JAMSTEC > Basic Research Area > Department of Solid Earth Geochemistry > Seminar Schedule > details

Department of Solid Earth Geochemistry

Seminar Schedule

[D-SEG seminar]

Date:
2014/12/11(Thursday)16:00-18:00
Place:
MMeeting Room at 3F, YOKOSUKA HQ
Speaker:
Masashi Ushioda(Tokyo Institute of Technology)
Title:
Condition of arc basaltic magma chamber based on experimental study: Case study on Miyakejima volcano
Abstract:
Miyakejima volcano, which is an active basaltic volcano, was chosen to understand evolution of arc volcano. In order to reveal the evolution of magma plumbing system of Miyakejima volcano, it is inevitable to understand its long-term evolution. Conditions of magma chamber in Ofunato Stage (4000-10000 yBP) was studied based on high-pressure experimental study and petrologic study on melt inclusions. Basalts in Ofunato Stage seem to be parental to rocks in later stages and the basalts in Ofunato Stage shows only limited compositional variation having no signs of magma mixing. It is the most suitable rocks in order to estimate the crystallization conditions in a magma chamber. Melting experiments on OFS (Ofunato scoria), one of the least fractionated Miyakejima basalt in the last 10,000 years, were carried out. Melt inclusions of phenocrysts in OFS scoria were also analyzed. In order to crystallize phenocrysts of OFS in the magma chamber, optimum P, T conditions and H2O content in melt were ~150 MPa and ~1100°C and ~3wt.% of H2O content in melt. From melt inclusion analyses, maximum H2O and CO2 content were 3.4 wt.% and 165 wt.ppm, respectively, and the saturated pressure of this melt was estimated to be ~150MPa (depth of ~6km). Magma plumbing system of Miyakejima volcano was simple in Ofunato Stage having a single magma chamber at around 6 km depth.
In a futher topic, a plagioclase phenocryst hygrometer was constructed, in order to estimate water content in arc basalts along NE Japan and Izu arcs. From the experiments on OFS most suitable runs to construct hygrometer were selected. As a result of the experiments, we found that the Ca/Na partition coefficient between plagioclase and hydrous basaltic melt increases linearly with increasing H2O content of melts. Based on these experiments we proposed a simple plagioclase hygrometer, which is expressed as a function of only H2O in melt without regard to P and T. Our hygrometer is useful to estimate water content in arc basaltic magma where precise conditions of crystallization (pressure, temperature, etc) are not known. Water content in pre-eruptive basaltic magma in Northeast Japan arc and Izu arc were estimated using a simple hygrometer developed. From literatures, we chose geochemical data sets of relatively primitive basaltic rocks (with no evidence of magma mixing) and most frequent Ca-rich plagioclase phenocrysts from 16 basaltic arc volcanoes, which includes both frontal-arc volcanoes and rear-arc volcanoes. In the 16 volcanoes studied, plagioclase phenocrysts of high anorthite content (An>90) are commonly observed, whereas plagioclase phenocrysts in rear arc volcanoes usually have lower anorthite content (90>An>80). In all volcanoes studied, the estimated H2O content of basaltic magma is at least 3 wt.% H2O or higher. Many volcanoes located on the volcanic front show about 5 wt.% H2O in magma whereas those from a rear-arc side are slightly lower in H2O content.

[D-SEG seminar]

Date:
2014/12/4(Thursday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Ryoko Senda
Title:
Selective sulphide assimilation will make an Os isotopic variation in lower oceanic crusts
Abstract:
The origin of the Mid-Ocean Ridge Basalts (MORB) are believed to be closely relate to the abyssal peridotites from trace element abundances and some isotope systematics. However, discrepancy between MORB and abyssal peridotites can be observed in Os isotope ratios. The isotopic variability of MORB is commonly ascribed to heterogeneities in their mantle source, such as recycled oceanic crust or subcontinental lithospheric mantle. Assimilation or alteration by sea water or weathering are also pointed out to change Os isotope ratios in MORB.
We will propose that the interaction between the abyssal peridotite (or mantle) and melts migrating through it might have shaped the Os isotopic composition of primitive lower crustal samples from the southernmost area of the Central Indian Ridge. These samples have radiogenic Os isotope signature and they are likely from selective assimilation of radiogenic sulfides from the reacted mantle. This mechanism can be able to generate melts mimicking the Os isotope variations in MORB without any different components than abyssal peridotite as their mantle source.

