Shimizu Nobumichi

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  • Preprint
    Cryptic variations in abyssal peridotite compositions : evidence for shallow-level melt infiltration in the oceanic lithosphere
    ( 2009-12-13) Warren, Jessica M. ; Shimizu, Nobumichi
    Ranges in clinopyroxene trace elements of 2-3 orders of magnitude occur over <2 cm distance in peridotite samples from the Atlantis II Fracture Zone on the Southwest Indian Ridge. This represents the smallest length-scale at which clinopyroxene trace element concentrations have been observed to vary in abyssal peridotites. Due to the absence of any accompanying veins or other macroscopic features of melt-rock interaction, these peridotites are interpreted as being the result of cryptic metasomatism by a low volume melt. The small length-scale of the variations, including porphyroclastic clinopyroxene grains of 2 mm diameter with an order of magnitude variation in light rare earth elements, precludes an ancient origin for these anomalies. Calculation of diffusive homogenization timescales suggests that for the trace element variations to be preserved, metasomatism occurred in the oceanic lithospheric mantle at 1000-1200°C and 10-20 km depth. This observation provides constraints for the on-axis thickness of the lithospheric mantle at an ultra-slow spreading ridge. Trace amounts of plagioclase are present in at least two of the metasomatized samples. Textural and trace element observations indicate that it formed following the trace element metasomatism, indicating that the mantle can be infiltrated multiple times by melt during the final stages of uplift at the ridge axis. The peridotites in this study are from two oceanic core complexes on the Atlantis II Fracture Zone. Our observations of multiple late-stage metasomatic events in the lithospheric mantle agree with current models and observations of melt intrusion into the mantle during oceanic core complex formation. These observations also indicate that heterogeneous lithospheric mantle can be created at ultra-slow spreading ridges.
  • Preprint
    CO2-rich komatiitic melt inclusions in Cr-spinels within beach sand from Gorgona Island, Colombia
    ( 2009-07) Shimizu, Kenji ; Shimizu, Nobumichi ; Komiya, Tsuyoshi ; Suzuki, Katsuhiko ; Maruyama, Shigenori ; Tatsumi, Yoshiyuki
    The volatile content of komatiite is a key to constrain the thermal and chemical evolution of the deep Earth. We report the volatile contents with major and trace element compositions of ~ 80 melt inclusions in chromian spinels (Cr-spinels) from beach sands on Gorgona Island, Colombia. Gorgona Island is a ~ 90 Ma volcanic island, where picrites and the youngest komatiites known on the Earth are present. Melt inclusions are classified into three types on the basis of their host Cr-spinel compositions: low Ti (P type), high Ti with high Cr# (K1 type) and high Ti with low Cr# (K2 type). Chemical variations of melt inclusions in the Cr-spinels cover all of the island's lava types. P-type inclusions mainly occur in the picrites, K1-type in high-TiO2 komatiites (some enriched basalts: E-basalts) and K2-type in low-TiO2 komatiites. The H2O and CO2 contents of melt inclusions within Cr-spinels from the beach sand are highly variable (H2O: 0.03–0.9 wt.%; CO2: 40–4000 ppm). Evaluation of volatile content is not entirely successful because of compositional alterations of the original melt by degassing, seawater/brine assimilation and post-entrapment modification of certain elements and volatiles. However, the occurrence of many melt inclusions with low H2O/K2O ratios indicates that H2O/K2O of Gorgona komatiite is not much different from that of modern mid-oceanic ridge basalt (MORB) or oceanic island basalt. Trend of CO2/Nb and Zr/Y ratios, accounted for by two-component mixing between the least degassed primary komatiite and low-CO2/Nb evolved basalt, allow us to estimate a primary CO2/Nb ratio of 4000 ± 2200 or a CO2 content of 0.16 ± 0.09 wt.%. The determined CO2/Nb ratio is unusually high, compared to that of MORB (530). Although the presence of CO2 in the Gorgona komatiite does not affect the magma generation temperature, CO2 degassing may have contributed to the eruption of high-density magmas. High CO2/Nb and the relatively anhydrous nature of Gorgona komatiite provide possible resolution to one aspect of the hydrous komatiite debate.
