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LPS XXVII
1007
LITHOSPHERIC BUOYANCY: IMPLICATIONS FOR GLOBAL
RESURFACING ON VENUS. E.M. Parmentier, J.W. Head, and P.C. Hess, Department
of Geological Sciences, Brown University, Providence, RI 029 12
Impact craters on Venus are consistent with a surface age of 300-500 Myr and their distribution
on the surface is indistinguishable from a random one. Despite the relatively young age and
volcanic origin of the surface, remarkably few craters are obviously flooded by volcanic deposits.
While a variety of interpretations of the Venusian cratering record have been proposed [1,2,3,4,5],
the simplest interpretation is a relatively rapid, global resurfacing about 300-500 Myr ago.
Mechanisms that could cause catastrophic and perhaps episodic global scale mantle overturn with
consequent decompression melting and volcanic resurfacing are thus important to understanding
the evolution of Venus.
Episodic instabilities of large horizontal scale, but not necessarily globally synchronous, are
seen in numerical simulations of thermal convection in a mantle containing endothermic phase
transitions [6,7]. This has been suggested as a mechanism for mantle overturn in Venus [8].
Episodic plate tectonics [9] has also been suggested for Venus although no remaining evidence of a
terrestrial style of plate tectonics has been found. In the plate tectonic hypothesis, a thick thermal
lithosphere develops that can account for the large inferred elastic plate thickness and large apparent
depths of compensation. But what keeps negatively buoyant thermal lithosphere floating on the
mantle for 300 Myr years or longer? Terrestrial oceanic lithosphere becomes negatively buoyant at
much younger ages of 30-50 Myr [lo].
Accumulation of a buoyant residual layer, depleted by the extraction of basaltic crust, at the top
of the Venusian mantle provides a possible mechanism to keep the lithosphere afloat and explain
global catastrophic overturn [ l l ] . As this layer thickens, increasing pressure at the top of
underlying convecting mantle reduces the degree of partial melting thus decreasing the
compositional buoyancy of newly added residual mantle. Increasing negative buoyancy due to
cooling eventually overcomes the positive compositional buoyancy leading to large-scale mantle
overturn. In this study we further examine the time required for the lithosphere to become
negatively buoyant.
For a given mantle potential temperature, melting curve, and latent heat of melting (600 kJ/kg),
the amount of melt generated by adiabatic decompression during a rapid mantle overturn can be
determined. For a reasonable melting curve, calculated crustal thickness as a function of mantle
potential temperature is given in Figure 1. To determine the net buoyancy of the lithosphere or
thermal boundary layer, we assume that basaltic crust transforms through granulite to eclogite with
increasing pressure. The pressure-temperature boundaries of the granulite field are taken from
[12]. Density increases linearly with increasing pressure in the granulite field from 3.0 gm/cm3 for
basalt to 3.6 gm/cm3 for eclogite. The temperature distribution in the thermal boundary layer as a
function of age is taken to be that due to simple conductive cooling. Neglecting the latent heat of
soldification of the crust must underestimate the temperature at any depth and so overestimate the
negative thermal buoyancy. Typical density distributions are shown in Figure 2 for three values of
the mantle potential temperature. Note that for a potential temperature of 17000C, a thick layer of
dense eclogite develops. For 15000C only lower density granulite is present while for 1300°C not
even granulite forms. In the mantle, the density is reduced by depletion of basalt which increases
linearly from the greatest depth of melting to the base of the crust.
The age at which the lithosphere contains zero net buoyancy is plotted in Figure 1 as a function
of mantle potential temperature. At younger or older ages the lithosphere is positively or negatively
buoyant, respectively. The maximum age of zero net buoyancy is about 300 Myr for a mantle
potential temperature of about 15000C corresponding to a crustal thickness of about 40 km. It
must be regarded as a concidence that this age concides almost exactly with lower bounds for the
estimated age of the Venusian surface. The calculated age depends at least weakly on parameters
such as the melting model, which is not precisely known for Venus. This simple anaylsis does
show that the buoyancy of the crust and depleted mantle layer are sufficient to keep the lithosphere
buoyant for time scales at least comparable to the present age the Venusian surface. Relative to the
much younger age of zero net buoyancy of the terrestrial oceanic lithosphere, this is accomplished
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1008
LITHOSPHERIC BUOYANCY ON VENUS. E.M. Parmentier, et al.
by having a thicker crust. It is interesting to note that recent gravity studies suggest a crustal
thickness in the range of 20-40 km [13] and that significant amounts of granulite begin to form at
the base of a 40 km thick crust. Gravitaitonal instability of dense granulite or eclogite may limit the
crustal thickness to about 40 krn.
References: [I] Strom, R.G., et al., J. Geophys. Res. 99, 10899, 1994. [2] Kirk, R.L.,et al, LPSC 26, 757,
1995. [3] Herrick, R.L., and R.J. Phillips, Icarus 111, 387, 1994. [4] Phillips, R.J., and V.L. Hansen, Ann. Rev.
Earth Planet. Sci. 22, 597, 1994. [5] Price, M., LPSC 26, 1143, 1995. [6] Machetel, P., and P. Weber, Nature
350, 55, 1991. [7] Tackley, P.J., et al., Nature 361, 699, 1993. [8] Steinbach, V., and D.A. Yuen, Geophys. Res.
Lett. 19, 2243, 1992. [9] Turcotte, D.L., J. Geophys. Res. 98, 17061, 1993. [lo] Oxburgh and Parmentier, J. Geol.
Soc. London 133, 343,1977. [ l l ] Parmentier and Hess, Geophys. Res. Lett. 19, 2015, 1992. [12] Ringwood,
Composition and Petrology of the Earth's Mantle, 1975. [13] Grimm, Icarus 112, 89, 1994.
Figure 1. Crustal
thickness (dashed)
and age of zero
lithospheric
buoyancy (solid)
as a function of
mantle potential
temperature.
mantle potential temperature ("C)
Figure 2.
Density as a
function of
depth at a
lithosphere are
of 300 Myr for
three values of
the mantle
potential
temperature.
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