Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3

Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author     

Distribution of moderately volatile trace elements in fine-grained chondrule rims in the unequilibrated CO3 chondrite, ALH A77307

The concentrations of Ni, Cu, Zn, Ga, Ge, and Se in five, fine-grained chondrule rims in the highly unequilibrated CO3 chondrite ALH A77307 (3.0) have been determined for the first time by synchrotron X-ray fluorescence (SXRF) microprobe at Brookhaven National Laboratory. These elements are especially useful for tracing the role of condensation and evaporation processes which occurred at moderate temperatures in the solar nebula. Understanding the distribution of moderately volatile elements between matrix and chondrules is extremely important for evaluating the different models for the volatile depletions in chondritic meteorites. The data show that the trace element chemistry of rims on different chondrules is remarkably similar, consistent with data obtained for the major and minor elements by electron microprobe. These results support the idea that rims are not genetically related to individual chondrules, but all sampled the same reservoir of homogeneously mixed dust. Of the trace elements analyzed, Zn and Ga show depletions relative to CI chondrite values, but in comparison with bulk CO chondrites all the elements are enriched by approximately 1.5 to 3.5 x CO. The abundance patterns for moderately volatile elements in ALH A77307 chondrule rims closely mimic those observed in the bulk chondrite, indicating that matrix is the major reservoir for these elements. The close matching of the patterns for the volatile depleted bulk chondrite and enriched matrix is especially striking for Na, which is anomalously depleted in ALH A77307 in comparison with average CO chondrite abundances. The depletion in Na is probably attributable to the effects of leaching in Antarctica. With the exception of Na, the volatile elements show a relatively smooth decrease in abundance as a function of condensation temperature, indicating that their behavior is largely controlled by their volatility.     

A Strontium and Neodymium Isotopic Investigation of the Fongen--Hyllingen Layered Intrusion, Norway

The Fongen—Hyllingen Intrusion, situated 60 km SE of Trondheim,Norway, is a synorogenic layered mafic intrusion of Caledonianage . The intrusion is divided into four evolutionary stages based on cryptic variations: StageI—a basal reversal; Stage II—unchanged mineral chemistryor slight normal evolution; Stage III—a gradual regression;Stage IV— a strong normal fractionation trend Magma replenishmentdominated during most of the crystallization, i.e. during StagesI, II and III Replenishing magma was more dense than resident,evolved magma, and continuing influx eventually caused a compositionallystratified magma column to form. Cryptic lateral variation isan important feature in the southern part of the complex andformed by in situ crystallization from a stratified magma alongan inclined floor, where modal layering formed parallel to thecrystallization front. Initial Sr- and Nd-isotopic ratios inthe cumulates vary as a result of assimilation of country rockand subsequent mixing between uncontaminated, replenishing magmaand contaminated, resident magma. The parental magma had a moderatelydepleted isotope composition, relative to Bulk Earth, with _Nd=584and Sr_i=070308, whereas the main contaminant was a partialmelt of metapelitic country rock with _Nd=-874 and Sr_i=07195(Sr_i is the initial ⁸⁷Sr/⁸⁶Sr). Sr_i in the analysed cumulatewhole-rock samples ranges from 070308 to 070535 and initial_Nd ranges from. 158 to 584. There is a strong correlationbetween mineralogical composition and isotopic trends in mostof the cumulates: the most primitive samples are the least contaminated,as reflected by relatively high ed and low Sr,, and more evolvedsamples have progressively lower eNi and higher Sry A gradualregression of several hundred metres thickness characterizesStage III; stratigraphically upwards mineral compositions becomemore primitive and isotope compositions more depleted (higher_Nd and lower Sr_i), implying a process of. progressive mixing-inof replenishing, primitive and uncontaminated magma. Magma influxin Stage III took place by fountaining, whereas magma additionwas more tranquil in the earlier stages. The fountaining influxentrained resident, relatively evolved and contaminated magma,resulting in a hybrid magma which ponded at the floor. Duringprolonged magma addition with concomitant crystallization, thelowermost magma layer was replaced by progressively more primitivehybrid magma, creating a gradual regression in the crystallizingcumulate sequence. A detailed two-dimensional study revealslateral variations in mineral compositions both at the baseand top of Stage III, whereas lateral variations in Sr- andNd-isotopic compositions are present at the top, but not atthe base. This implies that the lowest crystallizing part ofthe magma column was essentially isotopically homogeneous, butcompositionally stratified, before influx in Stage III. Isotopicgradients in the magma were strong close to the roof, wheremost of the assimilation occurred, and decreased downwards,merging into isotopically homogeneous magma. This stratifiedsystem was destroyed by turbulent mixing between replenishingand resident magma during fountaining influx in Stage III, anda new stratification was established with both an isotopic anda compositional gradient. After the final influx, crystallizationcontinued in an essentially closed system, in which the remainingmagma column eventually became homogenized, as magma layersmixed when their densities converged owing to release of buoyant,residual liquid during fractional crystallization. Corresponding author     

