4:00 PM - 4:15 PM
[SGC37-08] Volatiles in anhydrous mantle minerals from French Polynesian xenoliths
Keywords:mantle xenolith, volatiles
We measured volatile (H2O, F, S and Cl) and P2O5 contents of anhydrous minerals of olivine, orthopyroxene and clinopyroxene from French Polynesian mantle xenoliths using SIMS (CAMECA IMS-1280HR) at KOCHI, JAMSTEC. The mantle xenoliths from Tahiti and Moorea include 16 samples of harzburgite, 2 samples of pyroxenite, 4 samples of wehrlite and 22 samples of dunite. We particularly cared for measuring trace amount of H2O in these anhydrous minerals. The polished sample sections were embedded in indium metal; samples were kept in the sample chamber of SIMS for at least one week before the measurement; and the pressure of the analysis chamber was kept < 8 × 10-8 mbar by a cryo-pump to decrease the volatile background. We also embedded San Carlos olivine (~1 ppm H2O) in all sample mounts to monitor water background and reproducibility of signal intensity ratios (i.e., 12C/30Si, 16OH/30Si, 19F/30Si, 31P/30Si, 32S/30Si and 36Cl/30Si) in each sample. In this study, CO2, S and Cl contents of all minerals are close to their detection limit (2ppm for CO2, 0.02 ppm for S and Cl), so we used them to detect the surface contamination during measurement. The effective limits of quantification are ~1 ppm for H2O and ~0.2 ppm for fluorine.
H2O and F contents of olivine (N=243) are 0 ppm to 27 ppm and 0.1 ppm to 3.5 ppm, respectively. H2O and F contents of orthopyroxene (N=60) are 11 ppm to 260 ppm and 0.5 ppm to 107 ppm, respectively. H2O and F contents of clinopyroxene (N=73) are 18 ppm to 800 ppm and 2 ppm to 230 ppm, respectively. Olivine, orthopyroxene and clinopyroxene with H2O >10 ppm, >150 ppm and >300 ppm, respectively are mostly from dunite, pyroxenite or wehrlite samples and seemed to be hydrated by secondary processes, presumably reaction with magma (as their Mg# of minerals are ~80-84). Excluding these high water data, H2O and F contents of minerals broadly have positive correlation with the H2O/F ratios of ~4.
Shimizu et al. (Chemical Geology, 2019, v522, p283-) showed strong linear correlation between H2O and F contents of un-degassed, deep-marine basaltic glasses and indicated that this trend can be the mixing trend of depleted MORB upper mantle (H2O/F ~10) and FOZO deep mantle (H2O/F ~18). They also showed that oceanic island basalts (OIBs) with contribution of dehydrated recycle materials (i.e., HIMU, EM I and EM II) are enriched in F and have low H2O/F ratios (4 to 10). The H2O/F ratios of ~4 obtained in this study are significantly lower than those of MORB source mantle and are in the range of OIBs contributed by recycle materials. This study showed that the original ratio of H2O/F may be preserved in minerals in mantle xenoliths.
H2O and F contents of olivine (N=243) are 0 ppm to 27 ppm and 0.1 ppm to 3.5 ppm, respectively. H2O and F contents of orthopyroxene (N=60) are 11 ppm to 260 ppm and 0.5 ppm to 107 ppm, respectively. H2O and F contents of clinopyroxene (N=73) are 18 ppm to 800 ppm and 2 ppm to 230 ppm, respectively. Olivine, orthopyroxene and clinopyroxene with H2O >10 ppm, >150 ppm and >300 ppm, respectively are mostly from dunite, pyroxenite or wehrlite samples and seemed to be hydrated by secondary processes, presumably reaction with magma (as their Mg# of minerals are ~80-84). Excluding these high water data, H2O and F contents of minerals broadly have positive correlation with the H2O/F ratios of ~4.
Shimizu et al. (Chemical Geology, 2019, v522, p283-) showed strong linear correlation between H2O and F contents of un-degassed, deep-marine basaltic glasses and indicated that this trend can be the mixing trend of depleted MORB upper mantle (H2O/F ~10) and FOZO deep mantle (H2O/F ~18). They also showed that oceanic island basalts (OIBs) with contribution of dehydrated recycle materials (i.e., HIMU, EM I and EM II) are enriched in F and have low H2O/F ratios (4 to 10). The H2O/F ratios of ~4 obtained in this study are significantly lower than those of MORB source mantle and are in the range of OIBs contributed by recycle materials. This study showed that the original ratio of H2O/F may be preserved in minerals in mantle xenoliths.