Saxonian Erzgebirge Research Articles

Life-cycle analysis of coesite-bearing garnet

AbstractDetrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultra-high-pressure conditions, exhumation to Earth’s surface, erosion and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very-fine-sand fraction. To understand this imbalance, we analyse the role of source-rock lithology, inclusion size, inclusion frequency and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that: small coesite inclusions of dimension < 9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz; and garnet fracturing, fluid infiltration and the coesite-to-quartz transformation is a late process during exhumation taking place at c. 330°C. A model for the disintegration of coesite-bearing garnet enables the heterogeneous grain-size distribution to be explained by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration.

Comment to “Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation” by

• Coesite-bearing garnet found in tributaries have led to overestimation of UHP rocks. • Geodynamic models exist alternative to exhumation of a whole UHP unit. • UHP terrane in the Erzgebirge in Saxony consists of UHP rocks embedded in HP rocks.

Reply to comment on “Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation”

Reply to comment on “Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation”

Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation

Contrasting metamorphic conditions determined by chemical geothermobarometric investigations of ultrahigh-pressure (UHP) lenses surrounded by high-pressure (HP) and medium-pressure (MP) felsic country rocks are an enigmatic feature of UHP terranes. One of the major questions arising is whether the UHP lenses and the country rocks are a product of different peak metamorphic conditions corresponding to different maximum depth or whether country rocks also experienced UHP conditions but equilibrated and/or re-equilibrated at a different metamorphic stage. Here we address this question to the central Saxonian Erzgebirge in the northwestern Bohemian Massif, Germany. In order to screen the variety of garnet from lithologies occurring in the study area, we analyzed the detrital garnet record from seven modern stream sands. In addition to 700 inclusion-bearing garnet grains previously studied from the 125–250 μm grain-size fraction, we analyzed the 63–125 and 250–500 μm fractions and extended the dataset to overall 2100 inclusion-bearing grains. The new findings of coesite and diamond inclusions in several garnet grains, which are in compositional contrast to garnet of the known UHP lenses but match with those of the felsic country rocks, show that considerable parts of the country rocks underwent UHP metamorphism. Melt inclusions containing cristobalite, kokchetavite, and kumdykolite in garnet derived from the country rocks point to partial melting and re-equilibration during exhumation at HP/HT conditions. Although an amalgamation of rocks which reached different maximum depth may be responsible for some of the contrasting peak metamorphic conditions, the mineralogical evidence for UHP conditions in the felsic country rocks surrounding the UHP lenses proves a largely coherent slab subducted to UHP conditions. Furthermore, the presence of coesite in the subducting voluminous felsic crust and its transformation to quartz during exhumation have great implications for buoyancy development during the metamorphic cycle, which may explain the high exhumation rates of UHP terranes.

Diamond and coesite inclusions in detrital garnet of the Saxonian Erzgebirge, Germany

Abstract Local occurrences of coesite- and diamond-bearing rocks in the central Erzgebirge (northwestern Bohemian Massif, Germany) reveal ultrahigh-pressure (UHP) metamorphic conditions during the Variscan orogeny. Although UHP metamorphism supposedly affected a wider area, implying that rocks that equilibrated under UHP conditions occur dispersed in large volumes of high-pressure country-rock gneisses, mineralogical evidence is scarce. Here we have applied the new concept of capturing the distribution and characteristics of UHP rocks by analyzing inclusions in detrital garnet. Out of 700 inclusion-bearing garnets from seven modern sand samples from creeks draining the UHP area around the Saidenbach reservoir, we detected 26 garnets containing 46 mainly monomineralic coesite inclusions and 22 garnets containing 41 diamond inclusions. Combining these results with geochemical classification of the host garnets, we show (1) that coesite-bearing rocks are common and comprise eclogites as well as felsic gneisses, (2) that small inclusion size is a necessary precondition for the preservation of monomineralic coesite, and (3) for the first time, that diamond-bearing crustal rocks can be detected by analyzing the detrital record. Our results highlight the potential of this novel application of sedimentary provenance tools to UHP research, and the necessity of looking at the micrometer scale to find evidence in the form of preserved UHP minerals.

