Can the change in composition from basanite to nephelinite (i.e. increasing alkalis with decreasing SiO2) be explained in terms of fractional crystallisation, or only by a decreasing degree of partial melting? If the latter, does the silica content decrease because it is compatible and therefore isn't enriched in the melt?

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    $\begingroup$ If I'm not mistaken, the difference in the depth of melting. Melts formed at deeper depths and higher pressure have more alkalis and less silica, in general. Thus melting at shallow levels will form a basalt, whereas melting in the mantle will form an alkali liquid such as basanite or nephelinite. I don't have any references to back it up at the moment, but I think you can read some of the abstracts or papers you get from a quick Google Scholar search for an idea. $\endgroup$ – Gimelist May 1 '17 at 21:07

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