The effect of water on the P21/c to C2/c high-pressure phase transition in MgSiO3-clinopyroxene: implications for the mantle X-discontinuity
Abstract
The presence and distribution of hydrogen in Earth's mantle has intriguing implications for the geochemical evolution of liquid water on the planet. Bell and Rossman (1992) showed that pyroxenes of upper mantle origin typically contain ~200 to 500 wt ppm H2O. It is possible that in regions of lherzolitic or depleted harzburgitic mantle, the phase transition of Ca-poor orthoenstatite (OEN) to high-clinoenstatite (HCEN) may result in an observable seismic discontinuity at around 300 km-depth (Woodland, 1998). We devised a simple experiment to test the effect of water on the transition pressure of the P21/c low-clinoenstatite (LCEN) to the high-density C2/c HCEN in situ. A single crystal of dry MgSiO3 LCEN was placed adjacent to a second crystal of LCEN containing approximately 500 wt ppm H2O in a diamond-anvil cell. Raman spectroscopy was used to track the Si-O-Si bending vibrations from the pyroxenes, which display a characteristic doublet between 600 and 750 cm-1 for the P21/c structure, or a singlet for the C2/c structure (Ross and Reynard, 1999). On compression, the phase transition from LCEN to HCEN occurred at 7.9(1) GPa for the dry LCEN, but at only 5.8(1) GPa for the wet LCEN. On decompression, the hysteresis of the phase transition was reduced from ~3 GPa for the dry sample, to ~1.5 GPa for the hydrous sample. We studied upper-mantle discontinuity structures below the western U.S., South America, and in the Tonga subduction zone using receiver functions calculated from P-to-S converted arrivals. Within each region, there are multiple reflectors between 100 and 300 km depth, but their depths vary and not all of them are present at each station. In the western US, there is a sporadic and relatively weak reflector varying in depth between about 250 and 300 km depth. In S.A. along the Andean belt, a moderate to strong arrival is observed at around 290-300 km-depth, but below the craton strong arrivals appear at 245-280 km depth. At Tonga, the strongest reflector occurs at around 100 km depth, with only weak evidence for arrivals from 300 km. Mantle discontinuity structure in the upper 300 km of the mantle is considerably more complicated than for the transition zone, but if the effect of water on the LCEN to HCEN transition pressure observed in these experiments is comparable to that for OEN to HCEN in the mantle, any corresponding seismic discontinuity would be shifted to lower depths by about 40-60 km, or, extinguished altogether by widening of the divariant loop in the (Mg,Fe)SiO3 system.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.U53A..04J
- Keywords:
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- 1025 Composition of the mantle;
- 3600 MINERALOGY AND PETROLOGY (replaces;
- 3620 Crystal chemistry;
- 3630 Experimental mineralogy and petrology;
- 3934 Optical;
- infrared;
- and Raman spectroscopy