Daniel Ohnenstetter

Cathodoluminescence in geosciences

1 Cathodoluminescence in Geosciences: An Introduction.- 2 Physical Parameters for the Identification of Luminescence Centres in Minerals.- 3 Information Encoded in Cathodoluminescence Emission Spectra.- 4 Importance of Instrumental and Experimental Factors on the Interpretation of Cathodoluminescence Data from Wide Band Gap Materials.- 5 Systematic Cathodoluminescence Spectral Analysis of Synthetic Doped Minerals: Anhydrite, Apatite, Calcite, Fluorite, Scheelite and Zircon.- 6 The Status of the Standards Program of the Society for Luminescence Microscopy and Spectroscopy.- 7 Geologic Application of Cathodoluminescence of Silicates.- 8 Cathodoluminescence Microcharacterisation of Silicon Dioxide Polymorphs.- 9 Brittle Deformation in Sandstone Diagenesis as Revealed by Scanned Cathodoluminescence Imaging with Application to Characterization of Fractured Reservoirs.- 10 High-Resolution Cathodoluminescence Studies of Feldspar Minerals.- 11 Application of Cathodoluminescence to Carbonate Diagenesis.- 12 Cathodoluminescence of Carbonate Shells: Biochemical vs Diagenetic Process.- 13 Quantitative High Resolution Spectral Analysis of Mn2+ in Sedimentary Calcite.- 14 Systems of Interacting Luminescence Centers in Natural Diamonds: Laser-Induced Time-Resolved and Cathodoluminescence Spectroscopy.- 15 Use of Cathodoluminescence for U-Pb Zircon Dating by Ion Microprobe: Some Examples from the Western Alps.- 16 A Combination of Single Zircon Dating by TIMS and Cathodoluminescence Investigations on the Same Grain: The CLC-Method - U-Pb Geochronology For Metamorphic Rocks.- 17 Relevance of Cathodoluminescence for the Interpretation of U-Pb Zircon Ages, with an Example of an Application to a Study of Zircons from the Saxonian Granulite Complex, Germany.- 18 Cathodoluminescence in Applied Geosciences.- 19 Cathodoluminescence as a Tool in Gemstone Identification.

Two magmatic series with island arc affinities within the vourinos ophiolite

Within the Vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks, which constitute the earlier Krapa sequence and the younger Asprokambo sequence. The Asprokambo dyke basic magmas which are poor in incompatible elements and relatively Ni and Cr rich, bear some resemblance to very low Ti basalts (transitional to boninites) found in subduction related arcs or interarc basins. Krapa series magmas from sills, massive and pillow lavas are best equated with low-K tholeiites of island arc suites. Compositions of Al- and Ti- poor Cpx in lavas from both series are comparable to those in island arc basalts, the Asprokambo Cpx being richer in Ca and Cr than those from Krapa.The large volume of cumulates from the Krapa sequence displays the following crystallization order: Ol±Sp, Cpx, Pl±Opx, Mt. Periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates (wehrlite, Ol-clinopyroxenite and melagabbro). The upper cumulates, gabbronorite and leucogabbronorite with minor Mt-bearing gabbronorite, crystallized in the upper levels of a magma chamber which became progressively smaller with time. In the Asprokambo sequence, Ol+Sp, Opx, Cpx, PI and Amph are the successively crystallizing phases. The ortho to heteradcumulates consist of websterite, Pl-websterite, gabbronorite, amphibole bearing leuconorite, diorite and granophyre. In cumulates, especially in the lower Krapa sequence, significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism. Petrological features indicate that the evolution of the Krapa series is more compatible with an intermediate fractional/equilibrium crystallization history in an initially open system, whereas nearly perfect fractional crystallization in closed system may have occurred in the small Asprokambo magma chambers. Chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates. Significantly, the Asprokambo intrusives have igneous Mg-hornblende and vanadium bearing, chromian, aluminous titaniferous magnetite, crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks. Major and trace element modelling necessitates a two stage model for the petrogenesis of the Vourinos parental melts, involving high-degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids. The primary magmas parental to the Krapa and Asprokambo series could have been derived respectively by 20 and 30% equilibrium partial fusion of variably depleted lherzolitic sources, leaving residua having a harzburgitic to dunitic composition.

Na and Cr contents in clinopyroxenes from peridotites: A possible discriminant between “sub-continental” and “sub-oceanic” mantle

Abstract The compilation of data available in the literature and new analyses show that clinopyroxenes are significantly richer in Na and poorer in Cr in peridotites associated with high-grade metamorphic rocks than in ultramafites from oceanic environments, considered as “sub-continental” or “sub-oceanic” mantle, respectively. Two distinctive fields can be drawn in the Na-Cr plot. This fact is related to the large amount of basic magma provided by the oceanic mantle along the mid-oceanic ridges. Application of this Na-Cr diagram to clinopyroxenes from peridotites in orogenic belts and appearing as xenoliths in volcanic rocks and kimberlites (“nodules”) allows us to specify their origin, taking into consideration that the clinopyroxene composition is controlled by several factors each of which gives rise to a particular trend:P-T. conditions, mineral facies, partial melting and crystal fractionation, metasomatism. It appears that oceanic-type mantle may be found under continents in extensional areas having evolved towards rift systems, and in ophiolites. The latter exhibit different degree of depletion related to their formation in two main geotectonic situations: mid-oceanic ridges and active margin systems.

