Formation of dolomite in recent island-arc sediments due to gas-seawater-sediment interaction

Dolomite occurs as a primary precipitate that cements Recent volcaniclastic sands in the shallow-marine environment of Luise Harbor, Lihir Island, Pap ua New Guinea. Field observations and geochemical data suggest that dolomit e precipitates through complex; mineral reactions in response to discharge of gas (> 90% CO2) and heated seawater from hydrothermal vents in the seafl oor, Gas discharge causes the formation of shallow circulation cells, and c old seawater is drawn into the sediment, where it is heated to approximatel y 100 degreesC. Reaction between hydrothermal CO2 (g)! primary sediment gra ins, and heated seawater causes the precipitation of primary dolomite, Once temperatures are above 70 degreesC the Mg/Ca ratio in seawater increases b ecause CaSO4 precipitates, while at the same time the dissolution of iron o xide minerals increases the pH; both reactions favor dolomite precipitation . Ferrous iron reacts with H2S to form co-precipitated Fe-sulfide minerals. The dolomite is nearly stoichiometric and occurs as zoned inclusion-rich a nd inclusion-poor spherulitic cements, many of which are present as coalesc ed composite spherules, Oxygen isotope values for dolomite of -8.8 to -10.2 parts per thousand VPDB constrain the temperature of precipitation to be b etween about 77 to 110 degreesC, Strontium isotope ratios for dolomite (ave rage 0.70886) are close to those of modern seawater; their slightly less ra diogenic compositions implicate minor addition of magmatic strontium. Hydro thermal dolomite may develop in shallow. near-surface sediments in active t ectonic settings.