● IFREE MC セミナーのお知らせ
日　時：2013年5月30日（木）16:30-18:00
場　所：横須賀本部本館３階 大会議室
発表者：木村 純一（チームリーダー）

タイトル:
Magma genesis beneath arcs: Geochemical forward model perspectives

概要:
Subducted water is carried by downgoing oceanic plate slab and is released by dehydration/melting of the slab in prograde metamorphism. The released water migrates upwards and contributes melting of the mantle wedge to form primary arc magmas. The water is then captured by the arc magmas and eventually solidify/erupt to form arc crust and volcanoes.
Water, either in fluids or melts in both slab and mantle, promotes dissolution and mobilization of elements truncating material transfer in the subduction factory. Water also affects physical properties of the sub-arc slab and the mantle accelerating/decelerating mantle convection and seismicity. The role of water, however, should considerably differ between arcs due to difference in thermal structure.
In this seminar, I will demonstrate a coherent model that may explain the geophysical and geochemical roles of the water beneath the NE and the SW Japan arcs. Due to typical old-cold and hot-young subduction of different oceanic plates, comparisons of the two arcs provide two extreme endmember cases. I will mostly talk about dehydration profiles of the downgoing slabs based on the seismic/geophysical/geochemical data and examine the roles of the slab water to form geochemical variations found in the Quaternary arc magmas erupted on the two arcs. Arc Basalt Simulator version 4 (ABS4), a petrological/geochemical model developed for to describe the mass balance between the subduction slab inputs and the arc magma outputs, examines the subduction zone processes including behaviors of the water and 33 major/trace elements and Sr-Nd-Hf-Pb isotopes. Intensive/extensive parameters that govern those petrogenetic processes are also estimated by the model and are compared with geophysical observations.
The combined approach between geophysics and geochemistry provides new perspectives in both element behaviors and geophysics in the subduction factory.

●IFREE MC セミナーのお知らせ
日　時：2013年4月19日（金）16:30-18:00
場　所：横須賀本部本館（山側）1F　セミナー室
発表者：Marinah A. Ranaivoson (横浜国立大学）

タイトル:
Carbonatite and associated phlogopitite-pyroxenite intrusions along the Beraketa mega shear zone, in the Proterozoic southern Madagascar: Geochemical, O- and C- stable isotopes and Sr- and Nd-isotopic characteristics.

概要:
The basement of southern Madagascar consists of three distinct geologic domains (GAFBRG, 2008c,d) separated by the two N-S trending Ampanihy and Beraketa mega shear zones (Rolin, 1991; Pili et al., 1997; Martelat et al., 2000;De Wit et al., 2001. These domains constitute a poly-deformed crustal scale of amphibolite to granulite facies metamorphic rocks and characterized by different group of stratified and intrusive rocks (Tucker et al., 2011).
The terrain embraces myriad of mineralization, amongst others, the well-known apatite and phlogopite ore bodies. The later forms trinity with pyroxenite and carbonate rocks, mainly along the Beraketa mega shear zone which is hundreds kilometer long by tens kilometer wide and interpreted as a vertical lithospheric shear network probably rooted in the mantle (Pili et al., 1998, Boulevais et al.,1999). Apatite mineralization occurs within and in close spatial association with the trinity. They form an apparently continuous ore body of economic importance with respect to apatite mining. The Beraketa mega shear is then, thought to be a conduit of magmatic fluid/melt during the upwelling of the asthenospheric mantle, after the Pan African Orogeny, and the carbonate-bearing and associated rocks have been interpreted as metasomatized rocks (Pili et al., 1997). Pili et al.,(1997) interpreted metasomatism is related to mantle derived CO2-H2O fluid tapped along the lithospheric shear zones (Pili at al, 1997). However, recent study of Martin et al., (2010) and Morteani et al., (2012) proposed that these association represent crustal silico-carbonatite magmas.
Nevertheless, the petrogenetic links between these carbonate rocks and phlogopitite-pyroxenite is not well understood. In addition, the uncertainty exists on whether the carbonate rocks represent mantle derived melt or crustal melt.

