Slip, Stress, and Lattice Preferred Orientation in Ultramafic Xenoliths From Cerro Mercedes, Costa Rica

Ultramafic xenoliths are abundant in Quaternary lavas from Cerro Mercedes, Costa Rica, a small back-arc cone roughly 70 km behind the Central American volcanic front. These rare subduction zone xenoliths offer a glimpse into the deformation conditions in the mantle wedge behind an active subduction zone. Examination of more than 75 xenoliths reveals a lack of garnet, suggesting an origin depth of approximately 60-70 km. These samples are mainly lherzolites and harzburgites with some dunites and pyroxenites. The xenoliths examined thus far are coarse grained and have either a protogranular or porphyroclastic texture. The presence of both of these textures implies varying amounts of deformation between samples. In addition, many xenoliths have deformation textures such as undulose and kinked olivine grains and recrystallization around grain boundaries. By comparing the orientation of olivine crystallographic axes and kink-band boundaries on a Universal Stage Microscope, we have determined that slip is dominantly along the [100] crystallographic axis in the samples, with both [100](010) and [100](001) slip systems present. These systems match slip observed in other upper mantle samples and samples experimentally deformed at mantle conditions, indicating that the deformation features seen in the xenoliths were created at depth. Stress estimates taken from the recrystallized grain size and the size of subboundaries created by dislocations match well with published values for other xenoliths, reinforcing our interpretation that the deformation seen in these xenoliths occurred in the upper mantle. Currently, we are analyzing the lattice-preferred orientations of these xenoliths. Many xenoliths have strongly aligned kink band boundaries, and the alignment of these lineations implies that the xenoliths have a strong preferred orientation. The strength of the axis alignment, along with the relation between the crystallographic axes and the lineations, will help to reveal the conditions in the mantle wedge in Costa Rica. Since plate motions have not changed appreciably since the eruption of the xenoliths approximately 1 My ago, the degree of anisotropy and type of preferred orientation in the xenoliths likely reflects current conditions behind the Central American Arc.