Showing papers by "Eizo Nakamura published in 1997"

Across-arc variations of isotope and trace element compositions from Quaternary basaltic volcanic rocks in northeastern Japan: Implications for interaction between subducted oceanic slab and mantle wedge

Abstract: Isotopic compositions of Pb, Sr, and Nd and concentrations of trace elements were determined for Quaternary island arc basaltic rocks from northeastern Japan. Sr and Pb isotopic ratios decrease, and Nd isotopic ratios increase from the volcanic front toward the back arc. The isotopic compositions nearest the back arc side are nearly identical to those of mid-ocean ridge basalt (MORB). The high field strength elements and heavy rare earth elements show homogeneous and MORB-like characteristics. These observations indicate that the mantle wedge beneath northeastern Japan originally had a MORB-type mantle composition that was homogeneous across the arc. Pb isotope compositions show a mixing relationship between mantle wedge and oceanic sediments reflecting the introduction of subduction component into the mantle wedge, Across-arc isotopic variations were caused by interaction between MORB-type mantle wedge and the subducting slab, and the amount of subduction component correlates with the depth to the slab. The isotopic compositions of subduction component are expressed by bulk mixing of 15 wt % of oceanic sediment and 85 wt % of altered MORB. Inversion analyses of isotopic compositions using two-component mixing relationships show that the Sr/Nd and Pb/Nd ratios in subduction component decrease with increasing depth to the slab, while the Sr/Pb ratio is nearly constant. These changes can be explained only by a preferential discharge of the elements into the wedge mantle associated with continuous dehydration of the subducting slab. The present study further demonstrates that a very wide range of isotopic and elemental compositions in island arc magmas is a consequence of the interaction between subducting slab and mantle wedge without the involvement of an oceanic island basalts component, and the slab can carry water and supply a subduction component as a fluid to the overlying mantle wedge to depths exceeding 150 km.

Suppression of Matrix Effects in ICP‐MS by High Power Operation of ICP: Application to Precise Determination of Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U at ng g‐1 Levels in Milligram Silicate Samples

Abstract: We found that the suppression of signals for 88Sr, 140Ce and 238U in rock solution caused by rock matrix in ICP-MS (matrix effects) was reduced at high power operation (1.7 kW) of the ICP. To make the signal suppression by the matrix negligible, minimum dilution factors (DF) of the rock solution for Sr, Ce and U were 600, 400 and 113 at 1.1, 1.4 and 1.7 kW, respectively. Based on these findings, a rapid and precise determination method for Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U using FI (flow injection)-ICP-MS was developed. The amount of the sample solution required for FI-ICP-MS was 0.2 ml, so that 1.8 mg sample was sufficient for analysis with a detection limit of several ng g-1. Using this method, we determined the trace element concentrations in the USGS rock reference materials, DTS-1, PCC-1, BCR-1 and AGV-1, and the GSJ rock reference materials, JP-1, JB-1, -2, -3, JA-1, -2 and -3. The reproducibilities (RSD %) in replicate analyses (n=5) of BCR-1, AGV-1, JB-1, -2, -3, JA-1, -2, and -3 were < 6 %, and typically 2.5%. The difference between the average concentrations of this study for BCR-1 and those of the reference values were < 2%. Therefore, it was concluded that the method can give reliable data for trace elements in silicate rocks.

Determination of Boron in Silicate Samples by Direct Aspiration of Sample HF Solutions into ICPMS

Abstract: A rapid and precise technique for the determination of boron content in silicate rocks was developed by employing isotope dilution inductively coupled plasma mass spectrometry with a flow injection system (FI-ID-ICPMS). The sample was decomposed with HF and mannitol, dried to remove silicon as gas, and redissolved with HF to extract boron from the fluoride residue. Ultimately, the HF solution was directly aspirated into the ICPMS. The total procedural blank was 0.08-0.36 ng. No matrix-induced mass discrimination was observed in the rock solution for dilution factors of up to 110. The present method is capable of determining 1 μg g -l boron in a 1 mg silicate rock sample with blank correction of <20%. Replicate analyses of USGS rock standards, PCC-1, BCR-1, and AGV-1, and GSJ rock standards, JB-1, JB-2, JB-3, JA-1, JA-2, and JA-3, yielded reproducibilities (RSD) of <2%. These results were identical with those obtained by ID thermal ionization mass spectrometry within the analytical errors. These results indicate that the present method allows the precise determination of the boron content in a small sample of silicate materials with simple handling, resulting in the extremely low procedural blank of boron.

