Quantifying noble gas contamination during terrestrial alteration in Martian meteorites from Antarctica

We investigated exterior and interior subsamples from the Martian shergottite meteorites Allan Hills (ALH) A77005 and Roberts Massif (RBT) 04261 for secondary minerals, oxygen isotopes, Ar‐Ar, and noble gas signatures. Electron microprobe investigations revealed that RBT 04261 does not contain any visible alteration even in its most exterior fractures, whereas fracture fillings in ALHA77005 penetrate into the meteorite up to 300 μm, beyond which the fractures are devoid of secondary minerals. Light noble gases seem to be almost unaffected by terrestrially induced alteration in both meteorites. Thus, a shock metamorphic overprint of 30–35 GPa can be deduced from the helium measurements in RBT 04261. Oxygen isotopes also seem unaffected by terrestrially weathering and variations can easily be reconciled with the differences in modal mineralogy of the exterior and interior subsamples. The measurements on irradiated samples (Ar‐Ar) showed a clear Martian atmospheric contribution in ALHA77005, but this is less apparent in our sample of RBT 04261. Exterior and interior subsamples show slight differences in apparent ages, but the overall results are very similar between the two. In contrast, krypton and xenon are severely affected by terrestrial contamination, demonstrating the ubiquitous presence of elementally fractionated air in RBT 04261. Although seemingly contradictory, our results indicate that RBT 04261 was more affected by contamination than ALHA77005. We conclude that irrespective of on which planet the alteration occurred, exposure of Martian rocks to atmosphere (or brine) introduces noble gases with signatures elementally fractionated relative to the respective atmospheric composition into the rock, and relationships of that process with oxygen isotopes or mineralogical observations are not straightforward.     

International Heliophysical Year 2007: A Report from the UN/NASA Workshop Bangalore, India, 27 November–1 December 2006

The IHY Secretariat and the United Nations Basic Space Science Initiative (UNBSSI) assist scientists and engineers from all over the world in participating in the International Heliophysical Year (IHY) 2007. A major thrust of IHY/UNBSSI is to deploy arrays of small, inexpensive instruments such as magnetometers, radio telescopes, GPS receivers, all-sky cameras, etc. around the world to allow global measurements of ionospheric and heliospheric phenomena. The small instrument programme is envisioned as a partnership between instrument providers and instrument hosts in developing nations. The IHY/UNBSSI can facilitate the deployment of several of these networks world-wide. Existing data bases and relevant software tools will be identified to promote space science activities in developing nations. Extensive data on space science have been accumulated by a number of space missions. Similarly, long-term data bases are available from ground-based observations. These data can be utilized in ways different from originally intended for understanding the heliophysical processes. This paper provides a comprehensive overview of IHY/UNBSSI, its achievements, future plans, and outreach to the 192 Member States of the United Nations as recorded in the UN/NASA workshop in India.     

Chloritoid-Bearing Mineral Assemblages in High-Pressure Metapelites from the Bughea Complex, Leaota Massif (South Carpathians)

High-Mg chloritoid (X_Mg = 0·40–0·47) andrelatively high-Mg staurolite (X_Mg = 0·25–0·28)coexisting with kyanite and garnet were identified in a mica–garnet-richrock associated with very high-pressure eclogites in the BugheaComplex of the Leaota Massif (South Carpathians). Major andtrace element geochemical data for both fresh eclogites andassociated rocks which represent a metasomatic or retrogradealteration rind of the eclogites, indicate a pelitic precursor.Magnesian chloritoid was found as inclusions in garnet as partof a chloritoid–kyanite–garnet assemblage whichis indicative of high-pressure conditions. The host garnet showsa typically prograde chemical zoning pattern. The chloritoid-bearingassemblage is confined to the inner part of the garnet porphyroblasts,whereas the matrix assemblage in equilibrium with Mg-rich garnetrims has exceeded the thermal stability limit of chloritoid.Pressure–temperature pseudosections for simplified compositionsapproaching the rock bulk-chemistry show a high-pressure fieldfor the identified chloritoid-bearing assemblage in good agreementwith pressure–temperature estimates in the CFMASH andKCFMASH chemical subsystems using analysed mineral compositions.The derived pressure–temperature path is clockwise, indicatingoverprinting during exhumation from 1·8 GPa and 580°Cto 1·15 GPa and 620°C, at a water activity approachinga_H2O = 1. These conditions were attained in a subduction mélangeindicating transient thermal perturbations of a subduction channel. KEY WORDS: high-pressure metapelite; Mg-rich chloritoid; P–T path; P–T pseudosection; very high-pressure eclogite     

Closed-form evaluation and approximation considerations of the non-resonant thermonuclear reaction rate

By use of a highly efficient theory of generalized hypergeometric functions, we show that the non-resonant thermonuclear reaction rate can be evaluated in closed form. This new approach is advantageous to operate analytically with reaction rates The numerical computation of the rate may be performed with the aid of associated differential equation, a convergent series expansion for small values of the characteristic parameter and an asymptotic expansion for large values of the characteristic parameter.     

Evidence for low-temperature ultrapotassic siliceous fluids in subduction zone environments from experiments in the system K2O---MgO---Al2O3---SiO2---H2O (KMASH)

Experiments in the system K₂O---MgO---Al₂O₃---SiO₂---H₂O (KMASH) were undertaken with the piston-cylinder-apparatus to study the reactions:

1. (1) phengite±quartz+K,Mg-rich siliceous fluid=feldspar+phologopite+H₂O

2. (2) phengite+talc+K,Mg-rich siliceous fluid=phlogopite+quartz/coesite+H₂O

at temperatures between 400 and 700°C. The ultrapotassic fluid appearing at pressures above 15 kbar on the low-temperature sides of the corresponding reaction curves, which show positive dP/dT slopes, is probably supercritical. The P-T positions of the reactions are compatible with KMASH mineral reactions studied previously and with melting investigations in the KMASH system undertaken at temperatures higher than 700°C.

It is possible that natural rocks, chiefly K-rich metasediments subducted as minor portions of the oceanic crust, could give rise to low-temperature ultrapotassic fluids, mainly at temperatures between 300° to 600°C and pressures between 15 and 30 kbar. The ascending K-rich fluids would penetrate the overlying mantle to metasomatize it. After termination of the subduction process, heating of this mantle material, previously cooled by the subducted lithosphere, could lead to the formation of high-temperature K-rich magmas.     

P -T-t history of the Lower Austroalpine Nappe Complex in the “Tarntaler Berge” NW of the Tauern Window: implications for the geotectonic evolution of the central Eastern Alps

New petrologic and ⁴⁰Ar/³⁹Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence. Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression. Whole-rock ⁴⁰Ar/³⁹Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe, and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely the high-pressure metamorphism. The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais) Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to final continental collision. Received: 30 July 1996 / Accepted: 7 April 1997