Petrography, whole rock chemistry, and C-and O-stable isotope geochemistry have been performed to clarify the origin of the carbonate rocks and associated intrusions. In addition, Sr- and Nd-isotope geochemical analysis has been conducted for these rocks at IFREE, JAMSTEC. The carbonate rocks and pyroxenites contain relatively high LILE and moderate to high REE (up to 1000 ppm). Calcites from the carbonate rocks and those from the phlogopitite-pyroxenite intrusions have δ13CPDB = -6.32 to -0.916 and δ18OSMOW ranges from 10.30 to19.63 per mil.
K-Ar dating on the phlogopite crystals from the carbonate-pyroxenite-phlogopitite association yields ages of 460-500 Ma suggesting their emplacements postdated the Pan-African metamorphic event (>550 Ma) (Martelat et al., 2000).

Initial 87Sr/86Sr at 470 Ma ranges from 0.720593 to 0.763006 and 143Nd/144Nd from 0.511039 to 0.511958. The eSr(t) and eNd(t) exhibit extremely enriched isotopic signature suggesting that these are crustal melts derived from old enriched lower crustal regions. However, our C- and O- isotope compositions are distinctly different from that of the marbles of sedimentary origin. Instead, our data show a well-defined positive Rayleigh fractionation trend from the primary carbonatite box (Taylor et al., 1967; Keller and Hoefs, 1995), suggesting that these were derived from mantle derived magmas undergone isotopic fractionation. The enriched Sr and Nd isotopic composition of our rocks would reflect that these are anatextic melts of ancient lower crustal materials. This lower crustal melting would be induced by flushing CO2-rich mantle fluid along lithospheric mega shear.

●IFREE MC セミナーのお知らせ
日　時：2013年4月11日（木）16:30-18:00
場　所：横須賀本部本館（山側）1F　セミナー室
発表者：浜田 盛久 (IFREE 4F 研究員）

タイトル:
Trace amount of hydrogen in plagioclase as a hygrometer of hydrous arc basaltic magmas

概要
Quantifying H2O concentration in magmas, especially arc magmas, is critical to understand their generation, differentiation and eruption. Analyses of melt inclusions has been one of the most popular methods to quantify H2O dissolved in magmas. Besides analyses of melt inclusions, I will introduce that analyzing trace amount of hydrogen accomodated in phenocrysts (nominally anhydrous minerals,
NAMs) can be another useful method to understand magma processes involving H2O.

Among NAMs, plagioclase is the most abundant and common phenocrysts (NAMs) found in arc basaltic rocks. Recently, I determined partition coefficients of hydrogen between plagioclase and basaltic melt by equilibrating plagioclase with hydrous melt in order to use hydrogen in plagioclase as a proxy of H2O in melt (Hamada et al., 2013 EPSL). In order to achive this purpose, this study was performed in two parts. The first part of this study is analyses of plagioclase and low-H2O basaltic melt inclusions (0.3 wt% H2O) collected from the Indian mid-ocean ridge basalt (MORB). The second part of this study is hydrogen partitioning experiments between plagioclase and hydrous basaltic melt (0.8-5.5 wt% bulk H2O).

The obtained can be applied to better understand the 1986-1987 summit eruption of Izu- Oshima volcano (Hamada et al., 2011 EPSL). It is proposed that deep-seated island arc basalt magmas erupting from frontal-arc volcanoes are richer in H2O than previously thought, containing approximately 1 wt.% H2O based on analyses of “leaked” melt inclusions and phase equilibrium studies at "low-pressure conditions".

●IFREE MC セミナーのお知らせ
日　時：2013年3月28日（木）16:30-18:00
場　所：横須賀本部本館3F 大会議室
発表者：高橋俊郎(IFREE 4F 技術研究副主任)

タイトル:
Generation of TH and CA suite magmas at Chokai volcano in the NE Japan rear-arc

