Elsevier

Journal of Crystal Growth

Volume 458, 15 January 2017, Pages 72-79
Journal of Crystal Growth

A new heating stage for high Temperature/low fO2 conditions

https://doi.org/10.1016/j.jcrysgro.2016.11.043Get rights and content

Highlights

  • We designed a new heating stage able to run at T>2000 °C and very low fO2.

  • We homogenize magmatic inclusions with a good temperature control (±20 °C).

  • This protocol can produce whisker crystals of refractory compositions.

Abstract

Understanding the processes involved in the formation of intracrystalline inclusions can be valuable for both geological studies and industrial production. In view of this, we developed a new heating stage that can operate in extreme conditions. The use of tungsten as the heating material allows temperatures of over 2000 °C to be reached and also requires that experiments are run under reducing atmospheres. Small samples of metal are needed to calibrate the temperature for each experiment and the fO2 is achieved by a flow of mixed gases (CO, Ar, He). The first experiments run on this device highlight the good agreement between the different ways of estimating the temperature (by the amount of power delivered, the use of a thermocouple or by chemical composition), and a precision of ±20 °C is obtained for temperature determinations. As well as the homogenization of magmatic inclusions in ultramafic rocks, processes such as whisker crystal formation or transcrystalline migration of inclusions can be investigated using the new stage thanks to its very high maximum temperature and to the thermal gradients observed close to the heating wires. This new device looks to be a very promising tool that could easily be adapted for a range of studies by changing the nature and shape of the heating filaments.

Introduction

Inclusions, a type of three-dimensional macroscopic defect, are frequently found in crystals that have grown from a parental liquid. Their presence significantly decreases the purity of the crystalline products and can thus be problematic in industrial processes [1], [2]. However, if the formation of inclusions can be controlled then these defects might provide an opportunity to encapsulate liquids in crystalline containers [3], [4]. In the field of geology, magmatic inclusions are regarded as powerful tools for unraveling the evolution of basaltic magma compositions during their journey through the Earth's crust. For example, isolated in their host crystals, inclusions can preserve their volatile contents during ascent towards the surface and they can thus preserve the composition of the parental magma from which their host minerals grew. Several methods have been developed for studying the chemistry of inclusions, e.g. RAMAN and FTIR mainly for volatile content, but the most widely used are the electron microprobe for determining elemental bulk compositions and the secondary ion mass spectrometry for measuring isotopic compositions.

A general feature of these inclusions is that they frequently exhibit multiple phases — a glassy part, a bubble, and daughter minerals — that appear during cooling and after isolation of the inclusion from the residual magma. To obtain accurate bulk compositions of these inclusions, it is necessary to homogenize these micrometric magmatic systems, i.e. to reverse the phase changes that occurred during natural cooling [5]. This involves heating the inclusions until the bubbles and minerals dissolve into the melt, and then quenching them quickly enough to avoid the return of the different phases. This "homogenization temperature" also provides direct information on the crystallization temperature of the host crystal.

To achieve homogenization, two main protocols have been developed. The first consists in performing experiments in a vertical quenching furnace under a controlled atmosphere. This technique provides a high degree of thermal control, but multiple runs must be performed and a higher number of samples are therefore required. The second method uses high temperature heating stages coupled to an optical microscope [6], [7]. Changes in the magmatic inclusion during heating can be observed directly, which considerably reduces the number of runs required, but practical restrictions limit the types of conditions that can potentially be tested.

The two types of high-temperature (up to 1500 °C) heating stages that can be used are the Vernadsky stage and the Linkam stage. Vernadsky-type stages (see [7] for a description) use platinum wires and foils in the heating device, which forces the temperature and oxygen fugacity conditions to remain within limited domains. This type of stage may nominally reach 1600 °C before the platinum becomes too weak to hold the sample. Consequently, the maximum temperature can only be attained in pulses and cannot be maintained for a long experimental run, and the practical maximum continuous temperature is thus limited to about 1500 °C. For terrestrial samples, this is not a major issue since most of the rocks investigated by this method have compatible liquidus temperatures. However, ultramafic rocks such as komatiites, as well as refractory forsterites found in chondrites, the most abundant meteorite type, are thought to have undergone crystallization temperatures as high as 1500 or 1700 °C [8], [9], [10], [11].

