Understanding craton formation through their geochemical and

geophysical characteristicsA 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

Bostock 1998

Dipping Upper mantle reflector at 100 km indicates fossil subductions

Slave

o Spatial overlap of velocity discontinuity (left; from receiver functions) and conductive anomaly (right; from Magnitotelluric studies) indicates paleo-subduction interface

o Other receiver functions see the boundary too (et. Abt et al. 2010; Miller et al 2011; Yuan et al 2006)

Chen et al. 2009; Jones et al. 2003

P-wave receiver functions Electrical Resistivity

Griffin et al. Lithos, 2004

SlaveShallow Anisotropy shallow “red” layer = highly depleted chemical layer (Mg# 92%)

Slave Craton

Yuan and Romanowicz 2010

• To add:– Ages– More constraints on error/uncertainty– More geochemical data

Slave

Slave

Slave

van der Velden and Cook JGR 2005

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 = 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

Group 1

Group 2

Kaapvaal

Yuan & Romanowicz, AGU 2012 DI21A-2352

Group 1

Group 2

Kaapvaal

Anisotropy Direction

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 SlaveWorking 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