Experimental studies on the recovery process of deformed olivines and the mechanical state of the upper mantle

Abstract

Naturally deformed olivines in peridotites from Eifel, Horoman, Red Hill and San Carlos were annealed at 1300–1500°C under controlled oxygen fugacity for the purpose of clarifying the dislocation annihilation process. The decreasing rate of dislocation density was well approximated by the following equation: dρ dt = −10 2 exp{−110,000 (cal/mo)/RT}ρ 2 (cm −2 sec −1) where ρ is the dislocation density. Thus the half-life time of dislocation density in annealing process depends both on temperature and on initial dislocation density. The activation energy obtained is very similar to the value given by Kohlstedt and Goetze (1974) for the steady state creep of olivine. Two types of dislocation structures were observed. One is an equi-axed cellular network structure (cellular type) which probably formed in steady-state creep, and the other is a tangled structure (tangled type) probably formed in transient creep. Thus, if the annealing time is short enough and the dislocation structure is characteristic of steadystate creep (i.e., cellular type), then the differential stress acting within the upper mantle can be inferred from the dislocation density of the olivine in peridotite nodules. The annealing time is estimated for the olivines in peridotite nodules from Ichinomegata, northern Japan and the differential stress is estimated as 100 to 300 bars. The successive partial melting and crystallization process is inferred from coexistence of two types of olivines in peridotite.