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Pace of tectonic modes on Venus and 2Earth and atmospheric Argon 1 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 40 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 40 abundance of Ar of 1.4 +/- 0.46 e16kg indicates that Venusʼ atmosphere possesses only ~24% of 40 radiogenic Ar produced over its history. The difference between Earthʼs atmosphere - which 40 has approximately half of Earthʼs radiogenic Ar 40 contingent - and Venusʼ atmospheric Ar cannot be attributed to differences in atmospheric loss, as 36 non-radiogenic (ie. primordial) Ar is ~80 times more abundant in Venusʼ atmosphere than in Earths (Kaula, 1999). Instead in has been 40 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 40 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 40 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 40 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 40 Venus, and thus its Ar degassing rates. 40 The ratio in Ar degassing rate of 1.8 between mobile and episodic regimes is similar to the ratio 40 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. References Lenardic, A., Richards, M.A., Busse, F.-H. Depth-dependent rheology and the horizontal length scale of mantle convection. J. Geophys. Res. 111 (1996) Busse, F-.H., Richards, M.A., Lenardic, A. A simple model of high Prandtl and high Rayleigh number convection bounded by thin low-viscosity layers. Geophys. J. Int., 164 (1996), 160-167 Moresi. L., & Solomatov, VS., Mantle convection with a brittle lithosphere: thoughts on the global tectonic styles of the Earth and Venus, Geophys. J. Int. 133 (1998), 669-682. 40 40 Coltice, N., Albarede, F., Gillet, Ph., K- Ar constraints on recycling continental crust in the mantle, Science 288 (2000), 845-847. Bender, ML., Barnett, B., Dreyfus, G., Jouzel, J. & Porcelli, D., The contemporary degasing rate of 40 Ar from the solid Earth, PNAS 105 (2008), 82328237. Kaula, WM., Constraints on Venus evolution from radiogenic argon, Icarus 139 (1999), 32-39. von Zahn, U., S. Kumar, H. Niemann, and R. Prinn 1983. Composition of the Venus atmosphere. In Venus (D. M. Hunten, L. Colin, T. M. Donahue, and V. I. Moroz, Eds.), pp. 299–430. Univ. of Arizona Press, Tucson. Xie, S., & Tackley, P., Evolution of helium and argon isotopes in a convecting mantle, Phys. Earth Planet. Int. 146 (2004), 417-439. Hirschmann, MM.,Mantle solidus: experimental constraints and the effects of peridotite composition, G3 1, (2000), doi:10.1029/2000GC 000070. Jaques, AL., Green, DH., Anhydrous melting of peridotite at 0–15 Kb pressure and the genesis of tholeiitic basalts, Contributions to Mineralogy and Petrology, 73, (1980), 287-310. Mackwell, SJ., Zimmerman, ME., Kohlstedt, DL., High-temperature deformation of dry diabase with application to tectonics on Venus, JGR 103(B1) (1991), 975-984. Hess, PC., Head, JW., Derivation of primary magmas and melting of crustal materials on Venus: Some preliminary petrogenetic consideration, Earth, Moon and Planets, 50-51, (1990), 57-80. Nimmo, F., McKenzie, D., Volcanism and tectonics on Venus, Ann. Rev. Earth Planet. Sci. 26 (1998), 23-51