Geochronology of Metasomatic Events (Cha-6) In-situ Characterization of Chronometer Phases

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1 Geochronology of Metasomatic Events (Cha-6) In-situ Characterization of Chronometer Phases M.L. Williams, I.M. Villa w/ M.J. Jercinovic Univ. of Mass

2 Monazite: (LREE) PO 4 Common accessory phase in igneous, metamorphic and sedimentary rocks Major REE s: Ce, La, Nd Minor: Y, Si, Ca, Sm, Gd, Pr, Dy, Th : 100 s of ppm => 10 s of wt% U : 10 s of ppm => Several wt% Pb : Very little common Pb (ppb => several ppm)

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8 S1 S2

9 Methods Full-Section Search Mapping Full section Mg - Ce map (Migmatite, Grand Canyon)

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13 Canadian Shield

14 Metasomatism

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16 Legs Lake shear zone N Magnetic anomaly map Cent STZ With structures drawn over

17 East Lake Athabasca - Anhydrous felsic granulite Qtz Grt Kfs + Pl Sil

18 Legs Lake shear zone - Hydrated felsic granulites Qtz Pl Sil Bt Qtz Grt 3 mm

19 Legs Lake shear zone - Hydrated felsic granulites Retrograde assemblage: Grt +Bt + Crd + Sil + Pl + Qtz GPa, C Bt Crd Grt 0.5 mm Sil

20 Pseudosection hydrous felsic granulite GPa C

21 GPa P-T pseudosection for hydrated felsic granulite C

22 Pressure [GPa] Felsic granulites - P-MH2O pseudosection When? M H2O (1.5 wt% H 2 O)

23 Mapping-based approach - Full thin section X-ray maps Monazite distribution in retrograde felsic granulite Mg Ka Monazite Grt Bt

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26 Felsic granulites - Monazite dating results

27 Felsic granulites - Monazite dating results

28 Felsic granulite evolution

29 East Lake Athabasca region - Grease River shear zone

30 Felsic granulites Legs Lake shear zone Grease River shear zone 3 mm 2 mm

31 C S

32 Grease River shear zone - Syn-kinematic monazite mzt

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35 Assemblage: monazite, muscovite, albite, amorphous SiO 2 Reagents: CaF 2 Na 2 Si 2 O 5 Experimental conditions 4.5 kbar, 450 C for a Duration: 16 days. See: Budzyn (2009)

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37 M-Z 5m

38 From Harlov Coupled dissolution-reprecipitation is a well-established chemical reaction, driven by a minimization in the Gibbs free energy. In this process, a mineral phase, in the presence of a reactive fluid, is replaced either by an altered composition of the same phase or by an entirely new phase (Putnis, 2002).

39 Brazil Monazite Pristine Unaltered core Altered domains Oxide MX MX Weight Gr1 core Hi-Th-Rt LoThLo-Bk2 CaO P2O ThO UO Y2O La2O Ce2O Nd2O Pr2O Sm2O Gd2O Dy2O PbO Total

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52 Postsdam Formation (sandstone) 0.4 mm XPL PPL

53 Sample: Potsdam Formation (NY) Northern samples (LM & EC) Lake Ontario Adirondack Massif Eastern samples (PC & WFR) Southern sample (WNR, borehole) 60 miles NY state geological map (R.B. William et al., 1990)

54 Methodology: element mapping XPL Sm La Nd La Detrital core 50 µm Overgrowth Ca Ka Y La Th Ma

55 Low-grade xenotime Low-grade xenotime overgrowth on detrital zircon, more rare than monazite overgrowth Minute grain ~5-10 µm on zircon or along core-rim boundary of monazite Commonly detached from their support; metamorphic minerals filling the gap (chlorite, illite, quartz)

56 Quantitative analysis Full quantitative analysis of homogeneous domains; focus on U-Th-Pb analysis for microprobe age 3-10 analyses in each domain = one age Problem: low-grade monazite are poor in actinides: U < ppm Th ~ % Radiogenic Pb < 500 ppm Large error on age (5 to 20%) Monazite

57 Rosetta monazite! Sm La Nd La 1) Detrital Mnz 2) Deposition (diagenesis) 3) Salinic orogeny? 4) Neo-Acadian orogeny? 5) Alleghenian orogeny? Average chemistry of some key elements (a.p.f.u.) Component % Core Rim 1 Rim 2 Rim 3 Rim 4 Ca La Ce Nd Sm LREE HREE+Y Chr+Thr Rims vs core Ca - - La + + Ce + + Nd + Sm +

58 Age results: Monazite & xenotime rims Each homogeneous domain = one Gaussian probability curve Gaussian curves cumulated for each area (EC+LM = North, PC+WFR = East, WNR = South) D: Deposition time & diagenesis T: Taconic orogeny S: Salinic orogeny Ac: Acadian orogeny nac: Neo-Acadian orogeny Al: Alleghenian orogeny WARNING: possible excess 206 Pb from 230 Th (age < 500 Ma, high Th/U)

59 REE-phosphates: a fluid story Fluid inclusion study suggest increasing fluid salinity from 480 to 300 Ma In few samples, presence of sulfate or carbonate => SO 4 2 or CO 3 2 Leaching of detrital Mnz; S, Cl and C anion complex are good ligand for REE (e.g. Millero 1992; Haas et al. 1995), although ph dependent (higher solubility at low ph) Close relation between monazite rims and K-fsp. K-complex could possibly be used as a catalyst for LREE re-deposition by increasing the ph and diminishing the solubility of REE in fluid

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61 Collaborators: M. Jercinovic, K. Mahan, G. Dumond, S. Regan, J. Allaz (1), J. Baldwin, R. Flowers, S. Bowring (2), Karl Karlstrom (3), and Simon Hanmer (4), C. Kopf (5) 1 University of Massachusetts - Amherst 2 Massachusetts Institute of Technology, 3 University of New Mexico 4 Geological Survey of Canada 5 Mansfield University, PA