概要
Tholeiitic (TH) and Calc-alkaline (CA) suite lavas is coexisting at Quaternary volcanoes in NE Japan arc. Generally, CA lavas have evidence of magma mixing, e.g. disequilibrium phenocrystic assemblage such as olivine and quartz. Therefore, many previous works discussed that TH is evolved from mantle-derived basalt magma by fractional crystallization, and CA is generated by magma mixing between basic and acidic magmas, both derived from the common TH basalt though fractionation. However, based on Sr isotope micro-analysis of plagioclase phenocrysts in lavas from Zao and Azuma volcanoes at central area of NE Japan arc, Tatsumi et al. (2008) and Takahashi et al. (2012) argued that isotopically radiogenic TH basalt was formed by melting of the lower-crustal amphibolite and CA was formed by magma mixing of the unradiogenic mantle-derived basalt, the radiogenic crust-derived basalt and the differentiated magma which relates to basalt magmas.
The Quaternary Chokai volcano is located in the rear-arc side of the NE Japan. The Chokai volcano is a typical stratovolcano and the eruption activities are classified into Stage 1, 2, and 3 (Hayashi, 1984: Ban et al., 2001). Stage 1 lavas are olivine two-pyroxene basalt to two-pyroxene andesite, and the lavas contain phenocrystic minerals in equilibrium with the host magmas with occasional dusty plagioclase, which exhibits disequilibrium. Anorthite-contents (An%) of the plagioclase phenocryst cores show unimodal distribution, and An% decrease with increasing SiO2 in the host lavas (An90-80 for basalt, An70-50 for andesites). Stage 2 lavas are mostly amphibole-bearing olivine two-pyroxene andesite with a small amount of olivine two-pyroxene basalt. Stage 3 lavas are mostly olivine two-pyroxene andesite with two-pyroxene andesite. Most of the plagioclase phenocrysts in the Stage 2 and 3 lavas possess dusty zones or sieve textures. An% in these plagioclase core exhibits wide range (An50-80). Bulk-rock compositions of the Chokai lavas plot near the boundary between high-K and medium-K. On the FeO*/MgO vs. SiO2 diagram, Stage 1 lavas fall on TH, whereas Stage 2 and 3 lavas fall on TH fields. Stage 2 and 3 lavas collectively show straight trend on MgO vs. SiO2 plots and MgO content is higher than those in the Stage 1 lavas. Sr isotope compositions of the Stage 1 lavas gently increase with increasing SiO2 (0.70303-0.70341) contrasting to the steep increase shown by the Stage 2 and 3 lavas (0.70288-0.70342). Stage 1 geochemical trends can simply be explained by fractional crystallization of a basalt magma with minor crustal assimilation Stage 2 and 3 trends can be generated by mixing between basaltic and felsic magmas. Stage 1 parental basalt magma differs from Stage 2 and 3 basalt mixing end-member, chemically and isotopically.
The contrasting the petrological and geochemical characteristics of the Chokai TH (State 1) vs. CA (Stage 2 and 3) magmas are similar to those the Zao and Azuma TH vs. CA magmas found at the volcanic front of the NE Japan arc. Tatsumi et al. (2008) and Takahashi et al. (2012) have argued that TH basalt generated by melting of the lower crustal amphibolite, whereas CA basalt was formed by magma mixing between a mantle derived basalt and a felsic magma generated from TH basalt by fractional crystallization. The same mechanism would explain the TH and CA suite magmas at Chokai. The Sr isotopic composition of the Stage 1 TH lavas overlaps with that of amphibolite xenoliths from lower crustal depth beneath Ichinomegata volcano (0.7032-0.7051; Yamamoto and Takeda, 2008: Yamamoto et al., 2013). Yamamoto et al. (2013) discussed that the Oga volcanic rocks (e.g., Ichinomegata host pumice, Toga pumice and Kampu lavas) have undergone isotopic and geochemical changes during interaction with lower-crustal amphibolite. The geochemical features of the xenoliths from Ichinomegata and the TH basalt of Chokai volcano suggest that the Stage 1 TH basalt can be formed by a mantle-derived basalt magma (rear-arc medium-K basalt) assimilate to lower-crustal amphibolite and/or mixing between a mantle-derived basalt magma and amphibolite melt.

●IFREE MC セミナーのお知らせ
日　時：2013年3月21日（木）16:30-18:00
場　所：横須賀本部本館3F 大会議室
発表者：小澤 春香(IFREE 4C 技術研究副主任）

タイトル:
Decomposition of Fe3S above 250 GPa

概要:
Sulfur is thought to be a plausible light element in the core, and the phase relations in Fe-FeS binary system at the core pressure range are of great importance. Theoretical calculations have predicted a continuous solid solution between Fe and FeS at the inner core pressure (Alfe` et al., 2002), while previous experimental studies showed that the solubility of sulfur in solid Fe is limited and Fe−Fe3S system exhibits eutectic melting at least up to 220 GPa and 3300 K (e.g., Morard et al., 2008; Kamada et al., 2010).
Here we examined the subsolidus phase relations of iron-sulfur alloys up to 269 GPa and 2540 K on the basis of synchrotron X-ray diffraction (XRD) measurements in situ at high pressure and temperature using a laser-heated diamond-anvil cell. In addition, we also performed chemical analyses on recovered samples with an analytical transmission electron microscope. Mixed fine powder of Fe and FeS or Fe3S synthesized beforehand in a Kawai-type multi-anvil apparatus were used as starting materials. The results demonstrated the coexistence of hexagonal close-packed (hcp) Fe and tetragonal Fe3S phase up to 241 GPa and 2510 K. On the other hand, the XRD measurements performed above 250 GPa showed the coexistence of hcp + ordered bcc phases. Moreover, when we used the starting material containing 10 atm% S, only hcp phase was observed above 3860 K at 285 GPa. These results indicate that hcp Fe can incorporate substantial amount of sulfur, at least 10 atm% S above 285 GPa, which is high enough to account for the density deficit from pure Fe in the inner core.

