Natural olivine crystal-fabrics in the western Pacific convergence region: A new method to identify fabric type
Résumé
Crystallographic preferred orientations (CPOs) of olivine within natural peridotites are commonly depicted by pole figures for the [100], [010], and [001] axes, and they can be categorized into five well-known fabric types: A, B, C, D, and E. These fabric types can be related to olivine slip systems: A with (010)[100], B with (010)[001], C with (001)[001], D with {0kl}[100], and E with (001)[100]. In addition, an AG type is commonly found in nature, but its origin is controversial, and could involve several contributing factors such as complex slip systems, non-coaxial strain types, or the effects of melt during plastic flow. In this paper we present all of our olivine fabric database published previously as well as new data mostly from ocean floor, mainly for the convergent margin of the western Pacific region, and we introduce a new index named Fabric-Index Angle (FIA), which is related to the P-wave property of a single olivine crystal. The FIA can be used as an alternative to classifying the CPOs into the six fabric types, and it allows a set of CPOs to be expressed as a single angle in a range between −90° and 180°. The six olivine fabric types have unique values of FIA: 63° for A type, −28° for B type, 158° for C type, 90° for D type, 106° for E type, and 0° for AG type. We divided our olivine database into five tectonic groups: ophiolites, ridge peridotites, trench peridotites, peridotite xenoliths, and peridotites enclosed in high-pressure metamorphic rocks. Our results show that although our database is not yet large enough (except for trench peridotites) to define the characteristics of the five tectonic groups, the natural olivine fabrics vary in their range of FIA: 0° to 150° for the ophiolites, 40° to 80° for the ridge peridotites, −40° to 100° for the trench peridotites, 0° to 100° for the peridotite xenoliths, and −40° to 10° for the peridotites enclosed in high-pressure metamorphic rocks. The trench peridotites show a statistically unimodal distribution of FIA consisting of the high peak equivalent of the A type, but with some FIAs close to the AG and D types. The variations in the olivine fabrics in the trench peridotites could result from variations in deformation within the supra-subduction uppermost mantle, possibly related to evolution of the mantle since the subduction initiation of the Pacific plate.