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Diagenetic evolution of some modern and ancient cold seep-carbonates from East Coast Basin, New Zealand.

Cold seep-carbonates are the microbially mediated by-products of the anaerobic oxidation of methane (AOM) at seafloor cold seeps, and are widespread about modern continental margins and in the geologic record. Some modern and Miocene examples of cold seep-carbonates from the East Coast Basin, North Island, New Zealand have been analysed in this study, to characterise and determine their carbonate fabrics, elemental and mineralogical composition, and stable δ18O and δ13C isotope signatures, so as to provide insights into the diagenetic changes associated with the lithification and burial of seep-carbonates. The ancient samples were collected from the onshore middle Miocene Tauwhareparae (TWP) seep deposit, while the modern samples were obtained from the National Institute of Water and Atmosphere (NIWA) Cruise TAN0616 (November 2006) from Ritchie Ridge, offshore Hikurangi Margin. A paragenetic sequence of diagenetic events involving early aragonitic phases, followed by late calcitic phases is defined for the seep-carbonates. This sequence likely has relevance for understanding the fluid-cement histories of seep-carbonates more widely. Two main carbonate mineralogies occur in each of the sample groups - modern samples are aragonitic or dolomitic, while the ancient ones consist dominantly of either aragonite or calcite. Thus, aragonite common to both sample groups, and is interpreted to represent the initial primary carbonate precipitate in hydrocarbon seep provinces under specific fluid flux and local pore-water chemistry conditions. Aragonite morphologies range from microcrystalline carbonate ('micarb'), to acicular aragonites that may form botryoids or spherulites. Dolomite occurs in those modern samples which appear to constitute exhumed remnants of a former subsurface 'seep plumbing system', and so are strictly not true seabed 'seep-carbonates', but instead are part of the larger hydrocarbon seep province. Calcite in the ancient samples is either a product of alteration and neomorphic transformation of aragonite, or derives from late stage cementation from burial fluids. As a result of their formation processes, the calcites are generally recrystallised and have equant or 'cellular' textures. Stable δ13C and δ18O isotope cross-plots reveal a large spread of values for the sample groups. Ancient samples range from δ13C -8 to -50 PDB and δ18O -5.5 to +2 PDB. Modern samples have δ13C values from -6 to -41 PDB and δ18O values ranging from +2.6 to +6.7 PDB. The δ13C values suggest the majority of the methane that formed these seep-carbonates is of thermogenic origin, although some mixing from other carbon sources may have occurred. The positive δ18O signatures are suggestive of carbonate formation during dissociation of gas hydrates, while the negative values possibly indicate that some of the formation fluids were warmer than normal in the 17 - 30 C range.
Type of thesis
Ewen, S. M. (2009). Diagenetic evolution of some modern and ancient cold seep-carbonates from East Coast Basin, New Zealand. (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/2788
The University of Waikato
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