New type of crystallographic preferred orientation (CPO) of clinopyroxene in serpentinite

Abstract Diopside is a major clinopyroxene mineral present in the mantle peridotites and serpentinites. However, the crystallographic preferred orientation (CPO) of diopside and its deformation mechanism are poorly understood. Here, we present a new S1-type CPO diopside in deformed serpentinites from Monte San Petrone (Alpine Corsica, France). The new CPO is characterized by a strong alignment of the (100) poles subnormal and a girdle distribution of the [010] and [001] axes subparallel to the foliation. Orientation analysis showed that, during serpentinization, the (100) planes of diopside were topotactically related to those (001) of antigorite: (100)di // (001)atg. The dominant (001) basal glide of antigorite appears to cause the (100) poles of the surrounding diopsides to align parallel to the foliation, mainly through phase boundary sliding and grain rotation during dynamic recrystallization. The anomalous radial anisotropy of P-waves observed in the mantle wedges in south-western Japan and Alaska subduction zones can be attributed to the new CPO of diopside.