Reversals cannot be explained by closed-system crystal fractionation. Pressure and f O 2 variations, mixing between magma sheets in a stratified magma chamber or magma chamber replenishments are classically suggested to explain such compositional reversals and mineral disappearances. Magma mixing can also force the hybrid melt to leave the cotectic curve where a cumulus phase was crystallizing. Magma chamber replenishments and magma mixing between primitive and resident magmas are thus able to explain the reversals in mineral compositions plagioclase, olivine, Ca-rich pyroxene and magnetite as well as the temporary disappearance of Ca-rich pyroxene observed at the top of the cyclic units from MCU II.
A substantial decrease of the pressure during crystallization can change the location of the cotectic boundaries in basaltic systems, leading to the temporary disappearance of cumulus phases Cameron, , ; Osborn, Pressure decrease also causes the An content of plagioclase and the Mg-number of ferromagnesian minerals to increase Osborn, ; Panjasawatwong et al. Such a large pressure variation is unrealistic in a subsurface layered intrusion such as Sept Iles.
Oxygen fugacity largely influences the composition of ferromagnesian minerals, as a result of its control on the redox state of the magma.
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Progressive increase of f O 2 during crystallization could thus explain the reversals observed in ferromagnesian minerals within MCU II but cannot explain the reversals observed in plagioclase composition. Chromium reversals at cycle boundaries cannot thus be achieved by f O 2 increase.
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Intense convection in a stratified magma chamber can mix magmas from different compositional layers. Reversals in mineral composition can result from this magma mixing. Tegner et al. The upper boundary of each cycle displays reversals in mineral composition and cumulus apatite disappearance. Before cumulus magnetite saturation, fractional crystallization increases the density of the residual melt and a stratified magma chamber can develop.
When magnetite starts to crystallize, the melt density of the basal layer decreases. When the density becomes equal to that of the overlying layer, magmas from the two layers can mix by breakdown of the diffusive boundary Tegner et al. Reversals thus do not correlate with the appearance of a new cumulus phase, suggesting that no significant change in melt density is expected at the onset of the reversals.
Modes and compositions of plagioclase, olivine, Ca-rich pyroxene and magnetite, as well as the Sr-isotope compositions of plagioclase separates and whole-rock data from the Sept Iles layered series allow us to reach the following conclusions. The Sept Iles layered intrusion crystallized from a ferrobasaltic parent magma.
The sequence of crystallization in the Sept Iles layered series is: plagioclase and olivine, followed by magnetite and ilmenite, then Ca-rich pyroxene and finally apatite. Examination of phase equilibria in ferrobasalt systems Snyder et al. Mineral modes display cyclicity in the layered series; this is particularly well illustrated by the presence of two apatite-rich layers, one in the middle and the other in the upper part of the layered series. Three megacyclic units MCU have thus been identified in the layered series. Each MCU shows the same succession of cumulus assemblages.
MCU I displays a normal and uninterrupted differentiation trend of mineral evolution similar to that observed in smaller layered mafic igneous complexes, such as Skaergaard McBirney, and Kiglapait Morse, This differentiation trend is interrupted upwards by two large and many small reversals in mineral composition, resulting from magma chamber replenishment by undifferentiated primitive basaltic magma Fig.
Intense mixing between the newly injected and resident magma must have occurred to account for the gradual evolution of mineral compositions in reversals. Disappearance of apatite, Ca-rich pyroxene and locally Fe—Ti oxides is associated with these magma chamber replenishments. Schematic model of magma chamber replenishments in the Sept Iles layered intrusion. Six significant magma inputs have occurred during the crystallization of MCU II and led mineral to more primitive compositions. Lawrence River and do not crop out. Bologne, H. Bernhardt, C. Gilson and C. Pirard are thanked for the assistance with X-ray fluorescence, microprobe, Sr-isotopic and laser ablation inductively coupled plasma mass spectrometry LA-ICP-MS analyses.
We thank J. Duchesne for comments and discussions. Constructive reviews by A. McBirney and an anonymous reviewer were greatly appreciated. Google Scholar. Google Preview. One hundred and eighty-seven cumulate samples and four fine-grained mafic samples were selected for geochemical analysis. All the samples c. They were manually crushed with a hammer and milled in agate mortars. Major elements were measured on lithium tetra- and meta-borate fused glass discs, with matrix corrections following the Traill—Lachance algorithm.
Trace elements were measured on pressed powder pellets and data were corrected for matrix effects by Compton peak monitoring. Final purification was realized by HCl leaching of mineral separates to dissolve any grains of apatite, followed by intense cleaning of the mineral powder in ultrapure water and grinding in an agate mortar. Major and trace Sr, Ba element compositions were obtained by XRF on Li-fused glass discs and pressed powder pellets respectively. Olivine samples , Ca-rich pyroxene samples and orthopyroxene 27 samples compositions were obtained with a Cameca SX 50 electron microprobe at the University of Bochum Germany.
Analytical conditions were 15 kV for the accelerating voltage and 15 nA for the beam current. The major element compositions of 26 Ca-rich pyroxene separates were determined by XRF on fused glass discs. Bulk compositions were compared with the average compositions of in situ electron microprobe analyses and differences were found to be negligible. International Fe—Ti oxide reference materials as well as synthetic and in-house standards were used for calibration. Ten samples of ilmenite were separated by heavy liquids.
Strontium isotope compositions were analysed on 20 samples of plagioclase separates and on one whole-rock powder of country rock , by thermal ionization mass spectrometry TIMS at the Royal Museum for Central Africa Tervuren, Belgium.
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The solutions were dried and taken up with 6N HCl. The purity of the dissolutions was carefully checked. From this solution, an aliquot of c. The aliquot was then taken up by 2 ml 2N HNO 3 and loaded into an ion exchange column. A third of the Sr was loaded with phosphoric acid on a single outgassed Ta filament from the GV Sector 54 multi-collector mass spectrometer for measurement.
Analyses were calibrated using 29 Si as an internal standard isotope based on SiO 2 concentrations previously measured by XRF on plagioclase separates. NIST glass was used as the external standard, assuming the composition given by Pearce et al. Mineral modes Supplementary Data were determined in a selection of cumulate samples using two independent methods: point counting and least-squares linear regression.
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Point counting was carried out on polished thin-sections from samples. The data obtained in volume per cent were converted to weight per cent using the density of the minerals calculated from their chemical composition. For samples where mineral compositions have not been determined, densities were calculated using mineral compositions obtained by interpolation from the adjacent samples.
Mineral modes in weight per cent were also estimated using least-squares linear regression from the major element composition of the cumulus phases and whole-rocks samples; Supplementary Data. For samples where mineral analyses were not available, mineral compositions obtained by interpolation between adjacent samples were used. Both methods give consistent results. Oxford University Press is a department of the University of Oxford.
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E-mail: onamur ulg. Oxford Academic. Bernard Charlier. Michael J. Michael D. Jacqueline Vander Auwera.
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