Alfons van den Kerkhof

Fluid inclusion petrography

Abstract A procedure of fluid inclusion studies is proposed with emphasis on the criteria of selecting fluid inclusions for detailed (microthermometry and spectroscopic) analysis. An overview of descriptive and genetic classifications of fluid inclusions in single crystals and in massive rocks is given with the intention of further differentiating the commonly used terms ‘primary’ and ‘secondary’ fluid inclusions. Some principles of fluid inclusion modification are explained. Cathodoluminescence (CL) studies of quartz with the optical high-power CL-microscope and the electron microprobe provided with a CL detector are an important help in ‘fluid petrography’. CL textures are subdivided in primary, growth textures and a wide variety of secondary microtextures, which are in part induced by fluid inclusions. The latter is grouped in textures indicative of local lower crystal order (increasing defect structures) and microtextures indicative of local quartz healing (reduction of the defect structures). Microtextures showing the genetic relationship between fluid inclusions and the host mineral provide information about the possible post-entrapment changes of fluid inclusions and therewith testify their geological relevance.

The evolution of late-Hercynian granites and rhyolites documented by quartz – a review

The potential of igneous quartz for providing a better understanding of magmatic processes is demonstrated by studying late-Hercynian rhyolites and granites from central and western Europe. Cathodoluminescence (CL) reveals growth patterns and alteration structures within igneous quartz reflecting the magma crystallisation history. The relatively stable and blue-dominant CL of zoned phenocrysts is principally related to variations in the Ti concentration, which is a function of the crystallisation temperature. The Al/Ti ratio of igneous quartz increases with progressive magma differentiation, as Ti is more compatible, compared to Al, Li, K, Ge, B, Fe, P during magma evolution. The red-dominant CL of the anhedral groundmass quartz in granite is unstable during electron bombardment and associated with OH- and H 2 O-bearing lattice defects. Thus, CL properties of quartz are different for rocks formed from H 2 O-poor and H 2 O-rich melts. Both groundmass and phenocrysts in granites are rich in alteration structures as a result of interaction with deuteric fluids during cooling, whereas phenocrysts in extrusive rocks do not usually contain such structures. The combined study of trace elements along with the analysis of quartz textures and melt inclusion inventories may reveal detailed PTX-paths of granite magmas. This study shows that quartz is a sensitive indicator for physico-chemical changes during the evolution of silicarich magmas. Common growth textures show a wide variety in quartz phenocrysts in rhyolites and some granites. This paper presents a classification of textures, which formed as a result of heterogeneous intra-granular lattice defects and impurities. The alternation of growth and resorption microtextures reflects stepwise adiabatic and non-adiabatic magma ascent, temporary storage of magma in reservoirs and mixing with more mafic, hotter magma. The anhedral groundmass quartz overgrowing early-magmatic phenocrysts in granites is free of growth zoning.

Fluid Composition and Evolution Attending UHP Metamorphism: Study of Fluid Inclusions from Drill Cores, Southern Sulu Belt, Eastern China

Rocks from the first pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSDPPH1, 432 m), located in the eastern part of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, have been subjected to a coesite-eclogite-facies metamorphic event, followed by an amphibolite-facies overprint. Primary fluid inclusions occur in garnet, omphacite, and apatite from eclogite; in kyanite and in topaz from quartzite; and in garnet, epidote, and apatite from paragneiss. Secondary fluid inclusions are present in all lithologies. Fluid inclusions are absent from ultramafic rocks. Based on fluid compositions and textural criteria we distinguished: (1) low-salinity aqueouscarbonic inclusions in topaz from quartzite, which may have originated from a supracrustal protolith; (2) primary CaCl2-NaCl-rich brine inclusions in garnet and in omphacite from eclogite and in kyanite from quartzite, representing UHP metamorphic fluids; (3) high-salinity aqueous-carbonic inclusions in quartz from eclogite and quartzite, representing amphibolite-facies fluids; (4) aqueous fluids of low- and intermediate salinity trapped as primary inclusions in garnet, epidote (or allanite) and apatite from gneiss, or as secondary inclusions, representing amphibolite-facies and later retrograde fluids; (5) carbonic inclusions are distributed along transgranular fractures in quartz from quartzite, and probably represent the latest retrograde fluid. The diversity in fluid inclusion populations and compositions from different vertical depths suggests a closed fluid system without largescale fluid migration during UHP metamorphism. However, the common low- and medium-salinity inclusions in most rock types suggests that a water-dominated fluid from an external source infiltrated into the rock system during amphibolite-facies metamorphism, resulting in extensive retrogression of the UHP rocks.

