Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway

Fluid inclusions in garnet, kyanite and quartz from microdiamond-bearing granulites in the Western Gneiss Region, Norway, document a conspicuous fluid evolution as the rocks were exhumed following Caledonian high- and ultrahigh-pressure (HP–UHP) metamorphism. The most important of the various fluid mixtures and daughter minerals in these rocks are: (N₂ + CO₂ + magnesian calcite), (N₂ + CO₂ + CH₄ + graphite + magnesian calcite), (N₂ + CH₄), (N₂ + CH₄ + H₂O), (CO₂) and (H₂O + NaCl + CaCl₂ + nahcolite). Rutile also occurs in the N₂ + CO₂ inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions. Volatiles composed of N₂ + CO₂ + magnesian calcite characterise the ambient metamorphic environment between HP–UHP (peak) and early retrograde metamorphism. During progressive decompression, the mole fraction of N₂ increased in the fluid mixtures; as amphibolite-facies conditions were reached, CH₄ and later, H₂O, appeared in the fluids, concomitant with the disappearance of CO₂ and magnesian calcite. Graphite is ubiquitous in the host lithologies and fluid inclusions. Thermodynamic modelling of the metamorphic volatiles in a graphite-buffered C-O-H system demonstrates that the observed metamorphic volatile evolution was attainable only if the f _O2 increased from c. −3.5 (±0.3) to −0.8 (±0.3) log units relative to the FMQ oxygen buffer. External introduction of oxidising aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the f _O2. The oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental sediments previously subducted during an extended period of continental collision in conjunction with the Caledonian orogeny. Received: 15 December 1997 / Accepted: 25 May 1998     

Stony meteorite characterization by non‐destructive measurement of magnetic properties

Abstract— Four parameters of low‐field magnetic susceptibility (bulk value, frequency dependence, degree of anisotropy, and ellipsoid shape) have been determined for 321 stony meteorites from the National Collection of Canada. These parameters provide a basis for rapid, non‐destructive, and accurate meteorite classification as each meteorite class tends to have a distinct range of values. Chondrites show a clear trend of increasing bulk susceptibility from LL to L to H to E within the 3.6 to 5.6 logχ (in 10⁻⁹ m³/kg) range, reflecting increasing Fe‐Ni metal and Fe‐Ni sulfide content. Achondrite values range in logχ from 2.4 to 4.7 and primitive achondrites from 4.2 to 5.7. Frequency dependence is observed, using 19,000 Hz and 825 Hz, with variations in strength among meteorite classes and individual specimen dependence ranging from 1–25.6%. Degrees of anisotropy range from 1 to 53% with both oblate and prolate ellipsoids present. The aubrite class is marked by high degrees of anisotropy, low bulk magnetic susceptibility, and prolate fabric. Camel Donga is set apart from other eucrites, marked by higher bulk susceptibility, degree of anisotropy, and magnitude of oblate ellipsoid shape. The Shergotty, Nakhla, and Chassigny (SNC) meteorites show subclass distinction using frequency dependence and Chassigny is set apart with a relatively strong oblate fabric. The presence of both strong oblate and prolate fabrics among and within meteorite classes of chondritic and achondritic material points to a complex, multi‐mechanism origin for anisotropy, more so than previously thought, and likely dominated by impact processes in the later stages of stony parent body formation.     

Low δ18O eclogites from the Kokchetav massif, northern Kazakhstan

Oxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO₂‐laser fluorination methods. Systematic analyses yield extremely low δ¹⁸O for eclogites, as low as ?3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ¹⁸O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ¹⁸O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ¹⁸O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.     

USTÍ NAD ORLICI: A NEW L6 CHONDRITE FROM CZECHOSLOVAKIA

Ustí nad Orlici (Kerhartice), a meteorite which fell on June 12, 1963 in Czechoslovakia, is classified as a L6 chondrite. Compositions of olivine (Fa 23.4), orthopyroxene (Fs 20, Wo 1.3), plagioclase (Ab₈₅An₁₀Or₅) along with the bulk composition of the meteorite support this classification. Chromite compositions vary with grain size. Large chromites are higher in TiO₂ and lower in Fe³ than small chromites. This may indicate that either these two chromites formed initially under different fO₂ conditions, or that this difference resulted from different equilibration behaviors of both chromites as a function of grain size. The meteorite contains three distinct sulfide assemblages: 1) troilite-pentlandite, 2) troilite, pentlandite-cubanite-chalcopyrite-pyrrhotite-mackinawite, 3) troilite-tetrataenite-(Fe, Cu, Ni)_{1***. 02}S. These assemblages indicate equilibration down to temperatures close to 200 °C.     

Fabric at granodiorite-schist contact,Bear Island,Maryland

Summary The essential difference between the granodiorite and its wall rock fabric consists in the two well-defined deformation plans. The wall rock is a B-tectonite, the granodiorite an S-tectonite. Transport in the former was rotational and monoclinic, in the latter possibly by laminar flow parallel with the contact.The granite fabric could hardly be an in place fabric or replacement without differential movement. It would have retained the wall rock fabric, even if not the mineral composition. Transport in the granodiorite was presumably along the wall parallel S₄ and the contact. The present study is barely a beginning, but it is hoped that it can be continued in the near future and extended over a wider area.With 9 figures.     

The distribution of aluminum-26 in the early Solar System—A reappraisal

Abstract— A compilation of over 1500 Mg-isotopic analyses of Al-rich material from primitive solar system matter (meteorites) shows clearly that ²⁶Al existed live in the early Solar System. Excesses of ²⁶Mg observed in refractory inclusions are not the result of mixing of “fossil” interstellar ²⁶Mg with normal solar system Mg. Some material was present that contained little or no ²⁶Al, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chondrites accreted and probably was a minor component in the accretion regions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebular histories of CAIs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formation require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether ²⁶Al was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.