[D-SEG seminar]

Date:
2014/11/13(Thursday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Maria Luisa Tejada  (DSEG)
Title:
Probing the origin and tectonic evolution of a marginal basin, IODP Expedition 349, South China Sea
Abstract:
Expedition 349 is the first drilling expedition in the new International Ocean Discovery Program (IODP), which successfully drilled and cored 5 sites in the South China Sea (SCS), a marginal basin whose origin is largely debated. An important objective of the expedition is to obtain oceanic crust samples to better determine the timing of the initiation and cessation of spreading. The SCS oceanic crust was sampled for the first time at three of the Expedition 349 drill sites, U1431, U1433, and U1434, located near fossil spreading centers. In addition, large and frequent deep-sea turbidity currents and multiple seamount-derived volcaniclastic layers were discovered from the sedimentary sections overlying oceanic crust at these sites. I will be presenting the scientific objectives and some of the preliminary findings of this expedition.

[D-SEG seminar]

Date:
2014/10/30(Thursday)16:30-18:00
Place:
Lecture room at Guest House 2F, YOKOSUKA HQ
Speaker:
Takeshi Hanyu
Title:
Raivavae Island; HIMU study once more
Abstract:
So-called HIMU geochemical signature observed in some ocean island basalts has been believed by many geochemists to be a manifestation of ocean crust recycling caused by mantle-scale circulation. However, there exist considerable debate on the nature of the recycled sources and melting processes. We present new geochemical data for basalts from Raivavae in the northern Austral island chain in the South Pacific. Subaerial lavas on this island show binary geochemical characteristics, strong and weak HIMU signatures (say, more-radiogenic and less-radiogenic Pb isotopic compositions, respectively), providing us an oppourtunity to compare geochemical characteristics of strong HIMU basalts with those of weak HIMU basalts.
Our data together with previous data display correlation between major element, trace element, and isotopic compositions. As SiO2 decreases, CaO, MnO, incompatible element concentrations (e.g., Th, Nb), La/Yb, Nb/Zr, and 206Pb/204Pb increase, and 143Nd/144Nd decreases. This fact suggests that the HIMU signature appears more robust in the basalts that were formed by lower degree of melting. Because the strongest HIMU signature (i.e., the most radiogenic Pb isotopes) is associated with SiO2 < 42 wt %, the HIMU melt is likely produced by low degree melting of carbonated fertile peridotite or pyroxenite. On the other hand, basalts with weak HIMU signature (i.e., less-radiogenic Pb isotopes) can be formed by larger degree melting of the same source added by low-degree melt from the depleted mantle, the latter of which would be a melt from the ambient mantle or lithosphere reheated by an upwelling mantle plume.
We also present isotopic and trace element compositions of clinopyroxene (cpx) phenocrysts in the basalts. Sr, Nd, and Pb isotopic compositions of cpx overlap with those of host basalts, except for one sample. Cpx are mostly in equilibrium with host basalts in terms of trace element abundance, indicating that there is no cross-talk for cpx phenocrysts between basalts with the strong and weak HIMU signatures, even if these cpx were cumulate in origin. However, cpx show larger variations in Hf isotopes (and Sr isotopes in small extent) than whole rock basalts. The Hf isotopic variation is larger among cpx from the weak-HIMU basalts than those from the strong-HIMU basalts. We interpret that cpx recorded isotopic variability in the magmas at the time of cpx crystallization, and such variability reflects possible chemical heterogeneity in the depleted ambient mantle caused by previous depletion/enrichment process.

