The precipitation of aragonite from shallow-water hydrothermal fluids in a coral reef, Tutum Bay, Ambitle Island, Papua New Guinea

Abstract

The fringing reef in Tutum Bay on the west side of Ambitle Island, Papua New Guinea is the only presently known coral reef exposed to the extensive discharge of a hot, mineralized hydrothermal fluid. There, aragonite and ferrihydrite, a hydrous ferric oxide, are the prominent hydrothermal precipitates. Aragonite forms two distinct crystal habits, (a) euhedral (pseudo-hexagonal) crystals up to 2 cm long and (b) micro-crystals similar in appearance to “feather dendrite”. Aragonite encrusts dead coral fragments, volcaniclastic boulders and pebbles, and fills secondary fracture and remaining primary intergranular porosity within volcaniclastic arenite. The hydrothermal aragonite has a distinctively different isotopic composition when compared to “normal” shallow-water marine inorganic and organic carbonate that precipitated from seawater. This difference arises from precipitation at high temperature from a mixture of seawater and hydrothermal fluid that has lower 87Sr/86Sr and δ18O values than seawater. Based on a 87Sr/86Sr mixing model, aragonite precipitated from a hydrothermal fluid–seawater mixture of approximately 9:1. Precipitation from the hydrothermal solution is mainly caused by CO2 degassing, but mixing between hydrothermal fluid and seawater may have enhanced precipitation due to an increase in pH.

The δ13C of Tutum Bay hydrothermal aragonite ranges from 1.9‰ to 2.2‰ (VPDB). This range of values is in good agreement with experimental data [J. Phys. Chem. 72 (1968) 800; Geochim. Cosmochim. Acta 61 (1997) 3461], indicating that C-13-equilibirum has been reached during its formation. Values for δ18O range from 14.2‰ to 14.7‰ and calculated isotopic equilibrium temperatures are approximately 20 °C lower than directly measured vent fluids and those temperatures obtained from fluid inclusion experiments and the 87Sr/86Sr mixing model. This indicates that either oxygen isotope equilibrium was not attained or that the calcite–water fractionation factor for oxygen isotopes is not applicable for the precipitation of Tutum Bay hydrothermal aragonite.

Trace element concentrations, except for the REEs, Y and Sr are low. The REE patterns of aragonite are similar to those of Tutum Bay vent water, indicating the hydrothermal origin of the aragonite. Rare earth element concentrations are higher in the coarse than in the fine-grained aragonite, which might be caused by a change in precipitation rate and seawater mixing.