Showing papers by "Eizo Nakamura published in 2008"

Coupled Lu–Hf and Sm–Nd geochronology constrains garnet growth in ultra‐high‐pressure eclogites from the Dabie orogen

Abstract: Ultra-high-pressure eclogites from the Dabie orogen that formed over a range in temperatures (∼600 to > 700 °C) have been investigated with combined Lu–Hf and Sm–Nd geochronology. Three eclogites, sampled from Zhujiachong, Huangzhen and Shima, yield Lu–Hf ages of 240.0 ± 5.0, 224.4 ± 1.9 and 230.8 ± 5.0 Ma and corresponding Sm–Nd ages of 222.5 ± 5.0, 217.6 ± 6.1 and 224.2 ± 2.1 Ma respectively. Well-preserved prograde major- and trace-element zoning in garnet in the Zhujiachong eclogite suggests that the Lu–Hf age mostly reflects an early phase of garnet growth that continued over a time interval of c. 17.5 Myr. For the Huangzhen eclogite, despite preserved elemental growth zoning in garnet, textural study reveals that the Lu–Hf age is biased towards a later garnet growth episode rather than representing early growth. The narrow time interval of <6.6 Myr defined by the difference between Lu–Hf and Sm–Nd ages indicates a short final garnet growth episode and suggests a rapid cooling stage. By contrast, the rather flat element zoning in garnet in the Shima eclogite suggests that Lu–Hf and Sm–Nd ages for this sample have been reset by diffusion and are cooling ages. The new Lu–Hf ages point to an initiation of prograde metamorphism prior to c. 240 Ma for the Dabie orogen, while the exact peak metamorphic timing experienced by specific samples ranges between c. 230 to c. 220 Ma.

New sequential separation procedure for Sr, Nd and Pb isotope ratio measurement in geological material using MC-ICP-MS and TIMS

Abstract: A new 3-step sequential separation chemistry for Sr, Nd and Pb from silicate samples, which is suitable for isotope analysis by MC-ICP-MS as well as TIMS, has been developed. The chemistry is designed to minimize the number of evaporation steps, enabling high throughput especially when MC-ICP-MS is employed. The sample solution in 0.5 mol l-1 HNO3 after digestion with HF-HClO4, which is prepared for trace element analysis, can be directly used in the new chemistry. In the first column using Sr resin, Sr and Pb are collected. The recovered solution for Sr can be directly aspirated into MC-ICP-MS. The Pb solution is dried, re-dissolved, separated into two aliquots and measured by double spike MC-ICP-MS. Subsequently, LREEs are collected by the second column packed with cation exchange resin. Finally, Nd was purified from Sm using Ln resin in the third column. The recovered Nd solution can also be directly nebulized into MC-ICP-MS. The Sr and Nd isotope ratios for the standard material JB-3 (basalt) from the Geological Survey of Japan by MC-ICP-MS and TIMS were reported to verify the chemistry and mass spectrometry developed in this study.

Multiple Pulses of the Mantle Plume: Evidence from Tertiary Icelandic Lavas

Abstract: We present major and trace element concentrations and Sr^Nd^ Hf^Pb isotope data for the c. 13^2 Ma Tertiary lavas from eastern Iceland. Our new geochemical results, together with published geological, geochronological, geochemical and geophysical data, are used to evaluate temporal changes in mantle sources contributing to the Tertiary Icelandic magmatism and the relative roles of these sources in magma productivity. The trace element and radiogenic isotopic compositions clearly distinguish three distinct end-member components in the Tertiary magmatism. Temporal variations in lava geochemistry can be attributed to changes in the relative contributions of these three end-member components to the erupted magmas and correlated with temporal variations in magma productivity. The extrusion of lavas with geochemically and isotopically enriched compositions was particularly pronounced at � 13^12 and 8^7 Ma, coincident in time with higher magma productivity. However, the geochemical characteristics of the lavas are different during these two periods: the 13^12 Ma lavas have more radiogenic 176 Hf/ 177 Hf and less radiogenic 206 Pb/ 204 Pb than those erupted from 8 to 7 Ma. The eruption of relatively depleted lavas, at around 10 Ma and younger than 6� 5 Ma, is coincident with lower magma productivity. The correlation between the composition and productivity of the Tertiary lavas from eastern Iceland is probably due to periodic changes in the involvement of the enriched end-member component, followed by a gradation to periods dominated by the signature of the depleted end-member component and lower magma productivity, at an approximate frequency of 5 Myr.

