Monazite geochronology

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Monazite geochronology

• Introduction to monazite
• Monazite characteristics
• Advantages of the in-situ technique
• Examples!

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Monazite introduction

• Monazite phosphate LREE (PO)4
• typically dominated by the LREE Ce but all LREE
  may be present in minor amounts
• Most monazite contains Th, U, some HREE, Y, Ca,
  Si and Pb (mostly radiogenic)
• Monazite is widespread as accessory mineral in
• Felsic igneous rocks
• Mid to high grade metamorphic rocks of low Ca
  pelitic composition
• Monazite is useful as geochronometer
• due to crystal structure excl. Pb during
  formation
• crystal structure able to withstand high ? dosage
  and recoil damage due to wt levels of Th and U
  (strong P-O bonds?)

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Monazite (cont)

• Exchange vectors in monazite
• Th, U, Si and Ca controlled by solid solution
  vectors with end-members
• Huttonite (Th,U) SiO4
• via Th or U Si ? REE P substitution
• Brabanite (Th,U) Ca½ (PO4)
• via Th or U Ca ? 2REE substitution

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Monazite stability

• Monazite stability in pelites
• Above greenschist facies commonly present in
  pelitic bulk rock compositions (as low as
  chlorite zone)
• Present in LP contact aureoles, granulite
  migmatites, UHP pelites
• can survive diagenesis
• Metamorphic monazite derived from decomposition
  of
• e.g. allanite, titanite, apatite
• detrital monazite

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Monazite stability and growth cont.

• Many studies note formation of monazite is
  dependant on Ca/Al ratio of the host rock
• Low Ca/Al rocks favoured, monazite uncommon in
  meta-aluminous rocks
• Fitzsimons et al (2005) argues that
• Mnz growth favours intermediate Fe-Mg
  compositions

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Monazite thermometry

• Offers direct link with metamorphic temperatures
  and geochronology
• Monazite-xenotime and garnet-monazite (apatite)
  thermometry are recent calibrations
• Monazite-xenotime based on Y-REE miscibility gap
  between co-existing monazite and xenotime (e.g.
  Heinrich and co-workers)
• Monazite-garnet equilibrium (Pyle et al 2001)
• YAG ap qtz gross plag Y in mnz H2O
• Obvious important tool in rocks with co-existing
  equilibrated monazite, xenotime and garnet
  (apatite)

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Prograde monazite growth at low grade - garnet
zone

• Increasing Temp
• Garnet increases
• Monazite increases
• Xenotime decreases
• High Y garnet cores in equilibrium with xenotime
• Low Y garnet rims after xenotime consumption

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Prograde growth of monazite linking textures
with chemistry of major phases

• Garnet core 2450 ppm Y rim 65 ppm Y
• Core hosts monazite and xenotime
• Rims low-Y (xenotime absent)
• Prograde sequence of growth
• linking textural context with chemistry

After Pyle et al 2001
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Monazite growth at the staurolite-isograd

• Many studies note marked increase in monazite
  abundance during prograde metamorphism via of
  apatite ( allanite) breakdown (LREEs) at the
  st-in isograd via
• Garnet Chlorite Staurolite Biotite
• Breakdown of garnet at staurolite-in isograd P
  at the 100 ppm level
• Chlorite apatite monazite (e.g. Lanzirotti
  Hanson 1996)

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Monazite growth/breakdown after the staurolite
isograd

• Increasing temperature results in further
  decomposition of apatite, muscovite resulting in
  stabilisation of garnet biotite sillimanite
  assemblages ( 550-650 C)
• At the onset of low P granulite facies, monazite
  consumed by partial melting reactions such as
• Sil Bt (mnz) Crd Grt Kfs Melt
• Crystallisation of melt on cooling results in
  precipitation of new monazite

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Prograde monazite growth versus zircon
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Retrograde monazite breakdown (H2O)

• Breakdown of monazite from high-grade to
  low-amphibolite facies to form allanite-apatite-ep
  idote-thorite coronas
• Requires fluid influx

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Linking monazite stability to metamorphic (P-T)
paths

• Examination of textural context with major phases
  and mineral chemistry, sequential monazite
  growth/dissolution patterns and P-T paths can be
  established.
• Recommended reading (for example)
• Pyle Spear (2003) from New England, USA
• Kelsey et al (2007) from Rauer Islands, Antarctica

