Download Pace of tectonic modes on Venus and Earth and atmospheric Argon

Pace of tectonic modes on Venus
and 2Earth and atmospheric
Argon
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1
Tobias Höink , Craig O’Neill , Adrian Lenardic
1
( Department of Earth Science, Rice University, Houston, Texas, USA,
2
Department of Earth & Planetary Science, Macquarie University, Sydney, Australia)
Summary
Differences in tectonic histories of Venus and
Earth are controlled by different convective
stresses to which their planetary surfaces are
exposed. Convective stresses are in turn
controlled by the internal viscosity structure of a
planet. The tectonic history of Venus and Earth
can be understood using melting and degassing
models constrained by atmospheric abundances
of radiogenic Argon.
Pace of tectonic mode on Venus and Earth
Venus and Earth, although similar in many
characteristics, have experienced different modes
of global tectonics over their lifetime due to
differences in their respective styles of mantle
convection. Earthʼs lithosphere is fragmented into
several mobilized plates. Venusʼs lithosphere in
contrast is a single stagnant lid for extended
periods time, and periodically interrupted by
substantial subduction and tectonic activity (Moresi
and Solomatov, 1998).
Both Venus and Earth are expected to have depthdependent viscosity (a weak upper compared to
lower mantle) due to increasing pressure with
depth. One of the major differences between the
planets is their respective viscosity structure,
possibly due to the existence of water on Earth
and lack thereof on Venus. Earthʼs asthenosphere
is a thin low-viscosity layer beneath its lithospheric
plates. In contrast, Venusʼ sublithospheric
viscosity structure is better described by a lowviscosity upper mantle and a high-viscosity lower
mantle.
Since the discovery of plate tectonics it has been
suspected that the low viscosity layer beneath
Earthʼs plates may play a central role in facilitating
their continuous motions. This notion derives from
a body of evidence for a mechanically weak
asthenosphere and the intuitive idea that a low
viscosity layer would offer less resistance to plate
motion. The lack of mobile-lid plate tectonics on
Venus may therefore be associated with the lack
of a thin low-viscosity asthenosphere.
Here we use three-dimensional mantle convection
simulations to show that the thickness and
viscosity of a sublithospheric low-viscosity layer
have first-order control on determining the surface
manifestation of mantle convection, i.e. plate
tectonics. We demonstrate that the thickness and
viscosity of a low-viscosity channel directly
influence convective mantle stresses and
consequently control whether mantle convection
operates in mobile-lid (plate tectonic), stagnant-lid
(single plate planet) or episodic regimes
(Figure 1).
The tectonic style of a planet is governed by the
convective stress to which its surface is exposed.
Our results, as well as previous theoretical
predictions (Busse et al., 2006, Lenardic et al.,
2006) demonstrate that these stresses decrease
with asthenosphere viscosity and increase with
decreasing
low-viscosity
layer
thickness
(Figure 2).
Our results indicate that a thin low-viscosity
asthenosphere, as compared to a thicker upper
mantle of low viscosity, increases the stress at the
base of the lithosphere and, in turn, influences the
Pace of tectonic modes on Venus and Earth and atmospheric Argon: Höink, OʼNeill, Lenardic
(Figure 3). Stagnant-lid convection is the infiniteperiod limit, and mobile-lid convection is the case
of continuous overturn, i.e. the zero-period limit.
Fig 2: Surface yield stress versus nondimensional ratio of relative viscosity of the lowviscosity layer and cube of layer thickness,
demonstrate for low yield stresses that
increasing the thickness of the low-viscosity
layer can lead from mobile-lid tectonics to
stagnant-lid tectonics, and for high yield
stresses that decreasing the low-viscosity
channel thickness can lead from stagnant-lid
tectonics to mobile-lid tectonics.
global tectonic style of a terrestrial planet. The
strong dependence on thickness shows that the
long-standing concept that the asthenosphere
facilitates plate tectonics by "lubricating" the base
of plates is not strictly correct. More important is
the role it has on channelizing lateral mantle flow
and the associated effect on convective stress
levels.
The prediction that stress increases with
decreasing asthenospheric thickness provides a
step toward resolving the discrepancy between the
very low values of yield stress needed to generate
plate like behavior in previous mantle convection
simulations and the values determined from
laboratory experiments. Our results show that the
difference between a thin low viscosity layer
(asthenosphere) and a weak upper mantle is a
first-order one in terms of facilitating plate
tectonics, and provide insights into differences
between the Earth and its sister planet Venus.
The periodicity of overturn events in the episodic
regime is controlled by thickness and relative
viscosity of the sublithospheric low-viscosity layer
Fig 3: a) Nusselt number versus overturn times
for different simulations, in which only the nondimensional relative viscosity µA and thickness
dA of the low-viscosity layer varies. b) Map of
tectonic regimes spanned by period in overturn
3
times versus µA/dA . One overturn time for Earth
3
is 100 Ma. Note that for small µA/dA the period
is comparable with the time scale of secular
cooling, resulting in effective stagnant-lid
convection. Stagnant-lid convection is the
infinite-period limit, while mobile-lid convection,
the case of continuous overturn, is the zero
period limit.
