Seminar Schedule < Institute For Research on Earth Evolution

2010

● IFREE MC seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 9th, December (Thursday)
Speaker: Toshiro Takahashi (Mantle Evolution Research Team)

Title: Preliminary petrological study of Quaternary Chokai volcano -based on bulk Sr isotope ratio-

Abstract:
  The island arc magmatism is characterized by the coexistence of
tholeiitic series (TH) and calc-alkaline series (CA), and many works
have discussed their genetically related processes. Generally, magma
generation and evolution process of TH and CA have been thought
that the former is produced by fractional crystallization from
mantle-derived basalt magma and the latter is formed by magma mixing
between basic and felsic magmas. However, based on Sr isotope
micro-analyses of plagioclase phenocrysts in basalt and andesite
lavas from Zao volcano, Tatsumi et al. (2008) argued that TH basalt
was formed by melting of lower-crustal amphibolite, and CA was
formed by magma mixing between mantle-derived basalt magma and
the crust-derived basaltic andesite to dacite magmas. Then, We
studied another example from Azuma volcano in the NE Japan arc
which verifies our model. And, we conclude that the magma
generation and evolution model proposed by Tatsumi et al. (2008) is
applicable to the Azuma volcanic rocks. Next our object is Chokai
volcano in back arc side of NE Japan arc, and we are conducting a
detailed investigation of Chokai volcanic rocks now.
  The Quaternary Chokai volcano is located at the back arc side of
NE Honshu arc, and is a typical stratovolcano in Chokai volcanic
zone. Chokai volcano activity is divided into Stage 1 (0.6 Ma to
0.12 Ma) to Stage 3 (0.02 Ma to present) (Hayashi, 1984; Ban et
al., 2001). Stage 1 consists mainly of two-pyroxene andesite with
a minor amount of olivine two-pyroxene basalt. Stage 1 rocks
have not disequilibrium texture (Un-mixed rocks) or rarely have
plagioclase phenocryst which has dimly dusty zone (Less-mixed
rocks). Stage 2 are composed largely of olivine two-pyroxene
andesite with a small amount of olivine two-pyroxene basalt. Most
of them contain hornblende as phenocryst. Stage 3 rocks are
olivine two-pyroxene andesite and two-pyroxene andesite. The
almost plagioclase phenocryst in Stage 2 and 3 rocks has dusty
zone and sieve texture (Mixed rocks).
  Chokai volcanic rocks are plotted on boundary of high-K and
medium-K on the SiO2 vs. K2O diagram. On the FeO*/MgO vs.
SiO2 diagram, trend of Stage 1 rocks (Un-mixed and Less-Mixed
rocks) and Mixed rocks show the tholeiitic signature and calc-
alkaline series, respectively. The range of bulk Sr isotope ratio of
Un-mixed rocks, Less-mixed and Mixed rocks are very similar
(Un-mixed: 0.70316 to 0.70334, Less-mixed: 0.70303 to 0.70341,
Mixed: 0.70297 to 0.70342). But, Sr isotope ratio of Un-mixed and
Less-mixed rocks is constant or locks slightly ascent with
increasing SiO2, whereas Mixed rocks is distinctly ascent with
increasing SiO2.
  Petrographical and petrological feature of Chokai volcanic rocks
indicate that Stage 1 is produced by fractional crystallization from
basic magma (but, it has the possibility that magma mixing was
involved in magma evolution process of Less-mixed rocks) and
Stage 2 and 3 are formed by magma mixing between basic and
felsic magma. And, it is thought that parent magma of Stage 1 and
the basic end-member magma of Stage 2 and 3 have different
geochemical features. However, the origin of two basaltic magma
was not able to be specified at present. We need to study the
matter further by Sr isotope micro-analysis of plagioclase
phenocryst.

● IFREE 2C & IFREE 3 joint seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 25th November(Thursday)
Speaker: Kenji Shimizu (Earth Surface and Interior Co-Evolution Research Team)
Title: Chemical compositions of volcanic glasses from ~145Ma Shatsky Rise, Northwest Pacific Ocean

Abstruct:
Shatsky Rise is a Late Jurassic-Early Cretaceous oceanic
plateau located in the northwestern Pacific. It consists of
three major massifs: Tamu, Ori and Shirshov Massifs from
southwest to northeast. The Rise is 1000km x 200km in size,
similar to Japan or California State. Seafloor magnetic
anomalies around Shatsky Rise indicate that the massifs get
younger toward the northeast and have formed along the trace
of a triple junction at a mid- oceanic ridge. During the IODP
Expedition 324 (Sept.-Nov., 2009) five sites were drilled on
Shatsky Rise, two on Tamu Massif (Sites U1347 and U1348; east
flank and north flank), two on Ori Massif (Sites U1349 and
U1350; summit and east flank) and one on Shirshov Massif
(Site U1346; summit). Unaltered fresh volcanic glasses were
sampled from all sites except at Site U1349. Glass is preserved
as < 2cm thin rims of flow margins or occur as groundmass in
volcanic breccias. Major and trace element compositions of 115
volcanic glasses were analyzed by EPMA and LA-ICP-MS at IFREE,
and about half of them (57 samples) were selected for volatile
(H2O, CO2, F, Cl and S) analyses by SIMS at WHOI. All glasses
are sub-alkalic (tholeiitic) basalts, with MgO and SiO2 contents
ranging from 5 to 8.5 wt% and from 48.5 to 52 wt%, respectively.
Three types of magmas (high-TiO2, low-TiO2 and high-K2O) are
identified in this study, but most of the basaltic glasses
are high-TiO2 type. The trace element patterns of the high-TiO2
type glasses suggest that they formed by melting at deeper depths
within the garnet- stability field (~3GPa), whereas the low-TiO2
and high-K2O magmas may have formed by melting at shallower depths,
where spinel is stable (~1 GPa). However, the occurrence of
volcanic glasses with high Nb/U ratios (80-120 vs. 40-60 for
other glasses and MORB) from Site U1348 and high-K2O type from
Site U1350 indicate that the volcanic rocks of Shatsky Rise
require more than one mantle source. Volatile contents of
glasses from Site U1347 (H2O: 0.2-0.6 wt%, Cl: 100-550 ppm
and S: 250-700 ppm) can be explained by seawater assimilation,
whereas those of glasses from Site U1350 (H2O: 0.2-0.6 wt%,
Cl: 600-1200 ppm and S: 750-1500 ppm) can be explained by
assimilation of 40% NaCl brine with high S content. The CO2
contents of all volcanic glasses are low (<40 ppm),
indicating that the lavas were erupted and degassed in
shallow water (< 1000m).

● IFREE MC seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 11th, November (Thursday)
Speaker: T. Miyazaki and B.S. Vaglarov (IFREE 3B)

Title： Automated column separation for isotope analysis

Abstract：
Sr-Nd-Pb-Hf isotope ratios are fundamental forms of
geochemical information in earth science. The development of
precise analytical methods leads to increased number of
important samples we want to process. However, the
preparation for isotope analysis and especially column
separation, are still time consuming procedures compared to
the actual measurement with TIMS or MC-ICP-MS. For marked
increase of sample data in near future, it is necessary for
us to save valuable time particularly during sample preparation.

For this purpose, JAMSTEC and HOYUTEC Co. Ltd. developed the
fully automated open type column separation system" PAT. P.
In this seminar, I will talk about this new development in
detail.

● IFREE MC seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 28th, October (Thursday)
Speaker: Shigehiko Tateno (Tokyo Institute of Technology)

Title: Ultrahigh-pressure experiment: Journey to the center of the Earth

Abstract:
The Earth’s solid inner core, the deepest part of our
planet, is mainly composed of iron. Because the relevant
ultrahigh pressure and temperature conditions could only be
reproduced by dynamical shock-wave compression experiments,
the crystal structure of iron at the inner core has been
uncertain. Our static experiments show for the first time
that the hexagonal close-packed (hcp) structure is a stable
form of iron up to 377 gigapascals and 5700 Kelvin,
corresponding to the condition at the center of the Earth.
The inner core seismic anisotropy may be therefore caused by
the preferred orientation of the hcp phase.

● IFREE MC seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 7th, October (Thursday)
Speaker: Takeshi Hanyu (IFREE 3B)

Title: Noble gas systematics of the HIMU basalts; new evidence for recycling of the subducted slab and its impact on U, Th and K budget in the mantle

Abstract:
It has been demonstrated in the previous studies that HIMU
basalts show characteristically low 3He/4He relative to
MORBs and other OIBs, and this suggests that the HIMU
reservoir is related to the ancient subducted slab with high
(U+Th)/He. In order to further constrain the origin of the
HIMU reservoir, new He-Ne-Ar isotope data are presented for
the HIMU basalts from Cook-Austral Islands in the South
Pacific. Using 4He/21Ne as a monitor of elemental
fractionation by magma degassing, fractionation-corrected
4He/40Ar of the HIMU basalts (>10) are higher than those of
MORBs and other OIBs (2-5). 4He/40Ar can be converted to K/U
of the source, because 4He and 40Ar have been produce by
decay of U+Th and 40K, respectively. As a result, estimated
K/U of the HIMU reservoir (2000-4000) is significantly lower
than that of the depleted and primitive mantle (around
13000). Relatively low K/U of the HIMU reservoir is best
explained by the model that it involved the subducted
oceanic crust that had lost K relative to U through
subduction dehydration.
If the subducted oceanic crust, which can be a significant
host of U+Th but not of K, established an isolated reservoir
with no small volume, K/U of the bulk silicate Earth needs
modification from the previously estimated value. The mass
balance calculation demonstrates that if the oceanic crust
ever subducted into the mantle has been accumulated to
occupy the mantle by 6-9 % in volume, the K/U of bulk
silicate Earth is reduced from the previously estimated
value (13000) to 7000-9000. Furthermore, U and Th would be
unevenly distributed in the mantle and concentrated where
the subducted slabs have resided. These facts may require
re-consideration of existing models for not only geochemical
evolution but also thermal history of the mantle.

● IFREE MC seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 30th, September (Thursday)
Speaker: Hiroshi Kawabata (IFREE 3B)

Title: Two primary magmas derived from Samoan mantle plume: FOZO-type tholeiitic and EM2-type alkalic magmas

Abstract:
We examined alkaline volcanic rocks from Tutuila Island in
Samoa in order to constrain the origin of chemical
heterogeneity in mantle. Bulk rock compositions of Tutuila
volcanic rocks show a binary mixing array on the plots using
Sr-Nd-Pb-Hf isotopic and element ratios. One mixing
end-member magma with FOZO-type isotopic signature shows
higher Zr/Nb and lower LREE/HREE ratios than the other
end-member with EM2-type isotopic signature. Nature of the
two end-member magmas is further constrained by mineral
compositions and phenocryst assemblage. The EM2-type basalts
contain diopsidic clinopyroxene with high LREE/HREE ratios
together with olivine and/or plagioclase. In contrast, the
FOZO-type basalts include augitic clinopyroxene with low
LREE/HREE ratios together with orthopyroxene having a
reaction rim. Presence or absence of othopyroxene and the
discrimination of magma type using clinopyroxene
compositions indicate that the FOZO-type and EM2-type
end-member magmas are tholeiitic and alkalic basalt,
respectively. Geochemical modeling suggests that the
FOZO-type tholeiitic magma requires the involvement of
recycled oceanic crust in their source mantle.

● IFREE 2C & IFREE 3 joint seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 26th, August (Thursday)
Speaker: Jun-ichi Kimura
Title: Role of open system melting beneath mid ocean ridges: implications from the Red Hills peridotite, New Zealand.

Abstract:
Melting processes beneath mid ocean ridges (MORs) have been
a central research subject because oceanic lithosphere is
formed at MORs. Melting of a column of depleted MOR basalt
(MORB) source mantle (DMM) has been examined by varying melt
coalescence processes and spreading geometry. We here
propose a melting model that explains both the residual DMM
and MORB melt compositions. We use the Red Hills peridotite,
New Zealand, which represents the residual DMM (Sano &
Kimura, 2007; Journal of Petrology, 48, 113-139).
Fractional to batch melting in porous flow mechanism are
commonly used in such models, although the possibility of an
alternative mechanism, reactive flow (or open system
melting), has been much debated since 1990. Reactive flow
was developed in response to petrological observations made
on the MOHO-mantle transition zone in relatively undeformed
ophiolites. Here dunite channels, on various scales, occur
in a harzburgite matrix, with an association of wehrlite
mainly in the dunites. The dunite channels have been
interpreted to be formed by reactions between harzburgites
and melts and are ascribed to melt channels that were formed
by focused MORB melt flow. Melt focusing is a well-known
property that occurs in solid media during the formation of
corrosive melt flow channels. However, a difference in
chemical potential is required between the host harzburgites
and melts to form the channel structures in mantle
peridotite. Consumption of clinopyroxene by olivine
saturated MORB melt is one possibility; however, it is not
applicable to a harzburgite host consisting mostly of
olivine and orthopyroxene. Chemical stoichiometry of the
dunite forming reaction in harzburgite has not yet been
proved. Previously we have reported modes, major and trace
element compositions of bulk rock and minerals in the Red
Hills peridotite (Sano & Kimura, 2007), and discussed the
role of chromatographic (porous) melt flow in forming the
chemical variety observed in the harzburgite matrix.
However, the origin of dunite channels has still not been
well constrained.

Here we examine a two stage melting model that explains the
formation of the dunite channels. In the stage 1, we assume
the formation of a variably depleted harzburgite by
fractional melting in the garnet stability field (2.5 GPa),
and in the stage 2, reactions between various melts and
residual solids from stage 1 in the spinel stability field
(1 GPa) form dunite channels. The residual solids from stage
1 are silica oversaturated (orthopyroxene-rich) harzburgite
and the fractional melts are silica under-saturated basalts.
Therefore, a corrosive reaction between the solids and the
melts should occur when they meet at shallower depth during
stage 2. The upwelling velocity of the solids is in the
range of half-spreading rates (as much as 8 cm/y at ultra
fast spreading ridges), whereas U-Th disequilibrium
indicates melt transfer to be as much as 15 cm/y. Therefore,
melt upwelling is about twice as fast as solid upwelling in
the MOR system allowing disequilibrium transport of stage 1
melts and solids, which can then contribute to the stage 2
reaction between the stage 1 products. Based on the model,
mode, bulk rock major and trace element, and mineral major
and trace element compositions were calculated. The results
compare well to the observed compositions from the Red Hills
peridotite. Model melt compositions were also calculated and
these reproduce well the natural MORB melt compositions,
such as normal (N)-MORB and ultra depleted MORB, observed at
fast spreading ridges. Combined with the porous flow model
in the harzbugite matrix (Sano & Kimura, 2007), reactive
melt flow reasonably explains the origin of dunite channels
during MORB generation beneath MORs.

● IFREE 2C & IFREE 3 joint seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 5th, August (Thursday)
Speaker: Yoshihiko Tamura
Title：TWO PRIMARY BASALT MAGMATYPES FROM NORTHWEST ROTA-1 VOLCANO,MARIANA ARC

Abstract:
(I presented similar talk in JpGU meeting and IBM-2
workshop. In this talk, I will show more data, results and
revised figures, which include backarc basin basalts from
southern Mariana Trough and arc volcanoes of Mariana Central
Island Province. I will also show the recent Mariana cruise
summary of NT10-12.)

