TY - JOUR A1 - Wunder, Bernd A1 - Kutzschbach, Martin A1 - Hosse, Luisa A1 - Wilke, Franziska Daniela Helena A1 - Schertl, Hans-Peter A1 - Chopin, Christian T1 - Synthetic B-[4]-bearing dumortierite and natural B-[4]-free magnesiodumortierite from the Dora-Maira Massif BT - differences in boron coordination in response to ultrahigh pressure JF - European journal of mineralogy N2 - Dumortierite was synthesized in piston-cylinder experiments at 2.5-4.0 GPa, 650-700 degrees C in the Al2O3 -B2O3-SiO2-H2O (ABSH) system. Electron-microprobe (EMP) analyses reveal significant boron-excess (up to 0.26 B-[4] per formula unit, pfu) and silicon-deficiency relative to the ideal anhydrous dumortierite stoichiometry Al7BSi3O18 . The EMP data in conjunction with results from single-crystal Raman spectroscopy and powder X-ray diffraction provide evidence that silicon at the tetrahedral site is replaced by excess boron via the substitution Si-[4] <--> B-[4] + H. The Raman spectrum of synthetic dumortierite in the frequency region 2000 4000 cm(-1) comprises eight bands, of which six are located at frequencies below 3400 cm(-1). This points to strong hydrogen bonding, most likely O2-H center dot center dot center dot O7 and O7-H center dot center dot center dot O2, arising from a high number of octahedral vacancies at the All site and substitution of trivalent Al3+ and B3+ for Si4+ at Si1 and Si2 sites, causing decreasing acceptor-donor distances and lower incident valence at the acceptor oxygen. Contrary to the synthetic high-pressure ABSH-dumortierite, magnesiodumortierite from the Dora-Maira Massif, which is assumed to have formed at similar conditions (2.5-3.0 GPa, 700 degrees C), does not show any B-excess. Tourmaline shows an analogous behaviour in that magnesium-rich (e.g., dravitic) tourmaline formed at high pressure shows no or only minor amounts of tetrahedral boron, whereas natural aluminum-rich tourmaline and synthetic olenitic tourmaline formed at high pressures can incorporate significant amounts of tetrahedral boron. Two mechanisms might account for this discrepancy: (i) Structural avoidance of Mg-[6]-(OR3+)-R-[4] configurations in magnesiodumortierite due to charge deficieny at the oxygens O2 and O7 and strong local distortion of M1 due to decreased O2-O7 bond length, and/or (ii) decreasing fluid mobility of boron in Al-rich systems at high pressures. KW - dumortierite KW - magnesiodumortierite KW - Dora-Maira KW - ultrahigh-pressure (UHP) KW - ABSH-system KW - synthesis KW - tetrahedral boron KW - Raman spectroscopy Y1 - 2018 U6 - https://doi.org/10.1127/ejm/2018/0030-2742 SN - 0935-1221 SN - 1617-4011 VL - 30 IS - 3 SP - 471 EP - 483 PB - Schweizerbart CY - Stuttgart ER - TY - JOUR A1 - Ferrero, Silvio A1 - Godard, Gaston A1 - Palmeri, Rosaria A1 - Wunder, Bernd A1 - Cesare, Bernardo T1 - Partial melting of ultramafic granulites from Dronning Maud Land, Antarctica BT - constraints from melt inclusions and thermodynamic modeling JF - American mineralogist : an international journal of earth and planetary materials N2 - In the Pan-African belt of the Dronning Maud Land, Antarctica, crystallized melt inclusions (nanogranitoids) occur in garnet from ultramafic granulites. The granulites contain the peak assemblage pargasite+garnet+clinopyroxene with rare relict orthopyroxene and biotite, and retrograde symplectites at contacts between garnet and amphibole. Garnet contains two generations of melt inclusions. Type 1 inclusions, interpreted as primary, are isolated, < 10 mu m in size, and generally have negative crystal shapes. They contain kokchetavite, kumdykolite, and phlogopite, with quartz and zoisite as minor phases, and undevitrified glass was identified in one inclusion. Type 2 inclusions are < 30 mu m in size, secondary, and contain amphibole, feldspars, and zoisite. Type 2 inclusions appear to be the crystallization products of a melt that coexisted with an immiscible CO2-rich fluid. The nanogranitoids were re-homogenized after heating in a piston-cylinder in a series of four experiments to investigate their composition. The conditions ranged between 900 and 950 degrees C at 1.5-2.4 GPa. Type 1 inclusions are trachytic and ultrapotassic, whereas type 2 melts are dacitic to rhyolitic. Thermodynamic modeling of the ultramafic composition in the MnNCKFMASHTO system shows that anatexis occurred at the end of the prograde P-T path, between the solidus (at ca. 860 degrees C-1.4 GPa) and the peak conditions (at ca. 960 degrees C-1.7 GPa). The model melt composition is felsic and similar to that of type 1 inclusions, particularly when the melting