MG-FE2+ PARTITIONING INVOLVING TOURMALINE IN METAPELITIC ROCKS: BRINGING DISORDER FROM CHAOS

HENRY, Darrell J. and DUTROW, Barb L., Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, glhenr@unix1.sncc.lsu.edu and GUIDOTTI, Charles V., Dept. of Geology, University of Maine, Orono, ME 04469-5711

Tourmaline (tur) is a common constituent in metapelitic rocks and is stable from the lowest to highest grades of metamorphism. Furthermore, at any given metamorphic grade systematic intermineral partitioning of Mg and Fe between the tur rims and coexisting mafic minerals (KD = (Mg/Fe)tur/(Mg/Fe)pair) demonstrate that chemical equilibrium is typically attained. For instance, in NW Maine the staurolite zone metapelites attain well-defined KDs of KDtur-grt = 13.94± 0.72, KDtur-bt=1.986± 0.086 and KDtur-chl =1.728± 0.082. Ostensibly, tur should be an ideal member of a partitioning pair to monitor variations in temperature. However, upon examining the apparent temperature dependence of KDs in a series of metapelites an unusual behavior is revealed; the KDs generally increase with temperature up to the staurolite zone and then slightly decrease. This is counter to typical Mg-Fe partitioning between pairs of mafic minerals in that KDs approach a value of 1 as temperature increases, i.e. the coordination polyhedra of the coexisting minerals become more energetically equivalent as temperature increases.

This peculiar behavior can be understood by considering the polar nature and crystallographic constraints of tur. (1) Tur is a polar mineral that, at temperatures below staurolite zone conditions, can exhibit compositional polarity such that different compositions develop at the (+) and (-) poles along the c-axis. There is a tendency for the (+) c-pole to be enriched in Al and Mg relative to the (-) c-pole, with no change in Fe. Compositional polarity results in two possible KDs such that at the (+) c-pole, the pole with the most extensive growth at lower grades, the KD is less than that at the (-) c-pole. Compositional polarity diminishes with temperature such that the KD distinctions will also diminish. (2) Tur has two distinct octahedral sites, Y and Z, that are subject to disordering effects that permit Fe2+ and Mg to exhibit apparent decoupled behavior. Local short-range disordering involving Al and Mg on the Y and Z sites are generally related to local bond valence requirements that develop in conjunction with the incorporation of O2- for OH- at the O(1) site. Consequently, KD will also be a function of disordering associated with Al and O2- incorporation.

tourmaline, metapelite, partitioning, polar, disorder

Full reference: Henry, D. J., Dutrow, B. L. and Guidotti, C. V. (1997) Mg-Fe2+ partitioning involving tourmaline in metapelitic rocks: Bringing disorder from chaos. Geological Society of America Abstracts with Program, 29, 345.

[Back to Darrell Henry home page] [Back to Tourmaline Today]