Helene Brätz

A combined zircon SHRIMP and Sm-Nd isotope study of high-grade paragneisses from the Mid-German Crystalline Rise: evidence for northern Gondwanan and Grenvillian provenance

SHRIMP analyses of detrital zircon cores from high-grade metasediments from the Mid-German Crystalline Rise, which is situated between Eastern Avalonia in the NW and Saxo-Thuringia in the SE, yield mostly ages of c. 550 Ma and c. 2.06 Ga, with minor c. 1.0 and 2.4–2.9 Ga components. The sedimentary protoliths were deposited during the Late Proterozoic to Early Cambrian, probably prior to the break up of the northern Gondwana margin at 460–500 Ma. These data are consistent with the sediments’ high ϵNd values (0.9 to −3.0), which are comparable to those of well-documented Late Proterozoic sediments from other parts of Europe. The combined isotopic data suggest derivation of the sediments from at least three distinct crustal source regions. Dominant sources were the Avalonian–Cadomian orogenic belt (c. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West African and/or eastern Amazonian cratons (c. 45%). The Grenvillian belt was a minor source (c. 10%).

Olivine from spinel peridotite xenoliths: Hydroxyl incorporation and mineral composition

Abstract Traces of water in mantle minerals strongly influence mantle melting and viscosity that, in turn, governs large-scale processes like mantle convection, plate tectonics, and the stabilization of cratons. One way of estimating the mantle’s water content is by analyzing mantle xenoliths brought to the Earth’s surface. A major problem in estimating the mantle’s water budget from xenoliths arises from decompression-induced water loss during uplift. Mantle-derived xenoliths from numerous occurrences worldwide have been investigated with respect to water. However, little is known about water in the mantle beneath most parts of Europe and Asia. This study presents water contents for mantle olivine from Germany, Austria, Mongolia, and Nigeria and suggests a possibility to assess water loss. It also addresses the question whether or not water contents are related to olivine composition and/or the presence of coexisting amphibole. The highest water concentrations are present in olivine from the Eifel, Germany (up to 21 ppm H2O), whereas the Fichtelgebirge xenoliths, Germany, reveal the lowest contents (<1 ppm). All water-bearing olivines show three dominant infrared absorption bands: bands at 3572, 3525, and 3560 cm-1 and weaker bands at 3485, 3542, and 3597 cm-1. The peaks at 3572 and 3525 cm-1 are ascribed to Ti-related substitutions of H. Additional peaks, related to H substitutions involving trivalent cations, occur in the 3300-3400 cm-1 range. However, their intensity does not correlate with the content of trivalent cations. The olivines show a pronounced correlation of Al + Cr and Ca implying that incorporation of Al and Cr is governed by pressure and temperature and primarily attributed to Tschermak’s substitution. This study confirms that a coupled substitution involving Ti is the most important mode of water storage in shallow upper mantle olivine. The Ti content and the fraction of water bound to “Ti defects” are related and support the substitution model Ti4+ + 2H+ = Mg2+ + Si4+. Hence, the Ti content is a useful proxy to estimate the maximum amount of water incorporated by this substitution-providing a tool to approximate the degree of water loss. Five of the investigated olivines provide evidence for no or very little water loss. Their water content of 16-21 ppm is presumably typical for the depleted uppermost mantle. Twelve samples with <1-15 ppm may have lost between 36 and >80% of their original water. Olivine from amphibole-bearing spinel peridotite has relatively low water contents as well as low amounts of Ca, Al, Cr, Ti, and V. Particularly low Ca, Al, and Cr contents suggest fluid infiltration, amphibole formation, and re-equilibration of the whole assemblage at comparably low pressure and/or temperature and may explain the low water content of olivine. Infrared spectra with dominant peaks in the 3200-3300 cm-1 range (spectrum type E) are confined to olivine from amphibole-bearing peridotite.

Rare earth element geochemistry of columbite-group minerals: LA-ICP-MS data

Abstract New data on rare earth element (REE) concentrations and distribution patterns of columbite-tantalite minerals from Ta-ore provinces worldwide are presented. The REE geochemistry was studied by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Five major types of chondrite-normalized REE distribution patterns are defined for columbite-group minerals (CGM) from lithium-caesium- tantalum (LCT) pegmatites and rare-metal granites. Features to discriminate between the types include (1) the shape of the pattern (e.g. flat or concave), (2) calculated ratios between groups of the REE (e.g. heavy REEN/middle REEN), and (3) the presence and intensity of anomalies (e.g. Ce*, Eu*). Four pegmatites in central and southern Africa are used as case studies to discuss application of the types of REE patterns in individual deposits. The REE fractionation during progressive evolution of the melt in apegma tite body (either Nb → Ta or Fe → Mn fractionation lines, or both) results in smaller heavy REEN/middle REEN ratios whereas replacement of primary CGM by secondary CGM produces modifications in the light REEN patterns and the heavy REEN/middle REEN ratios also. Critical features of REE patterns such as highly variable heavy REEN/middle REEN ratios or striking differences in the appearance of Eu anomalies are discussed considering structural data of the host minerals and the differentiation behaviour of the pegmatitic melt. In general, CGM from each individual Ta-ore province are characterized by a predominance of one type of REE distribution pattern. Consequently, these patterns are suitable for tracing the origin of tantalum ore concentrates (e.g. as a forensic tool).

