Segregation vesicles, gas filter-pressing, and igneous differentation

Some vesicles in certain subaerialfìows of bàsalt and basaltic andesite are partìally filled with dark partly-glassy segregation material. The segregatìon matetial has about three times rhe concentrationof K40, Ti02, arrd P20S as the associated host rock. Therefore, the segregation materials are inferred to be fro~el!-~es~d~a~1iquj:c;isthat migratedJn~~ th: ve~icle~. Severalpossible processes.for fìlling the veeicles.are exàrnined within theconfext of'thecooling urne implied by heat conductìon, The preferred process is one in which the resìdual liquid mjgra;tes through a porous and permeable, but rigid, network of interloc'k:ing cryst~ls in response to a pressure .gradìeat generated by vapor saturated crystallisarion. As crystallization p'roc~eds:H2~ is concentrated in the residual.liquid and gas. This-produces ahigher pressure ìn the gas-poor mat~x than l:i'l the, vesicles and forces .liquid to migrate into tbe vesiclés. Textutal features suggestive of or consistent with the process include. (I) vesiculation of segregation material, (2) convex perforations of the .segtegation lining of vesicles, (3,).vuggy vcids in, the matrix, and (4) bubbles in residualglass. We suggest that fìlter pressmg by gas effervescence may petrologically be important to crustal deprhs as great.as several kilorneters.