Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand

Abstract

ABSTRACT Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle‐ductile transition, which has been raised to 5‐6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H 2 O + NaCl‐CO 2 mixtures at 200‐350d̀ C. Bulk fluid composition is 15‐20 mol% CO 2 and <4.3 total mol CH 4 + N 2 + Ar/100mol H 2 O. Water hydrogen isotopic ratio δD H2O in the fissure veins spans ‐29 to ‐68‰, δ 18 O H2O ‐0.7 to 8.5‰, and bulk carbon isotopic ratio δ 13 C ranges from ‐3.7 to ‐11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.