1 FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 2 Sr isotopic micro analyses of plagioclase in andesites from Zao Volcano, NE Japan Toshiro Takahashi 1 , Masako Yoshikawa 2 , Tomoyuki Shibata 2 , Yoshiyuki Tatsumi 3 and Nobumichi Shimizu 4 1 Research Program for Data and Sample Analyses, Institute for Research on Earth Evolution (IFREE) 2 Beppu Geothermal Research Laboratory (BGRL), Kyoto University 3 Research Program for Geochemical Evolution, Institute for Research on Earth Evolution (IFREE) 4 Department of Geology and Geophysics, Woods Hole Oceanographic Institution 1. Introduction Sr isotope ratio is basic and important geochemical tracer for studying magma generation and differentiation processes. The bulk chemical compositions of volcanic rocks represent the inte- grated effects of magmatic processes, such as partial melting, fractional crystallization, magma mixing and assimilation. Phenocrystic minerals in volcanic rocks crystallizing from the magma, on the other hand, could record such magmatic processes. Thus, chemical compositions and texture of phenocrystic minerals can provide key constraints on decoding magmatic processes. Davidson and Tepley [1997] devised the technique that samples the small portion of plagioclase phenocryst by using the micro- drill, and analyzed Sr isotope ratios using thermal ionization mass spectrometer (TIMS). Davidson and Tepley [1997], Tepley et al. [1999] and Tepley et al. [2000] investigated the variation in Sr isotope ratio inside the plagioclase phenocrysts in volcanic rocks by using such methods, and discussed magma evolution processes in a magma chamber. However, analytical precision of their Sr isotopic micro analysis was inferior to bulk Sr isotopic analysis though plagioclase having high Sr concentration (>1200ppm) is used. Tepley et al. [1999] reported that >5ng Sr was recovered from plagioclase using the their micro drilling technique. The magma genesis of coexisting tholeiitic series and calc- alkaline series has been one of the important issues on the island arc magmatism. The plagioclase is ubiquitously contained in the island arc volcanic rocks, and it is relatively early crystallizing phase in magma evolution process. The Sr isotopic micro analysis for plagioclase can be thought to become contributory application to the above issues. However, basalt to basaltic andesite of tholei- itic series (or low-K series) located on the volcanic front of the NE Japan arc has relatively low bulk Sr concentration (<300ppm) [e.g., Shibata and Nakamura, 1997], and it expects that Sr con- centration of plagioclase phenocryst contained in them is low (<1000ppm). Therefore, the precise isotope determination of trace amount of Sr is necessary to Sr isotopic micro analysis for these plagioclase phenocrysts. Herein, we reported improved analytical techniques for the iso- tope measurement of trace amount of Sr, and report results of Sr isotopic micro analysis of plagioclase phenocrysts in rocks from Zao volcano. 2. Analytical procedures Before the Sr isotopic micro analysis, major elements composi- tions of plagioclase phenocrysts were analyzed to investigate a compositional change of these inside. It were examined by an electron probe microanalyzer, JEOL SUPERPROBE JXA-8800 at Japan Marine Science & Technology Center (JAMSTEC), using analytical procedures described by Shukuno [2003]. The Sr isotopic micro analyses was performed by combined method of micro drilling and TIMS. The micro drilling is a sam- pling technique including a sample mechanically with a small drill and collecting the sample particles milled by it [e.g., Davidson and Tepley, 1997; Tepley et al., 1999]. Drilling for samples was carried out on the Merchantek ® MicroMill system installed by JAMSTEC at Beppu Geothermal Research Laboratory (BGRL), Kyoto University. This system is placed in the clean booth. The diameter at the tip of the drills used for sampling is 0.1 and 0.27mm. A drop of Milli-Q water (Millipore) is placed on the sampling point of the prepared sample, then, drilling is carried out. The drilled and powdered sample particles trapped into Milli- Q water is collected by pipette into cleaned Teflon PFA vial (Savillex ® ). Sample dissolution and chemical separation are done in the clean room (class 100) at BGRL. The sample dissolution, chemical separation and mass spectrometry basically followed Yoshikawa and Nakamura [1993], Yoshikawa et al. [2001] and Shibata et al. [2003]. Sr isotope measurements was carried out on the Triton TI ® (multicollector mass spectrometer) installed by JAMSTEC at BGRL. Sr sample solution was loaded on Re fila- ment with a Ta solution. This procedure basically followed Birck [1986] and Yoshikawa and Nakamura, [1993]. Ta solution is the ionization activator for Sr, and it is used in order to obtain stable and high intensity Sr ion beams. The 87 Sr/ 86 Sr ratios were normal- ized to 86 Sr/ 88 Sr ratio of 0.1194. Sr concentration of plagioclase phenocrysts were measured by a Cameca IMS 3f at Department of Geology and Geophysics, Woods Hole Oceanographic Institution, following procedures described by Shimizu and Hart [1982]. A standard material used for the calibration of trace element compositions was labradorite megacrysts. The concentrations of elements in a standard sample have been analyzed precisely by inductively coupled plasma mass spectrometry. Plagioclase phenocrysts in prepared sample were sputtered with primary O - beam, resulting in 5-8 μm beam diame- ter. The average of analytical error for Sr measurements as meas- ured by 2σ counting statistics was 1.8%. 3. Analytical precision of Sr isotope measurement The stable and high ion beam is necessary for precise analysis of isotopic ratios. However, it is challenging to simultaneously obtain stable and strong enough ion beam during Sr isotopic analysis for low-Sr sample. We observed the relationship between average 88 Sr ion beam intensity during individual measurements and analytical precision for 100ng and 5ng Sr samples (Fig. 1). Sr