IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_1B-O1

Room B1

Date/Time: July 23 8:45-9:15

Generation processes of magmas of large pyroclastic eruption cycles at Aso volcano,
SW Japan
Katsuya Kaneko1 , Takehiro Koyaguchi2
1

Graduate school of Human and Environmental Studies, Kyoto University, Japan, 2 Earthquake
Research Institute, The University of Tokyo, Japan
E-mail: katsuya@gaia.h.kyoto-u.ac.jp

Activities of Aso volcano, which has the largest caldera in SW Japan, are characterized by four large pyroclastic
eruption cycles (LPEC) with 102 km3 DRE and many minor eruptions between them after 300 ka. In order tounderstand magma plumbing system of Aso volcano, we investigated petrological and geochemical features ofmagmas of three LPECs with 20-30 ky interval between 141 ka and 90 ka.
In each of the three LPECs, the magmas were ejected from a gravitationally stratified magma chamber with a silicic
magma overlying a mafic magma. Important geochemical features of them are summarized as follows. (1) Variation
diagrams of incompatible element concentrations show same trends throughout the three LPEC. (2) The silicic andmafic magmas of each of the LPECs have the same Sr isotope ratio. (3) The Sr isotope ratio of each of the LPEC
is slightly but clearly different from the other LPECs. (4) Whole-rock Ni concentration of a silicic magma is equal
to or larger than that of a mafic magma in each of the LPECs. (5) K2 O contents of melts of the mafic magmas
in the three LPECs are same and those of the silicic magmas are also same. These geochimecal features of the
LPEC magmas indicate that the magmas of each of the LPECs were produced by distinct generation events fromthe other LPECs (features 3) and that the silicic and mafic magmas of each of the LPECs were generated from thesame source (feature 1) whereas they have no parent-daughter relationship on fractionation (feature 4).
Magma genesis of each of the three LPECs can be explained qualitatively as follows. The magmas were generated
by partial melting of gabbro in lower crust due to injections of hot magmas from the mantle. The mafic magma
was produced by partial melting with high degree near the hot magmas, followed by fractional crystallization which
decreased Ni concentration of the mafic magma. The silicic magma was generated by partial melting of the samegabbro with low degree relatively far from the hot magmas. The two magma batches segregated upward from thesource areas and formed a shallow layered magma chamber.
For the above magma genesis, segregation processes of the magmas from the source areas are important.
Although the Sr isotope ratios are slightly different between the three LPECs, the compositional trends on
incompatible elements do not change (feature 1). If the source gabbros of the three LPECs have almost the same
concentrations of highly incompatible elements, their concentrations of melt are approximate indices of degree ofpartial melting regardless of detail petrological processes. In that case, the feature 5 suggests that the silicic andmafic magmas segregated from the source areas at a certain degree of partial melting, respectively. Estimation ofchemical composition of the source materials is needed to obtain degree of partial melting in magma genesis andphysical constraints of the segregation processes.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

63

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_1B-O2

Room B1

Date/Time: July 23 9:15-9:30

Evolution of a large, hot, restless rhyolitic magma system at Laguna del Maule, Chile
Brad S Singer1 , Nathan L Andersen1 , Martyn Unsworth3 , Wes Hildreth2 , Judy Fierstein2 , Carlos
Cardona4 , Brian R Jicha1 , Nick Rogers5
1

University of Wisconsin Madison, USA, 2 U.S. Geological Survey, USA, 3 University of Alberta, Canada,
4
Observatorio Volcanologica de los Andes del Sur (OVDAS), Chile, 5 Open University, U.K.
E-mail: bsinger@geology.wisc.edu

The Laguna del Maule Volcanic Field (LdM) is the foremost example of post glacial rhyolitic volcanism in theSouthern Andes. New 40Ar/39Ar age determinations indicate that silicic eruptions began with deglaciation atabout 19 to 20 ka and persisted until less than about 2 ka. These rhyolitic and rhyodacitic domes and coulees
total 6.5 km3 and encircle the 23 by 16 km lake basin. This is not only the greatest concentration of post-glacial
rhyolite in the Andes, but to our knowledge there has been no comparable rhyolite flare up this recently anywhere
else on Earth. Colinear major and trace element variation suggests these lavas share a common evolutionary
history (Hildreth et al., 2010). Moreover, geodetic observations (InSAR and GPS) indicate that LdM is inflating at
a rate of 30 cm per year since 2007. Modeling predicts that an expanding magma body located at 5 km depth
is driving this inflation (Fournier et al., 2010; LeMevel et al., 2013). The distribution of high-silica rhyolite lavas
erupted 12 km apart on opposite sides of the lake within a few kyr of each other, magnetotelluric data showing an
extensive zone of low resistivity at 5 km depth, and numerous local volcano-tectonic and long period earthquakes
shallower than 5 km, suggest that the magma intrusion may represent only a portion of a much larger rhyolitic body,potentially of caldera-forming dimensions. Significant andesitic and dacitic volcanism is absent from the central
basin of LdM since the early post-glacial silicic flare up began suggesting that a large body of low density rhyolite
has blocked mafic magmas from reaching the surface. Temporal trends in the major element compositions of thephenocryst-poor and glassy rhyolite domes show the most evolved was erupted early in the post-glacial period
followed by slightly lower-silica rhyolites. Two oxide thermometry reveals that the rhyolites tapped highly oxidized
and hot melt stored at 760 to 850 oC and fO2 at Ni-NiO plus 1.25 log unit. Fractional crystallization modeling using
the rhyolite calibration of the MELTS algorithm (Gualda et al., 2012) reproduces the high-silica compositions froma basaltic parental magma. The preferred model predicts 86 percent crystallization while cooling from 1290 to 800
deg. C at a depth of 5 to 8 km resulting in a water content of 4-6 wt. percent in the residual high-silica magma. The
suite of recent LdM lavas lies on a single evolutionary pathway supporting a cogenetic origin. Our observations andmodeling are are consistent with a large active, hot, crystal-poor, rhyolitic body of melt underlying much of the LdM
lake basin. This body has the potential to fuel either a modest explosive eruption, or perhaps a caldera-forming
eruption.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

64

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_1B-O3

Room B1

Date/Time: July 23 9:30-9:45

Changing the metaphor: from magma chamber to magma reservoir
Katharine V Cashman1 , Guido Giordano2
1

University of Bristol, UK, 2 Universita Roma Tre, Italy
E-mail: glkvc@bristol.ac.uk

Two basic tenets of volcanology are: (1) volcanic eruptions are driven by exsolution of volatiles and (2) eruptionsof near-liquidus (crystal-poor) melts require prior segregation/assembling of that melt into a single coherent body,or chamber. The first tenet derives from the prevalence of highly vesicular pyroclasts expelled by explosive
eruptions coupled with clear evidence for syn-eruptive volatile loss during magma ascent and eruption coupledwith the large volume change accompanying gas exsolution and decompression. The second is supported by thehomogeneity of erupted magma compositions and physical properties erupted during single events, particularly
during early phases of large eruptions. There is growing evidence, however, that both assumptions should be
re-evaluated. First, not all explosively erupted pyroclasts are highly vesicular. Second, accumulation of verylarge volumes of crystal-poor melt is difficult to reconcile with (a) thermal models that suggest that large coherent
volumes of eruptible magma should represent transients within the crust, (b) evidence that most magma is stored
as crystal-rich mush, and (c) new data showing that erupted magma batches may be assembled shortly prior to
eruption.

The inadequacy of conventional models is highlighted by mafic explosive caldera-forming eruptions, wherestorage of large melt volumes is thermally implausible and explosive ejection of large volumes of poorly vesicularmagma is puzzling. To explain these eruptions, we suggest that melt is stored within, and erupted directly from,magma reservoirs, where we use the term reservoir in the sense used for water-, oil- and gas-bearing systems.
This model extends the concept of eruptible melt assembled from a rigid sponge to the idea that the sponge itself
may feed eruptions. Tapping an over-pressured network of melt pockets within a rigid crystal framework provides
an attractive model for several reasons: (1) it does not require a large (thermally and physically unstable) bodyof molten magma to be assembled prior to an eruption, but instead allows erupted magma to be stored within athermodynamically stable crystal mush; (2) it allows syn-eruption tapping of large melt volumes from within thereservoir through permeable networks established both prior to and during eruption; (3) decompression of an
over-pressured reservoir provides a physical mechanism for magma ascent and eruption that does not rely on
ascent-related gas exsolution as the only driving force; and (4) a reservoir model can link the timing of caldera
collapse directly to the strength of the reservoir framework. We then examine the conditions under which thismodel may apply more generally to eruptions of crystal-poor melt.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

65

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_1B-O4

Room B1

Date/Time: July 23 9:45-10:00

Amphibole barometry in rhyolitic systems: an experimental recalibration and application
to the Cappadocian ignimbrites (Turkey).
Etienne Medard1 , Jean-Luc Le Pennec1 , Justine Francomme1 , Abidin Temel2 , Francois Nauret1
1

Laboratoire Magmas et Volcans, Universite Blaise Pascal / CNRS / IRD, 63000 Clermont-Ferrand,
France, 2 Hacettepe University, Department of Geological Engineering, 06800, Beytepe-Ankara, Turkey
E-mail: E.Medard@opgc.univ-bpclermont.fr

Classical Al-in hornblende barometry relies on the hypothesis that the Al content in amphibole only depends
on pressure, through the Tschermack substitution MgSi=VI AlIV Al. When temperature variations are taken into
account, however, Al content in amphibole also varies through the edenite substitution _Si=NaIV Al. Accurate Al-in
hornblende barometers thus needs either to take into account the effect of temperature, or exclude the edenite
substitution. We have performed a detailed experimental study of amphibole compositions as a function of pressure
and temperature in a rhyolitic pumice from the Kizilkaya ignimbrite (Central Turkey). Samples were analyzed using
optimized microprobe conditions in order to reduce uncertainties on Al content. Our data, combined with existing
litterature data, show a very good pressure-dependence of the Tschermack substitution (correlation coefficient of0.95) for amphiboles in equilibrium with biotite, plagioclase and quartz, reproducing experimental pressures withan average error of 36 MPa in the 100-400 MPa range. More experiments are underway to improve the accuracyof the barometer.
The Cappadocia area of Central Turkey superbly exposes a succession of Neogene dacitic to rhyolitic ignimbrites
and fallout deposits, recording 10 Ma of magmatic activity. This provides an excellent example of long-lived, lowfrequency magmatic system, with a low average magma production rate (about 0.001 km3/a), but short-lived
large eruptions (up to 300 km3 for the Cemilkoy ignimbrite). Extensive fieldwork campaigns, followed by AMS
measurements, geochronology and geochemistry allow us to refine the characterization of the various units andprovide a framework for petrological studies. Most of the ignimbritic and fallout units share a very similar phenocrystmineralogy (plagioclase + biotite + amphibole + magnetite + quartz). The low-variance phenocryst assemblage of
those units makes them perfect target for our new Al-in amphibole barometer.
Amphibole-plagioclase thermometry of the six ignimbrite units and one plinian fallout unit investigated so far indicate
relatively stable pre-eruptive temperatures between 700 and 760 °C for most units. Pressure estimates indicatethat amphiboles crystallized in the upper crust, between 9 and 14 km depth, hinting at a constant depth of magma
storage beneath the Cappadocian ignimbrite field over the last 10 Ma. Detailed investigation of various samples of
one of the more recent unit, the Kizilkaya ignimbrite, produces a tight unimodal pressure distribution at 260 MPa(9.8 km), with a maximum data dispersion of 20 MPa (0.8 km), lower than the 2 sigma uncertainty of the barometer.
If the erupted reservoir was homogeneous, as suggested by the absence of magma mixing, and amphibole waspresent in the entire reservoir, the aspect ratio of the erupted volume would be relatively low, with a minimum
diameter of 7.5 km for a 1.5 km maximum height.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

66

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_1B-O5

Room B1

Date/Time: July 23 10:00-10:15

Experimental study of origin of silicic magmas in the primitive, intra-oceanic Tongan arc
Tracy Rushmer, Raul Brens, Simon Turner, Trevor Green, John Adam
Macquarie University, Australia
E-mail: Tracy.Rushmer@mq.edu.au

The origin of felsic magmas in intra-oceanic arc settings has been an active topic of debate. Currently, there
are two main, but very different, processes suggested: fractional crystallization of basaltic magma and partial
melting of lower crustal mafic amphibolites. The physical conditions, such as pressure, melt reactions, rates
of melting and fluid dynamics of melt extraction differ markedly between these two mechanisms. A number ofsophisticated numerical models of lower crustal amphibolite melting have been developed over the past decade.
Such models are becoming widely invoked so there is a need for their applicability to be tested. Fonualei is unusual
amongst subaerial volcanoes in the primitive Tongan arc because it has erupted dacitic vesicular lavas, tuffs andphreomagmatic deposits for the last 165 years and makes for an excellent natural laboratory. All of the products are
crystal-poor and formed from relatively low viscosity magmas inferred to have crystallized at high temperatures.
Major, trace element and isotopic data, along with experimental data from partial melting of amphibolites and phaseequilibria experiments on the basaltic andesites from Late have been combined to assess competing models forthe origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution by closed-system crystal
fractionation and an origin by simple direct partial melting of amphibolite cannot reproduce the data either. Instead,we develop a model in which the dacites reflect mixing between two dacitic magmas, both products of fractional
crystallization of basaltic-andesite magma. Mixing was efficient because the two magmas had similar temperatures
around. Low-pressure equilibration of the magmas has been suggested by the Late phase equilibria experiments.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

67

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A1_1B-O6

Room B1

Date/Time: July 23 10:15-10:30

Mantle origin of basalts parental to tholeiitic differentiation products in the AndeanSouthern Volcanic Zone (SVZ): Confirmed by U-series disequilibria and10 Be/9 Be
M. Dungan1 , L. Cooper2 , J. Schaefer3 , C. Langmuir4 , O. Reubi5 , S. Turner4 , S. Escrig6 , J. Jweda3 , G.
Goldstein3 , B. Bourdon7
1

Dept. Geological Sciences, University of Oregon, Eugene OR, USA, 2 Inst. Geochemistry andPetrology, ETH-Zurich, Switzerland, 3 Lamont-Doherty Geological Observatory, Columbia University, NY,
USA, 4 Dept. Earth and Planetary Sciences, Harvard University, Cambridge MA, USA, 5 Geociences,
University of Lausanne, Switzerland, 6 Faculty Geosciences and Env., Ecole Polytechnique Federale deLausanne, Switzerland, 7 Ecole Normale Superieure, Lyon, France
E-mail: michael.dungan@unige.chSome dominantly mafic volcanoes in a restricted part of the SVZ (37-41°S) have erupted basaltic andesites (52-56% SiO2) with high Ti, Fe, and V that are the products of tholeiitic (TH) differentiation related to closed-systemfractional crystallization and a high proportion of plagioclase in the fractionating assemblage. These volcanoes
have not produced voluminous andesite with strong calc-alkaline (CA) affinities. Abundant CA andesite anddacite at volcanoes from 36.8-33°S are products of variable combinations of high-P (wet) fractionation and crustal
assimilation. The most pronounced TH differentiates are young, crystal-poor lavas from Lonquimay (38.45°S:
2-3 ka to 1989; 53-64 % Si02). The most evolved have FeO∗/MgO 10, comparable to oceanic TH andesites.
Major and trace element systematics eliminate open-system processes during differentiation, but the keys to linking
these evolved magmas directly to mantle-derived parental basalts are U and Pa excesses, which we argue must
be mantle signals. Lonquimay lavas have among the highest 10 Be/9 Be we have measured at six volcanoes inthis arc segment. High 10 Be must also be mantle-derived. Nine Lonquimay magmas analyzed for U-Th-Pa-Ra
activities have (231 Pa/235 U) 1.75-1.85, which is typical of basaltic magmas at Antuco, Llaima, Villarrica, Puyehue,and Osorno ( 1.5-2.1). These values extend to the most evolved Lonquimay magmas, verifying an absence of
crustal contributions. Minor contamination (5-20%) of some historic eruptive products at Llaima is recorded bymajor and trace elements and diminished U, Ra, and Pa excesses. Elemental U/Th and (238 U/230 Th) are lower at
Lonquimay (0.31, 1.15; 53-64 % SiO2) than in uncontaminated basalts at nearby Llaima (38.7°S; 0.37, 1.25),
in accord with other distinctions. Evolved Lonquimay magmas have Sr-Nd-Pb isotopic compositions typical ofmantle-derived, uncontaminated basalts in this part of the SVZ (e.g., 0.7039-0.7040), which are closely similar interms of Sr and Nd isotopes to local Miocene granitoids. Inferred origins of TH magma series by partial meltingof crustal lithologies under special circumstances, as has been proposed for Zao and Azuma volcanoes in the NE
Japan Arc (Tatsumi et al., 2008; Takahashi et al., 2013), cannot be extrapolated to all TH differentiation trends incontinental arcs.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

68

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_1B-O7

Room B1

Date/Time: July 23 10:45-11:15

Tomography of crustal magma bodies
Jonathan M. Lees
University of North Carolina, Chapel Hill, USA
E-mail: jonathan.lees@unc.edu

Because of the simplicity of the blob model for magma accumulation in the crust, geophysicists have long been
intrigued by the potential signature a large magma body in the subsurface seismic imaging of volcano magma
chambers is now relatively common and has been explored at numerous sites. Distinct and clear seismic evidenceof large accumulations of molten material has proven elusive, perhaps as a result of poor resolution and lack of
critical data, or because the magma plexus does not conform to pre-conceived notions. Furthermore, technicaldifficulties in the inversion schemes associated with noise suppression may be masking larger anomalies expected
for high percent melt anomalies. Inherent blurring of seismic images, though, prevents detailed structural featuresfrom being isolated and associated with specific eruptive events. Examples illustrating inversion results at large
volcanoes like Kliuchevskoi and the recently erupted Sheveluch appear to be feeding from deep within the mantle,
well below crustal reservoirs where magma chambers are commonly imaged at eruption centers like Mt. St. Helens,Mt. Fuji, and Unzen. Typical crustal magma anomalies reside in the 7-15 km depth range and extend over from
one to tens of km in radius. Attenuation analysis holds some promise for advances in estimation of partial melt
percentages, although Q (quality factor) is elusive and difficult to constrain.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

69

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_1B-O8

Room B1

Date/Time: July 23 11:15-11:30

Deep magma feeding system of Fuji volcano, Japan
Eiichi Takahashi1 , Junichi Nakajima2 , Kenta Asano1
1

Department of Earth and Planetary Sciences, Tokyo Institute of Technology„ Japan, 2 Research Center
for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Japan
E-mail: etakahas@geo.titech.ac.jp

Fuji volcano is the largest among Japanese Quaternary volcanoes.

