Numerical models of magmatism in convecting mantle with temperature‐dependent viscosity and their implications for Venus and Earth

Abstract

Abstract. Numerical models are presented for magmatism in a convecting mantle that contains internal heat source. Mantle convection is modeled by a convection of binary eutectic material with Newtonian temperature-dependent rheology driven by thermal, compositional, and melt buoyancy as well as the buoyancy from the "660-km" phase transitions. Mantle magmatism is modeled by a gravitationally induced permeable flow of magma generated by pressure release melting through matrix. Magma mostly has the eutectic composition, that is, basaltic composition. The numerical models suggest that magmatism and mantle convection strongly influence each other to form a coupled system when the internal heating is sufficiently strong. Episodic magmatism takes place to chemically differentiate the mantle; basaltic materials occupy deeper part of the lower mantle, while magma residue occupies the uppermost mantle. Compositional buoyancy of the differentiated materials makes mantle convection sluggish except when magmatic activities take place. The style of magmatism and mantle convection depends on the viscosity contrast across the cold thermal boundary layer (TBL) along the top surface boundary. When the viscosity contrast is large, the fluid in the coldest part of top TBL becomes a stagnant lid and a conspicuous lateral heterogeneity arises in the upper mantle. Magmatism is induced by hot uprising diapirs originating in the top of the lower mantle. The diapirs induce convective circulation within the upper mantle when the lower mantle is rather cold but induce flushing event, that is, a massive flow across the 660-km phase boundary and the resulting vigorous magmatism, when the lower mantle becomes sufficiently hot owing to the internal heating. When the viscosity contrast is moderate, the fluid in the coldest part of top TBL behaves as a moving lid. Both the moving lid and hot uprising diapirs induce magmatic activity; the magmatic activity due to a moving lid resembles ridge volcanism. The mantle of Venus and that of the early Earth are suggested to have been on the regime modeled here, and the magmatic activity by flushing event is compared to the magmatism of volcanic plain formation on Venus.

Mantle Geochemical Geodynamics

“…However, while the possibility of primordial layering exists, most of the observed heterogeneity appears to be related to recycled material, so several models have studied the evolution of a mantle in which heterogeneity is continuously introduced over time, generally to simulate the differentiated components in subducted slabs (Gurnis, 1986a;Ogawa, 1988Ogawa, , 1993Ogawa, , 1997Ogawa, , 2000bOgawa, , 2003Christensen, 1989b;Christensen and Hofmann, 1994;Kameyama et al, 1996;Ogawa and Nakamura, 1998;Walzer and Hendel, 1999;Davies, 2002;Tackley and Xie, 2002;Ogawa, 2003;Nakagawa and Tackley, 2004b;van Thienen et al, 2004;Xie and Tackley, 2004b;Tackley, 2005a, 2005b;Tackley et al, 2005). However, while the possibility of primordial layering exists, most of the observed heterogeneity appears to be related to recycled material, so several models have studied the evolution of a mantle in which heterogeneity is continuously introduced over time, generally to simulate the differentiated components in subducted slabs (Gurnis, 1986a;Ogawa, 1988Ogawa, , 1993Ogawa, , 1997Ogawa, , 2000bOgawa, , 2003Christensen, 1989b;Christensen and Hofmann, 1994;Kameyama et al, 1996;Ogawa and Nakamura, 1998;Walzer and Hendel, 1999;Davies, 2002;Tackley and Xie, 2002;Ogawa, 2003;Nakagawa and Tackley, 2004b;…”

Rheological and petrological implications for a stagnant lid regime on Venus

“…Some authors regard this regime as either complementary to episodic global resurfacing (Fowler and O'Brien, 1996;Sleep, 2000) or as a change in convective regime following the last global resurfacing event (Reese et al, 1999). Whilst some lateral movements are expected in response to large-scale convection of the mantle (Grimm, 1994), such a lithosphere is not mobile in the terrestrial sense and is instead dominated by discrete small-scale plumes (Nimmo and McKenzie, 1998;Ogawa, 2000).…”

Water, Life, and Planetary Geodynamical Evolution

“…The latter data suggest an elastic lithosphere of up to 100 km thickness and a crust of 90 km thickness. The necessity of removing the heat in the absence of plate tectonics leads to episodic scenarios as have been proposed by Turcotte (1993), Turcotte (1995), Turcotte et al (1999), Reese et al (1999), andOgawa (2000). Some authors (Turcotte 1995;Phillips and Hansen 1998;Nimmo 2002;and most recently Van Thienen et al 2005) suggest that present Venus may actually be heating up.…”

“…The suggestion prompted a number of papers that speculated on an episodic thermal history of Venus with pulses of global activity followed by epochs of relative tranquillity like the present one (e.g. Turcotte 1995;Phillips and Hansen 1998;Reese et al 1999;Ogawa 2000;Nimmo 2002). The early conclusions from the cratering record have been challenged by e.g.…”