Complex behavior of magma-hydrothermal processes: Role of supercritical fluid

Magmas emplaced into the upper portions of the earth's crust are accompanie d by extensive hydrothermal activity. Hydrothermal activity is represented as a system of coupled processes that dissipate thermal, mechanical, and ch emical energy into the magma's lithocap, primarily by convection of HO-rich fluids. To investigate dynamical behavior of the system, a serial experime nt was undertaken in which T(t) and P(t) values are computed for a pluton l ocation during the time the region was subjected to near-critical hydrother mal convective flow. The consequent evolution of fluid buoyancy, del (x)rho (f), ion stability, Delta(G) over bar degrees, and fracture extension, del taL/L-0 during this time indicates that variations in density gradients inc rease smoothly until 70,000 yr then burst into frequent, approximate to 100 -yr oscillations. Oscillations first increase in magnitude then decrease. O scillatory behavior of state conditions derived from numerical experiments illustrate resonant effects in chemical equilibrium and fracture extension processes and show the sensitivity of the stable mineral assemblage to eith er of the competing chemical and mechanical transport processes. An oscilla tory zoned tourmaline that formed at near-critical conditions of H2O from t he Geysers Geothermal deposit appears to provide evidence of nonlinear syst ematics in hydrothermal activity. Mathematical analogs to this system demonstrate that processes in this syst em record their dynamical behavior in the supercritical region and suggest that alteration events are generated by the complex, "chaotic" behavior of these processes. This type of behavior appears to be further augmented by s trong divergence of H2O-fluid properties toward +/- infinity at commonly en countered state conditions in the shallow reaches of magma-hydrothermal act ivity. System behavior elucidated here arises from affording for connectivi ty of processes by numerical experiments of hydrothermal activity for a reg ion near the contact of a magma and its lithocap. The cumulative data from numerical experiments, equation-of-state (EOS) relationships, geologic and geochemical observations support the proposition that magma-hydrothermal pr ocesses should be thought of as complex dynamical systems whose behavior at state conditions near the supercritical region of the fluid is likely chao tic. Copyright (C) 2001 Elsevier Science Ltd.