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Vibrational Spectroscopy of Mantle Minerals

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Vibrational Spectroscopy of Mantle Minerals Mineral Spectroscopy: A Tribute to Roger G. Burns © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D; Dyar, C. McCammon and M. W. Schaefer Vibrational spectroscopy of mantle minerals 2 PAUL F. McMILLAN",2 RUSSELLJ, HEMLEy ,3 and PHILIPPEGILLET2 'Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, U,S.A 2Laboratoire de Sciences de la Terre, Ecole Normale Superieure des Sciences, 46 allee d'Italie, 69365 Lyon Cedex 07, France 3Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington D.C. 20015-1305, U.S.A. Abstract- Vibrational spectroscopy has been used extensively to characterize the high pressure minerals important in the Earth's mantle. The results of IR and Raman studies complement structural information obtained from diffraction experiments, for example in providing information on [6]- coordinated silicon in high-pressure phases such as stishovite and MgSi03 perovskite, on the tetrago- nal distortion of MgSi03 garnet, and on the presence of the inter-tetrahedral SiOSi linkage in /3_ Mg2Si04• Such measurements provide detailed information on phase transitions and tests of ab initio and empirical calculations of mineral properties. In situ studies at high pressure and/or temperature have been used to study soft-mode behaviour associated with displacive phase transitions, as demon- strated recently for stishovite. The presence of OH in natural minerals and synthetic candidate high- pressure phases has been demonstrated by IR spectroscopy, permitting the identification of potential storage sites for hydrogen in the mantle. In situ high P-T Raman spectroscopy has demonstrated the stability of magnesite (MgC03) to mantle temperatures and pressures, important in determining the carbon budget within the deep Earth. Micro-Raman and (more recently) micro-IR spectroscopies have proven very useful for non-destructive, in situ analysis of minerals in complex natural and experimental sample assemblages. Experimental vibrational data have been used to construct and constrain models for calculation of thermodynamic parameters such as heat capacity and entropy. These have been useful in establishing P-T phase diagrams for mantle mineral assemblages. We summarize these results and collect together vibrational spectra obtained to date for principal mineral phases of the mantle. We also discuss both limitations and future directions of the techniques. INTRODUCTION grains and in thin sections of multiphase natural samples as well as in experimental charges OUR KNOWLEDGEof the internal structure of the quenched from high pressures and temperatures. Earth has been largely gleaned from analysis of Finally, vibrational data have been used exten- seismic and other geophysical data, bounded by sively for modeling the thermodynamic properties cosmological constraints on its composition (AN- of mantle minerals. This has been an important part DERSON,1989; POIRIER, 1991; GILLET, 1995; BINA of this research because such vibrational data in and HEMLEY, 1996). Some direct sampling of man- many cases have provided the first, and in some tle minerals has been possible, from examination cases the only, constraints on the thermodynamic of ultramafic xenoliths brought to the surface in properties of such materials at high P-T conditions kimberlites or alkali basalts, or as inclusions in corresponding to the deep Earth. diamonds (SAUTTER and GILLET, 1994), However, We review here the uses of Raman and infrared most of our present understanding of mantle miner- vibrational spectroscopy in the study of mantle alogy is based on experimentally determined phase minerals, We begin with an overview of the kinds relations and thermophysical properties of candi- of information that can be obtained from such mea- date minerals and assemblages. Vibrational spec- surements and their relevance to the Earth's man- troscopy has played an important role in this work tle, We discuss in some detail the vibrational prop- (McMILLAN and HOFMEISTER, 1988; McMILLAN, erties of the major mantle minerals, summarizing 1989). Such measurements provide detailed infor- the available vibrational data for each phase. Our mation on structure and bonding in materials, motivation has been to collect the spectra obtained thereby complementing direct structural studies by to date in a single set of figures, for convenience of diffraction techniques and spectroscopy of elec- comparison and for identifying areas where further tronic transitions. Vibrational spectroscopy pro- work is needed. This then extends and comple- vides a straightforward means for identifying phase ments earlier and more general reviews of the ap- transitions, as well