The role of H 2 O in the genesis of andesite

1. The role of H2O in the genesis of andesite Kayla Iacovino NASA Space Grant Symposium April 19th, 2008 Thanks to: Dr. Gordon Moore, Dr. Kurt Roggensack Dr. John Holloway, and Depths of the Earth Lab

2. Outline • What is andesite, and why is it important? • Can andesite originate in the mantle? • Is H2O important? • Experimental methods • Results and implications

3. Defining andesite • Andesite • Type locality: Andes of S. America • Intermediate volcanic rock (58-62 wt% SiO2) • Associated with continental subduction zones

4. Subduction Zones and Andesites

5. Andesites from the mantle? • Typically, mantle magmas are basaltic (<52 wt% SiO2) • Silica-rich magmas not typically in chemical equilibrium with mantle minerals (olivine, orthopyroxene) • Evidence for mantle andesite • Andesite composition lava found with mantle xenoliths (Blatter & Carmichael, 1998) How can we explain this observation if we know that andesites are not in chemical equilibrium with mantle minerals?

6. Hypothesis • Water effects the andesite magma chemistry such that it can be in equilibrium with mantle minerals Mg2SiO4 + SiO2 = Mg2Si2O6 • Evidence • Hydrous minerals (hornblende) in xenoliths • Water brought by slab during subduction • Test hypothesis with high-pressure and temperature experiments (500MPa, 1190°C) Olivine melt Pyroxene

7. Methods The Piston Cylinder Non-End Loaded “Sandwich” The Assembly The Capsule AuPd ~ 0.25’’ The Piston Cylinder

8. Results • Several experiments with varying water contents • H2O contents estimated from probe totals melt opx bread xtallizing opx opx xtallizing ol melt 950x 350x Low H2O ~1 wt% High H2O ~3 wt%

9. Implications • Water does affect mineral-melt equilibrium • Crystallizing phases change with varying water content • Low water content experiments show no signs of crystallizing olivine • Presence of H2O (≥~3 wt%) can explain equilibrium between an andesitic melt and mantle minerals

10. Thank you