B. Spiering

The Granulite Terrane of the Nilgiri Hills (Southern India): Characterization of High-Grade Metamorphism

The Nilgiri Hills massif in South India exposes an oblique section through late-Archaean lower crust (25 to 35 km paleo-depth). It is predominantly composed of foliated to massive enderbitic granulites (plag + qtz + opx + gar + bio) which, based on their geochemical features, isotope systematics and field evidences are interpreted as an intensely metamorphosed sequence of short-lived psammitic sediments interlayered with andesitic to dacitic volcanogenic rocks. The deeper part of the crustal section has been repeatedly intruded by basaltic and picritic magmas now represented by numerous extended bodies, lenses and pods of gabbroic to anorthositic two-pyroxene-plagioclase rocks, ferroan garnetiferous pyroxene-plagioclase rocks and pyroxenites. This ensemble is cut by undeformed but metamorphosed late dolente dykes.

Experimental study of the Fe-Mg exchange between garnet and biotite; constraints on the mixing behavior and analysis of the cation-exchange mechanisms

Abstract New experimental data are presented for the Fe-Mg exchange between garnet and biotite in the temperature range 600-800 °C at 0.2 GPa. The Fe-Mg-Al mixing properties of biotite were evaluated and the garnet-biotite geothermometer was recalibrated. SEM observations and comparative laser granulometry show that solution-precipitation largely controls the cation exchange mechanism, involving about 50% of the mineral volume. Mass balance calculations emphasize the effectiveness of the experimental design: A high Gt/Bio ratio ensures that the garnet composition remains approximately constant and close to equilibrium, even if the entire garnet volume is not involved in the cation exchange. Progressively decreasing partition coefficients with decreasing Fe content of garnet indicate nonideal thermodynamic mixing behavior. The application of various garnet activity models support nearly ideal Fe-Mg mixing in garnet. The remaining nonidealities were attributed to nonideal Fe, Mg, and Al mixing in biotite as the initially binary biotite samples changed toward more aluminous compositions during the experiments. Adopting the standard state properties and the garnet-mixing model of Berman (1988, 1990), least square regressions reveal nearly ideal mixing of Fe and Mg in biotite with WFeMg = - 2.3 ± 1.6 kJ/mol, while the difference between Fe-Al and Mg-Al interactions yield ΔWAl = - 17.6 ± 2.4 kJ/mol (1 cation). This interaction parameter is strictly valid only for Tschermaksubstituted [6]Al in biotite according to the operational substitution. Application of the suggested garnet-biotite geothermometer reproduces well the reference temperatures of experimental and natural garnet biotite assemblages.

Petrologic phase equilibria and stable isotope fractionations of carbonate-silicate parageneses from granulite-grade rocks of Sri Lanka

Abstract The calc-silicate rocks and marbles of central and southern Sri Lanka can be characterized by three groups of parageneses: Cc+Dol+Phl±Fo±Spl±Di+Acc in the northern and eastern part of the Highland Complex, Di+Scp+Spn+Cc+Acc following to the west and Wo+Di+Scp+Spn±Cc+Acc in the most southwestern part. This distribution mirrors the compositional variation of the sedimentary precursors and the regional pressure gradient. During prograde metamorphism decarbonation with internal fluid buffering increased XCO2 to 0.75. Graphite precipitation at near-peak metamorphic conditions at constant oxygen fugacity reduced XCO2 to ∼0.5. The variation of δ18O ( +26 to +9‰ ) and δ13C ( +4 to −5‰ ) of carbonates can largely be explained by primary differences in the isotopic composition and in the silicate/carbonate contents of the protoliths. The particular light isotopic compositions are due to fractionation processes during decarbonation of the rocks. The dolomite-free scapolite-bearing rocks in general are isotopically lighter than dolomite-bearing parageneses. Calcite-graphite temperatures in the range of 750–900°C were calculated, using the experimentally derived thermometer of Scheele (1991). Calcite-silicate temperatures (150–900°C) were calculated with the increment method (Hoffbauer et al., 1994). These temperatures reflect near-peak equilibrium as well as different degrees of retrograde isotope resetting.

Iron in silicate glasses of granitic composition: a Mssbauer spectroscopic study

Abstract57Fe Mössbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M+-cations are available to balance the charge of both M+Fe3+O2 and M+AlO2 complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe3+ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe2+. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe2+ are depolymerized for increasing mean Z/r2 of the network modifiers, which do not stabilize M3+ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe2+ among these sites and thereby to an ordering in the network of melts.