Highly unradiogenic lead isotope ratios from the Horoman peridotite in Japan

Abstract: Peridotites from the Horoman massif have the least radiogenic lead isotope ratios reported from any mantle material, and unlike any inferred from the compositions of mid-ocean ridge basalts. These data hint at the existence of ancient mantle domains that are not sampled by mid-ocean ridge basalts. Basalts at mid-ocean ridges are generated by partial melting of the Earth’s upper mantle. As a result of this process, the upper mantle has become depleted over time in elements that are preferentially removed by melting1,2,3. Although mid-ocean-ridge basalts have traditionally been thought to reflect the chemical composition of such depleted mantle2,3,4,5,6,7, recent work has revealed the existence of domains in the upper mantle that are apparently not sampled by the basalts8. Here we present the lead (Pb), neodymium (Nd) and hafnium (Hf) isotope compositions of peridotites from the Horoman orogenic massif in Japan, which is considered to represent the residues of melting of the upper mantle. These peridotites exhibit the lowest Pb isotope ratios reported from any known mantle material, along with high Nd and Hf isotope ratios. These data suggest that chemical depletion of the peridotites occurred around a billion years ago, and that they represent ancient mantle domains that have escaped convective stirring and homogenization. We suggest that such domains—if abundant in the mantle—may constitute a hitherto unrecognized reservoir with highly unradiogenic lead.

Generation of Rear-arc Magmas Induced by Influx of Slab-derived Supercritical Liquids: Implications from Alkali Basalt Lavas from Rishiri Volcano, Kurile Arc

Abstract: . Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C. & Essling, A. M. (1971). Precision measurement of half-lives and specific activities of U and U. Physical Review C4, 1889^1906. Jicha, B. R., Singer, B. S., Brophy, J. G., Fournelle, J. H., Johnson, C. M., Beard, B. L., Lapen, T. J. & Mahlen, N. J. (2004). Variable impact of the subducted slab on Aleutian island arc magma sources: evidence from Sr, Nd, Pb, and Hf isotopes and trace element abundances. Journal of Petrology 45, 1845^1875. Jochum, K. P. & Verma, S. P. (1996). Extreme enrichment of Sb, Tl and other trace elements in altered MORB. Chemical Geology 130,

Metamorphic history and tectonic evolution of the Himalayan UHP eclogites in Kaghan valley, Pakistan

Abstract: Eclogites of the Kaghan valley, Pakistan Himalaya were investigated petrographically and geochemically. Based on petrography, geochemistry and mineral compositions, metamorphic history and a reasonable tectonic model are proposed. Eclogites exposed in the Kaghan valley are classified into two groups. Group I eclogites appear as massive and Group II are lens-type. Group I eclogites have a mineral assemblage of garnet, omphacitic clinopyroxene, quartz, symplectite with rare epidote and phengite. Accessory minerals include abundant zircon, rutile, ilmenite, and rare apatite. Group II eclogites have a mineral assemblage of garnet, omphacitic clinopyroxene, phengite, quartz/coesite, epidote, and symplectite. In accessory minerals rutile and ilmenite are common while zircon and apatite are rare. Different types of protolith are proposed for these eclogites. Group I eclogites have higher FeO and TiO2 contents and trace element contents, and seem to be derived from gabbroic protolith. Group II eclogites have lower FeO and TiO2 and trace element contents and were derived from basalts. Pressure-temperature-time path was constructed for the Kaghan valley eclogites using various mineral assemblages along with textural relationship and inclusions study. At least three distinct metamorphic stages were identified. The first stage is the prograde garnet growth stage deduced from the inclusion paragenesis in garnet core. The second stage records the ultrahigh-pressure metamorphic stage deduced from the presence of coesite inclusions in omphacitic clinopyroxene. The third stage is the decompression stage and is deduced from the quartz-albite-amphibole symplectite portions. These petrological and geochemical results combined with isotopic ages reported elsewhere indicate that basalts and associated gabbroic dikes were emplaced at about 267 Ma when the Indian plate was moving northward and passing above an unknown hot spot. The closure of the Tethys and initiation of collision of the Indian plate with the Kohistan-Ladakh Island Arc is reported from 65-50 Ma. After that the leading-edge of the Indian plate underwent eclogite facies metamorphism at 49 Ma and when it reached to depths of about 100 km, the ultrahigh-pressure metamorphic event took place at 46 Ma.

New preconcentration technique of Zr, Nb, Mo, Hf, Ta and W employing coprecipitation with Ti compounds: Its application to Lu-Hf system and sequential Pb-Sr-Nd-Sm separation

