A metamorphic history from micron-scale 207Pb/206Pb chronometry of Archean monazite

Abstract

Ion microprobe measurements of Pb isotope ratios in monazites have been obtained, in situ, from thin sections using the Cambridge ISOLAB 120. Molecular interferences are sufficiently resolved at an RP of 6500 to allow 2°7pb/Z°6pb dating of monazite with precisions as low as 4-5 Ma (2~r). The results presented here provide important information on the chronological history of the Late Archean metamorphism of the Wind River Range, Wyoming (USA).Matrix monazites and monazite inclusions in garnets from a metapelite from the northern Wind River Range have been analysed by SIMS. In a previous study peak metamorphic conditions (T = 800°C; P = 8 + 1 kb *) were estimated using inclusion assemblages in garnets from this same sample. Isolated monazite inclusions in garnet yield 2°7pb/Z°6pb age estimates of 2781 + 6 to 2809 + 10 Ma. Those along fractures yield lower ages (2603-2687 Ma) which are similar to TIMS and SIMS ages of matrix monazites. A single large (500 /xm) monazite grain locally preserves growth zoning, but has a recrystallised core and a resorbed (recrystallised?) rim. Age estimates for these three regions are 2788 _+ 9 Ma, 2663 _+ 4 and 2523 + 6 Ma, respectively. Thus the inclusion assemblages of Sharp and Essene * may record peak metamorphic conditions at ca. 2.8 Ga, and indicate a phase of metamorphism that predates by over 100 Ma the emplacement of the Bridget Batholith, the major lithologic component of the northern Wind River Range.The analysed monazite grains appear to preserve ca. 300 Ma history, even within a single grain. Monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth, even in high grade terranes where most chronometers are reset. The age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure. Instead, a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis.

Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland

“…Spear & Parrish 1996). This is consistent with the previous studies which show that monazite may preserve ages corresponding to peak temperatures up to granulite facies conditions (DeWolf et al 1993), and may record prograde growth ages (Vry et al 1996).…”

Timing of Proterozoic metamorphism in the southern Curnamona Province: implications for tectonic models and continental reconstructions ∗

“…Fluid-induced recrystallisation of monazite has been documented extensively (De Wolf et al 1993;Braun et al 1998;Crowley & Ghent 1999;Poitrasson et al 2000;Schandl & Gorton 2004). Total replacement of preexisting monazite during recrystallisation preferentially occurs along grain boundaries, within fractures, in inclusion-rich zones, and in fine-grained matrices (Ayers et al 1999;Crowley & Ghent 1999;Carson et al 2004).…”

“…can be interpreted as either growth ages or recrystallisation ages when fluid/rock ratios are interpreted to have been high. Furthermore, pristine monazite often exists when enclosed by silicate phases and in this situation is armoured from fluids even under relatively high fluid/rock ratios (De Wolf et al 1993;Hawkins & Bowring 1997;Braun et al 1998;Crowley & Ghent 1999;Montel et al 2000). Therefore, chemical ages recorded by monazite armoured by silicate phases constrain the maximum growth age of those silicate phases.…”

Ion microprobe U-Pb dating of monazite with about five micrometer spatial resolution

“…The commonest application of the SIMS U-Pb dating has been on zircons with 20~30 µm scale using the Sensitive High Resolution Ion MicroProbe (SHRIMP) instrument built at the Australian National University (Compston et al, 1984;Williams, 1998). The SIMS method has been also extended to several minerals such as monazite (DeWolf et al, 1993), perovskite (Ireland et al, 1990), titanite (Kinny et al, 1994) and apatite (Sano…”