{sub 3}{sup -}/Cl{sup -} of 0.62. Pore water extracted from core samples of the repository host rock has lower NO{sub 3}{sup -}/Cl{sup -}-ratios with an average value of 0.28 and a range over two orders of magnitude. Of all the components of the natural system, pore water has the largest compositional variability with an average CV of 62 percent, and thus, is the most difficult to characterize. Because pore water is extracted from dry-drilled core, its solute content may have been increased by evaporation during drilling, handling, storage, and extraction by ultracentrifugation. Further, microbial activity in the core during storage may reduce the concentration of NO{sub 3}{sup -} thus decreasing the NO{sub 3}{sup -}/Cl{sup -} ratio. Therefore, the more dilute pore water samples might be considered the most representative of native pore water with NO{sub 3}{sup -}/Cl{sup -} ratios close to unity or greater.

Z.E. Peterman; T.A. Oliver

2006-06-19

223

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

224

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

NASA Astrophysics Data System (ADS)

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

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

2006-10-01

225

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

226

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

227

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

228

A Model For Nonarrhenian Newtonian Viscosities For Multicomponent Melts  

NASA Astrophysics Data System (ADS)

Magma dynamics can control volcanic eruptions. Thus a description of the rheology of magma is an essential input variable for forward simulations of magmatic eruptions and for the interpretation of volcano monitoring data related to magma movements. Accurate quantification of magma rheology must be based on a robust model for the viscosity of the liquid. The first parameterizations of the viscosity of multicomponent silicate melts for geological purposes employed an Arrhenian dependence of the vis- cosity on temperature. Although a useful approximation over restricted ranges of tem- perature, the Arrhenian approximation leads to serious errors over larger temperature ranges. When such models are compared with the more complete viscosity data sets for multicomponent silicate liquids which have been coming on line in the past decade the discrepancy is apparent. As a response to this growing inadequacy of Arrhenian models, Hess and Dingwell (1996) developed the first empirical non-Arrhenian vis- cosity model for the binary system calcalkaline rhyolite water. Here, on the basis of ca. 350 viscosity data obtained on anhydrous melts, we present the first multicompo- nent nonArrhenian model. Based on 350 new determinations of Newtonian viscosities of multicomponent liquids, ranging from basanite through phonolite and trachyte, to dacite in the range of (10**0 to 10**12 Pa s) and regressions using the 3-parameter Tammann-Vogel-Fulcher equation, a relationship between the chemical composition of the multicomponent defined in terms of the mole fraction of network modifiers, yields a good predictive tool for the calculation of the viscosity of multicomponent silicate melts. This parameterisation is proposed as the first reliable non-Arrhenian description of the Newtonian viscosity of multicomponent silicate melts.

Giordano, D.; Dingwell, D. B.

229

The crystallization of shoshonitic to peralkaline trachyphonolitic magmas in a H2O-Cl-F-rich environment at Ischia (Italy), with implications for the feeder system of the Campania Plain volcanoes  

NASA Astrophysics Data System (ADS)

Bulk-rock and mineralogical characterization of massive samples (lava flows, lava domes, scoria, spatter), chosen to be representative of different activity periods and of the whole known compositional range of Ischia island (Roman Magmatic Province, Campanian district) is reported in this paper. The rocks vary in composition from shoshonites to peralkaline trachyphonolites. Crystallization started with Al-rich chromite inclusions and their host magnesian olivine in shoshonites and latites, and ended with låvenite, rinkite, kochite, hiortdahlite, hainite, Mn-aenigmatite, britholite and Ti-Zr-rich aegirine in the groundmass of the peralkaline trachyphonolites. Removal of feldspar-bearing assemblages (with calcic-to-sodic plagioclase in shoshonites and latites, sodic sanidine and anorthoclase in trachytes and trachyphonolites) is established throughout the compositional range, and is accompanied by interaction between variably evolved magmas (with their differing phenocryst assemblage), mostly in mafic and intermediate compositions. This led to enrichment in Mn, alkalis, Zr, Nb, REE, Rb, Th, U, Cl and F, and depletion in Mg, Fe, V, Ca, Ba, Sr and Eu in the most evolved magmas. The Ischian rocks have a tendency to sodic affinity and by peculiar mineral compositions and compositional trends, which do not indicate extremely oxidizing conditions. The Ischian rocks thus differ from the Phlegrean Fields analogues and the ultrapotassic, more silica undersaturated rocks of Somma-Vesuvius. Overall, variations in the chemical compositions of the rocks and their trends imply unrelated feeding systems and thus are inconsistent with the hypothesis of a common magma reservoir beneath the main volcanic areas of the Campanian Plain.

Melluso, L.; Morra, V.; Guarino, V.; de'Gennaro, R.; Franciosi, L.; Grifa, C.

2014-12-01

230

The Eastern Carpathians “ophiolites” (Romania): Remnants of a Triassic ocean  

NASA Astrophysics Data System (ADS)

Mesozoic ophiolitic and related rocks in the Eastern Carpathians occur in three areas, from north to south: Rar?u, H?ghima? and Per?ani Mts. They are found as blocks ranging from few metres to a few kilometers in size and as centimetre-sized in breccias, most likely embedded in the Late Barremian-Early Albian Wildflysch formation. Compositionally, they range from lherzolites and harzburgites to mafics such as FeTi gabbros, dolerites, basalts, and to andesites. The volcanics comprise highly-depleted basalts/andesites to enriched-type mid-ocean ridge basalts; additionally they include ocean island basalts and calc-alkaline basalts/andesites and trachytes. Based on paleontological evidence, their age is Middle to ?Late Triassic. They can be clearly compared with remnants of the Meliata-Hallstatt Ocean in the Western Carpathians, but do not match the Jurassic ophiolites and island arc volcanics in the Mure? Zone of the Southern Apuseni Mts. We propose a Triassic ocean connected with the Meliata-Hallstatt Ocean, between (a) the Bucovinian/Sub-Bucovinian continental crust, (b) the Infrabucovinian and finally (c) the Northern Apuseni microcontinents. This ocean closed in the Late Triassic to Early Jurassic causing close juxtaposition of all three microcontinents. An ophiolite complex together with ocean island basalts and calc-alkaline basalts/andesites remained from this ocean and was subsequently eroded and transported as blocks of different size into the Lower Cretaceous Wildflysch basin, together with blocks and clasts of limestones similar to the Mesozoic sedimentary sequences in the Northern Apuseni realm. The Wildflysch formation was thrust as an independent unit during the Albian over the Bucovinian Nappe in the Eastern Carpathians and the Northern Apuseni continental crust, respectively.

Hoeck, Volker; Ionescu, Corina; Balintoni, Ioan; Koller, Friedrich

2009-03-01

231

Influence of alteration on physical properties of volcanic rocks  

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

232

Geology and geochemistry of the Mount Riley-Mount Cox pluton, Dona Ana County, New Mexico  

SciTech Connect

The Mount Riley-Mount Cox area is comprised of a relatively homogeneous pluton of rhyodacite rising some 1600 feet above the La Mesa surface. The pluton, of apparent Tertiary age, intrudes Cretaceous sedimentary rocks and Tertiary ( ) latite and tuff. The rhyodacite is holocrystalline, light gray to pinkish gray, porphyritic to microporphyritic, and locally banded. Phenocrysts include hornblende, quartz, biotite, and calcite. The phenocrysts range in size from 0.2 to 2 mm and make up one to fifteen percent of the rock. The phenocrysts often display a glomerophyric texture within a trachytic groundmass. The groundmass ranges from cryptocrystalline to very fine grained and is composed of plagioclase, quartz, potassium feldspar, hornblende/biotite, and iron-oxide material. Locally, the rhyodacite displays millimeter-scale banding and a poikilitic texture consisting of quartz oikiocrysts and plagioclase chadocrysts. The rhyodacite averages 68.74%, SiO/sub 2/, 0.39% TiO/sub 2/, 16.40% Al/sub 2/O/sub 3/, 2.87% Fe/sub t/, 0.10% MnO, 1.21% MgO, 2.56% CaO, 3.79% Na/sub 2/O, and 3.96% K/sub 2/O. The rhyodacite is cut by veins and veinlets of brown to white calcite. The veins attain a maximum thickness of one meter, are locally bordered by calcite-cemented breccia zones, and locally include pyrite. The veins trend north or northwest, consistent with regional trends for the Rio Grande rift and the Texas Lineament, respectively. Sixty-five samples of rhyodacite, breccia, and vein were analyzed for 31 elements by emission-spectrographic methods. Trace-element data suggestive of hydrothermal mineralization was not recognized.

Zimbelman, D.R.; Siems, D.F.; Kilburn, J.E.; Hubert, A.E.

1985-01-01

233

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

234

Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern Gabon (Western Central Africa)  

NASA Astrophysics Data System (ADS)

We report new geochemical data for the volcanic and subvolcanic rocks associated with the evolution of the Francevillian basin of eastern Gabon during Paleoproterozoic times (c. 2.1-2 Ga). Filling of this basin has proceeded through four main sedimentary or volcano-sedimentary episodes, namely FA, FB, FC and FD. Volcanism started during the FB episode being present only in the northern part of the basin (Okondja sub-basin). This volcanism is ultramafic to trachytic in composition and displays a rather constant alkaline geochemical signature. This signature is typical of a within-plate environment, consistent with the rift-setting generally postulated for the Francevillian basin during the FB period. Following FB, the FC unit is 10-20 m-thick silicic horizon (jasper) attesting for a massive input of silica in the basin. Following FC, the FD unit is a c. 200-400 m-thick volcano-sedimentary sequence including felsic tuffs and epiclastic rocks. The geochemical signatures of these rocks are totally distinct from those of the FB alkaline lavas. High Th/Ta and La/Ta ratios attest for a calc-alkaline signature and slight fractionation between heavy rare-earth suggests melting at a rather low pressure. Such characteristics are comparable to those of felsic lavas associated with the Taupo zone of New Zealand, a modern ensialic back-arc basin. Following FD, the FE detrital unit is defined only in the Okondja region, probably associated with a late-stage collapse of the northern part of the basin. It is suggested that the alkaline to calc-alkaline volcanic transition reflects the evolution of the Francevillian basin from a diverging to a converging setting, in response to the onset of converging movements in the Eburnean Belt of Central Africa.

Thiéblemont, Denis; Bouton, Pascal; Préat, Alain; Goujou, Jean-Christian; Tegyey, Monique; Weber, Francis; Ebang Obiang, Michel; Joron, Jean Louis; Treuil, Michel

2014-11-01

235

On the anatomy of magma chamber and caldera collapse: The example of trachy-phonolitic explosive eruptions of the Roman Province (central Italy)  

NASA Astrophysics Data System (ADS)

Textural and compositional features of pyroclastic products erupted during caldera-forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit/vent systems. Here we report on ultrapotassic trachytic-phonolitic explosive eruptions of the Roman Province (central Italy), which show remarkable changes of textural features and glass compositions in the juvenile material, even if the bulk chemical composition is essentially constant. In each example, the lower eruption sequence contains whitish, crystal-poor (leucite-free), highly vesicular pumice, emplaced by early Plinian fallout and/or pyroclastic currents; upsection, the eruption sequence contains black, low porphyritic (sanidine + leucite-bearing), moderately vesicular, scoria or spatter, emplaced by major pyroclastic flows (red tuff with black scoria) and associated co-ignimbrite, coarse lithic-rich breccias. This suggests a shift from a central feeder conduit, tapping the central part of the magma chamber, to a ring fracture vent system, tapping the peripheral portions of the magma chamber, during caldera collapse. Key features of these evacuating magma chambers are the thermal and volatile concentration (Xvol) gradients that produce the observed textural and compositional spectrum of trachy-phonolitic rock types. In particular, the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different PH2O conditions. Both leucite-free and leucite-bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions, as a result of pre-eruptive conditions in the feeder magma, with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas.

Palladino, Danilo M.; Gaeta, Mario; Giaccio, Biagio; Sottili, Gianluca

2014-06-01

236

Petrogenetic evolution of the Early Miocene Alaçamda? volcano-plutonic complex, northwestern Turkey: implications for the geodynamic framework of the Aegean region  

NASA Astrophysics Data System (ADS)

Extensional-tectonic processes have generated extensive magmatic activity that produced volcanic/plutonic rocks along an E-W-trending belt across north-western Turkey; this belt includes granites and coeval volcanic rocks of the Alaçamda? volcano-plutonic complex. The petrogenesis of the Early Miocene Alaçamda? granitic and volcanic rocks are here investigated by means of whole-rock Sr-Nd isotopic data along with field, petrographic and whole-rock geochemical studies. Geological and geochemical data indicate two distinct granite facies having similar mineral assemblages, their major distinguishing characteristic being the presence or absence of porphyritic texture as defined by K-feldspar megacrysts. I-type Alaçamda? granitic stocks have monzogranitic-granodioritic compositions and contain a number of mafic microgranular enclaves of monzonitic, monzodioritic/monzogabbroic composition. Volcanic rocks occur as intrusions, domes, lava flows, dykes and volcanogenic sedimentary rocks having (first episode) andesitic and dacitic-trachyandesitic, and (second episode) dacitic, rhyolitic and trachytic-trachydacitic compositions. These granitic and volcanic rocks are metaluminous, high-K, and calc-alkaline in character. Chondrite-normalised rare earth element patterns vary only slightly such that all of the igneous rocks of the Alaçamda? have similar REE patterns. Primitive-mantle-normalised multi-element diagrams show that these granitic and volcanic rocks are strongly enriched in LILE and LREE pattern, high (87Sr/86Sr)i and low ? Nd( t) ratios suggesting Alaçamda? volcano-plutonic rocks to have been derived from hybrid magma that originated mixing of co-eval lower crustal-derived more felsic magma and enriched subcontinental lithospheric mantle-derived more mafic magmas during extensional processes, and the crustal material was more dominant than the mantle contribution. The Alaçamda? volcano-plutonic complex rocks may form by retreat of the Hellenic/Aegean subduction zone, coinciding with the early stages of back-arc extension that led to extensive metamorphic core-complex formation.