For the last 100kya, Fuji has erupted

dominantly basalt magma, but its eruption style changed (from debris flow and tephra dominant Ko-Fuji or OlderFuji, to lava flow dominant Shin-Fuji or Younger Fuji) at 15 kyaBP. The incompatible trace element compositionof the magma changed abruptly between Ko-Fuji and Shin-Fuji. The origin of the voluminous yet monotonousbasalt production and the simultaneous changes in volcanic style and magma chemistry in Fuji volcano have been
discussed but remain unanswered.
Here we report the seismic tomographic images of Fuji volcano for the first time, which reveal the existence ofstrong upwelling flow in the mantle and its connection to the voluminous lower crustal magma chamber. The velocity
perturbations in the upper mantle and lower crust beneath Fuji volcano are much greater than other volcanoes in
Izu arc (e.g., Hakone, Izu Oshima) and those in North Japan arc (e.g., Azuma, Iwate) suggesting higher degree ofmelting or larger volume of magma storage. Magma chamber is imaged in the lower crust 20-40 km depth and is
wider in NW section but is narrower in EW section.
We also carried out high-pressure melting experiments on Fuji Basalt (Hoei-IV, AD1707) and demonstrate thatits main magma chamber is located at ca.25km depth (Asano et al., this conference). The chemistry of Fuji
magma is buffered by a lower crustal AFC magma chamber located at 25-35km depth. Very frequent low frequency
earthquakes just above the magma chamber may be due to the injection of basalt magma and/or fluids (Ukawa,
2007). The total lack of silica-rich rocks (basaltic andesite and andesite) in Fuji volcano must be due to the special
location of the volcano. The plate boundary between the Eurasia plate and the subducting Philippine Sea plate
is located just beneath Fuji volcano ( 5 km depth). Large tectonic stress and deformation associated with theplate boundary inhibit the survival of a shallow level magma chamber, which would allow the evolution of basalt tosilica-rich magma (as observed in nearby volcanoes, e.g., Hakone, Izu Oshima).
The change in volcanic eruption style may be understood by assuming the existence of a glacier (or thick ice-cap)
during Ko-fuji. A landslide occurred at 17 kyBP and a large part of its volcanic edifice, including the summit of
Ko-fuji, has been lost. This landslide would have been triggered by melting of the glacier and should have caused
a significant deloading effect on the magma feeding system. Very large magma production with a peak at 10 kyBP
might be explained by this deloading effect.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

70

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_1B-O9

Room B1

Date/Time: July 23 11:30-11:45

Upper crustal structure of Newberry Volcano from P-wave tomography and finite
difference waveform modeling
Emilie Hooft1 , Matthew Beachly1 , Douglas Toomey1 , Gregory Waite2
1

Department of Geological Sciences, University of Oregon, USA, 2 Department of Geological andMining Engineering and Sciences, Michigan Technological University, USA
E-mail: emilie@uoregon.edu

We show that seismic tomography combined with waveform modeling can constrain the dimensions and melt
content of a magma body in the upper crust at Newberry Volcano, Oregon, USA. We obtain a P-wave tomographic
image by combining travel-time data collected in 2008 on a line of densely spaced seismometers with active-source
data collected in the 1980s. The tomographic analysis resolves a high-velocity intrusive ring complex surroundinga low-velocity caldera-fill zone at depths above 3 km and a broader high-velocity intrusive complex surrounding acentral low-velocity anomaly at greater depths (3-6 km). This second, upper-crustal low-velocity anomaly is poorly
resolved and resolution tests indicate that an unrealistically large, low-velocity body representing ∼60 km³ of melt
could be consistent with the available travel times. This uncertainty in magma volume is largely due to wavefront
healing of the first arriving seismic phase.
Here we show that we can more accurately constrain the magmatic system when, in addition to using arrival timesof the primary phase, we also include amplitude information as well as the timing and amplitude of secondary
phases that have interacted with the magmatic system.

The densely spaced 2008 Newberry seismic data

exhibit low amplitude first arrivals and an anomalous secondary P wave phase originating beneath the caldera.
Two-dimensional finite difference waveform modeling through the tomographically obtained velocity model does
not reproduce these observations. To reproduce these phases, we predict waveforms for models that include
synthetic low-velocity bodies and test possible magma chamber geometries and properties. Three classes ofmodels produce a transmitted P-phase consistent with the travel time and amplitude of the observed secondary
phase and also match the observed lower amplitude first arrivals. These models represent a graded mush region,a crystal-suspension region, and a melt sill above a thin mush region. The three possible magma chamber models
comprise a much narrower range of melt volumes (1.6-8.0 km³) than could be constrained by travel-time tomography
alone.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

71

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_1B-O10

Room B1

Date/Time: July 23 11:45-12:00

The New Tolbachik Fissure Eruption in Kamchatka, 27.XI 2012-2013. Earthquakes
Preceding the Event and First Estimates of its Duration
Sergey A. Fedotov2 , Alexey L. Sobisevich1 , Lidia B. Slavina1
1

Institute of the Physics of the Earth, Russian Academy of Sciences, Russia, 2 Instiute of Volcanology
and Seismology, Far East Branch, Russian Academy of Sciences, Russia
E-mail: alex@ifz.ru

Magmatic feeding system and mechanisms of activity of the Kliychevskaya Group of Volcanoes (KGV) areconsidered. The KGV is one of the most significant volcanic center in subduction zones. It is located in thenorthern part of the Kuril-Kamchatka volcanic belt and consists of: the Kliychevskoi basaltic stratovolcano (thelargest erupting volcano in Eurasia), the Bezymianny andesite volcano, the basaltic Hawaiian-type Plosky Tolbachik
volcano (PT) with summit caldera, the Tolbachinskaya zone of cinder cones and fissure eruptions (TZ) and the
Ushkovsky volcano.
In 1975-1976 the Great Tolbachik Fissure Eruption (GTFE) in TZ produced 2.2 km3 of basalts.
Development of the New Tolbachik Fissure Eruption (NTFE) started 27.XI 2012 is analysed, including: the locationof the feeding fissure and the two eruptive centers on it in TZ; effusive nature of eruption with lava flows of liquidalumina basalts produced during the first two months covering an area of more than 27 km2.
Remarkable features of the process of local seismic activation preceding the NTFE are considered. Increase in
the number of earthquakes under PT (at a depth of about 4 km near the peripheral magmatic chamber of PT) has
started in mid-2012 and continued up to the beginning of NTFE. It has culminated during the last two days before
NTFE in the form of a large swarm of small earthquakes that propagated towards to the surface. The swarm was
the forerunner of the impending eruption and highlighted the position and depth of magma origin of the NTFE .
It is demonstrated, that during the first months, until January 20, 2013, NTFE was the large fissure lava eruption,
slowly decreasing over time. Quantities of produced material and duration of such eruptions are subject fortheoretical estimations. The first rough estimates have been given for the NTFE on December 15, 2012 andon January 22, 2013 respectively.
The initial forecast has assumed continuation of the NTFE while the amount of volcanic products will not exceed
the amount of alumina basalts (500 million tons) potentially accumulated in the peripheral magmatic chambers of
the PT volcano. Thus in this case, an exponential decay of the eruptive activity could last up to 110 days.
According to preliminary estimates of volumes of erupted lava in the NTFE, available by mid-January 2013,
the number of produced basalts significantly exceeded the volume of excess magma, potentially located in the
peripheral magma chamber of the PT volcano. For the next forecast the redundant magma volume accumulated
up to the NTFE in magmatic systems throughout the KGV (2,100 million tons) has been considered as a possible
upper limit of the volume of eruptive products. In this case, the volume of lava produced by the NTFE may exceed
1 km3 and reach the amount of lava erupted in the Southern breakthrough of the GFTE in 1976. Thus, the NTFE
may continue for up to 300 days.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

72

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_1B-O11

Room B1

Date/Time: July 23 12:00-12:15

Low velocity structure beneath Kyushu island, Japan, inferred from Receiver functions.
Takuya Ueda1 , Hiroshi Takenaka1 , Takumi Murakoshi2 , Taro Okamoto3
1

Kyushu University, Japan, 2 National Defense Academy of Japan, Japan, 3 Tokyo Institute ofTechnology, Japan
E-mail: 2SC12120N@s.kyushu-u.ac.jp

The Kyushu island, Japan, there are many active volcanos, for example, Aso, Kirishima, and Sakurajima volcanosalong with the volcanic front, and Unzen volcano located far from the volcanic front. Information on structures
beneath active volcanos is very important, and many researchers have revealed the structures under the active
volcanos. Receiver function analysis is a useful tool to image the seismic velocity structures. We apply it toimage the Kyushu volcanic area. In this study, we use teleseismic records from Hi-net and F-net seismic stations
in Kyushu, which are supplies by the National Research Institute for Earth Science and Disaster Prevention.
When those seismic stations are located at the top or in the sedimentary layer, the records include strong effectof reverberation within the sedimentary layer, which makes the image of the structure unclear. To overcome
this problem, we exploit the modified S-wavevector receiver functions (SWV-RFs) [Takenaka and Murakoshi,
2010, SSJ]. The SWV-RFs is derived by deconvoluting the upgoing S-wave component with the upgoing P-wave
component of the records [Reading et al., 2003, GRL]. For suppressing the sedimentary layer effect, we virtually
move the seismic sensor to the top of the basement layer, and calculate the SWV-RFs at that location [Takenaka
and Murakoshi, 2010]. This method needs the structure model from the surface to the sensor location. We employ
the Integrated Velocity Structure Model by the Headquarters for Earthquake Research Promotion. We take several
cross sections in Kyushu island to map the calculated SWV-RFs. We then interpret the continental Moho, thePhilippine Sea Plate and low velocity regions in the mapped SWV-RFs. It can be seen that characteristic low
velocity regions beneath the crust around volcanic area, some of which may be related to magma. We will alsomodel some SWV-RF sections by the 2.5-D finite-difference method [Takenaka and Okamoto, 2012, InTech] toconfirm our imaging results.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

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IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_1B-O12

Room B1

Date/Time: July 23 12:15-12:30

Why we need ultra-deep drilling into IBM arc crust?
Yoshihiko Tamura1 , Yoshiyuki Tatsumi2
1

IFREE, JAMSTEC, Japan, 2 Kobe University, Japan
E-mail: tamuray@jamstec.go.jp

At first glance, intra-oceanic arcs do not appear to be the right place to study the production of andesitic magmas,
because (1) modern magmatism at the intra-oceanic Izu-Bonin-Mariana (IBM) arc is bimodal, with basalt andrhyolite predominating (Tamura and Tatsumi, 2002); and (2) turbidites sampled during Ocean Drilling Program(ODP) Leg 126 in the Izu-Bonin arc, which range in age from 0.1 to 31 Ma, are similarly bimodal (Gill et al., 1994),
suggesting that the bimodal volcanism has persisted throughout much of the arc’s history. Moreover, such bimodal
magmatism is not unique to the Izu-Bonin arc, with the 30-36.5 degrees S sector of the Kermadec arc, anotherexample of an intra-oceanic arc, also exhibiting it (Smith et al., 2003; 2006; Wright et al., 2006). So why and how
do we study the intra-oceanic arcs to solve the ’andesite problem’?
Closer inspection of the IBM arc remarkably reveals the presence of a significant volume of middle crust withseismic velocities of 6.0-6.8 km/s throughout the entire arc (Calvert et al., 2008; Kodaira et al., 2007a,b; Kodaira
et al., 2008; Kodaira et al., 2010; Takahashi et al., 2007; Takahashi et al., 2008; Takahashi et al., 2009). This isremarkable because these velocities are characteristic of a wide range of intermediate-felsic plutonic/metamorphic
rocks (Christensen and Mooney, 1995; Behn and Kelemen, 2003, Behn and Kelemen, 2006) and are similar to themean velocity of andesitic continental crust, such material would not be expected to be present on the basis of
the bimodal volcanism. Moreover, this crust is presently thickest beneath basaltic volcanoes and thinnest beneathrhyolitic volcanoes (Kodaira et al., 2007), which is another enigma.
One possible way to understand this phenomenon is to investigate arc crustal sections exposed on land in order toexamine the relationship between volcanic and plutonic rocks and the generation of andesitic magmas, as exposedarc crustal sections typically include middle crust composed of diorite to tonalite to granodiorite (e.g. Kawate andArima, 1998; Busby et al., 2006; DeBari and Greene, 2011). However, any continental crust we observe on thesurface of the Earth will have experienced deformation, metamorphism, and been otherwise processed, perhaps
several times from its creation in subduction zones to the present day, thus overprinting, resulting in the loss of, key
information that can provide clues to its genesis.
’Ultra-Deep Drilling into Arc Crust’ is the best way to sample unprocessed juvenile continental-type crust in order toobserve the active processes that produce the nuclei of new continental crust, and to examine the nature of juvenile
continental crust being generated at intra-oceanic arcs.

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74

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_1B-O13

Room B1

Date/Time: July 23 15:00-15:30

Along-arc geochemical and isotopic variations in Javanese volcanic rocks: constraints
on the crustal architecture of the Sunda arc, Indonesia
Heather Handley1 , Janne Blichert-Toft2 , Simon Turner1 , Colin Macpherson3 , Ralf Gertisser4
1

GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109,
Australia, 2 Laboratoire de Géologie de Lyon, Ecole Normale Superieure de Lyon, 69007 Lyon, France,
3
Department of Earth Sciences, University of Durham, Durham, DH1 3LE, UK, 4 School of Physical andGeographical Sciences, Keele University, Keele, ST5 5BG, UK
E-mail: heather.handley@mq.edu.au

Understanding the genesis of volcanic rocks in subduction zone settings is complicated by the multitude ofdifferentiation processes and source components that exert control on lava geochemistry. However, throughdetailed studies of individual magmatic systems it is possible to identify and establish the relative importanceand contributions of various potential source components and differentiation processes that modify composition,such as crustal contamination. Geochemical and isotopic constraints on the ’crustal’ end members involved inassimilation at individual volcanoes, can then be combined to gain insight into the nature of the arc crust on an
island- or arc-scale. Along-arc changes in lava geochemistry have long been recognised on Java in the Sundaarc, Indonesia, but debate still prevails over the cause of such variations and the relative importance of shallow(crustal) versus deep (subduction) contamination. We present new Pb isotope data for Javanese volcanoes, which,when combined with our recently published geochemical and isotopic data of Javanese volcanic rocks and resultsfrom other detailed geochemical studies, elucidate the potential changing nature of the arc crust and its control
on lava chemistry. In 207 Pb/204 Pb-206 Pb/204 Pb isotope space the Javanese volcanic data reveal two distinct trends.
One trend, which consists of one West Javanese and the Central Javanese volcanoes, exhibits a wide range in
207

Pb/204 Pb at relatively constant 206 Pb/204 Pb (a steep positive correlation) and is attributed to strong control bycrustal assimilation processes. The second trend shows a shallower positive correlation between 207 Pb/204 Pb
and 206 Pb/204 Pb and is consistent with source contamination by local sedimentary material on the down-goingplate. Sr isotope ratios of volcanic rocks generally increase from West to Central Java, showing a wide range
within individual volcanic centres and broadly correlating with inferred crustal thickness implying a strong, shallow
level control on isotopic composition. However, East Javanese volcanic rocks show significantly lower Sr and Pb
isotopic ratios and extremely restricted isotopic variation at individual volcanoes. Key trace element ratios combinedwith radiogenic isotopic data of Javanese volcanoes reveal three distinct trends, which roughly equate with thegeographical boundaries West, Central and East Java. These results provide evidence for major transitions in thecrustal architecture of Java.

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75

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_1B-O14

Room B1

Date/Time: July 23 15:30-15:45

Andesites and the continental crust
Jon P Davidson
Durham University, UK
E-mail: j.p.davidson@durham.ac.uk

The relationship between andesites and the continental crust occupies a special place in petrology.
Compositionally andesites resemble continental crust both in intermediate silica contents, and in their LREE
enrichment. The relationship between andesites and subduction is also compelling given that continental crust
seems to have formed predominantly at subduction zones. The association suggests that if we can establish how
andesites form we can, as a consequence, determine the origin of the continental crust.
One possibility is that andesites are the natural result of density filtering by the crust. Such a suggestion gains
support from consideration of tectonic environments such as the Andes. Here subduction related magmas risethrough continental crust at composite volcanoes, while to the east on the Altiplano, extension permits shoshonitic
and more mafic magmas to pass through the crust. The composite volcanoes of the Andes therefore seem tobe filtered or stewed in the crust. But there is perhaps a circular argument here if we are suggesting that the
manufacture of andesites actually requires a crustal filter. It may well be that andesites are self filtering.
The hydrous character of andesites at subduction zones means that they will be subject to prolific crystallisation.
For mafic primary or primitive magmas, olivine crystallisation will be promoted. This will lead to rapid decreases in
MgO and NiO and increases in silica, generating major element compositions similar to the crust. Nevertheless,olivine crystallisation alone cannot rotate REE patterns to match those of the crust. For this, and to satisfy Dy Yb
relationships, the involvement of amphibole appears to be required (either as a fractionate or a residue). Amphibole
crystallisation will similarly leverage silica contents to intermediate values. Note that some models advocating an
adakitic origin for the crust (high pressure wet melting of basalt) are more consistent with garnet fractionation,
which, in turn, does not fully satisfy the Dy Yb systematics of continental crust. So wet andesites may fractionate
olivine at first, but the most significant differentiation will likely involve plagioclase and amphibole fractionation,
producing continental crust like compositions via multiple mixing and cannibalisation of magma batches. Magmas
will contain significant crystal cargoes, and will be capable of filtering via both density and rheology.
A witness to this processs is provided by the crystal cargo itself, especially plagioclase, for which examinationof crystal core rim stratigraphy, in particular Sr isotopes and dissolution surfaces, invariably provides evidence ofmultiple open system cycling and mixing.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

76

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_1B-O15

Room B1

Date/Time: July 23 15:45-16:00

Sr-Nd-Pb-Hf isotopic study of Cretaceous to Paleogene granitic rocks from northeast
Japan
Nobutaka Tsuchiya1 , Shinichi Kagashima2 , Yuka Hirahara3 , Toshiro Takahashi3 , Ryoko Senda3 , Qing
Chang3 , Takashi Miyazaki3 , Jun-Ichi Kimura3
1

Department of Geology, Faculty of Education, Iwate University, Japan, 2 Department of Earth andEnvironmental Sciences, Yamagata University, Japan, 3 Institute for Research on Earth Evolution, Japan
Agency for Marine-Earth Science and Technology, Japan
E-mail: tsuchiya@iwate-u.ac.jp

The Japanese Islands represent a segment of a 500 Ma old subduction related orogen developed along the western
Pacific convergent margin, and most tectonic units are composed of late Paleozoic to Cenozoic accretionary
complexes and their high P/T metamorphic equivalents (e.g., Maruyama, 1997; Isozaki et al., 2010). Maruyama(1997) described the formation of the Japanese Islands has been taken as the standard model for an accretionary
orogeny. He also stated that the most important cause of the orogeny is the subduction of an oceanic ridge, bywhich the continental mass increases through the transfer of granitic melt from the subducting oceanic crust to theorogenic belt. On the other hand, Jahn (2010) described that the subduction-accretion complexes consisting of
granitic and sedimentary rocks in southwest Japan are composed mainly of recycled old continental crust.
We present Sr-Nd-Pb-Hf isotopic ratios for granitic rocks in northeast Japan. Kagami et al. (1999) describedthat the Honshu Arc can be divided into three groups based on their Sr-Nd isotope characteristics: the Kitakami,
North (Abukuma belt), and South (Ashio/Mino belts) Zones, in order of increasing Sr isotopic enrichment, with
Nd isotopic depletion from NE to SW. Among these zones, the Kitakami zone is characterized by the occurrenceof adakitic rocks probably related to Lower Cretaceous ridge subduction. Sr-Nd-Pb-Hf isotopic study are madefor granitic rocks from the Kitakami belts (Kitakami Mountains), the Abukuma belts (Shirakami Mountains, Obonai
area, Taihei Mountain, Sekiryo Mountains, and Abukuma Mountains), and the Ashio/Mino belts (Okutone area,
Tadami area, Okutadami area, Taisyaku Mountains, and Ashio Mountains). Newly isotopic data from these granitic
rocks show increasing enrichment of crustal component in order of the Kitakami, Abukuma, and Ashio/Mino belts.
Multi-isotope plots of these rocks indicate that the trend in variation could result from the mixing of depleted andenriched components. The depleted components are likely to originate from the magmatic flux related to theLower Cretaceous ridge subduction. On the other hand, the enriched components probably originate from the oldcontinental fragments of proto-Japan initially developed along the southeastern margin of the South China Block.

