PRELIMINARY EXPERIMENTAL DATA ON TRACE-ELEMENT PARTITIONING BETWEEN TOURMALINE AND SILICATE MELT

Tourmaline is a ubiquitous accessory mineral in magmatic deposits, especially those of evolved compositions. It displays a high compositional sensitivity to the composition of its host environment, as well as pressure, temperature and f(O2). These characteristics make tourmaline an ideal monitor of the physical and chemical conditions in its host silicate melt. Of particular interest is its potential to record the trace-element signature of the melt. However, to read this record requires knowledge of the partitioning behavior of trace elements between tourmaline and melt. Here, I present preliminary results of a partitioning experiment between tourmaline and a H 2O- and borosilicate-saturated melt at 800°C and 7.5 kbar, in the presence of B-bearing olivine, hercynitic spinel and ilmenite. Calculated partition-coefficients are invariably close to unity, which shows that tourmaline is unable to significantly fractionate trace elements. Partitioning is systematic and agrees with predictions based on lattice-strain theory, which allows element valence and element site-occupancy to be determined from partitioning data. I conclude that the limited range in partition coefficients results from the diversity of lattice sites in the tourmaline structure, which accommodates trace elements of widely different charge and radius. The composition of tourmaline thus can directly record the compositional signature of its host melt

Intersector element partitioning in tourmaline: a potentially powerful single crystal thermometer

Hourglass sector zoning, and related polar overgrowths, are common features of metamorphic tourmaline, developing as a result of variations in element preference on the different growth surfaces. For sector-zoned crystals, three domains are present for each growth zone (c+, c- and a), with compositional differences most distinct for Ca and Ti, and among c+ and c- sectors. Intersector differences vary, commonly showing decreasing fractionation from core to rim attributed to increasing metamorphic grade. Here we show that intersector element partitioning is temperature dependent and derive empirical geothermometers based on c+-c- and c+-a partitioning of Ca and Ti. These thermometers are applicable over a range of temperatures and bulk-rock compositions. Intersector partitioning is not affected by re-equilibration and records and preserves complete T-histories of individual tourmaline grains from prograde to peak and on to retrograde growth. Information on element mobility is preserved by tourmaline composition, because intersector partitioning is independent of element concentration. These factors make intersector partitioning an ideal tool to elucidate the thermal history of tourmaline grains and thus their host environment and tourmaline's refractory nature preserves these signatures even into the sedimentary record. © Springer-Verlag 2006

Using estimated thermodynamic properties to model accessory phases: the case of tourmaline

Accessory phases and minor components in minerals are commonly ignored in thermodynamic modelling. Such an approach seems unwarranted, as accessory phases can represent a significant element reservoir and minor components can substantially change their host mineral's stability field. However, a lack of thermodynamic data prohibits assessment of these effects. In this contribution, the polyhedron method is used to estimate the thermodynamic properties of tourmaline, a common and widespread accessory phase, stable over a range of P-T-X conditions. The polyhedron method allows Δ H, S, V, Cp and Vm (T, P) properties to be estimated from a linear stoichiometric summation over the fractional properties of its polyhedron constituents. To allow for estimates of tourmaline, fractional thermodynamic properties for B III and B IV polyhedra were derived. Mixing contributions to molar volume were evaluated and symmetrical mixing parameters derived for Al-Mg, Al-Fe and Al-Li interaction on tourmaline's Y-site and T-site Al-Si interaction. Evaluation of the estimated properties using experimental and natural equilibria between tourmaline and melts, minerals and hydrothermal fluids, shows that reliable semi-quantitative results are obtained. The boron contents in fluids coexisting with tourmaline are calculated to within an order of magnitude of measured content, and where anchor-points are available, agreement improves to within a factor of 2. Including tourmaline in petrogenetic modelling of metamorphic rocks indicates that its presence leads to disappearance of staurolite and garnet, among others, and modifies the XMg of coexisting phases, in line with observations on natural rocks. © 2007 Blackwell Publishing Ltd

Boron isotope and light element sector zoning in tourmaline: Implications for the formation of B-isotopic signatures

Hourglass sector zoning in metamorphic tourmaline is known to effectively fractionate the major and trace elements between different sectors, producing three distinct compositions at the growth surface. Here we show that the light elements Li, Be, H and B, as well as the δ11B signature are also affected by sector zoning. The sector enrichment in elements is controlled by a dual process of initial preference due to growth surface charge and morphology, followed by consecutive, charge-balance controlled enrichment. The light elements, especially B and H, appear to act as charge-balance cations in this mechanism, with a preferred B3+ for Si4+ and O2- for OH- substitution in the c- sector. A lighter δ11B signature (Δc+c- = 1.8 ± 0.6 ‰) accompanies this increase in BIV in the c- sector, in line with the lighter signatures for BIV compared to BIII in minerals. However, it exceeds this theoretical fractionation by a factor of 10. Although we have not identified a definite process that is responsible for this fractionation, it may be related to local variations in host medium along the growth surface resulting from preferred uptake of elements. These observations show that δ11B signatures in tourmaline are not an independent measure of its host environment, but also a function of mineral composition, and the 3-dimensional zoning variations herein. © 2007 Elsevier B.V. All rights reserved

