ALBERT

Description: Soil and lake sediments are important paleoclimate archives often forming a source-sink setting. To better understand magnetic properties in such settings, we studied red soil on low-magnetic bedrock and subrecent sediments of Caohai Lake (CL) in Heqing Basin, China. Red soil is the only important source material for the CL sediments, it is highly magnetic with susceptibilities (χ) of ~10−5 m3/kg. The red soil is dominated by pedogenic nano-magnetite (~10–15 nm) arranged in aggregates of ~100 nm, with particle interaction that causes a wide effective grain size distribution in the superparamagnetic (SP) range tailing into stable single-domain behavior. Transmission electron microscopy and broadband frequency χ(f) suggest partial disintegration of the aggregates and increased alteration of the nanoparticles to hematite during transfer of red soil material to CL. This shifts the domain state behavior to smaller effective magnetic grain sizes, resulting in lower χfd% and χ values, and a characteristic change of χ(f). The SP-stable single-domain distribution of the aggregates in red soil could be climate dependent, and the ratio of saturation remanence to χ is a potential bedrock-specific paleoclimate proxy reflecting it. Magnetic properties of the CL sediments are controlled by an assemblage of nanoparticle aggregates and larger-sized bedrock-derived magnetite. The results challenge the validity of the previous paleoclimate interpretation from the 168-m-long Core-HQ (900–30 ka) in Heqing Basin. Disintegration of aggregates could lead to SP behavior with low χfd% without extinction of individual magnetite nanoparticles, and the χfd%-based assumption of SP magnetite dissolution may be wrong.

Description: Zur Ermittlung von geochemischen Indizies der Monazite aus Zinnerzen und aus Graniten in Thailand, unter besonderer Berücksichtigung der wirtschaftlich wichtigen impregnativen Zinnvererzungen und Greisen, und zur Klärung der genetischen Beziehungen zwischen Zinnvererzungen und Graniten im Thai-Malaya Orogen, wurden in 192 akzessorisehen und detritischen Monaziten die Verteilungen von geochemischen Indizies La+Ce+Pr (Atom.-%) und Ce/(Nd+Y) (Atomquotient) und von Th und Ca untersucht. Die Anwendung von Monaziten als geochemische Indikatoren der wirtschaftlich wichtigen imprägnativen ZinnVererzungen und Greisen bzw. der als zinnführend bekannten Muskovit- und Turmalin-Graniten ist durch das typomorphe Auftreten von postmagmatischen Monaziten möglich und wurde durch geochemische Prospektionstests an detri tischen Monaziten aus Fluß- und Strandablagerungen in der näheren und weiteren Umgebung der imprägnativen Zinnlagerstätten und der Greisenlagerstätten verifiziert.

Description: Microprobe determinations of geochemical indices La+Ce+Pr (atom.-%) and atomic ratio Ce/(Nd+Y) and of thorium and calcium content have been carried out in 192 samples of accessory and detritaimonazites from Thailand in order to establish geochemical criteria for application of monazites as indicators for tin ore prospec ting and tin-bearing granites. The most important geochemical results are as following: Distribution functions of La+Ce+Pr indicated three main types of monazites: 1) Magmatic monazites characterized by arith. mean of 73,9 atomic percent and by variation range (x + 2s) from 69,5 to 78,3 atomic percent La+Ce+Pr. 2) Postmagmatic monazites characterized by arith. mean of 66,5 atomic percent and by variation range (x + 2s) from 63,0 to 69,9 atomic percent La+Ce+Pr. Both types of monazites occur associated in disseminated cassiterite deposits, in greisen deposits and in tin-bearing muscovite and tourmaline granites. Therefore, the occurence of both types of monazites or only of postmagmatic-monazites in heavy minerals concentrates from stream sediments can be used to allocate disseminated cassiterite and greisen mineralizations or tin-bearing muscovite and tourmaline granites. 3) Magmatic monazites characterized by extremely high values of La+Ce+Pr between 76,5 and 90,5 atomic percent. This type of monazites seems to be typical for Nb-Ta-Sn-pegmatites and accordingly can be used to locate pegmatitic tin and tantalum mineralizations or resulting placer deposits. Thorium and calcium can be used only very restricted as indicators. Because distribution functions of thorium and calcium contents show similarities between monazites from tin deposits and tin-barren granites.

