Japan-France cooperative study of mantle plumes by seismic observation and rock sampling on Polynesian seafloor

我们部署了10台宽带海底地震仪，以提高波利尼西亚地区地幔结构的地震分辨率，获得地幔柱的详细结构。我们用日本海洋技术公司的潜水器“SHINKAI-6500”对火山岩进行了取样调查。主要研究结果如下：1.我们确定了法属波利尼西亚下至200公里的上地幔S速度模型。各热点下方均存在低速异常，但不同热点下方低速异常的最大深度不同。在社会和麦当劳热点之下，缓慢的异常至少延伸到200公里。用接收函数法确定了地幔过渡带的厚度。我们只在学会热点下方探测到一个薄的过渡带，在其他台站下方没有发现明显的厚度异常，表明法属波利尼西亚从下地幔到上地幔的地幔柱并不广泛，而是局限于学会热点下方。我们确定P波速度结构下波利尼西亚旅行时层析成像使用P波到达时间和差分P波到达时间之间的站。在下地幔1000 km以上的深度存在一个广泛的慢速度异常。缓慢异常在1000至上地幔底部的深度处是狭窄的。上地幔的结构是目前的数据相对较差的解决。位于学会热点下方的下地幔中的一个狭窄的缓慢异常表明地幔柱正在从下地幔上升到学会热点。我们可以用“SHINKAI-6500”在库克群岛沿着的海底获得良好的岩石样品。海底岩石样品的成分与库克群岛陆地上获得的样品不同，表明岩浆来源随着热点火山的增长而改变。

We deployed 10 broadband ocean bottom seismographs to improve seismic resolution for mantle structure beneath the Polynesian region to obtain detailed structure of mantle plumes. We made a sampling survey of volcanic rocks with the submersible "SHINKAI-6500" of JAMSTEC. The obtained results are as follows.1. We determined the upper mantle S-velocity model beneath French Polynesia down to 200 km. Slow velocity anomalies were found beneath all the hot spots, while their maximum depths are different beneath different hot spots. Beneath the Society and McDonald hot spots, slow anomalies extend down at least to 200 km.2. Thickness of the mantle transition zone was determined by a receiver function method. We detected a thin transition zone only beneath the Society hot spot and did not find significant thickness anomalies beneath other stations, indicating that mantle plumes from the lower to upper mantle in French Polynesia are not broad but is localized beneath the Society hot spot.3. We determined P-velocity structure beneath Polynesia by travel time tomography using P-wave arrival times and differential P-wave arrival times among stations. There is a broad slow velocity anomaly at depths greater than 1000 km in the lower mantle. Slow anomalies are narrow at depths from 1000 to the bottom of the upper mantle. The upper mantle structure is relatively ill-resolved by the present data. A narrow slow anomaly in the lower mantle located beneath the Society hot spot suggests that a mantle plume is ascending from the lower mantle to the Society hot spot.4. We could obtain good rock samples with "SHINKAI-6500" on the seafloor along the Cook-Austral islands. Composition of the seafloor rock samples are distinct from those obtained on the land area of the Cook-Austral islands, suggesting a magma source was changed with a growth of a hot spot volcano.