Shimizu Nobumichi

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Shimizu
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  • Article
    CO2 content beneath northern Iceland and the variability of mantle carbon
    (Geological Society of America, 2017-11-16) Hauri, Erik H. ; Maclennan, John ; McKenzie, Dan ; Gronvold, Karl ; Oskarsson, Niels ; Shimizu, Nobumichi
    Primitive basalt melt inclusions from Borgarhraun, northern Iceland, display large correlated variations in CO2 and nonvolatile incompatible trace elements (ITEs) such as Nb, Th, Rb, and Ba. The average CO2/ITE ratios of the Borgarhraun melt inclusion population are precisely determined (e.g., CO2/Nb = 391 ± 16; 2σM [two standard errors of the mean], n = 161). These data, along with published data on five other populations of undegassed mid-oceanic ridge basalt (MORB) glasses and melt inclusions, demonstrate that upper mantle CO2/Ba and CO2/Rb are nearly homogeneous, while CO2/Nb and CO2/Th are broadly correlated with long-term indices of mantle heterogeneity reflected in Nd isotopes (143Nd/144Nd) in five of the six regions of the upper mantle examined thus far. Our results suggest that heterogeneous carbon contents of the upper mantle are long-lived features, and that average carbon abundances of the mantle sources of Atlantic MORB are higher by a factor of two than those of Pacific MORB. This observation is correlated with a similar distinction in water contents and trace elements characteristic of subduction fluids (Ba, Rb). We suggest that the upper mantle beneath the younger Atlantic Ocean basin contains components of hydrated and carbonated subduction-modified mantle from prior episodes of Iapetus subduction that were entrained and mixed into the upper mantle during opening of the Atlantic Ocean basin.
  • Article
    Crystallization depth beneath an oceanic detachment fault (ODP Hole 923A, Mid-Atlantic Ridge)
    (John Wiley & Sons, 2016-01-21) Lissenberg, C. Johan ; Rioux, Matthew ; MacLeod, Christopher J. ; Bowring, Samuel A. ; Shimizu, Nobumichi
    Oceanic detachment faults are increasingly recognized as playing an integral role in the seafloor spreading process at slow and ultraslow spreading mid-ocean ridges, with significant consequences for the architecture of the oceanic lithosphere. Although melt supply is considered to play a critical control on the formation and evolution of oceanic detachments, much less well understood is how melts and faults interact and influence each other. Few direct constraints on the locus and depth of melt emplacement in the vicinity of detachments are available. Gabbros drilled in ODP Hole 923A near the intersection of the Mid-Atlantic Ridge and the Kane transform fault (23°N; the MARK area) represent magmas emplaced into the footwall of such a detachment fault and unroofed by it. We here present U-Pb zircon dates for these gabbros and associated diorite veins which, when combined with a tectonic reconstruction of the area, allow us to calculate the depths at which the melts crystallized. Th-corrected single zircon U-Pb dates from three samples range from 1.138 ± 0.062 to 1.213 ± 0.021 Ma. We find a crystallization depth of 6.4 +1.7/−1.3 km, and estimate that the melts parental to the gabbros were initially emplaced up to 1.5 km deeper, at <8 km below the seafloor. The tectonic reconstruction implies that the detachment fault responsible for the exposure of the sampled sequence likely crossed the ridge axis at depth, suggesting that melt emplacement into the footwall of oceanic detachment faults is an important process. The deep emplacement depth we find associated with “detachment mode” spreading at ∼1.2 Ma appears to be significantly greater than the depth of magma reservoirs during the current “magmatic mode” of spreading in the area, suggesting that the northern MARK segment preserves a recent switch between two temporally distinct modes of spreading with fundamentally different lithospheric architecture.
  • Article
    Insights on coccolith chemistry from a new ion probe method for analysis of individually picked coccoliths
    (American Geophysical Union, 2007-06-30) Stoll, Heather M. ; Shimizu, Nobumichi ; Arevalos, Alicia ; Matell, Nora ; Banasiak, Adam ; Zeren, Seth
    The elemental chemistry of calcareous nannofossils may provide valuable information on past ocean conditions and coccolithophorid physiology, but artifacts from noncoccolith particles and from changing nannofossil assemblages may bias geochemical records from coccolith size fractions. We describe the first method for picking individual coccoliths using a tungsten needle in micromanipulator. Epoxy-mounted individuals and populations of coccoliths can be analyzed by secondary ion mass spectrometry (SIMS). For Paleocene sediments the technique distinguishes the high Sr/Ca ratios of coccoliths (0.3 to 2.8 mmol/mol) from low ratios in abiogenic calcite blades (0.1 mmol/mol). The large heterogeneity of Sr/Ca ratios among different genera suggests that primary geochemical differences have not been homogenized by diagenetic overgrowth and the thick massive coccoliths of the late Paleocene are a primary feature of biomineralization. Sr/Ca ratios for modern genera are on average higher than those of Paleogene genera but exhibit a comparable level of variability.