Description: thesis

Description: DFG, SUB Göttingen

Description: Wadsleyite can store significant amounts of H2O in its crystal structure as hydroxyl. However, H2O solubility in Fe‐bearing wadsleyite remains poorly constrained at mantle transition zone temperatures. Previous studies (e.g., Demouchy et al., 2005 [https://doi.org/10.2138/am.2005.1751]; Litasov et al., 2011 [https://doi.org/10.1007/s00269-010-0382-3]) focused primarily on Fe‐free systems, which do not represent the Earth's interior because Fe may affect the H2O solubility. Here, we investigated the temperature dependence of H2O solubility in Fe‐bearing and Fe‐free wadsleyite at 1500–2100 K. The results indicate that H2O solubility in Fe‐bearing wadsleyite is higher than in Fe‐free samples at 1800–1900 K, corresponding to transition zone geotherm, but there is no clear Fe content dependence in the Fe‐bearing samples. Wadsleyite can contain approximately 1.0 wt.% H2O at transition zone temperatures. The H2O solubility in wadsleyite is lower than ringwoodite along a plume geotherm, which may result in dehydration melting at the 520‐km discontinuity by upwelling flow in plumes.

Description: Plain Language Summary: The mantle transition zone at 410–660 km depth is a H2O sponge because wadsleyite and ringwoodite can contain large amounts of H2O in their crystal structures as hydroxyl defects. However, the solubility of H2O in Fe‐bearing wadsleyite is poorly constrained compared with ringwoodite and Fe‐free wadsleyite, which have been extensively investigated. The exact H2O storage capacity of the mantle transition zone, therefore, remains unknown. Here, we investigated the solubility of H2O in Fe‐bearing wadsleyite as a function of temperature. The results indicate that wadsleyite can store ∼1.0 wt.% H2O at transition zone temperatures and ∼0.65 wt.% along a plume geotherm. H2O solubility in wadsleyite is lower than that in ringwoodite in mantle plumes. A dehydration melting layer at the 520‐km discontinuity near plumes can, therefore, form via the phase transition from ringwoodite to wadsleyite under H2O‐saturated conditions driven by upwelling flow in mantle plumes.

Description: Key Points: H2O solubility in Fe‐bearing wadsleyite decreases with increasing temperature. Fe‐bearing wadsleyite can contain ∼1.0 wt.% H2O at mantle transition zone temperature. Dehydration melting could occur at the 520‐km discontinuity by upwelling flow near plumes.

Description: Deutsche Forschungsgemeinschaft

Description: Oxygen vacancies (OVs), that charge-balance the replacement of octahedrally coordinated Si4+ by Al3+ in the mineral bridgmanite, will influence transport properties in the lower mantle but little is known about their stability and local structure. Using 27Al nuclear magnetic resonance (NMR) spectroscopy we have characterized OVs within six aluminous bridgmanite samples. In the resulting NMR spectra sixfold, fivefold, and fourfold coordinated Al species are resolved, in addition to near eightfold coordinated Al substituting for Mg. Fivefold coordinated Al is formed by single OV sites but fourfold coordination must result from short range ordering of OVs, producing OV clusters that may form through migration into twin domain walls. Characterizing the occurrence of such OV structures is an important prerequisite for understanding how transport properties change with depth and composition in the lower mantle.

Description: Projectile–target interactions as a result of a large bolide impact are important issues, as abundant extraterrestrial material has been delivered to the Earth throughout its history. Here, we report results of shock-recovery experiments with a magnetite-quartz target rock positioned in an ARMCO iron container. Petrography, synchrotron-assisted X-ray powder diffraction, and micro-chemical analysis confirm the appearance of wüstite, fayalite, and iron in targets subjected to 30 GPa. The newly formed mineral phases occur along shock veins and melt pockets within the magnetite-quartz aggregates, as well as along intergranular fractures. We suggest that iron melt formed locally at the contact between ARMCO container and target, and intruded the sample causing melt corrosion at the rims of intensely fractured magnetite and quartz. The strongly reducing iron melt, in the form of μm-sized droplets, caused mainly a diffusion rim of wüstite with minor melt corrosion around magnetite. In contact with quartz, iron reacted to form an iron-enriched silicate melt, from which fayalite crystallized rapidly as dendritic grains. The temperatures required for these transformations are estimated between 1200 and 1600 °C, indicating extreme local temperature spikes during the 30 GPa shock pressure experiments.

