S.R. Sutton

X-ray microprobe analysis of iron oxidation states in silicates and oxides using X-ray absorption near edge structure (XANES)

Abstract Initial results are reported on a new microprobe technique for determining Fe 3+ ∑Fe ratios in Fe-bearing minerals. The technique is based on the energy shift of a pre-edge peak in X-ray absorption near-edge structure (XANES) spectra obtained using the synchrotron X-ray fluorescence microprobe. A linear relationship between pre-edge peak energy and iron oxidation state was observed for oxide and silicate standards. Reasonable iron oxidation state results were obtained for pallasitic olivine, acmite, altered magnetites and synthetic wustite. Direct measurements of Fe 3+ ∑Fe in coexisting phases is feasible using this technique.

Reduced chromium in olivine grains from lunar basalt 15555 - X-ray Absorption Near Edge Structure (XANES)

The oxidation state of Cr in 200 [mu]m regions within individual lunar olivine and pyroxene grains from lunar basalt 15555 was inferred using X-ray Absorption Near Edge Structure (XANES). Reference materials had previously been studied by optical absorption spectroscopy and included Cr-bearing borosilicate glasses synthesized under controlled oxygen fugacity and Cr-doped olivines. The energy dependence of XANES spectral features defined by these reference materials indicated that Cr is predominantly divalent in the lunar olivine and trivalent in the pyroxene. These results coupled with the apparent f[sub o[sub 2]]-independence of partitioning coefficients for Cr into olivine imply that the source magma was dominated by divalent Cr at the time of olivine crystallization. 29 refs., 8 figs., 1 tab.

An asteroidal breccia: The anatomy of a cluster IDP

Abstract We report results of a consortium study of a large interplanetary dust particle known as cluster L2008#5. This cluster is composed of fifty-three fragments (>5 pm in diameter) and several hundred fines ( Several methods were used to estimate the degree of heating that this cluster experienced. Variations in the inferred peak temperatures experienced by different fragments suggest that a thermal gradient was maintained. The cluster as a whole was not strongly heated; it is estimated to have a low earth-encounter velocity which is consistent with origin from an object in an asteroidal orbit rather than from a comet, which would most likely have a high entry velocity. Our conclusions show that cluster L2008#5 consists of a chemically and mineralogically diverse mixture of fragments. We believe that cluster L2008#5 represents a heterogeneous breccia and that it was most likely derived from an object in an asteroidal orbit. We also present an important cautionary note for attempts to interpret individual, small-sized 10–15 μm IDPs as representative of parent bodies. It is not unique that individual building blocks of IDPs, such as discrete olivine, pyroxene, sulfide grains, regions of carbonaceous material, and other noncrystalline material, are found in several fragments; however, it is unique that these building blocks are combined in various proportions in related IDPs from one large cluster particle.

High-pressure x-ray tomography microscope: Synchrotron computed microtomography at high pressure and temperature

A new apparatus has been developed for microtomography studies under high pressure. The pressure generation mechanism is based on the concept of the widely used Drickamer anvil apparatus, with two opposed anvils compressed inside a containment ring. Modifications are made with thin aluminum alloy containment rings to allow transmission of x rays. Pressures up to 8GPa have been generated with a hydraulic load of 25T. The modified Drickamer cell is supported by thrust bearings so that the entire pressure cell can be rotated under load. Spatial resolution of the high pressure tomography apparatus has been evaluated using a sample containing vitreous carbon spheres embedded in FeS matrix, with diameters ranging from 0.01to0.2mm. Spheres with diameters as small as 0.02mm were well resolved, with measured surface-to-volume ratios approaching theoretical values. The sample was then subject to a large shear strain field by twisting the top and bottom Drickamer anvils. Imaging analysis showed that detailed microst...

Strontium heterogeneity and speciation in coral aragonite: implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO3 (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 μm scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment.

Assessment of the uncertainties and limitations of quantitative elemental analysis of individual fluid inclusions using synchrotron X-ray fluorescence (SXRF)

Abstract Synchrotron X-ray Fluorescence (SXRF) analysis is a nondestructive analytical technique that provides compositional information from single fluid inclusions. A protocol for conducting quantitative analyses of metal concentrations in individual fluid inclusions has been developed. This has led to an understanding of the accuracy, precision, and detection limits of this technique, as well as the optimal shapes, sizes, and geometries required for reliable fluid inclusion analysis. Aqueous fluid inclusions containing known concentrations of SrCl 2 were synthesized for the development and the standardization of this technique. Strontium chloride was selected because it is highly soluble, its freezing-point depression is well known (allowing us to confirm the inclusion composition using microthermometric analyses), and the energetic Sr X-rays are only mildly attenuated by quartz. To confirm the composition of the synthetic standards, solutions were measured before and after each hydrothermal run using Atomic Absorption Spectroscopy (AAS), and the freezing-point depression for each fluid inclusion was measured. SXRF analyses were performed on beam line X26A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory using an 8 × 12 μm white X-ray beam. The analytical volume was calculated based on known beam dimensions and fluid inclusion geometry determined using a modified spindle stage. Elemental concentrations were determined by ratioing the Sr counts from an inclusion to the counts obtained from capillaries of known diameter containing similar solutions. Numerous inclusions from five samples, each with a different Sr concentration, were analyzed. Within a single population the mean is very close to the known concentration, but the precision is poor, with standard deviations (lσ) from 10–39% of the mean. Errors in determining the inclusion geometry are the main contributor to the poor precision. The poor precision requires that numerous inclusions within one population be analyzed and averaged to accurately estimate the metal concentration for that population. Selection of flat-lying, equant, regularly-shaped inclusions for analysis minimizes errors resulting from inclusion geometry if quantitative results are sought. The detection limit for Sr in synthetic fluid inclusions (typically 4–15 μm thick, and 5–100 pm below the upper polished surface) is approximately 2,000 ppm Sr.

