Interplay between Fluid Extraction Mechanisms and Antigorite Dehydration Reactions (Val Malenco, Italian Alps)

Abstract

We investigate the feedback between antigorite dehydration reactions and fluid extraction mechanisms through the analysis of field-scale structures and microtextures formed by antigorite dehydration reactions producing olivine and talc in the Bergell intrusion contact aureole at Alpe Zocca (Malenco Unit, Northern Italy). The reactions, which resulted in the replacement of serpentinites by metaperidotites, occurred under quasi-static conditions. The main reaction front, which defines the equilibrium isograd, is an ∼150 m wide zone composed of variably reacted rocks with an irregular distribution at scales ranging from a few centimeters to a few meters. Veins composed of the prograde mineral assemblage occur downstream (<100 m) of this front. They are often surrounded by centimeter- to decimeter-scale dehydration reaction zones that propagate into the serpentinite wall-rock. Olivine in the metaperidotites and partially reacted serpentinites has a crystallographic preferred orientation (CPO) correlated with the antigorite CPO, with [010]Ol axes parallel to [001]Atg. This CPO is accompanied by a shape-preferred orientation (SPO) that marks the foliation in both rock types. Olivine crystals in Ol–Tlc dehydration veins also have SPO and CPO that define a jackstraw texture within the plane of the vein. They are elongated parallel to [001] within the vein plane and have their [010]Ol axes normal to the vein. We interpret the olivine–talc assemblage in the veins as resulting from dehydration reactions at lower temperature than the equilibrium conditions owing to fluid extraction from the wall-rock into the veins. The jackstraw texture indicates fast kinetics, with the crystal orientation controlled by anisotropic growth under a fluid pressure gradient. We interpret the foliated metaperidotites as formed at near equilibrium conditions, with pervasive fluid extraction from the metaperidotite by viscous metamorphic compaction. Olivine CPO in these rocks may result from topotaxy, oriented growth in the presence of an anisotropic (foliation-controlled) fluid flow, and/or solid reorientation of the anisometric olivine crystals accommodated by the deformation of the weaker talc (± chlorite) matrix during compaction.

Hydrostatic Dehydration Fabrics of Antigorite at High Pressure and High Temperature: Implications for Trench Parallel Seismic Anisotropy at Convergent Plate Boundaries

“…However, these deformation mechanisms can be excluded in our experimental samples because the type-C like olivine CPOs are formed under hydrostatic conditions. 11a) in natural meta-peridotites (Clément et al, 2019;Nagaya et al, 2014), or by anisotropic growth with fast-growing axis parallel to the [001] ol in the jackstraw-textured veins (Clément et al, 2019). The (010)[h0l] olivine CPO has been suggested to form by topotactic growth of [010] ol //[001] atg (Clément et al, 2019 (Dilissen et al, 2018).…”

“…11a) in natural meta-peridotites (Clément et al, 2019;Nagaya et al, 2014), or by anisotropic growth with fast-growing axis parallel to the [001] ol in the jackstraw-textured veins (Clément et al, 2019). The (010)[h0l] olivine CPO has been suggested to form by topotactic growth of [010] ol //[001] atg (Clément et al, 2019 (Dilissen et al, 2018). Padrón-Navarta et al (2015) reported that the topotactic growth of [100] ol,opx //[001] atg induced a strong concentration of the [100] ol normal to the former foliation and a girdle distribution of the other two olivine crystallographic axes along the former foliation plane in both natural samples (Figure 11b) and the dehydration experiments on foliated antigorite (Figure 11c).…”

“…The [010] ol axes show weak concentrations normal to the foliation in "sealed" experiments and parallel to the lineation in "unsealed" experiments, respectively (Figures 8 and 9f). Normally, olivine displays an anisometric grain shape when it grows in the fluid or melts, characterized by a flat (010) plane or a short [010] axis due to the slow growth rate along this axis (Clément et al, 2019;Miyazaki et al, 2013;Wen et al, 2018).…”

“…Crystallographic preferred orientations (CPOs) of prograde minerals (olivine and/or enstatite) after antigorite dehydration have been documented in both dehydration experiments (Padrón‐Navarta et al., 2015) and natural samples (Clément et al., 2019; Dilissen et al., 2018; Nagaya et al., 2014; Padrón‐Navarta et al., 2010, 2015). The studies on natural samples revealed a topotactic relationship between antigorite and its dehydration products, that is, [100] ol //[100] atg , [010] ol //[001] atg , [001] ol //[010] atg (Dilissen et al., 2018; Nagaya et al., 2014) and [100] opx //[001] atg , [001] opx //[100] atg , [010] opx //[010] atg (Dilissen et al., 2018).…”

Time-resolved grain-scale 3D imaging of hydrofracturing in halite layers induced by gypsum dehydration and pore fluid pressure buildup

“…By using gypsum dehydration as an analogue for the dehydration of serpentine minerals (cf. Leclere et al, 2016Leclere et al, , 2018, the observations may also elucidate metamorphic developments in subduction zones that are often related to intermediate-depth seismicity (e.g., Miller et al, 2003, Jung et al, 2004, Plümper et al, 2017, Dilissen et al, 2018, Clément et al, 2019.…”