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

为了提高波利尼西亚地区地幔结构的地震分辨率，我们部署了10台宽带海底地震仪，以获得地幔柱的详细结构。利用JAMSTEC的“SHINKAI-6500”潜水器对火山岩进行了采样测量。得到的结果如下：1。我们确定了法属波利尼西亚地下200千米的上地幔s速度模型。所有热点下均存在慢速异常，但不同热点下的慢速异常最大深度不同。在社会和麦当劳热点下面，缓慢异常至少延伸到200公里以下。用接收函数法确定了地幔过渡带的厚度。我们只在社会热点下发现了一个薄的过渡带，而在其他站点下没有发现明显的厚度异常，这表明法属波利尼西亚从下地幔到上地幔的地幔柱并不宽，而是局限于社会热点下。我们利用纵波到达时间和各测点间的差分纵波到达时间，通过行时断层扫描确定了波利尼西亚地下的纵波速度结构。下地幔在深度大于1000 km处存在广泛的慢速异常。缓慢异常在1000至上地幔底部的深度很窄。根据目前的资料，上地幔结构的解析相对较差。位于社会热点下方的下地幔中一个狭窄的缓慢异常表明地幔柱正从下地幔上升到社会热点。我们可以在Cook-Austral群岛的海底使用“SHINKAI-6500”获得良好的岩石样本。海底岩石样本的组成与库克-奥斯特拉群岛陆地地区的岩石样本不同，表明岩浆来源随着热点火山的生长而改变。

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.