17:15 〜 18:45
[SGC32-P03] Hydrogen isotope measurements of basaltic glass with relatively low water content
キーワード：玄武岩ガラス、水素同位体比
Hydrogen isotope (D/H) ratios of volcanic glasses provide important constraints on the behavior and origin of water in the Earth’s interior. As water content of the many undegassed basaltic glass is > 0.5 wt%, Shimizu et al., (Geochemical Journal, v53, p195-, 2019) developed the rapid measurement (~6 minutes duration) of D/H ratios of a basaltic glass of H2O > 0.5 wt% within 10‰ errors (2sigma) with a Faraday cup and an electron multiplier (FC-EM) multi-detector setting by SIMS (CAMECA IMS-1280HR) at Kochi, JAMSTEC. In this study, we modified this method to determine D/H ratios of basaltic glass with a low water content of < 0.1 wt%.
A 20 keV and ~ 1nA defocused Cs+ ion beam of ~15 µm diameter was used. Negative secondary ions of 16OH- and 16OD- were measured by two electron multiplier (EM) detectors at an off-axis and an axial position, simultaneously. The mass resolving power of the off-axis EM was ~5000 with a slit of 250 µm in width to distinguish 16OH- from 17O-. For the axial EM, the mass resolving power was ~10000 to separate 16OD- from 18O-, 17OH-, and 16OHH-. After 20 s pre-sputtering and ~120 s auto-centering of 16OH- to the field and contrast apertures, 5 sec. × 300 cycles measurements were conducted (total analysis time for one spot is ~28 min). The reproducibility of the working standard EPR-G3 (a MORB glass, H2O= 0.22 wt%, dD=-65±5‰) was ~10‰ (2 sigma), comparable to the in-run precision of 2SE. EPR-G3 was analyzed 2 times for every 5, 6 samples. The OD/OH IMF and analytical error of 2 SD were calculated by averaging 8–10 data of EPR-G3. In the present condition, the D/H ratios of basaltic glass with 0.05 wt% water could be measured within 18‰ error (2SE). We also confirmed that D/H ratios of basaltic glasses with water contents of 0.1 to 0.3 wt% and dD of -120 to -40 ‰ determined by EM-EM multi-detector setting agreed with those by FC-EM multi-detector setting within errors.
A 20 keV and ~ 1nA defocused Cs+ ion beam of ~15 µm diameter was used. Negative secondary ions of 16OH- and 16OD- were measured by two electron multiplier (EM) detectors at an off-axis and an axial position, simultaneously. The mass resolving power of the off-axis EM was ~5000 with a slit of 250 µm in width to distinguish 16OH- from 17O-. For the axial EM, the mass resolving power was ~10000 to separate 16OD- from 18O-, 17OH-, and 16OHH-. After 20 s pre-sputtering and ~120 s auto-centering of 16OH- to the field and contrast apertures, 5 sec. × 300 cycles measurements were conducted (total analysis time for one spot is ~28 min). The reproducibility of the working standard EPR-G3 (a MORB glass, H2O= 0.22 wt%, dD=-65±5‰) was ~10‰ (2 sigma), comparable to the in-run precision of 2SE. EPR-G3 was analyzed 2 times for every 5, 6 samples. The OD/OH IMF and analytical error of 2 SD were calculated by averaging 8–10 data of EPR-G3. In the present condition, the D/H ratios of basaltic glass with 0.05 wt% water could be measured within 18‰ error (2SE). We also confirmed that D/H ratios of basaltic glasses with water contents of 0.1 to 0.3 wt% and dD of -120 to -40 ‰ determined by EM-EM multi-detector setting agreed with those by FC-EM multi-detector setting within errors.