The Transition Zone and Uppermost Lower Mantle in

Outline • All about water – Quick review of water in mantle – New

The Transition Zone • Melting at 410 km depth – Slow Layer Upon the

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

Bercovici and Karato 2003 • Upwelling “wet” transition zone mantle melts upon passage through

Limits… Transition width is limited by melt production

0° Dihedral Angle • Melt and olivine-basalt aggregate at 1 Gpa (Cmiral et al.

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

Strong 520/Weak 410 • 520 -km discontinuity: – Wadsleyite to Ringwoodite transition • Narrower,

Another Strong 520 -km Strong 520 coincident with LVL

Farallon Slab? Grand et al. , 1997 @ 30° N

Mid-Mantle Reflectors Grand et al. , 1997

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

Conclusions • SLUTZ: – Marker of locally wet transition zone – Slab-displaced TZ material

Conclusions • Frequent lower mantle reflectors – Associated with mid-mantle slabs – Signal of

Slides: 51

Download presentation

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

Sc. S Reverberation Method

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

Sc. Sn 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 410 • 520 -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. , 1997 @ 30° N

Mid-Mantle Reflectors

East-West Path

North-South Path

Mid-Mantle Reflectors

Mid-Mantle Reflectors Grand et al. , 1997

Sc. Sn Coverage

Two Mid-Mantle Reflectors

Paths in Coral Sea

Two Mid-Mantle Reflectors

“ 850 -km” Reflector

“ 1100 -km” Reflector

Hall and Spakman, 2002

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

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?