Experiments with Low Voltage Field Emission EPMA

We report results from 5-7 keV Field Emission EPMA experiments on selected natural minerals and synthetic materials to illustrate some strengths -- and pitfalls --of low keV FE-EPMA. In a silicate mineral in pseudotachylite from South Mountain, AZ (Goodwin, 1999), the spatial resolution (equation of Merlet & Llovet, 2012, with an 80 nm diameter beam) at 7 keV for Si Ka is calculated to be 588 nm, 391 nm for Ca Ka and 641 nm for Fe La. This pseudotachylite contains abundant 5-10 um sieve-textured crystals full of inclusions with low BSE intensity. Previous 15 keV work suggested the sieve phase was amphibole. At 7 keV, it is possible to identify the compositions of the submicron inclusions as SiO2 and a K-rich alumino-silicate phase; the host composition is epidote. The enhanced resolution of FE-EPMA reveals problems with some microanalytical standards. Vicenzi and Rose (2008) showed submicron inclusions in the Smithsonian Kakanui hornblende standard. Our 7 keV experiments show the ~400 nm inclusions consist of a silicate phase (glass?), Fe-Ti oxide and possibly a gas bubble, concentrated along planes or grain boundaries. SEM imaging of an inclusion analyzed with a focused FE beam shows radiating trails of debris on the hornblende host, consistent with residue from a popped vapor bubble in the inclusion. How should FE-EPMA handle standards that may have inclusions? Use a focused beam avoiding inclusions? Sometimes, perhaps. However, we used a defocused beam to "average" the phases. The results show little or no deviation from the published wet chemical analysis. Operation at reduced keV may require use of non-traditional X-ray lines (e.g. Gopon et al, 2013 for Fe Ll vs Fe La). Experiments at 5 keV were also performed upon a synthetic material enriched in Nd (Nd-Mg-Zn). Fischer & Baun (1967) demonstrated problems with the Ma/Mb lines of REE; we find that use of the Nd Mz line is necessary in order to achieve reasonable results in this material (98 wt% total, Nd 36 wt% vs 126 wt% total, Nd 67 wt% with Mb line). We also report on 7 and 15 keV time dependent intensity variations in Ir-coated calcite and dolomite and on the lack of interfering lines with 6 Å Ir

The Cougar Point tuff: Implications for thermochemical zonation and longevity of high-temperature, large-volume silicic magmas of the Miocene Yellowstone hotspot

The 12.7-10.5 Ma Cougar Point Tuff in southern Idaho, USA, consists of 10 large-volume (>10²-10³ km³ each), high-temperature (800-1000 °C), rhyolitic ash-flow tuffs erupted from the Bruneau-Jarbidge volcanic center of the Yellowstone hotspot. These tuffs provide evidence for compositional and thermal zonation in pre-eruptive rhyolite magma, and suggest the presence of a long-lived reservoir that was tapped by numerous large explosive eruptions. Pyroxene compositions exhibit discrete compositional modes with respect to Fe and Mg that define a linear spectrum punctuated by conspicuous gaps. Airfall glass compositions also cluster into modes, and the presence of multiple modes indicates tapping of different magma volumes during early phases of eruption. Equilibrium assemblages of pigeonite and augite are used to reconstruct compositional and thermal gradients in the pre-eruptive reservoir. The recurrence of identical compositional modes and of mineral pairs equilibrated at high temperatures in successive eruptive units is consistent with the persistence of their respective liquids in the magma reservoir. Recurrence intervals of identical modes range from 0.3 to 0.9 Myr and suggest possible magma residence times of similar duration. Eruption ages, magma temperatures, Nd isotopes, and pyroxene and glass compositions are consistent with a long-lived, dynamically evolving magma reservoir that was chemically and thermally zoned and composed of multiple discrete magma volumes

Pyroxene thermometry of rhyolite lavas of the Bruneau–Jarbidge eruptive center, Central Snake River Plain