●IFREE MC セミナーのお知らせ
日　時：2013年3月7日（木）16:30-18:00
場　所：横須賀本部本館3F 大会議室
発表者：谷 健一郎(IFREE 4D 技術研究副主任）

タイトル:
Oomurodashi Volcano

概要:
Oomurodashi is a bathymetric high located ~20 km south of Izu-Oshima. Using the 200 m bathymetric contour to define its outline, the diameter of Oomurodashi is ~20 km, similar in size to the Hachijyojima Volcano. Previous dredge surveys conducted on Oomurodashi had recovered fresh pumice clasts (Hamuro et al., 1983, Bull. ERI), but since then it has been ignored, largely because it has a vast flat-topped summit at 100 - 150 meters below sea level (mbsl), and has been regarded as inactive.

However, during cruise NT07-15 of R/V Natsushima in 2007, we conducted a dive survey using the remotely-operated vehicle (ROV) Hyper-Dolphin of the small crater, Oomuro Hole, located in the center of the flat-topped summit of the Oomurodashi. The heat flow measurement conducted on the floor of Oomuro Hole recorded an extremely high value of 4200 mW/m2. Furthermore, ROV observations revealed that the southwestern wall of Oomuro Hole consists of fresh rhyolitic lava flows.

These findings suggest that Oomurodashi is in fact an active silicic submarine volcano. To confirm this hypothesis, we conducted detailed geological and geophysical surveys of Oomurodashi in July-August 2012, again using the R/V Natsushima and ROV Hyper-Dolphin (cruise NT12-19). In addition to further ROV surveys, we carried out single-channel seismic (SCS) surveys across Oomurodashi in order to examine the shallow crustal structures beneath the current edifice.

The ROV surveys revealed numerous active hydrothermal vents on the floor of Oomuro Hole. The maximum water temperature measured at the hydrothermal vents reached 194C, almost equivalent to the boiling temperature of water at the ~200 mbsl water depths of the floor of Oomuro Hole. We also conducted comprehensive heat flow measurements across the floor of Oomuro Hole, with very high heat flows up to 29000 mW/m2 being detected. ROV observations revealed that the area surrounding Oomuro Hole on the flat-topped summit of Oomurodashi is covered by extensive fresh rhyolitic lava and pumice clasts with minimum biogenetic or manganese cover, suggesting recent eruption(s). Furthermore, the SCS surveys revealed the presence of a buried caldera-like structure, ~8 km in diameter, beneath the flat-topped summit of Oomurodashi.

These findings strongly indicate that Oomurodashi is an active silicic submarine volcano, with recent shallow-sea eruption(s) occurring from Oomuro Hole.　Since Oomurodashi is situated in a shallow-sea environment, in close proximity to the inhabited northern Izu Islands, further volcanological surveys are essential to understand the detailed volcanic history and potential hazards of this volcano.

We will present a summary of the surveys conducted during the NT07-15 and NT12-19 cruises, as well as the latest analytical results obtained from the samples.

●IFREE MC セミナーのお知らせ
日　時：2013年2月28日（木）16:30-18:00
場　所：横須賀本部本館(夏島側)１階 セミナー室
発表者：常 青 (IFREE 4B 技術研究主任）

タイトル:
Applications of laser ablation MC-ICP-MS with ion counting detectors in microanalysis of Pb isotopic analysis of low Pb reference glasses

概要:
Laser ablation (LA) multiple collector (MC) ICP-MS has been demonstrated as a powerful tool for in-situ Pb isotopic analysis in geochemical studies. To attain accurate and high precision isotopic analysis, the target materials however are restricted to either material with high content of interested elements (e.g. >500 ppm Pb in case of Pb isotopic determination) or large sampling area (>100 micrometer); the required precision for discussion often has to be compromised by spatial resolution. To improve detection efficiency we installed four Channeltron ion counters specified for determination of all isotopes of lead on our MC-ICPMS. Ion counter facilitates to measure small ion beam, but isotopic abundance ratios measured by them are critically affected by counting linearity, stability, efficiency calibration of each ion counter and laser ablation manner. In this talk we'll briefly introduce the assessing and optimizing operation parameters of LA-MC-ICP-MS, describe an analytical protocol for Pb isotope analysis using ion counters equipped on MC-ICPMS, and discuss Pb isotopic data obtained recently by using ~30 micrometer ablation spot for low Pb (1-10 ppm) reference glasses of USGS and Max Planck Institute for Chemistry (MPI-DING glasses).

● IFREE MC セミナーのお知らせ
日　時：2013年2月21日（木）16:30-18:00
場　所：横須賀本部本館3F 大会議室
発表者：仙田 量子 (IFREE 4B 技術研究副主任）

タイトル:
Variation of the Os isotope ratios in abyssal peridotites from mid-ocean ridges

概要:
Osmium isotope ratios of abyssal peridotites are of great interest to geoscientists because they provide direct information about the present-day Earth's mantle. This is based on the fact that Os is a highly siderophile and compatible element and so concentrates in the core in preference to the mantle, and in the mantle in preference to the crustal materials. In addition, Os is not susceptible to metasomatism and alteration by sea water. The Os isotope ratios of the peridotites calculate Os model age as a timing of melt extraction from mantle. I will explain about the definition of Os model ages and show the variation of the Os isotope ratios in abyssal peridotites whole rocks, and chromites and sulphides within abyssal peridotites from the Atlantic, Indian and Pacific Oceans. I will discuss about the origin of the Os and other platinum group elements in abyssal peridotites.

●IFREE MC セミナーのお知らせ
日　時：2013年2月7日（木）16:30-18:00
場　所：横須賀本部本館1階 セミナー室 (山側)
発表者：柏原輝彦 (IFREE 3C/SRRP, ポストドクトラル研究員)

タイトル:
Formation mechanisms of REY-rich mud on the seafloor investigated by Synchrotron Radiation X-ray analysis.

概要:
Recently, deep-sea mud in certain areas of the Pacific Ocean is gathering vigorous attention because of its great potential as a new REY (= rare earth elements and metal Y) resource. While geochemistry of REY in oceanic sediments has been widely discussed for many years, the causes of such high concentrations in several types of sediments have not yet been solved. The purpose of this study is to reveal the chemical form of REY in deep-sea mud by direct spectroscopic method, which is a piece of critical information to elucidate geochemical formation mechanisms of REY-rich mud. The findings can be a basis for providing clear direction for development of REY-rich mud as a resource, and further, decoding the history of ancient marine environment in marine sediments.
We revealed by XAFS analysis that (i) the dominant host phase of REY is apatite in all the sediments independent of REY concentrations, and (ii) Chemical forms of Fe, which could play important carrier of REY into sediment, are Fe (oxyhydr)oxides and/or clay minerals. We also found by mass accumulation rate (MAR) analysis that there are three factors responsible for the high concentration of REE: addition of Fe flux, addition of another flux containing abundant REE, and magnitude of dilution by biogenic carbonate. In the seminar, I will discuss the formation mechanisms of REY-rich mud based on the relationships among these findings.

●IFREE MC セミナーのお知らせ
日　時：2013年1月31日（木）16:30-18:00
場　所：横須賀本部本館3F 大会議室
発表者：ニコルス　アレクサンダー (IFREE 4A 研究員）

タイトル:
Volatiles in glasses from the Louisville seamounts:implications for paleoenvironments, paleomagnetic data and subsidence of underlying oceanic crust

概要:
The Louisville seamounts form a 4,300 km-long trail in the south Pacific, and are commonly attributed to a mantle plume located at the south-eastern end of the trail. IODP Expedition 330 (December 2010 − February 2011) drilled five of the older seamounts (50 to 77 Ma) at the north-western end. The main objective of the expedition was, using paleomagnetic and age data, to determine whether the Louisville mantle plume had moved with the Hawaiian mantle plume, which has moved 15deg south from 80 to 50 Ma, or, as mantle flow model calculations predict, the two plumes had moved relative to one another. Volcanic basement was reached at five drill sites on four of the seamounts. I will present new geochemical data for glass samples that were collected from four of these drill cores each on a different seamount. I will start by briefly showing the initial major and trace element data, before discussing in detail the volatile contents of the glasses and what this can tell us about where the glass formed and the implications this has for the paleomagnetic measurements and subsidence of the crust underlying the seamounts.