Pace of tectonic modes on Venus and Earth and atmospheric Argon: Höink, OʼNeill, Lenardic
Atmospheric Argon
An important constraint on the tectonic evolution of
a planet through time comes from melting and
degassing histories, which can be provided by
40
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radiogenic Ar. Produced by the decay of K in
the mantle or crust, it behaves incompatibly during
melting, and is lost from the mantle during melting
events (Coltice et al., 2000). As it diffuses from the
crustal reservoir on a timescale of the order of
~1Gyr (Bender et al., 2008), and is reasonably
heavy and inert, it should be stable in the
atmospheres of Earth & Venus over the lifetimes
of these planets (Kaula, 1999), and provide
constraints on a planetʼs volcanic history.
Venusʼs atmospheric argon was measured by
three mass spectrometers and two gas
chromatographs during the Pioneer mission, and
the results are presented in von Zahn et al. (1983).
Kaula (1999) summarises these observations and
the potassium abundance of Venera mission
samples, and concludes that the atmospheric
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abundance of Ar of 1.4 +/- 0.46 e16kg indicates
that Venusʼ atmosphere possesses only ~24% of
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radiogenic Ar produced over its history. The
difference between Earthʼs atmosphere - which
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has approximately half of Earthʼs radiogenic Ar
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contingent - and Venusʼ atmospheric Ar cannot
be attributed to differences in atmospheric loss, as
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non-radiogenic (ie. primordial) Ar is ~80 times
more abundant in Venusʼ atmosphere than in
Earths (Kaula, 1999). Instead in has been
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suggested that the differences in Ar abundances
between Earth and Venus may in fact reflect
differences in the degassing/volcanic history of
these planets (Kaula, 1999), or fundamental
differences in mantle viscosity (Xie & Tackley,
2004).
Here we test the effect of a planetʼs tectonic
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regime on its Ar degassing history, by simulating
melting and degassing in 2D mantle convection
models incorporating mobile plates and nearsurface yielding. Three end member regimes are
expected: 1) a mobile lid regime, whereby the
lithosphere actively participates in convective
overturn. An example is plate tectonics on Earth.
2) A stagnant lid regime, whereby convection
occurs under a rigid, immobile lithosphere. Such a
regime has been postulated to apply to Mars. 3)
An episodic regime, which is essentially an
Fig 4: a) Melt production by different tectonic
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models versus model time. b) Ar degassing
from same models versus model time. Note the
axis had been clipped to compare models over
an appropriate time interval (ie. one episodic
overturn).
oscillation between stagnant and mobile regimes.
Here rapid pulses of subduction and tectonic
activity are interspersed by long periods of
quiescence. Such a regime has been proposed for
Venus
(Moresi
and
Solomatov,
1998).
The generation of melt in these models varies
significantly depending on the tectonic regime.
Melting in the stagnant lid regime is hindered by
the presence of a thick, immobile lithosphere,
which prevents upwelling mantle from reaching the
surface. However, the inability of such systems to
efficiently loose heat means they generally have
elevated mantle temperatures compared to their
mobile-lid
counterparts,
which
encourages
extensive deep melting in high-Ra upper mantle
models (ie. Ra>~1e7). In contrast, the ability of
upwelling mantle to rise nearly to the surface in
mobile-lid models generates extensive melt in lowRa (Ra<1e7) upper mantle models (Figure 4).
Pace of tectonic modes on Venus and Earth and atmospheric Argon: Höink, OʼNeill, Lenardic
However, and somewhat counter-intuitively, at
higher basal Ra, melting rates may in fact fall due
to lower internal temperatures. In the episodic
regime, hot mantle temperatures and rapid plate
velocities result in widespread melting during
overturn episodes; however, in quiescent periods
melting rates are noticably subdued.
The sensitivity of the degree and depth range of
melting to the internal temperatures suggests a fair
degree of sensitivity to the solidus used, and this is
found to be the case. Experimental results suggest
that the peridotite solidus follows a third-order
polynomial, and such curved solidii have
significantly lower temperatures at depth than their
linearized counterparts (eg. Hirschmann, 2000;
Jaques and Green, 1980). This acts to enhance
melting, especially for hotter, stagnant lid models.
However, large differences (several hundred
degrees) also occur between anhydrous and
hydrous peridotite solidii, resulting in major
differences between hydrated and dehydrated
planets (Mackwell et al., 1998).
For lithospheric thickness comparable to Earth
(50-100km for overturning oceanic-like plates), the
equivalent upper mantle Ra is around 1e7. For
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this value, the relative Ar degassing rate, based
on the difference in melt production between
regimes, is 20 times greater for mobile lid than
stagnant lid convection, and 1.8 times greater than
episodic convection. These ratios vary for different
Ra and internal heating rates. For higher Ra,
stagnant lid regimes can produce more melt than
mobile lid regimes, though this requires
unrealistically low lithospheric thicknesses in these
models. In addition, a significant difference in the
solidus is expected between Earth and Venus
(Hess and Head, 1990; Nimmo and McKenzie),
primarily to due differences in water content in the
mantle, which would tend to depress melting on
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Venus, and thus its Ar degassing rates.
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The ratio in Ar degassing rate of 1.8 between
mobile and episodic regimes is similar to the ratio
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of percentage degassed atmospheric Ar on the
two planets (~50% for Earth, ~24% for Venus),
and suggests an important tectonic control on
atmospheric degassing on these planets.
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