Erkül, Sibel Tatar

2012-01-01

237

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

238

The last 40 ka tephrostratigraphic record of Lake Ohrid, Albania and Macedonia: a very distal archive for ash dispersal from Italian volcanoes  

NASA Astrophysics Data System (ADS)

A 1075 cm long core (Lz1120) was recovered in the south-eastern part of the Lake Ohrid (Republics of Macedonia and Albania) and sampled for identification of tephra layers. Magnetic susceptibility investigations show rather high magnetic values throughout the core, with peaks unrelated to the occurrence of tephra layers but instead to the relative abundance of detrital magnetic minerals in the sediment. Naked-eye inspection of the core allowed us to identify of two tephra layers, at 896-897 cm and 1070-1075 cm. Laboratory inspection of the grain-size fraction > 125 ?m allowed for the identification of a third cryptotephra at 310-315 cm. Major element analyses on glass shards of the tephra layers at 896-897 cm and 1070-1075 cm show a trachytic composition, and indicate a correlation with the regionally dispersed Y-3 and Y-5 tephra layers, dated at ca 30 and 39 cal ka BP. The cryptotephra at 310-315 cm has a mugearitic-benmoreitic composition, and was correlated with the FL eruption of Mt. Etna, dated at 3370 ± 70 cal yr BP. These ages are in agreement with five 14C AMS measurements carried out on plant remains and macrofossils from the lake sediments at different depths along the core. The recognition of distal tephra from Italian volcanoes allows us to link the Lake Ohrid succession to other archives located in the Mediterranean area and in eastern Europe. The benmoreitic-mugearitic tephra layer at 310-315 cm is the first recognition in the Balkan area of a distal ash deposit from a mid-intensity explosive eruption of Mt. Etna, as far as 600 km from the source.

Wagner, B.; Sulpizio, R.; Zanchetta, G.; Wulf, S.; Wessels, M.; Daut, G.; Nowaczyk, N.

2008-10-01

239

Chitinophaga qingshengii sp. nov., isolated from weathered rock surface.  

PubMed

A novel mineral-weathering bacterium was isolated from weathered rock (potassic trachyte) surfaces collected from Nanjing (Jiangsu, PR China). Cells of strain JN246(T) were Gram-stain-negative, rod-shaped and non-motile. Strain JN246(T) was aerobic, catalase- and oxidase-positive, and grew optimally at 28 °C and pH 7.0. On the basis of 16S rRNA gene sequence analysis, strain JN246(T) belonged to the genus Chitinophaga and the closest phylogenetic relatives were Chitinophaga eiseniae YC6729(T) (98.5% 16S rRNA gene sequence similarity), Chitinophaga terrae KP01(T) (96.8%), and Chitinophaga jiangningensis JN53(T) (96.3?%). The major respiratory quinone was MK-7 and the major polyamine was homospermidine. The major fatty acids were iso-C15:0, C16:1?5c, C16:0 and iso-C17:0 3-OH. The polar lipid profile of strain JN246(T) consisted of phosphatidylethanolamine, unknown aminolipids and unknown lipids. The genomic DNA G+C content of strain JN246(T) was 48.8 mol%. Based on the low level of DNA-DNA relatedness of strain JN246(T) (ranging from 22.6% to 42.4%) to the type strains of other species of the genus Chitinophaga and unique phenotypic characteristics, strain JN246(T) represents a novel species of the genus Chitinophaga, for which the name Chitinophaga qingshengii sp. nov. is proposed. The type strain is JN246(T) (?=?CCTCC AB 2014201(T)?=?JCM 30026(T)). PMID:25342110

Cheng, Cheng; Wang, Qi; He, Lin-Yan; Huang, Zhi; Sheng, Xia-Fang

2015-01-01

240

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

241

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

242

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

SciTech Connect

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

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

1993-03-01

243

Rb-Sr and oxygen isotopic study of alkalic rocks from the Trans-Pecos magmatic province, Texas: Implications for the petrogenesis and hydrothermal alteration of continental alkalic rocks  

SciTech Connect

Rb-Sr and O isotopic data for mid-Tertiary alkalic rocks from the Trans-Pecos magmatic province of west Texas demonstrate that hydrothermal alteration and fluid/rock (cation exchange) interactions have affected the isotope geochemistry of these rocks. Strontium and O isotopic data for late-stage minerals in an alkali basalt (hawaiite) still record two episodes of fluid/rock interactions. These data suggest that later meteoric fluids introduced Sr with a Cretaceous marine {sup 87}Sr/{sup 86}Sr ratio into minerals with significant cation exchange capacity. Dilute HCl leaching experiments demonstrate the removal of this labile or exchangeable Sr from the alkali basalt. Rb-Sr isotopic data for the leached alkali basalt and handpicked calcite record a crystallization age of 42 Ma, consistent with K-Ar data for an unaltered alkali basalt (hawaiite) dike from the same area (42.6 {plus minus} 1.3 Ma). Leaching experiments on one phonolite suggest the Sr isotopic variability in unleached phonolite and nepheline trachyte samples may be attributed to Sr in secondary calcite and zeolites, which have an upper Cretaceous marine {sup 87}Sr/{sup 86}Sr ratio. Rb-Sr isotopic data for leached phonolite and sanidine separate yield an age of 36.5 {plus minus} 0.8 Ma, within analytical uncertainty of a K-Ar biotite age (36.0 {plus minus} 1.1 Ma) of another phonolite. These leaching experiments demonstrate that the Rb-Sr isotopic systematics of hydrothermally-altered continental alkalic rocks may be significantly improved, providing more reliable geochronologic and isotopic tracer information necessary in constructing precise models of mantle sources.

Lambert, D.D.; Malek, D.J.; Dahl, D.A. (Texas Christian Univ., Fort Worth, TX (USA))

1988-10-01

244

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

245

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

PubMed

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

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

2013-06-01

246

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

PubMed

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

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

2012-08-15

247

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

248

Oxygen isotopes composition of sapphires from the French Massif Central: implications for the origin of gem corundum in basaltic fields  

NASA Astrophysics Data System (ADS)

Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ ( n = 22; mean ?18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < ?18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < ?18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ ( n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The ?18O values of 4.4-4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the ?18O values obtained for anorthoclase (7.7-7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a ?18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.

Giuliani, Gaston; Fallick, Anthony; Ohnenstetter, Daniel; Pegere, Guy

2009-02-01

249

Late Ordovician volcanism in Korea constrains the timing for breakup of Sino-Korean Craton from Gondwana  

NASA Astrophysics Data System (ADS)

In the early Paleozoic the Sino-Korean Craton (SKC) and South China Craton (SCC) were situated along the margin of east Gondwana. The SKC was connected to core Gondwana by an epeiric sea which was the site for deposition of lower Paleozoic sequences of SKC. The SKC and SCC may have drifted away from core Gondwana sometime during the mid-Paleozoic and would have been outboard microcontinents in the late Paleozoic, until they collided to form the East Asian continent in the Triassic. The breakup of SCC from Gondwana was suggested to have taken place at ?380 Ma, while no reliable suggestions have hitherto been made for breakup of SKC from Gondwana. This study presents a convincing evidence for breakup of SKC from Gondwana, based on the recognition of Late Ordovician volcanism in Korea. New SHRIMP U-Pb zircon ages, 445.0 ± 3.7 Ma and 452.5 ± 3.2 Ma, are obtained from trachytic rocks of the Ongnyeobong Formation of Taebaeksan Basin in Korea which occupied the marginal part of the SKC in the early Paleozoic. This Late Ordovician volcanism along with previous records of Ordovician volcanic activities along the western margin of the SKC is interpreted indicating the development of an incipient oceanic ridge. The oceanic ridge uplifted the SKC including the epeiric sea, which subsequently resulted in terminating the early Paleozoic sedimentation of the epeiric sea. The paucity of lower Paleozoic volcanic rocks across much of the SKC however suggests that the oceanic ridge did not extend into the epeiric sea. Instead, spreading of oceanic ridge entailed dextral movement of associated transform faults, which may have played a major role in breakup of SKC from mainland Gondwana by the end of Ordovician.

Cho, Deung-Lyong; Lee, Seung Ryeol; Koh, Hee Jae; Park, Jun-Beom; Armstrong, Richard; Choi, Duck K.

2014-12-01

250

Late Pleistocene zircon ages for intracaldera domes at Gölcük (Isparta, Turkey)  

NASA Astrophysics Data System (ADS)

Pleistocene to Quaternary volcanism in the Isparta region (SW Anatolia, Turkey) comprises potassic lavas and pyroclastic deposits, which are largely centered around Gölcük caldera. Trachytic intracaldera lava domes represent the latest eruptive event at Gölcük, and their eruption age is crucial for defining a minimum age for the preceding caldera-forming explosive eruption. Here, we present combined U-Th and (U-Th)/He zircon geochronological data for two intracaldera lava domes constraining their crystallization and eruption ages, respectively. U-Th zircon crystallization ages peak between ca. 15 and 25 ka. In rare instances U-Th zircon crystallization ages date back to ca. 59 and 136 ka. U-Th zircon crystallization ages also permit (U-Th)/He eruption ages from the same crystals to be individually corrected for uranium series decay chain disequilibrium, which is mainly due to the deficit of the intermediate daughter 230Th in zircon. Average disequilibrium-corrected (U-Th)/He zircon ages are 14.1 ± 0.5 and 12.9 ± 0.4 ka (1?). These ages are indistinguishable within analytical uncertainties suggesting that both lavas erupted quasi simultaneously. This contradicts published K-Ar ages that suggest an extended hiatus from ca. 52 to 24 ka between intracaldera dome eruptions. Evidence for protracted zircon crystallization over several thousands of years prior to eruption indicates the presence of a long-lived magma reservoir underneath Gölcük caldera. Implications of the revised eruptive geochronology presented here include younger ages for the latest effusive eruptions at Gölcük, and potentially also a more recent explosive eruption than previously assumed.

Schmitt, Axel K.; Danišík, Martin; Siebel, Wolfgang; Elitok, Ömer; Chang, Yu-Wei; Shen, Chuan-Chou

2014-10-01

251

Overview of Geothermal Development at Olkaria in Kenya  

SciTech Connect

The Olkaria geothermal field has been under continuous development since 1970. A feasibility study, completed in 1976, after six wells had been drilled and tested, indicated that development of the Olkaria field was feasible. The feasibility study was followed by production drilling and the construction of three 15 MW generating units. The first unit was brought on stream in July 1981, the second in December, 1982, and the third is scheduled to be completed in early 1985. The current output of 19 productive wells is equivalent to 46 MWe. Distribution of fumaroles and resistivity surveys indicate an areal extent of some 80 km{sup 2} for the Olkaria geothermal field. Gas chemistry of fumaroles indicates comparable underground temperatures over the whole field, 200-250{degrees}C. The capacity of the resource has been estimated to be 500-1000 MW electric for a production period of 25 years. Most of the drilling has been confined to a small part of the geothermal field. Here maximum recorded downhole temperature is 339{degrees}C and temperatures follow the boiling point curve with depth. A thin steam zone at 240{degrees}C is observed in the top of the reservoir at approximately 600-700 m depth. The reservoir fluid is dilute, of the sodium chloride type, contains chloride in the range of 200-700 ppm. The reservoir rocks consist of a sequence of near horizontal lavas and tuffs of trachytic composition, but basaltic andesites have also beenidentified. The drilled rocks at Olkaria are of relatively low permeability, the average yield of wells being equivalent to about 2.5 MWe. Exploratory drilling is presently in porgress in the Olkaria field, the aim being to locate new production areas withing the field. Three holes have been completed and the forth and last hole under the present plan is being drilled.

Svanbjornsson, Andres; Matthiasson, Jonas; Frimannsson, Hreinn; Arnorsson, Stefan; Rjornsson, Sveinbjorn; Stefansson, Valqarour; Samundsson, Kristjan

1983-12-15

252

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

NASA Astrophysics Data System (ADS)

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

Lendyel, P.; Koronovsky, N.

2013-12-01

253

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

254

Igneous Rocks of the East Pacific Rise: The alkali volcanic suite appear to be differentiated from a tholeiitic basalt extruded from the mantle.  

