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Lower crustal density estimation using the density-slowness relationship: a preliminary study
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The density of the Earths crust is an important parameter. Carlson and Raskin  and Carlsan and Herrick (1990] used an empirical approach an the relationship between density and seismic slowness to estimate the density of the oceanic crust. The properties of the lower continental crust are largely unknown because it cannot be sampled directly. Xenoliths and high-grade metamorphic terrains exposed at the surface provide localized samples. A number of researchers have used various methods to estimate the density of the lower crust. Woollard , for example, used the principle of isostasy to infer that the density of the lower continental crust is greater than the density of the oceanic crust. The objective of this study is to determine whether an empirical density-slowness relationship of the kind used by Carlson and Raskin (1984] can be used to estimate the density of the lower continental crust .which has more variable pressures, temperatures, and compositions than the oceanic crust. To use this method, the lower continental crust was defined to be at a depth greater than 18 km (pressures > 600 MPa) and temperatures greater than 400 C. Rock types or suites of rocks that are stable under these conditions are amphibolite-and granulite-facies metamorphic rocks. Velocity-density data was compiled from the literature for pressures greater than 600 MPa and linear fits of density on slowness were made. No correction was made for the effect of temperature. Densities were then estimated for a number of crustal seismic structure models:the Wind River Mountain, Ivrea, and Christensen's and Mooney's  average-crust model. The densities estimated using the density slowness method were then compared to the densities estimated by other methods. The conclusions of this study are: 1) The scatter in the data is due to compositional variation, not experimental error. 2) Although the P-wave velocity is affected by pressure, the pressure effect is not discernible. 3) There is no statistical difference between densities estimated using velocity and those using slowness. 4) The slowness densities are systematically lower than the densities estimated by other methods by about 0. 10 Mg M-3 , which is less than one standard deviation. 5) Correcting the laboratory velocities for the effect of temperature leads to much improved agreement between the slowness-model densities and densities estimated by othermeans.
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Jones, Gary Wayne (1996). Lower crustal density estimation using the density-slowness relationship: a preliminary study. Master's thesis, Texas A&M University. Available electronically from
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