Crystal bending, subgrain boundary development, and recrystallization in orthopyroxene during granulite-facies deformation

Abstract

A prominent feature of a granulite-facies shear zone from the Hidaka Main Zone (Japan) is the folding of orthopyroxene (opx) porphyroclasts. Dislocation density estimated by transmission electron microscope (TEM) and chemical etching in homogeneously folded domains is too low to account for the amplitude of crystallographic bending, leading us to propose a model similar to “flexural slip” folding, where folded layers are micrometer-wide opx layers between thin planar clinopyroxene (cpx) exsolutions. Extension (compression) in the extrados (intrados) of the folded layer is accommodated by dislocations at the cpx–opx interfaces. Alternatively to distributed deformation, crystal bending also localizes in grain boundaries (GBs), mostly oriented close to the (001) plane and with various misorientation angles but misorientation axes consistently close to the b-axis. For misorientation up to a few degrees, GBs were imaged as tilt walls composed of regularly spaced (100)[001] dislocations. For misorientation angles of 7°, individual dislocations are no longer visible, but high-resolution TEM (HRTEM) observation showed the partial continuity of opx tetrahedral chains through the boundary. For 21° misorientation, the two adjacent crystals are completely separated by an incoherent boundary. In spite of these atomic-scale variations, all GBs share orientation and rotation axis, suggesting a continuous process of misorientation by symmetric incorporation of (100)[001] dislocations. In addition to the dominant GBs perpendicular to the (100) plane, boundaries at low angle with (100) planes are also present, incorporating dislocations with a component of Burgers vector along the a-axis. The two kinds of boundaries combine to delimit subgrains, which progressively rotate with respect to host grains around the b-axis, eventually leading to recrystallization of large porphyroclasts.

Appendix

We estimated the length of opx and cpx unit cell c parameter for T = 800°C and P = 0.7 GPa as:

$$ c = c_{0} \left( {1 + 800\alpha_{\text{T}} + 0.7\alpha_{\text{P}} } \right) $$

where c ₀ is the value for ambient P and T and α_T and α_P express the thermal dilatation and compressibility coefficients. For opx, $\alpha_{\text{T}} \approx 0.16 \times 10^{ - 4} /^{ \circ } {\text{C}}$, irrespective of its composition (Frisillo and Buljan 1972; Sueno et al. 1976) and $\alpha_{\text{P}} \approx - 3.5 \times 10^{ - 3} /{\text{GPa}}$ (Angel and Hugh-Jones 1994 for enstatite). For cpx, $\alpha_{\text{T}} \approx 0.06 \times 10^{ - 4} /^{ \circ } {\text{C}}$ (Cameron et al. 1973 for either hedenbergite of diopside) and $\alpha_{\text{P}} \approx - 2.8 \times 10^{ - 3} /{\text{GPa}}$ (Levien and Prewitt 1981 for diopside).

In other words, pressure-dependent variations are similar for cpx and opx, while thermal expansion of opx is much larger than in cpx. In contrast, unit cell parameter c ₀ is consistently larger in cpx (Levien and Prewitt 1981; Salviulo, et al. 1997) than in opx (Angel and Hugh-Jones 1994; Smyth 1973; Sueno et al. 1976). To take into account the large compositional dependence of c ₀, we considered compositions as close as possible to ours (Raimbourg et al. 2008): For opx, Smyth (1973) reported $c_{0}^{\text{opx}} = 5.232\;{\text{\AA}}$, while for cpx, Salviulo et al. (1997) reported $c_{0}^{\text{cpx1}} = 5.26\;{\text{\AA}}$ and $c_{0}^{\text{cpx2}} = 5.28\;{\text{\AA}}$ for two samples of similar composition (VG3187 and VG3185Pc).

When considering the first value $c_{0}^{\text{cpx1}}$, parameter c has roughly the same length in opx and in cpx for T = 800°C and P = 0.7 GPa, i.e., no mismatch dislocations are required. For the same P–T conditions, when considering the second value $c_{0}^{\text{cpx2}}$, $c^{\text{cpx2}}$ is larger than $c^{\text{opx}}$ by ~0.2%, which implies dislocations spaced by 250 nm.

These calculations tend to show that $c^{\text{cpx}} > c^{\text{opx}}$ at high P and T, requiring emplacement of mismatch dislocations along the interface. Our own HRTEM observations of the polarity of regularly spaced dislocations along the interface of a lamellae in a domain devoid of bending show similarly that $c^{\text{cpx}} > c^{\text{opx}}$. On the other hand, the large uncertainties in unit cell parameter at ambient conditions, of the order of the pressure- and temperature-dependent variations, preclude to precisely assess the density of such mismatch dislocations and we retain therefore only the possible magnitude of dislocation spacing, as of the order of a few hundreds of nanometers.