Robert G. Berman

Monazite U–Pb and Th–Pb geochronology by ion microprobe, with an application to in situ dating of an Archean metasedimentary rock

Abstract Monazite grains from nine different Precambrian rock samples and having concordant isotope dilution U–Pb and Th–Pb ages were used to develop and evaluate techniques for U–Pb, Th–Pb, and Pb–Pb isotopic dating using an ion microprobe operating at high mass resolution ( R =5500, 1%). An unidentified isobar at ∼203.960 amu complicated the determination of 204 Pb in the monazites with higher Th, but it appeared to have been eliminated when using a slight energy offset. The 207 Pb*/ 206 Pb* ratios of individual spots were determined in 10 min analyses to ±0.3–0.5% (1 σ ) of the true values using a 20-μm-wide O 2 − primary beam, whereas the uncertainty in the mean of several spots on a single generation of monazite was typically ±0.1–0.4% (2 σ ). Instrumental bias in the measured Pb + /UO + and Pb + /ThO + values of individual analyses was monitored using linear relationships between 206 Pb + /UO + vs. UO 2 + /UO + and 208 Pb + /ThO + vs. UO 2 + /UO + . The bias appears to vary somewhat with monazite Th content, requiring the use of compositionally matched standards. Individual spot measurements of the 206 Pb/ 238 U and 208 Pb/ 232 Th ratios in unknown monazites of low to moderate Th can be determined to ±2% (1 σ ). The dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite-grade metapelite of Archean origin. Both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral (e.g., garnet, staurolite, plagioclase). Euhedral monazite grains armoured in low-CaO garnet and matrix plagioclase yielded an age of 2548±17 Ma, interpreted as dating an early episode of low-pressure metamorphism. Partially resorbed monazite within a high-CaO garnet rim yielded ages of ca. 2500 Ma, but are interpreted as inherited from the first event rather than syn-genetic with respect to the growth of this garnet. An age of 1755±30 Ma was obtained for anhedral monazite associated with late, chlorite-filled fractures, apparently reflecting post-tectonic retrogression.

Collisional Snowbird tectonic zone resurrected: Growth of Laurentia during the 1.9 Ga accretionary phase of the Hudsonian orogeny

The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre–1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia.

The stability of tremolite; new experimental data and a thermodynamic assessment

Abstract The equilibria: tremolite + forsterite = 2diopside + 5enstatite + H2O (1) tremolite + 3calcite + 2quartz = 5diopside + 3CO2 + H2O (4) have been reversed experimentally at Pfluid = PH₂O = 0.5 kbar, 1.0 kbar, and 5.0 kbar and at Pfluid = PH₂O + PCO₂ = 5 kbar, respectively. Starting materials consisted of natural tremolite (St. Gotthard, Switzerland) and quartz (Brazil), and synthetic calcite, forsterite, diopside, and enstatite mixed in stoichiometric proportions. Reaction direction was determined by comparing XRD patterns of reactant and product assemblages and by examining surface features of experimental products with an SEM. Our new experimental data for Equilibrium 1 are consistent with the natural-tremolite results of Skippen and McKinstry (1985), who used St. Gotthard tremolite, whereas the new bracket for Equilibrium 4 is ≈25 ℃ lower than that of Slaughter et al. (1975) who also used St. Gotthard tremolite. Comparison of our results with other studies indicates that use of the St. Gotthard tremolite in place of synthetic tremolite in the starting material displaces these equilibria toward higher temperatures by about 25 and 5 ℃. respectively. Tremolite stability differences reflected in these data, as well as m phase equilibrium data for nine additional equilibria involving synthetic and natural tremolite can be accounted for with a simple ideal on-site mixing model to describe tremolite compositional differences. Our analysis leads us to conclude, however, that tremolite growth in some experiments near the equilibrium boundary occurs with respect to metastable end-member pyroxenes used in starting materials, whereas pyroxene-stable half-brackets involve growth of stable pyroxene compositions. Thennodynamic properties for end-member tremolite, retrieved by mathematical programming analysis of the experimental phase equilibrium data with these assumptions, provide the most sound basis for prediction of calcic aniphi- bole stability relationships in natural assemblages, as well as improved calibration of quantitative amphibole geothermobarometers. Our success hi extracting consistent thermodynamic properties for end-member tremolite from experimental data obtained with both synthetic and natural tremolite. assuming the former to contain 10 mol% magnesiocum- mingtonite component (Jenkins 1987), can be taken either as support for the validity of this assumption or as an indication that chain multiplicity faults (Maresch et al. 1994) produce a similar degree of stabilization as this solid solution.