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 crystallization. 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 correlations among increasing cylinder abundance, increasing lava porosity, and increasing groundmass crystal size. These features suggest unusually high water contents in the magma before eruption. Although both 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-rhyolitic glass, Fe-Ti oxides, anorthoclase, apatite + fayalite &aegerine. Geothermometry indicates that the cylinders began forming at N 1 lOO-1075°C but ceased crystallizing at N 950°C. Pre-eruptive, high-temperature, iddingsite alteration of olivine phenocrysts in many lavas containing vesicle cylinders shows that the fo, of the magmas was extremely high at eruption ( N 10m4). After eruption, the fo, of the lavas fell dramatically to values of about lo-” and conditions paralleled the FMQ buffer to final crystallization. Because the iddingsite forms before eruption, the magmas may become relatively oxidizing by addition of meteoric water late in their evolution. Oxygen-18 analyses of four basalt-differentiate pairs suggest that meteoric water addition has occurred in some of the magmas. Field relations and thermal profiles of cooling lava flows limit the growth period of vesicle cylinders to l-5 days after flows of typical thickness (3-10 m) come to rest. Estimated viscosities of host lavas and frothy differentiate during cylinder growth are I lo6 and _ lo4 poise, respectively. Although an adequate quantitative model describing growth of vesicle cylinders does not exist, they apparently form by bubble nucleation and resulting density instability above the rising lower solidification front of the cooling flows. As the coalesced bubbles rise, residual melt and additional vapor migrate into the low-pressure, vertical discontinuity formed by tbe plume.