A NOVEL METHOD TO DETERMINE THE TRANSPORT LIFETIMES OF IGNEOUS INTRUSIONS

To understand magma transport, we often rely on magmatic intrusions that have frozen and been exhumed at the surface. These records are imperfect, in part because they show an integrated history of igneous activity covering a typically unknown amount of time. Here we implement a novel paleomagnetic and thermodynamic technique that can constrain the amount of time that an igneous intrusion is actively transporting magma. First, a paleomagnetic sampling transect is done in the wall-rock to determine which samples have exceeded their Curie or blocking temperatures (typically 550-580 °C for magnetite). Samples that have exceeded the Curie temperature will lose their original paleomagnetic directions, and long-lived intrusions will heat a greater volume of wall rock than short-lived intrusions. We then combine these results with a 1-D thermal conduction model to quantitatively estimate the duration of heat transport into wall rocks, parameterizing magmatic heating with a kinematic model for steady, possibly monotonically declining, magma flow within the intrusion. Inversions for magnetic reset distances in the context of this model thus estimate the ‘active lifetime’ of the intrusion.

We demonstrate the usefulness of this technique by applying it to 25 dike segments in the Columbia River Basalts and Central Atlantic Magmatic Province. We found that none of the dike segments were active for more than ~6 years, and many segments were active for a few months or less. We found no correlation between dike thickness, segment length, or paleodepth and dike lifetime. Within a single dike complex, individual segments can have large differences in reset distance (and hence active lifetime estimates) despite having the same composition and being in close proximity to each other. These results suggest that shallow magma transport is a highly heterogeneous process. We speculate that flow localization within dikes explains this heterogeneity and thus is common even in flood basalts with high magma flux rates.