Mathematics – Logic
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.3809p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 18, pp.3809-3833
Mathematics
Logic
2
Scientific paper
The circumstances of preservation of relict metamorphic minerals and fabrics which have subsequently been involved in later metamorphic cycles remain enigmatic because the kinetic parameters are not well understood. Textures and chemistry cannot always give direct information on the conditions attained in the successive events. We show in this study that 40 Ar/ 39 Ar laser studies can, in contrast, provide thermal history determinations and successfully discriminate between geological models. Two samples have been examined from the Seconda Zona Diorito Kinzigitica (IIDK) of the Sesia Zone, Italian Alps. A unit which preserves relict Hercynian metamorphic assemblages and is now located structurally above rocks which preserve Alpine eclogite and greenschist facies assemblages. The circumstances of this preservation remain controversial, with proposed models falling into two main categories. One hypothesis advocates that the IIDK was juxtaposed into its present position late in the Alpine orogeny and experienced no significant heating event. Alternatively the IIDK has been in its present position for the duration of the Alpine orogenic cycle and has been exposed to high temperatures but due to sluggish reaction kinetics, it failed to reequilibrate and develop Alpine metamorphic minerals. A high spatial resolution 40 Ar/ 39 Ar study using the ultraviolet laser ablation microprobe (UVLAMP) technique was undertaken with the aims of reassessing the metamorphic history of the IIDK and investigating the behavior of argon within the unit during its thermal evolution. 128 analyses have been obtained from ten profiles, between 60 and 275 m long, across biotite grains which show apparent ages ranging from 514 to 161 Ma within individual 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 explained by chemical alteration and require diffusion of excess argon into the grains. The age profiles show a remarkable fit to theoretically derived curves for volume diffusion of argon. Time integrated diffusion values obtained for the profiles fall between 2 × 10 -6 cm -6 and 5 × 10 -6 cm 2 ; this variation is interpreted as being due to spatial and temporal fluctuations in the availability of argon from the grain boundary network. Diffusion, and hence profile development, initiated at different times within the biotites, consequently producing the range of time integrated diffusion values. The temperatures at which the profiles developed have been estimated using published biotite diffusion parameters to model possible thermal histories. Using the assumptions 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°C. Possible geological models are proposed which document the thermal and excess argon history of the IIDK from the Hercynian through to the Alpine orogeny. The study shows the key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages.
Kelley Simon P.
Pickles C. S.
Reddy Sudhakar M.
Wheeler John
No associations
LandOfFree
Determination of high spatial resolution argon isotope variations in metamorphic biotites *1 does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Determination of high spatial resolution argon isotope variations in metamorphic biotites *1, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Determination of high spatial resolution argon isotope variations in metamorphic biotites *1 will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1225918