The 10 angstrom phase: a high-pressure expandable sheet silicate stable during subduction of hydrated lithosphere

H2O storage and release in deep subducting lithosphere is controlled by com plex reaction suites involving a variety of hydrous phases. As a result of its relatively large thermal stability and intermediate composition, the 10 Angstrom phase (Mg3Si4O10(OH)(2). nH(2)O) has been regarded as a relevant H2O reservoir in a wide range of rock compositions and mineral assemblages. High-pressure syntheses of the 10 Angstrom phase were carried out at 6.7 G Pa and 650 degreesC under fluid-saturated conditions in a Walker-type multi -anvil apparatus, from 5 min to 430 h. X-ray powder diffraction of large pl aty hexagonal crystals of the 10 Angstrom phase (up to 100 mum) were indexe d on the basis of a trioctahedral-type structure. Long-term run products (> 110 h) reveal sensitivity of the 10 Angstrom phase to treatment with aceton e leading to the appearance of diffractions at greater d-spacings (10.2-11. 6 Angstrom) with respect to the basal peak of the 10 Angstrom phase (9.64- 10.07 Angstrom). This swelling behavior is strongly related to synthesis ru n duration. The Raman spectrum of the 10 Angstrom 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 10 Angstrom phase exhibits three modes (909, 992 and 1058 cm(-1)), as compared to two in talc. The bending mode of water (v(2)) 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 10 Angstrom phase is interpreted as due to i ntercalation 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 hy dration of the 10 Angstrom phase. The fractions transformed from non-expand able to expandable fractions was fitted to the Avrami empirical law which s uggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions. The ability to accommodate variable amounts of H2O make s the 10 Angstrom phase a major H2O sink whenever a hydrous phase such as c hlorite and serpentine breaks down during prograde transformations in the s ubducted lithosphere. Under H2O-saturated conditions, a fully hydrated 10 A ngstrom phase occurs; when H2O-undersaturated conditions prevail, a H2O-def icient 10 Angstrom phase incorporates the volatile component available. The exchange capacity of interlayer molecules in the 10 Angstrom phase structu re opens new scenarios on the control of fluid compositions escaping from s ubducted slabs. (C) 2001 Elsevier Science B.V. All rights reserved.