  • Preprint
    C–O–H–S fluids and granitic magma : how S partitions and modifies CO2 concentrations of fluid-saturated felsic melt at 200 MPa
    ( 2011-03-10) Webster, James D. ; Goldoff, B. ; Shimizu, Nobumichi
    Hydrothermal volatile-solubility and partitioning experiments were conducted with fluid-saturated haplogranitic melt, H2O, CO2, and S in an internally heated pressure vessel at 900°C and 200 MPa; three additional experiments were conducted with iron-bearing melt. The run-product glasses were analyzed by electron microprobe, FTIR, and SIMS; and they contain ≤ 0.12 wt% S, ≤ 0.097 wt.% CO2, and ≤ 6.4 wt.% H2O. Apparent values of log ƒO2 for the experiments at run conditions were computed from the [(S6+)/(S6++S2-)] ratio of the glasses, and they range from NNO-0.4 to NNO+1.4. The C-O-H-S fluid compositions at run conditions were computed by mass balance, and they contained 22-99 mol% H2O, 0-78 mol% CO2, 0-12 mol% S, and < 3 wt% alkalis. Eight S-free experiments were conducted to determine the H2O and CO2 concentrations of melt and fluid compositions and to compare them with prior experimental results for C-O-H fluid-saturated rhyolite melt, and the agreement is excellent. Sulfur partitions very strongly in favor of fluid in all experiments, and the presence of S modifies the fluid compositions, and hence, the CO2 solubilities in coexisting felsic melt. The square of the mole fraction of H2O in melt increases in a linear fashion, from 0.05-0.25, with the H2O concentration of the fluid. The mole fraction of CO2 in melt increases linearly, from 0.0003-0.0045, with the CO2 concentration of C-O-H-S fluids. Interestingly, the CO2 concentration in melts, involving relatively reduced runs (log ƒO2 ≤ NNO+0.3) that contain 2.5-7 mol% S in the fluid, decreases significantly with increasing S in the system. This response to the changing fluid composition causes the H2O and CO2 solubility curve for C-O-H-S fluid-saturated haplogranitic melts at 200 MPa to shift to values near that modeled for C-O-H fluid-saturated, S-free rhyolite melt at 150 MPa. The concentration of S in haplogranitic melt increases in a linear fashion with increasing S in C-O-H-S fluids, but these data show significant dispersion that likely reflects the strong influence of ƒO2 on S speciation in melt and fluid. Importantly, the partitioning of S between fluid and melt does not vary with the (H2O/H2O+CO2) ratio of the fluid. The fluid-melt partition coefficients for H2O, CO2, and S and the atomic (C/S) ratios of the run-product fluids are virtually identical to thermodynamic constraints on volatile partitioning and the H, S, and C contents of pre-eruptive magmatic fluids and volcanic gases for subduction-related magmatic systems thus confirming our experiments are relevant to natural eruptive systems.
  • Preprint
    Protracted timescales of lower crustal growth at the fast-spreading East Pacific Rise
    ( 2011-12) Rioux, Matthew ; Lissenberg, C. Johan ; McLean, Noah M. ; Bowring, Samuel A. ; MacLeod, Christopher J. ; Hellebrand, Eric ; Shimizu, Nobumichi
    Formation of the oceanic crust at mid-ocean ridges is a fundamental component of plate tectonics. A majority of the crust at many ridges is composed of plutonic rocks that form by crystallization of mantle-derived magmas within the crust. Recent application of U/Pb dating to samples from in-situ oceanic crust has begun to provide exciting new insight into the timing, duration and distribution of magmatism during formation of the plutonic crust1-4. Previous studies have focused on samples from slow-spreading ridges, however, the time scales and processes of crustal growth are expected to vary with plate spreading rate. Here we present the first high-precision dates from plutonic crust formed at the fast-spreading East Pacific Rise (EPR). Individual zircon minerals yielded dates from 1.420–1.271 million years ago, with uncertainties of ± 0.006–0.081 million years. Within individual samples, zircons record a range of dates of up to ~0.124 million years, consistent with protracted crystallization or assimilation of older zircons from adjacent rocks. The variability in dates is comparable to data from the Vema lithospheric section on the Mid-Atlantic Ridge (MAR)3, suggesting that time scales of magmatic processes in the lower crust may be similar at slow- and fast-spreading ridges.