The Bruneau–Jarbidge eruptive center of the central Snake River Plain in southern Idaho, USA produced multiple rhyolite lava flows with volumes of <10 km³ to 200 km³ each from ~11.2 to 8.1 Ma, most of which\ud follow its climactic phase of large-volume explosive volcanism, represented by the Cougar Point Tuff, from 12.7 to 10.5 Ma. These lavas represent the waning stages of silicic volcanism at a major eruptive center of the Yellowstone hotspot track. Here we provide pyroxene compositions and thermometry results from several lavas that demonstrate that the demise of the silicic volcanic system was characterized by sustained, high pre-eruptive magma temperatures (mostly ≥950 °C) prior to the onset of exclusively basaltic volcanism at\ud the eruptive center. Pyroxenes display a variety of textures in single samples, including solitary euhedral crystals as well as glomerocrysts, crystal clots and annealed microgranular inclusions of pyroxene\ud ±magnetite± plagioclase. Pigeonite and augite crystals are unzoned, and there are no detectable differences in major and minor element compositions according to textural variety — mineral compositions in the\ud microgranular inclusions and crystal clots are identical to those of phenocrysts in the host lavas. In contrast to members of the preceding Cougar Point Tuff that host polymodal glass and mineral populations, pyroxene compositions in each of the lavas are characterized by single rather than multiple discrete compositional modes. Collectively, the lavas reproduce and extend the range of Fe–Mg pyroxene compositional modes observed in the Cougar Point Tuff to more Mg-rich varieties. The compositionally homogeneous populations of pyroxene in each of the lavas, as well as the lack of core-to-rim zonation in individual crystals suggest that individual eruptions each were fed by compositionally homogeneous magma reservoirs, and similarities with the Cougar Point Tuff suggest consanguinity of such reservoirs to those that supplied the polymodal Cougar Point Tuff. Pyroxene thermometry results obtained using QUILF equilibria yield pre-eruptive magma temperatures of 905 to 980 °C, and individual modes consistently record higher Ca content and higher temperatures than pyroxenes with equivalent Fe–Mg ratios in the preceding Cougar Point Tuff. As is the case with the Cougar Point Tuff, evidence for up-temperature zonation within single crystals that would be consistent with recycling of sub- or near-solidus material from antecedent magma reservoirs by rapid reheating is extremely rare. Also, the absence of intra-crystal zonation, particularly at crystal rims, is not\ud easily reconciled with cannibalization of caldera fill that subsided into pre-eruptive reservoirs. The textural, compositional and thermometric results rather are consistent with minor re-equilibration to higher\ud temperatures of the unerupted crystalline residue from the explosive phase of volcanism, or perhaps with newly generated magmas from source materials very similar to those for the Cougar Point Tuff. Collectively, the data suggest that most of the pyroxene compositional diversity that is represented by the tuffs and lavas was produced early in the history of the eruptive center and that compositions across this range were preserved or duplicated through much of its lifetime. Mineral compositions and thermometry of the multiple lavas suggest that unerupted magmas residual to the explosive phase of volcanism may have been stored at sustained, high temperatures subsequent to the explosive phase of volcanism. If so, such persistent high temperatures and large eruptive magma volumes likewise require an abundant and persistent supply of basalt magmas to the lower and/or mid-crust, consistent with the tectonic setting of a continental hotspot

NBS K409: A potential reference material for sub-micron X-ray resolution by EPMA

With advances in electron beam instrumentation, there has been a trend toward higher resolution electron gun sources for electron microprobes. JEOL has been marketing field-emission gun (FEG) microprobes since 2003 (JXA 8500F), and CAMECA introduced their SX5FE microprobe in 2011. However, there remain questions about the full utilization of such tight beams as those afforded by the FEG applied to common rock-forming minerals (e.g. silicates, oxides, carbonates, phosphates, glasses), because the desired improvement in X-ray spatial resolution for quantitative determination of the compositions of sub-micron size objects necessitates operation at lower accelerating voltages and use of low-energy X-ray lines. The physics of electron scatter and ionization energies under such conditions is of primary concern regarding the spatial resolution of field-emission EPMA. In the 1970s, the U.S. National Bureau of Standards (now National Institute of Standards and Technology) developed a series of glass reference materials for microanalysis. The two glasses K411 and K412 were certified in 1982 (Marinenko, 1982) and contain SiO2, MgO, CaO and FeO/Fe2O3, with K412 additionally containing Al2O3. Both glasses were independently characterized and each found to be homogeneous. The composition of K411 is equivalent to stoichiometric pyroxene (augite). Decades later, with interest in microanalysis of particles, microspheres (2-40 um) of K411 composition were developed (Marinenko et al., 2000). Recently, a vial of NBS glass "K409" was unearthed at the University of Wisconsin; it was apparently a "failed experiment" for a sodium-rich microanalysis standard (D. Newbury, pers. comm.) with a nominal composition of SiO2 (55 wt%), Al2O3 (15 wt%), FeO (20 wt%) and Na2O (10 wt%). Close inspection of this sample by SEM reveals a plenitude of equant euhedral iron oxide microlites ≤ 1000 nm in diameter. This "failed experiment" fortuitously created a potential standard for higher resolution X-ray microanalysis, which we document here for the first time. Our investigations of the K409 material using a JEOL 8530F include Si Kα, Fe Kα and Fe Lα line profiles several microns in length across the sharp grain boundaries of the Fe-oxides and provide some initial results for X-ray spatial resolution based on the lateral distance over which the intensity shifts between ~10 and 90 % of maximum. Spatial resolution for Si Kα improves from >500 nm at 10 keV to ~200-300 nm between 5-8 keV; resolution for Fe Kα improves from ~300 nm to ~200 nm with a drop from 10 to 8 keV. The Fe Lα line profile acquired at 5-7 keV likewise yields resolution in the ~200 nm range. Results of further examination of K409 and other materials (e.g. a Fe-Si couple) that constrain both beam and X-ray spatial resolution will be presented. References: Marinenko, R.B., 1982. Natl. Bur. Stand. (U.S.) Spec. Pub. 260-74 Marinenko, R.B. et al., 2000. Microsc. Microanal. 6 (6), 542-50

