Submarine Arc Volcanism in the Southern Mariana Arc: Results of Recent ROV studies

The submarine Diamante cross-arc volcanoes (~16°N) and the Sarigan-Zealandia Bank Multi-Volcano Complex (SZBMVC; ~16°45’N), north and south, respectively, of Anatahan Island in the southern Mariana Arc, were studied during several dives in June 2009 using the ROV Hyper-Dolphin, cruise NT09-08 (R/V Natsushima); neither has been studied in detail before. The data collected provide a new perspective on how the subduction factory operates to complement previous studies on other cross-arc volcanic chains in the Izu-Bonin-Mariana Arc. The Diamante complex consists of three major edifices, two cones (West and Central Diamante) and a more complex caldera-like edifice at the volcanic front (East Diamante). West and Central Diamante are basaltic volcanoes but East Diamante has a more complex history. Our studies indicate initial construction of a basaltic volcano. Magmatic evolution led to a violent caldera-forming and quieter dome-building events. Post-caldera quiescence allowed a carbonate platform to grow, now preserved on the eastern caldera wall. Felsic magma or hot rock provides a heat source for an active hydrothermal field associated with felsic domes in the caldera, which NOAA investigators discovered in 2004. A new type of hydrothermal deposit was discovered in the hydrothermal field, consisting of large sulfide-sulfate mounds topped by bulbous constructions of low-temperature Fe and Mn oxides. Vents on the mounds were observed to emit shimmering water. The SZBMVC consists of six closely spaced edifices whose loci are aligned along two parallel trends, one along the volcanic front (Zealandia Bank, Sarigan and South Sarigan), and one about 15 km west towards the rear-arc (Northwest Zealandia, West Zealandia and West Sarigan). Zealandia Bank dives revealed that, as with East Diamante, initial activity was basaltic and became more evolved with time. The western half of Zealandia Bank is dominated by felsic lavas centered on a small (~2 km diameter) caldera and resurgent dome. Samples included banded rhyodacite lavas, indicating magma mixing or mingling. Carbonates were also recovered, presumably deposited between eruptions. A single dive on Northwest Zealandia largely recovered dacitic rocks. Dives on West Zealandia and three small volcanoes between Zealandia Bank and Sarigan collected solely basaltic rocks. Interestingly, the olivine basalts from West Zealandia include ultramafic xenoliths, which may be derived from the upper mantle or lowermost crust and, if so, represent the first such material to be recovered in the Mariana Arc. NT09-08 dives show that felsic lavas are not restricted to subaerial volcanoes and volcanoes with large calderas in this region. Thus, the abundance of intermediate and felsic lavas is not controlled by volcano size and well developed magma reservoirs, but is a property of the region, suggesting that it reflects processes occurring at depth in the subduction zone in the Anatahan region.