SHOCK-WAVE EQUATION OF STATE OF SERPENTINE TO 150-GPA - IMPLICATIONS FOR THE OCCURRENCE OF WATER IN THE EARTHS 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 % H2O is consistent with seismically determined lower mantle density estimates. Greater amounts of H2O 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 H2O-rich magnesium silicate determined from shock equation of 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.