PRELIMINARY CUSTOM IRON FORMATION CALIBRATION FOR A PORTABLE X-RAY FLUORESCENCE INSTRUMENT USING THE NEOARCHEAN SOUDAN IRON FORMATION, MINNESOTA, USA

Portable x-ray fluorescence (pXRF) instruments offer the opportunity for real-time acquisition of geochemical data in the field. When used with a custom-built, rock-type specific calibration, it is possible to measure both major and trace element distributions in solid rock samples including outcrops and drill core. The goal of this work is to create an iron formation specific calibration designed to measure key elements that relate to signatures of clastic contamination, seawater, and hydrothermal fluids using the Bruker 5g Tracer pXRF instrument. The Soudan iron formation (NE, Minnesota, USA) was chosen as a test material for calibration development because of its mineralogical homogeneity (quartz, magnetite, hematite). We first measured major and trace elements using standard digestion techniques (ICP-OES, ICP-MS). Key elements of focus include Si, Fe, Ca, Mn, and Al (26.18 wt. %, 27.49 wt. %, 1.29 wt. %, 0.031 wt. %, and 0.034 wt. %, respectively). We performed several operational analyses with the pXRF to understand performance and usability. Geochemical data from the same aliquot of homogenized, powdered material measured by ICP-MS were used for comparison. Each test run used pressed powders analyzed every 5 minutes to assess possible instrument drift. Four preliminary tests looked at instrument warm-up and cool-down periods. The two warm-up tests include 70 runs taken over 3+ hours each, and two cool-down tests where the machine is shut off over increasing amounts of time over 13+ hours and 21 runs. These tests demonstrate a minimum 2.5 hour warm-up period for certain elements such as Si, Ca, and Ni. However, once the pXRF is warmed-up, no significant cooling trend appears for at least 1 hour making field usage plausible. We also tested element presence or absence using different internal calibrations. From the different internal calibrations, 48 out of 77 possible elements were identified, with several elements showing regular identification and comparable values to ICP-MS data (e.g. 30.29-32.70 wt. % Si, 22.79-27.74 wt. % Fe, 1.31-1.90 wt. % Ca, 0.01-0.05 wt. % Mn, and 0.05-0.10 wt. % Al). Future work will focus on the development of custom calibrations for powdered, hand sample, and outcrop materials.