Fine-Scale Shipboard Resolution Among Mafic Igneous Rock Sequences Recovered During Ocean Drilling: Quantitative PXRF Determination of Key Elements on Rock Surfaces and Powders During IODP Expedition 352

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While handheld energy dispersive X-ray spectrometers (pXRF) are designed and generally used for qualitative survey applications on land, we employed a handheld pXRF instrument to conduct rapid, quantitative elemental measurements on recovered mafic rock samples onboard the JOIDES Resolution over the eight weeks of the International Ocean Discovery Program (IODP) Expedition 352 to the Izu-Bonin forearc. Working curves were developed via pXRF measurements of a suite of geologic standard reference materials and characterized lavas compositionally similar to our drilling targets (basalts and boninites), which permitted accurate quantitative measurements for a range of major and trace elements on both sample powders and rock surfaces, with good agreement between pXRF and shipboard ICP-OES data (and subsequently with post-cruise XRF determinations) for most elements. pXRF performance on the ship was found to be optimal for elements between Z=19 and Z=40, yielding reproducible data for K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Rb, Sr, and Zr on both powdered samples and rock surfaces. Some 2000 pXRF determinations were made on igneous materials recovered during >1600 m of basement drilling on the Expedition.

While XRF and pXRF instrumentation have been sporadically employed on previous research cruises, IODP Expedition 352 is the first time the instrument has been used to establish a detailed, high-density chemostratigraphy of rock core surface measurements, allowing the recognition and resolution of chemically distinct eruptive units in near real-time, as well as larger-scale trends in compositional change potentially related to changes in mantle sources during subduction initiation magmatism. The rapid identification of geochemical trends vastly improved our selection of samples for shipboard and shore-based analysis, facilitated drilling decisions on the ship, and has permitted a more comprehensive interpretation of Expedition results.


Complete list of authors: Timothy Chapman, Marguerite Godard, Maria Kirchenbaur, Wendy R. Nelson, Julie Prytulak, Kenji Shimizu, Scott Whattam

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Geological Society of America Abstracts with Programs, v. 48, issue 7, no. 128-14