Ewa Słaby

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.

Sensitive high-resolution ion microprobe analysis of zircon reequilibrated by late magmatic fluids in a hybridized pluton

Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U–rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt, whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 ± 2 Ma indicates that fluid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years.

LREE distribution patterns in zoned alkali feldspar megacrysts from the Karkonosze pluton, Bohemian Massif – implications for parental magma composition

Abstract The elemental compositions of zoned alkali feldspar megacrysts from the Karkonosze pluton have been analysed and Pb isotope ratios determined using LA-ICP-MS, EMPA and TIMS. The results are used to interpret the magmatic environments within which they crystallized. Growth zones in the megacrysts show fluctuating trace element patterns that reflect a systematic relationship between incompatible LREE and compatible Ba. Chemical gradients between zones in the cores and rims of the megacrysts are not accompanied by significant variation in initial Pb isotope composition. The nucleation and crystallization of the megacrysts is interpreted as having occurred in an environment of magmatic hybridization caused by mixing of mantle and crustal components in which effective homogenization of the Pb isotope composition preceded the onset of megacryst growth. The concentrations of LREE in alkali feldspar zones were used to reconstruct hypothetical melt compositions. Some of the zones appear to have crystallized in an homogenous magmatic environment having clear geochemical affinities with end-member magmas in the Karkonosze pluton, whereas others crystallized in heterogeneous domains of magma hybridization. With the exception of Nd, zones crystallized in more homogeneous magma show LREE fractionation under near-equilibrium conditions. Trace element abundances of megacrysts grown in dynamic, homogeneous magmatic environments of the Karkonosze pluton occasionally deviate from the predicted patterns and show LREE impoverishment.

Formation of intracontinental basins in the opposite corners of the Tabas block as coeval structures controlled by transpressional faulting, Iran

The interaction of strike-slip faults in their restraining junctions allowed for the coeval formation of the Tabas and Abdoughi Basins and led to their inversion during the late Cenozoic. The intracontinental basins filled with Neogene and Quaternary deposits were controlled by large-scale dextral transpression along major faults that bounded the Tabas block, which is a part of the Central Iranian block. The anastomosing strike-slip fault pattern facilitated the development of both basins in opposite corners of the Tabas block. The subsided areas were formed as a result of interaction between the restraining junctions of strike-slip faults and thrusts. Flexural loading caused by the uplifted series of thrust sheets resulted in the depression of the opposite fault slabs, which permitted deposition of Neogene sediments. Deformation according to the “bookshelf” mechanism can be considered as a consequence of accommodation of the shortening of the area north of the Main Zagros thrust and externally imposed shearing along the Great Kavir (Doruneh) fault during the collision of the Arabian and Eurasian plates. Related processes of transpression and counterclockwise rotation of the tectonic blocks included in the Central Iranian block favored the interaction of strike-slip faults. The change of far-field stress and continuous transpression caused inversion of the basins and formation of Neogene folds in the northern and southern corners of the Tabas block. The geomorphic features observed along these strike-slip faults and on the thrust surfaces bounding the folds display their recent activity, consistent with present-day seismicity and geodetic measurements within the Central Iranian block.

Visualization of trace-element zoning in fluorapatite using BSE and CL imaging, and EPMA and μPIXE/μPIGE mapping

In this paper, zonation patterns of trace elements in fluorapatite are discussed that were visualized using four analytical techniques, namely back-scattered electrons (BSE) and cathodoluminescence (CL) imaging, electron probe micro-analysis (EPMA), and micro-proton-induced X-ray/gamma ray emission (μPIXE/μPIGE) mapping. Each method demonstrates the in-grain compositional variations in a slightly different way. Both BSE and CL provide qualitative data, and the internal textures are displayed in most detail. Additionally, CL points to specific elements enriched in certain growth zones. Qualitative EPMA maps show detailed zonation patterns for specific elements (with high spatial resolution), which are in general correspondence with the patterns observed in BSE and CL images. The μPIXE/μPIGE maps are fully quantitative and the detection limits are relatively low compared to EPMA mapping. In present spot measurements μPIXE demonstrates lower detection limits than EPMA, however, the latter could be considerably improved by extending the acquisition times. There is no significant overlap of REE (rare earth elements) peaks in the acquired μPIXE energy spectra, however, when multiple REEs are present with sufficiently high concentrations, peak deconvolution may pose some difficulties. Spatial resolution of μPIXE/μPIGE images is not sufficiently high to reflect minor textural features, which also result from the greater interaction depth of the proton beam. However, major growth zones are distinguishable. Even though each method has their advantages and limitations, when applied together, they provide an almost complete characterization of compositional variability in trace-element-bearing minerals.

