Liu

A hydrothermal system model driven by andesite magma melts

L. Liu¹* & R. Lowell²

Corresponding author: lei@gatech.edu
¹Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA, 30332
²Virginia Polytechnic Institute and State University, Department of Geosciences, Blacksburg, VA, 24061

Abstract:
Most hydrothermal systems at oceanic spreading centers are driven by basaltic magma; however, some of the hydrothermal fields within parts of the Lau Basin in the Southwest Pacific are dominated by the andesite. Because the viscosity of these magmas is significantly different, the thermal characteristics and evolution of hydrothermal systems driven by basalt and andesite may also be different. Basaltic magmas are very dry, hence viscosity depends mainly on the crystal content and to a lesser degree on temperature; however, the viscosity of andesitic magma is affected not only by crystallinity and temperature but also water content. Moreover, andesitic magma has greater viscosity than basaltic magma because of its higher SiO2 content (> 60%) and lower liquidus and solidus temperatures. We adapted the equation for the viscosity of andesitic magma in [Vetere et al., 2006], and incorporated this equation into our earlier model of magma convection [Liu and Lowell, 2009] in order to determine the different characteristics of magma-driven hydrothermal systems, from three types of magmas (basalt, dry andesite and 3wt% andesite). Higher viscosity andesitic magma convects less vigorously, which results in slower heat transport but also in a lower rate of decay of vent temperature. We might therefore expect hydrothermal systems driven by andesite to have a longer lifetime than those driven by basalt. Because magma replenishment is needed to maintain hydrothermal temperatures, however, the higher viscosity of andesitic magma may make magma replenishment more difficult and hydrothermal heat output and vent temperature may not remain steady state.

Keywords:
magma-hydrothermal heat transfer; magma-hydrothermal models; andesite

Contributions to integration and synthesis:
Time-series observations of hydrothermal vents along spreading centers reveal a close coupling between magma supply and hydrothermal fluxes. The submersible studies have the chemical, biology evolution of hydrothermal vent fluids. And the correlation between magma budget and hydrothermal flux has been confirmed by seismic data. Based on the premise that magma supply exerts a primary control on hydrothermal processes, we are coupling magma convection model of Liu and Lowell [2009] with the multiphase hydrothermal model FISHES [Lewis and Lowell, 2009] to have a better understanding of the temporal variability of thermal structure and vent salinity characteristics of hydrothermal vents. We also use our magma replenishment model to simulate the episodic magma supply. The coupled magma-hydrothermal models will help us visualize the relationship between magma convection and long-lived hydrothermal circulation, and have a better understanding of how long magma-driven hydrothermal circulation could last, the frequency of episodic magma injection needed to maintain steady state of hydrothermal venting, and the typical rate for magma injection without eruption. Heat flux and time series data on vent temperature, salinity, crustal deformation, and seismicity provide important model constraints and are needed to develop site-specific models.

References:
F. Vetere, H. Behrens, F. Holtz, and D.R. Neuville (2006), Viscosity of andesitic melts—new experimental data and a revised calculation model, Chem. Geol. 228, p. 233.
L. Liu, and R.P. Lowell (2009), Models of hydrothermal heat output from a convecting, crystallizing, replenished magma chamber beneath an oceanic spreading center, J. Geophys. Res., 114