Microstructures induced by the post-garnet transformation in mid-ocean ridge basalt (MORB)

Abstract:

The subduction of slabs is one of the key features of the Earth´s dynamics. Seismic studies revealed that slabs can sometimes even sink through the transition zone and penetrate into the lower mantle [1,2]. The introduction of dense basaltic material into the lower mantle is a potential source of chemical and seismic heterogeneities [3] and may affect the whole mantle convection [4]. While the phase relations and mineralogical transformations in subducted basaltic crust were studied over the past decades [5,6], little is known about the impact of these changes on the microstructure and hence, on the seismic signature of subducted basaltic crust in the mantle.

In this study, we provide the first experimental microstructure analysis of mid-ocean ridge basalt (MORB) to lower mantle pressures and particularly across the post-garnet transformation at ~25 GPa. We used synchrotron X-ray diffraction in a multigrain crystallography setup (MGC) [7] in a laser-heated diamond anvil cell (LH-DAC) to determine the microstructure in-situ at high temperature and high pressure. This is, to the best of our knowledge, the first application of the MGC technique to a complex multicomponent system that is an appropriate proxy for natural lithologies.

Our results show the evolution of grain positions, relative grain sizes, grain orientations and relative phase proportions of each individual component across the post-garnet transformation, including majoritic garnet and stishovite before the transition, and bridgmanite, CaSiO₃-perovskite, calcium-ferrite-type aluminous phase and stishovite after the transition. The effect of textures developed upon the transition on the seismic response of subducted MORB will be discussed.

[1]Fukao, Y., & Obayashi, M. (2013); [2]Goes, S., et al. (2017); [3]Kaneshima, S. & Helffrich, G. (2003); [4]Ballmer, M. et al. (2015); [5]Perrillat, J. P., et al. (2006); [6]Ricolleau, A., et al. (2010); [7]Sørensen, H. O., et al. (2012)