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Lecture 15

EPSC201 Lecture 15 Notes.doc

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Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 - Lecture 15 Notes Partial melting – minerals that melt at low temperatures melt into a liquid before minerals with high melt- ing points. If the mantle is in the range where only some melting occurs, partial melting occurs. The minerals with lower MPs melt and leave the mantle, forming a magma. The mineral that is last to melt is magnesium rich olivine. It dominates the mantle, and melts last. Olivine either has iron or magne- sium. The iron olivine melts at a lower temperature. In general, minerals with high silica content melt lower (feldspar and quartz). The early magma is enriched in potassium, sodium, and silica, because these are found in minerals with low MPs. The left over residue that doesn’t melt is called mafic residue and contains lots of magne- sium olivine. Temperature is on the y axis here. This chart shows which minerals melt the easiest. What we see is that as the silicates get more compli- cated, the temperature of crystalliza- tion increases (from tetrahedrons to silicate sheets). At the bottom, the minerals melt eas- ily, and have a low MP. At the top of the chart, the minerals have a high MP, and crystallize quickly. Calcium rich plagioclase crystallizes at a higher temperature then plagioclase with sodi- um. This mineral crystallizes over quite a interval, which is termed a continuous series. This chart can explain most of the igneous rocks in the world. In the scenario on the left, crystallization is starting to occur. Convection is occurring in the magma cham- ber. According to the Bowen series, the olivine will get converted to the pyroxene, which will eventually continue to be amphibole…etc. Gradually, the min- erals change into different minerals. Over time, the orange (say olivine) will be converted to the blue (say pyroxene). At the beginning of crystallization, there is only olivine being crystallized, but as time pro- gressed, there is a mixture being crystallized. However, as time progresses, the temperature cools. So by the time olivine is converted to another mineral, it may all be cooled into rock. As the magma cools, it either crystallizes, or forms a new mineral that has a lower crystallization point. That’s why on the Bowen Series, we see the arrows pointing to the bottom three minerals. Low degree of partial melting will make a rhyolite or granite. High degree of partial melting will make a basalt or gabbro. Medium amount will make an andeosite. Prof did a horrible job explaining frac- tional and equilibrium crystallization. Wikipedia answers…. The process of fractional crystallization is responsible for the bulk of differentia- tion that is occurs in igneous rocks. As ascending melts cool and react with country rock, those minerals in the melt that have the highest melting points or the lowest solubilities (like olivine and py- roxene) crystallize out first, leaving min- erals with the lowest melting points be- hind in the melt to freeze out last. Gravitative differentiation is the commonest form of fractionation, and results from the phenomenon that most solid minerals are denser than their parent melts. As denser crystals settle to the bottom of the magma body, they become segregated from the residual melt. Rocks that are formed by settling crystals are termed cumulates, and the rocks are often zoned, with the densest, first-formed crystals accumulated at the base of the magma chamber. Cumulates formed by the lighter crystals occasionally float to the top, with the lightest at the very top. This process produces layering in igneous rocks. Key point – large variety of magmas can be pro- duced from fractional crystallization and partial melt- ing Cold environments are dominated by physical weath- ering, because chemical reactions require heat t
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