Biomineralisation and geochemistry of hydrothermal vent fauna

Abstract

Hydrothermal ventfauna, 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. This dissertation describes 3 separate studies of ventfauna undergoing early mineralisation associated with active seafloor vent systems. Tube samples of the polychaete worm Alvinella pompejana collected from 13°N on the East Pacific Rise were studied to identify the processes occurring during early sulphide mineralisation. Iron sulphide mineralisation is present within the walls of the organic welling tube, and is induced by microbialfauna preserved within the tube micro-layers. A laminated tube structure wasformed, comprising alternating layers of tube material and microbially templated iron sulphide-rich interlayers. The iron monosulphides mackinawite and greigite, have been identified as intermediate phases that occur as precursor minerals during theformation ofpyrite. Later marcasite mineralisation is observed to form over some of the pyritised organic layers. Later stagemineralisation processes in a selection offossil tube structures recoveredfrom an extinct sulphide mound on the Southern East Pacific Rise were studied. The similarity to existingfaunal assemblages suggests that these structures were also associated with Alvinella sp. polychaete worms. Two generations of pyrite were identified along with a later stage chalcopyrite phase. Fine scale variability in tube wall geochemistry was demonstrated with electron micro-probe transects across the structures and this is inferred to reflect early mineralisation processes as identified in living polychaete worms. Sulphur isotope analyses of the different sulphide phases gave d^'*S values higher than any previously measured for chimney structures on a sediment-free ridge setting = 3.8-10.8%c). A two stage model is proposed whereby early pyrite / marcasite laminae originally laid down in association with biogenic tube structures, are replaced by euhedral pyrite, and later stage primary chalcopyrite precipitation. A study of Ridgea piscesae from the Juan de Fuca Ridge demonstrated that silicification processes proceed so rapidly that the vestimentiferan tubes can become mineralised during the worm's life cycle. The organic component of the worm tube is rapidly replaced by amorphous silica which undergoes dewatering toform opal-A. This mineralisation is then overgrown by barite laths, and microbially templated amorphous silica. With time, prograde mineralisation causes the further precipitation and consequent overprinting of the primary mineral suite with iron- and zinc-sulphides. Other observations from samples studied here include micro-tubes (-50 jjm length) associated with adult R. piscesae and a silicified mucus lining to inhabited worm tubes. Associated with the mucus lining is a ligament which runs along the inside of the tube, and is tentatively inferred to be a form of anchor, allowing the worm to rapidly retract into the tube when threatened. Once mineralisation has replaced the organic tube material there is scope for the tube structure to be preserved along with the host sulphide body. Understanding of the mechanisms of early sulphide and silica/barite mineralisation as inferred from detailed observations and measurements of tube worm samples is an important contribution to the growing field of hydrothermal palaeobiology.

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ORCID for Rachel Mills: orcid.org/0000-0002-9811-246X

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