Interface coupled dissolution-reprecipitation in garnet from subducted granulites and ultrahigh-pressure rocks revealed by phosphorous, sodium, and titanium zonation

Garnet zonation provides an unparalleled record of the pressure-temperature-time-fluid evolution of metamorphic rocks. At extreme temperature conditions >900 °C, however, most elements preserve little zonation due to intracrystalline diffusional relaxation. Under these conditions, slowly diffusing trace elements including P, Na, and Ti have the best chance of recording metamorphic histories. Here we map dramatic zoning patterns of these elements in subducted high-pressure felsic granulite (Saxon Granulite Massif) and ultrahigh-pressure diamondiferous “saidenbachite” (Saxonian Erzgebirge, Bohemian Massif). The results show that garnet replacement via interface coupled dissolution-reprecipitation can strongly affect garnet compositions in subduction zones and that P, Na, and Ti record burial and exhumation histories that are otherwise lost to diffusion. In these samples, P diffuses the slowest, and Ti the fastest.

Exsolution of rutile or apatite precipitates surrounding ruptured inclusions in garnet from UHT and UHP rocks

AbstractThis study describes halos of rutile ± apatite needles and/or plates centred on quartz (inferred former coesite) inclusions or multiphase inclusions in garnet. Both types of central inclusions are surrounded by cracks. The multiphase inclusions contain mica or carbonate minerals and are interpreted to represent decrepitated fluid inclusions. Examples from two localities are examined: (i) ultrahigh‐temperature (UHT) metapelitic gneisses from the southern end of the Central Maine Terrane in northeastern Connecticut, USA (rutile only) and (ii) ultrahigh‐pressure (UHP) diamondiferous saidenbachite from the Saxonian Erzgebirge (rutile and apatite). The needles of apatite or rutile are typically oriented in three directions within garnet. Chemical zonation in garnet shows clear Ti or P depletion halos corresponding spatially with the rutile or apatite inclusion halos. The radii of the inclusion halos, when measured from the centre of the central inclusions, are about two to several times the radii of the central inclusions. We propose that the inclusion halos of rutile ± apatite formed by exsolution out of Ti‐bearing and/or P‐bearing garnet during retrogression. Because of its strength, garnet can internally maintain higher pressures than the surrounding matrix. Nonetheless, if the inclusion pressure is significantly greater than the confining lithostatic pressure, then deformation of the host garnet can occur. During exhumation, high internal pressures relative to the matrix can result from retention of fluid entrapment pressure or a phase change with a large positive volume increase (e.g. coesite → quartz). The differential stress imposed on the garnet adjacent to the central inclusion preceding and during mechanical failure would create dislocations and other crystalline defects which are ideal sites for exsolved precipitates to nucleate and grow. The strain‐induced exsolution hypothesis is consistent with observations. (i) The radii of the halos are roughly consistent with the pressure vessel model or the multi‐anvil model for an inclusion, which predict that differential stress will drop off sharply away from the boundary of the central inclusion into the host garnet. (ii) The rutile or apatite inclusions formed in Ti‐ or P‐bearing garnet adjacent to ruptured central inclusions; rutile or apatite are rare or absent elsewhere in garnet, even in areas of high‐Ti or high‐P concentration. In addition, rutile is absent in areas surrounding the central inclusion that were depleted in Ti prior to rupturing (e.g. garnet rims that lost Ti during retrogression). Thus, both elevated Ti ± P concentrations and proximity to inclusions which deformed the host garnet were necessary for halo formation. (iii) Reintegrated garnet Ti or P contents in and adjacent to the halos obtained using wide‐beam electron probe microanalysis are consistent with local derivation of the Ti or P necessary to form rutile and apatite from garnet. Elevated Ti ± P concentrations in aluminous garnet are mostly found in UHT, HP/UHP, granulite and mantle environments. Consequently, the phenomena described will most likely occur in such settings.

Microstructure, composition and P–T conditions of rutile from diamondiferous gneiss of the Saxonian Erzgebirge, Germany

The chemical composition and microstructure of rutile grains in a ultra-high pressure metamorphic gneiss of the Saxonian Erzgebirge, Germany have been studied by Raman spectroscopy, SEM, EMPA and TEM. Rutile inclusions in garnet contain free dislocations, iron-enriched dislocations and exsolved ilmenite lamellae, while subgrain boundaries are observed in rutile grains of the rock matrix. The previously reported α-PbO2 type TiO2 phase could not be confirmed by our TEM observations. On the basis of Zr solubility in the rutile and the presence of microdiamonds, minimum metamorphic peak conditions of 3.95GPa and 915°C are estimated.

Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions

Tourmaline grains extrcted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora Maira, Western Alps (peak P - T conditions ~3.7 GPa, 750 °C), has a structural formula of X(Na0.90Ca0.05K0.01□0.04)Y(Mg1.78Al0.99Fe2+0.12Ti4+0.03□0.08)Z(Al5.10Mg0.90)(BO3)3TSi6.00O18V(OH)3W[(OH)0.72F0.28]. Dravite from Lago di Cignana, Western Alps, Italy (~2.7–2.9 GPa, 600–630 °C), has a formula of X(Na0.84Ca0.09K0.01□0.06)Y(Mg1.64Al0.79Fe2+0.48Mn2+0.06Ti4+0.02Ni0.02Zn0.01)Z(Al5.00Mg1.00)(BO3)3T(Si5.98Al0.02)O18V(OH)3W[(OH)0.65F0.35]. “Oxy-schorl” from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at >2.9 GPa, 870 °C, has a formula of X(Na0.86Ca0.02K0.02□0.10)Y(Al1.63Fe2+1.23Ti4+0.11Mg0.03Zn0.01) Z(Al5.05Mg0.95)(BO3)3T(Si5.96Al0.04)O18V(OH)3W[O0.81F0.10(OH)0.09]. There is no structural evidence for significant substitution of [4]Si by [4]Al or [4]B in the UHP tourmaline ( distances ~1.620 A), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high- T –low- P tourmaline, which typically has significant amounts of [4]Al. There is an excellent positive correlation ( r 2 = 1.00) between total [6]Al (i.e., YAl + ZAl) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation ( r 2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and YAl content ( r 2 = 1.00) that is best explained by the exchange vector YAlO(R2+F)−1. This is consistent with the W site (occupied either by F, O, or OH), being part of the YO6-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.

Decrepitated UHP fluid inclusions: about diverse phase assemblages and extreme decompression rates (Erzgebirge, Germany)

AbstractMinute polyphase inclusions in garnet of quartzofeldspathic rocks (saidenbachite) from the Saxonian Erzgebirge, Germany, contain microdiamond or graphite, phlogopite, quartz, paragonite, phengite and other minerals in minor amounts. These inclusions are interpreted to represent an original dense COH + silicate fluid, trapped in crystallizing garnet at depths of >150 km. Inspection of the inclusion population in a single garnet by SEM reveals two characteristic features: (i) The shape of most inclusions indicates healed radial cracks in the host garnet, and, thus, the buildup of a significant differential pressure ΔP, i.e. a contrast in pressure between the inclusion (Pi) and the host mineral (Pe). The mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of ∼750 ± 50 °C and pressures below 2.5 GPa. (ii) The diverse types of inclusions appear to be randomly distributed in the garnet host. Thus, the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet. Combining observations (i) and (ii), we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times, and thus, of distinct histories of Pi, as ΔP at decrepitation is primarily controlled by inclusion size and shape. Applying a flow law for dislocation creep of garnet, a low strength of garnet at 750 ± 50 °C for low geological strain rates is predicted. Thus, differential pressure should have been kept low (i.e. Pi≈ Pe) by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks. To attain the differential pressure (Pi >> Pe) required for catastrophic brittle failure of the garnet host, the decompression rate must have been extremely high. As a robust lower bound, a minimum exhumation rate on the order of 100 m year−1 is suggested, which corresponds to ascent rates of magma.

The nitrogen and methane gas occurrences of the Saxonian Erzgebirge and its adjacent areas geochemistry and origin

The nitrogen and methane gas occurrences of the Saxonian Erzgebirge and its adjacent areas geochemistry and origin

Unusual inclusions in garnet from the diamond-bearing gneiss of the Erzgebirge, Germany

