ACIDIC AND SULFATE-RICH HYDROTHERMAL FLUIDS FROM THE MANUS BACK-ARC BASIN, PAPUA-NEW-GUINEA

Hot (greater than or equal to 88-120 degrees C) and acidic (pH less th an or equal to 2.1) hydrothermal fluids rich in sulfate were discovere d venting in the DESMOS caldera (depth = 1926 m), eastern Manus back-a re basin, Bismarck Sea, surrounded by Papua New Guinea. The abundant s ulfate (greater than or equal to 32.8 mM, higher than the seawater val ue of 28 mM) with elemental sulfur deposition around the vents, and re markably low delta D(H2O) and delta(34)S(H2S) values (-8.1 parts per t housand and -5.6 parts per thousand, respectively), are suggestive of the incorporation of a magmatic fluid and the disproportionation of th e exsolved SO2 from a magma body. The DESMOS fluid may be similar in o rigin to the acidic sulfate-chloride hot springs associated with subae rial volcanic activity. In contrast to the typical hydrothermal end me mber Mg concentration of 0, the DESMOS fluids are rich in Mg (46-52 mM ), probably because of Mg dissolution by acid attack upon magnesium si licate minerals.