Growth model of a hydrothermal low-temperature Si-rich chimney: Example from the CDE hydrothermal field, Lau Basin

Abstract

The CDE hydrothermal field was first discovered during a Chinese cruise to the East Lau Basin Spreading Centre in 2007. Apart from significant amounts of loose Fe-Si-Mn (oxyhydr) oxide (referred to as oxide below) precipitates, a small Si-rich oxide chimney was also recovered on this cruise. In this study, we report on the mineralogical and geochemical analyses of this chimney and a model for its growth that has been developed. Based on the mineralogy and O isotope results, the chimney walls can be divided into four growth generations (layers) from the inner to the outer layers: amorphous opal and barite layer (precipitation temperature 68.5°C based on oxygen isotope determinations), a rod-like amorphous layer (precipitation temperature 39.6°C), a filamentous Fe-Si oxide layer, and an outer Fe-Mn oxide layer. Investigations based on SEM and EDS showed that neutrophilic Fe-oxidizing bacteria play an important role in the formation of this chimney, particularly in the outer two generations. In the first stage, the metabolic activity of the microbes results in the pervasive precipitation of the filamentous Fe-rich oxides inside a ring formed by some amorphous opal and barite; therefore, a loose porous layer forms. In the second stage, amorphous opal then precipitates inside this wall as a result of conductive cooling and gradually controls the mixing between the hydrothermal fluids and ambient seawaters. In the third stage, barite and some amorphous opal form from the higher temperature fluids at the summit of the chimney growth history. In the last stage, the chimney wall becomes thicker and denser and the exchange of hydrothermal fluids and seawater ceases. As a result, a Fe-Mn oxide layer precipitates onto the outer surface of the chimney wall as neutrophilic Fe-oxidizing bacteria reoccupy the surface of the chimney. This mineral sequence and the resultant growth generations are confirmed by the chemical characteristics of the chimney wall. Sr isotopes extracted from the Fe oxides of the four-generation wall generally show a decreasing trend of the 87Sr/86Sr ratios from the second layer to the inner layer (from 0.707008 to 0.705877) except for the outer layer (0.706502). The Sr isotope and chondrite normalized REE patterns of the corresponding bulk samples from the chimney wall also display a similar trend. Our study shows that the biogenic filament network plays a key role in the formation of the chimney in contrast to previous growth models of higher temperature chimneys, which often ignore the influence of biogenic factors.