Secondary CO2 fluid inclusion arrays in peridotite xenoliths from basalt are formed by the healing of CO2-filled microcracks. We have produced microcracks and deformation textures comparable to those in xenoliths by deforming peridotite in the presence of excess CO2 fluid at constant strain rate of 10-5 s-1, a confirming pressure of 1.0--1.5 GPa, and a temperature of 800¿--1050 ¿C. Plastic and microclastic deformation textures coexist, and healed microcracks extending out of (100) and (011) olivine deformation lamellae have the crystallographic orientation of microcracks nucleated on pileups of (100)<010> and (011)<100> edge dislocations. Healed microcracks in olivine grains without deformation lamellae tend to be in these orientations or in the orientation of microcracks nucleated on pileups of (110)<001> and (010)<001> edge dislocations. The same crystallographic orientation of healed microcracks is present in peridotite xenoliths from basalt.

The steady flow stress data of the experiments without added CO2 fluid fit a power law with a stress exponent n of 3 and a thermal activation coefficient of 410¿50 kJ mol-1. The maximum stress that can be supported by samples with added CO2 fluid is 100--300 MPa less than the flow stress of samples without added CO2 fluid. The reduction in mechanical strength correlates with an increase in the concentration of intragranular healed microcracks and an increase in the proportion of healed microcracks inclined to the maximum compressive stress direction. Strength and microcrack characteristics of experiments with CO2 performed at 1.5 GPa are similar to experiments without CO2 at 1.0 GPa. The dependence of strength and microcrack concentration on confining pressure suggests that the principal effect of the CO2 fluid is to reduce the effective confining pressure and to promote microclastic deformation mechanisms. The similarity of experimental textures to xenoliths suggests that CO2 fluid may produce semibrittle mechanical behavior in the regions of the lower lithosphere where xenoliths originate. ¿ American Geophysical Union 1987