FIELD CHARACTERIZATION OF FINE-GRAINED LAVAS AND VOLCANIC TEPHRA USING PORTABLE X-RAY FLUORESCENCE
A calibration based on known standard powders (BCR-2, BHVO-2, BIR-1, NIST-278, NIST-688) is able to recover compositions on several suites of natural lavas ranging from basalt to rhyolite as determined by ICP-MS (trace) and ICP-OES (major). Whole rock powders were analyzed under the vacuum at 15 keV for low mass elements and at 40 keV for elements heavier than Ca. Five replicates at 60 s each were analyzed for each sample, shaking the sample between each replicate to ensure representative mixing of the powder. Although the detector operates to <1 keV, we are unable to obtain reliable data for the Na K-alpha due to low signal and peak overlap with the Zn L-alpha line. P gives unreliable results due to significant energy overlap with the much larger Si K-beta peak. Accuracy is better than 5% RSD for Al, Si, Ca, Ti, Mn, Fe; better than 10% for K, Zn, Sr and Zr; and better than 20% for V, Cr, Ni, Cu, Rb and Y. Precision decreases with concentration, detection limits are ~25 ppm for trace elements.
Two types of field applications were tested with this calibration, fine-grained relatively homogeneous lavas from Volcan Baru, Panama and volcanic tephra from Newberry Volcano in Oregon. For field samples, major elements precision is better than 10% RSD and for most trace elements is < 20 %.