THE 10Α PHASE: A HIGH-PRESSURE EXPANDABLE SHEET SILICATE STABLE DURING SUBDUCTION OF HYDRATED LITHOSPHERE

Abstract

H2O storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases. As a result of its relatively large thermal stability and intermediate composition, the 10Α phase (Mg3Si4O10(OH)2.nH2O) has been regarded as a relevant H2O reservoir in a wide range of rock compositions and mineral assemblages. High-pressure syntheses of the 10Α phase were carried out at 6.7 GPa and 650°C under fluid-saturated conditions in a Walker-type multi-anvil apparatus, from 5 min to 430 h. X-ray powder diffraction of large platy hexagonal crystals of the 10Α phase (up to 100 μm) were indexed on the basis of a trioctahedral-type structure. Long-term run products (>110 h) reveal sensitivity of the 10Α phase to treatment with acetone leading to the appearance of diffractions at greater d-spacings (10.2-11.6 Α) with respect to the basal peak of the 10Α phase (9.64-10.07 Α). This swelling behavior is strongly related to synthesis run duration. The Raman spectrum of the 10Α phase at frequencies less than 800 cm-1 shows a strong similarity to talc. In the Si-O stretching region (800-1100 cm-1), the 10Α phase exhibits three modes (909, 992 and 1058 cm-1), as compared to two in talc. The bending mode of water (ν2) is found at 1593 cm-1. In the OH stretching region, peaks at 3593, 3622 and 3668 cm-1 were observed. The acetone treated sample shows a C-H stretching mode at 2923 cm-1 while the double bond C#O signal is absent. The swelling behavior of the 10Α phase is interpreted as due to intercalation of acetone with pre-existing interlayer water. The efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration. The relation between the response to acetone treatment and run duration is therefore interpreted as a time-dependent hydration of the 10Α phase. The fractions transformed from non-expandable to expandable fractions was fitted to the Avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions. The ability to accommodate variable amounts of H2O makes the 10Α phase a major H2O sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere. Under H2O-saturated conditions, a fully hydrated 10Α phase occurs; when H2O-undersaturated conditions prevail, a H2O-deficient 10Α phase incorporates the volatile component available. The exchange capacity of interlayer molecules in the 10Α phase structure opens new scenarios on the control of fluid compositions escaping from subducted slabs.