  • Article
    The Sr-87/Sr-86 and Nd-143/Nd-144 disequilibrium between Polynesian hot spot lavas and the clinopyroxenes they host : evidence complementing isotopic disequilibrium in melt inclusions
    (American Geophysical Union, 2009-03-11) Jackson, Matthew G. ; Hart, Stanley R. ; Shimizu, Nobumichi ; Blusztajn, Jerzy S.
    We report 87Sr/86Sr and 143Nd/144Nd data on clinopyroxenes recovered from 10 ocean island lavas from three different hot spots (Samoa, Society, and Cook-Austral island chains). The clinopyroxenes recovered from eight of the 10 lavas analyzed in this study exhibit 87Sr/86Sr disequilibrium with respect to the host lava. The 87Sr/86Sr ratios in clinopyroxene separates are 95–3146 ppm (0.0095–0.31%) different from their respective host whole rocks. Clinopyroxenes in three lavas have 143Nd/144Nd ratios that are 70–160 ppm (0.007–0.016%) different from the host lavas. The 87Sr/86Sr and 143Nd/144Nd disequilibrium in one lava (the oldest lava considered in this study, Mangaia sample MGA-B-47) can be attributed to posteruptive radiogenic ingrowth, but the isotope disequilibrium in the other, younger lavas cannot be explained by this mechanism. In five of the lava samples, two populations of clinopyroxene were isolated (black and green, separated by color). In four out of five of these samples, the 87Sr/86Sr ratios of the two clinopyroxene populations are isotopically different from each other. In addition to 87Sr/86Sr disequilibrium, the two clinopyroxene populations in one of the lavas (Tahaa sample TAA-B-26) have 143Nd/144Nd ratios that are ∼100 ppm different from each other. Given the resilience of clinopyroxene to seawater alteration and the likelihood that the Sr and Nd isotope composition of fresh clinopyroxene separates provides a faithful record of primary magmatic compositions, the clinopyroxene-clinopyroxene isotope disequilibrium in these four lavas provides strong evidence that a mechanism other than seawater alteration has generated the observed isotopic disequilibrium. This study confirms the isotopic diversity in ocean island lavas previously observed in olivine-hosted melt inclusions. For example, the Sr isotopic variability previously observed in olivine-hosted melt inclusions is mirrored by the isotopic diversity in clinopyroxenes isolated from many of the same Samoan lavas. The isotopic data from melt inclusions and clinopyroxenes are not consistent with shallow assimilation of sediment or with entrainment of xenocrystic clinopyroxene from the oceanic crust or upper mantle. Instead, the data are interpreted as reflecting isotopic heterogeneity in the mantle sources of the lavas. The isotopic diversity in clinopyroxenes and melt inclusions suggests that a single lava can host components derived from isotopically diverse source regions.
