P-T-time in high-grade rocks

1
P-T-time in high-grade rocks

• Daniela Rubatto Research School of Earth
  Sciences, The Australian National University,
  Canberra

AIM How to determine the petrological
significance (interpretation) of an U-Pb date in
high grade rocks
2
Content

• Metamorphism and geochronology the challenge
• Importance of U-Th-Pb dating in high-grade rocks
• Strengths and weaknesses
• Behaviour of zircon and monazite during prograde
  metamorphism
• Tools to link age to of P-T conditions
• Example Malenco granulite zircon monazite
• Example Duria peridotite
• Example Reynolds Range granulite

3
Metamorphic evolutions are complex

• Metamorphic rocks can be poly-metamorphic. Need
  to know which stage you are dating
• Rates can be calculated only if two or more
  points on a continuous P-T path are dated.

• Geochronology needs P-T-time correlations

4
Metamorphic minerals are complex (zoned)
GARNET in Zermatt eclogite
ALLANITE in migmatite
ZIRCON in granilite
To resolve this complexity microbeam dating is
the best solution
5
Why U,Th Pb by microbeam

• U,Th-Pb and Ar are the only
• geochronological system that we can
• analyze by microbeam (Ion micropobe or LA-ICPMS)
  
• Ar need a gas mass spectrometer (done via laser
  ablation)
• Other systems have isotopes very similar in mass
  and would require a much higher mass resolution
• 87Sr and 87Rb (versus 206Pb, 207Pb, 208Pb)
• Some isotopes have very low abundances and
  require much higher senstivities
• Sm-Nd chondrite values are 0.15 and 0.46 ppm
  1.5 ppm in garnet
• (Th-U are very incompatible and concentrated in
  accessory minerals at 100s ppm)

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U-Th-Pb needed for high-grade
Fission track apatite, zircon
P
Ar-Ar biotite
Ar-Ar muscovite, amphibole
Sm-Nd Lu-Hf grt, cpx
U-Pb zircon, monazite
High closure T (gt700-900C) allows dating
formation and not cooling ages and thus
high-grade metamorphism
T
600C
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Advantages of using U-Th-Pb

• Mineral containing U are abundant in metamorphic
  and magmatic rocks (zircon, monazite,
  titanite..)
• Robust accessory minerals that preserve zones
  that record the age of different stages or
  events.

Zircon in eclogite
Zircon with 3 growth stages
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Disadvantages of U-Th minerals

• These are accessory minerals zircon, monazite,
  titanite, allanite, baddeleyite, rutile....
• Small in size
• Little known about their metamorphic behaviour
  and in which reaction they participate. They are
  often refractory and do not participate.
• Limited P-T estimates (trace element thermometry)
• Textural relationships are uncertain
• Formation by several processes (metamorphic
  reactions, dissolution-precipitation, melt
  crystallisation, recrystallization, fluid
  percolation....) in a wide range of PT conditions

Link ages to conditions of formation
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U-Pb minerals Zircon
Forms in a variety of P-T conditions from
deep-sea floor alteration and diagenesis to
extreme metamorphism Most robust to chemical and
physical alteration
30µm
Magmatic zircon modified by LT sea-floor
fluids Spandler et al. 05
Rubatto Hermann, Elements 2007
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U-Pb minerals Zircon
Increasing metamorphism of zircon in
high-pressure rocks
Rubatto Hermann, Elements 2007
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U-Pb minerals Monazite (Ce,La,Th,Nd,Y) PO4
Changes with chemical composition!
Large stability field in pelitic compositions ?
on Ca-Al content. Found also in diagenetic
conditions and as detrital Less robust to
chemical and physical alteration than zircon
Corroded monazite in mid-grade metapelite
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U-Pb minerals Monazite
ZIRCON Reynolds Range, Australia (Rubatto et al.
01)
T
detrital core
inh
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How do monazite and zircon form during
metamorphism?