[D-SEG seminar]

Date:
2014/10/2(Thursday)16:30-18:00
Place:
Meeting Room at Guest House 2F, YOKOSUKA HQ
Speaker:
Takashi Miyazaki(DSEG)
Title:
Development and evaluation of a method for Sr-Nd-Pb-Hf isotope analyses of clinopyroxene samples
Abstract:
Whole rock Sr-Nd-Pb-Hf isotope analyses commonly employ the leaching technique to remove the effects of secondary alteration and contamination due to external factors. For rock samples from ocean, the leaching step is indispensable to eliminate the seawater alteration effects. Sample powders are usually treated with hydrochloric acid to dissolve and remove the altered and contaminated components of the rocks. Leaching conditions such as acid concentration, heating temperature, and heating time vary according to the sample condition. However, it is difficult to completely remove the alteration products, even after application of harsh leaching conditions. Thus, focus of some researches shifted to clinopyroxene (CPX) because of its early crystallization characteristics and its high resistance to alteration. Because of these properties, CPX is expected to preserve more primitive isotopic signatures than the leached whole rock samples. Using CPX samples from ocean island basalts (OIBs), Hanyu and Nakamura (2000) and Jackson et al. (2009) determined the Sr and Nd isotopic signature of mantle source for OIBs. In order to investigate the mantle end-member components from OIBs, our research group (in IFREE and D-SEG) analyzed OIB CPX for Pb and Hf isotopes as well as Sr and Nd isotopes. Successful Sr-Nd-Pb-Hf isotope analyses revealed the isotope signatures of CPX samples obtained from OIBs. Hanyu et al. (2011) (Polynesia region) and Hanyu et al. (2014) (St. Helena) published the Sr-Nd-Pb-Hf isotope data of CPX samples. Although the Pb concentration of CPX samples was extremely low relative to whole rock samples, i.e., Pb 0.018-0.026 ppm in CPX from Rurutu island (Hanyu et al., 2011), CPX data must as precise as whole rock data. Therefore, it was necessary to further optimize the method for CPX analysis (especially for Pb isotopes) to complete this research. In this seminar, I will present about the methods for Sr-Nd-Pb-Hf isotopic analysis for the CPX samples and evaluate the method using the isotope data from a recent project (Raivavae Island).

[D-SEG seminar]

