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)

Fig 1: The Rogaland anorthosite province showing the various units and particularly the BKSK intrusion with its two main lobes.

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 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 4: Variation of the V2O3 content of ilmenite vs. the stratigraphic succession

in the Bjerkreim lobe.

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.

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.

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.

The QUILF approach The QUILF algorithm (Andersen et al., 1993) obtained in BKSK (Fig. 3) clearly give temperatures (ca. 760°C) which are lower than the expected liquidus temperatures - e.g. in quartz mangerite zircon saturation indicate temperatures in the 880°- 920°C 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 1160°C 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.

Stratigraphic position in the sequence

Rock # NameTemperature

(at 5 kbar)Delta FMQ aSiO2 assemblage

73-77-6Two-pyroxene quartz

mangerite753°± 8°C 0.04± 0.15 1 (quartz)

Mt Usp39 (meas. on isolated grain); Ilm Hem07 (meas. on isolated grain); Augite En20Wo37 (meas.); Opx En26.4Wo1.8 (meas.)

769°C; 4.39 kbar -0,24 1 (quartz)Mt Usp39 ( meas. on isolated grain); Ilm Hem07 (meas. Hem03); Olivine Fo06 (meas.); Augite En15.5Wo36.8 (meas.)

761°C -0,26 1 (quartz) same except Olivine Fo08 and Augite En14

679° ± 13°C -0.25 ± 0.73 0,95

Mt Usp34, measured on isolated grain); Ilm Hem05 (calc.); Oliv. Fo7.6 (meas.); Aug. En16Wo35 (meas.); Pig. En17.5Wo9.6 (meas.); Opx. En17.8Wo1.8 (meas.)

860° (fixed) -0.24 ± 0.09 0.96 (fixed)

Mt Usp34 (meas.); Ilm Hem11 (calc.); Oliv. Fo11 (calc.); Aug. En16Wo35 (meas.); Pig. En17.5Wo12.9 (calc.); Opx. En17.8Wo4.7 (calc.)

80.14.6A Oxide-rich werhlite 909°C -0,43 0,73Mt Usp57 (meas.); Ilm Hem09 (meas. Hem03); Oliv. Fo32 (meas. Fo29); Aug. En31.7Wo39 (meas. En34.1Wo39)

66,216 Oxide-rich werhlite 897°C -0,420.75

(fixed)Mt Usp56 (meas.); Ilm Hem8.4 (calc.); Oliv. Fo29 (meas.); Aug. En31Wo39 (meas.)

80.27.1 Oxide-rich werhlite 895°C 0,32 0,65Mt Usp45 (meas. Usp25); Ilm Hem12 (meas. Hem03); Oliv. Fo50 (meas. Fo45); Aug. En40Wo41 (meas. En42Wo41)

64-82 Oxide-rich werhlite 893°C 0,6 0,63Mt Usp41 (meas.Usp31); Ilm Hem14 (meas. Hem03); Oliv. Fo56 (meas. Fo50); Aug. En42Wo42 (non meas.)

64-23 Troctolite 1036°C 1,27 0,65Mt Usp42 (meas. Usp9); Ilm Hem25 (meas.

Hem14); Oliv. Fo69 (meas. Fo70): Opx En75 (meas. En74)

64-107 Anorthosite 1150°C (fixed) >0.54±0.4 ? Hem18 (meas.)

Base of MCU IV

Mangerite zone

Mangeritic Transition Zone

Noritic Transition Zone

Quartz mangeritic upper part

TII

66-261Olivine quartz

mangerite

Olivine mangerite

The (Eu2+/Eu3+)plag variationThe 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.

The Vanadium behaviour in BKSKThe 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.

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.

Variation of DVmt/ilm in the BKSK intrusionDVmt/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 >10 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.

ConclusionsThe 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.