UPWARD AND DOWNWARD FLOW IN A CAMPTONITE DIKE

A 1.6-m-wide vertical camptonite dike, cutting the Holyoke Basalt on Higby Mt. in Middlefield CT, contains deformed vesicles indicating both upward and downward flow. The flow directions in this dike were determined from the shape and orientation of deformed vesicles and ocelli (felsic blobs). Near the margins of the dike, vesicles and ocelli form imbricated ellipses that indicate upward flow. Approximately 30 cm from the contacts, however, they reverse their imbrication direction, indicating the middle of the dike flowed downward. Toward the center of the dike, vesicles become spherical. This is to be expected, because the shear stress approaches zero at the center of a sheet of liquid in laminar flow. In a vertical section across the middle of the dike, the deformed vesicles consequently appear to form a parabolic drape. The vesicle size systematically increases from 0.1 mm near the margin, to ~15 mm at the center of the dike, indicating that they nucleated and grew in situ, rather than being transported in from elsewhere. Near the margins, vesicles have smooth, convex outward surfaces, indicating that they were still growing at the time of quenching. In the central part of the dike, however, their shape becomes convoluted and irregular, indicating that they were collapsing at the time of solidification. Assuming simple conductive cooling, the period of backflow constitutes 83% of the total solidification time of the dike. The reason for backflow is unknown but might have resulted from lateral spreading of the dike at a lower depth. Without the presence of vesicles in the dike, the backflow would have gone unrecognized. This raises the interesting question of whether backflow is common in other dikes, but goes unnoticed due to the lack of evidence.