Date:
2014/9/11 (Thursday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Takashi Yoshino (OKAYAMA UNIVERSITY)
Title:
Origin of conductivity anomalies in the mantle
Abstract:
Recent electromagnetic studies show that better models of conductivity profiles in the mantle up to the uppermost lower mantle. There are two major conductivity anomalies found in the mantle. Electrical conductivity of the oceanic upper mantle at ~100 km depth is very high, about 10-2 to 10-1 S/m and is characterized by strong anisotropic feature. This zone is known in seismology as the low velocity zone, corresponding to the high conductivity layer. On the other hand, recent electromagnetic studies have shown a great variety of lateral heterogeneities in conductivity structure of the mantle transition zone. Especially the transition zone where slab stagnates showed high conductivity anomaly. The origin of these conductivity anomalies will be discussed in this talk based on the laboratory conductivity measurement. Recent publications on electrical conductivity of hydrous olivine suggested that olivine with the maximum soluble H2O content at the top of the asthenosphere has lower conductivity less than 0.1 S/m, which is a typical value of conductivity anomaly observed in the oceanic mantle, and has weak anisotropic feature (e.g., Yoshino et al., 2006; 2009a; Poe et al., 2010; Du Frane and Tyburczy, 2012; Yang, 2012). The inability of hydrogen to account for the conductivity anomaly at the top of the asthenosphere requires an alternate explanation for the high conductivity beneath the young ocean floor. Partial melting has been considered as an attractive agent for substantially raising the conductivity in this region (Shankland and Waff, 1977), because basaltic melt has greater electrical conductivity (> 100.5 S/m) and high wetting properties. However, dry mantle peridotite cannot reach the solidus temperature at depth 100 km. Volatile components can dramatically reduce melting temperature, even if its amount is very small. Petrological considerations suggest that melts are thermodynamically stable in the low velocity zone (asthenosphere) beneath the oceanic lithosphere, because of trace amounts of volatile components such as H2O and CO2. Recent studies on conductivity measurement of volatile-bearing melt suggest that conductivity of melt dramatically increases with increasing volatile components (H2O: Ni et al., 2010a, b; CO2: Gaillard et al., 2008; Yoshino et al., 2010; 2012a). Because incipient melt includes higher amount of volatile components, conductivity enhancement by the partial melt is very effective at temperatures just above that of the volatile-bearing peridotite solidus (Silfe et al., 2014). In situ conductivity measurement of partial molten peridotite showed one order of magnitude difference between parallel and normal to shear direction under shear. Thus, conductivity anisotropy observed at the oceanic asthenosphere can be explained by anisotropic melt distribution due to elongation of melt pocket parallel to the shear direction.
The mantle beneath Philippine Sea and central China where descending slab stagnates at the transition zone has higher conductivity than that beneath central Europe or Canada. The conductivity data suggests that if we compare the global average model by Kelbert et al. (2009), the estimated water contents in wadsleyite are less than 0.1 wt.%. On the other hand, the conductivity anomaly zone where descending slab stagnates at the transition zone depth requires nearly 1 wt.% water in wadsleyite, if we assume lower temperature (< 300 K) of this region. However, the storage capacity of H2O in peridotite does not allow such higher water content in wadsleyite. The presence of very tiny amounts of supercritical fluid (0.1 vol.%) with high conductivity (102 S/m) can explain the conductivity values.

[D-SEG seminar]

Date:
2014/08/28 (Thursday) 16:30-18:00
Place:
Seminar Room at 1F, YOKOSUKA HQ
Speaker:
Dr. Norikatsu Akizawa (Kanazawa University)
Title:
Formation process of a suboceanic Moho and its hydrothermal alteration: an example from the northern Oman ophiplite.
Abstract:
We petrologically examined peridotitic rocks for about 72 m in depth beneath the layered gabbro along Wadi Fizh in the northern Oman ophiolite. We tentatively call the lower end of the layered gabbro section as "Moho" here. Wehrlites are predominant just beneath the Moho, and change to harzburgites through dunites in 10m in depth. Network-like gabbro sills increase in frequency within the dunite-wehrlite section, and some of the sills appear to be continuous to gabbros above the Moho. In the harzburgites, aggregates of orthopyroxene are gradually reduced in size, and spinels which are generally associated with the orthopyroxene aggregates gradually increase their idiomorphism upward to the Moho.
Mineral chemistry shows systematic variations upward to the Moho; (1) the Fo content (91 to 86) and NiO (0.4 to 0.2 wt%) of olivines decrease and (2) the Cr/(Cr + Al) atomic ratio (0.5 to 0.6), TiO2 content (nil to 1.5 wt %) and the Fe3+/(Cr + Al + Fe3+) atomic ratio (0.05 to 0.2) in spinels increase from the harzburgite through the dunite to the wehrlite. Trace-element characteristics of clinopyroxenes in dunites and wehrlites suggest an involvement of MORB-like melts.
Geological, petrographical and mineral chemical characteristics indicate that the petrologic gradient has formed by peridotite/magma (MORB-like) interaction including incongruent melting of orthopyroxene around the present Moho beneath a spreading center. The interaction was most remarkable at the Moho and decreased with a distance from the layered gabbro downward.
In the presentation, we will also show rocks recorded high-temperature hydrothermal activity around the Moho. The high-temperature hydrothermal fluid mobilized Cr as well as REEs (rare earth elements) from the uppermost mantle section to the lowermost crustal section, about 70 m in depth. We will discuss chemical characteristics of the fluid and mechanism of mobilizing Cr, REE and other elements.