  • Article
    Big-picture geochemistry from microanalyses - my four-decade odyssey in sims
    (European Association of Geochemistry, 2019-04) Shimizu, Nobumichi
    Secondary Ion Mass Spectrometry (SIMS) is now a well established analytical technique in geochemistry. Its developmental history goes back to the 1970s. Here, I tell the story of how I got involved in its applications to geochemistry in 1974 at the Institut de Physique du Globe in Paris (IPGP) with the Cameca IMS 300 instrument and my ensuing struggles with theories of secondary ion formation processes and the eventual development of the energy filtering approach as an effective method for suppressing molecular ion interferences in silicate minerals and glasses. The geochemical applications of the techniques that I developed with my colleagues at IPGP, Massachusetts Institute of Technology (MIT), and Woods Hole Oceanographic Institution (WHOI) are summarised in four different categories: (1) trace element zoning of phenocrysts and the kinetics of magmatic crystallisation processes, (2) trace element abundance patterns and geochemical processes in the mantle, (3) use of trace element abundances in magmatic processes in the mid-ocean ridge system, and (4) use of Sr/Ca ratios in biogenic carbonates in palaeoceanographic studies. Trace element zoning patterns observed in phenocrysts reveal that crystal growth in magmas can occur with non-equilibrium partitioning of trace elements at the crystal-melt interface. Trace element zoning patterns in augite phenocrysts from Gough Island also indicate repeated drastic changes in magma composition, suggesting a turbulent dynamic state of magma bodies beneath eruptive centres. Chondrite normalised rare earth element (REE) patterns measured in clinopyroxenes from mantle rocks (peridotites from the Horoman massif and xenoliths in basalts from both oceanic and continental localities) show strong evidence for melt-rock reactions, indicating that lithospheric peridotites depleted in incompatible elements by melt extraction often show evidence of having been subsequently enriched in these elements through melt-rock reaction. The distribution of Sr in garnet inclusions in peridotitic diamonds from South Africa and Siberia is highly heterogeneous over wide concentration ranges, suggesting growth of inclusion garnets as well as formation of these diamonds, occurred shortly before the diamonds were carried to the surface by kimberlite eruptions. Rare earth element and other trace element abundance patterns measured in clinopyroxenes in abyssal peridotites clearly indicate that melting and melt extraction processes beneath mid-ocean ridges is akin to fractional melting by which small degree melt fractions are progressively extracted from residues as the mantle decompresses and only later mixed, sometimes incompletely, to form mid-ocean ridge basalt lavas erupted on the ocean floor. However, meltrock reactions between residual peridotites and upwelling melt fractions could significantly alter trace element abundance patterns of originally residual clinopyroxenes. In situ analyses reveal very large trace element variations occurring on intra-mineral scales, suggesting that precipitation of clinopyroxene also occurred during melt-rock reactions. It is evident that abyssal peridotites contain complex geochemical histories beyond melt extraction via fractional melting. Sr/Ca variations in coral skeletons (aragonite) reveal strong effects of photosynthesis of symbiont algae in day time growth zones, while those in night time growth zones near centres of calcification record variations of sea surface temperatures (SST) in Porites lutea. In Astrangia poculata, which experience a large temperature range (-2 – 23 °C), non-symbiotic skeletons faithfully record temperature variations, while symbiotic skeletons display ontogenic effects of algal symbionts. Sr/Ca variations in coccolithophores across the Paleocene-Eocene Thermal Maximum (PETM) display different responses of individual species to the sudden greenhouse environment, and the severity of the response also is dependent on the oceanographic conditions of their habitats.
  • Article
    Globally elevated titanium, tantalum, and niobium (TITAN) in ocean island basalts with high 3He/4He
    (American Geophysical Union, 2008-04-17) Jackson, Matthew G. ; Hart, Stanley R. ; Saal, Alberto E. ; Shimizu, Nobumichi ; Kurz, Mark D. ; Blusztajn, Jerzy S. ; Skovgaard, Anna C.
    We report evidence for a global Ti, Ta, and Nb (TITAN) enriched reservoir sampled by ocean island basalts (OIBs) with high 3He/4He ratios, an isotopic signature associated with the deep mantle. Excesses of Ti (and to a lesser degree Nb and Ta) correlate remarkably well with 3He/4He in a data set of global OIBs, demonstrating that a major element signature is associated with the high 3He/4He mantle. Additionally, we find that OIBs with high 3He/4He ratios have moderately radiogenic 187Os/188Os (>0.135). The TITAN enrichment and radiogenic 187Os/188Os in high 3He/4He OIBs indicate that they are melts of a mantle domain that hosts a nonprimitive (nonchondritic) component. The observation of TITAN enrichment in the high 3He/4He mantle may be important in balancing the Earth's budget for the TITAN elements. Understanding the origin of the TITAN enrichment is important for constraining the evolution of the enigmatic high 3He/4He mantle domain.
  • Article
    Oxidising agents in sub-arc mantle melts link slab devolatilisation and arc magmas
    (Nature Publishing Group, 2018-08-29) Bénard, Antoine ; Klimm, Kevin ; Woodland, Alan B. ; Arculus, Richard J. ; Wilke, Max ; Botcharnikov, Roman ; Shimizu, Nobumichi ; Nebel, Oliver ; Rivard, Camille ; Ionov, Dmitri A.
    Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42− in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/∑Fe in spinel record a S6+–Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ34S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.