The range of oxygen fugacities attainable using the Vernadsky stage is also limited. While platinum can endure oxygen-rich conditions, even during high temperature runs, reducing atmospheres are much more problematic. In particular, the heating potential of the stage is considerably reduced by the progressive evaporation of platinum, leading first to errors in temperature estimations and eventually to furnace failure. The use of purified inert gases (Ar, He), often used to prevent oxidation of the samples, can help to run experiments in reducing conditions limiting this platinum evaporation. However, H2, which is an important reducing gas in cosmochemistry studies, is particularly efficient in weakening the platinum, thus prohibiting long experiments and drastically shortening the lifetime of this type of heater. Consequently, platinum-bearing heating devices are not fully adapted for the kind of experiments (high temperatures/low fO2) we want to run.

In the Linkam TS1500 stage, a 6-mm diameter ceramic cup is heated by surrounding platinum wires. The increased size of the sample holder allows for better observation of the different experimental phases and also facilitates the use of in-situ techniques (X-ray or Raman-based techniques, for example) [12], [13], [14]. However, the presence of Pt creates the same limitations as in the Vernadsky stage in terms of the maximum temperature and the possible fO2 conditions. Moreover, though the ceramic cup allows for better temperature stability and control, it also significantly limits the maximum cooling and heating rates that can be attained. In practical terms, this prevents the quenching of melt inclusions with low viscosity basaltic compositions (i.e. Mg- and Fe-rich but relatively Si-poor) after homogenization. Another Linkam stage, the TS1400XY may reach higher heating/cooling rates (up to 200 °C/min) but the maximum temperature is namely 1400 °C, which is too low for our applications [15].

Thus, while the Vernadsky heating stage appears to be a powerful tool for studying magmatic inclusions in most terrestrial rocks (e.g. [5], [7], [16], some samples (such as highly refractory rocks) remain outside its analytical reach. The alternative, the Linkam-type heating stage, is of interest for investigating the relative stability of different phases (e.g. [14], [17]) but its weak reactivity means that it is not suitable for homogenizing ultra-rich magnesium systems, one of the major goals of our study.

Thus, in view of extending our experimental capabilities, we aimed to design a new type of heating stage with the ability to heat samples to a temperature of at least 1800 °C in reducing atmospheres. Practical aspects such as the ability to perform rapid quenching and ease of use for installation or disassembly were also kept in mind during the conception of this new very-high temperature device.

Section snippets

Description of the stage

The new heating stage consists of a parallelepipedic aluminum block, 11 cm in length and width and 3 cm thick. Two quartz windows were fitted along the length of the frame to allow microscopic observations to be performed during experiments, and a water-cooling circulation system was emplaced in the sides of the block. Swagelok® quick-connects were used for the water and gas connections as these are designed for fluid circulation and are able to resist pressure variations. A schematic

fO2

Oxygen fugacity is a critical parameter in this device, first because tungsten is very efficiently oxidized at high temperature in an O-bearing atmosphere and, second, as some of the minerals that might be studied using this stage contain elements with different valence states (for example, Fe and Ti). Precise and accurate control of the oxygen concentration is therefore a prerequisite before every test or experimental run. To allow changes in fO2 to be monitored online, we installed a Setnag

Inclusion homogenization

The main objective in designing this device was to be able to reach the homogenization temperature of magmatic inclusions seated in refractory minerals such as chondritic forsterite. Due to the paucity of these natural objects, the first tests were conducted on synthetic samples. Inclusion-bearing forsterite crystals were grown according to the method described by [18], starting with a simplified CMAS (CaO-MgO-Al2O3-SiO2 system) composition (from [19]. The forsterite crystals were recovered

Temperature gradients

Fig. 3 shows the relationship between the power supplied to the stage and the temperature reached at the surface of the sample holder, as a function of the thickness of the disk. The slopes of these trends can be used to estimate the temperature expected for a particular thickness and power conditions (see Fig. 7).

It should be noted that these estimations are quite approximate, and that some errors may exceed 100 °C. Consequently, "in-situ" temperature calibrations using metal chips (Au, Pd and

Conclusion

We have designed a new heating stage that can reach extreme conditions – reducing atmospheres and very high temperatures – unattainable by previous generations of heating stage. Particular effort was made to make the device as easy to use as possible and fast connectors were used for the water and gas connections so that the device can be easily disconnected when changing the filaments or sample. These features drastically reduce the likelihood of an accident occurring when preparing the device

Acknowledgements

This work was funded by the Program National de Planétologie / CNRS-INSU, the Région Lorraine, and the Réseau National Cristech. This is CRPG contribution no. 2475.

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