[D-SEG seminar]

Date:
2014/08/21 (Thursday)16:30-18:00
Place:
Meeting Room on 2F,Marine Science Museum, YOKOSUKA
Speaker:
Dr. Alessio Sanfilippo (Kanazawa University)
Title:
Hybrid troctolites from oceanic ridges: reassessing the significance of bulk oceanic crust
Abstract:
The idea that hybridized mantle rocks can contribute to the oceanic crust composition has recently emerged thanks to studies on primitive (olivine-rich) troctolites. These rocks have been mostly found at slow spreading ridges or at their fossil analogues. However, similar rocks have been recently collected in the 25ºS area of the intermediate spreading Central Indian Ridge (CIR), and locally characterize the crust mantle boundary at fast spreading ridges, being an ubiquitous lithology under the oceanic plates. Structural and chemical studies on olivine-rich troctolites worldwide inferred a formation by melt-rock interaction, generally agreeing with a mantle origin for the pre-existing mineral matrix. However, the exact reaction process by which these rocks originate is still highly debated and their role on the bulk oceanic crust composition has been never defined. This talk discusses the process of formation of the olivine-rich troctolites and how it may affect the models of crustal accretion at spreading ridges. In particular, new textural and chemical evidence are used to infer that these crustal rocks formed through a direct (one-stage) conversion of a mantle peridotite. This evidence suggests that the significance of the bulk oceanic crust should be reassessed. When hybrid troctolites are included at crustal levels, the oceanic crust cannot be considered equal to the composition of the melt extracted from the mantle, but it results more primitive and importantly thicker.

[D-SEG seminar]

Date:
2014/07/24 (Monday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Yoshihiko Tamura(DRMACC)
Title:
Preliminary report of IODP Expedition 350 'Izu-Bonin-Mariana rear arc: the missing half of the subduction factory'
Abstract:
International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Ocean Drilling Program (ODP) Site 792, and at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layer that may record large-volume eruptions on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layer and enclosing sediments.

IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize "the missing half of the subduction factory." Numerous ODP/Integrated Ocean Drilling Program sites were previously drilled in the arc to fore-arc region, but this was the first to be drilled in the rear part of the Izu arc. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest IODP casing on record (1085.6 m) in Hole U1437E and drilled to 1804 mbsf.

[D-SEG seminar]

Date:
2014/7/10(Thursday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Junichi Kimura(DSEG)
Title:
Ocean Basalt Simulator version 1 (OBS1): Trace element mass balance in adiabatic melting of a pyroxenite-bearing peridotite
Abstract:
I present a new numerical mass balance model for estimating the source conditions of a primary ocean basalt from adiabatic melting of a pyroxenite-bearing peridotite. The Ocean Basalt Simulator version 1 (OBS1) uses a thermodynamic model of adiabatic melting of a pyroxenite-bearing peridotite with experimentally parameterized liquidus–solidus and melting intervals and thermodynamically/experimentally parameterized source mineralogy of pyroxenite and peridotite. OBS1 uses a sequence of adiabatic melting with pyroxenite melting, melt metasomatism in the host peridotite, and melting of the metasomatized peridotite. OBS1 explores (1) the fractions of peridotite and pyroxenite, (2) mantle potential temperature Tp, (3) depth of termination of melting Pmt, (4) degree of melting F, and (5) residual mode Xa of the sources. The model also examines the mass balance of 26 incompatible trace elements in the sources and in the generated basalt. OBS1 can also calculate Sr–Nd–Hf–Pb isotope compositions in the sources and in the melt. OBS1 is coded in an Excel spreadsheet and runs with VBA macros. Using OBS1, we examine the source conditions of the mid oceanic ridge basalts (MORBs), plume-influenced ocean ridge basalts in Galápagos, Loihi– Loa–Koolau basalts in the Hawaiian hotspot, high-mu (HIMU) basalt from the St. Helena hotspot, and Shatsky Rise and Jurassic Mikabu oceanic plateau basalts and picrites. OBS1 is a unique model that provides an opportunity to examine the source mantle conditions using incompatible trace elements and relevant isotopes, which are key factors in global mantle recycling.

