Fine-scale isotopic heterogeneities and fluids in the deep crust: a 40Ar/ 39Ar laser ablation and TEM study of muscovites from a granulite-eclogite transition zone

Abstract

Spectacular exposures on Holsnøy Island, western Norway, reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids. Stepwise and laser 40Ar/ 39Ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy (TEM) analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling. Muscovites with 0.10–0.17Na/(Na + K) yield 40Ar/ 39Ar plateau and integrated ages in the range 450–465 Ma, 20–35 Myr older than 40Ar/ 39Ar muscovite plateau ages from adjacent eclogite samples. Laser 40Ar/ 39Ar analyses on ∼ 75 μm spots across two single muscovite grains from these samples are approximately uniform at 444.2 ± 1.9 (21 spots) and 443.3 ± 1.7 Ma (26 spots). However, incremental heating of high sodic (0.18–0.32 (Na/(Na + K)) muscovite failed to yield a plateau. Laser 40Ar/ 39Ar analyses of 100 areas from two single grains from this sample reveal complex two-dimensional patterns with variable apparent ages in the range of 445–625 Ma. Electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to (001). All the TEM bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase, yet several of the selected area electron diffraction (SAED) analyses document locally intergrown muscovite and sodic muscovite, possibly in an exsolution relationship on a unit cell scale. Analytical electron microscopy (AEM) analyses obtained over a 5–100 nm scale on the same sample reveal great range and heterogeneity inNa/K, including pure muscovite, although no separate paragonite domains were found. In contrast, muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inNa/K at the microprobe level. These data provide evidence of variable fluid-mediated incorporation of excess 40Ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions. The data indicate that fine-scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface. Despite the influx of free fluids in the rocks at high pressure, the heterogeneities are best reconciled with local buffering and limited mobility of the fluids, at least during the final stages of equilibration.