APATITE AS A MONITOR OF VOLATILE ACTIVITY IN METAMORPHISM: THE HP/UHT BARRO-ALTO COMPLEX, GOIAS, BRAZIL AS AN EXAMPLE

Apatite is ubiquitous in metamorphic rocks.  It has been used to infer a variety of intensive/extensive properties including composition of metamorphic fluids, as a measure of distance from intrusions in contact environments, and to document the ingress of hydrothermal fluids. In spite of these varied uses, there are no data that relate Ap chemistry to metamorphic grade for a series of rocks of similar lithology over a wide range of P-T conditions. Here we report initial results of a study aimed in part to document this relationship, which has the potential to record changes in relation to the volatile phase during metamorphism. 

The Barro Alto complex (BA) is one of a group of three layered mafic-ultramafic complexes that make up part of the Neoproterozoic Brasilia Belt. The BA was metamorphosed to high-pressure (~ 8 kbar) and ultra-high temperature (>980^oC) during the Brasiliano Orogeny [Moraes & Fuck, 2000], with metamorphic grade decreasing through the BA and into the amphibolite facies volcano-sedimentary Juscelândia Sequence to the NW. Within the upper layered series of the BA, with increasing metamorphic grade, F in Ap increases (0.49 -> 0.95 pfu; all analyses by EPMA), whereas Cl (0.013 -> b.d.) and OH (0.42 -> 0.05; calc.) both decrease. Similarly, Fe, Mn, Sr, S and Y decrease, whereas Si increases, with metamorphic grade.  Cathodoluminescence reveals zoning with discrete cores in some Ap, but the chemical origin of the zoning is not within the sensitivity of the EPMA; a second kind of zoning is present in Ap, and  consists of increases in F, Fe, S and Sr toward grain edges, with concomitant decreases in Cl, Y and Mg, but these trends may be contradictory. Where Ap is present as inclusions in Grt, it has elevated F, OH, Fe, Sr and Si relative to matrix Ap. In migmatite, Ap in melanosome has higher F (0.64 vs. 0.49 pfu), and lower Cl (0.010 vs. 0.016) and OH (0.36 vs. 0.49). The trends in halogen concentrations in Ap (increase in F/Cl and F/OH, in conjunction with a decrease in Cl/OH) suggest: 1) Ap growth in the presence of a volatile undersaturated environment, followed by a volatile saturated environment, 2) down temperature crystallization of Ap (with Cl and F in a constant concentration reservoir), 3) changing stability of other halogen-bearing phases (e.g. Bt or Hb) during the prograde/retrograde path, or 4) influx of volatiles from an external source.