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Dear All All Kurt says is of course true, but, because you have to define components differently for the altered rock rather than the initial dunite, none of his examples involve a decrease in the number of degrees of freedom. In both cases, if more fluid flow occurs, one of the secondary phases will disappear and the number of degrees of freedom will increase. There are plenty of field examples that show this in marbles and ultramafic rocks. So really, we should say that infiltration metasomatism leads to an increase in the number of degrees of freedom, not a decrease in the number of phases. If you are baffled by that: the altered dunite started off with just olivine having constant ratios of Fe:Mg:Si (so they combine as 1 component). As soon as a trace of CO2-H2O fluid arrives (and some H2 is lost) it makes a phase which concentrates the Si (enstatite), a phase with just Mg (magnesite) and a phase with the Fe (magnetite). Now these 3 elements must be treated as separate components because their relative proportions are different in each mineral. So we have gone from a rock with 1 immobile component and 1 solid phase to a rock with 3 immobile components and 3 solid phases. For sure, it may have begun to experience metasomatism but at present it still has reserves of minerals available to react and modify the fluid composition to something defined by the rock. Eventually, if the rock is fortunate enough to experience even more fluid infiltration, all the olivine will go and the enstatite may then become carbonated. When the carbonation of enstatite is complete, the number of phases has been reduced by 1 because we now have another component acting as mobile (the Si is being leached out). And to get back to the original query which we lost sight of some time ago - there is some really nice recent work about alteration of sea floor rocks from Ron Frost (B.R. Frost if you are searching) and his collaborators. Bruce Professor Bruce Yardley School of Earth and Environment University of Leeds Leeds LS2 9JT, UK Tel: +44 (0)113 3435227 Fax: +44 (0)113 3435259 -----Original Message----- From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Kurt Bucher Sent: 12 October 2012 13:06 To: [log in to unmask] Subject: [geo-metamorphism] Metasomatism Dear All, The metasomatism topic seems to be attractive. It has, at least, attracted sevral responses that have been adressed directly to me personally. This is of course not the purpose of a mailbase discussion. Anyhow: I have labeled the photomicrograph that was attached to my last mail. On this picture you can see two extremely resorbed olivine grains (olivine 1 and olivine 2) from the original dunite. The metasomatic reaction products are Mg-carbonate (magnesite) and Mg-silicate (enstatite). The dark opaque phase is magnetite (Fe-oxyde). It does not participate in the reaction. It is irrelevant for Korzinsky games. The frozen-in reaction recorded by the texture is: Olivine + CO2 = Enstatite + Magnesite. The reaction, as shown, is an efficient natural CO2-sequestration reaction. The CO2 from the gas/fluid phase is transferred to a solid ( magnesite). However, this was not my major point. The point is, that the metasomatic rock contains more minerals than the starting material. Cheers Kurt