Synchrotron x‐ray fluorescence microprobe: Quantification and mapping of mixed valence state samples using micro‐XANES

The synchrotron x‐ray fluorescence microprobe is a valuable instrument for quantification and mapping of mixed valence state samples with high spatial resolution and elemental sensitivity. A method has been developed for quantifying the proportions of Fe2+ and Fe3+ with 100 μm spatial resolution and better than 100 ppm sensitivity using x‐ray absorption near‐edge structure (XANES). Applications of valence state mapping have been made to selenium in water‐saturated sediments and manganese associated with wheat roots attacked by the take‐all fungus.

Evidence of heterovalent europium in zoned Llallagua apatite using wavelength dispersive XANES

Abstract Eu L3 X-ray absorption near edge structure (XANES) spectroscopy was conducted with a wavelength dispersive (WDS) spectrometer to determine the valence state of europium in a natural, Mn-and REE-rich apatite from Llallagua, Bolivia. Europium exists in both the divalent and trivalent states in the Llallagua apatite with a Eu2+/Eu3+ ratio between 0.12 and 0.22. With the enhanced energy resolution of WDS the EuLα1 fluorescence line can be resolved from the MnKα line, allowing for significant reduction of background in the EuL3 absorption edge region and resolution of the Eu XAS, which is difficult by conventional methods because of fluorescence peak interference. The anomalous partitioning behavior of Eu in these samples (Rakovan and Reeder 1996) can be explained by the observed presence of Eu2+ and Eu3+ and is consistent with the suggested size effect on intrasectoral zoning of REEs in apatite.

Petrogenesis of the Zagami meteorite: Inferences from synchrotron X-ray (SXRF) microprobe and electron microprobe analyses of pyroxenes

Abstract The Zagami meteorite is a shergottite, a basalt rich in pigeonite, augite, and maskelynite glass (shocked plagioclase). It is one of the SNC meteorites, which are inferred to have formed on Mars. Major and minor element abundances in pyroxene, principally pigeonite, from newly available (UNM) samples were determined by electron microprobe; abundances of trace elements Ni, Cu, Zn, and Ga were obtained by synchrotron X-ray (SXRF) microprobe. Many pigeonite grains have normally zoned rims and reversely zoned cores, so that the most magnesian pigeonite forms rings or annuli in thin section view; in three dimensions, the most magnesian pigeonite may form tubes or hourglass shapes. The cores commonly contain magmatic inclusions. Other pigeonites have irregular or multiple magnesian regions, with magmatic inclusions among and between them. Abundances of Ni and Cr in the magnesian zones and cores are partially decoupled from abundances of other compatible (Mg) and incompatible (Fe, Ti) elements. The textures, chemical zoning, and element decouplings in the pigeonites are consistent with igneous crystal growth at moderate cooling rates, not isothermally or near equilibrium. From element partitioning and mass balance, the UNM Zagami is estimated to contain 20 ± 5% or less cumulus pyroxene (half augite and half pigeonite), half the proportion suggested by equilibrium distribution of Fe/Mg between pigeonite and melt. This inconsistency may be resolved if all pigeonite/melt element partitioning was affected by the rapid growth of the pigeonites. The composition of the parental magma for the UNM Zagami, calculated by mass balance on bulk and pyroxene compositions, is similar to the 1170°C melt of the BMNH Zagami, not the 1140°C melt inferred as parental by other workers. The inference that the pyroxene zoning patterns reflect only magmatic events is consistent with an igneous age of 180 m.y. for Zagami.

Micro-beam X-ray absorption and fluorescence spectroscopies at GSECARS: APS beamline 13ID.

GeoSoilEnviroCARS, sector 13 of the Advanced Photon Source at Argonne National Laboratory, provides a micro-beam facility for XAFS, XANES, XRE and x-ray diffraction studies. A KirkpatrickBaez mirror pair gives a focussed monochromatic undulator beam down to 1 x 1/zm with sufficient intensity for x-ray fluorescence mapping and extended XAFS of dilute systems at energies above 4KeV. Special emphasis for these facilities has been given to environmental and earth science studies, including dilute contaminants in natural sediments. As an example, XRF maps and EXAFS data taken with a 4pm by 7#m spot size are shown for the Pu LIII edge of a natural tuff exposed to a dilute aqueous solution of Pu. In addition, x-ray diffraction and scattering capabilities allow the study of surfaces and surface adsorbates with micron-sized beams using xray reflectivity, x-ray standing-waves, and grazing-incidence XAFS.