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77

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_1B-O16

Room B1

Date/Time: July 23 16:00-16:15

Geophysical Investigations of Cerro Negro Volcano, Nicaragua: Insights on magmatic
plumbing systems
Patricia G MacQueen, Glyn Williams-Jones, Jeffrey M Zurek
Department of Earth Sciences, Simon Fraser University, Canada
E-mail: pmacquee@sfu.ca

Cerro Negro near Léon, Nicaragua is a very young (163 years), relatively small basaltic cinder cone volcanothat has been unusually active during its short lifespan (recurrence interval 6-7 years). Multiple explosive eruptions
have deposited significant amounts of ash on Léon and the surrounding rural communities. While there have been anumber of studies investigating the geochemistry, stress regime, and hazard implications of the volcano, subsurface
structures have only been studied by shallow soil gas surveys. These studies have raised several questions as tothe proper classification of Cerro Negro and its place in the regional stress regime. Should Cerro Negro, very young
but very active, be considered a short lived, low intensity polygenetic cinder cone on the Cerro La Mula-Cerro Negroalignment, or as the early stages in the development of a longer lived, higher intensity stratovolcano? Is Cerro
Negro a separate entity from neighboring Las Pilas-El Hoyo with a separate magma source, or merely the newest
vent on the Las Pilas-El Hoyo volcanic complex? While surface features at Cerro Negro follow definite alignmentsthat correspond to the regional stress regime, is this pattern continued and controlled by similar mechanisms at
depth? In order to address these questions, we collected both gravity and magnetic data on Cerro Negro volcano
in February and March of 2012 and 2013 in an attempt to delineate deep structures at the volcano. The gravity
survey included a tight grid on Cerro Negro itself, an irregular grid of several stations on Las Pilas-el Hoyo, andseveral distal stations forming a ring approximately 24 x 31 km around Cerro Negro . Magnetic data was collectedalong several profiles within 3 km of Cerro Negro.
Analysis of gravity and magnetic data have revealed a marked NE-SW regional gravity trend, a 5 km3 positive
anomaly beneath the neighboring Las Pilas-El Hoyo volcanic complex, a NNW trending linear anomaly beneaththe Cerro Negro-Cerro La Mula alignment, and a 3 km3 positive anomaly to the southeast of Rota volcano. Thesefindings suggest that eruptions at Cerro Negro may be tapping a large magma reservoir beneath Las Pilas-El Hoyo,
implying that Cerro Negro should be considered the newest vent on the Las Pilas-El Hoyo volcanic complex. As
such, it is possible that the intensity of volcanic hazards at Cerro Negro may eventually increase in the future toresemble those pertaining to a stratovolcano.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

78

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_1B-O17

Room B1

Date/Time: July 23 16:15-16:30

Nature of Sub-volcanic Magma Chambers in Emeishan Province, China: Evidence from
Quantitative Textural Analysis of Plagioclase Megacrysts in the Giant Plagioclase
Basalts
Lilu Cheng, Ling Zeng
China University of Geosciences, China
E-mail: chenglilu1027@163.com

Sub-volcanic magma chambers in the Crust might be a widespread component of large flood provinces, howeverthese magma processes in Emeishan large igneous province (LIP) remain unclear.

The Giant Plagioclase

Basalts (GPB) which can reveal the sub-volcanic magma chambers’ presence widely occur in Emeishan LIP. The
plagioclase megacrysts are 2 -5 mm , have compositions ranging from An53 to An60 and are generally weakly
zoned, The GPBs are texturally classified into isolated coarest crystals(IC) and glomeroporphyritic crystals(GC)type. A completed Permian basaltic section in the middle Emeishan LIP is studied in order to further constrain
GPBs’ petrogenesis and sub-chambers magma process. The IC basalts and the GC basalts in this section areinterlayered, the GPB flows can be divided into five flows which are GC1 flow, IC1 flow, GC2 flow, IC2 flow andGC3 flow. The crystal size distributions(CSD) of these megacrysts mostly plot as almost straight lines on a classic
CSD diagram, and the characteristic lengths of the five flows are 1.54 mm, 2.99 mm, 1.70 mm,3.22 mm and 1.86
mm, respectively. Taking a simple assumption of plagioclase steady-state magma chamber models and simple
continuous growth at the rate of 10-10 mm/s, the results suggest the possible every flow magma residence times of
489.2 years, 946.7 years, 538.7 years, 985.9 years and 588.3 years, respectively. Using the thickness and durationof IC1 flow, GC2 flow and IC2 flow, we may get the megacryst-bearing magma eruption rate of 0.422 m/year,0.324
m/year and 0.350 m/year. However, the lack of crystals smaller than 2 mm suggests that coarsening may have been
involved because of temperature cycling. A possible history is as follows:(1) magmas derived from mantle start toaccumulate at crust, forming the sub-volcanic chambers;(2) the glomeroporphyritic crystals grow in higher-levelstatic sub-volcanic chamber, the isolated coarest crystals grow in lower-level sub-volcanic chamber which is filled
gradually with hot, new magma and coarsen in response to thermal cycling;(3) the five flows of GC type and IC type
are the products of volcanic magma cycle.If we take the duration of IC1 crystallization as aphyric basalts eruption
duration, we can give a 23.9-m-thick aphyric basalts flow rate of 0.049 m/year. Quantitative integration of texturaland field information for igneous rocks can contribute to our understanding of physical processes and petrogenesis
in the sub-volcanic chambers of Emeishan LIP.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

79

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_1B-O18

Room B1

Date/Time: July 23 16:30-16:45

Across arc variation of strontium isotopic and K2 O composition in the quaternary
volcanic rocks from west java: evidence for crustal assimilation and the involvement of
subducted components
Mirzam Abdurrachman1 , Masatsugu Yamamoto2 , Daizo Ishiyama2 , I Gusti Bagus Eddy Sucipta1 , EmmySuparka1 , Idham Andri Kurniawan1 , Putri Amalia1
1

Geological Engineering, Bandung Institute of Technology, Bandung, Indonesia, 2 Department ofGeosciences, Geotechnology, and Material Resource Engineering, Akita University, Japan
E-mail: mirzam@gc.itb.ac.id

Many studies in the West Java suggested that Sr isotopic ratios and K2 O contents increase from the volcanic front
to the back arc side volcanoes. In detail, however, when the normalized of K2 O wt% to SiO2 =55 wt% from theWest Java Arc (WJA) are closely examined and compared with the Northeast Japan Arc (N 38°-41°, NEJA), the
WJA shows rough across-arc variation and more diverse in K2 O especially in the volcanic front. For this reason, Sr
isotope, major and trace elements data from 54 volcanic rocks of Quaternary volcanoes from WJA were collected
and compared to 46 volcanic rocks of Quaternary volcanoes from NEJA. The increasing K2 O and decreasing of87

Sr/86 Sr ratios with distance from trench have been found in NEJA but there are rough and no across arc variationof K2 O and Sr isotopic ratios in WJA. Our study shows that the across arc variation of magma chemistry on the
WJA can be explained by crustal assimilation and the involvement of subducted components (e.g. altered oceanic
crust and subducted sediment).

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80

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P2_1B-O19

Room B1

Date/Time: July 23 17:00-17:30

Structure of the Tongariro Volcanic Complex Magmatic System, New Zealand: Insightsfrom Magnetotelluric Imaging
Graham J Hill1 , T. Grant Caldwell1 , Yasuo Ogawa2 , Hugh M Bibby1 , Stewart L Bennie1 , Edward A
Bertrand1 , Harry Keys3
1

GNS Science, New Zealand, 2 Tokyo Institute of Technology, Japan, 3 Department of Conservation,
New ZealandE-mail: g.hill@gns.cri.nz

A key step in understanding a volcanic system is the determination of the location and size of the magma reservoir
beneath the volcano. The electrical conductivity or resistivity of a magma reservoir containing an interconnected
melt fraction will be much more conductive than the surrounding host rock. Here we use the results from 136
magnetotelluric (MT) measurements to determine the location and structure of the magmatic system beneath the
Tongariro Volcanic Complex. 3-D inverse resistivity modelling of the MT data shows a narrow vertical conductive
zone located beneath Mount Ngauruhoe linked to but offset from a larger more conductive region about 4 km
beneath the surface centred under the north-eastern flanks of Mount Tongariro. The location of the recent eruption
and seismicity prior to the eruption occurred at the margin of this larger conductive region.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

81

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P2_1B-O20

Room B1

Date/Time: July 23 17:30-17:45

Magnetotelluric images of magma distribution beneath Volcan Uturuncu, Bolivia
Matthew J. Comeau1 , Martyn J. Unsworth1 , Faustino Ticona2
1

University of Alberta, CANADA, 2 Universidad Mayor de San Andres, Bolivia
E-mail: mjcomeau@ualberta.ca

The Altiplano-Puna magma body (APMB) is recognized as one of the largest magma bodies on Earth. Geophysicalstudies reveal a thin ultra-low velocity zone at a depth of about 20 km that is inferred to be 14-27 percent partial
melt and less than 1 km thick (1,2). The APMB was also noted to have a very low resistivity by early MT studies.
Furthermore, the APMB is spatially associated with the major ignimbrite eruptions of the Altiplano-Puna Volcanic
Complex (APVC). Volcan Uturuncu in Southern Bolivia is located near the centre of the APMB and has been
inflating over the past two decades at rates of 1-2 cm/year. It has been suggested that this represents a location
where pluton formation may be occurring in real time (3,4).

The PLUTONS project is making a comprehensive set of geological and geophysical measurements to define the
distribution of magma beneath Volcan Uturuncu, and also to understand the eruptive history. This has included
geological studies, seismic monitoring, and detailed geodetic measurements. Magnetotelluric (MT) data use
passive electromagnetic signals to image subsurface resistivity from the surface to the upper mantle. Electricalresistivity is an important property because it is sensitive to the presence of partial melt and hydrothermal fluids inthe crust.

An extensive MT data set was collected at Volcan Uturuncu in 2011 and 2012. Broadband MT data in thefrequency band 0.001-300 Hz were collected at 149 stations. These data are being analysed to generate both2-D and 3-D resistivity models of the subsurface. Initial analysis of the data has revealed a number of interesting
features. An anomalous region of low resistivity at a depth of about 20 km is detected and can clearly be identified
as the APMB. This region disappears as we move farther west. The upper boundary bulges upward below Volcan
Uturuncu. A number of low resistivity fingers can also be seen, which connect the APMB conductor to the surface
above, possibly representing past conduits with hydrothermal alteration, or shallow accumulations of magma
beneath Volcan Uturuncu.

1.

Zandt, G., M. Leidig, J. Chmielowski, D. Baumont, X. Yuan, Seismic detection and characterization of

the Altiplano-Puna magma body, Central Andes, Pure Appl. Geophys., 160, 2003.
2. Schilling, F. R., G. M. Partzsch, H. Brasse, G. Schwarz, Partial melting below the magmatic arc in the central
Andes deduced from geoelectromagnetic field experiments and laboratory data, PEPI, 103, 1997.
3. Pritchard, M.E. and M. Simons, An InSAR-based survey of volcanic deformation in the Southern Andes,
Geophys. Res. Lett., 31, 2004.
4. Sparks, R.S.J., C. B. Folkes, M.C.S. Humphreys, D. N. Barfod, J. Clavero, M. C. Sunagua, S.R. McNutt, andM.E. Pritchard, Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion, Am J Sci, 308,
2008.
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82

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P2_1B-O21

Room B1

Date/Time: July 23 17:45-18:00

Towards a sharper image of the Lazufre complex in the Central Andes by combining
independent data from magnetotellurics and InSAR
Heinrich Brasse1 , Ingmar Budach1 , Daniel Diaz2 , Thomas R Walter3
1

Fachrichtung Geophysik, Freie Universität Berlin, Germany, 2 Departamento de Geofisica, Universidad
de Chile, Santiago, Chile, 3 GFZ - Geoforschungszentrum Potsdam, Germany
E-mail: heinrich.brasse@fu-berlin.de

The Lazufre volcanic complex in the Central Andes constitutes one of the largest inflating volcanic systems in theworld. An InSAR time series produced by combining the ERS1, ERS2 and Envisat radar data archives allowed
to detect an uplift increasing to currently 3-4 cm/a, affecting an area of over 2000 sq km. Geodetic inversion
modeling hints at influx of magmatic material into the upper crust at approx. 10 km depth. Since partial melts aregood electrical conductors, such a system forms an ideal target for deep electromagnetic investigation, particularly
magnetotellurics (MT).

Preliminary long-period magnetotelluric soundings were carried out along a W-E running profile across the
structure from the Chilean Precordillera into the Argentinian Puna. Additional sites were placed around thecenter, although coverage was limited due to difficult accessibility and other logistical issues. Dimensionality
and strike analysis did not yield an unequivocal electrical preference direction; induction vectors clear hint at athree-dimensional subsurface conductivity distribution. Thus two-dimensional modeling needs to be regarded withcare. Nevertheless, the resulting model already shows a vast conductor beneath Lazufre, extending through theentire crust. This is corroborated by three-dimensional inversion: a high-conductivity zone rises obliquely fromthe upper mantle to a depth of approximately 10 km. Therefore, the depth of the roof of the high conductivity
region correlates well with the depth of the magmatic sill inferred from InSAR data. Although the mantle source isonly poorly resolved, we believe that melts originating from the upper mantle constitute the source of the volcanic
uplift. Other features of the model comprise a conductor associated with the Precordillera Fault System and thewell-conducting Puna; both structures are in accordance with earlier studies farther to the north.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

83

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P2_1B-O22

Room B1

Date/Time: July 23 18:00-18:15

Geophysical and laboratory constraints on the size and composition of crustal magma
bodiesMartyn J Unsworth1 , Leila Hashim2 , Matthew J Comeau1 , Fabrice Gaillard2
1

University of Alberta, Canada, 2 Universite d’Orleans, France
E-mail: unsworth@UAlberta.ca

A number of magnetotelluric (MT) and seismic studies have detected coincident zones of low resistivity, lowseismic velocity, and high attenuation in the mid-crust of active orogens. These have been detected in both
continent-continent collisions such as the Tibetan Plateau (1), and in the thickened crust formed above a
subduction zone in the Altiplano (2). These anomalies have been interpreted as zones of fluids, most likely acombination of aqueous fluids and partial melts.
In addition to causing volcanic eruptions, these zones of partial melt are key geodynamic features since they
weaken the crust and can control how the orogen deforms. However, the rheology is very sensitive to the amount ofmelt, its composition, and distribution. Geophysical data alone cannot define these parameters reliably. Laboratory
measurements of the electrical and rheological properties of melts provide invaluable constraints when determining
these properties. In this paper we analyse MT data from Southern Tibet and the Altiplano with available seismicand laboratory data to constrain the properties of mid-crustal magma systems.
In Southern Tibet, magnetotelluric studies have reported a high conductance in excess of 10000 S. This is the
product of the layer thickness and conductivity, and is the quantity most reliably determined from MT data analysis.
These observations were combined with laboratory data from partial melting experiments on metapelites (3) andpassive seismic studies. The results suggest that the pure melt phase has a higher resistivity than previously
thought, requiring a relatively thick layer of leucogranite partial melt.
Magnetotelluric and passive seismic data from the Southern Altiplano has detected a major magma body termed
the Altiplano-Puna Magma body. Seismic data gives a thickness and velocity constraint that can be combined withthe high conductance to yield revised estimates of the melt fraction and layer thickness.

1. Unsworth MJ, AG Jones, W Wei, G Marquis, S Gokarn, JE Spratt, Crustal rheology of the Himalaya andSouthern Tibet inferred from magnetotelluric data, Nature, 438, 78-81, doi:10.1038/nature04154, 2005.

2. Brasse H, P Lezaeta, V Rath, K Schwalenberg, W Soyer and V Haak, The Bolivian Altiplano Conductivity
Anomaly, Journal of Geophysical Research, 107, doi 10.1029/2001JB000391, 2002.

3.

Hashim L, F Gaillard, R Champallier, N Le Briton, L Arbaret, B Scaillet, Electrical conductivity, crustal

melting and strain localization beneath the Himalayan Belt, Abstract T51F-2672, presented at 2012 Fall Meeting,
American Geophysical Union, San Francisco, California, December 2012.

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IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
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3P2_1B-O23

Room B1

Date/Time: July 23 18:15-18:30

Accelerating unrest within a large rhyolitic magma system during 2007-2013 at Lagunadel Maule volcanic field, Chile
Helene Le Mevel1 , Kurt Feigl1 , Tabrez Ali1 , M. Loreto Cordova2 , Charles DeMets1 , Bradley S. Singer1
1

Department of Geoscience, University of Wisconsin-Madison, U.S.A., 2 Observatorio Volcanologico delos Andes del Sur (OVDAS), SERNAGEOMIN, Temuco, Chile
E-mail: lemevel@wisc.edu

The Laguna del Maule (LdM) volcanic field is remarkable for its unusual concentration of post-glacial rhyolitic lava
coulées and domes that were erupted between 20 to less than 2 thousand years ago. Covering more than 100
square kilometers, they erupted from 24 vents encircling a ∼20 km diameter lake basin on the range crest of theAndes. This recent concentration of crystal-poor rhyolite is unparalleled both in the Andes and globally. Geodetic
measurements at the LdM volcanic field show rapid uplift since 2007 over a ∼20km diameter area centered on thewestern portion of the young rhyolite domes. By quantifying this active deformation and its evolution with time, we
aim to investigate the storage conditions and dynamic processes in the underlying rhyolitic reservoir that drive theongoing inflation. Analyzing interferometric synthetic aperture radar (InSAR) data, we infer the following evolutionof the deformation: the rate of vertical displacement is negligible from 2003 to 2004, and then accelerates from atleast 200 mm/yr in 2007 to more than 300 mm/yr in 2012 (Feigl et al., 2013, in review). The deformation signal
is modeled as inflation of a 7.5 km by 5.5 km sill at a depth of 5 km, assuming a rectangular dislocation in a half
space with uniform elastic properties. The sill’s maximum rate of inflation is 51 ± 8×106 m³/yr in 2012. In total, thevolume increase of the source is about 160 million cubic meters since inflation began, no later than March 2007.
Three continuous GPS stations installed in April 2012 around the lake confirm this extraordinarily high vertical uplift
rate of more than 280 mm/yr and a significant radial expansion. As of January 2013, the rapid deformation persistsin the InSAR and GPS data. During this interval, the rate of deformation at Laguna del Maule is higher than at anyother volcano that is not actively erupting. For example, the remarkable recent inflation episodes at the Yellowstone(Chang et al., 2010) and Santorini calderas (Newman et al., 2012;Parks et al., 2012) exhibit rates two to five times
slower than at LdM. Using the dynamic models, we can calculate changes in the local stress field. In particular,a large increase in stress in the magma chamber roof might lead to initiation and/or reactivation of ring faults. A
finite element model is being developed to describe the configuration of the magmatic system and overpressure
responsible for the observed uplift. Several hypotheses are considered for driving the deformation, including: (1)an intrusion of basalt into the base of a melt-rich layer of rhyolite leading to heating, bubble growth and subsequent
increase pressure in the reservoir, and/or (2) the possibility of inflation of a hydrothermal system above the rhyolite
melt layer.

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85

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P2_1B-O24

Room B1

Date/Time: July 23 18:30-18:45

Structural control on magmatism along divergent and convergent plate boundaries
Valerio Acocella
Dipartimento Scienze Roma Tre, Italy
E-mail: acocella@uniroma3.it

Plate boundaries, characterized by seismic and volcanic activity, are the most active, unstable and hazardous
areas on Earth. Our knowledge on the structure and magmatic processes along plate boundaries has dramatically
improved in the last decade, allowing proposing a timely overview on the structural control on magmatism.
I first provide an overview of the main structural features along divergent and convergent plate boundaries, as wellas their relationships to volcanism. Such an overview is meant to summarize the current state of the art, offering acomprehensive perspective, highlighting tectono-magmatic relationships, similarities and differences in the various
settings.
I then propose an original and innovative frame to understand and study tectono-magmatic processes along plateboundaries, based on the two following major points. a) Tectonics and magmatism are both effective in controllingthe development of any plate boundary. Along divergent plate boundaries, regional tectonic extension may play amajor role in the early phases of rifting, at least in the upper crust; however, magmatism is the most effective way tospread plate boundaries apart along mature continental rifts and oceanic rifts. Along convergent plate boundaries,
at least three different processes may allow magma to play a crucial role in controlling the structure of volcanicarcs. This suggests that a significant part of the evolution of both divergent and convergent plate boundaries ismagma-induced. b) Magmatism along both divergent and convergent plate boundaries focuses along wedges,
or magmatic systems, usually controlled by a dominant volcano. Magmatic systems display specific geometricfeatures, suggesting that the linear mode of magmatic accretion of divergent plate boundaries and extensional arcs
is replaced by a punctiform mode of accretion in compressive arcs. Also, magmatic systems in each tectonic setting
are characterized by specific compositional features, as a function of the opening rate: these compositions range
from mafic (fast oceanic ridges), passing to bimodal, with a progressively higher amount of felsic component (fromslow spreading ridges to immature continental rifts), to reach andesitic terms (compressive arcs).

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86

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P1

Date/Time: July 24 Poster

Magma genesis of the acidic volcanism and association with basaltic volcanism in theintra-arc rift zone of the Izu volcanic arc, Japan
Satoru Haraguchi1 , Hidekazu Tokuyama2 , Teruaki Ishii3
1

School of Engineering, University of Tokyo, Japan, 2 Center for Advanced Marine Core Research,
Kochi University, Japan, 3 Fukada Geological Institute, Japan
E-mail: haraguti@sys.t.u-tokyo.ac.jp

The Izu volcanic arc extends over 550 km from the Izu Peninsula, Japan, to the Nishinoshima Trough or Sofugan
tectonic line. It is the northernmost segment of the Izu-Bonin-Mariana arc system, which is located at the easternside of the Philippine Sea Plate. The recent magmatism of the Izu arc is bimodal and characterized by basalt
and rhyolite (e.g. Tamura and Tatsumi 2002). In the southern Izu arc, volcanic front (VF) from the Aogashima to
the Torishima islands is characterized by submarine calderas and acidic volcanisms. The intra-arc rifting (IAR),
characterized by back-arc depressions, small volcanic knolls and ridges, is active in this region. The echelon
seamount chain (ESC), backarc side of the intra-arc rift zone, was active before 2.8Ma. Volcanic rocks wereobtained in 1995 during a research cruise of the R/V MOANA WAVE (Hawaii University, cruise MW9507) from VF
to ESC in the southern Izu arc, and that of the R/V TANSEI-MARU (University of Tokyo, cruise KT09-12) aroundthe Myojin Depression in the intra-arc rift zone. Geochemical variation of volcanic rocks and magma genesis werestudied by Hochstaedter et al. (2000, 2001), Machida et al (2008), etc. These studies focused magma and mantle
dynamics of basaltic volcanism in the wedge mantle. Acidic volcanic rocks were also dredged during the cruises
MW9507 and KT09-12 mainly from the IAR. However, studies of these acidic volcanics were rare. Herein, we
present petrographical and chemical analyses of these acidic rocks, and compare these results with those of otheracidic rocks in the Izu arc and lab experiments, and propose a model of magma genesis in a context of acidic
volcanism and association with basaltic arc volcanism.
The petrographical features of rhyolites exhibit massive or flow textures, and aphyric or rare phyric. Phenocrysts
are mainly plagioclase and quartz. Colored minerals are rare and observed mainly orthopyroxene. Amphibole
and biotite are not observed. The phenocryst and groundmass mineral compositions of rhyolites exhibit felsic
characteristics and narrow ranges. These mineral compositions are not overlapped on those of andesites andbasalts. Bulk composition of rhyolite shows depleted in the VF side and enriched in the backarc side. Especially,
HFSE systematics indicates the parent material of rhyolites was enriched in the backarc side.
Acidic volcanism in the Izu arc is considered to partial melting of arc middle to lower crust (e.g. Tamura and Tatsumi,2002) because rhyolite exhibits similar composition to melting experimental results of basaltic or andesitic parental
material under anhydrous, low pressure and low temperature (e.g. Shukuno et al., 2006). Compare to theseexperiments, we consider that volcanics in the rift zone was produced from decompressional melting of andesitic
middle crust under anhydrous melting, and this crust exhibits depleted in the front side and enriched in the reararcside caused by across-arc variation of basaltic volcanism.

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87

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P2

Date/Time: July 24 Poster

Petrology and geochemistry of java volcanoes, indonesia: a key for understanding thelithospheric interior beneath western sunda arc
Mirzam Abdurrachman, Febby Yudhi Pratama
Geological Engineering, Bandung Institute of Technology, Bandung 40132, Indonesia
E-mail: mirzam@gc.itb.ac.id

Phenocryst assemblage, Sr isotopic, K2 O and Nb concentration data from 28 Quaternary Java Volcanoes werecollected. The distinct lateral variations in the abundances of Sr isotopic ratios, K2 O and Nb are observed along
the Java but only rough lateral variation across the Java. Based on phenocryst appearance, Java Volcanoes canbe divided into 3 groups, which are: Muria group (biotite), Tangkuban Prahu group (amphibole and pyroxene) and
Papandayan group (pyroxene) which show inter fingering relation. From Krakatoa to Lamongan (West to East); K2 O
and Nb contents increase gradually. Whereas, Sr isotopic ratios decrease along the Java Arc, except: Papandayan
and Merapi Volcanoes. On the other hand there is only rough and no across arc variation for Sr isotopic ratios, K2 O
and Nb contents. Lateral and across arc variation of phenocryst assemblage, Sr isotopic, K2 O and Nb concentrationfrom Java possibly related to crustal thickness and composition, condition of magma’s pathway (conduit) and also
segmentation of subducted slab, creating condition that promote different degree of magma differentiation.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

88

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P3

Date/Time: July 24 Poster

The crustal structure in the cinder cones zone of Plosky Tolbachik volcano, Kamchatka
Yulia Kugaenko, Vadim Saltykov
Geophysical Survey of Russian Academy of Sciences, Kamchatkan Branch, Russia
E-mail: ku@emsd.ru

Regional zone of cinder cones of Plosky Tolbachik volcano (Tolbachik Dol) is situated in the southwest area of the
Klyuchevskaya group of volcanoes in Kamchatka. The most important historic eruption of this area is the Large
Tolbachik Fissure Eruption (LTFE) 1975-1976. Among the described big fissure eruptions of the world, the LTFE
is distinguished by a unique variety of phenomena and is considered as one of the largest basaltic eruption in the
Kuril-Kamchatka volcanic belt and one of the most studied volcanic eruptions. However, some aspects require
extended studies. In particular, these are the issues related to the structure and properties of shallow magmaticchambers and feeding channels, as well as the main peculiarity of the eruption - the change in the composition of
erupted basalts. In order to restore the deep structure of Tolbachik Dol a passive seismic technique - low-frequency
microseismic sounding method (Gorbatikov et al ., 2008) - was applied. In this technique surface Rayleigh waves ofdifferent frequencies are used as sounding signals. The method is based on the fact that the crustal heterogeneities
distort the spectrum of the low frequency microseismic field in the vicinity of the underground object. The spectral
amplitudes on the Earth’s surface decrease above high velocity heterogeneities and increase above low velocity
ones.
Registration of the wide frequency band microseismic field was carried out in Tolbachik Dol along 3 linear parallel
profiles (14-15 km in length, step of measurement 500 m) embedded transversely to the line of cinder cones throw
3 very much alike volcanic fissure vents of different ages: Northern Vent of LTFE, 1975; Vent-1004 occurred about
2000 years ago; Vent-Alaid occurred about 1500-1000 years ago.
The crust vertical cross-sections, reflecting the distribution of relative velocities of transverse seismic waves up to
20 km were constructed.
The detected structural heterogeneities were interpreted with consideration of previously received data of complex
studies of Tolbachik Dol and LTFE. We confirmed the existence of shallow magma chambers on the depth 2-3 km,
7-8 km and 15-20 km under Northern Vent of LTFE found during previous geophysical investigations. The same
objects were detected under Vent-1004 and Vent-Alaid too. So we assume the regularity in the configuration ofmagmatic system in the cinder cones area (in central part of Tolbachik Dol). A new result is localization of magma
conduits of the eruptions. Configuration of the conduits allows the possibility of magma supply to the surface fromvarious deep sources, and this is the possible reason for changes in the erupted basalt composition. The structureof the areal volcanism zone was shown in detail with obviously expressed change in the character of magma
intrusions from a consolidate basement to sedimentary crust: subvertical magmtic channels are replaced with asystem of inclined ones and sills.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

89

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P4

Date/Time: July 24 Poster

Petrology of felsic rocks dredged from the Myojin Seamount and the Myojin Rift in thenorth Izu-Bonin arc - Contribution of intra-oceanic subduction system to making
continental middle crust Takanori Yoshida1 , Satshi Okamura1 , Izumi Sakamoto2 , Yasuo Ikeda3 , Yoshiko Adachi4 , Moeru Kojima4 ,Makoto Sugawara5 , Ryuichi Shitahaku2
1

Hokkaidou University of Education, Sapporo, Japan, 2 Tokai University, Japan, 3 Hokkaidou University of
Education, Kushiro, Japan, 4 Niigata University, Japan, 5 Mitsubishi Materials Techno Corporation, Japan
E-mail: c12617yt@stu.hokkyodai.ac.jp

The Myojin Seamount, located in the volcanic front of the north Izu-Bonin arc, is a submarine composite volcano
with a large caldera. The Myojin Rift, western back arc side of the Myojin Seamount, exhibits graben structure rifting
at present. The volcanic rocks from the Myojin Seamount are comprised of basalt, andesite, dacite and rhyolite,
whereas the Myojin Rift is characterized by basalt and rhyolite assemblage. The felsic rocks from the Myojin
Seamount and Rift consist mostly of pumices with variable vesicularity and lesser amount of massive lava andplutonic rocks, which are divided into three suites on the basis of incompatible element and isotopic characteristics:
type 1 rocks with low Na, Zr, LREE, and high Ba, 87 Sr/86 Sr, type 2 rocks with low K, Rb, Ba, and high 87 Sr/86 Sr,
type 3 rocks with high Na, K, Rb, Zr, Nb, LREE, and low Ba, 87 Sr/86 Sr.
The type 1 felsic rocks occur in the Myojin Seamount of the volcanic front, the type 3 felsic rocks in the Myojin Riftside, and the type 2 felsic rocks overall from volcanic front to back arc. Isotopic compositions of basalts from thevolcanic front are similar to the type 1 and 2 felsic rocks, whereas those of basalts from the Myojin Rift are similar
to the type 3 felsic rocks. Geochemical signatures and occurrences of the felsic and basaltic rocks suggest thatthe type 1 felsic magma may be derived from the basaltic sources beneath the volcanic front, and the type 3 felsicmagma from the basaltic sources beneath the back arc. Isotopic compositions of the type 2 felsic rocks are similar
to the type 1 felsic rocks, however, the differences of major and trace elements between the type 1 and the type 2
felsic rocks can not be explained by different conditions from the common basaltic sources, such as variable fO2 (e.
g. Sission et al., 2005; Tatsumi and Suzuki, 2009). An alternative model of the type 2 felsic rocks is partial meltingof another source material, such as pre-rifting stage basaltic crust (e.g. the Oligocene middle crust of Tamura etal., 2009). The dispersed distribution of the type 2 felsic rocks from the volcanic front to the back arc is consistent
with the old pre-rifting stage lower crust model.
The Calc-alkaline andesite and dacite from the Myojin Seamount at the volcanic front have magma mixing
evidences, such as mixing trend of bulk chemistry, mingling texture under thin sections, and sieve texture ofplagioclase phenocrysts. This suggests that the intermediate composition volcanic rocks may be originated bymixing of the basaltic magma and the type 1 felsic magma beneath the volcanic front at the Myojin Seamount,which process contributes to make intermediate composition continental crust in the Izu-Bonin oceanic arc.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

90

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P5

Date/Time: July 24 Poster

Tomographic image of the crust beneath the Aira caldera in southern Kyushu
Hiroki Miyamachi1 , Paul Karson Alanis2 , Hiroshi Yakiwara1 , Takeshi Tameguri3 , Masato Iguchi3
1

Kagoshima University, Japan, 2 Philippine Institute of Volcanology and Seismology, Philippines, 3 Kyoto
University, Japan
E-mail: miya@sci.kagoshima-u.ac.jp

Associated with the Philippine Sea plate subduction, a nearly straight chain of active Quaternary volcanism runs
almost parallel to Nankai Trough in the central part of southern Kyushu. In this region, there are four large calderas:
the Kakuto, the Aira, the Ata and the Kikai calderas.
We applied the tomography method with a fine grid configuration to the P- and S-wave arrival times of 829
local earthquakes well observed at 101 stations in central and southern Kyushu, and revealed the detailed
three-dimensional seismic velocity structure of the crust, especially the region beneath the Aira caldera.
In a shallow range from 0 to 10 km depth, low velocity zones (LVZs) are found in the eastern part of Kyushu,
where basement rocks are made up of uplifted sedimentary marine terraces (Nakada et al., 2002). The LVZs arealso distributed along the volcanic front between Kirishima volcano to Kaimon volcano. Meanwhile, the hypocentersoccurred in the inland area are obviously concentrated in regions with a relatively higher velocity and a low Poisson’s
ratio at a depth of 10 km.
At 20 km depth, the most interesting feature is that a distinctly high Poisson’s ratio zone is located exactly beneath
the Aira caldera. This compacted zone also extends southward and connects the Aira caldera with the Ata caldera.
At 30 km depth, a small zone with high Poisson’s ratio appears to remain beneath the Aira caldera. This small zone
is assumed to be a portion of the distinctly high zone at 20 km depth. We also found that the velocity distribution in
the western region (Satsuma Peninsula) is quite different from that in the eastern region (Ohsumi Peninsula): thewestern region is characterized by high P- and S-wave velocities, while the eastern region is characterized by the
low velocities.
The anomaly with a very high Poisson’s ratio at 20 km depth beneath the Aira caldera possibly suggests thepresence of partial melts and the source of volcanism in the area. It is also found that LF earthquakes occur inthe lower crust in and around the Aira caldera. A deeper part of the focal zone of these LF earthquakes appears
to overlap the high Poisson’s ratio zone observed at 30 km depth. These facts lead us to postulate that magma
penetrating into the crust from the upper-most mantle may construct the high Poisson’s ratio zone at about 30 km
depth, and move upwards through the LF focal zone, and finally be stored at about 20 km depth.
Ishihara (1990) described the magma supply system of Sakurajima volcano wherein one magma reservoir is locatedat about 4 km depth just beneath Sakurajima volcano and another at 8 to 10 km depth beneath the Aira caldera.
Although space resolution in our tomography is insufficient to distinguish these magma reservoirs in the upper-mostcrust, our result puts forward a possibility of the deeper magma reservoir supplying two shallow magma reservoirs
previously found.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

91

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P6

Date/Time: July 24 Poster

The significance of two-pyroxene pseudo-decompression paths (PDPs) in mafic tointermediate arc magmas
Georg F Zellmer1 , Yoshiyuki Iizuka2 , Anja Moebis3 , Masaya Miyoshi4 , Hui-Ho Hsieh2
1Smithsonian Institution, U.S.A., 2 Academia Sinica, Taiwan, 3 Massey University, New Zealand,
4
University of Fukui, Japan
E-mail: gzellmer@earth.sinica.edu.tw

Two-pyroxene thermobarometry of a range of mafic to intermediate arc magmas from the Chile, Japan, New
Zealand, and Taiwan yield P-T paths that are approximately isothermal and range from mantle to upper crustalpressures. Using MELTS modeling, we show that such features can be successfully reproduced by combining
pyroxene crystals that formed at a range of crustal pressures but happen to be in Fe-Mg exchange equilibrium.
We refer to these P-T paths as "pseudo-decompression paths" (PDPs), because they give the illusion of rapid melt
ascent at constant temperature, whereas in reality they may have resulted from magma mixing or crystal uptake.
We show that many "microlites" in explosively erupted arc magmas are actually small crystal fragments, arguing forcrystal uptake rather than conventional magma mixing models. Further, while we cannot preclude magma mixing
as a process that may result in PDPs in theory, we show that high-Mg andesite (HMA) would likely be required as
one mixing component. While HMA magmas are being recognized in some volcanic arcs, they do not typically erupt
at the volcanoes we studied, again arguing for crystal uptake. We conclude that many mafic to intermediate arc
magmas (with the exception of remobilized porphyritic dome lavas) leave their source as aphyric melts and acquire
their crystal cargo en route to the surface from previously intruded plutons. Crystallization of the host melt itself islikely prevented due to rapid ascent and superheating. Ironically, the observed PDP’s may therefore actually mirror
the quasi-isothermal ascent paths of these magmas. Our results have profound implications for volcano monitoring
and for the compositional range of melts leaving the mantle wedge.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

92

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P7

Date/Time: July 24 Poster

Preliminary petrologic and geochemical observations of volcanic rocks associated withan area of active surface uplift in the Lazufre region, Andean Central Volcanic Zone
Alicia D Wilder, Todd C Feeley, Gary S Michelfelder
Montana State University, United States
E-mail: alicia.wilder@msu.montana.edu

The Lazufre region is an area of active surface uplift (~25°14’S) situated between two potentially active Quaternary
volcanic centers, Lastarria and Cordon del Azufre, in the Andean Central Volcanic Zone. InSAR observations show
signs of unrest with an elliptical deformation area 45 by 37 km with a maximum inflation rate of ~2.5 cm/yr. The
inferred depth of the magma chamber is 10 km below sea level. Given this depth, deformation and surface uplift
are unlikely to be solely hydrothermal in origin and imply active pluton growth. Whole rock K-Ar dates of lavas
from Cordon del Azufre place the most recent eruptions at 0.6-0.3 Ma ±0.3 Ma. The most recent eruptive activity
at Lastarria has been dated at ~0.5-0.1 Ma. Bombs and blocks of banded pumice are common on the surfacesof Lastarria lava flows indicating intermittent explosive and effusive eruptive activity. Eruptive products of Lazufre
are primarily andesites and dacites (59-67 wt.% SiO2 ) with plagioclase-pyroxene-amphibole-biotite-opaque-quartz
phenocryst assemblages. Many plagioclase phenocrysts in all composition rocks possess resorption textureswith finely-sieved, dusty zones near the margins of the crystals suggesting that magma mixing and mingling
occurred at a late stage after the phenocrysts formed but prior to or during eruption. Trends in major andtrace element concentrations suggest that crystallization-differentiation processes were also important processes
in the evolution of the magmas.

