| Title | One Source or Two? Implications for Interpreting Hydrothermal Fluids at the Field and Regional Scale in the Taupo Volcanic Zone, New Zealand |
|---|---|
| Authors | Blackstock, J. M., Horton, T. W., Gravley, D. M., Brown, K. L. |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Boron, Chlorine, Cesium, Lithium, Water-Rock Interaction, Taupo Volcanic Zone |
| Abstract | B, Cl, Cs and Li are commonly used in hydrothermal fluid studies as they are assumed to generally behave conservatively along complex flow paths. Over the past halfcentury, geothermal systems in the Taupo Volcanic Zone, New Zealand (TVZ) have been extensively studied and numerous inferences have been put forth describing the origin and flow paths through fluid geochemistry. Several studies in the TVZ have used elemental ratios (e.g. B/Cl and Li/Cs) to argue that hydrothermal fluids in the TVZ are derived from two spatially discrete sources: an arc-type fluid in the eastern TVZ and a rift-type fluid in the western TVZ. We reviewed selected geochemical data for hydrothermal fluids in the TVZ to date and show that two distinct B/Cl ratios are present in TVZ fluids. However, contrary to earlier work, our results do not exhibit an east-west spatial distribution of fluid chemistry. Rather, higher B/Cl ratios occur on both the eastern and western margins of the TVZ. We also observe that fluids in local fields (in both the eastern and western TVZ) can switch between the different B/Cl ratio types through time. This discrepancy most likely indicates that B is not behaving conservatively and B/Cl ratios of hydrothermal fluids in the TVZ are not directly linked to the current tectonomagmatic setting. Unlike the B/Cl ratios, the Li/Cs ratios of the same fluids are similar, irrespective of their geographical location, implying a similar parent-fluid across the TVZ. We suggest that ascending fluids in the TVZ hydrothermal systems originate from a geochemically similar parent-fluid at depth as indicated by Li/Cs ratios and through varying contributions of B from water-rock interaction, two B/Cl groups arise. Previous water-rock interaction studies of geologic formations in the TVZ and recent geophysical imaging of hydrothermal plumes in the TVZ support these interpretations. |