Description: Ferric iron can be incorporated into the crystal structure of bridgmanite by either oxygen vacancy substitution (MgFeO2.5 component) or charge-coupled substitution (FeFeO3 component) mechanisms. We investigated the concentrations of MgFeO2.5 and FeFeO3 in bridgmanite in the MgO-SiO2-Fe2O3 system at 27 GPa and 1700–2300 K using a multianvil apparatus. The FeFeO3 content increases from 1.6 to 7.6 mol.% and from 5.7 to 17.9 mol.% with and without coexistence of (Mg,Fe)O, respectively, with increasing temperature from 1700 to 2300 K. In contrast, the MgFeO2.5 content does not show clear temperature dependence, that is, ~2–3 and 〈 2 mol.% with and without the coexistence of (Mg,Fe)O, respectively. Therefore, the presence of (Mg,Fe)O enhances the oxygen vacancy substitution for Fe3+ in bridgmanite. It is predicted that Fe3+ is predominantly substituted following the oxygen vacancy mechanism in (Mg,Fe)O-saturated Al-free bridgmanite when Fe3+ is below ~0.025 pfu, whereas the charge-coupled mechanism occurs when Fe3+ 〉 0.025 pfu.

Description: Garnet is a prototypical mineral in metamorphic rocks because it commonly preserves chemical and textural features that can be used for untangling its metamorphic development. Large garnet porphyroblasts may show extremely complex internal structures as a result of a polycyclic growth history, deformation, and modification of growth structures by intra- and intercrystalline diffusion. The complex internal structure of garnet porphyroblasts from garnet–phengite schists (GPS) of the Zermatt area (Western Alps) has been successfully decoded. The centimetre-sized garnet porphyroblasts are composed of granulite facies garnet fragments overgrown by a younger generation of grossular-rich eclogite facies garnet. The early granulite facies garnet (G-Grt) formed from low-P, high-T metamorphism during a pre-Alpine orogenic event. The late garnet (E-Grt) is typical of high-pressure, low-temperature (HPLT) metamorphism and can be related to Alpine subduction of the schists. Thus, the garnet of the GPS are polycyclic (polymetamorphic). G-Grt formation occurred at ~670 MPa and 780°C, E-Grt formed at ~1.7 GPa and 530°C. The G-Grt is relatively rich in Prp and poor in Grs, while E-Grt is rich in Grs and poor in Prp. The Alm content (mol.%) of G-Grt is 68 of E-Grt 55. After formation of E-Grt between and around fragmented G-Grt at 530°C, the GPS have been further subducted and reached a maximum temperature of 580°C before exhumation started. Garnet composition profiles indicate that the initially very sharp contacts between the granulite facies fragments of G-Grt and fracture seals of HPLT garnet (E-Grt) have been modified by cation diffusion. The profiles suggest that Ca did not exchange at the scale of 1 µm, whereas Fe and Mg did efficiently diffuse at the derived maximum temperature of 580°C for the GPS at the scale of 7–8 µm. The Grt–Grt diffusion profiles resulted from spending c. 10 Ma at 530–580°C along the P–T–t path. The measured Grt composition profiles are consistent with diffusivities of log DMgFe = −25.8 m2/s from modelled diffusion profiles. Mg loss by diffusion from G-Grt is compensated by Fe gain by diffusion from E-Grt to maintain charge balance. This leads to a distinctive Fe concentration profile typical of uphill diffusion.

