in the genesis of andesite The role of H2O in the genesis of andesite Kayla Iacovino NASA Space Grant Symposium April 19th, 2008 Picture: Mt. Yasur Thanks to: Dr. Gordon Moore, Dr. Kurt Roggensack Dr. John Holloway, and Depths of the Earth Lab
Outline What is andesite, and why is it important? Can andesite originate in the mantle? Is H2O important? Experimental methods Results and implications
Defining andesite Andesite Type locality: Andes of S. America Intermediate volcanic rock (58-62 wt% SiO2) Associated with continental subduction zones
Subduction Zones and Andesites Water brought by subduction into the mantle.
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) Si rich = andesite. This observation is inconsistent with what we know about the chemistry. How can we explain this observation if we know that andesites are not in chemical equilibrium with mantle minerals?
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
Methods The Piston Cylinder Non-End Loaded “Sandwich” The Assembly The Capsule AuPd ~ 0.25’’ The Piston Cylinder
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%
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
Thank you