SIMS analysis and the hydrogen content in silicate melt

After recovering the DAC sample at ambient conditions, a cross-section at the center of the laser-heated portion was prepared parallel to the compression/laser-heating axis by using a dual-beam focused ion beam (FIB, Versa 3D^TM, FEI). Textural observation and preliminary compositional characterization were made with a field-emission-type scanning electron microscope (FE-SEM) and an energy-dispersive X-ray spectrometer (EDS) with a silicon drift detector (Bruker) in the dual-beam FIB system (Supplementary Fig. ³).

Subsequently the hydrogen content in a quenched silicate melt was determined with an isotope microscope system installed at the Hokkaido University, which is composed of CAMECA IMS-1270 SIMS and a two-dimensional ion detector, SCAPS (stacked CMOS-type active pixel sensor)⁴⁵. This system provides quantitative maps of secondary ions emitted from sample surface^46,47 (Fig. 1b), because the CMOS sensor exhibits a good linear relationship between an output voltage and the number of secondary ions⁴⁸). Therefore, we can quantify the abundance of each element from the intensity map.

Two-dimensional images of ¹H⁺, ²⁸Si⁺, and ⁴⁰Ca⁺ with 0.5–1.2 µm spatial resolution for ¹H⁺ were collected using the ¹⁶O⁻ primary beam (13 keV, 37 nA) that was focused to 20–30 μm in diameter and rastered across a 100 μm × 100 μm region on the sample surface. We set the contrast aperture to be 100 μm in diameter with the exit slit fully opened. In order to minimize the effect of adsorbed water on the sample surface, we employed the energy slit to be ±20 eV to select kinetic energy ranges of secondary ions from 80 to 120 eV by loading a sample offset voltage of −100 V. The pressure during measurements was 6.5–8.0 × 10⁻⁸ Pa. Secondary ion images of ¹H⁺, ²⁸Si⁺, and ⁴⁰Ca were obtained sequentially in the following order; ²⁸Si⁺, ¹H⁺, ²⁸Si⁺, ¹H⁺, ²⁸Si⁺, and ⁴⁰Ca⁺. Accumulation time was 500 and 100 s in each image, and two and three images were combined to calculate concentration maps for ¹H⁺ and ²⁸Si⁺, respectively.

Hydrogen concentration in the quenched silicate melt was quantified from the ¹H/²⁸Si intensity ratio using a calibration curve established by three silicate glasses with known H₂O concentrations (0.00–4.5 wt%)⁴⁹ (Supplementary Fig. ⁷). The analyses of these standards provide a linear relation between the count and known ¹H/²⁸Si ratios; a correlation coefficient R² = 0.996. The detection limit of H₂O is 15 ppm from its y-intercept. These standards were measured before, during, and after the analysis of each DAC sample. In order to reduce statistical errors for each DAC sample, regions of interest (ROIs) were selected on the quenched silicate melt. Combining the ¹H/²⁸Si intensity ratio with the Si content obtained by a field-emission-type electron probe micro-analyzer (FE-EPMA, see below), hydrogen (water) concentration in silicate melt was determined with ±2% to ±7% uncertainty, depending on its abundance (Supplementary Table ¹).