Geochemistry of Gases in the Alpine Fault, DFDP-2
Final Report Abstract
The key aim of the project was the systematic and comprehensive study of the composition of gases extracted on- and off-line from drill mud taken during the drilling campaign DFDP-2 through the Alpine Fault. Individual fluid systems above and below the fault were to be characterized in order to answer the question: Are there systematic compositional differences in the gas composition of the Indo-Australian and Pacific plates along the Alpine fault and does a hydraulic connection exist through the fault zone? Unfortunately, the target depth for DFDP-2, of 1300 m was not reached, nor was the active principal slip zone of the Alpine Fault, which was expected at ~1000 m depth, intercepted. Due to technical problems the DFDP-2 was abandoned at 893 m and did not reach the Australian Plate. Therefore, conclusions cannot be drawn about differences in the fluid systems of Australian and Pacific Plate as planned in the proposal. However, several gas influx zones could be identified with the continuous mud gas analyses. The identified discrete peaks with high gas concentrations indicate that gases enter the borehole at discrete depths but correlations with the lithology or depth trends could not be observed. Maximum methane concentrations measured during drilling were similar to concentrations in gases emitted from local spring waters. Variation in CH4 concentrations measured during drilling may be due to variability in permeability of the formation. Elemental and isotopic data (δ13C-CH4 of -47.9 ‰) indicate a rather thermogenic source of the gas. Hydrogen may be either artificially produced or derived from a biogenic source or radiolysis of water. Alternatively, hydrogen might be produced on fresh mineral surfaces created by tectonic processes. Hydrogen has been identified to occur in parts of the hole with enhanced permeability where fracture density is higher. During drilling relative low amounts of CO2 were detected within the drilling fluids probably due to the high pH of the drilling fluid. The 12C/13C-ratio of a gas sample from 236 m yields a δ13C-CO2 of -12.9 ‰ which is in the range of free gas from hot springs along the Alpine Fault (δ13C-CO2 of -7.2 – 18.6 ‰). Low levels of alpha-activity were observed during drilling, but only few discrete peaks with high Rn concentrations were identified. High radon activity and a low level of background gas suggest an active fluid flow through fractures and faults rather than pore-space derived gas homogeneously distributed throughout the bulk rock. The 3He/4He values show that a mantle signature, although small, is present throughout the borehole DFDP-2 indicating that the fault is a possible conduit for mantle fluids. Previous studies suggest there may be a 5 - 6 % mantle component in the vein forming fluids using helium isotope ratios in veins proximal to the Alpine Fault. Due to the fact that DFDP-2 did not penetrate through the Alpine Fault, it could not be clarified whether the high 3He/4He ratio of the Kotuku sampling site on the Australian side reflects a distinct fluid origin or is just a local feature.
Publications
- (2016): The fluid budget of a continental plate boundary fault: Quantification from the Alpine Fault, New Zealand. - Earth and Planetary Science Letters, 445, pp. 125-135
Menzies, C. D., Teagle, D. A. H., Niedermann, S., Cox, S. C., Craw, D., Zimmer, M., Cooper, M. J., Erzinger, J.
(See online at https://doi.org/10.1016/j.epsl.2016.03.046) - (2017): Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand. - New Zealand Journal of Geology and Geophysics, 60, 4, pp. 497-518
Toy, V. G., ..., Zimmer, M.
(See online at https://doi.org/10.1080/00288306.2017.1375533) - (2017): Extreme hydrothermal conditions at an active plate-bounding fault. - Nature, 546, pp. 137-140
Sutherland, R., ..., Zimmer, M.
(See online at https://doi.org/10.1038/nature22355) - (2017): Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone. - Geochemistry Geophysics Geosystems (G3), 18, 12, pp. 4709-4732
Townend, J., ..., Zimmer, M.
(See online at https://doi.org/10.1002/2017GC007202)
