Untitled Document
The redox state of terrestrial basalts determined by
V/Sc olivine-melt partitioning data
Guil Mallmann1, Hugh O'Neill1,
Frances Jenner1, Marc
Norman1, Steve Eggins1, Richard Arculus1 and
Chris Ballhaus2
1 Research School of Earth Sciences, Australian National University,
Canberra, ACT 0200, Australia
2 Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer
Schloss, Bonn 53115, Germany
Figure 1. Partition coefficients
obtained empirically for V and Sc between olivine phenocryst and
silicate melt (glass or matrix). The positive correlation between
is indicative of effects (possibly melt composition) other than oxygen
fugacity. The dashed lines, illustrating values of oxygen fugacity
relative to the QFM buffer, were calculated based on the experimental
partitioning data.
The dependence of the partitioning of V between olivine and silicate
melt () on oxygen fugacity was used to estimate directly the redox
state of primitive terrestrial basaltic and picritic magmas at that
stage in their evolution when they begin to crystallize olivine. The
effect of other variables was accounted for by rationing to ,
because the partitioning of Sc, a redox insensitive element having
approximately the same incompatibility at terrestrial oxygen fugacities,
is shown to depend rather similarly on melt composition.
The method
was calibrated on basaltic compositions equilibrated in the laboratory
(one atmosphere) at QFM and QFM-2.7 between 1300 and 1400°C. We demonstrated
that this method can be effective over the entire range of redox conditions
observed in geological and cosmochemical materials, and therefore may
serve as a universal redox indicator in olivine-phyric mafic volcanic
rocks. Our preliminary assessment indicates accuracy in relative oxygen
fugacity between 0.2 to 0.5 log units, but precision typically better
than ±0.2 log units.
The method was applied to 41 mid-ocean ridge (MORB),
25 ocean island (OIB), and 13 island arc (IAB) recent primitive basalts
and picrites. The data indicate that MORBs and OIBs record a very restrict
range of redox conditions, between QFM and QFM+1, with no clear distinction
between them. However, IABs record consistently more oxidizing conditions,
ranging from QFM+0.5 to QFM+3 (average at QFM+1.7). Except for MORBs,
for which the data cluster exactly on the maximum redox condition ever
reported, the results presented here are in good agreement with previous
estimations using various methods in minerals and melts.