Mid-infrared imaging of ultracompact H II regions

We discuss preliminary results of an 11.7 m imaging survey of ultracompact H II regions from the Wood and Churchwell radio survey. We find that that the morphologies of ionized gas and warm dust are often significantly different, indicating that an H II region classification scheme should be based on more than radio data.     

A verification of some methods to determine the fluxes of momentum,sensible heat,and water vapour using standard deviation and structure parameter of scalar meteorological quantities

A set of micro-meteorological data collected over a horizontal, uniform terrain (the plain of La Crau, France) in June 1987 is analysed. Conditions were predominantly sunny and arid, while due to the Mistral the wind speed could exceed 10 m/s. Verification of several methods to evaluate surface fluxes of heat, momentum and water vapour from the standard deviation of temperature, wind and specific humidity is presented. Also, a similar approach using the structure parameter of temperature is considered. These methods are all based on Monin-Obukhov (M-O) similarity theory. It is found that the standard deviation of temperature, vertical and horizontal wind speed as well as the structure parameter for temperature behave according to M-O similarity. It is shown that the sensible heat flux and friction velocity can be determined from a fast response thermometer and a cup anemometer. Also, it appears that the analytic solution of the set of governing equations as derived by the first author yields good results. M-O theory does not appear to work for the standard deviation of specific humidity. This may be due to the relative importance of large eddies.     

Experimental investigation and application of the equilibrium rutile + orthopyroxene = quartz + ilmenite

Equilibria in the Sirf (Silica-Ilmenite-Rutile-Ferrosilite) system: $${\text{SiO}}_{\text{2}} + ({\text{Mg,Fe}}){\text{TiO}}_{\text{3}} {\text{ + (Mg,Fe)SiO}}_{\text{3}} $$ have been calibrated in the range 800–1100° C and 12–26 kbar using a piston-cylinder apparatus to assess the potential of the equilibria for geobarometry in granulite facies assemblages that lack garnet. Thermodynamic calculations indicate that the two end-member equilibria involving quartz + geikielite = rutile + enstatite, and quartz + ilmenite = rutile + ferrosilite, are metastable. We therefore reversed equilibria over the compositional range Fs_40–70, using Ag₈₀Pd₂₀ capsules with \(f_{{\text{O}}_{\text{2}} } \) buffered at or near iron-wüstite. Ilmenite compositions coexisting with orthopyroxene are \(X_{{\text{MgTiO}}_{\text{3}} }^{{\text{Ilm}}} \) of 0.06 to 0.15 and \(X_{{\text{Fe}}_{\text{2}} {\text{O}}_{\text{3}} }^{{\text{Ilm}}} \) of 0.00 to 0.01, corresponding toK _D values of 13.3, 10.2, 9.0 and 8.0 (±0.5) at 800, 900, 1000 and 1100° C, respectively, whereK _D =(XMg/XFe)^Opx/(XMg/XFe)^Ilm. Pressures have been calculated using equilibria in the Sirf system for granulites from the Grenville Province of Ontario and for granulite facies xenoliths from central Mexico. Pressures are consistent with other well-calibrated geobarometers for orthopyroxeneilmenite pairs from two Mexican samples in which oxide textures appear to represent equilibrium. Geologically unreasonable pressures are obtained, however, where oxide textures are complex. Application of data from this study on the equilibrium distribution of iron and magnesium between ilmenite and orthopyroxene suggests that some ilmenite in deep crustal xenoliths is not equilibrated with coexisting pyroxene, while assemblages from exposed granulite terranes have reequilibrated during retrogression. The Sirf equilibria are sensitive to small changes in composition and may be used for determination of activity/composition (a/X) relations of orthopyroxene if an ilmenite model is specified. A symmetric regular solution model has been used for orthopyroxene in conjunction with activity models for ilmenite available from the literature to calculatea/X relations in orthopyroxene of intermediate composition. Data from this study indicate that FeSiO₃?MgSiO₃ orthopyroxene exhibits small, positive deviations from ideality over the range 800–1100°C.     

The concentration and isotopic composition of carbon in basaltic glasses from the Juan de Fuca Ridge, Pacific Ocean

The abundance and 13C/12C ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion/extraction technique. Carbon released during the first two combustion steps at 400-500 degrees C and 600-650 degrees C is interpreted to be secondary, and only the carbon recovered during a final combustion step at approximately 1200 degrees C is thought to be indigenous to the samples. For carbon released at approximately 1200 degrees C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with delta 13C values of -4.8 to -9.3%, whereas samples crushed to 38-63 microns or 63-90 microns yielded 56-103 ppm C with delta 13C values of -6.1 to -9.2%. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and delta 13C ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and delta 13C values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing.     

Improved Petrological Modal Analyses from X-ray Powder Diffraction Data by use of the Rietveld Method I. Selected Igneous, Volcanic, and Metamorphic Rocks

