Dear experts,
Crystal fractionation is usually an important process that controls chemical composition evolution of magmatic rocks. I want to know how great is the effect of crystal fractionation on SiO2 in a single igneous intrusion without magma mixing and/or contamination? Whether it can lead to more than 10% SiO2 content difference in a single igneous intrusion?
Hope for answers from experts.
With best regards.
Lu
Irrespective of crustal contamination, crystal fractionation by itself canl lead to increase in the silica (and commonly also volatile) content in the evolving liquid. We can refer to the classical studies of Bowen and his co-workers who used the term petrogey residua for the late liquids resulting from crystal fractionation of magmas. In tholeiitic system, ''petrogeny residue" would result into formation of granitic rocks. The classical reference is:
Tuttle OF & Bowen NL, 1958. Origin of granite in light of experimental studies in the system...Geol Soc America Memoir 74, 153 pp.
As Leonid suggested, there are many modern references on the subject.
Cheers, Qasim
Crystal fractionation leads to increase of silica.crystal fractionation may or may not be accompanied with crustal contamination.
We were modelled fraction crystallization in basalts and we found that SiO2 content is very much depends on water content in silicate melt. At water-rich conditions it will change SiO2 very much, but in dry conditions after degassing silica almost don't change with fractional crystallization. Probably it will also work in more silica-rich systems, needs to be checked. See also:
Churikova, T. G., Gordeychik, B. N., Iwamori, H., Nakamura, H., Ishizuka, O., Nishizawa, T., Haraguchi, S., Miyazaki, T., & Vaglarov, B. S. (2015). Petrological and geochemical evolution of the Tolbachik volcanic massif, Kamchatka, Russia. Journal of Volcanology and Geothermal Research, 307, 156-181.
Hi Lu,
the effect of crystal fractionation on the composition of the residual melt strongly depends on the composition of the primitive melt, on the composition of the crystallizing minerals and on the style of fractionation.
Of course the most silica-poor the fractionating assemblage, the most silica-rich the residual melt. An olivine-rich fractionating assemblage can drive to silica enrichment a residual melt much faster than a plagioclase-dominated assemblage. Being plagioclase a mineral with SiO2 content not very different from a basaltic primitive magma, its crystallization can lead in no substantial SiO2 enrichment. The evolved liquid will have lower Al and CaO, but not strongly different (usually exactly the same) Si than the original magma. On the other hand crystallization of silica-free minerals (e.g., magnetite) can lead to strong increase of Si in the evolved melt.
It is very simple to deal with this, using for example the "solver" routine in MS Excel. Please give a look on the attached file. I give it to my graduate students to calculate the hypothetical composition of the assemblages in a rock suite.
Cheers,
michele
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