Chapter 10 Ans

Chapter 10 Ans - Answers for Chapter 10: Mantle Melting and...

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Answers for Chapter 10: Mantle Melting and the Generation of Basaltic Magma 1. Name as many sources of mantle samples as you can. Ophiolite terranes, kimberlite xenoliths, xenoliths in basalts (particularly alkaline basalts), ocean dredge samples from near-ridge areas and fracture zones. 2. Use Figure 2.2c to describe the differences between dunite, harzburgite, and lherzolite. All are peridotites (ultramafic rocks with more than 40% olivine). Lherzolite contains at least 10% orthopyroxene and 10% clinopyroxene, harzburgite has 10-60% orthopyroxene and little or no clinopyroxene, and dunite contains at least 90% olivine. 3. What does Fig. 10.1 suggest about the relationship between dunite, harzburgite, and lherzolite samples? Lherzolite is probably “fertile” (undepleted) mantle from which basalt is extracted. Removing basalt (as partial melts) depletes lherzolite in basaltic components, resulting in a residual harzburgite, and, if basalt extraction is extensive enough, dunite. The linear array indicates that if basalt is remixed with the residual harzburgite or dunite the lherzolite can be regenerated. 4. What features in Fig. 10.2 suggest that mantle melting is not a “normal” process in the “typical” Earth? The normal geotherm never intersects the solidus, so melting temperatures are never reached at any depth. 1 15 10 5 0 0.0 0.2 0.4 0.6 0.8 Wt.% Al 2 O 3 Wt.% TiO 2 Dunite Harzburgite Lherzolite Partial Melting Residuum Tholeiitic basalt Figure 10.1 . From Brown and Mussett (1993). 1000 2000 3000 0 5 10 15 20 T o C P (GPa) 200 400 600 Garnet Lherzolite High-Pressure Phases Liquidus Solidus Geotherm Depth (km) Liquid Spinel Plagioclase Figure 10.2 . Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient.
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5. You described the REE diagrams on the left in the review questions in Chapter 9. Suppose you demonstrated that the two rocks represented primary melts. Using Fig. 10.2, what could you say about the depth of melting for each? Explain. The top diagram shows a negative europium anomaly, suggesting that plagioclase was a residual phase in the mantle source. Because the melt is primary, plagioclase fractionation by crystal settling from the rising melt is eliminated. Plagioclase is a stable lherzolite phase only under very shallow mantle conditions (less than about 40 km), so this is the probable origin of the liquid. The bottom diagram has very a steep REE slope, particularly HREE, suggesting garnet as a residual phase and a source deeper than about 80 km and shallower than about 400 km. 6. If melting is not a “normal” process, under what circumstances can it be accomplished? Note the
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Chapter 10 Ans - Answers for Chapter 10: Mantle Melting and...

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