The use of boron nitride to impose reduced redox conditions in experimental petrology

Boron nitride (BN) is a commonly used pressure-transmitting material in experimental petrology. It is often considered to be as inert as MgO or Al (sub 2) O (sub 3) , and its redox potential is seldomly discussed. It is generally implied that, when used as a capsule sleeve, BN may impose relatively reduced conditions, similar to the effect of the fayalite-magnetite-quartz (FMQ) buffer. However, sediment melting experiments performed at 1050 degrees C and 3 GPa with BN as the capsule sleeve, produced a hydrous rhyolitic melt with dissolved H (sub 2) S and CH (sub 4) (Li et al. 2021). The resulting f (sub O2) estimate is significantly more reduced than that for the magnetite-wuestite (MW)-buffered experiment where H (sub 2) S and CH (sub 4) were undetected (Li et al. 2021), possibly to the extent of the quartz-iron-fayalite (QIF) buffered conditions produced when BN is used as a capsule or crucible (Wendlandt et al. 1982). To establish an explanation for such a discrepancy, we have conducted further investigation to better constrain the f (sub O2) imposed by BN, when used as a capsule sleeve. Here we report results on analyses of Fe content in Au capsules, a comparative experiment using a QIF buffer and an experiment with an Fe-(Mg,Fe) O sensor for direct analysis of f (sub O2) . The calibration of the equilibrium between FeO in melt and Fe in the Au capsule, from Ratajeski and Sisson (1999) appears to be inadequate in constraining f (sub O2) for our experiments. However, we were able to obtain Fe diffusion coefficients in Au from the Fe diffusion profiles observed in the capsule of the Fe-(Mg,Fe)O sensor experiment, and both the inner and outer capsules of the MW-buffered experiment, with resulting values of 1 X 10 (super -13) m (super 2) /s, 3 X 10 (super -14) m (super 2) /s, and 5 X 10 (super -14) m (super 2) /s, respectively. The QIF-buffered and Fe-(Mg,Fe)O sensor experiments provide several lines of evidence supporting the observation that BN imposes QIF-like redox conditions. First, the Fe-(Mg,Fe)O sensor returned an f (sub O2) value of QIF. Second, the "apparent" partition coefficients between FeO content in melt and Fe in the Au capsules are similar between the BN experiment and the QIF-buffered experiment. Third, we observe CH (sub 4) and H (sub 2) O peaks with similar intensities in the Raman spectra of melts from these two experiments, suggesting similar H (sub 2) and thus O (sub 2) fugacity. As our experiments were performed on a cubic press with the experimental assembly encased in a pyrophyllite cube, we interpret that the significantly reduced conditions imposed by BN are likely due to high H (sub 2) O activity maintained by dehydration of pyrophyllite, which can be explained using the reaction 2BN + 3H (sub 2) O = B (sub 2) O (sub 3) + N (sub 2) + 3H (sub 2) . Lower H (sub 2) O activity will reduce or inhibit the oxidation of BN and its f (sub O2) buffering ability. If heat-treated, BN acts as a highly efficient H (sub 2) barrier, as shown by Truckenbrodt et al. (1997). Through our efforts to determine the f (sub O2) imposed by using BN as a capsule sleeve in our experimental assembly, we are able to demonstrate the reducing ability of BN as an assembly component and, furthermore, shed light on the process by which BN imposes such reduced f (sub O2) . We hereby present what we have learned during the course of this investigation in the hope that the effect of BN on f (sub O2) control is both recognized and further exploited in future experimental studies.