A fifty year perspective of magmatic evolution on Ruapehu Volcano, New Zealand: verification of open system behaviour in an arc volcano

Geochemical and petrological data for samples from well documented eruption s that occurred at Ruapehu volcano over the period 1945-1996 can be used to illustrate the complexity of short term geochemical variation in an are-ty pe volcano. Collectively, data from Ruapehu Volcano show trends with time o f increasing SiO2 abundance and rising Sr-87/Sr-86 ratios, consistent with broad control by assimilation and crystal fractionation processes (AFC). Ho wever, the magmas emplaced during the past fifty years show geochemical var iability that spans most of the range shown by lavas erupted over the entir e history of the volcano. Magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes as sociated with magma recharge events within the volcano. These complex trend s are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short-term ef fects that are imposed during recharge on the overall AFC trend. We show th at the temporal geochemical and petrographic variations among erupted magma s are modulated by processes of mixing and mingling between fresh magma fro m below and stagnant melt and entrained crystals from earlier events remain ing in the volcanic edifice, probably in dikes and sills. These processes a re probably replicated over longer time periods (hundreds to thousands of y ears) as melts arrested at different levels in the near surface conduit sys tem are progressively displaced by new magma batches. Are type volcanoes su ch as Ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or l ess continuous activity. This style of activity is difficult to predict thr ough the usual (seismicity, ground deformation, lake water geochemistry) vo lcano monitoring techniques, and petrology and geochemistry may provide the basis for an alternative strategy. (C) 1999 Elsevier Science B.V. All righ ts reserved.