degree is low (< 1 mol%), close to the solidus. However the modeling fails to reproduce the highly potassic signature of the melt and its low H2O content. The combination of petrology, melt inclusion study, and thermodynamic modeling supports the interpretation that melt was produced by anatexis of the ultramafic boudins near peak P-T conditions, and that type 1 inclusions contain the anatectic melt that was present during garnet growth. The felsic, ultrapotassic composition of the primary anatectic melts is compatible with low melting degrees in the presence of biotite and amphibole as reactants. KW - Nanogranitoids KW - partial melting KW - thermodynamic modeling KW - Antarctica KW - ultramafic granulites KW - kumdykolite KW - kokchetavite KW - High-Grade Metamorphism KW - Anatexis KW - and Granite Magmatism Y1 - 2018 U6 - https://doi.org/10.2138/am-2018-6214 SN - 0003-004X SN - 1945-3027 VL - 103 IS - 4 SP - 610 EP - 622 PB - Mineralogical Society of America CY - Chantilly ER - TY - JOUR A1 - Borghini, Alessia A1 - Ferrero, Silvio A1 - Wunder, Bernd A1 - Laurent, Oscar A1 - Ziemann, Martin Andreas T1 - Granitoid melt inclusions in orogenic peridotite and the origin of garnet clinopyroxenite JF - Geology N2 - Granitic melt inclusions were found in layers of garnet clinopyroxenites from orogenic peridotites hosted in high-pressure felsic granulites of the Granulitgebirge, central Europe. The inclusions are both glassy and crystallized, and occur as clusters in the garnet. Microstructural features suggest that the inclusions formed while garnet was growing as a peritectic phase, likely alongside clinopyroxene. The chemistry of the melt, in particular its trace element signature, shows a crustal contribution, probably due to the involvement of phengite in the melt-producing reaction, most likely in the presence of a fluid. The presence of a granitoid melt in mantle rocks may be the result of localized melting of a phengite-bearing protolith either already present in the peridotites or, more likely, within the local deeply subducted crustal units. In the latter case, the melt would have infiltrated the peridotites and generated pyroxenite via metasomatism. In either case, the presence of granitoid inclusions in orogenic peridotite provides direct evidence for a genetic connection between a high-pressure crustal melt and garnet pyroxenites. The in situ characterization of these remnants of natural melt provides direct quantitative constraints on (one of) the agents responsible for the interaction between crust and mantle. Y1 - 2018 U6 - https://doi.org/10.1130/G45316.1 SN - 0091-7613 SN - 1943-2682 VL - 46 IS - 11 SP - 1007 EP - 1010 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Ferrero, Silvio A1 - Borghini, Alessia A1 - Wunder, Bernd A1 - Walle, Markus A1 - Günter, Christina A1 - Ziemann, Martin Andreas T1 - A treasure chest full of nanogranitoids BT - an archive to investigate crustal melting in the Bohemian Massif JF - Metamorphic Geology: Microscale to Mountain Belts N2 - The central European Bohemian Massif has undergone over two centuries of scientific investigation which has made it a pivotal area for the development and testing of modern geological theories. The discovery of melt inclusions in high-grade rocks, either crystallized as nanogranitoids or as glassy inclusions, prompted the re-evaluation of the area with an ‘inclusionist’ eye. Melt inclusions have been identified in a wide range of rocks, including felsic/perpotassic granulites, migmatites, eclogites and garnet clinopyroxenites, all the result of melting events albeit over a wide range of pressure/temperature conditions (800–1000°C/0.5–5 GPa). This contribution provides an overview of such inclusions and discusses the qualitative and quantitative constraints they provide for melting processes, and the nature of melts and fluids involved in these processes. In particular, data on trace-element signatures of melt inclusions trapped at mantle depths are presented and discussed. Moreover, experimental re-homogenization of nanogranitoids provided microstructural criteria allowing assessment of the conditions at which melt and host are mutually stable during melting. Overall this work aims to provide guidelines and suggestions for petrologists wishing to explore the fascinating field of melt inclusions in metamorphic terranes worldwide, based on the newest discoveries from the still-enigmatic Bohemian Massif. Y1 - 2018 SN - 978-1-78620-400-4 U6 - https://doi.org/10.1144/SP478.19 SN - 0305-8719 VL - 478 SP - 13 EP - 38 PB - Geological Soc Publishing House CY - Bath ER -