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| Titel |
Tourmaline as a petrogenetic indicator mineral: where are we in 2011? |
| VerfasserIn |
Darrell Henry, Barbara Dutrow |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052947
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| Zusammenfassung |
| A once obscure and overlooked mineral, tourmaline has now become a primary contributor to
understanding petrogenetic history of nearly all rock types. In 1985, with the publication of
“Tourmaline as a petrogenetic indicator mineral: an example from the staurolite-grade
metapelites of NW Maine” (Am. Min, v.70, 1-15), tourmaline rose to relevance among the
suite of minerals that could be used to extract information about the rocks in which they
formed. Two of the significant findings were that tourmaline (1) exhibited systematic
element partitioning with other minerals that formed during metamorphism and
(2) inherited a distinctive chemical signature from the rock type that it developed.
Consequently, tourmaline could be an extremely useful petrogenetic indicator mineral, if
sufficient background information was attained so that the chemical signatures
of tourmaline could be properly interpreted. This required a significant amount
of allied investigations of tourmaline in disparate fields. Since 1985 tremendous
advancements have been made understanding of tourmaline including the following:
Crystallographic nature of tourmaline. The general structural formula of tourmaline is now
known to be: XY3 Z6(T6O18)(BO3)3V3W where the common ions at each site
are X = Na1+, Ca2+, K1+ and vacancy: Y = Fe2+, Mg2+, Al3+, Li1+, Fe3+ and
Cr3+; Z = Al3+, Fe3+, Mg2+ and Cr3+; T = Si4+, Al3+ and B3+; B = B3+; V
= OH1-and O2-; and W = OH1-, F1- and O2-. Several crystallographic and
chemical issues influence the stability range and its interaction with its petrologic
environment. (1) Tourmaline exhibits a wide array of heterovalent and homovalent coupled
substitutions. (2) Within the tourmaline structure F1- occurs only in the W site, and (3)
Short-range bond-valence requirements and order-disorder reactions control the type and
crystallographic positions of ions in the structure. With this knowledge a new nomenclature
for the tourmaline supergroup minerals has been established, with 18 approved
species. The extremely low diffusion rate and wide stability range of tourmaline is
likely related to these crystallographic features. In addition, chemical changes in
tourmaline are a function of both crystallographic constraints and the petrologic
environment e.g. F1- incorporation in the W site with different X-site occupancy.
Temperature-pressure-fluid stability range of tourmaline. Tourmaline is known to be stable
over most of the crustal PT conditions. Tourmaline has been found in diagenetic evaporitic
and hydrothermal environments that record temperatures of 900Ë C in experiments and is locally found in granulite-facies rocks. The upper P
stability is high – based on experiments tourmaline is stable up to 7 GPa and it
can be associated with high with high-P minerals such as coesite and diamond.
Within the extensive PT stability range, tourmaline is unstable in environments
that have fluids with high pH and selected aqueous species. Temperature and fluid
determination using tourmaline. Tourmaline is a crystallographically asymmetric
mineral that incorporates different concentrations of cations at opposite poles of
the crystal as a function of temperature. This has been empirically calibrated as a
thermometer that can be used to infer much of the thermal growth history of metamorphic
tourmaline. In addition, experiments have shown that X-site Na and Ca contents can
be related to the composition of the dissolved species in an aqueous fluid phase.
Isotopic signatures in tourmaline. B, O and H stable isotopes have been extensively
used for determination of T as well as the sources of the fluids that are associated
with tourmaline formation. With the advent of very sensitive mass spectrometers,
40Ar/39Ar systematics in tourmaline has been developed as a thermochronometer. |
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