Timing of Upper Carboniferous-Permian horst-basin formation and magmatism in the NW Thuringian Forest, central Germany: a review

Abstract During Late Carboniferous-Early Permian times dextral transtensional movements along the NW-trending Franconian Fault System and parallel faults caused complex block faulting in the Thuringian Forest region, Germany, accompanied by intense magmatism. This is well constrained by geochronological data (207Pb/206Pb single zircon, SHRIMP, 40Ar/39Ar mica, zircon fission-track ages), field relations, and the sedimentary record from the Ruhla Crystalline Complex (RCC) and surroundings. The combined dataset indicates that the Ruhla Crystalline Complex was faulted into three nearly N-S-trending segments, which underwent different exhumation histories during Late Carboniferous-Permian times. The central segment of the RCC was exhumed by several kilometres as a horst block, while the eastern and western segments subsided simultaneously, forming the basement to the Oberhof and Eisenach molasse basins, respectively. Late Carboniferous-Permian uplift of the central segment is constrained by 40Ar/39Ar cooling ages of 311 ± 3 (muscovite) and 294–288 ± 3 Ma (biotite), a weighted zircon fission-track age of 272 ± 7 Ma and overlying Zechstein sediments. In contrast, the eastern segment shows much older 40Ar/39Albiotite cooling ages between 336 ± 4 and 323 ± 3 Ma, was exposed at c. 300 Ma, and subsequently covered by molasse sediments and volcanic rocks between 300 and c. 275 Ma. A similar Late Carboniferous evolution is inferred for the western segment, as it is also overlain by Lower Permian volcanic rocks and has a 297 ± 29 Ma single zircon fission-track age. Simultaneous horst and basin formation is additionally constrained by granite pebbles in conglomerates of the Oberhof and Eisenach basins. These pebbles can partly be derived from granites in the central segment of the RCC. Age data and the orientation of granitoid bodies and dykes in the Ruhla Crystalline Complex and its surroundings provide evidence for the opening of NE-trending structures between 300 and 294 Ma, and formation or reactivation of W- to NW-trending structures between 290 and 275 Ma. Magmatic activity in the Thuringian Forest region may have been caused by widespread mantle upwelling in central Europe during the Late Carboniferous-Early Permian.

LA-ICP-MS micro-analysis of fluid inclusions in quartz using a commercial Merchantek 266 nm Nd:YAG laser a pilot study

The results of this pilot study show that using a commercial Merchantek 266 nm Nd:YAG laser system, laser ablation -inductively coupled plasma - mass spectrometric analysis of fluid inclusions in quartz is possible. Fifteen elements (Li, Na, K, Mg, Fe, As, Rb, Sr, Sn, Ba, Mo, U, W, Mn, and Pb) were determined for aqueous fluid inclusions from the Zinnwald Sn deposit (Ore Mountains, Germany). The concentrations of the measured major and trace elements are in good agreement with literature data for the same location. A first evaluation of the data shows that the high temperature inclusions (Th: 400 - 370 °C) contain high Fe and Na concentrations as well as Sn contents typical of tin-bearing fluids. A decrease in salinity from the high temperature fluid inclusions to low temperature secondary inclusions probably reflects a dilution trend of the magmatic fluid by external fluids.

Post-peak re-equilibration in a mafic gneiss from the KTB main hole: implications for the metamorphic evolution

Abstract Detailed microprobe investigations were carried out on mineral phases of a mafic garnet–biotite–hornblende and garnet–biotite gneiss from the KTB main hole. The results indicate that the complex zonation patterns of garnet porphyroblasts are mainly due to post-growth diffusional exchange, as constrained by theoretical modelling. Moreover, the chemical composition of biotite and hornblende enclosed in garnet were also changed by later diffusion and retrograde net-transfer reactions. Therefore, the garnet zonation in combination with mineral inclusions cannot be used to reconstruct the prograde metamorphic evolution or to estimate the maximum temperatures attained. However, by applying the GRISP geobarometer, minimum pressures of 8.5 kbar (at 700 °C) can be derived for the metamorphic peak, well conforming to previous estimates on the amphibolite-facies metamorphic stage of the KTB metamorphics. Mineral assemblages formed during the retrograde evolution allow to estimate P–T–X conditions of approximately 3 kbar, 450 °C and X(H2O)=0.9.