碱性长石在次固相下的微组构重组织:碱性长石流体相互作用

辫状微纹长石通过粗化、微孔隙和亚颗粒的形成 ,最终发展为脉状微纹长石和条纹长石 ,粗化沿着不规则的前锋从晶体边缘往内部推移和扩展。亚颗粒和微孔隙的形成极大地提高了碱性长石的反应性和岩石的渗透性。通过沿着晶体边缘的拱状褶边、平行褶边以及褶头的过渡带往整个晶体内的推移和“繁殖” ,辫状微纹长石最终改造为脉状微纹长石和条纹长石。水从褶边向晶体内部的扩散促进了褶边的粗化以及过渡带的发展。流体长石相互作用机制包括 :体积扩散、管道扩散、溶解再沉淀。碱性长石流体的氧同位素交换机制主要是溶解再沉淀。碱性长石在次固相下的微组构重组织发生于约 4 75～ 4 0 0℃的温度下 ,区块性条纹长石的形成温度更低。碱性长石的微组构重组织导致放射成因氩的局部和部分丢失 ,从而给出年轻的表面年龄。

MICROTEXTURAL REORGANIZATION OF ALKALI FELDSPAR DURING DEUTERIC ALTERATION:ALKALI FELDSPAR-FLUID INTERACTION

Vein microperthite and perthite have been formed via fluid mediated coarsening, formation of micropores and sub grains within braid microperthite. Coarsening develops and advances inwards from the irregular head on the grain boundary. The formation of sub grains and micropores greatly enhances the reactivity of alkali feldspar and rock permeability. The arched and parallel sided pleated rims, and the transitional zone above the pleat head advance and propagate through the entire crystal; the braid microperthite is eventually transformed to vein microperthite and perthite. Water diffusion from the pleated rim to the crystal's interior promotes the coarsening of pleated rim and development of the transitional zone. The mechanisms of alkali feldspar fluid interaction include volume diffusion, pipe diffusion, and dissolution reprecipitation. The mechanism of oxygen isotope exchange between alkali feldspar and fluid is mainly dissolution reprecipitation. Deuteric microtextural reorganization of alkali feldspar occurs at a temperature from about 475 to 400℃. However, the formation of patchy microperthite could continue to lower temperatures. Reorganization of braid microperthites into parallel sided pleated rims and veins tends to result in local loss of radiogenic argon when coherency is lost, thereby giving younger apparent ages.