Sulphide mineralisation in the deep sea hydrothermal vent polychaete, Alvinella pompejana: implications for fossil preservation

Hydrothermal vent fauna, particularly vestimentiferan and polychaete worm tubes, are occasionally preserved in the geological record. The early stages of mineralisation are particularly important in defining whether or not preservation will occur, and they are poorly understood. Tube samples of the polychaete worm Alvinella pompejana collected from 13°N on the East Pacific Rise have been studied to identify the processes occurring during early pyrite/marcasite mineralisation. Iron sulphide mineralisation is present within the walls of the organic dwelling tube, and is induced by microbial fauna preserved within the tube micro-layers. Various microorganisms were observed coating the inner tube surfaces, together with 10-100 μm-sized Fe- and Zn-sulphide particles precipitated from vent fluids. The microbes and particulate sulphides become entombed within the tube wall as further layers of organic material are secreted by the worm, during tube-building episodes. This results in a laminated tube structure being formed, composed of alternating layers of tube material and microbial/sulphide-rich interlayers. The microbial/sulphide layers provide a template for further mineralisation and replacement of the microbes with pyrite while degradation of the organic components occurs. The iron monosulphides mackinawite and greigite have been identified as intermediatory phases that occur as precursor minerals during the formation of pyrite. Later marcasite mineralisation is observed to form over some of the pyritised organic layers. Once mineralisation has replaced most of the organic tube material, the structure will then be preserved along with the host sulphide body. These observations enhance our understanding of the mechanisms of fossil pyritisation in fine-scaled organic structures throughout the geological record.