Plastic flow of omphacite in eclogites at temperatures below 500°C – implications for interplate coupling in subduction zones

Abstract

In subduction zones oceanic crust of the downgoing plate presumably forms a continuous interlayer between the upper and the lower plate. During subduction, the basaltic material is transformed to eclogite. Thus, the flow strength of eclogite must pose an upper bound to shear stresses across the plate boundary for the deeper levels of subduction zones. Up to now, experimental flow laws for wet diopside have been applied to predict the strength of eclogites. However, based on experimental calibration the material would be essentially undeformable by dislocation creep at 650 C. Even at 700 C a strain rate of 10 ±14 s ±1 would still imply a differential stress of ca. 150 MPa. Omphacite microstructures in eclogites from the Piemonte Zone, Western Alps, reveal shape-preferred orientation, subgrains and sutured high-angle grain boundaries due to migration recrystallization, and a pronounced crystallographic preferred orientation. These textures indicate that the omphacite was deformed by dislocation creep. In contrast, garnet forms rigid inclusions within the weaker pyroxene matrix. Fe±Mg exchange thermometry for garnet omphacite pairs indicates temperatures of 46550 C (Valle di Locana) and 47550 C (Vallone di Saint Marcel), for a pressure of 1.5 GPa. These results are not consistent with the predictions based on experimental flow laws for diopside. Our data suggest that the flow strength of sodic pyroxene is significantly lower than that of diopside. This finding is consistent with the homologous temperature concept, with the melting temperature of jadeite at 3 GPa being approximately 350 C lower than that of diopside. In addition, dynamic migration recrystallization of omphacite in the investigated samples can be linked to small chemical changes. This indicates that mobile grain boundaries formed an effective pathway for the exchange of ions and that this exchange was fast compared to the rate of boundary migration. Thus, the combined reduction of stored strain energy and chemical free energy provided the driving force for recrystallization. The potential effects of these chemical changes on flow strength remain to be explored.

Microstructures and crystallographic preferred orientations of omphacite in Alpine eclogites: implications for the exhumation of (ultra-) high-pressure units

“…1 and 5), by Philippot and van Roermund (1992), Mauler (2000) Mauler et al (2000), and Bascou et al (2001) from the Monte Viso ophiolite (Piemont-Ligurian origin) (Figs. 1 and 4) immediately above the Dora Maira Massif, and by Piepenbreier and Sto¨ckhert (2001) from the Zermatt-Saas ophiolite (Piemont-Ligurian origin) (Fig. 1) immediately above the Monte Rosa Nappe.…”

“…In L-type CPOs, the c-axes of omphacite show well defined clusters near the stretching lineation, the b-axes a girdle distribution perpendicular to the foliation plane. Both fabrics have been described from several (U)HP units by numerous authors (e.g., Buatier et al, 1991;Boundy et al, 1992;Philippot and van Roermund, 1992;Godard and van Roermund, 1995;Abalos, 1997;Mauler et al, 1998Mauler et al, , 2000Mauler et al, , 2001Mauler, 2000;Jin et al, 2001;Piepenbreier and Sto¨ckhert, 2001;Bascou et al, 2001Bascou et al, , 2002Brenker et al, 2002;Ji et al, 2003). Moreover, L-and transitional LS-type fabrics have been simulated by Bascou et al (2001Bascou et al ( , 2002 by viscoplastic self-consistent modeling, both for simple shear and pure shear, and transtension as well.…”

“…In eclogites, strain is mainly accommodated by omphacite. While omphacite develops a shape and Crystallographic Preferred Orientation (CPO, texture) (e.g., Godard and van Roermund, 1995;Abalos, 1997;Mauler et al, 1998Mauler et al, , 2001Jin et al, 2001;Piepenbreier and Sto¨ckhert, 2001;Bascou et al, 2001Bascou et al, , 2002Brenker et al, 2002), garnet is mostly unaffected by deformation (Brenker et al, 2002).…”

Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)

“…On the other hand, phase segregation and healed fractures also point to a key contribution to small-scale deformation processes from dissolution-precipitation, in addition to creep. Dissolution-precipitation may indeed be an important deformation mechanism within omphacite-rich domains, which are particularly subject to channelized fluid circulation (Piepenbreier and Stöckhert, 2001), or within garnets (as shown by the propagation of crack-tips;…”

Numerical study of the origin and stability of chemically distinct reservoirs deep in Earth's mantle

“…We set f to a constant value of 1 for F < 0.005 and to 10 for F > 0.05. For eclogite we use f = 0.1 because of the lower flow strength of omphacite [Piepenbreier and Stöckhert, 2001]. For the partial melting of basaltic crust (model run I-b, Table 4) we apply the solidus and liquidus of a hydrated tholeiite using data from Green [1982].…”