BUFFERED AND UNBUFFERED DIKE EMPLACEMENT ON EARTH AND VENUS - IMPLICATIONS FOR MAGMA RESERVOIR SIZE, DEPTH, AND RATE OF MAGMA REPLENISHMENT

Models of the emplacement of lateral dikes from magma chambers under c onstant (buffered) driving pressure conditions and declining (unbuffer ed) driving pressure conditions indicate that the two pressure scenari os lead to distinctly different styles of dike emplacement. In the unb uffered case, the lengths and widths of laterally emplaced dikes will be severely limited and the dike lengths will be highly dependent on c hamber size; this dependence suggests that average dike length can be used to infer the dimensions of the source magma reservoir. Probable e xamples on Earth of the unbuffered case are flanking rift zones on shi eld volcanoes such as the Hawaiian Kilauea East Rift Zone, in which th e dikes of average widths of less than a meter extend for several km f rom the central part of the edifice. In contrast, emplacement of later al dikes in the constant driving pressure (buffered) case can produce dikes which have sizes and widths which are very large, and are indepe ndent of chamber size. For relatively shallow magma chambers, buffered emplacement is expected to produce graben of relatively fixed length which are associated with eruptive fissures and long, large volume lav a flows. A decline in magma supply rate and loss of pressure buffering during the later stages of such eruptions may give rise to caldera fo rmation/collapse events. Deeper dikes are not likely to erupt but will produce surface graben of variable length. Therefore, edifices or dik e swarms which show an extremely wide variation in fracture or dike le ngths are likely to have been formed in buffered conditions. On Earth. the characteristics of many mafic-dike swarms suggest that they were emplaced in buffered conditions (e.g., the Mackenzie dike swarm in Can ada and some dikes within the Scottish Tertiary). On Venus. the distin ctive radial fractures and graben surrounding circular to oval feature s and edifices on many size scales and extending for hundreds to over a thousand km are candidates for dike emplacement in buffered conditio ns.