Julien M. Allaz

Trace Element Analyses by EMP: Pb-in-Monazite and New Multipoint Background Method

University of Oregon, CAMCOR, Eugene (OR), USA Precision in electron microprobe analysis is primarily a matter of counting statistics, and involves voltage, beam current, counting time, and geometric efficiency. Besides potential issues with dead-time correction and beam damage, counting statistics do not significantly affect the accuracy of major and minor element analyses. Accuracy chiefly depends on standards and matrix correction. Additional challenges with both accuracy and high precision appear when measuring trace elements [1]: 1) Analytical precision requires the use of high intensity monochromators, optimized PHA settings, high beam current, higher voltage, and/or lengthy count time. These latter requirements can lead to beam damage, surface contamination, and internal charge effects. 2) Characterization and measurement of the background become crucial for accurate results when the peak-to-background ratio approaches 1. 3) Accurate results require correction or avoidance of peak and background interferences, even for some 2

Late Cretaceous crustal hydration in the Colorado Plateau, USA, from xenolith petrology and monazite geochronology

Formation and subsequent modification processes of lower crust play important roles in evolution and rheological models for continental lithosphere. In the last two decades, numerous xenolith studies have documented metasomatism of continental mantle in the Rocky Mountain region of North America. However, much less attention has been paid to whether and to what extent these processes may have affected the crust. We address the nature and timing of hydrous alteration of extant deep Proterozoic crust in the Colorado Plateau through a petrological and geochronological study of a xenolith from the Red Mesa diatreme in the 30–20 Ma Navajo volcanic field. Fluid-related alteration is widespread in sample RM-21, with the main features recorded by breakdown effects in feldspar, garnet, and allanite, and the production of secondary assemblages characterized by lower Ca plagioclase (or albite) + muscovite + biotite + calcite + monazite + zoisite at estimated conditions of 0.8–0.7 GPa (or ∼27 km depth) and 480 °C. Th-Pb dating by ion probe reveals a period of monazite crystallization at 70–65 Ma, interpreted to reflect late alteration of the high-temperature metamorphic assemblage with hydrous fluid introduced by the shallowly subducting Farallon slab. Similar to previous suggestions involving mantle hydration, the growth of low-density hydrous phases at the expense of high-density, anhydrous minerals, which are abundant in unaltered Proterozoic crust, if sufficiently widespread, could have contributed to elevated topography in the Colorado Plateau and regionally across the Rocky Mountains and High Plains.

Transition metals in the transition zone: Crystal chemistry of minor element substitution in wadsleyite

Abstract As the most abundant solid phase at depths of 410–525 km, wadsleyite constitutes a large geochemical reservoir in the Earth. To better understand the implications of minor element substitution and cation ordering in wadsleyite, we have synthesized wadsleyites coexisting with pyroxenes with 2–3 wt% of either TiO2, Cr2O3, V2O3, CoO, NiO, or ZnO under hydrous conditions in separate experiments at 1300 °C and 15 GPa. We have refined the crystal structures of these wadsleyites by single-crystal X ray diffraction, analyzed the compositions by electron microprobe, and estimated M3 vacancy concentration from b/a cell-parameter ratios. According to the crystal structure refinements, Cr and V show strong preferences for M3 over M1 and M2 sites and significant substitution up to 2.9 at% at the tetrahedral site (T site). Ni, Co, and Zn show site preferences similar to those of Fe with M1≈ M3 > M2 > T. The avoidance of Ni, Co, and Fe for the M2 site in both wadsleyite and olivine appears to be partially controlled by crystal field stabilization energy (CFSE). The estimated CFSE values of Ni2+, Co2+, and Zn2+ at three distinct octahedral sites show a positive correlation with octahedral occupancy ratios [M2/(M1+M3)]. Ti substitutes primarily into the M3 octahedron, rather than M1, M2, or T sites. Ti, Cr, and V each have greater solubility in wadsleyite than in olivine. Therefore these transition metal cations may be enriched in a melt or an accessory phase if hydrous melting occurs on upward convection across the wadsleyite-olivine boundary and may be useful as indicators of high-pressure origin.

Focused Interest Group on Microanalytical Standards (FIGMAS): Assessing the Quality, Availability and Need for Standards in the Microanalytical Community

It has been recognized over the past years that different electron microprobe and scanning electron microprobe laboratories use different sets of standards or reference materials for quantitative analysis. Unfortunately, some of these standards either have become unavailable (e.g., some natural minerals from the Smithsonian Institution collection) or are only available to a restricted group of people (e.g., internal reference materials). Other synthetic materials are also available commercially or provided by other institutions and research centers. However, they sometimes lack either broad availability or acceptable characterization (e.g., NIST glasses, Corning glasses, Drake & Weill REE-glasses... [1,2]). Another important problem for the community is a clear assessment of standard quality (the “Good”, “Bad” and “Ugly” of Carpenter [3]): “good” homogeneous standards with accurate compositional information and without impurities or inclusions are rare, whereas “bad” standards, which lack good characterization, are more common. Individual lab managers do commonly examine their own standard collections to re-evaluate compositional homogeneity and test the accuracy of published compositions. Standards are also frequently re-analyzed at individual labs using various techniques, and therefore multiple accepted compositions for individual standards may exist.