Detailed mineralogical investigation of Australian natural zeolite using EPMA

Summary - Improved spatial resolution for quantitative analysis of zeolite particles <<20 μm can be achieved using field-emission electron probe microanalysis (FE-EPMA) under special conditions (10kV accelerating voltage, 5nA beam current, 5-μm defocused beam) and using the mean atomic number background method - Fine-grained Australian natural zeolites are identified as magnesium Heulandite/Clinoptilolite-Ca - Potential applications include: hydrogen separation processing and water treatment (under special conditions

Comprehensive mineralogical study of Australian zeolites.

Industrial applications of natural zeolites depend on their mineralogical, physical and chemical characteristics. Over the last 20 years, Australian natural zeolites have been investigated for use in various industrial applications. However, there are few, if any, mineral characterisation studies on Australian natural zeolites since the early 1990s that use modern techniques. In this study, a detailed mineralogical analysis was conducted on zeolite specimens from Avoca and Werris Creek deposits, located in Queensland and New-South-Wales, respectively, in Australia. Their physical properties, thermal behaviour and porosity, as well as mineral compositions were determined by conventional methods, including thermogravimetry, N2 adsorption/desorption, optical microscopy, XRF, in situ XRD, SEM/EDS and EPMA/WDS. High-precision, high-accuracy measurements of the chemical compositions of fine-grained zeolites (<20 μm) were performed in situ in thin section using accepted EPMA protocols and data reduction methods. The Australian zeolites were identified as magnesium heulandite/clinoptilolite-Ca which corrects interpretations from earlier studies.</p

Composition of glass from high-temperature rhyolite of the Snake River Plain Yellowstone hotspot track: implications for crustal melting

Large volume explosive eruptions comprising the members of the Cougar Point Tuff (≥ 7,000 km3) erupted from the Bruneau-Jarbidge volcanic center of the Yellowstone hotspot are melt-dominated with crystal contents ranging from 10-15%. Zircons from all of the units have low δ18O ratios indicative of a heterogeneous, hydrothermally altered protolith that underwent melting in the mid to upper crust as the result of injection of basalt. This study investigates detailed variations in melt compositions by measuring elemental concentrations of 51 elements in individual glass shards from basal airfall tuffs by EPMA and LA-ICP-MS. Some eruptive units contain two distinct compositional modes of glass, indicating the presence of discrete liquid volumes in the magma reservoir system at the time of eruption. Multi-modal behavior is also observed for minerals indicating that the erupted magmas are complex, non-equilibrium assemblages. The overall elemental pattern in glass is for younger units to become more mafic (higher Fe) and for zirconium saturation temperatures and εNd to likewise increase. A few elements (B, Rb, Cs, Th, U) decrease in concentration with Fe. Most trace elements however remain fairly constant in composition over the 2.2 myr eruption interval and exhibit no systematic variation with respect to Fe concentration (Li, Be, Zn, Nb and REE). Elements that increase systematically with Fe include Sr, Ba, Eu, P, Zr, Hf and Sc indicating feldspar, zircon and apatite control on the melt composition. Compositional variations can be modeled by batch melting, suggesting that magma reservoirs are constructed by incremental extraction of melts from a crystal-liquid mush with episodic eruption and replenishment. Fractionation may play a role in generating compositional diversity within units