 |
| Titel |
Experimental halogen partitioning between earth upper mantle minerals and silicate melt |
| VerfasserIn |
Bastian Joachim, Alison Pawley, Ian Lyon, Torsten Henkel, Ray Burgess, Christopher J. Ballentine |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072659
|
|
|
|
|
|
| Zusammenfassung |
| Owing to their incompatibility, halogens have similar geochemical properties to noble gases
in many systems and may therefore be used as key tracers of volatile transport
processes in the earth. Halogen fractionation may occur during partial melting of the
upper mantle, fractional crystallization or partitioning between immiscible fluids.
Experimental determination of the halogen partitioning behaviour is the basis for
the investigation of the concentration and distribution of halogens in the earth´s
mantle.
High P-T partition experiments were performed in a piston cylinder apparatus using a
model primitive mantle composition proposed by Jagoutz et al. (1979) simplified
to the four components CaO, MgO, A2lO3 and SiO2 (CMAS) according to the
procedure of O´Hara (1968). Defined small amounts of halogens (0.2 wt%) were
added as CaF2, CaCl2 and CaBr2. All experiments were first heated up to 1720Ë
C and then cooled slowly to the target temperature to guarantee growth of large
homogeneous crystals, following the method of Beyer et al. (2011). Pressures range
between 1.0 GPa and 2.5 GPa and final experimental temperatures between 1500Ë
C and 1600Ë C, thus representing partial melting conditions of the earth upper
mantle.
Back-scattered electron images of polished samples show euhedral, almost rectangular
forsterite grains or a mixture of euhedral forsterite and pyroxene grains with a side length of
up to 150 μm, which are embedded in a MORB-like melt.
Electron microprobe analysis reveals a homogeneous major element composition of the
forsterite and pyroxene single crystals as well as of the melt. Halogen mapping, measured via
Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS), shows no concentration
gradients within the minerals or within the melt. These observations suggest that the
experiments were performed at equilibrium conditions.
The fact that we were able to produce large pyroxene and forsterite crystals at equilibrium
conditions in a halogen doped system, which simulates partial melting and fractional
crystallization processes in the earth´s upper mantle, allows us to determine the partitioning
behaviour of fluorine, chlorine and bromine between silicate melts and pyroxene as well as
forsterite using TOF-SIMS. |
|
|
|
|
|
|