Primitive basalts are rarely found in arcs. The active NW
Rota-1 volcano in the Mariana arc has erupted near-primitive
lavas, which we have sampled with ROV Hyperdolphin (HPD).
Samples from the summit (HPD480) and eastern flank (HPD488)
include 17 magnesian basalts (51-52 wt % SiO2) having
7.5-9.5 wt % MgO and Mg# of 61-67, indicating little
fractionation. Olivine phenocrysts are as magnesian as Fo93
which contain 0.4 wt % NiO; Cr/(Cr+Al) of spinels are mostly
0.5-0.8, further indicating equilibrium with depleted
mantle. There are three petrographic groups: 1) cpx-olivine
basalts (COBs); 2) plagioclase-olivine basalts (POBs); and
3) porphyritic basalts. Zr/Y and Nb/Yb are higher in POBs
(3.1-3.2 and 1.2-1.5, respectively) than in COBs (Zr/Y =
2.8-3.0 and Nb/Yb = 0.7-0.9), suggesting that POBs formed
from lower degrees of mantle melting, or that the COB mantle
source was more depleted. On the other hand, COBs have Ba/Nb
(70-80) and Th/Nb (0.4-0.5) that are higher than for POBs
(Ba/Nb = 30-35 and Th/Nb = 0.1-0.2), and also have steeper
light Rare Earth element (LREE) enriched patterns,
suggesting that COBs have a greater subduction component
than POBs. The calculated primary basaltic magmas of NW
Rota-1 volcano (primary POB and COB magmas) indicate
segregation pressures of 1.5- 2 GPa (50-65 km deep). These
magmas were formed by 15-25 % melting of mantle peridotite
having Mg# ~89.5. These two basalt magmatypes are similar to
those found for Sumisu and Torishima volcanoes in the
Izu-Bonin arc, with COBs representing wetter and POBs
representing drier magmas, with subduction zone-derive melt
components coupled with the water contents.

● IFREE 2C & IFREE 3 joint seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 29th, July (Thursday)
Speaker: Ryuko Senda (IFREE 3B)
Title: Differences between boninite and tholeiite primary magmas in Izu-Bonin-Mariana arc from an Os isotope perspective

Abstract:
Arc primary magmas are important in understanding subduction systems,
and furthermore, they provide information on how elements behave
in the subduction system. To know about that, Os isotope ratios of
whole rocks and Chromium spinels (Cr-spinels) within boninites, a type of
high-Mg andesite, and tholeiites from Izu-Bonin-Mariana arc, were
determined.We used Cr-spinel as a tool to get primary information of
those magmas. This is because it crystallizes at an early stage of
fractional crystallization and thus is not as severely contaminated
by crustal materials during magma ascent.In addition, it is resistant
to weathering and highly concentrates Os. Os isotope ratios of
volcanic rocks provide further hints as to the origin of the magma
and also determine the magma’s history. In this presentation,
I will show you the difference between the primary boninite compositions
and primary tholeiite compositions in IBM arc, in terms of their Os isotope ratios.

The whole rock Os isotope ratios of both boninites and tholeiites are higher
than those of Cr-spinels.In both cases, this is likely to be due to the rock
assimilatin crustal materials with higher Os isotope ratios than the mantle
during magma ascent. Os isotope ratios of Cr-spinel from boninites are
similar to those of abyssal peridotites from the IBM (Parkinson et al, 1998,
/Science 281, 2011-2013/). This suggests that the Os in the boninite, primary
magma originates from depleted mantle, not from fertile mantle or subducted
materials.On the other hand, Os isotope ratio of Cr-spinel from tholeiite shows
higher than those from boninites. Crustal components were possibly
contributed to formation of the tholeiite magma. The difference in
Os isotope ratios between boninites and tholeiites shows that they have
a different origin and evolutionary history.

● IFREE 2C & IFREE 3 joint seminar ●
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 8th, July (Thursday)
Speaker: Yoshiyuki Tatsumi
Title: Anti-Continent Accreted at the Base of the Mantle and Recycled in Mantle Plumes

Abstruct:
The continental crust is a unique reservoir of light
elements in the solid Earth; it possesses an intermediate
composition and is believed to have been created along
volcanic arcs, major sites of terrestrial andesitic
magmatism. Mantle-derived arc magmas are, however, generally
mafic or basaltic. A simple mechanism to overcome this
apparent dilemma would be partial remelting of an initial
mafic arc crust by heat supplied from underplating basaltic
magmas to generate andesitic melts. In that case, a
consequence of continental crust formation should be the
corresponding production of refractory melting residue, here
referred to as 'anti-continent'. The anti-continent is
likely to detach from arc crust by density inversion and
descend into the upper mantle. High-pressure experiments
demonstrate that sinking anti-continent is, in contrast to
the subducting oceanic crust, always denser than the
surrounding mantle, suggesting that anti-continent founders
without stagnation at the upper-lower mantle boundary and
accumulates at the base of the mantle to form a 250km-thick
mass known as the D" layer. Geochemical modeling further
provides evidence for the contribution of the accreted
anti-continent to deep-seated hotspot sources. In
complementary processes, Earth creates buoyant continents
and dense anti-continents at the top and the base of the
mantle, respectively, and has recycled portions of
anti-continent in mantle plumes.

● IFREE MC & HQ joint special seminar ●
Place: International Exchanging Room at 5th Floor, YOKOSUKA HQ
Time: 16:00-17:30 on 1st, July (Thursday)
Speaker: David Selby (Senior Lecturer, Durham University, UK)
Title: Successes, Systematics, possible limitations, & unknowns: Insights into the world of Re-Os isotope petroleum geosciences

Abstruct:
How can we date the deposition of sedimentary rocks, and the time when oil
is generated from an oil shale? This talk will explore the background, application,
systematics and implications of the rhenium-osmium isotope system to sedimentary
and petroleum geosciences. Using case studies I will show how accurate and
precise dates can be obtained from organic-rich rocks and petroleum. I will also
explore and demonstrate the behavior of the rhenium and osmium systematics
in hydrocarbons. Additionally I will discuss how osmium isotope stratigraphy is
a powerful tool to evaluate paleo- oceanography, climate and the onset and
duration of Large Igneous Provinces.

