Spatial resolution, stepwise release: connecting the multi-isotope record with microchemistry and petrology

Most metamorphic reactions require dissolution/reprecipitation, i.e. water activity controls petrology in metamorphic minerals, and also the isotope record, as radiogenic isotopes (except 4He) do not diffuse faster than major elements forming the mineral structure [1]. Isotopic inheritance in relicts (i.e. slow diffusion) was observed in zircon, monazite, amphibole, K-feldspar, and micas. However, a priori there could be causes of isotope loss/exchange other than re¬crystallization. Temperature was proposed to play a role by changing diffusivity in geochronometers [2]. If diffusion has been the factor limiting isotopic (or chemical) closure, the concentration profile is bell-shaped. To ascertain if isotope transport in a sample was controlled by diffusion or recrystallization, spatial information is needed: only bell-shaped gradients are compatible with volume diffu¬sion. So far, in-situ dating never described bell-shaped isotope gra¬dients in patchily zoned minerals. On the contrary, patches are certain evidence of fluid-mediated local recrystallization, i.e. a guarantee of petrological, and therefore isotopic, disequilibrium. The geochronology of mixed diachronous phases is managed by two complementary “SR techniques”: spatially resolved analyses and stepwise release. SR techniques have shaped a better understanding of geochronology, as they reveal the elemental & isotopic composi¬tions on the subgrain scale, and allow the recognition and the chemical/ isotopic characterization of relicts and retrogression. In SR techniques, the chemical signature is also measured in the same analysis as the age. Electron microprobe “chemical” geochronology [3] offers the most complete microchemical characterization of intra-crystalline zonations. 39Ar-40Ar analyses yield the concentrations of three elements (K, Ca, Cl) and their ratios to radiogenic 40Ar. Why worry about the Ca/Cl ratio in a mineral, if the age is calculated from the Ar/K ratio? So as to unravel polyphase mixtures by comparing Cl/Ca/K signatures with independent microchemical data. When K-Ar ages of metamorphic minerals are older than we expect, they are brushed off as “excess Ar”. Much confusion can arise if we mix up “excess Ar” (40Ar gain) with “inherited Ar” (40Ar loss). Inherited Ar often correlates with Ca/Cl. In addition to just K-Ar, multi-isotope geochronology (Rb-Sr, Lu-Hf, etc) should be used. If Ar and Sr correlate, it’s due to Ar and Sr inheritance, not to excess Ar [4]. Overdetermined Rb-Sr isochrons [5] also can reveal inheritance. Thanks to submicroscopic petrology, isotopic inheritance can be put into context with petrogenetic disequilibria. Analytical advances allow dating of each mineral generation. This opened up a wealth of data on the P-T-A-X-d history of rocks. In the long run, this will improve our ability to develop credible numeric models. [1] Villa (1998) Terra Nova 10, 42-47 [2] Jäger (1967) Beitr Geol Karte Schweiz 134, 11-21 [3] Williams et al (2007) Ann Rev Earth Planet Sci 35, 137-175 [4] Villa et al (2006) J Volc Geoth Res 152, 20-50 [5] Glodny et al (2008) Geochim Cosmoch Acta 72, 506