Hawaii 3 - 1 HAWAII Lecture 3 Output from Mauna Loa and...

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Unformatted text preview: 1 HAWAII Lecture 3 Output from Mauna Loa and Kilauea have been very similar during historical times 2 Submarine Stage Emergent Stage Shield Building Stage Post-Shield Stage The evolutionary stages of Hawaiian volcanoes are consistent with a thermally zoned plume that is hotter and producing more melt in the plume center than at the margins. Thermal model of a mantle plume (Ribe & Christensen, 1991) Distance in 50 km intervals D e p t h k m Cartoon showing position of plume relative to volcanoes. Caution we have no idea where it is actually located or what its shape and radius are! 3 The common crystallization sequence is: olivine ( Mg-Cr spinel), olivine + clinopyroxene ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers. Note difference from MORB The common crystallization sequence is: olivine ( Mg-Cr spinel), olivine + clinopyroxene ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene Figure 7-2. After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers. Note difference from MORB 4 T( o C) = 23.0(MgO) + 1012 Therefore the temperature of a lava with 7% MgO will be:- 23.0 x 7.0 + 1012 = 1174 o C Relationship between MgO in glass and temperature for Mauna Loa (Montierth et al., 1995) Note how quickly cpx follows plagioclase Similarly for Kilauea T( o C) = 20.1(MgO) + 1014 (Helz and Thornber, 1987) The crystallization sequence is clearly shown by the rock chemistry. 1. Extensive olivine crystallization (and accumulation) 2. Followed by clinopyroxene and plagioclase crystallization o l i v i n e c p x + p l a g 5 Data for Mauna Loa compared with Kilauea There are three problems: 1. Why does Mauna Loa differ from Kilauea? 2. Why do we only see an olivine fractionation trend for Mauna Loa? Why are there so few fractionated lavas showing cpx and plag crystallization? 3. What is the composition of the parental or primary magma (olivine fractionation vs accumulation)? E Pigeonite field Cpx field Olivine field 1. Mauna Loa lavas first crystallize olivine 2. At E olivine is joined by Cpx (+ Plag) and pigeonite 3. E is a reaction point where olivine reacts with the melt to produce pigeonite. The magma composition is fixed at E until all olivine is consumed. Only then can the melt evolve further. 4. Recharge and mixing with new parental magma drags the melt composition back into the olivine field 5. The process is repeated over and over again 6. Therefore Mauna Loa rarely erupts fractionated lavas Note all crystallization paths from the olivine field (red arrows) lead to E 6 Where does the parental (primary?) magma plot on this olivine-control line?...
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Hawaii 3 - 1 HAWAII Lecture 3 Output from Mauna Loa and...

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