Age, petrochemistry, and origin of a REE-rich mineralization in the Longs Peak-St. Vrain batholith, near Jamestown, Colorado (U.S.A.)

Abstract An unusual rare earth element (REE) mineralization occurs at a locality known as the “Rusty Gold” within the anorogenic 1.4 Ga Longs Peak-St. Vrain monzo- to syenogranite Silver Plume-type intrusion near Jamestown, Colorado (U.S.A.). Irregular-shaped centimeter- to decimeter-sized mineralized pods and veins consist of zoned mineral assemblages dominated by fluorbritholite-(Ce) in a gray-colored core up to 10 cm thick, with monazite-(Ce), fluorite, and minor quartz, uraninite, and sulfides. The core zone is surrounded by a black, typically millimeter-thick allanite-(Ce) rim, with minor monazite-(Ce) in the inner part of that rim. Bastnäsite-(Ce), törnebohmite-(Ce), and cerite-(Ce) appear in a thin intermediate zone between core and rim, often just a few hundreds of micrometers wide. Electron microprobe analyses show that the overall REE content increases from rim to core with a disproportionate increase of heavy REE (∑HREE increases 10-fold from 0.2 to 2.1%) compared to light REE (∑LREE increases twofold from 21.3 to 44.3%). The fluorbritholite-(Ce) contains minor U, Th, Fe, Mn, and Sr (total 0.10 apfu), with Al, Mg, Na, K, Ti, Pb, S, and Cl below instrument detection limits. Cerite-(Ce) is a minor constituent of the thin zone between the inner rim and the core. The cerite-(Ce) is Fe-rich with low Ca, and minor Al, Mg, and Mn, whereas törnebohmite-(Ce) is Al-rich and Ca-poor. Monazite-(Ce) and uraninite U-Th-Pb microprobe ages yield 1.420(25) and 1.442(8) Ga, respectively, confirming a co-genetic relationship with the host ca. 1.42(3) Ga Longs Peak-St. Vrain granite. We suggest the origin of the REE mineralization is a F-rich and lanthanide-rich, either late-magmatic hydrothermal fluid or residual melt, derived from the granite. This late-stage liquid, when becoming progressively enriched in REE as it crystallized, could explain the observed concentric mineralogical and geochemical zoning.

Correction to: Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

In the version of the paper originally published the current address of author Jeremy T Ross was given incorrectly as Untied Status Marine Corp, 3351 Onslow Drive, Camp Lejeune, NC 28547, USA.

Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, εNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N <50 with significant negative Eu anomalies, which contrast with higher (La/Yb)N >100 and absence of an Eu anomaly in both the aplites and segregations. These data are consistent with the aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher εNd1.42Ga values for the aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.

Testing a New Electron Microprobe and Developing New Analytical Protocols

Since the development of the first electron probe microanalyzer (EPMA) [1], hardware and software have evolved considerably. Nowadays, it is possible to achieve high stability, precision and accuracy. Field emission (FE-) EPMA opens the door to sub-micron analysis at low (over-) voltage, the large-area monochromator and higher stability permit trace element analysis down to 1-10 ppm range, and Energy Dispersive Spectrometers (EDS) enable fast mineral identification, hyperspectral mapping, and good (standard-based) quantitative work. As the precision and stability of the instrument improve, more sensitive tests must be developed to ensure high quality analysis. This paper is an overview of the optimization and quality control of a new EPMA, and discusses the development of new analytical protocols, focusing on trace element analysis.

Focused Interest Group on Microanalytical Standards (FIGMAS): An Update

Reference materials and standards (RMS) play a crucial role in quantitative microbeam analysis, as they are the basis for instrumental calibration, data quality assurance (e.g., secondary standards) and interlaboratory comparison. Good standards that have been evaluated and certified for their homogeneity and reference composition are rare and only available from a handful of recognized providers. Whereas some crucial RMS have or will soon become unavailable, some researchers continue to develop and test new RMS [1-3], at times becoming more widely disseminated. Unfortunately, practical constraints make RMS development often difficult [4], and leave analysts with a set of “second choice” reference materials instead of certified standards. Such reference materials may suffer from various shortcomings such as questionable provenance, homogeneity or impurity issues (natural samples), contamination (synthetic materials), or incomplete or inaccurate reference compositions. Furthermore, variations between individual batches and/or updated reference compositions [1, 5]) can lead to confusion and the potential risk for mixing old and new values. Efforts have to be made to keep a record, evaluate, and guarantee the quantity of RMS for the next century.

Microanalytical Standards, Reference and Research Materials: Continuing the Effort toward Breaking the Accuracy Barrier

Reference materials and standards play a critical role in quantitative microbeam methods, as they are necessary for instrumental calibration, quality assurance (e.g. secondary standards) and inter-laboratory comparison. Certified materials (“good” standards) are available from a handful of providers, having been tested for micro-homogeneity and reference composition through independent methods and round robins. However, this process is time consuming and costly [1]. Hence, most reference materials in use are not formally certified, and many analysts use compilation values or their own preferred values.