Tourmaline compositions and textures: Reflections of the fluid phase

Abstract

Tourmaline uniquely records evidence of its geologic history in its composition and, if properly decoded, provides insight into the geologic environment of formation. Recent studies suggest that tourmaline not only retains chemical information on the host-rock environment, but also provides signatures on the fluid phase with which it interacted. Such signatures are embedded in its major-and minor-element compositions as well as in its isotopes. Some of these elemental signatures are qualitative, while others provide quantitative evaluation of the evolving fluid-phase compositions. Chemical fingerprints of interacting fluids are found as compositional variations in each of the tourmaline structural sites. Boron, a fluid-mobile element, can be used to monitor local release of boron from the breakdown of pre-existing B-bearing minerals or it can provide evidence for infiltration of external fluids. The exchange of Fe3+ for Al in deprotonated tourmalines along the oxy-dravite to povondraite join (O–P trend) is generally consistent with tourmaline formation coexisting with fluids in an oxidizing, saline environment. Fluorine can serve as a marker for F-bearing aqueous fluids, but the crystallochemical constraints associated with vacancies on the X site and the resulting increase in local charge on the Y site must be considered. In some cases, quantitative information derived from Na, Ca, K, and vacancies allow calculation of the compositions of select components in the co-existing fluids. While isotopes provide valuable insights into fluid-related processes, their signatures are beyond the scope considered here. Insights into geological processes in fluid-rich environments can be inferred by the presence of tourmaline as well as by tourmaline compositions. For example, tourmaline forming along shear zones suggests that enhanced permeability accompanied infiltration of Na and B-bearing fluids. Oscillatory-zoned tourmaline from geothermal systems captures chemical, thermal, and mechanical feedback from non-linear fluid behavior at critical conditions in the H2O-system. In other cases, textural, morphological and chemical changes in tourmaline mark the transition from magmatic-to-hydro-thermal conditions during the evolution of igneous systems. Consequently, signatures in tourmaline can be indicators of the fluid-phase composition and, in some cases, provide quantitative estimates of ion concentrations in the fluid in addition to its host rock environment. Capturing these chemical signals of fluids preserved in tourmaline extends its petrogenetic utility.