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Chapter 11 Ans

Chapter 11 Ans - Answers for Chapter 11 Diversification of...

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Answers for Chapter 11. Diversification of Magmas Simply put, diversification requires: 1) Two (or more) phases in which components are unequally distributed . 2) A physical process in which the phases are separated. When combined, these two processes allow geologic materials, such as rocks, melts, fluids, etc. to change composition. 1. Provide an example from previous chapters of a magmatic system that diversified. Specify the roles of parts (1) and (2) above in the diversification process. We discussed partial melting in many binary and ternary systems in Chapters 6 and 7. We also discussed partial melting with respect to major elements and trace elements in Chapters 8 and 9, and with respect to the mantle in Chapter 10. In any case, the melt formed is not the same composition as the solid residue, being enriched in the low-temperature (Chapters 6-7) or incompatible (Chapters 8-10) components (part 1 above). Then, typically due to its buoyancy and mobility, the melt is released from the residue (part 2 above). 2. Explain why some critical melt fraction (% melt generated) is required before any melt can be extracted from a melting source area. What factors play important roles in hindering melt escape? What principal factors drive melt separation? Enough melt is required to form an interconnected grain boundary network of sufficient thickness to enable some portion to avoid surface adhesion to the mineral grains and free itself from the surrounding solid enclosure. Low permeability, surface adhesion, and viscosity are the principal hindering factors, whereas buoyancy (density contrast between melt and residue) and compaction are important driving factors (as are low values for the limiting factors of course). 3. Use Appendix A to calculate the viscosity (using the method of Shaw) of the basalt in Table 8.1 if it were magma at 1200 o C. Use only H 2 O + and report your results in Pa sec and in poise. I got 4.79 x 10 3 Pa sec (4.79 x 10 4 poise) 1
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Gravity settling of early-forming minerals is the classically proposed mechanism for magmatic differentiation. For non-Newtonian fluids, the processes is best described by Stoke’s Law: V 2gr ( ) 9 2 = - ρ ρ η s l (Stokes’ Law) (11-1) where: V = the settling velocity (cm/sec) g = the acceleration due to gravity (980 cm/sec 2 for the Earth) r = the radius of a spherical particle (cm) ρ s = the density of the solid spherical particle (g/cm 3 ) ρ l = the density of the liquid (g/cm 3 ) η = the viscosity of the liquid (1 g/cm sec = 1 poise) 4. Calculate the settling velocity of a spherical clinopyroxene with a diameter of 5mm and a density of 3.4 g/cm 3 in a basaltic magma with a density of 2.6 g/cm 3 and a viscosity of 900 poise. Would gravity settling in such a situation be a conceivably important process? Explain why.
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