● IFREE 3 seminar ●
Place: Conference Room, the main building 3F, Yokosuka HQ
Date: Tuesday 29 June, 2010, 16：30～
Speaker: Felix van Aulock (University of Canterbury, New Zealand.)
Title: How to Hydrate Magma

Abstruct:
The solubility and speciation of water in volcanic rocks has
been constrained by a number of models based on experimental
data from either high (magmatic) or low temperatures
(alteration). However, shallow crustal processes often
involve temperatures below magmatic temperatures (i.e., the
glass transition) but well above usual surface temperatures,
and the processes in this regime involving water are poorly
understood. Recent experiments by Ben Kennedy at the
University of Canterbury have shown that magma can become
hydrated at these temperatures and pressures. In addition,
studies by Hugh Tuffen (University of Lancester, UK) show
that spherulite growth can increase the temperature and
water content of the surrounding melt. These processes could
lead to a "reactivation" of degassed magma and this will be
tested in a new pressure device at the University of
Canterbury in New Zealand and compared to natural examples
from volcanoes in New Zealand.

● IFREE MC seminar ●
Place: Conference Room, the main building 3F, Yokosuka HQ
Time: 16:30-18:00 on 10th, June (Thursday)
Speaker: Tatsuo Nozaki (Earth Surface and Interior Co-Evolution Research Team)
Title: A rapid determination method for Re and Os isotope compositions using MC-ICP-MS combined with sparging method

Abstruct:
We present a rapid determination method for Re and Os isotope compositions using MC-ICP-MS
(NEPTUNE) combined with a sparging method. The sparging method allows us to measure Os
isotope compositions just after acid sample digestion in a Carius tube without the most commonly
used but time-consuming solvent extraction by using CCl4 and HBr. Our NEPTUNE is equipped
with the multi-ion counter (MIC) system, enabling simultaneous measurement of a maximum of
four Os isotopes. Compared to negative thermal ionization mass spectrometry (N-TIMS) which is
now the most widely used for Re-Os analyses, sample throughput becomes several times higher.
Therefore, the Re and Os measurement of sedimentary rock samples by NEPTUNE combined with
the sparging method is expected to be a powerful tool to reconstruct the secular change of the
marine Os isotope compositions with high resolution, which possibly unravels the cause of paleo-
ocean global environmental change. In this presentation, we introduce the developed analytical
method of Re and Os isotopes and their geological application. The accuracy, reproducibility and
Re-Os data of GSJ geochemical reference samples (JCh-1 and JMS-2) measured by NEPTUNE
are also compared with those by N-TIMS.

● IFREE MC & HQ joint special seminar
Place: Lecture room at 5F, YOKOSUKA HQ
Time: 16:30-18:00 on 20th, May (Thursday)
Speaker: Maria Luisa G. Tejada (Associate Professor, the Univ. of the Philippines)
Title: The Platinum Group Element Signature of Oceanic Anoxic Event 1a:
No Evidence of a Bolide Trigger for Ontong Java Plateau Volcanism

Abstruct:
　　The large-volume eruption and the short-period of emplacement shown by the geochronological,
geological, and geophysical studies of the Ontong Java Plateau (OJP) require some explanation
beyond what is already known from plate tectonic processes. Plume impact, bolide impact, and their
variants have been proposed to account for the origin of these features. The two end member models
(bolide vs. plume) cannot be distinguished from one another based solely on the Os isotopic
compositions of the OJP lavas because both meteorites and mantle have unradiogenic Os isotope signature.
Additional evidence to test the models can be derived from the platinum group element (PGE) abundances
in sedimentary rocks deposited before and during the LIP emplacement.
　　We analyzed the PGE contents of the Selli Level horizon at Gorgo a Cerbara, central Italy,
which was previously analyzed for Os isotope composition, to further test the mantle plume vs. bolide
impact models for the origin of the OJP and the Early Cretaceous oceanic anoxic event,OAE1a. We found
that Ir concentrations are low compared to other known large impact horizons. Only one black shale bed
yielded high Ir concentration of 544 ppt but it is not at the base of the Selli Level interval.
In addition chondrite-normalized Os/Ir and Pt/Ir values of 1.4-21 and 7.7-31, respectively,
within the Selli Level horizon are nonchondritic, including the bed that yielded the highest concentration
of Re and other PGEs. Previous analysis of the Os isotopic compositions of the organic fractions of
these sedimentary units showed a major interval of unradiogenic Os isotopic compositions coinciding
with the Selli Level (Tejada et al., 2009).The absence of positive correlation between enrichment
of PGE and the unradiogenic Os isotopic compositions suggests that the OAE1a event represented
by the Selli Level horizon was triggered by a significant amount of mantle input into the marine environment
and not by a bolide impact. Highly variable Os/Ir ratios that are uncorrelated with initial 187Os/188Os
are very difficult to reconcile with the impact hypothesis. Our PGE data further corroborate
our previous results that favor a mantle plume origin for the OJP and that the main phase of
plateau volcanism triggered the OAE1a event.

Reference:
Tejada, M. L. G., Suzuki, K., Kuroda, J. ,Coccioni, R., Mahoney, J. J., Ohkouchi, N., Sakamoto, T.
and Tatsumi, Y., 2009. Ontong Java Plateau as a trigger for the Early Aptian oceanic anoxic event.
Geology, 37, 855-858.

