TOURMALINE AS A GEOCHEMICAL TAPE RECORDER IN METAMORPHIC ROCKS

Tourmaline has an almost unique potential to record and retain geochemical evidence of most of the prograde metamorphic history of the rock in which it forms, analogous to a geochemical tape recorder. Tourmaline is an ideal guide to metamorphism in that it (1) is stable over a wide range of temperatures (50-900ºC), pressures (1 bar to >60 kbar) and metamorphic fluid conditions, (2) develops in many metamorphic rock types, (3) is chemically responsive to changing thermal and baric conditions, metamorphic reactions and metamorphic fluids, and (4) retains any chemical zoning. Growth and compositional variability of tourmaline are subject to several interactive factors including (1) availability of B, (2) bulk composition, (3) local mineral assemblages, (4) composition of the fluid phase, (5) crystallographic constraints, (6) surface energy interactions and (7) PT conditions. The challenge lies in properly interpreting the chemical signal imprinted on the tourmaline so that it can be related to metamorphic processes. With the greatly-improved knowledge of the structure and crystal chemistry of tourmaline, it is now possible to make more robust petrologic and, ultimately, tectonic interpretations using tourmaline. Because tourmaline in metapelitic rocks generally forms as B becomes available, tourmaline growth corresponds either to the breakdown of B-bearing minerals (e.g. clay minerals or muscovite) or influx of B-bearing fluids. Tourmaline segments that were in equilibrium with progressively changing mineral assemblages along the PT path will generally bear a chemical imprint of metamorphic reactions that were taking place during tourmaline growth. Several features that bear on the interpretation of geochemical evolution of metamorphic rocks include (1) development of compositional polarity in tourmaline as a function of temperature, (2) intramineral element partitioning involving tourmaline compositions at opposing ends of the crystal, and its potential for low-temperature geothermometry, (3) intermineral element partitioning involving tourmaline and coexisting minerals, (4) complex chemical zoning in medium grade tourmaline, and their correlation to metamorphic reactions, and (5) chemical signatures that yield unique information on the composition of metamorphic fluids.