Manfred Bernroider

Hagendorfite (Na,Ca)MnFe2(PO4)3 from type locality Hagendorf (Bavaria, Germany) : crystal structure determination and 57Fe Mössbauer spectroscopy

The crystal structure of hagendorfite (Na,Ca)MnFe 2 (PO 4 ) 3 from type locality has been determined and it was found to be isostructural with alluaudite. It accepts space group symmetry C2/c both at room temperature and at 100 K. At room temperature the lattice parameters are a = 11.9721(9) A, b = 12.5988(8) A, c = 6.5029(5) A, β = 114.841(8)° with Z = 4. For reasons of comparison the structure of an alluaudite sensu strictu (s.s.) from Buranga (Rwanda) was re-determined. The specific arrangement of M(1) and M(2) octahedral sites and of P(1) and P(2) tetrahedral sites gives rise to two different channels aligned along the crystallographic c-axis, containing the A(1) and A(2)’ sites. In both compounds the A(1) site is fully occupied and shows a mixed occupation of Na + , Ca 2+ and Mn 2+ (hagendorfite) and Ca 2+ and Na + (alluaudite s.s.). The A(2)’ is fully occupied by Na + in hagendorfite and partly filled by 0.14 Na + atoms per formula unit in Buranga alluaudite. The structural topology of hagendorfite is described in detail and structural parameters are compared to alluaudite s.s. and to other, mostly synthetic compounds also crystallizing in the alluaudite structure type. 57 Fe Mossbauer spectroscopy shows that iron is exclusively in the trivalent state in alluaudite s.s. while in hagendorfite about 2/3 of the total iron are in the divalent state. The ferrous iron resonance absorption contribution appears to be broad and can only be refined with four different Fe 2+ subcomponents, which are all ascribed to the M(2) site. The four different spectroscopic signals presumably arise from different next nearest neighbour occupations of adjacent M(2) and A(1) sites.

Low-grade retrogression of a high-temperature metamorphic core complex: Naxos, Cyclades, Greece

Retrogressive deformation and metamorphism are often reported from the main low-angle shear zones and detachments of metamorphic core complexes, but their importance is not sufficiently emphasized for the footwall interior. In order to contribute to a better understanding of exhumation-related retrogression processes within and at the top of metamorphic core complexes, an integrated detailed microstructural, textural, 40 Ar/ 39 Ar geochronological, and thermobarometric study on the Naxos metamorphic core complex within the Aegean Sea is presented that provides a new perspective on low-grade retrogression during exhumation through shallow ductile levels. We found variable retrogressive deformation within the Naxos metamorphic core complex, which even pervasively affected significant portions of the migmatite-grade metamorphic core and remnant high-pressure areas of the metamorphic core complex, where retrogression led to pervasive formation of new fabrics within greenschist-facies metamorphic conditions during brittle-ductile transition. Within a continuum of retrogression, 40 Ar/ 39 Ar white mica dating allowed us to deduce three retrogressive ages at 16.52 ± 0.39 Ma (within the Naxos metamorphic core complex), 12.6 ± 0.28 Ma (Moutsounas detachment shear zone on the eastern boundary of the metamorphic core complex), and 10.43 ± 0.44 Ma to 8.40 ± 0.76 Ma (last ductile activity along the Naxos-Paros shear zone to the north of the metamorphic core complex). A further stage of retrogression at 12−11 Ma occurred along distinct low-angle normal faults within the middle Miocene Naxos Granite. Retrogressive microstructures, low-temperature calcite fabrics in marbles, and chloritization in metapelites (at temperatures of ∼350−130 °C) in the metamorphic core complex core resulted mainly from late-stage E-W shortening and folding. Late-stage flow of hydrous fluids resulted in resetting of fabrics and enhancement of ductile deformation. The middle−late Miocene retrogression events are also reflected by a similarly aged tectonic collapse basin in the hanging-wall unit above the detachment. The wide temporal range of retrogression within the Naxos metamorphic core complex coincides in age with retrogressive deformation within other metamorphic core complexes of the Aegean Sea. We interpret the long temporal range of retrogression to reflect outward, southwestward retreat of the subduction and sequential activation of major detachment zones.