These trends include consistent decreases in MgO (4.0-1.5 wt.%), CaO(6.4-3.5 wt.%), TiO2 (1.03-0.61 wt.%), and FeO (6.8-3.9 wt.%); variable increases in K2 O (2.3-4.1 wt.%), andlittle variation in Na2 O (2.9-3.6 wt.%) with increasing SiO2 contents. Large ion lithophile trace elements showenrichments beyond those that can be produced by crystallization-differentiation alone (Rb=103-291 ppm and
Th=18-66 ppm); while compatible trace elements are strongly depleted relative to inferred primitive mantle melts:
Sr (578-294 ppm), Cr (61-10 ppm), Ni (33-14 ppm). Closed system crystallization-differentiation processes alone
cannot be the only factors influencing magma compositions, however, as 87 Sr/86 Sr values are highly elevated(0.70651-0.70715). Collectively these data attest to a multitude of differentiation processes and magma sources
including crystallization-differentiation from more mafic magmas, melting and assimilation of older crustal rocks,
and magma mixing and mingling.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

93

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P8

Date/Time: July 24 Poster

8 Ma time series of explosive volcanism at the Central American Volcanic Arc (marine
tephras from ODP and DSDP sites)
Julie C Schindlbeck, Steffen Kutterolf, Armin Freundt
GEOMAR Helmholtz Center for Ocean Research Kiel, Germany
E-mail: jschindlbeck@geomar.de

The Central American Volcanic Arc (CAVA) is, and has been, one of the most active volcanic regions on earth andgenerated numerous Plinian eruptions along his 1200 km extension.
Numerous ash layers up to 8 Ma, which occur in ODP and DSDP cores of Legs 66, 67, 170, 202 and 205,
originated in Central America and southern Mexico. The best preserved tephra archive lies in the cores acrossthe ash distribution areas expected from dominant wind directions as identified by mapped fallout deposits. We
analyzed > 150 ash layers of all Legs for first detailed analysis of these sites to 1) built up a arc-wide recent to
Tertiary tephrostratigraphy and 2) evaluate how magmatism, and especially the influence of subduction parameters,
sediment input and crustal assimilation/interaction change with time at the CAVA.
The black to gray to white ash layers have commonly sharp contacts at the bottom and diffuse transitions toterrigenous and pelagic sediments at the top. Ash layer thicknesses range from 0.5 to 60 cm with typical grain sizes
from medium silt to coarse sand. The mineral assemblages are typical for arc volcanism (plagioclase, pyroxene,
hornblende, and olivine). The most evolved tephras also contain biotite.
Electron microprobe analyses of 1748 glass shards yield compositions ranging from basalt to rhyolite and trachyte.
Felsic ashes can be divided into seven compositional groups by means of silica and potassium contents. Definitionof source areas on land, but also their variations are based on major element geochemistry of glasses and minerals,
and trace element data from LA-ICP-MS analyses combined with Ar/Ar age dating. First results show that sourceareas of the ash layers are distributed along the entire CAVA, as well as at the Southern Mexican Arc. Making useof typical systematic CAVA along-arc variations of trace-element characteristics (Zr/Nb, Ba/La, Ce/Yb, La/Yb and
Ba/Zr) of the arc rocks with time is the main key to unravel the long-term changes in magmatic variability and its
causes along the arc in future.
Therefore, the marine tephra record provides important data for ongoing studies of CAVA volcanism: (a) dating ofundated land tephra by correlation with marine ashes and the ages derived by sedimentation rates and Ar/Ar dating;(b) regional stratigraphic correlations along the entire arc; (c) reconstruction of long-term changes in magmatic
evolution of volcanic complexes.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

94

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P9

Date/Time: July 24 Poster

Santorini volcano magma plumbing system: constraints from a combined experimental
and natural products study
Anita Cadoux1 , Joan Andujar1 , Bruno Scaillet1 , Tim Druitt2 , Etienne Deloule3
1

Institut des Sciences de la Terre d’Orleans, Universite d’Orleans, France, 2 Lab Magmas et Volcans,
Blaise Pascal University, Clermont-Ferrand, France, 3 Centre de Recherches Petrographiques etGeochimiques, Nancy, France
E-mail: acadoux@cnrs-orleans.fr

Santorini Volcano constitutes a serious hazard in the Aegean region of Greece. The recent 2011-2012 episodeof caldera unrest has focussed attention on the magma plumbing system of the volcano, and notably on thedepths of storage of different magma batches. We investigated the differentiation conditions of mafic-intermediatemagmas, as well as the pre-eruptive storage conditions of dacitic-rhyodacitic magmas prior to the four largest
Plinian eruptions of the volcano over the last 200 ka (Minoan, Cape Riva, Lower Pumice 2, Lower Pumice 1). The
intensive variables (P, T, fO2 , volatiles content) were determined by combining laboratory crystallization experimentswith study of the natural products (including volatiles in melt inclusions). Our results show that the magma plumbing
system is dominated by a large, long-lived (≥ 200 ka) storage region at about 8 km depth (2 kb), from which thesilicic magmas are derived. This region is fed by injections from a deeper, mafic reservoir located at the boundary
between the upper and lower crust (∼15 km; 4 kb). Mantle-derived basalts stall in the 15 km storage region, where
they fractionate to basaltic andesite (55 to 58 wt.% SiO2 ) with 2-3 wt.% H2 O through crystallization of about 60
wt.% of oliv, cpx, plag, mt/ilm and opx, at 1040-1000°C, 4 kbar, fO2 = FMQ-0.5. The basaltic-andesitic magmas
then ascent to about 8 km where they either (1) experience a last episode of equilibration prior to eruption, or (2)fractionate to produce dacitic to rhyodacitic liquids.
Prior to the major Plinian eruptions, the silicic magmas were stored at T = 850-900°C, under relatively reduced
conditions (∼FMQ or, ∆NNO = -0.9 to -0.1), with melts depleted in fluorine (∼500-700 ppm) and particularly insulphur (<100 ppm) but rich in water and chlorine (5-6 wt.%, ca. 2500-3500 ppm, respectively) probably co-existingwith a hydrosaline liquid, at pressure ≥ 2 kbar. The fact that Santorini dacites-rhyodacites have high enoughdissolved Cl for the melts to be saturated with hydrosaline liquid (and maybe vapor) before Plinian eruptions
has significant consequences on the Cl degassing budget, which was previously estimated as insignificant. A
re-evaluation is under progress and the results will be presented in our communication.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

95

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P10

Date/Time: July 24 Poster

The redox states of basaltic and boninitic magmas throughout the early stage of theBonin arc formation; Fe-K edge XANES study
Hidemi Ishibashi1 , Shoko Odake2 , Kyoko Kanayama3 , Morihisa Hamada4 , Hiroyuki Kagi5
1

Department of Geoscience, Faculty of Science, Shizuoka University, Japan, 2 GIA Tokyo, Japan,
Department of Earth Science, Graduate School of Natural Science and Technology, Kanazawa
University, Japan, 4 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Japan,
5
Geochemical Research Center, Graduate School of Science, the University of Tokyo, Japan3

E-mail: shishib@ipc.shizuoka.ac.jp

One of the significant features of arc magmas is their relatively oxidized nature compared to those from othertectonic settings. Previous studies discussed that the nature is attributed to relatively high oxygen fugacity (fO2 )conditions in their source mantle region, and accession of subducting slab-derived fluid may have raised fO2 ofthe MORB source-like mantle. However, it is unknown how the fO2 condition of melting region in mantle wedge
develops throughout the early stage of arc formation. In this study, we investigated the fO2 conditions of basaltic
and boninitic magmas erupted during the initial ca. 12 million years of the Bonin arc formation based on the Fe
valence states of quenched silicate glasses, which is a sensitive indicator of magmatic fO2 , to examine how fO2 oftheir source mantle region developed with time.
Fe-K edge micro-XANES (X-ray Absorption Near Edge Structure) measurements enable us to determine valence
state of Fe in silicate glass with spatial resolution of several microns. In this study, Fe-K edge XANES spectra were
measured for quenched silicate glasses included in pillow lavas and hyaloclastites of basalt and boninite from Boninarc, using a X-ray micro-beam system at Beam Line 4A in Photon Factory, KEK, Japan. Mole ratios of ferric to total
iron, Fe3+ /Fetotal , were determined from the spectra, and then the ratios were used to estimate fO2 of silicate melts.
Precision of our analyses was evaluated to be within standard deviation of 0.4 log unit in fO2 using basaltic andandesitic standard glasses synthesized at controlled fO2 conditions.
The measured samples include forearc basalt (FAB) collected at Bonin Ridge, which is the earliest volcanic productfrom Bonin arc and has MORB-like geochemical feature, erupted right after the initiation of subduction (ca. 51Ma),
boninites from Chichijima, Otojima, and Mukojima, erupted at ca. 44-48 Ma, and arc basalts from Anejima andHahajima, erupted at ca. 37-44Ma. The measured Fe3+ / Fetotal ratios of quenched glasses are ca. 0.20 for FAB,
0.17-0.24 for boninites from Chichijima, Otojima, and Mukojima, and 0.20-0.22 for arc basalts from Anejima andHahajima, respectively. All measured samples show Fe3+ / Fetotal ratios higher than that of MORB. fO2 of themeasured glasses are estimated to be near Ni-NiO buffer which is consistent with the range of arc magmas. The
results suggest that fO2 of magmas erupted at Bonin arc has been higher than that of MORB since the initiation ofarc volcanism. In addition, the similarity of fO2 among MORB-like FAB, boninites, and arc basalts implies that theoxidation process may be independent of accretion of subducting-slab derived aqueous fluid.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

96

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P11

Date/Time: July 24 Poster

Petrological and petrochemical investigations of Iwanoyama-Iyuzan volcanic chain in
Higashi-Izu monogenetic volcano group, Izu-Bonin volcanic arc, central Japan
Hiromichi Kanai1 , Yoji Arakawa2 , Yoshiyuki Tajima1 , Kei Ikehata2
1

Geosciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan,
2
Faculty of Life and Environmental Sciences, University of Tsukuba, Japan
E-mail: k2-kanai@geol.tsukuba.ac.jp

Higashi–Izu monogenetic volcano group consists of more than 75 volcanoes in northeastern part of Izu Peninsula
northern Izu–Bonin volcanic arc. These monogenetic volcanoes were interpreted to have formed mainly under
the tensional stress regime due to the several block movement (Koyama and Umino, 1991). Some of thesemonogenetic volcanoes forming volcanic chains (NW–SE in direction) with different space, time are somewhere
distributed (e.g., Hayakawa and Koyama, 1992; Koyama et al., 1995). These eruptions are thought to have occurredin 142.5 ka–2.7 ka (e.g., Hayakawa and Koyama, 1995). Erupted products of these monogenetic volcanoes havewide compositional variations, from basalt to rhyolite, and SiO2 wt.% of 48–73. Some generation processes havebeen provided for those variations. Andesitic magmas were formed by differentiation from basaltic magma, or
crustal contamination, while dacite–rhyolite magmas were by partial melting of granitic crustal materials (e.g.,
Hamuro, 1985; Miyajima, 1990; Suzuki, 2000). Recent melt inclusion study proposed a model that the chemicalvariability was attributed to the crystallization within magma chamber (Nichols et al., 2012).
Iwanoyama–Iyuzan volcanic chain is the youngest one in this region (2.7 ka), and also has wide compositionalvariations from basaltic andesite to dacite, even within one monogenetic volcano (e.g., Kurasawa, 1984; Suzuki,2000). These variations in this volcanic chain are not yet fully discussed with enough data. Therefore, the purposeof this study is to clarify their magma genesis, origin and evolution of magma in this volcanic chain based on detailed
petrographic, petrological and geochemical investigations.
The Iwanoyama, Ananoyama, Yahazuyama and Ananokubo in this chain are mostly composed of dacite and havesome different petrography and mineral chemistries. The dacites of former three volcanoes are characterized abimodal plagioclase anorthite content (An). High An group of plagioclases (An=92–96) indicates normal chemicalzoning, but low An group (An=56–70) are dominated, in most cases, by dusty–zoned, or reversely zoned texture.
Clinopyroxene has Mg# of around 80 both in core and rim, while orthopyroxenes are grouped into two: 70–72
and 76–80. Major and trace element characteristics give similar bimodal data distributions, which suggest somemixing including crustal materials as important process. Measured Sr isotope ratios (87Sr/86Sr) of Ananoyama
and Yahazuyama dacites range in 0.70325–0.70349, indicating inhomogeneous magma and exclude simple crystal
fractionation model. They probably reflect the mixing of basaltic magma and crustal material, or mixing of basaltic
magma and crustal–derived felsic magma. We will focus our discussion on the generation mechanism andcompositional variability within single volcano, or within 2.7 ka volcanic chain, and possibly the relationship with
regional tectonics.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

97

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P12

Date/Time: July 24 Poster

Evolution of volcanism and magmatism during initial arc stage of the Oman ophiolite
Yuki Kusano1 , Yoshiko Adachi2 , Susumu Umino1 , Sumio Miyashita2
1Kanazawa University, Japan, 2 Niigata University, Japan
E-mail: ykusano@staff.kanazawa-u.ac.jp

We present detailed volcanostratigraphy‚ petrology and geochemistry of juvenile arc tholeiite and subsequent boninite
magmatism from the Oman ophiolite. Volcanic sequences of the Oman Ophiolite are divided into a spreading stage V1(Geotimes‚ Lasail)‚ a subduction stage V2 (Alley‚ Cpx−phyric) and a pre−obduction alkaline stage V3 (Salahi)in ascending order (Alabaster et al.‚ 1982‚ Ernewein et al.‚ 1988‚ Umino et al.‚ 1990).
The V2 sequence considered to begin <2 Ma after the V1 (e.g. Hacker et al.‚ 1996) is a suitable site to investigate
magmatic and volcanic developing processes of an infant arc.
An 1110 m thick V2 sequence is divided into the lower 970 m (LV2) and upper 140 m (UV2) thick subsequencesby a 1.0 m thick sedimentary layer. Pahoehoe flows dominate in the lower part of the LV2‚ while the upper part consists
mainly of sheet flows with intervened few pelagic sediments‚ a fissure vent and a cylindrical plug. In addition to
the presence of feeder conduits‚ the flow−dominant lithofacies with a few thin sedimentary interbeds in the LV2 indicates thatthe study area was the center of a monogenetic volcano grown in a short period.
The UV2 is composed of sheet flows overlain by a 2.0 m thick subaqueous pyroclastic fall deposit. The LV2 consist ofarc tholeiite with orthopyroxene phenocrysts increasing in amount upward. The UV2 lavas are composed ofboninite containing olivine and clinopyroxene phenocrysts with plagioclase in the groundmass.
Successive orthopyroxene−bearing arc tholeiitic volcanism in LV2 followed by a relatively small amount of boninite lavas
in UV2 overlain by thick pelagic sediments suggests that the infant arc volcanism was short lived and terminated long before
the ophiolite obduction.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

98

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P13

Date/Time: July 24 Poster

New geochemical classification of global boninites
Kyoko Kanayama, Keitaro Kitamura, Susumu Umino
Department of Earth Sciences, Kanazawa University, Japan
E-mail: ka78ma@stu.kanazawa-u.ac.jp

Boninite is an important volcanic rock type associated with the initiation of a subduction zone. It is generally defined
as a variety of high-magnesian andesites with SiO2 >52 wt%, MgO >8 wt% and TiO2 <0.5 wt%. Compilation of the
global data on bulk geochemistry of boninites defined as such shows a broad compositional range consisting of
a number of regional trends which are characteristic to the individual volcanic suites, suggesting that the genetic
conditions of boninite magmas are highly variable dependent on the tectonomagmatic situations. Therefore,
re-evaluation of the classification scheme of global boninites is crucial to understand the genetic conditions ofboninite magmas and their relationships with the tectonomagmatic settings.