PubMed

The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte). In contrast, the more submerged parts of the Rise are largely composed of a tholeiitic basalt which has low concentrations of K, P, U, Th, Pb, and Ti. This tholeiitic basalt is either the predominant or the only magma generated in the earth's mantle under oceanic ridges and rises. It is at least 1000-fold more abundant than the alkali suite, which is probably derived from tholeiitic basalt by magmatic differentiation in and immediately below the larger volcanoes. Distinction of oceanic tholeiites from almost all continental tholeiites is possible on the simple basis of total potassium content, with the discontinuity at 0.3 to 0.5 percent K(2)O by weight. Oceanic tholeiites also are readily distinguished from some 19 out of 20 basalts of oceanic islands and seamount cappings by having less than 0.3 percent K(2)O by weight and more than 48 percent SiO(2). Deep drilling into oceanic volcanoes should, however, core basalts transitional between the oceanic tholeiites and the presumed derivative alkali basalts. The composition of the oceanic tholeiites suggests that the mantle under the East Pacific Rise contains less than 0.10 percent potassium oxide by weight; 0.1 part per million of uranium and 0.4 part of thorium; a potassium:rubidium ratio of about 1200 and a potassium: uranium ratio of about 10(4). PMID:17806796

Engel, A E; Engel, C G

1964-10-23

255

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

NASA Astrophysics Data System (ADS)

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

Condamine, Pierre; Médard, Etienne

2014-07-01

256

Geochemistry and petrogenesis of late Ediacaran (605-580 Ma) post-collisional alkaline rocks from the Katherina ring complex, south Sinai, Egypt  

NASA Astrophysics Data System (ADS)

The Katherina ring complex (KRC) in the central part of south Sinai, Egypt, is a typical ring complex of late Neoproterozoic age (605-580 Ma). It was developed during the final tectono-magmatic stage of the north Arabian-Nubian Shield (ANS) during evolution of the Pan-African crust. The KRC includes Katherina volcanics, subvolcanic bodies, ring dykes and Katherina granitic pluton. The Katherina volcanics represent the earliest stage of the KRC, which was subsequently followed by emplacement of the subvolcanic bodies and ring dykes. The Katherina granitic pluton depicts as the latest evolution stage of the KRC that intruded all the early formed rock units in the concerned area. The Katherina volcanics are essentially composed of rhyolites, ignimbrite, volcanic breccia and tuffs. Mineralogically, the peralkaline rhyolites contain sodic amphiboles and aegirine. The rhyolite whole rock chemistry has acmite-normative character. The subvolcanic bodies of the KRC are represented by peralkaline microgranite and porphyritic quartz syenite. The ring dykes are semicircular in shape and consist mainly of quartz syenite, quartz trachyte and trachybasalt rock types. The Katherina subvolcanic rocks, volcanic rocks as well as the ring dykes are alkaline or/and peralkaline in nature. The alkaline granitic pluton forms the inner core of the KRC, including the high mountainous areas of G. Abbas Pasha, G. Bab, G. Katherina and G. Musa. These mountains are made up of alkaline syenogranite and alkali feldspar granite. The mantle signature recorded in the KRC indicates a juvenile ANS crust partial melting process for the generation of this system. The evolution of the KRC rocks is mainly dominated by crystal fractionation and crustal contamination. Mineral geothermometry points to the high temperature character of the KRC, up to 700-1100 °C.

Azer, M. K.; Obeid, M. A.; Ren, M.

2014-10-01

257

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

PubMed

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

Mollel, Godwin F; Swisher, Carl C

2012-08-01

258

Natural background groundwater composition in the Azores archipelago (Portugal): A hydrogeochemical study and threshold value determination.  

PubMed

Groundwater discharges were sampled in selected springs from São Miguel (Furnas and Fogo trachytic central volcanoes) and Santa Maria islands (Azores, Portugal), in order to characterize natural background levels (NBLs) and proceed to the determination of threshold values (TVs). Besides being a key issue in order to fully assess the anthropogenic pressures, NBLs are also instrumental to derive TVs, therefore complying with requirements from the European Union Groundwater Directive. The composition of groundwater corresponds mainly to low mineralized Na-HCO3 to Na-Cl water types, the latter dominant in Santa Maria island, with a decreasing order of Na>Ca>Mg>K and Cl>HCO3>SO4>NO3 for cations and anion respectively. The majority of the samples are slightly acid to slightly alkaline (pH range of 5.45-7.43), and the electrical conductivity range between 180 and 1458?S/cm. Groundwater composition is controlled by two major drivers, addition of sea salts and dissolution of silicate minerals. Results shown that TVs established along the present study are in general in the lower rank when compared to the range of values proposed by the several EU member states, with the main exception of NO3, reflecting the impact of agriculture activities over water quality in the Azores, and lower than the national ones. The comparison between the estimated NBL and TV with values derived with another dataset from the Azores, usually higher, depicts the effect of a larger and diverse number of groundwater sources over calculations. On the other hand, all samples which show a contribution from volcanic/hydrothermal systems were excluded from the dataset, which explains why the derived NBLs and TVs are lower comparing to other active volcanic areas, which is also a conservative approach on a subject that has regulatory implications. PMID:25813965

Cruz, J V; Andrade, C

2015-07-01

259

Large cutting tools in the Danjiangkou Reservoir Region, central China.  

PubMed

Handaxe-bearing sites in China are currently known to occur in a number of alluvial basins, the best known being Dingcun, Bose and Luonan. Bose in the south and Luonan in central China on the northern margin of the Qinling Mountains are most familiar to English-speaking researchers. Here we document the Danjiangkou Reservoir Region (DRR) as another major area for large cutting tools (LCTs), located in central China on the southeastern edge of the Qinling Mountains. Large cutting tools are preserved in three terraces of the Han and Dan Rivers in Hubei and Henan Provinces, with dates from ca. 0.8 Ma (millions of years ago) (Terrace 4) to the first half of the Middle Pleistocene (Terrace 3), and possibly to the Late Pleistocene (Terrace 2). This paper reports on LCTs discovered in Terraces 3 and 2, with a majority from the older terrace (and one specimen from Terrace 4). Regional environments during the Middle Pleistocene were relatively warm, humid and stable. Despite the poor quality of raw materials (predominantly quartz phyllite and trachyte for the LCTs), good examples of both handaxes and cleavers are present, plus two types of picks. The LCT technology is compared and contrasted with other Asian industries and with the Acheulean. Overall the DRR LCTs show both technological and morphological similarities with Acheulean LCTs, with some differences that are mainly attributed to raw material properties, subsistence ecology, and 'cultural drift.' The DRR LCTs expand the range of morphological variability of the East Asian material and highlight the need for greater reliance on technological analysis and raw material evaluation for best comparison of Chinese assemblages with the Acheulean tradition. PMID:25223718

Kuman, Kathleen; Li, Chaorong; Li, Hao

2014-11-01

260

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

261

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

262

Off-ridge alkaline magmatism and seamount volcanoes in the Masirah island ophiolite, Oman  

NASA Astrophysics Data System (ADS)

The Masirah ophiolite offers an unique opportunity to study well preserved small seamount structures. Obducted seamounts have not been described up to now, and from the present-day ocean floor they are almost exclusively known from bathymetric studies. The thin oceanic crust of the Masirah ophiolite was formed at a ridge-transform intersect in Upper Jurassic time. It was overprinted and reworked by a major intra-oceanic tectono-magmatic event at mid-Cretaceous time, that has been well dated owing to the presence of interstratified sedimentary rocks (late Hauterivian to early Barremian, c. 130-125 Ma). This mid-Cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites. Volcanism occurred in a NW-SE extensional regime. Small, elongate submarine volcano structures (seamounts) developed within widespread alkalibasaltic pillow lava and pillow breccia deposits, which are interfingered with deep-marine pelagic sediments. The volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height. The seamounts are built up of basic to acid subvolcanic stock- or sheet-like intrusions, several generations of dikes, vent agglomerates and pyro- to epiclastic deposits. The latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment. In the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place. The alkaline rocks show a pronounced ocean island basalt (OIB) character indicating the considerable contribution of a mantle plume source (hotspot). As cause of the volcanism we propose a combination of original transform setting followed by drift past the Marion hotspot during the major plate tectonic reorganization between Greater India, Madagascar and Africa starting in mid-Cretaceous time.

Meyer, J.; Mercolli, I.; Immenhauser, A.

1996-12-01

263

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

USGS Publications Warehouse

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

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

1996-01-01

264

Zircon evidence for a ~200 k.y. supereruption-related thermal flare-up in the Miocene southern Black Mountains, western Arizona, USA  

NASA Astrophysics Data System (ADS)

The Silver Creek caldera (southern Black Mountains, western Arizona) is the source of the 18.8 Ma, >700 km3 Peach Spring Tuff (PST) supereruption, the largest eruption generated in the Colorado River Extensional Corridor (CREC) of the southwestern United States. Within and immediately surrounding the caldera is a sequence of volcanics and intrusions ranging in age from ~19 to 17 Ma. These units offer a record of magmatic processes prior to, during, and immediately following the PST eruption. To investigate the thermal evolution of the magmatic center that produced the PST, we applied a combination of Ti-in-zircon thermometry, zircon saturation thermometry, and high-precision U-Pb CA-TIMS zircon dating to representative pre- and post-supereruption volcanic and intrusive units from the caldera and its environs. Similar to intracaldera PST zircons, zircons from a pre-PST trachytic lava (19 Ma) and a post-PST caldera intrusion (18.8 Ma) yield exceptionally high-Ti concentrations (most >20 ppm, some up to nearly 60 ppm), corresponding to calculated temperatures that exceed 900 °C. In these units, Ti-in-zircon temperatures typically surpass zircon saturation temperatures (ZSTs), suggesting the entrainment of zircon that had grown in hotter environments within the magmatic system. Titanium concentrations in younger volcanic and intrusive units (~18.7-17.5 Ma) decline through time, corresponding to an average cooling rate of 10-3.5 °C/year. The ~200 k.y. thermal peak evident at Silver Creek caldera is spatially limited: elsewhere in the Miocene record of the northern CREC, Ti-in-zircon concentrations and ZSTs are much lower, suggesting that felsic magmas were generally substantially cooler.

McDowell, Susanne M.; Miller, Calvin F.; Mundil, Roland; Ferguson, Charles A.; Wooden, Joseph L.

2014-07-01

265

Igneous rocks of the East Pacific Rise  

USGS Publications Warehouse

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

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

1964-01-01

266

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

267

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

268

The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization  

NASA Astrophysics Data System (ADS)

The Early Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska, cross-cuts Paleozoic igneous and metasedimentary rocks of the Alexander terrane of the North American Cordillera and was emplaced during a rifting event. The complex is a circular body (~ 3 km in diameter) of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. The Bokan granites and related dikes have SiO2 from 72 to 78 wt.%, high iron (FeO (tot) ~ 3-4.5 wt.%) and alkali (8-10 wt.%) concentrations with high FeO(tot)/(FeO(tot) + MgO) ratios (typically > 0.95) and the molar Al2O3/(Na2O + K2O) ratio < 1. The granitic rocks are characterized by elevated contents of rare earth elements (REE), Th, U and high field strength elements (HFSE) and low contents of Ca, Sr, Ba and Eu, typical of peralkaline granites. The granites have high positive ?Nd values which are indicative of a mantle signature. The parent magma is inferred to be derived from an earlier metasomatized lithospheric mantle by low degrees of partial melting and generated the Bokan granitic melt through extensive fractional crystallization. The Bokan complex hosts significant rare-metal (REE, Y, U, Th, Nb) mineralization that is related to the late-stage crystallization history of the complex which involved the overlap of emplacement of felsic dikes, including pegmatite bodies, and generation of orthomagmatic fluids. The abundances of REE, HFSE, U and Th as well as Pb and Nd isotopic values of the pluton and dikes were modified by orthomagmatic hydrothermal fluids highly enriched in the strongly incompatible trace elements, which also escaped along zones of structural weakness to generate rare-metal mineralization. The latter was deposited in two stages: the first relates to the latest stage of magma emplacement and is associated with felsic dikes that intruded along the faults and shear deformations, whereas the second stage involved ingress of hydrothermal fluids that both remobilized and enriched the initial magmatic mineralization. Mineralization is mostly composed of “new” minerals. Fluorine complexing played a role during the transportation of REE and HFSE in hydrothermal fluids and oxygen isotopes in the granites and quartz veins negate the significant incursion of an external fluid and support a dominantly orthomagmatic hydrothermal system. Many other REE-HFSE deposits hosted by peralkaline felsic rocks (nepheline syenites, peralkaline granites and peralkaline trachytes) were formed by a similar two stage process.

Dostal, Jaroslav; Kontak, Daniel J.; Karl, Susan M.

