Technical Note
Cathodoluminescence of authigenic albite

Dedicated to H. Füchtbauer on the occasion of his 80th birthday
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Abstract

Authigenic albites in carbonate rocks typically grow in a high-grade diagenetic to low-grade metamorphic environment and often show Roc-Tourné-twinning sensu Füchtbauer. Based on an investigation of four Middle to South European occurrences, they show Mn2+- and Fe3+-activated cathodoluminescence (CL), as revealed by combined high resolution spectroscopy of cathodoluminescence emission (HRS-CL), electron paramagnetic resonance (EPR), and proton-induced X-ray emission (μPIXE).

Introduction

Authigenic albites, which commonly grow in carbonate rocks, have a higher angle between their optical axes (2vZ) compared to low-temperature albites of magmatic origin, and frequently show Roc-Tourné twinning (a combination of albite twins and X-Carlsbad twins) according to Füchtbauer, 1948, Füchtbauer, 1950. Regional studies have shown that the size of the albite crystals increases with increasing temperatures during diagenesis Patnaik and Füchtbauer, 1975, Richter, 1978, Brauckmann, 1984. Authigenic albite tends to be the nearly pure end member (0–0.4 mol% anorthite and 0–1 mol% orthoclase component) and is widespread in high-grade diagenetic to low-grade metamorphic carbonate rocks (e.g., Spötl et al., 1999). The cathodoluminescence (CL) properties of authigenic albites are poorly known up to now. Although previous workers (Kastner, 1971, Kastner and Siever, 1979, using cold-cathode equipment; Spötl et al., 1999, using hot-cathode equipment) found no luminescence of authigenic sodium feldspars, a weak luminescence was observed repeatedly with the hot-cathode microscope at Bochum. The observed colours were brown to dark brown in low-grade metamorphic Triassic formations of the central Alps near Reschensee (Südtirol; Richter and Zinkernagel, 1975), reddish-brown to bluish-violet in carbonates of the Muschelkalk in northwestern Germany, which are diagenetically overprinted to different degrees (Brauckmann, 1984), dark greenish-brown in middle Triassic carbonates of Hydra (Greece; Gillhaus et al., 1999). Blue and yellow CL colours in albitized feldspars (antiperthites) in high-grade diagenetic sandstones of the Cambrian Charny Group of Quebec were reported by Ogunyomi et al. (1981).

In this study, the results of a combined investigation of authigenic albites from four locations are presented using a hot-cathode CL-microscope and a high-resolution spectrometer (high resolution spectroscopy of cathodoluminescence emission, HRS-CL), the proton-induced X-ray emission (μPIXE) technique, and electron paramagnetic resonance (EPR) equipment.

Section snippets

Samples

The four investigated samples were taken from the central and southeast European localities given below.

Locality 1—Exploratory well, Ortland Z2 near Diepholz (northwestern Germany); shallow water bioarenites of the Upper Muschelkalk (Anisian Age); formerly studied by Brauckmann (1984); sample no. ORT 1698.

Locality 2—Coastal cliff in the bay of Zoodochou Pigis approximately 1 km west of Cape Surva (East Hydra, central Greece); dasycladacean arenites of the Upper Eros Limestone (Upper Anisian

Methods

The CL investigation was carried out using polished thin sections on the hot-cathode CL-device at Bochum in combination with a high-resolution spectral analysis equipment (Neuser et al., 1996). The spectrograph and detector technology with digital data processing (EG & G spectrograph with N2-cooled CCD camera) uses special light guide optics and allows a spatial resolution of approximately 30 μm in diameter. All CL-spectra were recorded at 14 keV. The maximum spectral resolution within the

Petrography

The authigenic albites in the samples from Ortland Z2 (loc. 1) and Hydra (loc. 2) reach a maximum size of about 200 μm. In the samples of southern Greece, maximum sizes are 1 mm (Rhodes) and 5 mm (W-Crete), respectively. The crystals of loc. 1, 2, and 3 show a predominantly tabular habitus (Fig. 1), while the albites of Rhodes have an elongated shape (Fig. 1b,e). The individual crystals typically show complex twins (Roc-Tourné twins, Fig. 1a,b) or albite twins. Untwinned albites are less

Conclusions

(1) Combined CL, EPR, and μPIXE investigations of four locations with different geological settings revealed for the first time the widespread occurrence of Mn2+ and Fe3+ as activators of CL in authigenic albite.

(2) HRS-CL has a high sensitivity with respect to trace element concentrations in albite.

(3) The formation of albite in carbonate-host rocks tends to be restricted to high-grade diagenetic and low-grade metamorphic conditions as suggested by previous sediment petrographic studies of

Acknowledgements

Technical advice from H. Völkel on the preparation of polished thin sections and from M. Oly as operator of the scanning electron microscope is greatly appreciated. Comments by R. Hesse on an early draft of the manuscript were very helpful.

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