Jens Götze

Spectral information from minerals relevant for luminescence dating

The paper reviews basic spectral features of luminescence from minerals used in dating and allied research. Luminescence production is a result of multiple interactions within the imperfect crystal lattice and spectral information is not limited to the emission of light. Results of spectral investigations of luminescence emission during thermal stimulation (TL) or optical stimulation (OSL) form the main part of the paper. However, information on luminescence excitation and light absorption spectroscopy is also presented and possible links between luminescence production in minerals and particular lattice defects are considered. Quartz and feldspars, the most commonly used minerals, receive special attention, but the review includes other materials such as polymineral fine-grained fractions from sediments, zircon, calcite and other salts (halite, sulfate), meteorites, flint, volcanic materials (obsidian, tephra), ceramics and metallurgical slags. Although a wide range of different luminescence emission wavebands occur, it can be shown that certain emissions dominate in particular materials. Basic dosimetric properties are often known just for single emission wavebands of a particular mineral, and are listed in this case. The paper also aims to provide a starting point and inspiration for the study of other TL and OSL emissions, with particular regard to their potential and suitability for dating and related dosimetry tasks. These investigations, involving palaeodose determination based on an emission waveband with known characteristics, need careful separation of the particular emission peak, which may be influenced by its behaviour during the dating procedure (sample preparation, irradiation, preheat treatments, luminescence measurements, etc.). Spectral information available in this context and some technical remarks on the experimental conditions will be given to pave the way for conventional TL or OSL measurements in luminescence dating and dosimetry using natural or semi-natural materials.

Chemistry, textures and physical properties of quartz - geological interpretation and technical application

Abstract Quartz is one of the most abundant minerals in the Earth’s crust and the most important silica mineral, occurring in large amounts in igneous, metamorphic and sedimentary rocks. The mineral is widely used as a raw material in several industrial applications. Because of its chemical composition (SiO2) and its specific properties, quartz can be used both as a bulk product (e.g. quartz sands in the glass or foundry industry) and as a high-tech material (e.g. piezo or optical quartz). Dependent on the specific conditions of either natural or synthetic formation, quartz can display typomorphic properties. Variations in crystal shape, specific micro-structure, trace element or isotope compositions, characteristic spectroscopic properties, etc. may be controlled by the genesis of the quartz involved. Accordingly, the defect structure of quartz is a fingerprint of its conditions of formation. A knowledge of the interrelation between quartz genesis and the specific properties developed at that time can be used both for the reconstruction of geological processes and for specific technical applications. Selected examples in the present study give an overview of how to analyse and use the specific information inherent in the mineral quartz.

Genetic significance of the trace element content in metamorphic and hydrothermal quartz: a reconnaissance study

Abstract A reconnaissance study on trace elements in metamorphic and hydrothermal quartz was carried out using quartz samples from the tin district Erzgebirge, Germany, the gold mineralization at Kasperske Hory, Czech Republic, and the gold-quartz vein deposits Muruntau and Myutenbai, Uzbekistan. A new method of sample preparation has been developed to prepare pure quartz samples by combining conventional hand-picking with microscopic and spectroscopic studies as well as acid wash/etch procedures. Preparation of monomineralic samples was followed by sample dissolution and measurement by ICP-MS. The metamorphic quartz has very low concentrations of Li (≤0.4 ppm), Al (≤30 ppm), K (≤35 ppm), Rb (≤50 ppb), Sr (≤0.3 ppm), and Y (≤15 ppb). Moreover, it is characterized by light rare earth element enriched lanthanide distribution patterns lacking Eu anomalies. The low element concentrations in metamorphic quartz are interpreted to result from recrystallization. Metamorphic quartz from alteration halos enveloping tin and gold deposits has distinctly different trace element signatures. These differences are related to the hydrothermal overprint of the pre-existing metamorphic quartz by the mineralizing fluids. Hydrothermally altered metamorphic quartz from tin deposits has elevated concentrations of Li (≥0.9 ppm), Al (≥50 ppm), K (≥45 ppm), Rb (≥250 ppb), and Y (≥40 ppb) whereas altered metamorphic quartz from gold deposits is characterized by elevated concentrations of Sr (≥0.5 ppm). The rare earth element distribution patterns of altered metamorphic quartz show variable enrichments of the heavy rare earth elements and frequently display positive Eu anomalies. Hydrothermal vein quartz from the gold deposits usually has elevated Al (≥50 ppm) and Sr (≥0.6 ppm) contents. The lanthanide distribution patterns exhibit variable enrichments of the heavy rare earth elements and commonly show positive Eu anomalies. The elevated Sr concentrations in the quartz from gold deposits are best explained by Sr release during feldspar alteration in the wall rocks. This mechanism may also account for the relative enrichment of Eu in the mineralizing fluids although other processes may not be unambiguously ruled out.