Mineral modes have been determined for specimens of eight rocktypes from CuK X-ray powder diffraction data using the Rietveldmethod. The samples include two granites, a granodiorite, adamellite,gabbro, basalt, trachyte, and two granulite-facies metamorphicrocks. Up to eight individual mineral components have been measuredin each sample (no glassy phases were observed), with a detectionlimit of {small tilde}1 wt.%, depending on the mineral assemblage.Marked variations in grain size (i.e., granite vs. trachyte)provide no difficulties for the X-ray method. The X-ray resultscompare very favourably with (1) optical modes determined forthe medium–coarse-grained samples by point counting, (2)normative calculations obtained using locally enhanced catanormand mesonorm software, and (3) corresponding Rietveld modesdetermined, for two samples, from neutron powder data. Wheredifferences occur, these are discussed in relation to the limitationsof each of the methods. The improved accuracy of the X-ray method is due primarily tothe incorporation of the full diffraction profile in the Rietveldanalysis calculations, and the elimination of preferred orientationby collecting the data from samples packed in glass capillaries(i.e., Debye–Scherrer mode). The good agreement of theX-ray and neutron modes shows that the usual problems encounteredwith microabsorption, extinction, and sampling are of littleconcern in these rocks. The results highlight one of the majoradvantages provided by Rietveld modal analysis over the moretraditional ‘reference intensity’ X-ray methods,namely, that the crystal chemistry (and thus the calibrationconstants) of the individual phases can be adjusted dynamicallyduring each individual analysis. This not only provides moreaccurate phase abundances, but also gives important supplementaryinformation about the mineralogy of the major components.     

Pressure Increases, the Formation of Chromite Seams, and the Development of the Ultramafic Series in the Stillwater Complex, Montana

This paper explores the hypothesis that chromite seams in theStillwater Complex formed in response to periodic increasesin total pressure in the chamber. Total pressure increased becauseof the positive V of nucleation of CO₂ bubbles in the melt andtheir subsequent rise through the magma chamber, during whichthe bubbles increased in volume by a factor of 4–6. Byanalogy with the pressure changes in the summit chambers ofKilauea and Krafla volcanoes, the maximum variation was 0•2–0•25kbar, or 5–10% of the total pressure in the Stillwaterchamber. An evaluation of the likelihood of fountaining andmixing of a new, primitive liquid that entered the chamber withthe somewhat more evolved liquid already in the chamber is basedupon calculations using observed and inferred velocities andflow rates of basaltic magmas moving through volcanic fissures.The calculations indicate that hot, dense magma would have oozed,rather than fountained into the chamber, and early mixing ofthe new and residual magmas that could have resulted in chromitecrystallizing alone did not take place. Mixing was an important process in the Stillwater magma chamber,however. After the new magma in the chamber underwent {smalltilde}5% fractional crystallization, its composition, temperature,and density approached those of the overlying liquid in thechamber and the liquids then mixed. If this process occurredmany times over the course of the development of the Ultramaficseries, a thick column of magma with orthopyroxene on its liquiduswould have been the result. Thus, the sequence of multiple injections,fractionation, and mixing with previously fractionated magmacould have been the mechanism that produced the thick bronzitecumulate layer (the Bronzitite zone) above the cyclic units.     

Electron paramagnetic resonance of Sc3+-stabilized CO 3 3− molecule-ion in natural calcite

Electron paramagnetic resonance (EPR) spectra of CO ₃ ^3– molecule-ions stabilized by Sc³⁺ in natural calcite were identified and studied at X-band frequencies and room temperature. The principal values of the g-tensor (g _xx= 1.9997, g _yy = 2.0030, g _zz = 1.9972) and the direction cosines of the g and A tensors for CO ₃ ^3– -Sc³⁺ center were found to be close to that for the well-known CO ₃ ^3– -Y³⁺ center. A quantitative comparison of different impurity contents in calcite samples and analysis of the intensities of forbidden transitions were used to identify Sc³⁺. An estimation of the unpaired electron spin density on the nuclei of paramagnetic centers confirms that both centers, CO ₃ ^3– -Sc³⁺ and CO ₃ ^3– -Y³⁺, have the same nature.     

Automatic equipment to monitor groundwater radon

Prototype instrumentation, able to automatically measure groundwater radon content variations, is presented. The equipment is made of stainless steel and has spherical valves with automatic and pneumatic control. The deemanation of the gases from the water is obtained by evacuating a suitable expansion chamber. The instrumentation can make discrete sampling ranging from 1 per hour to 1 per 99 hours. The equipment was tested in the laboratory: the efficiency was measured by means of a²⁶⁶Ra solution. A mean value of (0.65±0.07) count/s/Bq was obtained. A calibration test was carried out by comparing countings from the automatic equipment with those obtained by the standard laboratory cell. Results of an operational check over a period of approximately one year indicate that variations in radon at the calibration site are attributable more to meteorological than to tectonic causes.     

RADON DEPTH MIGRATION1

A depth migration method is presented that uses Radon-transformed common-source seismograms as input. It is shown that the Radon depth migration method can be extended to spatially varying velocity depth models by using asymptotic ray theory (ART) to construct wavefield continuation operators. These operators downward continue an incident receiver-array plane wave and an assumed point-source wavefield into the subsurface. The migration velocity model is constrained to have longer characteristic wavelengths than the dominant source wavelength such that the ART approximations for the continuation operators are valid. This method is used successfully to migrate two synthetic data examples:

• 1 a point diffractor, and
• 2 a dipping layer and syncline interface model.

It is shown that the Radon migration method has a computational advantage over the standard Kirchhoff migration method in that fewer rays are computed in a main memory implementation.     

Isotope and trace element evidence for three component mixing in the genesis of the North Luzon arc lavas (Philippines)

Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in ⁸⁷Sr/⁸⁶Sr (0.70327–0.70610), ¹⁴³Nd/¹⁴⁴Nd (0.51302–0.51229) and ²⁰⁸Pb^*/²⁰⁶Pb^* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high 8/4, and to a lesser extent high 7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS subduction component interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (<5%) of subducted S China Sea terrigenous sediment. The ⁸⁷Sr/⁸⁶Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁸Pb^*/²⁰⁶Pb^*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.