Boninite is usually a part of volcanic rock suites which forms a continuous fractionation trend from magnesian(MgO >20 wt%) boninite through less magnesian andesite to dacite and rhyolite. These regional fractionation
trends form subparallel curves on a SiO2 -MgO plot, namely boninite series, that differ from volcanic suites tosuites. We advocate to classify these boninite-series rocks into high- and low-Si boninites by a discrimination line
running through points of SiO2 = 55 wt% at MgO = 20 wt% and SiO2 = 59 wt% at MgO = 8 wt% on a SiO2 vs.
MgO plot. Boninites from Ogasawara (Bonin) Islands on the Izu-Ogasawara (Bonin)-Mariana forearc and western
Pacific ophiolites in Papua New Guinea and New Caledonia show compositional trends of high-Si boninite series
which are controlled by crystal fractionation of olivine and orthopyroxene. Whereas, boninites from Tonga arc,
DSDP Site 458 and Guam, and Neo-Tethys ophiolites such as Oman and Troodos show low-Si boninite series
trends controlled by olivine, orthopyroxene and clinopyroxene fractionation. Low-Si boninite-series rocks do not
evolve across the discriminate line by crystallization differentiation. Primary magmas of Low-Si boninites arecharacterized by enhanced LILEs and LREEs by slab-derived H2 O-rich fluids. Melting experiments of peridotites
have demonstrated that low-Si boninite-like melts with SiO2 <54 wt%, MgO <23 wt% could be produced under
1-2.5 GPa and dry and water-undersaturated conditions. On the contrary, SiO2 -rich (SiO2 >54 wt%) melts likehigh-Si boninites have never been produced by peridotite melting experiments. Instead, highly depleted REEs
and high Zr/Ti ratios of high-Si boninite magmas require slab-derived felsic melts that reacted with the depleted
harzburgite in the mantle wedge.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

99

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P14

Date/Time: July 24 Poster

Magma mixing mechanism of Fujiyama lava dome in Takahara volcano, northeast Japan
Yoshiyuki Tajima1 , Yoji Arakawa2 , Kei Ikehata2 , Hiromichi Kanai11Life and Environmental Sciences, University of Tsukuba, Japan, 2 Faculty of Life and Environmental
Sciences, University of Tsukuba, Japan
E-mail: y_tajima@geol.tsukuba.ac.jp

We carried out petrological and petrochemical studies on Fujiyama lava dome in northern part of Takahara volcano
which is a Quaternary stratovolcano located on volcanic front in the southern part of northeast Japan arc. Fujiyama
is mainly composed of dacites. Takahara volcano was presumed to have finished its volcanic activity ca. 100 ka(Inoue et al., 1994), but Fujiyama was assumed to have been formed at 6.5 ka (Okuno et al., 1997; Takashima,1999).
Magma mixing is accepted as one of the important processes leading to the formation of mafic inclusions (e.g.,
Eichelberger, 1975; Koyaguchi, 1986).

The presence of mafic inclusions was already reported in Fujiyama(Ikeshima and Aoki, 1962). However, there are no investigations for the mafic inclusions in Fujiyama, and theirpetrogenesis is not yet understood. Therefore, we tried to study to intend to understand genetic processes of thesemafic inclusions and magma mixing mechanism in Fujiyama lava dome.
The mafic inclusions in Fujiyama dacites (SiO2=67.4-70.4 wt.%) have andesitic composition (SiO2=60.7 wt.%).
They are dark in color and most inclusions have spherical to oblate in shape. In the boundary between host rockand inclusion, a few phenocrysts straddle the boundary. All inclusions have abundant vesicles. These featuresare similar to mafic inclusions reported from other volcanoes, and indicate that the inclusions were liquid when
they were entrained by the silicic magma (e.g., Heiken and Eichelberger, 1980; Bacon, 1986). The host rocks(dacite) contain phenocrysts of plagioclase, quartz, orthopyroxene, hornblende, Fe-Ti oxides and rarely augite. The
inclusions contain phenocrysts of plagioclase, orthopyroxene, augite, Fe-Ti oxides and rarely include hornblendeand quartz.
Whole-rock major and trace element composition of the inclusions are plotted between host dacites and basalts
erupted during the earliest activity in Takahara volcano. In general, chemical compositions have liner data trend
which may be evidence of magma mixing between dacitic and basaltic magma. Plagioclase and orthopyroxene
phenocryst cores in the host rocks and inclusions have a large variation in composition. Both rocks contain Ab-richplagioclase, An-rich plagioclase, Fe-rich orthopyroxene and Mg-rich orthopyroxene. Most of Ab-rich plagioclase
and Fe-rich orthopyroxene in the inclusions show reverse zoning. Sr isotopic ratio (87Sr/86Sr) of the inclusion isslightly lower than that of the host dacite.
From these results, it is presumed that magma mixing occurred between silicic magma and mafic magma in
Fujiyama. Silicic end-member might have been close to in composition to the host Fujiyama dacitic magma, andmafic end-member might have been basaltic magma having similar composition to the products erupted during theearliest stage in activity of Takahara volcano. Therefore, it is suggested that the mafic inclusions were formed fromresultant andesitic magma by the magma mixing.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

100

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P15

Date/Time: July 24 Poster

Origin of voluminous andesite - dacite in the Hohi Volcanic Zone : Role of basaltic
magma, SW Japan
Yoshiyuki HORIKAWA, Takashi NAGAO
Yamaguchi University, Japan
E-mail: m006wa@yamaguchi-u.ac.jp

Adakites are erupted related to subduction of the Philippine Sea Plate (PSP) on the Quaternary volcanic front of
the Chugoku region tothe norteast Kyushu from Pleistocene time (e.g. Kimura et al., 2005). However, in the Hohi
Volcanic Zone (HVZ) in northern Kyushu, a large amount of andesite - dacite continuous erupted from about 6
Ma, basaltis rare. Therefore, magmatism of the HVZ has been studied of a small amount of basalt (Nakada andKamata, 1991). So, it is important to clarification the origin oh andesite - dacite in the HVZ. It also can be to identify
the evolutionof PSP subduction. In this study, we have examined th Futago volcanoe group (2.0 -1.0 Ma), the Usa
volcanic rocks (6.6 -2.7 Ma) and the Hikosan volcanic rocks (4.8 -3.8 Ma) distributed in Hikosan from Kunisaki
peninsula located in the northern part of the HVZ.
The initial activity in the HVZ, High-Nb andesite with high-K2 O produced in the forearc side of the Usa volcanic
rocks of about 6 Ma. The trace element pattern of High-Nb andesite is similar to alkali basalt - basanite. It is
indicating that the alkali basalt - basanite magma may contribute to the genesisof the High-Nb andesite (Nagao etal., 2001). In addition, andesite - dacite of the Usa volcanic rocks are incmpatible element (K2 O, Nb, Zr) decreases
with increasing SiO2 , and continuously changes to the island arc andesite composition. this compositional trend can
not be wxplained by crystallization differentiation. this compositional change suggests High-Nb andesite magma
was AFC of the crust and/or mixing with felsic magma by melting of the lower - middle crust. Around 4.5 Ma,
primitive tholeiitic basalt erupted in a back-arc side Hikosa volcanic rocks, show OIB-like signature. Also andesite- dacite of Hikosa volcanic rocks, it consider to mixed with tholeiitic magma and felsic magama indicate a linear
compositional trend. In the Pleistocene, OIB-like tholeiitic basaltis including with mafic enclave in Futago volcane
group from 2 Ma. These are heated and remobilized mashed adakitic magama in the crust. But, in the Quaternary
volcanic front, volcanic activity dominated by the adakitic magma.
The important things are that the amount of andesite - dacite (Usa volcanic rocks : 83 km³; Hikosan volcanic rocks: 111 km³) are erupted at HVZ, it is the need for large amounts of basaltic magma is likely to be derived from fertile
mantle characteristic of the back-arc side of geochemical characteristics. therefor, there is a possibility that thefertile mantle wasa flowing up by 6 - 3 Maunder the mantle of the HVZ.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

101

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P16

Date/Time: July 24 Poster

Co-Variation of Sr-Nd-Pb-Hf Isotope Chemistries of the Quaternary Lavas and theBasement Granitoids, Northeastern Japan arc
Yuka Hirahara1 , Shinichi Kagashima2 , Masao Ban2 , Nobutaka Tsuchiya3 , Toshiro Takahashi1 ,Takeyoshi Yoshida4 , Takeshi Kuritani5 , Akihiko Fujinawa6 , Tsukasa Ohba7 , Shintaro Hayashi7 , Ryoko
Senda1 , Qing Chang1 , Takashi Miyazaki1 , Jun-Ichi Kimura1
1

JAMSTEC, Japan, 2 Yamagata Univ., Japan, 3 Iwate Univ., Japan, 4 Tohoku Univ., Japan, 5 Osaka city
Univ., Japan, 6 Ibaraki Univ., Japan, 7 Akita Univ., Japan
E-mail: hiraharay@jamstec.go.jp

Sr-Nd-Pb-Hf isotopic ratios were determined for the frontal-arc lavas and for the Cretaceous to Paleogene
granitoids occurring in the Northeastern Japan arc in order to examine the genetic relationships between the young
magmas and their basement rocks. Frontal-arc Nasu and rear-arc Chokai volcanic chains develop in response to
the - 90 to -180 km depth contours of Wadati-Benioff zone of the Pacific Plate slab. Previous studies showed that
the Sr-Nd isotopic compositions of the rear-arc lavas have a common depleted source, whereas the frontal-arc lavas
have various enriched sources with increasing Sr and decreasing Nd isotope ratios from north to south.The isotopic
variations of the frontal-arc lavas almost correlate with that in the basement granitoids, which also have increasingly
radiogenic Sr from north to south. We newly present Sr-Nd-Pb-Hf isotope ratios for the representative frontal-arc
lavas and the basement granitoids. Frontal-arc lavas examined were from Hakkoda, Akita-komagatake, Iwate
volcanoes from the northern area, Zao and Azuma from the central area, and Nasu and Takahara from the southernarea. Basement granitoids were analyzed for the Kitakami belt (north area), the Abukuma belt (central area),
and the Ashio/Mino belt (south area). All the Sr-Nd-Pb-Hf isotope data confirmed increasingly enriched sourcefrom north to south for both the Quaternary lavas and the basement granitoids. Multi-isotope plots show discretequasi-linear mixing arrays consisting of the lavas at each volcano suggesting mixing of endmember magmas fromdepleted and enriched sources. The depleted source is, the most likely, originated from a common Indian MORB
mantle affected by the slab components from the Pacific Plate. In contrast, the enriched sources for the lavas varyand the source at each volcano appears to correlate with the underlying basement granitoids in terms of isotope
geochemistry. This suggests either crustal assimilation of the upper crustal granitoids or deep assimilation of thelower crustal amphibolite which was the source of the basement granitoids. The isotopic evidence clarified that the
Quaternary lavas in the Northeast Japan arc were fundamentally derived for a common mantle source but severely
affected by the various crustal materials.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

102

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P17

Date/Time: July 24 Poster

Relationship between seismic structure and magma migration during the 2011-2012 El
Hierro (Canary Islands) eruption
Antonio Villasenor1 , Carmen del Fresno2 , Carmen Lopez2 , Adelina Geyer1 , Joan Marti1
1

Institute of Earth Sciences Jaume Almera, CSIC, Spain, 2 Observatorio Geofisico Central, Instituto
Geografico Nacional, Spain
E-mail: antonio@ictja.csic.es

The submarine eruption in El Hierro island started on 10 October 2011 and was preceded by intense seismic
activity that continued throughout the eruption and after it finished on March 2012. Seismicity was monitored
by a 9-station seismic network that allowed to locate more than 10,000 earthquakes. The distribution of theseearthquakes and stations is very suitable for studying the P-wave structure of the island down to 20 km withtravel-time local-earthquake tomography. Using a subset of well distributed and recorded earthquakes we haveobtained a three-dimensional P-wave velocity model of the island that is well resolved beneath its central andwestern part. The most prominent feature of this model is a high-velocity anomaly in the center of the islandthat at shallow depths coincides with the edifice of the Tanganasoga volcano. This anomaly dips to the south
extending from the surface to 10 km depth, and seems to have great importance in the migration of magma that fed
the 2011-2012 eruption. Two seismic zones converge at the base of this anomaly, while the seismicity inside theanomaly is very low. The first seismic zone dips very steeply (> 60 degrees) to the north and extends approximately
to 25 km depth below the El Golfo area. The second zone dips more gently to the south below El Julan, reachinga maximum depth of 20 km, coinciding with the center of the inflation determined from GPS data. This seems toindicate that magma ascended from the south and/or north deep seismic zones reaching the base of the anomaly
where its ascent was stopped. After this first period seismicity and presumably magma started to migrate south,always at the base of the high-velocity body, until it erupted 2 km offshore of the southern tip of the island. These
results are consistent with thermobarometric and petrologic studies of young erupted rocks in El Hierro that suggest
main fractionation of magma at 19-26 km depth, and also the existence of multi-stage ascent with small, isolated
magma pockets where mixing of distinct magma batches may occur.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

103

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P18

Date/Time: July 24 Poster

Geochemical study of the mafic rocks from Hokkaido, northern Japan: Spatial variationsin the wedge mantle and the magma-generation processes at an arc-arc junction
Ayumi Kosugi, Mitsuhiro Nakagawa
Departmant of Natural History Sciences, Graduate School of Science, Hokkaido University, Japan
E-mail: kosugi@mail.sci.hokudai.ac.jp

Hokkaido Island is a junction between the Northeast Japan (NEJ) and the Kuril arcs, where the extensive arc
volcanism has continued at least since late Miocene. We newly investigate the spatial geochemical variation ofmafic rocks (<1.7Ma). We focus on the rocks with SiO2 <54% for avoiding the effect of crustal assimilation. Basedon the spatial compositional variations of the volcanic rocks, the junction can be divided into four volcanic regions:the trench (T-SWH) and the rear (R-SWH) sides of the southwestern Hokkaido in NEJ arc, and the central Hokkaido(CH) and the eastern Hokkaido (EH) in Kuril arc. Across-arc compositional variations of the rocks, such as theincrease of K2 O contents from low-K to medium-K rocks toward the back-arc side, can be clearly recognizedin SWH and EH. The low-K rocks from these regions are characterized by the obvious Pb and Ba spikes inspidergrams and flat REE patters. However, low-K rocks in the EH show extreme negative anormaly of Nb andTa, and depletion of LREE. On the other hand, there exist no low-K rocks in CH. In addition, across-arc variations
are not clearly recognized. In a Nb/Y-Zr/Y diagram, the rocks from four regions show distinct linear and parallel
trends, respectively, and can be divided into three groups, T-SWH, R-SWH and CH, and EH. This indicates that
there exists compositional heterogeneity of the wedge mantle, which cannot be explained by the degree of melting
from a single mantle source. The degree of depletion in the mantle increases from T-SWH to EH. According to these
Nb/Y values at similar Zr/Y, we assume the Enriched-DMM (depleted MORB mantle), DMM and Depleted-DMM ofWorkman and Hart (2005) for T-SWH, R-SWH and CH, and EH as the source mantle compositions, respectively.
Then, degree of melting (F) for primary magmas in each region is calculated by the contents of fluid-immobile
elements. Next, using these F values, we determine a metasomatized mantle source compositions for each region.
Contents of fluid-mobile elements of a metasomatized mantle source show positive, various correlations in Th andU vs. F diagrams. These data indicate that there exists the difference of subduction components among theseregions.
Four volcanic regions in Hokkaido can be extended to the main parts of NEJ and Kuril arcs. In NEJ arc, twodistinct regions similar in Hokkaido continue to northern Honshu. Thus, the wedge mantles are distinct between
the trench and rear sides in NEJ arc. On the other hand, the rocks from Kuril Islands have similar or more depleted
composition compared with those from EH. Thus the wedge mantle slightly changes to become more depleted in
Kuril Islands. Although low-K tholeiitic rocks occur in trench-side volcanoes of both arcs, the origins of these rocks
are different. Low-K rocks are produced by lower degree of melting more depleted mantle in Kuril arc and by higher
degree of melting of enriched mantle in NEJ arc.

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104

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P19

Date/Time: July 24 Poster

Crustal structure beneath Aso caldera, Japan, as derived from receiver function analyses
of waveform data from densely distributed stations
Yuki Abe1 , Takahiro Ohkura1 , Takuo Shibutani2 , Kazuro Hirahara1 , Shin Yoshikawa1 , Hiroyuki Inoue1
1

Graduate School of Science, Kyoto University, Japan, 2 Disaster Prevention Research Institute, Kyoto
University, Japan
E-mail: abeyuki@aso.vgs.kyoto-u.ac.jp

Aso volcano, which is an active volcano, rises in the Kyushu district, Japan. It erupted over 600 km3 of pyroclastic
deposit 90 thousand years ago and formed a caldera with dimensions of 18 km by 25 km. After the huge eruption,
small eruptions formed the central cones. Abe et al. (2010, JVGR) estimated the crustal structure beneath thecaldera with receiver function (RF) analyses. They obtained a low velocity layer whose S-wave velocity is 2.4 km/s
at depths between 15 km and 21 km beneath the western part of the caldera. They did not estimate the crustal
structure beneath the eastern part because of the lack of seismic stations.
We set 5 stations in the eastern part of the caldera in July 2009, and started observation. We used waveform
data obtained at these temporal stations and permanent stations established in and around the caldera by Aso
Volcanological Laboratory, Kyoto Univ. and National Research Institute for Earth Science and Disaster Prevention,and examined the crustal structure beneath the whole of Aso caldera with RF analyses.
We used waveform data of teleseismic events (epicentral distances: 30-90 degrees, Magnitude: greater than 5.5)
for calculating RFs with the extended-time multitaper method (Shibutani et al., 2008, BSSA), and estimated theS-wave velocity structure with genetic algorithm inversion of the RFs (Shibutani et al., 1996, GRL).
We revealed that low velocity layers whose S-wave velocities are 2.2-2.6 km/s exist between 8 km and 28 km in
depth beneath Aso caldera except the northern and southern rim. Considering a study by Takei (2002, JGR),
we estimated from the S-wave velocity structure that these low velocity layers contain 15% of melt or 30% ofwater at maximum. If the low velocity layers contain melt, its amount is 300 km3 at maximum, which is about
the same amount as magma erupted at the last huge eruption: 200 km3 (Kaneko et al., 2007, JVGR). Beneaththe eastern flank of the central cones, a low velocity layer whose S-wave velocity is 2.6 km/s exists only at
depths of 8-15 km. Beneath the same area, a swarm of low frequency events exists at 15-25 km in depth
according to a hypocentral catalog by Japan Meteorological Agency, and Geographical Survey Institute (2004,
Report of Coordinating Committee for Prediction of Volcanic Eruption) detected a sill-like deformation source at
15 km in depth. Fluid movement just beneath the low velocity layer would cause low frequency earthquakes andcrustal deformation. We can expect that melt is intruding through the swarm of the low frequency earthquakes,
accumulating at the deformation source, and melting the crustal material above it.

