Shimizu Nobumichi

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Shimizu
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Nobumichi
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  • 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.
  • 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.
  • 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
    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
    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.
  • 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.
  • 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.