A FIFTY YEAR PERSPECTIVE OF MAGMATIC EVOLUTION ON RUAPEHU VOLCANO, NEW ZEALAND : VERIFICATION OF OPEN SYSTEM BEHAVIOUR IN AN ARC VOLCANO

Abstract Geochemical and petrological data for samples from well documented eruptions that occurred at Ruapehu volcano over the period 1945–1996 can be used to illustrate the complexity of short term geochemical variation in an arc-type volcano. Collectively, data from Ruapehu Volcano show trends with time of increasing SiO 2 abundance and rising 87 Sr/ 86 Sr ratios, consistent with broad control by assimilation and crystal fractionation processes (AFC). However, the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano. Magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano. These complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short-term effects that are imposed during recharge on the overall AFC trend. We show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice, probably in dikes and sills. These processes are probably replicated over longer time periods (hundreds to thousands of years) as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches. Arc type volcanoes such as Ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity. This style of activity is difficult to predict through the usual (seismicity, ground deformation, lake water geochemistry) volcano monitoring techniques, and petrology and geochemistry may provide the basis for an alternative strategy.