Monazite geochronology

1. Monazite geochronology *Introduction to monazite *Monazite characteristics *Advantages of the in-situ technique *Examples!

2. 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?)

3. Monazite (cont) • Exchange vectors in monazite • Th, U, Si and Ca controlled by solid solution vectors with end-members: • Huttonite (Th,U) SiO4 viaTh or U + Si ↔ REE + P substitution • Brabanite [(Th,U) Ca]½ (PO4) via Th or U + Ca ↔ 2REE substitution

4. 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

5. 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

6. 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)

7. Increasing Temp = Garnet increases Monazite increases Xenotime decreases High Y garnet cores in equilibrium with xenotime Low Y garnet rims after xenotime consumption Prograde monazite growth at low grade - garnet zone

8. ~463 °C YAG-mnz ~541 °C YAG-mnz 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

9. Monazite growth at the staurolite-isograd • Many studies note marked increase in monazite abundance during prograde metamorphism via of apatite (± allanite) breakdown (LREE’s) 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)

10. 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

11. Prograde monazite growth versus zircon

12. Retrograde monazite breakdown (+H2O) • Breakdown of monazite from high-grade to low-amphibolite facies to form allanite-apatite-epidote-thorite coronas • Requires fluid influx

13. 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

15. 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

16. 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. @ >800 °C • Initial Pb contents very low

17. Monazite “closure temperature”

18. 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)

19. 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!

20. 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

21. In-situ SHRIMPmethodology • 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.

22. 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

23. In-situ monazite U-Pb geochronology and datingfabrics: a cautionary tale from the Committee Bay granite-supracrustal belt GAC-MAC: 2003

24. Western Churchill Province

25. 2002 2001 2000

26. 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)

27. 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

28. 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

29. 2350 ± 8 Ma 1853 ± 8 Ma, 1777 ± 9 Ma “northern domain” samples 2344 ± 6 Ma, 1841 ± 5 Ma SW-56K In-situ monazite geochronology sites

30. In-situ monazite from SW-56K

31. In-situ monazite from SW-56K

32. In-situ monazite U-Pb data from SW-56K

33. 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

34. 2350 ± 8 Ma 1853 ± 8 Ma, 1777 ± 9 Ma “northern domain” samples

35. 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 – XC’s S1/S2, forms cleareuhedral small (<0.5 mm) grains. In textural equilibrium with crd • Crdalso overprints bio-sill S1/S2 fabric • P-T estimates of overprinting assemblage ~4.5 kbar and ~700 °C

36. S2 S2 grt1 S2 crd bio z7260 p sill grtII crd grtII S2

37. cordierite In-situ analysis of monazite inclusions from within type 1 & type 2 garnet, fabric-defining biotite and cordierite S2

38. In-situ monazite U-Pb data from 56O

39. U-Pb zircon data from leucosome

40. 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

41. 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……

42. 1.83 1.87 2.36 2.34 1.85 1.86 1.86 NO penetrative fractures around mnz

43. e.g. Skulski et al 2003 Roddick et al 1992 LeCheminant et al 1987

44. 1.76 1.86 1.86 1.86 BSE

45. 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

46. 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

47. 1.76 1.86 1.86 1.86 BSI SEI 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 • Fluid-mediated processes? • Fluid must have been in equilibrium with enclosing porphyroblast (no chemical zoning of host around fractures) • Fractures in grt must have been open at ca. 1.78 Ga • Porphyroblasts can act as limited “open systems” (e.g. Whitney 1996)

48. 1.76 1.86 1.86 1.86 BSE What does this mean? • New mnz growth?.... volume issue • Volume diffusion of Pb* probably not viable, as most studies indicate Pb diffusion in mnz is v. slow (re closure Temp) • Dissolution and/or re-precipitation processes likely esp. in presence of Ca-rich fluids (e.g. Seydoux-Guillaume et al 2002) • Mnz not assoc. with fractures do not show effects of ca. 1.78 Ga event. NO penetrative fractures around mnz 1.86

49. 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.