Abstract
Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth’s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flemsøya in the Nordøyane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0–2.5%, max. AVs = 0.6–2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist. Furthermore, we found that amphibole CPOs were the main contributors to the higher seismic anisotropies in some amphibole-rich samples. The average seismic velocities of Flem eclogite were greatly affected by the relative volume proportions of omphacite and amphibole. The Vp (8.00–8.33 km/s) and Vs (4.55–4.72 km/s) were remarkably larger than the hydrous-phase-bearing eclogite, blueschist, and gabbro, but lower than dry eclogite and peridotite. The Vp/Vs ratio was almost constant (avg. ≈ 1.765) among Flem eclogite, slightly larger than olivine-free dry eclogite, but similar to peridotite, indicating that an abundance of olivine is the source of their high Vp/Vs ratios. The Vp/Vs ratios of Flem eclogite were also higher than other (non-)retrograded eclogite and significantly lower than those of gabbro. The seismic features derived from the Flem eclogite can thus be used to distinguish olivine-rich eclogite from other common rock types (especially gabbro) in the deep continental crust or subduction channel when high-resolution seismic wave data are available.
Highlights
Eclogite is a unique high-pressure to ultrahigh-pressure (HP–UHP) metamorphic rock that commonly originates from the subduction of an oceanic or continental crust or derives from the thickening of an orogenic crust during a continental collision [1,2,3,4]
Based on the field occurrences and deformation microstructures, the Flem eclogite could be divided into massive eclogite (MEC) and foliated eclogite (FEC)
To calculate the seismic property of the MEC and FEC, the elastic stiffness, crystal-preferred orientation (CPO), densities, and volume proportions of their constituent minerals are required before inserting them into the Christoffel equation to solve for the seismic velocities (i.e., Vp, Vs1, and Vs2) along every direction in 3D space [48]
Summary
Eclogite is a unique high-pressure to ultrahigh-pressure (HP–UHP) metamorphic rock that commonly originates from the subduction of an oceanic or continental crust or derives from the thickening of an orogenic crust during a continental collision [1,2,3,4]. The eclogite in the Flem Gabbro from Flemsøya in the Western Gneiss Region (WGR), Norway, is exceptional due to its peculiar mineral assemblage of olivine and orthopyroxene, which were inherited from an olivine gabbro protolith [8,33] This eclogite body may once have been situated in a continental subduction channel (the interface between the subducted slab and the wedge of an overlying crust and mantle) or a continental root zone (around Moho) owing to continental subduction and collision [2]. It can provide a valuable opportunity to examine the seismic properties of this rare type of eclogite and its implications for the detectability of eclogite bodies in the deep crust
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