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| Titel |
Electrochemical effects of magmatic crystallisation: cyclic units of the Bushveld Igneous Complex |
| VerfasserIn |
Ilya V. Veksler, David L. Reid, Peter Dulski, Jakob K. Keiding, Robert B. Trumbull |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075668
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| Zusammenfassung |
| The Upper Critical Zone (UCZ) of the Bushveld Igneous Complex displays spectacular
layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence
of silicate cumulates in the order, from the bottom to the top, pyroxentite-norite-anorthosite.
Electron microprobe and laser ablation ICP-MS analyses of chromite and silicate cumulate
minerals in the cyclic units between the UG2 chromitite and the Merensky reef revealed
variations in major and trace element compositions that are difficult to reconcile with existing
models of cumulate mineral-melt evolution. The anomalies in mineral chemistry are best
developed at sharp contacts of chromitites with adjacent anorthosite and pyroxenitic
cumulates. At the contacts, major element characteristics of chromite composition change
abruptly from high and stable Mg/(Mg+Fe2+) and Fe2+/Fe3+ typical for cumulus
chromitites to variable and generally low values in chromite crystals disseminated
in silicate cumulates. Chromites from different types of cumulates also differ in
Sc, V, Ni and Zn contents. The abrupt changes in chromite composition mark the
contacts regardless of the thickness of the chromitite layer and estimated mass
proportions of chromite to intercumulus liquid. Chemical variations, which defy a simple
explanation, are also observed in plagioclase. In addition to previously revealed
inconsistency between chemical trends of cumulus plagioclase and orthopyroxene in the
UCZ cyclic units our study demonstrates that intercumulus, poikilitic plagioclase
cementing chromitite layers has anomalously low Li, K, Rb concentrations and K/La
values.
Summarising previous studies and the new trace element data we propose a model of
post-cumulus re-crystallisation leading to consolidation of a modally layered crystal-liquid
mush into a sequence of nearly monomineral layers of chromitites, pyroxenites and
anorthosites, which defines the cyclic units. The crucial element of the model is the
establishment of redox potential gradients at contacts between chromite-rich cumulates and
adjacent layers dominated by silicate minerals (plagioclase, olivine, orthopyroxene) due to
peritectic reactions between the crystals and intercumulus liquid. Because basaltic melts are
ionic electrolytes with Na+ as the main charge carrier, a redox potential gradient in such
conductive media should induce electrochemical migration of Na+ and other alkali ions.
Preliminary estimations based on the observed variations in ferric/ferrous iron ratios in
chromite from the Bushveld cyclic units show that the redox gradient and the resulting
electrochemical flux of Na+ ions would be sufficient to mobilize about one-third of the total
Na content of a metre-thick mush layer within 10 years. Electrochemical migration of Na is
expected to cause re-melting of previously formed cumulates and major changes in modal
mineral proportions, which eventually may lead to the formation of sharply divided
monomineral layers. The processes is enhanced by the presence of cumulus chromite
but, in principle, it may operate in any type of cumulates where ferrous and ferric
iron species are contrastingly distributed between crystalline and liquid phases.. |
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