● IFREE 2C & IFREE 3 joint seminar ●
Date: 2010/3/18 (Thursday) 16:30 ～
Place: International Conference Room, the main building 5F, Yokosuka HQ
Speaker: Dr.Tatsuro Adachi (National Institute of Polar Research)

Title: Metamorphic evolution and its implication for tectonic process in the central Sor Rondane Mountains, East Antarctica

Abstract:
  The Sor Rondane Mountains in the eastern Dronning Maud Land, East
Antarctica is inferred to be situated within an orogen which is a
collision boundary of Gondwana amalgamation at late Neoproterozoic to
early Cambrian. The mountains have been recognized as the key area for
the reconstruction of Gondwana super-continent and geological activity
during the formation of super-continent. In this context, the author has
carried out petrological and geochronological studies of the metamorphic
rocks in the central Sor Rondane Mountains.
  Metamorphic rocks in the central part of Sor Rondane Mountains are
classified into at least three types based on petrological
characteristics as follows: (1) Metamorphic rocks mainly from
Austkampane area preserve ca. 800 °C and 0.4-0.5 GPa peak metamorphic
condition followed by the decompression and subsequent isobaric cooling
accompanying with hydration (A-type). (2) Rocks from the Brattnipene
area preserve peak metamorphic temperature of ca. 800 °C similar-T with
Austkampane but relatively higher pressure at 0.7-0.8 GPa. They also
experienced compression after peak metamorphism and subsequent isobaric
cooling with hydration (B-type). (3) Rocks in Lunckeryggen and couple of
other areas preserve signatures of prograde metamorphism and
amphibolite-facies peak metamorphic condition (L-type).
  Rutile exsolution in quartz was found from the retrograde hydrated
gneisses widespread in central Sor Rondane Mountains. Recovered
pre-exsolution Ti concentration in quartz combined with Ti-in-quartz
thermometer indicates that >700-800 °C peak metamorphic conditions are
pervasive in the NE terrane (Austkampane, Brattnipene and Menipa areas).
On the other hand, <600 °C of the metamorphic condition is recognized in
the SW terrane (mainly Lunckeryggen area).
  U-Pb zircon SHRIMP and U-Th-Pb monazite EPMA datings suggest that
ca. 640-600 Ma event is the timing of peak granulite-facies metamorphism
for A- and B-type rocks and that ca. 570-550 Ma event is also detected
which being interpreted as timing of hydration. Contact metamorphism
locally recognized in A-type rocks occurs at ca. 500 Ma. Timing of
amphibolite-facies metamorphism of L-type rocks is ca. 550 Ma. L-type
rocks lack ca. 640-600 Ma events, indicating that A- and B-type rocks
and L-type rocks share the geological event after 550 Ma.
  L-type unit is structurally apparently overlain by B-type unit. The
boundary between B-type unit and L-type unit seems nearly-horizontal and
parallel to the foliation observed in both units. This implies that
nappe-like structure, A- and B-type units thrust up onto L-type unit, is
essentially prevailing in the central Sor Rondane Mountains. Such
nappe-like structure has been also proposed for EAAO related areas
including Mozambique, central Dronning Maud Land and Sri Lanka, and
nappe structure should be dominant tectonics in the EAAO region.

● IFREE 3 special seminar ●
Date: Tuesday 9 March, 2010, 16:30 ～
Place: Conference Room, the main building 3F, Yokosuka HQ
Speaker: Yan Lavallee (University of Munich)

Title: Cracking magmas

Abstract:
The failure of magma and onset of an explosive eruptive phase is
commonly preceded by accelerations in discharge rate and seismic activity.
As magma is forced through the ductile-brittle transition cracks nucleate,
propagate and coalesce, thereby releasing an increasing rate of seismic energy
when approaching catastrophic failure; this acceleration can in principle
be used to forecast the onset of explosive eruptive phase.
Herein, we present the rheology of highly crystalline magmas,
investigated using a high-temperature, high-pressure uniaxial
press equipped with a fast-acquisition acoustic emission system.
Magmas are pseudoplastic fluids, which exhibit a strong component of
shear thinning as their deformation accelerates
across the ductile to brittle transition. When deformation approaches
failure, we record acceleration in released AE energy, which can be used as
a proxy to forecast failure. Ultra-high resolution neutron computed
tomography of magmas that have been experimentally deformed within
the ductile-brittle transition reveal that the competition
between extensional and shear fracturing controls the time
available to accurately predict magma failure. Understanding
the mechanism by which shallow magma fragment will improve
our ability to predict the progression and styles of explosive eruptions.

● IFREE 2C & IFREE 3 joint seminar ●
Date: 4th March 2010 (Thu) 16:30 ～
Place:Seminar Room, the Annex 1F, Yokosuka HQ
Speaker: Shigeaki Ono (IFREE3C)

Title: Behavior of carbonates in the deep mantle

Abstract:
The behavior of carbonates is very important to understand the global
geochemical carbon cycle. Previous experimental works demonstrated that
the solubility of carbon in silicates is very small. This indicates most
of the Earth's carbon must be hosted by high-pressure phases of
carbonates. Therefore, we have investigated the stability and the phase
transition of carbonates using high-pressure experiments and the
first-principles calculations. Four new phases in Mg- and Ca-carbonates
were found to be stable at pressures corresponding to the lower mantle
conditions. We will mention the possibility of carbon dioxide release at
the very bottom of the mantle. The possibility of pressure-induced
decomposition of carbon dioxide will be discussed.

