Stable isotope evidence for magmatic fluid input during large-scale Na–Ca alteration in the Cloncurry Fe oxide Cu–Au district, NW Queensland, Australia

Sodic–calcic alteration is common in mineralized hydrothermal systems, yet the relative importance of igneous vs. basinal fluid sources remains controversial. One of the most extensive volumes of sodic–calcic rocks occurs near Cloncurry, NW Queensland, and was formed by overlapping hydrothermal systems that were active synchronously with emplacement of mid‐crustal batholithic granitoids (c. 1.55–1.50 Ga). Altered rocks contain albite–oligoclase, actinolite, diopside, titanite and magnetite. Alteration was localized by: (A) composite veins and breccias containing crystallized magma intimately intergrown with hydrothermal precipitates; (B) intrusions that host setting A veins and breccias; and (C) extensive breccia and vein systems linked to regional fault systems. Isotope analyses of actinolites in settings A and B indicate calculated δ¹⁸O_H2O (+8.2 to +10.6‰) and variably depleted δD_H2O (?130 to ?54‰) compared with typical magmatic fluids, whereas those from setting C typically indicate calculated δ¹⁸O_H2O (+8.0 to +12.8‰) and δD_H2O (?29 to ?99‰). The lowest δD_H2O values are interpreted as representing residual fluids after significant (> 90%) open‐system magmatic degassing. Overall the stable isotope, field, geochronological and geobarometric data suggest that these sodic–calcic alteration systems were formed by the episodic incursion of magmatic fluids that underwent minor isotopic modification as a result of varying degrees of interaction with country rocks.     

Pyroxene microstructure in the Northwest Africa 856 martian meteorite

Abstract— Transmission electron microscopy was used to examine pyroxene microstructure in the Northwest Africa (NWA) 856 martian meteorite to construct its cooling and shock histories. All pyroxenes contain strained coherent pigeonite/augite exsolution lamellae on (001). The average width and periodicity of lamellae are 80 and 400 nm, respectively, indicating a cooling rate below 0.1 °C/hr for the parent rock. Pigeonite and augite are topotactic, with strained coherent interfaces parallel to (001). The closure temperature for Ca‐Fe, Mg interdiffusion, estimated from the composition at the augite pigeonite interface, is about 700 °C. Tweed texture in augite reveals that a spinodal decomposition occurred. Locally, tweed evolved toward secondary pigeonite exsolutions on (001). Due to the decreasing diffusion rate with decreasing temperature, “M‐shaped” concentration profiles developed in augite lamellae. Pigeonite contains antiphase boundaries resulting from the C2/c to P2₁/c space group transition that occurred during cooling. The reconstructive phase transition from P2₁/c clinopyroxene to orthopyroxene did not occur. The deformation (shock) history of the meteorites is revealed by the presence of dislocations and mechanical twins. Dislocations are found in glide configuration, with the [001](100) glide system preferentially activated. They exhibit strong interaction with the strained augite/pigeonite interfaces and did not propagate over large distances. Twins are found to be almost all parallel to (100) and show moderate interaction with the augite/pigeonite interfaces. These twins are responsible for the plastic deformation of the pyroxene grains. Comparison with microstructure of shocked clinopyroxene (experimentally or naturally shocked) suggests that NWA 856 pyroxenes are not strongly shocked.     

The mid-infrared spectrum of the zodiacal and exozodiacal light

The zodiacal light is the dominant source of the mid-infrared sky brightness seen from Earth, and exozodiacal light is the dominant emission from planetary and debris systems around other stars. We observed the zodiacal light spectrum with the mid-infrared camera ISOCAM over the wavelength range 5-16 μm and a wide range of orientations relative to the Sun (solar elongations 68°-113°) and the ecliptic (plane to pole). The temperature in the ecliptic ranged from 269 K at solar elongation 68° to 244 K at 113°, and the polar temperature, characteristic of dust 1 AU from the Sun, is 274 K. The observed temperature is exactly as expected for large (>10 μm radius), low-albedo (<0.08), rapidly-rotating, gray particles 1 AU from the Sun. Smaller particles (<10 μm radius) radiate inefficiently in the infrared and are warmer than observed. We present theoretical models for a wide range of particle size distributions and compositions; it is evident that the zodiacal light is produced by particles in the 10-100 μm radius range. In addition to the continuum, we detect a weak excess in the 9-11 μm range, with an amplitude of 6% of the continuum. The shape of the feature can be matched by a mixture of silicates: amorphous forsterite/olivine provides most of the continuum and some of the 9-11 μm silicate feature, dirty crystalline olivine provides the red wing of the silicate feature (and a bump at 11.35 μm), and a hydrous silicate (montmorillonite) provides the blue wing of the silicate feature. The presence of hydrous silicate suggests the parent bodies of those particles were formed in the inner solar nebula. Large particles dominate the size distribution, but at least some small particles (radii ∼1 μm) are required to produce the silicate emission feature. The strength of the feature may vary spatially, with the strongest features being at the lowest solar elongations as well as at high ecliptic latitudes; if confirmed, this would imply that the dust properties change such that dust further from the Sun has a weaker silicate feature. To compare the properties of zodiacal dust to dust around other main sequence stars, we reanalyzed the exozodiacal light spectrum for β Pic to derive the shape of its silicate feature. The zodiacal and exozodiacal spectra are very different. The exozodiacal spectra are dominated by cold dust, with emission peaking in the far-infrared, while the zodiacal spectrum peaks around 20 μm. We removed the debris disk continuum from the spectra by fitting a blackbody with a different temperature for each aperture (ranging from 3.7″ to 27″); the resulting silicate spectra for β Pic are identical for all apertures, indicating that the silicate feature arises close to the star. The shape of the silicate feature from β Pic is nearly identical to that derived from the ISO spectrum of 51 Oph; both exozodiacal features are very different from that of the zodiacal light. The exozodiacal features are roughly triangular, peaking at 10.3 μm, while the zodiacal feature is more boxy, indicating a different mineralogy.     

Why we study the geological record for evidence of the solar oscillation about the galactic midplane

The Solar System oscillates about the plane defined by the disk of matter in our Galaxy. This oscillatory motion gives rise to a substantial modulation in the tidally induced flux of Oort cloud comets. An observational determination of the quasi-periodicity of this motion carries with it significant information about the population, distributions, dynamics and origins of short-period and long-period comets. An additional incentive for emphasizing such a study is the information about dark disk matter that a period can yield. If dark disk matter is completely negligible, the amplitude of the solar motion will be sufficiently large that the peak-to-trough flux ratio will be ≈ 2.5 and the plane-crossing period will exceed 40 Myr. Dark disk matter comparable in mass to bright disk matter and distributed in any manner is inconsistent with K-dwarf distributions and can be rejected as a working hypothesis. But if a modest fraction of the disk matter is dark and distributed like the interstellar medium, as is consistent with limits deduced from K-giant and K-dwarf velocity distributions, the peak-to-trough flux ratio can increase to a factor of 4 even though the solar z amplitude is decreased. In that case the period can be as little as 30 Myr and the implied Oort population is smaller by a factor of 3. We should carefully reconsider the geological record as a potential discriminator of these options.