Macrocryst Fe - Ti oxides in olivine melilitites from Namaqualand-Bushmanland South Africa Article

Haggerty, SE, Moore, AE, Erlank, AJ. (1985). Macrocryst Fe - Ti oxides in olivine melilitites from Namaqualand-Bushmanland South Africa . CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 91(2), 163-170. 10.1007/BF00377763

cited authors

  • Haggerty, SE; Moore, AE; Erlank, AJ

authors

abstract

  • Ilmenite macrocrysts in olivine melilitites from Namaqualand-Bushmanland, South Africa, have decomposed by subsolidus reduction to form oriented Mg-titanomagnetite along {0001} ilmenite planes. Residual ilmenite contains 10-11 wt% MgO, ∼1 wt% MnO, and ∼0.1 wt% Cr2O3. This macrocryst assemblage is mantled by an annulus of Mg-titanomagnetite, followed by an overgrowth of radiating magnetite + perovskite. Terminal compositions of these magnetites are similar to groundmass spinels, and to the outermost margins of magnetite macrocrysts that have very high Fe3+ core contents. The assemblages are remarkably similar to oxide intergrowths in kimberlites and an upper mantle derivation is proposed for ilmenite macrocrysts in these melilitites. Oxidation states in the source regions are also very similar, whether on-or off-craton, being slightly above FMQ (∼NNO), but reduced to FMQWM with the onset of decompression, volatile loss, and carbonate immiscibility. In the case of the melilitites, late stage, low pressure crystallization above NNO precipated abundant magnetite + perovskite. The oxide fO2 data are consistent with, and refine the fO2 estimates obtained previously for the behavior of Fe/Mg and Ni contents in olivine from the same suite of samples. © 1985 Springer-Verlag.

publication date

  • October 1, 1985

published in

Digital Object Identifier (DOI)

start page

  • 163

end page

  • 170

volume

  • 91

issue

  • 2