[D-SEG seminar]

Date:
2014/06/12 (Thursday)15:30-17:30
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker1:
Dr. Hirochika Sumino (The University of Tokyo)
Title:
Noble gas and halogen recycling at subduction zones
Abstract:
Recent findings of subducted halogens and noble gases with seawater and sedimentary pore-fluid signatures in exhumed mantle wedge peridotites (Sumino et al., 2010), as well as seawater-derived heavy noble gases (Ar, Kr, and Xe) in the convecting mantle (Holland & Ballentine 2006), provide observations that allow us to investigate the processes that control the return of volatile and highly incompatible elements into the mantle. To verify whether and how such subduction fluids modify the composition of the mantle beneath subduction zones, we are investigating noble gas and halogen compositions of olivines and mantle-derived xenoliths in arc lavas from Western-Pacific subduction zones (Izu, Kamchatka and N. Philippines) and those of seafloor sediments and basalts from NW margin of the Pacific plate. MORB-like 3He/4He and halogen ratios of the olivines indicate insignificant contributions to the arc magmas of radiogenic 4He and pore-fluid-like halogens, both of which are observed in the subduction fluids released from a slab at a depth of 100 km (Sumino et al., 2010). In contrast, mantle-derived xenoliths exhibit pore-fluid-like halogens and heavy noble gases but MORB-like He. The high I/Cl ratios of pelagic clays and radiolarian cherts can account for the enrichment of iodine in subduction fluids relative to sedimentary pore-fluids, whereas contributions of halogens and noble gases from altered oceanic basalts are limited. These results indicate that subduction and closed system retention of marine pore fluid occurs to depths of at least 100 km, necessitating a reassessment of the dominant transport mechanism and source of water in subduction zones. Further subduction of a small amount of marine pore fluid can account for the heavy noble gas composition observed in the convecting mantle (Sumino et al., 2010).
The mechanism by which the seawater-like noble gases are delivered to the convecting mantle remains to be elucidated. Serpentinized lithosphere of subducting slab is probably the best candidate, because serpentine contains significant amounts of Cl (up to 0.5 wt%). If the hydration of the lithosphere by pore fluids is operating in a closed system, subduction of the serpentinized lithosphere can transport pore-fluid derived noble gases and halogens into the deep mantle. This is supported by a recent observation of pore fluid-like noble gases and halogens in exhumed serpentinites (Kendrick et al., 2011).

[D-SEG seminar]

Date:
2014/5/19(Monday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker1:
Doctor Isobel Yeo (GEOMAR, Germany)
Title:
256 Shades of Grey: Segment Scale Volcanic Events on the Kolbeinsey Ridge from High-Resolution Sidescan Sonar
Abstract:
Description: The Kolbeinsey Ridge is an anomalously shallow section of the Mid-Atlantic Ridge between 66.8ºN and 71.5ºN and connects the Tjörnes and Jan Mayen fracture zones. The section North of 70.7ºN is known as Eggvin bank and was the subject of cruise POS436 in 2012. During this cruise a full bathymetric dataset was collected allowing the ridge axis to be identified and the ridge volcanology to be characterised for the first time. Additionally, extensive high-resolution sidescan sonar coverage of the ridge axis was achieved by the AUV (Autonomous Underwater Vehicle) Abyss. This sidescan sonar data contains not only information about the surface geology - the differences in the amplitude of the reflected wave can also be used to assess the sediment thickness and therefore the ages of the mapped lava flows. Using this new method we identify a number of flows with identical levels of sediment cover and therefore similar ages. These flows extend over more than 50km of the ridge axis and likely represent a single large volcanic event occurring in the last 10,000 years.

