FIELD CHARACTERIZATION OF FINE-GRAINED LAVAS AND VOLCANIC TEPHRA USING PORTABLE X-RAY FLUORESCENCE

We calibrate the Bruker Tracer III-SD pXRF with USGS standard powders for use in the field to determine major elements, excluding Na and P, and many trace elements. Traditional handheld XRF units suffer from an inability to analyze low mass elements and factory calibrations optimized for alloys that poorly reproduce silicate compositions. The Tracer-III uses a Rh X-ray tube, a silicon drift detector and has a spot size of 3x4 mm. Given this spot size fine-grained lavas and tephras are well suited to field analysis. A portable vacuum system that attains pressures of ~1 torr minimizes atmospheric absorption of low-energy characteristic electrons and allows the analysis of Mg.

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 %.