The presence of hydrous minerals or H
2O in the mantle wedge beneath northeastern Japan was investigated by comparing temperatures estimated from seismic
Q and heat flow. Applying the laboratory
Q results of dry peridotite onto the seismic anelasticity structures, the upper mantle temperatures were estimated. We then compared these temperatures with those obtained from observed heat flow. In the fore-arc region from the aseismic front to the wedge apex, the heat flow temperatures are much lower (-250-660°C lower) than the temperatures estimated from the seismic
Q of dry peridotite. This temperature difference indicates the presence of hydrous minerals or free-H
2O. On the other hand, no such temperature difference is found beneath the volcanic front to the Japan Sea coast area, indicating that the uppermost mantle wedge of this region is generally dry. Low
Q values (
Qs≅100) beneath this zone are due to high subsolidus temperatures (-1, 000°C) in the anhydrous mantle, and intermediate
Q values (
Qs≅550) beneath the aseismic front to the wedge apex are due to hydration of the low-temperature mantle (≤600°C). For island arc magmatism, we suggest that local enrichment of melt as well as H
2O occurs beneath volcanoes in the mostly dry mantle.
View full abstract