TOURMALINE AS A PETROGENETIC INDICATOR MINERAL IN THE HAUT-ALLIER METAMORPHIC SUITE, MASSIF CENTRAL, FRANCE

Tourmaline is a ubiquitous accessory phase in rocks spanning a wide range of P-T-X conditions; owing to its compositional sensitivity to variations in such conditions, it is potentially an ideal indicator mineral. Here, we apply a variety of tools to reconstruct the P-T-X history of the Variscan metamorphic basement of the central Massif Central in Prance using tourmaline compositions and zoning. Tourmaline thermometry, using the temperature dependence of element partitioning among sectors in individual grains, indicates that these grains record and preserve the full metamorphic history of their host rocks, from nucleation at 350°C along the prograde path to a peak at 650°C, and down to 450°C on the retrograde path. Using major- and trace-element compositions, we are able to recognize specific reactions and changes in mineral assemblage and use these to constrain the prograde path of these rocks. Combining P-T constraints with experimentally determined partitioning of Na between fluids and tourmaline provides a record of Na concentrations in the metamorphic fluid for the entire metamorphic path. These results show that tourmaline can provide a fully quantitative record of the P-T-X conditions in its host environment during growth. This is especially valuable for the prograde history, where most of its growth takes place, and for which few other reliable sources of information are available. Tourmaline, therefore, has the potential to become the prime indicator mineral for P-T-X conditions in a wide variety of geological environments

A new method to calculate end-member thermodynamic properties of minerals from their constituent polyhedra I: enthalpy, entropy and molar volume

The thermodynamic properties of silicate minerals can be described as a linear combination of the fractional properties of their constituent polyhedra. In contrast, given the thermodynamic properties of these polyhedra, the thermodynamic properties of minerals can be estimated, where only the crystallography of the mineral needs to be known. Such estimates are especially powerful for hypothetical mineral end-members or for minerals where experimental determination of their thermodynamic properties is difficult. In this contribution the fractional enthalpy, entropy and molar volume for 35 polyhedra have been determined using weighted multiple linear regression analysis on a data set of published mineral thermodynamic properties. The large number of polyhedra determined, allows calculation of a much larger variety of phases than was previously possible and the larger set of minerals used provides more confident fractional properties. The OH-bearing minerals have been described by partial and total hydroxide coordinated components, which gives better results than previous models and precludes the need of a S-V term to improve estimates of entropy. However, the fractional thermodynamic properties only give adequate results for silicate minerals and double oxides, and should therefore not be used to estimate the properties of other minerals. The thermodynamic properties of 'new' minerals are calculated from a linear stoichiometric combination of their constituent polyhedra, resulting in estimates generally with associated uncertainty of < 5%. The quality of such data appears to be of sufficient accuracy for thermodynamic modelling as shown for meta-bauxites from the Alps and the Aegean, where the effect of Zn on the P-T stability of staurolite can be both qualitatively and quantitatively reproduced. © 2005 Blackwell Publishing Ltd

TOURMALINE: AN IDEAL INDICATOR OF ITS HOST ENVIRONMENT

Tourmaline-supergroup minerals are ubiquitous accessory minerals in rocks of the Earth's crust. They can adjust their composition to suit a wide variety of environments, and therefore display a remarkable range in stability in terms of pressure, temperature, fluid composition, and host-rock composition. Because of this compositional sensitivity, tourmaline is an excellent indicator of the environmental conditions in its host. This is further enhanced by negligible diffusion up to high temperatures and a strongly refractory character during subsequent host-rock alteration and weathering, as well as mechanical transport of grains. Whereas most prior research on tourmaline has focused on chemical and crystallographic characterizations and systematics of the tourmaline-supergroup minerals, recent studies are shifting the focus to a quantitative reconstruction of environmental conditions in the host using a combination of structural, compositional and crystallographic characteristics of the tourmaline. This thematic issue, which follows a special session at the 2009 GAC-MAC-AGU meeting in Toronto, highlights these exciting advances; here we discuss some of the obstacles that will need to be overcome to insure the practical applicability of tourmaline. The papers presented in this thematic issue of The Canadian Mineralogist show that we are standing on the brink of a major breakthrough in the use of tourmaline as a quantitative indicator of the chemical and physical properties of its host environment these properties may well make tourmaline the prime mineral for this purpose