5.5 Discussion
5.5.2 Hydrogen partitioning
The partitioning of hydrogen between the sulfide and silicate melt is quantified by the molar partition coefficient, defined as
Dsulf/silicate
H = XHsulfide
XHsilicate
where XHis the mole fraction of H in the silicate or sulfide. In this designation no inference is made concerning the nature of the H species, which is certainly H2O in the silicate melt and may well also be in the sulfide. In this study, this value ranged between 0.1 and 0.4, suggesting nominally lithophile, rather than chalcophile behaviour, and showing little variation with oxygen fugacity or bulk water content. In figure 5.8, for the sulfide experiments conducted for this study, there does appear to be a slight increase in hydrogen content of the sulfide as oxygen fugacity decreases. When the partition coefficient is considered, however, that dependence disappears and DH appears relatively constant with fO2 (figure 5.12). The lack of fO2 dependence also makes it difficult to determine whether H or H2O is present in the sulfide phase. A lack of any dependence also raises questions as to whether the entire H content of the sulfide was captured by the quenched assemblages or whether H or H2O could have separated from the sulfide melt during quenching.
Figure 5.12: Silicate/sulfide partition coefficient for hydrogen determined for this study (blue symbols) plotted with those determined for metal/silicate (red squares, Clesi et al., 2018), including one sulfide (red circle). Diamond symbol denotes experiment with greater bulk water content added and run at a 1573K rather than 1673 K. Triangle denotes experiment from a separate study run at 3 GPa at 1823 K.
Experiments to investigate whether hydrogen could be an important light ele-ment in the core have suggested that at high pressures (above 3 GPa) a signif-icant amount of hydrogen could dissolve into metallic iron (Fukai and Suzuki, 1986; Okuchi, 1997; Iizuka-Oku et al., 2017). These partitioning experiments, however, were performed with starting materials that contained water con-tents of several weight percent, creating very high water fugacities within the experimental charge and thereby saturating the iron. In these experiments, the exsolution of hydrogen during quenching creates bubbles in the resulting metals. By determining the volume of these bubbles and using estimates of the hydrogen equation of state, it is possible to determine the original melt hydrogen contents. In the study of Okuchi (1997), for example, these reach approximately 40 atom % of the metal. Using X-ray diffraction and an esti-mate of the effect of H on the volume of FeS liquid, Shibazaki et al. (2011)
estimated that 9−10 mole percent H may enter the structure of solid FeS at 3 GPa. It must be appreciated however, that there are large uncertainties in these determinations. There is the potential for significant amounts of H to enter these phases but to be mainly lost during the crystallization that occurs on quenching.
Following this reasoning, the recent work of Clesi et al. (2018) determined metal/silicate partition coefficients for hydrogen using a similar procedure as the one described here, employing H contents that are more applicable to the plausible H2O content in a bulk silicate earth magma ocean, i.e. < 3000 ppm (Marty, 2012). Their resulting partition coefficients are plotted alongside those determined for a sulfide for this study in figure 5.13. As in the current experiments, it would seem that there is a finite and non trivial amount of H in both metal and sulfide. The idea is that by lowering the bulk H content of the experiment it may be possible to reach a metal or sulfide H content that can be entirely preserved in the crystallized solid phase. In general, there is good agreement between the datasets for both metal and sulfide, indicating that hydrogen partitioning between a silicate and a sulfide is similar to that between a silicate and metallic iron. Both suggest lithophile behaviour for hydrogen, particularly at lower pressures.
Figure 5.13:logDH between sulfide/silicate (this study, blue circles) and iron metal/silicate (Clesi et al., 2018, red squares). There is general agreement between the datasets; one of the data points from the previous study was a sulfide, rather than Fe metal (red circle). Diamond symbol denotes experi-ment with a higher added water content and run at 1573K, triangle denotes experiment from a separate study run at 1823 K.
It is possible that more hydrogen could have been dissolved in the sulfide during the experiment, and was lost upon quench crystallisation. Wykes and Mavrogenes (2005) used the depression of melting to propose H2O dissolved in sulphide melts, whereas at lower oxygen fugacities Buono and Walker (2015) proposed that H dissolved in them based on the same argument. Similarly, Shibazaki et al. (2011) proposed that 9−10 mole percent H may enter the structure of solid FeS at 3 GPa, based on X-ray diffraction measurements.
Our results allow only a minimum possible H content of the sulphide to be determined. Loss of H or H2O on crystallisation cannot be excluded. As ex-plained previously, the H contents vary between 0.2 and 0.7 atomic %, but over this range show no correlation with other potentially controlling parame-ters. The lack of variation in the H partition coefficient with fO2 also prevents the speciation of H in the sulphide from being assessed. This lack of correla-tion, which is also present in the results of Clesi et al., (2018) compounds the
suspicion that not all H was preserved.
Along the grain boundaries of the quenched crystalline monosulfide solid so-lution from our samples are crystals of pentlandite and FeSO. From textures and the Fe-Ni-S phase diagram, it is clear that MSS crystallized first and left a remaining melt which then crystallized pentlandite and FeSO. This melt might be expected to become concentrated in dissolved H or H2O. In some grain boundary areas, there are small void spaces, which may have been the result of a volatile-rich fluid phase exolving during quenching, as exhibited by H in metallic melts (Fukai and Suzuki, 1986; Okuchi, 1997). The voids are, however, relatively sparse and it is also possible that they are simply the result of some grains of pentlandite having been plucked and lost during sam-ple preparation (see figure 5.3). In addition, we note that the final quenched assemblages containing pentlandite are also rich in Pd from capsule contam-ination. H can dissolve readily in Pd, however there is insufficient Pd in the majority of the samples to account for the entire H content.
If the crystallised assemblage is capable of preserving 0.7 % H, then any par-tition coefficients based on sulphide H concentrations below this potential threshold might be considered to have preserved the entire sulphide H con-tent. However, here it must also be considered that the sulfide may contain both H and H2O at high temperatures, and only preserve the H2O or a com-ponent thereof. In a scenario where the bulk H2O content of the melt is raised until the entire H content of the sulphide cannot be preserved, then a plateau in Dshould be followed by a decrease inD. At H2O contents below the thresh-old, at which H can be preserved in the quenched sulphide, the plateau in D should occur due to the concentrations being below the Henry’s law limit.
Once the threshold is reached, however, D should drop, because H is being lost from the sulfide. Figure 5.14 does not reveal a plateau, but it does broadly indicate a drop in D as the melt H2O content increases, consistent with a threshold being overstepped. This would imply that the highest H partition coefficient obtained, i.e. ∼0.4, is still a minimum and further experiments at even lower H2O contents would be required to investigate if a plateau eventu-ally does arise. For this reason the results of Clesi et al., (2018) must also be considered with suspicion.
Figure 5.14: Sulfide/silicate H partition coefficient as a function of the melt H2O content. If the quenched sulfide reaches a threshold in the H that it can retain, then D should drop as the melt H2O content increases. This is in broad agreement with the data.