In this contribution, we present results from new diffusion couple experiments with two motivating factors in mind. First, the solvent‐normalized diffusivity can only be defined in situations where there is a large initial concentration gradient for the component of interest and an effective binary diffusion model is applicable. And yet, we are aware that large diffusive isotope effects can arise even in the absence of large initial concentration gradients. One such example is in ugandite‐rhyolite diffusion couple experiments where Ca isotopes were fractionated by ∼2‰ due to diffusive coupling of CaO with Al2O3 (Watkins et al., 2009). Such strong multicomponent diffusion effects warrant further investigation because they may contribute to isotope variations within and among minerals formed in high‐T settings. Second, the ratio Di/DSi tends to be lower and approach unity for elements that are present in major quantities because the net flux of a major element requires cooperative motion of the other major components of the liquid. The β factor for Li can be high because it diffuses fast, and it can diffuse fast because it is present in trace quantities. The same may be true for Ca; the β factor for Ca approaches that of Li in experiments where Ca is present in minor quantities (∼2 wt%; Watkins et al., 2011). These observations raise the question of whether the (typically) fast‐diffusing K2O component will behave like Li and have a high β factor or whether it will behave like other major elements and have a β factor closer to zero.
2.2. METHODS
2.2.1. Experiments
Two 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.
2.2.2. Electron Microprobe Analyses
Each diffusion couple was extracted and sectioned down its vertical axis, mounted in a 1‐inch epoxy puck, and polished to 0.25 μm. After mounting, we measured the length of the diffusion couples and note that each one had compressed from an initial length of 10.4 mm down to about 6.6 mm. Axis‐parallel major‐element diffusion profiles were measured from end‐to‐end with a JEOL JXA‐8200 SuperProbe at Lawrence Livermore National Laboratory using a 15 nA beam current rastered at 12000× magnification (12 μm × 9 μm beam dimensions) with an accelerating voltage of 15 kV. Sodium was measured first at each spot to mitigate effects of Na migration. All electron probe data are provided as an Excel file in the Electronic Supplement.
2.2.3. Ca Isotopic Measurements
After microprobe measurements, diffusion couples were sectioned into wafers, about 465 μm thick and weighing about 3 mg, using a Bico diamond wafer saw with blade thickness of 165 μm. The wafers were dissolved in a mixture of hydrofluoric and perchloric acid, dried at 165°C, redissolved in 5 mL 3N HNO3, aliquotted, mixed with a 42Ca‐48Ca double spike to correct for spectrometer‐induced mass discrimination (cf. Watkins, 2010), dried to a small bead, and the bead was redissolved in 100 μL 3N HNO3 for loading onto cation exchange columns. The Ca fraction was separated and collected by cation exchange chromotography using Eichrom Ca‐spec DGA resin. The non‐Ca fraction was saved for subsequent K isotope work. About 3 μg of purified Ca from each sample were loaded onto a zone‐refined Re filament, dried down, topped with 1 μL of 20% of H3PO4 acid and re‐dried.
Ca isotopes were measured by thermal ionization mass spectrometry (TIMS) at UC‐Berkeley on a Thermo‐Finnegan Triton TI with nine moveable Faraday collectors. For each sample, at least 100 isotope ratio measurements were made to reduce within‐run uncertainties to ± 0.04‰. At the time these data were collected in 2009, the long‐term uncertainty in the standard
