A NEW TYPE OF SECTOR ZONING IN TOURMALINE: INFERENCES AND APPLICATIONS

Tourmaline is a common accessory in magmatic, metamorphic and sedimentary rocks that occurs in bulk rock compositions including carbonates, MORB and pelites. Tourmaline owes this widespread occurrence to its large stability range (near surface conditions to 850°C and 3.7 GPa) and complex crystal chemistry. Tourmaline's complex structure allows interaction with many other minerals as well as (grain-boundary) fluids, and its composition thus represents a combined effect of P, T, paragenesis and fluid composition. The ability of tourmaline to 'sample' these conditions in its host environment has been used with varying success in ore exploration, thermobarometry and isotope studies.

An important feature of tourmaline is the ubiquitous chemical zoning, resulting from slow diffusion rates both parallel and perpendicular to the c-axis of the crystal. The presence of compositional zoning prohibits use of bulk tourmaline compositions and requires accurate information on growth history and equilibrium assemblages to interpret the results. In addition to concentric growth zoning we have identified a new type of sector zoning, which appears to be widespread in metamorphic tourmalines from the French Massif Central, Tauern Window and Syros, Greece.

This new sector zoning is an hourglass structure with three distinct compositional domains; ab, c+ and c- zones, where the latter two result from asymmetry in the crystal structure. These domains record differential uptake of elements on the growth faces and analyses of major and trace elements show that the inter-sector differences exceed growth-zoning variability by a factor of 2. Depending on the sector analysed, compositions can thus vary widely within one growth zone, which has important implications for the interpretation of any chemical analysis.

Since the different sectors grow in the same mineral assemblage and fluid composition, the inter-sector 'Kd' only depends on P and T. This potentially allows the use of sector zoning in tourmaline as a geothermobarometer, without many of the problems of conventional exchange thermobarometers. This type of geothermobarometry combined with growth zoning that preserves the P-T conditions of multiple growth episodes can make tourmaline a powerful tool to delineate the evolution of rocks in a wide variety of geological settings.