Characterisation of tobelite (NH4)Al2[AlSi3O10](OH)2 and ND4-tobelite (ND4)Al2[AlSi3O10](OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra

Tobelite (NH₄) Al₂ [AlSi₃O₁₀] (OH)₂, the ammonium analogue of muscovite, and its deuterated form ND₄-tobelite (ND₄) Al₂ [AlSi₃O₁₀] (OD)₂ have been synthesised at 600?°C and 200 and 500 Mpa using a well homogenised, stoichiometric SiO₂-Al₂O₃ oxide mix with Al₂O₃ in excess of 5 mol% and a 25% NH₃ solution whose relative abundance was such that the amount of NH₄ ⁺ stoichiometrically available was in excess of 50%. Characterisation of both tobelite and ND₄-tobelite using IR-spectroscopy, Rietveld refinement of X-ray powder diffraction data, and electron microprobe analysis indicate that, similar to K⁺ in muscovite, the NH₄ ⁺ or ND₄ ⁺ molecule occupies the interlayer site. IR absorption bands caused by NH₄ ⁺ and ND₄ ⁺ can be explained, to a very good approximation, on the basis of T_d symmetry. Nevertheless, substantial line broadening and the occurrence of shoulders indicate a deviation from ideal T_d symmetry. However, even at 77?K, no discrete splitting of the degenerate states could be confirmed. The OH stretching frequencies observed for synthetic tobelite are quite similar to those for muscovite, indicating that the replacement of K⁺ by NH₄ ⁺ has no effect. The low FWHH of the OH bands indicate that the hydroxyl groups are well ordered within the structure. Rietveld refinement of tobelite and ND₄-tobelite indicates that all samples synthesised consist of the 3 different mica polytypes which are typical of muscovite – namely 1M (C2/m), 2M ₁ (C2/c) and 2M ₂ (C2/c). Tobelite and ND₄-tobelite synthesised at 500 Mpa principally contain the 1M polytype, whereas the principle polytype for ND₄-tobelite synthesised at 200 Mpa, is 2M ₂. Rietveld refinement of X-ray diffraction spectra for tobelite synthesised at 200 Mpa was problematic due to the very broad FWHH of the X-ray peaks indicating poor crystallinity. In comparision to synthetic muscovite, the cell dimensions observed for tobelite and its deuterated analogue are quite similar except for the lattice constant c. Due to the larger radius of NH₄ ⁺ or ND₄ ⁺ compared to K⁺ cation, the c-direction is expanded form 10.275 Å in muscovite to approximately 10.540 Å in tobelite and ND₄-tobelite.     

http://www.sciencedirect.com/science/article/pii/S167498711400067X

The Proterozoic Bamble Sector, South Norway, is one of the world＇s classic amphiboliteto granulite- facies transition zones. It is characterized by a well-developed isograd sequence, with isolated ＇granulite-facies islands＇ in the amphibolite-facies portion of the transition zone. The area is notable for the discovery of C02-dominated fluid inclusions in the granolite-facies rocks by Jacques Touter in the late 1960＇s, which triggered discussion of the role of carbonic fluids during granulite genesis. The aim of this review is to provide an overview of the current state of knowledge of the Bamble Sector, with an emphasis on the Arendal-Froland-Nelaug-Tvedestrand area and off shore islands （most prominantly Tromay and Hisoy） where the transition zone is best developed. After a brief overview of the history of geological research and mining in the area, aspects of sedimentary, metamorphic and magmatic petrology of the Bamble Sector are discussed, including the role of fluids. Issues relevant to current geotectonic models for SW Scandinavia, directly related to the Bamble Sector, are discussed at the end of the review.     

Orthophosphate and biotite chemistry from orthopyroxene-bearing migmatites from California and South India: The role of a fluid-phase in the evolution of granulite-facies migmatites

Migmatites from Cone Peak, California, USA and the Satnur-Sangam road, Southern Karnataka, India contain coarser grained orthopyroxene-bearing leucosomes with subordinate biotite in finer grained hornblende-biotite-pyroxene-bearing hosts. At both localities the leucosomes are enriched in quartz and feldspar and have a higher ratio of pyroxene to hornblende + biotite compared to the host rocks. Biotite grains in leucosomes along the Satnur-Sangam road are concentrated at the margins of orthopyroxene grains and have lower abundances of Ti, Fe, and Cl and a higher abundance of F than biotite grains from the host rock. Fluorapatite grains in all rocks from both localities contain monazite inclusions similar to those produced experimentally by metasomatically induced dissolution and reprecipitation. Some fluorapatite grains at both localities are partially rimmed by allanite. The only compositional differences found between fluorapatite grains in the leucosomes and host rocks were higher concentrations of Cl in grains in leucosomes from Cone Peak. The mineralogies of the rocks suggest that the leucosomes formed by dehydration melting reactions that consumed feldspar, quartz, hornblende, and biotite and produced orthopyroxene. Allanite rims at the margins of fluorapatite grains may have formed by the later retrogression of monazite rims formed by incongruent dissolution of fluorapatite in the melt. Biotite grains at the margins of orthopyroxene crystals in the leucosomes from the Satnur-Sangam road apparently formed by retrogression of orthopyroxene upon the solidification of the anatectic melt. A similar high-grade retrogression did not affect orthopyroxene crystals at Cone Peak, indicating that H₂O was removed from the crystallizing leucosomes probably in a low H₂O activity fluid. Compositional differences between the paleosome and neosomes at Cone Peak are best explained by metasomatic interaction with concentrated brines while elevated Cl concentrations in fluorapatites in the leucosome suggest interaction with a Cl-bearing fluid. Brines may have been responsible for an exchange of elements between the host rock along the Satnur-Sangam road and zones of melt generation now marked by leucosomes, but fluid flow appears to have been less vigorous than at Cone Peak.     