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105

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P20

Date/Time: July 24 Poster

Origin of spatial compositional variations of volcanic rocks from Northern Kurile Islands:
Geochemical studies of active volcanoes on Paramushir, Atlasova, Antsiferova islands
and submarine volcanoes
Olga Bergal-Kuvikas1,2 , Mitsuhiro Nakagawa1 , Gennady Avdeiko2
1

Hokkaido University, Graduate School of Science, Department of Earth and Planetary Sciences,
Japan, 2 Institute of Volcanology and Seismology, Russia
E-mail: kuvikas@mail.sci.hokudai.ac.jp

The Northern Kurile Islands form the part of Kurile-Kamchatka volcanic arc. The Pacific plate has subductedbeneath the islands since the late Miocene to cause arc-type volcanism. We newly determined major and traceelement compositions, Sr-Nd isotopic variations of Quaternary rocks from 7 subarial and 3 submarine volcanoes.
Analysis of new and previous publications indicate that the Northern Kurile Islands belong to typical volcanic islandarc. About it there are indicated Ta, Nb minimum on the spider diagrams and naturally enriched of the LILE,LREE and depleted of the HFSE, HREE from front to back arc zone. Peculiarities of petrography and whole-rockchemistry enable us to divide all volcanoes into three main zones: frontal, intermediate and rear ones. Frontal zone
include Chikurachki, Tatarinova, Lomonosova, 1.3 volcanoes. The rocks are Ol-Cpx bearing Opx basaltic andesite.
Fuss, Antsiferova volcanic group and Ebeko volcano locates at the intermediate zone. Hbl-Cpx-Ol-bearing Opx
andesite are commonly characterized by the presence of hornblende phenocryst. Alaid, Grigoreva volcanic grouplocate at the rear zone. Ol-bearing Cpx basalts and basaltic andesite are typical. In addition, Alaid and Grigorev
volcanic group is characterized by the largest eruptive volume (150 km3 ). Frontal zone is characterized by lowest
contents of incompatible elements (Rb, Ba, K) and LREES (Nd, Ce). Isotopic variations have the highest value of143

Nd/144 Nd and 87 Sr/86 Sr as 0.7031-0.7034. In the opposite, rear and intermediate zones show narrower lower

contents of 143 Nd/144 Nd and 87 Sr/86 Sr as 0.7029-0.7031. The rocks of rear zone show highest contents of LILE
(K, Rb), LREES ( La, Gd, Nd, Sm) and HFSEE (Nb, Ta). Both 143 Nd/144 Nd and 87 Sr/86 Sr ratios of the rocks fromintermediate and frontal zones increase with increasing of silica contens. These suggest that andesitic and dacitic
rocks from these zones are possibly affected by crustal component. In contrast, crustal assimilation might be minor
process in the case of the rear zone, because basaltic rocks are predominant in the zone. Geochemical featuresof the mafic rocks investigate the spatial difference in magma sources of three zones. Rocks from rear zone aresystematically enriched in Nb/Y, Th/Yb, Ta/Yb, Nb/Yb, La/Yb ratios. These data are implied by the fact that magmain the rear zone more enriched with comparing depleted frontal zone. In addition, chemical variations of fluid-mobile
elements (Cs, Ba, U, Th, Sr) and immobile elements (Nd, Nb, Zr, Hf) of the mafic rocks will be explained by different
types of subduction components.
In summary, the following parameters have mainly affected the observed geochemical zonation across the arc in
the primary magma; variably depleted and enriched mantle source: the different type fluid flux from the slab to themantle wedge.

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106

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P21
40

Date/Time: July 24 Poster

Ar/39 Ar and U-Pb zircon ages provide new constraints on the history and magmatic
evolution of the central Aleutian arc
Brian R Jicha1 , Suzanne M Kay2 , Robert W Kay2 , Brad S Singer1
1

Department of Geoscience, University of Wisconsin-Madison, USA, 2 Department of Earth andAtmospheric Sciences, Cornell University, USA
E-mail: bjicha@geology.wisc.edu

New 40 Ar/39 Ar and U/Pb zircon ages from Adak, Kanaga, and Great Sitkin Islands supplement existinggeochronologic data and provide new information on the history and evolution of the central Aleutian arc. A clast
from a volcanic sequence in southeastern Adak in the Finger Bay Volcanic Formation, interpreted to be the oldest
unit on central Aleutian islands, yields an age of 38.19 + 0.53 Ma. This age is similar to previous ages from thisunit. However, all of the ages from the Finger Bay Volcanics are indistinguishable from the from 40 Ar/39 Ar and U/Pb
zircon ages of ca. 38 Ma from the tholeiitic Finger Bay pluton, which intrudes the Finger Bay Volcanics. So, either
the volcanism and plutonism were contemporaneous or the ages of the volcanics have been reset due to low grade
metamorphism during emplacement of the pluton. The latter scenario is likely because the metamorphic imprintof the pluton is similar to the observed overprint in the volcanics. It is also important to note that the Finger Bay
Volcanics on Adak are not related to the initiation of the central Aleutian arc as these rocks have meteoric not sea
water alteration and thus the arc must have been built above sea level when these lavas erupted. Lavas from theFinger Bay Volcanics on west-central Kanaga and southern Great Sitkin islands are much younger, ranging from
10.2 to 3.2 Ma, suggesting that previous geologic interpretations on these islands need to be re-evaluated. A new
40

Ar/39 Ar age of 34.35 + 0.05 Ma for a granodiorite in the calc-alkaline Hidden Bay pluton on Adak is in accord with

new U/Pb zircon ages in this sample, which also importantly shows no evidence of older zircons. Two mafic dikes
cutting the Hidden Bay pluton give ages of 33.36 + 0.27 Ma and 32.80 + 0.42 Ma. The central Adak Gannett Lake
pluton, which was assumed to have an age like the 14 Ma Kagalaska Island pluton to the east, yields an age of
31.68 + 0.06 Ma. Thus, the new age determinations indicate that calc-alkaline plutonism lasted for more than 3
Ma on Adak. The similarity of the trace element and isotopic signatures of the volcanic and plutonic rocks on Adakand Kanaga suggests that central Aleutian primitive arc magmas have changed little since at least 50 Ma except forincreases in Ba/La and Th/La ratios after 2 Ma, which are attributed to subduction of glacial sediments that flooded
the trench after 3 Ma. The near constancy of the mafic magma chemistry, the presence of calc-alkaline plutons
dominated by granodiorite after 35 Ma and a lack of low-K island arc tholeiites and boninites paints a very different
picture for the evolution of the central Aleutian arc than for western Pacific arcs.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

107

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P22

Date/Time: July 24 Poster

A petrological and isotopic study of volcanic rocks from Mt. Arakurayama and otherareas, North Fossa Magna, Central Japan
Akiyuki Narita, Masatsugu Yamamoto
Graduate School of Engineering and Resource Science, Akita University, Japan
E-mail: nrt.aky@gmail.com

The Arakurayama andesitic volcanic rocks (Pliocene) can be divided into the lower volcanic rocks, submarine fan
deposits (sandstone and siltstone of the Takafu formation), and the upper volcanic rocks. The lower volcanic rocks
consist mainly of hyaloclastite associated with pseudo-pillow lava and intraclastic siltstone. Some massive lavas
and agglomerate deposits including bombs are distributed in the central part of the lower volcanic rocks as proximal
facies. The upper volcanic rocks consist mainly of subaerial massive lavas.
The lower and upper volcanic rocks are andesitic in composition and contain phenocrysts of plagioclase, clinoand orthopyroxene, ± hornblende. The lower volcanic rocks have a wide range of bulk SiO2 contents (52-60
wt%) compared with the upper volcanic rocks (56-58 wt%), which may reflect contrasting P-T conditions ofcrystallization between the two magmas (i.e., of the lower and upper magmas, respectively). Geothermobarometry
using the chemical compositions of hornblende phenocrysts indicates that the lower magmas underwent polybaric
crystallization in deep (989-806 MPa, 1062-1034 °C) and shallow (374-174 MPa, 940-836 °C) chambers, whereas
the upper magmas crystallized in a single chamber at intermediate depths (789-387 MPa, 1033-928 °C). The
narrow range of SiO2 contents of the upper volcanic rocks indicates a simple fractional crystallization in a single
magma chamber, whereas the variable SiO2 contents of the lower volcanic rocks indicate complex magmatic
processes such as the mixing of magmas with different compositions in the deep and shallow magma chambers.
The Arakurayama volcanic rocks exhibit constant 143 Nd/144 Nd and variable 87 Sr/88 Sr ratios, showing a horizontal
trend in Nd-Sr isotopic systematics. This trend (i.e., the Arakurayama trend) is typical of volcanic rocks from theeastern and western margins of the Fossa Magna, including the Quaternary Myoko and Kurohime volcanoes (fromthe western margin, near the Itoigawa-Shizuoka Tectonic line), the Yoneyama area, and the Quaternary Naeba
volcano (from the eastern margin, near the Kashiwazaki-Chiba Tectonic line). A different trend is apparent for
igneous rocks from the central part of the Fossa Magna region, including Miocene-Pliocene granites and the Asama
volcano. This trend exhibits higher 143 Nd/144 Nd ratios than the Arakurayama trend. In a diagram of Nd-Sr isotopic
systematics, rock samples with the least radiogenic Sr isotopes in each trend are distributed in different partsof the mantle array field, indicating two different mantle components (i.e., pre-existing mantle and newly injected
asthenospheric mantle beneath the Fossa Magna area).

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108

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P23

Date/Time: July 24 Poster

Santorini Volcano plumbing system: constraints from melt inclusion volatile contents
Maxime MERCIER1 , Timothy H DRUITT1 , Etienne DELOULE2 , Nicolas CLUZEL1
1Laboratoire Magmas et Volcans, Blaise Pascal University, Clermont-Ferrand, FRANCE, 2 Centre deRecherches Petrographiques et Geochimiques (CRPG), Nancy, FRANCE
E-mail: m.mercier@opgc.univ-bpclermont.fr

Santorini Volcano has a long history of explosive volcanism, involving alternations of plinian and interplinian
eruptions, in two 180,000 year explosive cycles. At least four plinian eruptions generated calderas, the last occurringin the late 17th century BC (Minoan eruption).
We have measured H20 and CO2 contents of melt inclusions trapped in phenocrysts from the rhyodacitic productsof the Minoan eruption, from an eruption of primitive basalt discharged prior to the Minoan, and from the dacitic
products of the post-Minoan Kameni intracaldera volcano. The analyses were carried out by Secondary Ion Mass
Spectrometry and/or by micro-Raman spectroscopy on melt inclusions that were carefully chosen to ensure (1)complete isolation from the crystal exterior, and (2) an absence of any textural evidence for leakage.
Six melt inclusions in Fo85 olivines from the pre-Minoan basalt contain 2.8 to 3.2 wt percent H20 and 240 to
390 ppm CO2. Twenty eight melt inclusions trapped in plagioclase (An37-60) from the four explosive phases of the
Minoan eruption contain 3.3 to 6.1 wt percent H2O and 15 to 90 ppm CO2; the highest H2O contents are preservedin melt inclusions from the first eruptive phase, and the lowest from the fourth phase. Four inclusions in plagioclase(An48-56) in pumices from the 726 AD eruption of the Kameni volcano yielded 2.6 to 3.8 wt percent H2O and about
10 ppm CO2. All the analysed inclusions give H20+CO2 saturation pressures within a limited range (1.0 to 2.3 kb),
showing that all the magmas concerned were stored in the mid to upper crust prior to eruption. However, thesepressures are minimum estimates of total pressures, since they assume volatile saturation. Indeed the data for
the Minoan eruption suggest that the rhyodacitic magma was undersaturated in H20+CO2, as also concluded froma recent phase-equilibria study (Cadoux et al, 2013). The highest saturation pressure for Minoan magma comesfrom melt inclusions from the first eruptive phase, whereas the lowest pressures come from inclusions from thefourth phase. Assuming top-downward magma withdrawal during the eruption, the simplest interpretation is that
the Minoan rhyodacite was undersaturated in H2O+CO2 at all but perhaps the uppermost (first-erupted) levels.
Analyses of other units from the second explosive cycle are in progress, and will be presented as part of a growing
database on Santorini volatile contents and compositions.

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109

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P24

Date/Time: July 24 Poster

Three-dimensional seismic velocity structure of the upper crust beneath Kirishima
Volcanoes derived from local earthquake data
Hiroshi Yakiwara1 , Ryoma Mera2 , Syuichiro Hirano1 , Shigeru Nakao1 , Kazuhiko Goto1 , Hiroki
Miyamachi1
1Graduate School of Science and Engineering, Kagoshima University, Japan, 2 Faculty of Science,
Kagoshima University, Japan
E-mail: yakiwara@sci.kagoshima-u.ac.jp

Sub-Plinian and successive Vulcanian eruptions of Shinmoe-dake, Kirishima Volcanoes, started at January 2011.
Before the eruptions, inflation of the volcano edifice had been observed by GPS monitoring network (GEONET of
GJI). Combining the data of GEONET and temporal GPS observations, Nakao et al. (2012, submitted) located apressure source (Mogi model) at 10km depth beneath northwestern part of the volcanoes (5km northwest of the
Shinmoe-dake crater) for the period of magma accumulating process before the eruptions. Assuming incidence ofplane P waves from regional hypocenters and dividing a target volume into blocks of constant velocity, Yamamotoand Ida (1994) calculated P-wave velocity perturbations on each block. The other previous studies did not map thevelocity distributions deeper than about 5km depth by limitations of ray paths. The aims of the present research
are to show three dimensional P- and S-wave velocity models below 5km to 15km depth derived from data of local
earthquakes and to discuss the obtained seismic velocity structure and relation to the pressure source.
305 earthquakes with 15,221 P phases and 13,649 S phases recorded by 67 seismic stations (Kagoshima Univ.,
Kyushu Univ., JMA, and NEID in and around southern part of Kyushu) during the period from 2001 to 2012 wereselected to perform this analysis. In the 3-D inversion, we applied methods of grid model (Thurber, 1983), ray tracingwith Pseudo-bending (Um and Thurber, 1987), Parameter separation (Pavlis and Booker, 1980), and Damped Least
Squares (Aki and Lee, 1976). We also referenced results of checkerboard tests and diagonal elements of resolution
matrix (DERMs) to delineate velocity models of only areas where the relative reliable velocity distributions seemed
to be obtained.
As a result of the 3-D inversions, we obtained reliable P- and S-wave velocities at the depth range of 5-15km
beneath the area in and around the volcanoes. Characteristics of the velocity structure at 10km depth aresummarized as follows: (1) relative high P-wave velocities (high-Vp, 6.8-7.0km/s) distributed widely beneath thenorthwest, southwest, and southeast flanks of the volcanoes. The increases of Vp were 10-13 percent, (2)relative low P-wave velocities (low-Vp, 5.3-5.5km/s) areas, 11-15 percent decreases, were delineated beneath thewhole areas of the volcano edifices, (3) an obvious low S-wave velocity (low-Vs, 2.7-3.2km/s) area, 10-26 percent
decrease, located beneath the northwestern part of the volcanoes. The values of Vp/Vs for the characteristic low Pand S-velocity area were 1.9-2.1 (high-Vp/Vs). The obvious low velocity area contains the pressure source (Nakaoet al., 2012, submitted). These features, low-Vp, low-Vs, high-Vp/Vs, and containing the location of the pressuresource before the eruptions, suggest that a significant volume of magma accumulation existed at the low velocity
area and its environs.

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110

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P25

Date/Time: July 24 Poster

Ar-Ar dating for volcanic rocks from Bowers Ridge, Bering Sea at site U1342A andU1342D
Keiko Sato1 , Hiroshi Kawabata1 , Hironobu Hyodo2 , Hidenori Kumagai1 , David W. Scholl3 , Kozo
Takahashi4 , Katsuhiko Suzuki1
1

IFREE, JAMSTEC, JAPAN, 2 Research Institute of Natural Sciences, Okayama University of Science,
JAPAN, 3 U.S. Geological Survey, Menlo Park, University of Alaska„ USA, 4 Hokusei Gakuen University,
JAPAN
E-mail: keisato@jamstec.go.jp

Basement rocks were drilled down to ca. 42 m into the volcanic sequence directly underneath the sedimentary
section at Site U1342 on Bowers Ridge during the IODP Expedition 323 to the Bering Sea. This provided us an
opportunity to describe the details of the sequence and to decipher the virtually unknown origin and evolution of theBowers arc massif. There are two contrasting hypotheses for the origin of the arc, which include formation in the
Pacific Basin well to the south of its present location during the Cretaceous and in-situ formation within the Bering
Sea in Eocene.
The volcanic sequence recovered from Site U1342D was divided into six major lithological units: Unit 1, vesiculated
andesitic lava flow; Unit 2, interbedded volcanic sandstones and polymict volcanic conglomerates; Unit 3: monomict
volcanic conglomerates; Unit 4, interbedded volcanic sandstones and polymict volcanic conglomerates; Unit
5, monomict volcanic conglomerates; and Unit 6, polymict volcanic conglomerates. Units 3 and 4 represent
hydroclastic volcaniclastics, while units 2, 4, and 6 are epiclastic volcaniclastics (Kawabata et al., 2011). We
used the single grain Ar-Ar dating method by step-wise laser fusion for Unit 1 basaltic andesite rocks.
We collected basement volcanic rock samples from U1342A 9x section and U1342D 7x 19x sections. The
vesiculated porphyritic basaltic andesite at U1342A 9x and U1342D 7x- (Unit 1) and the porphyritic basaltic andesite
at U1342D 8x (Unit 1) are fresh samples and are good for single grain method of step-wise laser fusion Ar-Ar
dating. We distinguish for the first time two stage (age groups) of activity (34-32Ma and 28-26Ma) from our Ar-Ar
data, coupled with those from Wanke et al., (2012).

©IAVCEI 2013 Scientific Assembly. All rights reserved.

111

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P26

Date/Time: July 24 Poster"Hot Zone" development beneath a long-lived andesite stratovolcano: Temporal
evolution of the Mt. Taranaki (New Zealand) magmatic system
Anke V Zernack1 , Richard Price2 , Robert B Stewart1 , Shane J Cronin1
12Volcanic Risk Solutions, Massey University, Private Bag 11222, Palmerston North, New Zealand,
School of Science and Engineering, University of Waikato, Private Bag 3105, Hamilton, New ZealandE-mail: a.v.zernack@gmail.com

Mt. Taranaki (Egmont Volcano) in the western North Island of New Zealand is a high-K andesite volcano withan eruptive history extending over more than 200,000 years. Past petrological research has concentrated mainlyon the post-10 ka record of the modern edifice due to poor stratigraphic control of the older deposits. The earlier
history is recorded in 14 major pre-7 ka debris-avalanche deposits, which formed as a result of a catastrophic
collapse of the edifice at the time. The clast assemblages of 12 of these deposits were sampled to provide insights
into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form thebasis for a more complete chemo-stratigraphic analysis of the Mt. Taranaki volcanic succession.
Throughout the volcanic history of Mt. Taranaki, similar eruptive styles produced a similar range of lithologies,
indicating a long-term sustainability of the volcanic system.