)ratios. It is known that such geotherms may occur onlyin subduction and collision zones [2], and the collisiongeotectonic setting is favorable for the formation of dia-mond-bearing metamorphic complexes [3, 4].According to formal criteria (mineralogy, structure,and texture), the diamond-bearing rocks of metamor-phic complexes are classified as metamorphic. How-ever, some petrological, geochemical, and experimen-tal data of recent years indicated their possible partial orcomplete melting [5–12]. It should be noted that thereare still no reliable criteria for the identification of suchmelts. The reason is that the exhumation rate of ultra-high pressure (UHP) complexes is lower than the rate ofphase transformations, and metamorphic reactionsobliterate, therefore, evidence for melting. On the otherhand, the knowledge of the physical state of rocks isvery important, because it strongly affects the mecha-nisms of very important processes, including diamondformation and rock exhumation. Petrological and min-eralogical studies played a key role in the discovery ofdiamondiferous metamorphic complexes and can beused to solve these important problems.This paper reports a discovery of unusual inclusionsin garnet from the diamond-bearing garnet–mica gneiss ofthe Saxonian Erzgebirge. The morphology and phasecomposition of these inclusions could hardly be explainedwithin the model of solid-phase evolution only.GEOLOGIC SETTING AND PETROLOGY (LITERATURE DATA)The Erzgebirge composes a large anticlinorium inthe Variscan basement of the Bohemian Massif in Cen-tral Europe [12, 13]. The core of the complex is com-posed of migmatite-bearing gneisses and schists host-ing eclogite and peridotite lenses. Diamond-bearinggarnet–mica gneisses occur as lenses (up to severalhundred meters long) in the same sequence [14]. Theappearance of these rocks is indistinguishable from thatof rocks of the same name widespread in moderatepressure complexes. Diamonds in the gneisses of theErzgebirge were discovered by chance owing to prob-lems with thin section polishing [15]. Subsequently, inaddition to diamond, another lesser known indicator ofhigh pressures was found in these rocks, a polymor-phous modification of rutile—titanium dioxide with thestructural type of αPbO

δ13C signature of early graphite and subsequently formed microdiamond from the Saxonian Erzgebirge, Germany

AbstractMicrodiamond and graphite occur in quartzofeldspathic rocks (saidenbachites) of the central Erzgebirge in Saxony. Zircon from such rocks contains not only relatively high but also variable amounts of early formed graphite and later crystallized diamond, however no carbonate. Six purified zircon concentrates were analysed by mass spectrometry for δ13C(PDB) values which were between −24 and −33‰. The carbon inclusions resulted in C contents of the concentrates as high as 0.5 wt%. It is hypothesized that the observed δ13C signature results from organic material in the sedimentary protoliths of the saidenbachites probably deposited not earlier than Mid‐Devonian. This material was transformed to graphite during early metamorphism and subsequently to diamond after an anatectic event at great depths (>150 km). During these processes, the δ13C signature of the carbon did not (significantly) change.

Dating of zircon and monazite from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge – hints at burial and exhumation velocities

AbstractIn order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe analysing Pb, U and Th isotopes. The results were: (1) core zone – 21 analyses: Th/U ≤ 40.023 and 337.0±2.7 Ma (2σ, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone – 23 analyses: Th/U ≥ 50.037 and 336.8±2.8 Ma; and (3) rim zone – 12 analyses: Th/U = 0.015–0.038 (plus one analysis of 0.164) and 330.2±5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U ≥ 50.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4±2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7±8.0 (2σ) Ma was acquired from 113 analyses.By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continentalcrust (~1.8 GPa, 650ºC) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050ºC) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less.

On the origin of oriented rutile needles in garnet from UHP eclogites

AbstractAlthough oriented rutile needles in garnet have been reported from several ultrahigh‐pressure (UHP) rocks and considered to be important UHP indicators, their crystallographic features including growth habit and lattice correspondences with garnet host have never been properly characterized. This paper presents a detailed analytical electron microscopic (AEM) study on evenly distributed oriented rutile needles in garnet of two eclogitic rocks from Sulu. Some garnet in one UHP diamondiferous quartzofeldspathic rock from the Saxonian Erzgebirge, and in one high‐pressure (HP) felsic granulite from Bohemia also contain a few unevenly distributed oriented rutile needles. They have also been studied for the purpose of comparison. Despite different distribution patterns, AEM revealed that all rutile needles are oriented along the 〈111〉 directions of garnet with their lateral sides surrounded by the {110} planes of garnet, and that the growth directions of most needles are close to the normal of the {101} planes of rutile. No other specific crystallographic orientation relationships between rutile and garnet host were observed, and there is no pyroxene associated with rutile, as necessitated by the precipitation reaction of rutile in garnet as previously proposed. A simple solid‐state precipitation scenario for the formation of the rutile needles in garnet in these two eclogitic rocks is not justified. Three alternative mechanisms are considered for the formation of oriented rutile needles: (i) the rutile needles may be inherited from precursor minerals; (ii) the rutile needles may be formed by a dissolution–reprecipitation mechanism; and (iii) the rutile needles may be formed by cleaving and healing of garnet with rutile deposition. None of these mechanisms can fully explain the observations, although the first one is less likely and the third one is preferred. This study presents an example where the presence of oriented/aligned inclusions in minerals does not necessarily imply a precipitation origin.

