1. 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

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
Water brought by subduction into the mantle.

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)
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?

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