Rare earth element systematics of acidic geothermal waters from the Taupo Volcanic Zone, New Zealand

Concentrations of rare earth elements (REE) in acidic thermal waters from the Taupo Volcanic Zone, New Zealand have been determined. Acidity in waters from Rotokawa results from oxidation of H 2S upon encounter with oxygen-rich meteoric waters. The Ruapehu Crater lake is acidic due to input of magmatic volatiles such as HCl and SO 2. REE concentrations in both types of water are high, ranging from 10 − 3 to nearly 1 times chondrite. Chondrite-normalized patterns for heavy REE in waters from Rotokawa parallel those of their host rocks, suggesting little fractionation upon water–rock interaction. However, the light REE are depleted relative to their host rock, resulting in a “gull-wing” pattern. The hyperacidic Ruapehu Crater lake water generally does not have a “gull-wing” pattern. The patterns for these waters were invariant for several years prior to the September 1995 eruption, but changed substantially afterwards, generally decreasing in slope with time.