Mineralogy, geochemistry and petrogenesis of the V-Ti-bearing and chromiferous magnetite deposits hosted by Neoarchaean Channagiri Mafic-Ultramafic Complex, Western Dharwar Craton, India: Implications for emplacement in differentiated pulses

The Channagiri Mafic-Ultramafic Complex occupies lowermost section of the Neoarchaean Shimoga supracrustal group in the Western Dharwar Craton. It is a segmented body occupying the interdomal troughs of granitoids. The magnetite deposits occur in the northeastern portion; typically occupying the interface zone between gabbro and anorthositic. Mineralogically, the deposits are simple with abundant magnetite and ilmenite. Hogbomite is a consistent minor mineral. Magnetites are typically vanadiferous (0.7–1.25% V2O5). Ilmenite consistently analyses more MgO and MnO than coexisting magnetite. Chlorite, almost the only silicate present; lies in the range of ripidolite, corundophilite and sheridanite. The chromiferous suit occupying eastern side of Hanumalapur block (HPB) contains Fe-Cr-oxide analysing 37.8–11.9% Cr2O3 and 40.5–80% FeOt. In these too, chlorite, typically chromiferous (0.6–1.2% Cr2O3), is the most dominant silicate mineral. Geochemistry of V-Ti-magnetite is dominated by Fe, Ti and V with Al, Si, Mg and Mn contributing most of the remaining. Cr, Ni, Zn, Co, Cu, Ga and Sc dominate trace element geochemistry. The Cr-magnetite is high in Cr2O3 and PGE. Two separate cycles of mafic magmatism are distinguished in the CMUC. The first phase of first cycle, viz., melagabbro-gabbro, emplaced in the southeastern portion, is devoid of magnetite deposits. The second phase, an evolved ferrogabbroic magma emplaced in differentiated pulses, occupying northeastern portion of the complex, consists of melagabbro→gabbro-anorthosite→V-Ti magnetite→ferrogabbro sequence. Increase in oxygen fugacity facilitated deposition of V-Ti magnetite from ferrogabbroic magma pulse emplaced in late stages. The second cycle of chromiferous PGE mineralized suite comprises fine-grained ultramafite→alternation of pyroxinite-picrite→Crmagnetite sequence formed from fractionation of ferropicritic magma. HPB also includes >65m thick sill-like dioritic phase at the base of the ferriferous suit and a sinuous band of coarse-grained ultramafite enclosed within the chromiferous suit; both unrelated to the two mafic magmatic cycles.

Fossil eggshell cuticle elucidates dinosaur nesting ecology

The cuticle layer consisting mainly of lipids and hydroxyapatite (HAp) atop the mineralized avian eggshell is a protective structure that prevents the egg from dehydration and microbial invasions. Previous ornithological studies have revealed that the cuticle layer is also involved in modulating the reflectance of eggshells in addition to pigments (protoporphyrin and biliverdin). Thus, the cuticle layer represents a crucial trait that delivers ecological signals. While present in most modern birds, direct evidence for cuticle preservation in stem birds and non-avian dinosaurs is yet missing. Here we present the first direct and chemical evidence for the preservation of the cuticle layer on dinosaur eggshells. We analyze several theropod eggshells from various localities, including oviraptorid Macroolithus yaotunensis eggshells from the Late Cretaceous deposits of Henan, Jiangxi, and Guangdong in China and alvarezsaurid Triprismatoolithus eggshell from the Two Medicine Formation of Montana, United States, with the scanning electron microscope (SEM), electron probe micro-analysis (EPMA), and Raman spectroscopy (RS). The elemental analysis with EPMA shows high concentration of phosphorus at the boundary between the eggshell and sediment, representing the hydroxyapatitic cuticle layer (HAp). Depletion of phosphorus in sediment excludes the allochthonous origin of the phosphorus in these eggshells. The chemometric analysis of Raman spectra collected from fossil and extant eggs provides further supportive evidence for the cuticle preservation in oviraptorid and probable alvarezsaurid eggshells. In accordance with our previous discovery of pigments preserved in Cretaceous oviraptorid dinosaur eggshells, we validate the cuticle preservation on dinosaur eggshells through deep time and offer a yet unexplored resource for chemical studies targeting the evolution of dinosaur nesting ecology. Our study also suggests that the cuticle structure can be traced far back to maniraptoran dinosaurs and enhance their reproductive success in a warm and mesic habitat such as Montana and southern China during the Late Cretaceous.