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Description
Vesicle cylinders and pipe vesicles are two different features found in basalt lavas. Vesicle cylinders are associated with highly vesiculated basalt lavas which indicates that there must have been a high water content prior to the solidification of the melt. The source of this water is normally attributed to the degasing of magmatic (internal) volatiles. However, some workers suggest that an "external" water source may be involved in the genesis of some highly vesiculated lavas. Pipe vesicles that form near the base of many flows have been used as evidence for surfacewater vaporization into the overlying flow. However, pipe vesicles are also present in the interior of basalt pillows where they appear to terminate against the glassy selvages along the exterior of the pillows. This has been used as an argument against the incorporation of the "external" water into the pillow. In the attempt to resolve the respective roles of "internal" and "external" water in the vesiculation of a basaltic lavas, we examine the nature of an unusual basaltic lava flow in the San Quintin Volcanic Field (SQVF), Mexico. This flow was chosen because (1) it is highly vesiculated, (2) it contains vesicle cylinders, and (3) it overlies a thick pillow palagonite/hyaloclastite zone, indicating a potential source of "external" water. The flow has a diktytaxitic texture, which is unique when compared with the other SQVF flows which have intergranular textures and lack of vesicle cylinders. The diktytaxitic flow contains vertical vesicle cylinders that are evenly spaced and separated by an average of <20 cm. The cylinders have an average length of 25.3 cm and the average width of about 4.5 cm. The flow also contains horizontal vesicle "sheets" and vesicle "pods", both of which indicate the ponding of rising bubbles within the flow. The mineralogy of the vesicle cylinders differs slightly from that of the diktytaxitic host. The cylinders contain higher proportions of red-altered iddingsite, iron-oxide, and apatite. This mineralogy is consistent with the vesicle cylinders having crystallized from a volatile-rich fluid produced by differentiation of the host basalt. As the enriched fluid was drained from the host basalt, it left behind angular open spaces between previously crystallized plagioclase laths. This is the hallmark of diktytaxitic texture. There is no evidence that the vesicle cylinders are more abundant in the lower portion of the flow. Th.e field evidence and mineralogical evidence is consistent with the "internal" process of differentiation, unrelated to the "external" source of water that created the pillowpalagonite/hyaloclastite zone at the base of the diktytaxitic flow.
