Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.
Complete separation of Sr for the isotopic analysis of Mg-rich samples, such as ultramafic rocks and their constituent minerals, was established by adopting a combination of cationexchange chromatography in H+ form and pyridinium form with a DCTA complex using extremely small volumes of cation-exchange resin (1ml and 0.5ml respectively), This method made it possible to efficiently separate nanogram sizes of Sr from coexisting large amounts of Mg and Ca with a reduced total elution volume of only 24ml. The method also resulted in the reduction of procedural blanks for Sr and Rb to 32 and 25 pg, respectively. Applying static multicollection mass spectrometry with Ta-oxide as an ionization activator on a V-shaped W single filament, the isotopic composition of 10 to 20 ng Sr samples separated from 40 to 55mg of ultramafic rock standard (PCC-1) was determined with analytical precision of<0.005% and reproducibility of<0.006%. These precise analyses were performed with a high 88Sr+ ion beam intensity (>1.5×10-11 A). Our efficient chemical separation procedure also ensured the absence of Ca and Mg interference to Sr ionization. The Ta-oxide readily eliminated Rb prior to the Sr isotopic analysis in the mass spectrometer. This improved the analytical reliability of isotope dilution mass spectrometry (IDMS) for the simultaneous determination of Sr isotopic composition and concentration. The measured isotopic compositions of spiked PCC-1 agreed within error with those obtained from unspiked measurements, Rb/Sr ratio analyses for PCC-1 using IDMS provided analytical reproducibilities of better than 2%. These achievements indicate that our IDMS technique is capable of yielding trace Rb and Sr concentrations simultaneously with Sr isotopic composition in Mg-rich samples with an analytical reliability similar to that obtained from larger samples ( ?? 1 μg) of common silicate rock samples.
The Andaman Ophiolite, India, is located at the southeastern end of the Tethyan ophiolites. We examine petrology and mineralogy of two lherzolites and a completely serpentinized dunite associated with lherzolite from the middle Andaman Island. Major and trace element compositions of minerals in the lherzolites suggest their residual origin after low-degree of partial melting with less flux infiltration, and are similar to those of abyssal peridotites recovered from mid-ocean ridges. The dunite with spinels having low-Cr/(Cr + Al) ratio was formed by interaction between peridotite and mid-ocean ridge basalt-like melt. The 87Sr/86Sr and 143Nd/144Nd isotopic systematics of clinopyroxenes of the two lherzolites are consistent with MORB-type mantle source. Petrology and light rare earth element (LREE)-depleted patterns of clinopyroxene from the studied lhezolites are the same as those from some of the western Tethyan ophiolites. The age-corrected initial εNd values of the Tethyan lherzolite clinopyroxenes with LREE-depleted patterns are likely to be consistent with the depleted mantle evolution line.
The Oto-Zan lava in the Setouchi volcanic belt is composed of phenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids) and forms a composite lava flow. The phenocryst assemblages and element abundances change but Sr–Nd–Pb isotopic compositions are constant throughout the lava flow. The sanukitoid at the base is a high-Mg andesite (HMA) and contains Mg- and Ni-rich olivine and Cr-rich chromite, suggesting the emplacement of a mantle-derived hydrous (� 7w t %H 2O) HMA magma. However, Oto-Zan sanukitoids contain little H2O and are phenocryst-poor. The liquid lines of descent obtained for an Oto-Zan HMA at 0� 3GPa in the presence of 0� 7–2� 1w t %H 2O suggest that mixing of an HMA magma with a differentiated felsic melt can reasonably explain the petrographical and chemical characteristics of Oto-Zan sanukitoids. We propose a model whereby a hydrous HMA magma crystallizes extensively within the crust, resulting in the formation of an HMA pluton and causing liberation of H2O from the magma system. The HMA pluton, in which interstitial rhyolitic melts still remain, is then heated from the base by intrusion of a high-T basalt magma, forming an H2O-deficient HMA magma at the base of the pluton. During ascent, this secondary HMA magma entrains the overlying interstitial rhyolitic melt, resulting in variable self-mixing and formation of a zoned magma reservoir, comprising more felsic magmas upwards. More effective upwelling of more mafic, and hence less viscous, magmas through a propagated vent finally results in the emplacement of the composite lava flow.