Structure and luminescence characteristics of quartz from pegmatites

Abstract Samples of pegmatite quartz from several localities in Norway and Namibia were investigated by cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) measurements, and trace-element analysis (ICP-MS) to obtain information about their structure and luminescence characteristics. The defect structure and trace-element composition of the pegmatite quartz samples that were studied differ from those of quartz of other origin (hydrothermal, igneous and metamorphic quartz). EPR measurements reveal an almost complete lack of intrinsic lattice defects associated with O or Si vacancies (e.g., E’ center, O23- center), whereas some trace elements (Al, Ti, Ge, Li) are apparently enriched and form paramagnetic centers. The results indicate that there possibly is a redistribution of alkali ions during electron irradiation. The diamagnetic [AlO4/M+]0 center transforms into the paramagnetic [AlO4]0 center, while the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO4]0 and [TiO4]0 to form paramagnetic centers ([TiO4/Li+]0, [GeO4/Li+]0). These defects result in a specific luminescence behavior, which is very similar for all samples. In general, quartz from pegmatites shows homogeneous CL without growth zoning or internal structures suggesting constant physicochemical conditions during crystal growth. The CL emission is dominated by a transient bluish-green CL, which disappears after 60.100 s of electron irradiation. The two main emission bands centered at 505 nm (2.45 eV) and 390 nm (3.18 eV) are probably related to alkali-compensated, trace-element centers in the quartz structure. Other CL emission bands, which are characteristic features of igneous, metamorphic, or hydrothermal quartz (e.g., at 450 nm = 2.75 eV, 580 nm = 2.14 eV, 650 nm = 1.91 eV) are almost completely lacking. This fact indicates that the defects responsible for these CL emissions are absent in pegmatite quartz.

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt ☆

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.

Application of cathodoluminescence microscopy and spectroscopy in geosciences.

Cathodoluminescence (CL) microscopy and spectroscopy are luminescence techniques with widespread applications in geosciences. Many rock-forming and accessory minerals show CL characteristics, which can be successfully used in geoscientific research. One of the most spectacular applications is the visualization of growth textures and other internal structures that are not discernable with other analytical techniques. In addition, information from CL imaging and spectroscopy can be used for the reconstruction of processes of mineral formation and alteration to provide information about the real structure of minerals and materials, and to prove the presence and type of lattice incorporation of several trace elements. In the present article, an overview about CL properties of selected minerals is given, and several examples of applications discussed. The presented data illustrate that best results are achieved when luminescence studies are performed under standardized conditions and combined with other analytical techniques with high sensitivity and high spatial resolution.

High-Resolution Cathodoluminescence Studies of Feldspar Minerals

Feldspars are the most important rock-forming minerals occurring in igneous, metamorphic and sedimentary rocks. Cathodoluminescence (CL) of feldspars is an important tool in interpreting genetic conditions of rock formation and alteration (Marshall 1988). Furthermore, feldspars are widely used as dosimeters in dating geological and archaeological materials by thermally or optically stimulated luminescence (TL, OSL).

