Boron in granitic magmas: stability of tourmaline in equilibrium with biotite and cordierite |
| |
Authors: | Michael B Wolf David London |
| |
Institution: | (1) School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A. Fax: (405) 325–3140, US |
| |
Abstract: | Experiments at 750 °C, 200 MPa(H2O), a
(H2O)=1, and fO2∼Ni-NiO established that the equilibrium among tourmaline, biotite, cordierite, and melt (± spinel, aluminosilicate, or corundum)
occurs with ∼2 wt% B2O3 in strongly peraluminous melt with an aluminosity, measured by the parameter ASI, of >1.2. The experiments demonstrate the
relationship of tourmaline stability to the activity product of the tourmaline components boron and aluminum, which are inversely
related to one another. Tourmaline is unstable in metaluminous to mildly peraluminous melts (ASI <1.2) at 750 °C regardless
of their boron content. For a given aluminosity, addition of components such as F requires a greater boron content of melt
at this equilibrium. The stability of tourmaline increases with decreasing temperatures below 750 °C. At the inception of
melting, tourmaline breaks down incongruently to assemblages containing crystalline AFM silicates (biotite, cordierite, garnet,
sillimanite), aluminates (spinel, corundum), and B-enriched but Fe-Mg-poor melt. Granitic melts are likely to be undersaturated
in tourmaline from the start of their crystallization, and their initial boron contents will be limited by the abundance of
tourmaline in their source rocks. Quartzofeldspathic (gneissic, metapelitic) rocks that reached conditions of the granulite
facies and still contain (prograde) tourmaline are rare, and probably have never yielded a partial melt. Most leucogranitic
magmas will initially crystallize biotite, cordierite, or garnet, but not tourmaline. With crystallization, the Fe-Mg content
of melt decreases, and the B2O3 content increases until the tourmaline-biotite and/or tourmaline-cordierite (or garnet) equilibria are attained. The B2O3 content of melt is buffered as long as these equilibria continue to operate, but low initial Fe-Mg contents of the magmas
limit the quantity of boron that can be consumed by these reactions to <1 wt% B2O3. Normally, leucogranitic magmas contain insufficient Fe and Mg to conserve all boron as tourmaline and thus lose a large
fraction of magmatic boron to wallrocks. Leucogranites and pegmatites with tourmaline as an early and only AFM silicate mineral
probably contained >2 wt% B2O3 in their bulk magmas.
Received: 6 August 1996 / Accepted: 21 July 1997 |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|