2014-08-01

269

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

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

270

F-enriched magmas at Mt. Etna (Italy) and related volcanological implications  

NASA Astrophysics Data System (ADS)

The occurrence of halogen-bearing fluids in magmatic systems has been frequently recognized from fluid inclusion studies in volcanoes chiefly erupting evolved products (e.g., trachytes, rhyolites), but their role in basaltic magmas has been comparatively less investigated. Recent findings of F-bearing mineral phases, namely fluorophlogopite and fluorapatite, in a benmoreite lava flow related to ancient volcanic activity at Mt. Etna (Ellittico-Mongibello transition phase), indicate specific physical and chemical conditions of crystallization. Beyond their mineralogical interest, their occurrence, exceptional in Etnean products, could provide important clues for assessing some geochemical processes leading to fluorine enrichment in the feeding system as well as the possible role on the assessment of the eruptive styles. Textural evidence suggests a late-stage crystallization of F-bearing minerals, since fluorapatite is found in the groundmass and fluorophlogopite within lava vesicles. Furthermore, a limpid Si-rich glass, characterized by multi-stage deposition, has overgrown the fluorophlogopite crystals. Geochemical simulations demonstrated that differentiation processes, such as fractional crystallization or crustal assimilation, are not able to produce compositions consistent with those observed in the F-enriched lava. Conversely, we propose that non-traditional differentiation processes, ruled by elemental transfer (as halogen complexes/compounds) in gas phase, are able to account for the observed selective enrichments in some major and trace elements, and particularly of fluorine and other volatiles. The occurrence of Si-rich glass surrounding the fluorophlogopite crystals also fits its condensation under cooling conditions from halogens-complexes/compounds accompanied by separation of a F-rich component into the gas phase. Due to its depolymerizing effects, the anomalous F concentration in the thin lava flow bearing these minerals may also account for rather low yield strength. Furthermore, this elemental and volatile enrichment can strongly increase fluids pressure and consequently the magma overpressure. This matches the highly explosive dynamics of extrusion just observed for products emitted during the Ellittico-Mongibello transition phase. Our results, here focused on a specific eruptive phase, may have important implications for understanding some recent geochemical and volcanological developments occurred at Mt. Etna, providing hints to answer similar problems also at other basaltic volcanoes.

Nicotra, E.; Viccaro, M.; Ferlito, C.; Cristofolini, R.

2009-04-01

271

Triassic "adakitic" rocks in an extensional setting (North China): Melts from the cratonic lower crust  

NASA Astrophysics Data System (ADS)

Adakite was originally defined as a specific type of magmatic rock derived from melting of subducted oceanic plates (Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347 (6294), 662-665), producing unique chemical signatures with high Sr/Y and La/Yb. However, widespread occurrences of igneous rocks that are geochemically similar to the adakites, but from diverse tectonic settings, suggest that "adakitic" rocks may have a variety of origins. Late Triassic high Sr/Y lavas, mainly trachytes with minor pyroxene andesite and rhyolite, are found at Shuiquangou, in the Yanshanian fold-and-thrust belt on the northern margin of the North China Craton. Data on mineral chemistry, major and trace elements and Sr-Nd isotopes of whole rocks, and in situ U-Pb age and Hf-isotope analyses of zircons are reported here. The Shuiquangou volcanic rocks with high Sr/Y (> 72) and (La/Yb)N (> 24) also show enrichment in light rare-earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). They have low Ce/Pb (< 4.3) and Nb/U (< 4.8) and moderate (Gd/Yb)N (2.8-3.9). U-Pb dating of zircons yields concordant and lower-intercept ages of ~ 220 Ma, indicating that they erupted during the late Triassic. Concordant grains and an upper intercept age of ~ 2.50 Ga suggest that Neoarchean materials may have been involved in their petrogenesis. The relatively low initial 87Sr/86Sr (0.70529 to 0.70540) and negative ?Nd(t) (- 3.9 to - 9.9) of the these rocks, and the negative ?Hf(t) (- 8.6 to - 1.1) of their zircons, suggest that the magmas were derived by partial melting of the cratonic lower crust, induced by continuous magmatic underplating under an extensional regime following the southward subduction of the Paleo-Asian Ocean. Their high Sr/Y is inherited from their source, and does not necessarily imply melting at great depths (e.g., garnet-bearing lower crust). We suggest that partial melting of the ancient lower crust may be important for the petrogenesis of "adakitic" magmas in a continental extensional setting.

Ma, Qiang; Zheng, Jianping; Griffin, W. L.; Zhang, Ming; Tang, Huayun; Su, Yuping; Ping, Xianquan

2012-09-01

272

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

273

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

274

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

NASA Astrophysics Data System (ADS)

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

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

2010-10-01

275

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

276

Quaternary volcano-tectonic activity of the western rift margin in the Soddo region, southern Main Ethiopian Rift  

NASA Astrophysics Data System (ADS)

The Main Ethiopian Rift (MER) in East Africa is considered the ideal place to analyze the process of continental rifting since along its length it records all the different stages of rifting. Indeed, whereas the southern parts of the rift are believed to record the early stages of extension, with deformation localized at large boundary faults, the northern rift sectors are in incipient break-up stage, with focused tectono-magmatic activity at the rift axis. However, although the distribution and style of Quaternary volcano-tectonic deformation have been described in detail in both the northern and central rift sectors, knowledge of these characteristics is comparatively less constrained southwards. Analysis of fault distribution and comparison with analogue models, together with the characteristics of the seismic activity, suggest localized deformation at the rift margins during the Quaternary, with almost absent axial tectono-magmatic activity. However, geological data are currently still scattered in the area. In this contribution we present new geological and geochemical data from the western rift margin in the Soddo region, north of Lake Abaya, between latitudes 7°10' and 6°30'. Despite the lack of major fault escarpments, the area is characterized by numerous normal faults, which are typically sigmoidal in shape and en-echelon arranged; associated to these faults is a widespread volcanic activity, with recent volcanic centers (domes, scoria cones, fissures, etc.) aligned along and strongly interacting with normal faults. The volcanic activity is distinctly bimodal, with intermediate terms completely lacking. Erupted products are represented by alkali basalt, trachybasalts and rare tholeiitic basalts in the 45% to 49% SiO2 variation interval and alkali rhyolites with subordinate trachytes in 65%-74% range. The new field data coupled with new radiometric dating of faulted rocks suggest a Late Quaternary-Holocene age of the volcano-tectonic activity at this rift margin. This, together with a subordinate axial faulting, supports previous models that predict an along-axis, north to south decrease in rift maturity in the MER. Consistent with previous analogue models, the general fault architecture is coherent with the regional plate kinematics that gives rise to conditions of low-obliquity rifting. However, inversion of fault-slip data reveals local variations in the paleo-stress field that could result from local stress reorientations and/or influence of volcanic activity.

Corti, G.; Sani, F.; Agostini, S.; Philippon, M.; Sokoutis, D.; Willingshofer, E.; Abebe, T.

2012-04-01

277

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

278

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

279

Numerical Modeling of Thermal-Geochemical Processes in the Hydrothermal System of Pantelleria Island, Italy  

NASA Astrophysics Data System (ADS)

The island of Pantelleria, located in the Sicily Channel, Central Mediterranean, (about 100 km from Sicily and 70 km from Tunisia), represents the emergent part of a quiescent Quaternary volcano. It has been affected by an intense recent volcano-tectonic activity. The rocks outcropping on the island are mainly lavas and pyroclastic deposits, mostly represented by pantellerites and trachytes. Surface geothermal manifestations are diffused in Pantelleria, with fumaroles, mofettes and hot springs with temperatures up to 98 °C. Since the 60s’, a number of pre-feasibility studies was carried out on the island, which underwent an extensive geothermal exploration during the early 90s’. The results of the exploration revealed a very promising zone in the southern part of the island, where the main geothermal manifestations are concentrated. Temperatures above 250°C were measured in exploratory drillings down to 1100 m b.g.l. (well PPT1). The intrusion of pure seawater likely occurs throughout the island, and deep geothermal reservoir recharge appears to be predominantly of marine origin, though contributions from meteoric water are also possible. A conceptual model of the system has been set up on the basis of a wide set of geological, geophysical and geochemical data obtained during the surveys. The TOUGHREACT simulator was used for the water-rock reaction simulations. A plug-flow model has been used to perform non-isothermal calculations. Boundary thermal conditions have been calibrated according to heat flow measurements and well temperature data. Water-rock interactions have been evaluated in successive steps. First, the saturation states of minerals of sampled geothermal fluids were computed at reservoir temperatures to determine the mineral phases which would likely be dissolving or precipitating. The composition of “synthetic”, nearly-equilibrated waters likely occurring at depth within the reservoir has been then numerically reconstructed. Next, the evolution of fluid and rock compositions with time has been monitored and compared with “real-world” data. Numerical simulations were able to reproduce the mineralogical assemblage found in the reservoir and to highlight the role of CO2 degassing in controlling the geochemical evolution of the system. Fluid geochemistry can be likely ascribed to a mixing among seawater, freshwater and volcanic gas.

Bellani, S.; Gherardi, F.

2009-12-01

280

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

281

The Peralkaline Rhyolite Spectrum in Marie Byrd Land Volcanoes, West Antarctic Rift, and the Case for Polybaric Fractionation  

NASA Astrophysics Data System (ADS)

The Marie Byrd Land (MBL) volcanic province is associated with the rise of a large tectonomagmatic dome, over the past 25-30 m.y. Since 14-15 Ma, five volcanoes have produced pantellerites and three others have produced comendites. Together they display a range in SiO2, Al2O3, FeOt, and peralkalinity that is comparable with the full range of compositions in Pantelleria and the Kenyan and Ethiopian dome provinces. The comendite volcanoes occur on the east flank of the MBL dome, adjacent to volcanoes characterized by highly undersaturated phonolites (20-30% ne). The pantellerite volcanoes, with one exception, occur on the west flank, associated with less undersaturated phonolites (6-11% ne). These seem to represent two fairly distinct sub-provinces with perhaps slightly different plumbing systems. Isotopic and trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic spectrum, and the results of major and trace element modeling, all exclude crustal contamination and point to fractional crystallization as the controlling process in the origins of the felsic rocks. The availability of basalts in several stages of evolution, plus a variety of mugearites, benmoreites, and trachytes, have allowed us to model the evolution of felsic rocks in several stages. The best results, together with trace element and petrographic data, and supported by experimental work, suggest that polybaric fractionation in a multi-tiered plumbing system provides the most reasonable explanation of field, petrographic and geochemical characteristics. A plumbing system that favors prolonged kaersutite fractionation at the base of the crust for comendites (east flank) vs. one that favors comparatively short residency and little kaersutite fractionation at the base of the crust for pantellerites (west flank), seems to provide the best explanation for the differences in SiO2 and FeOt, and the wide spatial separation of these two rhyolite species. The models suggest further that, as a final step, peralkalinity evolved by fractionating a high proportion of plagioclase/clinopyroxene in shallow crustal magma chambers under very low PH2O.

Lemasurier, W. E.; Choi, S.; Mukasa, S. B.; Rogers, N. W.

2009-12-01

282

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

283

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

284

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

285

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

286

Unusual evolution of silica-under- and -oversaturated alkaline rocks in the Cenozoic Ambohimirahavavy Complex (Madagascar): Mineralogical and geochemical evidence  

NASA Astrophysics Data System (ADS)

The almost unknown Ambohimirahavavy ring complex in the Cenozoic alkaline province of northwestern Madagascar has recently attracted considerable interest because of the discovery of important rare-metal mineralization. The complex consists of arc-shaped bodies made up of silica-under- and -oversaturated syenites and extremely evolved peralkaline granitic dykes, as well as several mafic to felsic volcanic units, including basalt, phonolite and trachyte, all of which have an alkaline affinity. Uranium-lead zircon ages of 24.2 ± 0.6 Ma and 23.5 ± 6.8 Ma have been obtained for nepheline syenites and peralkaline granitic dykes, respectively, which, together with field data and ages of neighboring complexes, support emplacement controlled by regional lithospheric structures, rather than an evolving hot spot. Whole-rock major and trace-element and Sr-Nd isotopic data for the mafic suite suggest that the parental melt of this complex was generated by low degrees of melting of a metasomatized mantle source with residual amphibole. Fractional crystallization of this alkali basaltic melt likely produced the silica-undersaturated suite. We propose that the silica-oversaturated suite evolved from the undersaturated melt after contamination of the latter by crustal material. Further evolution to peralkaline compositions in both suites is attributed mainly to plagioclase and alkali feldspar segregation. Nepheline and feldspar compositions, as well as considerations of mineral equilibria among mafic silicates and Fe-Ti oxide minerals indicate crystallization temperatures of 1000 to 700 °C and an oxygen fugacity of 0.4 to 0.8 log units below the fayalite-magnetite-quartz (FMQ) buffer at 1 kbar for the silica-undersaturated melt, and temperatures of 860 to 570 °C and an oxygen fugacity of 1.5 to 3.8 log units below FMQ for the oversaturated syenitic melt. The undersaturated melt evolved towards a more peralkaline composition. Crystallization of arfvedsonite plus aegirine further reduced the melt the evolution of which ended with fluid exsolution. At late stages of crystallization, the oversaturated melt departed from the reducing trend of the undersaturated melt, evolving towards high oxygen fugacity. Very late exsolution of the fluid permitted concentration of the HFSE in the last stages of magmatic evolution with local production of low-temperature pegmatitic phases extremely enriched in these elements.

