Program > Browse abstracts by speaker > Hammouda Tahar

Carbonatites are stable only above ca. 2 GPa in the mantle (ca. 60 km depth) because of their dolomitic (CaMg(CO3)2) composition [1]. At the crossing of the carbonate ledge (at ca. 60 km) the sudden release of CO2 has the potential to fracture the wall rocks, inducing seismicity and high-energy eruptions.

We have performed analog experiments to investigate the consequences of the forceful release of CO2 and its fracturing potential, during controlled decompression.

The experiments were performed in the multianvil apparatus (MA) installed on 13-BM-D beamline of GSECARS facility at the Advances Photon Source [2] and in a Paris-Edinburgh (PE) press for combined AE and high-pressure x-ray microtomography (HPXMT) [3]. In the MA press, the transducers were positioned at the back of the six anvils, in order to permit acoustic emission location and characterize focal mechanisms. In the PE we positioned 2 transducers along the vertical axis of the press and attached 4 transducers on the lower PE anvil, slightly off the sample plane. A combination with x-ray diffraction is necessary to verify that the emissions coincide with transformations occurring in the sample.

We performed several runs on systems with pure CO2 or mixed H2O-CO2 as a fluid. In all cases, acoustic emissions were detected at the crossing of the carbonate ledge. Focal mechanism analysis confirms that an explosive component is present, confirming the potential for carbon-rich magmas to initiate fracturing at mantle P-T conditions. X-ray tomography performed in situ at high pressure and temperature confirms the formation and growth of bubbles, at the crossing of the carbonate ledge.

The eruption of CO2-rich magmas such as carbonatites and kimberlites have long been suspected to be very deeply rooted. This work documents the fracturing potential of these magmas at mantle depths.