Diffusivities of Rare Earth Elements and Ba in Magmatic Silicate Melts at High Pressures

Abstract: Diffusivities of 11 rare earth elements (REE) and Ba in jadeite (NaAlSi2O6) and diopside (CaMgSi2O6) melts have been measured at pressures between 10 and 20kbar. In jadeite melt, diffusivities of REE increase regularly with increasing their ionic radii, light REE (e.g., La and Ce) diffuse faster than heavy REE (e.g., Yb and Lu) by about 30%, Eu diffuses faster by factor of 8 to 12 than other REE under reducing conditions, and diffusivities of REE increase with increasing pressure at constant temperature. In diopside melt, diffusivities of REE are nearly the same as that of Ca and decrease with increasing pressure. The present study demonstrates that diffusion process in magma should change the relative abundances of REE and also generate Eu anomalies without involvement of plagioclase feldspar. This diffusion-induced process is a newly recognized mechanism of changing the relative REE abundances in magmas and generating Eu anomalies, and it must be taken into account in interpreting the REE abundance patterns of igneous rocks.

Sr, Nd and Pb isotope systematics of Akagi volcano, Northeast Japan: Implications for interaction between island arc magma and lower crust

Abstract: Isotopic compositions of Sr, Nd and Pb and REE concentrations have been determined for samples from“isotopically anomalous”Akagi volcano locating on the volcanic front of Northeast Japan Arc. The Sr isotopic compositions show a large variation with highly enriched isotope character (87Sr/86Sr=0.7060 to 0.7088) compared to those from other volcanoes on the front. The Nd isotopic compositions, ranging from eNd=-0.40 to -8.6, have a negative correlation with the Sr isotopic compositions. The Pb isotopic data along with Sr and Nd isotope systematics clearly indicate that the isotopic variations of Akagi volcano were caused by a two-component mixing between an end member isotopically similar to the primary magma on the volcanic front of Northeast Japan Arc and a lower crustal component. Chondrite-normalized REE patterns of Akagi samples show U-shaped, HREE-depleted pattern with positive Eu anomaly. Such REE features may have been developed by fractionation of amphibole formed by the reaction between a fluid-rich magma and clinopyroxene in granulitic lower crust. The fluid-rich magma could be originated from a highly metasomatized mantle wedge caused by the dehydration of oceanic slabs of the Pacific plate and the overlapping Philippine Sea Plate. Such a unique tectonic setting could result in higher water supply to the source region of magma than normal circumstances and thereby generating fluid-rich magma which could enhance assimilation of lower crust beneath Akagi volcano.

Sr, Nd and Pb Isotope Systematics of Akagi Volcano, Northeast Japan. Implications for Interaction between Island Arc Magma and Lower Crust.:Implications for Interaction between Island Arc Magma and Lower Crust

Abstract: Isotopic compositions of Sr, Nd and Pb and REE concentrations have been determined for samples from“isotopically anomalous”Akagi volcano locating on the volcanic front of Northeast Japan Arc. The Sr isotopic compositions show a large variation with highly enriched isotope character (87Sr/86Sr=0.7060 to 0.7088) compared to those from other volcanoes on the front. The Nd isotopic compositions, ranging from eNd=-0.40 to -8.6, have a negative correlation with the Sr isotopic compositions. The Pb isotopic data along with Sr and Nd isotope systematics clearly indicate that the isotopic variations of Akagi volcano were caused by a two-component mixing between an end member isotopically similar to the primary magma on the volcanic front of Northeast Japan Arc and a lower crustal component. Chondrite-normalized REE patterns of Akagi samples show U-shaped, HREE-depleted pattern with positive Eu anomaly. Such REE features may have been developed by fractionation of amphibole formed by the reaction between a fluid-rich magma and clinopyroxene in granulitic lower crust. The fluid-rich magma could be originated from a highly metasomatized mantle wedge caused by the dehydration of oceanic slabs of the Pacific plate and the overlapping Philippine Sea Plate. Such a unique tectonic setting could result in higher water supply to the source region of magma than normal circumstances and thereby generating fluid-rich magma which could enhance assimilation of lower crust beneath Akagi volcano.