Estrade, Guillaume; Béziat, Didier; Salvi, Stefano; Tiepolo, Massimo; Paquette, Jean-Louis; Rakotovao, Soatsitohaina

2014-10-01

287

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

288

A forward modeling approach to relate geophysical observables at active volcanoes to deep magma dynamics  

NASA Astrophysics Data System (ADS)

Geophysical signals usually recorded at active volcanoes mainly consist of i) seismicity - high frequency volcano-tectonic events, volcanic tremor, and LP, VLP, and ULP events, ii) ground displacement, and iii) gravity changes. These signals are inverted to constrain the characteristics of the underground signal source, usually under the simplifying assumptions of point source or small volume homogeneous source with simple geometry. We have instead designed a forward approach, that complements the more classical inverse approaches, whereby magma chamber dynamics are numerically solved for compressible-to-incompressible multi-component magmas in geometrically complex systems constituted by one or more magma chambers connected through dykes. Our new code, that we named GALES (GAlerkin LEast Squares), solves the complex time-space-dependent dynamics of convection and mixing of magmas with different composition and properties, and reveals patterns of overpressure much more complex than commonly assumed in inverse analyses. Time-space-dependent stress distributions computed along the rigid magma-wall boundaries are employed as boundary conditions in either numerical simulations of wave propagation through the rock system by taking into account wall rock heterogeneities and topographic surface, or semi-analytical solutions of the Green’s functions in homogeneous infinite space. Ground displacement computed at the topographic surface ranges from the seismic to the quasi-static frequency band. Density variations associated to the simulated magma convection dynamics are instead employed to determine the corresponding gravity change at the surface. Seismicity, ground deformation, and gravity changes associated to deep magma dynamics are therefore computed as a function of time at different points on the Earth’s surface. Performed numerical simulations involve cases with largely different magma/dyke size, geometry and depth, and magma compositions from basaltic to shoshonitic to phonolitic to trachytic with largely different H2O and CO2 contents. All of our results concur to suggest that ultra long period (hundreds of s) ground oscillations are a diagnostic character of magma convection dynamics. Variable patterns of gravity changes and magmatic overpressure, either positive or negative, are associated to different system configurations, with prolate shallow chambers preferentially showing pressure increase, and oblate geometries pressure decrease. Our results suggest that a forward modelling approach can be successfully employed to relate the complex underground magma dynamics to observable geophysical signals, to test the simple assumptions commonly employed in inverse analyses, and finally to get to a consistent picture of the global volcano dynamics for scientific and volcanic hazard evaluation purposes.

Montagna, C. P.; Longo, A.; Papale, P.; Vassalli, M.; Saccorotti, G.; Cassioli, A.

2010-12-01

289

Cannibalism of olivine-rich cumulate xenoliths during the 1998 eruption of Piton de la Fournaise (La Réunion hotspot): Implications for the generation of magma diversity  

NASA Astrophysics Data System (ADS)

Contrasting with its unusual isotopic homogeneity compared to other hotspot volcanoes, Piton de la Fournaise has produced a large diversity of basaltic magmas over its 0.5 Ma history: picrites and two types of transitional basalts with distinct petrological and chemical compositions. A minor group of evolved basalts (anomalous group of basalts or AGB) is enriched in both compatible (Mg, Fe, Ti, Cr, and Ni) and incompatible (K, Th, and La) elements and depleted in Ca and Si relative to the dominant group of evolved basalts. The 1998 eruption simultaneously produced the two basaltic types at two distinct vents (Hudson vent: AGB, Kapor vent: common basalt) but from the same feeding conduit. Glasses of both magmas are close in composition and belong to the single differentiation trend defined by all 1998-2007 glass compositions. Thermodynamic model (MELTS code) shows that AGB-type magmas cannot be produced by high pressure (> 1 GPa) clinopyroxene fractionation as previously proposed and that all melts of the 1998-2007 activity period are produced by low pressure (< 800 MPa) crystal fractionation from the most primitive basalt (MgO ~ 9%). Modal composition of 1998 lavas (mass balance calculation and SEM image analysis) and olivine crystal composition show that Hudson lavas have assimilated significant fractions of olivine xenocrysts contrary to Kapor lavas. In addition, the higher incompatible element contents of Hudson lavas suggest contamination by a differentiated (trachytic) melt. All AGB share the following characteristics: (i) evolved glass compositions, (ii) 5-10% olivine xenocrysts, and (iii) vents located in a narrow region at the summit of the edifice. They are interpreted as the result of the assimilation of olivine-rich xenoliths either by evolved melts or by basaltic melts contaminated by low fractions of differentiated melts produced from interstitial glass frequently coating cumulates minerals or resulting from partial melting of cumulates bearing pyroxene or plagioclase (wehrlitic to gabbroic cumulates). The scarcity of AGB magmas is attributed to their shallow transfer path in rarely intruded lateral zones of Piton de la Fournaise volcano: wehrlitic to gabbroic cumulates bodies are either heterogeneously distributed within the edifice or have been depleted in low melting point components in the 'Rift Zone' where most of the recent eruptive events are emplaced. These results emphasize the exceptional chemical homogeneity of the primary basaltic melt involved in volcanic activity of Piton de la Fournaise hotspot for 0.5 Ma and the increasingly recognized role of magma-wall rock interactions in erupted magma compositions.

Salaün, A.; Villemant, B.; Semet, M. P.; Staudacher, T.

2010-12-01

290

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

NASA Astrophysics Data System (ADS)

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

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

1980-11-01

291

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

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

292

St. Helena Revisited: Characteristics and Origin of the Type HIMU OIB  

NASA Astrophysics Data System (ADS)

Saint Helena, South Atlantic Ocean (15° 57' S, 5° 43' W), is the type HIMU hotspot oceanic island, the basalt - hawaiite - mugearite - benmoreite - trachyte volcanic suite having anomalously high 206Pb/204Pb of 20.4-20.9. Eruptions were from two main centres, the older Northeastern Volcanic Centre (NEVC) and the younger Southwestern Volcanic Centre (SWVC), the latter of which is divided into Lower Shield (LS), Main Shield (MS), Upper Shield (US), Late Extrusive Phase (LEP), and Late Intrusive Phase (LIP) episodes. Whole rock K-Ar ages suggest that subaerial volcanism spanned a period of 7 million years, from 14 Ma to 7 Ma, but new, precise, 40Ar/39Ar ages bracket the subaerial volcanism between 10.15 ± 0.13 Ma (a basal flow in the NEVC sequence) and 8.13 ± 0.08 Ma (a LEP lava flow). Within the mafic (basalt and hawaiite) lava flows, spanning approximately 2 million years of eruptive activity, there is only limited geochemical variation; for example, Zr/Nb averages 4.5 and only ranges from 4.0 to 5.2. Nonetheless, within some stratigraphic sequences (the SWVC Main Shield) there are subtle trends of, for example, decreasing Zr/Nb and increasing P/Nb, K/Nb, and Rb/Nb with stratigraphic height. These trends are interpreted to record a decreasing degree of melting through time rather than a variation in mixing proportions of discrete mantle source components. Increasing compositional diversity through time, and an increasing proportion of more evolved lava compositions, reflects an increasing degree of crystal fractionation. Although not apparent from whole rock major and trace element variations, phenocryst compositions suggest an important role for magma mixing in the origin of the volcanic suite. Both the trace element and isotopic (Pb, Sr, Nd, Hf, Os) characteristics of St. Helena volcanic rocks, and of HIMU OIB in general, are consistent with an ultimate origin from recycled ancient lithosphere, although the physical location of the source region (deep mantle plume; heterogeneous shallow upper mantle) is unresolved. The temporal and compositional relationships of volcanism in the St. Helena - Cameroon Line trend are not, however, consistent with a simple model of the passage of the African plate over a deep mantle plume.

Weaver, B.

2006-12-01

293

Impact of volcanism on the evolution of Lake Van (eastern Anatolia) III: Periodic (Nemrut) vs. episodic (Süphan) explosive eruptions and climate forcing reflected in a tephra gap between ca. 14 ka and ca. 30 ka  

NASA Astrophysics Data System (ADS)

Fifteen Lateglacial to Holocene rhyolitic, dominantly primary tephra layers piston-cored and drilled (ICDP Paleovan drilling project) in western Lake Van (eastern Anatolia, Turkey) were precisely correlated to either of the two adjacent and active large volcanoes Nemrut and Süphan based on shard textures, mineralogy and mineral and glass compositions. The young peralkaline (comenditic to pantelleritic) primary rhyolitic Nemrut tephras are characterized by anorthoclase, hedenbergitic to augitic clinopyroxene, fayalitic olivine, minor quartz, and rare accessory chevkinite and zircon. Phenocrysts in subalkaline primary rhyolitic Süphan tephras are chiefly oligoclase-labradorite, with minor K-rich sanidine in some, biotite, amphibole, hypersthene, rare augitic clinopyroxene, relatively common allanite and rare zircon. Two contrasting explosive eruptive modes are distinguished from each other: episodic (Süphan) and periodic (Nemrut). The Lateglacial Süphan tephra swarm covers a short time interval of ca. 338 years between ca. 13,078 vy BP and 12,740 vy BP, eruptions having occurred statistically every ca. 42 years with especially short intervals between V-11 (reworked) and V-14. Causes for the strongly episodic Süphan explosive behavior might include seismic triggering of a volcano-magma system unable to erupt explosively without the benefit of external triggering, as reflected in pervasive faulting preceding the Süphan tephra swarm. Seismic triggering may have caused the rise of more mafic ("trachyandesitic") parent magma, heating near-surface pockets of highly evolved magma - that might have formed silicic domes during this stage of volcano evolution - resulting in ascent and finally explosive fragmentation of magma essentially by external factors, probably significantly enhanced by magma-water/ice interaction. Explosive eruptions of the Nemrut volcano system, interpreted to be underlain by a large fractionating magma reservoir, follow a more periodic mode of (a) long-term relatively constant supply of parent magma, (b) evolution by low pressure crystal fractionation resulting in sporadic relatively low-volume eruption of trachytic and minor rhyolitic magmas, (c) evolution of a large magma reservoir to the point of highly explosive large-volume peralkaline rhyolitic Plinian eruptions at temporal intervals of ca. 20-40 ky, some accompanied by ignimbrites and inferred caldera collapse. A striking tephra gap between ca. 14 ka and ca. 30 ka, i.e. during glacial climate conditions, is postulated to be due to climate-forcing via lithosphere unloading following deglaciation.

Schmincke, Hans-Ulrich; Sumita, Mari

2014-09-01

294

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

295

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

296

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

297

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

298

The Geochemistry of Feldspar, Fresh Glass, and Melt Inclusions Within Feldspar as Potential Tools for Correlating Fresh and Altered Tuffs at Olduvai Gorge, Tanzania  

NASA Astrophysics Data System (ADS)

Olduvai Gorge, Tanzania, preserves a 100-m sequence of Plio-Pleistocene volcaniclastic sediments interbedded with at least 10 predominantly trachytic tephra layers. These tephra were deposited in environments including a saline-alkaline lake, volcaniclastic alluvial fan, and wetlands, and hence degrees of alteration vary. Fresh glass is sometimes preserved, but is usually altered to zeolite, limiting its use for geochemical analysis for stratigraphic correlation. In the absence of fresh glass, feldspar and melt inclusion compositions can be used. Samples of five major Bed I tuffs (Tuffs IB through IF) were collected from sites up to 15 km apart in various environments. Major and minor element analyses of glass shards, feldspar, and melt inclusions within feldspar from each tuff (electron microprobe) were supplemented by trace element analysis of feldspar and shards from select samples (Synchrotron XRF microprobe). At least two samples (20-60 grains) of each tuff were analyzed. Tuffs IB through IE can be distinguished using feldspar composition (mean compositions: IB= An4Ab63Or33, IC= An11Ab67Or22, ID= An30Ab61Or9, IE= An12Ab64Or24). Tuff IF feldspars are more variable and partially overlap the composition of Tuff IC, but 28% of the grains analyzed from IF contain significant Ba (1 to 4%) and Sr (up to 900 ppmw). These high Ba feldspars may suggest multiple eruptive sources for Tuff IF. Where glass shards were altered, fresh glass was preserved as inclusions in feldspar. The shard composition varies more within each sample than the feldspar composition, though most of the tuffs can be distinguished based on the concentrations of Ca and Al in the fresh shards (CaO% IB < 1, ID > 1.4 , IE = 1.2; Al2O3 % IB, ID, IE <17, IF > 18.5). The inclusions within the feldspars of Tuffs IB and IE are consistent with the compositions of the associated shards and cluster together tightly (especially in Ti and Ca), suggesting that inclusions could be used to correlate these tuffs. The compositions of the inclusions from Tuffs IC and ID are more widely variable and overlap, limiting their usefulness for identification. At Olduvai, feldspar composition currently provides the best method for identifying each tuff, for inclusion composition is too variable in some tuffs.

McHenry, L. J.

