Hydrogen content

Hydrogen contents of the silicate glasses and quenched sulfide liquids was subsequently determined with elastic recoil detection analysis (ERDA), at the nuclear microprobe at CEA Saclay, France. Details of this method can be found in section 2.2.5. For an ERDA analysis, great care must be taken with sample preparation. Samples must be mirror-polished; due to the extreme tilt angle required for analysis (75^◦), any topography of the sample could block the ejected protons from reaching the detector. In addition, epoxy, glues, and most solvents contain H and so care must be taken to ensure that no contam-ination can be introduced to the sample from these sources. For this study, samples were embedded in indium and then mounted in epoxy. The indium thus provided a 1-3 mm buffer zone between the sample and the epoxy, en-suring that the analysis results would not be contaminated with a signal from the epoxy.

As described in section 2.2.5, the nuclear microprobe uses a focused beam of ⁴He⁺ for the analysis. Two types of measurements are required. For the first, the incident beam is normal to the sample and chemical data is collected with the particle induced X-ray emission (PIXE) and Rutherford backscatter (RBS) detector. The RBS data also includes information on the charge on the samples surface, which is directly proportional to the number of incident particles, which is important for the interpretation of the ERDA spectrum. The second measurement is made with the incident beam at 75^◦ to the sample, which is necessary to allow the ejected protons to be collected by the detector (see figure 2.8).

The beam was scanned over the sample to create maps, which were 50×50 µm², except in one case which was 30×50 µm², in order to obtain spectra that are a more statistical representation of the bulk sample. For most samples separate maps were taken of both the silicate glass and the sulfide, but in two

cases maps spanning both phases were taken, and the appropriate regions were extracted from the maps during data analysis (see figure 5.6). ERDA data acquisition for sample V1045b was not possible, due to surface irregularities of the sulfide.

For analysis, the software RISMIN (Daudin et al., 2003) was employed to ex-tract the RBS and ERDA spectra from the data (this process is discussed further in section 2.2.5 and Bureau et al., 2009). These spectra were then processed with the software SIMNRA (Mayer, 1999). Table 5.6 contains the normalised atom % compositions, including H, of all analysed samples.

Figure 5.6: The steps of processing ERDA data. (A) Backscattered electron image, with the region that was analysed highlighted. (B) X-ray map of sil-icon, clearly showing the two phases. Outlined is the area from which the RBS and ERDA spectra were extracted for the silicate. (C) RBS spectrum of of the silicate glass. The edges result from the different elements, but the overall intensity is a direct function of the number of incident particles. (D) ERDA spectrum, used to determine the number of protons ejected from the sample.

Table 5.6:Atom % composition

1043a V1043b V1044a V1044b V1045b V1054 silicate SiO2 47.27 45.82 47.21 44.96 49.30 51.84

TiO2 1.13 1.05 1.13 1.08 1.25 1.23

Al2O3 8.22 7.80 8.10 7.79 8.57 8.91

FeO 4.06 9.13 7.17 14.27 13.67 9.09

MgO 24.33 22.30 21.80 18.32 12.30 13.64 CaO 12.00 11.12 11.65 10.77 11.85 12.30

Na2O 2.76 2.53 2.68 2.51 2.79 2.69

SO3 0.36 0.48 0.44 0.75 0.51 0.35

NiO 0.006 0.002 0.006 0.008 0.004 0.006

V2O3 0.22 0.23 0.24 0.24 0.26 0.27

MoO3 0.01 0.01 0.01 0.04 0.01 0.01

H 1.13 2.07 0.93 1.61 1.74 1.57

sulfide Si 0.16 0.02 0.06 0.05 0.06 0.05

Fe 43.21 47.16 44.92 46.43 47.46 46.01

S 48.89 45.32 46.83 43.74 43.94 48.53

O 2.24 5.63 4.17 7.47 6.01 3.27

In order to provide an independent evaluation of the ERDA results with a more conventional technique, the water contents of the silicate glasses was also determined with fourier-transform infrared spectroscopy (FTIR) at the Bayerisches Geoinstitut. Absorption peaks resulting from the stretch of the O-H bond, observed atλ= 2.8µm, for the glasses were used to calculate water contents using the method described in Stolper (1982):

c= 18.02A dρε

where cis the weight fraction of water, A is the height of the absorption peak, d is the thickness of the sample (cm, samples were thinned to 100 µm), ρ is the density of the sample (estimated at 3000 g/l), and ε is an extinction coeffi-cient, determined by Stolper (1982) to be 67 l/mol cm. The calculated water contents are in table 5.7. The results are in good agreement with the water

content of the glasses determined by ERDA, as is shown in figure 5.7. The only sample for which the two methods do not agree is V1054, for which FTIR returns a much larger value. It is possible that this is a result of accidental surface contamination of the sample during storage or preparation for FTIR measurements.

Table 5.7: FTIR-determined water content (wt ppm H₂O) compared to ERDA results

Sample Peak height FTIR ERDA

V1043a 0.5060 4725 4856

V1043b 0.912 8516 8760

V1044a 0.487 4547 3925

V1044b 0.780 7284 6590

V1045b 0.887 8283 7165

V1054 1.145 10692 6618

Figure 5.7:Weight ppm of H₂O in the silicate glasses of the samples as deter-mined by both ERDA and FTIR. In general there is good agreement between the two methods, except for sample V1054, for which the value determined by FTIR was much higher than the ERDA result.

The hydrogen contents of the sulfides are plotted in figure 5.8, as a function of oxygen fugacity, Fe/S ratio of the sulfide, FeO content of the coexisting silicate,

and pressure. For comparison, the data of Clesi et al. (2018) obtained on quenched metallic liquids at pressures between 5 - 21 GPa and temperatures between 2020 - 2775 K is also plotted. Although the concentrations of H vary up to close to 1 atomic % there does not appear to be a consistent trend that explains this variation. Although there might appear to be a weak fO2

dependence, this does not persist once the partitioning with the silicate melt is also considered, as discussed later. The amount of H measured in the sulfide is similar to that found in quenched iron melts by Clesi et al. (2018) at similar conditions.

Figure 5.8: Mole fraction of hydrogen in the sulfide as a function of Fe/S ratio in the sulfide, fO2, pressure, and mole fraction FeO in the coexisting silicate. Data from metal/silicate partitioning experiments (Clesi et al., 2018) is shown for comparison. For all plots, the diamond symbol denotes the experiment with greater bulk water and a lower temperature equilibration.

Triangle symbols denote a data point from a separate study, performed at 3 GPa and 1823 K.