DETERMINATION OF HIGH-SPATIAL-RESOLUTION ARGON ISOTOPE VARIATIONS IN METAMORPHIC BIOTITES

The circumstances of preservation of relict metamorphic minerals and f abrics which have subsequently been involved in later metamorphic cycl es remain enigmatic because the kinetic parameters are not well unders tood. Textures and chemistry cannot always give direct information on the conditions attained in the successive events. We show in this stud y that Ar-40/Ar-39 laser studies can, in contrast, provide thermal his tory determinations and successfully discriminate between geological m odels. Two samples have been examined from the Seconda Zona Diorito Ki nzigitica (IIDK) of the Sesia Zone, Italian Alps. A unit which preserv es relict Hercynian metamorphic assemblages and is now located structu rally above rocks which preserve Alpine eclogite and greenschist facie s assemblages. The circumstances of this preservation remain controver sial, with proposed models falling into two main categories. One hypot hesis advocates that the IIDK was juxtaposed into its present position late in the Alpine orogeny and experienced no significant heating eve nt. Alternatively the IIDK has been in its present position for the du ration of the Alpine orogenic cycle and has been exposed to high tempe ratures but due to sluggish reaction kinetics, it failed to reequilibr ate and develop Alpine metamorphic minerals. A high spatial resolution Ar-40/Ar-39 study using the ultraviolet laser ablation microprobe (UV LAMP) technique was undertaken with the aims of reassessing the metamo rphic history of the IIDK and investigating the behavior of argon with in the unit during its thermal evolution. 128 analyses have been obtai ned from ten profiles, between 60 and 275 mu m long, across biotite gr ains which show apparent ages ranging from 514 to 161 Ma within indivi dual crystals. These apparent age variations are systematic according to location in the grain, with the oldest ages at the grain edges and the youngest in the cores. These apparent age variations can not be ex plained by chemical alteration and require diffusion of excess argon i nto the grains, The age profiles show a remarkable fit to theoreticall y derived curves for volume diffusion of argon. Time integrated diffus ion values obtained for the profiles fall between 2 X 10(-6) cm(2) and 5 X 10(-6) cm(2); this variation is interpreted as being due to spati al and temporal fluctuations in the availability of argon from the gra in boundary network. Diffusion, and hence profile development, initiat ed at different times within the biotites, consequently producing the range of time integrated diffusion values. The temperatures at which t he profiles developed have been estimated using published biotite diff usion parameters to model possible thermal histories. Using the assump tions of either a short thermal pulse event or continuous cooling from high temperatures, the models suggest that the profiles developed at temperatures less than 300 degrees C. Possible geological models are p roposed which document the thermal and excess argon history of the IID K from the Hercynian through to the Alpine orogeny. The study shows th e key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages. Copyright (C) 1997 Elsevier Science Ltd.