Water in quartz? - A comparison of naturally and experimentally deformed crystals

In order to study the influence of water on the deformation of quartz, a series of high PT experiments (Pc up to 1500 MPa and T up to 900°C ) were carried out in a solid medium Griggs apparatus using a quartz single crystal containing a large number of fluid inclusions. FTIR spectroscopy was used to determine water contents. In the undeformed material, the H2O rich fluid inclusions show a large range in size (50 μm < d < 700 μm) and an extremely heterogeneous spatial distribution. Adjacent to the fluid inclusions the crystal is essentially dry (< 100 H/10^6 Si). The absorption spectra show no evidence for intra-crystalline H2O. H2O is only detected in the fluid inclusions (broad absorption band indicating molecular water). When samples were being brought up to experimental conditions, P and T remained close to the fluid inclusion isochore. After deformation, the inclusions are homogeneously distributed throughout the sample and dramatically reduced in size (d < 0.1 μm). Areas with high density of very small fluid inclusions (H2O content ≤ 3000 H/10^6 Si) correlate with high deformation (dislocation glide). The absorption spectra display a discrete peak, indicating OH- bonding in the quartz lattice. Naturally deformed quartz grains in the Truzzo granite (Alps, Northern Italy) are dynamically recrystallized during amphibolite facies conditions by subgrain rotation and grain boundary migration (dislocation creep). The recrystallized grain size (200 < d < 750 μm) indicates low differential stresses of 5-30 MPa. Microstructural observations clearly show that fluid inclusion originally contained in magmatic quartz are expelled during grain boundary migration leaving the recrystallized grains essentially dry with water contents comparable to Brazil quartz (< 200 H/10^6Si). In experiments, the release of H2O from fluid inclusions is considered an important process for crystal plastic deformation. Fluid inclusion rupture, micro cracking and the fast crack healing promote the distribution of H2O in the quartz crystal and thus influence the strength of the material The comparison of both situations, - experimental deformation of wetted, single crystal quartz by dislocation glide and natural deformation of dry quartz with wet grain boundaries by dislocation creep - raises a number of questions. (1) Assuming that wet grain boundaries control the mechanical behavior of quartz (recovery) implies that the presence of intra-crystalline water is not critical. (2) Assuming that intra-crystalline water is crucial and - at the same time - measuring very low intra-crystalline water content in nature implies that either the water is lost (i.e. was transient) or else even the low contents of 200 H/10^6Si are sufficient to enable intra-crystalline plasticity.