Shock-induced CO_2 loss from CaCO_3; implications for early planetary atmospheres

Abstract

We report new results of shock recovery experiments on single crystal calcite. Recovered samples are subjected to thermogravimetric analysis. This yields the maximum amount of post-shock CO_2, the decarbonization interval, ΔT, and the energy of association (or vaporization), ΔEV, for the removal of remaining CO_2 in shock-loaded calcite. Comparison of post-shock CO_2 with that initially present determines shock-induced CO_2 loss as a function of shock pressure. Incipient to complete CO_2 loss occurs over a pressure range of ∼ 10to∼ 70GPa. The latter pressure should be considered a lower bound. Comparable to results on hydrous minerals, ΔT and ΔEV decrease systematically with increasing shock pressure. This indicates that shock loading leads to both the removal of structural volatiles and weakening of bonds between the volatile species and remainder of the crystal lattice. Optical and scanning electron microscopy (SEM) reveal structural changes, which are related to the shock-loading. Comparable to previous findings on shocked antigorite is the occurrence of dark, diffuse areas, which can be resolved as highly vesicular areas as observed with a scanning electron microscope. These areas are interpreted as representing quenched partial melts, into which shock-released CO_2 has been injected. The experimental results are used to place bonds on models of impact production of CO_2 during accretion of the terrestrial planets.

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