A fully automated open-column resin-bed chemical-separation system, named COLUMNSPIDER, has been developed. The system consists of a programmable micropipetting robot that dispenses chemical reagents and sample solutions into an open-column resin bed for elemental separation. After the initial set up of resin columns, chemical reagents, and beakers for the separated chemical components, all separation procedures are automated. As many as ten samples can be eluted in parallel in a single automated run. Many separation procedures, such as radiogenic isotope ratio analyses for Sr and Nd, involve the use of multiple column separations with different resin columns, chemical reagents, and beakers of various volumes. COLUMNSPIDER completes these separations using multiple runs. Programmable functions, including the positioning of the micropipetter, reagent volume, and elution time, enable flexible operation. Optimized movements for solution take-up and high-efficiency column flushing allow the system to perform as precisely as when carried out manually by a skilled operator. Procedural blanks, examined for COLUMNSPIDER separations of Sr, Nd, and Pb, are low and negligible. The measured Sr, Nd, and Pb isotope ratios for JB-2 and Nd isotope ratios for JB-3 and BCR-2 rock standards all fall within the ranges reported previously in high-accuracy analyses. COLUMNSPIDER is a versatile tool for the efficient elemental separation of igneous rock samples, a process that is both labor intensive and time consuming.
Late Pliocene-Early Pleistocene igneous rocks of Yoneyama Formation from the northern Fossa Magna region, central Japan, consist of basaltic to andesitic rocks and small intrusive rocks; they contain frequently hornblende (Hbl) gabbroic xenoliths and Hbl xenocrysts. Based on field data, together with petrographic, geochemical, and geochlonological descriptions, the volcanism comprised 5 stages. The rocks at the Ogamidake, the 1st and 3rd stages are tholeiitic rock series (TH), whereas calc-alkalic rock series (CA) are dominated at the 2nd and 4th stages. All rocks are characterized by high-K content and contain pargasitic Hbl phenocrysts in both rock series. Estimation using Ca-amphibole geobarometer suggests that Hbls have crystallized at depths of lower crust. Existence of Hbl and high An content of plagioclase (~ An90) in both rock series imply that both magmas are rich in H2O. Estimated H2O contents are ~ 5 wt% for both TH and CA magmas. Based on mineral texture, P-T estimation and major-trace elements modeling, we infer that cryptic fractionation of Hbl can produce the TH magma trend. Our model is incompatible with general model that TH magma originate from anhydrous or low H2O content magma.
The subduction factory processes raw materials such as oceanic sediments and basaltic crust, selectively extracts particular subduction components and manufactures magmas, their solidified materials and continental crust as products. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic arc lower crust are transported down to the deep mantle modified their compositions and ultimately recycled as mantle plumes. Andesite composes the bulk continental crust and therefore is the major product in the subduction factory. Two types of andesites, calk-alkalic and tholeiitic series, are commonly recognized in a single arc volcano. We propose a new mechanism for production of these two magma series on the basis of data obtained by Sr isotopic micro-analyses of plagioclase in volcanic rocks from Zao Volcano, NE Japan. Tholeiitic magmas having constant and enriched isotopic signatures are produced by anatexis of the preexisting mafic lower crust, whereas calc-alkalic magmas, having compositions similar to the bulk continental crust, are products of mixing a mantle-derived, hence isotopically depleted, basaltic magma and crust-derived felsic tholeiites.
Although slab-derived fluid significantly affects melt generation and dynamics within subduction zones, its amount and distribution are not sufficiently constrained at present. Herein, we use isotopic systematics of arc volcanic rocks, subducting materials, and intrinsic mantle components prior to metasomatism, to quantify the contribution of the slab-derived fluid that metasomatizes the overlying mantle wedge beneath the entire area of Japan arcs. Simultaneous application of several multivariate statistical analyses (clustering analysis and principal/independent component analyses) to the isotopic data set allows Japan arcs to be broadly divided into eastern and western parts at the first order. Moreover, a clear higher-order inter-arc segmentation is observed, together with some intra-arc variations that possibly correspond to the heterogeneity of incoming plates. Inter-arc segmentation is shown to be primarily controlled by the geometrical parameters of the slab and the arc (e.g., subduction of a single plate or double plates beneath either oceanic or continental crust), which results in differences between mantle wedge and slab thermal conditions. Accordingly, the Kuril and Izu arcs, which have thin arc crusts (~20 km), exhibit the lowest extent of slab-derived fluid addition (0.1 wt%) to the mantle wedge, while the NE Japan arc, with a thicker arc crust (up to 36 km), features a higher value of 0.2 wt%, although the slab thermal parameters for these three arcs are essentially the same. The Central Japan arc shows the highest extent of slab-derived fluid addition (>1.0 wt%) because of the overlapping subduction of Pacific and Philippine Sea slabs, while the SW Japan and Ryukyu arcs feature moderate values of ~0.5 wt%. Moreover, a clear exotic plume zone and spots are observed in SW Japan and the Japan Sea. In addition to the variability of slab-derived fluid composition, the intrinsic mantle composition (before slab-derived fluid–induced metasomatism) shows a clear along-arc variation that is possibly caused by a large-scale mantle flow from the continental side. Thus, slab-derived fluid addition and mantle composition variability equally contribute to inter-arc segmentation, which highlights the importance of both local and regional thermal flow structures of slab-mantle systems.
We report a procedure that separates Nd from silicate rock samples using cation exchange chromatography allowing 143Nd/144Nd ratio analyses to be performed by thermal ionization mass spectrometry (TIMS). The procedure obtains the Nd fraction using a two-step column separation. First, a rare earth element (REE) fraction is separated via a cation exchange column with HCl as the eluent. Then, Nd is separated from this REE fraction using a second cation exchange column with α-hydroxy isobutric acid (HIBA) as the eluent. The procedural Nd blanks measured for each step are very low and negligible for 143Nd/144Nd ratios in natural rock samples. The Nd yields are high, with more than 90 % of Nd recovered during each step. Repeated analysis of 143Nd/144Nd ratios in the JNdi-1 standard solution gave a mean value of 0.512096 ± 0.000014 (2 SD, n = 144, RSD % = 0.0014), which is in good agreement with those reported in previous studies. The 143Nd/144Nd ratio obtained for the corrected JNdi-1 fractions using the technique in our laboratory was 0.512094 ± 0.000011 (2 SD, n = 4). We also measured 143Nd/144Nd ratios in several geochemical igneous rock reference materials (JB-la, JB-2, JA-1, and JR-1) producing mean values consistent with available data. These results indicate that the technique for Nd isotope analysis employed at our laboratory provides reliable and accurate data for geochemical studies.
