Compressibility of Chondrodite, Mg₅(SiO₄)₂(OH,F)₂ up to 9.6 GPa: The Effect of F/OH Substitution on the Bulk Modulus

Alexandra Friedrich (alexandra@kristall.erdw.ethz.ch)¹, Martin Kunz (kunz@kristall.erdw.ethz.ch)¹, Ronald Miletich (ronald@kristall.erdw.ethz.ch)¹ & George A. Lager (galager@louisville.edu)²

¹ Laboratory of Crystallography, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland

² Department of Geography and Geosciences, University of Louisville, Louisville, Kentucky 40292, U.S.A.

Chondrodite, a hydrous magnesium silicate, may serve as a possible host for water in the Earth's upper mantle. With the exception of Ti(Mn)-rich samples, naturally-occurring humite minerals always contain both F and OH. F is thought to stabilize the crystal structure through reduction of the H-H repulsion. Therefore, it is important to study the effects of F/OH substitution and hydrogen bonding on the physical properties, especially under pressure conditions representative of the Earth's upper mantle.

Natural, F-bearing chondrodite from the Tilley Foster mine, Brewster, NY, with OH/(OH+F)=0.44 was investigated at high pressure by single-crystal X-ray diffraction using an ETH-design diamond anvil cell. P-V data were collected up to 9.6 GPa using quartz as internal pressure calibrant, and fitted with a third-order Birch-Murnaghan equation of state. The axial compressibilities are: a-axis: a_0,calc=4.7327(1) Å, K(a)_0,T=459(3) GPa, K(a)_T´=28.4(1.0); b-axis: b_0,calc=10.2754 Å, K(b)_0,T=300.4(1.5) GPa, K(b)_T´=28.1(1.0) and c-axis: c_0,calc=7.8758(3) Å, K(c)_0,T=316(2) GPa, K(c)_T´=16.3(7) (space group P2₁/b).

A bulk modulus of K_0,T=117.5(4) GPa was derived from volume data with a pressure derivative of K_T´=5.45(10). This result is consistent with the bulk modulus (K_S=118.4(1.6) GPa) determined from Brillouin-scattering of a sample from the same locality [OH/(OH+F)=0.68; Sinogeikin and Bass, 1999]. A value of K_0,T=115.50(55) GPa and K_T´=4.95(16) has been reported for synthetic OH-chondrodite (Ross and Crichton, 1999). Faust and Knittle (1994) investigated the compressional behaviour of a F-rich chondrodite (OH/(OH+F)=0.27) resulting in K_0,T=136(±9) GPa and K_T´=3.7(±0.4). Reconsidering their data and fitting an equation of state with K_T´ set to 4.9-5.5, bulk moduli ranging from 118 to 125 are obtained. These data suggest that F reduces the compressibility of chondrodite. Structural investigations at high pressures will be carried out in order to investigate the variation of the hydrogen-bond strength with pressure and its influence on the high-pressure behaviour.

Sinogeikin SV & Bass JD, Phys.Chem.Minerals, 26, 297-303, (1999).

Ross NL & Crichton W, American Geophysical Union Meeting, San Francisco, December, (1999).

Faust J & Knittle E, Geophys. Res. Letters, 21, 1935-1938, (1994).

Melting Phase Relations and Element Partitioning in Peridotite between 20-26 GPa

Daniel Frost (dan.frost@uni-bayreuth.de)¹ & Reidar Tronnes (r.g.tronnes@toyen.uio.no)²

¹ Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, Germany

² Mineralogical-Geological Museum, University of Oslo, Norway

Peridotite phase relations and element partition coefficients along the mantle solidus are required in order to understand the earliest differentiation of the mantle, which may have occurred during the crystallization of a magma ocean. In addition, although melt extraction in the present mantle occurs mostly at pressures below 5 GPa, hotter plumes rising through the Archean mantle may have experienced initial melting in the upper part of the lower mantle. Melting experiments conducted in the multianvil apparatus at pressures >10 GPa have been plagued by large thermal gradients across the samples (>200°C/mm). In this study, melting relations in the 20-26 GPa range have been determined using larger volume 18/8 (octahedral assembly edge length/tungsten carbide cube truncation length in mm) and 10/4 type multianvil assemblies with the use of 1000 and 5000 tonne presses, respectively.

Oxide mixes of two different peridotite compositions, KLB-1 and pyrolite, were used in this study. A series of trace elements (REE, Rb, Sr, Ba, Sc, Y, Zr, Nb, Hf, Pb, Th, U) were added to the oxide mixes at the 50-500 ppm level, facilitating the analyses of resulting phases by ion micro probe. For pyrolite composition at 22 GPa the sequence of crystallization observed is garnet followed by magnesiowüstite with a solidus temperature of approximately 2170°C. At 23 GPa garnet appears on the liquidus followed by magnesiowüstite and then perovskite. At 24 GPa the sequence is magnesiowüstite, followed by garnet and perovskite with a solidus temperature of approximately 2200°C. Over this pressure range Ca-perovskite forms just below the solidus but would appear to scavenge the majority of the added trace elements in the charge. Similar experiments on KLB-1 composition at 24 GPa reveal a solidus temperature approximately 100°C higher than that of pyrolite.