  • Preprint
    An experimental study of the grain-scale processes of peridotite melting : implications for major and trace element distribution during equilibrium and disequilibrium melting
    ( 2007-12) Lo Cascio, Mauro ; Liang, Yan ; Shimizu, Nobumichi ; Hess, Paul C.
    The grain-scale processes of peridotite melting were examined at 1340°C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P– T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal melt interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxenemelt partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements in residual minerals reequilibrate with their surrounding melt.
  • Preprint
    Sulfur isotope fractionation between fluid and andesitic melt : an experimental study
    ( 2014-07) Fiege, Adrian ; Holtz, Francois ; Shimizu, Nobumichi ; Mandeville, Charles W. ; Behrens, Harald ; Knipping, Jaayke L.
    Glasses produced from decompression experiments conducted by Fiege et al. (2014a) were used to investigate the fractionation of sulfur isotopes between fluid and andesitic melt upon magma degassing. Starting materials were synthetic glasses with a composition close to a Krakatau dacitic andesite. The glasses contained 4.55 to 7.95 wt% H2O, ~140 to 2700 ppm sulfur (S), and 0 to 1000 ppm chlorine (Cl). The experiments were carried out in internally heated pressure vessels (IHPV) at 1030°C and oxygen fugacities (fO2) ranging from QFM+0.8 log units up to QFM+4.2 log units (QFM: quartz-fayalite-magnetite buffer). The decompression experiments were conducted by releasing pressure (P) continuously from ~400 MPa to final P of 150, 100, 70 and 30 MPa. The decompression rate (r) ranged from 0.01 to 0.17 MPa/s. The samples were annealed for 0 to 72 h (annealing time, tA) at the final P and quenched rapidly from 1030°C to room temperature (T). The decompression led to the formation of a S-bearing aqueous fluid phase due to the relatively large fluid-melt partitioning coefficients of S. Secondary ion mass spectrometry (SIMS) was used to determine the isotopic composition of the glasses before and after decompression. Mass balance calculations were applied to estimate the gas-melt S isotope fractionation factor αg-m. No detectable effect of r and tA on αg-m was observed. However, SIMS data revealed a remarkable increase of αg-m from ~0.9985 ± 0.0007 at >QFM+3 to ~1.0042 ± 0.0042 at ~QFM+1. Noteworthy, the isotopic fractionation at reducing conditions was about an order of magnitude larger than predicted by previous works. Based on our experimental results and on previous findings for S speciation in fluid and silicate melt a new model predicting the effect of fO2 on αg-m (or Δ34S g-m) in andesitic systems at 1030°C is proposed. Our experimental results as well as our modeling are of high importance for the interpretation of S isotope signatures in natural samples (e.g., melt inclusions or volcanic gases).
  • Preprint
    Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas
    ( 2011-02) Helo, Christoph ; Longpre, Marc-Antoine ; Shimizu, Nobumichi ; Clague, David A. ; Stix, John
    The abundance of volatile compounds, and particularly 18 CO2, in the upper oceanic mantle affects the style of volcanic eruptions. At mid-ocean ridges, eruptions are generally dominated by the gentle effusion of basaltic lavas with a low volatile content. But, explosive volcanism has been documented at some ocean spreading centres1-3, indicative of abundant volatile compounds. Estimates of the initial CO2 concentration of primary magmas can be used to constrain the CO2 content of the upper oceanic mantle, but these estimates vary greatly4,5. Here we present ion microprobe measurements of the CO2 content of basaltic melt trapped in plagioclase crystals. The crystals are derived from volcanic ash deposits erupted explosively at Axial Seamount, Juan de Fuca Ridge, in the northeast Pacific Ocean. We report unusually high CO2 concentrations of up to 9,160 ppm, which indicate that the upper oceanic mantle is more enriched in carbon than previously thought. And we furthermore suggest that CO2 fluxes along mid-ocean ridges4,5 vary significantly. Our results demonstrate that elevated fluxes of CO2 from the upper oceanic mantle can drive explosive eruptions at mid-ocean ridges.
  • Preprint
    The boron and lithium isotopic composition of mid-ocean ridge basalts and the mantle
    ( 2017-03-17) Marschall, Horst R. ; Wanless, V. Dorsey ; Shimizu, Nobumichi ; Pogge von Strandmann, Philip ; Elliott, Tim ; Monteleone, Brian D.
