the ascent of kimberlite: taxicabs for...
TRANSCRIPT
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THE ASCENT of KIMBERLITE:
Taxicabs for Diamonds
Professor Kelly Russell Volcanology & Petrology Laboratory
University of British Columbia, Vancouver
2012 Saturday Morning Lectures Series at TRIUMF
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ORGANIZATION
Diamond Basics
Mantle Basics
Kimberlite Basics
THE ISSUE
Answer
Proof of Concept
A NEW THEORY
QUESTION:
What Allows for Ascent of Diamondiferous Kimberlite?
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Diamond Basics
Carbon Polymorphs (C)
Graphite (Hexagonal)
Planar layered structure
Soft Mineral 2/10
Diamond (Cubic)
Cubic structure
Hard Mineral 10/10
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Diamond Basics
Diamond Size (Carat = 0.2 gm : Origin Carob seed)
603 ct.119 ct. 83 ct.39 ct. 100 ct. 13 ct.
$12 m
~2 gm
~120 gm
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Diamond Basics
Diamond Size
215 carat29 carat
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Diamond BasicsStability of Carbon Polymorphs (C)
Pre
ssu
re,
Kb
!
Temperature, oC!
43!
!
!47!!
!51!
!
!55!
!
!63!1200 1400 ! !1600!
Graphite!Diamond!
Dep
th,
km
!
120!
160!
200!
Courtesy Maya Kopylova (2011)
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Mantle Basics
Where are the Carbon Polymorphs (C)
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Mantle Basics
Where are the Carbon Polymorphs (C)
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Mantle Basics
OceanContinent
Where are the Carbon Polymorphs (C)
Courtesy Maya Kopylova (2011)
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Kimberlite Basics
Courtesy Maya Kopylova (2011)
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Canadian Kimberlite Bodies & Mines
Courtesy Maya Kopylova (2011)
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Kimberlite Basics
Volcano & Pipe
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A154S
A154NA418
V: 6.2 MCM
SA: 245882 m2
D: 715 m
V: 5.6 MCM
SA: 214621 m2
D: 600 m
V: 5.9 MCM
SA: 206327 m2
D: 435 m
Diavik Kimberlite Mine
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Diavik: A154N/S
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GOAL: A simple mechanism for kimberlite ascent
THE ISSUE
The Problem
Factoids
The Answer
Proof of Concept
A NEW THEORY
Helmstaedt & Gurney, 1995
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BASIC FACTOIDS :
1 ) The primary composition of kimberlite melts
especially volatile contents, remains unmeasured.
FOV = 7.8mm
Volatile enriched
Low η
Fragile
(poor glass formers)
High Reactivity
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BASIC FACTOIDS:
2 ) Olivine xenocrysts show early reduction in size and rounding;
but later olivine crystallization is expressed as overgrowths.
Macrocryst: Core and jacket
FOV = 1 mm
ROUND Core
EUHEDRAL Jacket
FOV = 1 mm
Brett & Russell (2009)
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Peridotitic mantle : OL > OPX > Cpx > Gar (Sp)
BASIC FACTOIDS:
3 ) Kimberlite contains abundant xenocrysts of Ol,
Cpx, Gar, ... BUT .. ORTHOPYROXENE is rare
O
O
OO
OO
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BASIC FACTOIDS :
4 ) Kimberlite magmas have high solids content (> 25%)
and ascend through > 200 km of mantle lithosphere.
Ascent is fast
Ascent is continuous
Buoyancy =Volatiles
Deep seated Volatiles
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P
BASIC FACTOIDS:
5 ) CO2 (+ H2O) solubility in silicic magmas is limited;
precludes extraordinary sequestration of volatiles in melt.
Silicic Melts to Kimberlite
COMPILED IN Brooker et al. (2011)
Volatiles for Buoyancy
W & H 2007 Model
Exsolution > 2 GPa
Fluid-filled crack tips
120 km Cracks?
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• Volatile contents need to be HIGH (at depth)
• How HIGH?
• High enough to induce vesiculation within mantle
at the greatest depths of sampling (see geotherms)
• High enough to support rapid ascent
- preserves diamond
- carries many & large xenoliths (ol +opx+cpx+gar)
- mechanically mills xenoliths & megacrysts
- no early crystallization (overgrowths & phenos?)
Kimberlite - Ascent
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HOW?
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ANOTHER FACTOID:
6 ) CO2 solubility in carbonate melts is limited only by melt
stoichiometry; Na-carbonate melt > 40% dissolved CO2
Brooker et al. (2011)
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• ALL kimberlites start as carbonatic melts
• Carbonatic melts have stoichiometric CO2 contents
• Melts enter and sample cratonic mantle lithosphere
• Peridotite disaggregates - OPX dissolves preferentially
• Carbonate melt + MgSiO3 = EFFERVESCENCE
THE INDUCTIVE IDEA
Assimilation-Induced Foaming
The Mechanism for Kimberlite Ascent
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SiO2
So
lub
ilit
y C
O2 ?
Kimberlite
Field
0%
OPX
Assimilation(rapid & preferential)
CO2 ReleaseChemically Driven Exsolution
Olivine
saturation
30-20%
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DEDUCTIVE TESTS OF IDEA
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OPX
[from xenoliths]
Finely Powdered
(0-40% by Wt.)
NaCO3
[Tf = 851°C]
NEW HIGH-T WEIGHT LOSS EXPERIMENTS
Super-liquidus Conditions at T>1050°C for > 1hour
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Experiments demonstrate:
1) long-term stability of dissolved CO2 (~40 wt%) in
Na-carbonate melt (i.e., no degassing)
2) Opx dissolves rapidly and promotes immediate
effervescence of CO2
3) Amount of Opx introduced dictates extent of
decarbonation
4) Timescales << Ascent rates
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MECHANICAL MIXTURES
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MECHANICAL MIXTURES
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RESULTS: Transient Experiments
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RESULTS: Decarbonation Experiments
Mech
Mix
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RESULTS: Melts from Chemical Mixtures
K
Ol
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1) Carbonate melts enter mantle lithosphere (OPX-rich)
2) Dissolve Opx preferentially (a_SiO2) Opx>>Cpx, Gar, Ol
3) Causes spontaneous deep-seated effervescence
4) Exsolved fluid provides buoyancy
5) Melt is driven to higher SiO2 contents (i.e., Kimberlite)
6) Allows for late saturation of melt with Olivine
7) Continued assimilation = more vesiculation = more cargo
8) ΔP & expansion of fluid adds to (does not drive) buoyancy
Kimberlite Ascent
(THE ANSWER – FINALLY)
Kimberlite: Assimilation-Fueled Ascent of Carbonatite