THE DEVELOPMENT OF COMPONENTRY ISOTOPIC MICROSAMPLING: THE DAVIDSON YEARS (Invited Presentation)

In his quest to more fully understand volcanic systems, Dr. Jon Davidson sought new methodologies to glean more information from the components of volcanic rocks than from traditional sources of information. Whole-rock and whole-crystal isotopic analyses are common, traditional analytical techniques in the Geological Sciences, however, Jon recognized that data from these techniques potentially represented averaged values. His vision was to access and interrogate individual zones in crystals thus allowing researchers to more thoroughly constrain the petrogenetic histories of volcanic and plutonic systems. Consequently, Jon and his students spearheaded the development of techniques that allowed for isotopic fingerprinting of magmatic components at the crystal and sub-crystal scale in conjunction with established methodologies.

These new techniques involved micromilling growth zones of crystals in situ and low-blank chemical processing. Isotopic microsampling, when used in concert with Nomarski Interferometry (NDIC) to enhance compositionally dependent textural features of crystals, and electron microprobe analyses to determine compositional variations in target crystals, provided a combined textural, chemical and isotopic relationship that was used to diagnose open system processes such as magma mixing, contamination and recharge.

A variety of volcanic and plutonic systems were addressed and new hypotheses tested under Davidson’s guidance during these developmental years. From these studies, it became clear that mineral phases are not always in isotopic equilibrium with other co-existing phases and/or their own host groundmass. Significantly, it was found that intracrystalline isotopic heterogeneities are common, reflecting crystal growth in isotopically changing magmatic systems. This shift in capabilities and collection of more detailed observations has led to significant advances in our understanding of magmatic evolutionary processes.