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2.2. METHODS 2.2.1. Experiments
ОглавлениеTwo rock compositions used for the diffusion couple experiments were chosen on the basis of being as different from each other as possible while having similar CaO but different K2O. The compositions that best matched these criteria are a high‐CaO rhyolite with 70 wt% SiO2 and a phonolite with 55 wt% SiO2 (Table 2.1). The rhyolite was supplied by Shaun Brown and comes from Chuginadak Island in the Aleutians (Sample ID FMI‐6; Yogodzinski et al., 2010) and the phonolite was collected in 2002 by JMW from the northern Black Hills tertiary magmatic province. The two compositions have nearly identical CaO (2.9 wt% versus 3.0 wt%) and the large difference in SiO2 is balanced mostly by the extremely high K2O (10.4 wt%) and high Al2O3 (19 wt%) of the phonolite (Figure 2.1).
Approximately 0.055 grams of rock powder from each sample were tamped into a graphite capsule, with the higher density phonolite on the bottom to ensure gravitational stability. The capsule was capped with a graphite plug and graphite lid and loaded into a standard 3/4‐inch piston‐cylinder assembly (Fig. 2.2). The assembly was cold‐pressurized to 12.8 kbar and then brought to 1450°C at a ramp rate of 150°C/min. During the ramp, pressure increased initially due to thermal expansion but ultimately decreased to nearly the target pressure of 10 kbar and was brought to the target pressure through manual adjustments. Once the target temperature was reached, it was held at constant temperature (± 2°C) and pressure (± 0.2 kbar) for the dwell time. To end a run, the power was turned off and the sample cooled to below the glass transition within a few seconds and to 130°C in about 30 seconds.
Table 2.1 Major element composition of starting materials (fused glasses) measured by electron microprobe.
| Oxide | Rhyolite (n=13 spots) | Phonolite (n=14 spots) |
|---|---|---|
| SiO2 | 70.06 | 54.88 |
| Al2O3 | 14.75 | 19.10 |
| CaO | 2.88 | 3.01 |
| FeO | 3.93 | 4.93 |
| MgO | 0.89 | 1.38 |
| K2O | 2.86 | 10.43 |
| Na2O | 5.39 | 5.47 |
| TiO2 | 0.59 | 0.90 |
| P2O5 | 0.10 | 0.35 |
| MnO | 0.08 | 0.10 |
| Total | 101.09 | 100.27 |
Figure 2.1 Alkali‐silica diagram showing the coordinates of the two starting materials used in the diffusion couple experiments.
