Researcn of across-arc geochemical variation in the Izu arc lavas and material transfer between slab and mantle wedge

1.从日本伊豆弧的17个地区（富士-青岛）的火山中采集了168个熔岩，主要是玄武岩和玄武安山岩.这些样品覆盖了俯冲板片的深度在150公里和210公里之间。主要和微量元素的分析新鲜，相对原始的熔岩（56个样品）被选为化学分析。用X射线荧光光谱分析了主要元素组成。采用石墨炉原子吸收光谱法和电感耦合等离子体原子发射光谱法分别测定了样品中Ba、Rb、Sr、Pb、Be、Li和B、Nb、Zr、Y的含量。为了进行比较，还分析了来自千岛群岛、堪察加半岛和马里亚纳群岛弧的熔岩。建立了离子色谱法测定稀土元素的方法.结果与讨论伊豆岛弧熔岩的B/Nb、Pb/Nb和Ba/Nb比值具有明显的跨岛弧变化，且随板片深度的增加而系统地降低。板片源流体对地幔交代作用的程度在海沟侧为B[相近或相等]Ba>Pb[相近或相等]Rb>K>Li[相近或相等]Be，在弧后侧为Ba>Rb>Pb[相近或相等]B[相近或相等]K>Li[相近或相等]Be。在千岛-堪察加岛弧中也观察到类似的趋势，这表明这可能代表了岛弧的一般趋势。这些数据表明，微量元素组成的流体从板释放的变化与增加板深度。流体成分的这种变化意味着至少有两种含水矿物在不同的温度和压力下脱水，脱水发生连续的，而不是episodically.A异常的跨弧变化附近的俯冲-转换边界在堪察加弧最北端，这表明特别大的流体流入该地区。

1. Sample collection168 lavas, predominantly basalts and basaltic andesites, were collected from volcanoes of seventeen regions (Fuji - Aogashima) in the Izu arc, Japan. These samples cover the depth of the subducting slab between 150 km and 210 km.2. Analyzes of major and trace elementsFresh, relatively primitive lavas (56 samples) were selected for chemical analyzes. Major element compositions were analyzed by XRF. Ba, Rb, Sr, Pb, Be and Li concentrations and B, Nb, Zr and Y concentrations were determined by GF-AAS and ICP-AES, respectively. For comparison, lavas from the Kurile, Kamchatka and Mariana arcs were also analyzed. A technique for measuring rare earth elements using ion chromatography was developed.3. Results and discussionB/Nb, Pb/Nb and Ba/Nb ratios of the Izu arc lavas show clear across-arc variations, in which the ratios all systematically decrease with increasing the slab depth. The extent of mantle metasomatism by the slab-derived aqueous fluid is B【similar or equal】Ba>Pb【similar or equal】Rb>K>Li【similar or equal】Be at the trench side and Ba>Rb>Pb【similar or equal】B【similar or equal】K>Li【similar or equal】Be at the back-arc side. Similar trends are observed in the Kurile-Kamchatka arc, suggesting that this may represent the general trend in island arcs. These data indicate that the trace element composition of the fluid liberated from the slab changes with increasing slab depth. This change in fluid composition implies that at least two hydrous minerals dehydrating at different temperatures and pressures are involved, and the dehydration takes place continuously rather than episodically.An anomalous across-arc variation is observed near the subduction-transform boundary in the northernmost Kamchatka arc, suggesting particularly large fluid influx beneath this region.