Transcript Reconciling geophysical and geochemical data to

Understanding craton formation
through their geochemical and
geophysical characteristics
A Preliminary Report
CIDER 2012 Lithosphere Group
Presenters: Huaiyu Yuan & Pierre Bouilhol
Group: Cathleen Doherty, Erica Emry, Beth Paulson,
Mingming Li, Doug Wiens
Getting Started….
• History of the project
• Who’s involved
– 8 members of the group
– 4 geophysicists, 3 geochemists, 1
geodynamicist
Goals of Project
• First order observations:
– Layered vs. non-layered, corresponding to differences in
composition.
– Why are these cratons are so different? but still all cratons?
• 3 Cratons
– try to bring together existing geophysical observations
– new geophysical evidence where needed (Beth SRF study)
– try to add existing geochemical database of xenoliths and crustal
rocks.
• Main goal: understand the differences between cratons,
which would ultimately help us to better understand their
formation…
Focus Sites
• Three cratons:
– Slave
– Kaapvaal
– North Atlantic
• Slave & Kaapvaal are both well-studied,
lots of geophysical & geochemical
datasets
• North Atlantic has some xenolith data, but
few geophysical experiments
Slave
Dipping Upper mantle reflector at 100 km indicates fossil subductions
Bostock 1998
Slave
P-wave receiver functions
Electrical Resistivity
Chen et al. 2009; Jones et al. 2003
o
o
Spatial overlap of velocity discontinuity (left; from receiver functions) and
conductive anomaly (right; from Magnitotelluric studies) indicates paleosubduction interface
Other receiver functions see the boundary too (et. Abt et al. 2010; Miller et al
2011; Yuan et al 2006)
Slave
Shallow Anisotropy shallow “red” layer = highly depleted chemical layer (Mg# 92%)
Slave Craton
Griffin et al. Lithos, 2004
Yuan and Romanowicz 2010
Slave
• To add:
– Ages
– More constraints on error/uncertainty
– More geochemical data
Slave
Slave
fossil subductions indicated by
“dipping upper mantle reflectors”
from LithoProbe project in
many places
van der Velden and Cook JGR 2005
Subduction trench (suture) parallel
van der Velden and Cook JGR 2005
=
Shallow Anisotropy Direction
Yuan et al. 2011
North Atlantic:
• New data from Receiver Function
North Atlantic Craton
Limited # of SRFs: indicating presence of layering in the shallow upper mantle around 100 km depth.
Shear-wave receiver functions in the North Atlantic craton
North Atlantic Craton
Chemical Layering from olivine Mg #: shallow, highly depleted ver. bottom less depleted
North Atlantic craton is consistent with North American craton in general
• N. Atlantic Craton seems to show
geophysical similarities with Slave craton
Kaapvaal
• Kaapvaal is
different from
Slave and North
Atlantic craton
• No evidence for
layering within
the lithosphere
Kaapvaal
Group 2
Group 1
Yuan & Romanowicz, AGU 2012 DI21A-2352
Kaapvaal
Group 2
Anisotropy Direction
Group
1
Shear Velocity Variation
Yuan & Romanowicz, AGU 2012 DI21A-2352
Kaapvaal
• No obvious conductivity
layering in Kaapvaal
Evans et al, JGR, 2011
Kaapval
• Shear velocity w/ depth,
no indication of layering
• MT also no indication of
layering (not
pictured…yet)
• Mg # changes
significantly at ~175 km
• Receiver function at
170-180 km (Hansen et
al, 2009; Kind et al,
2012 AGU)
Kaapvaal
• Working hypothesis for Kaapvaal
Slave & North Atlantic?
Kaapvaal?
Lee, Annu. Rev. Earth Planet. Sci. 2011
Working hypothesis for Slave
Ongoing Work….
• Refine Geophysical data, include similar
datasets/observations for the three cratons
more receiver functions can be added
• Significant work for geochemistry
– Compile Re/Os for the lithosphere, compare
it with TDM from Sm/Nd and Hf of the crust.
– A closer look to the C and O isotopic
composition of the diamonds.
Numerical modeling: how does
subduction influence stability of
Craton roots?
• Motivation: dehydration of slabs releases water,
which is carried further away with regional
convection and influences the rheology and
composition of Craton roots.
Stable Craton
Destroyed Craton
Thank you