● IFREE 3A special seminar ●
Date: Monday 1 March, 2010, 16:00～ 17:00
Place: Seminar Room, the Annex 1F, Yokosuka HQ
Speaker:Dereje Ayalew (Addis Ababa University)

Title: Mantle contributions to the genesis of Ethiopian rhyolites

Abstract:
The Ethiopian continental flood basalt (CFB) province is one of the
youngest (30 Ma), best-exposed continental flood basalt provinces (3
x 105 km3). It was formed as the result of the impingement of the
Afar mantle plume beneath the Ethiopian lithosphere. This province is
composed of bimodal basalt-rhyolite suite with a lack of rocks of
intermediate composition. The rhyolites are found throughout the
volcanic stratigraphy as ash layers between basaltic lava flows, or
as individual eruptive units comparable in volume to individual mafic
lava units. They become thicker towards the uppermost part of the
lava sequences. The volume of the rhyolite is estimated to be at
least 6 x 104km3, which represents 20% of that of the trap basalts.
Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-
augite, ilmenite-Ti-magnetite, richterite, and eckermanite) suggests
temperatures in the range of 740 to 900°C. Their trace element
and isotope (Sr, Nd, O) signatures (high 143Nd/144Nd and low 87Sr/
86Sr ratios) are clearly different from those of rhyolites from other
CFB provinces and those of typical crustal melts. The Ethiopian
rhyolites are among the most isotopically primitive rhyolites and
exhibit more mantle isotopic signature. Their major and trace element
patterns suggest that they are likely to be derived from fractional
crystallization of basaltic magmas similar in composition to the
exposed flood basalts with only limited crustal contribution. The
Ethiopian rhyoltic ignimbrites are one of the best examples in our
planet, which show entirely mantle contributions to rhyolite genesis.

● IFREE 2C & IFREE 3 joint seminar ●
Date: 25th February 2010 (Thu) 16:30 ～
Place: Seminar Room, the Annex 1F, Yokosuka HQ
Speaker: Hiroshi Shukuno (IFREE3)

Title:Magmatism in the northern end of the Mariana Trough: implication for spinel chemistries

Abstract:
The northern end of the Mariana Trough lies on area between the northern
most part of Mariana arc and the southern end of Izu-Ogasawara arc. This
area is characterized by activities of K-rich and shoshonitic lavas,
referred to as the alkalic volcano province (AVP). Back-arc and
intra-arc rifting forms the Mariana Trough just south of the AVP. In
NT06-08 cruise using R/V Natsushima, we had dredge sampling along the
West Mariana Ridge, the northern tip of the Mariana Trough, in which the
rift propagation occurs. I will talk about mineral chemistry of the West
Mariana Ridge basalts with geochemical considerations.

The West Mariana Ridge samples are medium-K to shoshonitic lavas. The
basaltic rocks in the West Mariana Ridge have spinel inclusions hosted
in olivine phenocrysts. These spinel chemistries indicate resemblance to
signature of back-arc basin basalts and MORB. Pb isotope analyses
present that the West Mariana ridge lavas form a single trend on Pb-Pb
isotope plots. The end members of this trend are (1) lower 206Pb/204Pb
and high delta 207/204, and (2) high 206Pb/204Pb and low delta 207/204.
These components could correspond to subducted pelagic sediments and
subducted HIMU seamounts, respectively. The spinel chemistries of the
West Mariana Ridge basalts have correlation with eruption ages and
several geochemical data, such as Pb isotope ratios and Ce/Pb ratio.
These petrochemical and geochemical features could be explained by
difference of mantle depletion with time coupled with isotopical source
heterogeneities, which are caused by variable contributions of several
metasomatic components, such as subducted pelagic sediments, subducted
HIMU-like volcanics and enriched mantle.

● IFREE 2C & IFREE 3 joint seminar ●
Date: 4th February (Thu) 16:30 ～
Place: Seminar Room, the Annex 1F, Yokosuka HQ
Speaker: Jun-Ichi Kimura (IFREE3) cooperation with Hiroshi Kawabata (IFREE3B)

Title: A forward simulation model (OBS version 1) of the adiabatic melting of mantle peridotite and recycled material: implications for trace element and isotopic variations in Hawaiian island basalts

