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Evolution of oxygen fugacity with crystallization
in the Bjerkreim-Sokndal layered intrusion
(Rogaland, Norway)
J-C DUCHESNE, B CHARLIER J VANDER
AUWERA Department of Geology, University of
Liège, Bat. B20, B-4000 Sart Tilman (Belgium)
The (Eu2/Eu3)plag variation The Eu2/Eu3 ratio
is not directly measurable by analytical methods
at the level of concentrations in most rocks and
minerals. An approximate method has been
suggested by Philpotts (1970). Sr having the same
charge and nearly the same ionic radius as Eu2
is used as a proxy for Eu2 after correction
according to the Lattice Strain Model of Blundy
and Wood (1994). Total Eu is measured in pairs of
plagioclase and apatite In BKSK the
(Eu2/Eu3)plag has been calculated in MCU IIIe,
IVe, IVf and TZ for which REE data are available
(Charlier, 2001 Roelandts and Duchesne, 1979).
It varies from 26 at the base of MCUIVe to 162 in
the Transition Zone.
Fig 5 DVmt/ilm vs. (Eu2/Eu3)plag in the Grader
deposit, the Fedorivka layered intrusion and the
Bjerkreim lobe of BKSK. The dashed line is the
overall correlation (with r 0.84). The plain
line refers to the Bjerkreim lobe. Note that
sample 57 greatly influences the slope of the
linear relationship.
The Vanadium behaviour in BKSK The V2O3 content
in ilmenite from BKSK shows interesting variation
in the stratigraphic succession (Fig. 4) (i) in
IA, ilmenite is not a liquidus mineral (but
crystallized from the trapped liquid). It
slightly differs from that in IB where it is a
liquidus mineral (ii) similar contents appear in
IB, IIc, IIIc, IVa and IVc, where ilmenite is the
only oxide mineral (no magnetite). The bulk
DVcum/liq thus remains close to unit and this
leads to a DV ilm/melt 8, following the
cotectic proportions of Duchesne and Charlier
(2005) (iii) in the Bjerkreim lobe, there is a
continuous decrease of V contents from IVc to the
TZ, through IVe and IVf (iv) IVc appears as an
accident in the evolution (v) in the TZ and
overlying units the V contents remains low and
does not vary much.
Fig 1 The Rogaland anorthosite province showing
the various units and particularly the BKSK
intrusion with its two main lobes.
Variation of DVmt/ilm in the BKSK
intrusion DVmt/ilm vs. the V content in ilmenite
is plotted in Fig. 6. The ilmenite content can be
taken as a proxy for the degree of
crystallization of the intrusion, as shown in
Fig. 4. Fig. 6 shows that an overall increase of
DVmt/ilm from values ca. 5 to values gt10 in the
MCU IVf is followed by a clear decrease in the TZ
and the acidic upper part. Where the QUILF
approach has been used to reconstruct the
initial compositions of the oxides,. it can be
seen that, as a first approximation, the effect
of re-equilibration can be neglected (arrow). A
significant difference in the evolution appears
between the two lobes of the intrusion (Fig. 1).
This would mean that the evolution of fO2 was not
homogeneous in the whole magma chamber, possibly
due to different enclosing rocks (anorthosites in
the Sokndal lobe, migmatitic gneisses in the
Bjerkreim lobe). The decrease of the DVmt/ilm
value from the top of MCU IVf to the quartz
mangeritic unit also suggests an increase in fO2
though it might be partly due to the temperature
decrease and to the changing composition of the
melt. A new influx of acidic magma on top of the
layered series has been demonstrated by (Duchesne
and Wilmart, 1997). This influx of a magma at a
higher fO2 than the residing melt might have
risen the fO2 in the melt constituting the quartz
mangerites and also in the underlying mangerite
and TZ cumulates. Conclusions The fO2 evolution
in BKSK is more complex than the classical
decrease characteristics of a close system
crystallization. Several evidence show that fO2
increases in the upper part of the intrusion. The
DVmt/ilm appears correlated to (Eu2/Eu3)plag
and is thus a potential oxybarometer. Our results
tend to show that it is less sensitive to
subsolidus re-equilibration than the major
elements in the Fe-Ti oxide minerals.