Abstract: A new preconcentration technique of Zr, Nb, Mo, Hf, Ta and W has been invented employing coprecipiation with Ti compounds. Silicate samples were digested by HF with addition of Ti. Subsequent drying with HClO4 resulting in complete elimination of fluorine produces Ti compounds (oxide/hydroxides). The sample was then dissolved with dilute nitric acid and centrifuged, and the residual Ti compounds were collected. The Ti compounds forming from 2 mg of Ti without matrix elements concentrate 84-98% of Zr, Nb, Mo, Hf, Ta and W. In 20 mg basaltic and 50 mg peridotitic matrices, the addition of 1 mg of Ti was appropriate and gave yields of 46-69 and 54-79%, respectively, for Zr, Nb, Mo, Hf, Ta and W. Exploiting this preconcentration, sequential separation protocols of Hf, Pb, Sr, Lu, Nd and Sm for isotope analysis are developed with total blanks of 16, 11, 60, 2.4, 3 and 0.4 pg, respectively. The method is suitable for Lu-Hf system studies with capability of simultaneous separation of Pb, Sr, Nd and Sm from the same sample digest as well as isotopic anomaly studies of Zr, Mo and W. As application examples, accurate Lu-Hf data and Pb isotope ratios were obtained for basalt (JB-3) and peridotite (JP-1) by MC-ICP-MS.

Crystal-size distribution and composition of garnets in eclogites from the Dabie orogen, central China

Abstract: Crystal-size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation, growth environment, and the metamorphic evolution of their host rocks. Episodic nucleation and growth histories for garnets in eclogites from the Dabie orogen are revealed by variations in crystal size, geochemical zoning, and mineralogy of inclusions in garnets. The studied garnets show pseudo-lognormal crystal-size distributions (CSDs), prograde chemical zoning patterns indexed by mineral inclusions, and are mainly distributed at random. Constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns. The CSD shapes were evaluated in terms of a size-dependent proportionate growth model and a thermally accelerated, diffusion-controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet. Initial increasing nucleation rate followed by a subsequent, medial stage of nearly constant growth and finally declining nucleation rate is inferred from the CSD data. This study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through CSD shapes of minerals in metamorphic rocks.

Gravity-induced diffusion of isotope atoms in monoatomic solid Se

Abstract: A strong gravitational field resulted in the gravity-induced diffusion (sedimentation) of isotope atoms in monoatomic solid Se The layer crystalline morphology consisting of three zones of the fine-grained crystals, the long crystals and feather-shaped crystals grown parallel to gravity direction appeared in the specimen ultracentrifuged at (08–1)×106 G and at 190 °C Change in the concentration ratio 82Se/76Se of >08% was observed in the grown crystalline region These results show an evidence for the sedimentation of substitutional atoms in solids via self-diffusion, and suggest the possibility of application to the control of impurity and crystalline states as well as to isotope separation

A Hercynite-rich Inclusion in the Vigarano CV3 Chondrite

Abstract: An unusual hercynite-rich inclusion was found in the Vigarano CV3 chondrite. Pure and coarse-grained hercynite crystals occupy more than 50 vol.% of the inclusion, and it contains small amount of melilite (1 vol.%) and Mg-bearing spinel. Moreover, there is calcium monoaluminate CaAl2O4, which has been found only in a refractory inclusion of a CH chondrite [1, 2], in the core of this inclusion. However, it consists of grossite, corundum, and perovskite, which are common in normal refractory inclusions. Therefore, it obviously has a close relationship with normal refractory inclusions. The crystallization sequence of the hercynite-rich inclusion is consistent with equilibrium condensation sequence in the solar nebular gas, except for corundum. It is possible that corundum was produced by a reaction between grossite and FeO-bearing fluids during aqueous alteration on the parent body. The oxygen isotopic compositions of the minerals lie along the mass-dependent terrestrial fractionation line rather than the massindependent fractionation line obtained from normal CAIs in carbonaceous chondrites. Hercynite and calcium monoaluminate is the lightest phases in this inclusion (δ17O ~ -12‰, δ18O ~ -20‰), heavier than spinels in normal refractory inclusions. The hercynite-rich inclusion is indicative of unusual nebular conditions where it formed.

Isotope Fluctuation in Indium-Lead Alloy Induced by Solid Centrifugation

Abstract: The atomic-scale graded structure of In-Pb alloy was formed by an ultracentrifuge experiment under conditions that a gravitational field of 0.81 × 106G for 100 hours at 150°C (solid state) in our previous study. The isotope fluctuation on this sample was measured using secondary ion mass spectrometer (SIMS). The 115In/113In isotope ratio changed with positive gradient in the direction of centrifugal force approximately 1.2%. The isotope ratio fluctuation of centrifuged sample was 4 times larger than that of starting sample (<0.3%). This showed that the isotope fractionation effect was induced by solid centrifugation in this alloy, although achieved concentration gradients were small.

Isotope Separation by Condensed Matter Centrifugation: Sedimentation of Isotope Atoms in Se

Abstract: The sedimentation of isotope atoms has been realized in monoatomic liquid and solid Se. It was observed that the concentration ratio 82Se/76Se increased by % level in the specimen ultracentrifuged at 0.7–0.9 million G under liquid and solid states. The present result is evidence of sedimentation of substitutional atoms in condensed matter via self-diffusion, and suggests its possible application to isotope separation by condensed matter centrifugation.