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Monazite growth and decomposition

• Monazite participants in metamorphic reactions
• The exact role of major metamorphic phases in
  monazite production and breakdown is complex and
  still not well understood
• Fluids and metasomatism not discussed here
• Use of trace elements is a rapidly developing
  field for establishing the interaction between
  major and accessory phases
• The ability to use monazite as a geochronometer
  where textural constraints are available permit
  development of P-T-t paths

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Monazite as a geochronometer

• Contains abundant Th (up to 10 wt, ThO) and U
  (0.5-1.0 wt UO)
• Useful as U-Pb and Th-Pb chronometer using
  SHRIMP, or TIMS techniques
• Widespread use in chemical or total Pb methods
  (e.g. EMP)
• Very low Pb (and other species) diffusion rates
  resulting in high closure temp est. _at_ gt800 C
• Initial Pb contents very low

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Monazite closure temperature
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EMP dating techniques

• EMP variety of methodologies employed but all
  rely of measurement of total Pb
• Limited by ability to resolve low levels of Pb
  against background x-ray spectra, low signal to
  noise, background correction critical and no
  general agreement
• Also problems with x-ray interference
• for example Y-L? with Pb-M? peaks (and if beam
  excites adjacent K phases then K peaks with U
  peaks)

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EMP monazite cont.

• Development of EMP specially configured for
  monazite by Mike Williams at UMass, optimised for
  usage at low acc. voltages (10 Kv, but very high
  current up to 500 nA) limited excitation volume
  in target.
• Improved spectrometers and improved counting
  hardware and software
• Claimed accuracy of 5 Ma!

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High Res. Ion Probe method (SHRIMP)

• Similar methodology as for zircon
• Some compositional effects (matrix effects)
  reported
• e.g. Th contents (Stern Berman 2000)
• Need for compositional matching of std with
  unknowns for high Th monazite
• not always observed (e.g. Rubatto et al 2001)
• Unknown isotope at 204 amu, interferes with
  204Pb measurement
• Thought to be a complex ion proportional to Th
  content
• Effects greatly reduced by energy filtering to
  remove low energy ions

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In-situ SHRIMP methodology

• Selected monazites cored from polished thin
  sections, petrographic context retained (e.g.
  Rayner Stern 2002).
• Cores mounted in a 25mm dia. epoxy disc with
  pre-polished monazite std and Au coated.
• The technique for obtaining age data is largely
  irrelevant (EMP/SHRIMP), it is the in-situ
  approach that offers the critical advantage as
  textural context is retained.

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In-situ U-Pb analysis using SHRIMP

• Small cores from polished thin sections
  containing monazite
• Targets pre-selected by prior SEM and optical
  petrology
• Mounted in SHRIMP epoxy puck together with mnz
  std
• Petrographic context preserved

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In-situ monazite U-Pb geochronology and dating
fabrics a cautionary tale from the Committee
Bay granite-supracrustal belt
GAC-MAC 2003
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Western Churchill Province
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2002
2001
2000
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Committee Bay granite supracrustal belt

• Neoarchean 2.73-2.70 Ga supracrustals (Prince
  Albert Group)
• spinifex-textured komatiite, komatiitic basalts
  and rare pillow basalts interbedded with
  cross-bedded quartzites
• pelites and psammites
• Felsic tuffs and volcanogenic sedimentary
  horizons
• Voluminous 2.61-2.58 Ga tonalitic to
  monzogranitic plutons (U-Pb zircon, TIMS and
  SHRIMP)
• 1.82 Ga post-tectonic Hudsonian monzogranites
  (U-Pb zircon, TIMS and SHRIMP)

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Regional Structure

• dominated by NE-trending regional fabric
• interpreted as transposition S1/S2 fabric
• S1 rarely observed, locally identified in F2 fold
  hinges, and in areas of low D2 strain (e.g.
  SW-region)
• S1/S2 regional fabric - axial planar to
  asymmetric tight upright to overturned F2 folds,
  directed to NW.
• D2 structures locus of Au mineralisation
• S2 deflected by E-striking dextral shear zones

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Committee Bay Integrated Geoscience Project
The northern domain Upper-amphibolite migmatitic
paragneiss, lacks volcanogenic sedimentary
associations of the lower PAG and central domain
The Walker Lake intrusive complex Highly
magnetic, ksp augen 2.60 Ga granodiorites and
1.82 Ga Hudsonian monzogranites
The central domain Greenschist to
mid-amphibolite facies volcano-sedimentary
successions (the Prince Albert Group PAG) assoc.
with voluminous 2.60 Ga felsic intrusives
50km
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In-situ monazite geochronology sites
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In-situ monazite from SW-56K
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In-situ monazite from SW-56K
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In-situ monazite U-Pb data from SW-56K
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Interpretation?

• Matrix grains, aligned in S2 have been reset,
  recrystallised or grown during ca 1.84 Ga event
  (the Trans Hudson event), which may date
  formation of the S2 fabric.

• In absence of other constraints one could argue
  that ca. 2.35 Ga is age of S1 fabric development
  (Arrowsmith event), that is preserved in garnet
  and staurolite porphyroblasts that armour
  monazite from external events

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Petrology of Northern domain pelites

• Sill-bio locally defines outcrop S1/S2 fabric
• Two garnet types
• Type I, inclusion-rich, (pyrite, qtz, bio, sill,
  mz/zirc) corroded by plag and crd. Large
  (1-15mm). Texturally early - wrapped by S1/S2
• Type II, texturally late XCs S1/S2, forms
  clear euhedral small (lt0.5 mm) grains. In
  textural equilibrium with crd
• Crd also overprints bio-sill S1/S2 fabric
• P-T estimates of overprinting assemblage 4.5
  kbar and 700 C

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S2
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In-situ monazite U-Pb data from 56O
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U-Pb zircon data from leucosome
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Results from Northern domain

• U-Pb zircon SHRIMP analysis indicate
• Major episode of zircon growth and/or
  recrystallisation at 1.85 Ga (which collaborates
  with monazite data)
• No zircon ages at 1.78 Ga or clear indication of
  an event at 2.35 Ga

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Interpretation of results from Northern Domain

• Several possibilities exist for the
  textural-chronological evolution of the Northern
  Domain based on these observations
• The following cartoon sequence is one
  possibility

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e.g. Skulski et al 2003 Roddick et al 1992
LeCheminant et al 1987
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Interpretation?

• Monazite of 1.85 Ga within grt1 suggests grt1
  growth must be no older than 1.85 Ga
• S1/S2 must have developed between 1.85 Ga
  (maximum age of grt1) and the age of x-cing dykes
  at 1.82 Ga.
• Consistent with the timing of growth of aligned
  monazite in SW region at 1.84 Ga.
• Major period of monazite and v. low Th/U zircon
  growth at 1.85-1.84 Ga suggests high-grade
  tectono-metamorphism at this time along the
  Committee Bay Belt

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Take home messages from "Northern domain"

• ca. 1.78 Ga monazite aligned within S2 fabric
  from "Northern domain" are unrelated to fabric
  formation

• Mnz inclusions within garnet not connected by
  penetrative fractures to exterior are ca. 1.85 Ga
  or older, whereas, Mnz inlcusions connected to
  exterior show ca. 1.78 Ga disturbance

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What does this mean?

• In order to account for mnz resetting within
  grt (or other) porphyroblasts

also reported by other mnz in-situ studies
e.g Montel et al. 2000 Zhu et al. 1997 DeWolf
et al. 1993

1. Fluid-mediated processes?
2. Fluid must have been in equilibrium with
   enclosing porphyroblast (no chemical zoning of
   host around fractures)
3. Fractures in grt must have been open at ca. 1.78
   Ga
4. Porphyroblasts can act as limited open systems
   (e.g. Whitney 1996)

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What does this mean?

1. New mnz growth?.... volume issue
2. Volume diffusion of Pb probably not viable, as
   most studies indicate Pb diffusion in mnz is v.
   slow (re closure Temp)
3. Dissolution and/or re-precipitation processes
   likely esp. in presence of Ca-rich fluids (e.g.
   Seydoux-Guillaume et al 2002)
4. Mnz not assoc. with fractures do not show effects
   of ca. 1.78 Ga event.

NO penetrative fractures around mnz
1.86
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Further details on the Committee Bay example

• Carson C. J. et al. (2004) Canadian Journal of
  Earth Sciences, 41(9), 1049-1076.
• Berman R. G. et al. (2005) Canadian Mineralogist,
  43, 409-442.