3.4 Discussion
4.2.3 Run conditions
Three approximately 1 month long diffusion experiments were run. With very long experiments such as these there were some pauses to the experiment, however this should
91 have minimal effect on the diffusion coefficient. The temperature of all the experiments was logged by computer, so an exact calculation of seconds at temperature is possible. The final run time of all experiments is summarised in Table 16. Details of pauses in each experiment are discussed in the following sections.
Table 16: Final run time for diffusion experiments.
Diffusion experiment
Plagioclase
compositions Melt Temperature Seconds Hours days
Simple System An67, An96 CM & GH & TM 1290°C 2,490,880 691.91 28.8 Basaltic and
Andesitic Melts An58, An66 AGV & BIR 1190°C 2,799,269 777.57 32.4 Basaltic Melt An60, An68 BF 1190°C 3,011,047 836.4 34.9
All experiments had a plagioclase of roughly An67 in composition. This will allow for comparison between the same plagioclase composition at 2 temperatures and between 5 diffusant compositions.
Simple system diffusion
The buffer assemblies were pasted on the polished faces of the An67 and An95 natural, gem quality plagioclase crystal (Figure 42). The crystals were hung in a wire cage for 692 hours at 1290°C and an atmosphere of QFM, (94% CO2 +6% CO).
After 125 hours, there was a power outage which quenched the experiment. The experiment was at room temperature for a few days before being re-started. As diffusion rate is highly dependent on temperature, it can be assumed that this very short amount of time at low temperature would not affect the final diffusion rate.
At the end of the experiment, the experimental charges were quenched by dropping into a beaker of water.
It is important to note that sodium is highly volatile in at high temperatures and 1 atm pressure. This could cause a change in the sodium content over the length of the experiment.
The buffer-plagioclase interface was imaged using back-scattered electron imaging to visually determine if there was dissolution or re-growth (Figure 45).
Most of the experiments experienced a small amount of melting in the buffer assemblage. Anorthite-I (TM) and Labradorite-II (GH) experience high amounts of dissolution at the interface. These two experiments are omitted from all results.
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After the experiment was completed, the buffers were removed from the crystals by using a spatula to gently push the buffer from the polished surface. In the case of anorthite-I, the buffer was welded onto the crystal surface and was unable to be retrieved.
Figure 45: Edges of simple system diffusion experiments. Labradorite-II and Anorthite-I have significant dissolution so are not included in the results.
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Figure 46: Buffer assemblages measured by XRD
Figure 47: Temperature log for simple system diffusion
The buffer assemblages were then ground in acetone and an X-ray powder diffractometer was used to determine the crystalline buffer assemblages. The main peaks are shown in Figure 46 and the full assemblage noted in Table 17.
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Table 17: Phases in buffer assemblages in each experiment. Experiments with strike-through text represent high levels of dissolution.
System Experiment Main Phases Possible Phases CASN
Labradorite-I Labradorite, Corundum, Mullite
Labradorite-II Anorthite, Gehlenite, Hibonite, Melt Corundum Labradorite-III Labradorite, Corundum, Mullite, Melt Pyrophyllite CAS
Anorthite-I Labradorite + melt
Anorthite-II Anorthite, Gehlenite, Hibonite Corundum Anorthite-III Anorthite, Corundum, Mullite Antigorite
Changing plagioclase composition
The buffers for the labradorite experiments gained sodium, in comparison to the calculated starting mix. The buffer for the anorthite experiments had no measurable (by qualitative EDS) sodium at the end of the experiment. This suggests that the there was some sodium loss in this system. Also, measurements from the EPMA show some changes in CaO through the plagioclase (Figure 48).
Figure 48: The change of minor and major elements in plagioclase toward the diffusive contact. Anorthite number change <2 units between core and edge.
This change is not visible in the raw counts for calcium on the LA-ICP-MS due to signal noise and makes very little difference when the profile is normalised to Si29 rather than Ca43. Even though there has been some sodium and calcium changes in the plagioclase, the minor element diffusion as measured by EPMA and LA-ICP-MS are very similar. This confirms that the change in calcium does not affect the profiles measured by LA- ICP-MS.
