Shock Wave Equation of State of Serpentine to 150 Gpa: Implications for the Occurrence of Water in the Earth's Lower Mantle

The shock equation of state of serpentine has been determined to 150 GPa. Four distinct regions occur along the Hugoniot: a low-pressure phase, a mixed phase region, a high-pressure phase, and a very high pressure phase. The low-pressure phase (LPP) exists under shock pressures to about 40 GPa. This material exhibits shock properties that are partially consistent with those of low pressure serpentine, but steep release paths and a low value of K'= 2.77 suggest transformation to another, possibly amorphous, assemblage. Thermodynamic calculations indicate that under equilibrium conditions, serpentine would decompose to oxides plus water at conditions below 10 GPa along the Hugoniot. A mixed phase region begins at 40 GPa with complete transition to a high-pressure phase occurring by about 55 GPa. The high-pressure phase (HPP) occurs at shock pressures between 55 GPa and 125 GPa. Model Hugoniots based on perovskite plus periclase plus water and brucite plus periclase plus stishovite reproduce the serpentine HPP Hugoniot within experimental error, so definitive identification of the HPP as a distinct hydrous mineral phase or as a free water containing mixture is not possible. Above 125 GPa a transition to a very compressible phase, possibly a hydrous partial melt, occurs. The serpentine HPP Hugoniot is about 15-20 % less dense than the Earth's lower mantle. Models of the lower mantle based on shock equations of state for olivine, pyroxene, and serpentine indicate that for an atomic Mg/(Mg+Fe) ratio of 0.80, the presence of 2 wt % H20 is consistent with seismically determined lower mantle density estimates. Greater amounts of H20 can be accommodated if accompanied by an increase in Fe content. Calculated Hugoniot sound speeds of the serpentine HPP, although poorly constrained, are broadly consistent with lower mantle sound speeds. Thus the high-pressure density and sound speed of an H20-rich magnesium silicate determined from shock equation ot state experiments indicate that the observed seismic properties of the lower mantle allow the existence of several weight percent of water in the lower mantle. 1Now at Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Federal Republic of Germany. Copyright 1991 by the American Geophysical Union. Paper number 91JB01573. 0148-0227/91/91JB-01573505.00 Introduction The discovery and characterization of hydrous magnesium silicates that are stable to at least the pressures and temperatures of the Earth's upper mantle raise the question of their stability and existence as H20 reservoirs within the Earth's lower mantle [Ringwood and Major, 1967; Yamamoto and Akimoto, 1977; Akaogi and Akimoto, 1980; Liu, 1986, 1987; Finger et al., 1989]. Occurrence of significant amounts of H20 in the Earth's interior would strongly influence the petrological and thermal evolution of the planet [e.g., Liu, 1987; Finger et al., 1989; McGovern and Schubert, 1989]. However, detailed phase relations and the ultimate pressuretemperature stability range of these phases have yet to be determined. Also currently lacking are equation of state data that would allow comparison with Earth density and seismic velocity models. Shock wave studies can constrain the equation of state of high-pressure minerals. The shock and release properties of hydrous minerals are also of interest in the study of meteorite and planetesimal impacts, impactinduced devolatilization, and atmospheric evolution [Zahnle et al., 1988; Ahrens et al., 1989]. We have performed shock Hugoniot and release experiments at shock pressures up to 150 GPa on polycrystalline, magnesium end-member serpentine to determine its high pressure equation of state. The results are compared to global density and seismic velocity profiles to constrain the possible H20 content of the Earth's lower mantle. The Hugoniot to 88 GPa of a serpentinized rock from VerMyen, Italy, has been determined by McQueen (compiled by Marsh [1980]). This material has an initial density of 2.80 g/cm 3 and has a total Mg/[Mg+Fe] ratio of 0.9, whereas pure Mg end-member serpentine has a density of 2.50-2.55 g/cm3. Optical microscopic examination of the Ver-Myen serpentine reveals that it is an incompletely metamorphosed ultramafic rock; a number of the individual mineral grains comprising the rock consist of olivine and orthopyroxene cores surrounded by serpentine. Thus the H20 content is not known and is not uniformly distributed. This material undergoes a transition to a mixed phase at about 40 GPa on the Hugoniot that is complete by about 70 GPa. To determine the shock equation of state of pure, magnesium end-member serpentine, we examined the shock properties of three polycrystalline serpentines: (1) a lizardite serpentine found near Globe, Arizona, (2) an antigorite serpentine from Thurman, New York, and (3) a chrysotile serpentine from Quebec, Canada.