ERUPTION TIMESCALES OF THE COLUMBIA RIVER BASALTS

While significant progress has been made in recent years to understand the timing of flood basalt generation, one question remains extremely difficult to answer: How long do individual flood basalt eruptions last? The answer to this question has major implications for the geochronology, melt transport, melt generation, and ultimately the origin of the Columbia River Basalts (CRBs).

To roughly determine the timescales of CRB eruptions, we used magneto-thermometry on six CRB dikes. This under-utilized 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 2-D conductive cooling model, to determine a ‘lifetime’ of the dike. This method can be used for igneous dikes of any width, any paleo-depth above the brittle-ductile transition, and in any host rock. This method can also distinguish individual thermal pules from composite dikes.

We report results from six CRB dikes in the Chief Joseph dike swarm. These dikes range in thickness from 1-10m, and are hosted in basalt and marble. Our results show that shallow CRB dikes feeding eruptions of 500-1000 km³ have a lifetime of less than two years. These eruption timescales are in agreement with previous studies that suggest rapid emplacement rates, such as Reidel’s (1998) study of the Umatilla Member.