UNDERSTANDING MAGMA TRANSPORT VIA THE BAKED CONTACT TEST: AN EXAMPLE FROM THE COLUMBIA RIVER BASALTS

Since the baked contact test was first introduced, it has been primarily used to understand paleomagnetic directions from wall-rock around intrusions. However, the baked contact test can also inform us about the intrusions themselves. Here we implement a novel combination of the baked contact test and thermal modeling to constrain the amount of time that Columbia River Basalt (CRB) dikes were actively transporting magma. Long-lived dikes have a larger thermal aureole in the host-rock than short-lived dikes, and the baked contact test can be used to constrain the size of this thermal footprint in a variety of host-rock types. We then combine these results with a 1-D thermal conduction model to quantitatively estimate the duration of heating, and thus the duration of dike activity.This ‘magnetic geothermometer’ approach was applied to 26 CRB dikes exposed at shallow levels, ranging in width from 8 - 60 m, and spanning six formations. We found that no dike was active for more than 5 years, and most segments were active for a few months at most. These results are supported by co-located thermochronologic data (apatite & zircon U-Th-He, apatite & zircon fission track, biotite 40Ar/39Ar) on a subset of the sampled dike segments. Notably, we found a correlation between the amount of partially melted wall-rock and the inferred duration of dike heating, but no correlation among factors such as dike thickness, strike, paleodepth, and dip. Going forward, this magnetic geothermometer approach can be used to determine the active lifetimes of most igneous intrusions, and thereby advance our understanding of magmatic systems.