The Origin and Significance of Reverse Zoning in Melilite from Allende Type B Inclusions

In many Type B Allende inclusions, melilite is reversely-zoned over restricted portions of each crystal. Textural relationships and the results of dynamic crystallization experiments suggest that the reverselyzoned intervals in these Type melilites result from the co-precipitation of melilite with clinopyroxene from a melt, prior to the onset of anorthite precipitation. When clinopyroxene begins to precipitate, the Al/Mg ratio of the melt rises, causing the crystallizing melilite to become more gehlenitic, an effect which is negated by crystallization of anorthite. Because the equilibrium crystallization sequence in these liquids is anorthite before pyroxene, melilite reverse zoning can occur only when anorthite nucleation is suppressed relative to pyroxene. This has been achieved in our experiments at cooling rates as low as 0.5°C/hour. Our experiments further indicate, however, that reverse zoning does not form at cooling rates ≥50°C/hour , probably because the clinopyroxene becomes too Al-rich to drive up the Al/Mg ratio of the liquid. Type inclusions with reversely-zoned melilites must have cooled at rates greater than those at which anorthite begins to crystallize before clinopyroxene but <50°C/hour. Such rates are far too slow for the Type droplets to have cooled by radiation into a nebular gas but are much faster than the cooling rate of the solar nebula itself. One possibility is that Type B's formed in local hot regions within the nebula, where their cooling rate was equal to that of their surrounding gas. Other possibilities are that their cooling rates reflect their movement along nebular temperature gradients or the influence of a heat source. The sun or viscous drag on inclusions as they moved through the nebular gas are potential candidates for such heat sources

Zoning patterns of Fe and V in spinel from a type B Ca-Al-rich inclusion: Constraints on subsolidus thermal history

We obtained two-dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS-34 with a 4–5 μm resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS-34 contained V-rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ~8 μm and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600–700°C, this leads to a time scale for calcium-aluminum-rich inclusion (CAI) alteration in the range of decades to centuries

Materials Older Than Ca-Al-Rich Inclusions

Concentrations of refractory lithophile elements (RLEs) in Type B1 Ca-Al-rich inclusions (CAIs) are dominated by submicron inclusions, which are potentially relict grains older than the host CAI. These objects are so common, albeit tiny, that it is often very difficult to find a “pure” melilite

XANES and Mg isotopic analyses of spinels in Ca-Al-rich inclusions: Evidence for formation under oxidizing conditions

Ti valence measurements in MgAl_2O_4 spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti^(+4). Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti^(+3)-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti^(+4)-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ^(25)Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ^(25)Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ^(25)Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti^(+4) concentrations in spinel

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements

Electron probe microanalyser measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~100 µg g⁻¹ level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimise electron beam current density, a rastered 50 × 100 µm electron beam was adequate and minimised sensitivity to small Al‐rich contaminants. Reproducible analyses of eleven SIMS cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 µg g⁻¹. The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: Imilac, Eagle Station and Springwater. Our focus was on Al in olivine; but our technique could be refined to give accurate electron probe measurements for other contamination‐sensitive trace elements. For solar wind it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements

Electron probe microanalyser measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~100 µg g⁻¹ level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimise electron beam current density, a rastered 50 × 100 µm electron beam was adequate and minimised sensitivity to small Al‐rich contaminants. Reproducible analyses of eleven SIMS cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 µg g⁻¹. The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: Imilac, Eagle Station and Springwater. Our focus was on Al in olivine; but our technique could be refined to give accurate electron probe measurements for other contamination‐sensitive trace elements. For solar wind it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio

A Critical Examination of the X-Wind Model for Chondrule and Calcium-rich, Aluminum-rich Inclusion Formation and Radionuclide Production

Meteoritic data, especially regarding chondrules and calcium-rich, aluminum-rich inclusions (CAIs), and isotopic evidence for short-lived radionuclides (SLRs) in the solar nebula, potentially can constrain how planetary systems form. Intepretation of these data demands an astrophysical model, and the "X-wind" model of Shu et al. (1996) and collaborators has been advanced to explain the origin of chondrules, CAIs and SLRs. It posits that chondrules and CAIs were thermally processed < 0.1 AU from the protostar, then flung by a magnetocentrifugal outflow to the 2-3 AU region to be incorporated into chondrites. Here we critically examine key assumptions and predictions of the X-wind model. We find a number of internal inconsistencies: theory and observation show no solid material exists at 0.1 AU; particles at 0.1 AU cannot escape being accreted into the star; particles at 0.1 AU will collide at speeds high enough to destroy them; thermal sputtering will prevent growth of particles; and launching of particles in magnetocentrifugal outflows is not modeled, and may not be possible. We also identify a number of incorrect predictions of the X-wind model: the oxygen fugacity where CAIs form is orders of magnitude too oxidizing; chondrule cooling rates are orders of magnitude lower than those experienced by barred olivine chondrules; chondrule-matrix complementarity is not predicted; and the SLRs are not produced in their observed proportions. We conclude that the X-wind model is not relevant to chondrule and CAI formation and SLR production. We discuss more plausible models for chondrule and CAI formation and SLR production.Comment: Accepted for publication in The Astrophysical Journa

Root canal morphology of primary maxillary second molars:a micro-computed tomography analysis

Aim Successful endodontic treatment of primary teeth requires comprehensive knowledge and understanding of root canal morphology. The purpose of this study was to investigate the root canal configurations of primary maxillary second molars using micro-computed tomography. Methods Extracted human primary maxillary second molars (n = 57) were scanned using micro-computed tomography and reconstructed to produce three-dimensional models. Each root canal system was analysed qualitatively according to Vertucci's classification. Results 22.8% (n = 13) of the sample presented with the fusion of the disto-buccal and palatal roots; of these, Type V was the most prevalent classification. For teeth with three separate roots (n = 44), the most common root canal type was Type 1 for the palatal canal (100%) and disto-buccal canal (77.3%) and Type V for the mesio-buccal canal (36.4%). Overall, 7% (n = 4) of mesio-buccal canals were 'unclassifiable'. Conclusion The root canal systems of primary maxillary second molars were not only complex but had a range of configurations that may contribute to unfavourable clinical outcomes after endodontic treatment

Ion Implants as Matrix-Appropriate Calibrators for Geochemical Ion Probe Analyses

Ion microprobe elemental and isotopic determinations can be precise but difficult to quantify. Error is introduced when the reference material and the sample to be analysed have different compositions. Mitigation of such “matrix effects” is possible using ion implants. If a compositionally homogeneous reference material is available which is “matrix-appropriate,” i.e., close in major element composition to the sample to be analysed, but having an unknown concentration of the element, E, to be determined, ion implantation can be used to introduce a known amount of an E isotope, calibrating the E concentration and producing a matrix-appropriate calibrator. Nominal implant fluences (ions cm^(−2)) are inaccurate by amounts up to approximately 30%. However, ion implantation gives uniform fluences over large areas, thus it is possible to “co-implant” an additional reference material of any bulk composition having known amounts of E, independently calibrating the implant fluence. Isotope-ratio measurement standards can be produced by implanting two different isotopes, but permil level precision requires post-implant calibration of the implant isotopic ratio. Examples discussed include: (1) standardising Li in melilite; (2) calibrating a ^(25)Mg implant fluence using NIST SRM 617 glass; and (3) using Si co-implanted with ^(25)Mg alongside NIST SRM 617 to produce a calibrated measurement of Mg in Si