However, sample suites from the studied

debris-avalanche deposits show a progressive enrichment in K2 O and LILE, reflecting a gradual evolution tohigh-K andesite. The early (pre-100 ka) magmatic system produced a wide range of compositions, including
relatively primitive basalts and basaltic andesites with a more distinct mantle signature. These rocks containphenocryst assemblages that indicate crystallisation within the lower crust or mantle, including a broad range ofclinopyroxene compositions, high-Al2 O3 hornblende, olivine and phlogopite. The compositional variation observed
for the more primitive compositions also reflect a heterogeneous mantle source for primary Taranaki magmas.
A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage
low-pressure mineral phases, such as high-TiO2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradualshift to more evolved magmas with time.
These results are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated
within a lower crustal "hot zone", which formed as a result of repeated intrusions of primitive melts into the lower
crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin andcold, allowing primitive magmas to rise rapidly through the crust without significant interaction and modification.
As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted,
and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and
LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in theupper crust where the hot-zone melts were further modified by fractional crystallisation and magma mixing andmingling.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

112

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P27

Date/Time: July 24 Poster

Crustal structure of preexisting Mesozoic basement beneath the Northern Izu-Bonin Arc
Kenichiro Tani1 , Osamu Ishizuka2 , Hayato Ueda3 , Hiroshi Shukuno1 , Yuka Hirahara1 , Alexander R.L.
Nichols1 , Kenji Horie4 , Daniel J. Dunkley5 , Akira Ishikawa6 , Tomoaki Morishita7
1

Japan Agency for Marine-Earth Science and Technology, Japan, 2 Geological Survey of Japan/AIST,Japan, 3 Hirosaki University, Japan, 4 National Institute of Polar Research, Japan, 5 Curtin University,
Australia, 6 University of Tokyo, Japan, 7 Kanazawa University, Japan
E-mail: kentani@jamstec.go.jp

The Izu-Bonin Arc is widely regarded to be a typical intra-oceanic arc, with the oceanic Pacific Plate subducting
beneath the Philippine Sea Plate, an evolving complex of active and inactive arcs and back-arc basins. However,
little is known about the proto-Philippine Sea Plate, which existed along with the Pacific Plate at the time of
subduction initiation and before the formation of back-arc basins.
To investigate the crustal structures of the proto-Philippine Sea Plate, we conducted manned-submersible
SHINKAI6500 surveys during cruise YK10-04 of the R/V YOKOSUKA in April 2010 at the Daito Ridges. The
Daito Ridges comprise the northwestern Philippine Sea Plate along with what are regarded as remnants of the
proto-Philippine Sea Plate. Submersible observations and rock sampling revealed that the Daito Ridges expose
deep crustal sections of gabbroic, granitic, and metamorphic rocks, along with volcanic rocks ranging from basalt
to andesite. Jurassic to Cretaceous magmatic zircon U-Pb ages have been obtained from the plutonic rocks,
and whole-rock geochemistry of the igneous rocks indicates arc origins. Furthermore, mafic schist collectedfrom the Daito Ridge has experienced amphibolite facies metamorphism, with phase assemblages suggestingthat the crust was thicker than 20 km at the time. These finds show that the Daito Ridges represent developed
crustal sections of the Mesozoic arc that comprises part of the proto-Philippine Sea Plate, and, together with thetectonic reconstruction of the proto-Philippine Sea Plate (Deschamps and Lallemand 2002, JGR), they suggest that
subduction of the Izu-Bonin Arc initiated at the continental margin of the Southeast Asia, possibly correlating with
the Mesozoic island-arc and ophiolite complexes exposed in the southwest Pacific margins, such as those in thePhilippine Islands.
Furthermore, detrital zircon ages from volcaniclastic sandstones collected from northern Izu-Bonin forearc,
counterpart of the Daito Ridges, yield Mesozoic to Paleozoic ages, indicating that similar Mesozoic basement
may even exist beneath the present Izu-Bonin Arc. To confirm this hypothesis, we have conducted a SHINKAI6500
survey on the landward slope of the northern Izu-Bonin Trench during cruise YK11-07 of the R/V YOKOSUKA in
September 2011. The collected samples are dominantly andesite with two diorite samples, and preliminary zircon
U-Pb dating of the diorite sample yielded Cretaceous ( 100 Ma) magmatic age as well as abundant Paleozoic toProterozoic detrital zircons. Preliminary whole-rock geochemistry of the andesite and diorite samples show clear
arc-signatures, confirming that a preexisting basement composed of Mesozoic arc crust underlies at least part ofthe present northern Izu-Bonin Arc.
These new insights on the crustal structure of the proto-Philippine Sea Plate and the discovery of preexistingMesozoic arc basement beneath the Izu-Bonin Arc raise serious doubts about the intra-oceanic nature of the
Izu-Bonin Arc.

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113

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P28

Date/Time: July 24 Poster

Imaging rapidly deforming ocean island volcanoes in the western Galápagos
archipelago, Ecuador
Gabrielle Tepp1 , Cynthia Ebinger2 , Mario Ruiz3 , Manaloh Belachew2
1

Department of Physics and Astronomy, University of Rochester, USA, 2 Department of Earth andEnviromental Sciences, University of Rochester, USA, 3 IGEPN, Ecuador
E-mail: gtepp@pas.rochester.edu

Using finite-difference body-wave tomography methods to determine 3D seismic velocity structure, we imaged, forthe first time, the plumbing system of Sierra Negra volcano, Galapagos, as well as parts of the island platform.
This volcanic chain overlies a hotspot track and includes some of the fastest deforming volcanoes in the world,
making this an ideal location for studying shield volcano plumbing systems. We inverted P- and S-wave arrivals
recorded on a 15-station temporary array, SIGNET, installed between July 2009-June 2011 and used a minimum
1-D velocity model constrained by offshore refraction studies. Owing in part to seismicity from nearby volcanoes
in the chain, as well as the frequent earthquakes along the caldera ring fault system, the model resolution is good
between depths of 3km and 15.5 km beneath Sierra Negra, and adjoining Cerro Azul. Our results indicate thata shallow melt sill below the caldera, interpreted from geodetic data, must be shallower than ∼4 km subsurface.
The 10 km wide Sierra Negra caldera and ring fault systems are underlain by two small (<50 km³) low-velocity
zones, one just east of the caldera and one to the west. Another small (∼70 km³), shallow low-velocity zone isimaged beneath a line of eruptive centers with shallow earthquakes. We image a high-velocity zone around the siteof a lower crustal magma intrusion in June 2010. Here, the steepest velocity gradients are ringed by the intense
earthquake swarms, suggesting that the intrusion occurred at a petrological and/or thermal boundary. The crustal
velocity beneath the northern flanks of Sierra Negra and the platform between Alcedo volcano to the north is lower
than the initial 1-D model. Although resolution is poor at depths greater than ∼15 km, we find no evidence for avelocity increase marking the base of the crust, suggesting that the crust is thicker than implied from earlier gravity
models. Our results, interpreted in light of results of geodetic and petrological studies, provide tests of models forthe construction and evolution of ocean island volcanoes, and help constrain the processes of oceanic platform
construction above mantle plumes.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

114

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P29

Date/Time: July 24 Poster

Mingling-induced crystallization from Saruana Lava, Chokai, Tohoku Japan, and theorigins of end member magmas
Tsukasa Ohba1 , Hajime Oikawa1 , Shintaro Hayashi1 , Toshiro Takahashi2 , Yuka Hirahara2 , Masao Ban3 ,Akihiko Fujinawa4 , Jun-Ichi Kimura2 , Takashi Miyazaki2 , Qhan Ching2
1

Akita University, JPN, 2 IFREE/JAMSTEC, JPN, 3 Yamagata University, JPN, 4 Ibaraki University, JPN
E-mail: t-ohba@gipc.akita-u.ac.jp

Crystallization process during magma mixing was investigated for Saruana lava, West Chokai, Tohoku Japan,
by extensive petrological analyses on phenocryst minerals. Examined rocks are basaltic andesite containing
phenocrysts of cpx, opx, olivine, plagioclase, magnetite, spinel, apatite, and minor hornblende. Magma mixing issuggested by petrological evidence such as: disequilibrium relation of Fe-Mg partitioning in mafic minerals, complex
chemical zoning, dissolution textures, heterogeneous compositions in a single lava, etc. Basic cores of mafic
minerals, calcic part of plagioclase, and chromian spinel were derived from the mafic end member magma (highMg-basalt), whereas Mg-poor pyroxene cores, sodic plagioclase cores, magnetite, and apatite were from the felsic
end member (crystal-rich andesite). Each phenocryst crystallized in at least two different host magmas in terms oflarge variations in Mg-values of mafic minerals and An in plagioclase. In a cpx crystal that have an Mg-rich core(Mg-values:85-87), the core is surrounded by an oscillatory-zoned intermediate margin with polygonal growth bands
surrounded by an Mg-poorer thin rim. In an Mg-poor-cored pyroxene, a round Mg-poor core (Mg-values:65-67) issurrounded by an Mg-richer margin of which zoning pattern is similar to the Mg-rich-cored cpx. The oscillatory
zoning was formed in the cooled and partially hybridized mafic magma during the thermal equilibration between
the magmas. The shape of the core is rounded and this implies dissolution of the Mg-poor pyroxenes by heating of
the host felsic magma or by incorporation of the Mg-poor phenocryst into a hot mafic magma. All complex zoning
profiles of the orthopyroxene and the plagioclase phenocrysts can be accounted for by the similar process withthat for cpx. Whole-rock chemistry and isotopic compositions provided further insight into chemical natures of end
member magmas and their origins. Major element composition of the felsic end member is similar to that of theunderlying lava, which is a typical calc-alkaline andesite. The mafic end member is an Mg-rich high-K basalt of
which composition is similar to that of the basaltic lava occurred in a drillhole near the vent of Saruana lava. Sr,
Nd, Pb, and Hf isotopic analyses imply that the source of the basaltic magma is as depleted as MORB source. The
isotopic nature of the source mantle is different from those for the volcanic front where the source mantle is moreenriched.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

115

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P30

Date/Time: July 24 Poster

The initiation of crustal recycling processes in island arcs: insights from the Soufriere
Volcanic Centre, Saint Lucia
Jan Lindsay1 , Axel Schmitt2 , Robert Trumbull3 , Daniel Stockli4 , Philip Shane1 , Tracy Howe1
1

School of Environment, The University of Auckland, Auckland, New Zealand, 2 Department of Earthand Space Sciences, University of California Los Angeles, USA, 3 GFZ German Research Centre forGeosciences, Telegrafenberg, 14473 Potsdam, Germany, 4 Department of Geological Sciences, Jackson
School of Geosciences, University of Texas at Austin, USA
E-mail: j.lindsay@auckland.ac.nz

Extensive intermediate pyroclastic flow deposits, lava flows, domes and block-and-ash-flow deposits from the
Soufriere Volcanic Complex (SVC), Saint Lucia, collectively represent a conspicuous shift in composition anderuption style for this island arc segment. Compositionally, these dominantly silicic (61.6 to 67.7 wt. percent SiO2)
SVC deposits are distinct from older more mafic deposits in Saint Lucia. They contain accessory zircons, and manyof the most recent rocks have phenocrystic quartz. SVC deposits occur in and around the Qualibou Depression,a c. 10 km diameter wide sector collapse structure of poorly constrained age. Here, we re-investigate the onsetof silicic volcanism in Saint Lucia, and the relationship between individual SVC pyroclastic flows and the sector
collapse through (U-Th)/He, U-Th and U-Pb zircon chronostratigraphy, aided by mineralogical and geochemicalcorrelation.

The oldest SVC units comprise pyroclastic deposits at Micoud (640+/-8 ka), Bellevue (264+/-8 ka), AnseJohn (104+/-4 ka) and La Pointe (59.8+/-2.1 ka), and at Anse Noir and Piaye (undated). These were all previouslygrouped with or associated with the so-called Choiseul tuff, whereas our chronology demonstrates that the units
represent individual periods of activity spanning a range of ages. Choiseul pumice at the type locality has yielded a(U-Th)/He zircon age of 515+/-61 ka, overlapping with the age for Morne Tabac (532+/-42 ka), a dome truncated by
the depression escarpment. Younger SVC units comprise Morne Bonin (273+/-15 ka), Gros Piton and Petit Piton(71+/-3 ka and 109+/-4 ka, resp.), Belfond (13.6+/-0.4 ka) and Terre Blanche (15.3+/-0.4 ka). The younger units areclearly domes within the Qualibou Depression whereas the nature of Morne Bonin (dome or mega-block) remains
poorly constrained. Belfond and Terre Blanche have whole rock geochemistry and mineral assemblages similar to
the Belfond pyroclastic flow deposit, the most recent widespread pyroclastic flow with nearly-concordant (U-Th)/He
and 14C ages of 20 ka. The Belfond and Terre Blanche domes possibly represent late-erupted degassed portionsof the magma that produced the Belfond pyroclastic flow.

The geochemical characteristics and similar zircon age distributions of the silicic lava domes and pyroclastic
flows of the SVC suggest that these share a common magma source beneath the Qualibou depression. The
onset of activity from this proto-Qualibou centre occured much earlier than previously thought (by c. 500 k.a.), andmarks a change from largely quartz- and zircon-free andesitic magmatism to more evolved magma compositions.
We interpret crystal recycling in SVC magmas, and the presence of co-genetic plutonic enclaves, to indicate theprogressive build-up of a mid-crustal batholith, and thus a major change in the crustal structure of this island arc
segment.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

116

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P31

Date/Time: July 24 Poster

Primary boninite magmas explored from melt inclusions
Keitaro Kitamura, Susumu Umino, Kyoko Kanayama
Department of Earth Sciences, Kanazawa University, Japan
E-mail: keitarou@stu.kanazawa-u.ac.jp

Boninite melt inclusions were analyzed in Cr-spinel collected from beach sand from Mukojima, Chichijima, Anijima
and Ototojima, Bonin Islands. The common constituents of the inclusions are quenched glass, daughter minerals
which grew after trapping, and euhedral crystals of olivine, orthopyroxene and clinopyroxene that were trapped withthe surrounding melt, and shrinkage bubbles. We classified the inclusions into 6 types (Type-A–F) based on the
combination of the constituents and their textures. The difference in these inclusion types inherited from the melt
compositions and the cooling history dependent on the mode of occurrence of the host rocks. Because of thehomogeneity of the glass in inclusions and the host spinel adjacent to the inclusion walls, the bulk compositionsof quench glass with or without quench crystals (Type-A, B, E and F) are considered to represent the liquidcompositions when captured by the host spinel. Major and trace element compositions of melt inclusions showa wide range of boninitic compositions with SiO2 53-63 wt.%, TiO2 0.02-0.25 wt.%, Al2 O3 6-13 wt.%, MgO 8-23
wt.%, CaO 4-11 wt.%, Zr/Ti 0.01-0.04, Gd/Lu 2.4-7.2 and Gd/La 0.24-0.87. We classified the compositions of melt
inclusions into five types. BIC (Boninite Inclusion Compositions)-1 is characterized by a low-SiO2 trend and belongs
to high-CaO boninite series. It has a subhorizontal chondrite-normalized pattern with high REE abundances. Thesefeatures are similar to low-Si boninite by Kanayama et al. (2012). BIC-2 is characterized by relatively high CaO
content and low Zr/Ti, which resembles to high-Si type 2 boninite by Kanayama et al. (2012). BIC-3 shows a low-Ca
and low-SiO2 trend. BIC-4 and BIC-5 are typical low-Ca boninite with high Zr/Ti ratios and high SiO2 contents.
However, BIC-4 has a lower Al2 O3 content than BIC-5 and a U-shaped chondrite-normalized REE pattern and ismostly sampled from Mukojima. On the other hand, BIC-5 shows a characteristic V-shaped chondrite-normalized
REE pattern. In general, BIC-types show a systematic relationship with the host Cr-spinel compositions. BIC-1,
2 and 3 are hosted by low-Cr# spinel, whereas BIC-4 and 5 are hosted by high-Cr# spinel. I have estimated thepressures and temperatures for the most primitive melt inclusions in each BIC type to have been in equilibrium withthe mantle peridotite under anhydrous or hydrous conditions by using the olivine-liquid geothermometer by Putirka
et al. (2007) and the olivine-orthopyroxene-liquid geobarometer by Putirka (2008). As a result, it is shown that
BIC-1 type was generated in higher pressure condition than other BIC types.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

117

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P32

Date/Time: July 24 Poster

None of us belong: crystal chemistry of andesites in Dominica, Lesser Antilles
Tracy M Howe1 , Jan M Lindsay1 , Phil A Shane1 , Axel K Schmitt2 , Danny Stockli3
1

University of Auckland, New Zealand, 2 University of California, Los Angeles, United States, 3 Universityof Texas, Austin, United States
E-mail: t.howe@auckland.ac.nz

As the simplest subduction setting, island arcs provide a unique chance to study the magmatic processes involvedin andesitic eruptions. Located in the centre of the Lesser Antilles arc, Dominica has been the site of andesitic
volcanism for >500,000 years. Eruptions from the island produce dominantly porphyritic andesite with a phenocryst
assemblage of plagioclase (An54-92), clinopyroxene (En64-69), orthopyroxene (En52-58) and Fe-Ti oxides withless common amphibole and olivine. Microprobe spot and profile analyses were undertaken on all phenocryst
phases in order to constrain petrogenetic and ascent processes. While single plagioclase and amphibole profiles
within individual units lack core to rim zonation, each mineral population within the units displays significant chemicalvariability. Combined with the lack of equilibrium between phenocrysts and melt, this indicates a mineral-melt
decoupling which may be the result of magma mixing or the accumulation of crystal cargo during ascent from source
to surface. The homogenous crystal profiles seen in all mineral phases across all units suggest the existence of
locally homogeneous magma reservoirs in the crust. Amphibole pressures and the rhyolitic nature of matrix glasssuggest that such reservoirs were relatively shallow ( 3-5 km). Model temperatures calculated for individual phases
and equilibrium phase pairs also support mineral-melt decoupling, with Fe-Ti oxides (avg. 800°C), amphiboles(avg. 750°C), and high-An plagioclase (avg. 950°C) displaying different temperatures. Our findings suggest thatthe phenocryst assemblage in the Dominican andesites is the result of near-equilibrium crystallization in evolved
and primitive melts, followed by subsequent mixing which resulted in a non-equilibrium crystal cargo. U-Th zircon
geochronology provides further evidence for recycling, with most lava samples showing multiple crystallization
events prior to eruption. This work provides evidence for widespread disequilibrium between magmas and theircrystal cargo, which can be explained by a long-lived crystal mush beneath the silicic centers in Dominica. These
centers, which only recently emerged compared to the overall longevity of the arc, thus may indicate the initiationof a batholithic crust of continental affinity in an island arc.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

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