●IFREE MC セミナーのお知らせ
日　時：1月24日（木）16:00-18:00
場　所：横須賀本部本館3F 大会議室
発表者１： 佐藤 壮（IFREE 1D　技術研究副主任）

タイトル１:
Crustal structure from the Yamato Basin to the eastern margin of the Japan Sea deduced from a seismic survey

概要１:
The Japan Sea is one of very well studied back-arc basins in the northwestern Pacific. In the eastern margin of the Japan Sea, some destructive earthquakes occurred and the fault-fold belts developed by the deformation of the extension by the opening of the Japan Sea during the late Oligocene and the shortening since the late Pliocene (e.g., Sato, 1994). The seismic crustal model, however, has been inadequate to elucidate the detailed opening model of the Japan Sea and the relation between the occurrence of these earthquakes and the deformation in fault-fold belts in this margin. To understand the formation style and this relation of the earthquakes and deformation around the Yamato Basin in the eastern margin of the Japan Sea, the seismic survey using ocean bottom seismographs (OBSs), an airgun array, and a multi-channel hydrophone streamer were undertaken in this area. For this study, we will present the model from the Yamato Basin off Awa-shima to the coastal of the northeastern Japan Island Arc including the source area of the 1964 Niigata Earthquake.
The crustal thickness of the Yamato Basin and the Sado Ridge off Awa-shima are about 16 and 23 km, respectively. The crustal thickness of this Basin is less than that of a typical continental crust and greater than that of a typical oceanic crust. On the other hand, the crust of the Sado Ridge is similar to the island arc crust in the northeastern Japan Arc. From the velocity gradient, the crust of the Yamato Basin is divided to two parts; one is upper part having the steep gentle velocity gradient and the other is the lower part having the gentle gradient. These upper and lower parts have about 5 and 8 km thick, respectively. In the Yamato Basin, there is a little in the part of 5.5-6.4 km/s of the P-wave which corresponds to the island arc upper crust. Moreover, the lowermost lower crust from the central to western part in this Basin off Awa-shima has about 7.3 km/s. This high velocity may show that the mantle temperature was slight high during the formation of the Yamato Basin.


発表者２：平原 由香（IFREE 4F 技術研究副主任)
(共同発表者: 木村純一, 仙田量子, 常青, 宮崎隆, 高橋俊郎
(IFREE/JAMSTEC))

タイトル２:
Geochemical variations in the Japan Sea back-arc basin basalts controlled by the roles of adiabatic melting of hydrous mantle metasomatized by various subduction components

概要２:
Two types of Middle Miocene back-arc basin basalts (BABB)erupted during opening of the Japan Sea were recovered from the Yamato Basin by Ocean Drilling Program Legs 127 and 128. These are depleted- and enriched-type basalts in terms of concentrations of incompatible trace element and radiogenic isotope compositions. Both basalt types show evidence of the addition of subduction components to the their source mantle. The depleted-type basalts are derived from depleted mantle source similar to Indian-type mid ocean ridge basalts (MORB) in terms of Sr-Nd-Hf-Pb isotopic compositions. Although they have a lower abundance of highly incompatible elements than in normal MORB (N-MORB), highly fluid mobile elements such as Ba, Pb, and Sr are elevated (e.g. high Ba/Nb, Pb/Ce, Sr/Nd ratios). The enriched-type basalts have elevated concentrations of highly incompatible trace elements with enriched source mantle isotopic compositions. Although the trace element compositions of the enriched-type basalts are somewhat similar to those in the Quaternary rear-arc basalts (e.g., depleted in Nb-Ta), they differ from the rear-arc basalts by more elevated moderately incompatible trace elements and by having positive Zr-Hf anomalies. The isotope and trace element characteristics suggest that the depleted-type basalts were originated from a depleted mantle, which was metasomatized by fluid-mobile elements such as Ba, Sr, and Pb derived perhaps from subducted slab, whereas the source mantle of the enriched-type basalts was originated from the mixing of the depleted mantle with a subducted Pacific sediments (solid-solid mixing). Hf-Nd isotope data indicates a sedimentary component of sand-rich deep-sea turbidites with low Nd/Hf (owing to the presence of zircon). Bulk sediment addition, rather than sediment melt/fluid addition can account for the enrichment in the source mantle of the enriched-type basalts.
A forward model on adiabatic melting of a hydrous metasomatized mantle was examined with varying parameters of
(1) mantle potential temperature (Tp/C), (2) initial H2O content (H2O(i)/wt%), melting termination depth (Dmt/GPa), and sediment flux fraction (Fsed/wt%) mixed with the source peridotite. The calculation results suggest that conditions of Fsed = 1.2 wt%, H2O(i) = 0.01−0.12 wt%, Tp = 1200−1290 (C), final melting degree of F = 0.07 at depth of Dmt = 0.8−1.4 GPa explain the trace element abundances in the enriched basalts. In contrast, the depleted-type basalts can form at the conditions of Fsed = 0.0 wt%, H2O(i) = 0.00−0.08 wt%, Tp = 1340−1410 (C), F = 0.12−0.15 at depth of Dmt = 1.4−1.7 GPa. The melting conditions for the depleted-type basalts are deeper and hotter than those for primary N-MORB (H2O(i) = 0.01− 0.10 wt%, Tp = 1230−1330 (C), Dmt = 0.7−1.4 GPa, F = 0.10−0.12) calculated by the same method. This is consistent with the depletion of total rare earth elements (REEs) and heavy rare earth elements (HREEs) with higher MgO in the depleted-type basalts. The enriched basalts are derived from a different source and can form at shallower depth and lower Tp and F suggesting a heterogeneous source mantle with regard to chemistry and the melting regime.

