Vanished diamondiferous root beneath the Southern

Superior Province

Christine MillerMaster’s Candidate, UBC

Maya KopylovaDepartment of Earth and Ocean Sciences, UBC

John RyderDianor Resources Inc.

Outline

• Study Area• Samples/Methods• Results:

• Carbon Isotopes • Inclusion Chemistry• Thermobarometry

• Origin of Diamonds• Thermal Regime• Destruction of the Diamondiferous Root

Study Area

(Kopylova et al, 2011)

(Kaminsky et al, 2002)(Heaman and Kjarsgaard, 2000)

Samples/Methods: Diamonds

65 diamonds of variable size, color, resorption and morphology

Analysis:

• Carbon Isotopes

Results: Carbon Isotopes

Mantle range

Metaconglomerate Diamonds (N=14)

Eclogitic

Peridotitic

• 1-21 inclusions in each diamond (avg. 5)• Colors: purple, colorless, brown/

black • <100-500m in size• Morphology dominantly diamond controlled

Analysis:

• Polishing to expose inclusions

• Electron microprobe

Samples/Methods: Inclusions

Results: Inclusion Chemistry

• Polished and exposed inclusions in 46 diamonds

• Microprobe analyses of 173 inclusions

Garnet

(N=19)

(Gurney and Zweistra 1995; Grutter et al. 2006)

Harzburgitic Lherzolitic

Chromite

(N=94)(Gurney and Zweistra 1995; Sobolev et al 2004)

Average FeO ~ 14 wt%

Olivine/Orthopyroxene

Average Al2O3 ~0.6 wt%OPX

OLV

Mineral Equilibration

• Mg# orthopyroxene (94) > Mg# olivine (93)

• Low Al content in orthopyroxene (>1.5 wt%) = garnet peridotite

• High Fe in chromite = garnet peridotite

Mineral phases are well equilibrated and suitable for

thermobarometry

Origin within garnet facies peridotite (i.e. spinel-garnet

or garnet only)(Brey and Kohler 1990; Boyd et al. 1997)

Results: ThermobarometryThermometers: • O’Neill and Wood (1979): garnet-olivine/ 1055-1232°C @ 50 kbar• Ryan et al. (1996): Zn-in-chromite/ 993-1558°C

Barometers:• Grutter et al. (2006): 35-49 kbar (41 mW/m2)• Girnis and Brey (1999): 55-58 kbar @ 1000-1100°C

Sample Wsc13:

• garnet-olivine-orthopyroxene• 9 PT pairings: 5 thermometers, 2 barometers

(Gurney and Zweistra 1995; Grutter et al. 2006)

(Kennedy and Kennedy, 1976; O’Neill, 1981; Rudnick et al, 1998; Girnis and Brey, 1999)

39

41Minimum

LithosphericDepth

• Dominantly octahedral morphology/ peridotitic minerals

• Mineral chemistry combined with carbon isotopes = depleted Harzburgite host

• Cool thermal regime (39-41 mW/m2) and deep LAB (~190 km)

Tectonic Origin/ Thermal Regime

Origin in Pre-2.7 Ga Cratonic Root

(Stachel and Harris 2008)

Proterozoic Kimberlite• Barren kimberlite ~50 km NE of metaconglomerate

• 1.1 Ga (Kaminsky et al., 2002)

Jurassic Kimberlite• Max diamond grade ~0.02 ct/t (Brummer 1992, Vicker, unpublished data)

• ~156 Ma, (Heaman and Kjarsgaard 2000)

Present Day

T @ Moho

Archean: 39-41 mW/m2

LAB ≥190 km

Jurassic: 42-44 mW/m2

LAB ~145 km

Current: 41-42 mW/m2

LAB <150 km

Archean: 39-41 mW/m2

LAB ≥190 km

Proterozoic: 45-46 mW/m2

LAB ~150 km

Current: 41-42 mW/m2

LAB <150 km

Thermal Evolution/Lithosphere Thickness

Wawa Opatica

Heating

(Faure et al. 2011)

Destruction of the diamondiferous cratonic root

WA

OP

Dharwar Craton

(Kumar et al. 2007)

(Zhang et al. 2011)

North China Craton

Conclusions:

1. Harzburgitic mantle host

2. Cool, deep Archean lithosphere in the diamond stability field prior to 2.7 Ga

3. Minor heating of the mantle lithosphere from the Archean to present day

4. Thinning of the Southern Superior lithospheric root removing it from the diamond stability field

(Faure et al. 2011)

Current Thermal State

• Archean average (41 mW/m2), Superior average (42 mW/m2)

•Variable heat flow measurements within subprovinces• Heat flow not affected by crustal thickness or age, only composition (Mareschal et al. 2000)

• Remove crustal component to get mantle heat flow

4828

4246

Dharwar Craton, India

• Past lithospheric root in the DSF• Partial destruction/thinning• Modern seismic studies reveal lithospheric thickness of ≤100 km or less

(Kumar et al. 2007)

North China Craton

• Weakening of lithosphere through subduction-related hydration

• Lithospheric folding during Mesozoic causes dripping or delamination of weakened lithosphere

(Zhang et al. 2011)

(Kusky et al. 2007)

• Complete removal of lithospheric root beneath Eastern Block

(Kennedy and Kennedy, 1976; Rudnick et al, 1998; Girnis and Brey, 1999)