2001-12-01

299

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

PubMed

Mineral waters were monthly sampled in selected springs from Furnas and Fogo trachytic central volcanoes (São Miguel, Azores, Portugal). Water temperatures between 15.1 °C and 90.2 °C, characterize poorly mineralized waters of Na-HCO? and Na-HCO?-Cl types. According to the spring location, two watersheds were selected in each volcano to evaluate solute fluxes and chemical weathering rates (Fogo volcano: Ribeira Grande river - RRG; Furnas volcano: Ribeira Quente river - RRQ). Na, Mg, K and Ca fluxes in groundwater represented 43%, 60%, 46% and 57% of the total (subsurface plus surface fluxes) in RRG, and respectively 43%, 53%, 46% and 49% in RRQ. Average HCO? flux is ten times higher in RRQ (130.1 × 10(6) mol/yr) compared to RRG (13.8 × 10(6) mol/yr), reflecting the volcano degassing and the lower ratio between groundwater and river water fluxes. Based on these values, total CO?-consumption by weathering ranged from 1.5 × 10(6)mol/km(2)/yr (RRG) to 4 × 10(6)mol/km(2)/yr (RRQ). TDS load varied between 3772 t/yr (RRG) and 15388 t/yr (RRQ), and the ratio between values in groundwater and in surface water is respectively equal to 0.72 and 2.04. The associated chemical weathering rates in groundwater were 206 t/km(2)/yr (RRG) and 399 t/km(2)/yr (RRQ). Coupled to river water, these values indicate that total chemical weathering rates are respectively equal to 493 t/km(2)/yr and 594 t/km(2)/yr. A similar approach developed for the entire archipelago showed that the chemical weathering rates due to groundwater are in the range from 33 to 321 t/km(2)/yr, being partially controlled by the age of the islands. Results of the present study point out to the need to consider groundwater solute fluxes due to weathering when establishing geochemical budgets. PMID:23933431

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

2014-01-01

300

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

301

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

302

Crustal recycling beneath continental arcs: silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California  

NASA Astrophysics Data System (ADS)

We describe silica-rich (up to ˜69.5% SiO 2) glass inclusions trapped as grain boundary films and within-grain pockets in ultramafic xenoliths hosted by Pliocene basalts from the Sierra Nevada, California. The ultramafic xenoliths are lherzolites which equilibrated in the Sierra Nevada upper mantle at 1150-1180°C and ˜1.4-1.8 GPa. The glass inclusions have trachytic compositions, similar to previously described silicic melts from mantle xenoliths [1-9]. We have determined the Sr and Nd isotope compositions of the grain boundary films using a leaching technique, and calculated the glass isotopic compositions. The glass 87Sr 86Sr (0.7077-0.7085) and 143Nd 144Nd (?0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions ( N/b nNb n*˜0.15 ), enrichment of light rare earth elements ( L/a nYb n? 50 ), and the presence of negative Eu anomalies ( E/u nEu n*? 0.7-0.86 ), indicating a crustal origin for the melt source. The Nd isotope ratios ( ? Nd? -4 ) are inconsistent with an oceanic crust as the source for these former melts. The source rocks must have been continental materials recycled in the mantle, either foundered lower crust or subducted sediment. Low Rb/Sr (0.036-0.077) and high Sr/Nd (>35) ratios observed in the glasse are suggesting a lower crustal source. The Sierra Nevada lowermost crust (amphibole-bearing garnet pyroxenites and other dense Mesozoic cumulate mafic-ultramafic rocks), as defined by studies of older, Miocene xenolith-bearing volcanic rocks from the same area [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229-8244], has isotopic compositions similar to the glass inclusions. Geologic [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229-8244] and geophysical [G. Zandt, S. Ruppert, EOS Trans. AGU 77 (1996) 831] evidence indicate that the Sierra Nevada has lost its eclogitic arc root, probably by foundering in the mantle. We propose here that the silica-rich glasses were formed by low percent partial melting of the dense, cold Sierran batholithic lowermost crust during root delamination. Further tests need to be aimed at addressing the viability of the main alternative to our interpretation, i.e. derivation of glasses from melting subducted sediments.

Ducea, Mihai; Saleeby, Jason

1998-03-01

303

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

304

The occurrence of a complete continental rift type of volcanic rocks suite along the Yerer-Tullu Wellel Volcano Tectonic Lineament, Central Ethiopia  

NASA Astrophysics Data System (ADS)

The Yerer-Tullu Wellel Volcano-tectonic Lineament (YTVL) is an E-W trending fault system or aborted rift that intercepts the Main Ethiopian Rift (MER) at Debre Zeyt (Bishoftu)/Yerer, in the eastern periphery of Addis Ababa. The structure is in correspondence with the westward extension of the southern margin of the Gulf of Aden rift. The YTVL extends for more than 500 km with a very clear northern fault margin, between Addis Ababa and Ambo known as the “Ambo Fault”. The southern margin is indicated by an E-W trending segmented lineaments at the latitude of about N 8°30?, the Bedele-Metu being the most clear segment. In between these limits there are several evolved central volcanoes and cinder cones. The central volcanoes range in age from 12 to 7 Ma in the western most (Tullu Wellel) and gradually the upper limit get younger towards East to less than 1 Ma in the Wenchi and Debre Zeyt (Bishoftu) areas. These volcanic products cover the whole spectrum of a continental rift volcanic rocks suite: (1) in the eastern zone (Yerer-Bishoftu) the suite is silica over-saturated, ranging in composition from transitional basalt to peralkaline rhyolite, (2) moving westwards, between Wechacha and Wenchi, the rocks suite is silica saturated ranging in composition from alkali basalt to trachyte, (3) further West between Ijaji-Konchi and Nekemt the rocks suite is silica under-saturated ranging in composition from basanite to phonolite. Crossing the Dedessa lineament, the Tullu Wellel rocks appear to be silica saturated. Within a single suite fractional crystallization is the predominant evolutional process even in the silica over-saturated suite. The westwards progressive silica under-saturation and increase in alkalinity (except for the Tullu Wellel volcanic centers) is interpreted by the gradual deepening of an anomalous mantle where partial fusion took place. Therefore, as distance increases from the MER junction to the West, the amount of melt on the upper mantle was gradually reduced and became more alkaline but poorer in silica.

Abebe Adhana, Tsegaye

2014-11-01

305

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

NASA Astrophysics Data System (ADS)

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

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

1993-08-01

306

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

NASA Astrophysics Data System (ADS)

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

Shane, Phil; Hoverd, Joy

2002-04-01

307

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

308

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

NASA Astrophysics Data System (ADS)

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

Tripoli, Barbara; Cordonnier, Benoit; Ulmer, Peter

2014-05-01

309

Changes in Silica and Alumina Saturation in Melting Experiments on a Natural Syenodiorite: 9-20 Kilobars With Added H2O  

NASA Astrophysics Data System (ADS)

I have conducted piston-cylinder melting experiments on syenodiorite from the border zone of Rattlesnake Mountain sill in Big Bend National Park, Texas. The sample matches the bulk composition of the intruding magma: a silica-undersaturated (5% normative ne), metaluminous trachy-andesite. Experiments were run from 9-20 kb with added 0, 2, 4, and 8 wt.% H2O, and from 950° to 1165° C. Glass compositions are dominated by two similar yet distinct trends in SiO2-(K2O+Na2O) space. One trend includes runs at 15 and 18 kb and spans basaltic trachy-andesite to trachyte and is dominated by amphibole crystallization. These melts are mostly silica-undersaturated or unsaturated and metaluminous or slightly peraluminous. The other trend includes runs at 9, 15, and 20 kb, spans from trachy- andesite to rhyolite, and is marked by biotite crystallization with or without amphibole. These samples are quartz-normative and most are significantly peraluminous (molar Al/(2Ca+K+Na) > 1.1). The latter trend culminates at 20 kb and 1125° C, 2 and 4% added H2O, with a peraluminous, quartz-normative rhyolitic melt in equilibrium with Na-rich biotite, Na-rich amphibole, albitic feldspar, clinopyroxene, garnet, ilmenite, and apatite. Garnet and clinopyroxene occur in some experiments of both trends. The experimental charges experienced Fe-loss to the Au capsule and glasses show volatilization of Na during beam analysis. Each factor results in up to 30% relative loss. Melt (glass) compositions are constrained by mass-balance calculations and corrections are made for Fe loss and Na volatilization. In the case of the above-mentioned rhyolitic glasses, these losses would not change the classification of lithology, silica- saturation, or alumina-saturation. At 15 kb and 1125° C, ilmenite is a liquidus phase with 4 wt.% H2O and apatite is a liquidus phase coexisting with a vapor phase at 8 wt.% H2O. Apatite and ilmenite are the sole phases at 20 kb, 1125° C, and 8 wt.% H2O.

Schrader, C. M.

2008-12-01

310

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

311

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

NASA Astrophysics Data System (ADS)

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

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

312

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

USGS Publications Warehouse

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

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

2002-01-01

313

Dyke Swarms in Southeastern British Columbia: Mineralogical and Geochemical Evidence for Emplacement of Multiple Magma Types During Orogenic Collapse  

NASA Astrophysics Data System (ADS)

Eocene dyke swarms in southeastern British Columbia provide an important record of the tectonic and magmatic history of the Cordillera following orogenic collapse. New field mapping, petrographic, and geochemical data is presented for a swarm of more than thirty dykes located near the mining town of Trail, B.C. Detailed field mapping revealed that individual dykes are highly diverse, both in composition and morphology. As a group, the dykes trend northwest (average strike of 338 degrees) and dip steeply to the southwest. Their average thickness is approximately 1.5m, with a range from 4.5m to less than 1cm. Three sub-parallel dykes were mapped for a length of 2km, and exhibit irregularities in their form such as branching and offshoots that follow fractures in the country rock. Thin-section analysis shows a wide variety of rock types within the swarm, including: micro-quartz syenite, micro-syenite, micro-monzonite, latite, basalt, basaltic andesite, and lamprophyre. Texturally, these samples are consistently porphyritic and partially altered to chlorite and sericite. This alteration commonly occurs in concentric rims around phenocrysts. The samples are typically intergranular, although some display trachytic texture. Whole-rock geochemistry shows that the dykes have a wide range in composition, with SiO2 between 76.45 wt.% and 45.15 wt.% and MgO between 0.13 wt.% and 13.16 wt.%. The results also revealed that one dyke has very high values of Ni (430 ppm), Cr (1420 ppm), and Co (50 ppm), giving it a fairly primitive composition. Harker diagrams and trace element plots show three distinct groups: mafic calc-alkaline dykes, felsic calc- alkaline dykes, and minette lamprophyres. The felsic dykes are characterized by negative Eu and Sr anomalies suggesting fractionation of plagioclase feldspar, as well as pronounced negative P and Ti anomalies. The minettes are enriched in LILE and depleted in HSFE relative to the mafic dykes. The three groups do not appear to be related through magma differentiation processes, and it is suggested that they originate from separate magma sources. This has important implications for magma generation and heat flow in an extensional stress regime in an orogenic belt, and the potential interaction of different reservoirs in the crust. The findings from this study suggest extension in an ENE-WSW oriented pull-apart direction during Eocene time in southeastern B.C., and the concurrent emplacement of several different magma types perpendicular to extension.

Freeman, M.; Owen, J. P.; Hoskin, P. W.

2009-05-01

314

Chronology of magmatism and Eurekean deformation in the High-Arctic Large Igneous Province: 40Ar-39Ar age of the Kap Washington Volcanics, North Greenland  

NASA Astrophysics Data System (ADS)

Continental rift magmatism and seafloor spreading anomalies along the edges of Greenland provide a history of the tectonic evolution in the North Atlantic and Arctic regions. Here we report new 40Ar-39Ar ages for the alkaline volcanics of the Kap Washington Volcanic Province outcropping at the northern tip of Greenland. Amphibole from two closely related lapilli tuff units on the Kap Kane peninsula yields an age of c. 70 Ma. This is interpreted as the emplacement age of continental rift-magmatism. Whole-rock ages from three comenditic trachyte flows with large and abundant alkali feldspar phenocrysts from the same volcanic packages yield ages of 49-47 Ma. These ages are interpreted as a result of resetting during compressional tectonism resulting in northwards thrusting of older Palaeozoic metasediments over the Kap Washington Volcanics during the Eurekean Orogeny. Microperthitic exsolutions (patch-type with almost pure albite and orthoclase end-members) in alkali feldspars confirm that thermal overprinting exceeded the ~150°C required to reset K-Ar in alkali feldspar. Likewise, Rb-Sr errorchrons and scattered initial Sr-isotope compositions indicate a regional hydrothermal fluid event at c. 50 Ma. The new 40Ar-39Ar age, together with new U-Pb chronology (Thorarinsson et al., this session), for Kap Washington volcanism adds further evidence for protracted alkaline, continental-type volcanism from 92 to 61 Ma in the High-Arctic Large Igneous Province exposed in North Greenland and the Canadian Arctic islands. We interpret such prolonged alkaline volcanism as the result of continental extension associated with northwards propagation of seafloor spreading in the Labrador Sea-Baffin Bay system between North America and Greenland. The shift to compressional tectonics resulting in the Eurekean Orogeny at 48-47 Ma is explained by a change in the relative movement of Greenland in consequence of: (i) a shift to spreading east and north of Greenland (Northeast Atlantic-Arctic system) and (ii) right-lateral displacement on the transform fault linking the Northeast Atlantic and Arctic spreading systems.

Holm, P. M.; Tegner, C.; Storey, M.; Zhao, X.; Faurschou Knudsen, M.; Thorarinsson, S. B.; Lo, C.

