TTG formation via sluggish, drip-like subduction on the early Earth

Abstract:

The tectonic processes responsible for the formation of the early Earth’s felsic crust can inform the global regime of mantle convection operating at this time. Early Earth felsic crust is predominantly composed of tonalite-trondhjemite-granodiorte (TTG) and other similar felsic rocks. These rocks form by melting hydrated basalt at depths on the order of 30-50 km and temperatures ranging from ~800-1000° C. However, there is considerable debate about whether early felsic crust formed in subduction zone or intraplate settings. Crucially, hydrated basalt must reach the conditions where it melts before experiencing metamorphic dehydration and losing its water. Regional scale models of subduction have found that at the high mantle temperatures expected in the Archean, slabs sink quickly and steeply through the mantle, such that they dehydrate before experiencing melting.

Here, global scale models of mantle convection with grain damage, a mechanism for generating mobile lid convection via grain size reduction, at early Earth thermal conditions are presented. These models show that a sluggish, drip-like style of subduction emerges with increasing mantle temperature and internal heating rate, due to periodic tearing, or necking, of the slab. This sluggish style of convection leads to significant heating at shallow depths. I use scaling laws developed from the numerical models to assess whether, and under what conditions, melting of the downgoing crust can occur and form TTGs. Critical factors are the slab sinking speed, and the thickness of the overriding lithosphere, which controls the depth at which the slab interacts with the hot mantle wedge. I find that for slow enough subduction speeds with a thin enough overriding lithosphere, the subducting crust can melt before experiencing dehydration.