Southeastern Section - 67th Annual Meeting - 2018

Paper No. 21-5
Presentation Time: 9:20 AM


ASHLEY, Aaron Wolfgang, School of the Earth, Ocean, and Environment, University of South Carolina, 701 Sumter Street, EWS 617, Columbia, SC 29208, BIZIMIS, Michael, School of Earth, Ocean and the Environment, University of South Carolina, Columbia, SC 29208, PESLIER, Anne H., Jacobs Technology, JETS, NASA-Johnson Space Center, Houston, TX 77058 and JACKSON, Matthew G., Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106

Water is highly influential in Earth’s mantle, affecting key geodynamic processes like melting, rheology, electrical and thermal conductivity, and seismic wave propagation. Critically, its distribution in the oceanic upper mantle is mostly known indirectly from mantle melts (e.g., erupted lavas and melt inclusions), whereas data collected directly from mantle samples (peridotites or pyroxenites) is limited2,3. Generally, MORB are “drier” than OIB, while OIB water concentrations are variable down to individual hotspot scales. This indicates heterogeneous water concentrations in the mantle, likely controlled by recycling. Mantle xenoliths provide a more direct way of sampling the upper mantle, having equilibrated deeper in the lithosphere. Here we present water concentration data (by FTIR) on minerals from peridotite xenoliths from Savai’i of the Samoan hotspot. Carbonatitic metasomatism is thought to have affected these samples1, allowing investigation into the effect of carbonatitic metasomatism on the water distribution of the lithosphere. The olivine (OLIV) and orthopyroxene (OPX) have high Mg#s (~0.915 and ~0.922) and spinel (SP) has high Cr#s (up to 0.602) suggesting high degrees of melt depletion, consistent with near complete absence of clinopyroxene. Polarized FTIR spectra reveal low water contents in OPX and OLIV (24-89 and 0-3 ppm weight H2O respectively) relative to xenoliths from the Hawaiian hotspot, consistent with high degrees of melt. Detailed microprobe analyses in SP-PYX-OLIV symplectites show low Mg# values (~0.840 in OLIV and ~0.870 in OPX) suggesting reaction with melts and consistent with metasomatism. FTIR spectra also reveal CO2 peaks in OLIV and OPX inclusions, qualitatively consistent with carbonatitic metasomatism. Our data support a proposed link between high CO2 – low H2O activity during carbonatitic metasomatism in the lithosphere4, consistent with the presence of sparse apatite and absence of hydrous phases1.


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  2. Peslier, A. H., & Bizimis, M. (2015). Geochemistry, Geophysics, Geosystems, 1-22.
  3. Warren, J., & Hauri, E. (2014). Geophys. Res. Solid Earth, 119, 1851-1881. doi:10.1002/2013JB010328
  4. Yang, X., et al. (2014) Earth Planet. Sci. Lett. 403, 37-47.