HR: 1330h
AN: V52A-0418    [PDF]
TI: Boron Isotope Systematics of New Zealand Hydrothermal Systems
AU: * Aggarwal, J
EM: jaggarwal@es.ucsc.edu
AF: Keck Isotope Laboratory Department of Earth Sciences, University of California, Santa Cruz 1156 High Street, Santa Cruz, CA 95060 United States
AB: Recent analytical developments in multiple collector ICP-MS have allowed for the rapid and precise determination of B isotope ratios (Aggarwal et al 2003). This technique has been applied to the New Zealand environment to better understand the recycling of B in such a geologically diverse environment. Geothermal fluid samples have been collected from hot springs along the Alpine fault in the South Island, and extensively across the Taupo Volcanic Zone (TVZ), and the east coast of the North Island. These samples have been purified and analysed by an Axiom multiple collector ICP-MS with a direct injection nebuliser. Sample sizes were typically 200ng B, and analysed in duplicate giving an external precision of $<$0.2$\permil$. The data group into three. Samples from the Taupo Volcanic Zone from both geothermal wells and hot springs show $\delta$$^{11}$B values between -9$\permil$ and 0$\permil$ and similar Cl/B. These $\delta$$^{11}$B values are independent of temperature and reflect subtle differences in the isotopic composition of the hydrothermal systems and their host rock. This is consistent with other data from the Ngawha geothermal system in the north of the North Island that have $\delta$$^{11}$B values of -3.5$\permil$. This is in contrast to most other arc magmas that have higher $\delta$$^{11}$B values. The TVZ $\delta$$^{11}$B values are similar to those from Iceland (Aggarwal et al 2000) where B is sourced from the mantle suggesting that B could also be derived from mantle. It is however more likely that since the TVZ $\delta$$^{11}$B values are similar to those from Ngawha in the far north of the North Island, the sources would also be similar indicating that B is crustally derived. In contrast hydrothermal fluids from the east coast of the North Island show $\delta$$^{11}$B values from 18 $\permil$ to 37$\permil$. Cl/B ratios in these fluids are different to that of seawater indicating a minimal seawater involvement. Instead, it is likely that the high $\delta$$^{11}$B values are a result of the addition of fluids with a high $\delta$$^{11}$B value possibly from the dewatering of the adjacent accretionary prism during subduction. Samples from the third group come from a wide range of localities show a substantial spread of $\delta$$^{11}$B values (20$\permil$ to -2$\permil$). This range is perhaps a result of adsorption of $^{10}$B in the cooler fluids and the consequent fractionation of B isotopes from the crustal $\delta$$^{11}$B values seen in the TVZ and in Ngawha. It conclusion, B isotope measurements from New Zealand indicate that the Taupo Volcanic Zone is dominated by crustally derived B whereas on the east coast, B is influenced by B being dewatered from the subducted accretionary prism. Elsewhere B is strongly fractionated from the crustal signatures seen at Ngawha and in the Taupo Volcanic Zone.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting