The mechanism of myrmekite formation deduced from steady-diffusion modeling based on petrography: Case study of the Okueyama granitic body, Kyushu, Japan

Myrmekite is an intergrowth texture consisting of vermicular quartz and albitic plagioclase (Ab93An7 in this study), typically occurring between K-feldspar and plagioclase. It occurs ubiquitously in both metamorphic and granitic rocks; however, its genesis has been an enigma. This paper describes myrmekite's petrography and discusses its genesis from the Okueyama granitic body (OKG), which is a young (14 Ma) granite in Southwest Japan with no evidence of deformation after solidification. The genesis of a newly observed texture, the ‘reaction rim’, will be also discussed in relation to myrmekite. The reaction rim is an albite layer (Ab95An5) with no vermicular quartz between K-feldspar and plagioclase, and it occasionally makes a composite texture with myrmekite. Both myrmekite and the reaction rim are accompanied by a diffusive boundary layer (Olg-layer) with a mean composition of oligoclase (Ab75An25) in the rim of neighboring plagioclase rim.The overall reactions in an open system for the formation of myrmekite and that for the reaction rim are derived based on the following two models: 1) one based on the assumption of conservation of solid volume with arbitrarily specified closure components, and 2) the other based on the assumption of closure of AlO3/2 together with an arbitrarily specified volume factor. Steady diffusion modeling in an open system based on the overall reaction thus derived defines the stability field of myrmekite and of the reaction rim in terms of the ratios of phenomenological coefficients (L-ratios). The steady diffusion models for the above two models have essentially the same features. Myrmekite is stable for large values (> 10) of LAlAl/LCaCa, for moderate values of LAlAl/LSiSi, and for only small values (< 1) of LAlAl/LNaNa. In the case of the reaction rim, the stability field is much wider in a plot of LAlAl/LCaCa vs. LAlAl/LNaNa, and its dependence on LAlAl/LSiSi is stronger than that of myrmekite. The reaction rim is stable only for large values of LAlAl/LCaCa, which is consistent with the case of myrmekite. Exchange cycles for myrmekite and the reaction rim show that the essential formation mechanism is albitization of K-feldspar:KAlSi3O8 + NaO1/2 = NaAlSi3O8 + KO1/2,which is coupled with albitization of plagioclase via diffusive transport of NaO1/2 and SiO2:CaAl2Si2O8 + NaO1/2 + SiO2 = NaAlSi3O8 + CaO + AlO3/2.Formation of myrmekite requires more SiO2 than development of the reaction rim; some of the SiO2 is given by decomposition of K-feldspar and some is supplied from the environment to the boundary between K-feldspar and plagioclase.