• 1- Dissolution-precipitation from existing zircon
  and monazite
• Crystallization from melt when Zr or LREE
  saturation is reached
• See for example recent works of Kelsey and Clark
  
• Ostwald ripening
• Recrystallization in sub-solidus

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Behaviour during prograde metamorphism
ZIRCON Cooma, Australia (Williams 01)
T
Chemical control
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Behaviour during prograde metamorphism
MONAZITE Cooma, Australia (Williams 01)
T
Melanosome
Metapsammite
sillimanite
Chemical control
Leucosome
Metapelite
inherited
sillimanite
And zone reaction dissolution
Bt zone no reaction
Migmatite Complete new growth
Sil zone Dissolutiongrowth
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Behaviour during prograde metamorphism
Rubatto et al 01
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How do monazite and zircon form during
metamorphism?

• 2- Metamorphic reactions (sub solidus)
• Zr-garnet sill quartz gt
• cordierite zircon (Fraser et al. 97)
• (decompression)
• allanite apatite gt monazite (Bea Montero
  99, Catlos et al. 02, Janots et al. 08.)
• (prograde metamorphism)
• garnetchloritemuscovite gt
• staurolitebiotitequartzmonazite
• (Spear Pyle 02, Kohn Malloy 04)
• (prograde amphibolite-facies metamorphism)

Chemical control (Wing et al. 03, Fitzsimons et
al. 05)
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Do not assume..
Do not assume closure temperatures Do not
assume the conditions at which a mineral
formed RELATE MINERAL ZONE DATED TO METAMORPHIC
CONDITIONS

• Tools
• P-T estimate using the dated mineral
• Chemistry of dated minerals, trace elements
• Textural relationships, Inclusions
• Metamorphic reactions

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Tools for linking age to P-T

• 1. Inclusions and textural relationships
• Inclusions of diagnostic minerals
• Diamond in zircon zircon dates UHP
• Inclusions in major minerals
• Monazite in HT garnet monazite date the HT (??)
• BUT they are not always present
• of difficult interpretation
• accessory can be restitic/refractory

Diamond in zircon
Monazite in garnet
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Textural relationships
core
rim
matrix
Harts Range amphibolite-granulite
garnet
Matrix monazite 480 and 460 Ma
460 Ma
460 Ma
480 Ma
monazite
560 Ma
560 Ma
monazite
monazite
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Textural relationships
Limpopo granulite
Metamorphic zircon
Textural relationships can be very informative,
but accessory minerals are often refractory and
coexistence cannot be proved with textural
relationships only
Inh. zircon
garnet
Buick et al. 06
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Tool for linking age to P-T

• 2. Metamorphic reactions
• Can we put P-T on metamorphic reactions involving
  accessory ? Mainly inferred and maybe directly
  in some cases for monazite/allanite

Finger and Krenn, Lithos 2007
Gabudianu et al.
Mainly in sub-solidus, at HT dissolution-precipit
ation is still more common
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Tool for linking age to P-T

• 3. Trace elements
• Minerals with high
• trace element content

Mineral/chondrite
Zircon/chondrite

• Composition changes
• with paragenesis
• HREE ? garnet
• Eu-anomaly ? feldspars

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Zircon/monazite - garnet trace elements

• Three key observations
• Zircon/Monazite TE composition changes with the
  presence of garnet, thus garnet influences zircon
  chemistry
• Garnet TE chemistry varies during metamorphism
• The diffusion of TE in zircon and monazite is
  irrelevant even at granulite-facies conditions
  (lower than Pb, Cherniak et al. 1997) therefore
  their TE composition will reflect growth
  conditions

Zircon
Garnet
By relating zircon/monazite and garnet TE
composition we can relate formation ages to
metamorphic conditions
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Case study 1Malenco granulite multiple
zircon and monazite growth the importance of
REE composition and petrology
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Val Malenco, Eastern Central Alps
Granulite facies metamorphism due to intrusion of
gabbro
Relics of a Permian crust-mantle section
2
Partial melting in Metapelites gt Grt, Ky-rich
restites
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Malenco P-T path
Peak at T 800-850C P 10 kbar Near isobaric
cooling to T 600C P 8.5 kbar
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Malenco granulite, Alps

• ZIRCON has 3 distinct metamorphic domains

• Zoning pattern
• Age
• 2812 Ma
• 2693 Ma
• 258 4 Ma

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Malenco granulite

• ZIRCON 3 distinct metamorphic domains

LuN/SmN

• Zoning pattern
• Age
• Trace element composition
• Th/U composition
• Inclusions

