APATITE AS A HALOGEN TRACER IN THE MIGMATITE-GRANITE COMPLEX OF SOUTHERN MAINE

Apatite is a minor mineral that plays a major role in fluorine and chlorine cycling in granitic systems. The concentrations of halogens in apatite reflect halogen content of evolving magmatic systems, so apatite may be used as a tracer. This study uses apatite to examine halogen fractionation during the penecontemporaneous parts of the crustal melting process. We use a well-studied sample from a suite of migmatites from southern Maine for this investigation. Migmatites are witness to some part of the melting process, and so understanding their chemical compositions helps us reconstruct the generation, segregation, and transport of crustal magmas.

We analyzed apatite in situ with wavelength dispersive electron probe microanalysis using a Cameca SX5. This study concentrates on apatite from two distinct parts of a single sample from the Devonian migmatite-granite complex, taken in Cumberland, Maine. In the migmatite we identify a leucosome dominated by igneous-texture feldspar and quartz and more gneissose mesosomes, enriched in biotite and muscovite. Previous bulk chemical analysis of the two parts of these migmatite samples suggest that the leucosome represents the early-crystallized feldspar that clogged a thermally decaying melt escape network.

Apatite grains from both parts of the migmatite range from euhedral to subhedral, slightly elongate with diameters ranging from 0.04-0.25 mm. In some samples they occur as clusters of crystals. Multiple apatite grains (total of 115 spot analyses) from the leucosome yield ranges of F and Cl of 3.03-4.98 wt. % (avg. = 3.93 wt. %) and 0.057-0.28 wt. % (avg. = 0.17 wt. %), respectively. Fluorine and Cl concentrations of multiple grains (279 spots) from the mesosome were 3.05-4.93 wt. % (avg. = 4.01 wt. %) and 0.004-0.067 wt. % (avg. = 0.027 wt. %) respectively. The average F/Cl for grains from the leucosome is 28 whereas mesosome grains average 209, similar to the mean value for apatite from two-mica granite from this area (c. 331), which are similarly Cl-poor. The difference in ratios between leucosome and mesosome suggests a variation in melt composition at the different stages of melting and melt segregation in this rock.