Abstract:
The role of recycled material in mantle peridotite has become increasingly
important in understanding the geochemical variations exhibited by ocean
island basalts (OIB). Variations in radiogenic isotopes, incompatible trace
elements, and transition metals in OIBs have suggested the involvement of
fertile clinopyroxinite or garnetite mineralogy in the source peridotite
(PERID). These fertile materials are believed to be derived from either
altered or unaltered recycled oceanic crust (ROC). However, quantitative
examination of the element behavior is difficult as, because there are at
least two different phases (ROC and PERID), the melting regime is complex;
simple adiabatic melting is not applicable. Melting experiments have
revealed how major and trace element partition between ROC and PERID, and
the basalt melts they generate. A thermodynamic model (Phipps-Morgan J.,
2001, G-Cubed, 2, doi.2000GC00049) has been proposed, and the behavior of
trace element and isotopes in OIBs has been discussed assuming simple mixing
between ROC and PERID melts (Ito G. and Mahoney J.J., 2005a & b, EPSL, 230,
29-46, 47-63). Melt transport in the two phase system is not simple due to
(a) complex changes in residual mineralogy in both ROC and PERID along the
P-T path and (b) reactive/ non-reactive behavior between the ROC melt and
host PERID. In order to investigate the element behavior in the ROC-PERID
system, we have developed a simulation program, Ocean Basalt Simulator
version 1 (OBS1). OBS1 includes Phipps-Morgan (2001)'s thermodynamic
equations and parameterizations of residual mineral phases in ROC (MORB) and
PERID (PM) between 6 and 1 GPa using the pMELTS algorithm. The calculated
degree of partial melting (F) of ROC and PERID at a given P-T in the
two-phase adiabatic melting regime is used to calculate the residual
mineralogy (Xa) in both ROC and PERID at steps in P. Melting of ROC occurs
earlier due to a lower liquidus temperature. The generated ROC partial melt
metasomatizes the host PERID or pond in ROC. PERID melting is suppressed
compared to the single-phase system because latent-heat is consumed by ROC
melting. The first OIB melt from the two-phase system is generated by
melting of PERID. Generated melts from the two-phase system are accumulated
and provide the source basalt for OIB. These processes are simulated by
discrete incremental batch melting calculation sequences for ROC and PERID.
The role of mantle porosity is also examined by altering the extracted melt
fractions from the two melting systems, with the residual melt fraction
ranging from 1 (batch melting) to 0 (fractional melting). Melt accumulation
form (1) melting of the metasomatized PERID and (2) simple mixing between
PERID and ROC melts are two extreme cases for the primary OIB. The behavior
of incompatible trace elements and Sr-Nd-Hf-Pb isotopes were investigated
using the OBS1 program. The program reproduces (a) the convex-up
incompatible multi-element plot patterns, (b) correlation between Fo, Mn,
and Ni in olivines, and (c) the complex behavior (sigmoidal to curvilinear
shapes etc.) generated by isotope mixing, all exhibited by OIBs. The result
suggests that the simple mixing between PERID melt and ROC melt used in the
OBS1 program is likely for that of the OIB source. Although OBS1 has been
developed for OIBs, it is also applicable to adiabatic melting in an area of
ridge-plume interaction, and in the island arc source mantle, which may
include fertile materials such as slab melts or buoyant solid slab
materials.

● IFREE 2C & IFREE 3 joint seminar ●
Date: 28th January (Thu) 16:30～
Place: Seminar Room, the Annex 1F, Yokosuka HQ
Speaker: Satoshi Saito (IFREE 3C)

Title: Laboratory measurements of Vp and Vs in an olivine gabbro up to 1.0 GPa at room temperature: Roles of dry-open cracks on the elastic properties of crustal rocks

Abstract:
In order to determine the influence of dry-open cracks on the elastic
properties of crustal rocks, we developed a new experimental system
for simultaneous measurement of compressional-wave (Vp) and shear-
wave (Vs) velocities of an open crack-bearing rock sample up to 1.0
GPa at the room temperature. We measured Vp and Vs of an olivine
gabbro sample from the Oman Ophiolite. The measured Vp and Vs
markedly increase at lower pressures, and gradually increase at
higher pressures. The results are attributed to the decrease of open
crack abundances during pressurization and complete closure of cracks
at higher pressures. Based on the Vp and Vs data, we evaluated the
changes in Poisson ratio's, bulk density, compressibility,
volume, and porosity (total volume of open cracks) of the rock as a
function of pressure. The present results illustrate that the
development of dry open cracks in crustal rocks significantly lower
their Vp, Vp/Vs and Poisson's ratio. The data imply that the
crustal seismogenic regions distributed at shallow crustal level,
which are characterized by relatively lower Vp, Vp/Vs and
Poisson's ratio, consist of rocks having high abundance of dry
open-cracks.

● IFREE 2C & IFREE 3 joint seminar ●
Date: 8th January (Fri) 16:30 ～
Place: Seminar Room, the Annex 1F, Yokosuka HQ
Speaker: Kenji Shimizu (IFREE 2C)

Title1: Preliminary results of IODP Expedition 324; Testing plume and plate models of ocean plateau formation at Shatsky Rise, northwest Pacific Ocean

Abstract1:
There were IODP expedition 324 from Sept. 5 to Nov. 5, 2009 to drill
~145 Ma large igneous province (LIP) of Shatsky Rise Formation. During
the expedition, we drilled five holes to constrain evolutions of the
LIP. In this seminar, I will briefly present the preliminary results
of the expedition and show many photos from the expedition.

Title2: A new method for decomposing silicate rocks by flux-free fusion (FFF)

Abstract2:
We present a new method for decomposing silicate rocks by flux-free
fusion in preparation for whole-rock trace element analysis (Sc, Rb,
Sr, Y, Zr, Nb, Cs, Ba, rare earth elements and Hf) that is especially
applicable for zircon-bearing felsic rocks. The method was verified by
analyses of reference materials for mafic (JB-1a, JB-2, JGb-1) and
felsic rocks (JG-3, JR-3, JSd-1, GSP-2, G-2). Pellets of powdered
sample (up to 500 mg) without flux were weighed and put in a clean
platinum crucible. The samples were then fused in a Siliconit tube
furnace and quenched to room temperature. We determined optimum
conditions for fusion of granitic rock to be heating for 2-3 min at
1600 °C. The fused glass in the platinum crucible after heating was
decomposed using HF and HClO4 in a Teflon beaker. Decomposed and
diluted sample solutions were analysed using a quadrupole inductively
coupled plasma-mass spectrometer. Replicate analyses (n = 4 or 5) of
reference materials revealed that analytical uncertainties were
generally less than 3% for all elements except Zr and Hf (~6%) of
JG-3. These higher uncertainties may be attributed to sample
heterogeneity. Our analytical results for reference materials agreed
well with recommended concentrations and recently published
concentrations, indicating complete decomposition of our rock samples
during fusion.