3. Methods
3.1 Experimental Approach
Because most natural igneous melts contain some sulfur, the results of this project can inform more accurate interpretations of the redox and thermal histories of volcanic, plutonic, and mantle rocks on the Earth and other terrestrial planets. For example, constraints on the temperature-dependence of equation (1) and accurate placement of the transitions of S2- to S6+ and Fe2+ to Fe3+ as a function of ƒO2 are critical for evaluating the role of S in modulating the ƒO2 in the mantle wedge proximal to subducting and devolatilizing slabs of oceanic crust (e.g., Klimm et al. 2012), the effect of S degassing on the ƒO2 of magmas (e.g., Gaillard and Scaillet 2009, Gaillard et el. 2011, Moussallam et al. 2016, Edmonds & Woods 2018), and the genesis of porphyry ore deposits (e.g., Tang et al. 2020).
Homogenization and cooling experiments were run in a 1-atm Deltech vertical gas-mixing furnace, using H2-CO2 gas mixes to control ƒO2, which was calibrated and monitored using a Y2O3-stabilized ZrO2 oxygen sensor. The ƒO2 sensor was calibrated to the Fe-FeO reaction and was accurate to within 0.1 log units of the accepted value (Huebner 1971). Based on monitoring the sensor output over the experimental duration the precision was ±0.03 log units of ƒO2. Four different values of ƒO2 were used in the experiments, expressed relative to the fayalite-magnetite-quartz buffer (FMQ; referenced to the data of O’Neill 1987): ƒO2 = FMQ-1 (“HX”, n=15), FMQ+0.35 (“SF”, n=27), FMQ+2.8 (“HTOX”, n=2), and FMQ+4.7 (“H3X”, n=4) where n refers to the number of inclusions that were recovered and polished. The ƒO2 chosen for the SF experiments, FMQ+0.35, was based on the average Fe3+/Fe2+ measured by XANES in two natural Papakōlea melt inclusions, which ranged from Fe3+/Fe2+ = 0.20 (HIGS21) to 0.25 (HIGS14), from which ƒO2 was calculated using the expression of Kress & Carmichael (1991). Based on T = 1225
°C, the Fe3+/Fe2+ measured in the natural inclusions and the model of Nash et al. (2019), the liquids in the SF experiments were expected to have a fraction of total S present as S6+
(S6+/ST) = 0.55-0.88, or, based on the nominal experimental ƒO2 (FMQ+0.35) and the model of Jugo et al. 2010, S6+/ST = 0.04. Note that the high ƒO2 experiment (H3X) was run in pure CO2 gas, which at 1225 °C results in a CO-CO2 gas mix whose ƒO2 is outside the stability region for the Fo89.8(±1.9) olivines in the starting materials (Nitsan 1974). Two olivines recovered from this experiment were used as a “reversal” experiment and rehomogenized for 24 hr at 1225 °C at FMQ-1 in H2-CO2 gases (experiments H3XR).
Temperature (T) was recorded using a Type-S thermocouple, which was calibrated at the melting point of gold, and which was hung in the furnace hotspot alongside MgO
buckets that held the inclusion-bearing olivines. In each experiment, the MgO buckets were gradually lowered into the furnace hotspot and held isothermally at 1225 °C for 24 hr at the target ƒO2. The isothermal dwell period was designed to homogenize the melt inclusions and to equilibrate these inclusions with the ƒO2 imposed by the furnace gases at the exterior of the host olivines (Gaetani et al. 2012; Bucholz et al. 2013). During the cooling stages of the experiments, temperature was recorded digitally at a resolution of one measurement per second by attaching the in-situ Type-S thermocouple to a temperature logger.
