A New Method for Determining the Timescales of Magma Transport in Exhumed Intrusions, Applied to Columbia River Flood Basalt Dikes

Abstract:

At most volcanoes our understanding of eruption mechanics, hazards, and magma evolution relies heavily on records of past eruptions. However, the duration of any pre-historic eruption is a difficult variable to constrain. We have developed a paleomagnetic technique that can constrain the amount of time that a feeder dike is actively transporting magma. We apply this technique to the ca. 16 Ma Columbia River Flood Basalts (CRBs), where eruption durations are unknown but are needed to understand associated climate impacts. To constrain the timescales of flood basalt eruptions, we used magnetic geothermometry on eight CRB dikes. This technique exploits the magnetic properties of wall rock to constrain the maximum temperatures that are reached when heated by a dike or other intrusion. We then combine these results with a thermal conduction model to estimate the duration of heat transport into wall rocks and thus the active ‘lifetime’ of the dike. This method can be used for igneous dikes of any width, and in any host rock. It can also be applied to other intrusions such as sills and plutons. Our results suggest that CRB dikes feeding eruptions of 500-2500 km³ have a lifetime of less than five years. Most dikes appear to have an active lifetime of weeks-days, while others have lifetimes of ~3-5 years. Focusing of magma transport within the dikes may explain this bimodal distribution. As the dike cools, thermoviscous feedbacks and magma solidification produce focused magma channels within the dike. This could allow small portions of the dike to remain active long after most of the dike has solidified. A significant lateral component of flow is also required to reconcile mass balance. This is consistent with the ~100s km length and distributed venting of some CRB dike systems such as the Roza dikes.