Age and petrographic characterization of volcanic ash from ODP Leg 198 and Leg 126 sites

In this study of volcanic ash retrieved from Shatsky Rise during Ocean Drilling Program Leg 198, the texture and composition of the volcanic components (glass and crystals) were used to fingerprint ash layers for detailed correlation. Correlations among ash layers in holes drilled at the same site as well as between sites, including sites on different parts (highs) of the rise, were tested. Although high-to-high correlations failed, intrahigh correlations were more successful. Our data suggest a significantly different source for some pyroclastic debris, especially at Site 1208, possibly associated with pumice rafts carried northward from the Izu-Bonin arc by the Kuroshio Current. Other ashes are consistent with rhyolitic to dacitic air fall ash from Asian arc volcanoes. We were not able to texturally distinguish between air fall ash and pumice-raft fallout but suspect that the latter is associated with higher percentages of vesiculated ash components, as we demonstrate occur in more proximal Izu-Bonin pyroclastic deposits.本研究针对大洋钻探计划（Ocean Drilling Program）第198航次自Shatsky海隆（Shatsky Rise）获取的火山灰样品展开分析，通过火山组分（玻璃质与晶体）的结构与组成特征，采用指纹识别法厘定火山灰层以开展精细对比。研究对同一钻探位点钻井孔内的火山灰层，以及不同位点（包括该海隆不同构造高地（highs）上的位点）间的火山灰层相关性进行了检验。尽管不同高地间的跨位点对比未取得理想效果，但同一高地内部的对比则效果更佳。本研究数据表明，部分火山碎屑物质的物源存在显著差异，尤以1208站位为甚，其物源可能与黑潮（Kuroshio Current）自伊豆-小笠原弧（Izu-Bonin arc）向北搬运的浮石筏（pumice rafts）相关。其余火山灰则与亚洲弧系火山产出的流纹质至英安质降落火山灰特征一致。我们无法通过结构特征区分降落火山灰与浮石筏沉降的火山灰，但推测后者的具气泡构造的火山碎屑组分占比更高，这一推测与我们在更近源的伊豆-小笠原弧火山碎屑沉积中观测到的现象相符。