Co2+-Cu2+ substitution in bieberite solid-solution series, (Co1-xCux)SO4·7H2O, 0.00 ≤ x ≤ 0.46: Synthesis, single-crystal structure analysis, and optical spectroscopy

Abstract Single crystals of Cu-substituted bieberite (Co1-xCux)SO4·7 H2O have been synthesized from aqueous solution by the evaporation method. It is shown that the solubility of Cu2+ in bieberite is limited to 0.46 atoms per formula unit (apfu). Chalcantite CuSO4·5H2O only accommodates a very small amount of ~0.03 Co2+ atoms. Structure analysis by X-ray diffraction of single crystals along the join CoSO4·7H2O-CuSO4·5H2O showed that the hepta-hydrate phases are monoclinic with P21/c symmetry at room temperature, whereas the penta-hydrate phases are triclinic, P1̅. Within the Co1-xCuxSO4·7H2O series (0 ≤ x ≤ 0.46), lattice parameters are distinctly altered by the Cu2+ substitution, however, the observed changes cannot be ascribed to different ionic radii of Co2+ and Cu2+ but are due to an increasing distortion of the lattice by increasing Cu2+ content. Cu2+ is not distributed randomly over the two possible crystallographic metal sites but exclusively enters the M2 site. While all the oxygen atoms coordinating the M1 site act as donors of hydrogen bonding to the sulfate tetrahedron, one oxygen atom at the M2 site is a hydrogen bond acceptor. This may cause the slight tetragonal elongation of the M2 coordination polyhedron, which favors the preferential occupation of this site by Cu2+. Optical spectra show typical features of both the Co2+ and the Cu2+ absorption signatures. While in Co2+-rich hepta-hydrate phases Co2+ absorption bands arising from both M1 and M2 sites are detected, there are only bands arising from the M1 site in Co0.54Cu0.46SO4·7H2O. This supports the observation of selective substitution of Cu2+ at the elongated M2 octahedral site. Cu2+ absorption bands can be assigned on the basis of D4h symmetry with increasing splitting of the Cu2+, 2T2g, and 2Eg energy levels with increasing Cu2+ content.

Manganoan rockbridgeite Fe4.32Mn0.62Zn0.06(PO4)3(OH)5: structure analysis and 57Fe Mössbauer spectroscopy.

The structure of the basic iron phosphate rockbridgeite [iron manganese zinc tris(phosphate) pentahydroxide] was reinvestigated with special emphasis on the cation distribution deduced from new X-ray and 57Fe Mössbauer data. Rockbridgeite is orthorhombic, space group Cmcm, and shows three different Fe sites, one with 2/m symmetry, another with m symmetry and the third in a general position. One phosphate group has the P atom on a site with m symmetry, while the other has the P atom at a site with mm symmetry. Two Fe sites are fully occupied by ferric iron, while Mn3+ and Fe2+ are situated at a third, principally Fe, site. Structural data, bond-valence sums and polyhedral distortion parameters suggest a new interpretation of the rockbridgeite 57Fe Mössbauer spectrum.

Geochemical and isotopic evidence for Carboniferous rifting: mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

Abstract In this paper, we present detailed field observations, chronological, geochemical and Sr–Nd isotopic data and discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes exhibit a cumulate to foliated texture and strike NW–SE, parallel to the main trend of the region. The 40Ar/39Ar amphibole age of 321.32 ± 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through ca. 550 °C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units, the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized into those with light REE-enriched patterns [(La/Yb)N = 8.32–9.28] and others with a rather flat REE pattern [(La/Yb)N = 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)PM ratios. The alkaline mafic dykes were formed within an enriched mantle source at depths of ˃ 90 km, generating a suite of alkaline basalts. In comparison, the subalkaline mafic dykes were formed within more depleted mantle source at depths of ˂ 90 km. The subalkaline mafic dyke is characterized by 87Sr/86Sr ratio of 0.706 and positive ɛNd(t) value of + 0.77, whereas 87Sr/86Sr ratio of 0.708 and ɛNd(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the emplacement of the Jurassic Darijune gabbro.

