T154 Geochemical and Petrologic Forensics in the study of Earth’s Magmatism: A Tribute to the Distinguished Career of Jon Davidson (1959–2016) (Posters)

Monday, 23 October 2017
9:00 AM - 6:30 PM
Halls 4EF

PXRF: PACKED WITH POTENTIAL - PROTOCOLS FOR BASALTIC SAMPLES

Paper number: T154-7
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Portable XRF (pXRF) units are becoming more readily available and their usage within geoscience is increasing, but to what extent can we rely on these devices as research tools? This study investigates the best protocols for producing accurate datasets for volcanic rocks. Seven billets of basaltic lava flows of varying thickness were selected, and analyzed using best use practices for the device (an Olympus Vanta Innov-X, M Series). Controls on sample results came from previous XRF analysis, and associated LOI corrections and normalization. pXRF best use practices were developed in accordance with guidance from Olympus and previous studies.

A key aim for practical use of XRF datasets is to classify rock units, and for volcanic rocks this is done using a Total Alkali–Silica (TAS) Diagram. However, pXRF results are devoid of Na2O (a key oxide), as this is below the detection spectrum of the instrument. To counter this lack of data, direct relationships were calculated using the relative relationship between Na2O and K2O, and regression models were then created for specific samples and for basaltic rocks with an SiO2 content ranging from 41-57 weight percent. Calculated results were then normalized, and compared to previous XRF results. TAS-specific regression models (defined by TAS classification) produced more accurate results than the broader basaltic rock regression model (defined by samples with an SiO2 content of 41-57 wt%), which in most cases correctly determined rock type.

Thickness of billets did not affect results, with the minimum tested at 1.15cm (just above the minimum suggested thickness). In comparison to calculated LOI from XRF, no relationship can be established between pXRF data and XRF-derived LOI, indicating that locked in water content remains stable during testing and does not influence data collection. To obtain accurate results, regression models specific to rock type are suggested to calibrate pXRF datasets. However, broader SiO2-defined regression lines can be used to derive elements below detection limits.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-297511

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