Investigations Into the Behaviour of Argon Isotopes During Metamorphism: Reaction Fronts and Diffusion Profiles

C. S. Pickles Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK

S. P. Kelley Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

S. M. Reddy Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK

J. Wheeler Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK

Within orogenic belts, units are often seen which preserve relicts of earlier metamorphic events. Often it is difficult to assess the reasons why these units fail to show evidence for subsequent metamorphic events. Possible reasons for this effect include: lack of heating during the later orogenic event (perhaps having been brought into a final structural position during late stage juxtaposition) or sluggish reaction kinetics possibly caused by lack of grain boundary fluid. Application of the 40Ar/39Ar laserprobe technique to rocks within orogenic belts may be used to assess the temperature-time history of particular units and resolve some of the above problems. In this study we present data from the Seconda Zona Diorito Kinzigitica (IIDK), the upper element of the Sesia Zone of the Italian Western Alps. This unit represents part of the Austroalpine continental crust and preserves relict Hercynian amphibolite facies metamorphism with typical assemblages consisting of biotite - sillimanite - garnet - plagioclase - quartz. The IIDK is currently situated structurally above units preserving Alpine eclogite and greenschist facies metamorphism but itself shows
only limited locally developed greenschist and blueschist assemblages for which metamorphic temperature estimates are poorly constrained. We aim to constrain the thermochronological history of the IIDK from late Hercynian to Alpine times.

Ultra violet (UV) laser extraction offers a significant advance in the approach to 40Ar/39Ar dating because the technique has two important advantages over other laser methods as; (i) it provides high spatial resolution (<15µm), and (ii) it can be applied to minerals such as quartz and feldspar which are transparent to visible and near infra red lasers. Biotite grains from a Hercynian biotite - plagioclase - quartz pegmatite have been examined in situ by machining rectangular pits (10 x 60µm) at a gradually increasing distance from the grain margin towards the centre of the grain, parallel to the cleavage trace. Grains in contact with both quartz and plagioclase feldspar have been studied. Those grains in contact with feldspar show the following characteristics: (i) Images using SEM atomic number contrast show a change in composition of the grain close to the margins. (ii) Argon isotope profiles show increased 37Ar/39Ar and 38Ar/39Ar ratios toward their edges, reflecting relative increases in the amount of calcium and chlorine with respect to potassium close to the grain margin. (iii) When plotted on a correlation diagram those points at the grain margin, showing enhanced Ca and Cl, fall on a mixing line between atmospheric argon and an excess component, whereas those closer to the centre of the grain plot on a mixing line between the same excess argon component with a 40Ar/36Ar ratio of around 800 and a possible radiogenic component.

It is suggested that the data for the biotite grains adjacent to plagioclase reflect the existence of a reaction front between biotite and plagioclase producing a phase with increased amounts of calcium and chlorine at the interface between the two. The complex intermixing of an excess argon diffusion profile and meteoric fluids associated with late stage alteration is clearly shown in the Ar-Ar data. The incorporation of atmospheric argon at the grain edge overprints a pre-existing excess argon diffusion profile produced earlier in the history of the grain. Modelling the combined alteration/excess argon diffusion profile at a high spatial resolution, together with information from other samples in the IIDK, allows us to constrain aspects of the thermal and excess argon history of the unit, from late Hercynian to Alpine times. Moreover, the presence of a narrow, late stage atmospheric alteration, such that seen in the IIDK sample may explain a range of 40Ar/39Ar age spectra seen in stepped heating experiment on micas with complex metamorphic histories.