THE NEAR-UBIQUITY OF OPEN-SYSTEM MAGMATIC PROCESSES (Invited Presentation)

Up to and including the early 1970’s, igneous petrologists were concerned almost exclusively with the genesis of magmas by partial melting and their subsequent evolution by fractional crystallization (granitizers notwithstanding). Think about it: (1) What is the probability that primitive basaltic magma will ascend, rapidly or otherwise, through continental crust (particularly if it is under compression) without some interaction with crust that will alter its composition (pejorative ‘contamination’), and (2) What is the probability that basaltic magmas will ascend through the crust via well-established conduit systems without some blending with resident magmas. As large, long-lived magmatic systems exist by virtue of repeated magma recharge, concerns about thermal consequences are diminished.

This contribution honors Jon Davidson’s dedication to elucidating open-system processes by, among other things, developing the Crystal Isotope Stratigraphy method, which remains the most reliable proof of such processes. If inverting magma chemistry to the compositions of mantle sources is the objective, rigorously interrogating the relevant magmatic system, from the scale of the volcano (whole-rock analyses based on high-density sampling) to the petrographic scale (crystals, matrix glass, melt inclusions) will improve the probability that the least modified sample will provide the best-possible starting point for such an exercise. This presentation will focus on examples of open magmatic systems from the East Pacific Rise, the Taos Plateau volcanic field, and arc volcanoes of the Andean Southern Volcanic Zone (Central Chile). The identification and impact of open-system processes on favoring calc-alkaline evolution trends over tholeiitic trends, completely independent of tectonic setting, will serve as an underlying theme.