Geochemistry of enclaves and host granitoids from the kashan granitoid complex, central iran: Implications for enclave generation by interaction of cogenetic magmas

The major and trace elements and Sr-Nd-Pb isotopes of Miocene host granitoid rocks and their mafic microgranular enclaves (MMEs) were studied to understand the petrogenesis of MMEs in the Kashan complex, which is part of the Urumieh-Dokhtar magmatic belt (Iran). The host rocks consist of quartz-diorite and tonalite associated with a dioritic intrusion. The enclaves show microgranular texture and the same mineralogy as their respective host with plagioclase, quartz and biotite. MMEs have a diorite to quartz-diorite composition and show geochemical characteristics mostly between their granitoid host and the diorite intrusion. Chondrite-normalized REE patterns of all samples are moderately fractionated [(La/Yb)N=2.1 to 12.9]. The MMEs display in part small negative Eu anomalies (Eu/Eu*=0.54 to 0.99), with enrichment of LILE and depletion of HFSE. The enclaves show emplacement depth of ~4 to 6 km which is comparable with the host rocks. Moreover, the Hornblende-plagioclase equilibrium temprature of MMEs yields average temperatures of 795°C which is slightly higher than the host ones. Identical mineral compositions and Nd-Sr-Pb isotopic features of MME-host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in the Kashan plutons. Additionally, the geochemical and isotopic investigations of host and dioritic intrusions suggest a common source for their genesis. A thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower crust to generate Kashan granitoid rocks.

The Oligocene Reifnitz tonalite (Austria) and its host rocks: implications for Cretaceous and Oligocene–Neogene tectonics of the south-eastern Eastern Alps

Abstract In the south-eastern Eastern Alps, the Reifnitz tonalite intruded into the Austroalpine metamorphic basement of the Wörthersee half-window exposed north of the Sarmatian–Pliocene flexural Klagenfurt basin. The Reifnitz tonalite is dated for the first time, and yields a laser ICP-MS U–Pb zircon age of 30.72±0.30 Ma. The (U–Th–Sm)/He apatite age of the tonalite is 27.6 ± 1.8 Ma implying rapid Late Oligocene cooling of the tonalite to ca. 60 °C. The Reifnitz tonalite intruded into a retrogressed amphibolite-grade metamorphic basement with a metamorphic overprint of Cretaceous age (40Ar/39Ar white mica plateau age of 90.7 ± 1.6 Ma). This fact indicates that pervasive Alpine metamorphism of Cretaceous age extends southwards almost up to the Periadriatic fault. Based on the exhumation and erosion history of the Reifnitz tonalite and the hosting Wörthersee half window formed by the Wörthersee anticline, the age of gentle folding of Austroalpine units in the south-eastern part of the Eastern Alps is likely of Oligocene age. North of the Wörthersee antiform, Upper Cretaceous–Eocene, Oligocene and Miocene sedimentary rocks of the Krappfeld basin are preserved in a gentle synform, suggesting that the top of the Krappfeld basin has always been near the Earth’s surface since the Late Cretaceous. The new data imply, therefore, that the Reifnitz tonalite is part of a post-30 Ma antiform, which was likely exhumed, uplifted and eroded in two steps. In the first step, which is dated to ca. 31–27 Ma, rapid cooling to ca. 60 °C and exhumation occurred in an E–W trending antiform, which formed as a result of a regional N–S compression. In the second step of the Sarmatian–Pliocene age a final exhumation occurred in the peripheral bulge in response to the lithospheric flexure in front of the overriding North Karawanken thrust sheet. The Klagenfurt basin developed as a flexural basin at the northern front of the North Karawanken, which represent a transpressive thrust sheet of a positive flower structure related to the final activity along the Periadriatic fault. In the Eastern Alps, on a large scale, the distribution of Periadriatic plutons and volcanics seems to monitor a northward or eastward shift of magmatic activity, with the main phase of intrusions ca. 30 Ma at the fault itself.