HR: 1340h
AN: P33A-0997    [Abstracts]
TI: Pore Water Convection in Carbonaceous Chondrite Planetesimals
AU: * Travis, B J
EM: bjtravis@lanl.gov
AF: Los Alamos National Laboratory, Earth & Environmental Sciences Div., EES-2, MS-T003, Los Alamos, NM 87545 United States
AU: Schubert, G
EM: schubert@ucla.edu
AF: Dept. of Earth & Space Sciences, Institute of Geophysics & Planetary Physics, University of California, Los Angeles, CA 90095 United States
AB: Chondritic meteorites are so named because they nearly all contain chondrules - small spherules of olivine and pyroxene that condensed and crystallized in the solar nebula and then combined with other material to form a matrix. Their parent bodies did not differentiate, i.e., form a crust and a core. Carbonaceous chondrites (CCs) derived from undifferentiated icy planetesimals. Asteroids of the inner solar system are probably present-day representatives of the early planetesimals. CCs exhibit liquid water-rock interactions. CCs contain small but significant amounts of radiogenic elements (e.g., $^{26}$Al), sufficient to warm up an initially cold planetesimal. A warmed-up phase could last millions of years. During the warmed-up phase, liquid water will form, and could evolve into a hydrothermal convective flow. Flowing water will affect the evolution of minerals. We report on results of a numerical study of the thermal evolution of CCs, considering the major factors that control heating history and possible flow, namely: permeability, radiogenic element content, and planetesimal radius. We determine the time sequence of thermal processes, length of time for a convective phase and patterns of flow, amount of fluid flow throughout the planetesimals, and sensitivity of evolution to primary parameters. We use the MAGHNUM code to simulate 3-D dynamic freezing and thawing and flow of water in a self-gravitating, permeable spherical body. Governing equations are Darcy's law, mass conservation, energy conservation, and equation of state for water and ice. We have simulated the evolution of heating, melting of ice, subsequent flow and eventual re-freezing for several examples of CC planetesimals. For a reference simulation, we use typical values from meteorite analyses: 20 % porosity, 1 darcy permeability (~10$^{-12}$ m$^{2}$), 3x10$^{-8}$ wt fraction of $^{26}$Al, rock density of 3000 kg/m$^{3}$, rock specific heat of 1000 J/kg/K, body radius of 50 km, solid rock thermal conductivity of 3 W/m/K. For the initial temperature, we use 170 K, assume a constant exterior temperature of 170 K, and apply a radiation surface temperature boundary condition. We then consider variations from the reference case for three variables: permeability (10 darcys), radius (80 km) and radiogenic heat content (50 % increase). Our simulations demonstrate that hydrothermal convection should occur for a range of parameter values and would last for several millions of years. In all of the simulations, radiogenic heating creates a water phase in about 0.6 Myr. The liquid phase lasts at least 4, to over 20 Myr, depending on the case. The center warms to peak temperatures of 360 to 450 K. Convection starts after sufficient cooling at the outer regions (but inside the outer frozen shell) has occurred to create a sufficiently strong radial temperature gradient. In these simulations, boiling does not occur, but, for a time, the systems are not far from that state. In all the simulations the convection is characterized by a mix of plumes and sheets, with plumes sharply defined for the more strongly convecting cases (10 darcys, and 50% increased heating cases). Roughly half the interior experiences water fluxes of 100--200 pore volumes. High pore volume flux facilitates extensive chemical reactions.
DE: 5114 Permeability and porosity
DE: 5418 Heat flow
DE: 6020 Ice
DE: 6055 Surfaces and interiors
DE: 6205 Asteroids and meteoroids
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting