Lecture Fimall Possible New Essay Questions for the Mineralogy Use the phase dia

Question

Lecture Fimall Possible New Essay Questions for the Mineralogy Use the phase diagrams below to help you answer the following questions 400 20406080100 Ma SO forsterte Fe SO ayaite uppose some olivine or pyroxene crystallizes from a mafic magma. The remaining magma then moves upward leaving the crystals behind. If this process repeats, how will the magmas composition change? Campos hon uil oa isver in iron aund lower i m The Earth's upper mantle is roughly basaltic (mafic) in composition and acts as a source region for magmas that move up into the crust. Partial crystallization alluded to in a commonly occurs. If olivine crystals are left behind in the mantle and the magmas then crystallize in the crust, what effect does this have on the relative compositions of the crust and mantle? Assume olivine crystallized between 1700 and 1600° C. (No calculations are required) w Suppose magma in bafter leaving the mantle crystallizes plagioclase between 1500° and 1400° C in the lower crust and the magma rises to the upper crust leaving the plagioclase behind. What effect will this have on the relative composition of the upper and lower crust Suppose olivine begins to crystallize at high temperature. It grows larger as cooling continues. If the cooling is too rapid for the olivine to remain in equilibrium, growth rings c zoning will develop. How will the composition of the center of the grain compare to the composition of the margins. Compared to the compol the mamins

Explanation / Answer

1. The given phase diagram is showing the complete solid solution between Forsterite(Mg2SiO4) and Fayalite(Fe2SiO4). When the cooling takes place, melt touches the liquidus curve and cystal starts to form. The first form crystal will be Mg rich. Then the remaining magma moves upward leaving the cystals behind. If this process repeats, the magma will be richer in Fe and lowerr in Mg as Mg easily incorporates into crystals than Fe.

2.If the given condition meets, again the mantle will be higher in Mg as olivinecystals are left behind there and the crust will be lesser in Mg, but the composition will move towards felsic magma. So, after olivine, pyroxene followed by amphibole minerals will be formed according to the Bowen's reaction series.

3.After crystallising plagioclase in the lower crust if the magma rises to the upper crust, then the composition of lower crust will be higher in Ca and Na as plagioclase composition varies in between Albite(NaAlSi2O8) and Anorthite(CaAl2Si2O8) and the upper crust will be higher in Si element.

4. With continuous cooling growth rings and zoning is formed in the olivine crystal. As Mg in olivine is formed at higher temperature and with cooling olivine becomes enriched in Fe, hence the center of the zoning will be higher in Mg surrounded by Fe margin.

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