  • Article
    B content and Si/C ratios from cultured diatoms (Thalassiosira pseudonana and Thalassiosira weissflogii) : relationship to seawater pH and diatom carbon acquisition
    (Elsevier, 2013-06-18) Mejia, Luz Maria ; Isensee, Kirsten ; Mendez-Vicente, Ana ; Pisonero, Jorge ; Shimizu, Nobumichi ; Gonzalez, Cristina ; Monteleone, Brian D. ; Stoll, Heather M.
    Despite the importance of diatoms in regulating climate and the existence of large opal-containing sediments in key air-ocean exchange areas, most geochemical proxy records are based on carbonates. Among them, Boron (B) content and isotopic composition have been widely used to reconstruct pH from foraminifera and coral fossils. We assessed the possibility of a pH/CO2 seawater concentration control on B content in diatom opal to determine whether or not frustule B concentrations could be used as a pH proxy or to clarify algae physiological responses to acidifying pH. We cultured two well-studied diatom species, Thalassiosira pseudonana and Thalassiosira weissflogii at varying pH conditions and determined Si and C quotas. Frustule B content was measured by both laser-ablation inductively coupled mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS/ion probe). For both species, frustules grown at higher pH have higher B contents and higher Si requirements per fixed C. If this trend is representative of diatom silicification in a future more acidic ocean, it could contribute to changes in the efficiency of diatom ballasting and C export, as well as changes in the contribution of diatoms relative to other phytoplankton groups in Si-limited regions. If B enters the cell through the same transporter employed for HCO3− uptake, an increased HCO3− requirement with decreasing CO2 concentrations (higher pH), and higher B(OH)4/HCO3− ratios would explain the observed increase in frustule B content with increasing pH. The mechanism of B transport from the site of uptake to the site of silica deposition is unknown, but may occur via silicon transport vesicles, in which B(OH)4− may be imported for B detoxification and/or as part of a pH regulation strategy either though Na-dependent B(OH)4−/Cl− antiport or B(OH)4−/H+ antiport. B deposition in the silica matrix may occur via substitution of a B(OH)4− for a negatively charged SiO− formed during silicification. With the current analytical precision, B content of frustules is unlikely to resolve ocean pH with a precision of paleoceanographic interest. However, if frustule B content was controlled mainly by HCO3− uptake for photosynthesis, which appears to show a threshold behavior, then measurements of B content might reveal the varying importance of active HCO3− acquisition mechanisms of diatoms in the past.
  • Article
    Geochemical stages at Jasper Seamount and the origin of intraplate volcanoes
    (American Geophysical Union, 2009-02-03) Konter, Jasper G. ; Staudigel, Hubert ; Blichert-Toft, Janne ; Hanan, B. B. ; Polve, M. ; Davies, G. R. ; Shimizu, Nobumichi ; Schiffman, P.
    Ocean intraplate volcanoes (OIVs) are formed in a sequence of stages, from large to small, that involve a systematic progression in mantle melting in terms of volumes and melt fractions with concomitant distinct mantle source signatures. The Hawaiian volcanoes are the best-known example of this type of evolution, even though they are extraordinarily large. We explore the Pb-Sr-Nd-Hf isotopic evolution of much smaller OIVs in the Fieberling-Guadalupe Seamount Trail (FGST) and small, near-ridge generated seamounts in the same region. In particular, we investigate whether we can extend the Hawaiian models to Jasper Seamount in the FGST, which displays three distinct volcanic stages. Each stage has characteristic variations in Pb-Sr-Nd-Hf isotopic composition and trace element enrichment that are remarkably similar to the systematics observed in Hawaii: (1) The most voluminous, basal “shield building” stage, the Flank Transitional Series (FTS), displays slightly isotopically enriched compositions compared to the common component C and the least enriched trace elements (143Nd/144Nd: 0.512866–0.512909, 206Pb/204Pb: 18.904–19.054; La/Sm: 3.71–4.82). (2) The younger and substantially less voluminous Flank Alkalic Series (FAS) is comparatively depleted in Sr, Nd, and Hf isotope compositions plotting on the side of C, near the least extreme values for the Austral Islands and St. Helena. Trace elements are highly enriched (143Nd/144Nd: 0.512912–0.512948, 206Pb/204Pb: 19.959–20.185; La/Sm: 9.24). (3) The Summit Alkalic Series (SAS) displays the most depleted Sr, Nd, and Hf isotope ratios and is very close in isotopic composition to the nearby near-ridge seamounts but with highly enriched trace elements (143Nd/144Nd: 0.512999–0.513050, 206Pb/204Pb: 19.080–19.237; La/Sm: 5.73–8.61). These data fit well with proposed multicomponent melting models for Hawaii, where source lithology controls melt productivity. We examine the effect of melting a source with dry peridotite, wet peridotite, and pyroxenite, calculating melt productivity functions with depth to evaluate the effect of potential temperature and lithospheric thickness. This type of melting model appears to explain the isotopic variation in a range of small to large OIVs, in particular for OIVs occurring far from the complicating effects of plate boundaries and continental crust, constraining their geodynamic origin.
  • Article
    Exploring B/Ca as a pH proxy in bivalves : relationships between Mytilus californianus B/Ca and environmental data from the northeast Pacific
    (Copernicus Publications on behalf of the European Geosciences Union, 2011-09-13) McCoy, S. J. ; Robinson, Laura F. ; Pfister, Catherine A. ; Wootton, J. T. ; Shimizu, Nobumichi
    A distinct gap in our ability to understand changes in coastal biology that may be associated with recent ocean acidification is the paucity of directly measured ocean environmental parameters at coastal sites in recent decades. Thus, many researchers have turned to sclerochronological reconstructions of water chemistry to document the historical seawater environment. In this study, we explore the relationships between B/Ca and pH to test the feasibility of B/Ca measured on the ion probe as a pH proxy in the California mussel, Mytilus californianus. Heterogeneity in a range of ion microprobe standards is assessed, leading to reproducible B/Ca ratios at the 5% level. The B/Ca data exhibit large excursions during winter months, which are particularly pronounced during the severe winters of 2004–2005 and 2005–2006. Furthermore, B/Ca ratios are offset in different parts of the skeleton that calcified at the same time. We compare the M. californianus B/Ca record to directly measured environmental data during mussel growth from the period of 1999–2009 to examine whether seawater chemistry or temperature plays a role in controlling shell B/Ca. A suite of growth rate models based on measured temperature are compared to the B/Ca data to optimise the potential fit of B/Ca to pH. Despite sampling conditions that were well-suited to testing a pH control on B/Ca, including a close proximity to an environmental record, a distinct change in pH at the sampling locale, and a growth model designed to optimise the correlations between seawater pH and shell B/Ca, we do not see a strong correlations between pH and shell B/Ca (maximum coefficient of determination, r2, of 0.207). Instead, our data indicate a strong biological control on B/Ca as observed in some other carbonate-forming organisms.
  • Article
    Productivity response of calcareous nannoplankton to Eocene Thermal Maximum 2 (ETM2)
    (Copernicus Publications on behalf of the European Geosciences Union, 2012-05-31) Dedert, M. ; Stoll, Heather M. ; Kroon, Dick ; Shimizu, Nobumichi ; Kanamaru, K. ; Ziveri, Patrizia
    The Early Eocene Thermal Maximum 2 (ETM2) at ~53.7 Ma is one of multiple hyperthermal events that followed the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). The negative carbon excursion and deep ocean carbonate dissolution which occurred during the event imply that a substantial amount (103 Gt) of carbon (C) was added to the ocean-atmosphere system, consequently increasing atmospheric CO2(pCO2). This makes the event relevant to the current scenario of anthropogenic CO2 additions and global change. Resulting changes in ocean stratification and pH, as well as changes in exogenic cycles which supply nutrients to the ocean, may have affected the productivity of marine phytoplankton, especially calcifying phytoplankton. Changes in productivity, in turn, may affect the rate of sequestration of excess CO2 in the deep ocean and sediments. In order to reconstruct the productivity response by calcareous nannoplankton to ETM2 in the South Atlantic (Site 1265) and North Pacific (Site 1209), we employ the coccolith Sr/Ca productivity proxy with analysis of well-preserved picked monogeneric populations by ion probe supplemented by analysis of various size fractions of nannofossil sediments by ICP-AES. The former technique of measuring Sr/Ca in selected nannofossil populations using the ion probe circumvents possible contamination with secondary calcite. Avoiding such contamination is important for an accurate interpretation of the nannoplankton productivity record, since diagenetic processes can bias the productivity signal, as we demonstrate for Sr/Ca measurements in the fine (<20 μm) and other size fractions obtained from bulk sediments from Site 1265. At this site, the paleoproductivity signal as reconstructed from the Sr/Ca appears to be governed by cyclic changes, possibly orbital forcing, resulting in a 20–30% variability in Sr/Ca in dominant genera as obtained by ion probe. The ~13 to 21% increase in Sr/Ca above the cyclic background conditions as measured by ion probe in dominating genera may result from a slightly elevated productivity during ETM2. This high productivity phase is probably the result of enhanced nutrient supply either from land or from upwelling. The ion probe results show that calcareous nannoplankton productivity was not reduced by environmental conditions accompanying ETM2 at Site 1265, but imply an overall sustained productivity and potentially a small productivity increase during the extreme climatic conditions of ETM2 in this portion of the South Atlantic. However, in the open oceanic setting of Site 1209, a significant decrease in dominant genera Sr/Ca is observed, indicating reduced productivity.
