The Broadlands-Ohaaki geothermal system, New Zealand Part 1. Strontium isotope distribution in well BrO-29

Well BrO-29 at the Broadlands-Ohaaki geothermal system, New Zealand, penetr ates a mixed volcanic/sedimentary rock sequence whose Sr-87/Sr-86 ratios di splay linear distributions when plotted against 1/Sr values. These linear d istributions do not indicate binary mixing between unaltered lithological u nits, and a single fluid endmember. The linear distribution in the volcanic /pyroclastic sequences is related to the permeability of the host rocks, th e degree of leaching of Sr-86 rich primary plagioclase laths, and secondary crystallisation of the groundmass by a recharge fluid represented by modif ied Taupo rainwater. The greywacke/argillite basement from the same well di splays a separate linear isotopic trend when their Sr-87/Sr-86 ratios are p lotted against 1/Sr values, in addition to yielding a Sr-87/Sr-86 vs. Rb/Sr errorchron with an age of 88 +/- 6 Ma BP (MSWD = 9.48). Based upon support ing petrographic observations, the two isotopic trends in the greywacke/arg illite basement are believed to be an inherited metamorphic feature, from t he Rangitata Orogeny (141 +/- 3 Ma), which has been modified by geothermal fluids preferentially leaching Sr-87 and Rb-87 from primary micas. (C) 2000 Elsevier Science B.V. All rights reserved.