[D-SEG Seminar]

Date:
2014/5/15(Thursday)16:30-18:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker1:
KENTA UEKI(DSEG)
Title:
Thermodynamic calculation for multi pressure melting phase relation of mantle peridotite
Abstract:
We present a new thermodynamic model for the calculation of melting phase relation of anhydrous spinel lherzolite at 1-2.5 GPa. The model is constructed based on a total energy minimization algorithm for multi phase and multi component system and a thermodynamic configuration to describe a molar Gibbs free energy of silicate melt presented by Ueki and Iwamori [2013]. The algorithm conducts a total energy minimization by searching a set of molar content of end-components that minimizes the Gibbs free energy of the system at given pressure, temperature and bulk composition. The model is developed for the calculation in a SiO2-Al2O3-FeO-Fe3O4-MgO-CaO system, which includes silicate melt, olivine, clino-pyroxene, ortho-pyroxene, and spinel as possible phases. Thermodynamic parameters for silicate melt end-member components are newly calibrated with an expanded calibration data source in terms of pressure. We calibrate dV and dCp, these are differences of molar volume and molar specific heat between melt and solid end-member components. Our thermodynamic model reproduces some important features of multi pressure melting phase relations of spinel lherzolite, and will be useful for the modeling of polybaric mantle melting in various natural settings.
Speaker2:
CHIAKI TOYAMA(DSEG)
Title:
Geochemical classification of kimberlites and their xenoliths using I/Br ratios
Abstract:
Recently, halogens are revealed to be one of the powerful tracers for water cycling in subduction zones (e.g., Sumino et al., 2010). In the previous conference, we reported analytical method and some data for Cl, Br and I in kimberlites from South Africa, Greenland, Canada, Brazil, Russia and China, and found that the I/Br ratios of kimberlites are classified into two groups. In this study, additional samples of kimberlites and mantle-derived xenoliths collected from South Africa and Russia were analyzed to investigate the halogen characteristics and their origins in the kimberlite source regions. We analyzed halogens by using the pyrohydrolysis method (Muramatsu & Wedepohl 1998) combined with ICP-MS and ion chromatography. The kimberlite and xenolith samples from South Africa, Greenland, Canada and Brazil (Group S) showed high I/Br ratios (about 1 × 10-1). The value is fairly similar to that of CI chondrite (I/Br ratio: about 1 × 10-1 [Anders & Ebihara 1982]), suggesting these kimberlites preserve the characteristics of primordial halogen in the mantle from which the kimberlite magmas were formed. In contrast, both Chinese and Russian kimberlite and xenolith samples (Group C) showed low I/Br ratios (about 6 × 10-3). Similarly low I/Br ratios have been observed in fluid inclusions in eclogites derived from seawater-altered oceanic crust (Svensen et al., 2001) and in seawater associated with halite precipitation (Zherebtsova & Volkova 1966). This suggests an involvement of seawater-derived halogens having low I/Br ratios in the source regions of the Group C kimberlites. Low I/Br ratios found in xenoliths also indicate possible subduction-related metasomatism on the halogen composition of the subcontinental lithospheric mantle.
Speaker3:
Iona McIntosh (JSPS)
Title:
Bubble growth and resorption in magma: insights from dissolved water distributions in volcanic glass
Abstract:
Volcanic eruptions are driven by the growth of gas bubbles in magma, which grow and shrink as volatile species exsolve from and dissolve back into the melt in response to changes in the local environment, particularly in pressure and temperature. This movement of volatiles, particularly water, is recorded in the concentration profiles preserved in the glass around vesicles. This study presents water distributions, obtained using SIMS-calibrated BSEM imaging and FTIR spectroscopy, in experimentally-vesiculated volcanic glasses created under known pressure and temperature conditions. These data reveal that samples experienced partial bubble resorption during the quench to glass at the end of the experiments, as a result of increasing water solubility with decreasing temperature. This quench resorption can significantly reduce bubble volumes, creating characteristic resorption textures, and is likely to occur in a wide range of experimental and natural conditions. Crucially, water speciation data offer a way both to identify samples affected by quench resorption, and to reconstruct previous bubble growth history.