The diagram (right) is a SiO_2 versus CaO bi-variate diagram (sometimes called a

Question

The diagram (right) is a SiO_2 versus CaO bi-variate diagram (sometimes called a Harker diagram) that may be used to plot the composition of igneous rocks. On the diagram is a hypothetical magma composition. If anorthite feldspar (CaAl_2 Si_2 O_8) was to crystallize in the magma and begin to sink, forming a cumulate layer at the bottom of the magma chamber, using the diagram predict where you think the composition of the anorthite-rock would plot relative to the original magma Plot where you would predict the composition of the new magma (after the anorthite had been removed) would be relative to the original magma composition? Describe and explain a process that may cause the anorthite the sink in the magma chamber

Explanation / Answer

A) Anorthite are found in mafic igneous rocks which are low in silica;however the earlier magma composition has very lopw silica content in it due to the presense of CaO which is known to neutralize the effect of acidic SiO2.Thus the formation of Anorthite rock is likely to increase the silica in the magma composition relatively and the plot would be drawn above the prent plot line.

C) Anorthites are denser than the magma and also a solid is hevier than a liquid,therefore whwn it crystalizes,it sinks to the bottom of the magma chamber

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