Description: Little is known about water in nominally anhydrous minerals of orogenic garnet peridotite and enclosed metabasic rocks. This study is focused on peridotite-hosted eclogite and garnetite (metarodingite) from the Erzgebirge (EG), Germany, and the Lepontine Alps (LA), Switzerland. Newly discovered, peridotite-hosted eclogite in the Erzgebirge occurs in the same ultra-high pressure (UHP) unit as gneiss-hosted coesite eclogite, from which it is petrologically indistinguishable. Garnet is present in all mafic and ultramafic high pressure (HP) rocks providing for an ideal proxy to compare the H2O content of the different rock types. Garnet composition is very similar in EG and LA samples and depends on the rock type. Garnet from garnetite, compared to eclogite, contains more CaO (garnetite: 10.5–16.5 wt%; eclogite: 5–11 wt%) and is also characterized by an anomalous REE distribution. In contrast, the infrared (IR) spectra of garnet from both rock types reveal the same OH absorption bands that are also identical to those of previously studied peridotitic garnet from the same locations. Two groups of IR bands, SW I (3,650 ± 10 cm−1) and SW II (3,570–3,630 cm−1) are ascribed to structural hydroxyl (colloquially ‘water’). A third, broad band is present in about half of the analysed garnet domains and related to molecular water (MW) in submicroscopic fluid inclusions. The primary content of structural H2O, preserved in garnet domains without fluid inclusions (and MW bands), varies systematically—depending on both the location and the rock type. Garnet from EG rocks contains more water compared to LA samples, and garnet from garnetite (EG: 121–241 wt.ppm H2O; LA: 23–46 wt.ppm) hosts more water than eclogitic garnet (EG: 84 wt.ppm; LA: 4–11 wt.ppm). Higher contents of structural water (SW) are observed in domains with molecular water, in which the SW II band (being not restricted to HP conditions) is simultaneously enhanced. This implies that fluid influx during decompression not only led to fluid inclusions but also favoured the uptake of secondary SW. The results signify that garnet from all EG and LA samples was originally H2O-undersaturated. Combining the data from eclogite, garnetite and previously studied peridotite, H2O and CaO are positively correlated, pointing to the same degree of H2O-undersaturation at peak metamorphism in all rock types. This ubiquitous water-deficiency cannot be reconciled with the derivation of any of these rocks from the lowermost part of the mantle wedge that was in contact with the subducting plate. This agrees with the previously inferred abyssal origin for part of the rocks from the LA (Cima di Gagnone). A similar origin has to be invoked for the Erzgebirge UHP unit. We suggest that all mafic and ultramafic rocks of this unit not only shared the same metamorphic evolution but also a common protolith origin, most probably on the ocean floor. This inference is supported by the presence of peridotite-hosted garnetite, representing metamorphosed rodingite.

Description: We report the stability and solubility of the FeAlO3 component in bridgmanite based on phase relations in the system MgSiO3-FeAlO3 at 27 GPa and 2000 K using a multi-anvil apparatus combined with in situ synchrotron X-ray diffraction measurements. The results demonstrate that the FeAlO3 component dominates Fe3+ and Al3+ substitution in bridgmanite, although trace amounts of oxygen- and Mg-site vacancy components are also present. Bridgmanite with more than 40 mol% FeAlO3 transforms into the LiNbO3-type phase upon decompression. The FeAlO3 end-member decomposes into corundum and hematite and does not form single-phase bridgmanite. We determined the maximum solubility of the FeAlO3 component in bridgmanite at 27 GPa and 2000 K to be 67 mol%, which is significantly higher than previously reported values (25–36 mol%). We determined the partial molar volume (27.9 mol/cm3) and bulk modulus (197 GPa) of hypothetical FeAlO3 bridgmanite, which are significantly higher and lower than those of AlAlO3 and FeSiO3 bridgmanite, respectively. The non-ideality of MgSiO3-FeAlO3 solid solution (W = 13 kJ/mol, where W is the interaction parameter) is significantly larger than that for MgSiO3-AlAlO3 (5 kJ/mol) and MgSiO3-FeSiO3 (3 kJ/mol) solid solutions. The rapid decrease in abundance of the MgAlO2.5 component in bridgmanite with increasing pressure is enhanced by the presence of the FeAlO3 component. The FeAlO3 content in pyrolite and mid-ocean ridge basalt is far below its solubility limit in bridgmanite and provides new insight into the mineralogy of the lower mantle.

Description: New Cu isotope data obtained on chalcopyrite from the Black Mountain and the Broken Hill deposits in the medium- to high-grade metamorphic Aggeneys-Gamsberg ore district (South Africa) require a revision of our understanding of the genesis of metamorphic Broken Hill-type massive sulphide deposits. Chalcopyrite from both deposits revealed unusually wide ranges in δ65Cu (−2.41 to 2.84‰ NIST 976 standard) in combination with distinctly positive mean values (0.27 and 0.94‰, respectively). This is interpreted to reflect derivation from various silicate and oxide precursor minerals in which Cu occurred in higher oxidation states. Together with the observation of a typical supergene base metal distribution within the deposits and their spatial association with an unconformity only meters above the ore horizon, our new data are best explained by supergene oxidation of originally possibly SEDEX deposits prior to metamorphic sulphide formation, between the Okiepian (1,210–1,180 Ma) and Klondikean (1,040–1,020 Ma) orogenic events.