Natural olivine crystal-fabrics in the western Pacific convergent region

Abstract:

Crystallographic preferred orientations (CPOs) within a natural peridotite commonly consist three pole figures for [100], [010] and [001] axes and are categorized into the well-known five olivine fabric types: A, B, C, D and E, which are easily connected with olivine slip systems: A as (010)[100], B as (010)[001], C as (001)[001], D as {0kl}[100] and E as (001)[100]. The five fabric types have been discussed on flow stress, water contents and pressure effect in mantle. In addition, AG type has also been proposed in recognition of its common occurrence in nature. The development of AG-type is not clear and could require some factors such as complex slip systems, non-coaxial strain types or the effect of melt during plastic flow. Here, we present our olivine fabric database mainly for the convergent margin in the western Pacific region. We introduce a new index named fabric-index angle (FIA) related to P-wave property of a single olivine crystal instead of a tentative classification of CPOs into the six fabric types, so that a set of CPOs can be expressed as a single angle in a range between -90° and 180°. The six olivine fabric types can have unique 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 result shows that although our database is not yet large enough except the trench peridotites to define the characteristic of the five tectonic groups, the natural olivine fabrics vary in a range of the 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 statistically bimodal distribution of FIA consisting of the high peak equivalent to A type and the low peak of FIA close to AG type. The variation of the olivine fabrics in the trench peridotites could result from variation of strain types in the supra-subduction uppermost mantle related to the mantle evolution during the subduction initiation of the Pacific plate.