Petrology and Geochemistry of Igneous Rocks collected in Association with ROV Investigations of Three Hydrothermal Sites in the Mariana Arc: NW Rota-1, E. Diamante, and NW Eifuku

Dives with the remotely operated vehicle ROPOS in March/April 2004 discovered magmatic degassing and hydrothermal activity at three important sites in the submarine Mariana arc: NW Rota-1 (14°36'N, 144°46'E), E. Diamante (15°56'N, 145°41'E), and NW Eifuku (21°29'N, 144°03'E). The first two volcanoes are part of the Mariana Southern Seamount Province whereas NW Eifuku is part of the Northern Seamount Province. These hydrothermal sites are associated with the summit regions (NW Rota-1, NW Eifuku) or the resurgent dome of a caldera (E. Diamante). NW Rota-1 appears to be volcanically active, whereas E. Diamante is thought to be extinct. NW Eifuku has not been sampled previously, but vigorous CO2 bubbling from the Champagne site indicates degassing of magma. Lavas previously analyzed from the edifice flanks of E. Diamante are fractionated basalts and basaltic andesites. All samples belong to a medium-K calcalkaline suite, similar to most Mariana arc lavas. Major element compositions for these three suites span the entire fractionation range observed for IBM arc lavas. NW Rota-1 lavas are vesicular OL-CPX-plag porphyritic basalts and basaltic andesites (51-54 per cent silica) and NW Eifuku samples are vesicular OL-CPX-PLAG basalts (50-51per cent silica). E. Diamante samples are QZ-PLAG porphyritic rhyodacites (66-75 per cent silica); we cannot be sure whether these samples represent lavas or hypabyssal intrusive rocks. Abundant vesicles in NW-Rota-1 basalts is expected, given eruption depths of 500m, but abundant vesicles in NW Eifuku lavas, erupted at 1600m, is more surprising. NW Rota-1 (Mg# = 57 to 66) and NW Eifuku basalts (Mg# = 58-66) are unusually primitive for IBM arc lavas. These basalts are approximately saturated in silica. NW Rota-1 has Na6 = 2.5 whereas NW Eifuku has Na6 = 2.1, indicating significantly higher degrees of melting. Studies of mineralogic, trace element, and isotopic compositions are still in progress.