Geochemistry of agates: a trace element and stable isotope study

Abstract Agate samples of acidic, intermediate and basic volcanics from 18 localities around the world and of different age (Precambrian to Tertiary) were studied by trace element and stable isotope analysis to provide information about the process of agate formation and the origin of mineral-forming fluids. Trace element data are similar for agates from acidic and basic volcanics. The general chondrite-normalized REE distribution pattern is characterized by a slope from La to Lu with enriched LREE and a positive Eu anomaly in some samples. The similarity in the shape of the REE patterns between agates and the parent volcanic rocks suggests that the elements are mobilized by circulating fluids during syn- and postvolcanic alteration of the volcanic wall rocks. Observed positive Eu anomalies in agates probably originate from feldspar alteration. Deuterium and oxygen isotope analyses of agates and associated quartz incrustations (δD: −44‰ to −130‰; δ18O: +16.4‰ to 33.4‰) reveal variations in isotopic composition between samples of different localities but also within single agate samples (up to 10‰ for δ18O). In general, oxygen isotope compositions become heavier as volcanic host rocks grow more acidic. Furthermore, agates have higher δ18O values than associated quartz incrustations. Variations within single agate samples can be explained either by kinetic effects during isotope fractionation (e.g., the formation of agate from a noncrystalline precursor) or by mixing processes of meteoric and magmatic fluids. Remarkably high δ18O values of the parent volcanic rocks (up to+19.5‰) suggest that the circulation of 18O enriched hydrothermal fluids originated from heated meteoric water and/or residual magmatic fluids. This conclusion is supported by δ13C and δ18O data of paragenetic calcite. The temperature of agate formation was calculated for different fluid compositions, and they indicate a temperature range of ca. 50°C to 250°C.

Feldspar crystallization under magma-mixing conditions shown by cathodoluminescence and geochemical modelling – a case study from the Karkonosze pluton (SW Poland)

Abstract Feldspars from the Karkonosze pluton (SW Poland) display many features compatible with magma mixing. The mixing hypothesis has been tested using a geochemical mass balance law resulting in two possible paths of magma hybridization. Based on the results of the geochemical calculation, feldspar samples have been chosen along both mixing lines for cathodoluminescence (CL) investigation which was used as the main tool for the reconstruction of their crystallization path. Changes in the conditions of nucleation and crystallization of the feldspars as well as their movement within the magma chamber have been recognized due to different luminescence characteristics. These changes in the conditions of crystallization obtained by CL allow a precise determination of the genetic affinity of the samples to more mafic or more felsic environments. The results of the present study proved CL to be a valuable tool for the study of crystal-growth morphologies in a dynamic, turbulent environment and also as a geochemical tool for the reconstruction of various petrogenetic mechanisms (e.g. magma hybridization). Accordingly, the combination of CL with geochemical modelling provides corresponding information about magma evolution in an open system.

Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands

Eight sample sets — raw quartz sand, quartz fraction, < 2-μm clay fraction, 2–20-μm fraction and heavy minerals — of the Cretaceous quartz sand deposits of Haltern and Weferlingen (central Germany) and the Tertiary quartz sand of Frechen (western Germany) were studied to characterize the distribution of REE and trace elements and to get genetic information about provenance and sedimentologic history of the sediments. The quartz sand samples from different localities have rather similar trace-element distribution and REE patterns with enriched LREE and negative Eu anomalies. However, these relations can be modified by local changes in syn- or postsedimentary geochemical conditions. Adsorption of REE and trace elements on humic substances under reducing conditions and postsedimentary weathering in the quartz sand of Weferlingen resulted in significantly modified element abundances. The concentration of trace elements is most important in the < 2-μm fraction and in the heavy-mineral fraction where they may be enriched by a factor of up to 100. The chondrite-normalized REE distribution of the < 2-μm fraction is quite similar to those in the North American Shale (NAS) with average Eu/Sm ratios of 0.21 and (LaYb)cn of 6.1. Low trace-element abundances in the < 2- and 2–20-μm fractions were attributed to quartz dilution and quantified. Among the common minerals in the heavy-mineral fractions, especially zircon and the Fe/1bTi-oxides (rutile, ilmenite, magnetite) influence the trace-element distribution. Investigations concerning the provenance of the quartz sands illustrate that the host sediments have a rather homogeneous geochemical signature due to their sedimentological history. Taking into account Th/Ta variations the quartz sands of Weferlingen and Haltern can be distinguished from the Frechen quartz sand. Additionally, discriminations based on element ratios within the quartz fraction (Th/Sc, Ba/Sc, Ba/Co, Cs/Sc or Li/Sc) seem to be promising.