Abundances of minor and trace elements in olivine are increasingly used as petrogenetic indicators for mantle source lithologies, mantle metasomatism history, mantle potential temperatures, and magmatic differentiation. As it is common for olivine to be complexly zoned on a fine-scale, high precision analytical methods for EPMA (electron microprobe microanalyzer, or Electron Microprobe) trace element analysis under high spatial resolution have been developed. However, previous studies have focused more on analytical precision with fewer efforts in examining the accuracy of the data. In this study, we used the Cameca SXFive field emission (FE) EPMA to fully evaluate the effects of beam settings, background offsets and background regression models, and primary calibration standards on the data accuracy of 10 key petrogenetic elements (Na, Al, P, Ca, Ti, Cr, Mn, Co, Ni, and Zn) using MongOlSh11–2 olivine as a reference material. Our results indicate that high voltage, high beam current and long counting time not only improve data precision, but also improve data accuracy, especially on elements with low P/B (peak/background) ratios such as Zn and Cr. Importantly, careful background offsets and background regression models need to be obtained via high resolution WDS relative scans or step scans on each target element. Special care needs to be paid to Co element analysis to avoid or correct for peak interference of Fe Kβ. Among 10 minor and trace elements, exponential background regression models need to be applied to Al, Mn, and Ti elements, whereas other elements require linear background regression. Furthermore, to avoid Al and Zn surface contamination due to alumina polishing or brass presence, ultrasonic cleaning between each intermediate polishing steps and plasma cleaning immediately prior to EPMA experiments is highly recommended. Micro-inclusions such as chromite and spinel in olivine or adjacent Ca-rich phases need to be avoided to minimize primary or secondary fluorescence-related contamination on Al, Cr, or Ca. As a volatile element, Na element needs to be analyzed first with appropriate counting time to minimize the Na loss under high beam conditions. It needs mentioning that major elements (Mg, Fe, and Si) are best analyzed using MongOlSh11–2 or San Carlos olivine as primary standards for calibrations, which can yield more accurate data for both major elements and trace elements because of the improved matrix- corrections. Using our recommended analytical protocols, we have successfully discriminated “depleted” mantle olivine cores from an EMORB in northern East Pacific Rise (EPR) via Ca, Ti, Ni, Co, and Mn abundances. Our olivine data from Siqueiros Transform and the nearby 8◦20′ N seamounts also help reveal a metasomatized peridotite mantle beneath the northern EPR. Overall, the protocols proposed in this study can serve as a guide for accurate EPMA olivine trace element analyses, which potentially contributes to the efforts of fostering a comparable olivine database worldwide.
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A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K‐Feldspar Reference Materials
Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K‐feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre‐ to micrometre‐scale for Si, Al and Na with < 2000 μg g‐1total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA‐ICP‐MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K‐feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with > 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K‐feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified.
more » « less- PAR ID:
- 10433308
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Geostandards and Geoanalytical Research
- Volume:
- 47
- Issue:
- 4
- ISSN:
- 1639-4488
- Format(s):
- Medium: X Size: p. 907-929
- Size(s):
- p. 907-929
- Sponsoring Org:
- National Science Foundation
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