We have used zircon geochronology to parameterise time series in chemical compositions of volcanic whole-rocks and of phenocrysts. The latter are, in turn, used to obtain time series in calculated temperature, oxygen fugacity, and wt% H₂O dissolved in the magma, as described by Rohrlach and Loucks (RSES Annual Report, 2000). The time series reveal several major cycles of magma replenishment and differentiation over a period of ~ 8 Myr, as shown in the accompanying figure. This reservoir longevity and continuity in magmatic evolution over several million years, afforded by the hot, deep environment of entrapment, facilitates cyclic build-up of incompatible magmatic volatile components in residual melt fractions over time intervals which are an order of magnitude longer than those in shallow upper-crustal sub-volcanic magma chambers.

Ratios of incompatible on compatible elements in detrital zircons of Late Miocene to Recent age on the flanks of the Tampakan stratovolcano reveal cyclical increases in incompatible element concentrations within the melts from which the zircons crystallised (Figure 4b). The cyclic advancement and retreat of these element ratios is superimposed on a long-term trend to highly evolved incompatible-element-rich compositions. During this 8 million year period, erupted magmas progressively became more fractionated, as defined by increasing silica content of the melts (Figure 4a). They became increasingly water-rich as melts progressed from ~ 2.9 wt/% H₂O to ~ 7.3 wt/% H₂O (Figure 4c), and became progressively more chlorine-rich as defined by chlorine contents recorded in apatite phenocrysts. Modelling of major-, trace- and rare-earth-element trends in Late Miocene to Recent igneous rock samples from the Tampakan district reveal several magmatic cycles wherein hornblende became increasingly dominant within each fractionating sequence, with hornblende supplanting pyroxene in the crystallisation sequence at progressively earlier intervals in successive magmatic cycles. This Late Miocene to Recent period of increasing volatile contents in the long-lived, Moho-level magma reservoir, sampled by successive stratovolcanoes via a series of small and transient upper crustal chambers, is coeval with a 7 million year episode in which the amount of plate convergence in excess of that not accommodated by subduction was partitioned into compressive intra-crustal folding, thrusting and thickening (Figure 4d).

These geochemical data are interpreted to reflect a long-term build-up of magmatic volatile components within a sub-crustal magma chamber over several million years. Multiple replenishments of primitive magma from the mantle added successive aliquots of volatile components such as water, sulphur and chlorine, to a long-lived magmatic reservoir in which the horizontal compressive stress in the lower crust inhibits escape of buoyant volatile-enriched residual melt fractions by sub-vertical dyke propagation from the deep reservoir. In that case, the residual melt fraction from a cycle of magmatic differentiation can pass on its accumulation of incompatible components (H₂O, Cl, SO₃) to the next generation of replenishing mantle-derived magma. Through a succession of such differentiation and replenishment cycles, the residual melts may eventually ramp up to exceptional concentrations of incompatibles. High metallogenic fertility is strongly facilitated by high concentrations in magmatic chlorine and water. Because H₂O solubility in silicate melts is chiefly a function of pressure, the more H₂O-rich a magma is, the deeper it starts exsolving a magmatic hydrothermal fluid. Aqueous fluids exsolved deeper are denser and have much higher partition coefficients of metal-chloride salts into the exsolving fluid, so the metal scavenging efficiency of fluids from parental melt is expected to increase with depth of exsolution.