2009-12-01

315

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

316

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

317

Age of Magmatism and Eurekan Deformation in North Greenland  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

318

Modes and times of caldera resurgence: The < 10 ka evolution of Ischia Caldera, Italy, from high-precision archaeomagnetic dating  

NASA Astrophysics Data System (ADS)

Ischia is a well exposed and densely populated late Quaternary caldera in the Campanian magmatic province of Italy. Ischia Caldera experienced an average uplift rate of 3.3 cm/year in the last ca. 30 ka and is still actively resurging. During the last 10 ka, coeval with the resurgence, a volcanic field of alkali-trachytic to trachyandesitic lava domes, lava flows, tuff and scoria rings, and pumice cones developed, mainly on the eastern sector of the caldera, along both resurgence-related faults and regional NNW- and NE-striking faults. In order to improve both our understanding of the recent volcanic history and the evaluation of future risks on Ischia Island, a high-precision archaeomagnetic dating method was applied to the products of 12 volcanic centres with probable age < 10 ka. Accurate paleofield directions with a median 95% confidence angle of 1.47 were measured on 277 lava and spatter samples (25 sites). Coupled with the reference curves for secular variation in the western Mediterranean sea (during the last 3000 years) and eastern Europe (from 3000 to 8000 years ago), ages of between 4100 BC and 355 AD were obtained. These archaeomagnetic data were supported by a volcanologic and stratigraphic survey and are consistent with written sources, archaeological findings, and previous isotopic (K/Ar and 14C) ages. Archaeomagnetic and other geochronological data, as well as stratigraphic constraints, show that, during the studied time interval, Ischia volcanism occurred in five periods separated by phases of quiescence and coeval with earthquake and landslide events. This fact suggests a pulsating mode of uplifting and deformations of the Ischia resurgence. During the two oldest periods of activity (7200-6800 BC and 4100-2300 BC), resurgence probably produced a dome-shaped structure. Location and geometry of vents suggest the occurrence of magma uprise along the fractures produced by bending of the overburden crustal block. Most of magma was emplaced as intrusions at the interior of the resurgent block, whereas volcanism was represented by very viscous, differentiated, and crystallized lavas that emplaced as domes and high aspect-ratio flows. The resurgent dome caused recurrent lateral collapses that removed about 2.5 km 3 of rocks. During the three youngest periods of activity (1800-1000 BC; 650 BC -355 AD; and 1302 AD), resurgence affected a fault-bounded, asymmetric block. This resulted from both (a) hydrostatic rebound of the crustal block after removal of material involved into huge slope instability triggering an increase in uplift rate, and (b) new influxes of less evolved magma batches into the shallow reservoir that, in turn, favoured the intense volcanism of the last 4000 years.

Vezzoli, Luigina; Principe, Claudia; Malfatti, Jonas; Arrighi, Simone; Tanguy, Jean-Claude; Le Goff, Maxime

2009-10-01

319

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe  

NASA Astrophysics Data System (ADS)

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African-European collision plate boundary. We find that Pantelleria fluids contain a CO 2-He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (˜100°C) aquifer of mixed marine-meteoric water. The measured 3He/ 4He ratios and ? 13C of both the free gases (4.5-7.3 Ra and -5.8 to -4.2‰, respectively) and dissolved helium and carbon in waters (1.0-6.3 Ra and -7.1 to -0.9‰), together with their covariation with the He/CO 2 ratio, constrain a 3He/ 4He ratio of 7.3±0.1 Ra and a ? 13C of ca. -4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U-Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) ? 13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, ? 13C=-3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African-Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or 'less contaminated', 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/ 4He ratio but higher than MORB CO 2/ 3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU-EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.

Parello, F.; Allard, P.; D'Alessandro, W.; Federico, C.; Jean-Baptiste, P.; Catani, O.

2000-08-01

320

Regional Variation in the Timing of Cessation of Laramide Folding, Uplift, and Post Flat-Slab Ignimbrite Flare Ups in West Texas  

NASA Astrophysics Data System (ADS)

Tertiary volcanism in the Trans-Pecos region of West Texas is thought to be related to: 1) a major Farallon buoyant object-related slab flattening period resulting in early arc volcanic activity progressively shifting eastward to New Mexico and West Texas, 2) a subsequent slab-steepening/roll back or slab break away period resulting in renewed and extensive volcanism and ignimbrite flare ups across the region, and 3) final minor volcanism related to Basin and Range extension. Evidence for these tectonic induced volcanic periods can be observed in volcanic compositional variation and the timing and style of volcanism and deformation. This includes highly varied pre-folding mafic alkaline and calc-alkane volcanic and intrusive rocks from ~46Ma (Eocene) to ~32Ma (Oligocene). This is followed by broadly distributed, post-folding calc-alkaline volcanism with subsidiary alkaline mafic volcanism during subsequent regionally varying slab steepening or break away periods (38Ma to 28Ma). Later, smaller-scale, almost exclusively mafic volcanism is associated with Basin and Range extension that occurred between 25 to 2Ma (Barker, 1987). This study attempts to refine regional differences in the cessation of Laramide folding and episodic slab-asthenosphere influenced volcanism in West Texas by utilizing LA-ICP-MS U/Pb zircon geochronology. Ages have been measured for volcanic rock samples that occur both above and below the angular unconformity, including: the Eagle Mountain Lower Rhyolite, an unnamed tuff and trachyte unit from the Garren Group in the Indio Mountains, the Davis Mountains Huelster Fm. and Petan Basalts, the Chinati Mountains Morita Ranch Fm., the Tascotal Fm. From the Alamito Creek and Green Valley area, the Vieja Group from the Rio Grande River area, and the Big Bend National Park South Rim Fm. and Chisos Fm.. In addition, ages for detrital zircons have been obtained in silicilastics below the unconformity within Tertiary Black Peaks, Hannold Hill, and Canoe Formations to constrain the youngest zircon ages in each, as well as to constrain the range of igneous rock ages sampled from proximal fluvial source regions. Samples have been dated in an attempt to more precisely constrain the ages of the cessation of Laramide shortening and ignimbrite flare ups after flat-slab subduction across West Texas. Based on our preliminary results, we suggest cessation of shortening associated with Laramide-style deformation and uplift in the northern Trans-Pecos region ceased by ~38 Ma (Eocene) and persisted in the south e.g., in the Big Bend region until ~31 Ma (Oligocene). These differences may be explained by fundamental changes in the North American plate basal lithospheric structure from north to south in the region.

Davidson, M. E.; Casey, J.; Lapen, T. J.

2013-12-01

321

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

322

Cretaceous seamounts along the continent ocean transition of the Iberian margin: U Pb ages and Pb Sr Hf isotopes  

NASA Astrophysics Data System (ADS)

To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the "Tore Madeira" cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2 ?) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for 206Pb/ 204Pb, 15.544-15.828 for 207Pb/ 204Pb, 38.750-39.936 for 208Pb/ 204Pb, 0.70231-0.70340 for 87Sr/ 86Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high 207Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30° anti-clockwise rotation of Iberia.

Merle, Renaud; Schärer, Urs; Girardeau, Jacques; Cornen, Guy

2006-10-01

323

Alkalic rocks and resources of thorium and associated elements in the Powderhorn District, Gunnison County, Colorado  

USGS Publications Warehouse

Alkalic igneous rocks and related concentrations of thorium, niobium, rare-earth elements, titanium, and other elements have long been known in the Powderhorn mining district and have been explored intermittently for several decades. The deposits formed chiefly about 570 m.y. (million years) ago in latest Precambrian or Early Cambrian time. They were emplaced in lower Proterozoic (Proterozoic X) metasedimentary, metavolcanic, and plutonic rocks. The complex of alkalic rocks of Iron Hill occupies 31 km 2 (square kilometers) and is composed of pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite, in order of generally decreasing age. Fenite occurs in a zone, in places more than 0.6 km (kilometer) wide, around a large part of the margin of the complex and adjacent to alkalic dikes intruding Precambrian host rock. The alkalic rocks have a radioactivity, chiefly due to thorium, greater than that of the surrounding Powderhorn Granite (Proterozoic X) and metamorphic rocks. The pyroxenite, uncompahgrite, ijolite, and nepheline syenite, which form more than 80 percent of the complex, have fairly uniform radioactivity. Radioactivity in the carbonatite stock, carbonatite dikes, and the carbonatite-pyroxenite mixed rock zone, however, generally exceeds that in the other rocks of the complex. The thorium concentrations in the Powderhorn district occur in six types of deposits: thorite veins, a large massive carbonatite body, carbonatite dikes, trachyte dikes, magnetite-ilmeniteperovskite dikes or segregations, and disseminations in small, anomalously radioactive plutons chiefly of granite or quartz syenite that are older than rocks of the alkalic complex. The highest grade thorium concentrations in the district are in veins that commonly occur in steeply dipping, crosscutting shear or breccia zones in the Precambrian rocks. They range in thickness from a centimeter or less to 5 m (meters) and are as much as 1 km long. The thorite veins are composed chiefly of potassic feldspar, white to smoky quartz, calcite, barite, goethite, and hematite, and also contain thorite, jasper, magnetite, pyrite, galena, chalcopyrite, sphalerite, synchysite, apatite, fluorite, biotite, sodic amphibole, rutile, monazite, bastnaesite, and vanadinite. The Th0 2 content of the thorite veins ranges from less than 0.01 percent to as much as 4.9 percent in high-grade samples. The Th0 2 content is generally less than 1 percent, however, and is only 0.05 to 0.1 percent in many of the veins examined in the district. Samples of the dolomitic carbonatite of Iron Hill mostly range from 3 to 145 ppm (parts per million) thorium. Thirty samples of the carbonatite dikes, the most radioactive rocks within the complex of Iron Hill, contain about 30 to 3,200 ppm thorium and a trace to about 1.5 percent rare-earth oxides. The magnetite-ilmenite-perovskite rocks have a radioactivity of 2 to 12 times the background of Precambrian granite that is attributable chiefly to thorium substitution for calcium in the perovskite. In two analyses the perovskite contains 0.12 and 0.15 percent Th0 2 . Trachyte dikes as much as 25 m thick cut the Precambrian rocks; their radioactivity is generally about two to four times the background of typical Precambrian granite, is locally higher, but is low relative to other types of thorium concentrations. A finegrained granite that is anomalously radioactive occurs in thick, dikelike plutons as much as 1.2 km wide, or more. The thorium content varies widely within the granite bodies. Eight samples of the granite contain 32 to 281 ppm thorium (averaging 115 ppm). The economic potential of thorium in the Powderhorn district is related in part to other elements such as niobium, titanium, iron, and rare earths. The proportions of niobium and rare earths to thorium vary in different parts of the district. Within the carbonatite body of Iron Hill, the Nb 2 0 5 content greatly exceeds Th0 2 , but the Th0 2 -Nb 2 0 5

Olson, J.C.; Hedlund, D.C.

1981-01-01

324

Mathematical and numerical modelling of fractional crystallization coupled with chemical exchanges and differential magma-solid transport in magma chambers  

NASA Astrophysics Data System (ADS)

The knowledge of the chemical evolution of magmas is a major concern in geochemistry and petrology. The jumps (or discontinuities) of chemical composition observed in volcanic series from the same province are also the subject of many studies. In particular the phenomenon of "Daly gap" (Daly 1910, 1925), the name given to the jump in chemical composition between the mafic rocks (basalt) and felsic rocks (trachyte, rhyolite, phonolite), corresponding to the absence or scarcity of rocks of intermediate composition (andesite), in both ocean and continental series. Some authors explain these compositional jumps thanks to the intervention of various geological phenomena which follow in time. For example, when a magma chamber turns from a closed to an open system, the lava of a specific composition is ejected to the surface, favoring the rise of the lightest, the most volatile-rich and the less sticky magmas to the surface of the earth (Geist et al., 1995, Thompson et al., 2001). The various explanations offered, although they agree satisfactorily with the natural data, most often lead us away from basic phenomena of melting / solidification, relative migration and chemical equilibrium between solid and liquid and involve various additional phenomena. In our study, we propose a numerical modelling of the crystallization of a closed magma chamber. The physical and mathematical model distinguishes three main classes of processes occurring simultaneously: - heat transfer and solidification, - relative migration between the solid and the liquid magma, - chemical reactions between the two (solid and liquid) phases. Writing the partial differential equations with dimensionless numbers makes two parameters appear, they express the respective ratios of the solidification velocity on the transport velocity, and the kinetics of chemical exchange on the transport velocity. The speed of relative movement between the solid and the liquid, the solidification velocity and the chemical partition law between the solid and the liquid are assumed to be known; this last one may be non-linear and apply to major elements. The model is written for one chemical component. It is splitted into two submodels, the crystallization/sedimentation model and the reactive transport model. The first is expressed by an hyperbolic partial differential equation and is solved by a three-point scheme, the second is solved using non-centered schemes. The computing program is written in Fortran 90, it is then validated by theoretical methods such as the method of characteristic curves, analytical calculations or by qualitative considerations. Numerical simulations show that, for some values of the dimensionless parameters and for some shapes of the chemical partition curves, the chemical composition of the magma chamber can be non homogeneous, particularly bimodal (two values of concentrations are preferred), starting from homogeneous initial conditions. The degree of this bimodality notably depends on the shape of the chemical partition law. This model provides an intellectual framework to discuss the phenomena responsible for the variety of composition of magmatic rocks, particularly in the same province. It shows in particular that the coupling between three elementary phenomena, internal to the magma chamber, is enough to account for the bimodality or more generally the appearance of discontinuities in chemical compositions, without involving additional phenomenon.

Lakhssassi, Morad; Guy, Bernard; Cottin, Jean-Yves; Touboul, Eric

2010-05-01

325

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

326

Preliminary Geology of Gareloi Volcano, Western Aleutian Islands (Alaska)  

NASA Astrophysics Data System (ADS)

Gareloi Island consists of Gareloi volcano (1573 m elevation), and is located nearly 2000 km west of Anchorage and 120 km west of Adak in the western Aleutian (Andreanof) Islands. A geologic mapping operation was combined with the installation of a seismic monitoring network in September of 2003 by the Alaska Volcano Observatory. This work provided the first direct observations of Gareloi volcano since Robert Coats' four-day visit in 1945. Gareloi volcano is a stratovolcano 10 km by 8 km in diameter at its base with two summit craters separated by a narrow saddle. The southern crater is a 300-m-wide amphitheater formed by the partial collapse of its southern crater wall, and contains several active fumaroles. The northern crater is enclosed, although the intra-crater eruptive stratigraphy is abruptly interrupted by near-vertical local unconformities on the northwest wall, suggesting the occurrence of a sector collapse sometime in the past. Gareloi volcano is principally composed of intercalated trachytic lava flows, ranging from 0.5 m to more than 10 m in thickness. Two prominent valleys composed of thick lava flow packages on the SW flank are clearly U-shaped, suggesting that the oldest sequence of lava flows is of at least late Pleistocene age. Lavas erupted during the Pleistocene and Holocene range from basaltic trachyandesite to basaltic andesite in composition and contain plagioclase and clinopyroxene, with minor olivine, and rare hornblende. An explosive eruption in 1929 formed a SSE trending fissure of thirteen aligned craters, ranging from 80 to 1600 m in diameter. These craters extend from sea level up to the amphitheater of the southern crater (1160 m). Fall deposits from the 1929 eruption are interbedded with thin, laterally discontinuous pyroclastic flow deposits that are mainly limited to the island's southeastern flanks. Despite an abrupt change in color from light beige pumice clasts at the base of the 1929 fall deposit to black scoria at the top, the unit is homogeneous trachyandesite. Following the explosive phase of the eruption, 4 blocky trachyandesite lava flows emerged from craters below 600 m asl. All 1929 eruptive products contain plagioclase and clinopyroxene with scarce olivine. An effusive eruption during the 1980's from the center of the south crater amphitheater produced an elaborate blocky lava flow that extends 800 m in elevation down the SE flank. This lava flow is basaltic trachyandesite, and contains abundant coarsely sieved plagioclase phenocrysts with minor clinopyroxene and olivine. The majority of Gareloi lavas contain anomalously high concentrations of K, Na, and Rb and low concentrations of Mg compared to reported findings from other Aleutian lavas, including those of the western portion of the arc. This suggests that Gareloi magmas may be unique with respect to their source region and possibly storage conditions compared to other Aleutian volcanoes.

Browne, B. L.; Coombs, M.; Larsen, J.

2004-12-01

327

Insights Into the Origin of the Longest-lived Hotspot in the Pacific: Clues from the Tuvalus  

NASA Astrophysics Data System (ADS)

Insights Into the Origin of the Longest-lived Hotspot in the Pacific: Clues from the Tuvalus Anthony A.P. Koppers1, Jasper G. Konter2, Matthew G. Jackson3 1College of Earth, Ocean and Atmospheric Sciences, Oregon State University 2Dept. Geological Sciences, University of Texas at El Paso 3Dept. Earth Science, University of California, Santa Barbara Several prominent, long-lived volcanic chains stand out as bathymetric features on the Pacific plate. Several of these hotspot chains are long-lived, and thought to be fed by buoyantly upwelling mantle plumes. In the North Pacific, the Hawaiian hotspot has been continuously erupting for 85 Ma, and exhibits a sharp bend at ~50-47 Ma. Similarly, the Louisville hotspot, located in the South Pacific, exhibits volcanic activity going back to 76 Ma, but unlike Hawaii, the Louisville hotspot exhibits a more gradual change in orientation at ~50 Ma. The disparity between the traces of these two prominent hotspots in the Pacific, and the suggested plume source motion for Hawaii, as well as the observation that their respective hotspots traces are subducted at a relatively young age, prompted a thorough investigation of the Pacific hotpots in search of third, longer-lived hotspot that can be compared to Hawaii and Louisville. We suggest that the hotspot anchored to Rurutu, located in the Austral Islands, is the longest-lived (>100 Ma and up to 120 Ma at least) in the Pacific and will provide a third long-lived hotspot trace that will both inform upon and extend current plate motion models in the Pacific. Plate motion models predict that the ~50 Ma bend for the Rurutu hotspot is located where the Tuvalu Islands and Samoan Seamounts intersect, and the modeled trace of the Rurutu hotspot continues up through the Tuvalu and Gilbert Islands. Additionally, the Rurutu hotspot has a radiogenic Pb-isotopic (HIMU) signature, compared to the radiogenic Sr-isotopic signature of Samoa. Therefore, the unique geochemical signature of the Rurutu hotspot, together with its predicted hotspot track, make it relatively straightforward to test whether Tuvalu Islands represent the Rurutu hotspot right before its Hawaii-Emperor Bend. Initial radiogenic isotopic data obtained on deeply-dredged samples from the Tuvalu Islands showed that the islands belong to the HIMU geochemical taxonomy, which is consistent with an origin at the Rurutu hotspot. Here we present preliminary data on lavas from the 35-day dredging expedition aboard the R/V Roger Revelle (Expedition RR1310 from July 22 to August 25, 2013, starting in Guam and ending in Fiji). In total more than 25 separate seamounts and atolls were dredged in the Tuvalu Islands, and relatively fresh, dateable (by Ar-Ar) lavas were recovered from most dredges. Sample compositions range from olivine (+/- clinopyroxene)-rich basalts to plagioclase, amphibole and/or biotite-bearing trachytes/phonolites that were analyzed on-board by LIBS (Laser Induced Breakdown Spectroscopy).

Koppers, A. A.; Konter, J. G.; Jackson, M. G.

2013-12-01

328

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

329

Origin of the Alkaline Post-Erosional Volcanism on the Island of Mauritius  

NASA Astrophysics Data System (ADS)

Mauritius is the penultimate island of the Reunion mantle plume. Three episodes of eruptive activity has been recognized on this island: the Older Series, the Intermediate Series, and the Younger Series. The Older Series represent solidified lavas that form the shield volcano. The Intermediate Series and the Younger Series are categorized as post-erosional volcanism. Our new 40Ar/39Ar ages show that the construction of the Mauritius shield was well underway by 8.9 Ma. The shield-stage ended about 4.75 Ma, with the intrusion of trachytes (McDougall and Chamalaun, 1969). The Intermediate Series lavas subsequently erupted between 3.5 Ma and 1.66 Ma. This was followed by a hiatus of more than 0.6 million years. The hiatus ended with eruption of the Younger Series lavas, which continued until nearly the present. We found that the hiatus between the Intermediate and Younger Series was shorter than was previously believed, but appears to be real. While outcrops of the Intermediate Series are restricted to the southwestern area of the island, we found that the Intermediate Series lavas are present beneath Younger Series lava flows in drill cores throughout the rest of the island. The overall evolution of Mauritius resembles that of Hawaii, but there are some significant differences between them. The Older Series lavas on Mauritius are transitional between alkali basalt and tholeiite, different from the tholeiitic composition of shield lavas on Hawaii. Like Hawaii, the post-erosional volcanics have more 'depleted' isotopic signatures than shield-stage lavas. Unlike Hawaii, the post-erosional volcanism was interrupted by a long hiatus and the post-erosional lavas do not show strong silica undersaturation or strong enrichment in incompatible elements. Instead, the post-erosional lavas are only slightly less silica-saturated than the shield-building lavas and are less incompatible-element enriched. Our new isotope data show that the post-erosional lavas could be a mixture of the Reunion mantle plume and the depleted mantle source. We propose the during the early shield stage, melts from the Reunion plume rise into the cold oceanic lithosphere where they react with depleted mantle peridotite to form pyroxenite or eclogite veins. The post-erosional volcanism magmas form when the pyroxenite or eclogite veins are later melted by heat conducted into the lithosphere from the plume below. Mauritius is built on a 65 Ma old and 75 km thick oceanic lithosphere. Our simple calculations show that the conductive heat from the plume will raise the temperature of the lowermost 12 km of the lithosphere up to the pyroxenite solidus within 9 Ma, remelting the veins. Lithospheric flexure due to the construction of the next volcano in the volcanic chain (Reunion Island in the case of Mauritius) may dictate whether melts of the veins can reach the surface. Differences in post-erosional magma compositions between Mauritius and Hawaii may reflect the significantly thicker lithosphere beneath Hawaii and the greater heat flux provided by the Hawaiian plume.

Chen, C.; White, W. M.

2010-12-01

330

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

NASA Astrophysics Data System (ADS)

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

Kuehn, C.; Guest, B.

2012-12-01

331

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

332

The migration of volcanic Centers in the South Hangai volcanic area (Mongolia) in Late Mesozoic-Cenozoic  

NASA Astrophysics Data System (ADS)

The South Hangai volcanic area spans Southern and Central Mongolia. It is the most protracted intraplate magmatic area in Central and East Asia. The volcanic fields of the area extend from the southern boundary of Mongolia to the Hangai highland and its nearest surrounding. Systematic geochronological, paleovolcanological, and isotopic-geochemical studies of sequences of different age in different parts of this area allow us to reconstruct the volcanic evolution of this region. It was established that volcanic activity occurred in several pulses separated by short time intervals, with systematic migration of its volcanic centers. The following stages of volcanic activity were distinguished: end of the Jurassic to beginning of the Early Cretaceous (150-135 Ma), beginning to middle of the Early Cretaceous (135-120 Ma), middle to the end of the Early Cretaceous (120-115 Ma), end of the Early Cretaceous (115-102 Ma), end of the Early Cretaceous to the beginning of the Late Cretaceous (102-90 Ma), end of the Late Cretaceous (88-71 Ma), Paleocene-Early Eocene (62-47 Ma), Early Oligocene (38-31 Ma), Late Oligocene-Early Miocene (30-22 Ma), Middle Miocene (17-12 Ma), Late Miocene-Pliocene (10-2.1 Ma), Pleistocene (1.25-0.2 Ma), and Holocene (<0.01 Ma). Each stage, in turn, was subdivided into individual volcanic pulses responsible for the formation of individual volcanic fields or their groups. Thus, the history of the area is traced beginning in the Late Jurassic to the present time. Asynchronous volcanic pulses are typically spatially separated, which is related to the migration of magma conduits in the course of formation of volcanic area. A loop-like chain of volcanic fields more than 800 km long was detected. An observed shift in volcanic centers is considered as a trace of the South Hangai hot spot, which was expressed in the structure of lithosphere as the latter moved over the mantle plume. The volcanic rocks of the area comprise basanites, trachybasalts, predominant trachybasaltic andesites, as well as more rare phonotephrites, basalts, nephelinites, and carbonatites; trachyandesites, trachytes, and trachyrhyolites occur only in the Late Mesozoic. All rocks are characterized by distinctly expressed K specifics. Geochemical data indicate that main volcanic rocks of all age groups have similar abundance of incompatible trace elements. Their distribution patterns are close to that of OIB basalts, but differ in more fractionated REE pattern, higher LILE contents (Rb, Ba, Sr), and lower contents of such elements as Th, U, HREE (with the exception of Late Mesozoic rocks having higher HREE). All except the Late Mesozoic rocks display a prominent Ta-Nb maximum. In the Sr-Nd isotopic diagram, the rocks of the area define two trends. One trend involves the oldest Late Mesozoic rocks of the area. It strikes from EM-II type field to the PREMA. Cenozoic rocks fall between fields of PREMA and EM-I type. Geological (intracontinental) position of this volcanism and OIB-like composition of the volcanic products of the South Hangai area testify that its geodynamic setting was controlled by mantle plumes. This suggestion is consistent with paleomagnetic data, as well as with results of gravimetric and seismological studies, which revealed that the base of lithosphere is locally penetrated by asthenospheric fingers, which reach the crust bottom at a depth of about 50 km (Zorin et al., 2003). Thus, these fingers of hot mantle (asthenosphere) presumably controlled the volcanic activity of the region. Zorin Yu.A., Turutanov E.Kh., Mordvinova V.V. et al. The Baikal rift zone: the effect of mantle plumes on older structure // Tectonophysics. 2003. V. 371. P. 153-173.

Kudryashova, E. A.; Yarmolyuk, V. V.; Lebedev, V. A.; Kozlovsky, A. M.; Savatenkov, V. M.

2009-04-01

333

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

334

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

335

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

NASA Astrophysics Data System (ADS)

Results of a magnetic survey of the volcanic structure of Socorro Island in the Revillagigedo Archipielago are presented. Socorro is part of a group of seamounts and oceanic islands built by volcanic activity at the northern end of the Mathematician ridge and intersection with the Clarion and Rivera fracture zones. Subaerial volcanic activity is characterized by alkaline and pe