Experimental Na/K exchange between alkali feldspar and an NaCl–KCl salt melt: chemically induced fracturing and element partitioning

The exchange of Na⁺ and K⁺ between alkali feldspar and a NaCl–KCl salt melt has been investigated experimentally. Run conditions were at ambient pressure and 850 °C as well as 1,000 °C. Cation exchange occurred by interdiffusion of Na⁺ and K⁺ on the feldspar sub-lattice, while the Si–Al framework remained unaffected. Due to the compositional dependence of the lattice parameters compositional heterogeneities resulting from Na⁺/K⁺ interdiffusion induced coherency stress and associated fracturing. Depending on the sense of chemical shift, different crack patterns developed. For the geometrically most regular case that developed when potassic alkali feldspar was shifted toward more sodium-rich compositions, a prominent set of cracks corresponding to tension cracks opened perpendicular to the direction of maximum tensile stress and did not follow any of the feldspar cleavage planes. The critical stress needed to initiate fracturing in a general direction of the feldspar lattice was estimated at ≤0.35 GPa. Fracturing provided fast pathways for penetration of salt melt or vapor into grain interiors enhancing overall cation exchange. The Na/K partitioning between feldspar and the salt melt attained equilibrium values in the exchanged portions of the grains allowing for extraction of the alkali feldspar mixing properties.     

Formation of Al-rich titanite (CaTiSiO4O–CaAlSiO4OH) reaction rims on ilmenite in metamorphic rocks as a function of fH2O and fO2

Reaction rims of titanite on ilmenite are described in samples from four terranes of amphibolite-facies metapelites and amphibolites namely the Tamil Nadu area, southern India; the Val Strona area of the Ivrea-Verbano Zone, northern Italy, the Bamble Sector, southern Norway, and the northwestern Austroalpine Ötztal Complex. The titanite rims, and hence the stability of titanite (CaTiSiO₄O) and Al–OH titanite, i.e. vuaganatite (hypothetical end-member CaAlSiO₄OH), are discussed in the light of fH₂O- and fO₂-buffered equilibria involving clinopyroxene, amphibole, biotite, ilmenite, magnetite, and quartz in the systems CaO–FeO/Fe₂O₃–TiO₂–SiO₂–H₂O–O₂ (CFTSH) and CaO–FeO/Fe₂O₃–Al₂O₃–SiO₂–H₂O–O₂ (CFASH) present in each of the examples. Textural evidence suggests that titanite reaction rims on ilmenite in rocks from Tamil Nadu, Val Strona, and the Bamble Sector originated most likely due to hydration reactions such as clinopyroxene + ilmenite + quartz + H₂O = amphibole + titanite and oxidation reactions such as amphibole + ilmenite + O₂ = titanite + magnetite + quartz + H₂O during amphibolite-facies metamorphism, or, as in the case of the Ötztal Complex, during a subsequent greenschist-facies overprint. Overstepping of these reactions requires fH₂O and fO₂ to be high for titanite formation, which is also in accordance with equilibria involving Al–OH titanite. This study shows that, in addition to P, T, bulk–rock composition and composition of the coexisting fluid, fO₂ and fH₂O also play an important role in the formation of Al-bearing titanite during amphibolite- and greenschist-facies metamorphism.     

Petrology and mineralogy of granulite-facies mafic xenoliths (Sardinia, Italy): Evidence for KCl metasomatism in the lower crust

Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust (T = 850–900 °C, P = 0.8–1.0 GPa). They consist of orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An = 50–66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene + plagioclase (An = 56–72 mol%) + Fe–Ti oxides ± K-feldspar ± biotite ± fluorapatite ± Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase–plagioclase and plagioclase–pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase–plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16–33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase–K-feldspar grain boundary are also observed. Small amounts of biotite (TiO₂ = 4.7–6.5 wt.%; F = 0.24–1.19 wt.%; Cl = 0.04–0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl = 6–50% of the total F + Cl + OH). These lines of evidence suggest that formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high-T recrystallisation of relatively dry rocks.     

Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India

Chemical trends from north (amphibolite facies) to south (granulitefacies) along a 95 km traverse in Tamil Nadu, Southern India,include: whole-rock depletion of Rb, Cs, Th and U, enrichmentin Ti and F, and depletion in Fe and Mn in biotite and amphibole;increases in Al and decreases in Mn in orthopyroxene; enrichmentof fluorapatite in F. K-feldspar blebs are widespread alongquartz–plagioclase grain boundaries, and could indicateeither partial melting or metasomatism. In the northernmostportion of the traverse the principal rare earth element (REE)-bearingminerals are allanite and titanite. South of a clinopyroxeneisograd, monazite grains independent of fluorapatite are themajor REE- and Th-bearing phase. Further south independent monaziteis rare but Th-free monazite inclusions are common in fluorapatite.During prograde metamorphism, independent monazite was replacedby REE-rich fluorapatite in which the monazite inclusions laterformed. The loss of independent monazite was accompanied bya loss of whole-rock Th and possibly a small depletion in lightREE. Most mineralogical features along the traverse can be accountedfor by progressive dehydration and oxidation reactions. Trace-elementdepletion is best explained by the action of an externally derivedlow H₂O activity brine migrating from a source at greater depth,possibly preceded or accompanied by partial melting. KEY WORDS: granulite facies; charnockite; metasomatism; Archean; Tamil Nadu, India; fluorapatite; monazite; allanite; titanite; biotite