Cordierite and Its Retrograde Breakdown Products As
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Jörn C. Ogiermann Cordierite and its retrograde breakdown products as monitors of fluid-rock interaction during retrograde path metamorphism: case studies in the Schwarzwald and the Bayerische Wald (Variscan belt, Germany) Dissertation 2002, Heidelberg Dissertation submitted to the Combined Faculties for the Natural Sciences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germany for the degree of Doctor of Natural Sciences presented by Diplom-Geol.: Jörn Christian Ogiermann born in: Offenbach/Main Oral examination: 13. December, 2002 Cordierite and its retrograde breakdown products as monitors of fluid-rock interaction during retrograde path metamorphism: case studies in the Schwarzwald and the Bayerische Wald (Variscan belt, Germany) Referees: Prof. Dr. Angelika Kalt Prof. Dr. Rainer Altherr Abstract Ogiermann, J.C. Abstract Retrograde breakdown products of cordierite are commonly termed pinite, but their modes, compositions and formation conditions are only poorly known. The author carried out a systematic study on pinitised cordierite from Variscan high-temperature metamorphic pelites of the Schwarzwald and the Bayerische Wald using electron microprobe (EM), transmission electron microscope (TEM), scanning electron microscope (SEM), x-ray diffraction (XRD), petrographic microscope and Fourier Transformation Infrared Spectroscopy (FTIR) investigations. On the basis of composition, phase assemblage, textural position and grain size four pinite types (border, mat, fissure, isotropic type) were distinguished that formed by distinct allochemical processes under different pressure-temperature conditions at different times. They probably represent general features of cordierite breakdown. Border-type (b-type) pinite consists of muscovite and biotite, formed at 350 - 550 °C from a K+-bearing fluid, most likely derived from the breakdown of K-feldspar to muscovite and quartz. B-type pinitisation may be related to granite intrusion in the Carboniferous. Mat-type (m-type) pinite encompasses chlorite-muscovite pinite and complex m-type pinite, bearing clay minerals, and is in general considered to represent the alteration of cordierite by an alkli-bearing fluid. Petrogenetic grids define an upper P-T limit muscovite of around 550 500°C . - TEM revealed that complex clay mineral-bearing m-type pinite of this study has the principal constituents chlorite, berthierine, I/S R1 and I/S R0 mixed- layers, Na/K-illite and random chlorite/berthierine mixed-layers. The observed features are similar to those of non-equilibrium assemblages from early to late diagenetic parts of burial metamorphic sequences as complex m-type pinite lacks stable equilibrium and should have been formed much below 200 °C. In this case, its genesis is a complex two-stage stage process, with chlorite and I/S R1 representing the primary product of m-type pinitisation. Secondary berthierine and I/S R0 are formed by the replacement of chlorite and I/S R1 via a dissolution/precipitation process driven by the infiltration of an additional very low-grade fluid. M-type pinitisation could be related to low-temperature meteoric alteration of granites. F-type pinites form alteration veins penetrating intact cordierite. They are filled with tiny (< 20 - 250 Å in thickness), randomly oriented and randomly related, highly imperfect crystals, floating in an amorphous matrix. The principal crystals or packets of layers detected by TEM are berthierine, illite and/or I/S R1, smectite and chlorite, mixed-layers including 7/14 Å, 10/14 Å, I/S and complex mixed-layering of 7 Å, 10 Å and 14Å layers. Randomness in orientation of f-type pinite crystallites points to in situ crystallisation of packets of layers Abstract Ogiermann, J.C. directly either from a solution present in alteration veins or from a pre-existing gel-like material as more stable secondary phases. The occurrence of corresponding optical and chemical zonation pattern as further characteristics of f-type pinite let the author assume that corresponding processes of leaching and repolymerisation acting in discrete band-like zones are the principle mechanism of f-type pinitisation. Lack of perfect and homogeneous structures and enhanced thickness of individual packets of layers indicate f-type pinitisation to result from cordierite alteration at very low-grade conditions (weathering), perhaps at surface conditions, with Permian-Mesozoic sediments as possible Ca source. I-type pinite is in most cases in direct contact to intact cordierite, is fairly homogenous in BSE images, isotropic under crossed polars, lacks optical zonation patterns and occurs sporadically. I-type pinite shows similarities to hydrated glassy layers described from feldspar alteration. Compared to cordierite, i-type pinite is depleted in most elements but networkforming species (Si and Al) and potential interlayer cations with K and Ca being most abundant (Ca > K). A dramatic fall in Si counting