HR: 11:20h
AN: MR32A-05 INVITED    [Abstracts]
TI: Important yet Overlooked Topological Mechanisms of Compression in Silicate Liquids at Low Pressure (0-5 GPa).
AU: * Lange, R A
EM: becky@umich.edu
AF: University of Michigan, Dept. of Geological Sciences 1100 N. University Ave., Ann Arbor, MI 48109-1005, United States
AB: The compressibility of silicate liquids has long been known to be larger than corresponding solids (especially pronounced at low pressures; 0-5 GPa), with important consequences for the increase in the density difference between partial melts vs. crystalline mantle with decreasing pressure, which in turn has consequences for decompressional mantle melting and an increased buoyancy drive for partial melts. Despite the importance of the enhanced compressibility of liquids relative to solids at low pressures, its cause has often been overlooked. Instead, most of the focus on understanding the structural mechanisms of liquid compressibility has been limited to Al³⁺ and Si⁴⁺ coordination change. Here, a discussion of the importance of topological mechanisms of compression (and expansion) in liquids at low pressure is presented. The principal mechanisms of compression for minerals (also available to liquids) involve changes in either bond lengths or T-O-T bond angles. Minerals may also undergo an abrupt phase transition (usually first-order) to a higher-density structure with pressure, which corresponds either to an abrupt change in topology (e.g., quartz to coesite) and/or cation coordination (e.g., coesite to stishovite), both of which can induce large changes in density. In contrast, liquids may undergo continuous and gradual changes in topology (network connectivity) and/or cation coordination, which requires bonds to be broken and reformed and thus reflects the dynamic character of liquids in contrast to solids. Thus, not only changes in cation coordination, but also changes in topology is a mechanism of compression uniquely available to liquids. The magnitude of these topological mechanisms of compression can be seen by comparing the compressibility of liquid KAlSi₃O₈ to that of sanidine as a function of pressure. At one bar, the compressibility of liquid KAlSi₃O₈ is 264% larger than that of its crystalline equivalent; this difference diminishes to a 19% difference by 6.5 GPa. There is little evidence for coordination change of either Al³⁺ or Si⁴⁺ in KAlSi₃O₈ liquid at low pressure, so that the enhanced liquid compressibility clearly involves topological mechanisms (e.g., changing from a tridymite to a feldspar topology) rather than one based on Si/Al coordination change. This mechanism of compression rapidly increases with decreasing pressure and facilitates the large density difference between liquid and crystal at one bar. Another example of the importance of topological mechanisms of compression (and expansion) in liquids is seen with the volume of fusion (18%) of diopside at one bar. The 18% increase in liquid volume cannot be explained by a change in Si⁴⁺ coordination number or by mechanisms available to crystalline diopside. In fact, crystalline diopside would have to be heated by more than 2000 degrees above the one-bar melting temperature in order to increase its volume by 18%. Thus, the only plausible explanation for the large volume of fusion is a change in average topology (e.g., from a pyroxene to a pyroxenoid topology). As liquid CaMgSi₂O₆ is then compressed with increasing pressure, a wide range of denser topologies, including one similar to pyroxene, can be accessed; this mechanism contributes substantially to the compressibility of liquid CaMgSi₂O₆ at low pressure. Other examples and the consequences for the efficient extraction of oceanic and continental crust out of the uppermost mantle will be given.
DE: 1037 Magma genesis and partial melting (3619)
DE: 3640 Igneous petrology
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting