TEMPERATURE CONSTRAINTS ON THE GINKGO FLOW OF THE COLUMBIA RIVER BASALT GROUP

This study provides the first quantitative estimate of heat loss for a Columbia River Basalt Group flow. A glass composition-based geothermo meter was experimentally calibrated for a composition representative o f the 500-km-long Ginkgo flow of the Columbia River Basalt Group to me asure temperature change during transport. Melting experiments were co nducted on a bulk sample at 1 atm between 1200 and 1050 degrees C. Nat ural glass was sampled from the margin of a feeder dike near Kahlotus, Washington, and from pillow basalt at distances of 120 km (Vantage, W ashington), 350 km (Molalla, Oregon), and 370 km (Portland, Oregon). G inkgo basalt was also sampled at its distal end at Yaquina Head, Orego n (500 km). Comparison of the glass MgO content, K2O in plagioclase, a nd measured crystallinities in the experimental charges and natural sa mples tightly constrains the minimum flow temperature to 1085 +/- 5 de grees C. Glass and plagioclase compositions indicate an upper temperat ure of 1095 +/- 5 degrees C; thus the maximum temperature decrease alo ng the flow axis of the Ginkgo is 20 degrees C, suggesting cooling rat es of 0.02-0.04 degrees C/km. These cooling rates, substantially lower than rates observed in active and historic flows, are inconsistent wi th turbulent flow models. Calculated melt temperatures and viscosities of 240-750 Pa.s allow emplacement either as a fast laminar flow under an insulating crust or as a slower, inflated flow.