In order to evaluate the temperature and cooling-rate dependence of the Fe-S electron transfer in experimental melt inclusions, three cooling schemes were designed for the SF experiments (homogenized at FMQ+0.35). All the HX experiments described in this chapter and several (20) of the SF experimental inclusions were homogenized for 24 hr and then drop quenched directly from the hotspot at 1225 °C into water – these are referred to as homogenization experiments in that they experienced the highest quench rates attainable using this experimental setup (Xu and Zhang 2002), except perhaps by using colder water. Two different types of cooling experiments were run: Type-B thermocouple-controlled cooling rate and displacement cooling. The controlled cooling rate experiments (SF3 and SF4) were run by programming the Type B control thermocouple placed near the furnace elements to cool as quickly as possible (setting the cooling rate to 5000 °C/hr); however, because of limitations on how fast the furnace is capable of conducting heat out of the hotspot, the maximum cooling rate achieved was a factor of two lower. For experiment SF3, which was cooled from 1225-1150°C a linear fit to the thermocouple time vs. temperature data gives a cooling rate of 2900 °C/hr; for
experiment SF4, which was cooled from 1225-1000°C the linear cooling rate is ~1900
°C/hr. Although the program was set to linear cooling, the cooling path is curved (Figure 1B), indicating that cooling was dominated by conduction of heat out of the furnace hotspot and not by the reduced power delivered to the furnace elements. The remaining experiments were run using the displacement method where the ceramic quench rod from which the olivine-bearing buckets were suspended was manually displaced out of the furnace hotspot at 1225 °C into a cooler part of the furnace in less than two seconds. The vertical thermal profile of the furnace tube was measured by equilibrating the Type-S thermocouple at different heights above and below the hotspot (Figure 1A). Once displaced into a cooler region of the furnace, the olivines cooled via conduction in the thermal gradient of the furnace, and the MgO buckets were drop quenched into water at different times along these cooling paths. Experiment SF10 was displaced to ~1150 °C and quenched at the same time after initiation of cooling as the controlled cooling rate experiment SF3 (also quenched at 1150°C), in order to compare two different cooling paths quenched at nearly the same temperature. Experiments SF4 (controlled cooling and quench at 1000 °C) and SF6, SF7, and SF9 (displacement) were designed in a similar manner, and the latter set of three displacement experiments were displaced to and quenched from ambient furnace temperatures of 950-981°C. For the very short duration experiment SF13, the thermocouple rod was displaced far from the hotspot to a region with ambient T = 600
°C and the experiment was quenched at 900 °C; this served as an intermediate between the rapidly cooled experiments that were quenched at 1000 °C and the homogenization experiments that were drop quenched directly into water from 1225 °C. A summary of the experimental conditions for the
Figure 1. (A) Thermal profile of the Deltech 1- atm furnace. Gray squares are Type-S thermocouple measurements taken at different heights above and below the hotspot. Black curve is a polynomial fit to the thermocouple data. The arrows indicate the direction that experimental samples traveled using the displacement method. (B) Digitally logged thermocouple measurements during experimental cooling. Red diamonds indicate homogenization experiments that were drop quenched directly from the furnace hotspot.
Green circles are experiments that were cooled using the Type-B control thermocouple. Blue squares are displacement method experiments.
The red X on SF6 indicates that the inclusions in this experiment partially or nearly completely crystalline. The ambient T in the furnace at the final position of the quench rod in experiment SF13 was 600 °C.
A
B
A
experimental inclusions that were analyzed for XANES are reported in Table 1; additional information on the HX experiments can be found in Chapter 1 (Saper & Stolper 2020).
Note that the diffusion model described in Chapter 1, given the cooling rates and large sizes of the melt inclusions, predicts that chemical gradients due to olivine crystallization on the inclusion walls were not expected to reach near the inclusion centers in these experiments, except possibly for experiment SF4. This is important because it allows for different cooling rates while holding the melt compositions roughly constant at the inclusion centers, where the XANES analyses were planned to be taken, in attempt to reduce systematic sources of error due to variable melt composition. Thus this experimental setup yields a means to vary ƒO2, cooling rate, and quench temperature in S-bearing olivine-hosted melt inclusions, while keeping the melt compositions at their centers constant; in theory this type of approach can be used to explore both the T-dependence and kinetics of the Fe-S electron exchange reaction. However, I note here that the quenched experimental inclusions had little if any sulfur present as S6+ (consistent with Jugo et al.
2010, but also possibly due to the low Fe3+/Fe2+ measured in the glasses compared to the nominal ƒO2 of the experiments, see section 4.4) and so any effects of S6+ reduction and Fe2+ oxidation (the inferred down-T direction of equation (1) by Nash et al. 2019) during cooling were likely to be small, if at all measurable. Cooling rate experiments at higher ƒO2 were planned, however in 2020 the CoVID-19 pandemic meant that the availability of the XANES beamline in the near future became unlikely (especially considering I had been piggybacking on other researcher’s sessions), and so in the interest of finishing my Ph.D.
those experiments were abandoned for the meantime. In retrospect, to test the results of Nash et al. (2019), the SF experiments should have been conducted at slightly higher ƒO2,
guided by Jugo et al. (2010), to ensure that S6+ was present – this is explored more fully in section 5.