  • Article
    An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions
    (American Geophysical Union, 2009-12-17) Warren, Jessica M. ; Shimizu, Nobumichi ; Sakaguchi, C. ; Dick, Henry J. B. ; Nakamura, E.
    Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143Nd/144Nd and 87Sr/86Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.
  • Article
    Volatile (F and Cl) concentrations in Iwate olivine-hosted melt inclusions indicating low-temperature subduction
    (Springer, 2014-08-01) Rose-Koga, Estelle F. ; Koga, Kenneth T. ; Hamada, Morihisa ; Helouis, Thomas ; Whitehouse, Martin J. ; Shimizu, Nobumichi
    Investigation of olivine-hosted melt inclusions provides information about the abundance of volatile elements that are often lost during subaerial eruptions of lavas. We have measured the abundances of H2O, CO2, F, Cl, and S as well as Pb isotopes in 29 melt inclusions in the scoria of the 1686 eruption of the Iwate volcano, a frontal-arc volcano in the northeast Japan arc. Pb Isotope compositions identify that Iwate magma is derived from a mixture of depleted mantle, subducted basalt, and sediment. Systematics of F in comparison to MORB and other arc magma indicates that (1) the slab surface temperature must be among the lowest on Earth and (2) hydrous minerals, such as amphibole, humites, and/or mica, must be present as residual phases during the dehydration of the slab.
  • Article
    Chalcophile behavior of thallium during MORB melting and implications for the sulfur content of the mantle
    (John Wiley & Sons, 2014-12-18) Nielsen, Sune G. ; Shimizu, Nobumichi ; Lee, Cin-Ty A. ; Behn, Mark D.
    We present new laser ablation ICP-MS trace element concentration data for 28 elements in 97 mid-ocean ridge basalt (MORB) glasses that cover all major spreading centers as well as Tl concentration data for all mineral phases in five lherzolites from the Lherz massif, France. The ratio between the elements thallium (Tl) and cerium (Ce) is nearly constant in MORB, providing evidence that the depleted MORB mantle (DMM) has uniform Ce/Tl. Lherzolite mineral data reveal that sulfides are heterogeneous and contain between 23 and 430 ng/g of Tl while all other minerals contain Tl below the analytical detection limit of ∼1 ng/g. We argue that Tl in MORB is controlled by residual sulfide during mantle melting. To investigate the observed relationship between Tl and Ce, we conduct models of fractional mantle melting, which show that the constant Ce/Tl in MORB is only reproduced if the ratio between clinopyroxene and sulfide in the upper mantle varies by less than 10%. In addition, the rate of melting for these two phases must be nearly identical as otherwise melt depletion and refertilization processes would lead to Ce/Tl fractionation. These model results allow us to establish a relationship for the sulfur content of DMM: [S]DMM = SCSS × Mcpx /Rcpx, where SCSS is the sulfur concentration of a silicate melt at sulfide saturation, Rcpx is the melt reaction coefficient, and Mcpx is the modal abundance of clinopyroxene in the DMM. Using this equation, we calculate that the average upper mantle sulfur concentration is 195 ± 45 μg/g.
  • Article
    Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle : a new window provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook Islands
    (John Wiley & Sons, 2014-11-28) Cabral, Rita A. ; Jackson, Matthew G. ; Koga, Kenneth T. ; Rose-Koga, Estelle F. ; Hauri, Erik H. ; Whitehouse, Martin J. ; Price, Allison A. ; Day, James M. D. ; Shimizu, Nobumichi ; Kelley, Katherine A.
    Mangaia hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high µ = 238U/204Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H2O loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to ∼200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02–0.08).