rates occurred during electron beam exposure at standard conditions, suggesting that i-type pinites may represent amorphous gel- like highly hydrated material with a high portion of low-energy H-H bonds. This assumption is confirmed by the occurrence of fissures penetrating i-type pinite, which are strongly suspected to represent dehydration shrinkage fissures as they do not continue into adjacent material and do not show a preferred orientation. Unfortunately, TEM could not give further evidence. Therefore, the phase inventory and structural state of i-type pinite remain unclear. I- type pinitisation is probably a leaching process at very low temperatures (weathering) with network-modifying species being preferentially dissolved. H2O and CO2 contents of partly pinitised cordierite were determined by in situ FTIR measurements in order to detect possible late-stage modifications of the cordierite channel compositions, possibly related to pinitisation. Despite the fact that all samples record infiltration of several hydrothermal aqueous fluids by the alteration of cordierite to various types of pinite, a post-peak re-equilibration with hydrous fluids can be ruled out. The volatile contents of all investigated samples were found to be consistent with equilibration with a H2O undersaturated melt, during high-grade metamorphism. Therefore, a systematic change of cordierite channel volatile contents as a key step within the process of pinitisation is not indicated. Nevertheless, the crystallographic control of f-type pinitisation implies that the channel structure determines the c axis to be the preferred direction of cordierite dissolution/leaching. Contents 1. INTRODUCTION 7 1.1 Cordierite and pinite 7 1.2 Role of the structural channels in pinitisation 9 1.3 Aims and concept of this study 11 2. GEOLOGICAL SETTING AND SAMPLES 13 2.1 The Schwarzwald 13 2.2 The Bayerische Wald 16 2.3 Samples 18 3. ANALYTICAL TECHNIQUES 19 3.1 Transmission electron microscopy 19 3.1.1 The electron source and the electron-optical system 20 3.1.2 Image contrast 20 3.1.3 Resolution and lens aberrations 20 3.1.4 Electron diffraction 21 3.1.5 Analytical electron microscopy 23 3.1.6 Sample preparation and beam damage 23 3.2 Electron microprobe 24 3.2.1 The electron source and the electron-optical system 25 3.2.2 Electron microprobe analysis 26 3.2.3 Destruction of material 27 3.2.4 Sample preparation 27 3.3 Scanning electron microscope 27 3.3.1 Secondary electrons 28 3.3.2 Backscattered electrons 28 3.4 X-ray diffraction 29 3.4.1 Oriented powdered samples 30 3.4.2 Intensity of a reflection 30 3.5 Fourier-Transform infrared spectroscopy (FTIR) 31 3.5.1 Theory 31 3.5.2 FTIR spectrometer 32 4. EM, SEM AND PETROGRAPHIC MICROSCOPY: PETROGRAPHY, CLASSIFICATION AND CHEMICAL CHARACTERISATION OF RETROGRADE CORDIERITE BREAKDOWN PRODUCTS IN GNEISSES AND MIGMATITES OF THE SCHWARZWALD AND THE BAYERISCHE WALD, GERMANY 35 4.1 Abstract 35 4.2 Introduction 35 4.3 Sample selection and geological setting 38 4.4 Analytical techniques 40 4.5 Microscopic characterisation and BSE imaging of cordierite breakdown products 41 4.5.1 Border-type pinite (b-type) 41 4.5.2 Fissure-type (f-type) and isotropic-type pinite (i-type) 41 4.5.3 Mat-type pinite (m-type) 42 4.5.4 Samples Al-20 and To-2 43 4.5.5 Textural relationship between the pinite types 43 4.6 Composition of retrograde breakdown products 47 4.6.1 Primary and secondary biotite 47 4.6.2 Muscovite and muscovite-rich domains 48 4.6.3 Na-rich domains 50 4.6.4 Chlorite 51 4.6.5 F-type pinite 52 4.6.6 I-type pinite and fi-type 56 4.6.7 M-type pinite 56 4.7 Pinitisation processes 58 4.7.1 B-type pinitisation 59 4.7.2 M-type pinitisation 60 4.7.3 F-type and i-type pinitisation 61 4.8 Pinitisation processes in the regional context 63 4.9 Summary and conclusions 65 5.TEM & XRD: CHARACTERISATION OF VERY FINE-GRAINED PINITE FROM THE SCHWARZWALD, GERMANY 67 5.1 Abstract 67 5.2 Introduction 67 5.3 Samples and pinite types 69 5.4 Methods 71 5.5 X-ray diffraction 72 5.6 TEM observations 74 5.6.1 M-type pinite 74 5.6.2 F-type pinite 80 5.6.3 I-type pinite 87 5.7 AEM data 88 5.7.1 M-type pinite 88 5.7.2 F-type pinite 92 5.8 Discussion 93 5.8.1 Genesis of m-type pinite 94 5.8.2 Genesis of f-type pinite 95 5.8.3 Relationship between f-type and m-type pinite 97 5.9 Summary and conclusions 97 6. FTIR: THE ROLE OF THE STRUCTURAL CHANNELS IN THE PROCESS OF PINITISATION 99 6.1 Abstract 99 6.2 Introduction 99 6.3 Samples, geological settings and pinite types 101 6.4 Analytical techniques 103 6.5 Cordierite compositions 105 6.5.1 Major element concentrations 105 6.5.2 Cordierite intra-grain FTIR spectra 105 6.5.3 Pinite FTIR spectra 106 6.5.4 FTIR spectra of outermost cordierite rims 107 6.6 Discussion 108 6.6.1 Incorporation of volatiles in cordierite 108 6.6.2 Behaviour of volatile contents at low temperatures 112 6.6.3 Volatile contents of cordierite from granulitic and migmatitc terranes 112 6.6.4 Significance of the cordierite volatile contents determined in this study 112 6.7 Conclusions 118 7.