VESICLE CYLINDERS IN VAPOR-DIFFERENTIATED BASALT FLOWS

Vesicle cylinders are vertical pipes filled with bubbles and residual melt that differentiate from diktytaxitic basalt flows during crystall ization. They grow from about 0.25 m above the base of the flow to the bottom of the chilled flow top. Field relations limit their growth to the period between cessation of lava movement and deep penetration of columnar joints. Basalts containing vesicle cylinders show positive c orrelations among increasing cylinder abundance, increasing lava poros ity, and increasing groundmass crystal size. These features suggest un usually high water contents in the magma before eruption. Although bot h Vesicle cylinders and host lava are ''basaltic'', the cylinders are enriched in elements not removed by the initial crystallization of the host: Fe, Mn, Ti, Na, K, P and many incompatible trace elements. The last residues to solidify within the cylinders consist of dacitic-rhyo litic glass, Fe-Ti oxides, anorthoclase, apatite +/- fayalite +/- aege rine. Geothermometry indicates that the cylinders began forming at sim ilar to 1100-1075 degrees C but ceased crystallizing at similar to 950 degrees C. Pre-eruptive, high-temperature, iddingsite alteration of o livine phenocrysts in many lavas containing vesicle cylinders shows th at the f(O2) of the magmas was extremely high at eruption (similar to 10(-4)). After eruption, the f(O2) of the lavas fell dramatically to v alues of about 10(-11) and conditions paralleled the FMQ buffer to fin al crystallization. Because the iddingsite forms before eruption, the magmas may become relatively oxidizing by addition of meteoric water l ate in their evolution. Oxygen-18 analyses of four basalt-differentiat e pairs suggest that meteoric water addition has occurred in some of t he magmas. Field relations and thermal profiles of cooling lava flows limit the growth period of vesicle cylinders to 1-5 days after flows o f typical thickness (3-10 m) come to rest. Estimated viscosities of ho st lavas and frothy differentiate during cylinder growth are less than or equal to 10(6) and similar to 10(4) poise, respectively. Although an adequate quantitative model describing growth of vesicle cylinders does not exist, they apparently form by bubble nucleation and resultin g density instability above the rising lower solidification front of t he cooling flows. As the coalesced bubbles rise, residual melt and add itional vapor migrate into the low-pressure, vertical discontinuity fo rmed by the plume.