Structural control of tertiary Au-Ag-bearing breccias in an extensional environment,Nelson area,Southern Nevada,USA

Gold-silver mineralization in the Nelson area of southern Nevada was controlled by structures associated with intrusion of an east-west oriented pluton. Flatlying breccias formed during intrusion have allowed passive flooding of highly permeable zones and deposition of mineralized quartz and calcite. Steep fractures were formed in the pluton and immediate country rock during cooling, and later reactivated by north-south extension. These fractures have channelled fluids, and some have been the sites of hydrothermal eruptions which produced further brecciation and deposition of mineralized quartzcalcite veins. The mineralizing fluid was water which was boiling at or near 100 °C. The calcite deposited by this water has ¹³C = –5.4 to –7.1, and ¹⁸O = +5.8 to +11.3, and the water was probably meteoric in origin. Mineralization had an epithermal style, with strong local structural control, rather than deep-sourced regional detachment-related hydrothermal origin.     

Association of orogenic activity with the Ordovician radiation of marine life

The Ordovician radiation of marine life was among the most substantial pulses of diversification in Earth history and coincided in time with a major increase in the global level of orogenic activity. To investigate a possible causal link between these two patterns, the geographic distributions of 6576 individual appearances of Ordovician vician genera around the world were evaluated with respect to their proximity to probable centers of orogeny (foreland basins). Results indicate that these genera, which belonged to an array of higher taxa that diversified in the Middle and Late Ordovician (trilobites, brachiopods, bivalves, gastropods, monoplacophorans), were far more diverse in, and adjacent to, foreland basins than they were in areas farther removed from orogenic activity (carbonate platforms). This suggests an association of orogeny with diversification at that time.     

Molecular indicators for palaeoenvironmental change in a Messinian evaporitic sequence (Vena del Gesso, Italy). II: High-resolution variations in abundances and 13C contents of free and sulphur-bound carbon skeletons in a single marl bed

The extractable organic matter of 10 immature samples from a marl bed of one evaporitic cycle of the Vena del Gesso sediments (Gessoso-solfifera Fm., Messinian, Italy) was analyzed quantitatively for free hydrocarbons and organic sulphur compounds. Nickel boride was used as a desulphurizing agent to recover sulphur-bound lipids from the polar and asphaltene fractions. Carbon isotopic compositions (delta vs PDB) of free hydrocarbons and of S-bound hydrocarbons were also measured. Relationships between these carbon skeletons, precursor biolipids, and the organisms producing them could then be examined. Concentrations of S-bound lipids and free hydrocarbons and their delta values were plotted vs depth in the marl bed and the profiles were interpreted in terms of variations in source organisms, 13 C contents of the carbon source, and environmentally induced changes in isotopic fractionation. The overall range of delta values measured was 24.7%, from -11.6% for a component derived from green sulphur bacteria (Chlorobiaceae) to -36.3% for a lipid derived from purple sulphur bacteria (Chromatiaceae). Deconvolution of mixtures of components deriving from multiple sources (green and purple sulphur bacteria, coccolithophorids, microalgae and higher plants) was sometimes possible because both quantitative and isotopic data were available and because either the free or S-bound pool sometimes appeared to contain material from a single source. Several free n-alkanes and S-bound lipids appeared to be specific products of upper-water-column primary producers (i.e. algae and cyanobacteria). Others derived from anaerobic photoautotrophs and from heterotrophic protozoa (ciliates), which apparently fed partly on Chlorobiaceae. Four groups of n-alkanes produced by algae or cyanobacteria were also recognized based on systematic variations of abundance and isotopic composition with depth. For hydrocarbons probably derived from microalgae, isotopic variations are well correlated with those of total organic carbon. A resistant aliphatic biomacromolecule produced by microalgae is, therefore, probably an important component of the kerogen. These variations reflect changes in the depositional environment and early diagenetic transformations. Changes in the concentrations of S-bound lipids induced by variations in conditions favourable for sulphurization were discriminated from those related to variations in primary producer assemblages. The water column of the lagoonal basin was stratified and photic zone anoxia occurred during the early and middle stages of marl deposition. During the last stage of the marl deposition the stratification collapsed due to a significant shallowing of the water column. Contributions from anaerobic photoautotrophs were apparently associated with variations in depth of the chemocline.     

