Discovery Of An Extensive Hydrothermal Sulfide/Sulfate Mounds Field In East Diamante Caldera, Mariana Volcanic Arc

An elongate field of hydrothermal mounds was discovered along the NE flank of a cluster of resurgent dacite domes in East Diamante Caldera using the ROV Hyper-Dolphin aboard the R.V. Natsushima in June 2009 and July 2010. East Diamante seamount lies about 80 km north of Saipan and is the northernmost volcano of the Southern Seamount Province of the Mariana magmatic arc. East Diamante is an irregular caldera about 10 km x 4 km that is breached on the north and south sides. The caldera floor has a maximum water depth of about 700 m. After caldera collapse, dacitic domes intruded into the center of the caldera providing the heat source for production and circulation of hydrothermal fluids that generated the large mounds field and two nearby chimney fields, one active and one inactive, found in 2004 during a NOAA Ring-of-Fire cruise. The mounds field is more than 100 m long and about 25-30 m wide and occurs along a NE-SW rift valley at water depths of about 365-400 m b.s.l. Individual hydrothermal mounds and ridges along this trend vary in size and the bases of the mounds are buried beneath hydrothermal sediment so that only minimum dimensions can be determined. Mounds are typically 1-3 m tall and 0.5-2 m wide, with lengths of about 3 to more than 5 m. The sulfide/sulfate mounds are layered and an iron- and manganese-oxide subsidiary mound venting low-temperature fluids caps some of them. Some mounds also support inactive sulfide/sulfate chimneys and spires; chimneys rarely occur as independent structures within the mounds field. The mounds are composed primarily of barite layers and sphalerite (high cadmium, low iron) plus galena layers with up to 470 ppm silver and 3 ppm gold. The subsidiary mounds are composed of 7A manganate and goethite that occur around a delicate network of 2-10 mm diameter anastomosing channels. Similar oxides cover the seabed throughout the mounds field and precipitated from diffuse fluid flow throughout the region, but formed by both diffuse and focused flow on top of the layered mounds. Several age dates for one mound show the layered section to have formed about 4,000 years ago while the subsidiary oxides formed during the past 4 years. Mineralization appears to be controlled by proximity to the NE-SW rift or fracture zone. Focused flow occurred along a line source rather than a point source that is typical of most hydrothermal chimney fields. Diffuse flow occurred adjacent to the rift and through the sulfide/sulfate mounds deposited atop the conduit. This field may be in a waning stage of activity with higher temperature fluids having been involved in construction of the main mounds several millennia ago. Alternatively, the present low-temperature activity may represent rejuvenation of the system.