Experimental data and other case studies are
needed to strengthen this empirical approach.
The Bjerkreim-Sokndal layered intrusion (BKSK,
Fig. 1) is made up of a Layered Series of
cumulates, organized in several macrocyclic units
(MCU) (Fig. 2). MCU IV comprises a complete
series of rock types with leucotroctolites
passing upwards to olivine-free norites and
gabbronorites. This unit passes to a thin
Transition Zone TZ (in which olivine reappears),
which is itself topped by mangeritic and quartz
mangeritic units.
Fig 2 Cumulate stratigraphy in the Bjerkreim
lobe of the BKSK intrusion. The layered series is
divided in several megacyclic units (MCU)
subdivided into a sequence of zones (a-f),
defined by the presence or absence of certain
index minerals (from Meyer et al. 2002).
Fig 4 Variation of the V2O3 content of ilmenite
vs. the stratigraphic succession in the Bjerkreim
lobe.
The QUILF approach  The QUILF algorithm (Andersen
et al., 1993) obtained in BKSK (Fig. 3) clearly
give temperatures (ca. 760C) which are lower
than the expected liquidus temperatures - e.g. in
quartz mangerite zircon saturation indicate
temperatures in the 880- 920C range (Duchesne
and Wilmart, 1997). The only plausible T- fO2
conditions are those calculated for the
leucotroctolite in MCU IVb which show ?FMQ 1.3.
Interestingly a minimum value ?FMQ 0.54 0.4 is
obtained following Sauerzapf et al. (2008) and
assuming a temperature of 1160C in MCUIVa
(ilmenite being the only Fe-Ti oxide, there is no
subsolidus readjustment with magnetite!).
Nevertheless, at the recorded equilibrium
temperatures, a slight decrease in fO2 is
observed (1.2 log units of ?FMQ) followed by an
increase of 0.2 log units in mangerite and quartz
mangerite.
Variation of DVmt/ilm with (Eu2/Eu3)plag and
with fO2 The relative proportions of the
different Vn ionic species V3, V4 and V5 are
sensitive to fO2 variations. Toplis and Corgne
(2002) have shown that V content in magnetite can
be used as an oxybarometer provided the V content
of the parental magma is known. Duchesne et al.
(2007) have shown that the partition coefficient
of (V3 V4 V5) between magnetite and
ilmenite (DVmt/ilm ) varies with fO2. Large
variations of DVmt/ilm are indeed observed
between two Fe-Ti ore bodies related to
anorthosite massifs. The first is the Grader
deposit (Havre Saint Pierre, Québec) (Charlier et
al., 2008) in which fO2 is estimated at ?FMQ
ca. 1.5 log units (Lattard et al., 2005). The
second ore body is the Fedorivka layered
intrusion (Korosten plutonic complex, Ukraine
(Duchesne et al., 2006). The fO2 is estimated
following the QUILF method and varies from ?FMQ
0.7 to -1.4 log units. DVmt/Ilm measured in
Grader, Fedorivka and BKSK (after correction for
subsolidus re-equilibration) are plotted against
(Eu2/Eu3)plag on Fig. 5. The overall
correlation is a rough linear relationship (r
0.84, Fig. 5). Each massif nervertheless gives
somewhat different trends, Fedorivka and BKSK
showing an increase of both parameters with fO2..
A gross evaluation of the fO2 variation in BKSK
would be ca. ?FMQ 1 at the base of MCUIV to
?FMQ 0 in the transition zone TZ.
Fig 3 Calculation of T and fO2 values close to
liquidus using the QUILF algorithm (Andersen et
al. 1994) and Lattard et al. (2005) approach in
the BKSK intrusion.
Fig 6 DVmt/ilm vs. V2O3 in ilmenite. The
variation in the Sokndal lobe appears different
from that in the Bjerkreim lobe (see text). When
possible (see Fig. 3), the calculated liquidus
value of DV is indicated by an arrow.