ARE STABLE METAL ISOTOPE RATIOS RELATED TO SPATIAL POSITION IN MAGMA BODIES?

It is now established that stable isotope ratios of Si and Fe change systematically with differentiation index for a variety of igneous suites. The process causing these isotopic variations is debated. One proposal is that these isotopic variations reflect the process of thermal diffusion, isotopic fractionation occurring within a fluid lying in an imposed thermal gradient. In this process, heavier isotopes are always enriched at the colder end of the gradient. The size of the isotopic fractionation by thermal diffusion does not depend on the magnitude of the gradient but instead simply reflects the offset in temperatures between 2 locations. Temperature gradient experiments involving partially molten silicates show that thermal diffusion isotopic signatures of a melt are rapidly transferred to coexisting crystals. Thus, if differentiation involves crystal-melt mushes lying in the thermal boundary of the upper crust over 10⁵-10⁶ yr durations, thermal diffusion signatures might be expected. Notably, many igneous bodies, both intrusions and magma reservoirs before eruption, show compositional zoning consistent with the crust imposing a thermal condition on the body and with lowest temperature materials forming in the more shallow, colder crust while higher temperature materials form below.

I will present Si and Fe isotopic ratios for spatially located samples within a variety of upper crustal environments, showing that in all locations examined, systematic changes to heavier isotopic compositions occurs toward positions that should have been colder during the magmatic process. Examples described will include 2 diorite to granite intrusions (Cordillera Paine, Patagonia and Aztec Wash, Nevada), zoned silicic volcanic system (Cedar Butte, ID), a layered mafic intrusion capped with granophyre (Sonju Lake Intrusion, MN), and an ophiolite plagiogranite sequence (Troodos, Cyprus).