Multiphase modeling of contact metamorphic systems and application to transitional geomagnetic fields

Abstract

We present a numerical model that improves our capability to simulate multiphase, non-isothermal flow in variably saturated porous and fractured media at magmatic temperatures and shallow crustal pressures. Simulations of heat and fluid flow in variably saturated host rock near a magmatic intrusion provide insight into contact metamorphic processes, including dryout, condensation, and resaturation effects and implications for host-rock alteration. The numerical code, an enhanced version of FEHM, uses a finite-element/finite-volume technique incorporating implicit Newton–Raphson iteration to solve non-linear conservation equations for mass and energy, using thermodynamic properties of water and air in the ranges 10°C≤ T≤1500°C, 0.00123≤ P≤1000 MPa and 10°C≤ T≤1500°C, 0.00123≤ P≤22 MPa, respectively. The study area is located at Paiute Ridge, eastern Nevada Test Site, Nevada, USA, where hypabyssal mafic intrusions were emplaced at about 8.5–8.6 Ma (Ar/Ar age estimate) and cooled contemporaneously with part of a geomagnetic field reversal, inferred from paleomagnetic data from over 100 sites in intrusions and remagnetized host ash-flow tuffs. We used a radial model of heat flow and multiphase pore fluid flow adjacent to a 1200°C intrusion to characterize the thermal evolution of the contact metamorphic system. For likely initial pore saturations of 0.4–0.6, an expanding dryout zone near the intrusion and a condensation zone of enhanced saturation ( S≤0.8) extends 150–400 m from the intrusion. Host-rock temperatures reach 800°C near the contact and cool below 100°C within 2000 yr after emplacement, two to four times faster than predicted by a simple conduction model. The thermal history of the system is very sensitive to initial saturation. The multiphase thermal model allows bounds to be placed on the rate of change of the transitional part of the geomagnetic field during the field reversal recorded at Paiute Ridge. We assume that magnetization acquisition took place during the life of the thermal system that developed in the intrusions and contact rocks and that the paleomagnetic data provide a quasi-continuous record of the transitional part of the reversal. Sites in intrusions and thermally annealed ash-flow tuffs reveal subtle yet systematic variations in paleomagnetic directions. We combine the directional data with robust thermal (temperature/time) models to estimate the rate of change of the geomagnetic field. Modeled times of 140–290 yr and 215–440 yr for the duration of magnetization acquisition at two different sites correspond to estimated rates of change of 0.06–0.13°/yr for the field during the transitional part of the reversal.