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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
Samples/Methods: Diamonds
65 diamonds of variable size, color, resorption and morphology
Analysis:
• Carbon Isotopes
• 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
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Jurassic Kimberlite• Max diamond grade ~0.02 ct/t (Brummer 1992, Vicker, unpublished data)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
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