Presenter(s): Olivia Andersen – Geological Sciences
Faculty Mentor(s): Ray Weldon
Poster 68
Research Area: Natural Science
Funding: Presidential Undergraduate Research Scholars Program (PURS Program)
Central Oregon is made up of numerous basalt flows and dikes, having been produced from multiple volcanic events. While much of the basalt in the region has been mapped Picture Gorge Basalt of the Columbia River Flood Basalts, the local dikes may have a different source than the type area Picture Gorge Basalt. To further correlate or distinguish the local dikes from the basalt flows in the Central Oregon region, we used petrographic features observed from thin sections and compared the geochemistry of two dike samples to the geochemistry of various local basalt samples.
From the X-Ray Fluorescence (XRF) data of each sample, we produced plots that showed relationships and trends between our samples. There are three clusters in our plots. One cluster contains the dike samples and one of the PGB samples. These samples are abundant in TiO2, P2O5, SiO2 and K2O while having low percentages of MgO, Al2O3, and CaO. The second cluster consists of five PGB samples, with low measures of TiO2, P2O5, SiO2 and K2O while containing higher traces of MgO, Al2O3 and CaO. The last two basalt samples makes a third cluster of data points, which generally falls in the middle of the trend line created by the two clusters mentioned earlier.
The Petrographic features observed further explain the geochemistry. The dike and NR1 samples are a mixture of glass, albite, and few orthoclase grains, resulting in higher oxide content of SiO2, K2O and Na2O were as the cluster of five PGB samples have higher percentages of large pyroxene grains and fewer plagioclase grains and glass, resulting in higher traces of MgO, Al2O3 and CaO.
All samples plot within Grande Ronde Basalt boundaries, which have high silica content and MgO contents ranging from 2.5- 6.5 wt% and TiO2 ranging from 1.6-2.8 wt% (Reidel 2013). Plotting TiO2, P2O5, MgO and K2O offer the most reliable indicators used to identify and distinguish flows (Reidel 2016, Hooper 2000). To further our study, we compared the geochemistry of the PGB to other flow events of the CRFB, such as the Steens and Imnaha basalts. Both the Steens and Imnaha basalts plot below the Grande Ronde Basalt on an alkali vs. silica plot (Camp 2013), indicating that the dikes in Central Oregon are indeed Picture Gorge Basalt.