as the mechanism of such trans- plications of Raman and IR spectroscopy to Earth formations (e.g. soft modes and order parameters). and planetary sciences (REMLEY et al., 1987a; Minerals can be analyzed both as microscopic McMILLAN and HOFMEISTER, 1988; McMILLAN, 175 176 P. F. McMillan, R. J. Hemley and P. Gillett 1989; SHARMA, 1989, 1990). We also discuss vi- CO2 and F-, Cl- and S-bearing species degassing brational spectroscopic studies of the role of water from deep within the Earth, or returning via sub- and carbon in the mantle. A brief description of duction processes (THOMPSON, 1992; BELL and future directions is given in the concluding re- ROSSMAN, 1992a; GASPARIK,1993; GILLET, 1993a; marks. The review is limited to crystalline miner- CARROLLand HOLLOWAY,1994). In addition, this als, Vibrational spectroscopy has demonstrated that region is characterized by partial melting, so that large structural changes take place in silicate the properties of molten silicates are important in glasses at high pressures (and by inference melts), the upper mantle, and perhaps even deeper within but this is beyond the scope of the present article the Earth (WILLIAMS, 1990; ITO and KATSURA, and is reviewed elsewhere (McMILLAN and WOLF, 1992; WOLF and McMILLAN, 1995). The transition 1995). We also discuss results of theoretical studies zone is dominated by the mineralogy of refractory of the vibrational dynamics of mantle minerals; ultramafic phases including the ,B-(wadsleyite, or excellent reviews of theoretical techniques and the "modified spinel") and y-(ringwoodite, spinel) calculation of other properties have been written forms of (Mg,Fe)zSi04, along with the garnet and by COHEN (1994), BUKOWINSKI(1994), and CHlZ- perhaps ilmenite phases of MgSi03 (GASPARIK, MESKHYAet al., (1994), 1990). Both MgSi03 ilmenite and garnet contain [6]-coordinate silicon, and at least the majorite gar- GENERAL CONSIDERATIONS net will exist as a solid solution with Mgz+, Fe2+ and Ca2+ substituting on the large ([12]-coordinate) Overview of mantle mineralogy 4 H site, Al3+, Si + and Fe on the octahedral site, and The global structure of the mantle is defined by Si4+ and AlH on tetrahedral sites (IRIFUNE, 1994; its major seismic discontinuities near 410 km and KESSONet al., 1994). All of these nominally anhy- 660 km, which delineate the regions known as the drous mantle minerals can contain significant upper mantle (20-410 km), transition zone (410- amounts of dissolved H as "defect" OH species 660 km), and lower mantle (660-2900 km) (Fig. (BELL and ROSSMAN, 1992a; GILLET, 1993a). In 1). Based on extensive laboratory studies and theo- addition, several high pressure hydrous phases, par- retical modeling over the past two decades, it is ticularly within the system MgO-SiOz-H20, likely that the dominant mineral phase in the lower may be present in the transition zone (KANZAKI, mantle is a silicate perovskite with composition 1991; PREWITTand FINGER, 1992). near (Mg., 9Feo.l)Si03, probably containing up to several per cent Alz03 component, which co-exists Structure and bonding with (Mg,Fe)O (magnesiowustite) with the rock salt (Bl) structure, and a nearly end-member Although it is not a technique for direct structure determination, vibrational spectroscopy gives im- CaSi03 perovskite (WILLIAMSet al., 1989; ANDER- SON, 1989; HEMLEY and COHEN, 1992; IRIFUNE, portant insight into bonding in materials and struc- 1994). Free silica may be present as stishovite in ture-property relations, thereby complementing basaltic regions trapped near the top of the lower x-ray and neutron diffraction methods. Morever, mantle from subducted oceanic crust (KESSON et vibrational spectroscopy has revealed transitions al., 1994), which could give rise to aluminous and structural changes that were not apparent from phases (IRIFUNE et al., 1991; YAGI et al., 1994; diffraction measurements (KINGMA et al., 1995), AHMED-ZAID and MADON, 1995). A free SiOz although in other cases, such claims have not been phase near the base of the lower mantle has also supported by subsequent, and more accurate, direct been proposed (KNITTLEand JEANLOZ,1991; JEAN- structural investigations (e.g., DOWNSet al., 1996). LOZ, 1993). The mineralogies of the transition zone Measurements as a function of pressure and/or tem- and upper mantle are likely to be more complex perature permit detailed identification of response (ANDERSON,1989; GASPARIK,1990; GILLET, 1995). of the structure to these thermodynamic variables The upper mantle is dominated by ultramafic min- ti.e., changes in compression mechanisms in the erals, principally olivine, pyroxene and garnet absence of a thermodynamic phase transition). Vi- phases, but substantial amounts of plagioclase feld- brational spectroscopy has played an important
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