    A global selection of 56 mid-ocean ridge basalt (MORB) glasses were analysed for Li and B abundances and isotopic compositions. Analytical accuracy and precision of analyses constitute an improvement over previously published MORB data and allow a more detailed discussion of the Li and B systematics of the crust-mantle system. Refined estimates for primitive mantle abundances ([Li]=1.39±0.10[Li]=1.39±0.10 μg/g and [B]=0.19±0.02[B]=0.19±0.02 μg/g) and depleted mantle abundances ([Li]=1.20±0.10[Li]=1.20±0.10 μg/g and [B]=0.077±0.010[B]=0.077±0.010 μg/g) are presented based on mass balance and on partial melting models that utilise observed element ratios in MORB. Assimilation of seawater (or brine) or seawater-altered material beneath the ridge, identified by high Cl/KCl/K, causes significant elevation of MORB δ11Bδ11B and variable elevation in δ7Liδ7Li. The B isotope ratio is, hence, identified as a reliable indicator of assimilation in MORB and values higher than −6‰ are strongly indicative of shallow contamination of the magma. The global set of samples investigated here were produced at various degrees of partial melting and include depleted and enriched MORB from slow and fast-spreading ridge segments with a range of radiogenic isotope signatures and trace element compositions. Uncontaminated (low-Cl/KCl/K) MORB show no significant boron isotope variation at the current level of analytical precision, and hence a homogenous B isotopic composition of δ11B=-7.1±0.9‰δ11B=-7.1±0.9‰ (mean of six ridge segments; 2SD). Boron isotope fractionation during mantle melting and basalt fractionation likely is small, and this δ11Bδ11B value reflects the B isotopic composition of the depleted mantle and the bulk silicate Earth, probably within ±0.4‰. Our sample set shows a mean δ7Li=+3.5±1.0‰δ7Li=+3.5±1.0‰ (mean of five ridge segments; 2SD), excluding high-Cl/KCl/K samples. A significant variation of 1.0–1.5‰ exists among various ridge segments and among samples within individual ridge segments, but this variation is unrelated to differentiation, assimilation or mantle source indicators, such as radiogenic isotopes or trace elements. It, therefore, seems likely that kinetic fractionation of Li isotopes during magma extraction, transport and storage may generate δ7Liδ7Li excursions in MORB. No mantle heterogeneities, such as those generated by deeply recycled subducted materials, are invoked in the interpretation of the Li and B isotope data presented here, in contrast to previous work on smaller data sets. Lithium and boron budgets for the silicate Earth are presented that are based on isotope and element mass balance. A refined estimate for the B isotopic composition of the bulk continental crust is given as δ11B=-9.1±2.4‰δ11B=-9.1±2.4‰. Mass balance allows the existence of recycled B reservoirs in the deep mantle, but these are not required. However, mass balance among the crust, sediments and seawater shows enrichment of 6Li6Li in the surface reservoirs, which requires the existence of 7Li7Li-enriched material in the mantle. This may have formed by the subduction of altered oceanic crust since the Archaean.
  • Preprint
    Effect of fluorine on near-liquidus phase equilibria of an Fe–Mg rich basalt
    ( 2012-03-26) Filiberto, Justin ; Wood, Justin ; Dasgupta, Rajdeep ; Shimizu, Nobumichi ; Le, Loan ; Treiman, Allan H.
    Volatile species (H2O, CO2, F, Cl, etc) have important effects on the formation and crystallization history of basaltic magmas. Here, we have experimentally investigated the effects of F on phase equilibria of Fe-Mg-rich basalt. Our results show that fluorine has large effects on the liquidus temperature and the chemistry of crystallizing minerals. Compared to the F-free system, addition of ~2 wt.% F moves the olivine-pigeonite liquidus point down ~2 kbar and 95 °C (from 12 kbar, 1375 °C to 10 kbar, 1280 °C). With increasing fluorine concentrations, dramatically increases for both pyroxene and olivine, suggesting that fluorine in basaltic magmas complexes primarily with MgO. Complexing with MgO in the melt decreases its MgO activity, and forces the crystallizing minerals to greater Fe/Mg, and so increases . Models of basalt generation, where the magma is fluorine-rich, need to include the effect of not only water but fluorine on liquidus depression and minerals crystallizing/melting. Our results suggest that fluorine may significantly aid in the petrogenesis of silica-poor, alkali-rich magmas in the Earth and Mars.