Geochemical signatures of Variscan eclogites from the Saxonian Erzgebirge, central Europe

From the abundant metre to km-sized eclogite bodies in the Variscan crystalline complex of the Saxonian Erzgebirge we have investigated 19 samples from the ultrahigh pressure area at the Saidenbach reservoir. Twenty-two samples were from the south-western Erzgebirge, and from occurrences located only some km away from the reservoir. These samples were analysed for major and trace elements using X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). The non-Saidenbach eclogites (SiO 2=49–53 wt%) can be derived from N-mid-ocean ridge basalts (MORBs) partially transitional to P-MORBs (e.g., (Nb) N: 3–36; (Sr) N: 4–17; (La/Sm) N<1.5 (in most instances <0.7) and (Sm/Yb) N around 1.2). Eclogites from the Saidenbach reservoir (SiO 2=49–61 wt%) are characterised by (Nb) N: 20–170; (Sr) N: 9–43; (La/Sm) N: 1.2–3.0; (Sm/Yb) N: 1.4–8.8, and a clear negative Eu anomaly for the Si-rich samples, thus, being significantly different from the other investigated eclogites. These signatures point to protoliths related to within plate igneous rocks. However, we also discuss the possibilities of (1) protoliths related to a magmatic arc along an active continental margin and (2) the formation by melting of crustal material in the deep mantle and final crystallisation in the lowermost continental crust similar to the adjacent diamondiferous quartzofeldspathic rocks. Due to the specific geochemical signatures of eclogites in the Saidenbach area including other facts, this ultrahigh pressure region is believed to represent a section of lowermost crust not outcropping in other portions of the Saxonian Erzgebirge.

Fluid‐mediated modification of garnet interiors under ultrahigh‐pressure conditions

AbstractWe report the results of a novel experimental study on eclogitic garnets with abundant inclusions of clinozoisite, quartz and rutile subjected to temperatures (T) of 800–1100 °C and a pressure (P) of 4 GPa, representative of ultrahigh‐pressure (UHP) metamorphic terranes such as the Kokchetav massif, Saxonian Erzgebirge, etc. The experiments reveal extremely rapid recrystallization and partial melting of garnet interiors controlled by fluids liberated from the breakdown of the hydrous mineral inclusions. The traditional assumption that inclusions of minerals and primary fluid inclusions should be representative of the peak or even earlier metamorphic history cannot be strictly applied in this case. We argue that inclusions in UHP garnets may mirror P–T conditions postdating growth of the host crystal or even P–T conditions never actually experienced by the rock itself. The above modification of garnet interiors produces a typical patchy microstructure that occurs in natural eclogitic garnet from the diamond‐bearing UHP Kokchetav massif.

Ilmenite exsolution in olivine from the serpentinite body at Zöblitz, Saxonian Erzgebirge – microstructural evidence using EBSD

AbstractIn the large serpentinite body at Zöblitz, which is part of the Gneiss-Eclogite Unit of the Saxonian Erzgebirge, ilmenite rods have been detected in olivine. Although these rods are ≤1 μm wide, they can be unequivocally identified as ilmenite using electron backscatter diffraction. We also applied this method to prove that ilmenite is topotactically intergrown with the olivine host ([100]ol ‖ [001]ilm). This relation was found previously, e.g. for ilmenite exsolved from olivine of garnet peridotite from Alpe Arami, Swiss Alps. The degree of this exsolution phenomenon in the Alpe Arami rocks was taken as an indication of the original formation of a Ti-bearing mineral at depths of ∽300 km. As in the olivines of the serpentinite from Zöblitz, the density of the ilmenite rods is significantly less than in the rock from Alpe Arami. We think that our observation is compatible with previous P-T estimates of close to 4 GPa and 1000–1100°C for the ultramafic bodies in the central Saxonian Erzgebirge.

