The Transition Zone and Uppermost Lower Mantle in Subduction Zones Justin Revenaugh Anna Courtier Geology and Geophysics University of Minnesota

Outline All about water –Quick review of water in mantle –New seismic constraints –Conclusions

Hirschmann 2006

Reverberative Interval

ScS Reverberation Method

The Transition Zone Melting at 410 km depth –Slow Layer Upon the Transition Zone –SLUTZ The 520-km story

ScS n Coverage

Paths Without LVL

Paths With LVL

LVL Geography Yes No

Problems! Melt layer is too thick! Not obvious upwelling zones

Bercovici and Karato 2003 Upwelling “wet” transition zone mantle melts upon passage through “410-km” discontinuity. Calculations ignore dynamics

Limits… Transition width is limited by melt production

0° Dihedral Angle Melt and olivine-basalt aggregate at 1 Gpa (Cmiral et al., 1998) Silicate melt and olivine at P > 7 to 8 GPa (Karato et al., 2005) Fully wetting thin films; gravitationally stable?

Wet But Not Too Wet If transition zones aren’t near capacity, is there anyplace that is?

Obayashi et al., EPSL 2006

Paths Examined

Paths With Melt

Melt Profiles

Courtier and Revenaugh, 2006

Hirschmann et al., 2006

Strong 520/Weak km discontinuity: –Wadsleyite to Ringwoodite transition Narrower, “brighter” when wet

Another Strong 520-km Strong 520 coincident with LVL

Niu and Kawakatsu, 1997

Niu, et al. 2002

Reverberation Paths

Path 2 Eastern US/Gulf of Mexico

Path 3 Mid-Continent/Gulf of Mexico

Farallon Slab? Grand et al., 30° N

Mid-Mantle Reflectors

East-West Path

North-South Path

Mid-Mantle Reflectors

Grand et al., 1997

ScS n Coverage

Two Mid-Mantle Reflectors

Paths in Coral Sea

Two Mid-Mantle Reflectors

“850-km” Reflector

“1100-km” Reflector

Hall and Spakman, 2002

Hall and Spakman, 2002

Shieh et al. [1998] Getting Water Into the LM

Conclusions SLUTZ: –Marker of locally wet transition zone –Slab-displaced TZ material pushed through 410 –Below maximum UM storage capacity. 520: –Another marker of locally wet TZ

Conclusions Frequent lower mantle reflectors –Associated with mid-mantle slabs –Signal of dewatering? –Decarbonatization? Lower mantle “wet pools” –Triggers for LIPs? –Agents of eustasy? –Chemical filters?