280 Ma
260 Ma
270 Ma
Th/U
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Accessory trace element composition
Zircon
Mon 1 279 Ma
Mon 2 270 Ma
Mon 3 257 Ma
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Malenco granulite

• GARNET 5 growth zones distinct in trace elements

retrograde subsolidus
prograde melt
subsolidus
melt
No zoning in major elements
Ms Pg Bt Ky
Ms Bt
Supported by -inclusions -texture
Modified from Hermann and Rubatto, 2003
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Correlation zircon-garnet growth

• Trace element partitioning
• Equilibrium between 280 Ma zircon and garnet 1
• Equilibrium between 270 Ma zircon and garnet 2

• In line with
• inclusions

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Malenco temperature-time path
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Case study 2Duria peridotite zircon
recrystallization at subsolidus zircon forming
during decompression (exhumation rates)
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The Duria Peridotite
Europe
Africa (Adria)

• Located in the Central Alps at the convergence
  between Europe and Africa.
• The Central Alps underwent subduction (some
  units) in the Eocene and then Barrovian
  metamorphism in the Oligocene
• The intrusion of the Bergell pluton at 30-32 Ma
  cuts the nappe stacking

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Duria peridotite metamorphism

• Complex metamorphic evolution
• Peak metamorphism at 830C and 28 kbar (gar, cpx,
  opx, ol peridotite)
• Near isothermal decompression at 20 kbar with
  crustal fluid infiltration (spinel, cpx, amp)
• Retrogression to 700C and 7-10 kbar
  (symplectites opx, spn, amp)
• Serpentinization

3
2
1
1
1
1
4
2
4
VITAL to link dating to metamorphism
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Duria peridotite zircon

• Large zircons with 2 distinct domains of very
  similar age
• Domain 1 oscillatory zoned, metamorphic
  inclusions, high trace element contents.
• Domain 2 cross cutting, unzoned, metamorphic
  inclusions, low trace element contents.

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Duria peridotite zircon ages

• Domain 1
• Oscillatory zoned formed by melts or fluids
• Inclusions of amphibole and pyroxene as in the
  recrystallized matrix (lt20 kbar at 800C)
• Ti thermometry in zircon 850C
• Age 34.2 0.2 Ma
• Date decompression
• and crustal fluid influx

• Domain 2
• Irregular unzoned domains formed by sub-solidus
  recrystallization
• Ti-thermometry in zircon 690C
• Poor in REE (lower temperature) and steep HREE
  (garnet breakdown)
• Inclusions of matrix minerals as in the matrix
  (preserved from domain1)
• Age 32.9 0.3 Ma
• Date decompression at symplectitic stage (7-10
  Kbar)

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Duria peridotite P-T-time path

• Rapid exhumation in cm/year (10km/Ma)
• Rapid cooling of 100C/Ma

Zircon does not date the peak nor the rock
crystallization!
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Case study 3Stafford granulite multiple
zircon and monazite growth chemical variations
in monazite linking to garnet growth
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Mt Stafford granulite

• Metasediments (pelites-psammites)
• Metamorphic grade from amphibolite to granulite
  with variable of melt
• Peak conditions 850C and 4 Kbar

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Stafford granulite, Australia
10-15 melt
650
inherited
inherited
20-30 melt
800
T
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Linking age to P-T Monazite REE chemistry
Stafford granulite, central AU
Variations in Eu anomaly indicates feldspar
crystallization HREE variation indicates garnet
growth Variation in Th/U is related to zircon
growth The trace element
composition of accessory minerals can be related
to paragenesis
Rubatto et al. J Petrol 06
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Stafford protracted garnet growth
Monazite and zircon growth related to garnet
growth on the basis of - textures - trace
element composition
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Conclusions

• For metamorphic processes complexity is the rule.
  
• Formation of zircon and monazite (and other
  accessory) can occur over a variety of
  conditions.
• Acquire and age is only the first step.
• A combination of petrology and other
  micro-analytical techniques are required to
  interpret the age in terms of geological events.

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Conclusions