●IFREE MC セミナーのお知らせ
日時：1月17日（木）16:30-18:00
場所：横須賀本部3階，大会議室

発表者1：Dereje Ayalew (Addis Ababa University, Ethiopia)
タイトル1： Picrites from the 30 Ma Ethiopian flood basalt province
要旨1：
The Ethiopian continental flood basalt province was formed during the initial impact of the Afar mantle plume and prior to the separation of the Arabia from Africa. Picrites from the 30 Ma Ethiopian flood basalt province are member of the HT2 magma series as defined by Pik et al. (1998). The lavas are Mg-rich with MgO contents up to 20 wt.%. Samples with MgO > 15 wt.% are rich in olivine and clinopyroxene and have probably been affected by crystal fractionation. They are also characterized by low Al2O3 contents (7-9 wt.% at 15 wt.% MgO). The rocks are unusually Ti-rich with 3-4 wt.% TiO2 at 15 wt.% MgO. Their composition indicates that these picrites could represent primary melts. Picrites with 15 wt.% MgO have Ni ~ 500 ppm, Cr ~ 1000 ppm and Co ~ 75 ppm, all close to values expected for primary melts of a peridotitic source. The composition of the picrites reflect melting at considerable depth ~ 5 GPa, i.e., ~ 150 km and at elevated temperature (> 1600 ºC), consistent with a plume origin. The picrites are characterized by 3He/4He ratios up to 19R/Ra, implying an origin in the deep mantle. They have high Fe/Mn ratios (65 to 78) and unradiogenic 187Os/188Os ratios (< 0.127), which argue against the entrainment of core material in the source of the Afar mantle plume. The unradiogenic osmium isotopes also preclude a significant contribution from ancient recycled oceanic crust.

