magma mixing wall rock assimilation

موضوع: magma mixing wall rock assimilation 23rd نوفمبر 2010, 11:23 pm

Liquid unmixing, magma mixing, wall-rock assimilation

FC seems the dominant process for magma differenciation. However, its global relevance can be questioned:
- Need for huge amounts of cumulate;
- Difficult to operate in a viscous (i.e., Si-rich) magma;
- No “hard” evidence for large-scale magma chamber.

Other possible processes?
I. Liquid unmixing: a process of restricted application

In some phase diagrams, field with two liquids. During cooling, one liquid separates in an emulsion of two magmas. No obvious “everyday life” analogy for that…

However, this appears to be restricted to either (1) unlikely T conditions or (2) specific compositions, e.g. possibly some Fe-rich basalts; silicate-sulfide liquids; alkaline magmas (silicate-carbonates separation). Plus, probably, iron-silicate (during Earth formation).
II. Magma mixing: Good evidence but how generally applicable?

Common observation in felsic rocks: inclusions of more mafic material (commonly dioritic), with
- either quenched, or (sometimes and) diffuse margin
- lobed, pillow-like contacts
- K-spars in the diorite (shouldn’t exist here!), sometimes crossing the border, often reacted/resorbed.

Suggests mechanical mixing (“mingling”) of two actually poorly miscible magmas. Commonly taken as a proof for magma mixing, the granite and its enclaves representing end-members of the mixing, largely on geochemical grounds.
III. Assimilation: Often cryptic

Sometimes, good evidence for rock-wall material dispersed in a pluton, possibly partially molten. Possible reactions between the solid and the liquid can alter the liquid’s chemistry.

But… Problem of heat supply. Melting solids requires heat, which is only present in limited amount in a magma, which is generally not much hotter than its solidus: melting/assimilation of solids must therefore result in quick cooling and eventual solidification of the melt.

A possible solution: AFC or “Assimilation & FC”. Heat released by FC is used to melt the solids.

Even more commonly, no direct evidence for assimilation is found; it is used as an explanation for some geochemical (often isotopic) characteristics of the melts (“contamination”).