2.3.1 Experiments in the 1atm gas mixing furnace
Approximately 0.05g of the sample powder was weighed and mixed with polyethylene oxide and adhered on a platinum wire. These experimental charges were hung from a circular platinum “chandelier” suspended from an alumina ring and placed in the hot spot of a vertical tube furnace equipped for gas mixing.
The chandelier was loaded into the furnace tube at 600°C. The temperature was ramped up at 6°C/min from 600°C to T1; 100 °C above the target temperature then cooled at 0.1°C/min to the desired run temperature (T2) and held for between 48 and 160 hours. The temperature was controlled using a type B thermocouple external to the furnace tube and was measured using a second type B thermocouple inside the alumina rod from which the chandelier is suspended.
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To quench, the experimental charge was dropped from the furnace into a beaker of water. The beads were then cut from the chandelier and mounted in epoxy and polished with diamond paste of successively smaller grit size to 1μm for analysis.
2.3.2 High Pressure Experiments
For the pressurized experiments, 3.5mm diameter platinum tubing was cut to less than 8mm length and crimped, welded and flattened on one end to create a bucket. The bucket was weighed and packed with powder. Once the powder is 2mm from full, the bucket is crimped, welded and flattened to create a platinum capsule with the sample contained within. This sample is then weighed again and the weight of the power calculated. The capsule is placed in the piston cylinder assembly (Figure 4) and the assembly inserted into the large holder (Figure 9A).
Figure 9: The piston cylinder apparatus. A) The holder of the sample B) the piston and ram
The piston (Figure 9B) is driven up into the sample and the resulting pressure measured in psi. The pressure has been calibrated to allow for simple conversion of psi to kbar. After at least 1000 psi of pressure is placed on the sample, the heating stage is started. This heating is controlled by a type B thermocouple, encased in a mullite sheath and with 5mm alumina at the end. This thermocouple is placed 1mm from the top of the platinum capsule.
The temperature is ramped up at 9000 °C/h to 100 °C above the target temperature and held for 2 hours. The pressure by this stage is at the target pressure and will remain for the entirety of the experiment. The temperature is cooled at 6 °C/h to the target
21 temperature (T2) and held for 48 hours. During the cooling stage, the pressure relaxes. To ensure constant pressure during crystal growth, programmable piston cylinder apparatus was used whenever available, that automatically pumped up the pressure when lost. The automated piston cylinders usually maintain the pressure within 200 psi of the target. At the end of the experiment, the current to the experiment was cut and the pressure released which will cause the experiment to quench. After decompression the capsule was broken out of its casing, mounted in epoxy and polished to 1 µm.
In the case of the Fe-containing runs at pressure; 10 wt. % (of the total powder weight) of Pt2O was packed into the bottom of each capsule. This will create a finite buffer for the reaction PtO2 Pt (metal) + O2 and creates a highly oxidising environment. This causes all the Fe to exist as Fe2O3 (Fe3+). This creates an end member of Fe3+ partitioning that is not obscured by combination Fe2+ and Fe3+ partitioning.
2.3.3 Growing plagioclase and clinopyroxene
Attempts to grow crystals at a constant temperature were unsuccessful or too small for analysis by LA-ICP-MS. Similarly, heating the charge to 100°C above the target temperature and dropping to the target temperature as quickly as possible did not result in large minerals.
Figure 10: Experiments quenched after cooling from T2 to T1 with no dwell. Bright areas are platinum
loops. CAS) microcrystalline plagioclase in a quenched glass. These microcrystal grew during the cooling stage between 11477-1374°C. CMAS) An experimental charge showing no crystal growth during the cooling stage from 1415-1333 °C.
The best method for growing large plagioclase was heating to T1; roughly 100 degrees above the crystallisation temperature and cooling at 6oC/h to T2; the final temperature and holding for more than 48h. This heating and cooling allows for all nucleation sites to be
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homogenised and promote growth over nucleation, which results in large, homogeneous plagioclase (Tsuchiyama, 1983). Anorthite experiments quenched after the end of this cooling stage (no dwell) show either no crystal formation or thousands of microscopic plagioclase laths (Figure 10).
This proves that the plagioclase growth occurs during T2 and not during the cooling stage. Although this technique allows for the growth of large, homogenous plagioclase, the diopside can be highly zoned.
An investigation on rare earth element partitioning in anorthite grown during cooling
Crystals in most of these experiments are likely to have grown at their liquidus temperature (during the T2 dwell stage), rather than during the T1-T2 cooling stage. As not every experiment was investigated to ensure no crystal growth at the end of T2 the effect of cooling rate on the partitioning of the rare earth elements in anorthite is tested. This will allow for estimation of the error induced if minerals happened to grow during the cooling stage.
Figure 11: BSEI (Back scattered electron image) of anorthite + melt experimental charges; composition CMAS12. Samples cooled at different rates to 30 degrees past liquidus to ensure crystal growth during the cooling stage. Experiments cooled from 1415-1300 °C
23 The temperature was reduced past the liquidus (30°C cooler than the actual experimental temperature) between 1oC/h and 24oC/h to ensure plagioclase growth during the cooling stage.
Fast cooling rates cause the anorthite to grow as very thin laths, with the 24oC/h cooling rate too thin to analyse precisely (Figure 11). The cooling rates were compared to 6 °C/h (Figure 11b). There is an approximate 10% increase in partition coefficient with an increase in cooling rate.
Figure 12: Partitioning of the rare earth elements in the CMAS system, cooled at different rates from 1415-1300 °C (i.e. 30°C past the liquids temperature). A) The partition coefficients of the rare earth elements against their ionic radius B) The difference between the partitioning coefficients in relation to 6 °C/h cooling rate.
This experimental test shows that the cooling rate of minerals can affect the partitioning of trace elements. This emphasises the importance of these experiments being carried out at the liquidus temperature of these minerals. If the minerals were grown during the cooling stage, the partition coefficients could be drastically changed.
Zoning in Clinopyroxene
Clinopyroxene is notoriously difficult to synthesise in as large, homogenous crystal. This is especially true when dealing with aluminium-bearing systems.
There is complex zoning of aluminium, iron and the rare earth elements in many of the clinopyroxene grains. This zonation can rarely be seen in the backscattered electron image, but is clearly evident in element maps (Figure 13).
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Figure 13: RGB combined image of zonation in cpx in sample C5505 in system CMASNF. Ca=red, Fe=green, Al=blue. Bright yellow corresponds to high aluminium and iron clinopyroxene. Image
mapped by EPMA. cpx- clinopyroxene, Pl – plagioclase, melt – quenched glass
For each experiment that synthesises aluminous diopside, the relative standard deviation (RSD) for the Al2O3 analysis is calculated. This is an indication of the amount of zoning in the clinopyroxene grains.