発表者2：Minyahl Teferi Desta (Tohoku University)
タイトル2：A comparative petrological study of the Siberian and Ethiopian Large Igneous Provinces (LIPs) and a case study on Triassic mafic rocks in Chukotka, NE Russia
要旨2：
This study is mainly targeted to find the possible eastern marginal extension of Siberian LIP and to compare them with the central Siberian LIP and is also aimed to compare and contrast the geochemical and petrological characteristics of Siberian LIP (~ 250 Ma) with the Ethiopian LIP (~ 30 Ma) to consider the mantle and crustal processes in view of magmatic diversity among those LIPs. A review of previous geochemical data from the Siberian and Ethiopian LIP confirms notable differences in their major and trace element compositions. Siberian LIP comprises a variety of rocks (such as basalts, basaltic andesites, picrites and meimechites) with a wide range of SiO2 (40-62 wt. %). In contrast, Ethiopian LIP is characterized by bimodal volcanism with the absence of intermediate rock. The Ethiopian high-Ti basalts and picrites have higher TiO2 (3-6 wt. %), lower CaO/Al2O3 (0.5-1.5) and MgO (5-26 wt. %) than the Siberian high-Ti picrites and meimechites (2-4, 1.8-2.3 and 13-36 wt. % respectively). Siberian LIP shows more significant depletion in HFSE (mainly Nb) and higher La/Sm ratios than Ethiopian LIP. This may suggest contamination of Siberian LIP magma by crustal component. Triassic volcanic and intrusive rock samples are collected in 2009 from the Chukotka province (Northeast Russia), where geographically far to the east from the central Siberian flood basalt province. The petrography of the studied samples includes basaltic rocks (i.e. hornblende basalt, lamprophyre, pyroxene basalt, and ankaramite) and gabbroic rocks (i.e. amphibole-bearing type-I and amphibole free type-II). Clinopyroxenes (diopside to augite) both from basalts and gabbros show only low-Ti (<1 wt. %) characteristics. Amphiboles are pargasite in lamprophyres and type-I gabbros, tschermakite in hornblende-basalt, edenite in ankaramite and edenite-hornblende in type-II gabbros. Plagioclase ranges from An100 to An45.6 in type-I gabbros, An54.5 to An0.9 in type-II gabbros and An73.4 to An1.3 in basaltic rocks. Type-I gabbros are characterized by low TiO2, FeO* and high CaO, Y/Nb and Y/Zr. In contrast type-II gabbros exhibit high TiO2 and FeO* and low CaO, Al2O3, Y/Nb and Y/Zr. These differences may suggest that type-I gabbros derived from a more hydrous magma than type-II. The Triassic basalts and type-I gabbros of Chukotka province resembles the low-Ti suits of Siberian LIP with the presence of HFSE depletion (Nb and Ti) and high H2O content of the magma to crystallize abundant hornblende. Thus, the Triassic mafic rock in Chukotka resembles the Siberian LIP rocks and it is possible that the LIP magmatism outspread far from the trap.

発表者3：Erdenesaikhan Ganbat (Tohoku University)
タイトル3：Geochemistry and petrology of greenstones from the Erdenetsogt formation, central Mongolia: New evidence for a middle Paleozoic mantle plume
要旨3：
This research presents evidence of the plume-related formation of greenstones from the Erdenetsogt Formation (EF) hosted by Tsetserleg terrene of middle Paleozoic Hangay-Hentey accretionary complex, which is central part of the Central Asian Orogenic Belt (CAOB). Our new data can support middle Paleozoic evolution of plume-related magmatism of the paleo-oceanic plate and its accretionary processes, which significantly contributed to the continental growth of CAOB. EF is mainly made up of intensively deformed oceanic plate stratigraphy such as radiolarian/ribbon chert, red and green quartzites (metachert), fine to medium-grained sandstone, siltstone with minor amount of limestone, and some basaltic greenstones which have been discovered lately during geological mapping in the south Hangay region. The upper unit of this formation was dated by late Devonian conodonts from the chert. However, the chemistry of greenstones in this region has not been studied yet except brief study by Orolmaa (2008). The studied samples include metabasalts, dolerites, microgabbros, and occasional meta-picrites with mostly subophitic, intersertal, and partly intergranular textures. The common assembledge of phenocrysts is clinopyroxene (well preserved in all greenstones) and plagioclase (replaced by albite). Also completely altered olivine containing relict Cr-spinel observed from metapicrite and porphyritic metabasalt. The composition of the primary clinopyroxene (augite) and Cr-spinel were analyzed by electron microprobe and bulk rock major and trace elements by XRF. On the basis of Ca +Na vs. Ti and Ca vs. Ti+Cr discrimination diagrams, clinopyroxenes display tholeiitic magma type and non-orogenic affinity. The relict spinels are 20-250 m in size. The Al2O3 wt.% vs. TiO2 wt.% variation of the spinels corresponds to the transitional affinity of OIB and MORB. The Cr and Mg numbers of the spinels show very limited values ranging from 55 to 68 and from 0.3 to 0.6, respectively, and implies highly depleted (high melting degree) magma compare with MORB, and more identical to spinel from Hawaiian tholeiitic basalts. From those facts, it is concluded that the Cr-spinel of greenstones may have been derived from a mantle plume source. Furthermore, notable exceptions of the Hangay greenstones are very low ratios of Nb/Zr and Zr/Y (0.05-0.08 and 0.2-0.5, respectively), whereas Hentey basalts show HIMU characteristic (Tsukada, 2006). The greenstones are slightly enriched in LREE and TiO2 (1.6-2.2 wt.%). We suggest that greenstones in Erdenetsogt formation may have been formed as plume-related oceanic island (hotspot or oceanic plateau) within paleo-oceanic plate located between the Siberian and the North China Cratons, and then accreted to the active continental margin of Siberian Craton during middle to late Paleozoic. This setting is analogous to the present southwest Pacific realm.