Oxygen isotope composition as a tracer for the origins of rubies and sapphires

Oxygen isotopic compositions of rubies and sapphires from 106 deposits worldwide, as well as heated natural corundum, have been measured in this study. Artificially heated corundums have the same oxygen isotopic composition as unheated material. The 18O/16O ratio of natural corundum is a good indicator of its geological environment of formation. The consistently restricted δ18O range found for each type of deposit is explained by host-rock buffering during fluid-rock interaction. The δ18O constrains the geological source of the major type of gem-quality rubies sold on the market and brings new insight to gems found in placers. High-quality blue sapphires from Kashmir, Andranondambo, and Sri Lanka have specific oxygen isotopic ranges, but they overlap those of Mogok in Myanmar. Combined with traditional gemology techniques, oxygen isotope analysis will contribute toward defining the origin of some commercial high-value blue sapphires, especially from Kashmir.

The trace element geochemistry of Corsican ophiolites

Ultramafic rocks, gabbros, plagiogranites, dolerites and basaltic dikes and pillows lavas of the ophiolite slices of eastern Corsica have been metamorphosed in both oceanic and orogenic environments. The trace element content of the metabasaltic rocks which exhibit a tholeiitic differentiation trend towards a ferrobasaltic composition, is similar to that of oceanic basalts. The cumulate sequence is interpreted to have been formed by fractional crystallization from an olivine-tholeiite magma. The troctolites representing cumulates derived from melts representing liquid fractions in the range F=1-0.85, gabbros from melts with F=0.85-0.45, and ferrogabbros and plagiogranites from melts with F<0.45. An oceanic environment with moderate spreading rates and magmatic processes similar to present-day normal oceanic ridge segments is considered the more probable original setting of the Corsican ophiolites.

CO2–H2S–COS–S8–AlO(OH)-bearing fluid inclusions in ruby from marble-hosted deposits in Luc Yen area, North Vietnam

Abstract Fluid inclusions have been studied in rubies from marble-hosted deposits in the Luc Yen mining district, North Vietnam. Raman and Infrared spectroscopy combined with microthermometry investigations on primary and secondary fluid inclusions provided evidence of CO 2 –H 2 S–COS–S 8 –AlO(OH)-bearing fluids with native sulphur and diaspore daughter minerals, without visible water. Diaspore appeared as a nonvisible film of 2–3 μm coating the wall of the fluid inclusion cavity and native sulphur was generally nucleated during Raman irradiation. The presence of diaspore and COS in the fluid inclusions indicates that water was present in the paleofluid. The mole fractions of H 2 O and CO 2 are around 10 −2 and the expected concentration of water in the fluid inclusions is in the 1–10 mol.% range. Crush-leach identified sulphates and chlorides that are assigned to the presence of anhydrite and Na–Ca–Cl salts found by SEM in the ruby crystals. The CO 2 -rich inclusions which do not coexist with an aqueous phase by immiscibility process demonstrate that ruby grew from this CO 2 -rich and water-poor fluid at equilibrium with Na–Ca–Cl salts. Thermal reduction of evaporitic sulphates based on an initial assemblage of anhydrite, calcite and graphite, originating from the metamorphism of organic matter is proposed to explain the original fluid chemistry of these marble-hosted rubies. The marbles acted as a closed system and the carbonic composition of the parent fluids in ruby indicates that aluminium can be transported in CO 2 -rich fluids at high P – T .

Systematic Cathodoluminescence Spectral Analysis of Synthetic Doped Minerals: Anhydrite, Apatite, Calcite, Fluorite, Scheelite and Zircon

As the geochemical behavior of the rare earth elements (REE) is of increasing interest in geology (see the reviews edited by Lipin and McKay 1989 and Jones et al. 1996), the main REE minerals in most igneous, metamorphic and sedimentary rocks are now being studied in detail. REE are major or trace constituents in many minerals (Burt 1989). Besides the specific REE mineral assemblages which occur in alkaline, peralkaline and carbonatitic rocks (Vlassov 1966; Burt 1989; Mariano 1989; Larsen 1996; Taylor and Pollard 1996; Wall and Mariano 1996; Khomyakov 1996), more common accessory minerals such as zircon, apatite, anhydrite, carbonates and fluorites are also REE carriers and play an important role in petrologic processes. The major application of the REE studies is the melt mineral partition coefficient, used to model igneous petrogenetic processes. This will depend on the compatibility of the REE in major minerals occurring in late differentiated stages such as apatite and zircon (McKay 1989). Most of the REE are well known to be luminescence activators (Pringsheim 1949; Levrenz 1950; Monod-Herzen 1966; Diecke 1968; Marfunin 1979; Marshall 1988, Waychunas 1988). In order to interpret the cathodoluminescence (CL) emissions of natural REE bearing minerals, it is essential to compare their CL spectra to those of synthetic minerals.