Powder- and single crystal Mössbauer spectroscopy on synthetic fayalite

Mössbauer measurements on synthetic iron orthosilicate Fe₂SiO₄ (fayalite) were carried out in the antiferromagnetic spin state below T _N 65 K. The Mössbauer parameters isomer shift , inner magnetic field H(0), angle between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter were determined as a function of temperature. These parameters could be attributed to the two crystallographic sites M1 and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments. H(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T = 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous Mössbauer refinements published elsewhere.     

Mineralogical and geochemical compositions of modern bivalve shells from the Mediterranean coast of Egypt

The stable isotopic composition (¹³C and ¹⁸O) and elemental (Sr and Mg) of marine molluscs are presented for Carditacea and Solenacea shells collected off the Mediterranean coast of Egypt. Based on shell microstructures and mineralogy, the bivalve shells are preserved in their original mineralogy and chemistry.The Sr and Mg concentrations of the bivalve shells have mean values of 1960 ppm and 226 ppm respectively. The stable isotopic composition generally show high values of ¹⁸O and ¹³C. The ¹⁸O values range from +0.1 to –1.8 PDB and most shells are highly enriched in¹³C; averaging +2.5 PDB. These elemental and isotopic signatures are analogous to modern marine bivalves from other localities.The oxygen and carbon isotopes, together with the calculated temperatures, suggest that the aragonitic bivalve shells were precipitated in isotopic equilibrium from warm marine waters.     

The age of the Kagenfels granite (northern Vosges) and its bearing on the intrusion scheme of late Variscan granitoids

In the Saxothuringian part of the Vosges (France), a first series of Variscan plutonic rocks (diorites to granites) has been intruded by several younger granites. Rocks of both the older generations have been cross-cut by the late orogenic Kagenfels granite. The averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 Ma, respectively, whereas Rb-Sr and K-Ar dates around 290 Ma have been reported for the Kagenfels granite. Because of the unlikely large age hiatus, a redetermination of the intrusion age of the Kagenfels granite formation appeared to be irrevocable. The newly obtained mineral ages on the Kagenfels granite (K-Ar and ⁴⁰Ar/³⁹Ar biotite ages as well as single zircon radiogenic ²⁰⁷Pb/²⁰⁶Pb data: 331 ± 5 Ma) are about 40 Ma older than the previous results. They are interpreted as giving the time of emplacement of the Kagenfels granite during the latest Visan. The mineral ages of the earlier plutonic rocks in this part of the Variscan Orogeny in all probability are not significantly different from their ages of intrusion. Therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the Lower-Upper Carboniferous boundary.     

A search for ultra-high energy counterparts to gamma-ray bursts

A small air shower array operating over many years has been used to search for ultra-high energy (UHE) gamma radiation ( 50 TeV) associated with gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each burst are presented for seven GRBs located with zenith angles < 20°. A 4.3 excess over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8%. If this effect is real then cosmological models are excluded for this burst because of absorption of UHE gamma rays by the intergalactic radiation fields.