Clinopyroxene trace element chemistry as a proxy for magma compositional variations in the Izu Bonin rear arc over the last 15 million years

This study presents major and trace element chemistry of clinopyroxene (CPX) in 0-15 Ma core material recovered from Site U1437 during IODP Expedition 350. Because no fresh glass is present in the core samples older than 4.4 Ma, and hence there is no way to directly determine magma compositions, my study presents the development of a novel method for calculating liquid compositions from CPX grains in volcaniclastic sediments using distribution coefficients and trace elements measured in CPX. Geochemical data from CPX grains was acquired using a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). These data were used to calculate trace element ratios for intervals in each of the core’s seven lithostratigraphic units. Calculated ratios from Unit I and II (\u3c 4.4 Ma) were compared to measured ratios in coexisting glass to test the validity of the method and showed that the calculated melt chemistries can be used to track magma compositions through time. In turn, the calculated melt chemistries were used to evaluate competing hypotheses regarding the rear arc formation. The calculated melt chemistries for locally-sourced samples showed a change with increasing age from a rear arc seamount chain (RASC) signature (average La/Yb = 3.6 ±0.30 for Units II-V) (4-9 Ma) to a rift-related signature (average La/Yb = 2.0 ±0.16 for Units VI and VII) (9-15 Ma). These older active rift signatures agree with bulk rock chemistry of volcanic clasts recovered from Units VI and VII. These results show that there was a pronounced change in magma source characteristics at around 10 Ma. However, calculated melt chemistries of a small subset of Unit III and Unit V intervals were more enigmatic, reflecting a volcanic front signature with an overall average La/Yb of 0.52 ± 0.042. The progressive change in melt chemistries from a RASC signature to a rift-related signature with depth through the core as seen in the locally sourced intervals suggests a “from the middle” hypothesis for the RASC formation. We interpret the change in signature was due to slab roll-back; the location of site U1437 resided within a rift-related environment during the Miocene, presumably just behind the volcanic front, then became the site of the modern RASC in the present, even further removed from the volcanic front. The few intervals with volcanic front signatures are hypothesized to indicate either 1) reworking of Oligocene volcanic front material that may underly the modern RASC at a depth greater than was sampled by the IODP rear arc core; or 2) continued sampling of volcanic front type magmas in this region even as the volcanic front migrated trench ward

Identification, isolation, and characterization of a novel type of Fukushima-derived microparticle

In the course of the Fukushima nuclear accident, radionuclides were released in various forms, including so-called radiocesium-bearing microparticles (CsMP). So far, four types of CsMP were described: Type A is smaller in size ( 100 μm). In this work, we present a novel type of CsMP (proclaimed Type E). Three particles of Type E were extracted from a contaminated blade of grass that was sampled 1.5 km from the Fukushima Daiichi nuclear power plant in late 2011. They were located using autoradiography, isolated using an optical microscope and micromanipulator, and characterized using scanning electron microscopy, energy dispersive x-ray spectroscopy, and low-level gamma-ray spectrometry. Type E CsMPs are 10–20 μm in size and exhibit an unusually low and barely detectable 137Cs activity of only ≤ 10 mBq per particle. Their brittle and fragile character may indicate a high surface tension

Oxygen Isotope and Fluorine Impurity Signatures during the Conversion of Uranium Ore Concentrates to Nuclear Fuel

Within the front end of the nuclear fuel cycle, many processes impart forensic signatures. Oxygen-stable isotopes (δ18O values) of uranium-bearing materials have been theorized to provide the processing and geolocational signatures of interdicted materials. However, this signature has been minimally utilized due to a limited understanding of how oxygen isotopes are influenced during uranium processing. This study explores oxygen isotope exchange and fractionation between magnesium diuranate (MDU), ammonium diuranate (ADU), and uranyl fluoride (UO2F2) with steam (water vapor) during their reduction to UOx. The MDU was precipitated from two water sources, one enriched and one depleted in 18O. The UO2F2 was precipitated from a single water source and either directly reduced or converted to ADU prior to reduction. All MDU, ADU, and UO2F2 were reduced to UOx in a 10% hydrogen/90% nitrogen atmosphere that was dry or included steam. Powder X-ray diffraction (p-XRD) was used to verify the composition of materials after reduction as mixtures of primarily U3O8, U4O9, and UO2 with trace magnesium and fluorine phases in UOx from MDU and UO2F2, respectively. The bulk oxygen isotope composition of UOx from MDU was analyzed using fluorination to remove the lattice-bound oxygen, and then O2 was subsequently analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope compositions of the ADU, UO2F2, and the resulting UOx were analyzed by large geometry secondary ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU, ADU, and UO2F2 experienced significant oxygen isotope exchange, and the resulting δ18O values of UOx approached the values of the steam. When reduced without steam, the δ18O values of converted ADU, U3O8, and UOx products remained similar to those of the UO2F2 starting material. LG-SIMS isotope mapping of F impurity abundances and distributions showed that direct steam-assisted reduction from UO2F2 significantly removed F impurities while dry reduction from UO2F2 led to the formation of UOx that was enhanced in F impurities. In addition, when UO2F2 was processed via precipitation to ADU and calcination to U3O8, F impurities were largely removed, and reductions to UOx with and without steam each had low F impurities. Overall, these findings show promise for combining multiple signatures to predict the process history during the conversion of uranium ore concentrates to nuclear fuel