Cathodoluminescence, fluid inclusion and stable C–O isotope study of tectonic breccias from thrusting plane of a thin-skinned calcareous nappe

Basal hydraulic breccias of alpine thin-skinned Muráň nappe were investigated by means of cathodoluminescence petrography, stable isotope geochemistry and fluid inclusions analysis. Our study reveals an unusual dynamic fluid regime along basal thrust plane during final episode of the nappe emplacement over its metamorphic substratum. Basal thrusting fluids enriched in 18O, silica, alumina, alkalies and phosphates were generated in the underlying metamorphosed basement at epizonal conditions corresponding to the temperatures of 400–450°C. The fluids fluxed the tectonized nappe base, leached evaporite-bearing formations in hangingwall, whereby becoming oversaturated with sulphates and chlorides. The fluids further modified their composition by dedolomitization and isotopic exchange with the host carbonatic cataclasites. Newly formed mineral assemblage of quartz, phlogopite, albite, potassium feldspar, apatite, dravite tourmaline and anhydrite precipitated from these fluids on cooling down to 180–200°C. Finally, the cataclastic mush was cemented by calcite at ambient anchizonal conditions. Recurrent fluid injections as described above probably enhanced the final motion of the Muráň nappe.

Analysis of Low Element Concentrations in Quartz by Electron Microprobe

Although quartz is one of the most abundant minerals in many rock types, it has not been the focus of in situ quantitative chemical analysis by electron microprobe for a long time. This was simply due to its high purity. Since cathodoluminescence observations reveal a great variety of complex structures within quartz, in situ chemical analysis methods like laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), secondary ion mass spectrometry (SIMS), and electron microprobe (EMP) applied to quartz have received increasing interest from geoscientists. Although the concentrations of many trace elements in quartz are far below the detection limits of an electron microprobe, Al, K, Ti, and Fe are, amongst others, suitable candidates for quantification. The advantage of EMP analysis over other methods is its high spatial resolution combined with high accuracy. Monte Carlo simulations of the elements listed above in a quartz matrix indicate sampling depths of <2.7 μm for 99% of the acquired X-ray photons. Sampling volumes range from 75 to 250 μm3, and depend on excitation energy and defocusing of the electron beam. Unfortunately, beam-induced damage of the quartz lattice limits the use of high beam currents and focused beams. Irradiation induced damage strongly influences the low energy X-ray lines like Al-Kα. The beam sensitivity of the various quartz samples needs to be frequently tested and the analysis protocol must be adapted according to this signal behaviour. To minimise the effect, we propose dividing the measurement of Al into several subsets. Furthermore, exact investigation of the curvature of the background signal is required to avoid systematic errors. Secondary fluorescence of adjacent minerals is an often-neglected problem of trace element analysis by EMP. Joined crystals of pure quartz connected to TiO2, ilmenite (FeTiO3) and sanidine (KAlSi3O8) were used to quantify the effect of secondary fluorescence in quartz. Depending on the location of the disturbing phase, along the “line of sight” of the spectrometer or perpendicular to it, measurable effects above the detection limits can be recognized at distances up to 40 μm for Al, 60 μm for K, 200 μm for Ti and 220 μm for Fe. Depending on the position of the spectrometer relative to an adjacent phase, secondary fluorescence effects vary for Ti and Fe even at larger distances, which has to be taken into account when very low concentrations need to be detected. This effect complicates the application of empirical corrections for secondary fluorescence near phase boundaries. Setting of specific elements on multiple different spectrometers will increase the statistical certainty and can point to secondary fluorescence effects. Using our analysis protocol, detection limits of <10–15 μg g−1 for the elements Al, K, Ti and Fe in quartz can be achieved.