Dumortierite from the Gfohl unit, Lower Austria: chemistry, structure, and infra-red spectroscopy

For dumortierite from Gfohl, Lower Austria, crystal structure refinement in combination with chemical analyses gives the optimized formula (Al 0.78 □ 0.12 Mg 0.09 Ti 0.01 ) (Al 5.70 □ 0.20 Ti 0.06 Fe 0.04 ) (Si 2.85 Al 0.15 ) B O 16 [O 1.17 (OH) 0.81 F 0.02 ], with a = 4.6900(3), b = 11.7875(6), c = 20.1823(11) A. For dumortierite from Weisenkirchen in der Wachau, Lower Austria, crystal structure refinement in combination with the chemical analyses gives the optimized formula (Al 0.74 □ 0.13 Mg 0.12 Ti 0.01 ) (Al 5.70 □ 0.20 Ti 0.08 Fe 0.02 ) (Si 2.83 Al 0.17 ) B O 16 [O 1.14 (OH) 0.85 F 0.01 ], with a = 4.6948(2), b = 11.8037(5), c = 20.2106(8) A. Complete chemical characterizations, including light elements (B, H, Li, Be), were combined for the first time with high-quality structure refinements with R -values in the range 0.018–0.019, allowing assignment of site-occupancies with confidence. The optimizations demonstrate that all Mg occupies the M 1 sites. Mg substitution at M 1 plays a prevailing role in the high number of absorption bands. Contrary to M 2, M 3 and M 4, the M 1 site has substantial vacancies. Whereas the average M-O distances of the M 2 and M 3 sites are similar in each sample, the M-O distances of the M 4 sites in both samples are distinctly lower than M 2 and M 3. This is a clear indication that, unlike the M 2 and M 3 sites, the M 4 sites contain almost no vacancies and no significant substitutions of Fe or Ti. Significant amounts of tetrahedrally-coordinated A1 were only found at the T 1 sites (0.15 apfu ) but not at the T 2 sites. The MgO content, ∼0.7–0.8 wt%, is relatively high for “normal” dumortierite, but similar to dumortierites from the Saxonian Erzgebirge, Germany. Both dumortierite samples from the Gfohl unit contain relatively high OH contents (0.81–0.85 pfu ), and measureable amounts of both F and BeO, with 0.05–0.06 wt% and 0.01–0.02 wt%, respectively. The compositions of these dumortierite crystals from different localities are very similar and may therefore reflect similar PT conditions and fluids during crystallization in these pegmatites of the same geological unit (Gfohl unit). FTIR spectroscopy reveals spectra in the 4000–3000 cm −1 domain that are markedly different from those of dumortierite samples of different origin. The lack of a band at 3675 cm −1 in the sample from Gfohl and the importance of the 3696 cm −1 band in both investigated dumortierite samples suggest a possible ordering of Mg and □ at the M 1 site, in accord with the conclusions from the structure study.

A Low-Variance Mineral Assemblage with Talc and Phengite in an Eclogite from the Saxonian Erzgebirge, Central Europe, and its P-T Evolution

A light-coloured, fine-grained eclogite sample from near the village of Hammerunterwiesenthal in the Erzgebirge (NW Bohemian Massif ) preserves the low-variance mineral assemblage of garnet, omphacite, phengite, talc, amphibole, clinozoisite, quartz, rutile, and accessory phases. Porphyroblasts of amphibole, clinozoisite, and phengite formed during a late stage (III) of metamorphism. Paragonite joined the assemblage late in this stage (IIIb). The chemical zonation of the minerals was carefully studied. Various geothermobarometric methods were applied, especially involving phengite and talc. The constrained P–T path for the eclogite starts at about 480 C and 25 kbar (stage Ib), followed by a significant temperature rise (stage II) at slightly increasing pressure. At the peak P–T conditions of 720 C and 27 kbar, blastesis of amphibole, clinozoisite, and phengite was caused by infiltrating hydrous fluids. The resulting density reduction may have allowed buoyant uplift of the eclogite. Subsequently, significant cooling occurred at high pressures. Stage IIIb is characterized by P–T conditions around 520 C and 18 kbar at reduced water activities. This unusual late P–T evolution might explain the freshness of the eclogite, including the preservation of chemical zonation on the micrometre scale.