CHOMIK: a multi-method approach for studying Phobos

CHOMIK is the name of a penetrator constructed for sampling and retrieval of Phobos surface material. It formed an integral part of the Phobos Sample Return Mission. In this paper we present its construction and intended mode of operation, since the concept is still viable for future missions either to Phobos or to other small bodies of similar dimensions. We take Phobos as an example to describe the science case for such an instrument and how it might be utilized to resolve important open issues regarding the origin of the Martian moons. Concerning the latter, we place emphasis on measurement techniques and analysis tools for mapping trace element concentrations in returned sample.

Origin and evolution of volatiles in the Central Europe late Variscan granitoids, using the example of the Strzegom-Sobótka Massif, SW Poland

The results of the new Electron Microprobe Analysis of apatite, hornblende and biotite crystals of the hornblende-biotite variety of the Strzegom-Sobótka granite indicate that these rocks experienced several phases during their evolution. During the first phase, the melting of the continental crust was caused by heating from basaltic magma. Nevertheless, the system was closed, and there is no evidence of the influence of any mafic component. The volatile compositions of apatite crystals recorded the signature of this phase. Afterwards, small quantities of mafic material were delivered into the crystallizing melt by ascending basaltic magma. The volatile composition of hornblende, together with dramatic changes in the F/Cl ratios between apatite and hornblende, indicates that the injection of mafic material occurred between the crystallization of both minerals. During the crystallization of hornblende, the system was closed. During the last episode, the volatile composition recorded by biotite indicates that the system opened again. This is represented by the decrease of Cl in the melt. Moreover, the estimated F concentrations in the melt range from ~2000 to ~3000 ppm and do not change significantly within the crystallization interval, whereas the Cl concentrations decreased from 1470-900 ppm at 829-768 °C to as low as 100 ppm at 710-650 °C, most likely due to the continuous exsolution of aqueous fluid from silicate melt or the crystallization of Cl-bearing minerals.

Self-Similar Pattern of Crystal Growth from Heterogeneous Magmas: 3D Depiction of LA-ICP-MS Data

Crystals grown from mixed magmas are characterized by extreme geochemical heterogeneity. The system is self-similar which is reflected in a complex pattern of element distribution in the crystal. New tools are required to show the complexity. 3D depiction (digital concentration-distribution models DC-DMs) combined with fractal statistics is an ideal tool for the identification and description of any subsequent change occurring due to the chaotic processes. LA-ICP-MS analysis gives simultaneous information on the concentration of many elements from the same analysed crystal volume. Thus the data collected are an ideal basis for the calculation of both DC-DMs and fractals. Simultaneous information retrieved by LA-ICP -MS on both compatible and incompatible elements and further data processing allow the determination of the process dynamics in terms of element behavior: antipersistent/persistent, being incorporated according to Henry’s Law or beyond it. The multi-method approach can be used for any system showing geochemical variability.

Protomylonite evolution potentially revealed by the 3D depiction and fractal analysis of chemical data from a feldspar

Abstract An alkali feldspar megacryst from a protomylonite has been studied using laser ablation-ICP-mass spectrometry combined with cathodoluminescence imaging, Raman spectroscopy, and electron probe microanalysis. The aim was to determine the original (magmatic) geochemical pattern of the crystal and the changes introduced by protomylonitization. Digital concentration-distribution models, derivative gradient models, and fractal statistics, e.g., Hurst-exponent values are used in a novel way to reveal subtle changes in the trace-element composition of the feldspar. Formation of the crystal is reflected in a slightly chaotic trace-element (Ba, Sr, and Rb) distribution pattern that is more or less characterized by continuous development from a fairly homogeneous environment. Derivative gradient models demonstrate a microdomain pattern. Fractal statistics show that element behavior was changeable, with Ba and Sr always more persistent (continuing) and Rb always less persistent, with the latter showing a tendency to migrate. The variations in the Hurst exponent are, however, too large to be explained by magmatic differentiation alone. The observed element behavior may be explained by structural changes revealed by Raman spectroscopy and CL. In high-strain domains, T–O–T modes become stronger for Si–O–Al than Al–O–Al linkages. Increasing amounts of Al–O−–Al defects are demonstrated by cathodoluminescence. Both may result from small-distance diffusion creep, making the crystal geochemical pattern slightly patchy. In turn, the marginal part of the megacryst has a mosaic of randomly orientated, newly crystallized K-feldspars. The re-growth is confirmed by trace-element distribution patterns and fractal statistics which identify an abrupt change in the transformation environment. The novel set of tools used in this study reveals a complicated history of megacryst formation and transformation that otherwise would be difficult to unravel and decipher.