Basalt petrology, zircon ages and sapphire genesis from Dak Nong, southern Vietnam

Abstract Basalts associated with sapphire deposits are widespread in Europe, Africa, Asia and Australia. In Vietnam, blue, green and yellow sapphires are recovered from eluvial and alluvial placers hosted in basaltic fields of the Dak Lak Province. Two distinct basalt suites are recognized in this field: a tholeiitic suite without any xenocrysts and an alkaline suite with mantle and lower crustal xenocrysts. The sapphires are enriched in Fe (0.43 to 1.26 wt.%), with moderate contents of Cr (33−1582 ppm), Ti (35−1080 ppm), Ga (149−308 ppm) and V (28−438 ppm) and they are poor in Zn and Mg. Their O- isotope compositions range from 6.0 to 6.9% and are not in equilibrium with the alkali basalts which have δ18O values between 5.0 and 5.7%. The U-Pb dating of zircons recovered from the basaltic placers provides evidence of two eruptional events: one at ~6.5 Ma followed by another one at ~1 Ma. The petrography of the basalts and the oxygen isotopic composition of the sapphires suggest that the sapphires are xenocrysts and that they crystallized in a deep magma chamber, at the lower continental lithosphere and the upper mantle boundary, in evolved melts issued from the fractionation of alkali basaltic magmas contaminated with lower crustal fluids.

Microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta-anorthosite

The very fine-grained (≃1 μm) polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the Harris meta-anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested. The intrusive nature of the veins is shown by their geometry and internal structures. Clasts, which are almost always only plagioclase, are generally concentrated towards the centres of veins. Flow of tens of millimetres can account for this concentration in millimetre-thick veins as a result of the Bagnold effect. The veins are generally thin (5 μm to 5 mm or more), are frequently zoned and always contain transparent granules of high relief (probably Al-rich pyroxene) and opaque granules of magnetite up to a few micrometres in size. The granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures. In some veins, spherulitic or bow-tie textures occur. The coarsest textures are found in the centres of the thickest veins. The groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 μm in thin veins or in the margins of thicker veins. This fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0.2–1.5 μm, which show little sign of deformation. The local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins, the variation being within the compositional space defined by the minerals of the host rock. This shows that homogenization of the pseudotachylite has occurred. In cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock. This was produced by segregation of the granules, pyroxene being absent from the host rock. The plagioclase in the pseudotachylite is more disordered than that in the host rock. All the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine-grained and intensely strained rock powder. The physical state on and after intrusion was that of a melt and injection was followed by crystallization. The melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation. The fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys. It thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow-tie textures.

Ar–Ar ages in phlogopites from marble-hosted ruby deposits in northern Vietnam: evidence for Cenozoic ruby formation

Abstract Ruby growth in phlogopite-bearing marbles has been indirectly dated using the 40 Ar/ 39 Ar laser stepwise heating technique on purified syngenetic phlogopite and other micas from ruby deposits in Yen Bai, Luc Yen and Quy Chau mining districts, in northern Vietnam. The principal results indicate the following. (1) Across the Red River shear zone, the phlogopites from the Yen Bai deposits yielded Miocene cooling ages between 23.2 and 24.4 Ma identical to those previously published using the same dating method on magmatic and metamorphic rocks from the Day Nui Con Voi range. (2) Luc Yen ruby deposits in the Lo Gam zone, on the eastern flank of the Red River shear zone, yielded Oligocene cooling 40 Ar/ 39 Ar mica ages between 30.8 and 34.0 Ma. Regarding the age of ruby crystallisation itself, the most plausible hypothesis is that all rubies in both zones formed during the period 40 to 35 Ma. Diachronism of cooling in adjacent zones leads to the conclusion that around 35 Ma, the ductile deformation in the Lo Gam zone ended and the ruby-bearing marbles cooled rapidly while the high-temperature deformation remained in the Red River shear zone, resulting in cooling through blocking temperature, some 15 Ma later. (3) The Quy Chau ruby deposit is restricted to the Quy Chau shear zone that bounds the eastern part of the Oligocene–Miocene Bu Khang dome. Phlogopite and biotite samples reveal Miocene cooling ages between 21 and 22.5 Ma which are minimum ages for ruby formation. These ages could be linked with the end of the extension of the Bu Khang dome. Vietnamese ruby formation is linked to Cenozoic tectonics resulting from continental collision between the Asian and Eurasian plates as for other marble-hosted ruby deposits in Central and Southeast Asia.