95 It is possible, however, that the changes in CaO affects the diffusion rates of the trace elements themselves (Costa et al., 2003). The most significant diffusion of CaO occurs in experiment An70-II (Gehlenite + hibonite buffer) which changes from an anorthite number of An69 in the crystal to An75 at the rim due to dissolution, so this experiment is not included in the results. It is well known that higher anorthite numbers allow for slower diffusion for all elements (Cherniak, 2010) so this could cause slower than expected profiles measured from this experiment. The other simple system experiments vary by an anorthite number of <2 between the core and the rim, which will have less of an effect.
Diffusion from basaltic melt BF: Experiment I
The first experiment run was An60 and An68 plagioclase with their equilibrium basaltic melt (BF). This composition is loosely based on the BCR2g USGS standard. These experiments are named as a combination of their equilibrium melt and anorthite composition; BF60 and BF68.
The gasses were set to fO2= -8.5 by mixing 95% CO2 and 5% CO, then the experiment was ramped up from 600oC to 900oC at 6oC/min and held for 6 hours to ensure proper oxidation of all of the iron and other multi-valent elements. Then, due to a computing error, the furnace was ramped to 1178oC and held for 10.61 hrs, after which it was ramped at 6oC/min to the target temperature of 1190oC.
Figure 49: Temperature log for diffusion 1 experiment. Spikes (both positive and negative) are due to movement in the connection between the computer and the thermocouple.
The internal type-b thermocouple recorded a reduction in temperature over the length of the experiment which was most likely due to contamination of the thermocouple wires (Figure 49).
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After 35 days, the gases ran out unexpectedly. The experiment was ceased by ramping down to 900oC (to avoid the plagioclase buckets shattering from sudden temperature shock) then dropping the charges out of the furnace, into an empty beaker. This slow cooling allowed the melt to crystalize, which caused the interface to be obscured. The result of this experiment was a plagioclase crystal with a well of crystallised melt. The crystals in the melt consist of plagioclase, clinopyroxene and a melt (Figure 50).
The total time for this experiment at temperatures greater than 1180°C is 3,011,047 seconds.
Figure 50: Experiment I – BF60 and BF68. Melt pool crystallised into plagioclase, clinopyroxene and melt. Growth of plagioclase assumed, so rim is regarded as where the melt penetrates deepest (drawn example) Rim of plagioclase growth on BF60, approximately 18μm. Rim of plagioclase growth on BF68, ~33μm.
Diffusion from basaltic (BIR) and andesitic (AGV) type melts: Experiment II and III
The second set of experiments consisted of plagioclase of composition An58 and An66 at 1190°C and QFM for 34 days. The composition of the melt in these experiments are based on the USGS standards BIR-1; Icelandic basalt and AGV-2; Oregon Andesite. The
97 following experiments are named for their melt compositions and anorthite content of the plagioclase bucket; AGV58, AGV66, BIR58, BIR66.
Once again, due to the length of this experiment there were a number of un-expected condition changes, all of which should have a negligible effect on the diffusion. The second experiment started with 4 different buckets (AGV58, AGV66, BIR58, and BIR66) however within the first hour, the thermocouple broke causing the experiment to be aborted. Removing the crystals from the furnace caused visible cracks in the crystal. The crystal were put back in the furnace for 24 hours and then slowly cooled and removed once again in an attempt to see if the cracks would heal; they did not. The experiment was duplicated and it was decided that all 8 crystals (4 old, 4 new) could be placed in the furnace together.
Unfortunately, after 6 days the power failed at the university, causing the experiment to quench, however no visible cracks formed. These 8 crystals were re-started and ran again for 28 days at 1190°C and QFM.
Figure 51: Diffusion experiment II and III. Cumulative experimental time at experimental temperature.
The final run time for these experiments is given by recording all the time the experiment was at a temperature great than 1185°C. The first 4 buckets had a total run time of 797.94 hours or 33.25 days, while the second set of 4 buckets were at temperature for 777.57 hours or 32.40 days.
At the end of these experiments, the experimental charges were dropped from the furnace into water to ensure the melt quenched.
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The first set of buckets (Experiment II) had many cracks that allowed the melt to infiltrate and makes obtaining a smooth diffusion profile almost impossible. The buckets from the second set (experiment III) had cracks that are not filled with melt and most likely formed during the quench.
Experiment BIR58 experienced high amounts of dissolution (Figure 52) and is omitted from any further results.
Figure 52: Backscattered electron image (BSEI) of quenched diffusion experiment III. Sample BIR58 has experienced significant dissolution and is not used to gather diffusion data.