Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6436384 | Chemical Geology | 2015 | 13 Pages |
â¢U-Pb dates and HREE indicate monazite recrystallization at UHP.â¢UHP monazites have high Sr and non-radiogenic Pb.â¢High Sr-Eu2 +-Pb may result from feldspar breakdown at high pressure.
Monazite U-Pb and trace-element data were gathered from six high- to ultrahigh-pressure (UHP) samples from the Western Gneiss Region, Norway, using LASS (laser-ablation split-stream ICP-MS) to investigate variations in monazite composition during high-pressure metamorphism. The UHP monazites were found to contain up to 7600Â ppm Sr, 110Â ppm non-radiogenic Pb, relatively weak negative Eu anomalies, and Y concentrations as low as 500Â ppm. Amphibolite-facies monazite that rims the UHP monazite in one sample contains Y concentrations up to 1.6Â wt.%, Sr as low as 13Â ppm, and no detectable non-radiogenic Pb. The UHP monazite composition (high Sr-Pb) is interpreted to result from growth in the absence of feldspar, possibly aided by increased compatibility of Sr-Pb-Eu2Â + in the monazite crystal structure at high pressure. Sr in monazite as a proxy for feldspar stability may be a useful tool not only in studying high-pressure metamorphism, but also in determining timescales of melting and crystallization, when the amount of feldspar changes over time.